Bridging Rhodium–Iron Borataaminocarbyne Complexes Formed by Intramolecular Isonitrile–Borane Coordination

Reaction of [(TXPB)­Rh­(μ-CO)2Fe­(CO)­Cp] (1; TXPB = 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene) with both aryl and alkyl isonitriles formed [(TXPB)­Rh­(μ-CO)­(μ-CNR)­Fe­(CO)­Cp] (R = C6H4Cl-p (2), Xyl (3), nBu (4); Xyl = 2,6-dimethylphenyl), in which the CNRBR3 unit is best described as a borataaminocarbyne ligand with single bonds between carbon and nitrogen and between nitrogen and boron. The carbon atom of the borataaminocarbyne ligand is doubly bound to iron and singly bound to rhodium. For comparison, borane-free [(TXPH)­Rh­(μ-CO)2Fe­(CO)­Cp] (5; TXPH = 2,7-di-tert-butyl-4-diphenylphosphino-9,9-dimethylthioxanthene) was prepared from [(TXPH)­RhCl­(CO)] and K­[CpFe­(CO)2], and reaction of 5 with either 1 or 2 equiv of CNC6H4Cl-p afforded [(TXPH)­Rh­(CO)­(μ-CNC6H4Cl-p)2Fe­(CO)­Cp] (6), which contains two μ-isonitrile ligands and two terminal carbonyl ligands: one bound to iron and the other to rhodium. The C–N stretching frequency for 6 is 124 cm–1 higher than that for the TXPB CNC6H4Cl-p complex 2, and the Rh–(μ-CNR) and Fe–(μ-CNR) distances are notably longer than those in 2, and the C–N distance is shorter.