Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

Article abstract of DOI:10.1021/jacs.7b01722

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl₂)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl₂)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh₂)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl₂ and CPh₂), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), M?ssbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field M?ssbauer experiments, which show very large quadrupole splittings (QS, ΔE_Q), determined the S = 0 electronic configuration. More importantly, combined structural and M?ssbauer studies, especially the comparison with the low spin iron(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN^-) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).

Full text of DOI:10.1021/jacs.7b01722

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Communication
The Electronic Configuration and Ligand Nature of Five Coordinate
Iron Porphyrin Carbene Complexes: An Experimental Study
Yulong Liu, Wei Xu, Jing Zhang, William Fuller, Charles E. Schulz, and Jianfeng Li
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Journal of the American Chemical Society
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The Electronic Configuration and Ligand Nature of Five Coordinate
Iron Porphyrin Carbene Complexes: An Experimental Study
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Yulong Liu,† Wei Xu,‡ Jing Zhang,‡ William Fuller,§ Charles E. Schulz,§ and Jianfeng Li*,†
†College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Yanqi Lake,
Huairou District, Beijing 101408, China.
‡Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, Chi-
na.
§Department of Physics, Knox College, Galesburg, Illinois 61401, USA
Supporting Information Placeholder
porphyrin).14 However, a ground state of an open-shell singlet
ABSTRACT: The five coordinate iron porphyrin carbene com-
with two antiferromagnetically coupled electrons residing on the
iron(III) and carbene (–1) ligand was proposed. The authors em-
phasized that the bonding properties are remarkably analogous
to those of the ferric heme superoxide complexes originating
from Fe(II) and O2.15 The latter has been an intriguing problem
for many years that has attracted the attentions of many groups
(including ours16).17
plexes
[Fe(TPP)(CCl2)]
(TPP
=
tetraphenylporphyrin),
[Fe(TTP)(CCl2)] (TTP = tetratolylporphyrin) and [Fe(TFPP)(CPh2)]
(TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types
of carbene ligands (CCl2 and CPh2), have been investigated by
single crystal X-ray, XANES (X-ray Absorption Near Edge Spec-
troscopy), Mössbauer, NMR and UV-vis spectroscopies. The
XANES unambiguously suggested the iron(II) oxidation state of
the complexes. The multi-temperature and high magnetic field
Mössbauer experiments, which show very large quadrupole split-
tings (QS, ΔEQ), determined the S = 0 electronic configuration.
More importantly, the combined structural and Mössbauer stud-
ies, especially the comparison with the low spin iron(II) porphyrin
complexes with strong diatomic ligands (CS, CO and CN−) re-
vealed the covalent bond nature of the carbene ligands. A corre-
lation between the iron isomer shifts (IS, δ) and the axial bond
distances is established for the first time for these donor carbon
ligands (:C–R).
The experimental investigations on the electronic structures of
iron porphyrin carbenes also appear controversial. The oxidation
state of the iron has not been definitely determined. Carbenes
have been considered as carbon analogs of the porphyrin-iron-
oxo species ([(Porph)(FeIV)=O]) on the basis of similarity in the
electronic spectra.18 The vibrational spectral study had assigned
[Fe(TPP)(CCl2)] as a low spin iron(II) species;19 while iron(IV) was
then suggested for the same complex because of the oxidation-
state sensitive band at 1370 cm–1.20 The first Mössbauer study on
the iron porphyrin carbene was reported in 1983.12 An Fe(IV) was
indicated for [Fe(TPP)(CCl2)] based on the rather small isomer
shift (δ = 0.10 mm/s) at 131 K. Recently, for the same reason
Fe(IV) was suggested for [Fe(TFPP)(CPh2)].6b Again, a measure-
ment at only one temperature was done (288 K, δ = 0.03 mm/s).
Although IS is a good indicator of the oxidation state,21 excep-
tions have been found for species with strong ligand interac-
tions.22 Hence, experimental investigations with confidence in
the assignment of the iron oxidation and spin state appear highly
desired.
The strong interest in the nature of iron porphyrin carbene
complexes is directly related to the excellent catalytic perfor-
mance of cytochrome P450 enzymes for numerous biochemical
reactions.1 Formation of carbene complexes of cytochrome P450
has been suggested in the reductive metabolism of polyhalogen-
ated compounds,2 oxidative metabolism of benzodioxole deriva-
tives3 and in propylene epoxidation.4 The first iron (porphyrin)
carbene complex [Fe(TPP)(CCl2)] was reported in 1977.5 Now,
iron porphyrin carbenes have been identified as versatile cata-
lysts in various chemical reactions (e.g. cyclopropanation6 and N–
H insertion7).8
Here we report three iron porphyrin carbene complexes
[Fe(TPP)(CCl2)], [Fe(TTP)(CCl2)] and [Fe(TFPP)(CPh2)], which were
characterized by various spectroscopies. The ORTEP diagrams of
the three structures are given in Figure 1 and S1. The 24-atom
mean planes and the orientations of the axial ligands are availa-
ble in Figure S2. As can be seen, the [Fe(TFPP)(CPh2)]6b and the
new [Fe(TFPP)(CPh2)]·1.5C6H6, both arylcarbene structures show
ruffled conformation, which is contrasted to the saddled confor-
mation of the dichlorocarbene [Fe(TPP)(CCl2)] and
[Fe(TTP)(CCl2)]. The difference between the two types of carbene
can also be seen from the key structural parameters, which are
given in Table 1. The CPh2 is more sterically bulky than CCl2. To
avoid the steric interactions with the peripheral phenyl groups,
CPh2 tends to align itself along the Fe–NP vectors which results in
Although of the significance, the electronic structures of iron
porphyrin carbenes, as either reaction intermediates9 or isolated
products,10 remain elusive. Theoretical calculations have been
applied to this area. Zhang and coworkers claimed the predomi-
nant resonance structure of singlet FeII←{:C(X)Y}0,11 which is
contrasted to the more generally accepted FeIV={C(X)Y}2–.6b,12 The
same group subsequently extended their results that all of the
transition states and the resulting complexes have closed-shell
singlet ground states.13 Recently, Shaik and coworkers reported
the electronic structure of [Fe(Porph)(SCH3)(CHCO2Et)]− (Porph =
a small dihedral angle to the closest NP–Fe–Ccarbene plane (
ϕ =
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Table 1. Selected Structural Parameters of Iron Porphyrin Carbene Complexes and Related Species
1
2
a,b
a,b
a,c
complex
Δ24
Δ4
Fe–Ca
(Fe–Np)av
Fe–La
Fe–C–Xd
φd,e
τd,f
refg
3
4
5
6
7
8
9
Carbene Complexes
1.972(6)
[Fe(TPP)(CCl2)]
0.17
0.19
0.30
0.29
0.23
0.20
0.20
0.23
0.24
0.22
1.726(3)
1.7295(15)
1.789(2)
1.767(3)
1.689(3)
1.83(3)
124.68(17), 124.69(17)
124.34(9), 125.23(9)
122.11(15), 124.27(16)
122.3(2), 126.1(3)
176.7(3)
40.81
40.30
17.14
13.88
1.6
0.0
2.3
2.7
NAh
NAh
0.0
tw
tw
[Fe(TTP)(CCl2)]
1.980(9)
[Fe(TFPP)(CPh2)]·1.5C6H6
[Fe(TFPP)(CPh2)]
1.9847(9)
1.966(3)
tw
6b
27
28
6b
[Fe(TPP)(C=C(p-ClC6H4)2)]
[Fe(TPP)(CCl2)(H2O)]
[Fe(TFPP)(CPh2)(1-MeIm)]
1.984(1)
1.984(4)
1.973(4)
2.13(3)
2.168(4)
NAh
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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60
0.12
0.10
1.827(5)
124.2(4), 124.8(4)
19.36
Related Complexes
1.988(2)
23
23
29
25
[Fe(OEP)(CO)]
0.20
0.22
0.23
0.23
0.20
0.20
0.22
0.17
1.7140(11)
1.7077(13)
1.662(3)
177.20(8)
177.20(11)
176.3(2)
3.8
2.4
~4
[Fe(OEP)(CO)]·C6H6
[Fe(OEP)(CS)]
1.984(3)
1.982(5)
[K(222)][Fe(TPP)(CN)](100K)
1.8783(10)
1.986(7)
177.19(10)
6.3
aValues in angstroms. Displacement of iron atom from the 24-atom (Δ24) or the four pyrrole nitrogen atoms (Δ4) mean plane. The positive numbers indicate a
displacement towards the Cligand. cAverage distance between the iron and porphyrin nitrogen atoms. dAngle values in degrees. eDihedral angle between the carbene
ligand plane (which is denoted by the Ccarbene atom and the two bonded atoms) and the plane of the closest Np–Fe–Ccarbene. fThe tilt of the Fe–C vector off the nor-
mal to the 24-atom mean plane. gtw = this work. hNA = not available.
b
Fe–Ccarbene distances of the two [Fe(TFPP)(CPh2)] structures are
also consistent with the steric effect of CPh2. The key structural
parameters of the well-studied, low spin [Fe(II)(Porph)(XY)]
(XY=CO,23 CS24 and CN− 25) are also given in Table 1. It is seen the
Δ24 (and Δ4), and the (Fe–Np)av distances of the carbene complex-
es are in the range of 0.17~0.30 and 1.96~1.99 Å. Both parame-
ters, which are sensitive to the electronic configuration of the
central metal,26 are similar to those of the [Fe(Porph)(XY)] ana-
logues. Except [Fe(TPP)(C=C(p-ClC6H4)2)], all the carbene com-
plexes show Fe–C–X angles ~120°, which is consistent to the sp2
hybridized carbon atoms and in contrast with the XY ligands
which bond to heme in a linear fashion (~177°).
K-edge XANES have been done to determine the iron oxidation
states of the carbene complexes. [Fe(TPP)(CS)]24 and [Fe(TPP)Cl]
were also prepared and tested as references. The normalized
(top) and the first derivative (bottom panel) of the XANES spectra
are given in Figure 2. It is seen both the pre-edge features A’ and
the first absorption edge (7112.9 eV) of [Fe(TPP)(CCl2)] and
[Fe(TFPP)(CPh2)] are identical to those of the ferrous
[Fe(TPP)(CS)], suggesting the same oxidation states. This position
is consistent with calculations on the Fe(II) complexes with
square-pyramidal geometry.30 In contrast, the peaks of
[Fe(TPP)Cl] which shift towards higher energy due to the higher
valence state of iron(III) appeared at 7113.9 eV.31 Hence, the
finger print analysis determined the iron(II) oxidation state of
[Fe(TPP)(CCl2)] and [Fe(TFPP)(CPh2)].
Figure 1. Side-on ORTEP diagrams of (a) [Fe(TPP)(CCl2)], (b) [Fe(TTP)(CCl2)]
and (c) [Fe(TFPP)(CPh2)]. Thermal ellipsoids of all atoms are contoured at the
50% probability level. Hydrogen atoms and solvent molecules are not shown
for clarity.
13~17°). This is compared to the large
ϕ angle (~40°) of the CCl2
ligands which nearly bisect the NP–Fe–NP angles. The relatively
large iron out of plane displacements (Δ24 and Δ4) and the longer
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Journal of the American Chemical Society
Table 2. Selected Mössbauer Parameters for Iron Porphy-
rin Carbene Complexes and Related Species
1
2
3
ꢇ
ꢇ
ꢂꢅ ꢈꢆ which would contribute to the negative QS is presumed
4
because of the electron deficient nature of the TFPP macrocycle
5
ꢇ
ꢇ
that induced less density of the ꢄꢅ ꢈꢆ orbital.
6
7
ꢊ ꢂꢁ ꢋ ꢂꢅꢆ ꢊ ꢌꢍ ꢎꢂꢅꢁ ꢋ ꢂꢆꢁꢏꢐ (1)
ꢇ
ꢇ
ꢇ
ꢀ = kꢉꢂ
ꢁꢁ
ꢅ ꢈꢆ
It has been suggested that the isomer shifts of the iron will de-
crease both with an increase in ligand σ donation and iron -back
8
9
10
11
12
π
donation to the ligand.38 At the entire temperature range the IS
of [Fe(TPP)(CCl2)] are approximately half those of
[Fe(TFPP)(CPh2)], which suggests the CCl2 bonds more strongly to
iron than does the CPh2. This is in agreement with the relatively
shorter axial bond distance of [Fe(TPP)(CCl2)] as well as the larger
Figure 3. 4.2 K Mössbauer spectra of [Fe(TPP)(CCl2)] (top) and
[Fe(TFPP)(CPh2)] (bottom) recorded under 9T applied magnetic field.
13
14
15
16
17
18
19
20
21
22
23
24
25
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Two types of carbene complexes were further studied by
Mössbauer spectroscopy on the solid state to determine the
electron configurations. In Table 2, the isomer shifts and quadru-
pole splittings of the known porphyrin carbene complexes are
given. Also given are the Mössbauer parameters of the
[Fe(Porph)(XY)] (XY=CO, CS, CN–) analogues. As it is seen, alt-
hough the isomer shifts of the current samples show apparent
temperature dependence (~50% change) with smaller isomer
shifts at higher temperatures due to the second order Doppler
shift,32 all the values (0.02~0.19) are between those of the fer-
rous [Fe(Porph)(XY)] (−0.03~0.37 mm/s). The current samples
show unusually large magnitude of the quadrupole splitting (|ΔEQ|
= 2.2~2.5 mm/s), which are larger than all the known low spin
iron(II) heme complexes. The large |ΔEQ|, however, are compara-
ble to those of the five- (2.24(1))33 and six-coordinate
(−2.15~−2.31 mm/s) oxyheme complexes.34 It is interesting to
note that both dioxygen and carbene show bent ligand geometry
(Fe–O–O=122~132°).16
η
value of 0.90. This is also in accordance with the experimental
difficulty in isolating a CPh2 derivative with a simple porphyrin
such as TPP.6b Interestingly, a similar pattern is seen in the
[Fe(Porph)(XY)] complexes. The CS has been considered as a bet-
ter σ-donor and π
-acceptor than CO;24 and both are stronger
than the cyanide which is a typical σ donor. This sequence is well
consistent with the axial distance and IS, that the stronger bond-
ing corresponds to shorter bond distances and smaller IS. In fig-
ure 4, the relationship between the axial Fe–C distances and the
isomer shifts is plotted. All the five ligands have lone pair of elec-
trons on the donor carbon atoms (:C–R). The correlations be-
tween the two parameters for the diatomic ligands and/or car-
bene ligands are clearly evident. We note this is the first time
that such experimental data have been marshalled for a possible
correlation. The axial distances of the two carbene complexes are
between 1.73~1.79 Å, closer to those of the carbonyl
(1.7140(11)) and thiocarbonyl (1.662(3)), but shorter than that of
the cyano analogue (1.8783(10) Å). [Fe(TPP)(CS)] and
[Fe(TPP)(CCl2)] showed similarly significant Raman bands (1368
The two samples were further studied under strong magnetic
fields (3, 6 and 9 T) which gives more insight into the ligand na-
ture. An illustration of the fits to spectra is given in Figure 3; in-
formation on all fits are found in the Table S2. The high field re-
sults definitely assigned the low spin S = 0 states as determined
by high-quality fits to applied-field experiments. The fits suggest
1
and 1369 cm– ) suggesting their similarly strong
π-acceptor lig-
ands.19 This is consistent with the similar IS and QS values of the
two complexes (Table 2).
amm·s–1, bHMF = high magnetic field.
high rhombicity in the electric field gradient (EFG) with
η being
0.90 (CCl2) and 0.60 (CPh2). Both values are fairly large for a low
spin species, and compared to ~0.2 and 0.5 for oxyheme35 and
cytochrome
c
respectively.34b Interestinly, [Fe(TFPP)(CPh2)]
shows negative QS which is consistent to the DFT prediction,11
but opposite to that of [Fe(TPP)(CCl2)].36 The principal compo-
nent of the EFG, ꢀ is approximately given by the Equation 1,
ꢁꢁ
where k is the scaling factor and the ꢂꢃ values are the effective
populations of the appropriate 3d orbitals.37 As ꢄꢅꢆ is a non-
bonding orbital, the contribution to ꢀ comes from the imbal-
ꢁꢁ
ance in electron densities in the other d orbitals. A smaller
Figure 2. Comparison of the Fe K-edge XANES (top) and its first derivative
(bottom) of the carbene and reference complexes.
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Brault, D.; Rougee, M. J. Chem. Soc., Chem. Commun. 1977, 648-649.
(6) (a) Wolf, J. R.; Hamaker, C. G.; Djukic, J.-P.; Kodadek, T.; Woo, L. K.
J. Am. Chem. Soc. 1995, 117, 9194-9199. (b) Li, Y.; Huang, J.-S.; Zhou, Z.-
In summary, we have characterized two types of five coordi-
nate iron porphyrin carbene complexes. The solid state XANES
and Mössbauer studies unambiguously determined the iron(II) S
= 0 states of the products. A combined investigation of the crys-
tal structural data and the Mössbauer parameters, including the
comparison with the diatomic ligands (CS, CO and CN–), present a
clear picture of the covalent bond nature of the carbene ligands.
In particular, a correlation has been established for the first time
between the iron isomer shifts and the axial distances of the :C–R
ligated porphyrin complexes. The work shall push forward the
understanding on the nature of the carbene ligands.
1
2
3
4
5
6
7
8
a
Complex
T, K
Carbene complexes
ꢀEQ
δa
ref
[Fe(TPP)(CCl2)]
295
275
225
175
125
75
2.23
0.02
0.04
0.06
0.08
0.09
0.10
0.10
0.10
tw
tw
tw
tw
tw
tw
tw
tw
2.26
2.26
2.27
2.28
2.29
2.29
+2.30
= 0.90
2.28
2.41
2.44
2.45
2.46
2.47
2.48
2.48
–2.48
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
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ASSOCIATED CONTENT
Supporting Information
25
b
[Fe(TPP)(CCl2)]HMF
4.2
η
tw
12
Materials and methods, complete structural details for three
complexes, thermal ellipsoid diagrams and mean plane diagrams
of the three structures, multi-temperature and high magnetic
field Mössbauer spectra and tables of [Fe(TPP)(CCl2)] and
[Fe(TPP)(CCl2)]
131
300
250
200
150
100
50
0.10
0.09
0.15
0.16
0.18
0.18
0.19
0.19
0.18
[Fe(TFPP)(CPh2)]
tw
tw
tw
tw
tw
tw
tw
tw
1
[Fe(TFPP)(CPh2)], details of XANES, UV-vis spectra, H NMR and
13C NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
35
b
[Fe(TFPP)(CPh2)]HMF
4.2
η
= 0.60
2.34
tw
6b
*jfli@ucas.ac.cn
Notes
The authors declare no competing financial interests.
[Fe(TFPP)(CPh2)]
288
0.03
0.10
11
–2.37(S = 0)
[Fe(II)(Porph)(XY)]
24
24
23
23
25
ACKNOWLEDGMENT
[Fe(OEP)(CS)]
[Fe(OEP)(CO)]
293
4.2
298
4.2
25
1.95
–0.03
0.08
0.14
0.27
0.37
1.93
1.81
1.84
1.83
We thank the CAS Hundred Talent Program and the National
Natural Science Foundation of China (Grant no. 21371167 to J.L.)
REFERENCES
[K(222)][Fe(TPP)(CN)]
Oxyiron porphyrin complexes
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39
PCN-224FeO2
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0.29
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