Metallacumulenylidene complexes in the [(η5-C5Me5)(η2-dppe)Fe(=C(=C)nR2)]X (n = 0, 1, 2) series: investigations of the iron-carbon bonding by Mössbauer spectroscopy and X-ray analyses

Abstract

The synthesis of the iron allenylidene complexes [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=C=C(Ph)Ph)][X] ( 5a , X = PF 6 , 95%; 5b , X = BPh 4 , 91%; dppe = 1,2-bis(diphenylphosphino)ethane) was achieved by reacting the complex (η 5 -C 5 Me 5 )(η 2 -dppe)FeCl ( 10 ) with 1 equiv of 1,1-diphenyl-prop-2-yn-1-ol in methanol in the presence of KPF 6 or NaBPh 4 . Surprisingly, when the reaction was carried out in the presence of the tetraphenylborate anion, the final product contained both 5b and the hydroxyvinylidene [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=C(H)C(OH)(Ph) 2 )][BPh 4 ] ( 14b ) in the 1:1 ratio. Further treatment of the mixture with Amberlyst 15 in methanol provided the allenylidene 5b as a pure sample. The allenylidene complexes [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=C=C(Me)Ph)][PF 6 ] ( 6 ) and [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=C=C(Me)Et)][PF 6 ] ( 7 ) were prepared according to the same procedure and they were isolated as purple powders in 90% yield. The X-ray crystal structures were determined for the vinylidene complexes [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=CH 2 )][PF 6 ] ( 3 ) and [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=C=C(Ph)H)][PF 6 ] ( 4 ), and the allenylidene derivative 5a . In the homogeneous series of complexes [(η 5 -C 5 Me 5 )(η 2 -dppe)Fe(=(C) n (R)R’)][PF 6 ], ( n = 1, R = H, R′ = Me, X = PF 6 , 1; n =1, R = H, R’ = OMe, X = PF 6 , 2a; n = 1, R = H, R’ = OMe, X = CF 3 OSO 2 , 2b; n = 2, R = R′ = H, X = PF 6 , 3; n = 2, R = H, R′ = Ph, X = PF 6 , 4; n = 3, R = R′ = Ph, X = PF 6 , 5a; n = 3, R = R′ = Ph, X = BPh 4 , 5b; n = 3, R = Me, R′ = Ph, X = PF 6 , 6; n = 3, R = Me, R′ = Et, X = PF 6 , 7; n = 3, R = Me, R′ = OMe, X = BPh 4 , 8 ), an empiric relationship between the Mössbauer parameters, δ and QS , was found. This observation would indicate that the positive charge on the iron nucleus decreases with the Fe=C bond order. Moreover, in this series of iron cumulenylidene derivatives, comparison of the variation of the metal–carbon bond distances determined by X-ray analyses with the Mössbauer QS values allows the observation of a linear correlation ( R = 0.99).