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

• DOI: 10.1039/c1dt10902b
• Publication Date: 2011-08-18T08:30:57Z
• Authors: Mani Balamurugan; Ramasamy Mayilmur...; Eringathodi Suresh; Mallayan Palanian...
• Keywords: Octahedral molecular geometry

Several mononuclear Ni(II) complexes of the type [Ni(L)(CH3CN)2](BPh4)21–7, 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 characterized using CHN analysis, UV-Visible spectroscopy mass spectrometry. The single-crystal X-ray structures [Ni(L1)(CH3CN)(H2O)](ClO4)21a, [Ni(L2)(CH3CN)2](BPh4)22, [Ni(L3)(CH3CN)2](BPh4)23 [Ni(L4)(CH3CN)2](BPh4)24 determined. All these possess distorted octahedral coordination geometry in which coordinated to four nitrogen atoms ligands two CH3CN (2, 3, 4) or one H2O (1a) are located cis positions. Ni–Npy bond distances (2.054(2)–2.078(3) Å) 1a, 2 3 shorter than Ni–Namine bonds (2.127(2)–2.196(3) because sp2 sp3 hybridizations pyridyl tertiary amine nitrogens respectively. In Ni–Nim (2.040(5) (2.074(4) due stronger imidazole compared with pyridine donor. dichloromethane/acetonitrile solvent mixture, all an geometry, revealed by characteristic field bands visible region. They efficiently catalyze hydroxylation alkanes when m-CPBA used oxidant turnover number (TON) range 340–620 good alcohol selectivity for cyclohexane (A/K, 5–9). By replacing donors TPA weakly coordinating –NMe2 –NEt2 donor atom catalytic activity decreases slightly no change selectivity. contrast, upon strongly σ-bonding imidazolyl sterically demanding quinolyl/benzimidazolyl donor, both decrease, possibly destabilization intermediate [(4N)(CH3CN)Ni–O˙]+ radical species. Adamantane selectively (3°/2°, 12–17) oxidized 1-adamantanol, 2-adamantanol 2-adamantanone while cumene 2-phenyl-2-propanol. contrast oxidation, incorporation hindering around leads high 3°/2° adamantane oxidation. A linear correlation between metal–ligand covalency parameter (β) has observed.