Hofmann-Like Frameworks Fe(2-methylpyrazine)n[M(CN)2]2 (M = Au, Ag): Spin-Crossover Defined by the Precious Metal

Abstract

Hofmann-like cyanometalates constitute a large class of spin-crossover iron(II) complexes with variable switching properties. However, it is not yet clearly understood how the temperature and cooperativity of a spin transition are influenced by their structure. In this paper, we report the synthesis and crystal structures of the metal–organic coordination polymers {Fe^II(Mepz)[Au^I(CN)₂]₂} ([Au]) and {Fe^II(Mepz)₂[Ag^I(CN)₂]₂} ([Ag]), where Mepz = 2-methylpyrazine, along with characterization of their spin-state behavior by variable-temperature SQUID magnetometry and Mössbauer spectroscopy. The compounds are built of cyanoheterometallic layers, which are pillared by the bridging Mepz ligands in [Au] but separated in [Ag]. The complex [Au] exhibits an incomplete stepped spin transition as a function of the temperature with T_SCO1 = 170 K and T_SCO2 = 308 K for the two subsequent steps. In contrast, the complex [Ag] attains the high-spin state over the whole temperature range. In the crystal structure of [Ag], weak interlayer contacts (Ag−π, Me−π, and Ag–N) are found that may be responsible for an unusual axial elongation of the FeN₆ polyhedra. We propose that this structural distortion contributes to the trapping of iron in its high-spin state.