The Effect of Remote Donor Substituents on the Properties of Alkoxy and Amino Fischer Carbene Complexes of Tungsten

Abstract

Tungsten Fischer ethoxy‐ and dimethylaminocarbene complexes [W{C(X)(C6H4‐4‐R}(CO)5] (X = OEt: series a; or X = NMe2: series b) are synthesized from phenyl substrates containing remote tertiary amino substituents R’2N with R’ = Me (2), Ph (3) or C6H4Br‐4 (4). The π‐delocalization and carbene‐stabilizing effect of the distant tertiary amine donor substituent are investigated by NMR and IR spectroscopy, electrochemistry and quantum chemical calculations. A significant transfer of electron density from the remote 4‐R’2N substituent to the carbene C atom in the ethoxycarbene complexes is supported by NMR data and the solid‐state structure of 2a. This is strongly attenuated in the amino‐substituted carbene complex 2c and irrelevant in the dimethylaminocarbene complexes of series b, where the electron demand of the electrophilic carbene center is satisfied by the directly attached dimethylamino substituent. Quantum chemical calculations and IR spectroelectrochemistry on complexes 1a‐4a verify carbene‐centered reductions and a ligand based oxidation of complex 2b.