The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Two new NHC–Cu(i)-[κ2-SNS] complexes were synthesized to directly compare the bifunctional catalytic activity of a hard amido vs. a soft thiolate donor.

The kinetics of hydride transfer from Re( R bpy)-(CO) 3 H (bpy = 4,4′-R-2,2′-bipyridine; R = OMe, t Bu, Me, H, Br, COOMe, CF 3 ) to CO 2 and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re( R bpy)(CO) 3 H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

A combined experimental and mechanistic study of the chemoselective hydroboration of carbonyls by the paramagnetic bis-amido Mn­[SMeNSMe]2 complex (1) is described. The catalyst allows for room-temperature hydroboration of carbonyls at low catalyst loadings (0.1 mol %) and reaction times (<30 min). A series of mechanistic studies highlight the significance of bifunctional amido bis­(thioether) ligand L to the success of the reaction, insight otherwise difficult to attain in paramagnetic systems. Kinetic studies using variable time normalization analysis revealed no unusual reaction kinetics, indicating the absence of side reactions. A borylated analogue of L was observed and characterized via mass spectrometry. Density functional theory (DFT) calculations showed that thioether hemilability of L is crucial during catalysis for providing the active coordinating site. Also, the frequently proposed Mn–H intermediate was found not to be the active species responsible for catalysis. Instead, an inner-sphere reaction pathway with carbonyl coordination to the metal center and amido-promoted B–H reactivity is proposed to be operative.

The reaction of CFH and HC═CHSiMe with catalytic [PrIm]Ni(η-HC═CHSiMe) (1b) exclusively forms the C-H silylation product CFSiMe with ethylene as a byproduct ([PrIm] = 1,3-di(isopropyl)imidazole-2-ylidene). Catalytic C-H bond silylation is facile with partially fluorinated aromatic substrates containing two ortho fluorine substituents adjacent to the C-H bond and 1,2,3,4-tetrafluorobenzene. Less fluorinated substrates react slower. Under the same reaction conditions, catalytic [IPr]Ni(η-HC═CHSiMe) (1a) ([IPr] = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene) provided only the alkene hydroarylation product CFCHCHSiMe. Mechanistic studies reveal that the C-H activation and β-Si elimination steps are reversible under catalytic conditions with both catalysts 1a and 1b. With catalytic 1a, reversible ethylene loss after β-Si elimination was also observed despite its inability to catalyze C-H silylation; the reductive elimination step to form the silylation product is much slower than reductive elimination to form the alkene hydroarylation product. Reversible ethylene loss was not observed with 1b, which suggests that the rate-limiting step in the reaction is neither C-H activation nor β-Si elimination but either ethylene loss or reductive elimination of cis-disposed aryl and SiMe moieties.