Boron–boron J coupling constants are unique probes of electronic structure: a solid-state NMR and molecular orbital study

Abstract

Diboron compounds are a part of a relatively unexplored, yet immensely useful, class of compounds. Their main use is for β-boration reactions where a boron center is rendered nucleophilic with the use of a metal catalyst or a Lewis base (alkoxide, amine, or NHC) to form a sp²–sp³ diboron compound. The reactivity of these reagents is largely dictated by the nature of the B–B bond (strength and polarity); however, no experimental methods have been used to directly probe both of these quantities. We demonstrate that unprecedented experimental information regarding the B–B bond may be obtained using ¹¹B solid-state NMR spectroscopy. For example, the ¹¹B quadrupolar coupling constants can be understood on the basis of the polarization of the B–B bond. ¹¹B double-quantum-filtered (DQF) J-resolved NMR spectroscopy was applied to easily and accurately measure J(¹¹B,¹¹B) coupling constants with high precision. These are shown to be well correlated with the orbital energy of the B–B σ-bonding natural bond orbital as well as the hybridisation states of the boron atoms in the bond. An increase in the p character of the bond by electron-donating ligands or via the formation of a sp²–sp³ diboron compound weakens the bond, increases the bond length, and decreases the J(¹¹B,¹¹B) coupling constants. These experiments provide a detailed experimental characterization of the B–B bond and may be useful in understanding the reactivity of diboron compounds and in designing new systems. The potential applicability of ¹¹B DQF J-resolved NMR spectroscopy towards analyzing complex mixtures of diboron compounds and towards measuring ¹¹B J coupling across multiple intervening bonds is also investigated and shows much promise.