Structure, EPR and UV–VIS and IR spectroelectrochemistry of reversibly reducible compounds [(C5Me5)IrCl(L)](PF6), L = 1,3-dimethyllumazine or 1,3-dimethylalloxazine

Abstract

In an attempt to model possible metal–pterin or metal–flavin ligand interactions the title compounds have been prepared and their structure, spectroscopy and spectroelectrochemistry (EPR, IR, UV–VIS) compared with results for the corresponding rhodium analogues. The X-ray crystal structure of [(C₅Me₅)IrCl(DML)](PF₆), DML = 1,3-dimethyllumazine, shows O⁴/N⁵ chelate coordination of iridium with a shorter bond to the nitrogen centre [2.103(4)vs. 2.186(3)Å], a slightly more unsymmetrical situation than that found in the rhodium analogue. Electrochemical studies (cyclic voltammetry, polarography) showed largely reversible one-electron reduction processes to radical species [(C₅Me₅)IrCl(L)]˙ which allowed us to investigate the EPR, IR and UV–VIS response to the reduction. Although the EPR spectra of the iridium compounds remained unresolved, the low g factors (g < 2) and particular line shapes revealed small but significant contributions from the heavy-metal centre. Long-wavelength absorptions at ca. 600 nm and low-energy shifts of the carbonyl stretching bands confirm the formulation [(C₅Me₅)Ir^IIICl(L^–1)]˙ for the electronic situation.