Angewandte
Chemie
HS) spin crossover. At 400 K, the value of cMT
(2.96 cm3 KmolÀ1) is close to that expected for the HS state,
but the lack of a significant plateau suggests that the transition
is not quite complete at this temperature. The spin-state
transition occurs over a large temperature range (ca. 175–
400 K) and is reversible; both ascending and descending
temperature measurements are shown in Figure 2, and no
hysteresis was observed. The transition temperature T1/2
(defined as temperature at which complexes show a popula-
tion of 50% in the HS state) of this gradually proceeding spin
transition is about 265 K. Figure 2 also plots the observed
time-averaged quadrupole splitting value against temper-
ature; the strong correlation between the quadrupole splitting
and the susceptibility is clear.
To gain a better understanding of the thermodynamic
behavior of the spin states, density functional theory was
employed (see the Supporting Information).[12] At low
temperature, only the low-spin S = 0 state was thermodynami-
cally accessible. With increasing temperature the S = 2 state
became significant, and a spin-crossover event is predicted to
occur near 325 K (Figure S1 in the Supporting Information),
in good agreement with the value of 265 K from experiment.
The intermediate-spin S = 1 state was disfavored over the
entire temperature range explored.
state transition is not quite complete. Also completely
consonant with expectation are the increases in the displace-
ment of the iron center from the mean plane of the four
nitrogen plane.
The anisotropic thermal parameters also show evidence of
the spin crossover. As expected, the magnitude of all atomic
anisotropic displacement parameters increases with increas-
ing temperature. However, the cyanide carbon atom shows
different behavior over the temperature range. The thermal
parameters at 100 and 400 K are close to isotropic, consistent
with a single carbon atom site, whereas at intermediate
temperatures with substantial populations of two spin states
and differing carbon sites, the thermal parameters are much
À
more prolate, with elongation along the Fe C bond direction.
À
Importantly, the C N bond length in all structures remains
nearly constant, as expected if only CNÀ atoms occupy two
sites.
Additional evidence for the spin crossover comes from
temperature-dependent IR spectroscopy, which has the
advantage of a shorter time scale (10À13 s) and thus can
detect both spin isomers. Measurements at 296 K, as either
À
nujol mulls or KBr pellets, show two distinct n(C N)
frequencies at 2070 and 2105 cmÀ1, with the first being
stronger (see the Supporting Information) On cooling, the
2105 cmÀ1 peak gradually decreases in intensity, while the
2070 cmÀ1 peak increases in intensity. At 150–160 K, the
stretch at 2105 cmÀ1 disappears, and thus it can be assigned to
the HS stretch. A similar pattern of temperature-dependent
azide stretches was observed in a 5/2–3/2 spin-crossover
complex.[14]
We have also investigated the temperature-dependent
structure of the iron complex, as changes in metal–donor
separations, along with changes in magnetic properties, are a
hallmark of spin-state transitions. Structures were determined
at 100 K (two crystals) and at 296, 325, and 400 K.[13] A change
from a LS to a HS state in the five-coordinate complex is
À
expected to lead to increases in the axial Fe C separation, the
In coordination chemistry, cyanide and CO are deeply
entrenched as strong-field ligands.[15,16] Recently, Miller and
co-workers showed that [(NEt4)3][CrII(CN)5][17] is a distorted
trigonal bipyramidal complex that is not low-spin. Two
different theoretical calculations[18] suggested that the HS
state results from the buildup of electrostatic (ligand–ligand)
repulsions and not from the ligand field of cyanide per se; the
cyanide ligand is behaving as a strong-field ligand in this
chromium complex. However, [K(222)][Fe(tpp)(CN)] repre-
sents a case in which the CNÀ ligand should unequivocally
lead to LS species. That it does not strongly demonstrates the
weaker-field nature of cyanide, even in a case where p back-
bonding should be maximized.
In summary, the synthesis and characterization of the first
cyanoiron(II) porphyrinate, [K(222)][Fe(tpp)(CN)], is pre-
sented. It forms a LS-to-HS crossover complex; coordination
of a single axial cyanide ligand does not generate a sufficiently
strong ligand field to ensure a low-spin complex under all
conditions.[19] This finding is in distinct contrast to the
analogous five-coordinate CO complex, which is low-spin
under all known conditions.
equatorial Fe–Np bond lengths, and the displacement of the
iron atom from the mean porphinato plane. The results are
summarized in the ORTEP drawings given in Figure 3; for
simplicity only the cyanide group and FeN4 porphyrin core are
À
shown. The Fe C bond elongates by 0.23 ꢀ (Figure 3), which
is amongst the largest changes in bond lengths that have been
observed for iron(II) spin-crossover compounds.[11b] This
change takes place in part because the axial and equatorial
bond-length increases must be asymmetric owing to the
macrocyclic constraints of the porphyrin ring; note that Fe
Np bond length has increased by 0.103 ꢀ over the same
À
À
temperature range. The average Fe Np bond length at 100 K
(1.986 (7) ꢀ) is that for a pure LS state, whereas the 400 K
value (2.089 (8) ꢀ) is slightly less than expected for an anionic
HS iron(II) complex, consistent with the idea that the spin-
Received: August 21, 2008
Published online: November 6, 2008
Figure 3. ORTEP diagrams of the core porphyrin atoms (Fe and four
pyrrole N atoms) and the cyanide groups of [K(222)][Fe(tpp)(CN)] at
different temperatures. Axial ligand and average equatorial bond
lengths are given as well as the iron displacement from the mean N4
plane. Thermal ellipsoids are set at the 50% probability level.
Keywords: cyanides · iron · magnetic properties · porphyrinoids ·
spin crossover
.
Angew. Chem. Int. Ed. 2008, 47, 10144 –10146
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Li, Jianfeng
