Effect of the Phosphine Steric and Electronic Profile on the Rh-Promoted Dehydrocoupling of Phosphine–Boranes

Abstract

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine–boranes H₃B·PR₂H [R = 3,5-(CF₃)₂C₆H₃; p-(CF₃)C₆H₄; p-(OMe)C₆H₄; adamantyl, Ad] and H₃B·PCyH₂ to form the metal-bound linear diboraphosphines H₃B·PR₂BH₂·PR₂H and H₃B·PRHBH₂·PRH₂, respectively, are reported. Reaction of [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] [L = Ph₂P(CH₂)₃PPh₂, Ar^F = 3,5-(CF₃)₂C₆H₃] with 2 equiv of H₃B·PR₂H affords [Rh(L)(H)(σ,η-PR₂BH₃)(η¹-H₃B·PR₂H)][BAr^F₄]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η²-PR₂·BH₂PR₂·BH₃)][BAr^F₄]. With electron-withdrawing groups on the phosphine–borane there is the parallel formation of the products of B–P cleavage, [Rh(L)(PR₂H)₂][BAr^F₄], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H₃B·P(Ad)₂H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine–borane complex is formed, [Rh(L){η²-H₃B·P(Ad)₂H}][BAr^F₄], that undergoes B–P bond cleavage to give [Rh(L){η¹-H₃B·P(Ad)₂H}{P(Ad)₂H}][BAr^F₄]. The relative rates of dehydrocoupling of H₃B·PR₂H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P–B bond cleavage. H₃B·PCyH₂ undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P–H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.

Short abstract

The effect of changing the steric and electronic profile of the phosphine of primary and secondary phosphine−boranes H₃B·PR₂H (R = H, aryl, adamantyl) on the Rh-catalyzed dehydrocoupling to form metal-bound linear diboraphosphines is reported.

Introduction

The development of efficient catalytic methods for the formation of bonds between main group elements is of considerable interest for the continued development of main group chemistry. Such processes enable new discoveries to be made in the promising application areas that main group species are now occupying, such as high performance polymers, emissive materials, etch resists for lithography, and precursors to ceramic thin films or devices.¹⁻⁶ However, the development of this field lags substantially behind the advances made in catalytic C–C and C–X bond formation, for which there are now a myriad of efficient ways to promote such unions that are important for the construction of new molecules. Catalytic dehydrocoupling^5,7,8 of amine– and phosphine–boranes is one method that has emerged for the formation of B–N and B–P bonds, and development in the area has been spurred on by the potential for ammonia–borane to act as a hydrogen carrying vector.⁹⁻¹¹ In addition, polymeric materials that can arise from dehydropolymerization of primary analogues are also of significant interest as they are valence isoelectronic with technologically ubiquitous polyolefins. Although the metal catalyzed formation of polyaminoboranes has attracted recent attention,¹²⁻¹⁸ catalytic routes to polyphosphinoboranes have also been known since 1999.¹⁹ Perhaps the best example is that of the [Rh(COD)₂][OTf] catalyzed dehydrocoupling of secondary, H₃B·PR₂H, and primary, H₃B·PRH₂, phosphine–boranes to give oligomeric and polymeric materials (Scheme 1).¹⁹⁻²¹

Scheme 1. Phosphine–Borane Dehydrocoupling.

In contrast to amine–borane dehydrocoupling,^{8,10,15,22−24} the mechanism of catalytic dehydrocoupling of phosphine–boranes has received less attention. Although initial reports demonstrated that catalysis using [Rh(COD)₂][OTf] was a homogeneous process (i.e., not colloidal),²⁵ there has been only sporadic further work on elucidating the mechanistic details.²⁶⁻²⁹ Progress has no doubt been slowed due to the fact that the reaction conditions reported for phosphine–borane dehydrocoupling often require melt conditions, thus making interrogation of the catalytic cycle problematic. Recently, we have reported that the Rh(I) complexes [Rh(P^tBu₂H)₂(η⁶-FC₆H₅)][BAr^F₄],³⁰ and [Rh(L)(η⁶-FC₆H₅)][BAr^F₄],³¹ [L = Ph₂P(CH₂)₃PPh₂] are particularly well-suited to the study of the dehydrocoupling mechanism of secondary phosphine–boranes in solvents such as fluorobenzene; and on the basis of the observation of intermediates, kinetic studies, and H/D exchange experiments we have proposed a catalytic cycle for the dehydrocoupling of H₃B·PR₂H (R= Ph, ^tBu; Scheme 2). For this cycle, intermediate species were isolated, but their structures could not be confirmed by X-ray crystallography. In particular for R = Ph, a β-B-agostic σ complex B, and the product of dehydrocoupling F, that is proposed to sit off cycle, could be isolated and spectroscopically characterized. Under stoichiometric conditions the observation that B transforms into F on gentle heating allowed for kinetic parameters to be determined that suggested that the rate-determining step(s) for dehydrocoupling were located within the transformations B to D. In solution phase the turnover limiting step for catalysis is proposed to be the displacement of the linear diboraphosphine product (i.e., F to A), although under the melt conditions used for efficient catalysis this may well be different. Further insight comes from the observations that for R = ^tBu the barrier to dehydrocoupling is higher (70 °C versus 25 °C for reaction), P–H activation appears also to be a higher energy process, different intermediates (A and E) are observed, and the turnover limiting process in catalysis is now suggested to be the P–H activation/dehydrocoupling steps. Prior work has demonstrated a similar difference in relative rates of dehydrocoupling of secondary H₃B·PR₂H [R = p-(CF₃)C₆H₄, Ph, ^tBu, ⁱBu] and primary H₃B·PRH₂ [R = Ph, ^tBu, ⁱBu] phosphine–boranes using the [Rh(COD)₂][OTf] catalyst, and this was suggested to be due to a combination of steric and electronic (relative P–H bond strengths) factors,^21,32,33 although the mechanism of dehydrocoupling of phosphine–boranes using this catalyst is currently not known.^20,25,30 Interestingly, the related dehydrogenation of aryl amine–boranes shows that the activity of the N–H bond is such that spontaneous dehydrocoupling occurs in the absence of catalyst, with electron-withdrawing aryl groups [p-(CF₃)C₆H₄] undergoing faster reaction than electron-donating [p-(OMe)C₆H₄].³⁴ Very recent work has shown that paramagnetic Ti(III) centers might also be involved in dehydrocoupling of phosphine– and amine–boranes when using Cp₂Ti-based catalysts,³⁵ while oligomerization of base-stabilized phosphino–boranes at Cp₂Ti centers has been described.²⁹ Likely decomposition routes in Rh-systems for phosphine–borane dehydrocoupling to form bis(phosphine)boronium salts have also recently been discussed.³⁶

Scheme 2. Proposed Catalytic Cycle for the Dehydrocoupling of H₃B·PR₂H To Give H₃B·PR₂BH₂·PR₂H.

[BAr^F₄]⁻ anions are not shown.

In this Article, we report an extension of our investigations into the mechanism of phosphine–borane dehydrocoupling using the {Rh(Ph₂P(CH₂)₃PPh₂)}⁺ fragment, by varying the electronic and steric profile of the secondary phosphine–boranes H₃B·PR₂H [R = 3,5-(CF₃)₂C₆H₃; p-(CF₃)C₆H₄; p-(OMe)C₆H₄; adamantyl], as well as investigations with the primary phosphine–borane H₃B·PCyH₂. Dehydrocoupling forms the corresponding metal–bound linear diboraphosphines H₃B·PR₂BH₂·PR₂H and H₃B·PRHBH₂·PRH₂, respectively. These studies provide insight into the determining role of the electronics and sterics of the phosphine–borane in the dehydrocoupling process, as well as providing as yet unreported examples of the solid-state structures of the intermediates related to the catalytic cycle. We also report for the first time the partial control of diastereoselectivity in dehydrocoupling of primary phosphine–boranes, that can additionally be biased by use of a chiral chelating phosphine on the rhodium center.

Results and Discussion

Phosphine–Borane and Diboraphosphine Starting Materials

A range of secondary phosphine–boranes with differing electronic and steric properties have been used in this study (1, 2, 3, and 4, Figure 1), which also provide comparison with the previously reported Ph, 6, and ^tBu, 7, analogues.³¹ The primary phosphine–borane 5 has also been used.³⁷ Compounds 2(33) and 3(38) are known adducts and offer electron-withdrawing and donating aryl groups, respectively. Bis-CF₃-substituted 1 is a new complex and offers an alternative to 2. The synthesis of adamantyl-substituted phosphine, 4, an analogue of 7, has been reported in the patent literature.³⁹ Compared with the ^tButyl group, adamantyl has a greater steric bulk due to its larger volume and rigid structure.^40,41 The new linear diboraphosphines, 10–13, have also been synthesized to aid in the identification of final dehydrocoupling products. Complexes 10–12 are synthesized by a Rh-catalyzed process from the corresponding phosphine–boranes, while primary phosphine containing 13 has been synthesized in good isolated yield (85%) by addition of [NBu₄][BH₄] to the bis(phopshine)boronium [(CyH₂P)₂BH₂]Br.³⁶

Figure 1.

Phosphine–boranes 1–7 and diboraphosphines 8–13.

Stoichiometric Dehydrocoupling of Secondary Phosphine–Boranes

Addition of 2 equiv of 1 to [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] [L = Ph₂P(CH₂)₃PPh₂] in 1,2-F₂C₆H₄ solution at 25 °C rapidly (on time of mixing) resulted in the formation of [Rh(L)(H)(σ,η-PR₂BH₃)(η¹-H₃B·PR₂H)][BAr^F₄], 14 [R = 3,5-(CF₃)₂(C₆H₃), Scheme 3], which was characterized by NMR spectroscopy, ESI-MS (electrospray ionization mass spectrometry), and single crystal X-ray diffraction. Likewise, the use of 2 equiv of phosphine–borane 2 or 3 results in the formation of the analogous complexes 15 [R = p-(CF₃)C₆H₄] and 16 [R = p-(OMe)C₆H₄], respectively, which were fully characterized using solution techniques. All these complexes proceed to dehydrocouple (vide infra), and only for 14 was an analytically pure crystalline solid obtained. Even so, dissolution of crystalline material of 14 resulted in the observation of small amounts (approximately 5–10%) of the associated dehydrocoupling product in the solution NMR spectra after short periods of time. Complexes 15 and 16 could only be isolated as oils, but their characterization by NMR spectroscopy and ESI-MS was fully consistent with their formulation.

Scheme 3. Synthesis of Complexes 14, 15, and 16.

[BAr^F₄]⁻ anions are not shown.

The solution NMR spectra for 14, 15, and 16 are very similar to those previously reported for [Rh(L)(H)(σ,η-PPh₂BH₃)(η¹-H₃B·PPh₂H)][BAr^F₄] (i.e., B, Scheme 2(31)), and data for 14 is discussed in detail. The ³¹P{¹H} NMR spectrum of 14 shows four different phosphorus environments. Two of the resonances are broadened significantly compared to the other two, suggesting these phosphorus atoms are bound to a quadrupolar boron center. One of these shows both a large trans PP coupling [J(PP) 244 Hz] and coupling to ¹⁰³Rh [J(RhP) 75 Hz], while the other is a broad singlet. The other two signals are sharper and are assigned to the two ³¹P environments of the Ph₂P(CH₂)₃PPh₂ ligand. One of these sharper resonances [δ 29.5, ddd, J(RhP) 130, J(PP) 35, J(PP) 21 Hz] is assigned to the phosphorus atom trans to the weakly bound β-B-agostic interaction on the basis of the larger ¹⁰³Rh coupling constant, while the other signal [δ 11.3, ddd, J(RhP) 103, J(PP) 244, J(PP) 35 Hz] is assigned to the phosphorus atom trans to the coordinated phosphido ligand. In the ¹H NMR spectrum of 14 one broad, relative integral 3H, signal is observed at δ −0.78, indicative of a Rh···H₃B σ interaction in which the B–H bonds are undergoing rapid site exchange on the NMR spectroscopic time scale between terminal and bridging sites.⁴² A broad, relative integral 1H, resonance at δ −6.12 is assigned to a static β-B-agostic B–H interaction. Cooling of the solution to 0 °C led to the resolution of this signal as doublet [J(PH) = 65 Hz], fully consistent with its trans disposition to a phosphine. The remaining BH(terminal) signals are not observed, and it is likely they are coincident with the {CH₂}₃ signals. A sharper signal at δ −16.21, relative integral 1H, is assigned to a metal–hydride resonance, in which the coupling to both ¹⁰³Rh and ³¹P is clearly small and unresolved. The PH group is observed at δ 5.81 that collapses into a singlet in the ¹H{³¹P} NMR spectrum. The ¹¹B NMR spectrum shows a broad signal centered at δ −39.8, which is not shifted significantly from that of free phosphine–borane 1 (δ −42.0). This is assigned to a coincidence of the η¹ β-B–H···Rh agostic and σ Rh···H₃B signals, as has been noted previously.^31,43 Complexes 15 and 16 have similar ¹H, ¹¹B, and ³¹P NMR spectra, and thus we assign very similar structures.

Crystals of complex 14 of suitable quality for analysis by X-ray diffraction were obtained by layering of a 1,2-F₂C₆H₄ solution with pentane at −26 °C. The structure of 14 in the solid-state (Figure 2) is fully consistent with the structure deduced from the solution NMR spectroscopic data. The formally Rh(III) center adopts a pseudo-octahedral geometry, with the chelating phosphine ligand and the hydride located on one of the faces of the octahedron. Two of the three remaining coordination sites are occupied by a phosphine–borane unit that has undergone P–H activation, and is bound to the metal via a phosphido bond [Rh1–P3, 2.3045(10) Å] and a β-B-agostic bond [Rh1–B1, 2.515(4) Å]. The other phosphine–borane unit occupies the last coordination site via a σ η¹-Rh···H–B interaction.⁴² All the hydrides (B–H and Rh–H) were located in the final difference map. The structure is in full accord with the solution NMR spectroscopic data, confirming the spectroscopic assignments that have been made previously.³¹ β-B-agostic interactions are known,^35,44,45 and we have recently reported [Rh(κ¹,η-PPh₂BH₂·PPh₃)(PPh₃)₂][BAr^F₄] in which a base-stabilized phosphine–borane adopts a β-B-agostic interaction with the Rh-center.³⁶ σ phosphine–boranes are also known,^42,46,47 and bimetallic complexes showing both B-agostic and σ borane coordination modes have been reported.⁴⁸ Compared to a Rh(I) complex that shows a bidentate η²-coordination mode for the σ borane, [Rh(P^tBu₂H)₂(η²-H₃B·P^tBu₂H)][BAr^F₄],³⁰ the Rh···B distance for the η¹-interaction in 14 is considerably longer [2.188(3) Å versus 2.740(4) Å, respectively], consistent with this different binding motif. Similar changes in M···B distance have been noted on moving between η¹ and η² coordination modes in chelating phosphine–boranes.⁴³

Figure 2.

Molecular structure of the cation of 14. Displacement ellipsoids are drawn at 50% probability level. Some hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh1–P1, 2.778(10); Rh1–P2, 2.3163(9); Rh1–P3, 2.3045(10); P3–B1, 1.913(4); P4–B2, 1.918(4); Rh1···B1, 2.515(4); Rh1···B2, 2.740(4); Rh1–P3–B1, 72.54(14); Rh1–B2–P4, 121.3(2).

Complexes 14–16 undergo spontaneous dehydrocoupling (25 °C) to form products of the general formula [Rh(L)H(σ,η²-PR₂·BH₂PR₂·BH₃)][BAr^F₄]: 17, R = 3,5-(CF₃)₂C₆H₃; 18, R = p-(CF₃)C₆H₄; 19, R = p-(OMe)C₆H₄ (Scheme 4). This process also results in the liberation of H₂ (observed, ¹H NMR spectroscopy). For 17 and 18 there are additional products formed, assigned as [Rh(L)(PR₂H)₂][BAr^F₄], 21 and 22, respectively, on the basis of NMR spectroscopic data. These complexes are formed in parallel to 17 and 18, as preformed 17 (vide infra) does not proceed to form 21. Complex 21 has been independently prepared by addition of two equivalents of HP((CF₃)₂C₆H₃)₂ to [Rh(L)(η⁶-FC₆H₅)][BAr^F₄].

Scheme 4. Dehydrocoupling of Complexes 14–16.

[BAr^F₄]⁻ anions are not shown. Time = 6 h 17/21, 18/22 (25 °C); 8 h 16/19 (35 °C).

This mixture of products observed for the electron-withdrawing phosphine substituents (i.e., 1 and 2) contrasts with that found for when R = Ph³¹ and p-(OMe)C₆H₄, which yield the dehydrocoupled (e.g., 19 and F, Scheme 2) product in essentially quantitative form (∼95% by ³¹P{¹H} NMR spectroscopy). Complex 17 has been synthesized cleanly from direct addition of the preformed dehydrocoupled diboraphosphine product, 10, to [Rh(L)(η⁶-FC₆H₅)][BAr^F₄], Scheme 5. It was from this reaction that material of 17 suitable for single crystal X-ray diffraction was obtained.

Scheme 5. Synthesis of 17 by Direct Addition of the Linear Diboraphosphine 10.

[BAr^F₄]⁻ anions are not shown.

Figure 3 shows the solid-state structure of 17, in which the diboraphosphine acts as a chelate to the Rh(III) center, via a phosphido group and two B-agostic interactions: [Rh(L)H(σ,η²-PR₂·BH₂PR₂·BH₃)][BAr^F₄] [R = 3,5-(CF₃)₂C₆H₃]. All the hydride ligands (B–H and Rh–H) were located in the final difference map. The Rh(III) center has pseudo-octahedral geometry, in which the oligomeric phosphine–borane is bound tridentate to the metal through η²-BH₂···Rh [B2–Rh1, 2.280(5) Å] and phosphido [P3–Rh1, 2.3925(10) Å] interactions. The hydride ligand is positioned trans to one of the B–H···Rh interactions. The Rh···B distance is considerably shorter than those observed in 14, consistent with the η²-bidentate binding mode of the borane. This distance is similar to others reported for chelating phosphine–borane complexes with Rh.⁴⁹⁻⁵²

Figure 3.

Molecular structure of the cation of 17. Displacement ellipsoids are drawn at 50% probability level. Some hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh1–P1, 2.3241(11); Rh1–P2, 2.2650(11); Rh1–P3, 2.3925(10); Rh1···B2, 2.280(5); Rh1–P3–B1, 110.88(15); B1–P4–B2, 107.5(2).

The NMR spectroscopic data for 17 are fully consistent with the solid-state structure being retained in solution and are also very similar to that reported for the analogous complex formed from the deydrocoupling of 6 (R = Ph).³¹ The ³¹P{¹H} NMR spectrum shows four different phosphorus environments. Two of these signals are well-resolved and show coupling to ¹⁰³Rh, δ 46.6 [J(RhP) 111 Hz] and δ 12.8 [J(RhP) 91 Hz], and are attributed to the chelating phosphine ligand. One of these signals (δ 12.8) also shows large ³¹P–³¹P coupling [J(PP) 260 Hz] suggesting a trans position relative to the phosphido center. The other two environments are broad, typical of those observed when coupling to a quadrupolar boron center. For one of these trans J(PP) coupling is also observed. The ¹H NMR spectrum shows three different broad, relative integral 1H, environments assigned to the BH₃ moiety [δ −4.54, −1.20, and 4.37]. This indicates that the BH₃ unit is not undergoing exchange on the NMR spectroscopic time scale, as noted previously for similar η²-M···H₃B systems.^31,43,50,52 The Rh–H signal is observed at δ −13.98 as a sharper signal, although this also shows unresolved coupling. The ¹¹B NMR spectrum shows two different environments [δ −27.1 and 0.21] for the two boron atoms present in the diboraphosphine, with the latter assigned to the η²-H₃B unit on the basis of the large downfield shift from free ligand (Δδ = +36.8).⁴³ Spectroscopic data for complexes 18 and 19, that are produced by the direct dehydrocoupling route are similar, although for 18 this is also formed as a mixture with 22.

The dehydrocoupling reaction (i.e., 14 to 17) shows a dependence on the substituents on the phosphine. For electron-withdrawing aryl groups (e.g., p-CF₃), it is faster when compared with electron rich groups (i.e., p-OMe). Following these processes in situ using NMR spectroscopy demonstrated that these dehydrocoupling reactions follow a first order rate profile for the consumption of the starting material over at least three half-lives (see Supporting Information): 1 3 h (25 °C); 2 3 h (25 °C); 6 14 h (25 °C);³¹3 8 h (35 °C), ∼120 h (25 °C). That the parallel products 21 and 22 are formed in approximately equal ratio to the dehydrocoupled product (17, 18, respectively)) suggests that k₁ ≈ k₂ (Scheme 4). In addition to this parallel process, direct comparison of the rate constants is further complicated by the fact that 16 → 19 required heating to 35 °C to make the reaction run over a convenient time scale for analysis by NMR spectroscopy. Nevertheless these relative rates reflect previous observations on the rate of catalytic dehydrocoupling when the electronics of a system are changed, in as much as electron-withdrawing groups promote the reaction.²¹ Interestingly, for all the aryl complexes initial P–H activation to form a phosphido hydride complex (i.e., 14) is very rapid, occurring on time of mixing. This suggests that for aryl-substituted phosphine–boranes it is not initial P–H activation that is rate-determining for the dehydrocoupling event, as we have commented on for R = Ph.³¹ In this study we suggested that B–H activation/reorganization in intermediates such as B (Scheme 2) prior to P–B bond formation might be the rate limiting process.³¹ This might well be promoted by a weaker B–H bond, and calculations on analogous H₃B·L (L = Lewis base) systems show that the B–H bond is considerably weaker when there are electron-withdrawing groups on the Lewis base.⁵³ However, we cannot rule out that the relative P–H bond strengths in intermediates such as 14 also might play a role, or that there is a change in the rate determining step on changing the phosphine–borane ligand, as the intimate details of the mechanism leading to P–B formation still remain to be resolved. The observation that for an electron-withdrawing phosphine there is a significant proportion of parallel product formed that results from P–B bond cleavage is consistent with the weakening of the P–B bond with increasingly electron-withdrawing aryl substiutents.^8,54 P–B bond cleavage has been noted previously in σ phosphine–borane complexes to give either simple adducts⁴⁷ or further reaction to yield bis(phosphine)boronium salts.³⁰

Prior to the formation of the parallel product 21 (R = 3,5-(CF₃)₂C₆H₃) an intermediate is observed that has been characterized by ¹H and ³¹P{¹H} NMR spectroscopy as [Rh(L)H(σ,η-PR₂·BH₃)(PR₂H)][BAr^F₄] 20, i.e., a complex that sits directly between 14 and 21 by loss of one “BH₃” fragment (Scheme 6). Complex 20 results from P–B bond cleavage, formally of the σ-H₃B·PR₂H ligand, to afford a complex with a β-B-agostic interaction from a phosphide borane ligand (as for 14) and a simple PR₂H ligand trans to a hydride. Complex 20 was not isolated in pure form, being observed alongside 14 and the final products 17/21. However, after 2 h reaction a significant proportion of 20 is present (∼20% by ³¹P NMR spectroscopy), allowing for its identification aided by comparison with the NMR spectroscopic data for 14 (Supporting Information). In particular four environments are observed in the ³¹P NMR spectrum, with only one of these broadened significantly by coupling to quadrupolar boron. This signal also shows a large, mutual, trans J(PP) coupling with another phosphine environment. In the high-field region of the ¹H NMR spectrum a broad doublet is observed at δ −7.06 [J(HP) = 76 Hz] which is assigned to the β-B-agostic interaction, while there is a relatively sharper one at δ −9.61 [J(HP) = 165 Hz] assigned to Rh–H, and again ¹⁰³Rh coupling is not resolved. These assignments were confirmed by ¹H{³¹P}, ¹H{¹¹B}, and ¹H/³¹P correlation experiments.

Scheme 6. Formation of the Parallel Products 17 and 21 from 14.

[BAr^F₄]⁻ anions are not shown.

Addition of 2 equiv of the bulky and electron rich phosphine–borane H₃B·P(adamantyl)₂H, 4, to [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] in 1,2-F₂C₆H₄ solution at 25 °C rapidly results in a color change from orange to purple and the formation of the new σ bound Rh(I) phosphine–borane complex [Rh(L)(η²-H₃B·P(adamantyl)₂H)][BAr^F₄], 23, which was characterized in situ by NMR spectroscopy. This complex could not be isolated as it undergoes further reaction, by P–B bond cleavage at room temperature, to form 24 (Scheme 7). Addition of 1 equiv of 4 resulted in a final mixture of 24 and [Rh(L)(η⁶-FC₆H₅)][BAr^F₄].

Scheme 7. Synthesis of Complex 24 by Direct and Indirect Routes.

[BAr^F₄]⁻ anions are not shown.

The ¹H NMR spectrum of complex 23 immediately after preparation shows a broad, relative integral 3H, signal at δ −1.36 characteristic of a σ-bound phosphine–borane that is undergoing site exchange between the coordinated and uncoordinated B–H environments.⁴² Two signals are observed in the ³¹P{¹H} NMR spectrum, in a 2:1 ratio at δ 35.1 [J(RhP) 167 Hz] and δ 30.1 (br). Over time (1 h), complex 23 disappears to be replaced by a new complex that has been characterized by NMR spectroscopy and a solid-state X-ray diffraction experiment as [Rh(L)(PHR₂)(η¹–H₃B·PHR₂)][BAr^F₄] (24, R = adamantyl). Figure 4 shows the structure of the cation present in 24 in the solid-state. A Rh(I) center is in a pseudo-square-planar geometry with a chelating ligand, and the other two coordination sites are occupied by P(adamantyl)₂H and a η¹-H₃B·P(adamantyl)₂H [Rh···B, 2.457(7) Å] ligands, respectively. The BH and PH hydrogen atoms were located in the final difference map. The solution NMR spectroscopic data for 24 are fully consistent with the solid-state structure, and in particular the trans disposition of P1 and P3, and the η¹-H₃B·PR₂H ligand.

Figure 4.

Molecular structure of the cation of 24. Displacement ellipsoids are drawn at 50% probability level. Some hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Rh1–P1, 2.2262(16); Rh1–P2, 2.2861(16); Rh1–P3, 2.3568(15); Rh1···B1, 2.457(7); B1–P4, 1.936(7).

A significant amount of P–B bond cleavage product is thus observed for both electron poor aryl phosphine–boranes (e.g., 14) and very bulky electron rich phosphine–boranes (e.g., 24), but not the electron rich aryl phosphine 3 or H₃B·PPh₂H (6).³¹ Interestingly we have recently reported that for H₃B·P^tBu₂H P–B bond cleavage is also observed during dehydrocoupling catalysis being accompanied by a further dehydrocoupling step, through which bis(phosphine)boronium salts are ultimately formed.^30,36 Similar complexes can be prepared on rhodium using H₃B·PPh₂H and PPh₃ under stoichiometeric conditions.³⁶ One suggested mechanism for this process is the reaction of a short-lived phosphino–borane (or its masked equivalent) with coordinated phosphine, not dissimilar to the mechanism suggested for the formation of diaminoboranes from amine–boranes and amines catalyzed by alkaline earth catalysts.⁵⁵ Complexes 20 and 24 serve as models for intermediates in this process [Rh(III) and Rh(I), respectively], although we do not observe the formation of corresponding bis(phosphine)boronium salts in this case.

Stoichiometric Dehydrocoupling of Primary Phosphine–Boranes

The dehydrocoupling of primary phosphine–boranes can yield polyphosphinoboranes, rather than the simple oligomers observed with secondary phosphine–boranes (Scheme 1). With an appreciation of the intermediate metal complexes formed with secondary phosphine–boranes from this and previous work,^30,31,36 it was of interest to explore whether the proposed dehydrocoupling mechanism for secondary phosphine–boranes using [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] could be applied to primary analogues. Such insight into the mechanism of dehydropolymerization of phosphine–boranes is important, as these processes currently remain unresolved due to the melt conditions employed that make following intermediates or kinetics problematic.^20,28,33

In situ investigations using stoichiometric quantities of primary phosphine–boranes H₃B·PPhH₂ resulted in immediate reaction when combined with [Rh(L)(η⁶-FC₆H₅)][BAr^F₄], but a number of products were formed which we have not been able to convincingly characterize. This mixture of species observed is in contrast with H₃B·PPh₂H where single products are formed analogous to 14–16.³¹ However, reaction of [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] with a slight excess of H₃B·PCyH₂ (5) in 1,2-F₂C₆H₄ solution at 25 °C led to the instantaneous formation of only two complexes in a 1:1 ratio, 25a and 25b, [Rh(L)H(σ,η-PCyH·BH₃)(η¹-H₃B·PCyH₂)][BAr^F₄], as a proposed diastereomeric pair (Scheme 8). This stereoisomerism comes from P–H activation at the prochiral primary phosphine. These new products are directly analogous to those formed with secondary phosphine–boranes (i.e., 14), and the NMR spectroscopic data match closely. The ³¹P{¹H} NMR spectrum from this reaction shows 8 resonances, in addition to a broad peak at δ −35.5 due to excess phosphine–borane, as each diastereomer contains four distinct phosphorus environments. Signals centered at δ 31.7 and 30.5 are assigned to one of the chelating phosphine ligand ³¹P environments in each diastereoisomer, and show characteristic J(RhP) coupling constants consistent with a Rh(III) center. Complex overlapping multiplets at δ 11.8 [2 × ddd] represent the resonances for both diastereomers of the second chelated phosphorus center, which is trans to the phosphide position, displaying a large trans PP coupling constant [J(PP) ∼ 200 Hz] in addition to coupling to ¹⁰³Rh and cis-³¹P. The remaining 4 signals are broad indicating the phosphorus centers are bound to a quadrupolar ¹¹B nucleus. Of these, peaks at δ −11.0 and −32.1 are assigned to the phosphide centers of each diastereomer trans to the chelating phosphine [J(PP) ∼200 Hz], and resonances at δ −39.8 and δ −44.2 as assigned to phosphorus centers in the σ-bound phosphine–borane unit. These large differences in chemical shift of the phosphido signal (Δδ 21.2) might reflect significant local difference in steric pressure between 25a and 25b at this group. Interestingly, a much smaller difference is observed with the dehydrocoupled products (26a/b, Δδ 3.5) in which the phosphide group is part of a chelate ring. The ¹H NMR spectrum does not have the necessary resolution to separate out the diastereomers in the hydride region, with broad resonances observed at δ −2.3 (3 H, BH₃), δ −7.9 (1 H, Rh–H–B), δ −17.5 (Rh–H).

Scheme 8. Synthesis of 25a, 25b, and the Dehydrocoupled Products 26a and 26b.

[BAr^F₄]⁻ anions are not shown.

Complexes 25a/b cannot be isolated in pure form, and characterization by NMR spectroscopy is best performed on freshly prepared samples, as after 1 h (25 °C) they have undergone dehydrocoupling to give a mixture of two resolvable diastereomers 26a and 26b, with one of the diastereomers present in a significantly larger amount ∼6:1 (Scheme 8), indicating that the dehydrocoupling step occurs with some stereocontrol.⁵⁶ The decomposition product [Rh(L)(PH₂Cy)₂]⁺, analogous to 21/22, was also observed. NMR spectroscopic and ESI-MS analysis suggests that the dehydrocoupling products formed are direct analogues of 17. This mixture of diastereomers can also be synthesized cleanly by direct reaction of [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] with the preformed diaboraphosphine H₃B·PCyHBH₂·PCyH₂ (13) in 1,2-F₂C₆H₄ solution at 25 °C (Figure 5 for the solid-state structure). Immediate measurement of the ³¹P{¹H} NMR spectrum after mixing showed clean conversion to complexes 26a and 26b in an approximate 1:1 ratio, interestingly different from the 1:6 ratio observed from dehydrocoupling.

Figure 5.

Molecular structure of 13. Displacement ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (deg): P1–B1 1.9267(18), B1–P2 1.9381(18), P2–B2 1.926(2); P1–B1–P2 108.34(9), B1–P2–B2 113.32(9).

Resonances in the ³¹P{¹H} NMR spectrum of 26 can, again, be assigned aided by reference to those of structurally characterized 17. Peaks centered at δ 37.9 and 34.5 result from the chelated phosphorus trans to the B-agostic site, while the signals for the phosphorus trans to the phosphido group overlap at δ 10.7, and display characteristic J(PP) trans coupling [255 Hz]. The broad resonances of the diboraphosphine are observed at δ 19.8 and 16.2 for the phosphido center [J(PP) 255] and δ −14.9 and −16.6 ppm for the remaining site. The high-field region of the ¹H NMR spectrum of 26a/26b shows a slight downfield shift of the Rh–H hydride resonance to δ −16.1, when compared to 25a/25b, while the η²-BH₂···Rh units are observed as two broadened resonances at δ −2.98 (1H) and δ −5.98 (1H). For these hydride signals the separate signals are not resolved for each diasteroisomer, although each resonance is rather asymmetric suggesting two overlapping environments.

A ³¹P{¹H} NMR spectrum taken of this mixture after 18 h at 25 °C showed a significant change in the ratios of the diastereomers 26a/26b (Scheme 9). The peaks for one isomer at [δ 34.5, 16.2, 10.7, and −14.9] have reduced relative area, giving an approximate ratio of 6:1 for the two diastereoisomers. This ratio is similar to that found from direct dehydrocoupling in 25a/25b after 1 h (vide supra), underscoring the stereocontrol occurring in the P–B bond forming process. Leaving this solution for one week resulted in no significant change to this ratio, suggesting equilibrium had been reached. We suggest that the mechanism for equilibration involves reductive elimination of the phosphido and hydride ligands to form a Rh(I) σ phosphine–borane complex,³⁰ similar to E in Scheme 2, which then undergoes rapid oxidative addition of the other P–H bond. This must be a reversible process, leading to a thermodynamic ratio of the diastereoisomers and the resulting selectivity. Unfortunately we were unable to deduce the stereochemistry of the preferred isomer using ROESY experiments or a solid-state structure. However, inspection of models leads us to propose that the thermodynamic product is likely to have the cyclohexyl group pointing away from the chelating phosphine ligand’s phenyl groups, i.e., 26b. That these diastereoisomers are a result of the metal activation of the prochiral terminal P–H bonds in 13 is shown by addition of an excess of dppe to 26a/b.⁵⁶ This affords [Rh(dppe)(L)][BAr^F₄]³¹ with the concomitant formation of free 13 (Scheme 9).

Scheme 9. Change in Diastereoisomeric Ratio and Release of the Diboraphosphine.

[BAr^F₄]⁻ anions are not shown.

We have briefly explored the use of a chiral metal/ligand fragment in dehydrocoupling, [Rh(BDPP)]⁺ [S,S-BDPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane]. This chiral ligand was chosen as electronically and sterically (i.e., bite angle) it is similar to Ph₂P(CH₂)₃PPh₂. Addition of H₃B·PCyH₂, 5, to [Rh(BDPP)(η⁶-FC₆H₅)][BAr^F₄] results in the immediate formation of two diastereoisomers of [Rh(BDPP)H(σ,η-PCyHBH₃)(η¹-H₃B·PCyH₂)][BAr^F₄], 27, in a 3:1 ratio (Scheme 10). Although we are unable to comment on the absolute configuration of these isomers, it is interesting to note that this is biased away from the 1:1 ratio observed in the achiral system. Compounds 27a/b proceed on to dehydrocouple to form diastereoisomers of [Rh(BDPP)H(σ,η²-PRH·BH₂PRH·BH₃)][BAr^F₄], 28, 1:5:3:0 ratio. The same mixture of diasteroisomers can be formed by direct addition of 13 to [Rh(BDPP)(η⁶-FC₆H₅)][BAr^F₄]. Initially a 2:1:2:1 ratio of 4 isomers is observed, that changes to a 5:1:3:0 ratio after 18 h. We are unable to comment in more detail on the conformation of these isomers, although the observation of stereocontrol in the direct dehydrocoupling is similar to that observed for the achiral system. Addition of excess dppe to this mixture forms a product identified by ESI-MS as [Rh(BDPP)(dppe)]⁺ and free 13 (by ³¹P and ¹¹B NMR spectroscopy). We have not explored whether there is enantiocontrol at the central {PCyH unit} arising from this PB coupling event on release from the metal.

Scheme 10. Use of a Chiral Ligand in Dehydrocoupling.

[BAr^F₄]⁻ anions are not shown.

For these experiments with H₃B·PCyH₂ it is interesting to note that P–H activation is rapid and reversible with the Rh(I) precursor. This is in contrast to results obtained with secondary phosphine–boranes H₃B·P^tBu₂H and H₃B·P^tBu₂BH₂·P^tBu₂H, which on addition to [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] gave the corresponding Rh(I) σ-phosphine–borane complexes with no P–H activation.³¹ Such selectivity for primary over secondary phosphines in P–H activation at a metal center has been described previously for both phosphine⁵⁷ and phosphine–borane ligands.²⁷ In particular it has been shown that addition of H₃B·PPhH₂ to Pt(PEt₃)₂H(PPh₂·BH₃) results in exchange of the metal bound phosphide complex to give the primary phosphido–borane complex.²⁶ Here it was suggested that the greater thermodynamic driving force for formation of the primary phosphido–borane complex comes from steric effects, as M–P bonds to smaller primary phosphido ligands are likely to be stronger.

Catalytic Dehydrocoupling of Secondary Phosphine–Boranes

Under the standard catalytic melt conditions (90 °C, 5 mol %),²⁰ [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] will dehydrocouple the secondary aryl phosphine–boranes used in this study to form the corresponding linear diboraphosphines 10–12, although we have not explored in detail the temporal evolution of these systems due to the problems associated with directly interrogating the melt. However, trends can be observed. For electron-withdrawing groups (1 and 2), complete consumption of starting material occurs in 4 h (Table 1). The reaction at this temperature is not selective, and although the main product is the linear diboraphosphine, there are products that we tentatively identify as the cyclic oligomers (BH₂PR₂)_n (n = 3, 4).^20,33 Our results are broadly in line with the previously reported catalyzed dehydrocoupling of 2 using [Rh(COD)Cl]₂, which, at a slightly lower temperature (60 °C, 16 h, melt), affords the linear diboraphosphine product in 69% isolated yield, while at 100 °C only the cyclic oligomers are isolated. The mechanism of formation of the higher cyclic oligomers, (BH₂PR₂)_n, remains to be resolved.²⁰ For electron-donating 3 the reaction is slower using the [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] catalyst (8 h) but overall is more selective. For R = Ph we have previously shown that [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] catalyzes dehydocoupling to give the corresponding linear diboraphosphine in greater than 95% conversion after 4 h.³¹ For secondary phosphine–boranes, H₃B·PPh₂H thus offers balance between overall rate and selectivity.

Table 1. Conversion of H₃B·PR₂H with Time^a.

H3B·PR2H	time/h	H3B·PR2H/%	H3B·PR2BH2PR2H/%	(BH2PR2)n/%
1	1	10	55	<5
	4	<5	45	10
	8	<5	35	50
2	1	10	70	10
	4	<5	70	15
	8	<5	70	15
3	1	50	30	<5
	4	30	45	5
	8	20	60	5

^aR = aryl, see Figure 1. [Cat.] = [Rh(L)(η⁶-FC₆H₅)][BAr^F₄], L = Ph₂P(CH₂)₃PPh₂. Conversions calculated from ³¹P{¹H} NMR spectra. Conditions: [Rh(L)(η⁶-FC₆H₅)][BAr^F₄], 5 mol %, 90°C, melt.

Given the product distributions and likely decomposition pathways in the melt it is inappropriate to comment in detail on the nature of the rate-determining steps during catalysis under these conditions. However, on the basis of the solution studies, P–B bond formation, (dehydrocoupling) is faster with electron-withdrawing groups. The temporal differences in observed product conversion in the melt could reflect a difference in the rate of the P–B bond forming event, or alternatively, they could reflect the ease at which the bound product is substituted on the metal center, i.e., a turnover limiting step. To probe this latter scenario, reaction between 19 (aryl-OMe) and diboraphosphine 11 (aryl-CF₃) to form 18 and free 12 demonstrates that an equilibrium is established slightly in favor of 18 (Scheme 11). This suggests that there is not a strong inherent difference in binding strengths between the two products, with the implication being that the observed rate differences in the melt arise from the dehydrocoupling step. Although this is different from what is observed in solution at room temperature, in which release of the product is likely the turnover limiting step, it is consistent with the high local concentration of H₃B·PR₂H that being under melt conditions (90 °C) would promote such a substitution.

Scheme 11. Competition Experiments between Linear Diboraphosphines.

[BAr^F₄]⁻ anions are not shown.

Catalytic Dehydrocoupling of Primary Phosphine–Boranes

[Rh(L)(η⁶-FC₆H₅)][BAr^F₄] also acts as a catalyst for the dehydrocoupling of primary phosphine–boranes. Under melt conditions (90 °C, 5 mol %, 4 h) H₃B·PPhH₂ is dehydrocoupled to give a major product which is identified by ³¹P NMR spectroscopy as being polymeric (BH₂PPhH)_n by comparison with previously reported^19,20 data for purified material coming from the [Rh(COD)₂][OTf] catalyzed process [δ −49.3, d, J(PH) ∼350 Hz, 1,2–F₂C₆H₄; lit.: δ −48.9, δ, J(PH) 360 Hz, CDCl₃]. There were also other species observed ∼δ −55, which could be reduced in relative concentration (to ∼10%) by precipitation into hexanes. Such species have been previously suggested to be short-chain oligomers.²⁰ Interestingly, these proposed shorter chain oligomers are present in a greater proportion at shorter reaction times, which might suggest that polycondensation is occurring to give higher molecular weight polymer. Under non-melt conditions²⁰ (toluene heated to reflux, 0.5 mol %, 16 h) these shorter oligomers are by far the dominant species (Supporting Information). It thus appears that a high local concentration of phosphine–borane is necessary for productive dehydropolymerization. Positive mode ESI-MS (electrospray mass spectrometry) of the melt reaction product demonstrated polymerization, showing repeat units of [H(PPhHBH₂)_nPPhH₂]⁺ up to n = 10 (Supporting Information). Similar analyses have been reported for amine–borane dehydropolymerization.^12,14,58 That these polymers are terminated by {PPhH₂} groups rather than {BH₃} is confirmed by inspection of the corresponding isotopomer patterns. This formulation also argues against cyclic oligomers being observed by ESI-MS, and presumably the additional phosphine arises from P–B bond cleavage. Use of H₃B·PCyH₂ under these conditions afforded significantly more complex mixtures that we were unable to resolve.

Conclusions

The solid-state structures of the intermediates in the dehydrocoupling of secondary phosphine–boranes using the {Rh(Ph₂PCH₂CH₂CH₂PPh₂)}⁺ fragment have been determined. This demonstrates that the complex that precedes dehydrocoupling to form a linear diboraphosphine has σ bound and P–H activated phosphine–borane ligands, while the product has a linear diboraphosphine bound to the metal center. For aryl phosphine–boranes, electron-withdrawing groups (CF₃) promote stoichiometric dehydrocoupling faster than for more electron-donating (OMe) groups. This increase in rate is accompanied by a significant degree of parallel and competitive P–B bond cleavage to afford metal complexes with two monodentate phosphine ligands, which we suggest is due to a weakening of the P–B bond with electron-withdrawing aryl groups. These systems also turnover catalytically under melt conditions, with the overall rate of conversion broadly following the relative dehydrocoupling rates observed in the stoichiometric studies, suggesting that the dehydrocoupling step under melt conditions might also be the turnover limiting step. P–B bond cleavage also occurs for very bulky electron rich adamantyl phosphine–boranes, to such an extent that stoichiometric dehydrocoupling is not observed. For this phosphine–borane we suggest that sterics play a role in this process.

A significant observation is that, for primary phosphine–boranes, which are precursors to polyphosphinoboranes, use of the {Rh(Ph₂PCH₂CH₂CH₂PPh₂)}⁺ fragment results in some apparent diastereoselectivity in the dehydrocoupling step, at least in the stoichiometric reactions that produce metal-bound diboraphosphines. Such selectivity could well have implications in the control of the stereochemistry of polymer that would result from further insertion events. A significant future challenge is to harness any inherent bias in each dehydrocoupling insertion step productively while also developing the necessary spectroscopic and physical characterization markers to interrogate the oligomer and polymer stereochemistry.

Experimental Section

All manipulations, unless otherwise stated, were performed under an atmosphere of argon, using standard Schlenk and glovebox techniques. Glassware was oven-dried at 130 °C overnight and flamed under vacuum prior to use. Hexane and pentane were dried using a Grubbs type solvent purification system (MBraun SPS-800) and degassed by successive freeze–pump–thaw cycles.⁵⁹ CD₂Cl₂, C₆H₅F, and 1,2-F₂C₆H₄ were distilled under vacuum from CaH₂ and stored over 3 Å molecular sieves, 1,2-F₂C₆H₄ was stirred over alumina for 2 h prior to drying. Bis-1,3-(diphenylphosphino)propane (dpp3) and (2S,4S)-2,4-bis(diphenylphosphino)pentane (BDPP) were purchased from Aldrich. [Rh(nbd)Cl]₂⁶⁰ and [Rh(nbd)(dpp3)][BAr^F₄]¹⁶ were prepared as previously described. (4-Methoxyphenyl)₂HP·BH₃ (3), (adamantyl)₂HP·BH₃ (4), and CyH₂P·BH₃ (5) were prepared by the same method as Me₃P·BH₃⁶¹ but with the phosphine changed. (4-Trifluoromethylphenyl)₂PH·BH₃ (2) and (3,5-bis(trifluoromethyl)phenyl)₂PCl were prepared according to literature procedures reported by Clark et al.³³ NMR spectra were recorded on a Bruker AVD 500 MHz spectrometer at room temperature unless otherwise stated. In 1,2-C₆H₄F₂, ¹H NMR spectra were referenced to the center of the downfield solvent multiplet (δ 7.07), and ³¹P and ¹¹B NMR spectra were referenced against 85% H₃PO₄ (external) and BF₃·OEt₂ (external), respectively. The spectrometer was prelocked and preshimmed using a C₆D₆ (0.1 mL) and 1,2-C₆H₄F₂ (0.3 mL) sample. Chemical shifts are quoted in ppm and coupling constants in Hz. ESI-MS were recorded on a Bruker micrOTOF instrument.⁶² In all ESI-MS spectra there was a good fit to both the principal molecular ion and the overall isotopic distribution. Signals in the ³¹P{¹H} NMR spectra were integrated relative to those in similar environments (i.e., Rh–P or B–P) to obtain the relative ratios of products, and data was acquired with a pulse repetition time of 1 s. This avoids potential problems with different relaxation times for different phosphorus environments. Nevertheless, the quoted relative ratios based upon this data should be treated as qualitative rather than quantitative.

Synthesis and Characterization of New Complexes

Synthesis of H₃B·PR₂H [R = 3,5-Bis(trifluoromethyl)phenyl] (1)

A solution of (3,5-bis(trifluoromethyl)phenyl)₂PCl (1.48 g, 3.0 mmol) in diethyl ether (5 mL) was added dropwise to a diethyl ether (20 mL) suspension of LiBH₄ (0.070 g, 3.21 mmol) cooled to 5 °C with an ice bath. The mixture became cloudy immediately and was allowed to stir for 30 min. The diethyl ether was removed in vacuo, and the residue was dissolved in hexanes (30 mL) and filtered through Celite. The hexanes were reduced in vacuo to ∼10 mL, and the solution was placed in the freezer (−20 °C) overnight yielding colorless crystals. Excess hexanes were decanted, and crystals were dried to afford a fine white powder which was subsequently washed with 2 × 3 mL of cold hexanes. Removal of all volatiles under reduced pressure yielded 630 mg of fine white powder (1).

¹H NMR (300 MHz, CDCl₃): δ 8.14 (br s, 1 H, p-Ar-H), 8.09 (br s, 2 H, o-Ar-H), 6.58 (dm, ¹J_HP = 388 Hz, 1 H, PH), 0.3–2.0 (br m, 3 H, BH). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ 4.7 (br s, PH). ¹¹B{¹H} NMR (160 MHz, CDCl₃): δ −41.7 (br s, BH₃). ¹⁹F NMR (282 MHz, CDCl₃): δ −62.9 (s, CF₃). EI-MS (70 eV) m/z (%): 458 (62) [M⁺ – BH₃]. Anal. Found: C 40.71%, H 2.02%. Calcd for C₁₆H₁₀BF₁₂P: C 40.68%, H 2.14%.

Synthesis of (Adamantyl)₂PH·BH₃ (4)

(Adamantyl)₂PH·BH₃ was prepared under the same conditions as Me₃P·BH₃⁶¹ but with (adamantyl)₂PH instead of PMe₃.

¹H NMR (300 MHz, CDCl₃): δ 3.61 (dm, 1 H, ¹J_HP = 379 Hz, PH), 2.11 to 1.83 (30 H, adamantyl-H), 0.41 to −0.15 (br m, 3 H, BH). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ 40.1 (br m, PH). ¹¹B{¹H} NMR (160 MHz, CDCl₃): δ −44.8 (br d, BH₃). Anal. Found: C 75.78%, H 10.71%. Calcd for C₂₀H₃₄BP: C 75.89%, H 10.84%.

Synthesis of H₃B·PR₂BH₂·PR₂H [R = 3,5-Bis(trifluoromethyl)phenyl (10); 4-Trifluoromethylphenyl (11); 4-Methoxyphenyl (12)]

A Youngs flask charged with 0.25 mmol of R₂PH·BH₃ (118 mg of 1, 84 mg of 2, 65 mg of 3) and 5 mol % of [Rh(dpp3)(C₆H₅F)][BAr^F₄] (18.4 mg, 0.0125 mmol) was heated to 90 °C for 4 h (10 and 11) or 8 h (12) in melt conditions. The resulting solids were washed with n-hexane and recrystallized from a mixture of diethyl ether and hexane at −18 °C (10 30 mg, 25%; 11 22 mg, 26%; 12 32 mg, 49%).

Details follow for 10. ¹H NMR (300 MHz, CDCl₃): δ 8.09 to 7.89 (12 H, Ar–H), 7.32 (dm, ¹J_HP = 412 Hz, 1 H, PH), 2.45 (br m, 2 H, BH₂), 1.11 (br m, 3 H, BH₃). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ −1.7 (br s, PHR₂), −14.0 (br s, PR₂). ¹¹B{¹H} NMR (160 MHz, CDCl₃): δ −33.2 (br s, BH₂), −37.7 (br s, BH₃). Anal. Found: C 40.90%, H 1.83%. Calcd for C₃₂H₁₈B₂F₂₄P₂: C 40.76%, H 1.93%.

Details follow for 11. ¹H NMR (500 MHz, CD₂Cl₂): δ 7.77 to 7.52 (16 H, Ar–H), 7.04 (dt, ¹J_HP = 426 Hz, ³J_HH = 7.8 Hz, 1 H, PH), 2.37 (br m, 2 H, BH₂), 1.02 (br m, 3 H, BH₃). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ −3.5 (br s, PHR₂), −15.4 (br s, PR₂). ¹¹B{¹H} NMR (160 MHz, CD₂Cl₂): δ −33.7 (br s, BH₂), −37.6 (br s, BH₃).

Details follow for 12. ¹H NMR (500 MHz, CD₂Cl₂): δ 7.64 to 6.77 (16 H, Ar–H), 6.69 (dt, ¹J_HP = 415 Hz, ³J_HH = 6.6 Hz, 1 H, PH), 3.84 (s, 6 H, CH₃), 3.80 (s, 6 H, CH₃), 2.14 (br m, 2 H, BH₂), 0.96 (br m, 3 H, BH₃). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ −7.6 (br s, PHR₂), −22.1 (br s, PR₂). ¹¹B{¹H} NMR (160 MHz, CD₂Cl₂): δ −33.3 (br s, BH₂), −37.1 (br s, BH₃).

Synthesis of [Rh(dpp3)H(PR₂·BH₃)(H₃B·PHR₂)][BAr^F₄] [R = 3,5-Bis(trifluoromethyl)phenyl (14); 4-Trifluoromethylphenyl (15); 4-Methoxyphenyl (16)]

To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 2 equiv of H₃B·PR₂H (32 mg of 1, 23 mg of 2, 18 mg of 3 0.068 mmol) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred at room temperature 10 min, and a change in color from pale orange to bright yellow was observed. Complexes 15 and 16 were isolated as yellow oils, and characterized in situ by NMR spectroscopy and ESI-MS. Complex 14 could be crystallized at −24 °C in the freezer inside the glovebox (yield 29.6 mg, 37%). Complexes 15 and 16 could not be isolated cleanly, and attempts to do so led to intractable mixtures of 15 and 16 with 18 and 19, respectively.

Details follow for 14. Slow diffusion of pentane (10 mL) over a solution of 14 in 1,2-F₂C₆H₄ at −24 °C afforded yellow crystals (one of which was employed for an X-ray diffraction study).

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 5.81 (d, ¹J_HP = 435 Hz, 1 H, PH), 3.12–0.81 (8 H, 3CH₂ dpp3 + BH₂), −0.78 (br, 3 H, BH₃), −6.12 (br, 1 H, BH–Rh), −16.21 (s, 1 H, Rh–H). Signals from the aromatics were not observed due to being overlapped by signals from 1,2-F₂C₆H₄.

¹H NMR (500 MHz, 1,2-F₂C₆H₄, selected data at 0 °C): δ −6.12 (d, ¹J_HP = 65 Hz, 1 H, BH–Rh). ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 29.5 (ddd, J_PRh = 130 Hz, J_PP(cis)= 35 Hz, J_PP(cis) = 21 Hz, Ph₂P(CH₂)₃PPh₂), 11.3 (ddd, J_PP(trans) = 244 Hz, J_PRh = 103 Hz, J_PP(cis) = 35 Hz, Ph₂P(CH₂)₃PPh₂), −0.7 (dd, J_PP(trans) = 248 Hz, J_PRh = 75 Hz, Rh-PR₂BH₃), −2.62 (br s, PHR₂BH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ −6.2 (BAr^F₄), −39.8 (br, 2 × BH₃). ESI-MS (1,2-C₆H₄F₂, 60 °C) positive ion: m/z = 1431.07 (calcd 1431.07, M⁺ – 2BH₃). Anal. Found: C 46.82%, H 2.39%. Calcd for C₉₁H₅₈B₃F₄₈P₄Rh: C 47.02%, H 2.52%.

Details follow for 15. ¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 4.90 (d, J_HP = 414 Hz, PH), 3.01–1.10 (8 H, 3CH₂ dpp3 + BH₂), −1.18 (br, 3H, BH₃), −6.95 (d, J_HP = 78 Hz, 1 H, BH–Rh), −16.51 (s, 1 H, Rh–H). Signals from the aromatics were not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 27.3 (ddd, J_PRh = 127 Hz, J_PP(cis) = 37 Hz, J_PP(cis) = 16 Hz, Ph₂P(CH₂)₃PPh₂), 11.8 (ddd, J_PP(trans) = 231 Hz, J_PRh = 100 Hz, J_PP(cis) = 37 Hz, Ph₂P(CH₂)₃PPh₂), −0.3 (dd, J_PP(trans) = 231 Hz, J_PRh = 70 Hz, Rh-PR₂BH₃), −5.96 (br s, PHR₂BH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ −6.2 (BAr^F₄) −39.9 (br, 2 × BH₃). ESI-MS (1,2-C₆H₄F₂, 60 °C) positive ion: m/z = 1159.13 (calcd 1159.13, M⁺ – 2BH₃).

Details follow for 16. ¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 4.93 (d, J_HP = 409 Hz, 1 H, PH), 3.77 (s, 6 H, −OCH₃), 3.69 (s, 6 H, −OCH₃), 3.16–0.58 (8 H, 3CH₂ dpp3 + BH₂), −1.11 (br, 3 H, BH₃), −6.53 (d, J_HP = 73 Hz, 1 H, BH–Rh), −16.49 (s, 1 H, Rh–H). Signals from the aromatics were not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 27.3 (ddd, J_PRh = 132 Hz, J_PP(cis) = 37 Hz, J_PP(cis) = 12 Hz, Ph₂P(CH₂)₃PPh₂), 9.5 (ddd, J_PP(trans) = 227 Hz, J_PRh = 98 Hz, J_PP(cis) = 37 Hz, Ph₂P(CH₂)₃PPh₂), 0.7 (br dd, J_PP(trans) = 227 Hz, J_PRh = 65 Hz, Rh-PR₂BH₃), −11.2 (br s, PHR₂BH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ −6.2 (BAr^F₄) −38.7 to −49.8 (br, 2 × BH₃). ESI-MS (1,2-F₂C₆H₄, 60 °C) positive ion: m/z = 1021.25 (calcd 1021.25, M⁺ – BH₃), 914.19 (calcd 914.20, M⁺ – BH₃ – C₆H₄OMe), 900.17 (calcd 900.17, M⁺ – 2BH₃, −C₆H₄OMe), 775.17 (calcd 775.17, M⁺ – (MeOC₆H₄)₂HP·BH₃).

Synthesis of [Rh(dpp3)H(PR₂·BH₂PR₂·BH₃)][BAr^F₄] [R = 3,5-Bis(trifluoromethyl)phenyl (17); 4-Trifluoromethylphenyl (18); 4-Methoxyphenyl (19)]

Method A follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 2 equiv of H₃B·PR₂H (32 mg of 1, 23 mg of 2, 18 mg of 3 0.068 mmol) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred at room temperature for 24 h. The formation of H₂ gas is also observed. Complex 19 was isolated as yellow oil (37 mg, 61%). Complexes 17 and 18 could not be isolated cleanly; they were observed with 22 and 23, respectively.

Method B follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 1 equiv of 10 (32 mg, 0.068 mmol) was added 1,2-F₂C₆H₄ (4 mL). Complex 17 was isolated as yellow solid (65 mg, 82%).

Details follow for 17. Slow diffusion of pentane (10 mL) over a solution of 17 in 1,2-F₂C₆H₄ at −24 °C afforded yellow crystals (one of which was employed for an X-ray diffraction study).

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8H, BAr^F₄), 7.69 (s, 4H, BAr^F₄), 4.40 (vbr, 1H, BH) 3.10–2.12 (8H, 3CH₂ dpp3 + BH₂), −1.20 (vbr, 1H, BH), −4.54 (vbr, 1H, BH), −13.98 (s, 1H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 46.6 (dd, J_Rh–P= 111, J_P²_–P¹_(cis)= 36, Ph₂P(CH₂)₃PPh₂), 29.5 (m, J_P–P(trans) = 260, Rh-PR₂BH₃PR₂HBH₃), 12.8 (ddd,, J_P–P(trans) = 260, J_Rh–P= 91, J_P–P(cis) = 33, Ph₂P(CH₂)₃PPh₂), −2.7 (s, Rh-PR₂BH₃PR₂HBH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ 0.21 (br), −6.2 (s, BAr^F₄), −27.1 (br). ESI-MS (1,2-F₂C₆H₄, 60 °C) positive ion: m/z = 1457.09 (calcd 1457.12, M⁺). Anal. Found: C 47.15%, H 2.34%. Calcd for C₉₁H₅₆B₃F₄₈P₄Rh: C 47.07%, H 2.43%.

Details follow for 18. ¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 4.24 (v br, 1 H, BH), 2.61–1.72 (8 H, 3CH₂ dpp3 + BH₂), −1.29 (v br, 1 H, BH), −4.65 (v br, 1 H, BH), −14.90 (s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 42.5 (dd, J_PRh = 106 Hz, J_PP(cis) = 34 Hz, Ph₂P(CH₂)₃PPh₂), 28.6 (m, J_PP(trans) = 272 Hz, Rh-PR₂BH₃PR₂HBH₃), 15.3 (ddd, J_PP(trans) = 272 Hz, J_PRh = 101 Hz, J_PP(cis) = 33 Hz, Ph₂P(CH₂)₃PPh₂), −6.4 (s, Rh-PR₂BH₃PR₂HBH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ 0.4 (br), −6.2 (s, BAr^F₄), −25.9 (br). ESI-MS (1,2-F₂C₆H₄) positive ion: m/z = 1185.17 (calcd 1185.18, M⁺).

Details follow for 19. ¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 3.77 (s, 3 H, −OCH₃), 3.76 (s, 3 H, −OCH₃), 3.69 (s, 6 H, −OCH₃), 2.85–1.72 (8 H, 3CH₂ dpp3 + BH₂), −1.03 (v br, 1 H, BH), −4.00 (v br, 1 H, BH), −14.55 (s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 42.7 (ddd, J_PRh = 109 Hz, J_PP(cis) = 35 Hz, J_PP(cis) = 12 Hz Ph₂P(CH₂)₃PPh₂), 28.1 (br m, J_PP(trans) = 279 Hz, Rh-PR₂BH₃PR₂HBH₃), 12.5 (ddd, J_PP(trans) = 279 Hz, J_PRh = 88 Hz, J_PP(cis) = 12 Hz, Ph₂P(CH₂)₃PPh₂), −11.7 (br s, Rh-PR₂BH₃PR₂HBH₃). ¹¹B{¹H} NMR (160 MHz, C₆H₄F₂): δ 3.42 (br), −6.2 (s, BAr^F₄), −27.7 (br). ESI-MS (1,2-F₂C₆H₄) positive ion: m/z = 1033.27 (calcd 1033.27, M⁺).

Synthesis of [Rh(dpp3)(PHR₂)₂][BAr^F₄] [R = 3,5-Bis(trifluoromethylphenyl) (21)]

Method A follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 2 equiv of H₃B·PR₂H (32 mg of 1) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred at room temperature for 24 h. The formation of H₂ (gas) is also observed. Complex 21 could not be isolated cleanly as 17 was also observed.

Method B follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (20 mg, 0.034 mmol) and 2 equiv of PHR₂ (31 mg, 0.068 mmol, R = 3,5-bis(trifluoromethyl)phenyl) was added 1,2-F₂C₆H₄ (1 mL). After stirring for 10 min the solution was evaporated to dryness and the solid washed with pentane (2 mL). Complex 21 was isolated as yellow solid (yield 17.8 mg, 57%).

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.33 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 6.41 (dm, J_HP = 375 Hz, 2 H, PH), 2.62 (br, 4 H, 2 CH₂ dpp3), 2.17 (m, 2 H CH₂, dpp3). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 9.6 (m, 2P, AA′BB′M), 5.5 (m, 2P, AA′BB′M). ESI-MS (1,2-F₂C₆H₄) positive ion: m/z = 1431.04 (calcd 1431.07, M⁺). Anal. Found: C 47.71%, H 2.21%. Calcd for C₉₁H₅₂BF₄₈P₄Rh: C 47.60%, H 2.28%.

[Rh(dpp3)(PHR₂)₂][BAr^F₄] [R = 4-Trifluoromethylphenyl (22)]

Complex 22 was characterized by in situ NMR spectroscopy. ¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.33 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 5.91 (dm, J_HP = 359 Hz, 2 H, PH), 2.57 (br, 4 H, 2 × CH₂ dpp3), 2.06 (m, 2 H × CH₂, dpp3). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 12.2 (m, 2P, AA′BB′M), 5.2 (m, 2P, AA′BB′M).

Synthesis of [Rh(dpp3)(η²-H₃B·PR₂H)][BAr^F₄] [R = Adamantyl (23)]

To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and H₃B·PR₂H (4) (11 mg, 0.034 mmol) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred at room temperature for 1 min, and a change in color from pale orange to purple was observed. Complex 23 could not be isolated because further reaction to form 24 occurred.

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.68 (s, 4 H, BAr^F₄), 3.45 (d, 1 H, ¹J_HP = 380 Hz, B-PH), 2.52–1.12 (36 H, adamantyl-H + dpp3 CH₂), −1.36 (br, 3 H, BH₃). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 35.1 (d, J_PRh = 167 Hz, dpp3), 30.5 (br, B–P). ¹¹B NMR (160 MHz, 1,2-F₂C₆H₄): δ −0.8 (br), −6.0 (s, BAr^F₄). ESI-MS (1,2-F₂C₆H₄) positive ion: m/z = 629.09 (calcd 629.08, [Rh(dpp3)(C₆H₄F₂)]⁺). The weakly bound σ-complex could not be observed.

Synthesis of [Rh(dpp3)(PR₂H)(H₃B·PR₂H)][BAr^F₄] [R = Adamantyl (24)]

Method A follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 2 equiv of H₃B·PR₂H (4) (22 mg, 0.068 mmol) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred at room temperature for 24 h. Complex 24 was isolated as orange solid.

Method B follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 1 equiv of PHR₂ (10 mg, 0.034 mmol) was added 1,2-F₂C₆H₄ (5 mL). The solution was stirred for 5 min, and then 1 equiv of H₃B·PR₂H (4) (11 mg, 0.034 mmol) was added. The solution was stirred for 5 min, and complex 24 was isolated as orange solid (yield 48 mg, 71%). Slow diffusion of pentane (10 mL) into a solution of 24 in 1,2-F₂C₆H₄ at −24 °C afforded yellow crystals (one of which was employed for X-ray diffraction studies).

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.33 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 3.30 (d, ¹J_HP = 362 Hz, 1 H, B-PH), 2.87 (d, ¹J_HP = 412 Hz, 1 H, PH), 2.45–1.56 (66 H, dpp3 CH₂ and adamantyl-H), −0.24 (br, 3 H, BH₃). ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 60.4 (ddd, J_PP(trans) = 266 Hz, J_PRh = 142 Hz, J_PP(cis) = 30 Hz, Ph₂P(CH₂)₃PPh₂), 30.5 (s, PHR₂BH₃), 22.8 (dd, J_PRh = 163 Hz, J_PP(cis) = 30 Hz, PPh₂(CH₂)₃PPh₂), 6.6 (ddd, J_PP(trans) = 270 Hz, J_PRh = 144 Hz, J_PP(cis) = 30 Hz, Rh-PR₂BH₃). ¹¹B{¹H} NMR (160 MHz, 1,2-F₂C₆H₄): δ −6.0 (s, BAr^F₄), −42.2 (br, BH₃). ESI-MS (1,2-F₂C₆H₄) positive ion: m/z = 956.30 (unidentified fragment). Anal. Found: C 59.38%, H 4.99%. Calcd for C₉₉H₁₀₃B₂F₂₄P₄Rh: C 59.50%, H 5.20%.

Synthesis of [Rh(dpp3)H(PCyH·BH₃)(H₃B·PCyH₂)][BAr^F₄] (25a and 25b)

To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) was added 1,2-F₂C₆H₄ (5 mL). A 2 equiv portion of H₃B·PH₂Cy (5) (0.68 mL, 0.1 M solution in 1,2-F₂C₆H₄, 0.068 mmol) was then added. The solution was stirred at room temperature for 1 min, and a change in color from orange to pale yellow was observed. Complexes 25a and 25b were observed as an approximate 1:1 ratio of isomers and were characterized in situ by NMR spectroscopy. Complexes 25a and 25b could not be isolated as they reacted quickly to form complexes 26a and 26b. The ³¹P{¹H} NMR spectrum of this reaction mixture indicates that 2 diastereomers are present; while we were able to identify the 2 sets of 4 resonances each (labeled † and §, based on coupling constants and approximate integrations) it was not possible to determine which set of signals belonged to which diastereomer. See Figure S4, Supporting Information, for more detail.

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 2.73–0.32 (32 H, 3 CH₂ dpp3 + BH₂, CyH, PH), −2.29 (v br, 3 H, BH), −7.92 (br d, 1 H, BH-Rh), −17.51 (s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 31.7 (dm, J_PRh = 134 Hz, Ph₂P^1†(CH₂)₃PPh₂), 30.5 (dm, J_PRh = 129 Hz, Ph₂P^1§(CH₂)₃PPh₂), 11.8 (overlapping ddd, J_PP(trans) = approximately 200 Hz, J_PRh = approximately 104 Hz, J_PP(cis) = approximately 25 Hz, Ph₂P(CH₂)₃P^2†§Ph₂), −11.0 (br d, J_PP(trans) = approximately 200 Hz, Rh-P^3§HCy-B), −32.1 (br d, J_PP(trans) = approximately 200 Hz, Rh-P^3†HCy-B), −39.8 (br s, Rh–H₃BP^4†H₂Cy), −44.2 (br s, Rh–H₃BP^4§H₂Cy). ESI-MS (1,2-F₂C₆H₄, 60 °C) positive ion: m/z = 747.20 (calcd 747.21, M⁺ – 2BH₃).

Synthesis of [(CyH₂P)₂BH₂]Br

To a stirred solution of PCyH₂ (3.40 mL, 10% wt in hexane, 2.0 mmol) in dichloromethane (20 mL) was added BrH₂B·SMe₂ (1.0 mL, 1.0 M in CH₂Cl₂, 1.0 mmol) and the solution stirred at room temperature for 2 h. The resulting colorless solution was concentrated in vacuo to approximately 3 mL, and diethyl ether (30 mL) was added to precipitate a white solid. The solvent was decanted off and the solid washed with a further 10 mL of diethyl ether. The solid was redissolved in dichloromethane, filtered, and recrystallized from a mixture of dichloromethane and diethyl ether at −18 °C to yield white crystals (first crop 0.190 g, second crop 0.041 g, overall yield 71%). At room temperature in CD₂Cl₂ [(CyH₂P)₂BH₂]Br undergoes a degenerate exchange reaction; [(CyH₂P)₂BH₂]Br is in equilibrium with CyH₂PBH₂Br + PH₂Cy. This exchange process does not occur at −60 °C on the NMR time scale, and each of these species can be observed. In the solid-state the complex exists as [(CyH₂P)₂BH₂]Br.

¹H NMR (500 MHz, −60 °C, CD₂Cl₂): δ 5.46 (dm, ¹J_HP = 429 Hz, [(CyH₂P)₂BH₂]⁺), 4.75 (dm, ¹J_HP = 388 Hz, CyH₂PBH₂Br), 2.52 (dm, ¹J_HP = 196 Hz, CyH₂P), 2.37–1.16 (CyH and BH). ³¹P{¹H} NMR (202 MHz, −60 °C, CD₂Cl₂): δ −35.5 (br s, [(CyH₂P)₂BH₂]⁺), −38.2 (br s, CyH₂PBH₂Br), −107.5 (s, CyH₂P). Anal. Found: C 44.35%, H 8.74%. Calcd for C₁₂H₂₈BBrP₂: C 44.30%, H 8.68%.

Synthesis of CyH₂P·BH₂PCyH·BH₃ (13)

[(CyH₂P)₂BH₂]Br (0.150 g, 0.461 mmol) (prepared as above) and [NⁿBu₄][BH₄] (0.119 g, 0.461 mmol) were added to a Schlenk tube. Dichloromethane (10 mL) was added, and effervescence was observed; the solution was stirred at room temperature for 1 h. The solvent was removed in vacuo, and n-hexane (20 mL) was added to the white solid. The solution was filtered to remove [NⁿBu₄]Br and concentrated to approximately 2 mL. Storage of this solution for 16 h at −18 °C yielded colorless crystals of 13 (0.102 g, 86%).

¹H NMR (500 MHz, 25 °C, CD₂Cl₂): δ 4.68 (dm, ¹J_HP = 386 Hz, CyH₂P·BH₂PHCy·BH₃), 3.50 (dm, ¹J_HP = 326 Hz, CyH₂P·BH₂PHCy·BH₃), 2.17–0.02 (CyH and BH). ³¹P{¹H} NMR (202 MHz, 25 °C, CD₂Cl₂): δ −36.6 (br, CyH₂P·BH₂PHCy·BH₃), −45.1 (br, CyH₂P·BH₂PHCy·BH₃). Anal. Found: C 55.79%, H 11.82%. Calcd for C₁₂H₃₀B₂P₂: C 55.77%, H 11.71%.

Synthesis of [Rh(dpp3)H(PHCy·BH₂PHCy·BH₃)][BAr^F₄] (26a and 26b)

Method A follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) was added 1,2-F₂C₆H₄ (5 mL) followed by 2 equiv of H₃B·PCyH₂ (5) (0.68 mL, 0.1 M solution in 1,2-F₂C₆H₄, 0.068 mmol). The solution was stirred at room temperature for 12 h. The formation of H₂ is also observed. Complexes 26a and 26b were characterized as a mixture in solution by NMR spectroscopy and ESI-MS. The ³¹P{¹H} NMR spectrum of this reaction mixture indicates that 2 diastereomers are present; we were able to identify the 2 sets of 4 resonances each (labeled 26a and 26b, based on coupling constants and approximate integrations) and tentatively assigned the individual diastereomers (Scheme S5, Supporting Information) by inspection of a model.

Method B follows. To a Youngs flask charged with [Rh(dpp3)(C₆H₅F)][BAr^F₄] (50 mg, 0.034 mmol) and 1 equiv of CyH₂P·BH₂PCyH·BH₃ (13) (8.8 mg, 0.034 mmol) was added 1,2-F₂C₆H₄ (4 mL). Complexes 26a and 26b were characterized as a mixture in solution by NMR spectroscopy and ESI-MS.

¹H NMR (500 MHz, 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 4.35–3.01 (PH and BH), 2.92–0.92 (30 H, 3CH₂ dpp3, BH₂, CyH), −2.98 (br, 1 H, BH-Rh), −5.98 (br, 1 H, BH-Rh), −16.08 (s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from 1,2-F₂C₆H₄. ³¹P{¹H} NMR (202 MHz, 1,2-F₂C₆H₄): δ 37.9 (dm, J_PRh = 102 Hz, Ph₂P^1b(CH₂)₃PPh₂), 34.5 (dm, J_PRh = 102 Hz, Ph₂P^1a(CH₂)₃PPh₂), 19.8 (br d, J_PP(trans) = approximately 255 Hz, Rh-P^3bHCyB), 16.2 (br d, J_PP(trans) = approximately 255 Hz, Rh-P^3aHCy-B), 10.7 (overlapping dm, J_PP(trans) = 255 Hz, J_PRh = 88 Hz, J_PP(cis) = approximately 25 Hz, Ph₂P(CH₂)₃P^2abPh₂), −14.9 (br s, Rh-PHCyBH₂P^4aHCyBH₃), −16.6 (br s, Rh-PHCyBH₂P^4bHCyBH₃). ESI-MS (1,2-F₂C₆H₄, 60 °C) positive ion: m/z = 773.26 (calcd 773.24, M⁺).

Synthesis of [Rh(BDPP)(nbd)][BAr^F₄]

[Rh(nbd)₂][BAr^F₄] was synthesized by an adaptation of the preparation of [Rh(cod)₂][BAr^F₄].⁶³ [Rh(nbd)₂][BAr^F₄] (0.100 g, 0.0869 mmol) was dissolved in CH₂Cl₂ (10 mL) to produce a deep red solution. (2S,4S)-2,4-Bis(diphenylphosphino)pentane (BDPP) (0.0383 g, 0.0869 mmol) was added, and a color change to red was observed. The solution was stirred for 1 h at room temperature before the solvent was removed in vacuo. n-Pentane (20 mL) was added, and the solution was sonicated to produce an orange powder. The solvent was decanted and the solid washed with pentane (2 × 20 mL). The product was dried in vacuo and isolated (yield 0.102 g, 78%).

¹H NMR (300 MHz, CD₂Cl₂): δ 7.72 (s, 8 H, BAr^F₄), 7.56 (s, 4 H, BAr^F₄), 7.70–7.36 (m, P-Ph), 4.86 (m, 2 H, =C—H), 4.30 (m, 2 H, =C—H), 3.90 (m, 2 H, nbd C—H), 2.77 (m, 2 H, P—C—H), 1.84 (tt, ³J_HH = 6.45 Hz, ³J_HP = 19.95 Hz, 2 H, CH₂), 1.58 (m, 2 H, nbd CH₂), 1.13 (m, 6 H, CH₃). ³¹P{¹H} NMR (121.6 MHz, CD₂Cl₂): δ 27.4 (d, J_PRh = 149 Hz). Anal. Found: C 54.32%, H 3.18%. Calcd for C₆₈H₅₀BF₂₄P₂Rh: C 54.47%, H 3.36%.

Synthesis of [Rh(BDPP)(C₆H₅F)][BAr^F₄]

[Rh(BDPP)(nbd)][BAr^F₄] prepared as above (0.020 g, 0.0133 mmol) was added to a high-pressure NMR tube and dissolved in fluorobenzene (0.5 mL). The sample was degassed by the freeze–pump–thaw method and hydrogen gas (4 atm) introduced. The sample was mixed for 30 min and then degassed by the freeze–pump–thaw method and placed under argon.

¹H NMR (500 MHz, C₆H₅F): δ 8.32 (s, 8 H, BAr^F₄), 7.61 (s, 4 H, BAr^F₄), 5.63 (m, 2 H, Ar–H), 5.43 (m, 2 H, Ar–H), 5.10 (m, 1 H, Ar–H), 2.38 (m, 2 H, P-CH), 1.40 (m, 2 H, CH₂), 0.78 (m, 6 H, CH₃). Signals from P-Ph not observed due to being overlapped by signals from C₆H₅F. Signals from norbornane (from hydrogenation of norbornadiene) can also be observed at δ 2.12 (m, 2 H, nba C–H), 1.40 (m, 2 H, nba CH₂, overlapped), 1.11 (m, 2 H, nba CH₂). ³¹P{¹H} NMR (202 MHz, C₆H₅F): δ 39.9 (d, J_PRh = 194 Hz). ESI-MS (C₆H₅F, 60 °C) positive ion: m/z = 639.13 (calcd 639.12, M⁺).

Synthesis of [Rh(BDPP)H(PCyH·BH₃)(H₃B·PCyH₂)][BAr^F₄] (27a and 27b)

[Rh(BDPP)(C₆H₅F)][BAr^F₄] (26) was prepared in a high-pressure NMR tube as above. To this was added 2 equiv of H₃B·PH₂Cy (5) (0.27 mL, 0.1 M solution in 1,2-F₂C₆H₄, 0.027 mmol) and the solution mixed for 1 min. Complexes 27a and 27b were observed as ratio of isomers and were characterized in situ by NMR spectroscopy. Complexes 27a and 27b could not be isolated as they reacted quickly to form complexes of 28; signals for 28 can be observed in both ¹H and ³¹P{¹H} NMR spectra of 27a and 27b which, along with the presence of H₂, show that the complexes rapidly undergo dehydrocoupling to form complexes of 28. The ³¹P{¹H} NMR spectrum of this reaction mixture indicates that 2 diastereomers are present; while we were able to identify the 2 sets of 4 resonances (labeled † and §, based on coupling constants and approximate integrations), it was not possible to determine which set of signals belonged to which diastereomer. See Figure S6, Supporting Information.

¹H NMR (500 MHz, C₆H₅F + 1,2-F₂C₆H₄): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 4.19 to 0.32 (CH₃, CH₂, CH, BDPP, BH₂, CyH, PH), −1.80 to −3.31 (v br, 3 H, BH₃), −7.54 to −8.86 (br d, 1 H, BH-Rh), −17.73 (br s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from C₆H₅F and 1,2-F₂C₆H₄. H₂ can be observed in the NMR spectrum as a sharp singlet at δ 4.52 ppm suggesting that some dehydrocoupling to complex 28 has occurred. This is further evidenced by the small Rh–H hydride signal at δ −16.07 ppm which is observed for complex 28. ³¹P{¹H} NMR (202 MHz, C₆H₅F + 1,2-F₂C₆H₄): δ 42.5 (dm, J_PRh = 129 Hz, Ph₂P^1†(CH₂)₃PPh₂), 32.6 (dm, J_PRh = 119 Hz, Ph₂P^1§(CH₂)₃PPh₂), 28.3 (overlapping ddd, J_PP(trans) = approximately 208 Hz, J_PRh = approximately 98 Hz, J_PP(cis) = approximately 32 Hz, Ph₂P(CH₂)₃P^2†§Ph₂), 3.6 (br d, J_PP(trans) = approximately 208 Hz, Rh-P^3§HCy-B), −12.1 (br d, J_PP(trans) = approximately 208 Hz, Rh-P^3†HCy-B), −42.1 (br s, Rh–H₃BP^4§H₂Cy), −44.2 (br s, Rh–H₃BP^4†H₂Cy).

Synthesis of [Rh(BDPP)H(PCyH·BH₂PCyH·BH₃)][BAr^F₄] (28a, 28b, 28c, and 28d)

Method A follows. [Rh(BDPP)(C₆H₅F)][BAr^F₄] was prepared on an NMR scale from [Rh(BDPP)(nbd)][BAr^F₄] (0.020 g, 0.0133 mmol) as above. This solution was transferred by cannula to an NMR tube containing CyH₂P·BH₂PHCy·BH₃ (13) (3.4 mg, 0.0133 mmol), and the solution was mixed briefly to yield a pale yellow solution.

Method B follows. [Rh(BDPP)(C₆H₅F)][BAr^F₄] was prepared on an NMR scale from [Rh(BDPP)(nbd)][BAr^F₄] (0.020 g, 0.0133 mmol) as above. To this was added 2 equiv of H₃B·PH₂Cy (5) (0.27 mL, 0.1 M solution in 1,2-C₆H₄F₂, 0.027 mmol) and the solution mixed for 18 h.

The ³¹P{¹H} NMR spectrum arising from method A indicates that 4 diastereomers are present; while we were able to identify some of the signals from the 4 sets of 4 resonances (labeled †, §, $, and & based on coupling constants and approximate integrations), it was not possible to determine which set of signals belonged to which of the diastereomers. See Figure S7, Supporting Information. Method B affords, essentially, only one diastereoisomer.

¹H NMR (500 MHz, C₆H₅F): δ 8.32 (s, 8 H, BAr^F₄), 7.69 (s, 4 H, BAr^F₄), 5.47–2.46 (PH), 2.11–0.63 (CH, CH₂, and CH₃ BDPP, BH₂, CyH), −3.21 (br, 1 H, BHRh), −5.65 to −6.85 (br, 1 H, BHRh), −16.09, −16.20, and −17.04 (s, 1 H, Rh–H). Signals from aromatics not observed due to being overlapped by signals from C₆H₅F and 1,2-C₆H₄F₂. ³¹P{¹H} NMR (202 MHz, C₆H₅F): δ 58.9 (ddd, J_PRh = 107 Hz, J_PP(cis) = 30 Hz, J_PP(cis) = 12 Hz, Ph₂P^1§(CH₂)₃PPh₂), 58.0 (dm, J_PRh = 102 Hz, Ph₂P^1†(CH₂)₃PPh₂), 46.6 (dm, J_PRh = 100 Hz, Ph₂P^1$ (CH₂)₃PPh₂), 43.6 (dm, J_PRh = 102 Hz, Ph₂P^1&(CH₂)₃PPh₂), 28.7 (ddd, J_PP(trans) = 254 Hz, J_PRh = 90 Hz, J_PP(cis) = 32 Hz, Ph₂P(CH₂)₃P^2$Ph₂), 27.5 (ddd, J_PP(trans) = 250 Hz, J_PRh = 92 Hz, J_PP(cis) = 32 Hz, Ph₂P(CH₂)₃P^2&Ph₂), 24.8–17.7 (overlapping m, J_PP(trans) = approximately 254 Hz, J_PRh = approximately 87 Hz, J_PP(cis) = approximately 26 Hz, Ph₂P(CH₂)₃P^2†§Ph₂ and Rh-P³HCy-B), −13.5 to −20.6 (br s of 4 isomers, Rh-PHCyBH₂P⁴HCyBH₃). ESI-MS (1,2-C₆H₄F₂, 60 °C) positive ion: m/z = 801.29 (calcd 801.29, M⁺).

In order to establish that dehydrocoupling had occurred when method B was used to form complexes 28, excess (10 equiv) of 1,2-bis(diphenylphosphino)ethane was added to the reaction mixture to release the dehydrocoupled product, CyH₂P·BH₂PHCy·BH₃, from the metal center. The ³¹P{¹H} NMR spectrum showed two broad signals at δ −37.9 and −43.9 which are in agreement with those found for compound 13.

Attempted Polymerization of PhH₂P·BH₃ in Solution

In a procedure similar to that reported by Manners et al.,²⁰ PhH₂P·BH₃ (0.248 g, 2.0 mmol) was dissolved in toluene (10 mL) either in the presence of [Rh(dpp3)(C₆H₅F)][BAr^F₄] (14.7 mg, 0.01 mmol) or with no catalyst present. The solution was heated to reflux for 16 h before cooling to room temperature. The solution was concentrated in vacuo and added to stirred hexane (100 mL) to produce a white precipitate. The solvent was decanted and the solid washed with hexane (2 × 50 mL). The solid was dried in vacuo and isolated in air (yield 0.110 g Rh catalyzed, 0.101 g uncatalyzed). The ³¹P{¹H} NMR spectrum of the isolated solids produced by the different methods are very similar with several very broad peaks from δ −45 to −57 and very broad peaks of lower intensity from δ −72 to −87 . See Figure S8, Supporting Information. This is in agreement with the results obtained by Manners et al. for uncatalyzed polymerization of PhH₂P·BH₃.²⁰

Melt Polymerization of PhH₂P·BH₃

A Youngs flask charged with PhH₂P·BH₃ (31 mg, 0.25 mmol) and 5 mol % of [Rh(dpp3)(C₆H₅F)][BAr^F₄] (18.4 mg, 0.0125 mmol) was heated to 90 °C for 4 h in melt conditions. The resulting solid was dissolved in 1,2-difluorobenzene and analyzed by NMR spectroscopy. The ³¹P{¹H} NMR spectrum shows a peak at δ −49.3 ppm in agreement with that observed by Manners et al.⁷ for polymeric material and a lower intensity resonance at δ −55.0 ppm. See Supporting Information Figure S9. In the ³¹P{¹H} NMR spectrum the peak at δ −49.3 ppm split into a broad doublet with a J_PH coupling constant of approximately 350 Hz. Analysis by ESI-MS of the reaction mixture showed a repeating pattern corresponding to the polymeric repeat unit −[PhHP·BH₂]–, see Figure S10, Supporting Information.

Supporting Information Available

Further experimental and characterization details, including selected NMR data and X-ray crystallographic data (including data in CIF format). This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center (CCDC) and can be obtained via www.ccdc.cam.ac.uk/data_request/cif.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.