Light-initiated reversible conversion of macrocyclic endoperoxides derived from half-sandwich rhodium-based metallarectangles

Abstract

Although reversible photo-dimerization or oxygenation of anthracene and its derivatives is a common reaction, light-initiated reversible conversion of endoperoxide organometallic frameworks has only rarely been addressed. Herein, a series of tetranuclear organometallic macrocycles, [Cp*₂Rh₂(μ-C₂O₄-κO)]₂(BP4VA)₂(OTf)₄ (4), [Cp*₂Rh₂(BiBzIm)]₂(BP4VA)₂(OTf)₄ (5), and [Cp*₂Rh₂(DHBQ)]₂(BP4VA)₂(OTf)₄ (6), were obtained in good yields from the reactions of the binuclear half-sandwich rhodium precursors [Cp*₂Rh₂(μ-C₂O₄-κO)Cl₂] (1), [Cp*₂Rh₂(BiBzIm)Cl₂] (2), and [Cp*₂Rh₂(DHBQ)Cl₂] (3) with the 9,10-bis((E)-2-(pyrid-4-yl)vinyl)anthracene (BP4VA) ligand. The photochemical reaction of these metallarectangles was investigated by NMR and UV/vis spectroscopy. We have demonstrated that complexes 4, 5, and 6 can be reversibly and nearly quantitatively converted to the macrocyclic endoperoxides 4-O₂, 5-O₂, and 6-O₂. Meanwhile, the structure of the endoperoxide photoproducts was unambiguously confirmed by ¹H/¹³C NMR spectroscopy, IR spectroscopy, elemental analyses, and X-ray crystallography.