Spin crossover in 2D iron(ii) phthalazine cyanometallic complexes

Abstract

Two new 2D spin-crossover (SCO) analogues of Hofmann clathrates of composition [Fe(phth)₂M^II(CN)₄] (where phth = phthalazine; M^II = Pd, Pt) have been synthesized and their structures and switchable behaviour have been characterized. Single-crystal X-ray analysis reveals that the Pt and Pd derivatives contain Fe^II centres equatorially surrounded by four equivalent μ₄-[M^II(CN)₄]²⁻ groups. Two crystallographically equivalent phthalazine (phth) ligands occupy the axial positions of each Fe^II site, completing its octahedral coordination environment. The stabilization of these structures is realized via supramolecular C–H⋯M interactions and π–π stacking. Temperature-dependent magnetic susceptibility measurements showed that Pt (T_1/2↓ = 211 K and T_1/2↑ = 218 K) and Pd (T_1/2↓ = 202 K and T_1/2↑ = 207 K) derivatives display cooperative spin crossover with narrow thermal hysteresis loops. In addition, spin crossover in these complexes was characterized by optical measurements, differential scanning calorimetry, and IR and Raman spectroscopy. This research shows that the use of phthalazine leads to the production of new SCO systems with attractive transition characteristics and opens up new perspectives for the design of switchable complexes based on fused bicyclic azine ligands.