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 two equivalents of Na[H2B(mp)2] to [Pt(μ-Cl)(COEOMe)]2 (where COEOMe = 8-methoxycyclooct-4-en-1-ide) in the presence of two equivalents of a tertiary phosphine {PR3 = PPh3, PCy3, PCyp3, P(o-tol)3, PPh2(o-tol) and PPh2(2-(3-methyl)indolyl)} leads to the formation of the complexes [Pt{κ3-S,B,S-HB(mp)2}(PR3)] (1 ‒ 6). Addition of two equivalents of Na[H2B(mp)2] to [Pd(μ-Cl)(COEOMe)]2 in the presence of two equivalents of a tertiary phosphine {PR3 = PPh3, PCy3, PCyp3, and PPh2(o-tol)} leads to the formation of the complexes [Pd{κ3-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 serve to act as a “hydride acceptor” facilitating the ultimate transformation of the borohydride based ligand, [H2B(mp)2]‒ to the corresponding secondary borane 3-S,B,S coordinated HB(mp)2 pincer ligand. The complexes [Pd{κ3-S,B,S-HB(mp)2}(PR3)] and [Pt{κ3-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) Å for [Pd{κ3-S,B,S-HB(mp)2}(PPh2(o-tol))] and 2.076(10) Å, for [Pt{κ3-S,B,S-HB(mp)2}(PCy3)], respectively (the shorter distances of two independent complexes in the unit cells of both structures).