Diversity of Metal−Ligand Interactions in Halide (X = I, Br, Cl, F) and Halide-Free Ambiphilic Ligand Rhodium Complexes

Reaction of the neutral ambiphilic ligand 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene (TXPB) with [{Rh(mu-Cl)(CO)(2)}(2)] yields [RhCl(CO)(TXPB)] (1) (Emslie et al. Organometallics 2006, 25, 5835). Complex 1 is square planar with the TXPB ligand bound to rhodium via the phosphine and thioether donors (these are features common to complexes 2-5, vide infra). Treatment of 1 with Me(3)SiBr and Me(3)SiI allowed for halide substitution to afford [RhBr(CO)(TXPB)] (2) and [RhI(CO)(TXPB)] (3), respectively. The halide co-ligands in complexes 1 and 2 form a strong bridging interaction between rhodium and the borane group in TXPB. The presence of stronger borane-halide coordination in 1 is clearly illustrated by an (11)B NMR chemical shift of 12 ppm versus 27 ppm in 2. In contrast, the iodide ligand in 3 forms only a weak bridging interaction to boron, leading to a B...I distance of 3.125(7) A, and an (11)B NMR chemical shift of 56 ppm (versus 69 ppm for free TXPB). A lower carbonyl stretching frequency in 3 (2002 cm(-1)) versus 1 or 2 (2008 and 2013 cm(-1), respectively) could be attributed to weakening of the Rh-X bond in 1 and 2 as a consequence of halide-borane coordination and/or a shorter Rh-S bond in complex 3. [Rh(CO)(TXPB-F)] (4) and the halide-free cation [Rh(CO)(TXPB)][PF(6)] (5) were accessed by reaction of 1 with [NMe(4)]F and Tl[PF(6)], respectively. Complex 4 is zwitterionic with fluoride bound to boron [(11)B NMR delta 4 ppm; B-F = 1.445(6) A; Rh...F = 3.261(3) A] and an eta(2)-interaction between the cationic rhodium center and the ipso- and ortho-carbon atoms of a B-phenyl ring in TXPB-F. By contrast, rhodium in 5 engages in an eta(2)-interaction with boron and the ipso-carbon of one B-phenyl ring; Rh-B and Rh-C(ipso) bond lengths in 5 are 2.557(3) and 2.362(2) A, respectively. The long Rh-B distance and an (11)B NMR chemical shift of 57 ppm are consistent with only a weak Rh-B interaction in 5, and a CO stretching frequency of 2028 cm(-1) (Nujol), versus 2004-2013 cm(-1) for complexes 1-4, is indicative of greatly reduced electron density in 5, relative to 1-4.