Thermal and photochemical transformations of 2-methylthiothiophene in triosmium clusters: photoinduced migrations of MeS from carbon to metal and back again

2-Methylthiothiophene (2-MeSC4H3S) oxidatively adds to [Os-3(CO)(10)(MeCN)( 2)] with cleavage of the C-H bond at the 3-position to give [Os-3(mu-H)(mu- MeSC4H2S)(CO)(10)] 1, the X-ray structure of which shows that the MeS group is coordinated to osmium through the S atom while the thiophene ring is co ordinated to osmium through the 3-carbon atom. Only one invertomer at sulfu r is observed in solution and in the crystal the Me group is exo. Thermal t reatment of 1 in the dark gives only one product, [Os-3(mu-H)(mu(3)-MesC(4) H(2)S)(CO)(9)] 2 (X-ray structure), derived by loss of a CO from the Os(CO) (4) unit of 1 with concomitant eta(2)-coordination of the thiophene ring of bridging MeSC4H2S at the third metal atom. Whereas thermal reaction in the dark leads only to C-H cleavage products, visible irradiation at room temp erature leads to various products derived by migration of the MeS group. Th us thermal treatment of 1 in daylight for 2 h gave 2, together with an isom er 3. Cluster 2 converts at room temperature to 3 in daylight while thermal treatment of 2 in the nark (125 degrees C) gave no reaction. Isomer 3 of [ Os-3(mu-H)(mu(3)-MeSC4H2S)(CO)(9)] (X-ray structure) is closely related to 2 except that the MeS group and the Os-C sigma-bond have interchanged sites at the thiophene ring between the 2- and the 3-positions. Visible irradiat ion of 1 at room temperature for 3 days in daylight gave further chemical c hange leading to two bridging thienyl clusters, [Os-3(mu-MeS)(mu-2-C4H2S)(C O)(10)] 4 and [Os-3(mu-MeS)(mu(3)-2-C4H2S)(CO)(9)] 5. Cluster 5 is the ulti mate product of daylight irradiation of any of the clusters 1 to 4. (C) 199 9 Elsevier Science S.A. All rights reserved.