Boron-Oxygen Triple Play

By using platinum as a stabilizing influence, a team of inorganic chemists led by Holger Braunschweig of Germany’s University of Würzburg has created just the right set of electronic conditions to achieve one of chemistry’s few remaining bonding feats: creating an isolable compound with a Boron-Oxygen triple bond (Science 2010, 328, 345). Besides being the first metal complex with a boron mon­oxide ligand, the platinum oxoboryl complex and others like it could be useful as chemical building blocks or in catalysis.

Boron is known for its electron deficiency and its propensity to compensate by participating in multicentered bonds that let it draw electron density from neighboring atoms. But chemists have rarely enticed boron to participate in double or triple bonds.

Lai-Sheng Wang of Brown University and coworkers previously found that fleeting gas-phase species such as Au₂B≡O⁻ have a B≡O bond that is as strong as the triple bonds in electronically equivalent CN⁻ and CO complexes. “We wondered why no metal complexes with oxoboryl ligands had been synthesized when CN⁻ and CO are ubiquitous ligands in inorganic chemistry,” Wang says. Braunschweig’s team shows that “the trick appears to be stabilization of B≡O through significant platinum-boron covalent bonding,” he says.

Braunschweig’s group previously showed how a metal center can be used to stabilize the B≡N bond in imino­boryl complexes. Building on that work, Braunschweig, Krzysztof Radacki, and Achim Schneider made the B≡O complex by treating Pt(PR₃)₂, where R is cyclohexyl, with Br₂BOSi(CH₃)₃ in toluene at room temperature. The resulting intermediate eliminates BrSi(CH₃)₃, leaving behind B≡O as a novel ligand in (PR₃)₂BrPtB≡O.

The complex is unusually stable to heat and light, and even when the team treated it with ammonium thiophenylate, the thiophenyl group bypassed the B≡O ligand and opted to exchange with the bromine ligand, forming an oxoboryl thiophenyl complex.

Armin Berndt of Philipps University, in Marburg, Germany, an expert on multiple bonding and aromaticity in boron compounds, says achieving the B≡O bond is like a “Sahnehäubchen”—getting whipped cream “on top of a masterful series of studies on the stabilization of unusual boron species by metal complexation.”