Determination of Labile Chiral Supramolecular Ion Pairs by Chromatographic NMR Spectroscopy

Abstract

The existence of supramolecular complexes in solution and particularly of ion pairs has been neatly highlighted by NMR diffusometry. [1] Nonetheless, intriguing cases that defy this simple and effective approach are isomeric mixtures. NMR diffusometry relies on and requires measurable differences in the hydrodynamic radius. Isomers can be identified by this method if their shape differs significantly, [2] but otherwise remains a challenge, particularly because of their tendency to produce highly overlapping 1 H NMR spectra. Improvements to the resolution of diffusion-ordered spectroscopy (DOSY) have been proposed by addition of a soluble matrix capable of either amplifying the diffusion differences [3] or of reducing the spectral degeneracy. [4] For instance, epimers could be characterized by DOSY following the addition of cyclodextrin. [3f] Chromatographic NMR [5] is a version of matrix-assisted DOSY, the separation capabilities of which are modulated and boosted by addition of a solid chromatographic phase to the mixture under study, assisted by high-resolution magicangle spinning (HRMAS) to recover resolution. [6] In favorable cases, it has been demonstrated that this technique may be even capable to outperform liquid chromatography (LC), [6b] because of a larger flexibility on the choice of the liquid/solid phase ratio, which has direct effects on the resolution of the method. [7] In the following, we demonstrate that silica-enhanced diffusion-ordered NMR spectroscopy (DOSY) is capable of resolving the spectra of isomeric mixtures of ion pair supramolecular species, chiral metal(II) tris(diimine) complexes ML 3 , [8] and TRISPHAT counterions, even for cases that elude classic liquid chromatography.Previously, it was shown that chiral TRISPHAT anion 1 (tris(tetrachlorobenzenediolato)phosphate(V), Figure 1) is configurationally stable [9] and that its D or L enantiomers can be readily synthesized and separated on large scale. [10] When associated with ruthenium and iron tris(diimine) complexes, this anion is a valuable resolving and asymmetry-inducing reagent, respectively. [11,12] The asymmetric interactions occurring between the ions can be characterized and quantified by circular dichroism (CD) and by NMR spectroscopy as anions 1 are in addition good chiral solvating agents. [13] The first step in the present study was to reproduce by chromatographic NMR spectroscopy the diastereomeric separation of the mixture of [Ru(Me 2 bpy) 3 ] 2 enantiomers (Me 2 bpy = 4,4'-dimethyl-2,2'-bipyridine) paired to anions D-1, which have been previously shown to be separable by TLC on bare silica gel. [12a, 14, 15] The 1 H NMR spectrum of the two diastereomeric ion pairs gives rise to two well distinct sets of peaks in the aromatic region (Figure 2 A). Integration reveals that in this case the mixture has a diastereomeric ratio (d.r.) of 3.5:1 in favor of the heterochiral pair [L-2][D-1] 2 over homochiral species [D-2][D-1] 2 . The spectra of the diastereomerically pure species, which were separated by chromatographic...