Magnetic and structural studies of novel tetranickel hydroxamates

Abstract

The dinickel(II) compound [Ni 2(μ-OAc) 2(OAc) 2(μ-H 2O)(asy·dmen) 2]·2.5H 2O, 1; undergoes facile reaction in a 1:2 molar ratio with benzohydroxamic acid (BHA) in ethanol to give the novel nickel(II) tetranuclear hydroxamate complex [Ni 4(μ-OAc) 3(μ-BA) 3(asy·dmen) 3][OTf] 2·H 2O, 2, in which the bridging acetates, bridging two nickel atoms in 1, undergo a carboxylate shift from the μ 2-η 1:η 1 bridging mode of binding to the μ 3-η 1:η 2 bridging three nickel atoms in the tetramer. The structure of complex 2 was determined by single-crystal X-ray crystallography. The two monodentate acetates, water and two bidentate bridging acetates of two moles of complex 1 are replaced by three monodentate bridging acetates and three benzohydroxamates. Three nickel atoms in the tetramer, Ni(2), Ni(3) and Ni(4) are in a N 2O 4 octahedral environment, while the fourth nickel atom Ni(1) is in an O(6) octahedral environment. The Ni–Ni separations are Ni(1)–Ni(2) = 3.108 Å, Ni(1)–Ni(3) = 3.104 Å and Ni(1)–Ni(4) = 3.110 Å, which are longer than previously studied in dinuclear urease inhibited models but shorter than in the nickel(II) tetrameric glutarohydroxamate complex [Ni 4(μ-OAc) 2(μ-gluA 2) 2(tmen) 4][OTf] 2, isolated and characterized previously in this laboratory. Magnetic studies of the tetrameric complex show that the four Ni(II) ions are ferromagnetically coupled, leading to a total ground spin state S T = 4. Three analogous tetranuclear nickel hydroxamates were prepared from AHA and BHA and the appropriate dinuclear complex with either sy·dmen or asy·dmen as capping ligands.