Bistable Hofmann-Type Fe Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {Fe(L)[M(CN)]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the M derivatives (M = Pd and Pt) with sulfur-rich axial ligands (L = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for Pt-methyl and Pt/Pd-ethyl derivatives, while the Pd-methyl derivative undergoes a much less cooperative multistep SCO. Excluding Pt-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the Fe center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the Pd-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.