A new monoanionic dithiolene ligand is found in Tp*MoO(S2BMOQO). A combination of x-ray crystallography, electronic absorption and resonance Raman spectroscopies, and bonding calculations reveal that the monoanionic dithiolene ligand possesses considerable thiolate-thione character resulting from admixture of an intraligand charge transfer excited state into the ground state wavefunction. The unusual dithiolene exhibits a highly versatile donor-acceptor character that dramatically lowers the Mo(IV/V) redox couple and points to a potentially non-innocent role of the pterin fragment in pyranopterin Mo enzymes.

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.