The use of manganese in homogeneous hydrogenation catalysis has been a recent focus in the pursuit of more environmentally benign base metal catalysts. It has great promise with its unique...

trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)2, trans-RuH2(PPh­(OEt)2)4, and mer-IrH3(PiPr2CH2pyCH2PiPr2) provide M–H force constants and H–M–H interaction force constants that increase as FeII < RuII < IrIII. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsym H–M–H and antisymmetric νasym H–M–H vibrational modes of hydrides that are mutually trans in d6 octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm–1, while the 3d metals have only 90 cm–1. This has been verified experimentally for 3d, 4d, and 5d transition-metal trans-dihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh­(OEt)2)4 (from the literature) and trans-FeH2(PPh2CH2PPh2)2 and mer-IrH3(PiPr2CH2pyCH2PiPr2) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Δνt −235 cm–1) allows us to improve the simple equation reported previously, which allows a better estimate of νM–H.

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a CH/OH bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting.

Phosphorus-donor “arms” are readily added to amines in order to enable sturdy base metal coordination.

A neutral azole precursor to a protic N-heterocyclic carbene (pNHC) ligand, 6-((4,5-diphenyl-1H-imidazol-1-yl)methyl)-2,2′-bipyridine (3), was prepared from 6-(bromomethyl)-2,2′-bipyridine (2) and 4,5-diphenylimidazole. Complex [RuCl-(pNHC-bpy)(PPh 3 ) 2 ](PF 6 ) (4) bearing a protic, bipyridinetethered NHC ligand was prepared by refluxing 3 with RuCl 2 (PPh 3 ) 3 and KPF 6 in methanol and was characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, elemental analysis, and a single-crystal X-ray diffraction study. The hydrido complex [RuH(pNHC-bpy)(PPh 3 ) 2 ](PF 6 ) (5) was prepared by reaction of 4 with NaBH 4 in ethanol and characterized by NMR and Fourier transform infrared spectroscopy. Complex 5 was used as the catalyst (0.1 mol % loading) in the transfer hydrogenation of a range of alkyl/aryl ketones in basic iso-propanol at 60 °C. Bulky alkyl groups or ortho-substituted aryl groups at the ketones slowed down or inhibited the catalytic transformation. The addition of an excess of triphenylphosphine also slowed the catalysis, providing an indication for a mechanism involving phosphine dissociation, while the addition of an excess of elemental mercury had only a small effect on the conversion. The importance of potassium cations in the mechanism is consistent with the observation of reduced catalytic conversion when [2,2,2]-cryptand was present or when 1,8-diazabicyclo[5.4.0]undec-7-ene was used as the base. A plausible homogeneous catalysis mechanism involving the innersphere addition of hydride to the substrate in the transition state TS1 is supported by density functional theory calculations where the potassium ion has replaced the hydrogen atom of the N−H group in a protic NHC.