Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established