"Weakly coordinating anions" such as tetraarylborates are ubiquitous in applications of inorganic and organometallic chemistry, with great industrial importance. In this work, we probe the ion-pairing ability of these weakly coordinating anions using the highly sensitive chromium(VI) nitrido bis(diisopropylamido) system NCr(N-i-Pr-2)(2)X, with one variable coordination site (X). This system is being used in the quantification of ligand donor ability to high-valent metal centers and has simply been called the ligand donor parameter (LDP). The donor ability of the variable ligand can be measured by solution-state rotational barrier studies-via NMR spectroscopy. If the variable ligand is neutral, the chromium complex is cationic, NCr(N-i-Pr-2)(2)L}(+), with its pendant anion. Despite the weakly coordinating nature of the counteranions employed, a significant amount of ion pairing has been noted in solution, the result of which is substantial enough to perturb the sensitive LDP measurement. These effects have been noted for many commonly used counteranions, including hexafluoroantimonate(V), hexafluorophosphate(V), tetraphenylborate, and tetrakis(bis(3,5-trifluoromethyl)phenyl)borate (BArF24). Using diffusion ordered (DOSY) and rotating-frame Overhauser effect (ROESY) NMR spectroscopy and LDP values, we have shown, predictably, that the extent of ion pairing is solvent dependent and appears to be minimized by increasing the dielectric constant of the NMR solvent utilized. Additionally, we have gained insight into differences in the nature of ion pairing dependent upon the identity of the weakly coordinating anion employed. It was found that the tetraarylborate anions appear to be fully ion paired in CDCl3 but affect amido rotation less in comparison to other anions. We postulate that the smaller effect on the internal rearrangement by these fluorinated tetraarylborate anions is due to a lack of specificity in the interaction with the cation rather than a lack of ion pairing, which may be a general feature of these anions.
“弱配位阴离子”(如四芳基
硼酸)在无机和有机
金属
化学的应用中无处不在,具有重要的工业价值。在本研究中,我们使用高度敏感的六价
铬的液
氨二(异丙基)
氨基系统 NCr(N-i-Pr-2)₂X(其中X为一个可变配位位点)来探查这些弱配位阴离子的离子结合能力。该系统用于量化
配体向高价
金属中心的供体能力,简称为
配体供体参数(LDP)。通过溶液态旋转屏障研究(借助NMR光谱),可以测量可变
配体的供体能力。如果可变
配体为中性,则
铬复合物呈阳离子态,NCr(N-i-Pr-2)₂L}⁺,并带有其悬挂的阴离子。尽管所用的反离子具有弱配位特性,但溶液中观察到显著的离子结合,其结果足以干扰敏感的LDP测量。这一现象在多种常用反离子中得到观察,包括六
氟锑酸根(V)、
六氟磷酸根(V)、四苯基
硼酸根、以及四(双(3,5-二
氟甲基)苯基)
硼酸根(BArF24)。通过扩散有序谱(
DOSY)和旋转框架核奥瑟效应(ROESY)NMR光谱,结合LDP值,我们可预测地得出结论:离子结合的程度取决于溶剂,并且通过增加NMR溶剂的介电常数,可以最大限度地减少离子结合。此外,我们还深入了解了不同弱配位阴离子在离子结合性质上的差异。研究发现,四芳基
硼酸盐阴离子在CDCl₃中似乎完全离子结合,但与其他阴离子相比,对
氨基旋转的影响较小。我们推断,这些含
氟四芳基
硼酸盐阴离子对内部重排较小的影响是由于与阳离子的相互作用缺乏特异性,而不是由于缺乏离子结合,这可能是这些阴离子的一种普遍特性。