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

Tungsten Fischer ethoxy‐ and dimethylaminocarbene complexes [W{C(X)(C 6 H 4 ‐4‐R}(CO) 5 ] (X=OEt: series a ; or X=NMe 2 : series b ) are synthesized from phenyl substrates containing remote tertiary amino substituents R’ 2 N with R’=Me ( 2 ), Ph ( 3 ) or C 6 H 4 Br‐4 ( 4 ). The π‐delocalization and carbene‐stabilizing effects 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’ 2 N substituent to the carbene C atom in the ethoxycarbene complexes is supported by NMR data and the solid‐state structure of 2   a . This is strongly attenuated in the amino‐substituted carbene complex 2   c 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 1   a‐ ‐ 4   a verify carbene‐centered reductions and a ligand based oxidation of complex 2   b .