Redox Noninnocence of Nitrosoarene Ligands in Transition Metal Complexes

Studies on the coordination of nitrosoarene (ArNO) ligands to late-transition metals are used to provide the first definition of the geometric, spectroscopic, and computational parameters associated with a PhNO electron-transfer series. Experimentally, the Pd complexes PdCl2(PhNO)2, PdL2(PhNO)2, and PdL2(TolNO) (L = CNArDipp2; ArDipp2 = 2,6-(2,6- i Pr2C6H3)2-C6H3) are characterized as containing (PhNO)0, (PhNO)•1–, and (TolNO)2– ligands, respectively, and the structural and spectroscopic changes associated with this electron transfer series provide the basis for an extensive computational study of these and related ArNO-containing late-transition metal complexes. Most notable from the results is the unambiguous characterization of the ground state electronic structure of PdL2(PhNO)2, found to be the first isolable, transition metal ion complex containing an η1-N-bound π-nitrosoarene radical anion. In addition to the electron transfer series, the synthesis and characterization of the Fe complex [Fe(TIM)(NCCH3)(PhNO)][(PF6)2] (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) allows for comparison of the geometric and spectroscopic features associated with metal-to-ligand π-backbonding as opposed to (PhNO)•1– formation. Throughout these series of complexes, the N–O, M–N, and C–N bond distances as well as the N–O stretching frequencies and the planarity of the ArNO ligands provided distinct parameters for each ligand oxidation state. Together, these data provide a delineation of the factors needed for evaluating the oxidation state of nitrosoarene ligands bound to transition metals in varying coordination modes.