Regiospecificity in reactions of alkynes and phosphines with the phosphido-bridged iron–cobalt complex [(OC)4Fe(μ-PPh2)Co(CO)3]

Abstract The iron–cobalt phosphido-bridged complex [(OC) 4 Fe(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 1 CCR 2 (R 1 =R 2 =CO 2 Me, Ph; R 1 =H, R 2 =Ph), has been undertaken. In all cases, five-membered ferracycle-containing products of the type [(OC) 3 Fe{μ-PPh 2 C(O)CR 1 CR 2 }Co(CO) 3 ] (R 1 =R 2 =CO 2 Me ( 2a ), Ph ( 2c ); R 1 =H, R 2 =Ph ( 2b )), are initially obtained in which a molecule of CO and a molecule of R 1 C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) 3 Fe{μ-PPh 2 C(CO 2 Me)C(CO 2 Me)}Co(CO) 3 ] ( 3a ). Similar thermolysis of 2b results not only in the related decarbonylation product [(OC) 3 Fe(μ-PPh 2 CHCPh)Co(CO) 3 ] ( 3b ), but additionally in three other products all in low yield, namely the regioisomer of 3b , [(OC) 3 Fe(μ-PPh 2 CPhCH)Co(CO) 3 ] ( 4b ), the aldehyde-substituted complex [(OC) 3 Fe{μ-PPh 2 C(CHO)CPh}Co(CO) 3 ] ( 5b ) and the five-membered ferracycle-containing species [(OC) 3 Fe{μ-PPh 2 CHCPhC(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 2 P)(OC) 2 Fe{μ-PPh 2 C(O)C(CO 2 Me)C(CO 2 Me)}(μ-CO)Co(CO) 2 ] ( 7a ) and [{(MeO) 3 P}(OC) 2 Fe{μ-PPh 2 C(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 2 H to give [(OC) 3 Fe{μ-PPh 2 C(CO 2 Me)C(CO 2 Me)}Co(CO) 2 (L)] (L=PPhMe 2 ( 8a ), PPh 2 H ( 9a )). Thermolysis of the secondary phosphine-substituted complex 9a results in phosphorushydrogen bond cleavage to give [(OC) 3 Fe{μ-PPh 2 C(CO 2 Me)CH(CO 2 Me)}(μ-PPh 2 )Co(CO) 2 ] ( 10a ). Single-crystal X-ray diffraction studies have been performed on complexes 7b , 8a and 10a .