Synthesis of diiron bridging allenylidene complexes Cp*Fe-2(2)(mu-C=C=CR1R2)(mu-CO)(CO)(2) (Cp*=eta(5)-C5Me5) via nucleophilic addition to the diiron ethynediyl complex (mu-C C)[FeCp*(CO)(2)](2) and their conversion to cationic mu-vinylcarbyne and mu-vinylidene complexes

A variety of diiron bridging allenylidene complexes, Cp*Fe-2(2)((mu-C=C=(CRR2)-R-1)(mu-CO)(CO)2 (3; R-1, R-2 = H, H (a); Me, Me (b); n-Bu, n-Bu (c); H, Ph (d); H, t-Bu (e); Me, Ph (f); Me, n-Bu (g); t-Bu, Me (h)) are prepared by (i) addition of an excess amount of nucleophile (RLi, LiHBEt3) to the diiron mu-ethynediyl, complex (mu-C equivalent to C)[FeCp*(CO)(2)](2) (1) in portions (one-pot synthesis) or (ii) nucleophilic addition to the diiron mu-acylvinylidene complex Cp*Fe-2(2)[mu-C=C(H)-C(=O)R-1](mu-CO)(CO)(2) (2), which is also obtained from 1 (two-step synthesis). On the other hand, addition of an excess amount of nucleophile in one portion produces diacylvinylidene complexes Cp*Fe-2(2)[mu-C=C{C(=O)-R}(2)](CO)(2)(mu-CO) (4). Hybridization of the Fe-2-[mu-C=C=(CRR2)-R-1] moiety in 3 is similar to that in organic allene molecules, as revealed by X-ray crystallographic and spectroscopic analysis. Formation of 3 instead of the mu-alkenylvinylidene complex 4 (when either of R-1 or R-2 bears an alpha-hydrogen atom) can be interpreted in terms of steric repulsion between the bridging ligand and the Cp* ligands. In addition, the C-C coupling observed during the formation of 3 also proceeds in mononuclear iron acetylide complexes, (eta(5)-C5R5)Fe(CO)(2)-C equivalent to C-Ph (12) (R = H (a), Me (b)), upon treatment with nucleophiles (Nu) to give enone PhCH=CHC(=O)-Nu (13) and alkenyl complexes, (eta(5)-C5R5)Fe(CO)(PPh3)-C[C(=O)-Nu]=C(H)Ph (15) (in the presence of PPh3). The mu-allenylidene complexes 3 turn out to be amphoteric. Protonation of 3 takes place at the beta-carbon atom of the mu-allenylidene bridge to give cationic mu-vinylcarbyne species Cp*(2)-Fe2([)mu-{C-C(H)-C}(+RR2)-R-1](mu-CO)(CO)(2) (11; R-1, R-2 = H, H, (a); Me, Me (b); H, Ph (c)), which have been characterized by NMR (delta(C)(C-alpha) > 450) and X-ray crystallography (11c . BF4). Subsequent nucleophilic addition gives a variety of functionalized mu-vinylidene complexes Cp*Fe-2(2)[mu-C=C(H)-CR(1)R(2)Nu](mu-CO)(CO)(2) (18) through addition to the gamma-carbon atom. In contrast, nucleophilic addition to 3 takes place at the gamma-carbon atom, and mu-vinylidene complex 18 (reaction with LiHBEt3) or cyclic product 21 (reaction with n-BuLi) are obtained after protonolysis. Thus, it has been clarified that the electrophile and nucleophile attack the Band gamma-carbon atoms of the bridging allenylidene ligand, respectively. The configuration of the bridging ligand (X) and the Fe auxiliary Ligands (Cp* and CO) in a series of diiron bridging hydrocarbyl complexes, Cp*Fe-2(2)(mu-X)(mu-CO)(CO)(2) (X = allenylidene, vinylidene, and alkylidene), is also discussed on the basis of the molecular structure of 3a-c, 4, 18e,h,k, and a mu-alkylidene complex Cp*Fe-2(2)[mu-C(H)CH2CH3](mu-CO)(CO)(2) (19a), which are obtained in the present study. The dinuclear complexes relieve the steric repulsion among the X and Cp* ligands by a combination of a stretching and twisting of the X part and precession of the Cp* ligands.