The Chemistry of Chromium Sandwich Complexes

Abstract

<p>A series of complexes (C₆H₆)Cr(C₆F₅x), where x = CHO, CH(OH)CH₃, COCH₃ or CO₂Et, have been prepared via the lithium reagent (C₆H₆)Cr(C₆F₅Li). The dilithio derivative (C₆F₄Li₂)Cr(C₆H₆) forms a stable di-ester, but the bis(trimethylsilyl) derivative is very labile. Attempts to synthesise chromarenes with alkenyl substituents are described. The chemistry and air stability of symmetric chromarenes (Ar₂Cr) and the unsymmetric chromarenes of type ArCr(C₆H₆) can be correlated with the sum of the Hammett σⅿ parameters of the substituents. It is proposed that the unsymmetrical chromarenes exhibit an internally compensating stabilization effect analogous to that of the chromium tricarbonyl moiety. This postulate is supported by NMR and polargraphic evidence. Pentafluorochromarenyl diphenyl phosphine has been prepared and its structure determined by X-ray crystallography; its NMR spectra and reaction with [Rh(CO)₂Cl]₂ are also discussed. The cocondensation reaction of nickel vapor and 6,6-dimethylfulvene produces a mixture of 4,4,8,8-tetramethyl-1,4,5,8- (and 1,4,7,8-) tetra-hydro-sym-indacenes, which were identified X-ray crystallographically. Although this dimerization is formally achieveable by a (6+6) cycloaddition, it is believed to proceed via ligand coupling on a nickel template.</p>