New monomeric cobalt(II) and zinc(II) complexes of a mixed N,S(alkylthiolate) ligand: Model complexes of (His)(His)(Cys) metalloprotein active sites

The new N2S(alkylthiolate) ligand 2-methyl-1-[methyl-(2-pyridin-2-ylethyl)amino]propane-2-thiolate, PATH (1), has been prepared and reacted with zinc(II) and cobalt(II) to give the monomeric complexes [(PATH)ZnBr] (2), [(PATH)ZnNCS] (3), [(PATH)CoBr] (4), and [(PATH)CoNCS] (5). The molecular structures of 4 and 5 have been determined by X-ray diffraction. Each complex displays a distorted tetrahedral geometry at the metal center, with the PATH ligand providing the N2S(alkylthiolate) donors. These complexes are close structural mimics of the active site of metalloproteins with a His(2)Cys-M-II site such as that found in peptide deformylase. Complexes 4 and 5 are the first examples of crystallographically characterized Co-II complexes with an N2SL (L not equal N,S) donor set. Only one diastereomer for 2-5 is observed in the solid state, and simple molecular mechanics (Chem3D) calculations suggest this isomer is stable because of a favorable ligand conformation. NMR studies in the case of Zn-II and UV-vis studies in the case of Coll provide strong evidence that their solid-state structures are retained in solution. Cyclic voltammetry reveals processes for both the Co-II/I (4, - 1.51 V; 5, - 1.49 V) and Co-III/II (4, + 0.9 V; 5, + 0.9 V) couples. The UV-vis data for the cobalt complexes are consistent with a monomeric, four-coordinate geometry regardless of the nature of the solvent (i.e., donating (MeOH, CH3CN) vs nondonating (CH2Cl2)) and are compared with other cobalt complexes as well as cobalt-substituted His(2)Cys metalloproteins (peptide deformylase and blue-copper proteins). In addition, reaction of the bromide complexes 2 and 4 with hydroxide anion leads to the formation of 1:1 hydroxide:M-II complexes which have been characterized in situ by H-1 NMR and UV-vis spectroscopy, respectively.