Variable coordination chemistry of the phospha(III)guanidinate anion; application as a metal-functionalised phosphine ligand

Abstract

Structural investigation of Li-complexes of the phospha(III)guanidinate anion [Ph₂PC{NⁱPr}₂]⁻ revealed variable coordination to lithium; synthesis of the dimethyl aluminium compound, (Ph₂PC{NⁱPr}₂)AlMe₂, which behaves as a metal-functionalised phosphine ligand towards platinum, is reported.

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Tertiary phosphines, PR₃, have played a central role in the development of coordination chemistry and are currently employed as ancillary ligands in many catalytic transformations.¹ Amongst the properties that make them so appealing is the ease with which their steric and electronic properties can be systematically varied through introduction of different R-substituents at the phosphorus atom. With a view to developing a novel class of phosphine ligand, we have initiated a study of the ligating properties of phospha(III)guanidinate anions, [R₂PC{NR′}₂]⁻. In contrast to their conventional nitrogen analogues where delocalisation across the central ‘CN₃’ core of the molecule is normally observed,² retention of the lone-pair at phosphorus is predicted, due to unfavourable overlap with the sp²-hybridised carbon. Thus in addition to N,N′-bonding (type I),³ participation of the P-centre in either an N,P- (type II),⁴ or ambidentate (type III) coordination mode is possible. The latter situation may be regarded as a new type of metal-functionalised phosphine ligand, where in addition to derivitisation directly at the nitrogen and phosphorus atoms, the potential for facile modification of the steric and electronic properties may be further achieved through the incorporation of a broad range of ‘ML_n’ fragments at the amidinate moiety.

Reaction of Ph₂PLi with ⁱPrN=C=NⁱPr in THF proceeded via insertion of the carbodiimide into the P–Li bond, to afford the lithium salt [Ph₂PC{NⁱPr}₂Li] (1). Combustion analysis of the bulk sample indicated formation of the bis-THF adduct 1(THF)₂;‡ however, X-ray structural data of crystals that formed upon removal of solvent from the reaction mixture showed formation of the monosolvated, centrosymmetric dimer, [1(THF)]₂ (Fig. 1),§ in which the ligand adopts a μ-η¹,η²-bridge between two lithium centres, unique in phosphaguanidinate chemistry.⁵ The core of the molecule consists of three, 4-membered ring-systems in a folded ‘ladder-type’ arrangement (angle between CN₂Li and N₂Li₂ planes = 124.4°). Bond lengths are consistent with unequal delocalisation across the ‘CN₂’ moiety, where the longer C(1)–N(2) distance reflects a charge localisation at the nitrogen atom, as a consequence of incorporation in the Li₂N₂ ring.^5a The C(1)–P distance [1.915(2) Å] and pyramidal geometry at the phosphorus [Σ_angles = 315.61°] are consistent with a single P–C bond and retention of the lone pair at phosphorus.

Fig. 1 Molecular structure of 1 (thermal ellipsoids 30%; ′: −x, −y + 1, −z). Selected bond lengths (Å) and angles (°): C(1)–N(1) 1.316(3); C(1)–N(2) 1.339(3); C(1)–P 1.915(2); N(1)–Li(1) 2.024(5); N(2)–Li(1) 2.175(5); N(2)–Li(1′) 2.078(5); N(1)–Li(1)–N(2) 65.28(15); Li(1)–N(2)–Li(1′) 70.30(19); N(1)–C(1)–N(2) 117.3(2).

Addition of TMEDA to a toluene solution of 1 afforded colourless crystals which analysed as Ph₂PC{NⁱPr}₂Li(TMEDA) (2).‡ X-Ray diffraction data revealed that a major structural rearrangement had occurred in 2, to afford the first structurally characterised example of a complex containing type II bonding (Fig. 2).§ The C–N bond distances indicate a considerably more localised bonding situation than for 1, with the C(1)–N(2) distance [1.299(4) Å] close to the value found for C=N double bonds. As a consequence, the N(1)–Li bond [1.927(5) Å] is appreciably shorter than in 1, and overall the CN₂PLi core of the molecule is essentially planar.⁶ It is noteworthy that, although the mean P–Li bond distance in 2 [2.611 Å] is comparable to those reported in the closely related lithium phosphide complex [Ph₂PLi(TMEDA)]₂ (2.523–2.629 Å),⁷ severe distortion from ideal tetrahedral geometry results at both the P- and Li-atoms [angles at Li: 66.54–140.3°; angles at P: 74.49–139.88°], suggesting that such an interaction is weak in compound 2.

Fig. 2 Molecular structure of 2 (thermal ellipsoids 30%). Selected bond lengths (Å) and angles (°) for one of two independent molecules in the unit cell. C(1)–N(1) 1.354(4); C(1)–N(2) 1.299(4); C(1)–P 1.892(3); N(1)–Li 1.927(5); P–Li 2.625(5); N(1)–Li–P 66.54(16); Li–P–C(1) 74.49(14); N(1)–C(1)–N(2) 126.4(3).

Compounds 1 and 2 reacted with AlMe₂Cl to generate the aluminium dimethyl compound, (Ph₂PC{NⁱPr}₂)AlMe₂ (3).‡ X-Ray analysis, revealed a monomeric complex incorporating type I bonding of the phospha(III)guanidinate ligand (Fig. 3).§ The planar, four-membered metallacycle displays metrical parameters typical for conventional aluminium amidinate complexes,⁸ with carbon–nitrogen bond distances commensurate with delocalised bonding across the ‘CN₂’ moiety. The carbon–phosphorus distance [1.869(2) Å] is significantly shorter than in 1, which, in conjunction with a reduction in the Σ_angles at phosphorus [309.63°] suggests greater delocalisation of the P-lone pair into the ring system, a likely consequence of the greater electronegativity of Al vs. Li. In solution however, free rotation around the C–P bond is observed upon cooling to low temperature (198 K), indicated by equivalent ⁱPr resonances (¹H NMR spectroscopy), suggesting that the P-lone pair may remain available for coordination to an additional metal fragment (type III bonding).

Fig. 3 Molecular structure of 3 (thermal ellipsoids 30%). Selected bond lengths (Å) and angles (°): C(1)–N(1) 1.337(3); C(1)–N(2) 1.336(3); C(1)–P 1.869(2); N(1)–Al 1.928(2); N(2)–Al 1.9270(19). N(1)–Al–N(2) 68.95(8); N(1)–C(1)–N(2) 109.43(19).

Accordingly, the NMR scale reaction between two equivalents of 3 and PtMe₂(cod) indicated that quantitative displacement of the diene had occurred, resulting in formation of the mixed Pt/Al compound, [(Ph₂PC{NⁱPr}₂)AlMe₂]₂PtMe₂ (4, Scheme 1). ³¹P NMR spectroscopy was able to confirm coordination of 3via the phosphorus atom (¹J_PtP = 1780 Hz) and the presence of two phosphine ligands was established by the presence of a triplet in the ¹⁹⁵Pt NMR spectrum. Despite the inability to observe a peak in the ²⁷Al NMR spectrum (presumably due to quadrapolar broadening), both the ¹H and ¹³C NMR spectra exhibited characteristic high field peaks for the Al-Me groups (δ −0.15 and −8.7 respectively), clearly showing retention of the aluminium methyl moiety in the resultant complex. In addition, the EI⁺ mass spectrum exhibited a signal corresponding to the molecular ion with loss of one methyl group. The overall geometry at platinum was established as cis-square planar through simulation of the platinum methyl (¹H and ¹³C),¶ and the PCN₂ signals of the phospha(III)guanidinate ligand (¹³C),¶ in the proton and carbon NMR spectra.⁹

Scheme 1 Reagents and conditions: (i) 2 equiv. 3, C₆D₆.

In summary we have demonstrated the highly versatile coordination chemistry exhibited by the phospha(III)guanidinate anion [Ph₂PC{NⁱPr}₂]⁻ in lithium and aluminium complexes, including the first structurally characterised examples of μ-η¹,η²-bridging (1) and N,P-chelating modes (2). Formation of the aluminium complex (Ph₂PC{NⁱPr}₂)AlMe₂ (3) was achieved by salt metathesis, and it was demonstrated that the resultant complex retained sufficient electron density at the P-atom to coordinate to a ‘PtMe₂’ fragment as a novel class of metal-functionalised phosphine ligand in [Ph₂PC{NⁱPr}₂AlMe₂]₂PtMe₂ (4). NMR spectroscopic methods were used to establish a cis-square planar geometry at platinum in 4.

The University of Sussex is thanked for financial support. Dr Anthony G. Avent is thanked for the ¹⁹⁵Pt NMR measurement and simulations; Dr M. S. Hill is acknowledged for the donation of PtMe₂(cod).

Notes and references

1. B. Cornils and W. A. Herrmann, in, Applied Homogeneous Catalysis with Organometallic Compounds: A Comprehensive Handbook in Two Volumes, ed. B. Cornils and W. A. Herrmann, VCH Publishers, New York, 1996.
2. P. J. Bailey and S. Pace, Coord. Chem. Rev., 2001, 214, 91.
3. E. Hey-Hawkins and F. Lindenberg, Z. Naturforsch., 1993, 48b, 951.
4. D. H. M. W. Thewissen, H. P. M. M. Ambrosius, H. L. M. Van Gaal and J. J. Steggerda, J. Organomet. Chem., 1980, 192, 101.
5. Related structures have been observed in lithium benzamidinate derivatives. (a) J. Barker, D. Barr, N. D. R. Barnett, W. Clegg, I. Cragg-Hine, M. G. Davidson, R. P. Davis, S. M. Hodgson, J. A. K. Howard, M. Kilner, C. W. Lehmann, I. Lopez-Solera, R. E. Mulvey, P. R. Raithby and R. Snaith, J. Chem. Soc., Dalton Trans., 1997, 951–955; (b) D. Stalke, M. Wedler and F. T. Edelmann, J. Organomet. Chem., 1992, 431, C1; (c) F. A. Cotton, S. C. Haefner, J. H. Matonic, X. Wang and C. A. Murillo, Polyhedron, 1997, 16, 541.
6. Mean deviation from CN₂PLi plane = 0.026 and 0.070 Å for each independent molecule.
7. R. E. Mulvey, K. Wade, D. R. Armstrong, G. T. Walker, R. Snaith, W. Clegg and D. Reed, Polyhedron, 1987, 6, 987.
8. M. P. Coles, D. C. Swenson, R. F. Jordan and V. G. Young, Jr., Organometallics, 1997, 16, 5183.
9. E. O. Greaves, R. Bruce and P. M. Maitlis, J. Chem. Soc., Chem. Commun., 1967, 860.

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Footnotes

† Electronic supplementary information (ESI) available: simulated and observed NMR spectra. See http://www.rsc.org/suppdata/cc/b2/b209026k/

‡ Selected data for 1(THF)₂: Yield 72%. Anal. Calc. for C₂₇H₄₀N₂LiO₂P: C, 70.11; H, 8.72; N, 6.06. Found C, 70.12; H, 8.69; N, 6.13%. Selected data for 2: Yield 91%. Anal. Calc. for C₂₅H₄₀N₄LiP: C, 73.86; H, 9.92; N,6.89. Found C, 73.79; H, 9.79; N, 6.81%. Selected data for 3: Yield 64%. Anal. Calc. for C₂₁H₃₀N₂AlP: C, 68.46; H, 8.21; N, 7.60. Found: C, 68.53; H, 8.27; N, 7.52%. NMR (C₆D₆, 298 K): ¹H (300 MHz), δ 7.46 (m., 4H, PPh₂), 7.00 (m, 6H, PPh₂), 3.81 (d sept, ³J_HH = 6 Hz, ⁴J_PH = 4 Hz, 2H, CHMe₂), 0.89 (d, ³J_HH = 6 Hz, 12H, CHMe₂), −0.16 (s, 6H, AlMe₂). ¹³C (75.5 MHz), δ 172.7 (d, ¹J_PC = 51 Hz, PCN₂), 132.7 (d, J_PC = 50 Hz, C₆H₅), 132.9 (d, J_PC = 19 Hz, C₆H₅), 129.2 (d, J_PC = 7 Hz, C₆H₅), 129.0 (s, C₆H₅), 46.7 (d, ³J_PC = 13 Hz, CHMe₂), 25.4 (CHMe₂), −9.1 (br, AlMe₂). ³¹P (121.5 MHz) δ −19.8. Selected data for 4: NMR (C₆D₆, 298 K): ¹H (300 MHz), δ 7.79 (m, 8H, PPh₂), 6.79 (m, 12H, PPh₂), 3.81 (sept, J_HH = 5.9, 4H, CHMe₂), 1.64 (m, J_PtH = 69.9, J_PH = 6.0, 6H, PtMe₂), 0.80 (d, br, J_HH = 5.2, 24H, CHMe₂), −0.15 (s, 12H, AlMe₂). ¹³C NMR (75.5 MHz), δ 167.6 (m, PCN₂), 136.0 (m, PPh₂), 131.4 (PPh₂), 130.8 (PPh₂), 128.2 (PPh₂), 45.9 (m, CHMe₂), 25.2 (CHMe₂), 8.6 (m, J_PtC = 633 Hz, J_PC = 98 Hz, J_PC = 8 Hz, PtMe), −8.7 (s, br, AlMe₂). ³¹P NMR (121.5 MHz), δ 27.8 (J_PtP = 1780 Hz). ¹⁹⁵Pt NMR (107 MHz) δ −4881 (J_PtP = 1780 Hz). Mass spec. (EI⁺, m/z): 946 [M-Me]⁺, 930 [M-Me₂]⁺, 874 [M-AlMe₄]⁺.

§ Crystal data: for C₄₆H₆₄Li₂N₄O₂P₂ ([1(THF)]₂): M = 780.83, monoclinic, P2₁/n (no. 14), a = 9.9739(7), b = 19.6236(13), c = 11.6860(8) Å, β = 94.531(3)°, V = 2280.1(3) ų, T = 173(2) K, Z = 2, μ(Mo-Kα) = 0.14 mm⁻¹, independent reflections = 3937 (R_int = 0.088), R1 [for 3047 reflections with I > 2σ(I)] = 0.063, wR2 (all data) = 0.177. Crystal data: for C₂₅H₄₀LiN₄P (2): M = 434.52, triclinic, P1̄ (no. 2), a = 9.5099(2), b = 11.9348(2), c = 28.6122(6) Å, α = 98.125(1), β = 75.908(1), γ = 122.665(3)°, V = 2651.37(9) ų, T = 173(2) K, Z = 4, μ(Mo-Kα) = 0.12 mm⁻¹, independent reflections = 7271 (R_int = 0.065), R1 [for 5713 reflections with I > 2σ(I)] = 0.058, wR2 (all data) = 0.156. Crystal data for C₂₁H₃₀AlN₂P (3): M = 368.42, monoclinic, P2₁/n (no. 14), a = 16.0218(9), b = 8.3303(6), c = 17.6419(9) Å, β = 107.443(4)°, V = 2246.3(2) ų, T = 173(2) K, Z = 4, μ(Mo-Kα) = 0.17 mm⁻¹, independent reflections = 3095 (R_int = 0.041), R1 [for 2623 reflections with I > 2σ(I)] = 0.041, wR2 (all data) = 0.101. CCDC reference numbers 182193, 182194 and 193950. See http://www.rsc.org/suppdata/cc/b2/b209026k/ for crystallographic data in CIF or other electronic format.

¶ Coupling constants (Hz) for simulated NMR data: PtCH₃ (A₃A′₃ part of an A₃A′₃XX′ spin system) J_XY = ± 7; J_AX = J_A′X′ = −8.7; J_AX′ = J_A′X = +13.2; J_AA′ = 1.4. PtCH₃ (A part of AXX′ spin system) J_XY = ± 7; J_AX = 96.4; J_AX′ = 7.4. PCN₂ (A part of AXY spin system) Δδ_XY = 0.08 ppm; J_XY = ± 7; J_AX = 14.9; J_AY = 3.0.

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