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

Abstract

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]∗2 (4) which is also dimeric. The formation of 2 proceeded through the intermediate monomeric [CyNC (H) NCy]2 V(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]2 V (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]3V (6). In the case of the very bulky benzamidinate ligand, similar monomeric complexes [Me3SiNC (Ph) NSiMe3]2 V (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) Å, β = 114.37 (4) °, Z = 4; 2: C52H92N8V2, monoclinic, P21, a = 11.671 (3), b = 10.371 (2), c = 22.645 (3) Å, β = 100.65 (1)°, Z = 2; 5c: C34H62N4Si4O2V, orthorhombic, Pbcn, a = 18.133 (7), b = 12.504 (6), c = 17.428 (4) Å, Z = 4; 6: C42H75N6V, monclinic P21/c, c = 22.777 (4) Å,β = 103.04 (2)°, Z = 4; 7: C28H50N4VCl, triclinic, P-1, a = 11.132 (4), b = 13.630 (4), c = 10.276 (2) Åα= 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) Å, α = 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) Å, α =104.85 (3), β =113.29 (4), γ = 66.80 (4)°, Z = 2.