Red Phosphorescence in RuII Complexes of a Tridentate N-Heterocyclic Carbene Ligand Incorporating Tetrahydropyrimidine

A new N-heterocyclic carbene ligand N,N'-bis(2-pyridyl)-tetrahydropyrimidinium hexafluorophosphate (1) has been synthesized and characterized. Its homoleptic Ru-II complex 2 exhibits a more facile oxidation (+1.17 V vs. SCE) relative to the prototypical [Ru(tpy)(2)](2+) (+1.32 V vs. SCE) complex (tpy = 2,2':6',2 ''-terpyridine). The heteroleptic Ru-II complex 3, incorporating 1 and 4'-p-tolyl-tpy, also displays a more positive oxidising potential (+1.28 V vs. SCE) than [Ru(tpy)(2)](2+). The X-ray crystal structures of complexes 2 and 3 confirm the meridional tridentate coordination of ligand 1 to RuII. The Ru-C bond length is shorter in heteroleptic complex 3 [1.901(7) angstrom] than in homoleptic complex 2 [1.969(2) angstrom and 1.972(3) angstrom]. The complexes display broad metal-to-ligand charge-transfer absorption bands in the visible region (2: lambda(max) = 440 and 470 nm; 3: lambda(max) = 440 and 473 nm), and their solid-state emission is redshifted (2: lambda(max) = 753 nm; 3: lambda(max) = 735 nm) considerably relative to that of [Ru(tpy)(2)](2+) (lambda(max) = 620 nm).