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

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*Rh-2(2)(mu-C2O4-kappa O)] 2(BP4VA)(2)(OTf)(4) (4), [Cp*Rh-2(2)(BiBzIm)](2)(BP4VA)(2)(OTf)(4) (5), and [Cp*Rh-2(2)(DHBQ)](2)(BP4VA)(2)(OTf)(4) (6), were obtained in good yields from the reactions of the binuclear half-sandwich rhodium precursors [Cp*Rh-2(2)(mu-C2O4-kappa O)Cl-2] (1), [Cp*Rh-2(2)(BiBzIm)Cl-2] (2), and [Cp*Rh-2(2)(DHBQ)Cl-2] (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-O-2, 5-O-2, and 6-O-2. Meanwhile, the structure of the endoperoxide photoproducts was unambiguously confirmed by H-1/C-13 NMR spectroscopy, IR spectroscopy, elemental analyses, and X-ray crystallography.