Di-, Tri-, and Tetranuclear Zinc Hydroxamate Complexes as Structural Models for the Inhibition of Zinc Hydrolases by Hydroxamic Acids

Attempts to produce Zn analogues of the structural model complexes [M2(μ-O2CR)2(O2CR)2(μ-H2O)(tmen)2] (M = Ni, Co, Mn; R = CH3, C(CH3)3, CF3) by the reaction of a series of zinc carboxylates with N,N,N‘,N‘-tetramethylethylenediamine (tmen), resulted in the mononuclear complexes [Zn(OAc)2(tmen)] (1) and [Zn(crot)2(tmen)]·0.5H2O (2) for R = CH3 and (CH)2CH3, respectively, and the dinuclear complexes [Zn2(μ-piv)2(piv)2(μ-H2O)(tmen)2] (3) and [Zn2(μ-OAcF)2(OAcF)2(μ-H2O)(tmen)2] (4) for R = C(CH3)3 and CF3, respectively. In contrast to the analogous imidazole series, i.e., [M2(μ-O2CR)2(O2CR)2(μ-H2O)(Im)4] (M = Ni, Co, Mn; R = CH3, C(CH3)3, CF3), zinc carboxylates react with imidazole to give only the mononuclear complexes [Zn(OAc)2(Im)2] (5), [Zn(crot)2(Im)2]·H2O (6), [Zn(piv)2(Im)2]·0.5H2O (7), and [Zn(OAcF)2(Im)2] (8). Reaction of 1, 2, and 3 with either acetohydroxamic acid (AHA) or benzohydroxamic acid (BHA) gives the dinuclear complexes [Zn2(O2CR)3(R‘A)(tmen)], where R‘A = acetohydroxamate (AA) (9, 10, 11) or benzohydroxamate (BA) (13, 14, 15). In these complexes, the zinc atoms are bridged by a single hydroxamate and two carboxylates, with a capping tmen ligand on one zinc and a monodentate carboxylate bonded to the second zinc atom. This composition models closely the observed structure of the active site of the p-iodo-d-phenylalanine hydroxamic acid inhibited Aeromonas proteolytica aminopeptidase enzyme. In contrast, 4 reacts with AHA to give [Zn2(OAcF)3(tmen)2(AA)] (12) with an additional tmen ligand so that both Zn atoms are 6-coordinate, whereas reaction with BHA gives the trinuclear complex [Zn3(OAcF)4(tmen)2(BA)2] (16). Reactions of 3 and 4 with glutarodihydroxamic acid (GluH2A2) produce the tetranuclear complexes [Zn4(piv)6(tmen)4(GluA2)] (18) and [Zn4(OAcF)6(tmen)4(GluA2)] (19).