LABILE SR'C(O)CRCH-BRIDGED DICOBALT COMPLEXES - SYNTHESIS, STRUCTURESAND (CO)-C-13 LABELING STUDIES

Three alkyne-bridged dicobalt carbonyl complexes, [Co-2(mu-RCCH)(CO)(6 )] (R=H la, Me Ib, CH,OH Ic) have been reacted with Ph2P(SPh) and with the new thiophosphine ligands, Ph2P(SR') (R' = Bu-n, Bu-t), to give, initially, both mono-substituted [Co-2(mu-RCCH)(CO)(5){PPh2(SR')}] [(R ' = Ph; R = H 2a, Me 2b, CH2OH 2c), (R' = Bu-n; R= H 3a, Me 3b, CH2OH 3c), (R' = Bu-t; R= H 4a, Me 4b)] and bis-substituted [Co-2(mu-RCCH)(C O)(4){ PPh2(SPh)}(2)] [(R' = Ph; R = H 5a, Me 5b, CH2OH 5c), (R' = Bu- n; R = H 6a, Me 6b, CH2OH 6c), (R'=Bu-t; R=H 7a, Me 7b)] products. The rmolysis of complexes 2-4 gives [Co-2(mu-PPh2){mu-SR'C(O) CRCH}(CO)(4) ] [(R' = Ph; R = H 8a, Me 8b, CH2OH Sc), (R' = Bu-n; R = H 9a, Me 9b), (R' = Bu-t; R= H 10a, R= Me 10b)] in which sulphur-phosphorus bond cl eavage and sulphur-carbon bond formation at the dicobalt centre have o ccurred to generate SR'C(O)CRCH ligands incorporated into 5-membered r ing metallacycles. The transformations are regiospecific with the bulk y substituents (R = H) in all cases being located adjacent to the CO m oiety of the metallacycle. Treatment of the complex [Co-2(mu-PPh2){mu- SPhC(O)CHCH}(CO)(4)] 8a with (CO)-C-13 and monitoring of the reaction via C-13-{H-1}-NMR spectroscopy reveals the formation of a selectively labelled species, Ph2){mu-(SPhC)-C-13(O)CHCH}((CO)-C-13)(2)(CO)(2)], in which the ketonic bridging carbonyl signal has been enhanced, while only two of the four signals due to the terminal carbonyl groups have been similarly enhanced. The mechanistic implications of this labelli ng study are discussed and pathways postulated for the reversible frag mentation of the SR'C(O)CRCH-bridged dicobalt systems. Single crystal X-ray diffraction studies have been performed on [Co-2(mu-HCCH)(CO)(4) {PPh2(SBun)}(2)] 6a, revealing axially substituted thiophosphine group s and on [Co-2(mu-PPh2){mu-sBu(n)C(O)CHCH}(CO)(4)] 9a, in which an alm ost planar Co-S-C-C=C metallacycle-coordinated to the other cobalt is observed. (C) 1998 Elsevier Science Ltd. All rights reserved.