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 FeII-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
MII(CN)42– (MII = 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
FeII 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 MII = Ni (1), Pd (2), Pt
(3); X = Me and MII = Ni (4),
Pd (5)) or one 2X-pyrazine and one water molecule (X
= I and MII = Ni (7), Pd (8),
Pt (9)), or, alternatively, two distinct FeII positions with either two pyrazines or two water molecules (X =
Me and MII = Pt (6)) are observed. Temperature
behavior of magnetic susceptibility indicates that all compounds bearing
FeN6 units (1–6) display
cooperative spin transition, while FeII ions in N5O or N4O2 surrounding are high spin (HS). Structural
changes in the nearest FeII 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 1–9 (see abstract figure). Effects
of a pyrazine substituent and MII nature on the hyperfine
parameters in both spin states are established.