Spin-crossover modulation via single-crystal to single-crystal photochemical [2+2] reaction in Hofmann-type frameworks

This study reports the first modulation of spin-crossover (SCO) behavior via a photochemical [2 + 2] cycloaddition reaction. Here we construct two no-solvent Fe(II)-Ag(I) bimetallic Hofmann-type frameworks, [Fe(4-spy)(2){Ag(CN)(2)} 2] (1) and [Fe(2,4-bpe)(2){Ag(CN)(2)}(2)] (2) (4-spy = 4-styrylpyridine, 2,4-bpe = trans-1-(2-pyridyl)-2-(4-pyridyl) ethylene). For 1, the dimerization of 4-spy results in a single-crystal to single-crystal (SCSC) transformation from 2D interdigitated layers to a 3D interpenetrated structure. Additionally, a 3D / 3D structural transformation accompanied with Ag(I)-N bond breaking is achieved via the photochemical cycloaddition reaction of 2,4-bpe in 2. More importantly, both the spin transition temperatures and the SCO character are effectively modulated; thus, this approach provides a new strategy for constructing photo-responsive SCO magnetic materials.