Reactivity of iron(II) 5,10,15,20-tetraaryl-21-oxaporphyrin with arylmagnesium bromide: Formation of paramagnetic six-coordinate complexes with two axial aryl groups

Article abstract of DOI:10.1021/ic0495463

Coordination of σ-aryl carbanions by chloroiron(II) 5,20-ditolyl-10,15-diphenyl-21-oxaporphyrin (ODTDPP)Fe^IICl has been followed by ¹H NMR spectroscopy. Addition of pentafluorophenyl Grignard reagent (C₆F₅)MgBr to the toluene solution of (ODTDPP)Fe^IICl in the absence of dioxygen at 205 K resulted in the formation of the high-spin (ODTDPP)-Fe^II(C₆F₅). The titration of (ODTDPP)Fe^IICl with a solution of (C ₆H₅)MgBr carried at 205 K yields a rare six-coordinate species which binds two σ-aryl ligands [(ODTDPP)Fe^II(C ₆H₅)₂]^-. Warming of the [(ODTDPP)Fe^II(C₆H₅)₂] solution above 270 K results in the decomposition to mono-σ-phenyliron species (ODTDPP)Fe^IIC₆H₅). Controlled oxidation of [(ODTDPP)Fe^II(C₆H₅)₂]^- with Br₂ affords (ODTDPP)Fe^II(C₆H ₅)Br, which demonstrates a typical ¹H NMR pattern of low-spin σ-aryl iron(III) porphyrin. The considered oxidation mechanism involves the (ODTDPP)Fe^III(C₆H₅)₂ species, which is readily reduced to the iron(I) 21-oxaporphyrin, followed by oxidation with Br₂ and replacement of one bromide anion by aryl substituent. The ¹H NMR spectra of paramagnetic iron complexes have been examined in detail. Functional group assignments have been made with the use of selective deuteration. The peculiar ¹H NMR spectral features of [(ODTDPP)Fe^II(p-CH₃C₆H₄) ₂]^- (σ-p-tolyl: ortho, 30.8; meta, 53.6; para-CH₃,42.1; furan: -16.0 β-H pyrrole: -27.5, -34.3, -41.8 ppm, at 205 K) are without a parallel to any iron(II) porphyrin or heteroporphyrin and indicate a profound alteration of the electronic structure of iron(II) porphyrin upon the coordination of two σ-aryls.

Full text of DOI:10.1021/ic0495463

Inorg. Chem. 2004, 43, 5564−5571
Reactivity of Iron(II) 5,10,15,20-Tetraaryl-21-oxaporphyrin with
Arylmagnesium Bromide: Formation of Paramagnetic Six-Coordinate
Complexes with Two Axial Aryl Groups
Miłosz Pawlicki and Lechosław Latos-Graz3yn´ski*
Department of Chemistry, UniVersity of Wrocław 14 F. Joliot-Curie Street,
Wrocław 50 383, Poland
Received April 6, 2004
II
Coordination of σ-aryl carbanions by chloroiron(II) 5,20-ditolyl-10,15-diphenyl-21-oxaporphyrin (ODTDPP)Fe Cl has
been followed by 1H NMR spectroscopy. Addition of pentafluorophenyl Grignard reagent (C F )MgBr to the toluene
6
5
II
solution of (ODTDPP)Fe Cl in the absence of dioxygen at 205 K resulted in the formation of the high-spin (ODTDPP)-
II
II
Fe (C F ). The titration of (ODTDPP)Fe Cl with a solution of (C H )MgBr carried at 205 K yields a rare six-coordinate
6
5
6 5
species which binds two σ-aryl ligands [(ODTDPP)Fe (C H ) ]-. Warming of the [(ODTDPP)Fe (C H ) ]- solution
above 270 K results in the decomposition to mono-σ-phenyliron species (ODTDPP)Fe (C H ). Controlled oxidation
of [(ODTDPP)Fe (C H ) ]- with Br2 affords (ODTDPP)Fe (C H )Br, which demonstrates a typical 1H NMR pattern
II
II
6
5 2
6 5 2
II
6
5
II
III
6
5 2
6 5
III
of low-spin σ-aryl iron(III) porphyrin. The considered oxidation mechanism involves the (ODTDPP)Fe (C H ) species,
6
5 2
which is readily reduced to the iron(I) 21-oxaporphyrin, followed by oxidation with Br2 and replacement of one
bromide anion by aryl substituent. The 1H NMR spectra of paramagnetic iron complexes have been examined in
detail. Functional group assignments have been made with the use of selective deuteration. The peculiar 1H NMR
II
spectral features of [(ODTDPP)Fe (p-CH C H ) ]- (σ-p-tolyl: ortho, 30.8; meta, 53.6; para-CH , 42.1; furan: −16.0;
3
6
4 2
3
â-H pyrrole: −27.5, −34.3, −41.8 ppm, at 205 K) are without a parallel to any iron(II) porphyrin or heteroporphyrin
and indicate a profound alteration of the electronic structure of iron(II) porphyrin upon the coordination of two
σ-aryls.
Introduction
little attention has been given to iron heteroporphyrins despite
the extensive search for suitable porphyrins and metallopor-
phyrins to act as biomimetic models of the multiple
fundamental biochemical roles played by iron protoporphyrin
IX.20 In the first case studied it was found that high-spin
The coordination chemistry of core-modified porphyrins
(21-oxaporphyrin, 21-thiaporphyrin, and 21-selenaporphyrin)
has been explored for a number of metal ions.1-19 Relatively
* Author to whom the correspondence should be addressed. E-mail:
LLG@wchuwr.chem.uni.wroc.pl.
(1) Latos-Graz˘yn´ski, L. Core Modified Heteroanalogues of Porphyrins
and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000;
Vol. 2, Chapter 14, pp 361-416.
(9) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1992, 31, 5231.
(10) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Pacholska, E. Inorg. Chem.
1994, 33, 1992.
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M.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1994, 33, 192.
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M.; Balch, A. L. Inorg. Chem. 1996, 35, 566.
(2) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. J. J. Chem. Soc. (C)
1971, 3681.
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Am. Chem. Soc. 1987, 109, 4428.
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(4) Latos-Graz˘yn´ski, L.; Lisowski, J.; Olmstead, M. M.; Balch, A. L.
Inorg. Chem. 1989, 28, 3328.
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Acta 1998, 273, 184.
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1989, 161, 153.
(6) Latos-Graz˘yn´ski, L.; Lisowski, J.; Olmstead, M. M.; Balch, A. L.
Inorg. Chem. 1989, 28, 1183.
(16) Sridevi, B.; Narayanan, S. J.; Srinivasan, A.; Chandrashekar, T. K.;
Subramanianan, J. J. Chem. Soc., Dalton Trans. 1998, 1979.
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31, 1933.
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5564 Inorganic Chemistry, Vol. 43, No. 18, 2004
10.1021/ic0495463 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/12/2004

ReactiWity of (ODTDPP)FeIICl with σ-Aryl Carbanions
sponding model complexes such as (TPP)FeIIIPh (TPP is the
dianion of meso-tetraphenylporphyrin) can be prepared by
the reaction of a Grignard reagent with (TPP)FeIIICl.26-29
These compounds can be easily oxidized to form mono-
phenyl iron(IV) porphyrins.30 Alternatively, the addition of
Grignard reagents to iron(III) porphyrin π-cation radicals
results in an analogous iron(IV) species.31 The synthesis and
characterization of σ-bonded aryl low-spin iron(III) por-
phycene were also reported.32
Chart 1
The reaction of verdoheme (OEOP)FeIIIBr2 (OEOP is the
monoanion of octaethyl-5-oxaporphyrin) with Grignard
reagents resulted in the replacement of one bromide ligand
by an aryl to yield (OEOP)FeIIIBr(Ph).33 An unprecedented
six-coordinate verdoheme complex [(OEOP)Fe(Ph)2]- that
coordinates two aryl ligands was detected starting from
(OEOP)FeIIBr. Actually, this species was also obtained from
(OEOP)FeIIIBr2 in the presence of the Grignard reagent in
excess, which acts also as the reducing reagent.
iron(II) 21-thiaporphyrin is a five-coordinate complex where
the thiophene ring coordinates in a side-on fashion.6 Sub-
sequently, the iron complexes of 5,10,15,20-tetraphenyl-21-
oxaporphyrin (OTPP)H were investigated (Chart 1).19
The charge of the ligand and the size of the coordination
cavity of 21-oxaporphyrin resemble those of N-alkylporphy-
rins, for which the coordination of iron was extensively
explored.21 2-Aza-21-carba-5,10,15,20-tetraarylporphyrin (21-
CTPPH2)H, which can be formally treated as a core-modified
porphyrin or carbaporphyrin, and its derivatives revealed a
remarkable tendency to stabilize peculiar organoiron(II) and
organoiron(III) compounds.22-24
Considering the dimension of the coordinating core and
the feasible in-plane coordination of the furan ring, the 21-
oxaporphyrin seems to provide the most suitable environment
in the group of heteroporphyrins to explore iron chemistry.
In comparison to regular iron porphyrins, the properties of
iron 21-oxaporphyrin are expected to reflect the influence
of the decreased anionic charge and the replacement of one
of the nitrogens by an oxygen atom. In our previous papers,
we have studied the coordination of iron by 5,10,15,20-
tetraphenyl-21-oxaporphyrin (Chart 1).19 The skeleton of
iron(III) 21-oxaporphyrin (OTPP)FeIIICl2 is essentially planar.
The furan ring coordinates in the η1 fashion through the
oxygen atom, which acquires trigonal geometry, with two
axial chloride ligands coordinated apically. One-electron
reduction of (OTPP)FeIIICl2 produced the high-spin five-
coordinated (OTPP)FeIICl complex 1. Moreover, titration of
(OTPP)FeIIICl2 or (OTPP)FeIICl with n-BuLi resulted in the
formation of the diamagnetic (OTPP)FeII(n-Bu), which
decomposes via a homolytic cleavage of the iron-carbon
bond to produce iron(I) 21-oxaporphyrin (OTPP)FeI.19
It is known that iron(III) porphyrins with phenyl groups
as axial ligands are paramagnetic intermediates formed
during heme degradation by aryl hydrazines.25 The corre-
1H NMR spectroscopy was shown to be a definitive
method for detecting and characterizing iron porphyrins,34,35
N-substituted iron porphyrins,21,36-39 and hemoproteins on
different coordination/oxidation states.40,41 The essential
1
progress in H NMR of paramagnetic hemoproteins can be
related to the search for suitable spectroscopic models that
mimic the properties encountered in hemoproteins for a given
electronic state. Typically the general match between the sign
of the chemical shifts in model systems and hemoproteins
has been determined for the majority of iron porphyrin
ground electronic states. In this paper, we have focused on
(26) Cocolios, P.; Lagrange, G.; Guilard, R. J. Organomet. Chem. 1983,
253, 65.
(27) Cocolios, P.; Laviron, E.; Guilard, R. J. J. Organomet. Chem. 1982,
228, C39.
(28) Ogoshi, H.; Sugimoto, H.; Yoshida, Z.-I.; Kobayashi, H.; Sakai, H.;
Maeda, Y. J. Organomet. Chem. 1982, 234, 185.
(29) Guilard, R.; van Caemelbecke, E.; Tabard, A.; Kadish, K. M. Synthesis,
Spectroscopy and Electrochemical Properties of Porphyrins with Metal-
Carbon. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 3,
Chapter 21, pp 295-345.
(30) Balch, A. L.; Renner, M. W. J. Am. Chem. Soc. 1986, 108, 2603.
(31) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Rachlewicz, K. Magn. Reson.
Chem. 1993, 31, S47.
(32) Kadish, K. M.; Tabard, A.; Van Caemelbecke, E.; Aukauloo, A. M.;
Richard, P.; Guilard, R. Inorg. Chem. 1998, 37, 6168.
(33) Lord, P.; Latos-Graz˘yn´ski, L.; Balch, A. L. Inorg. Chem. 2002, 41,
1011-1014.
(34) Walker, F. A. Proton NMR and EPR Spectroscopy of Paramagnetic
Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
5, Chapter 36, pp 81-183.
(35) Walker, F. A. Inorg. Chem. 2003, 42, 4526.
(36) Balch, A. L.; Cornman, C. R.; Latos-Graz˘yn´ski, L.; Renner, M. W. J.
Am. Chem. Soc. 1992, 114, 2230.
(37) Balch, A. L.; La Mar, G. N.; Latos-Graz˘yn´ski, L.; Renner, M. W.
Inorg. Chem. 1985, 24, 2432.
(38) Balch, A. L.; Chan, Y.-W.; La Mar, G. N.; Latos-Graz˘yn´ski, L.;
(20) Kadish, K. M., Smith, K. M., Guilard, R., Eds. Biochemistry and
Binding. In The Porphyrin Handbook; Academic Press: New York,
2000; Vol. IV.
Renner, M. W. Inorg. Chem. 1985, 24, 1437.
(39) Wysłouch, A.; Latos-Graz˘yn´ski, L.; Grzeszczuk, M.; Drabent, K.;
Bartczak, T. J. Chem. Soc., Chem. Commun. 1988, 1377.
(40) La Mar, G. N.; Satterlee, J. D.; De Ropp, J. S. Nuclear Magnetic
Resonance of Hemoproteins. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2000; Vol. 5, Chapter 37, pp 185-298.
(41) Banci, L.; Bertini, I.; Luchinat, C.; Turano, P. Solutions Structures of
Hemoproteins. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
5, Chapter 39, pp 323-350.
(21) Balch, A. L.; Cornman, C. R.; Latos-Graz˘yn´ski, L.; Olmstead, M. M.
J. Am. Chem. Soc. 1990, 112, 7552.
(22) Chen, W.-C.; Hung, C.-H. Inorg. Chem. 2001, 40, 5070.
(23) Hung, C.-H.; Chen, W.-C.; Lee, G.-H.; Peng, S.-M. Chem. Commun.
2002, 1516.
(24) Rachlewicz, K.; Wang, S.-L.; Ko, J.-L.; Hung, C.-H.; Latos-Graz˘yn´ski,
L. J. Am. Chem. Soc. 2004, 126, 4420-4431.
(25) Ortiz de Montellano, P.; Kerr, D. E. Biochemistry 1985, 24, 1147.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5565

Pawlicki and Latos-Graz3yn´ski
reagents). The effect of the ether can be seen by comparing
trace A of Figure 1 with trace B, which shows the sample
to which the first portion of the titrant has been added. The
presence of ether results in the shift of all resonances (Figure
1, trace B), and their positions remain constant in the further
titration steps. Actually, in the independent experiment it has
been demonstrated that the addition of diethyl ether alone
causes identical spectroscopic changes. The detected trans-
formation is ascribed to the formation of high-spin (ODTDPP)-
FeIICl(Et2O) 1-Et2O, favored by the low-temperature con-
ditions of the experiment. Ethers do not act as strong ligands
toward iron porphyrins. Nevertheless the coordination of
tetrahydrofuran to acetylide iron(III) porphyrins was previ-
ously reported.42 Gradual addition of the Grignard reagent
also produces a set of new resonances, already seen in trace
B as the minor component, which differs from 1 or 1-Et2O
only by the shift values, affording a straightforward assign-
ment of 21-oxaporphyrin resonances of (ODTDPP)FeII(C6F5)
2. Once formed, the species is stable in the whole investi-
gated temperature range (203-270 K). Thus the spectro-
scopic evidence is clearly consistent with the replacement
of the chloride ligand by the apically σ-coordinating C6F5
group. Presumably the molecule of ether is coordinated in
the trans position to the carbanion. It has been shown
Figure 1. 1H NMR spectra of (ODTDPP)FeIICl 1 (toluene-d8, 205 K)
after addition of (C6F5)MgBr in diethyl ether: (A) 0 equiv; (B) 0.1 equiv;
(C) 0.4 equiv. The resonances of 1-Et2O are marked with 9; gradually
appearing signals of 2 are marked with b. Trace D shows the 1H NMR
spectrum of 2. Peak assignments: pyrr, pyrrole resonances; f, furan
resonance.
-
previously that the C6F5 anion tends to stabilize the high-
-
spin σ-aryliron(III) porphyrins contrary to C6H5 , where the
low-spin ground electronic state of iron(III) is preferred.32,43,44
Here, these preferences are extended to the iron(II) oxidation
level as seen for the (ODTDPP)FeII(C6F5)-(ODTDPP)FeII-
(C6H5) pair (see below).
1
σ-aryl coordination by iron(II) 21-oxaporphyrin using H
NMR as a spectroscopic probe. In particular, we have
detected that the coordination of two axial σ-aryls to the iron-
(II) 21-oxaporphyrin switches the isotropic shift signs for
all â-pyrrole positions, suggesting the presence of the less-
common ground electronic state of iron(II).
The titration of (ODTDPP)FeIICl 1 with a solution of (p-
CH3C6H4)MgBr in diethyl ether instead of (C6F5)MgBr yields
a completely different picture as illustrated in Figure 2.
Initially 1-Et2O has been detected. Upon subsequent addition
of the titrant, the spectrum of 1-Et2O gradually disappears,
and the spectrum of a new species, designated as [(ODTDPP)-
FeII(p-CH3C6H4)2]- 3-Me is observed as seen in trace A of
Figure 2. The new species is stable to the addition of further
quantities of arylmagnesium bromide. The resonances of
3-Me have been assigned on the basis of their relative
integrated intensities and comparison with 1H NMR spectra
of the specifically deuterated sample [(ODTDPP-d6)FeII(p-
CH3C6H4)2]- 3-Me-d6 and [(ODTDPP)FeII(p-CH3C6H4)2]-
Results and Discussion
Addition of aryl Grignard reagents (C6H5)MgBr, (C6D5)-
MgBr, (C6F5)MgBr, (p-CH3C6H4)MgBr, and (4-F-C6H4)-
MgBr to the toluene-d8 solutions (205 K) of (ODTDPP)FeIICl
1 resulted in the formation of paramagnetic organoiron
species, which have been detected and subsequently char-
acterized by 1H NMR. The 1H NMR resonance assignments
have been carried out on the basis of relative intensities and
line width analysis. The use of specifically deuterated
substrates, (ODTDPP-d6)FeIICl (deuterated at pyrrole posi-
tions) and (C6D5)MgBr, allowed the assignment of â-H and
σ-coordinated phenyl resonances. In each appropriate case
the stoichiometry of the σ-phenyl adducts has been unam-
biguously determined by a careful comparison of resonance
intensities assigned to the equatorial macrocycle and σ-aryl
protons, respectively.
1
3-Me. The species 3 presents the H NMR pattern without
a parallel to any iron porphyrin or heteroporphyrin, as clearly
seen by comparison of the data gathered in Table 1. The
bis(σ-aryl) adduct 3 shows strongly upfield shifted pyrrole
and furan resonances. The acquisition of a COSY spectrum
for [(ODTDPP)FeII(p-CH3C6H5)2]- 3-Me has allowed the
pyrrole resonances to be paired. A cross-peak is observed
between the pyrrole resonances at -27.6 and -34.4 ppm
1
Figure 1 shows the H NMR spectrum that was obtained
at 205 K during the titration of a solution of (ODTDPP)-
FeIICl 1 in toluene-d8 with a solution of (C6F5)MgBr in
diethyl ether. The titration reflects the combination of two
factors: the iron/pentafluorophenyl ratio and the solvent
composition. Understanding of this process is of particular
importance since the σ-aryliron complex is typically prepared
in the presence of diethyl ether (the solvent for Grignard
(42) Balch, A. L.; Latos-Graz˘yn´ski, L.; Noll, B. C.; Phillips, S. L. Inorg.
Chem. 1993, 32, 1124.
(43) Tabard, A.; Cocolios, P.; Lagrange, G.; Gerardin, R.; Hubsch, J.;
Lecomte, C.; Zarembowitch, J.; Guilard, R. Inorg. Chem. 1988, 27,
110.
(44) Kadish, K. M.; Van Caemelbecke, E.; Gueletii, E.; Fukuzumi, S.;
Miyamoto, K.; Suenobu, T.; Tabard, A.; Guilard, R. Inorg. Chem.
1998, 37, 1759.
5566 Inorganic Chemistry, Vol. 43, No. 18, 2004

ReactiWity of (ODTDPP)FeIICl with σ-Aryl Carbanions
Table 1. Chemical Shifts of Organoiron Porphyrin Complexesa
σ-aryls
compound
furan
pyrrole
o
m
p
ref
(ODTDPP)FeIICl 1
40.5
46.9
46.4
-15.3
-17.2
-16.0
-14.8
48.9, 38.8, -15.6
56.4, 38.7, -15.5
(ODTDPP)FeIICl(Et2O) 1-Et2O
(ODTDPP)FeII(C6F5) 2
58.9, 35.5, -23.1
[(ODTDPP)FeII(C6H5)2]- 3-Hb
-26.5, -32.7, -39.9
-33.2,-34.0,-42.9
-27.5, -34.3, -41.8
-26, -32.2, -39.2
32.3
14.1
30.8
52.2
46.7
53.6
51.9
51.2
5.84
14.5
170e
-4.47
43
20
23
-71
-67
51.65
5.22
-1.8
-
42.1
42.2
[(ODTDPP)FeII(4-F-C6H5)2]- 3-F
[(ODTDPP)FeII(p-MeC6H5)2]- 3-Me
[(ODTDPP)FeII(C6H5)(p-MeC6H5)2]- 3-(H,Me)b
33.7
27.2
(ODTDPP)FeII(C6H5) 4
(ODTDPP)FeIII(C6H5)Br 5c
(STPP)NiII(C6H5)d
9.09, 8.94, 8.79, 8.76
nol
6.02
-10.0
-58.0f
-31.35
-22.9, -28.0, -42.3
87.0, 35.4, 2.0
-143.05
-100.2
-37.8
616e
76e
9
14
33
46
46
30
30
47
47
[(O2TPP)NiII(C6H5)2]b
[(OEOP)Fe(C6H5)2]- d,g
(TTP)FeIII(C6H5)h,i
5.5e
-6.5
-111
-113
10
-30
89
-113
114
-28
(TPP)FeIII(p-Tol)h,b
-30
(TTP)FeIV(C6H5)Brh,i
(TPP)FeIV(p-Tol)Brh,b
(TPP)RuIII(C6H5)j,k
-60
-63
-290
-89.53
1.31
-30.94
8.35
-57.47
5.63
h,j
(TPP)RuIV(C6H5)2
e
a All values were obtained at 205 K (toluene-d8) unless otherwise indicated, b 215 K, c 243 K, d 203 K, 2H NMR, f Thiophene, g CD2Cl2, h CDCl3,
i 223 K, j RT, k Benzene. l no, not observed. This signal is expected in the region of intense, solvent signals (1-2 ppm).
Figure 2. 1H NMR spectra of (A) [(ODTDPP)FeII(p-CH3C6H4)2]- 3-Me
(1H NMR, toluene-d8, 205 K); (B) [(ODTDPP-d6)FeII(p-CH3C6H4)2]- 3-d6
(1H NMR, toluene, 205 K). Peak labels: pyrr, pyrrole resonances; f, furan
resonance; o, m, p-CH3, resonances of ortho, meta, and para signals of σ-p-
tolyl rings.
Figure 3. Curie plots (chemical shifts vs 1/T) for pyrrole, furan, and σ-(p-
tolyl) of [(ODTDPP)FeII(p-CH3C6H5)2]- 3-Me in toluene-d8.
downfield position of p-CH3 complies with the unusual
spectroscopic pattern of the σ-coordinated aryl ligand in
3-Me. The other Grignard reagents applied in titration lead
to the analogus bis-aryl complexes [(ODTDPP)FeII(C6H5)2]-
3-H and [(ODTDPP)FeII(4-F-C6H4)2]- 3-F as confirmed by
(205 K), which have been assigned to the unequivalent,
vicinal coupled H(7)-H(8) (H(17)-H(18)) pairs of protons.
The most upfield pyrrole resonance at -41.8 (205 K) has
been assigned to H(12), H(13) by default. The ortho and
meta proton resonances of σ-tolyl are located in the low-
field part of the spectrum at 30.9 and 53.7 ppm (205 K).
The assumption of metal-centered paramagnetic relaxation
yields to the anticipation that the line width for the various
protons will be proportional to r-6 where the r is the distance
from the iron to the proton in question.45 Thus these two
resonances could be differentiated by the line widths, which
are expected to be larger for the ortho resonance due its
proximity to the iron. The resonance observed at 42.2 ppm
(205 K) was assigned to the p-CH3 group by comparison of
the relative intensities of the axial ligand resonances. The
1
the similarity of their H NMR spectra (Table 1).
1
A plot of the chemical shifts of the H NMR resonances
of [(ODTDPP)FeII(p-Tol)2]- 3-Me versus 1/T are linear
within the accessible temperature range (from 203 to 273
K) over which 3-Me could be observed (Figure 3).
The pattern of resonances seen in Figure 2 is consistent
with the formation of [(ODTDPP)FeII(p-CH3C6H4)2]- 3-Me
as shown in Scheme 1. Consideration of the integrated
intensities of the σ-aryl resonances and the integrated
intensities of the pyrrole and furan resonances indicates that
there are two aryl groups axially bound to the iron(II) 21-
oxaporphyrin. To confirm this point, the experiments with
(C6H5)MgBr, (p-CH3C6H4)MgBr, and a mixture of (C6H5)-
MgBr and (p-CH3C6H4)MgBr (1:1 molar ratio) were per-
formed. All spectra were collected at the same temperature
with similar concentrations of reagents. The results are shown
(45) Swift, T. J. The Paramagnetic Linewidth. In NMR of Paramagnetic
Molecules. Principles and Applications; La Mar, G. N., Horrocks, W.
D., Jr., Holm, R. H., Eds.; Academic Press: New York, 1973; Chapter
2, pp 53-83.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5567

Pawlicki and Latos-Graz3yn´ski
Scheme 1. Coordination of σ-Aryl Ligand by Iron(II)
21-Oxaporphyrin
Figure 4. 1H NMR spectra of (A) [(ODTDPP)FeII(C6H5)2]- 3-H; (C)
[(ODTDPP)FeII(p-CH3C6H4)2]- 3-Me; and (B) a mixture of [(ODTDPP)FeII-
(C6H5)2]- 3-H, [(ODTDPP)FeII(p-CH3C6H4)2]- 3-Me, and [(ODTDPP)FeII-
(C6H5)(p-CH3C6H4)2]- 3-(H,Me) at 215 K. In trace B, the resonances of
the mixed-ligand complex, [(ODTDPP)FeII(C6H5)(p-CH3C6H4)2]- 3-(H,Me)-
are marked. Inset in trace B presents the selected region where the presence
of three lines assigned to three different species is clearly observed.
in Figure 4. Trace A shows the spectrum of [(ODTDPP)FeII-
(C6H5)2]- 3-H while trace C shows the corresponding portion
of the spectrum of [(ODTDPP)FeII(p-CH3C6H4)2]- 3-Me.
Trace B shows the spectrum obtained by adding a
1:1 mixture of (C6H5)MgBr and (p-CH3C6H4)MgBr to
(ODTDPP)FeIICl. In addition to the resonances of 3-H and
3-Me, there are new resonances assigned to the mixed-ligand
species, [(ODTDPP)FeII(C6H5)(p-CH3C6H4)2]- 3-(H,Me).
The signals of 3-(H,Me) include a methyl resonance at 42.2
ppm (215 K) for the p-tolyl group, two meta resonances (51.9
and 51.2 ppm, 215 K), and two ortho resonances (33.7 and
27.2 ppm, 215 K). The mixed species 3-(H,Me) contains two
different σ-aryl ligands. Consequently, a single meta (ortho)
resonances of 3-Me (3-H) is replaced for 3-(H,Me) by two
meta (ortho) resonances of the same intensity coming from
the σ-phenyl and σ-(p-tolyl) of 3-(H,Me), respectively. A
complete set of pyrrole and furan resonances has been also
assigned to 3-(H,Me) at the upfield region of the spectrum
(Figure 4, trace B).
Figure 5. Upfield region of 1H NMR spectrum of [(ODTDPP)FeIII(C6H5)-
Br] 5 (toluene-d8, 243 K) obtained after addition of Br2 to [(ODTDPP)FeII-
(C6H5)2]- 3-H. Inset presents the downfield region of spectra with meta
signal of the axial phenyl. Peak labels: pyrr, pyrrole resonances; f, furan
resonance; o, m, p, resonances of ortho, meta, and para signals of σ-phenyl
ring.
Warming of a solution of [(ODTDPP)FeII(C6H5)2]- 3-H
above 270 K results in the disappearance of the resonances
of this complex in a matter of minutes. The decomposition
product has been identified as the diamagnetic (ODTDPP)-
FeII(C6H5) 4. Three resonances are expected for a σ-co-
ordinated phenyl, but only two of them are observed. That
resonances are upfield shifted by the macrocyclic ring
currents (Table 1), similarly as in metalloporphyrins bearing
an axial aryl ligand.47 The pyrrole and furan resonances are
located at 8.79, 8.76 (H2(H3), H12(H13)) 9.09, 8.94
(H7(H18), H8(H17)) ppm. Integration of the â-H and the
axial ligand resonances indicates that one axial ligand is
present in the iron(II) complex (Table 1). The coordination
of σ-aryl ligand by iron(II) 21-oxaporphyrin examined in
this study are summarized in Scheme 1.
The newly detected [(ODTDPP)FeII(C6H5)2]- 3-H is un-
stable in solution and requires protection from atmospheric
dioxygen. Controlled oxidation of [(ODTDPP)FeII(C6H5)2]-
with Br2 yields [(ODTDPP)FeIII(C6H5)Br] 5, which presents
1
the typical H NMR features of a low-spin σ-aryl iron(III)
porphyrin (Figure 5).29,34
Paradoxically, the oxidation reaction mechanism may
involve the reduction step to generate the known iron(I)
oxaporphyrin intermediate as shown in Scheme 2. Hypo-
thetically, one-electron oxidation yields (ODTDPP)FeIII-
(C6H5)2, which is expected to be readily reduced in a two-
(46) Balch, A. L.; Renner, M. W. Inorg. Chem. 1986, 25, 303.
(47) Ke, M.; Sishta, C.; James, B. R.; Dolphin, D.; Sparapany, J. W.; Ibers,
J. A. Inorg. Chem. 1991, 20, 4766.
5568 Inorganic Chemistry, Vol. 43, No. 18, 2004

ReactiWity of (ODTDPP)FeIICl with σ-Aryl Carbanions
Scheme 2. Oxidation of 3-H
of 1H shifts does not correspond to any previously observed
σ-aryliron(III) porphyrins. Another possibility could include
Fe(I), which is d7, but the known spectroscopic features of
(ODTDPP)FeI and the fact that the iron(I) species does not
coordinate any axial ligand exclude such a possibility.19
Concluding, only the iron(II) oxidation state is consistent
with the detected reactivity of [(ODTDPP)FeII(C6H5)2]- 3-H.
1H NMR studies of high-spin iron(II) porphyrins demon-
strated that the dipolar contribution to the paramagnetic shift
is rather small.38,49-54 The contact shift pattern is consistent
2
1
1
1
2
with the high-spin electronic structure (dxy) (dxz) (dyz) (dz ) -
1
2
2
(dx -y ) and a dominance of a σ-delocalization mechanism.
An additional contribution of π-delocalization mechanism
in the porphyrin ring was also noted.53,54 Usually the
indication of the significant role of the contact shift for
tetraarylporphyrins can be derived from the shift pattern of
meso-aryl resonances. It was shown that in the paramagnetic
complexes of porphyrins and their analogues this pattern
depends on whether the contact or dipolar shift dominates.34
In particular, the contact shift mediated by π orbitals of the
meso-aryl results in a characteristic sign alteration of the
shifts experienced by the ortho, meta, and para protons of
the meso-phenyls. In the present case the shift pattern
displayed by the identified meso substituent signals (5.55
ppm (ortho) 5.90 ppm (para), other resonances are hidden
under the intense solvent signals) confirm that the dipolar
contribution is negligible. To show qualitatively that the
observed spread of shifts is not a result of dipolar contribu-
tion, we have assumed a purely dipolar shift of σ-aryl
resonances and performed geometric factor calculations
adjusting the orientation of the main magnetic axis along
Fe-Cipso. The predicted order of the paramagnetic shifts:
ortho > meta > p-CH3 is clearly inconsistent with the
experimental spectra.
electron process. Eventually in the presence of Br2 and the
Grignard reagent this reaction stops at the formation of 5.
While the spectroscopic properties of 3-H are consistent
with the [(ODTDPP)FeII(C6H5)2]- stoichiometry, the elec-
tronic structure of the compound remains to be discussed.
Generally, the straightforward and unambiguous correla-
1
The pattern of H NMR spectra of 3 is unlike those of any
1
tion between the H NMR pattern and the molecular and
other paramagnetic σ-aryliron porphyrin and σ-aryliron
heteroporphyrins now known.19,26,29,30,34,46,48 Note that
[(ODTDPP)FeII(C6H5)2]- can be prepared starting with the
high-spin iron(II) 21-oxaporphyrin (ODTDPP)FeIIBr. Alter-
natively, the stepwise addition of the aryl Grignard to
(ODTDPP)FeIIIBr2 originally results in the formation of
(ODTDPP)FeIIBr species, which is then converted into
[(ODTDPP)FeII(C6H5)2]- 3-H. Significantly, the treatment
of (ODTDPP)FeIIIBr2 with two equivalents of PhMgBr yields
(ODTDPP)FeI directly and cleanly at 205 K characterized
electronic structure has been determined for paramagnetic
metalloporphyrins and metalloheteroporphyrins containing
the σ-aryl ligand.19,26,29,34,46,48 Usually, the protons of a phenyl
σ-bounded to a five-coordinated Ni(II) display substantial
downfield contact shifts (Table 1), which are indicative of
the σ-contact contribution and is consistent with the
2
2
2
1
1
2
2
2
(dxy) (dxz) (dyz) (dz ) (dx -y ) ground electronic state and the
9,14,15,55
2
presence of the unpaired electron on the dz orbital.
The isotropic shifts of the equatorial ligand are also
dominated by the same effect revealing resonances in the
0-60 ppm region. The examination of the isotropic shifts
1
by distinct H NMR spectrum as described in detail previ-
ously.19 We have also found that the iron(I) 21-oxaporphyrin
(ODTDPP)FeI could not be detected once (ODTDPP)FeIIBr
has been used as a substrate in titration with aryl Grignard
reagents. Significantly the heterolytic dissociation of
[(ODTDPP)FeII(C6H5)2]- yields (ODTDPP)FeII(C6H5), and
the redox reactions are clearly not a part of the chemistry
that occurs here. It is unlikely that Fe(III) is involved since
we definitely observe reduction occurring during the addition
of the Grignard reagent to (ODTDPP)FeIIIBr2. The pattern
2
3
0
2
for low-spin σ-phenyliron(III), (dxy) (dxz,dyz) (dz ) -
(dx -y )
0 26,29,34,46,48; σ-phenyliron(IV), tetraarylporphyrins
2
2
((dxy) (dxz) (dyz) (dz ) (dx -y )
0 30; and σ-phenylruthenium(IV)
2
1
1
0
2
2
2
octaethylporphyrin, (dxy) (dxz) (dyz) (dz ) (dx -y )
0 47 indicated
2
1
1
0
2
2
2
a large amount of π-spin density at the axial phenyl ligand
(49) Goff, H. M.; La Mar, G. N. J. Am. Chem. Soc. 1977, 99, 6599.
(50) Mispelter, J.; Momenteau, M.; Lhoste, J. M. Biochimie 1981, 63, 911.
(51) Latos-Graz˘yn´ski, L. Biochimie 1983, 65, 143.
(52) Medhi, O. K.; Mazumdar, S.; Mitra, S. Inorg. Chem. 1989, 28, 3243.
(53) Yu, B.-S.; Goff, H. M. J. Am. Chem. Soc. 1989, 111, 6558.
(54) Shin, K.; Kramer, K.; Goff, H. M. Inorg. Chem. 1987, 26, 4103.
(55) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 2000, 39, 5639.
(48) Kadish, K. M.; Tabard, A.; Lee, W.; Liu, Y. H.; Ratti, C.; Guilard, R.
Inorg. Chem. 1991, 30, 1542.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5569

Pawlicki and Latos-Graz3yn´ski
because of overlapping of dxz and/or dyz orbitals with occupied
of an intermediate spin state for six-coordinate iron(II)
complexes was also reported and explained in terms of a
major distortion from the octahedral symmetry and degen-
π orbitals of axial aryl(s). Thus, the σ-aryl contains unpaired
2
electron(s), but the dz orbital is not occupied. Some
contribution of the σ-mechanism was also suggested.9 The
marked upfield shifts for ortho and para resonances and the
downfield shift for the meta is typical of such a mechanism.34
eracy of one-electron dxz, dyz, and dz orbitals.58 Although
2
the available NMR data do not permit further characterization
of the electronic state of the thermally unstable σ-aryl(II)
21-oxaporphyrin 3, they clearly identify the peculiar ground
electronic state of iron(II) porphyrins, which definitely
requires extensive theoretical studies.
1
Qualitatively the H NMR pattern detected for 3 can be
viewed as a hybrid of two extremes described above,
suggesting the possibility of coexistence of σ and π delo-
calization mechanisms. For that reason, the relatively small
downfield isotropic shifts of the σ-p-tolyl protons (60-20
ppm), which decrease in the peculiar order of meta > p-CH3
> ortho, have been determined for 3-Me. Contrary to p-CH3
of the 3-Me signal, the para-H resonance of 3-H is located
in an upfield position. Thus the replacement of the proton
by the methyl group produces the contact shift of the opposite
sign, as expected for the π-delocalization mechanism. The
peculiar spectral features of 3 indicate a profound alteration
of the electronic structure of iron(II) ion upon the coordina-
tion of two σ-phenyls (Figures 2 and 4, Table 1). The upfield
positions of the â-H macrocycle resonances imply the
domination of the π spin delocalization mechanism that
involves the π-orbital framework of 21-oxaporphyrin.34 The
pattern is indicative of a negligible σ-contact contribution
and is consistent with the absence of unpaired electron on
Now we can return to the discussion of the two other bis-
(σ-phenyl) paramagnetic metalloporphyrin systems where the
electronic state, as characterized by 1H NMR spectroscopy,
revealed the unusual features in comparison to the “regular”
σ-monoaryl species. Thus (O2TPP)NiII(C6H5)2 presents the
strongly upfield position of â-H pyrrole (-143.1 ppm) and
furan (-31.35 ppm) resonances accompanied by rather minor
isotropic shifts of σ-aryls (ortho 5.5, meta, -4.47 ppm, 243
K).14 The pattern of σ-phenyl resonances assigned for the
six-coordinate verdoheme complex [(OEOP)Fe(Ph)2]- (ortho,
-6.5, meta, 43; para, 10 ppm, 203 K) resembles the one
determined in this work for [(ODTDPP)FeII(C6H5)2]-.33
Consequently, the comparison of the spectroscopic properties
of [(ODTDPP)FeII(C6H5)2]- and [(OEOP)Fe(C6H5)2]- seems
to favor the less-common ground electronic state of iron(II)
in the verdoheme case as well.
2
2
the dx -y orbital. Hence, the iron(II) d orbitals that contribute
to the single occupied molecular orbitals must be those of
π-symmetry (i.e., dxz and dyz). An interpretation of this spin
delocalization requires a change of the metal ion ground state
In conclusion, the modification of the porphyrin core by
the introduction of an oxygen atom seems to be crucial for
stabilization of the organometallic derivatives of iron(II). The
nature of the equatorial ligand seems to be instrumental in
determining the stoichiometry of organoiron(II) species
2
1
1
1
1
2
2
2
from (dxy) (dxz) (dyz) (dz ) (dx -y ) that is common for high-
1
spin iron(II) macrocycles including the monoaryl adduct 2
trapped at low temperature. The H NMR spectral pattern
2
2
1
1
0
2
2
2
to involve sizable contributions of (dxy) (dx -y ) (dyz) (dxz) (dz ) ,
of 3 reveals the less-common ground electronic state of iron-
(II) porphyrins. Previously the variations of the spin density
sings of â-H resonances were reported for five-coordinate
high-spin core modified iron(II) porphyrins including N-alkyl
porphyrins.38,49 This work documents the peculiar case where
the axial coordination to iron(II) switches the sign of spin
density for each â-position. Thus the unique 1H NMR
spectrum of 3 provides a new entry to the increasing
collection of paramagnetic iron(II) porphyrins.
The recent reports on high-spin iron(II) hemoproteins are
of particular interest in light of our findings. These studies
provided a detailed assignment of resonances of the heme
pocket for Rhodobacter capsulatus ferrocytochrome c′,59
ferrous Aplysia limacine,60 and ferrous mamalian myoglo-
bins.61 Consequently, these allowed a firm separations of the
contact and dipolar shifts. Significantly, the sign alternations
of spin densities at the adjacent pyrolic â-positions have been
unambiguously established for ferrocytochrome c′ 59 and A.
limacine myoglobin.60 In contrast, the uniformly positive spin
density at all â-pyrrole positions was determined for ferrous
mamalian myoglobins.61 As discussed above, the available
2
2
1
1
2
1
2
1
1
1
2
2
2
2
2
(dxy) (dx -y ) (dyz,dxz) (dz ) (S ) 1), (dx -y ) (dz ) (dxy) (dyz) (dxz)
(S ) 2), or (dx -y ) (dz ) (dxy) (dyz) (dxz) (S ) 2) electronic
configurations for 3, respectively. Alternatively, one can
consider three electronic structures involving the dz orbital,
and left unoccupied dx -y one: (dxy) (dyz) (dxz) (dz ) (dx -y ) ,
2
1
2
1
1
1
2
2
2
2
1
1
2
0
2
2
2
2
2
2
1
2
1
0
2
2
1
1
0
2
2
2
2
2
2
(dxy) (dyz) (dxz) (dz ) (dx -y ) , (dxy) (dyz) (dxz) (dz ) (dx -y ) (S
) 1). Such a profound transformation of the ground
electronic state cannot be accounted for merely by a choice
of σ- or π-donating properties of the aryl ligand but results
primarily from the simultaneous coordination of two σ-aryl
ligands. One can note that the coordination of a single
aryl yields predictably high-spin (ODTDPP)FeII(C6F5) 2
and diamagnetic (ODTDPP)FeII(C6H5) 4.56 Consistently
(ODTDPP)FeIII(C6H5)Br 5 presents the typical spectrum of
low-spin σ-aryliron(III) porphyrins.26,29,34,46,48 Stabilization
of the less-common paramagnetic ground electronic state of
the six-coordinate iron(II) can be rationalized on the basis
of effective and simultaneous σ- and π-aryl donation to the
2
iron. As a result the dxz, dyz, and dz orbitals acquire similar
energy, affording one of the electronic configurations
described above. For the sake of comparison, it is worth to
admit that four-coordinate iron(II) porphyrins favor the
(58) Figg, D. C.; Herber, R. H.; Felner, I. Inorg. Chem. 1991, 30, 2535.
(59) Tsan, P.; Caffrey, M.; Lawson Daku, L.; Cusanovich, M.; Marion,
D.; Gans, P. J. Am. Chem. Soc. 1999, 121, 1795.
2
2
electronic configuration (dxy) (dz ) (dyz,dxz)2.57 The existence
2
(60) Ma, D.; Musto, R.; Smith, K. M.; La Mar, G. N. J. Am. Chem. Soc.
2003, 125, 8494.
(61) Bougault, C. M.; Dou, Y.; Ikeda-Saito, M.; Langry, K. C.; Smith, K.
M.; La Mar, G. N. J. Am. Chem. Soc. 1998, 120, 2113.
(56) Tahiri, M.; Doppelt, P.; Fischer, J.; Weiss, R. Inorg. Chem. 1988, 27,
2897.
(57) Goff, H. M.; La Mar, G. N. J. Am. Chem. Soc. 1977, 99, 6606.
5570 Inorganic Chemistry, Vol. 43, No. 18, 2004

ReactiWity of (ODTDPP)FeIICl with σ-Aryl Carbanions
the case of low-temperature studies, the sample was removed from
the glovebox and cooled to 195 K in the ethanol bath, which was
chilled by the addition of sufficient amount of liquid nitrogen to
reach 195 K. Subsequently aliquots of a 1 M solution of phenyl-
magnesium bromide (or other Grignard reagent in diethyl ether)
were introduced into the sample through the microsyringe. The
sample was shaken in the cold bath and transferred to the precooled
and typical spectroscopic modelsshigh-spin five coordinate
iron(II) porphyrinssdemonstrate the low-field positions of
â-H and methyl resonances as clearly found in deoxy
myoglobins.61 The compounds considered in this paper made
it clear that the upfield â-H contact shift is evidently
acceptable for axially coordinate iron(II) porphyrins.
1
NMR probe, and the progress of the reaction was followed by H
NMR spectroscopy.
Experimental Section
Preparation of Compounds. The iron(II) 21-oxaporphyrins
(ODTDPP)FeIICl 1 and (ODTDPP-d6)FeIICl 1-d6 were prepared as
described previously.19 Grignard reagents (C6H5)MgBr, (C6D5)MgBr
(C6F5)MgBr, (p-CH3C6H4)MgBr, and (4-F-C6H4)MgBr as solutions
in diethyl ether (Aldrich) have been used as received.
Instrumentation. All NMR spectra were recorded on a Bruker
Avance 500 spectrometer operating in the quadrature mode. An
exponential window function has been applied to improve the
signal-to-noise ratio. The residual 1H resonances of the deuterated
solvents were used as secondary references.
Phenylmagnesium Bromide Addition to 1. Samples for spec-
troscopic study were prepared in dioxygen-free solvents under
dinitrogen atmosphere in a glovebox. Sample concentration for the
iron complexes were in the range 1-3 mM. The sample was placed
in a 5 mm NMR tube and carefully sealed with a septum cap. In
Acknowledgment. This work was supported by the State
Committee for Scientific Research of Poland (KBN) under
Grant 4 T09A 147 22.
IC0495463
Inorganic Chemistry, Vol. 43, No. 18, 2004 5571