Pseudotetrahedral manganese complexes supported by the anionic tris(phosphino)borate ligand [PhBP^iPr₃]

Abstract

This paper presents aspects of the coordination chemistry of mono- and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand, [PhBP^iPr₃] (where [PhBP^iPr₃] = [PhB(CH₂PⁱPr₂)₃]⁻). The Mn(II) halide complexes, [PhBP^iPr₃]MnCl (1 and [PhBP^iPr₃]MnI (2), have been characterized by X-ray diffraction, SQUID magnetometry, and EPR spectroscopy. Compound 2 serves as a precursor to a series of Mn azide, alkyl, and amide species: [PhBP^iPr₃]Mn(N₃) (3), [PhBP^iPr₃]Mn(CH₂Ph) (4), [PhBP^iPr₃]Mn(Me) (5), [PhBP^iPr₃]Mn(NH(2,6-ⁱPr₂-C₆H₃)) (6), [PhBP^iPr₃]Mn(dbabh) (7), and [PhBP^iPr₃]Mn(1-Ph(isoindolate)) (8). The complexes 2 – 8 feature a divalent-metal center and are pseudotetrahedral. They collectively represent an uncommon structural motif for low-coordinate, polyphosphine-supported Mn complexes. Two Mn(I) species have also been prepared. These include the Tl-Mn adduct, [PhBP^iPr₃]Tl-MnBr(CO)₄ (9), and the octahedral complex [PhBP^iPr₃]Mn(CN^tBu)₃ (10). Some of our initial synthetic efforts to generate [PhBP^iPr₃]Mn≡N_x species are briefly described, as are DFT studies that probe the electronic viability of these types of multiply bonded target structures.

I. Introduction

Transition metal complexes containing metal-ligand multiple bonds figure prominently in coordination chemistry, especially in atom- and group-transfer processes.^{1, 2} Metal species of the types L_nM=E and L_nM≡E, where E = O, NR, CR₂, can act as intermediates in numerous catalytic reactions including epoxidation,^3-6 aziridination,^7-10 cyclopropanation,^11-13 olefin metathesis,^14-17 and C-H bond insertion.^{10, 18} Besides having broad synthetic utility, metal-to-ligand multiply bonded species also serve as biomimetic models of active sites in metalloenzymes.^19-22 Until a few years ago, examples of stable complexes featuring metal-to-ligand multiple bonds were rare for the later metals,^23-25 especially for those in the first-row.²⁶ While the paucity of these compounds was typically attributed to an inherent incompatibility of high d-electron counts with multiply bonded π-donor substituents,¹ low-coordinate geometries have provided one strategic path to circumvent this limitation. The recent rise in literature reports of well-defined L_nM=E and L_nM≡E systems for the metals M = Fe,^27-33 Co,^34-38 Ni,^39-43 and Cu^{44, 45} strongly attests to the viability of these types of species for the first-row, mid-to-late metals.

Manganese compounds featuring multiply bonded terminal functionalities such as nitrides and imides are far more abundant than their later first-row counterparts.⁴⁶ For example, Mn(V) nitrides are well known to be stabilized by various ligand auxiliaries, including porphyrins,^47-51 macrocyclic amines,^{52, 53} cyanides,⁵⁴ and Schiff bases.^55-58 In contrast, Mn(VI) and Mn(VII) nitrides are rare. Wieghardt and coworkers prepared two examples of Mn(VI) nitrides in situ, but neither compound has been fully characterized.^{52, 54} Hursthouse and coworkers isolated the sole example of a Mn(VII) nitride, [Mn(N)(N^tBu)₃]²⁻, which is supported by imido ligands.⁵⁹ Other relevant manganese imides from the Hursthouse group include species of the type Mn^VII(N^tBu)₃X, as well as the homoleptic compound [Mn^VI(N^tBu)₄]²⁻.^60-62 Terminal Mn(V) imides are also stabilized by corroles, as recently reported by Abu-Omar and coworkers.^63-65 Finally, elegant work from Groves and Carreira strongly implicates Mn(V) acylimides (generated in situ from the nitrides) as effective group-transfer catalysts to olefin and enol ether substrates.^{47, 55, 66}

Our group has been investigating the tris(phosphino)borate (generally denoted as [BP₃]) and hybrid bis(phosphino)pyrazolylborate ([BP₂(pz)]) ligands as scaffolds for M≡E linkages. Our endeavors have led to the characterization of stable Co(III)³⁶ and Fe(II/III) imides, ^{27, 29} as well as distinctive examples of Fe(IV) imides and nitrides.^{28, 67} Motivated by these results, we wish to extend this chemistry to other mid-to-late first-row metals such as Ni⁶⁸ and Mn. In particular, we are interested in whether [BP₃]Mn≡N_x species (d³ and d⁴) are electronically accessible, and in defining synthetic methods for their generation. In pursuit of this task, we began to study aspects of the fundamental coordination chemistry of previously unexplored [BP₃]Mn-systems. In this first study, we report on the synthesis and characterization of a family of monovalent- and divalent-manganese compounds supported by the [PhBP^iPr₃] ligand (where [PhBP^iPr₃] = [PhB(CH₂PⁱPr₂)₃]⁻), some of which might ultimately serve as precursors for the preparation of [BP₃]Mn≡N_x species. Complementary theoretical studies are described for hypothetical imide [PhBP^iPr₃]Mn(N^tBu) and nitride [PhBP^iPr₃]Mn(N) systems. These species appear to be plausible target structures based upon electronic considerations. While studies towards well-defined [BP₃]Mn≡N_x systems are ongoing, the four-coordinate [PhBP^iPr₃]Mn(II) species that are described here represent structurally unusual examples of low-coordinate manganese complexes supported by polyphosphine ligands.

II. Results and Discussion

IIa. Synthesis and Characterization of [PhBP^iPr₃]Mn(II) Halides

The Mn(II) halides [PhBP^iPr₃]MnCl (1) and [PhBP^iPr₃]MnI (2) were readily prepared by mixing the thallium reagent [PhBP^iPr₃]Tl with MnX₂ (where X = Cl or I) (Scheme 1).⁶⁹ This methodology has been used previously by us to prepare related compounds of divalent Fe, Ru, Co, and Ni.^{68, 70, 71} The halides 1 and 2 are colorless in solution and can be crystallized from benzene/petroleum ether. Compound 1 provided light pink blocks in low yield (33 %), while 2 was obtained as colorless blocks in moderate yield (62 %). Because of its higher crystalline yield, compound 2 was used as the preferred starting material throughout the research described herein.

Scheme 1.

The halides 1 and 2 were characterized by various techniques including X-ray crystallography, SQUID magnetometry, and EPR spectroscopy. The solid-state structures of 1 and 2 are shown in Figures 1 and 2. The chloride complex 1 crystallizes as a mixture of its monomer (1a) and its dimer {[PhBP^iPr₃]Mn(μ-Cl)}₂ (1b), whereas the iodide complex 2 crystallizes exclusively as a monomer. A similar monomer-dimer equilibrium was observed for [PhBP₃]CoX systems in solution (where X = Cl or Br).⁷¹ Solid-state structures of monomeric 1a and 2 are pseudotetrahedral with approximate three-fold symmetry about the axis containing B, Mn, and X (I-Mn-B = 176.6°, Cl-Mn-B = 175.5°). The Mn-P bond distances are nearly equal, as well as the P-Mn-P bond angles. For example, monomer 1a has Mn-P distances of 2.53 Å and P-Mn-P angles that are near 90° (90.6 to 92.6°). Likewise, compound 2 exhibits a narrow spread in its Mn-P bond distances (2.524 to 2.538 Å) and P-Mn-P angles (90.7 to 91.4°). The Mn-P bond distances are not unusual for high-spin Mn(II),⁷² but are significantly longer than those of tris(phosphino)borate complexes of later first-row transition metal ions. The series of structurally characterized chloride complexes, [PhBP^iPr₃]MCl for M = Mn, Fe, Co, and Ni, show decreasing M-P bond distances, which reflect both their decreasing ionic radii across the series as well as their total spin S.

Figure 1.

Displacement ellipsoid representations (50%) of (A) [PhBP^iPr₃]MnCl (1a) and (B)[PhBP^iPr₃]MnI (2). Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg) for 1a: Mn–Cl 2.2687(7), Mn–P1 2.5300(6), Cl–Mn–P1 125.38(2), Mn–P2 2.5298(6), Mn–P3 2.5289(6), Cl–Mn–P2 120.22(2), Cl–Mn–P3 126.14(2), P1–Mn–P2 90.55(2), P2–Mn–P3 92.64(2), P1–Mn–P3 92.22(2). For 2: Mn–I 2.6394(4), Mn–P1 2.5351(7), Mn–P2 2.5238(7), Mn–P3 2.5379(6), P1–Mn–I 127.19(2), P2–Mn–I 122.27(2), P3–Mn–I 124.09(2), P1–Mn–P2 91.39(2), P2–Mn–P3 90.68(2), P1–Mn–P3 90.95(2).

Figure 2.

Displacement ellipsoid representations (50%) of {[PhBP^iPr₃]Mn(μ-Cl)}₂ (1b) (top). Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg) for 1b: Mn–ClA 2.4625(9), Mn–ClA 2.5327(9), Mn–ClB 2.4630(9), Mn–ClB 2.525(1), Mn–P1 2.6162(7), Mn–P2 2.6132(7), Mn–P3A 2.552(4), Mn–P3B 2.580(4), P1–Mn–P2 86.38(2), P3A–Mn–P1 87.13(7), P3B–Mn–P1 94.03(7), P3A–Mn–P2 92.55(8), P3B–Mn–P2 86.26(7). View of the core of 1b showing both positions of the disordered chloride and P3 atom (below). The angle between the Mn₂(ClA)₂ and Mn₂(ClB)₂ planes is 41°.

The dimer {[PhBP^iPr₃]Mn(μ-Cl)}₂ (1b) crystallizes in the triclinic crystal system Pī and features a Mn₂Cl₂ core that contains an inversion center. The chloride and one Pⁱ Pr₂ unit are disordered, but both can be independently refined in two positions of nearly equal populations. The solid-state structure of one conformer is shown in Figure 2 (top). A simplified structure containing all of the disordered atoms in the core is shown below it. In the dimer, the Mn center is five-coordinate and roughly square pyramidal. Two phosphorus atoms and two chlorides comprise the base, with an axial phosphine at the apex of the pyramid. The Mn-P bond distances in the dimer are even more elongated relative to the monomer 1a (Mn-P = 2.613, 2.616, and 2.552 Å). One of these bonds (Mn-P3) is slightly shorter than the other two and corresponds to the Mn-P_ax bond. The Mn-Cl bonds in the dimer are also significantly longer (2.463 and 2.533 Å) compared to the monomer (2.269 Å).

Magnetic susceptibility (SQUID) data were obtained on polycrystalline samples of 1 and 2. The μ_eff versus T plots are nearly identical and show very little temperature dependence (Figure 3A). From 60 to 300 K, the μ_eff values average 5.94 μ_B for 1 (calculated as a monomer) and 5.97 μ_B for 2. The μ_eff value for 1 is identical to its solution magnetic moment of 5.9(1) in benzene, where we presume the monomeric form is favored over the dimer form (Evans method, 3 runs).^{73, 74} These values are consistent with 5 unpaired electrons per Mn, verifying the high-spin configuration of these [PhBP^iPr₃]MnX species. The proximity of μ_eff to the spin-only value (5.92 μ_B) further suggests that the ground states of 1 and 2 are orbitally nondegenerate as expected for a ground-state electronic configuration where all the d-orbitals are singly populated (⁶S). The χ_MT versus T plots for 1 and 2 are also similar, but distinct at very low temperatures. From 4 to 75 K, χ_MT dips slightly for 2, perhaps due to intermolecular paramagnetic quenching (Figure 3B).⁷⁵ The χ_MT versus T plot for 1, however, shows a gradual increase in χ_MT in the same temperature range. The small rise in χ_MT from 4.43 to 4.71 cm³ K/mol might be indicative of weak ferromagnetic coupling. Similarly weak coupling has been observed in another Mn(μ-Cl) dimer, and might thus arise from the presence of the dimer 1b.⁷⁶

Figure 3.

(A) SQUID plot of μ_eff (μ_B) versus and (B) χ_MT (cm³K/mol) versus T for [PhBP^iPr₃]MnCl, 1, (■), and [PhBP^iPr₃]MnI, 2, (□). An expansion of the χ_MT vs. T plot at low temperatures (4 to 30 K) is shown in the inset.

EPR spectra were also recorded on the halides in methyl-THF glass at 8 K. The iodide 2 has an axial spectrum with transitions at ∼120 and 330 mT (Figure 4A). The observed g values, ∼ 6 and 2, match the calculated transitions for the m_s = ± ½ states of a S = 5/2 paramagnet with E/D ∼ 0. Hyperfine structure due to the ⁵⁵Mn nucleus (I = 5/2) is evident in the g = 2 signal, which shows a six-line splitting with A ∼ 63 G. The octahedral complexes MnX₂(dmpe)₂ (where X = Br, I) show nearly identical EPR signatures.⁷⁷ Unlike 2, the chloride 1 has an EPR spectrum with numerous transitions occurring from 0 to 1000 mT (Figure 4B). The complicated spectrum is difficult to interpret and is tentatively attributed to a superposition of the monomer (∼ 130 and 360 mT) and dimer forms of 1. The dimer, an S = 5 paramagnet, could itself give rise to multiple transitions. For both 1 and 2, hyperfine coupling to the ³¹P nucleus (I = ½) is not observed, suggesting that the spin density is tightly localized on Mn and therefore indicative of attenuated covalent character at the Mn-P bond by comparison to other [BP₃]M paramagnets we have studied.^{27, 71, 78}

Figure 4.

EPR spectra of glassy methyl-THF solutions of (A) [PhBP^iPr₃]MnI (2) and (B) [PhBP^iPr₃]MnCl (1) at 8 K. Instrumental parameters: ν = 9.38 GHz, modulation frequency = 100 kHz, modulation amplitude = 5 G, microwave power = 0.064 mW, conversion time = 81.9 ms, time constant = 20.5 ms, sweep time = 83.9 ms, 3 scans.

Compounds 1 and 2 were also studied by NMR spectroscopy and electrochemistry, but these data proved relatively uninformative. The ¹H NMR spectra contained a few resolvable resonances and those that could be resolved were unusually broad by comparison to related [PhBP^iPr₃]FeX and [PhBP^iPr₃]CoX systems. The cyclic voltammograms of 1 and 2 did not reveal any reversible features (0.35 M [ⁿBu₄N][PF₆] in THF). The lack of any reversible redox couples also contrasts the analogous halide complexes of Fe, Co, and Ni, all of which exhibit at least pseudo-reversible M^II/M^I redox couples.

Surprisingly, a survey of the CCDC revealed only three structures of monomeric four-coordinate Mn halide complexes with one or two phosphine donor(s).^{72, 79, 80} No such structures with three phosphine donors were found. Even monomeric five-coordinate polyphosphine-ligated Mn halides are few in number, with only three structurally characterized examples.^81-83 Low-coordinate Mn halides are also known for the related tridentate ligand tris(pyrazoyl)borate (Tp). A handful of monomeric four-coordinate manganese structures featuring these ligands have been reported.^84-88

IIb. Synthesis and Characterization of [PhBP^iPr₃]Mn(II) Azide, Alkyls, and Amides

A series of divalent-manganese complexes featuring other X-type ligands were readily prepared from the iodide 2 via metathesis (Scheme 2). For example, addition of excess sodium azide to 2 produced off-white [PhBP^iPr₃]Mn(N₃) (3) in good yield (73 %). Compound 3 features a terminal azide ligand with a characteristic ν(N₃) stretch at 2070 cm⁻¹ (KBr, THF). Its solid-state structure is shown in Figure 5. Not surprisingly, the pseudohalide 3 exhibits a similar molecular geometry to the monomeric halides 1 and 2. In a CCDC search of all structurally characterized Mn complexes containing a terminal azide, the mean Mn-N_azide bond distance is 2.190 Å with a standard deviation of 0.096 Å. The Mn-N bond distance of 2.046(3) Å in 3 is therefore unusually short. Manganese complexes featuring similarly short Mn-N bond distances for terminal azides are typically in the oxidation state +3.^{53, 89-92}

Scheme 2.

Figure 5.

Displacement ellipsoid representations (50%) of [PhBP^iPr₃]Mn(N₃) (3) and [PhBP^iPr₃]Mn(CH₂Ph) (4). For clarity, hydrogen atoms have been omitted and only the phosphorous atoms of the ligand are shown. For 3, only one of the two molecules in the asymmetric unit is shown. Selected bond distances (Å) and angles (deg) for 3: Mn–N1 2.046(3), Mn–P1 2.534(1), Mn–P2 2.522(1), Mn–P3 2.521(2), N1–N2 1.112(5), N2–N3 1.202(6), N2–N1–Mn 128.8(3), P1–Mn–P2 92.33(4), P2–Mn–P3 93.21(5), P1–Mn–P3 91.04(5), N1–Mn–P1 123.1(1), N1–Mn–P2 125.7(1), N1–Mn–P3 122.2(1). For 4: Mn–C28 2.138(2), Mn–P1 2.5679(4), Mn–P2 2.5507(5), Mn–P3 2.5620(5), P1–Mn–P2 91.46(1), P2–Mn–P3 89.44(1), P1–Mn–P3 89.90(1), C28–Mn–P1 117.86(5), C28–Mn–P2 126.42(5), C28–Mn–P3 130.59(5), Mn–C28–C29 111.5(1).

Manganese alkyl compounds were also readily prepared from the addition of alkyl Gringards PhCH₂MgBr and MeMgBr to 2 at −90° in THF. The bright yellow complex [PhBP^iPr₃]Mn(CH₂Ph) (4) and pale yellow complex [PhBP^iPr₃]Mn(Me) (5) are soluble in hydrocarbons, and can be crystallized from cyclopentane at −30 °C. The benzyl complex 4 was characterized by X-ray diffraction, and its solid-state structure is shown in Figure 5. Related examples of low-coordinate monomeric organomanganese compounds are known and include trigonal (β-diketiminato)MnR and tetrahedral (P₂)MnR₂ species, where P₂ represents bidentate phosphine or two PR₃ ligands.^93-95 Despite the low valence electron count of these Mn systems, rather little has been reported concerning their reactivity patterns. One interesting exception concerns tetrahedral Mn(II) dialkyl compounds supported by diimine ligands that readily undergo alkyl migration to the diimine ligand at ambient temperatures.⁹⁶ We have found that the [PhBP^iPr₃]-supported manganese alkyls 4 and 5 are thermally stable up to 65 °C even under an atmosphere of dihydrogen or CO. This sharply contrasts the related iron(II) alkyls [PhBP^iPr₃]Fe-R, which lose alkane upon exposure to H₂ and bind CO readily.⁹⁷

Manganese amides were generated by the addition of lithium and potassium amides to 2 at −90° in THF (Scheme 3). In the metathesis reactions with lithium amides, a common side-product was the free ligand [PhBP^iPr₃][Li], easily identified by ³¹P NMR spectroscopy. Replacement of lithium by potassium minimizes the liberation of the [PhBP^iPr₃] anion. For example, 2,6-diispropylaniline can be deprotonated with KH and added to 2 to produce yellow [PhBP^iPr₃]Mn(NH(2,6-ⁱPr₂-C₆H₃)) (6) in high yield (90 %).

Scheme 3.

This strategy was also employed to install 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (Hdbabh) as the amide functionality. The lithium amide Li(dbabh) is a particularly interesting reagent to investigate because it has been successfully exploited as an N-atom transfer reagent to both Cr and Fe centers with concomitant loss of anthracene.^{28, 98} Methathesis of 2 and Li(dbabh) resulted in a mixture of the amide compound [PhBP^iPr₃]Mn(dbabh) (7), the bridged amide complex {(Hdbabh)MnI(μ-N-dbabh)}₂ (see Supporting Information), and the ligand [PhBP^iPr₃][Li]. Though the yield of the desired compound 7 was quite low, yellow crystals of 7 could be cleanly chosen from the colorless crystals of the byproducts. Attempts to improve the yield of 7 by using K(dbabh) were unsuccessful. Deprotonation of Hdbabh with KH or benzyl potassium afforded a bright yellow-green solution; upon addition of 2 at −90°C, the reaction solution turned orange. Red-orange crystals were grown from this solution that were clearly indicative of a species different from 7. An X-ray diffraction study revealed the product to be a different Mn amide, [PhBP^iPr₃]Mn(1-Ph(isoindolate)) (8), an isomer of 7 wherein one of the five-membered rings has opened by an overall C–C bond cleavage. This decomposition most likely occurs during the deprotonation stage of the reaction since the solution turns brilliant yellow-green upon addition of the potassium base, whereas solutions of Li(dbabh) are pale pink.

The solid state structures of the Mn(II) amides 6, 7, and 8 are shown in Figure 6. Collectively, these amides are similar to the other pseudotetrahedral, high-spin [PhBP^iPr₃]Mn(II) species reported here in that they have typical Mn-P bond distances (ca. 2.55 Å) and P-Mn-P bond angles (ca. 90°). The apical N atom of 7 lies on the Mn-B axis, whereas compounds 6 and 8 exhibit a slightly canted N atom with N-Mn-B angles of 156° and 167° respectively. The Mn-N bond distances follow the order: 7 (1.948(1) Å) < 6 (1.997(2) Å) < 8 (2.054(2) Å). This trend likely reflects the relative steric repulsion between the amide groups and the isopropyl groups of the [PhBP^iPr₃] ligand.

Figure 6.

Displacement ellipsoid representations (50%) of [PhBP^iPr₃]Mn(NH(2,6-ⁱPr₂-C₆H₃)) (6), [PhBP^iPr₃]Mn(dbabh) (7), and [PhBP^iPr₃]Mn(1-Ph(isoindolate)) (8). For clarity, hydrogen atoms (except N-H) have been omitted and only the phosphorous atoms of the ligand are shown. Selected bond distances (Å) and angles (deg) for 6: Mn–N 1.997(2), Mn–P1 2.5877(6), Mn–P2 2.5813(6), Mn–P3 2.5637(6), P1–Mn–P2 87.96(2), P2–Mn–P3 89.95(2), P1–Mn–P3 93.11(2), N–Mn–P1 144.77(5), N–Mn–P2 105.73(5), N–Mn–P3 118.63(5). For 7: Mn–N 1.948(1), Mn–P1 2.5357(5), Mn–P2 2.5473(5), Mn–P3 2.5503(5), P1–Mn–P2 90.46(2), P2–Mn–P3 89.51(2), P1–Mn–P3 90.71(2), N–Mn–P1 126.19(4), N–Mn–P2 126.13(4), N–Mn–P3 122.92(4). For 8: Mn–N 2.054(2), Mn–P1 2.5308(7), Mn–P2 2.5550(7), Mn–P3 2.5718(6), P1–Mn–P2 95.13(2), P1–Mn–P3 89.54(2), P2–Mn–P3 90.09(2), N–Mn–P1 111.98(5), N–Mn–P2 136.31(5), N–Mn–P3 122.30(5).

The Mn(II) alkyls and amides were studied by cyclic voltammetry in THF with 0.35 M [ⁿBu₄N][PF₆] as the supporting electrolyte and Ag/AgNO₃ as the reference electrode. Interestingly, amide 7 is distinct amongst this series of complexes in that it exhibits a reversible 1-electron redox couple centered at −0.26 V (vs. Fc/Fc⁺, scan rate = 50 mV/s), indicative of a Mn^II/Mn^III redox event (Figure 7). The amides 6 and 8 exhibit irreversible oxidative events near this same potential.

Figure 7.

Cyclic voltammagram of [PhBP^iPr₃]Mn(dbabh) (7) recorded at a scan rate of 50 mV/s in THF with 0.35 M [ⁿBu₄][PF₆] as the supporting electrolyte. Potentials are internally referenced to Fc/Fc⁺.

IIc. Synthesis and Characterization of [PhBP^iPr₃]Mn(I) Compounds

Monovalent-manganese compounds may prove to be valuable precursors to Mn≡NR multiply bonded species in future studies. Indeed, oxidative nitrene transfer to Fe(I) and Co(I) precursors has proven highly effective in the synthesis of related Fe≡NR, Co≡NR, and Co=NR species.^{29, 34-36, 99, 100} While monovalent-manganese complexes are numerous, they typically are octahedral 18-electron species stabilized by carbonyl ligands.^101-105 Low-coordinate and unsaturated Mn(I) compounds, on the other hand, are uncommon. Notable examples of this latter class include a three-coordinate Mn(I) dimer supported by bulky β-diketiminate ligands and several five-coordinate Mn compounds of the type (P^P)₂ Mn(CO)⁺ (where P^P is a bidentate phosphine).^106-109

To pursue a low-coordinate Mn(I) precursor, [PhBP^iPr₃]Tl and MnBr(CO)₅ were heated in benzene at 50 °C for several hours. The resulting yellow solution contained some TlBr precipitate, but to our surprise, the major product was a Tl-Mn adduct, [PhBP^iPr₃]Tl-MnBr(CO)₄ (9), which was isolated in 39 % crystalline yield. Compound 9 is diamagnetic and exhibits a doublet in the ³¹P NMR spectrum (45.4 ppm, J_{Tl-P 3} = 8060 Hz) that is shifted relative to [PhBP^iPr₃]Tl (24.3 ppm, J_Tl-P = 5913 Hz). The solid-state structure of 9 is shown in Figure 8. There are several crystal structures featuring a Tl-Mn bond, and the Tl-Mn bond of 2.6437(4) Å is similar to those reported previously.^{110, 111}

Figure 8.

Displacement ellipsoid representations (50%) of [PhBP^iPr₃]Tl–MnBr(CO)₄ (9). For clarity, hydrogen atoms have been omitted. Selected bond distances (Å) and angles (deg) for 9: Mn–Tl 2.6437(4), Mn–C28 1.913(3), Mn–C29 1.799(3), Mn–C30 1.851(3), Mn–C31 1.823(3), Mn–Br 2.5299(4), C31–Mn–Tl 172.74(8), Br–Mn–Tl 86.11(1), C29–Mn–C28 92.6(1), C29–Mn–C30 91.1(1), C28–Mn–Br 85.39(7), C29–Mn–Br 177.98(8), C30–Mn–Br 90.94(8), C31–Mn–Br 86.73(8).

Prolonged heating of a benzene solution of 9 resulted in its decomposition. By ³¹P NMR spectroscopy, one major product forms upon thermolysis, characterized by a broad peak at ∼50 ppm. An IR spectrum revealed two sharp stretches at 1999 and 1906 cm⁻¹ (KBr/THF). We propose this species to be the Mn(I) tricarbonyl compound, [PhBP^iPr₃]Mn(CO)₃ (Scheme 4). Indeed, the isoelectronic compound, [PhBP^iPr₃]Mn(CN^tBu)₃ (10), can be prepared by the reduction of [PhBP^iPr₃]MnI with sodium napthalenide followed by addition of excess CN^tBu. Compound 10 is isolated in pure form but in low yield (22 %) due in part to the competitive formation of [PhBP^iPr₃]Na.¹¹² Similarly, reduction of [PhBP^iPr₃]MnI with sodium napthalenide under CO produces the presumed degradation product of 9, [PhBP^iPr₃]Mn(CO)₃. Unfortunately, this species has not been isolated in pure form. The solid-state structure of 10 confirms its assignment as a monovalent-Mn complex with three isocyanides (Figure 9). Relative to the high-spin Mn(II) species, the Mn-P bonds (2.36 to 2.38 Å) are more than 0.15 Å shorter. The P-Mn-P bond angles are also slightly contracted (86.5 − 89.2°). Compound 10 is also characterized by a reversible redox process at −0.66 V in its cyclic voltammagram, shown in Figure 10 (vs. Fc/Fc⁺, scan rate = 100 mV/s).

Scheme 4.

Figure 9.

Displacement ellipsoid representations (50%) of [PhBP^iPr₃]Mn(CN^tBu)₃ (10) from the side (A) and top (B) views. For clarity, hydrogen atoms have been omitted and only the phosphorous atoms of the ligand are shown. Selected bond distances (Å) and angles (deg) for 10: Mn–C28 1.870(1), Mn–C33 1.864(1), Mn–C38 1.835(1), N1–C28 1.176(2), N2–C33 1.176(2), N3–C38 1.185(2), Mn–P1 2.3597(4), Mn–P2 2.3831(4), Mn–P3 2.3655(3), P1–Mn–P2 88.59(1), P2–Mn–P3 86.49(1), P1–Mn–P3 89.17(1), C33–Mn–C28 83.75(5), C38–Mn–C28 85.26(5), C38–Mn–C33 89.12(5).

Figure 10.

Cyclic voltammagram of [PhBP^iPr₃]Mn(CN^tBu)₃ (10) recorded at a scan rate of 100 mV/s in THF with 0.35 M [ⁿBu₄][PF₆] as the supporting electrolyte. Potentials are internally referenced to Fc/Fc⁺.

IId. DFT Models of Mn=N^tBu and Mn≡N species

DFT studies were undertaken to obtain some insight on the electronic viability of two species, [PhBP^iPr₃]Mn^III≡N^tBu and [PhBP^iPr₃]Mn^IV≡N. We have previously used DFT methods to probe the electronic structures of [BP₃]M species with terminally bound N_x-functionalities that are strongly π-donating, for example [PhBP₃]Co^III≡N(p-tolyl), [PhBP₃]Fe^III≡N^tBu, and [PhBP^iPr₃]Fe^IV≡N.^{29, 36, 28}

For the low-spin Fe and Co imides, a general d-orbital splitting diagram has been forwarded based upon these DFT studies. The d-orbitals, which transform as a₁ + 2 e under idealized three-fold symmetry, split into two high-lying and three lower lying orbitals. The higher energy e-set, which in simplest terms can be thought of as comprising d_xz and d_yz, is destabilized by σ^* and π^* interactions with the phosphines and imide functionality, respectively. The remaining orbitals, of predominant d_z2 (a₁) and d_xy and d_x2-y2 (e) character, lie at lower energy and are predominantly non-bonding in nature.²⁷

If we apply this qualitative d-orbital picture to the hypothetical d⁴ imide, [PhBP^iPr₃]Mn≡N^tBu, the two ground states low-spin (S = 0) and intermediate-spin (S = 1) need to be considered. Recently, we have reported the characterization of an unusual Fe^IV imide that exhibits a triplet ground state.⁶⁷ Because this species is isoelectronic to the hypothetical Mn(III) imide under consideration here, we might expect the latter to be a triplet as well. To test this from a theoretical standpoint, the imide [PhBP^iPr₃]Mn≡N^tBu was optimized as a singlet and a triplet using the Jaguar computational program (B3LYP/LACVP^**). The input geometry was derived from the crystal structure coordinates of [PhBP^iPr₃]Fe(NAd), where the metal and N_x-functionality were changed to Mn and N^tBu respectively. A geometry optimization was then performed using a spin-unrestricted calculation and conducted without any symmetry constraints. (See Supporting Information)

The predicted structures for the singlet and triplet states of [PhBP^iPr₃]Mn≡N^tBu differ quite remarkably from one another (Figure 11). The optimized singlet structure has a bent Mn-N-C angle (145°) with expanded P-Mn-P angles (97−98°), whereas the optimized triplet structure features a linear Mn-N-C angle (179°) with characteristic P-Mn-P angles (90−92°). The triplet state exhibits what appears to be a more likely geometry, in accord with the fact that it is predicted by DFT to be more energetically stable by 0.74 eV than the singlet state.

Figure 11.

DFT-optimized structures (Jaguar 5.0, B3LYP/LACVP**) for (A) singlet [PhBP^iPr₃]Mn(N^tBu) and (B) triplet [PhBP^iPr₃]Mn(N^tBu). Selected bond distances (Å) and angles (deg) for (A): Mn–N 1.63; Mn–P 2.28, 2.36, 2.36; Mn–N–C 144.8; P–Mn–P 96.8, 97.2, 97.7; N–Mn–P 104.7, 126.2, 127.2. For (B): Mn–N 1.66; Mn–P 2.40, 2.44, 2.45; Mn–N–C 179.2; P–Mn–P 89.5, 89.8, 91.5; N–Mn–P 121.9, 126.2, 127.0.

Based on DFT studies, the known nitride [PhBP^iPr₃]Fe^IV≡N has a splitting diagram that differs from the structurally analogous imides in that the a₁ orbital of dz² parentage is more appreciably destabilized by sigma antibonding interactions with the nitride functionality and the phosphine donors.²⁸ This situation leaves only two orbitals at very low energy, giving rise to the singlet ground state of [PhBP^iPr₃]Fe^IV≡N. Based on this splitting scheme, the hypothetical Mn nitride [PhBP^iPr₃]Mn≡N might be expected to populate a doublet ground state. To ascertain whether this would be the case, the Mn nitride [PhBP^iPr₃]Mn(N) was modeled as both an S = 1/2 and S = 3/2 system. In the optimized quartet structure, the nitride atom is severely tilted off the Mn-B axis by 30° (Figure 12A) in what appears to be an unfavorable geometry. The P-Mn-P bond angles (90 to 91°) and Mn-P bond distances (2.48 to 2.60 Å) are, however, typical of the pseudotetrahedral [PhBP^iPr₃]Mn(II) complexes described here. The Mn-N bond length of 1.62 Å seems excessively long for known Mn≡N species, which typically have Mn-N bond lengths in the range of 1.50 to 1.55 Å. These distortions in the optimized quartet structure presumably arise from population of the antibonding d_z2 orbital.

Figure 12.

DFT-optimized structures (Jaguar 5.0, B3LYP/LACVP**) for (A) doublet [PhBP^iPr₃]Mn(N) and (B) quartet [PhBP^iPr₃]Mn(N). Selected bond distances (Å) and angles (deg) for (A): Mn–N 1.50; Mn–P 2.35, 2.41, 2.45; N–Mn–P 112.1, 122.3, 127.5; P–Mn–P 94.8, 95.6, 97.2. For (B): Mn–N 1.62; Mn–P 2.48, 2.49, 2.60; N–Mn–P 103.9, 109.3, 155.3; P–Mn–P 89.9, 90.5, 91.2.

Relative to the quartet state, the doublet state is energetically preferred by 0.41 eV and exhibits what appears to be a more reasonable geometry (Figure 12B). The nitride atom is canted off the Mn-B axis by only 9° and binds tightly to Mn with a Mn-N bond length of 1.50 Å. The P-Mn-P bond angles are slightly expanded (95 to 97°), approaching a more tetrahedral geometry. Similar parameters were found in the DFT optimized structure of [PhBP^iPr₃]Fe^IV≡N, which features expanded P-Fe-P bond angles (99−101°) and an Fe-N bond length of 1.49 Å. It is the combination of these two structural parameters that renders the a₁ (dz²) orbital more antibonding for the metal nitrides than in the case of the related metal imides.

III. Concluding Remarks

With this report, we have extended our study of the coordination chemistry of first-row mid-to-late transistion metals supported by electron-releasing [BP₃] ligands. As with Fe, Co, and Ni, the [PhBP^iPr₃] scaffold readily supports Mn(II) complexes in a pseudotetrahedral geometry. The preparative significance of the systems described herein is best underscored by noting how uncommon low-coordinate manganese complexes supported by polyphosphines are. The high donor strength and anionic charge of the [PhBP^iPr₃] ligand may be advantageous over polyphosphine ligands in general by helping to attenuate problematic phosphine dissociation. These two ligand properties are not always sufficient, however, as evident from the observed propensity to substitute manganese by an alkali metal cation or by thallium in the [PhBP^iPr₃] binding pocket under certain metathetical exchange reactions. Fortunately, the halide [PhBP^iPr₃]MnI does serve as a sufficiently useful reagent for most metathetical reactions, allowing the preparation of [PhBP^iPr₃]Mn(II) azide, alkyl, and amide species.

DFT calculations of the hypothetical Mn(III) imide [PhBP^iPr₃]Mn(N^tBu) and nitride [PhBP^iPr₃]Mn(N) structures suggests that their preferred ground states will be triplet and doublet, respectively. While the preparation of Mn≡N_x species of these types has, at this stage, only been cursorily canvassed it is worth noting a few experimental observations. For instance, we have explored whether the Mn(I) complex 10 might undergo facile oxidative group transfer in the presence of organic aryl and alkylazides. We have also canvassed conditions that might have effected the oxidative deprotonation of the anilide species 6.⁴² Finally, attempts to generate a Mn(IV) nitride species, including thermolysis and photolysis of the amide complex [PhBP^iPr₃]Mn(dbabh) 7, as well as photolysis of the Mn azide 3, have been explored. As yet, none of these straightforward approaches have provided a tractable Mn≡N_x species. Therefore, while the preparation of well-defined low-valent Mn complexes with multiply bonded linkages remains a challenge, it is one that should be surmountable. Our hope is that manganese imides and nitrides of these types will benefit from synthetic access to unsaturated Mn(I) precursors, such as a four-coordinate [PhBP^iPr₃]Mn-PR₃ species. Oxidative nitrene or nitride group transfer might then prove synthetically more fruitful. Efforts along these lines are part of ongoing studies in our labs.

Experimental Section

All manipulations were carried out using standard Schlenk or glovebox techniques under a dinitrogen atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thorough sparging with N₂ gas followed by passage through an activated alumina column. Nonhalogenated solvents were tested with a standard purple solution of benzophenone ketyl in tetrahydrofuran to confirm effective oxygen and moisture removal. Deuterated solvents were purchased from Cambridge Isotopes Laboratories, Inc. and were degassed and stored over activated 3-Å molecular sieves prior to use. The compounds [PhBP^iPr₃]Tl,⁷⁰ benzyl potassium,¹¹³ and Li(dbabh)⁹⁸ were prepared according to literature procedures. The reagents 2,6-diisopropyl aniline and ^tBuNC were dried over sieves prior to use. Sodium azide was carefully dried by heating at 50 °C under vacuum for two days. (CAUTION: Azides are potentially explosive and should generally be stored away from heat and ignition sources.) Potassium hydride was crushed into a fine powder, washed liberally with petroleum ether, and stored at −30 °C. All other reagents were purchased from commercial vendors and used without further purification. Elemental analyses were performed by Desert Analytics, Tucson, AZ. Varian 300 MHz spectrometers were used to record the ¹H NMR and ³¹P NMR spectra at ambient temperature. ¹H chemical shifts were referenced to residual solvent, while ³¹P NMR chemical shifts were referenced to 85% H₃PO₄ at δ0 ppm. IR measurements were obtained with a KBr solution cell using a Bio-Rad Excalibur FTS 3000 spectrometer controlled by Bio-Rad Merlin Software (v. 2.97) set at 4 cm⁻¹ resolution. Electrochemical measurements were recorded in a glovebox under a dinitrogen atmosphere using a BAS CV 100W (Bioanalytical Systems Inc., West Lafayette, IN). A glassy carbon working electrode, a platinum wire auxiliary-electrode, and an Ag/AgNO₃ non-aqueous reference electrode were used in the electrochemical studies. X-ray diffraction studies were carried out in the Beckman Institute Crystallographic Facility on a Bruker Smart 1000 CCD diffractometer. Crystals were mounted on a glass fiber with Paratone-N oil.

EPR Measurements

X-band EPR measurements were recorded using a Bruker EMX spectrometer (controlled by Bruker Win EPR software version 3.0) equipped with a rectangular cavity working in the TE₁₀₂ mode. Low temperature measurements were conducted with an Oxford continuous-flow helium cryostat. Accurate frequency values were recorded from a frequency counter on the microwave bridge. Solution spectra were acquired in 1-methyltetrahydrofuran. Samples were prepared in a glovebox under dinitrogen in quartz EPR tubes equipped with ground glass joints.

Magnetic Measurements

Magnetic measurements were recorded using a Quantum Designs SQUID magnetometer running Magnetic Property Measurement System Rev. 2 software. Data were recorded at 5000 G. The sample was suspended in the magnetometer in a plastic straw sealed under nitrogen with Lilly No. 4 gel caps. Loaded samples were centered within the magnetometer using the DC centering scan at 35 K and 5000 G. Data were acquired at 4−30 K (one data point every 2 K) and 30−300 K (one data point every 5 K). The magnetic susceptibility was adjusted for diamagnetic contributions using the constitutive corrections of Pascal's constants. The molar magnetic susceptibility (χ_m) was calculated by converting the calculated magnetic susceptibility (χ) obtained from the magnetometer to a molar susceptibility (using the multiplication factor {(molecular weight)/[(sample weight)(field strength)]}). Effective magnetic moments were calculated using eqn (1).

$${\mu }_{\mathrm{eff}}={\left(7.997{\chi }_{\mathrm{m}}T\right)}^{1∕2}$$

Computational Methods

All calculations were performed using the Jaguar 5.0 program package (Jaguar 5.0, Schrodinger, LLC, Portland, OR, 2002). The calculation employed the hybrid DFT functional B3LYP and the basis set LACVP**. Input coordinates for the geometry optimizations were derived from the solid state structure of [PhBP^iPr₃]Fe≡NAd, where Fe was replaced by Mn.⁹⁹ For the Mn imide, the adamantyl group was truncated to a tert-butyl group; and for the nitride, the adamantyl group was removed completely. Spin states and molecular charges were imposed as described in the text. The calculations were spin unrestricted, and no symmetry constraints were used. The default values for geometry and SCF iteraction cutoffs were used, and all structures converged under these criteria.

Synthesis of [PhBP^iPr₃]MnCl (1)

A THF solution (18 mL) of [PhBP^iPr₃]Tl (250.0 mg, 36.46 mmol) was added dropwise to a stirring suspension of MnCl₂ (46.3 mg, 36.36 mmol). The reaction mixture was stirred for 24 h, and filtered through a Celite pad. The colorless solution was concentrated under reduced pressure to dryness to yield an off-white solid. After washing the solid liberally with petroleum ether, the product was extracted into benzene. Vapor diffusion of petroleum ether into the benzene solution resulted in pale pink blocks (80 mg, 33% yield). ¹H NMR (C₆D₆, 300 MHz) δ ∼20 (v. br), 10.3 (br), 8.1 (br). Anal. Calcd for C₂₇H₅₃BClMnP₃: C 56.71; H 9.34; N 0. Found: C 56.43; H 9.44; N 0.19.

[PhBP^iPr₃]MnI (2)

A THF solution (20 mL) of [PhBP^iPr₃]Tl (284.7 mg, 41.52 mmol) was added dropwise to a stirring suspension of MnI₂ (129.5 mg, 41.52 mmol). The reaction mixture was stirred for 12 h, and filtered through a Celite pad. The colorless solution was concentrated under reduced pressure to dryness to yield an off-white solid. After washing the solid liberally with petroleum ether, the product was extracted into hot benzene. Vapor diffusion of petroleum ether into the benzene solution resulted in colorless crystals (172 mg, 62% yield). ¹H-NMR (C₆D₆, 300 MHz) δ 21 (v. br), 8.8 (br), 6.6 (br). Anal. Calcd for C₂₇H₅₃BIMnP₃: C 48.89; H 8.05; N 0. Found: C 49.12; H 7.54; N < 0.05.

[PhBP^iPr₃]Mn(N₃) (3)

A THF solution (15 mL) of [PhBP^iPr₃]MnI (406.0 mg, 0.612 mmol) was added to a suspension of NaN₃ (203.0 mg, 3.06 mmol). The reaction mixture was stirred for 12 h, and filtered through a Celite pad. The cream-colored filtrate was concentrated under reduced pressure to dryness and extracted with benzene (258 mg, 73 % yield). Single crystals suitable for X-ray diffraction were grown from vapor diffusion of petroleum ether into the benzene solution. ¹H NMR (C₆D₆, 300 MHz) δ 11 (v. br), 8.3 (br). Anal. Calcd for C₂₇H₅₃BMnN₃P₃: C 56.07; H 9.24; N 7.26. Found: C 55.67; H 8.85; N 7.34.

[PhBP^iPr₃]Mn(CH₂Ph) (4)

An Et₂O solution of benzylmagnesium bromide (107 μL, 1.41 M) was added dropwise to a THF solution of [PhBP^iPr₃]MnI (100.0 mg, 0.151 mmol) chilled to −90 °C. The solution was allowed to warm slowly to rt. The yellow solution was filtered through a Celite pad, concentrated under reduced pressure to dryness, and extracted with petroleum ether (83.4 mg, 88 % yield). Yellow block crystals suitable for X-ray diffraction were grown from cyclopentane at −30 °C. ¹H-NMR (C₆D₆, 300 MHz) δ 16 (v. br), 10.3 (br), 8.3 (br). Anal. Calcd for C₃₄H₆₀BMnP₃: C 65.08; H 9.64; N 0. Found: C 64.98; H 9.32; N <0.05.

[PhBP^iPr₃]Mn(Me) (5)

An Et₂O solution of methylmagnesium bromide (53.5 μL, 2.82 M) was added dropwise to a THF solution of [PhBP^iPr₃]MnI (100.0 mg, 0.151 mmol) chilled to −90 °C. The solution was allowed to warm slowly to rt. The pale yellow solution was filtered through a Celite pad, concentrated under reduced pressure to dryness, and extracted with petroleum ether (74.1 mg, 89 % crude yield). Crystals were grown from cyclopentane at −30 °C (42.3 mg, 51 % yield). ¹H NMR (C₆D₆, 300 MHz) δ ∼ 17 (v. br), 10.1 (br), 8.3 (br). Anal. Calcd for C₂₈H₅₆BMnP₃: C 60.99; H 10.24; N 0. Found: C 60.85; H 9.94; N < 0.05.

[PhBP^iPr₃]Mn(NH(2,6-ⁱPr₂-C₆H₃)) (6)

A THF solution (8 mL) of 2,6-diisopropylaniline (25.1 mg, 0.142 mmol) was added dropwise to a suspension of KH (5.7 mg, 0.142 mmol) chilled to −90 °C. The solution was allowed to warm slowly to rt. After stirring for 12 h, the solution was rechilled to −90 °C and a THF solution (8 mL) of [PhBP^iPr₃]MnI (93.9 mg, 0.142 mmol) was added dropwise. After slowly warming to rt, the bright yellow reaction was concentrated under reduced pressure to dryness. The product can be extracted with petroleum ether, filtered through a Celite pad to remove salts, and concentrated under reduced pressure to afford yellow solids (90.9 mg, 90 % yield). Single crystals suitable for X-ray diffraction were grown from TMS₂O/petroleum ether at −30 °C. ¹H NMR (C₆D₆, 300 MHz) δ 16.3 (v. br), 10.7 (br), 8.1 (br). Anal. Calcd for C₃₉H₇₁BMnNP₃: C 65.73; H 10.04; N 1.97. Found: C 65.81; H 9.71; N 1.88.

[PhBP^iPr₃]Mn(dbabh) (7)

A THF solution (5 mL) of [PhBP^iPr₃]MnI (142.2 mg, 0.214 mmol) was added dropwise to a Et₂O suspension (10 mL) of Li(dbabh) (42.7 mg, 0.214 mmol) chilled to −90 °C. The solution was allowed to warm slowly to rt. The yellow solution was filtered through a Celite pad, concentrated under reduced pressure to dryness, and extracted with hot Et₂O. Cooling the ether solution to −30 °C afforded yellow blocks (49.7 mg, 32 % yield). ¹H NMR (C₆D₆, 300 MHz) δ 21 (v. br), 10.3 (br), 8.1 (br). Anal. Calcd for C₄₁H₆₃BMnNP₃: C 67.59; H 8.72; N 1.92. Found: C 66.28; H 8.40; N 1.77.

[PhBP^iPr₃]Mn(1-Ph(isoindolate)) (8)

A THF solution (10 mL) of benzyl potassium (40.0 mg, 0.307 mmol) was added to a solution of Hdbabh (59.4 mg, 0.307 mmol) chilled to −90 °C. The solution was allowed to warm slowly to rt. After stirring for 24 h, the solution was rechilled to −90 °C and a THF solution (10 mL) of [PhBP^iPr₃]MnI (203.9 mg, 0.307 mmol) was added dropwise. After slowly warming to rt, the orange solution was filtered through a Celite pad, concentrated under reduced pressure to dryness, and washed with petroleum ether, and extracted with hot Et₂O. Cooling the ether solution to − 30 °C afforded red-orange crystals (71.8 mg, 35 % yield). ¹H NMR (C₆D₆, 300 MHz) δ 11 (v. br), 8.2 (br). Anal. Calcd for C₄₁H₆₃BMnNP₃: C 67.59; H 8.72; N 1.92. Found: C 66.34; H 9.83; N 1.82.

[PhBP^iPr₃]Tl-MnBr(CO)₄ (9)

[PhBP^iPr₃]Tl (150.0 mg, 0.219 mmol) and MnBr(CO)₅ (73.6 mg, 0.262 mmol) were mixed together in 10 mL of benzene. The reaction mixture was heated at 50 °C for 7 h. After cooling, the solution was concentrated under reduced pressure to dryness, extracted with benzene, filtered through a glass wool pipette, and concentrated to dryness. The yellow solids were washed liberally with petroleum ether and extracted with benzene. Vapor diffusion of petroleum ether into the solution afforded yellow-orange crystals (79.2 mg, 39 % yield). ¹H NMR (C₆D₆, 300 MHz) δ 7.76 (d, J = 6.9 Hz, 2H, H_o of Ph), 7.53 (t, J = 6.9 Hz, 2H, H_m of Ph), 7.30 (t, J = 6.9 Hz, 1H, H_p of Ph), 2.00 (m, 6H, CH₂P), 1.26 (m, 6H, CHMeMe′), 1.09 (d, J = 6.9 Hz, 18H, CHMeMe′), 1.01 (d, J = 6.9 Hz, 18H, CHMeMe′). ³¹P NMR (C₆D₆, 121 MHz) δ 45.4 (d, J_Tl-P = 8060 Hz). Anal. Calcd for C₃₁H₅₃BBrMnO₄P₃Tl: C 39.92; H 5.73; N 0. Found: C 40.35; H 5.83; N 0.06.

[PhBP^iPr₃]Mn(CN^tBu)₃ (10)

A THF solution (5 mL) of sodium naphthalenide (0.237 mmol, prepared by stirring excess Na and stoichiometric naphthalene in THF for 2 h, filter prior to use) was added to a stirring solution of [PhBP^iPr₃]MnI (150.0 mg, 0.226 mmol). ^tButylisocyanide (43.7 μL, 0.724 mmol) was then quickly added to the mixture. After 11 h, the reaction mixture was concentrated under reduced pressure to dryness, extracted with Et₂O, and filtered through a glass wool pipette. The solution was again concentrated under reduced pressure to dryness, extracted with petroleum ether, and filtered through a glass wool pipette. Cooling of the solution to −30 °C afforded pale yellow crystals (39.4 mg, 22 % yield). ¹H NMR (C₆D₆, 500 MHz) δ 8.15 (br, 2H), 7.63 (br, 2H), 7.34 (br, 1H), 2.19 (br, 6H), 1.54 (br, 18H), 1.39 (br, 18H), 1.27 (s, 27H), 0.94 (br, 6H). ³¹P NMR (C₆D₆, 121 MHz) δ 55.6 (br). Anal. Calcd for C₄₂H₈₀BMnN₃P₃: C 64.20; H 10.26; N 5.35. Found: C 63.84; H 10.10; N 5.22. IR (KBr/THF): ν(cm⁻¹) 2048 (br).

Table 1.

X-ray Diffraction Experimental Details for compounds 1a, 1b, 2−4, and 6−10.

	1a	1b	2	3	4
chemical formula	0.98C27H53BP3Mn Cl·0.02 Tl C27H53BP3	C54H106B2Cl2Mn2P6	C27H53BP3MnI·C6H6	0.77C27H53BP3Mn N3·0.05 TlC27H53BP3·0.18 C27H53BP3MnCl	C34H60BMnP3
formula weight	573.66	1143.61	741.36	1165.21	627.48
T (K)	100	100	100	100	100
λ (Å)	0.71073	0.71073	0.71073	0.71073	0.71073
a (Å)	14.493(2)	10.950(2)	9.5424(8)	11.946(2)	9.4120(6)
b (Å)	12.928(2)	11.718(2)	9.9006(8)	34.586(7)	11.8746(8)
c (Å)	34.850(5)	13.432(3)	19.668(2)	15.423(3)	16.736(1)
α (°)	90	68.000(3)	89.780(1)	90	90
β (°)	100.344(3)	76.424(4)	89.920(1)	90	105.605(1)
γ (°)	90	78.528(3)	82.662(1)	90	90
V (Å3)	6424(2)	1541.5(5)	1842.9(3)	6372(2)	1801.6(2)
Space group	C2/c (# 15)	P-1 (# 2)	P-1 (# 2)	Pnc2 (# 30)	P21 (# 4)
Z	8	1	2	4	2
Dcalcd (g/cm3)	1.186	1.232	1.336	1.215	1.157
μ (cm−1)	7.3	6.9	13.5	8.3	5.2
R1, wR2 (I>2σ(I))a	0.0422, 0.0720	0.0356, 0.0674	0.0305, 0.0586	0.0938, 0.0783	0.0328, 0.0570

	6	7	8	9	10
chemical formula	C39H71BMnNP3	C41H63BMnNP3	C41H63BMnNP3·0.50C4H10O	C31H53BBrMnO4P3Tl	C42H80BMnN3P3
formula weight	712.63	728.58	765.64	932.67	785.75
T (K)	100	100	98	100	100
λ (Å)	0.71073	0.71073	0.71073	0.71073	0.71073
a (Å)	10.690(1)	10.4294(9)	19.6218(9)	10.2574(5)	11.2996(7)
b (Å)	11.249(1)	16.812(1)	77.737(4)	18.6613(8)	17.578(1)
c (Å)	18.718(2)	23.393(2)	11.2629(6)	19.9243(9)	23.108(1)
α (°)	98.393(2)	90	90	90	90
β (°)	101.614(2)	97.476(1)	90	100.590(1)	93.660(2)
γ (°)	107.482(2)	90	90	90	90
V (Å3)	2050.8(4)	4066.7(6)	17180(2)	3748.9(3)	4580.4(5)
Space group	P-1 (# 2)	P21/c (# 14)	Fdd2 (# 43)	P21/n (# 14)	P21/n (# 14)
Z	2	4	16	4	4
Dcalcd (g/cm3)	1.154	1.190	1.184	1.652	1.139
μ (cm−1)	4.7	4.7	4.5	58.6	4.2
R1, wR2 (I>2σ(I))a	0.0419, 0.0694	0.0409, 0.0724	0.0514, 0.0731	0.0355, 0.0532	0.0437, 0.0707

^aR₁ = Σ∥F_o| - |F_c∥ / Σ|F_o|, wR2 = {Σ[w(F_o² — F_c²)²] / Σ[w(F_o²)²]}^1/2