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(μ-Cl)­(COE^OMe)]₂ (where COE^OMe = 8-methoxycyclooct-4-en-1-ide) in the presence of 2 equiv of a tertiary phosphine (PR₃ = PPh₃, PCy₃, PCyp₃, P­(<i>o</i>-tol)₃, PPh₂(<i>o</i>-tol), PPh₂(2-(3-methyl)­indolyl)) leads to the formation of the complexes [Pt­{κ³-<i>S,B,S</i>-HB­(mp)₂}­(PR₃)] (<b>1</b>–<b>6</b>). Addition of 2 equiv of Na­[H₂B­(mp)₂] to [Pd­(μ-Cl)­(COE^OMe)]₂ in the presence of 2 equiv of a tertiary phosphine (PR₃ = PPh₃, PCy₃, PCyp₃, PPh₂(<i>o</i>-tol)) leads to the formation of the complexes [Pd­{κ³-<i>S,B,S</i>-HB­(mp)₂}­(PR₃)] (<b>7</b>–<b>10</b>). It was also demonstrated that the synthesis of the palladium complex <b>7</b> 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 COE^OMe 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 κ³-<i>S,B,S</i> coordinated HB­(mp)₂ pincer ligand. The complexes [Pd­{κ³-<i>S,B,S</i>-HB­(mp)₂}­(PR₃)] and [Pt­{κ³-<i>S,B,S</i>-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) Å for [Pd­{κ³-<i>S,B,S</i>-HB­(mp)₂}­(PPh₂(<i>o</i>-tol))] and 2.076(10) Å for [Pt­{κ³-<i>S,B,S</i>-HB­(mp)₂}­(PCy₃)], respectively (the shorter distances of two independent complexes in the unit cells of both structures)