Mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation using m-chloroperbenzoic acid as oxidant

• Mixed ligand nickel(II) complexes 1–5 have been isolated.
• X-ray structure of 2 possesses distorted octahedral coordination geometry.
• Ligand field spectroscopy reveal octahedral geometry for 1–5 in solution.
• Catalytic efficiency depends on Lewis acidity, π-back bonding and ligand denticity.

A new family of nickel(II) complexes of the type [Ni(PA)(L)(CH3CN)n]BPh41–5, where n = 1, 2, H(PA) is 2-picolinic acid and L is N,N′-tetramethylethylenediamine (L1) 1, N,N′,N″-pentamethyldiethylenetriamine (L2) 2, 2,2′-bipyridine (L3) 3, 1,10-phenanthroline (L4) 4 or 2,9-dimethyl-1,10-phenanthroline (L5) 5, has been isolated and characterized using CHN analysis, UV–Vis spectroscopy and ESI-MS. The complex [Ni(PA)(L2)(CH3CN)](BPh4) 2 possesses a distorted octahedral coordination geometry in which Ni(II) is chelated to 2-picolinate anion and L2. DFT calculations show that trans isomers of 3–5 are more stable than cis isomers by ca. 4.0 kJ/mol. In contrast, cis-1 is more stable than trans-1 by 15.8 kJ/mol. The complexes catalyze the hydroxylation of cyclohexane efficiently in presence of m-CPBA as oxidant with 244–569 turnover numbers and good alcohol selectivity (A/K, 3.4–7.0). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with varying bond selectivity (3°/2°, 9.3–14.2) while cumene is selectively oxidized to 2-phenyl-2-propanol. Upon replacing bidentate L1 by tridentate L2 or strongly π-back bonding phen the catalytic activity increases. In contrast, phen is replaced by non-planar bpy or 2,9-dmp with sterically hindering methyl groups the catalytic activity decreases. Thus ligand denticity, Lewis acidity of Ni(II) center and π-back bonding determine the catalytic activity.

A series of mixed ligand nickel(II) complexes has been isolated and studied as catalysts for alkane hydroxylation reaction using m-CPBA as oxidant. The π-back bonding ability of the heterocyclic nitrogen facilitates the formation and stabilization of high-valent nickel-oxo intermediate species leading to enhancement in catalysis.Figure optionsDownload as PowerPoint slide