O2-Dependent Aliphatic Carbon-Carbon Bond Cleavage Reactivity in a Ni(II) Enolate Complex Having a Hydrogen Bond Donor Microenvironment; Comparison with a Hydrophobic Analogue

A mononuclear Ni(II) complex having an acireductone type ligand, and supported by the bnpapa (N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) ligand, [(bnpapa)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (14). has been prepared and characterized by elemental analysis, H-1 NMR, FTIR, and UV-vis, To gain insight into the H-1 NMR features of 14, the air stable analogue complexes [(bnpapa)Ni(CH3C(O)CHC(O)CH3)]ClO4 (16) and [(bnpapa)Ni(ONHC(O)CH3)]ClO4 (17) were prepared and characterized by X-ray crystallography, H-1 NMR, FTIR, UV-vis, mass spectrometry, and solution conductivity measurements. Compounds 16 and 17 are 1:1 electrolyte species in CH3CN. H-1 and H-2 NMR studies of 14, 16, and 17 and deuterated analogues revealed that the complexes having six-membered chelate rings for the exogenous ligand (14 and 16) do not have a plane of symmetry within the solvated cation and thus exhibit more complicated H-1 NMR spectra. Compound 17, as well as other simple Ni(II) complexes of the bnpapa ligand (e.g., [(bnpapa)Ni(ClO4)(CH3CN)]ClO4 (18) and [(bnpapaNi)(2)(mu-Cl)(2)](ClO4)(2) (19)). exhibit H-1 NMR spectra consistent with the presence of a plane of symmetry within the cation. Treatment of [(bnpapa)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (14) with O-2 results in aliphatic carbon-carbon bond cleavage within the acireductone-type ligand and the formation of [(bnpapa)Ni(O2CPh)]ClO4 (9), benzoic acid, benzil, and CO. Use of O-18(2) in the reaction gives high levels of incorporation (>80%) of one labeled oxygen atom into 9 and benzoic acid. The product mixture and level of O-18 incorporation in this reaction is different than that exhibited by the analogue supported the hydrophobic 6-Ph(2)TPA ligand, [(6-Ph(2)TPA)Ni(PhC(O)C(OH.)C(O)Ph)]ClO4 (2). We propose that this difference is due to variations in the reactivity of bnpapa- and 6-Ph(2)TPA-ligated Ni(II) complexes with triketone and/or peroxide species produced in the reaction pathway.