METAL PI-COMPLEXES OF BENZENE-DERIVATIVES - GERMANIUM IN THE PERIPHERY OF BIS(BENZENE)VANADIUM AND BIS(BENZENE)CHROMIUM - SYNTHESIS AND STRUCTURE OF NEW HETERAMETALLOCYCLOPHANES

Mono-and digerma[n]metallocyclophanes (n = 1, 2) [(eta(6),eta(6)'-dime thyldiphenylgermane)M] (M = Cr, 17; V, 18(.)), [(eta(6),eta(6)'-tetrap henylgermane)M] (M = Cr, 20; V, 21(.)), and ramethyl-1,2-di-eta(6),eta (6)'-phenyldigermane)Cr] (24) were synthesized by means of lithiation and subsequent reaction with dichlorodimethylgermane or dichlorodiphen ylgermane. Metal-ligand cocondensation of bromodimethylphenylgermane w ith chromium atoms followed by reductive coupling with lithium naphtha lide gave the digermane complex 24. Additionally, the nonbridged deriv atives [bis(trimethylgermyl-eta(6)-benzene)M] (M = Cr, 10d; V, 11(.)), trimethylgermyl-eta(6)-benzene)(eta(6)-benzene)Cr] (10m), and [bis(tr iphenylgermyl-eta(6)-benzene)M] (M = Cr, 14; V, 15(.)) were prepared a nd characterized by H-1- and C-13-NMR (10m, 10d, 14, 17, 20), cyclic v oltammetry (CV) (10d, 11(.), 14, 15(.), 18(.), 21(.), 24), and EPR spe ctroscopy (10d(.+) 11(.), 14(.+), 15(.), 18(.), 21(.), 24(.+)). Crysta ls of 20 were subjected to a structure determination by X-ray diffract ion, which disclosed a bending of the sandwich axis from linearity by 14.4 degrees. The strain exerted on the coordinated benzene ring force s a pyramidal structure on the ipso-C leading to a markedly shielded C -13-NMR resonance. An EPR spectroscopic investigation of the vanadium derivatives reveals an increasing metal to ligand spin delocalization and the appearance of orthorhombic g and A tensors on bending the sand wich axis. While the redox potentials are virtually unaffected, when p assing from the unstrained vanadium complexes 11(.) and 15(.) to the g erma[1]vanadocyclophanes 18(.) and 21(.), the transient monocationic s pecies of the latter are destabilized dramatically. No evidence of rin g-opening polymerization was observed on heating 18(.) to 165 degrees C. Instead, metal-ligand cleavage occurs.