P-Atom Transfer from Phosphaethynolate to an Alkylidyne

Abstract

A low-spin and mononuclear vanadium complex, (^Menacnac)V(CO)(η²–P≡C^tBu) (2) (^Menacnac⁻ = [ArNC(CH₃)]₂CH, Ar = 2,6-ⁱPr₂C₆H₃), was prepared upon treatment of the vanadium neopentylidyne complex (^Menacnac)V≡C^tBu(OTf) (1) with Na(OCP)(diox)_2.5 (diox = 1,4-dioxane) while the isoelectronic ate-complex [Na(15–crown–5)]{([ArNC(CH₂)]CH[C(CH₃)NAr])V(CO)(η²–P≡C^tBu)} (4), was obtained via the reaction of Na(OCP)(diox)_2.5 and ([ArNC(CH₂)]CH[C(CH₃)NAr])V≡C^tBu(OEt₂) (3) in the presence of crown–ether. Computational studies suggest the P–atom transfer to proceed by [2+2]–cycloaddition of the P≡C bond across the V≡C^tBu moiety, followed by a reductive decarbonylation to form V–C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of Density Functional Theory (DFT) calculations, UV-Vis and NMR spectroscopies, cyclic voltammetry, X-ray Absorption Spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]^‒ and an alkylidyne ligand.

Graphical Abstract

The now popular ambidentate phosphaethynolate anion,^[1] [PCO]⁻, can not only coordinate like the ubiquitous isocyanate ligand but can also selectively deliver a P–atom to transition metals complexes including Ti^[2], W^[3], Re^[4], Ir^[5], Ni^[6], Pt,^[7] Cu^[5] and Au^[5], as well as main group compounds composed of Ge^[8] and Sn^[9] concurrently with CO extrusion. In some exceptional cases, the oxophilicity of the metal ion can instead deoxygenate [PCO]⁻, thus affording the metal–oxide and cyaphide (CP⁻) as the final fragments.^[10] [PCO]⁻ can also deliver a phosphorus atom to various electrophilic organic moieties, such as imidazolium,^[11] silyldiium,^[12] cyclopropenium,^[13] chlorodiazaphospholidine,^[14] cyclotrisilenes,^[15] boroles,^[16] and acyl chlorides,^[17] inter alia.^[18] Likewise, unsaturated organic molecules can engage the [PCO]⁻ anion to give cyclooligomerizations that produce P–containing heterocycles,^[19] wherein the reaction generally proceeds through [2+2], [3+2], or [4+2] cycloaddition pathways.^[19f]

In the context of early–transition metals, [PCO]⁻ has been demonstrated to insert into strained zirconium carbon bonds (benzyne complex) to generate a zirconio–phosphaalkene moiety, which then can hydrolyze to produce benzoylphosphine, PhC(O)PH₂, along with the presumed oxo side–product [Cp₂ZrO]_x.^[20] Other than this one example with a strained Zr−C bond, the chemistry of the [PCO]⁻ anion with reactive early–transition metals possessing metal–carbon multiple bonds has remained unexplored. Hence, we desired to obtain some insight into the reactivity between [PCO]⁻ and a highly polarized and basic alkylidyne moiety with a redox–active metal center, namely the complexes (^Menacnac)V≡C^tBu(OTf) (1)^[21] and ([ArNC(CH₂)]CH[C(CH₃)NAr]) V≡C^tBu(OEt₂) (3).^[22] In spite of the hard nature and oxophilicity of vanadium center in 1 and 3, the V≡C^tBu fragment in these systems engages in a [2+2]-cycloaddition involving the P–C multiple bond, followed by a decarbonylation step that furnishes a V–carbonyl and side-bound phosphaalkyne. To the best of our knowledge, this is the first example of sodium phosphaethynolate as a P–atom transfer reagent to an alkylidyne to produce a phosphaalkyne complex,^[23] and could provide entry to an arsenal of metal-stabilized phosphaalkynes with less bulky groups^[24] without detrimental pathways such as oligomerization.^[25]

Treatment of the vanadium alkylidyne complex 1 with one equivalent of Na(OCP)(dioxane)_2.5 in THF over a period of 1 hour resulted in a color change from dark green to a lighter shade of green with Na(OTf) precipitation. The resulting diamagnetic vanadium complex was isolated at 91% yield and characterized as (^Menacnac)V(CO)(η²–PC^tBu) (2) based on a combination of NMR and IR spectroscopic techniques (Scheme 1a). Accordingly, complex 2 displays a ν_CO stretch at 1832 cm⁻¹ consistent with a carbonyl ligand experiencing significant back bonding when compared to free CO (2143 cm⁻¹). The ¹H NMR spectrum revealed a C_s symmetric complex due to the presence of two inequivalent ⁱPr methine moieties at 5.42 and 0.27 ppm (in addition to four diastereotopic ⁱPr methyls), and one β–methine resonance at 4.83 ppm belonging to the β–diketiminate ligand. The two ⁱPr methine groups differ by more than 5 ppm, suggesting that the anisotropy of the β–diketiminate ligand has a substantial influence on their chemical shift. Further assignments of these groups and the sp² hybridized CH hydrogens were derived from a combination of ¹H–¹H COSY, 1H–¹³C{¹H} HSQC and HMBC NMR spectroscopic experiments. In addition, the ³¹P{¹H} and ⁵¹V NMR spectra evinced a highly downfield and broad resonance at 479.8 ppm (Δν_1/2 = 576 Hz) and 2572.3 ppm (Δν_1/2 = 1358 Hz), respectively, consistent with the presence of single V and P chemical environments.

Scheme 1.

Synthesis of complexes 2 and 4, from addition of Na(OCP)(diox)_2.5 to 1 and 3, respectively.

Figure 1a displays the centrosymmetric single-crystal solid–state structure of compound 2, confirming the C_s symmetry established by solution NMR spectral data, but also revealing the unprecedented P–atom transfer from the phosphaethynolate to the α–carbon of the neopentylidyne fragment. Evidently, the symmetry of this molecule is defined by the plane in which the carbonyl C≡O and phosphaalkyne C≡P atoms coincide while bisecting the β–diketiminate ligand. Notably, the phosphaalkyne ligand possesses a slightly elongated P−C bond (1.661(2) Å; Table 1) when compared to a free ^tBuC≡P (1.542(2) Å)^[26], seemingly due to some π back–bonding interaction with the vanadium center (vide infra, Figure 2, HOMO‒1 orbital). The solid–state structure of 2 also shows the ⁱPr methine groups to be situated above and below the ring–current provided by the β–diketiminate ligand, which might be responsible for the different anisotropically induced local magnetic field observed in the ¹H NMR spectrum. The molecular orbitals obtained from DFT calculations, at B3LYP-D3/cc-pVTZ(-f)/LACV3P//6–31G**/LACVP level of theory are shown in Figure 2, and indicate that the occupied but orthogonal d_xy and d_xz orbitals in a tetrahedral ligand field engage in back donation with the π* orbitals of the carbonyl and phosphaalkyne ligands, thus, engendering the elongation of the CO (1.163(2) Å vs. free CO 1.128(2) Å^[27]) and PC bonds (vide supra). The two π back–bonding orbitals, HOMO at −4.748 and HOMO–1 at −5.707 eV, include π* contributions from carbonyl and phosphaalkyne ligands respectively, indicating that the vanadium center is formally reduced by 4 electrons from the initial V^V–neopentylidyne fragment in 1 (Figure S29a–b).

Figure 1.

XRD studies of complexes 2 (a) and 4 (b) with thermal ellipsoids at 50% probability. H–atoms (execpt salient hydrogens to discussions) and the co-crystallized pentane in compound 4 are excluded for clarity.

Table 1.

Salient geometrical and spectroscopic metrics for compounds 2 and 4.

Compounds	2	4
τ5	0.01	0.04
P≡CtBu (Å)	1.661(2)	1.653(2)
P−Ccarbonyl (Å)	2.535(2)	2.384(2)
V−P (Å)	2.350(1)	2.359(1)
V−CtBu (Å)	1.970(1)	2.003(2)
V−Ccarbonyl (Å)	1.903(1)	1.848(2)
C carbonyl ≡O (Å)	1.163(2)	1.196(3)
P−V−CtBu ( °)	44.1(4)	43.5(8)
P−V−Ccarbonyl (°)	72.2(9)	67.7(9)
V−C≡Ocarbonyl (°)	177.5(1)	174.8(2)
31 P (ppm)	479 (exp.) / 495 (calc.)	362 (exp.) / 473 (calc.)
51 V (ppm)	2572 (exp.) / 2994 (calc.)	687 (exp.) / 990 (calc.)
νCO (cm−1)	1832	1685

Figure 2.

Molecular orbitals of 2 representing two π back–bonding interactions (See Figure S28 for 4⁻). Isodensity value = 0.05 a.u. DFT: B3LYP-D3/cc-pVTZ(-f)/LAC3VP//6–31G**/LACVP.

To further study the electrochemical properties of 2, we conducted some cyclic voltammetry experiments (Figure 3). The square–wave voltammetry of a solution of 2 in 1,2–difluorobenzene with 0.108 M [ⁿBu₄N][PF₆] as the electrolyte (Figure S25 and S26) showed two 1e⁻ redox events centered at −1.04 V and 0.78 V vs. Fc^0/+ (0.0 V). Furthermore, as the cyclic voltammograms in Figure 3a impart, the first cathodic event at −1.04 V is fully reversible, which is corroborated by the linearity of the Randles–Ševčík plot (Figure 3b). The reversibility at varying scan rates suggests the reduction of 2 to proceed with minimal structural rearrangement. In fact, DFT calculations predict the reduction and oxidation steps to occur at −0.96 V and 0.67 V, respectively, which is in reasonable agreement with the experimental results. The reduction and oxidation events culminate with Mulliken spin densities of 1.25 and 1.16 on the vanadium centers, respectively, suggesting that these one-electron redox events are more metal centric. The oxidation wave at 0.78 V is irreversible at 0.1 V/s scan rate, however, the cathodic wave gains intensity with an increased scan rate (Figure S24). This is likely attributable to the short life of the oxidized species.^[28] Notably, DFT optimized geometry of the oxidized species 2⁺ has a relatively short CO bond (1.144 Å), indicating the foreseeable weakening of π back–bonding interaction with the vanadium center upon oxidation. Consequently, computations suggest that the dissociation of CO is only 10.7 kcal/mol endergonic from 2⁺, a plausible reason for irreversible oxidation of 2.

Figure 3.

(a) Cyclic Voltammogram of 2 in 1,2–difluorobenzene with 0.108 M [ⁿBu₄N][PF₆]. Full voltammogram with 0.1 V/s scan rate and inset dashed CVs of the reversible redox event (I) with various scan rates. (b) Randles–Ševčík plot for redox event (I).

The electronic absorption spectrum of 2 correlates well with the calculated electronic excitations (Figure 4). Time-dependent density functional theory (TD-DFT) calculations were carried out with restricted B3LYP-D3/6-31G**/LACVP level of theory.^[29] The singlet excited states S_n, where n = 1, 2 and 3, represent electronic transitions from the HOMO to unoccupied orbitals that mainly constitute contributions from vanadium centered d_z², d_x²-y², and d_yz orbitals, respectively. The calculated excitation energies 1078 (S1), 610 (S2), and 586 nm (S3) correspond to the d–d transitions and are in good agreement with the experimental absorbances centered at 1054, 655, and 608 nm, respectively. Thus, the photochemical excitation calculations suggest that 2 has a more metal centric d⁴ electronic configuration in accord with a low-valent state.

Figure 4.

UV–Vis spectra of 2 (6.16 ×10⁻⁴ M) in pentane and 4 (8.23×10⁻⁴ M) in THF. The glitch at 800 nm is caused by the grating changeover of the UV–Vis instrument, respectively. The acceptor orbitals of the d–d transitions are shown with isodensity of 0.05 a.u. S4 transition involves

To understand if salt metathesis is required in the transfer of [PCO]⁻ to the V–center, we turned our attention to the etherate-alkylidyne complex 3.^[22] Following a similar protocol to prepare 2, we introduced Na(OCP)(dioxane)_2.5 to 3, which resulted in the formation of a salt–like complex in nearly quantitative yield when the reaction mixture was treated with the crown-ether 15−crown−5 to encapsulate the Na⁺ counter ion. The resulting complex was characterized as diamagnetic ate-complex [Na(15–crown–5)]{([ArNC(CH₂)]CH[C(CH₃)NAr])V(CO)(η²–PC^tBu)} (4) and such species possesses a more red–shifted CO stretch at 1691 cm⁻¹ when compared to 2, indicating more significant π back–bonding between vanadium and carbonyl ligand alongside the interaction of Na⁺ with the oxygen atom of carbonyl. The ¹H NMR spectrum shows retention of bis-anilido ligand, manifested by the presence of two diastereotopic hydrogens for the methylene group (3.00 and 3.58 ppm). The loss of C₂ symmetry in the chelating ligand is demonstrated by one resonance for the methine γ–CH at 5.41 ppm in combination with four different methine resonances for the ⁱPr groups on the aryl motifs (5.23, 4.74, 2.11, and 1.24 ppm). As in the case of 2, the chemical shifts of the four ⁱPr groups range from 5.22 to 1.25 ppm, presumably due to a similar anisotropically induced local magnetic field that shields and deshields the ⁱPr methines above and below the bis-anilido ligand. Using a combination of multinuclear and 2D NMR experiments (¹H–¹H COSY, ¹H–¹³C{¹H} HSQC and ¹H–¹³C{¹H} HMBC), we correlated all resonances in the ¹H NMR spectrum to their respective carbon resonance, and established the hybridization of each with the aid of ¹H–¹³C{¹H} HSQC and ¹³C{¹H} DEPT–135 spectroscopic studies. Notably, the phosphaalkyne resonance has a broad feature (Δν_1/2 = 251 Hz) at 362.00 ppm in ³¹P NMR spectrum, whereas the vanadium center resonates as a broad peak (Δν_1/2 = 1821 Hz) at 687.00 ppm in ⁵¹V NMR spectrum. These spectral features are significantly upfield shifted when compared to the ³¹P and ⁵¹V resonances for 2, unveiling the discrepancy in electronics between 2 and 4. This disparity in NMR chemical shifts is likely attributable to the paramagnetic shielding (σ^P) contribution as UV–Vis–NIR spectroscopic studies also verified the low-lying excited electronic state of 2 (S1 transition at 9487 cm⁻¹, Figure 4) vs. 4 (S1 transition at 14535 cm⁻¹, Figure S17).^[30] DFT–calculated NMR spectroscopic shifts also correlate well to the upfield shifts for 4, as noted in Table 1. Furthermore, both the UV–Vis absorption spectrum and the vertical excitation calculations showcase significant blue–shift in the energy of d–d transitions for compound 4, reflecting its electronic divergence from 2 (Figure 4 and Table S2).

Figure 1b shows the single-crystal X-ray structure of 4, clearly depicting the side–on bound ^tBuC≡P oriented in the same imaginary plane as the CO ligand. The V−P and V−C(^tBu) bond distances of 2.359(1) Å and 2.003(2) Å in 4 are both similar to the parameters of 2 (Table 1). The carbonyl C−O distance in 4 (1.196(3) Å) is longer than in 2 (1.163(2) Å), in accord with more π back–bonding implied by the IR spectrum. Likewise, the slightly shorter P≡C distance of 1.653(2) Å reflects that the metal center is more engaged in π back–bonding with the carbonyl group than the phosphaalkyne moiety in 4 when compared to 2. The stronger π back–bonding from vanadium in 4 stems from greater electron donation from the anilido ligand, which is manifested by the shorter V−N distances of 1.936(2) and 1.937(2) Å in 4 vs. 1.977(1) Å and 1.969(1) Å in 2. Another salient feature is how the Na⁺ counter–ion is partially encapsulated by the crown–ether, since it binds to the carbonyl oxygen atom (Na−O, 2.257(2) Å).

Figure 5a shows the reaction energy profile derived from DFT calculations. In the first step, the [PCO]⁻ displaces the triflate and ether ligands from 1 and 3 to adopt an η²–coordination mode through the P–C bond in 1a and 3a, respectively. These structures are 4.3 and 2.8 kcal/mol more favorable than the isomeric coordination through the O-atom (Figure S27). Using the natural bond orbital analysis, Wang and coworkers^[31] suggested that the HOMO of [PCO]⁻ anion is composed of π–bonding interaction between C and P atoms with a lone pair located on the O atom. In spite of that, the preference for η²(P−C) coordination is rather surprising given the hard and Lewis acidic nature of the vanadium^V ion. Nonetheless, the addition of [PCO]⁻ to these complexes traverses through a [2+2] cycloaddition step, which unexpectedly leads to the cleavage of the P–C bond and P–atom transfer to the V≡C^tBu fragment. As the putative 4-membered vanadacycle could not be located as a stationary point after the cycloaddition step, it is very likely that electronic reorganization, as shown in Figure 5b, facilitates a synchronous P–atom transfer and decarbonylation. Thus, the pathway to cycloaddition products, 1b and 3b, face overall energy barriers of 11.4 and 9.1 kcal/mol, respectively. Finally, the ring inversion of the bidentate diamide ligand in 1b and 3b will form highly thermodynamically stable compounds 2 and 4⁻ at −23.6 and −24.9 kcal/mol, respectively, with the carbonyl ligand residing on the equatorial position.

Figure 5.

a) Gibbs energy profile, in kcal/mol, for the P-atom transfer reaction of 1 (red) and 3 (blue) calculated with B3LYP-D3/cc-pVTZ(-f)/LACV3P//6–31G**/LACVP level of DFT. The counter-ion Na(15-crown-5) was omitted for the reaction pathway from 3. b) Proposed electronic reorganization following the transition state, 1a-TS.

To further probe the valence state of the vanadium ion in 2 and 4 we turned to X-ray absorption near edge structure (XANES). The proposed coordination environments are almost identical between 2 and 4, and as expected, the spectra are similar (Figure 6). It is understood that 3d transition metal pre-edge 1s-3d transitions are dipole forbidden, but they become allowed through metal 4p-3d mixing in non-centrosymmetric environments. Non-octahedral geometries (e.g. tetrahedral or square pyramidal) and higher oxidation states (lower d-band occupancy) are two contributors to higher pre-edge intensity, but bond length and covalency also have significant effects on the energy and intensity of XANES features by modulating the extent of p-d mixing.^[32] The pre-edge peak energies and areas were fit to compare the effects of oxidation state and coordination environment (Table 2). NH₄VO₃, V₂O₅, and V₂O₃ are included to provide a connection to other work as calibration and fitting methods may differ (Figure S32). The XANES for (THF)V^III(Mesityl)₃ and (NacNac)V^III(CH₃)₂^[33] are a good match to 2 and 4, while also revealing the effect of oxidation state on the pre-edge area of T_d vanadium compounds.^[32a] Based on pre-peak and edge energies of these four spectra, one may clearly infer a non-centrosymmetric V^III for 2 and 4. However, the pre-peak areas are significantly lower than the comparable T_d V^III compounds, which in turn, are much smaller than the T_d vanadium^V compound. One possibility consistent with the spectra is 5-coordinate V^III, which would be expected to have somewhat lower pre-peak area. On the other hand, DFT calculations predict π back-bonding for the phosphaalkyne ligand, as illustrated in Figure 2, which indicates the V bonding may effectively be pseudo-tetrahedral instead of highly distorted square-pyramidal. If that is the case, then the smaller pre-peak area suggests lower valent vanadium.

Figure 6.

X-ray Absorption Near Edge Structure (XANES) of 2 (red) and 4 (blue) in comparison with [V^III] references in tetrahedral (Td) geometry. Inset: Pre-edge XANES of 2 and 4, zoomed in. Mes = Mesityl.

Table 2.

XANES pre-edge peak centroids and areas (fit procedure given in SI)

Compound	O.S.	Geometry	Centroid Energy (eV)	Area (eV)
NH 4 VO 3	+5	Td	5468.82(6)	2.29(7)
V 2 O 5	+5	Sq. Pyr.	5469.48(5)	1.64(6)
(nacnac)V(CH 3 ) 2	+3	Td	5465.81(4)	1.39(6)
(THF)V(mesityl) 3	+3	Td	5465.80(6)	1.32(5)
V 2 O 3	+3	Oh	5467.96(6)	0.52(3)
2			5466.20(4)	1.12(5)
4			5466.29(4)	1.11(5)

In conclusion, we have showcased the reactivity of sodium phosphaethynolate, Na(OCP), with vanadium alkylidynes ((^Menacnac)V≡C^tBu(OTf) (1) and ([ArNC(CH₂)]CH[C-(CH₃)NAr])VC^tBu(OEt₂) (3)), where metathesis of V≡C and P≡C multiple bonds takes place, followed by carbonyl migration to the metal, overall manifesting a direct P–atom transfer from Na(OCP) to an alkylidyne ligand to yield vanadium–bound phosphaalkyne complexes. Mechanistically, the P–atom transfer proceeds via [2+2]–cycloaddition of the P≡C bond across the V≡C^tBu moiety, followed by reduction of V ion to form V–C≡O bond and an η²–phosphalkyne. Comparison of XANES spectra of these compounds with several standards revealed the low-valent nature of the V centers. In addition, UV–Vis analysis, supported by TD-DFT calculations, show three d–d transitions also suggesting the presence of a low-valent vanadium center.