Coordinatively Unsaturated Hydridoruthenium(II) Complexes of N-Heterocyclic Carbenes

The saturated imidazolin-2-ylidene compounds RNC 3 H 4 NR, with R = mesityl (SIMes), 2,6-diisopropylphenyl, were synthesized by use of diamines that were prepared by the hydrogenation of the corresponding diimines catalyzed by RuHCl( R -binap)( R, R -dach)/KO t Bu in toluene. The ligands IMes (MesNC 3 H 2 NMes) and SIMes react with RuHCl(PPh 3 ) 3 to give RuH(SIMes-H)(PPh 3 ) 2 ( 1 ) and RuH(IMes-H)(PPh 3 ) 2 ( 2 ), respectively, where there is cyclometalation of a C−H bond of an ortho Ar−CH 3 group. The reaction of RuHCl(PPh 3 ) 3 with I t Bu ( t BuNC 3 H 2 N t Bu) produces I t Bu·HCl and a red solution that, upon reaction with H 2, produces the dihydride Ru(H) 2 (I t Bu)(PPh 3 ) 2 as a mixture of the two isomers 3a (trans PPh 3 ligands) and 3b (cis PPh 3 ). These isomers have an agostic C−H bond from a methyl group. There is an interesting windshield wiper exchange of coordinated tert -butyl groups occurring in isomer 3a, as monitored in solution by VT NMR with a free energy of activation of 11.8 kcal/mol. The reaction of 2 with CO at 20 °C produces ( OC -34)-RuH(CO)(PPh 3 ) 2 (IMes-H) ( 4 ), while at 68 °C it forms the Ru(0) complex Ru(CO) 3 (PPh 3 )(IMes) ( 5 ). The reaction of 2 with phenol results in the formation of RuH(η 5 -C 6 H 5 O·HOPh)(IMes)(PPh 3 ) ( 6 ). The structures of complexes 1, 3a, 5, and 6 have been characterized by single-crystal X-ray diffraction.