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

The reaction of VCl2(TMEDA)(2) and of VCl3(THF)(3) with two equivalent s of formamidinate lithium salts, respectively, yielded dimeric {[CyNC (H)NCy]V-2}(2) (2), with a very short V-V multiple bond and {[CyNC(H)N Cy]2VCl}(2) (4) which is also dimeric. The formation of 2 proceeded th rough the intermediate monomeric [CyNC(H)NCy]V-2(TMEDA) (1) which was isolated and fully characterized. The dinuclear structure was reversib ly cleaved by treatment with pyridine forming the monomeric [CyNC(H)NC y]V-2(Py)(2) (3). Conversely, similar reactions with acetamidinate ani on gave only the monomeric [CyNC(Me)NCy]V-2(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) c ompound [CyNC(Me)NCy]V-3 (6). In the case of the very bulky benzamidin ate ligand, similar monomeric complexes [Me(3)SiNC(Ph)NSiMe(3)]V-2(THF )(2) (5c) and [Me(3)SiNC(Ph)NSiMe(3)]2VCl (10) were obtained. However, attempts to form dinuclear species via THF dissociation from 5c or re duction of 10 gave a novel dinitrogen complex {[Me(3)SiNC(Ph)NSiMe(3)] V-2}, (8). Crystal data are as follows. 1: C32H62N6V, monoclinic, P2(1 )/a, a=18.085(3), b=10.737(2), c=18.721(2) Angstrom, beta=114.37(4)deg rees, Z=4; 2: C52H92N8V2, monoclinic, P2(1), a=11.671(3), b=10.371(2), c=22.645(3) Angstrom, beta=100.65(1)degrees, Z=2; 5c: C34H62N4Si4O2V, orthorhombic, Pbcn, a=18.133(7), b=12.504(6), c=17.428(4) Angstrom, Z =4; 6: C42H75N6V, monoclinic, P2(1)/c, a=17.35(1), b=10.854(2), c=22.7 77(4) Angstrom, beta=103.04(2)degrees, Z=4; 7: C28H50N4VCl, triclinic, P-1, a=11.132(4), b=13.630(4), c=10.276(2) Angstrom, alpha=95.92(2), beta=98.38(2), gamma=109.29(3)degrees, Z=2; 8: C52H92N10V2Si8, triclin ic, P-1, a=14.181(2), b=18.713(2), c=13.257(2) Angstrom, alpha=99.48(1 ), beta=94.13(2), gamma=91.87(2)degrees, Z=2; 10:C26H46N4VClSi4, tricl inic, P-1, a=12.781(5), b=13.128(4), c=12.126(5) Angstrom, alpha=104.8 5(3), beta=113.29(4), gamma=66.80(4)degrees, Z=2.