Red Phosphorescence in Ru II 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 Ru II . 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 : λ max = 440 and 470 nm; 3 : λ max = 440 and 473 nm), and their solid‐state emission is redshifted ( 2 : λ max = 753 nm; 3 : λ max = 735 nm) considerably relative to that of [Ru(tpy) 2 ] 2+ ( λ max = 620 nm).