RESONANCE RAMAN SIGNATURES OF OXOMOLYBDENUM THIOLATE AND DITHIOLENE MODELS OF MOLYBDENUM PROTEINS

Resonance Raman (RR) data are reported for molybdenum compounds having ligands related to those which may be present at the active site of Mo redox enzymes: MoVI02(dttd) (1) (dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane), MoIV(S2C2(C02Me)2)32-(2), and MolvO(S2C2(C02Me)2)22-(3). For 1 RR bands are observed at 922/865 and 375/356 cm−1and are assigned to symmetric/asymmetric Mo=0 and Mo—S(thiolate) stretchings, respectively. Excitation profiles (EP’s) show the 922-cm−1Mo=0 band to reach maximum enhancement in resonance with a strong 410-nm electronic absorption of 1, which is assigned to an O—Mo charge-transfer (CT) transition. In contrast, the 356-cm−1Mo—S band EP has a dispersive shape with a maximum at 520 nm and a dip at 480 nm. This behavior is indicative of interference between scattering contributions from the strong 410-nm transition and a weaker transition at ~480 nm, attributed to thiolate-Mo CT. For 2, the 647.1-nm-excited RR spectrum in resonance with a broad ~650-nm electronic absorption, shows a strong band at 365 cm−1and a weaker one at 702 cm−1, assigned to Mo—S and C—S stretching modes, respectively. Weak enhancement is also observed for bands at 1475, 1488, and 1525 cm−1, one or more of which may arise from C=C stretching of the dithiolene ring. Replacement of a dithiolene ligand by an oxo ligand (giving 3) produces marked RR changes. The Mo—S and C=C stretches, now at 393 and 1535 cm−1, are seen with violet excitation, 406.7 nm, while yellow excitation (568.2 nm) reveals the Mo=0 stretch, at 910 cm−1, albeit weakly. The altered RR pattern implies a substantial electronic rearrangement, which is also reflected in the shifted ground-state vibrational frequencies. © 1990, American Chemical Society. All rights reserved.