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

Abstract

AbstractA new N‐heterocyclic carbene ligand N,N′‐bis(2‐pyridyl)‐tetrahydropyrimidinium hexafluorophosphate (1) has been synthesized and characterized. Its homoleptic RuII 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 RuII 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) Å] than in homoleptic complex 2 [1.969(2) Å and 1.972(3) Å]. The complexes display broad metal‐to‐ligand charge‐transfer absorption bands in the visible region (2: λmax = 440 and 470 nm; 3: λmax = 440 and 473 nm), and their solid‐state emission is redshifted (2: λmax = 753 nm; 3: λmax = 735 nm) considerably relative to that of [Ru(tpy)2]2+ (λmax = 620 nm).