Synthesis and characterization of doubly bonded and quadruply bridged trans/anti and trans/syn-(eta(5)-RC5H4)(2)M-2(mu-Se)(2)(mu-SePh)(2) (M=Mo, W) isomers

Reactions of a mixture of doubly bonded and double-bridged trans/anti and t ranslsyn-(eta(5)-RC5H4)(2)M-2(CO)(2)(mu-SePh)(2) isomers with elemental sel enium afford a series of novel doubly bonded and quadruply bridged isomers, trans/anti- (eta(5)-RC5H4)(2)M-2(mu-Se)(2)(mu-SePh)(2) (1a, M=Mo, R=H; 2a, M=Mo, R=MeO2C; 3a, M=Mo, R=EtO2C; 4a, M=W, R=MeO2C; 5a, M=W, R=EtO2C) and trans/syn-(eta(5)- RC5H4)(2)M-2(mu-Se)(2)(mu-SePh)(2) (1b, M=Mo, R=H; 2b, M =Mo, R=MeO2C; 3b, M=Mo, R=EtO2C; 4b, M=W, R=MeO2C; 5b, M=W, R=EtO2C). Such reactions were found to involve configuration inversions of the phenyl grou ps bonded to selenium atoms from syn to anti or vice versa. Thus, the react ion of single isomer trans/syn-(eta(5)-C5H5)(2)Mo-2(CO)(2)(mu-SePh)(2) with elemental selenium in tetrahydrofuran (THF) or toluene at reflux gave sing le trans/anti isomer 1a, whereas trans/anti-(eta(5)-EtO2CC5H4)(2)Mo-2(CO)(2 )(mu-SePh)(2) reacted with selenium powder under similar conditions gave si ngle trans/syn isomer 3b. Except for isomer 1b (previously prepared by anot her route), all of the above quadruply bridged isomers have been characteri zed by elemental analysis, MS, IR and H-1 NMR spectroscopies, as well as pa rtly by C-13 NMR and Se-77 NMR spectroscopies. (C) 1999 Elsevier Science Lt d. All rights reserved.