High-frequency EPR study of reduced diruthenium and dirhenium polypyridine complexes based on the 1,2,4,5-tetrazine radical bridge

The radical complexes {(mu-L)[Ru(bpy)(2)](2)}(.3+), {(mu-bmtz)[Ru(cym)Cl](2)}(.+) and {(mu-L)[Re(CO)(3)Cl](2)}(.-), where L are 3,6-disubstituted 1,2,4,5-tetrazines such as 3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine (bmtz) and cym = p-cymene, were studied by X-band EPR in fluid solution and by 285 GHz EPR in glassy frozen solution. A comparison with other transition metal complexes (Cu, Rh, Os, Ir, Pt) involving tetrazine radical ligands reveals that the g anisotropy reflects (i) the pi acceptor effect of the tetrazine substituents, (ii) the competition from ancillary pi acceptor ligands for back donation from the metal, and (iii) the spin-orbit coupling contributions from the transition metal.