Combined solution calorimetric and quantum mechanics studies of reactions involving saturated and unsaturated N-heterocyclic carbene (NHC) ligands show that the difference in their relative bond dissociation energies is very small (1 kcal·mol-1). Structural and computational studies reveal small metric parameter differences. These observations in conjunction with relative reactivity profiles of NHC-modified ruthenium-based olefin metathesis catalysts suggest that very small changes in the donor properties of the NHC ligands can translate into significant differences in catalytic properties.

The synthesis and characterization by X-ray crystallography of the complexes [Ir(cod)-(py)(L)]PF 6 (L ) IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (2); L ) IPr, 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene (3); L ) ICy, 1,3-bis(cyclohexyl)imidazol-2ylidene ( 4)) are reported. Complexes 2-4 have been employed as catalysts for transfer hydrogenation reactions from 2-propanol to a number of unsaturated substrates and their activity compared with that of the related cationic iridium(I) species [Ir(cod)(py)(SIMes)]-PF 6 (1), [Ir(cod)(py)(PCy 3 )]PF 6 (5), and complexes formed in situ from [Ir(cod)(py) 2 ]PF 6 and diazabutadienes (RNdCHCHdNR, DAB-R; R ) cyclohexyl, DAB-Cy; R ) 2,4,6-trimethylphenyl, DAB-Mes; R ) adamantyl, DAB-Ad; R ) 2,4,6-trimethoxyphenyl, DAB-trimethoxyphenyl). All complexes tested were found to be active catalysts for transfer hydrogenation of ketones, with complex 4 displaying the highest activity. Complex 4 also exhibits moderate activity toward simple olefins and an aromatic nitro compound.

[structure: see text]. A Pd(OAc)2/diazabutadiene system has been developed for the catalytic cross-coupling of aryl halides with arylboronic acids. A combination of the diazabutadiene DAB-Cy (1, N,N'-dicyclohexyl-1,4-dizabutadiene) and Pd(OAc)2 was found to form an excellent catalyst for the Suzuki-Miyaura cross-coupling of various aryl bromides and activated aryl chlorides with arylboronic acids.

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