Group 10 Transition-Metal Complexes of an Ambiphilic PSB-ligand: Investigations into η3(BCC)-Triarylborane Coordination

Reaction of 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene (TXPB) with [PdCl2(COD)] resulted in the formation of [PdCl(mu-Cl)(TXPB)] (3), which can be reduced in a stepwise fashion, forming [Pd(TXPB)] (2) via [{Pd-1(mu-Cl)(TXPB)}(2)] (4). Dinuclear 4 could also be prepared through a comproportionation reaction of palladium(II) complex [PdCl(mu-Cl)(TXPB)] (3) with either [Pd(TXPB)] (2) or [Pd(dba)(TXPB)] (5). In complexes 3 and 4, the TXPB ligand is bound to palladium via the phosphine and thioether donors, with a chloride anion bridging between the metal and the borane unit of TXPB. By contrast, the TXPB ligand in 2 is bound to palladium not only via the phosphine and thioether donors but also through a Pd-(eta(3)-BAr3) linkage involving boron and the ipso- and ortho-carbon atoms of one B-phenyl ring. The analogous nickel complex, [Ni(TXPB)] (6) also proved accessible by direct reaction of [Ni(COD)(2)] With TXPB. In both 2 and 6, short distances (2.02-2.33 angstrom) between the metal and the B-C-ipso-C-ortho unit of TXPB and B-11 NMR signals shifted 38-39 ppm to lower frequency of free TXPB confirm the presence of a strong M-{eta(3)(BCC)-BAr3) interaction. Reaction of either [Pd-3(dvds)(3)] (dvds = 1,3-divinyltetramethyldisiloxane) with TXPB or complex 2 with dvds resulted in rapid formation of [(kappa(1)-TXPB)Pd(eta(2):eta(2)-dvds)] (7). The platinum analogue of complex 7, [(kappa(1)-TXPB)Pt(eta(2):eta(2)-dvds)] (8), was also prepared by reaction of [Pt(COD)(2)] with dvds, followed by TXPB. In both 7 and 8, the metal is trigonal planar as a result of eta(2):eta(2)-coordination to dvds and bonding only to the phosphine group of TXPB. To assess the potential for a ligand with the same structural characteristics as TXPB to coordinate via three eta(1)-interactions, the phosphine analogue of TXPB; 2,7-di-tert-butyl-4,5-bis(diphenylphosphino)-9,9-dimethylthioxanthene (Thioxantphos) was prepared, and reaction with [PtX2(COD)] resulted in the clean formation of [PtX(Thioxantphos)]X where X = Cl (9) and I (10). These complexes are square planar with the Thioxantphos ligand coordinated through three eta(1)-interactions, confirming the steric accessibility of more traditional kappa(3)-coordination in 4,5-disubstituted thioxanthene ligands such as Thioxantphos and TXPB.