Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Abstract

Self-assembled supramolecular host-guest complexes have been characterized by electrospray ionization mass spectrometry. The spectra obtained by use of a Q-TOF instrument equipped with a Z-spray ion source show primarily the 3-and 4-charge states of the assemblies. The assemblies have the general formula [guest ʚ Ga 4 L 6 ] 11Ϫ where L represents the chelating bidentate catechol ligand 1,5-bis(2=,3=-dihydroxy-benzamido)naphthalene and guests are tetramethyl ammonium (Me 4 N ϩ ), tetraethyl ammonium (Et 4 N ϩ ), tetra-n-propyl ammonium (Pr 4 N ϩ ) and decamethylcobaltocenium (Cp* 2 Co ϩ ) cations. For the first time, the mass spectrum of the empty assembly [Ga 4 L 6 ] 12Ϫ is reported. This article also reports that provided the electrospray ion source is capable of preserving noncovalent interactions, it is possible to observe host-guest complexes containing both weak binding guests as well as sterically demanding guests in the mass spectra. The present data suggest that electrospray mass spectrometry is a powerful tool for characterization of supramolecular host-guest complexes. ( I n the area of supramolecular chemistry, metal-ligand interactions have been exploited to design highly symmetric closed shell structures that selfassemble from simple metal and ligand components. The formation of discrete molecular architectures relies on labile coordinative bonds that allow for self-correction and exclusive formation of the thermodynamic product. A great variety of self-assembled structures of different molecular sizes and shapes is present in the literature [1][2][3][4][5]. Here, we focus on contributions to this field made by Raymond and coworkers [2, 6 -10]. The common theme in these metal-ligand assemblies is the presence of chelating bidentate catechol amide ligands, which bear certain geometric constraints upon coordination to octahedral metal centers resulting in M 4 L 6 coordination tetrahedra (Figure 1). The complexes are highly charged, rendering them soluble in polar solvents such as water and methanol, yet contain hydrophobic cavities of variable sizes. The presence of these hydrophobic cavities affords rich host-guest chemistry, enabling encapsulation of monocationic guest molecules [2, 6 -8]. Several analytical methods are necessary to efficiently characterize these host-guest complexes. 1 H-NMR spectroscopy shows remarkable upfield shifts in the proton signals of encapsulated guests within the cavities [2]. However, since the structures exhibit high point symmetry and the metal components are often paramagnetic, complete characterization by NMR is not always possible. Although solid-state structures may, in the best of cases, be determined by single crystal X-ray crystallography, intrinsically labile metal-ligand interactions do not always allow for unambiguous characterization of solution state structures. Additionally, mass spectrometry is more sensitive than NMR spectroscopy and allows analysis of lower concentrations, thus observing species with solubility properties insufficient for NMR measureme...

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