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

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(Mepz)[Au(CN)]} () and {Fe(Mepz)[Ag(CN)]} (), 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 but separated in . The complex exhibits an incomplete stepped spin transition as a function of the temperature with = 170 K and = 308 K for the two subsequent steps. In contrast, the complex attains the high-spin state over the whole temperature range. In the crystal structure of , weak interlayer contacts (Ag-π, Me-π, and Ag-N) are found that may be responsible for an unusual axial elongation of the FeN polyhedra. We propose that this structural distortion contributes to the trapping of iron in its high-spin state.