Dynamic Disorder and Electronic Structures of Electron-Precise Dianionic Diboranes: Insights from Solid-State Multinuclear Magnetic Resonance Spectroscopy

The J(11B,11B) coupling
constants of various salts of the electron-precise hexacyanodiborane(6)
dianion, [B2(CN)6]2–, were
obtained using 11B double-quantum-filtered (DQF) J-resolved solid-state nuclear magnetic resonance (SSNMR)
spectroscopy. Our results show that the magnitude of the DQF J splitting is influenced by both the crystallographic symmetry
of the system and the presence of dynamics. The splittings are amplified
by a factor of 3 as compared to the corresponding theoretical J coupling constants for cases where (1) there is an absence
of dynamics but the boron pairs are crystallographically equivalent
or (2) the boron pairs are crystallographically inequivalent but are
rendered magnetically equivalent on the time scale of the experiment
due to dynamic disorder, which was identified by 11B and 13C SSNMR experiments. Consequently, molecular motions need
to be taken into consideration when interpreting the results of DQF J-resolved experiments, and conversely, these experiments
may be used to identify dynamic disorder. Variable-temperature NMR
data support the notion of three different motional processes with
correlation times ranging from 102 to 106 s–1 over the temperature range of 248–306 K. When
molecular motion and crystallographic symmetry are both accounted
for, the J(11B,11B) coupling
constants for various [B2(CN)6]2– salts were measured to range from 29.4 to 35.8 Hz, and their electronic
origins were determined using natural localized molecular orbital
and natural bond orbital analyses. The coupling constants were found
to strongly correlate to the hybridization states of the boron orbitals
that form the B–B bonds and to the strength of the B–B
bonds. This study provides a novel tool to study dynamics in ordered
and disordered solids and provides new perspectives on electron-precise
dianionic diboranes featuring two-center–two-electron bonds
in the context of related compounds featuring multiply and singly
bonded boron spin pairs.