Photochemical [2+2] reaction-tuned spin-crossover cooperativity in an Fe(II)-Ag(I) bimetallic Hofmann-type framework

Photo-responsive spin-crossover (SCO) compounds are particularly appealing due to their adjustable wavelength/energy of light and the capability for remote switching. Herein, we introduce the 3-styrylpyridine (3-spy) ligand into a 2D Fe(II)-Ag(I) bimetallic Hofmann-type framework to construct a photo-responsive SCO compound [Fe(3-spy)₂{Ag(CN)₂}₂] (1). Upon irradiation at 365 nm, the criss-crossed C=C double bonds of 3-spy ligands in 1 undergo a pedal-like conformational motion and a photochemical [2+2] cycloaddition reaction, leading to the structural disorder but resulting in a more regular coordination geometry around the SCO center in the cyclobutane product [Fe(hh-rctt-1,2-ppcb){Ag(CN)₂}₂] (2) (hh-rctt-1,2-ppcb = hh-rctt-1,2-bis(3-pyridyl)-3,4-bis(phenyl)cyclobutane, r = reference group, c = cis and t = trans, hh = head-to-head). These structural changes exert opposite influences on SCO cooperativity and the ligand field of SCO center. Studies of SCO behavior under different irradiation times revealed that the SCO cooperative behavior gradually shifts from a two-step SCO in 1 to the incomplete and gradual one-step SCO in 2. Notably, T_1/2, which represent the intermediate spin state where γ_HS = 0.5, remains almost unchanged, indicating only the SCO cooperativity is significantly modulated in this unique photochemical [2+2] reaction-integrated SCO system. Moreover, upon irradiation at 254 nm, partial stereoselective cleavage of the cyclobutane moieties in 2 was observed, suggesting the potential for constructing photo-switchable SCO materials.