The reactions of tricobalt alkylidyne complexes

The alkylidyne-capped complexes [Co₃(μ₃-CR)(CO)₉] (R = Me 1a, CO₂Me 1b) react with PPh₂CH=CH₂ in heptane at 308 K to give the phosphine-substituted complexes [Co₃(μ₃-CR)(CO)₈(PPh₂CH=CH₂)] (R = Me 2a, CO₂Me 2b), [Co₃(μ₃-CR)-(CO)₇(PPh₂CH=CH₂)₂] (R = Me 3a, CO₂Me 3b) and, in the case of R = CO₂Me, the tris-substituted complex [Co₃(μ₃-CCO₂Me)(CO)₆(PPh₂CH=CH₂)₃] 4. Heating of complexes 2a and 2b at 343 K results in loss of a CO group and coordination of the vinyl moiety of the PPh₂CH=CH₂ ligand to afford [Co₃(μ₃-CR)(μ-PPh₂CH=CH₂)(CO)₇] (R = Me 5a, CO₂Me 5b). In contrast, reaction of [Co₃(μ₃-CR)(CO)₉] (R = Me 1a, CO₂Me 1b) with cis-Ph₂PCH=CHPPh₂ (dppee) gives [Co₃(μ₃-CR)(μ-Ph₂PCH=CHPPh₂)(CO)₇] (R = Me 6a, CO₂Me 6b) in which the dppee ligand bridges two adjacent cobalt atoms via both phosphino moieties. Nuclear magnetic resonance studies on 5a and 5b indicate that two isomeric structures in a ratio of 7:3 are present in solution at 293 K. Conversion of [Co₃(μ₃-CR)(μ-PPh₂CH=CH₂)(CO)₇] 5a and 5b back to [Co₃(μ₃-CR)(CO)₈(PPh₂CH=CH₂)] 2a and 2b can be achieved by purging with CO at 293 K while further purging at 343 K results in conversion to [Co₃(μ₃-CR)(CO)₉] 1a and 1b. The structure of [Co₃(μ₃-CCO₂Me)(μ-Ph₂PCH=CHPPh₂)(CO)₇] 6b was determined by single-crystal X-ray diffraction. All the complexes have been characterized spectroscopically and by elemental analysis.