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

Abstract

Herein, 6-(1,2,3-triazol-1-ylmethyl)coumarin (L1), which can combine molecule photoreactivity and spin-crossover activity in Fe(II) complexes, was prepared by reacting 6-(bromomethyl)coumarin with sodium salt 1,2,3-triazole. L1 forms mononuclear systems [Fe(L1)₆](ClO₄)₂·nCH₃CN (n = 0, 2). The 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 ligand molecules for access to a third fragment located between them. The distances between the double bonds of the lactone rings are equal to 3.70 and 3.92 Å, and light irradiation at a wavelength of 365 nm results in [2 + 2] cycloaddition. Although photoconversion can proceed in two directions, this competition ultimately yields a two-dimensional polymer layer with a frustrated topology. The resulting complex (1c) also exhibits spin crossover, showing a slight shift to lower temperatures (T_1/2 = 141 K). The non-solvated complex (2) exhibits a two-step spin-crossover (T¹_1/2 = 83 K and T²_1/2 = 62 K). In contrast to 1, only one type of ligand pair exists for which topochemical requirements are fulfilled. In effect, a regular (2,2) polymeric layer is formed as a result of the [2 + 2] photocycloaddition (λ = 365 nm). This results in an unprecedented shift in the spin crossover to higher temperatures up to 197 K. The photoconversion product (2c) is characterized by the presence of strong strains involving dimerized ligand molecules, particularly cyclobutane rings. DFT modelling based on the structure of the dimerized ligand reveals a significant difference in this strain, and the energetic effect of it (stress) is more than 240 kJ mol⁻¹ higher for the ligands corresponding to the complex in the HS state. This is consistent with the observed 114 K shift of the transition temperature upon transformation from the initial mononuclear complex to the photoconverted 2D structure.