Synthesis and Characterization of Platinum and Palladium Complexes Featuring a Rare Secondary Borane Pincer Motif

The synthesis and characterization of a series of platinum and palladium complexes containing a secondary borane Z-class ligand supported by 2-mercaptopyridine heterocycles is reported herein. Addition of 2 equiv of Na[H2B(mp)(2)] to [Pt(mu-Cl)(COEOMe)](2) (where COEOMe = 8-methoxycydooct-4-en-1-ide) in the presence of 2 equiv of a tertiary phosphine (PR3 = PPh3, PCy3, PCyp(3), P(o-tol)(3), PPh2(o-tol), PPh2(2-(3-methyl)indolyl)) leads to the formation of the complexes [Pt{kappa(3)-S,B,S-HB(mp)(2)}(PR3)] (1- 6). Addition of 2 equiv of Na[H2B(mp)(2)] to [Pd(mu- Cl)(COEOMe)](2) in the presence of 2 equiv of a tertiary phosphine (PR3 = PPh3, PCy3, PCyp(3), PPh2(o-tol)) leads to the formation of the complexes [Pd{kappa(3)-S,-B,S-HB(mp)(2)}(PR3)] (7-10). It was also demonstrated that the synthesis of the palladium complex 7 could be achieved from the palladium precursor [PdCl(Me)(COD)] (where COD = 1,5-cyclooctadiene) as an alternative synthetic strategy. In the above reactions, either the COEOMe fragment or the methyl fragment serves to act as a hydride acceptor facilitating the ultimate transformation of the borohydride-based ligand [H2B(mp)(2)](-) to the corresponding secondary borane kappa(3)-S,B,S coordinated HB(mp)(2) pincer ligand. The complexes [Pd{kappa(3)-S,B,S-HB(mp)(2)}(PR3)] and [Pt{kappa(3)-S,B,S-HB(mp)(2)}(PR3)] are rare examples of metal-borane complexes where one hydrogen substituent remains at the boron center. These compounds have particularly short palladium-and platinum-boron distances, the shortest of the structurally characterized compounds being 2.067(6) angstrom for [Pd{(kappa(3)-S,B,S-HB(mp)(2)}(PPh2(o-tol))] and 2.076(10) angstrom for [Pt{kappa(3)-S,B,S-HB(mp)(2)}(PCy3)], respectively (the shorter distances of two independent complexes in the unit cells of both structures).