Synthesis and Characterization of Terpyridine-Supported Boron Cations: Evidence for Pentacoordination at Boron

Abstract

Hypervalent boron centers are proposed to be key intermediates in many stoichiometric and catalytic reactions. However, structurally characterized examples remain rare. We have isolated two new borocations with formal charges of 1+ and 2+. Because the dicationic complex displays evidence of pentacoordination at the boron center, we conclude that the interaction is predominantly electrostatic and is a result of the highly electrophilic dicationic boron atom.

Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

“…Much weaker interactions are observed for the boron complexes than for the other triel systems. [23] Two new borocations with formal charges of 1 + and 2 + and with hypervalent boron centers were also isolated. Interactions in aluminum complexes are characterized by meaningful electrostatic contributions, whereas for gallium complexes, the most important electron charge shift is observed as a result of complexation.…”

“…This is because of the lower acidity of the B center relative to that of the other triel centers, and it may be the result of a stronger backbonding effect for BF 3 than for the other triel trifluorides. [23] Pentacoordinate and tetracoordinate carbon and boron compounds were characterized by X-ray crystal structure analysis. Analysis of the geometry of the triel complexes and of the Z···N interactions (triel bonds) in these complexes is based on ab initio calculations as well as on the quantum theory of atoms in molecules and the natural bond orbitals method.…”

Mono- and Polynuclear Boron Dications

“…21 The 11 B NMR resonance of [2][OTf] 2 observed at 7.43 ppm is independent of temperature, showing the absence of liberation of the ligand from the boron center in the solution. The quasi-pentacoordination at the boron center is further corroborated with single-crystal X-ray diffraction analysis of [2][OTf] 2 .…”

“…It is proposed that both steric and π-donating abilities of the Lewis base control the balance between the dimer and monomer. Nevertheless, the dicationic [21] 2+ and [22] 2+ still serve as functional equivalents of primary borenium in the boron cationmediated CH borylation.…”

“…Ghadwal et al took a similar approach to synthesize the NHO analogues of [21] 2+ . NHO-BH 3 complexes can be transformed into the corresponding hydride-bridged diboron dication [23] 2+ with one equivalent of Tf 2 NH (Figure 4).…”