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 Mossbauer spectroscopy and X-ray analyses

The synthesis of the iron allenylidene complexes [(eta(5)-C5Me5)(eta(2)-dppe)Fe(=C=C=C(Ph)Ph)] [X] (5a, X = PF6, 95%; 5b, X = BPh4, 91%; dppe = 1,2-bis(diphenylphosphino)ethane) was achieved by reacting the complex (eta(5)-C5Me5)(eta(2)-dppe)FeCl (10) with 1 equiv of 1,1-diphenyl-prop-2-yn-1-ol in methanol in the presence of KPF6 orNaBPh(4), Surprisingly, when the reaction was carried out in the presence of the tetraphenylborate anion, the final product contained both 5b and the hydroxyvinylidene [(eta(5)-C5Me5)(eta(2) -dppe)Fe(=C=C(H)C(OH)(Ph)(2))][BPh4] (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 [(eta(5)-C5Me5)(eta(2)- dppe)Fe(=C=C=C(Me)Ph)] [PF6] (6) and [(eta(5)-C5Me5)(eta(2)-dppe)Fe(=C=C=C(Me)Et)] [PF6] (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 [(eta(5)-C5Me5)(eta(2)-dppe)Fe(=C=CH2)] [PF6] (3) and [(eta(5)-C5Me5)(eta(2)-dppe)Fe(=C=C(Ph)H)][PF6] (4), and the allenylidene derivative 5a. In the homogeneous series of complexes [(eta(5)-C5Me5)(eta(2) -dppe)Fe(=(C)(n)(R)R')][PF6], (n = 1, R = H, R' = Me, X = PF6, 1; n = 1, R = H, R '= OMe, X = PF6, 2a; n = 1, R = H, R' = OMe, X = CF3OSO2, 2b; n = 2, R = R' = H, X = PF6,3; n = 2, R = H, R'= Ph, X = PF6,4; n = 3, R = R'= Ph, X = PF6,5a; n = 3, R = R'= Ph, X = BPh4,5b; n 3, R = Me, R'= Ph, X = PF6,6; n = 3, R = Me, R'= Et, X = PF6,7; n = 3, R = Me, R'= OMe, X = BPh4,8), an empiric relationship between the Mossbauer parameters, delta 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 Mossbauer QS values allows the observation of a linear correlation (R = 0.99).