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

Abstract

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions. Although these effects are widely appreciated in synthesis and catalysis, probing and controlling anion–cation interactions remains a significant challenge. Here we report the synthesis, characterisation and reactivity of the IMP anions, a family of anions with a coordinating ability that can be tuned for a given application. The anions are robust, compatible with both strongly basic and acidic media, suitable for isolation of unstable organometallic species, and effective as counteranions for homogeneous catalysis. IMP anions are prepared in two steps: deprotonation of substituted 2-phenylimidazoles with NaH, followed by addition of 2 equiv. B(C₆F₅)₃. The anions prepared feature a range of functionality, including nitro, ester, amide, amine and alcohol groups. Based on the spectroscopic properties of [Pd(IPr)(C(O)C₉H₆N)] [IMP-R], the coordinating ability of [IMP-R]⁻ ranges between BF₄⁻ and BAr^F₄⁻, depending on the polarity of the R group. Gold complexes of type [L-Au-L′][IMP-R] have been isolated and characterised, resulting in the first X-ray structure of a (η²-diphenylacetylene)Au complex. [(tBuXPhos)Au(MeCN)][IMP-R] catalyses [2 + 2] cyclisation of alkenes and alkynes, as well as the hydroalkoxylation of alkynes. Unlike SbF₆⁻ and BArF₄⁻, the [IMP-H]⁻ and [IMP-CF₃]⁻ salts are sufficiently soluble to efficiently promote cyclisations in toluene with [(tBuXPhos)Au(MeCN)]⁺.