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

Abstract

Photoresponsive spin-crossover (SCO) compounds are particularly appealing due to precise adjustability of light wavelength/energy 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 photoresponsive SCO compound [Fe(3-spy)₂{Ag(CN)₂}₂] (1). Upon irradiation at 365 nm, the criss-cross CC double bonds of 3-spy ligands in 1 undergo a pedal-like conformational motion and a photochemical [2 + 2] cycloaddition reaction, leading to 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 the 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 an incomplete and gradual one-step SCO in 2. Notably, T_1/2, which represents the intermediate spin state where γ_HS = 0.5, remains almost unchanged, indicating that 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 photoswitchable SCO materials.