Homobimetallic hydride and dihydrogen complexes of ruthenium bearing N-heterocyclic carbene ligands

Abstract

A series of new homobimetallic hydride complexes of ruthenium bearing N-heterocyclic carbene ligand of the type [{RuHCl(IMes)(PPh₃)(CO)}₂ (4,4´-bpy)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) (4,4´-bpy = 4,4´-bipyridine) (2), [{RuHCl(IMes)(PPh₃)(CO)}₂ (4,4´-dpyen)] (4,4´-dpyen = 1,2-bis(4-pyridyl)ethylene) (4), and [{RuHCl(IMes)(PPh₃)(CO)} ₂ (4,4´-dpyan)] (4,4´-dpyan = 1,2-bis(4-pyridyl)ethane) (5) has been synthesized and characterized starting from [RuHCl(IMes)(PPh₃)(CO)] (1). The pyridyl-based ligands, (4,4´-bpy), (4,4´-dpyen), and (4,4´-dpyan) in these complexes act as the bridging ligands between the two ruthenium fragments. The bridging ligand in these bimetallic complexes is very labile. Reaction of these complexes with PMe₃ led to extensive substitution of not only the bridging ligand but also the PPh₃ ligands. Reaction of complexes 2, 4, and 5 with HOTf at low temperatures result in the formation of the corresponding dihydrogen complexes, [{RuCl(η²-H₂)(IMes)(PPh₃)(CO)}₂(4,4´-bpy)][OTf]₂ (9), [{RuCl(η²-H₂)(IMes)(PPh₃)(CO)}₂(4,4´-dpyen)][OTf]₂ (10), and [{RuCl(η²-H₂)(IMes)(PPh₃)(CO)}₂ (4,4´-dpyan)][OTf] ₂ (11), respectively. These are the first examples of homobimetallic ruthenium dihydrogen complexes bearing NHC ligands. The intact nature of the Hsingle bondH bond in these derivatives was established by variable temperature ¹H (400 MHz) spin-lattice relaxation time (T₁, ms) measurements and from the H, D coupling constants of the corresponding HD isotopomers. The H—H distances (d_HH) in these complexes fall in the range 0.95–0.98 Å. Hydrogen evolution takes place upon warming the dihydrogen complexes generated at low temperatures, indicating the labile nature of the H₂ ligand in these complexes. The X-ray crystal structure of [{RuHCl(IMes)(PPh₃)(CO)}₂(4,4´-bpy)] complex (2) has been determined.