Octahedral non-heme oxo and non-oxo Fe(IV) complexes: An experimental/theoretical comparison

Article abstract of DOI:10.1021/ja063590v

Electron-transfer series are described for three ferric complexes of the pentadentate ligand 4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane-1-acetate (Me₃cyclam-acetate) with axial chloride, fluoride, and azide ligands. These complexes can

Full text of DOI:10.1021/ja063590v

Published on Web 09/22/2006
Octahedral Non-Heme Oxo and Non-Oxo Fe(IV) Complexes:
An Experimental/Theoretical Comparison
†
‡
John F. Berry, Eckhard Bill, Eberhard Bothe, Frank Neese, and Karl Wieghardt*
Contribution from the Max-Planck-Institut f u¨ r Bioanorganische Chemie, Stiftstrasse 34-36,
D-45470 M u¨ lheim an der Ruhr, Germany
Received May 23, 2006; E-mail: wieghardt@mpi-muelheim.mpg.de
Abstract: Electron-transfer series are described for three ferric complexes of the pentadentate ligand 4,8,11-
trimethyl-1,4,8,11-tetraazacyclotetradecane-1-acetate (Me cyclam-acetate) with axial chloride, fluoride, and
3
azide ligands. These complexes can all be reduced coulometrically to their Fe(II) analogs and oxidized
reversibly to the corresponding Fe(IV) species. The Fe(II), Fe(III), and Fe(IV) species have been studied
spectroscopically and their UV-vis, M o¨ ssbauer, EPR, and IR spectra are presented. The fluoro species
n+
[
(
3
(Me cyclam-acetate)FeF] (n ) 0, 1, 2) have been studied computationally using density functional theory
2+
DFT), and the electronic structure of the Fe(IV) dication [(Me
that of the isoelectronic Fe(IV) oxo cation [(Me
3
cyclam-acetate)FeF] is compared with
cyclam-acetate)FeO] ; the different properties of the two
+
3
species are mainly due to the significantly covalent FedO π bonds in the latter.
Introduction
highly oxidized corrolate complexes, some of which have been
7
claimed as examples of Fe(IV) complexes, but corroleate
Iron is the most prominent transition metal in biological
systems and is known to play an active role in the catalytic
cycles of many metalloenzymes. By far, the most common
oxidation states of iron in proteins are the +2 and +3 states,
though higher oxidation states (+4 and +5) are often proposed
for specific intermediates in oxygen-activating enzymes and
8
ligands are also fairly easily oxidized, and it is believed that
1
these complexes are, in fact, Fe(III) complexes of corrolate
9
,10
radicals.
In non-heme iron systems, the ligands bound to iron are
generally considered to be redox-innocent, and intermediates
containing Fe(IV) or Fe(V) have been postulated, and a high-
spin Fe(IV)-oxo intermediate has been observed and studied
spectroscopically in the case of the enzyme taurine/R-ketoglu-
2
-5
model systems.
Because of these proposals, the chemistry
of heme and non-heme iron in its high valent states is currently
of intense interest, and synthetic routes to stable forms of these
species have been sought.
1
1
tarate dioxygenase. In addition to work done by other groups
1
2-17
in the synthesis and characterization of Fe(IV) complexes,
A problem encountered in the chemistry of heme complexes
in their higher oxidation states is that the porphyrin ligand itself
can also be oxidized forming a π radical. In fact, in the class of
oxidizing enzymes known as cytochromes P450, intermediates
known as “Compound I” are believed to be the most active
species in the catalytic cycle ultimately responsible for the
hydroxylation of C-H bonds in substrates and are also believed
to contain an Fe(IV)-oxo unit coordinated to an oxidized
18
our laboratory has produced evidence for mono- and di-
19
18,20
nuclear Fe(IV) complexes, Fe(V) complexes,
and recently
21
even Fe(VI) species. For the mononuclear complexes, we have
(7) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41, 5433.
(
8) Shen, J.; Shao, J.; Ou, Z.; E, W.; Koszarna, B.; Gryko, D. T.; Kadish, K.
M. Inorg. Chem. 2006, 45, 2251.
(9) Nardis, S.; Paolesse, R.; Licoccia, S.; Fronczek, F. R.; Vicente, M. G. H.;
Shokhireva, T. K.; Cai, S.; Walker, F. A. Inorg. Chem. 2005, 44, 7030.
10) Walker, F. A.; Licoccia, S.; Paolesse, R. J. Inorg. Biochem. 2006, 100,
810.
(
(
6
porphyrin radical. In such species, the oxidation state of the
iron would actually be higher if it were not for the oxidation of
the porphyrin. Similar considerations have led to ambiguity in
11) Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M.; Krebs, C.
Biochemistry 2003, 42, 7497.
(12) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252.
(
13) Cummins, C. C.; Schrock, R. R. Inorg. Chem. 1994, 33, 395.
†
(14) Chanda, A.; Popescu, D.-L.; de Oliveira, F. T.; Bominaar, E. L.; Ryabov,
Current address: Department of Chemistry, University of Wisconsins
A. D.; M u¨ nck, E.; Collins, T. J. J. Inorg. Biochem. 2006, 100, 606.
Madison, 1101 University Avenue, Madison, WI 53706.
(
15) Jensen, M. P.; Costas, M.; Ho, R. Y. N.; Kaizer, J.; Payeras, A. M. I.;
M u¨ nck, E.; Que, L., Jr.; Rohde, J. U.; Stubna, A. J. Am. Chem. Soc. 2005,
127, 10512.
‡
Current address: Institut f u¨ r Physicalische und Theoretische Chemie,
Universit a¨ t Bonn, D-53115 Bonn, Germany.
(
(
(
(
(
(
1) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry; University
(16) Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X. P.; M u¨ nck, E.; Que,
L., Jr.; Bakac, A. Angew. Chem., Int. Ed. 2005, 44, 6871.
(17) Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X.; M u¨ nck, E.; Que, L.,
Jr.; Bakac, A. Angew. Chem., Int. Ed. 2006, 45, 340.
(18) Grapperhaus, C. A.; Mienert, B.; Bill, E.; Weyherm u¨ ller, T.; Wieghardt,
K. Inorg. Chem. 2000, 39, 5306.
(19) Slep, L. D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyherm u¨ ller, T.; Bill,
E.; Bothe, E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 15554.
(20) Meyer, K.; Bill, E.; Mienert, B.; Weyherm u¨ ller, T.; Wieghardt, K. J. Am.
Chem. Soc. 1999, 121, 4859.
(21) Berry, J. F.; Bill, E.; Bothe, E.; George, S. D.; Mienert, B.; Neese, F.;
Wieghardt, K. Science 2006, 312, 1937.
Science Books: Mill Valley, CA, 1994.
2) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Muller, J.; Lippard,
S. J. Angew. Chem., Int. Ed. 2001, 40, 2782.
3) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004,
1
04, 939.
4) Bassan, A.; Blomberg, M. R. A.; Siegbahn, P. E. M.; Que, L., Jr. Angew.
Chem., Int. Ed. 2005, 44, 2939.
5) See the following special issue dedicated to Fe(IV) chemistry: Ghosh, A.
J. Inorg. Biochem. 2006, 100, 419.
6) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV. 1996,
9
6, 2841.
10.1021/ja063590v CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 13515-13528
9
13515

A R T I C L E S
Scheme 1
Berry et al.
can be reversibly oxidized to their corresponding Fe(IV) species
(
such oxidations are irreversible for cyclam-acetate com-
24
plexes). Third, the mononuclear azido-Fe(IV) complex [(Me3-
cyclam-acetate)FeN3](PF6)2 (hereafter designated 3ox) has been
found to be photoactive producing the diamagnetic nitrido-
VI
2+
Fe(VI) species [(Me3cyclam-acetate)Fe N] upon irradiation
21
with 650 nm light in acetonitrile solution. Thus, the coordina-
tion chemistry of the Me3cyclam-acetate ligand with iron
extends access to high-valent Fe(IV) f Fe(VI).
In studying the first observation, an understanding of the
effects (mainly steric and electronic effects) responsible for the
spin state of ferric compounds of these ligands has been
2
5
reached, owing in large part to the work done by the
2
6
Meyerstein group on alkylated and nonalkylated macrocyclic
ligands and their complexes. The second observation allows us
to gain unique insights into the nature of iron in its tetravalent
state as the ligand field is changed in a controlled manner. This
2+
is possible by studying [(Me3cyclam-acetate)FeX] species
-
-
-
with various X anions (XdCl , N3 , F ), which is the basis of
the experimental part of this report. The effect of a terminal
oxo ligand on the electronic structure is also probed in this report
utilized the supporting tetradentate ligand cyclam (see Scheme
2+
theoretically by comparing [(Me3cyclam-acetate)FeF] with
1) because of its properties as a spectroscopically and redox-
+
the isoelectronic [(Me3cyclam-acetate)FeO] species, which
innocent ligand, which can coordinate iron with four equatorial
nitrogen donors akin to the coordination mode of a porphyrin
ligand.
A high-valent Fe(V)-nitrido species formulated as [(cyclam)-
FeN(N3)]ClO4 was characterized by our group,20 as well as a
similar Fe(V)-nitrido complex [(cyclam-acetate)FeN]PF6 hav-
ing the modified cyclam-acetate ligand,18 which has been
characterized in more detail than the former complex. In
particular, it should be noted that the original reports of
2+ 18
is similar to [(cyclam-acetate)FeO]
and the handful of Fe-
(
IV)-oxo complexes which have been characterized in the Que
1
5-17,23,27-31
lab.
Finally, the third observation has enabled the
preparation and spectroscopic characterization of the second
2
1
Fe(VI) complex yet to be prepared.
Results and Discussion
Synthesis. The oxo-bridged diiron(III) species (Me3cyclam-
acetate)Fe-O-FeCl3 is a convenient starting material for the
[
(cyclam)FeN(N3)]ClO4 and [(cyclam-acetate)FeN]PF6 had
3
synthesis of other complexes of the Me cyclam-acetate ligand.
claimed both of the complexes to be high spin (S ) /2), whereas
further study of the latter complex has conclusively shown it
to be low-spin with a nearly orbitally degenerate doublet ground
state. Using the cyclam-acetate ligand, we also reported that
the complex [(cyclam-acetate)Fe(OTf)]PF6 reacts with ozone
to produce a very reactive green species which was formulated
3
As shown in Scheme 2, the -(µ-O)FeCl3 moiety is easily
hydrolyzed and removed as rust and the axial ligand can be
replaced by chloride, fluoride, or azide. Interestingly, if the
hydrolysis is performed in a concentrated solution of KPF6,
precipitation of the bright-yellow chloro complex [(Me3cyclam-
acetate)FeCl]PF6, 1, occurs, whereas if a dilute solution of KPF6
is used, only rust precipitates and if the resulting yellow filtrate
is allowed to stand, crystals of the brilliant-yellow fluoro
complex [(Me3cyclam-acetate)FeF]PF6, 2, form over the course
of several days (this time allows the slow acid-catalyzed
2
2
as an Fe(IV)-oxo species ([(cyclam-acetate)FeO]PF6) based on
its M o¨ ssbauer spectrum.18 After this work, the group of Que
reported that the corrosponding Fe(IV)-oxo complex of the
ligand Me4cyclam ([(Me4cyclam)FeO(NCCH3)](OTf)2) was
stable enough that it could be crystallized and its crystal structure
2
3
hydrolysis of the PF anions to occur). It was subsequently found
has been obtained.
6
that 2 can be formed in much better yields and purity by heating
Because the addition of methyl groups to the cyclam ligand
has been shown to be an effective method of stabilizing high
valent iron species, the addition of methyl groups to the
cyclam-acetate ligand so as to produce Me3cyclam-acetate is
a logical extension of our previous work in this area with the
original aim of further stabilizing Fe(IV) and Fe(V) species. In
using the Me3cyclam-acetate ligand, three surprising discover-
ies were made. First, the Fe(III) complexes of Me3cyclam-
acetate were found to be high spin (whereas trans complexes
an aqueous solution of 1 in air, which accelerates the hydrolysis
of the PF6 anions. After a few minutes of heating, a rusty brown
precipitate appears in the flask, and the resulting filtrate is
brilliant-yellow in color and yields crystals of 2 upon standing.
Using a concentrated solution of KPF6 and NaN3, the azido
(
25) Berry, J. F.; Bill, E.; Garcia-Serres, R.; Neese, F.; Weyherm u¨ ller, T.;
Wieghardt, K. Inorg. Chem. 2006, 45, 2027.
(26) Meyerstein, D. Coord. Chem. ReV. 1999, 186, 141.
(
27) Lim, M. H.; Rohde, J. U.; Stubna, A.; Bukowski, M. R.; Costas, M.; Ho,
2
4
of cyclam-acetate and cyclam itself are always low spin).
Second, octahedral Fe(III) complexes of Me3cyclam-acetate
R. Y. N.; M u¨ nck, E.; Nam, W.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 3665.
(28) Bukowski, M. R.; Koehntop, K. D.; Stubna, A.; Bominaar, E. L.; Halfen,
J. A.; M u¨ nck, E.; Nam, W.; Que, L., Jr. Science 2005, 310, 1000.
(
(
(
22) Aliaga-Alcalde, N.; George, S. D.; Mienert, B.; Bill, E.; Wieghardt, K.;
(29) Sastri, C. V.; Park, M. J.; Ohta, T.; Jackson, T. A.; Stubna, A.; Seo, M. S.;
Lee, J.; Kim, J.; Kitagawa, T.; M u¨ nck, E.; Que, L., Jr.; Nam, W. J. Am.
Chem. Soc. 2005, 127, 12494.
(30) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.; Woodrum, N. L.; Cramer, C.
J.; Que, L., Jr. Angew. Chem., Int. Ed. 2005, 44, 3690.
(31) Rohde, J. U.; Que, L., Jr. Angew. Chem., Int. Ed. 2005, 44, 2255.
Neese, F. Angew. Chem., Int. Ed. 2005, 44, 2908.
23) Rohde, J. U.; In, J. H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.;
Stubna, A.; M u¨ nck, E.; Nam, W.; Que, L., Jr. Science 2003, 299, 1037.
24) Berry, J. F.; Bill, E.; Bothe, E.; Weyherm u¨ ller, T.; Wieghardt, K. J. Am.
Chem. Soc. 2005, 127, 11550.
13516 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
Table 1. M o¨ ssbauer Parameters from Zero-Field Spectra of
Frozen Acetonitrile Solutions of the Complexes
, mm s-
1
∆E
Q
, mm s-
1
T, K
ref.
compound
δ
1
1
1
2
2
2
3
3
3
3
red
1.08
1.10
0.08
0.39
1.11
0.02
0.33
0.35
1.08
0.11
2.75
3.49
2.40
0.95
3.71
2.43
2.21
0.84
3.17
1.92
80
80
80
4.2
80
80
80
200
80
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25
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24
24
24
24
red - MeCN bound
ox
a
red
ox
- low spin
- high spin
red
ox
80
Figure 1. Magnetic susceptibility data for 1 from 2 to 300 K, which have
been corrected for intrinsic diamagnetism as well as temperature-independent
paramagnetism (TIP). The solid line represents a least-squares fit of the
data based on a spin Hamiltonian simulation. (Inset) Fitting parameters.
a
This measurement was done on a solid sample.
-1
M o¨ ssbauer data, which both suggest a D value of ∼ -4.5 cm
-
1
(
vide infra). By fixing the D value to -4.5 cm and allowing
Scheme 2
a Weiss constant to be refined, a successful fit can be obtained,
which is the fit shown in Figure 1. The Weiss constant of -1.4
-
1
cm is most likely due to Fe‚‚‚Fe antiferromagnetic coupling
between iron centers in a chain structure.
H ) BS gâ BB + BS ‚D ‚ BS
(1)
The M o¨ ssbauer parameters of all of the species described in
this paper are given in Tables 1 and 2 for spectra measured
either without or with an applied magnetic field, respectively.
The M o¨ ssbauer spectrum of a solid sample of 1 at 80 K and no
externally applied magnetic field shows considerable spin
relaxation with rates that are intermediate to the time scale of
the nuclear transition and Larmor precession. The spectrum
consists of a broad asymmetric trough from which the isomer
shift (δ) and quadrupole splitting (∆EQ) are not readily deduced.
To determine these parameters, the M o¨ ssbauer spectrum of the
solid was measured at 4.2 K in a strong applied magnetic field
of 7.0 T (see Figure 2a). The spectrum is well resolved but has
only weak magnetic splitting typical of a diamagnetic species
showing no internal field at the iron nucleus. The values of δ
and ∆EQ could be determined from a simulation with the usual
nuclear Hamiltonian and the applied field to be 0.38 and -1.45
complex [(Me3cyclam-acetate)FeN3]PF6, 3, forms as a deep-
red precipitate.24 Compound 3 is light sensitive and solutions
II
bleach with photoreduction to [(Me3cyclam-acetate)Fe -
+
21
(
NCCH3)] if left in ambient light for even a few hours, so it
is important that 3 is crystallized in the dark.
-1
mm s , respectively, and the asymmetry parameter of the
Characterization of the Fe(III) Species. The crystal struc-
tures and some of the spectroscopic properties of 2 and 3 have
been previously reported, which have shown the molecules to
adopt the trans-III conformation of the cyclam ligand as shown
in Scheme 2.24
electric field gradient (EFG) tensor, η, is 0.3. The parameters
are rather unusual for high-spin Fe(III) coordination compounds;
particularly the high quadrupole splitting is remarkable and
largely exceeds the values found for the other ferric high-spin
species in Table 2. The M o¨ ssbauer parameters instead resemble
those found for µ-oxo bridged di-iron complexes like the (Me3-
The spectroscopic properties of 1 in the solid state, for which
good crystals could not be obtained, are in agreement with the
same structural formulation. The magnetic susceptibility of 1
is shown in Figure 1, where a temperature-independent value
-1
cyclam-acetate)FeOFeCl3 starting complex (δ ) 0.44 mm s ,
-
1
|∆EQ| ) 1.46 mm s ), which suggests that the carboxylate
should function as a bridging ligand in a chain structure and
that at least one of the Fe-O(carboxylate) bonds is quite strong.
This may explain also the diamagnetism of the sample found
in the spectrum measured at liquid helium temperature. We
emphasize that the presence of a true µ-oxo dimer motif can be
excluded because such a bridging ligand would mediate strong
-
1
of T ) 4.36 emu K mol is seen above 50 K, which is in
5
agreement with the spin only value for an S ) /2 ground state:
-
1
øT ) (1/2)(S(S+1)) ) 4.375 emu K mol . Below 50 K, there
is a steep decrease in øT, and the value falls below 2 emu K
-
1
mol at 2 K, which exceeds the field saturation behavior
expected for the applied field of B ) 1 T, and thus indicates
either zero-field splitting or weak intermolecular spin coupling
or both. A fit of the data using the spin Hamiltonian given in
eq 1 (g is the Land e´ factor, D is the axial zero-field splitting
parameter, â is the Bohr magneton, B is the magnetic field,
and S is the spin operator), which only accounts for axial zero-
2
exchange interaction between the ferric ions of the order 10
-
1
cm , in contrast to the weak interaction found in the magnetic
data as well as the paramagnetic relaxation observed in the
M o¨ ssbauer spectrum recorded at 80 K.
Crystals of 1 are poorly diffracting, and only a rough structure
was determined, which is so poor that the data will not be
presented here. Nevertheless, the connectivity which appears
in the structure is shown in Scheme 3 and it should be noted
-
1
field splitting, results in a value of D ) (10 cm , but this
was found to be too large to be in agreement with the EPR and
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13517

A R T I C L E S
Berry et al.
ref.
Table 2. Parameters from Applied Field M o¨ ssbauer Measurements of Complexes in Acetonitrile Solution at 4.2 K
, mm s-
1
∆E
, mm s-1,
η
D, cm-1, E/D
(R,
â,
γ
), degd
g
x
, g
y
, g
z
(Axx, Ayy, Azz), T
compound
δ
Q
1
1
2
2
2
3
3
3
3
3
(solid)b,c
0.38
0.48
0.39
1.11
0.02
0.34
0.41
0.34
0.42
0.12
-1.45, 0.3
-0.40, 0.7
-0.95, 0.40
+3.71, 0.2
+2.45, 0.2
-2.28, 0.2
-0.93, 0
-2.43, 0.3
-0.32, 0
+1.92, 0.1
n.d.
n.d.
11, 111, 0
n.d.
n.d.
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24
-4.8(4), 0.10
+0.4(2), 0.12
-2.0(2), 0.20
+22.(1), 0
-
2.00
2.00
2.00
2.00
1.78, 2.26, 2.70
2.00
1.78, 2.26, 2.70
2.00
2.00
-22.0, -22.0, -19.5
-21.3, -21.3, -21.0
-15.5, -22.5, -13.4
-20.4, -19.1, -2.5e
-31.6, +24.8, -49.1
-20.1g
-
red
ox
0, -8, 0
-
- low spinb
0, 66, 0
-
b
f
- high spin
- low spin
- high spin
ox
+4.0(5), 0.3
-
0, 69, 0
-
-50.2, +47.4, -5.8
f
-20.8g
+2.0(2), 0.3
+24.(1), 0.02
-
-15.4, -17.8, -2.3
a
-1
The experimental errors are typically 0.02 mms for δ and ∆EQ; 0.01 for E/D; 10° for R,â,γ; 0.01 for g values; and 0.1 for A values. Those for D are
given in brackets. This measurement was done on a solid sample. In solid samples, 1 does not have an apical Cl ligand. d These are Euler angles
describing rotation of the EFG tensor with respect to the principal axes of the zero-field splitting tensor. The Azz value is not defined. Taken from the EPR
measurement of 3 in solution (see Figure 4). Constrained to isotropic values.
b
c
-
e
f
g
applied magnetic fields of 4.0 and 7.0 T (Figure 2b,c). These
spectra show large paramagnetic hyperfine patterns typical of
ferric high-spin ions with large zero-field splitting and were
5
fitted using the spin-Hamiltonian given in eq 2 with S ) /2.
2
z
1
3
2
2
H ) gâ BB BS + D BS - S(S + 1) + E/D( BS - BS ) +
[
x
y
]
BS ‚ A ‚ BI - g â BB BI + H (2)
N
N
Q
Here, g, â, B, S, and D are the same as described in eq 1; E is
the rhombicity term of the zero-field splitting tensor, A is the
hyperfine coupling tensor, I is the nuclear spin operator, gN is
5
7
the nuclear g factor (for Fe), âN is the nuclear magneton, and
HQ is the Hamiltonian describing the quadrupole interaction in
the excited-state of the iron (its main parameters being ∆EQ
and η). The wide split hyperfine pattern with narrow lines and
distinct intensity ratios reveals the presence of a strong internal
field of about 46 T at the M o¨ ssbauer nucleus and an “easy axis
of magnetization”. This is typical of a paramagnetic complex
with small rhombicity and a large negative D value, or a fully
rhombic system. Because the rhombicity parameter of 1 is found
to be small from the EPR spectra, E/D ) 0.1 (see below), the
axial parameter D could be reliably determined from the field
dependence of the M o¨ ssbauer spectra at 3 and 7 T. The
simulations performed in the limit of fast spin relaxation yield
Figure 2. M o¨ ssbauer spectra of solid (A) and acetonitrile solutions (B) of
taken in applied magnetic field at the field strengths given at 4.2 K. The
solid lines represent simulations of the spectra with S ) 0 for (A) and S )
1
5
/
2 for (B) and parameters given in Table 2.
Scheme 3
-
1
D ) -4.8 cm , as well as the components of the hyperfine
coupling tensor A/gNâN ) (-22.0, -22.0, -19.5) T, with the
anisotropy constraint to axial symmetry. The latter values
represent a nearly isotropic hyperfine tensor that is in agreement
with the assignment of high-spin Fe(III) in which each d orbital
contains one unpaired electron. Finally, the values of δ, ∆EQ,
-
1
and η determined from these spectra are 0.48 mm s , -0.40
-
1
mm s , and 0.7, respectively, and are in the range expected
for high-spin Fe(III) complexes.
The EPR spectrum of 1 in butyronitrile solution at 10 K is
qualitatively similar to that of the corresponding fluoro complex
(2) and consists of a widely split and broadened S ) /2 signal
that it appears that the molecules form chains with bridging
carboxylate groups, according to the spectroscopic observations.
The chloride ions and hexafluorophosphate anions are separated
from the positively charged chains.
5
due to large zero-field splitting exceeding the Zeeman splitting
(D . gâB). Resolved derivative signals are observed at effective
g values of 8.0, 5.6, and 4.0, which can be assigned to the | S,
To break up the antiferromagnetic effects of the chain
structure and determine the properties of the individual mol-
ecules, the M o¨ ssbauer spectrum of 1 was measured in aceto-
5
1
mS > ) | /2, ( /2 > Kramers doublet (geff ) 8 and 4) and the
5
3
| /2, ( /2 > Kramers doublet (geff ) 5.6) for a system with a
small rhombicity parameter of E/D ≈ 0.1. The relative intensities
at various temperatures in the range 4.2-20 K indicate a
negative axial zero-field splitting parameter. The spectrum in
Figure S1 was simulated similarly to that of 2, and the zero-
57
nitrile solution (∼0.1 mM, 40% enriched with Fe). Again, the
M o¨ ssbauer spectrum at 80 K with no externally applied field
was not conducive to interpretation because of intermediate spin
relaxation, so two spectra were measured at 4.2 K at externally
13518 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
Figure 4. X-band EPR spectrum of 3 in frozen acetonitrile solution in the
presence of excess NBu4PF6 at 20 K. The major g values are labeled: those
for the high-spin species in red, and those for the low-spin species in brown.
Figure 3. Variable temperature magnetic susceptibility of 3 plotted as øT
vs T. The black circles represent the data and the solid line represents a fit
of the data as described in the text with the fitting parameters given in the
inset. The dashed line is a measure of how much of the sample is high spin
based on the simulation and corresponds to the scale on the right-hand side
of the plot.
range. The functions FHS and FLS were not refined, and their
corresponding g and D values were fixed to the values given
above. The optimized parameters were ∆H, TC, and PHS, which
refined to the values given in the inset to Figure 3. The enthalpy
field splitting parameters D ) -4.0 and E/D ) 0.12 were
determined, which are similar to those determined by M o¨ ssbauer
spectroscopy (vide supra).
-
1
difference is ∆H ) 813 J mol and may be used to calculate
the ∆S of the spin crossover process (∆H ) T ∆S), which is
The properties of 2 have been previously reported25 as have
C
-
1
-1
some of the properties of 3,24 although we present here a more
7.5 J mol
K . It should be noted that 62% of the sample
does not undergo spin crossover and remains high spin in the
entire temperature range, and this agrees well with the M o¨ ss-
bauer spectrum of the solid at 4.2 K (vide infra). Although it is
unusual that a portion of the sample remains high spin, it is not
without precedent, and several examples of similar systems are
detailed discussion of 3, with particular emphasis on its spin-
crossover behavior. The unusual temperature dependence of øT
found for 3 (Figure 3) is reproducibly observed in several
experiments and is a result of spin crossover. At room
-
1
temperature, the øT value is 4.3 emu K mol , very close to
3
2-37
5
known in the literature.
Compound 3 shows a sharp EPR spectrum in frozen aceto-
the spin-only value expected for an S ) /2 system (4.375 emu
-
1
K mol ). This value drops fairly steadily as the temperature is
-
1
nitrile solution in the presence of NBu PF . The spectrum,
decreased, ultimately reaching a value of ∼ 2 emu K mol at
the lowest temperatures measured (2 K). This behavior can be
modeled as arising from a variety of different sources such as
zero-field splitting or intermolecular spin pairing, and indeed a
hypothetical fit of the magnetic susceptibility data with the spin
Hamiltonian given in eq 1 treating only axial zero-field splitting
gives a reasonable fit of the data, but a zero-field splitting value
4
6
shown in Figure 4, can be thought of as having two main
components, one having signals at geff ) 9.0, 4.4, and 4.0, and
the other having signals at g ) 2.70, 2.26, and 1.78. The g
values of the latter species are in good agreement with a low-
spin iron(III) formulation (and have a larger anisotropic spread
than the g values of [(cyclam-acetate)FeN3]PF6, which are 2.55,
2.26, and 1.91), whereas the higher geff values of the former
species clearly indicate a species with a higher total spin, and
-
1
D which is unphysically large (D ) ∼50 cm ).
The fact that a spin-crossover phenomenon is involved is best
seen in the EPR and M o¨ ssbauer spectra of the compound (vide
infra), in which both high-spin and low-spin components are
seen at low temperatures. On the basis of this information, it is
reasonable to simulate the susceptibility data in Figure 3 with
an ideal solution spin crossover model which also includes the
zero-field splitting of the high-spin state, as in eq 3.
3
8
these may be assigned to the high-spin species.
It is remarkable that both high- and low-spin species are
observed in the solution spectra because it means that: a. 3
undergoes spin crossover in frozen solution (not just in the solid
state), b. as was seen in the solid-state susceptibility measure-
ments, the spin crossover is incomplete and a good portion of
the sample remains high spin at low temperatures, and c.
relaxation between the two spin states is slow relative to the
FHS - FLS
øT ) (1 - PHS)
+ FLS + P F
HS HS
∆
R T
H 1
1
1
+ exp
-
(
)
(32) Terzis, A.; Filippakis, S.; Mentzafos, D.; Petrouleas, V.; Malliaris, A. Inorg.
Chem. 1984, 23, 334.
(
(
TC))
(3)
(
33) Timken, M. D.; Hendrickson, D. N.; Sinn, E. Inorg. Chem. 1985, 24, 3947.
34) Chun, H. P.; Bill, E.; Weyherm u¨ ller, T.; Wieghardt, K. Inorg. Chem. 2003,
42, 5612.
(
Here, FHS is a function simulating the high-spin species based
(
35) Chun, H.; Weyherm u¨ ller, T.; Bill, E.; Wieghardt, K. Angew. Chem., Int.
Ed. 2001, 40, 2489.
-
1
on eq 1 with g ) 2.00, and D ) 4.00 cm (also including a
temperature-independent paramagnetism component of 2.14 ×
(
36) Ryabova, N. A.; Ponomarev, V. I.; Zelentsov, V. V.; Atovmyan, L. O.
Kristallografiya 1982, 27, 279.
-
3
-1
1
0
emu K mol ), FLS is a function simulating the low-spin
(37) Ryabova, N. A.; Ponomarev, V. I.; Zelentsov, V. V.; Atovmyan, L. O.
Kristallografiya 1982, 27, 81.
species with g ) 2.00, ∆H is the enthalpy difference between
the high- and low-spin states, R is the gas constant, TC is the
critical temperature (i.e., the temperature at which equal amounts
of high- and low-spin species are present in the portion of the
sample that undergoes spin crossover), and PHS is the amount
of the sample that remains high spin over the whole temperature
(
38) Simulation of the high-spin species has thus far not been possible because
the signals at 9.0, 4.4, and 4.0 do not correspond to any of the expected
5
transitions based on the rhombogram for an S ) /
2
system, though they
are somewhat near the transitions expected for the fully rhombic case (E/D
) 0.33 having geff ) 9.6 and 4.3). It is also possible that these signals
5
arise from a weakly coupled dimer of spin /
2
centers, though preliminary
simulations are not conclusive. A Q-band spectrum of this species was
also measured, but the general shape of the spectrum is the same.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13519

A R T I C L E S
Berry et al.
are negative, implying that the shape of the EFG tensor is
elongated along the O-Fe-X axis of the molecule. This is in
contrast to what is seen for ferric high-spin porphyrin species,
where the sign of ∆EQ is nearly always positive, an effect which
we believe to be due to the π bonding capability of porphyrin
ligands which Me3cyclam-acetate lacks.
With the high-spin subspectrum well defined, it is possible
1
to simulate the minor subspectrum with an S ) /2 model using
the g values taken from the EPR spectrum and the δ and ∆EQ
values from the zero-field spectrum, which allows the asym-
metry parameter of the EFG tensor, η, and the hyperfine splitting
tensor components to be refined. Like its low-spin non-
methylated cyclam-acetate analog, [(cyclam-acetate)FeN3]PF6,
-
1
the ∆EQ value of 3 is negative (∆EQ ) -2.28 mm s , η ) 0.2
-
1
for 3; ∆EQ ) -2.53 mm s , η ) 0.40 for [(cyclam-acetate)-
1
8
FeN3]PF6 ), and the hyperfine tensor components are highly
anisotropic ranging from -49.1 to +24.8 T in 3. In [(cyclam-
acetate)FeN3]PF6, the range is from -41.2 to +35.0 T,18 and
in both complexes the Ayy component is positive and the other
two components are negative.
An applied field M o¨ ssbauer spectrum of 3 was also taken on
a frozen solution of 3 in acetonitrile at 4.2 K (Figure 5, bottom),
which is similar in complexity to the spectrum of the solid.
Again, both high- and low-spin components are observed,
although the ratio of the two species here is roughly 30:70. For
the high-spin subspectrum, the refined parameters (Table 2) are
all quite similar to those observed in the solid except that the
value of ∆EQ appears to be decreased. In the low-spin
subspectrum, however, the values of the A tensor vary more
(from -50.17 to + 47.44 T), though the general pattern that
Ayy > 0; Axx, Azz < 0 is still true.
Figure 5. M o¨ ssbauer spectra of solid 3 (A) and in acetonitrile solutions
(
B) taken in applied magnetic field at the field strengths given at 4.2 K.
5
The lines represent simulations of the spectra with spin S ) /2 for the
1
high-spin component (dashed-dotted line, labeled h.s.), S ) /2 for the low-
spin component (dashed line, labeled l.s.), and parameters as given in Table
2
. The high-spin contribution is calculated in the slow-relaxation limit in
the solid (A), 67% relative intensity, and in fast relaxation in solution (B),
0% relative intensity. From the low-spin component, spin relaxation was
3
found to be fast in the solid (A) but slow for the solution (B).
time scale of EPR (and also relative to the M o¨ ssbauer time scale
The spectroscopic data for the low-spin species warrants
further discussion. The anisotropy of the g values may be
interpreted in terms of the ligand field model developed by
(Vide infra)).
Both high- and low-spin species are seen in the low-
temperature M o¨ ssbauer spectra of 3 taken either on the solid or
on a frozen solution. At 200 K, an acetonitrile solution of 3
shows a single asymmetric quadrupole doublet (with ap-
3
9
40
5
Griffith and Taylor for description of the 3d low-spin
configuration in octahedral symmetry. Analysis of the g values
2
3
yields a ground state with ∼ 97% (dxy) (dxz, dyz) character and
-
1
-1
proximately δ ) 0.35 mm s , ∆EQ ) 0.84 mm s
)
in this analysis we have chosen the y-axis to be the unique axis
characteristic of a high-spin ferric species, but at 80 K, two
doublets are observed which were deconvoluted, one having
the same M o¨ ssbauer parameters as above and the other having
(implying a hole in the dyz orbital). The splitting ∆/λ between
the (dxz, dyz) and dxy orbitals is 4.58, with a rhombic splitting
between the dxz and dyz orbitals of V/λ ) 2.82, where λ is the
spin orbit coupling parameter. The resulting valence contribution
to the EFG arising from the dyz orbital has its main component
along the y direction. This agrees with the spin Hamiltonian
analysis of the magnetic M o¨ ssbauer spectrum in which the Euler
angle â ≈ 70° is found, which is close to the expected value of
â ) 90° to rotate Vzz (the main component of the EFG tensor)
into the y-axis. The spin-dipolar contribution to the anisotropy
of the magnetic hyperfine tensor A may also be predicted within
this ionic picture. The estimate yields a positive component for
Ayy and negative values for Axx and Azz which fits the
experimental result. Moreover, this general picture agrees with
the previous analysis of [(cyclam-acetate)FeCl]PF6 and [(cy-
clam-acetate)FeN3]PF6, though the t2g splitting in low-spin 3
is less rhombic than for the non-methylated azido species (∆/λ
-
1
a slightly lower δ of 0.33 mm s and a larger ∆EQ of 2.21
-
1
24
mm s , both indicative of low-spin Fe(III). An applied-field
(7 T) M o¨ ssbauer spectrum of solid 3 at 4.2 K was obtained
which shows a complex pattern (Figure 5, top). Knowing the
ratio of high- and low-spin components from the susceptibility
measurement, it is possible to deconvolute this M o¨ ssbauer
spectrum into two parts, one for the high-spin species (account-
ing for roughly 60% of the total iron in the sample) and one
for the low-spin species (accounting for the remaining 40%).
The six most intense lines in the spectrum can be easily
ascribed to the high-spin component by their large magnetic
splitting indicating an internal field of about 45 T, and by
comparison with the applied field M o¨ ssbauer spectra of 1 and
2
. Spin Hamiltonian analysis of this subspectrum yields the
parameters given in Table 2, and it should be noted here that δ
)
5.7, V/λ ) 3.9) and resembles more the nonmethylated chloro
-
1
and ∆EQ of 0.41 and -0.93 mm s , respectively, agree well
with the formulation as high-spin Fe(III) and are similar to the
values found in the M o¨ ssbauer spectra of 1 and 2. The same is
true for the hyperfine tensor components. It should also be noted
that the ∆EQ values for all three high-spin species 1, 2, and 3
complex (∆/λ ) 4.84, V/λ ) 2.39). The difference in the two
azido species likely arises from a difference in the Fe-N-N
(
39) Griffith, J. S. Proc. R. Soc. London, Ser. A. 1956, 235, 23.
(40) Taylor, C. P. S. Bichimica Biophysica Acta. 1977, 491, 137.
13520 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
Figure 6. Cyclic voltammograms of 1, 2, and 3 in acetonitrile with 0.1 M
NBu4PF6 supporting electrolyte. The numbers next to each wave are their
corresponding E1/2 values in V vs ferrocene/ferrocenium.
Figure 7. Electronic spectra of 2red, 2, and 2ox obtined during controlled
potential coulometry.
a
Table 3. Electrochemical Data for Cyclam-Acetate and
Me3cyclam-Acetate Complexes
the Fe(IV) chloro complex (vide infra). Coulometric reduction
and oxidation of 1, 2, and 3 allow study of the Fe(II) and Fe-
Fe2
+/
3+
Fe3+/4+
compound
E
1/
2
, V
E
1
/
2
, V
ref.
(
IV) species by various techniques, the results of which are
[
(cyclam-acetate)FeCl]PF6
-0.640
-0.750
1.330
irreversible)
0.990
irreversible)
18
18
detailed below.
(
[
(cyclam-acetate)FeN3]PF6
Characterization of the Fe(II) Species. The reduction
products of 1, 2, and 3 were investigated by spectroelectro-
chemistry in the UV-vis region. The electronic spectra for 3red,
(
1
2
3
-0.290
-0.614
-0.360
1.380
1.230
1.150
this work
this work
24
2
4
3, and 3ox have been previously reported, and those for 2red,
2
, and 2ox are shown in Figure 7. As seen from these spectra,
a
Solvent: MeCN; all potentials are referenced vs the ferrocene/
solutions of 2red and 3red are colorless, and 1red is no
ferrocenium couple.
different. The lack of color indicates that the bands originating
5
5
from the T2gf Eg transition of the ferrous ion must occur at
1000 nm. Also, any charge-transfer bands from the acetate
angle in the two species; in 3, the additional methyl groups may
not allow this angle to become as acute as is found in [(cyclam-
acetate)FeN3]PF6 (132 °).
Electrochemically, 1, 2, and 3 behave similarly in acetonitrile
solutions. Their respective cyclic voltammograms are shown
in Figure 6, and their redox potentials are collected in Table 3
along with those of the analogous cyclam-acetate complexes.
>
or axial groups to the iron are shifted significantly into the UV.
Compounds 2 and 3 were also studied by infrared spectro-
electrochemistry because in both compounds, very intense and
important stretches are found in regions of the spectrum which
are not obscured by the solvent (acetonitrile) or electrolyte
(tetrabutylammonium hexafluorophosphate). Compounds 2 and
Notably, whereas the complexes of cyclam-acetate show only
irreversible waves corresponding to the Fe3
+/4+
process, this
-1
3 have carbonyl stretches at 1680 and 1677 cm , respectively.
-
1
process is reversible for all three complexes of Me3cyclam-
acetate. Compounds 1 and 3 are more easily reduced than their
non-methylated analogs by 350-400 mV, which is in agreement
with the difference in spin state between the two species and
agrees with the observations of the Meyerstein group on the
weaker donor strength of N-alkylated cyclam ligands. In
contrast to the greater stability of the ferrous species in the
complexes of Me3cyclam-acetate, which is a thermodynamic
In compound 3, an intense band at 2090 cm is also present
which is the azido stretch. Infrared spectra of 3red, 3, and 3ox
are shown in Figure 8, which display both of these stretches as
well as some of the absorptions of the solvent and electrolyte,
which emphasize the limitations of these measurements. The
values highlighted in Figure 8 for the N3 and CdO stretching
are also given in Table 4 along with the corresponding values
from 2 and its derivatives. In both cases, the CdO stretch shifts
to lower frequency upon reduction to Fe(II), which is in
agreement with there being a weaker Fe-O bonding interaction
in the ferrous state and therefore more π-delocalization within
the carboxylate moiety. The azido stretch in 3red is also
observed at lower frequency than the corresponding band in 3,
indicating stronger π interactions between the azide ligand and
the ferrous ion than is observed in the case of the ferric ion.
The reduction products of 1, 2, and 3 have also been studied
by M o¨ ssbauer spectroscopy by using electrochemically reduced
2
6
3+/4+
effect, the reversibility of the Fe
represents a kinetic stability of the Fe4 species.
couple in these complexes
+
Of the three methylated complexes, the fluoro complex 2 is
the most difficult to reduce, which agrees well with the fact
that the fluoride ion, a very hard base, prefers to form a strong
bond to the ferric ion, which is a harder acid than the ferrous
ion. The oxidation potentials increase in the series 3 > 2 > 1,
which may be considered a measure of how well each of the
three axial ligands stabilizes the tetravalent iron center. The fact
that the oxidation potential of the chloro complex is the highest
may also in part be due to steric crowding. The chloro anion
may not be able to form a short bond to the iron because it
could be hindered by the methyl groups. Indeed, we believe
this to be the main reason for the relative kinetic instability of
5
7
samples of 1, 2, and 3 enriched 40% in Fe. Reduction of 1 is
complex. The M o¨ ssbauer spectrum of coulometrically reduced
1 shows two overlapped doublets (Figure S2) having the
-
1
following parameters (see Table 1): δ1 ) 1.08 mm s , ∆EQ1
-
1
-1
-1
) 2.75 mm s ; δ2 ) 1.10 mm s , ∆EQ2 ) 3.49 mm s .
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13521

A R T I C L E S
Berry et al.
Scheme 4
Figure 8. Infrared spectra of 3red, 3, and 3ox (with the charge n ) 0, 1,
-
1
or 2, respectively) in the region between 1400 and 2300 cm taken during
chronoamperometry to generate the Fe(II) and Fe(IV) species. The dominant
shown in Figure S3 along with a spectrum of the same sample
measured in an applied magnetic field of 7 T. This was
simulated using the spin Hamiltonian analysis (see eq 2) for S
-
1
bands at 2300, 2250, and 1500-1400 cm are absorptions of the solvent
and electrolyte, shown to emphasize the limited window of the system.
Numbers next to each band are the frequencies of the peaks, and a diagram
of the molecule is shown in the inset, highlighting the two different
absorptions seen.
)
2, resulting in the parametrized values given in Table 2. The
42
electronic g values were fixed to be equal to 2.
Characterization of the Fe(IV) Species. Coulometric oxida-
tion of 1 at a potential of ∼1.5 V vs ferrocene/ferrocenium was
monitored by the changes in the UV-vis spectrum. For a simple
Table 4. IR Data for 2red, 2, 2ox, 3red, 3, and 3ox
1
N
3
stretch, cm-
1
C
d
O stretch, cm-
reference
-
electron-transfer process (e.g., 1 f 1ox + e ) the changes in
2
2
2
3
3
3
red
1630
1680
1753
1633
1677
1732
this work
this work
this work
24
24
24
the UV-vis spectrum should be isosbestic during the entire
coulometry as the spectrum of 1 disappears and the spectrum
of 1ox appears. In practice, however, isosbestic behavior is seen
during the first ∼100-200 s of the coulometry, but the newly
appearing band at 460 nm thereafter loses intensity nonisos-
bestically. From this experiment it is apparent that the oxidation
product 1ox is not stable and decomposes during the time of
the coulometry (even though the coulometry was carried out at
ox
red
2068
2090
2037
ox
Both species may be assigned to high-spin Fe(II) complexes
due to their large isomer shifts and quadrupole splittings.
Because the two species have similar M o¨ ssbauer parameters,
they are also presumed to be structurally very similar, and it is
proposed that both are six-coordinate and that one species
contains an axial chloro ligand whereas in the other, this ligand
has been replaced by acetonitrile.41 This proposal, however,
presents the problem of determining which species is actually
-
25 °C). Thus, it was not possible to produce a pure sample of
1
ox for spectroscopic study, but an aliquot of the reaction
mixture taken during the coulometry but before decomposition
was frozen and a M o¨ ssbauer spectrum was measured at 80 K
(see Figure S4) where two species are clearly observable. The
main signal is broad and ill defined and is typical of the high-
1
red and which is the acetonitrile-bound species. To determine
spin ferric species 1, but a sharp quadrupole doublet is also
this, reduction of 1 was undertaken in acetonitrile with a mixture
of electrolytes. A 0.1 M electrolyte solution was used, but
instead of using NBu4PF6 as before, a mixture of 9:1 NBu4PF6
and NBu4Cl was used, which provides a large excess of chloride
ions in solution. The M o¨ ssbauer spectrum of the resulting
reduced species shows only one major signal having the same
parameters as species 1 above, clearly identifying this species
as 1red.
-1
observed with the parameters δ ) 0.08 mm s and ∆EQ )
-1
2
.40 mm s . These are assigned as the Fe(IV) species 1ox.
Notably, the quadrupole splitting is somewhat larger than that
-1 24
observed for 3ox (for which ∆EQ ) 1.92 mm s ).
We propose two possible reasons for the instability of 1ox.
The first is that the chloride ion may be an ineffective axial
ligand for the complex because of its size. As discussed above,
the chloride ion can be fairly easily removed in the ferrous and
ferric complexes and may not be able to bind closely to the
Fe(IV) center because of the steric effects of the methyl groups.
Another consideration is that the electrode potential is very high
for the coulometric process, and if the equilibrium shown at
the top of Scheme 4 produces any free chloride ions in solution,
these may be oxidized to chlorine either by the electrode or
perhaps by 1ox itself. These points are interesting considering
the stability of a chloro-Fe(IV) complex, synthesized by Collins
The reactions involved in the above discussion are represented
in Scheme 4, which also shows that a pre-equilibrium between
and an acetonitrile-bound Fe(III) species may also occur, but
1
to only a small extent. The loss of chloride in 1red is believed
to be due mainly to steric reasons, but also due to the fact the
Cl-Fe(II) bond will be weaker than the Cl-Fe(III) bond for
electrostatic reasons.
In contrast to this behavior, reduction of 2 and 3 give cleanly
one product as seen in their M o¨ ssbauer spectra. The spectrum
of 3red was previously reported and is consistent with the
formulation of high-spin Fe(II), and the spectrum of 2red is
43
et al. and of (corrole)FeCl species may may contain the Fe(IV)
7
ion.
(
42) The A and D values determined in this fit are not unambiguously defined
(
41) An alternative proposal is that one species is five-coordinate, but this should
lead to a much larger quadrupole splitting whereas the quadrupole splittings
observed are similar to those of the six-coordinate species 2red and 3red.
and should be considered to be approximately correct.
(43) Collins, T. J.; Kostka, K. L.; M u¨ nck, E.; Uffelman, E. S. J. Am. Chem.
Soc. 1990, 112, 5637.
13522 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
in the solid lines in Figure 9 is the result, but it is unfortunately
not a unique fit because the value of Azz is undefined (any value
for Azz results in an identical simulation of the spectrum) and
4
5
D is correlated with E/D. Thus, only the relative magnitude
of D can be determined for 2ox, but this is in good agreement
with the value determined for 3ox.
The spectroscopic details of 3ox have been described previ-
ously; it is an octahedral Fe(IV) species with an S ) 1 ground
state which has several absorptions in the UV-vis region at
∼
330, 430, 540, and 650 nm. Interestingly, when 3ox is
irrradiated with laser light at 650 nm in frozen acetonitrile
solution, the yellow diamagnetic dication [(Me3cyclam-ac-
2
+
2
etate)FeN] is formed, which is a genuine Fe(VI) species (d ,
S ) 0) via photooxidation producing also one equivalent of
2
1
dinitrogen.
Calculations on the Electron-Transfer Series [(Me3cyclam-
0/+/2+
25
acetate)FeF]
. We previously reported calculations using
density functional theory (DFT) on the cation from 2, namely,
Figure 9. M o¨ ssbauer spectra of 2ox in acetonitrile solution taken in applied
magnetic field at the field strengths given at 4.2 K (top three) and 15 K
+
[
(Me3cyclam-acetate)FeF] . These results shall be compared
0
(bottom trace). The solid lines represent spin Hamiltonian simulations for
here with new results for [(Me3cyclam-acetate)FeF] and [(Me3-
2+
S ) 1, assuming fast spin relaxation with the parameters given in Table 2.
cyclam-acetate)FeF] to gain more insight into the geometric
and electronic structures of these species. The optimized
geometry for [(Me3cyclam-acetate)FeF] was taken as a
starting point for geometry optimizations of the ferrous and
Coulometric oxidation of 2 yields a pale-orange solution,44
the electronic spectrum of which is shown in Figure 7. Here,
the intense bands in the visible region at 420 and 360 nm are
likely charge-transfer bands (acetate to iron charge transfer)
which are low enough in energy to be observed in the visible
region. They are assigned as such due to the fact that they occur
in 1ox, 2ox, and 3ox and therefore are not altered when the
axial ligand is exchanged. The decrease in energy of these
charge-transfer bands correlates nicely with the expected
shortening of the Fe-O distance (vide infra) in the complex as
the oxidation state increases. The observed shifts in the CdO
stretching band in the IR spectrum (Table 4) also agree well
with this interpretation, as will be discussed further below.
The M o¨ ssbauer spectrum of a frozen acetonitrile solution of
+
Fe(IV) species, and the main geometric results are collected
46
together in Table 5. The calculated geometry of the ferric
species is in good agreement with the known structure of 2,
with the exception that the Fe-N distances are overestimated
25
by the calculation; therefore, reasonable agreement between
the calculated and actual geometries of the Fe(II) and Fe(IV)
species is expected, even though the exact structures of these
are not yet known. As expected, all of the iron-ligand bond
distances decrease upon increasing the oxidation state of the
iron due to the increased charge on the metal. All of the
calculated structures can be described as consisting of an
octahedrally coordinated iron center with an axially compressed
ligand surrounding. The Fe-F bond is in each case the shortest
metal-ligand bond in the molecule and becomes shorter by 0.07
to 0.10 Å per increase in the oxidation state of the iron resulting
in a steady increase in the calculated Fe-F bond order (Table
6). The corresponding change in the Fe-O distance is 0.13 to
0.15 Å, and the change in the average Fe-N distances is not
as drastic between the ferrous and ferric complexes but is
calculated to be ∼0.10 Å from ferric to Fe(IV). Interestingly,
the average Fe-N distance never falls below 2.1 Å, even in
the low-spin Fe(IV) structure, which is likely a result of the
steric effects of the methyl groups in the complex that cause
the Fe-N bonds to be longer than one would expect. This is
nicely shown by geometry optimization of the corrosponding
Fe(IV) fluoro complex of the cyclam-acetate ligand (a hypo-
thetical molecule), which has N-H groups instead of N-Me
groups. This model converges to a structure having Fe-N bond
distances of 2.053 Å, 0.06 Å shorter than in the methylated
complex.
2
ox prepared coulometrically consists of a single doublet with
-
1
-1
δ ) 0.02 mm s and ∆EQ ) 2.43 mm s , which agree with
the assignment of the oxidation as being iron centered. Compar-
ing these data to those of 1ox and 3ox, the isomer shift values
of the complexes decrease in the series as the axial ligand is
changed: F < Cl < N3. Because the isomer shift is directly
related to the electron density at the iron nucleus, this may be
taken as an indication of the π donating ability of the axial
ligands F, Cl, and N3. Spectra of 2ox were also measured in
applied magnetic fields of 1, 4, and 7 T at 4.2 K and also an
additional spectrum at 15 K and 7 T was measured (see Figure
9
). The 4.2 K spectra show only limited magnetic splitting,
indicating an integer spin ground state with fairly large positive
zero-field splitting (resulting in an mS ) 0 state lowest in
energy). Because 2ox could be either high spin (S ) 2) or low
spin (S ) 1), and both of these possibilities may result in an mS
)
0 ground state, both possibilities were employed to fit the
data. It was found that only the S ) 1 model could account for
the splitting of the left line of the quadrupole doublet into three
lines, so the S ) 2 model was discounted. The fourth spectrum
at 15 K was then measured so that a more precise determination
of the zero-field splitting tensor could be made. The fit shown
(45) In addition to the parameters from the axial model listed in Table 2, an
-1
alternate simulation with D ) 24.7 cm , E/D ) 0.33 (the rhombic limit)
is also possible. In this case, the A values change to {-28.4, -16.0, -3.6}
T.
(
46) The calculations were performed using two different basis sets of double-ú
and triple-ú quality. Though changing the basis set results only in minor
differences in the resulting geometries, for practical purposes, the results
of only the triple-ú calculations shall be used in our discussion.
(
44) In this case, it was found to be necessary to perform the oxidation at -40
°
C and in a glassy carbon vessel to prevent decomposition.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13523

A R T I C L E S
Table 5. Geometric Data for Calculated Structures of the [(Me3cyclam-Acetate)FeF] Series, n ) 0, 1, 2
Berry et al.
a
n+
cyclam
(
Me
3
cyclam
−
[(Me
3
−
[(Me
3
cyclam
−
+
[(Me
3
cyclam−
acetate)FeF]+
acetate)FeF]2
acetate)FeF]2
+
acetate)FeF
(exp.)b
S
)
2
S
) 5
/
b
S
)
1
S ) 2
2
2
Fe-F, Å
1.848(2)
2.148[2]
1.979(2)
175.5(1)
0.099
1.933
2.248
2.087
172.1
0.132
1.871
2.221
1.959
168.6
0.079
1.766
2.115
1.800
174.8
0.036
0
1.766
2.201
1.803
Fe-Nav, Å
Fe-O, Å
O-Fe-F, deg
172.9
0.071
+50
c
∆
d(Fe-N4), Å
-
1
E, rel., kJ mol
a
Reported values are from calculations with BP86/TZVP. Geometries using SV(P) given in Table S1. b Data taken from ref 25. c This is the distance of
the iron atom above the plane formed by the four amine nitrogen atoms.
Table 6. Calculated and Experimental Spectroscopic Properties of 2, 2red, and 2ox
(
Me
3
cyclam
−
[(Me
3
cyclam
−
[(Me
3
cyclam−
2red
acetate)FeF
2
acetate)FeF]+
2ox
acetate)FeF]2
+
2
S + 1
5
-
-
1630
2.00
-2.00, 0.20
1.11
+3.71, 0.15
5
3.66
0.5684
1663
2.034
-1.74, 0.045
0.99
6
-
-
6
4.18
0.6783
1706
2.010
+0.898, 0.037
0.43
-1.22, 0.29
3
-
-
3
2.14
0.8599
1770
2.044
+5.62, 0.029
0.086
+2.31, 0.04
5
Fe 3d spin density
3.57
0.8569
a
Fe-F bond order
ν(CdO), cm
giso
-
1
1682
2.00
+0.4, 0.12
0.39
1753
1765
b
2.00b
2.015
-
1
D, cm , E/D
+21.9, 0
0.02
+2.43, 0
+10.3, 0.324
-
1
δ, mm s
0.102
∆
EQ, mm s-1, η
+2.83, 0.136
-0.95, 0.4
+0.474, 0.528
{-24.6, -25.7,
-31.7}
A, MHz
{-21.4, -31.0, {-21.54, -38.41, {-29.4, -29.4, {-31.3, -31.9, {-15.2, -13.8, {-4.1, -33.0,
-
c
18.5}
-39.36}
-29.0}
-32.0}
0 }
-33.3}
a
Calculated from L o¨ wdin analysis. b Fixed in simulation of the M o¨ ssbauer spectra. c Value could not be determined.
In the crystal structure of 2, the position of the iron atom is
somewhat out of the equatorial plane formed by the four
nitrogen atoms of the cyclam ring, pulled 0.099 Å out of this
plane away from the acetate ligand (as calculated from a dummy
position at the center of the mean plane described by the four
N atoms). This distortion is likely due to the mismatch between
the cavity size of the Me3cyclam-acetate ligand and the ionic
exception that the average Fe-N distance of 2.201 Å is nearly
0.1 Å longer for the former, as has been previously noted in
calculations of low-and high-spin Fe(IV)dO species.4
7,48
This
is easily understood considering a simple ligand field model
4
for the d Fe(IV) ion, which in an axially compressed octahedral
2
1
1
geometry will have the configuration (d ) (d ) (d ) for the
xy
xz
yz
1
1
1
2
2 1
xy
xz
yz
x -y
low-spin state and (d ) (d ) (d ) (d ) for the high-spin state,
radius of the ferric ion. The calculated structures of
where now there is an electron in the d
x2-y2
orbital that is
+
(
[
Me3cyclam-acetate)FeF, [(Me3cyclam-acetate)FeF] and
strongly antibonding with respect to the equatorial nitrogen
atoms. Energetically, the high-spin state is calculated 50 kJ/
mol above the ground triplet state (calculated from B3LYP).
(Me3cyclam-acetate)FeF]2+ reproduce this distortion, which
shall be called ∆d(Fe-N4), and the value of ∆d(Fe-N4) for
the calculated ferric structure (0.079 Å) is in good quantitative
agreement with the crystal structure of 2 (∆d(Fe-N4) ) 0.099
Å). The calculated values of ∆d(Fe-N4) increase as the
oxidation state of the iron is lowered, and hence as the ionic
radius of the iron ion increases. In the ferrous species, the Fe-N
bond distances increase by only 0.02 Å as compared to the ferric
species, which may indicate the limit of how much the cyclam
ring can stretch to accommodate the large radius of the high-
spin ferrous ion, which is consequently pushed significantly out
of the N4 plane by 0.13 Å. The Fe(IV) ion is much smaller,
and fits well into the cavity of the Me3cyclam-acetate ligand
with only a small ∆d(Fe-N4) of 0.036 Å. It should be
mentioned that the crystal structure of 3 shows no distortion,
but this may be an artifact due to the fact that the iron atom in
The above trend in the orbital population as well as the trends
for the ferric and ferrous species are summarized in Figure 10,
which shows a semi-qualitative energy diagram (energies are
based on the calculated energies of quasi-restricted orbitals for
48
each case ) of the d orbitals and their occupancies across the
series from high-spin Fe(II) to low-spin Fe(III), high-spin Fe(III),
low-spin Fe(IV), and high-spin Fe(IV). UHF natural orbitals
for the low-spin Fe(IV) species are shown in the right side of
the Figure.
For all species, the iron d orbitals have energies which
2
2
2
increase in the order dxy < dxz ≈ dyz < dx -y < dz , as expected
from ligand field theory. Another qualitative prediction from
ligand field theory is that as the oxidation state of the metal is
increased, so too is the value of 10Dq. This is also in good
agreement with the results seen in Figure 10, as the energy
3
lies on a crystallographic center of inversion and is therefore
required to have a ∆d(Fe-N4) of 0 Å.
2
separation between the dxy and dz orbitals increases by 14%
An advantage of DFT is the ability to calculate the properties
of hypothetical molecules or real molecules with a different
electronic structure from that observed in their ground state.
between high-spin Fe(II) and high-spin Fe(III), and by 38%
between high-spin Fe(III) and low-spin Fe(IV). This result
correlates well with the more dramatic change in the iron-ligand
Using this approach, the Fe(IV) species [(Me3cyclam-acetate)-
III/IV
FeF]2+ was calculated in the unobserved high-spin state (S )
bond distances between the Fe
couple as compared to the
II/III
Fe
couple.
2
), so that more information about the electronic structure and
spin energetics of octahedral Fe(IV) complexes could be gained.
Geometrically, the structure of high-spin [(Me3cyclam-acetate)-
FeF]2 is similar to the analogous low-spin structure, with the
(
47) Neese, F. J. Inorg. Biochem. 2006, 100, 716.
(
48) Sch o¨ neboom, J. C.; Neese, F.; Thiel, W. J. Am. Chem. Soc. 2005, 127,
+
5840.
13524 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
Figure 10. Energies of the 3d orbitals and electron configurations as a function of oxidation state and spin state calculated for the [(Me3cyclam-acetate)-
n+
2+
FeF] species (left). Contour plots of the valence 3d orbitals calculated from [(Me3cyclam-acetate)FeF] showing their interactions with various ligand
orbitals are shown to the right.
Plots of the iron 3d orbitals from [(Me3cyclam-acetate)-
FeF]2 are also shown in Figure 10, where it is clearly seen
that they all have significant interactions with the orbitals of
the ligands. There is a small but significant interaction of the
dxy orbital with p orbitals of the two axial ligands, which causes
a tilting of the dxy orbital slightly out of the N4 plane. More
significant are the antibonding π interactions between the axial
ligands and the dxz and dyz orbitals, which cause significant
+
-
1
energy difference (∼4000 cm ) between the dxy orbital and
2
2
the nearly degenerate dxz, dyz set. The dx -y orbital interacts
with sigma orbitals of the four equatorial nitrogen atoms, and
2
the dz orbital interacts with sigma orbitals of all of the ligand
3
Figure 11. Plot of the CdO stretching frequencies of [(Me cyclam-
n+
+
acetate)FeF] , n ) 0,1,2, as well as [(Me3cyclam-acetate)FeO] against
the calculated Fe-O bond distances between the iron ion and the carboxylate
oxygen atom in each species. Both experimental and calculated CdO
stretching frequencies are given, and the solid and dashed lines represent
the results of linear least-squares fits.
atoms and is therefore highest in energy.
DFT was also used to calculate spectroscopic properties of
the three members of the electron-transfer series [(Me3cyclam-
acetate)FeF]
0
/+/2+
for comparison with experiment to show
convincingly that all of the redox reactions are metal based and
that 2ox indeed represents a genuine example of an octahedral
Fe(IV) complex. These properties are summarized in Table 6
and compared to experimental values. It should be noted that
the calculated properties of 2 have already been reported and
discussed, and are shown here to facilitate comparison with the
properties of 2red and 2ox.
Calculation of g and D tensors was also performed to compare
with the experimental data for 2, 2red, and 2ox. In general,
good agreement between calculated and experimental values is
observed. In particular, it should be noted that the negative sign
of the D for 2red, which is unusual because high-spin ferrous
porphyrin species such as reduced cytochrome P450 and
49
hemoglobin have positive D values, is also suggested by the
The trend in the CdO stretching frequencies of the [(Me3-
cyclam-acetate)FeF] series is remarkably well reproduced
calculation. The calculated D value of [(Me cyclam-acetate)-
FeF] is also in reasonable agreement with the value obtained
3
n+
2+
by the calculations, though it should be noted that all of the
for 2ox, since the sign is correct though the calculated value is
48
calculated CdO stretching frequencies are uniformly 20-30
too low, due to reasons discussed more thoroughly elsewhere.
-1
-1
cm higher than the experimental values. The CdO stretching
frequencies correlate linearly with the calculated Fe-O distance
to the bound carboxylate arm, and this correlation is shown in
Figure 11 for the set of both calculated and experimental CdO
stretching frequencies. Also shown is the calculated CdO stretch
Both 2ox and 3ox have relatively large D values (>20 cm ),
which appears to be a hallmark of S ) 1 Fe(IV) species as
similar results have been found in various ferryl complexes as
well as an Fe(IV) complex synthesized by the Collins group
having a macrocyclic equatorial ligand and two axial isonitrile
2
+
50
for an Fe(IV)-oxo species, [(Me3cyclam-acetate)FeO] (vide
infra) which also fits into this correlation very well. It appears
that this correlation is electrostatic in origin and can be explained
simply that as the Fe-O bond becomes stronger, the O-CdO
acetate group has less π delocalization and therefore the CdO
bond is more like a true double bond. This is also reflected in
the CdO bond orders calculated from L o¨ wdin analysis of (Me3-
ligands. The large D values in these species have been
attributed to spin orbit coupling of excited states, and in
particular the low-lying S ) 2 excited-state into the ground
4
8
state.
M o¨ ssbauer parameters and hyperfine coupling tensors may
also be calculated using DFT with good accuracy.5
1,52
The
(
49) Debrunner, P. G. In Iron Porphyrins Part III; Lever, A. B. P., Gray, H. B.,
+
cyclam-acetate)FeF, [(Me3cyclam-acetate)FeF] , and [(Me3-
Eds.; VCH: Weinheim, 1989; Vol. III, pp 137.
2+
(50) Collins, T. J.; Fox, B. G.; Hu, Z. G.; Kostka, K. L.; M u¨ nck, E.; Rickard,
C. E. F.; Wright, L. J. J. Am. Chem. Soc. 1992, 114, 8724.
(51) Neese, F. Inorg. Chim. Acta. 2002, 337, 181.
cyclam-acetate)FeF] , which increase from 2.11 to 2.21 to
.29.
2
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13525

A R T I C L E S
Table 7. Calculated Properties of [(Me3cyclam-Acetate)FeO]+
Berry et al.
a
S
)
1
S ) 2
FedO, Å
1.666
2.103
1.945
176.0
0.085
0
1.663
2.200
1.933
175.1
0.121
+11
Fe-Nav, Å
Fe-O, Å
O-Fe-F, deg
∆
d(Fe-N4), Å
-
1
E, rel., kJ mol
Fe 3d spin density
1.34
3.13
L o¨ wdin FedO bond order
ν(CdO), cm
1.5491
1717
2.018
+5.15, 0.007
0.128
-0.243, 0.623
{-4.07, -29.80, -31.25}
1.5622
1713
2.013
+6.975, 0.01
0.18
-1.587, 0.111
{-24.63, -26.45, -42.06}
-
1
giso
-
1
D, cm , E/D
-
1
δ, mm s
EQ, mm s-1, η
A, MHz
∆
a
Geometries were calculated using BP86/TZVP. Geometries using SV(P) are given in Table S2.
Table 8. M o¨ ssbauer and Spin Hamiltonian Parameters for (FeO)2 Speciesa
+
[
(cyclam
−
[(Me
4
cyclam)FeO
)](OTf)
[(BPMCN)-
FeO(NCR)]2
[(TPA)FeO(X)]2
+
(NCCH
[(TMCS)FeO]+
+
2 5
[(H O)
FeO]2
+
acetate)FeO]OTf
3
2
2
S +1
3
3
3
3
3
5
-
1
b
D, cm , E/D
δ, mm s
∆
Axx, Ayy, Azz, MHz
reference
+23, 0
0.01
1.37, 0.8
-32, -32, -14
18
+28, 0
0.01
0.92, 0.9
-32.4, -32.4, -6.9
27
+28, 0
0.17
+35, 0
0.19
-0.22, 0
-23, -22, -5
28
+19, 0.15
0.10
1.75, 0.8
-30, -23, -2
27
+9.7, -
0.38
-
1
EQ, mm s-1, η
+1.24, 0.5
-0.33, 0
b
-34, -28, -4
- , -28, -28
23
16,17
a
Structures for abbreviated ligands are given in the references. b Value not determined.
calculated isomer shifts and quadrupole splittings of the
observation further supports our assignment of 2ox as being a
low-spin Fe(IV) complex rather than being high-spin.
n+
[(Me3cyclam-acetate)FeF] series agree well with the values
obtained experimentally for 2, 2red, and 2ox. In particular, the
Fe(IV)-Fluoro vs Fe(IV)-Oxo: Isoelectronic Cousins. We
change in sign of ∆EQ for the Fe(III) species is well reproduced
3+
2+
may consider the (FeF) moiety to be isoelectronic to (FeO) .
Because there is now a considerable wealth of spectroscopic
and structural data for Fe(IV)-oxo species due to recent synthetic
2
5
as has been previously mentioned. Hyperfine constants
5
7
describing the interaction of the unpaired spin with the Fe
nuclear spin have also been calculated, though it should be
mentioned that the experimental values for 2red and 2ox are
not very well defined, and therefore quantitative comparison is
not appropriate. Qualitatively speaking, however, the basic
pattern of the A values calculated for [(Me3cyclam-acetate)-
FeF]2 (all three A values being negative, one small and two
large and nearly equal) is similar to what has been observed
for ferryl species, as well as in 3ox, where the A values are
more well defined.
23
breakthroughs in the Que lab, it is pertinent to compare the
electronic structure and properties of [(Me3cyclam-acetate)-
2+
FeF] with those of the corresponding oxo complex, [(Me3-
+
cyclam-acetate)FeO] , which may be considered as an analog
of the complex [(cyclam-acetate)FeO]OTf which was previ-
+
18
ously reported in our group. Geometric and spectroscopic
properties of the hypothetical cation [(Me3cyclam-acetate)-
+
FeO] have been calculated and are presented in Table 7 and
2+
shall be here compared to the properties of known (FeO)
Spectroscopic properties were also calculated for the hypo-
thetical high-spin [(Me3cyclam-acetate)FeF] dication (S )
) to compare with what is known for the low-spin Fe(IV)
species which have been collected in Table 8.
2+
+
The calculated geometry of [(Me3cyclam-acetate)FeO] is
in good agreement with the known structural data on FeO
2
2+
species. The CdO stretch, g value, and isomer shift are not
species; in particular the FedO bond distance of 1.67 Å is very
close to that observed in the crystal structure of [(Me4cyclam)-
FeO(NCCH3)](OTf)2, 1.65 Å. This distance is interestingly
shorter than the calculated Fe-F distance for 2ox by ∼0.1 Å,
emphasizing the difference in bond orders for the two species.
The calculated FedO bond order, 1.55, is considerably greater
than the Fe-F bond order, 0.86, clearly reflecting more π
bonding in the ferryl species. The Fe-O bond to the acetate
arm in the ferryl complex is elongated by 0.15 Å as compared
to the Fe(IV)-fluoro species, reflecting the greater trans influence
of the short FedO multiple bond.
very different from what is calculated for the low-spin species,
-1
but the calculated D value of 10.3 cm for the high-spin species
is nearly twice as large as that of the low-spin species and is
rhombic rather than axial. Although this D value is significantly
larger than that reported for the five-coordinate high-spin Fe(IV)
-
1 53
complex of Collins (-3.7 cm ), it agrees well with the D
2
+
value reported for the ferryl aquo ion [(H2O)5FeO] (+9.7
-
1 16,17
cm ).
One major difference between high- and low-spin
2+
[
(Me3cyclam-acetate)FeF] is that the calculated quadrupole
splitting of +0.47 mm s for the high-spin species is much
smaller than the calculated value for the low-spin species (+2.31
-
1
As seen in Table 8, a large positive zero-field splitting value
-
1
-1
mm s ) and from the observed value (+2.43 mm s ). This
-1
(
>10 cm ) is typical of ferryl species. Indeed, all of the non-
oxo Fe(IV) species presented here share this characteristic, and
this is reproduced in the calculation of both the Fe(IV)-fluoro
and -oxo species to the same degree of accuracy. In both
(
52) Sinnecker, S.; Slep, L. D.; Bill, E.; Neese, F. Inorg. Chem. 2005, 44, 2245.
53) Kostka, K. L.; Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard, C. E.
F.; Wright, L. J.; M u¨ nck, E. J. Am. Chem. Soc. 1993, 115, 6746.
(
13526 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006

Non-Heme Oxo and Non-Oxo Fe(IV) Complexes
A R T I C L E S
species, an axial D of ∼+5 cm-1 is calculated, which is a
surprising result because theoretical analysis of zero-field
splittings in ferryl species reveals that the triplet-quintet energy
gap is one of the major factors contributing to the large D value.
S ) 1 state.54 This is also remarkably smaller than the calculated
-1
energy difference between low- and high-spin 2ox (50 kJ mol )
and is so small that the S ) 1 and S ) 2 states of this complex
should be considered to be very close in energy. We currently
plan to synthesize the complex to determine the nature of the
ground state experimentally.
3+
In the case of the FeF species, however, this energy difference
-
1
(
50 kJ mol ) is computed to be much larger than in the case
2+
-1
of the FeO species (11 kJ mol ) which should lead to a D
value in the former ion which is much smaller than in the ferryl
species. We suggest that the unusually large D value for the
fluoro complex is due to the fact that the covalency of the Fe-F
bond is much lower than that of the FedO bond, which allows
more efficient spin-orbit coupling contributions to the D value.
Conclusions
A new family of non-oxo Fe(IV) complexes is presented and
characterized by spectroelectrochemistry, M o¨ ssbauer spectros-
copy, and DFT calculations. The calculated and spectroscopi-
cally observed changes in 1, 2, and 3 upon oxidation and
reduction are consistent with the presence of iron-centered redox
processes. This gives us a unique opportunity to observe Fe-
(II), Fe(III), and Fe(IV) complexes with essentially the same
ligand sphere. In all cases, the complexes become more highly
colored as the oxidation state of the iron increases, as is expected
due to the shortening of the metal-ligand bond distances (and
hence the lowering in energy of LMCT bands in the electronic
spectrum). For the series 2red/2/2ox, the calculated structures
indicate a contraction of all of the iron-ligand bond distances
3+
The hyperfine tensor values are also similar for both FeF and
2
+
FeO species because for both the Fe(IV)-fluoro and -oxo
species, A values of A11 , A22 ≈ A33 are calculated and this
pattern fits well the experimental examples (Table 6 and 8).
From the survey of M o¨ ssbauer parameters of low-spin ferryl
species (Table 8), the quadrupole splittings can vary over a wide
range (from -0.22 to +1.24 mm s , though often the sign is
not well defined because of the large asymmetry parameter η)
whereas the isomer shifts generally are limited to the range of
-
1
-1
2
1
1
0
.01 to 0.19 mm s . Thus, the quadrupole splitting parameter
upon increasing oxidation number, and a (dxy) (dxz) (dyz) ground
state configuration is found for 2ox. The Fe(IV)-oxo species
is apparently very sensitive to the coordination sphere of the
molecules, but not in a readily predictable way. This contrasts
with the octahedral non-oxo Fe(IV) species presented here where
a large positive ∆EQ value is observed. Because in both oxo-
+
[(Me3cyclam-acetate)FeO] is isoelectronic to the cation of 2ox
but is different from the fluoro complex in many ways, all of
which are related to the increased covalency of the FedO double
bond in the ferryl species. This strong covalent bond affects
the Fe(IV) center by elongating the iron-ligand bond trans to
the FedO bond, slightly increasing the isomer shift, significantly
decreasing the quadrupole splitting, and increasing the separation
between the dxy orbital and the dxz and dyz (which are strongly
π antibonding with respect to the oxygen atom). The last effect
causes weak electronic transitions to appear in the absorption
spectrum of ferryl species in the range from 500 to 1100 nm,
which are absent in the case of 2ox.
2
1
1
and non-oxo species the (dxy) (dxz) (dyz) configuration leads to
the same valence contribution to the electric field gradient (EFG)
tensor (which should lead to a ∆EQ value that is large and
positive), the effect of the more covalent FedO double bond is
to decrease the ∆EQ value significantly.
2
1
1
In fact, it is unfair to assign the (dxy) (dxz) (dyz) configuration
to Fe(IV)-oxo species because the dxz and dyz orbitals form strong
covalent π bonds to p orbitals of the oxygen atom. The singly
occupied orbitals in ferryl species are essentially Fe-O π*
orbitals having 60.3% Fe 3d character and 34.8% O p character
Experimental
-
1
and these are raised in energy by ∼15 000 cm above the
General. The complex (Me
prepared according to the previously reported procedure. [(Me
cyclam-acetate)FeF]PF can be prepared from (Me cyclam-acetate)-
Fe-O-FeCl
as previously reported,25 but we also report here a new
3
3
cyclam-acetate)Fe-O-FeCl was
doubly occupied dxy orbital, similar to what has been found from
2
5
3
-
2+ 54
DFT analysis of [(Me4cyclam)FeO] . This large splitting
6
3
within the t2g level (which we shall call Π) is not nearly as
3
-
1
large in 2ox, in which a splitting of only 4,500 cm between
the dxy and dxz, dyz occurs. This result agrees well with the
differences in the electronic spectra of 2ox and typical ferryl
species. The latter species typically have a number of bands
method below that leads to better purity. All solvents and reagents were
used as received.
3
[(Me cyclam-acetate)FeCl]PF
6 3
(1). Solid (Me cyclam-acetate)-
Fe-O-FeCl (0.100 g, 0.188 mmol) was mixed with a solution of
3
-
1
-1
with ꢀ < 500 M cm in the region from 500 to 1100 nm,
whereas this region in the spectrum of 2ox is empty. These
bands for the ferryl species have been assigned as d f d
6
KPF (0.208 g, 1.13 mmol) in 5 mL of water for 14 h, causing the
slow precipitation of a bright-yellow powder. This was collected by
filtration and dried in air to yield 61 mg (61%) of the product. IR (KBr,
-
1
5
4
cm ): 3454 m, br, 2906 w, 1682 s (CdO), 1456 m, 1340 w, 1297 m,
transitions, arising in the visible to near IR range due to the
large value of Π. In 2ox, the corresponding bands will be much
higher in energy due to both the smaller value of Π and the
1
5
PF
105, w, 1089 w, 996 w, 955 m, 923 w, 839 vs (PF
6
), 744 w, 714 w,
+
57 s. ESI+ mass spectrum (m/z, amu): 390 (M-PF
6
) , 355.3 (M-
+
6
-Cl) . Anal. calcd for C15
H N
31 4
6
FeClPF : C 33.63, H 5.83, N
-
1
value of 10Dq of around 30 000 cm and would undoubtedly
1
0.46%. Found: C 34.25, H 5.75, N 10.24%.
(Me cyclam-acetate)FeF]PF (2). A solution of [(Me
acetate)FeCl]PF (0.056 g, 0.105 mmol) in 20 mL of water was heated
be masked by the very intense charge-transfer bands from 350-
[
3
6
3
cyclam-
4
50 nm.
6
+
The high-spin (S ) 2) state of [(Me3cyclam-acetate)FeO]
gently to 80° for a few minutes during which time a light-orange
precipitate (rust) was observed. The mixture was filtered to give a
brilliant-yellow solution, which was concentrated and allowed to
evaporate slowly in air. After 1 week, large crystals of the fluoro
was also calculated and was found to be only 11 kJ mol- higher
1
in energy than the S ) 1 ground state, which is a remarkable
2+
difference to the case of [(Me4cyclam)FeO] , in which the high-
-
1
complex had grown, which were collected and washed with Et
2
O.
spin state was calculated to be 54 kJ mol in energy above the
Yield: 20 mg, 37%. Complete evaporation of the solution yields a
-
1
messy mixture. IR (KBr, cm ): 3455 w, br, 2969, w, 1682 s (CdO),
(
54) Decker, A.; Rohde, J. U.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc.
2
004, 126, 5378.
1470 m, 1338 w, 1298 s, 1262 w, 1154 w, 1107 w, 1092 w, 1059 w,
J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006 13527

A R T I C L E S
Berry et al.
1
023 w, 969 w, 918 w, 841 vs (PF
6
), 822 s, 746 w, 712 w, 558 s, 543
Computations. The species (Me
3
cyclam-acetate)FeF (S ) 2),
+
cyclam-acetate)FeF] (S ) 1
+
5
m. ESI+ mass spectrm (m/z, amu): 374 (M-PF
FePF
.10, N 10.58%.
(Me cyclam-acetate)FeN
acetateFe-O-FeCl (0.300 g, 0.563 mmol) and 1.5 mL of MeOH was
added solid NaN (0.366 g, 5.63 mmol), resulting in a deep-red mixture,
to which a solution of KPF (0.622 g, 3.38 mmol) in 15 mL of water
6
) . Anal. calcd for
[(Me
3
/
2
2+
and S ) /
2
, data previously
25
C
6
15
H O N
31 2 4
7
: C 34.70, H 6.02, N 10.79%. Found: C 34.80, H
reported ), [(Me cyclam-acetate)FeF] (S ) 1, 2), and [(Me cyclam-
acetate)FeO] (S ) 1, 2) were used as models for density functional
3
3
+
[
3
3 6 3
]PF (3). To a mixture of Me cyclam-
theory calculations, which were performed using the ORCA program
55
cyclam-acetate)FeF]+ (S )
3
package. The initial geometry of [(Me
3
3
5
/
) was taken from its crystal structure as described previously and
25
2
6
optimized using the BP86 functional and using the SV(P) and TZVP
was added. The resulting red mixture was stirred in the dark for 14 h,
resulting in a red solid and red supernatant solution. The solid was
collected by filtration, washed with Et
acetonitrile to give a brilliant red-orange solution. This was allowed to
evaporate slowly in the dark to yield a crop of large red crystals, which
were collected, washed with EtOH and Et
56,57
basis sets.
These optimized geometries were used as starting
geometries for (Me
FeF] (S ) 1, 2), whose geometries were optimized in the same way.
3
cyclam-acetate)FeF and [(Me cyclam-acetate)-
3
2
O, dried, and extracted with
2+
2+
The optimized geometries of [(Me cyclam-acetate)FeF] were used
3
as starting geometries for the geometry optimization of [(Me
3
cyclam-
2
O, and dried in air. Yield:
+
acetate)FeO] . For all structures, the triple-ú optimized geometries were
used for calculations of the total energy and all properties. Vibrational
frequencies were calculated using the BP86 functional using a numerical
differentiation of analytic gradients with an increment of 0.005 Bohr.
1
75 mg, 57%. The compound is somewhat light sensitive in solution.
-
1
IR (KBr, cm ): 3432 s, br, 2090 vs (N
3
), 1671 s (CdO), 1465 m,
367 m, 1296 m, 1023 w, 968 w, 956 w, 916 w, 844 s (PF ), 744 w,
) . Anal. calcd
: C 33.22, H 5.76, N 18.08%. Found: C 33.10,
1
5
6
+
6
58 m. ESI+ mass spectrum (m/z, amu): 397.2 (M-PF
FePF
H 5.23, N 17.24%.
2+
In the high-spin [(Me cyclam-acetate)FeF] dication, a negative
3
for C15
H O N
31 2 7
6
-
1
vibrational frequency of -9 cm was observed, but the geometry was
not reoptimized because the frequency is small and does not correspond
to an important vibrational mode of the molecule and since the
calculated molecule is a hypothetical one. In all other cases, no negative
frequencies were observed, indicating that the structures correspond
to potential energy minima. Zero-point vibrational energies and thermal
corrections were taken from these frequency calculations. Total energies
were calculated with the B3LYP functional. M o¨ ssbauer parameters
(including magnetic hyperfine tensors) were calculated at the B3LYP
5
7
]. A sample of 57Fe foil (32 mg, 0.56 mmol) was
[
NEt ][ FeCl
4 4
dissolved in hot concentrated HCl (over Pt foil, to expedite the process).
The resulting yellow solution was cooled to room temperature, and
the solvent was removed in a rotary evaporator yielding an orange solid
residue. This was dissolved in a minimal amount of EtOH (∼1 mL),
4
and added to a solution of excess NEt Cl, which was also dissolved in
a minimal amount of EtOH (∼1 mL). Upon mixing, a soft-yellow
precipitate appeared, and the mixture was stirred for ∼30 min to ensure
complete reaction. The solid was collected by filtration, washed with
Et
compounds were prepared using a 40:60 ratio of [NEt
normal [NEt ][FeCl ]. ([NEt ][FeCl ] is the starting material used to
synthesize (Me cyclam-acetate)Fe-O-FeCl .)
Physical Measurements. IR spectra were recorded in the range
5
1
level using the triple-ú basis set and an expanded CP(PPP) basis set
for iron, as described previously.51 The g tensors and D tensors were
2
O, and dried in air. Yield: 154 mg, 83%. Isotopically enriched
5
7
48,58-60
4
][ FeCl
4
] and
calculated according to previously developed procedures.
were visualized using the Molekel program.61
Orbitals
4
4
4
4
3
3
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support. J.F.B. thanks the Alexander von
Humboldt Foundation for a postdoctoral fellowship.
-
1
4
00-4000 cm on a Perkin-Elmer 2000 FT-IR spectrometer on
samples pressed into KBr discs. ESI mass spectra were obtained on a
Finnigan MAT 95 spectrometer. Elemental analyses were done by the
H. Kolbe Mikroanalytisches Laboratorium in M u¨ lheim an der Ruhr,
Germany. Variable temperature magnetic susceptibilities were measured
on a Quantum Design SQUID magnetometer in the range 2-300 K at
an applied external field of 1000 G. Data points were corrected for
intrinsic diamagnetism of the sample, the sample holder, and also for
temperature-independent paramagnetism. X-Band EPR spectra were
recorded at 10 K on a Bruker ESP 300E spectrometer equipped with
a helium-flow cryostat (Oxford Instruments ESR 910). M o¨ ssbauer
spectra were recorded on an alternating constant-acceleration spec-
trometer with a minimum line width of 0.24 mm s . The sample
temperature was maintained by an Oxford Instruments Variox cryostat
for zero-field measurements, or by an Oxford Instruments M o¨ ssbauer-
Spectromag 2000 cryostat, which was used for measurements in applied
magnetic fields with the field oriented perpendicular to the γ source.
Isomer shifts (δ) are referenced against iron metal at 300 K.
Supporting Information Available: Tables S1 and S2;
Figures S1-S4. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA063590V
(55) Neese, F. ORCA - an ab initio, DFT, and Semiempirical Electronic Structure
Package, 2.4, revision 29; Max-Planck-Institut f u¨ r Strahlenchemie: M u¨ l-
heim, Germany, 2003.
(56) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
57) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
58) Neese, F. J. Chem. Phys. 2001, 115, 11080.
-
1
(
(
(59) Neese, F.; Solomon, E. I. Inorg. Chem. 1998, 37, 6568.
(60) Ray, K.; Begum, A.; Weyherm u¨ ller, T.; Piligkos, S.; van Slageren, J.; Neese,
F.; Wieghardt, K. J. Am. Chem. Soc. 2005, 127, 4403.
(
61) Fl u¨ kiger, P.; L u¨ thi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.0; Swiss
Center for Scientific Computing: Manno, Swizerland, 2000.
13528 J. AM. CHEM. SOC.
9
VOL. 128, NO. 41, 2006