Electrochemistry of Fischer alkoxycarbene complexes of chromium: The use of density functional theory to predict and understand oxidation and reduction potentials

The electrochemical behaviour of a series of Fischer ethoxycarbene complexes of the type [(CO) 5 Cr C(OEt)R] with R = 2-thienyl ( 1 ), 2-furyl ( 2 ), 2-(N-methylpyrrolyl) ( 3 ), N-methyl-2-(2′-thienyl)pyrrole) ( 4 ) and 2,2′-thienylfuran ( 5 ), is investigated by means of cyclic voltammetry. Results show that the first one electron reduction process is sensitive to the energy, shape and distribution of the LUMO orbital, leading to a linear relationship between the formal reduction potential and the LUMO energy: E 0′ (C carbene ) = –0.70 E LUMO –3.44 ( R 2 = 1.00) which is valid over a large potential range. The dimeric heteroarene substituents of 4 and 5 lead to enhanced stabilization of the reduced complexes 4 and 5 , making another one electron reduction possible. The formal reduction potential, E 0′ (Cr) of the first oxidation process of 1 – 5 lays within a narrow potential range of 0.43–0.50 V vs . Fc/Fc + , is Cr-based and is mainly sensitive to the electrophilic character of the heteroarene ring directly attached to the carbene carbon.