Multimetal complexes of Fischer carbenes

Abstract

Fischer carbene complexes of the Group VI transition metals (Cr, Mo and W) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized and studied both in solution and in the solid state. For the complexes of the type [M(CO)5{C(OR)R’}], the substituents chosen included (hetero)aromatic (benzene or thiophene) rings π-bonded to a chromium tricarbonyl fragment or ferrocene as the R’-substituent, while the OR-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1 (M = Cr, R = Et, R’ = Fc), 2 (M = W, R = Et, R’ = Fc), 5 (M = Cr, R = TiCp2Cl, R’ = Fc), 6 (M = W, R = TiCp2Cl, R’ = Fc), 7 (M = Mo, R = TiCp2Cl, R’ = Fc), 12 (M = Cr, R = TiCp2Cl, R’ = 2-thienyl) and 13 (M = Cr, R = TiCp2Cl, R’ = [Cr(CO) 3 (η 6-phenyl)]). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, also resulted, in most cases, in the formation of the novel biscarbene complexes with bridging ferrocen- 1,1’-diyl carbene ligands [μ-Fe{C5H4C(OEt)M(CO) sub>5}2] (3: M = Cr, 4: M = W) or the unusual bimetallacyclic bridged biscarbene complexes [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2-C,C’}{CM(CO) 5}2] (8: M = Cr, 9: M = W, 10: M = Mo). It was attempted to prepare the mixed heteronuclear biscarbene complex 11 [W(CO) 5C{μ-TiCp2O2- O,O’}{μ-Fe(C5H4)2-C,C’}CCr(CO) 5], however the complex could not be fully characterized. The investigation was expanded to include Group VII transition metals Mn and Re, and using the same methodology, the manganese complexes isolated included [MnCp(CO2{C(OR)Fc}] (22: R = Et, 24: R = TiCp2Cl), 23 [μ- Fe{C5H4C(OEt)MnCp(CO) 2}2] and 25 [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2- C,C’}[CMnCp(CO) 2}2]. The different reactivity of the binary dirhenium decacarbonyl precursor complex, compared to that of the Group VI complexes, resulted in the formation of a range of complexes. The target compounds [Re2 (CO) 9{C(OR)Fc}] (26: R = Et, 31: R = TiCp2Cl), 27 [μ-Fe{C5H4C(OEt)Re2 (CO) 9}2] and 33 [{μ- TiCp2O2-O,O’}{μ-Fe(C5H4)2-C,C’}[CRe2 (CO) 9}2] were isolated displaying a variety of different geometric isomers. In addition, acyl (30) and aldehyde (32) decomposition products, as well as hydrido (29), and hydrido acyl hydroxycarbene (34) complexes and the unique dichloro-bridged biscarbene complex (28) were also characterized. Most of these complexes displayed Re-Re bond breaking, and two probable mechanisms, either radical or ionic, were proposed involving either hydrogen transfer or protonation followed by hydrolysis. Finally, the structural features and their relevance to bonding in the carbene cluster compounds of the Group VI transition metals were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry. The best results were obtained from the cyclic voltammetric studies, where the localized metal centre’s oxidation potential correlated to both the calculated HOMO energy, and the effect of both the heteroatom substituent and the (hetero)arene substituent, as well as different combinations of the above.