Study of Molybdenum(4+) Quinoxalyldithiolenes as Models for the Noninnocent Pyranopterin in the Molybdenum Cofactor

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• 作者：Kelly G. Matz ; Regina P. Mtei ; Rebecca Rothstein ; Martin L. Kirk ; Sharon J. Nieter Burgmayer
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：October 17, 2011
• 年：2011
• 卷：50
• 期：20
• 页码：9804-9815
• 全文大小：1188K
• 年卷期：v.50,no.20(October 17, 2011)
• ISSN：1520-510X

文摘

A model system for the molybdenum cofactor has been developed that illustrates the noninnocent behavior of an N-heterocycle appended to a dithiolene chelate on molybdenum. The pyranopterin of the molybdenum cofactor is modeled by a quinoxalyldithiolene ligand (S₂BMOQO) formed from the reaction of molybdenum tetrasulfide and quinoxalylalkyne. The resulting complexes TEA[Tp*MoX(S₂BMOQO)] [1, X = S; 3, X = O; TEA = tetraethylammonium; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] undergo a dehydration-driven intramolecular cyclization within quinoxalyldithiolene, forming Tp*MoX(pyrrolo-S₂BMOQO) (2, X = S; 4, X = O). 4 can be oxidized by one electron to produce the molybdenum(5+) complex 5. In a preliminary report of this work, evidence from X-ray crystallography, electronic absorption and resonance Raman spectroscopies, and density functional theory (DFT) bonding calculations revealed that 4 possesses an unusual asymmetric dithiolene chelate with significant thione鈥搕hiolate character. The results described here provide a detailed description of the reaction conditions that lead to the formation of 4. Data from cyclic voltammetry, additional DFT calculations, and several spectroscopic methods (IR, electronic absorption, resonance Raman, and electron paramagnetic resonance) have been used to characterize the properties of members in this suite of five Mo(S₂BMOQO) complexes and further substantiate the highly electron-withdrawing character of the pyrrolo-S₂BMOQO ligand in 2, 4, and 5. This study of the unique noninnocent ligand S₂BMOQO provides examples of the roles that the N-heterocycle pterin can play as an essential part of the molybdenum cofactor. The versatile nature of a dithiolene appended by heterocycles may aid in modulating the redox processes of the molybdenum center during the course of enzyme catalysis.