Regiospecificity in reactions of alkynes and phosphines with the phosphido-bridged iron–cobalt complex [(OC) 4Fe(μ-PPh 2)Co(CO) 3

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The iron–cobalt phosphido-bridged complex [(OC) 4Fe(μ-PPh 2)Co(CO) 3] ( 1) can be prepared conveniently and in good yield from the reaction of [Co 2(μ-PPh 2) 2(CO) 6] with [Fe(CO) 5]. A study of the reactivity of 1 towards symmetrical and unsymmetrical alkynes, R 1CCR 2 (R 1=R 2=CO 2Me, Ph; R 1=H, R 2=Ph), has been undertaken. In all cases, five-membered ferracycle-containing products of the type [(OC) 3Fe{μ-PPh 2C(O)CR 1CR 2}Co(CO) 3] (R 1=R 2=CO 2Me ( 2a), Ph ( 2c); R 1=H, R 2=Ph ( 2b)), are initially obtained in which a molecule of CO and a molecule of R 1CCR 2 have been inserted regiospecifically into a CoP bond in 1. Decarbonylation of 2a occurs during its preparation or in low yield on its thermolysis to give the four-membered ferracyclic species [(OC) 3Fe{μ-PPh 2C(CO 2Me)C(CO 2Me)}Co(CO) 3] ( 3a). Similar thermolysis of 2b results not only in the related decarbonylation product [(OC) 3Fe(μ-PPh 2CHCPh)Co(CO) 3] ( 3b), but additionally in three other products all in low yield, namely the regioisomer of 3b, [(OC) 3Fe(μ-PPh 2CPhCH)Co(CO) 3] ( 4b), the aldehyde-substituted complex [(OC) 3Fe{μ-PPh 2C(CHO)CPh}Co(CO) 3] ( 5b) and the five-membered ferracycle-containing species [(OC) 3Fe{μ-PPh 2CHCPhC(O)}Co(CO) 3] ( 6b). Treatment of 2a and 2b with PPhMe 2 and P(OMe) 3 results in substitution of an iron-bound carbonyl group to give, respectively, [(PhMe 2P)(OC) 2Fe{μ-PPh 2C(O)C(CO 2Me)C(CO 2Me)}(μ-CO)Co(CO) 2] ( 7a) and [{(MeO) 3P}(OC) 2Fe{μ-PPh 2C(O)CHCPh}(μ-CO)Co(CO) 2] ( 7b) in high yield. In contrast, substitution of a cobalt-bound carbonyl is achieved on reaction of 3a with PPhMe 2 or PPh 2H to give [(OC) 3Fe{μ-PPh 2C(CO 2Me)C(CO 2Me)}Co(CO) 2(L)] (L=PPhMe 2 ( 8a), PPh 2H ( 9a)). Thermolysis of the secondary phosphine-substituted complex 9a results in phosphorushydrogen bond cleavage to give [(OC) 3Fe{μ-PPh 2C(CO 2Me)CH(CO 2Me)}(μ-PPh 2)Co(CO) 2] ( 10a). Single-crystal X-ray diffraction studies have been performed on complexes 7b, 8a and 10a.