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 (S(2)BMOQO) formed from the reaction of molybdenum tetrasulfide and quinoxalylalkyne. The resulting complexes TEA[Tp*MoX(S(2)BMOQO)] [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-S(2)BMOQO) (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-thiolate 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(S(2)BMOQO) complexes and further substantiate the highly electron-withdrawing character of the pyrrolo-S(2)BMOQO ligand in 2, 4, and 5. This study of the unique noninnocent ligand S(2)BMOQO 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.
17 Figures and Tables
Figure 1. Valence-bond description of dithiolene ligand forms. Twoelectron oxidation of the ene-1,2-dithiolate leads to oxidized forms described by 1,2-dithione and 1,2-dithiete Lewis structures.
Table 1. IR Data for 1 5 and Related Tp*Mo(dX) Dithiolene Complexesa
Figure 2. Pyranopterin dithiolene ligand in the molybdenum cofactor, Moco, the catalytic site in molybdenum enzymes.
Table 2. Electronic Spectral Data for Oxo- and Sulfidomolybdenum Dithiolene Compounds
Figure 3. Reaction of molybdenum tetrasulfide and an alkyne (BMOQO) forming a quinoxalyldithiolene complex, 1, at ambient temperature in path a and a pyrroloquinoxalyldithiolene complex, 2, under moderate heat in path b.
Table 3. Comparison of the Solvent Polarity andWavelength in Increasingly Polar Solutions of 4
Figure 4. Proposed mechanism for intramolecular cyclization of complex 1, producing pyrrolodithiolene in 2.
Table 4. Electrochemical Data for Oxo- and Sulfidomolybdenum Quinoxalyl- and Pterinyldithiolene compoundsa
Figure 5. Competition for π donation from oxo and dithiolene orbitals to the Mo dyz orbital.
Figure 6. Top: Room temperature solution isotropic EPR spectrum of 5. Bottom: 75 K EPR spectrum of 5. All data were collected in CH2Cl2.
Figure 7. Electronic spectrum of 3 in ACN with 2% H2O.
Figure 9. Calculated EDDM that details the nature of the intraligand transition in 2 (red, electron density loss in transition; green, electron density gain in transition).
Figure 10. Electronic spectra of 4 (red line) and 5 (black line) in ACN.
Figure 11. Calculated EDDM that details the nature of the intraligand transition in 5 (red, electron density loss in transition; green, electron density gain in transition.
Figure 14. Mo5+/4+ reduction potentials for Tp*MoO(dithiolene) complexes in ACN referenced to ferrocenium/ferrocene. Potentials are taken from the literature (dithiolene, Mo5+/4+, V: bdt, 0.84;49 qdt, 0.62;49 dmac, 0.35;44 S2PEPP, 0.4427).
Figure 15. (a) Asymmetry in the bond distances of dithiolene chelate in 4. (b) Key resonance structures for the asymmetric dithiolene in 4.
Figure 16. Dihedral fold angle in 4. The red plane is calculated through the atoms Mo S1,S2 of the dithiolene chelate, and the green plane is calculated through dithiolene atoms S1, S2, C1, and C2.
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