Transcript Spin States of highly deformed iron(III) - Extra Materials

Spin States of Highly Deformed Iron(III)
Porphyrin Complexes Studied by
57Fe Mössbauer Spectroscopy
Mikio Nakamuraa,b,c and Masashi Takahashid
a Department
of Chemistry, School of Medicine, Toho University
Ota-ku, Tokyo 143–8510, Japan
b Research Center for Materials with Integrated Properties, Toho University
Funabashi, Chiba 274-8510, Japan
c Division of Chemistry, Graduate School of Science, Toho University
Funabashi, Chiba 274-8510, Japan
d Department of Chemistry, Faculty of Science, Toho University
Funabashi, Chiba 274-8510, Japan
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Introduction: spin state and deformation of porphyrin ring
dz2
dx2-y2
ruffled deformation
low spin state
saddled deformation
high spin state
intermediate spin state Fig. 2 Two modes of deformation of porphyrin ring:
atoms shown by open circles are above the least
S = 1/2
S = 5/2
S = 3/2
square plane, and those by the filled circles are below
Fig. 1. Spin states adopted by Fe(III)-porphyrin complexes
the plane.
Elucidation of the electronic structures and spin states of iron porphyrin complexes is of importance to
understand the function of naturally occurring heme proteins. A number of investigations have been carried out
to this end using many physicochemical methods. Among them iron-57 Mössbauer spectroscopy is a powerful
and appropriate tool.
The spin states of iron(III) in porphyrin complexes are controlled by the nature and number of the axial ligands
through the strength of the ligand field splitting (Fig. 1). The deformation of the porphyrin ring, which induces
certain specific interactions between the iron d orbitals and the porphyrin ring, also has considerable influence
on the spin state [ref. 1]. A number of naturally occurring heme proteins exhibit porphyrin ring deformation.
Hence the elucidation of the deformation effect is an important subject. Two modes of deformation are wellknown: ruffle and saddle (Fig. 2), which result in different effects on the electronic configuration. In this paper
we will focus mainly on FeIII-OETPP complexes, which adopt the saddled deformation. The abbreviations of the
porphyrins and ligands appearing in this paper are listed in the last slide.
Ref. 1. M. Nakamura, Coord. Chem. Rev. 250 (2006) 2271–2294.
Spin states of Fe(III) porphyrin complexes
Fig. 3 shows the spectra of saddle-type Fe(III) OETPP
complexes: [Fe(OETPP)Cl], [Fe(OETPP)(thf)2]ClO4
and [Fe(OETPP)(Him)2]ClO4.
[Fe(OETPP)Cl] has a small quadrupole splitting (DEQ)
value and is assigned to the S = 5/2 state, while
[Fe(OETPP)(Him)2]ClO4 has a larger DEQ value and a
rather small isomer shift (d) value corresponding to an
S = 1/2 state. [Fe(OETPP)(thf)2]ClO4 shows a very
large DEQ and rather large d values, indicating a large
imbalance in d electron population, and is assigned to
the S = 3/2 state.
Five-coordinate porphyrin complexes with an axial
halide ligand usually show the S = 5/2 state.
Interestingly the saddled OETPP complex with an
iodide ion in the axial position has a large DEQ value
(3.05 mm s–1 at 77 K), indicating the S = 3/2 state [ref.
2]. This demonstrates that control of the spin state
using the axial ligand is easier in non-planar porphyrin
complexes. Indeed the porphycene complex,
[Fe(EtioPc)I] also adopts the S = 3/2 state [ref. 3].
[Fe(OETPP)Cl]
S = 5/2
[Fe(OETPP)(thf)2]ClO4
S = 3/2
[Fe(OETPP)(Him)2]ClO4
S = 1/2
Fig. 3. 57Fe Mössbauer spectra of OETPP complexes
at room temperature in different spin states.
Ref. 2. M. Nakamura et al., Chem. Commun., (2002) 1198–1199.
Ref. 3. Y. Ohgo et al., Inorg. Chem., 41 (2002) 4527–4529.
Spin states of [Fe(OETPP)L2]ClO4
Fig. 4 demonstrates that the ligand field
strength of the axial ligand L determines
the spin state. Strongly splitting ligands
[imidazole (Him) and 4dimethylaminopyridine (dmap)] lead to the
pure S = 1/2 state, while the weakly
splitting ligand tetrahydrofuran (thf) gives
a pure S = 3/2 spin state. Intermediate
strength ligands such as pyridine (py) and
4-cyanopyridine (4-CNpy) result in new
spin-crossover complexes as discussed
later [ref. 4]. Interestingly the importance
of the ring deformation is also confirmed
in the case of the ruffled deformation:
[Fe(TEtPrP)(thf)2]ClO4 also has the S = 3/2
state (d = 0.24 and DEQ = 3.80 mm s–1) [ref.
5].
Ref. 4. T. Ikeue et al, Angew. Chem. Int. Ed.
40 (2001) 2617 – 2620.
Ref. 5. T. Sakai et al., J. Am. Chem. Soc.,
125 (2003) 13028 – 13029.
L = Him,
d = 0.18, DEQ = 1.82 mm s–1
S = 1/2
L = dmap,
d = 0.19, DEQ = 2.21 mm s–1
S = 1/2
L = py,
d = 0.32, DEQ = 2.76 mm s–1
S = 3/2–1/2
L = 4-CNpy,
d = 0.37, DEQ = 3.26 mm s–1
S = 3/2
L = thf,
d = 0.41, DEQ = 3.65 mm s–1
S = 3/2
Fig. 4 Mössbauer spectra at room temperature of sixcoordinate complexes [Fe(OETPP)L2]ClO4.
Spin-crossover in [Fe(OETPP)L2]ClO4 (L = py , 4-CNpy)
The Mössbauer spectra of 4-CNpy and py show very
interesting temperature dependence [ref. 4]. In both
complexes the spectra change as the temperature is
lowered. The 4-CNpy complex exhibits a new doublet
below 230 K and the relative intensities for the site
increase on decreasing the temperature. The d and DEQ
values of the new site (blue) are 0.20 and 2.70 mm s–1 at
77 K, while those of the other site (green) are 0.57 and
3.03 mm s–1. This clearly indicates that two spin states
co-exist at 77 K and implies a spin-crossover between
S=3/2 and S=1/2.
Although no new peak is observed in the py complex,
both d and DEQ values decrease on lowering the
temperature (Fig. 8 in a later slide), reaching values of
0.25 and 2.29 mm s–1, respectively at 77 K. This is
obviously the low spin state. Thus the py complex is also
a spin-crossover complex. The difference in the
Mössbauer behaviour of the complexes is due to the time
scale of the spin-crossover transition.
Fig. 5 Mössbauer spectra of py and 4-CNpy
complexes at room temperature and 77 K.
Magnetic moments and 13C nmr chemical shifts of [Fe(OETPP)L2]ClO4
L = thf
L = 4-CNpy
L = py
L = dmap
L = Him
L = dmap
L = Him
L = py
L = 4-CNpy
Fig. 6. Temperature dependence of magnetic
moments of [Fe(OETPP)L2]ClO4 in the
crystalline state (a) and in CH2Cl2 solution (b).
Fig. 7. Temperature dependence of 13C
chemical shifts of the meso 13C atoms in
CD2Cl2 solution.
Spin-crossover in py and 4-CNpy complexes is confirmed by other physicochemical methods [ref.4]: the magnetic
moments of py and 4-CNpy complexes are definitely those of spin-crossover complexes (Fig. 6). Interestingly the
magnetic moments determined by the Evans method in CH 2Cl2 indicate that spin-crossover occurs in the py
complexes (Fig. 6 inset). Furthermore, the 13C chemical shifts of the meso carbon atoms in CD2Cl2 also show spincrossover for the py complex (Fig. 7). DtrsH and DtrsS of the py complex are estimated to be 16.9 kJ mol –1 and 66.6 J
K–1 mol–1 from the chemical shift values. Thus spin-crossover between intermediate and low spin states in the py
and 4-CNpy complexes is definite.
Spin states of [Fe(OMTPP)L2]ClO4
4.0
OETPP-Py (S=3/2– 1/2)
3.0
/
OETPP-DMAP (S = 1/2)
B

eff
OMTPP-Py (S =1/2)
OMTPP-DMAP (S = 1/2)
2.0
1.0
0
Fig. 8 Temperature dependence of DEQ of
OMTPP and OETPP complexes.
100
200
300
Temperature/K
Fig. 9 Temperature dependence of magnetic moments of
OMTPP and OETPP complexes.
We extend the discussion to the methyl analogues [Fe(OMTPP)L2]+ with py and dmap [ref. 6]. Mössbauer spectra
indicate that both py and dmap complexes maintain the S = 1/2 state over the temperature range 77 – 300 K (Fig. 8),
and this is confirmed by SQUID magnetometry (Fig. 9). These results unexpectedly suggest that the spin states in
the OMTPP complexes are quite different from those of the structurally related OETPP complexes. Furthermore, the
magnetic moments measured for the solution samples confusingly show that the OMTPP-py complex behaves as a
spin-crossover complex just like the OETPP-py complex [ref. 7].
Ref. 6. Y. Ohgo et al., Eur. J. Inorg. Chem. (2004) 798–809. Ref. 7. T. Ikeue et al., Inorg. Chem. 42 (2003) 5560–5571.
Origin of difference in spin-crossover behaviour
[Fe(OMTPP)(py)2]ClO4
V = 19.81 Å3
Fe–Nax = 2.058 Å
Fe–Np = 1.963 Å
d = 1.406 g cm–3
V = 18.77 Å3
Fe–Nax = 2.024 Å
Fe–Np = 1.973 Å
d = 1.460 g cm–3
[Fe(OETPP)(py)2]ClO4
V = 32.08 Å3
Fe–Nax = 2.201 Å
Fe–Np = 1.985 Å
d = 1.296 g cm–3
V = 23.19 Å3
Fe–Nax = 1.993 Å
Fe–Np = 1.957 Å
d = 1.388 g cm–3
Fig. 10 Temperature-dependent orientation change of the pyridine ligands in the cavities of
[Fe(OMTPP)(py)2]ClO4 and [Fe(OETPP)(py)2]ClO4 from 298 K (left) to 80 K (right).
The key is the diference in the molecular structures, especially in the cavity around the axial ligand [ref. 6]. While
the Fe–Nax lengths of OMTPP-py hardly change with temperature, those of OETPP-py contract on lowering the
temperature. This difference is induced by a difference in the molecular packing: OETPP-py molecules are more
loosely packed than OMTPP-py molecules. This leads to a large contraction of the cavity size (V) of OETPP-py on
cooling. Thus we can conclude that the large cavity around the axial ligands is essential and important for the spincrossover process.
Abbreviations
porphyrins
Me
Me
Et
Ph
Ph
N
H
Me
Ph
Me
Me
N
H
Me
Ph
Ph
Me
(OMTPP)H2
2,3,7,8,12,13,17,18-octamethyl5,10, 15,20-tetraphyenylporphyrin
Et
Et
Me
Me
H
N
Et
Ph
Ph
N
C H E t2
E t2H C
Et
(OETPP)H2
2,3,7,8,12,13,17,18-octaethyl-5,10,
15,20-tetraphyenylporphyrin
(TEtPrP)H2
5,10,15,20-tetrakis(1-ethylpropyl)porphyrin
axial ligands
H 3C
N
H
N
N
H
N
Et
Me
N
H
N
Et
C H E t2
Et
N
H
N
E t2H C
Ph
Et
N
N
Et
N
N
CH3
CN
N
H
N
N
H
N
N
N
O
H im
py
dm ap
4 -C N p y
th f
Me
Me
Et
Et
(EtioPc)H2
2,7,12,17-tetraethyl-3,6,11,18tetramethylporphycene