Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The
transition between spin states in d-block metal complexes has
important ramifications for their structure and reactivity, with applications
ranging from information storage materials to understanding catalytic
activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover
compounds for several transition metals in the periodic table, but
this has mostly been limited to coordinatively saturated metal centers
in octahedral ligand environments. Spin-crossover complexes with low
coordination numbers are much rarer. Here we report a series of four-coordinate,
(pseudo)­tetrahedral Fe­(II) complexes with formazanate ligands and
demonstrate how electronic substituent effects can be used to modulate
the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo
spin-crossover in solution with T1/2 above
room temperature (300–368 K). While structural analysis by
X-ray crystallography shows that the majority of these compounds are
low-spin in the solid state (and remain unchanged upon heating), we
find that packing effects can override this preference and give rise
to either rigorously high-spin (6) or gradual spin-crossover
behavior (5) also in the solid state. Density functional
theory calculations are used to delineate the empirical trends in
solution spin-crossover thermodynamics. In all cases, the stabilization
of the low-spin state is due to the π-acceptor properties of
the formazanate ligand, resulting in an “inverted” ligand
field, with an approximate “two-over-three” splitting
of the d-orbitals and a high degree of metal–ligand covalency
due to metal → ligand π-backdonation. The computational
data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is
present at the C–Ar or N–Ar rings, which is ascribed
to the opposing effect on metal–ligand σ- and π-bonding.