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

Abstract

A model system for the molybdenum cofactor has been developed that illustrates the non-innocent behavior of a N-heterocycle appended to a dithiolene chelate on molybdenum. The pyranopterin of the molybdenum cofactor is modeled by a quinoxalyl-dithiolene ligand (S2BMOQO) formed from reaction of a molybdenum tetrasulfide and a quinoxalyl alkyne. 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 the quinoxalyl-dithiolene forming Tp*MoX(pyrrolo-S2BMOQO) (2, X= S; 4, X= O). 4 can be oxidized by one electron to produce the Mo(5+) complex 5. In a preliminary report of this work evidence from X-ray crystallography, electronic absorption and resonance Raman spectroscopies, and DFT bonding calculations revealed that 4 possesses an unusual asymmetric dithiolene chelate with significant thione-thiolate character. The results described here provide a detailed description of the reaction conditions that lead to formation of 4. Data from cyclic voltammetry, additional DFT calculations and several spectroscopic methods (infrared, electronic, resonance Raman, 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 and 4. This study of the unique non-innocent 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 dithiolene appended by heterocycles may aid in modulating the redox processes of the molybdenum center during the course of enzyme catalysis.

Pyranopterin conformation defines the function of molybdenum and tungsten enzymes

“…5B). In support of this, a monoanionic, thiol-thione type dithiolene was recently observed in a Mo-dithiolene model complex that possesses a donor-acceptor dithiolene ligand bound to a reduced Mo(IV) ion (14). The accessibility of a thiol-thione dihydro pyranopterin chelate (Fig.…”

“…This non-innocence extends the redox cycling of the metal through its VI/V/IV oxidation states to include two electron redox reactions of the dithiolene ligand via access to the reduced ene-dithiolate, radical dithiolene, and oxidized dithione forms (12)(13)(14). Inspection of the tetrahydro pyranopterin structure reveals the potential for additional oxidation states, such as the quinonoid 4a,10a-dihydro form (Fig.…”

Solvent-Dependent Pyranopterin Cyclization in Molybdenum Cofactor Model Complexes

“…Quinoxaline rotation precedes an intramolecular cyclization process to form a pyrrolo-dithiolene ligand. 42,43 …”

Pyranopterin Coordination Controls Molybdenum Electrochemistry in Escherichia coli Nitrate Reductase

“…If the role of the pyranopterin dithiolene chelates is merely to contribute to a molybdenum-binding scaffold at the active site, then variants of the target residues studied herein should have little or no effect on enzyme function. If, however, the noninnocence reported for model compounds extends beyond the dithiolene chelate and into the heterocyclic ring system of the pyanopterin (13)(14)(15)20), variants of pyranopterin-coordinating residues should have a dramatic effect on molybdenum electrochemistry and catalysis.…”