Methylation of Tethered Thiolates in [(bme-daco)Zn](2) and [(bme-daco)Cd](2) as a Model of Zinc Sulfur-Methylation Proteins.
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abstract
The dimeric dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctanato]zinc(II), [(bme-daco)Zn](2) or Zn-1, and its cadmium analogue, Cd-1, were investigated as models for the active site of zinc-dependent methylation proteins. The key issue addressed was whether alkylation of a thiolate in a relatively rigid tetradentate ligand would result in coordination of the thioether product to the metal. On the basis of (1)H and (13)C NMR spectroscopy and similar reactivity toward alkylating agents, the newly synthesized cadmium complex, Cd-1, is proposed to be isostructural with the previously reported Zn-1 complex, which is known from X-ray crystallography to be dimeric in the solid state (Tuntulani, T.; Reibenspies, J. H.; Farmer, P. J.; Darensbourg, M. Y. Inorg. Chem.1992, 31, 3497). Iodomethane reacts with Zn-1 in hot CH(3)OH/CH(3)CN to produce a thioether which dissociates, replaced by coordination of iodide in the pseudotetrahedral complex, (Me(2)bme-daco)ZnI(2) or Zn-2. Complex Zn-2 crystallizes in the triclinic P&onemacr; space group with a = 7.911(2) , b = 10.675(2) , c = 12.394(2) , alpha = 75.270(10) degrees, beta = 75.270(10) degrees, gamma = 82.12(2) degrees, V = 998.270 (3), and Z = 2. An analogous reaction was observed for the cadmium derivative, Cd-1, which displays a (1)H NMR spectrum identical to that of Zn-2. In attempts to promote thioether binding, the iodide was displaced by addition of AgBF(4) to solutions of Zn-2 or the BF(4)(-) analogue was synthesized directly from Zn(BF(4))(2) and methylated ligand, Me(2)bme-daco, to yield Zn-3. Similar reactions with the cadmium analogue yielded a product identified as Cd-3 that was indistinguishable from Zn-3 by (1)H NMR. The (113)Cd NMR spectra of Cd-3 displayed a single resonance at 88 ppm consistent with a hard donor environment and inconsistent with sulfur binding. As a further attempt to induce thioether binding to zinc, the macrocyclization reagent 1,3-dibromopropane was added to Zn-1. The resulting product, [BrZn(macrocycle)](+), was only slightly soluble in pyridine and identified by +FAB/MS as the desired macrocyclic product with a large amount of free macrocycle ligand. Recrystallization from pyridine/ether resulted in loss of the zinc as Zn(py)(2)Br(2), which was obtained as colorless crystals and characterized by X-ray crystallography. Complex Zn(py)(2)Br(2) crystallizes in the monoclinic P2(1)/c space group with a = 8.534(2) , b = 18.316(4) , c = 8.461(2) , beta = 101.07(3) degrees, V = 1297.9(5) (3), and Z = 4.
