Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand.

Ruth Patchett,Alfredo Vargas,Adrian B Chaplin,Richard C Knighton,James D Mattock

doi:10.1021/acs.inorgchem.7b02441

Ruth Patchett, Alfredo Vargas + Show 3 more

Open Access

https://doi.org/10.1021/acs.inorgchem.7b02441

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Abstract

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

Highlights

Electrostatic forces between ions are the strongest noncovalent bonding interactions encountered in supramolecular chemistry.Correspondingly, the attraction between oppositely charged components features widely throughout the host−guest chemistry of biological and synthetic systems.1 repulsion between identically charged host and guest molecules is significantly destabilizing, and unsurprisingly well-defined supramolecular complexes featuring such unfavorable interactions are rare.2,3As part of our ongoing work exploring the coordination chemistry of calix[4]arene-based ligands,4 we serendipitously discovered that cationic rhodium and iridium complexes 1, bearing bis(imidazole-2-thione)-functionalized calix[4]arene ligand 2, show significant uptake of potassium cations (Chart1)
Potassium Binding by Rhodium and Iridium-Based Hosts (COD = 1,5-Cyclooctadiene) interactions associated with the potassium cation binding, the host−guest chemistry of 1 is contrasted with that of bis(imidazole-2-thione) 2 and calix[4]arene 3 (Figure 1)
The potassium adduct was isolated in high yield by crystallization

Summary

■ INTRODUCTION

Electrostatic forces between ions are the strongest noncovalent bonding interactions encountered in supramolecular chemistry. Potassium Binding by Rhodium and Iridium-Based Hosts (COD = 1,5-Cyclooctadiene) interactions associated with the potassium cation binding, the host−guest chemistry of 1 is contrasted with that of bis(imidazole-2-thione) 2 and calix[4]arene 3 (Figure 1). Formation of a 1:1 potassium adduct of upper-rim-unfunctionalized calix[4]arene 3 was established in CD2Cl2 solution (slow host−guest exchange at 298 K, 400 MHz) and crystalline 3⊃K+ was subsequently obtained in 80% isolated yield. 3⊃K+ is Together only weakly exergonic the combined solution Consistent with this reasoning, 1,3-diisopropyl4,5-dimethylimidazole-2-thione (IiPr2Me2S) does not form a potassium complex upon standing in a suspension of K[BArF4] in CD2Cl2 at 298 K.11. Hirshfeld charges for 1 highlight greater charge differences between the sulfur and rhodium (−0.12/+0.19) than between the sulfur and iridium (−0.09/+0.07), which presumably accounts for the slightly less unfavorable electrostatic term in the EDA of 1a⊃K+, compared to 1b⊃K+, and correspondingly the marginally different binding affinities of the transition-metal-based hosts

■ SUMMARY AND OUTLOOK

■ ACKNOWLEDGMENTS

■ REFERENCES