Preparations and characterizations of homodinuclear complexes containing flyover bridge or cyclopentadienone ligands ; X-ray crystal structures of Co-2(CO)(3)(PPh3){mu-C(SiMe3)=CH-CH=C(SiMe3)-CH=C(SiMe3)}, Co-2(CO)(2)(mu-CO)(2)(eta(4)-2,5-bistrimethylsilylcyclopentadienone)(2) and 2,5-bis(trimethylsilanyl)cyclopenta-2,4-dien-1-one

The reaction of Co-2(CO)(5)(PPh3)(mu-HC=C-SiMe3 (Ib) with 2.5 equivalent am ount of trimethylsilylacetylene in refluxed THF yielded two homodinuclear c omplexes containing 'flyover bridge' ligands, Co-2(CO)(3)(PPh3){mu-C(SiMe3) =CH-CH=C(SiMe3)-CH=C(SiMe3)} (2a) and a small amount of Co-2(CO)(2)(mu-CO)( 2)(eta(4)-2,5-bistrimethylsilylcyclopentadienone)(2) (4). The yield of (4) was improved, while using about five equivalent amount of trimethylsilylace tylene. The crystal structures of (2a) and (4) were determined. The structu re of (2a) can be regarded as a dimetallic frame contains three linked HC=C SiMe3 groups that are arranged in a 'flyover-bridge' geometry. Trimethylsil yl groups are attached to carbon atoms of the flyover bridge ligand at posi tions 1, 3, and 6. The ligand might be seen as the composition of two allyl groups, C1-C2-C3 and C4-C5-C6, which are joined by a single bond between C 3 and C4. Each allyl group is bonded to one cobalt atom. 1,2,4-Tris-trimeth ysilanylbenzene was obtained from the degradation of the compound (2a). The crystal structure of (4) shows that two cyclopentadienone ligands, two ter minal and two bridged carbonyls were found to be coordinated to both metal centers. The formation of the two cyclopentadienones is believed to take pl ace via the transition metal-mediated cyclization of two alkynes with one c arbon monoxide. 3,5-Bis(trimethylsilanyl)cyclopenta-2,4-dien-1-one (5) was obtained from the degradation of (4) by exposing to air in solution. The fo rmation of (5) is believed to take place via the Diels-Alder type cyclizati on of two released cyclopentadienone ligands from (4). (C) 1999 Elsevier Sc ience S.A. All rights reserved.