[2+2] photocycloaddition induces 114 K high-temperature shift of spin crossover transition - the role of the cycloaddition induced strain

Abstract

6-(1,2,3-triazol-1-ylmethyl)coumarin (L1), able to combine in Fe(II) complex a molecule photoreactivity and a spin crossover activity, was prepared in the reaction of 6-(bromomethyl)coumarin with sodium salt of 1,2,3-triazole. L1 forms mononuclear systems [Fe(L1)₆](ClO₄)₂·nCH₃CN (n = 0, 2). Solvated form (1) exhibits complete, one step spin crossover at T_1/2 = 155 K. In 1 there is a competition between two coumarin fragments from two ligands molecules for an access to a third one located between them. Distances between double bonds of lactone rings are equal to 3.70 and 3.92 Å and irradiation with a light of wavelength 365 nm results in [2+2] cycloaddition. Due to the competition photoconversion occurrs in two directions and results in two dimensional polymeric layer of frustrated topology. The resulted complex (1c) also exhibits spin crossover, shifted to slightly lower temperatures (T_1/2 = 141 K). The non-solvated complex (2) exhibits two-step spin crossover (T¹_1/2 = 83 K, T²_1/2 = 62 K). In contrast to 1, there exists only one type of ligands pair for which topochemical requirements are fulfilled. In effect a regular (2,2) polymeric layer is formed as a result of [2+2] photocycloaddition (λ = 365 nm). It results in an unprecedented shift of spin crossover to higher temperatures up to 197 K. The product of the photoconversion (2c) is characterized by a presence of strong strains involving dimerized ligand molecules, particularly cyclobutane rings. The DFT modelling based on the structure of the dimerized ligand reveals a significant difference in this strain, the energetic effect of it (stress) being more than 240 kJmol^-1 higher for the ligands corresponding to the complex in the HS state in line with the observed 114 K shift of transition temperature on going from the initial mononuclear complex to the 2D structure of the photoconverted one.