Differentially Substituted Acyclic Diaminocarbene Ligands Display Conformation-Dependent Donicities

Abstract

Complexes of the type [(L)Ir(COD)Cl] and [(L)Ir(CO)2Cl] (L = N,N′-dimesityl-N,N′-dimethylformamidin-2-ylidene (3) and N,N′-bis(2,6-di-isopropylphenyl)-N,N′-dimethylformamidin-2-ylidene (4); COD = cis,cis-1,5-cyclooctadiene) were synthesized and studied in solution as well as in the solid state. While the acyclic diaminocarbene (ADC) ligand in [(3)Ir(COD)Cl] adopted a conformation in which the N-aryl substituents were anti with respect to the coordinated metal, the respective Ir carbonyl complex was prepared as separable isomers ([(anti-3)Ir(CO)2Cl] and [(amphi-3)Ir(CO)2Cl]). The ADC ligands in [(4)Ir(COD)Cl] and [(4)Ir(CO)2Cl] adopted exclusively amphi conformations, where one N-aryl substituent was oriented toward the coordinated metal and the other was oriented away. The Tolman electronic parameter (TEP) for anti-3 (2047.8 cm−1) was derived from the carbonyl stretching energy (νCO) of the aforementioned Ir(CO)2Cl complex and was found to be larger than the TEPs calculated for amphi-3 (2044.4 cm−1) and 4 (2044.0 cm−1). Likewise, the oxidation potential of [(anti-3)Ir(CO)2Cl], as measured by cyclic voltammetry, was found to be significantly higher (1.57 V) than the analogous oxidation potentials measured for [(amphi-3)Ir(CO)2Cl] (1.26 V) and [(4)Ir(CO)2Cl] (1.24 V).