Synthesis, structure and electrochemical properties of tris-picolinate complexes of rhodium and iridium

Abstract

Reaction of picolinic acid (Hpic) with rhodium trichloride or iridium trichloride affords tris-picolinate complexes of the type [M(pic)₃] (M = Rh, Ir). The crystal structures of both complexes have been determined by X-ray diffraction. In both complexes, the picolinate ligands are coordinated to the metal center as bidentate N,O-donors forming five-membered chelate rings. There is one molecule of water of crystallization per molecule of each [M(pic)₃] complex. The water molecule is hydrogen bonded to the carboxylate fragments of two adjacent [M(pic)₃] molecules and thus acts as a bridge between the individual complex molecules. The complexes are diamagnetic (low-spin d⁶, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on the [M(pic)₃] complexes shows a M(III)-M(IV) oxidation near 1.0 V versus SCE and a ligand-centered reductive response near -1.0 V versus SCE.