Control of light-promoted [2+2] cycloaddition reactions by a remote ancillary regulatory group that is covalently attached to rhenium rectangles

Abstract

The high-yielding self-assembly of three neutral rhenium(I) rectangles, [Re(2)(CO)(6)(L)(bpe)](2) (1 a, L=2,2'-biimidazolate (biim); 1 b, L=2,2'-bisbenzimidazolate (bbim); 1 c, L=2,2'-bis(4,5-dimethylimidazolate) (bdmim); bpe=trans-1,2-bis(4-pyridyl)ethylene), under hydrothermal conditions is described. The rectangles were structurally characterized by spectroscopic techniques and further confirmed by single-crystal X-ray diffraction. Upon irradiation with a Hg lamp at 365 nm, the bpe ligands of rectangles 1 a and 1 b underwent [2+2] photocycloaddition reactions to produce [{(Re(CO)(3))(2)L}(2)(4,4'-tpcb)(2)] (2 a, L=biim; 2 b, L=bbim; 4,4'-tpcb=1,2,3,4-tetrakis(4-pyridyl)cyclobutane) through a single-crystal-to-single-crystal (SCSC) transformation. However, rectangle 1 c, which contained methyl groups on the 2,2'-biimidazolate ligand, failed to undergo cycloaddition, even after prolonged irradiation. This result indicates that the light-induced cycloaddition reaction can be preferentially controlled by the remote regulatory substituents, which are attached onto the same backbone of the rectangle complex. This transformation is the first reported utilization of a remote ancillary regulatory ligand that is covalently attached onto a coordination compound to control the [2+2] cycloaddition reaction.