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

The saturated imidazolin-2-ylidene compounds RNC3H4NR, 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)/KOtBu
in toluene. The ligands IMes (MesNC3H2NMes) and SIMes react with RuHCl(PPh3)3 to give
RuH(SIMes-H)(PPh3)2 (1) and RuH(IMes-H)(PPh3)2 (2), respectively, where there is cyclometalation of a C−H bond of an ortho Ar−CH3 group. The reaction of RuHCl(PPh3)3 with
ItBu (tBuNC3H2NtBu) produces ItBu·HCl and a red solution that, upon reaction with H2,
produces the dihydride Ru(H)2(ItBu)(PPh3)2 as a mixture of the two isomers 3a (trans PPh3
ligands) and 3b (cis PPh3). 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)(PPh3)2(IMes-H)
(4), while at 68 °C it forms the Ru(0) complex Ru(CO)3(PPh3)(IMes) (5). The reaction of 2
with phenol results in the formation of RuH(η5-C6H5O·HOPh)(IMes)(PPh3) (6). The structures
of complexes 1, 3a, 5, and 6 have been characterized by single-crystal X-ray diffraction.