Nickel(II) complexes of tripodal 4N ligands as catalysts for alkane oxidation using m-CPBA as oxidant: ligand stereoelectronic effects on catalysis

Several mononuclear Ni(II) complexes of the type [Ni(L)(CH3CN)(2)](BPh4)(2) 1-7, where L is a tetradentate tripodal 4N ligand such as N,N-dimethyl-N', N'-bis(pyrid-2-ylmethyl) ethane-1,2-diamine (L1), N,N-diethyl-N',N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N, N-dimethyl-N'-(1-methyl-1H- imidazol-2-ylmethyl)-N'-(pyrid-2-ylmethyl)ethane-1,2-diamine (L3), N, N-dimethyl-N', N'-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L4), N,N-dimethyl-N', N'-bis(quinolin-2- ylmethyl)ethane-1,2-diamine (L5), tris(benzimidazol-2-ylmethyl)amine (L6) and tris(pyrid-2-ylmethyl)amine (L7), have been isolated and characterized using CHN analysis, UV-Visible spectroscopy and mass spectrometry. The single-crystal X-ray structures of the complexes [Ni(L1)(CH3CN)(H2O)](ClO4)(2) 1a, [Ni(L2)(CH3CN)(2)](BPh4)(2) 2, [Ni(L3)(CH3CN)(2)](BPh4)(2) 3 and [Ni(L4)(CH3CN)(2)](BPh4)(2) 4 have been determined. All these complexes possess a distorted octahedral coordination geometry in which Ni(II) is coordinated to four nitrogen atoms of the tetradentate ligands and two CH3CN (2, 3, 4) or one H2O and one CH3CN (1a) are located in cis positions. The Ni-N-py bond distances (2.054(2)-2.078(3) angstrom) in 1a, 2 and 3 are shorter than the Ni-N-amine bonds (2.127(2)-2.196(3) angstrom) because of sp(2) and sp(3) hybridizations of the pyridyl and tertiary amine nitrogens respectively. In 3 the Ni-N-im bond (2.040(5) angstrom) is shorter than the Ni-N-py bond (2.074(4) angstrom) due to the stronger coordination of imidazole compared with the pyridine donor. In dichloromethane/acetonitrile solvent mixture, all the Ni(II) complexes possess an octahedral coordination geometry, as revealed by the characteristic ligand field bands in the visible region. They efficiently catalyze the hydroxylation of alkanes when m-CPBA is used as oxidant with turnover number (TON) in the range of 340-620 and good alcohol selectivity for cyclohexane (A/K, 5-9). By replacing one of the pyridyl donors in TPA by a weakly coordinating NMe2 or -NEt2 donor nitrogen atom the catalytic activity decreases slightly with no change in the selectivity. In contrast, upon replacing the pyridyl nitrogen donor by the strongly sigma-bonding imidazolyl or sterically demanding quinolyl/ benzimidazolyl nitrogen donor, both the catalytic activity and selectivity decrease, possibly due to destabilization of the intermediate [(4N)(CH3CN)Ni-O-center dot](+) radical species. Adamantane is selectively (3 degrees /2 degrees, 12-17) oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone while cumene is selectively oxidized to 2-phenyl-2-propanol. In contrast to cyclohexane oxidation, the incorporation of sterically hindering quinolyl/benzimidazolyl donors around Ni(II) leads to a high 3 degrees /2 degrees bond selectivity for adamantane oxidation. A linear correlation between the metal-ligand covalency parameter (beta) and the turnover number has been observed.