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

Abstract

Reaction of 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene (TXPB) with [PdCl2(COD)] resulted in the formation of [PdCl(μ-Cl)(TXPB)] (3), which can be reduced in a stepwise fashion, forming [Pd(TXPB)] (2) via [{PdI(μ-Cl)(TXPB)}2] (4). Dinuclear 4 could also be prepared through a comproportionation reaction of palladium(II) complex [PdCl(μ-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-(η3-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 Å) between the metal and the B−Cipso−Cortho unit of TXPB and 11B NMR signals shifted 38−39 ppm to lower frequency of free TXPB confirm the presence of a strong M−{η3(BCC)-BAr3} interaction. Reaction of either [Pd2(dvds)3] (dvds = 1,3-divinyltetramethyldisiloxane) with TXPB or complex 2 with dvds resulted in rapid formation of [(κ1-TXPB)Pd(η2:η2-dvds)] (7). The platinum analogue of complex 7, [(κ1-TXPB)Pt(η2:η2-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 η2:η2-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 η1-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 η1-interactions, confirming the steric accessibility of more traditional κ3-coordination in 4,5-disubstituted thioxanthene ligands such as Thioxantphos and TXPB.