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{κ3SBS-HB­(mp)2}­(PPh3)] and [Pd­{κ3SBS-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-κ3S,B,S coordination mode with short platinum– and palladium–boron
distances: 2.098(4) and 2.091(3) Å, respectively (the shorter
distances of two independent complexes in the unit cells of both structures).
The complexes [Pd­{κ3S,B,S-HB­(mp)2}­(PPh3)] and [Pt­{κ3S,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.