The role of ligand steric hindrance in determining the stability of very short V-V contacts. Preparation and characterization of a series of V(II) and V(III) amidinates

Article abstract of DOI:10.1016/0020-1693(95)04757-3

The reaction of VCl₂(TMEDA)₂ and of VCl₃(THF)₃ with two equivalents of formamidinate lithium salts, respectively, yielded dimeric {[CyNC(H)NCy]₂V}₂ (2), with a very short V-V multiple bond and {[CyNC(H)NCy]₂VCl}₂ (4) which is also dimeric. The formation of 2 proceeded through the intermediate monomeric [CyNC(H)NCy]₂V(TMEDA) (1) which was isolated and fully characterized. The dinuclear structure was reversibly cleaved by treatment with pyridine forming the monomeric [CyNC(H)NCy]₂V(Py)₂ (3). Conversely, similar reactions with acetamidinate anion gave only the monomeric [CyNC(Me)NCy]₂V(THF)₂ (5a) and [CyNC(Me)NCy]₂VCl (7), respectively. Attempts to form a dinuclear structure by either removal of THF from 5a or reduction of 7 gave only the V(III) compound [CyNC(Me)NCy]₂V (6). In the case of the very bulky benzamidinate ligand, similar monomeric complexes [Me₃SiNC-(Ph)NSiMe₃]₂V (THF)₂ (5c) and [Me₃SiNC(Ph)NSiMe₃]₂ VCl (10) were obtained. However, attempts to form dinuclear species via THF dissociation from 5c or reduction of 10 gave a novel dinitrogen complex {[Me₃SiNC(Ph)NSiMe₃]₂V}₂ (8). Crystal data are as follows. 1: C₃₂H₆₂N₆V, monoclinic, P2₁/a, a = 18.085(3), b = 10.737(2), c = 18.721(2) A, β=114.37(4)°, Z=4; 2: C₅₂H₉₂N₈V₂, monoclinic, P2₁, a =11.671(3), b =10.371(2), c = 22.645(3) A, β =100.65(1)°, Z=2; 5c: C₃₄H₆₂N₄Si₄O₂V, orthorhombic, Pbcn, a = 18.133(7), b = 12.504(6), c = 17.428(4) A, Z = 4; 6: C₄₂H₇₅N₆V, monoclinic, P2₁/c, a = 17.35(1), b = 10.854(2), c = 22.777(4) A, β =103.04(2)°, Z = 4; 7: C₂₈H₅₀N₄VCl, triclinic, P-1, a = 11.132(4), b = 13.630(4), c = 10.276(2) A, α = 95.92(2), β = 98.38(2), γ = 109.29(3)°, Z = 2; 8: C₅₂H₉₂N₁₀V₂Si₈, triclinic, P-1, a = 14.181(2), b = 18.713(2), c = 13.257(2) A, a = 99.48(1), β = 94.13(2), γ =91.87(2)°, Z = 2; 10: C₂₆H₄₆N₄VClSi₄, triclinic, P-1, a = 12.781(5), b = 13.128(4), c = 12.126(5) A, a = 104.85(3), β = 113.29(4), γ = 66.80(4)°, Z = 2.