Utilizing the 8-Methoxycyclooct-4-en-1-ide Unit As a Hydrogen Atom Acceptor en Route to "Metal-Borane Pincers"

The synthesis and characterization of palladium and platinum complexes containing the neutral ligand HB(mp)(2) (where mp = 2-mercaptopyridyl) are presented. Addition of 2 equiv of Na[H2B(mp)(2)] to [M(Cl)(COEOMe)](2) (where M = Pt, Pd; COEOMe = 8-methoxycyclooct-4-en-1-ide) in the presence of 2 equiv of PPh3 leads to the formation of the metal borane pincer complexes [Pt{kappa(SBS)-S-3-HB(mp)(2)}(PPh3)] and [Pd(kappa(SBS)-S-3-HB(mp)(2)}(PPh3)]. In these reactions, a hydrogen migration reaction occurs from the borohydride ligand to the metal center, eventually leading to the elimination of the COEOMe unit from the metal center. X-ray crystallographic characterization of the two isostructural complexes reveals a rare mer-kappa S-3,B,S coordination mode with short platinum- and palladium-boron distances: 2.098(4) and 2.091(3) angstrom, respectively (the shorter distances of two independent complexes in the unit cells of both structures). The complexes [Pd{kappa S-3,B,S-HB(mp)(2)}(PPh3)] and [Pt{kappa S-3,B,S-HB(mp)(2)}(PPh3)] are the first examples of metal-borane complexes featuring a pincer-type coordination where one hydrogen substituent remains at the boron center.