Electron Exchange Along the Tercyclopentadienyltrimetallic Scaffold: Kinetics, Equilibria, and Bond Strengths

Abstract

___Electron transfer is fundamental to chemical reactivity.[1] A prototype example in organometallic chemistry is the attack of an anionic metal center on a metal-metal bond,Such is not cleanly found, however, the outcome of this mechanistically complex transformation usually being controlled by thermodynamic factors through electron redistribution to eventually provide M-M + M' -.[2] We report the first direct observation of such equilibria for cyclopentadienyl (Cp) metals by NMR, mechanistic studies, and the utilization of the data in the estimation of CpM-M'Cp bond strengths. Key to these results is the employment of the tercyclopentadienyl ligand as a frame on which to render the desired transformations intramolecular and regiospecific (Scheme 1). [3] ___The required materials were made according to described protocols (Table 1), [3,4] with a minor modification for the preparation of the anions 1-4, 7, 8, and 9, which, to avoid (competing) reduction of the metal-metal bond, were generated using NaH or (for 9) tert-BuOK. Structural proof of all new compounds rested on spectral data, conversion to stable methyl derivatives (in the case of the anions), and X-ray analyses of 3 and 4.Particularly diagnostic were the 1 H NMR spectra, which could be assigned, as appropriate, by comparison with known systems or known substructures, [3] by exploiting the effect of the anisotropy of a metal-metal bond on the δ values of the 1