Weakly Coordinating yet Ion Paired: Anion Effects on an Internal Rearrangement

Abstract

“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-Pr2)2X, 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-Pr2)2L}+, 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.

Acceleration of CO2 insertion into metal hydrides: ligand, Lewis acid, and solvent effects on reaction kinetics

“…We suggest that this is related to differences in ion-pairing, i.e . NaBArF 4 is more dissociated than NaOTf in THF, resulting in an increased effective concentration of Na + , 18 as opposed to the anion having an explicit role in the reaction. Ion-pairing effects also potentially complicate direct comparison between cations.…”

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

“…Similar hydrogen‐bonding interactions between organometallic cations and BAr F ‐type anions B(C 6 F 5 ) 4 − or [B{C 6 H 3 (3,5‐CF 3 ) 2 } 4 ] − have been identified on previous occasions, including Cp 2 Fe + B(C 6 F 5 ) 4 − , or can be identified upon close inspection of their crystal structures . Although they are very weakly coordinating, they can nevertheless form ion pairs, particularly in solvents with low dielectric constants . In the crystal lattice, cations and anions form tubular arrangements along the b axis of the unit cell.…”

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

“…Several NMR techinques are available for quantification if the key resonances are observed in situ. 15,[45][46][47] Granular adjustments to anion coordinating ability could reveal potential roles of counteranion during catalysis, offering a broader understanding of underlying mechanisms.…”