Steric and Electronic Effects of Metal-Containing Substituents in Fischer Carbene Complexes of Chromium

The Fischer carbene complexes of chromium pentacarbonyl with one or two different metal-containing substituents were synthesized and studied in solution and in the solid state. The dimetallic complexes [Cr(CO)5{C(OTiCp2Cl)(2-BT)}] (2) (BT = benzo[b]thienyl) and [Cr(CO)5{C(OEt)((η6-2-BT)Cr(CO)3)}] (3) and trimetallic complexes [Cr(CO)5{C(OTiCp2Cl)((η6-2-BT)Cr(CO)3)}] (4) and [Cr(CO)5{C(OTiCp2Cl)Fc}] (5) (Fc = ferrocenyl) were prepared and systematically studied with respect to the reference complex [Cr(CO)5{C(OEt)(2-BT)}] (1) for the importance of the metal substituents in influencing carbene ligand reactivity. It was clear from the crystal structure determination of 4 that most of the unoccupied space was found around the benzo[b]thienyl substituent. Hence, it was also possible to synthesize the analogous more crowded trimetallic carbene complex 5 containing an electron-donating ferrocenyl instead of the [Cr(η6-2-BT)(CO)3] substituent. Dilithiated ferrocene reacts with 2 equiv of chromium hexacarbonyl, which after alkylation with Et3OBF4 affords the ferrocen-1,1′-diyl-bridged biscarbene [(Cr(CO)5)2{μ2-C2(OEt)2Fe(C5H4)2-C,C′)}] (7), while metalation with TiCp2Cl2 resulted in the formation of the novel ferrocene-titanocene-bridged biscarbene complex [(Cr(CO)5)2{μ2-C2(O2TiCp2-O,O′)(Fe(C5H4)2-C,C′)}] (6).