Liquid coordination complexes of Lewis acidic metal chlorides: Lewis acidity and insights into speciation

Abstract

Coordination complexes of Lewis acidic metal chlorides AlCl, GaCl, InCl, SbCl, SnCl, SnCl, ZnCl and TiCl with trioctylphosphine (P) and trioctylphosphine oxide (PO) were synthesised. All compounds formed liquid coordination complexes (LCCs) at ambient temperature, although decomposition via a redox mechanism was detected in some cases. The Lewis acidity of the metal chlorides (measured in 1,2-dichloroethane solutions) and the LCCs (measured neat) was quantified by using the Gutmann acceptor number (AN) approach. In general, LCCs were equally or more Lewis acidic than the corresponding metal chlorides. The AN values were compared with the catalytic activity of selected LCCs in a model Diels-Alder reaction. Insight into speciation of LCCs was gained using multinuclear NMR spectroscopy, revealing that most LCCs comprised charge-neutral complexes rather than ionic ones. The relationship between the speciation, Lewis acidity (AN scale) and catalytic activity is discussed in detail. This approach reveals several new, promising catalytic systems, such as PO-InCl, with Lewis acidity enhanced compared to chloroindate ionic liquids, and PO-TiCl, with hydrolytic stability enhanced with respect to neat TiCl.

Protic Ionic Liquids Based on Oligomeric Anions [(HSO4)(H2SO4)x]− (x = 0, 1, or 2) for a Clean ϵ-Caprolactam Synthesis

“…5 provides comparison of the -caprolactam yields (circles) obtained using different PILs under the same reaction conditions, and acidity of each PIL, expressed as an acceptor number, AN (histogram bars). [25] There is a certain correlation between the AN values determined for the PILs in our earlier work [25,32,33] and the yield of the -caprolactam. PILs with χ H2SO4 = 0.50 are not catalytically active, which corresponds to their weak-to-medium acidity (AN = 65.5-96.5).…”

Hydrophobic Deep Eutectic Solvents Incorporating Trioctylphosphine Oxide: Advanced Liquid Extractants

“…however, unlike static hydrogen-bonding enabling solid-state co-crystal formation, the dynamic nature of hydrogen-bonds here probably lead to disruptively competition with lattice packing forces to induce the liquid formation. It is worth noting that these changes in chemical shift (∆δ 31 P = 10-11) are much smaller than have been observed for interactions of TOPO with Lewis acid metal salts forming, for example, liquid coordination complexes 29 or when TOPO coordinates to hydrated protons, 54 where the 31 P chemical shifts have been reported to change by up to nearly 40 ppm.…”

“…The scope for formation of liquid eutectic solvents has subsequently been expanded to a range of terpenes with monocarboxylic acids 27 and, interestingly, also to make use of mixtures of carboxylic acids to access eutectic liquid compositions. 28 Building on previous experience using TOPO as a Lewis basic (hydrogen bond-accepting) ligand to generate liquid coordination complexes [29][30][31] with tunable Lewis acid properties we were interested in studying whether trioctylphosphine oxide (TOPO) could be used as a hydrogen-bond accepting component to produce DES and whether a liquid DES formed in this way could be used as an extractant phase containing high phosphine oxide concentrations.…”

Frustrated Lewis pairs in ionic liquids and molecular solvents – a neutron scattering and NMR study of encounter complexes

“…78% of the phosphine saw a change in electronic environment. On average, when phosphines form adducts with strong Lewis acids, their 31 P NMR signal shifted downfield by about Δδ 31P = +20 ppm; 14 for example, triphenylphosphine adduct with BH 3 has Δδ 31P = +26 ppm, compared to free triphenylphosphine. 15 Therefore, this change where the 31 P nuclei is slightly shielded, rather than deshielded, does not suggest adduct formation, but a different type of interaction.…”