Metal π Complexes of Benzene Derivatives. Germanium in the Periphery of Bis(benzene)vanadium and Bis(benzene)chromium. Synthesis and Structure of New Heterametallocyclophanes^,¹

Abstract

Mono- and digerma[ n ]metallocyclophanes ( n = 1, 2) [(η 6,η 6 ‘ -dimethyldiphenylgermane)M] (M = Cr, 17; V, 18 • ), [(η 6,η 6 ‘ -tetraphenylgermane)M] (M = Cr, 20; V, 21 • ), and [(1,1,2,2-tetramethyl-1,2-di-η 6,η 6 ‘ -phenyldigermane)Cr] ( 24 ) were synthesized by means of lithiation and subsequent reaction with dichlorodimethylgermane or dichlorodiphenylgermane. Metal−ligand cocondensation of bromodimethylphenylgermane with chromium atoms followed by reductive coupling with lithium naphthalide gave the digermane complex 24 . Additionally, the nonbridged derivatives [bis(trimethylgermyl-η 6 -benzene)M] (M = Cr, 10d; V, 11 • ), [(trimethylgermyl-η 6 -benzene)(η 6 -benzene)Cr] ( 10m ), and [bis(triphenylgermyl-η 6 -benzene)M] (M = Cr, 14; V, 15 • ) were prepared and characterized by 1 H- and 13 C-NMR ( 10m, 10d, 14, 17, 20 ), cyclic voltammetry (CV) ( 10d, 11 •, 14, 15 •, 18 •, 21 •, 24 ), and EPR spectroscopy ( 10d • +, 11 •, 14 • +, 15 •, 18 •, 21 •, 24 • + ). Crystals of 20 were subjected to a structure determination by X-ray diffraction, which disclosed a bending of the sandwich axis from linearity by 14.4°. The strain exerted on the coordinated benzene ring forces a pyramidal structure on the ipso-C leading to a markedly shielded 13 C-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 sandwich axis. While the redox potentials are virtually unaffected, when passing from the unstrained vanadium complexes 11 • and 15 • to the germa[1]vanadocyclophanes 18 • and 21 •, the transient monocationic species of the latter are destabilized dramatically. No evidence of ring-opening polymerization was observed on heating 18 • to 165 °C. Instead, metal−ligand cleavage occurs.