Structural variations in dinuclear model hydrolases and hydroxamate inhibitor models: synthetic, spectroscopic and structural studies

Reactions of the structural model hydrolases [M2(OAc)4(H2O)(Im)4]; M=Mn (E′); M=Co (D′); M=Ni (B′) and [M2(OPiv)4(H2O)(tmen)2]; M=Mn (E″); M=Co (D″); M=Ni (B″) with a number of hydroxamic acids, RHA (aceto- (R=CH3), benzo- (R = C6H5) and N-phenylacetohydroxamic acid (NPhAHA)) gave a series of hydroxamate dibridged complexes [M2(OAc)(RA)2(Im)4][OTf] and [M2(OPiv)(RA)2(tmen)2][OTf]; M=Co, Ni, in which the bridging hydroxamates exhibit a novel bonding mode in which the deprotonated hydroxamate hydroxyl bridges the two metal centres only. The formation of this type of structure by NPhAHA is the first example involving a secondary hydroxamic acid. These complexes are good structural models of the acetohydroxamate-inhibited C319A variant of Klebsiella aerogenes urease (KAU) and their structures are close to those previously reported for complexes containing tmen capping ligands. Reaction with glutarodihydroxamic acid leads to hydroxylamine elimination and formation of a dimer containing deprotonated N-hydroxyglutarimide as bridging ligand but in this case the structure contains pentacoordinated Co(II) and only one bridging acetate in contrast to the tmen-based series where the analogous complex contains hexacoordinated Co(II) and two bridging acetates. Reaction of [Mn2(OAc)2(μ-OAc)2(μ-H2O)(tmen)2] with acetohydroxamic acid (AHA) gave the first structurally characterized manganese hydroxamate, [Mn2(OAc)3(AA)(tmen)2] with the same bridging/chelating mode of hydroxamate bonding as in the analogous cobalt and nickel complexes, although only one bridging hydroxamate occurs in the manganese complex in contrast to the two bridging hydroxamates in the cobalt and nickel complexes. The isolation of the dimanganese hydroxamate bridged complex suggests that hydroxamic acids may also inhibit the dimanganese based metallohydrolase, arginase.