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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry