Department of Chemistry and Biomolecular Sciences & Centre for Catalysis Research and Innovation, University of Ottawa , Ottawa, Ontario, Canada K1N6N5.
Institut für Anorganische Chemie, Institut für nachhaltige Chemie & Katalyse mit Bor (ICB), Julius-Maximilians-Universität Würzburg , Am Hubland, 97074 Würzburg, Germany.
J Am Chem Soc. 2017 Jun 21;139(24):8200-8211. doi: 10.1021/jacs.7b01783. Epub 2017 Jun 6.
The J(B,B) coupling constants of various salts of the electron-precise hexacyanodiborane(6) dianion, [B(CN)], were obtained using B 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 B and C 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 10 to 10 s over the temperature range of 248-306 K. When molecular motion and crystallographic symmetry are both accounted for, the J(B,B) coupling constants for various [B(CN)] 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.
各种电子精确六氰合二硼（6）二阴离子[B(CN)]盐的 J(B,B)偶合常数是通过 B 双量子滤波（DQF）J 分辨固态核磁共振（SSNMR）光谱获得的。我们的结果表明，DQF J 分裂的大小受到系统的晶体对称性和动力学的影响。与相应的理论 J 偶合常数相比，在以下两种情况下，DQF J 分裂的大小会放大 3 倍：(1) 不存在动力学，但硼对在晶体学上是等价的，或 (2) 硼对在晶体学上是不等价的，但由于动态无序而在实验时间尺度上变得磁等价，这种动态无序是通过 B 和 C 的 SSNMR 实验来识别的。因此，在解释 DQF J 分辨实验的结果时，需要考虑分子运动，反之，这些实验也可以用来识别动态无序。变温 NMR 数据支持存在三种不同的分子运动过程的观点，在 248-306 K 的温度范围内，相关时间从 10 到 10 s 不等。当考虑分子运动和晶体对称性时，各种[B(CN)]盐的 J(B,B)偶合常数测量值在 29.4 到 35.8 Hz 之间，其电子起源是通过自然局域分子轨道和自然键轨道分析确定的。结果表明，偶合常数与形成 B-B 键的硼轨道的杂化状态以及 B-B 键的强度密切相关。这项研究为研究有序和无序固体中的动力学提供了一种新的工具，并为具有双中心双电子键的电子精确二阴离子二硼烷提供了新的视角，同时也为具有多重和单键硼自旋对的相关化合物提供了新的视角。
