Synthesis, Characterization, and Structural Studies of Multimetallic Ferrocenyl Carbene Complexes of Group VII Transition Metals

Fischer carbene complexes of the group VII transition metals (Mn and Re) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized. The structural features and their relevance to bonding in the carbene multimetal compounds were investigated, as they represent indicators of possible reactivity sites in polymetallic carbene assemblies. For complexes of the type [MLx{C(OR)R′}] (MLx = MnCp(CO)2 or Re2(CO)9), ferrocenyl (Fc) was chosen as the R′ substituent, while the OR substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1a (MLx = MnCp(CO)2, R = Et, R′ = Fc), 2a (MLx = MnCp(CO)2, R = TiCp2Cl, R′ = Fc), 3a (MLx = Re2(CO)9, R = Et, R′ = Fc), and 4a (MLx = Re2(CO)9, R = TiCp2Cl, R′ = Fc). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, resulted in formation of the novel biscarbene complexes with bridging ferrocen-1,1′-diyl (Fc′) substituents [{π-Fe(C5H4)2-C,C′}{C(OEt)MLx}2] (1b, MLx = MnCp(CO)2; 3b, MLx = Re2(CO)9) or the unusual bimetallacyclic bridged biscarbene complexes [{π-TiCp2O2−O,O′}{π-Fe(C5H4)2-C,C′}{CMLx}2] (2b, MLx = MnCp(CO)2; 4b, MLx = Re2(CO)9). The target compounds that were isolated displayed a variety of different geometric isomers and conformations. The greater reactivity of the binary dirhenium acylates in solution, compared to that of the cyclopentadienyl manganese acylate, resulted in a complex reaction mixture. Although the stabilization of hydroxycarbene or hydrido-acyl intermediates of dirhenium carbonyls could not be achieved, their existence in solution was confirmed by the isolation of [(π-H)2−(Re(CO)4{C(O)Fc})2] (8), the unique dichloro-bridged biscarbene complex fac-[(π-Cl)2-(Re(CO)3{C(OEt)Fc})2] (6), the known hydrido complex [Re3(CO)14H] (5), the acyl complex [Re(CO)5{C(O)Fc}] (7), and the aldehyde-functionalized eq-[Re2(CO)9{C(OTiCp2Cl)(Fc′CHO)}] (9).