The δ in 18 + δ Electron Complexes: Importance of the Metal/Ligand Interface for the Substitutional Reactivity of “Re(0)” Complexes (α-diimine^-)Re^I(CO)₃(X)

Abstract

The factors determining the electron transfer-induced halide labilization in complexes (α-diimine)Re(CO)3(Hal), Hal = Cl and Br, were systematically studied via EPR and cyclic voltammetry in the presence of substituting ligands such as triphenylphosphine, cyanide, or acetonitrile. The α-diimines employed were the four isomeric bidiazines (bdz) 3,3‘-bipyridazine, 2,2‘-bipyrazine, and 2,2‘- and 4,4‘-bipyrimidine and the nonaromatic α-diimines 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene (dab) and 1,3-di-tert-butylsulfurdiimine (sdi). For comparison, the complexes (L)Re(CO3)Cl, L = 2,2‘-bipyridine, 1,4,7,10-tetraazaphenanthrene, and η2-2,2‘,2‘‘-terpyridine, and the new cationic species [(bdz)Re(CO)3(CH3CN)]+ were also investigated. In a further experiment, in situ EPR spectroelectrochemistry was employed to study the primary paramagnetic intermediates during the reduction of the prototype compound, (bpy)Re(CO)3Cl, under a CO2 atmosphere. The susceptibility to substitution was found to be dependent not on the redox potential but on the π molecular orbital coefficients at the metal-coordinating nitrogen centers which are reflected by 14N, 185,187Re, and 31P EPR coupling constants. The most labile systems were thus found among the complexes of the small dab and sdi ligands, despite their facile reduction. In contrast, the complexes of these nonaromatic compounds showed an electrochemically reversible one-electron oxidation which, in comparison to the absorption maximum, allowed us to estimate contributions to the reorganization energy of the MLCT excited state in two cases. For the reductive labilization, it is primarily the small but variable and EPR-detectable ligand-to-metal electron (spin) transfer at the metal/ligand interface which determines the extent of activation in 18 + δ valence electron intermediates.