The inhibition of iridium-promoted water oxidation catalysis (WOC) by cucurbit[n]urils

Abstract

A series of iridacycles bearing π-bonded moieties of variable electron-withdrawing capabilities were tested for their ability to promote water oxidation catalysis (WOC) in the presence of high loading in a sacrificial oxidant, under conditions chosen for optimal dioxygen production. This report shows that none of these complexes performs differently than monometallic iridacycles and that the π-bonded moiety does not affect the overall rate of O₂ production. Furthermore, it is shown that cucurbituril macrocycles significantly inhibit the production of dioxygen independently of the nature of the Cp*Ir(III)-based catalyst used to perform WOC. Theoretical first-principles based DFT-D3 investigations including a complete treatment of solvation with COSMO and COSMO-RS treatments supported by ITC analyses suggest that concealment of the catalyst by curcurbit[7]uril could occur by non-covalent interaction of the Cp*Ir moiety in the hydrophobic pocket of the cavitand. For other cavitands of smaller inner cavity diameter, inclusion may not be the main mode of inhibition. Assuming the intervention of the putative Ir(IV)–oxyl biradical of a Cp*Ir^IV(O)(H₂O)₂ species like suggested by many authors, inhibition of WOC by inclusion would probably result from unfavourable coulombic interactions between water and the inclusion complex.