Molecular structure and linkage isomerism of cis-[Ru(bipy)2{trans-Cr(cyclam)(CN)2}2]4+(bipy = 2,2′-bipyridine, cyclam = 1,4,8,11-tetraazacyclotetradecane)

Abstract

The molecular structure of the trinuclear complex cis-[Ru(bipy)₂{trans-Cr(cyclam)(CN)₂}₂]⁴⁺(bipy = 2,2′-bipyridine, cyclam = 1,4,8,11-tetraazacyclotetradecane) has been determined by X-ray diffraction. The co-ordination octahedra of the ruthenium(II) and chromium(III) units are staggered, and a modest deviation from linearity is observed in the Ru–(NC)–Cr units. The binding mode of the bridging cyanides, not obtainable from the crystallographic data, was demonstrated to be –Ru^II–NC–Cr^III– by infrared spectroscopy. This binding mode is imposed by synthesis, but is not the thermodynamically stable one. Indeed, linkage isomerization of the bridging ligands can be induced by heating the complex in the solid state (several hours at 150 °C). The flip of the bridging cyanide is clearly detectable by infrared spectroscopy. The photophysical properties of the –Ru^II–CN–Cr^III– linkage isomer, featuring efficient transfer of excitation energy from the Ru(bipy)₂²⁺ chromophore to the emitting chromium(III) fragment, have been studied.