As possible models for the coordination of molybdenum by an unsaturated thiolate-thioether ligand in the molybdenum-pterin cofactor of molybdenum enzymes, Mo02(dttd), MoOCl(dttd), and MoO(PPh2Et)(dttd) (dttdH2 = 2,3;8,9-dibenzo-1,4,7,10-tetrathiadecane) have been synthesized. The structure of Mo02(dttd) has been determined by X-ray crystallography. The compound crystallizes in space group P21/c, with a = 14.34 (2) A, b = 8.168 (6) A, c = 14.507 (8) A, 0 - 103.42 (8)°, and Z = 4. Mo02(dttd) is six-coordinate with substantial distortion from octahedral geometry. The two thiolate S atoms are trans to one another (Mo-Sav= 2.402 (7) A) and cis to the terminal oxo groups. The thioether S atoms are approximately trans to the terminal oxo groups and exhibit much longer Mo-S distances (2.687 (6) A). Mo02(dttd) undergoes a one-electron reversible reduction on the cyclic voltammogram time scale, but is reduced irreversibly in the presence of Et4NCl in a two-electron step to [MoOCl(dttd)]- on the coulometric time scale; [MoOCl(dttd)]- in turn is reversibly oxidized to the EPR active monomer MoOCl(dttd). MoOCl(dttd) undergoes a reversible one-electron reduction to [MoOCl(dttd)]-, but cannot be oxidized to Mo02(dttd) in the voltage range used. MoO(PPh2Et)(dttd) appears to lose the PPh2Et group in DMF/Et4NCl solution, exhibiting electrochemical behavior identical with that of [MoOCl(dttd)]“. Reaction of Mo02(dttd) with 1 equiv of PPh2Et in DMF in the presence of Et4NCl gives [MoOCl(dttd)]-, while subsequent addition of Me2SO to the solution regenerates Mo02(dttd). The dimer, Mo203(dttd)2, has also been obtained. It is reversibly reduced in a 1.00 electron/Mo process to [MoOCl(dttd)]- and reversibly oxidized in a 0.50 electron/Mo process to a mixture of equal amounts of Mo02(dttd) and MoOCl(dttd) in DMF in the presence of Et4NCl. The EPR parameters of MoOCl(dttd) and the electrochemical parameters of the various complexes are reported.
ASJC Scopus subject areas
- Colloid and Surface Chemistry