Five- and six-coordinate adducts of nitrosamines with ferric porphyrins: Structural models for the type II interactions of nitrosamines with ferric cytochrome P450

Article abstract of DOI:10.1021/ic901751z

Nitrosamines are well-known for their toxic and carcinogenic properties. The metabolic activation of nitrosamines occurs via interaction with the heme-containing cytochrome P450 enzymes. We report the preparation and structural characterization of a number of nitrosamine adducts of synthetic iron porphyrins. The reactions of the cations [(por)Fe(THF)₂]ClO ₄ (por = TPP, TTP, OEP) with dialkylnitrosamines (R₂NNO; R₂ = Me₂, Et₂, (cyclo-CH₂) ₄, (cyclo-CH₂)₅, (PhCH₂) ₂) in toluene generate the six-coordinate high-spin (S = 5/2) [(por)Fe(ONNR₂)₂]ClO₄ compounds and a five-coordinate intermediate-spin (S = 3/2) [(OEP)Fe(ONNMe₂)]ClO ₄ derivative in 57-72% yields (TPP = 5,10,15,20- tetraphenylporphyrinato dianion, TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion). The N-O and N-N vibrations of the coordinated nitrosamine groups in [(por)Fe(ONNR ₂)₂]ClO₄ occur in the 1239-1271 cm^-1 range. Three of the six-coordinate [(por)Fe(ONNR₂) ₂]ClO₄ compounds and one five-coordinate [(OEP)Fe(ONNMe₂)]ClO₄ compound have been characterized by single crystal X-ray crystallography. All the nitrosamine ligands in these complexes bind to the ferric centers via a sole η¹-O binding mode. No arylnitrosamine adducts were obtained from the reactions of the precursor compounds [(por)Fe(THF)₂]ClO₄ with three arylnitrosamines (Ph₂NNO, Ph(Me)NNO, Ph(Et)NNO). However, prolonged exposure of [(por)Fe(THF)₂]ClO₄ to these arylnitrosamines resulted in the formation of the known five-coordinate (por)Fe(NO) derivatives. The latter (por)Fe(NO) compounds were obtained more readily by the reactions of the three arylnitrosamines with the four-coordinate (por)Fe^II precursors.

Full text of DOI:10.1021/ic901751z

Inorg. Chem. 2010, 49, 4405–4419 4405
DOI: 10.1021/ic901751z
Five- and Six-Coordinate Adducts of Nitrosamines with Ferric Porphyrins:
Structural Models for the Type II Interactions of Nitrosamines with Ferric
Cytochrome P450
Nan Xu,† Lauren E. Goodrich,‡ Nicolai Lehnert,*,‡ Douglas R. Powell,† and George B. Richter-Addo*,†
†Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman,
Oklahoma 73019, and ‡Department of Chemistry, University of Michigan, 930 North University Avenue,
Ann Arbor, Michigan 48109
Received September 2, 2009
Nitrosamines are well-known for their toxic and carcinogenic properties. The metabolic activation of nitrosamines
occurs via interaction with the heme-containing cytochrome P450 enzymes. We report the preparation and structural
characterization of a number of nitrosamine adducts of synthetic iron porphyrins. The reactions of the cations
[(por)Fe(THF)2]ClO4 (por = TPP, TTP, OEP) with dialkylnitrosamines (R2NNO; R2 = Me2, Et2, (cyclo-CH2)4, (cyclo-
CH2)5, (PhCH2)2) in toluene generate the six-coordinate high-spin (S = 5/2) [(por)Fe(ONNR2)2]ClO4 compounds
and a five-coordinate intermediate-spin (S = 3/2) [(OEP)Fe(ONNMe2)]ClO4 derivative in 57-72% yields (TPP =
5,10,15,20-tetraphenylporphyrinato dianion, TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, OEP = 2,3,7,8,12,-
13,17,18-octaethylporphyrinato dianion). The N-O and N-N vibrations of the coordinated nitrosamine groups in
[(por)Fe(ONNR2)2]ClO4 occur in the 1239-1271 cm-1 range. Three of the six-coordinate [(por)Fe(ONNR2)2]ClO4
compounds and one five-coordinate [(OEP)Fe(ONNMe2)]ClO4 compound have been characterized by single crystal
X-ray crystallography. All the nitrosamine ligands in these complexes bind to the ferric centers via a sole η1-O binding
mode. No arylnitrosamine adducts were obtained from the reactions of the precursor compounds [(por)Fe(THF)2]ClO4
with three arylnitrosamines (Ph2NNO, Ph(Me)NNO, Ph(Et)NNO). However, prolonged exposure of [(por)Fe(THF)2]ClO4
to these arylnitrosamines resulted in the formation of the known five-coordinate (por)Fe(NO) derivatives. The latter
(por)Fe(NO) compounds were obtained more readily by the reactions of the three arylnitrosamines with the four-
coordinate (por)FeII precursors.
Introduction
nitrosation of amines in the acidic stomach,5 or by the
macrophage-assisted nitrosation of amines.6-8
Nitrosamines (R2N-NdO; R = alkyl, aryl) belong to the
class of N-nitroso compounds. These include the simple
dialkylnitrosamines such as dimethylnitrosamine and
diethylnitrosamine, and elaborate examples such as nitro-
soureas and nitrosonicotine.1,2 Nitrosamines occur in a wide
range of environments such as cigarette smoke, beer, saliva,
and in rubber nipples for feeding bottles.3,4 Endogenous
sources of nitrosamines are generally from the in vivo
Nitrosamines are generally considered to be carcinogenic.
They require metabolic activation by the heme-containing
enzyme cytochrome P450 in the liver, lung, or nasal tissues
to exert their carcinogenic effects.1,9-12 The interaction of
nitrosamines with the active site of cytochrome P450 can
occur via two main paths. Several years ago, Appel and co-
workers demonstrated that nitrosamines bind to the active
*To whom correspondence should be addressed. E-mail: lehnertn@
umich.edu (N.L.), grichteraddo@ou.edu (G.B.R.-A.).
(1) Nitrosamines and Related N-Nitroso Compounds. Chemistry and Bio-
chemistry; Loeppky, R. N., Michejda, C. J., Eds.; American Chemical Society:
Washington, D.C., 1994; Vol. 553.
(2) Nitrosamines: Toxicology and Microbiology; Hill, M. J., Ed.; VCH Ellis
Horwood Ltd.: Chichester, England, 1988.
(3) Lijinsky, W. Chemistry and Biology of N-Nitroso Compounds;
Cambridge University Press: Cambridge, 1992.
(4) Tannenbaum, S. R.; Archer, M. C.; Wishnok, J. S.; Bishop, W. W.
J. Nat. Cancer Inst. 1978, 60, 251–253.
(6) Iyengar, R.; Stuehr, D. J.; Marletta, M. A. Proc. Natl. Acad. Sci.,
U.S.A. 1987, 84, 6369–6373.
(7) Wu, Y. N.; Brouet, I.; Calmels, S.; Bartsch, H.; Ohshima, H.
Carcinogenesis 1993, 14, 7–10.
(8) Ohshima, H.; Tsuda, M.; Adachi, H.; Ogura, T.; Sugimura, T.; Esumi,
H. Carcinogenesis 1991, 12, 1217–1220.
(9) Human Cytochrome P450 Enzymes, 3rd ed.; Guengerich, F. P., Ed.;
Kluwer Academic/Plenum: New York, 2005.
(10) Yang, C. S.; Smith, T. J. Adv. Exp. Med. Biol. 1996, 387, 385–394.
(11) Ioannides, C.; Gibson, G. G. In Safety Evaluation of Nitrosatable
Drugs and Chemicals; Gibson, G. G., Ioannides, C., Eds.; Taylor and Francis:
London, 1981; pp 257-278.
(5) Lintas, C.; Clark, A.; Fox, J.; Tannenbaum, S. R.; Newberne, P. M.
Carcinogenesis 1982, 3, 161–165.
(12) Hecht, S. S. Chem. Res. Toxicol. 1998, 11, 559–603.
r
2010 American Chemical Society
Published on Web 04/14/2010
pubs.acs.org/IC

4406 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Figure 2. Crystallographically determined binding modes of nitrosa-
mines to metal centers.
Figure 1. Type I and Type II interactions of nitrosamines with the active
site of cytochrome P450.
from nucleophilic attack of primary amines on the coordi-
nated nitrosyl ligand in K[IrCl5NO].23,24
To further probe the interactions of nitrosamines with
heme, we embarked on a systematic study of the reactions of
nitrosamines with both ferric and ferrous porphyrins, with
the goal of establishing the conditions that are suitable for
simple adduct formation of the Type II class and the condi-
tions that promote denitrosation of the nitrosamines. In this
article, we report the successful preparation, isolation, and
crystallographic characterization of non-cyclic and cyclic
dialkylnitrosamine adducts of ferric porphyrins. We show
that both six-coordinate and five-coordinate complexes can
be obtained. This allows, for the first time, for a comparison
between the geometric parameters of bound dialkylnitrosa-
mines to ferric porphyrins as a function of nitrosamine alkyl
substitution, and a comparison of the reactivity of dialkyl
and aryl nitrosamines with iron porphyrins. In addition,
low-temperature electron paramagnetic resonance (EPR)
spectroscopy has been employed on solid state and frozen
solution samples to ascertain the spin states of these ferric
nitrosamine complexes. To the best of our knowledge, these
compounds, together with our previously reported [(TPP)-
Fe(ONNEt2)2]þ compound,21,22 represent the only structural
models of the Type II interactions between nitrosamines and
heme known to date.
site of cytochrome P450 through both Type I and Type II
mechanisms as shown in Figure 1.13
The Type I interaction is typical of cytochrome P450
monooxygenase systems in their interaction with substrates,
where the nitrosamine binds to the protein pocket site near
the iron center in preparation for its R-hydroxylation and
subsequent chemical modification to generate highly reactive
organic species. The Type II interaction places the nitrosa-
mine in direct contact with the iron center in cytochrome
P450. Despite this important observation by Appel and
co-workers, very little is known about the Type II heme-
nitrosamine interaction.
The reactions of dialkylnitrosamines with synthetic iron
and manganese porphyrins in the presence of oxidants have
yielded decomposition products such as the aldehydes,
presumably via initial R-hydroxylation of the nitrosamine.14,15
Interestingly, exposure of phenobarbital-treated mouse liver to
diethylnitrosamine resulted in the formation of the N-alkylated
green pigment N-hydroxyethylprotoporphyrin IX.16
Nitrosamines have been shown to bind to metal centers via
one of four binding modes as illustrated in Figure 2.
The binding mode A was the first to be established by
X-ray crystallography in 1968, and this was for a copper
complex oligomer of the form {(Me2NNO)CuCl2}n.17,18 The
binding mode B was established in 1980 for a Pd complex of
the form Pd{N(O)N(Me)C6H4}Cl(PPh3).19 Although bind-
ing mode C had been proposed for a series of Lewis acid
adducts of nitrosamines (based on IR and 1H NMR spectro-
scopy),20 no metal-nitrosamine complex with this binding
mode was reported until our preparation and crystallization
in 1995 of a diethylnitrosamine adduct of a ferric porphyrin,
namely, [(TPP)Fe(ONNEt2)2]ClO4 (TPP = tetraphenylpor-
phyrinato dianion).21,22 Recently, Doctorovich and co-workers
have confirmed the binding mode D (Figure 2) that results
Experimental Section
All reactions were performed under an atmosphere of
prepurified nitrogen using standard Schlenk glassware and/
or in an Innovative Technology Labmaster 100 Drybox.
Solutions for spectral studies were also prepared under a
nitrogen atmosphere. Solvents were distilled from appropri-
ate drying agents under nitrogen just prior to use: CH2Cl2
(CaH2), hexane (CaH2), toluene (Na).
Chemicals. N-nitrosodimethylamine (Me2NNO, 99%),
N-nitrosodiethylamine (Et2NNO, >99%), N-nitrosodiphe-
nylamine (Ph2NNO, >97.0%), AgClO4 (97%), were pur-
chased from Aldrich Chemical Co. and used as received.
N-nitrosopiperidine ((cyclo-CH2)5NNO, 99%) and N-nitro-
sopyrrolidine ((cyclo-CH2)4NNO, 99%) were purchased from
(13) Appel, K. E.; Ruf, H. H.; Mahr, B.; Schwarz, M.; Rickart, R.; Kunz,
W. Chem.-Biol. Interact. 1979, 28, 17–33.
22
Fluka. [(TPP)FeIII(THF)2]ClO4 and N-nitrosodibenzyla-
mine ((PhCH2)2NNO25 were prepared by published proce-
dures. N-ethyl-N-nitrosoaniline (Ph(Et)NNO), N-methyl-N-
nitrosoaniline (Ph(Me)NNO) and the nitroso 15N-labeled
compounds Et2N15NO, (cyclo-CH2)4N15NO and (PhCH2)2-
N15NO were prepared in a manner similar to that used to
prepare the unlabeled (PhCH2)2NNO,25 but using Na15NO2
instead of NaNO2 in the nitrosation reactions. Nitric oxide
(98%, Matheson Gas) was passed through KOH pellets and a
cold trap (dry ice/acetone) to remove higher nitrogen oxides.
Caution! Nitrosamines (R2NNO) are generally considered to be
toxic; thus, they were handled with extreme care in a Drybox or in
(14) Mochizuki, M.; Okochi, E.; Shimoda, K.; Ito, K. In Nitrosamines and
Related N-Nitroso Compounds; Loeppky, R. N., Michejda, C. J., Eds.; American
Chemical Society: Washington, D.C., 1994; Chapter 113, pp 158-168.
(15) Okochi, E.; Mochizuki, M. Chem. Pharm. Bull. 1995, 43, 2173–2176.
(16) White, I. N. H.; Smith, A. G.; Farmer, P. B. Biochem. J. 1983, 212,
599–608.
(17) Klement, U. Acta Crystallogr. 1969, B25, 2460–2465.
(18) Klement, U.; Schmidpeter, A. Angew. Chem., Int. Ed. Engl. 1968, 7,
470.
(19) Constable, A. G.; McDonald, W. S.; Shaw, B. L. J. Chem. Soc.,
Dalton Trans. 1980, 2282–2287.
(20) Schmidpeter, A. Chem. Ber. 1963, 96, 3275–3279.
(21) Yi, G.-B.; Khan, M. A.; Richter-Addo, G. B. J. Am. Chem. Soc.
1995, 117, 7850–7851.
(22) Chen, L.; Yi, G.-B.; Wang, L.-S.; Dharmawardana, U. R.; Dart,
A. C.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem. 1998, 37, 4677–4688.
(23) Doctorovich, F.; Di Salvo, F.; Escola, N.; Trapani, C.; Shimon, L.
Organometallics 2005, 24, 4707–4709.
(24) Di Salvo, F.; Estrin, D. A.; Leitus, G.; Doctorovich, F. Organome-
tallics 2008, 27, 1985–1995.
(25) Zolfigol, M. A. Synth. Commun. 1999, 29, 905–910.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4407
Schlenk glassware to minimize exposure to either the vapor or the
material. Detailed procedures for the disposal of carcinogenic
nitrosamines have been published.26 Because of potential explo-
sion and fire hazards, the perchlorate compounds were prepared in
small amounts and handled carefully.
that used to prepare the six-coordinate compound [(TPP)-
Fe(ONNMe2)2]ClO4 except that the OEP macrocycle was used
in place of TPP. Yield: 68%. Anal. Calcd for C38H50O5-
N6ClFe 0.1CH2Cl2 0.5C6H14: C, 60.67; H, 7.08; N, 10.33;
3
3
Cl, 5.23. Found: C, 59.42; H, 6.57; N, 10.48; Cl, 4.84. IR
(KBr, cm-1): υNO/NN 1239 m, υClO 1091 br, 623 s.
Instrumentation. Infrared spectra were recorded on a Bio-Rad
FT-155 FTIR spectrometer. EPR spectra were recorded on
a Bruker X-band EMX spectrometer equipped with an Oxford
Instruments liquid helium cryostat. The solution EPR spectra
were obtained on ∼2 mM frozen toluene solutions using
∼20 mW microwave power and 100 kHz field modulation with
the amplitude set to 1 G. EPR spectra were simulated using the
program SpinCount (version 2.2.40) written by Prof. Michael
Hendrich, Carnegie Mellon University. This program uses the
following Spin Hamiltonian to describe exchange interactions:
H = JAB SA SB; i.e., J > 0 implies antiferromagnetic coupling.
4
Preparation of [(TPP)Fe(NO)(ONNMe2)]ClO4. Method I.
Nitric oxide was bubbled through a CH2Cl2 solution (15 mL) of
[(TPP)Fe(ONNMe2)2]ClO4 (50 mg, 0.055 mmol) for 2 min.
During this time, the color of the mixture changed from
red-purple to bright-red. The volume of solution was reduced
to ∼5 mL, and hexane (20 mL) was added. A dark purple
crystalline solid formed after ∼30 min. The solid was collected
by filtration, washed with hexane (2 ꢀ 15 mL), and dried in
vacuo to give the bright red [(TPP)Fe(NO)(ONNMe2)]ClO4
(0.023 g, 0.028 mmol, 56% isolated yield). IR (NaCl, cm-1):
3
3
υ
NO 1922 m, υNO/NN 1245 m, υClO 1098 s, 1074 m, 620 s.
Method II. Crystals of [(TPP)Fe(ONNMe2)2]ClO4 in a vial
Preparation of [(TPP)Fe(ONNMe2)2]ClO4. To a toluene
solution (20 mL) of [(TPP)Fe(THF)2]ClO4 (0.042 g, 0.048
mmol) was added Me2NNO (0.11 mL, 1.5 mmol). The mixture
was stirred for 45 min, during which time the color of the
solution changed from brown-purple to red-purple. The solvent
was reduced to ∼10 mL, and hexane (30 mL) was added. The
resulting solution was placed in a freezer (-22 ꢀC) overnight.
The violet crystalline solid that formed was collected by filtra-
tion, washed with hexane (2 ꢀ 15 mL), and dried in vacuo to give
[(TPP)Fe(ONNMe2)2]ClO4 (0.029 g, 0.032 mmol, 66% isolated
yield). Slow evaporation of a CH2Cl2/hexane (3:1 ratio; 5 mL)
solution of the product at room temperature gave suitable
crystals for X-ray diffraction studies. Anal. Calcd for C48H36-
O6N8ClFe 0.05CH2Cl2 0.6C6H14: C, 64.08; H, 4.63; N, 11.57;
4
under an N2 atmosphere were exposed to NO gas for 5 h. The
vial was then purged with N2 to remove unreacted NO. An IR
spectroscopic analysis of the product identified it as the bright
red crystalline [(TPP)Fe(NO)(ONNMe2)]ClO4. IR (KBr, cm-1):
υ
NO 1910 m, υNO/NN 1253 m; also, υClO 1117 m, 1101 s, 1072 m,
4
1061 w, 623 m. The crystals obtained under these conditions,
unfortunately, did not diffract X-rays.
Preparation of [(TPP)Fe(NO)(ONNEt2)]ClO4. This com-
pound was prepared as described previously in reference 22
(i.e., by Method I above). Yield: 39%. IR (NaCl, cm-1): υNO
1923 m, υNO/NN 1267 m; also υClO 1096 s, 1075 m, 622 m.
4
[(TPP)Fe(NO)(O15NNEt2)]ClO4.. IR (NaCl, cm-1): υNO
3
3
Cl, 4.03. Found: C, 62.55; H, 4.30; N, 11.23; Cl, 3.91. IR
1924 m, υ N 1248 m.
15NO/N15
(KBr, cm-1): υNO/NN 1256 m; also υClO 1119 m, 1102 s, 1073 m,
1061 w, 623 m.
Reaction of Ferrous (TPP)FeII with Ph2NNO. The four-
coordinate compound (TPP)FeII (0.090 mg, 0.135 mmol) was
prepared in situ by the reduction of equimolar (TPP)FeCl in
CH2Cl2 by Zn amalgam. The solution was filtered into a Schlenk
tube, and solid Ph2NNO (0.029 g, 0.145 mmol) was added to
the stirred solution. The mixture was stirred for an additional
10 min during which time the color of the solution changed from
red to dark red. The solvent was reduced to ∼5 mL and hexane
(30 mL) was added, and the solution placed in a freezer (-22 ꢀC)
overnight. The resulting dark red solid was collected by filtra-
tion and dried in vacuo to give the known five-coordinate
compound (TPP)Fe(NO) (0.036 g, 0.052 mmol, 38% isolated
yield). IR (KBr, cm-1): υNO 1701 m.
4
The other bis-nitrosamine complexes were generated simi-
larly using the respective porphyrin macrocycles and nitrosa-
mines (TTP = tetratolylporphyrinato dianion):
[(TTP)Fe(ONNMe2)2]ClO4. Yield: 60%. IR (KBr, cm-1):
υNO/NN 1255 m; also υClO 1117 m, 1101 s, 1073 m, 1062 m,
623 m.
[(TPP)Fe(ONNEt2)2]ClO4. This compound was prepared as
4
previously described in reference 22. IR (NaCl, cm-1): υNO/NN
1271 m; also υClO 1098 s, 1074 m, 623 m.
4
[(TPP)Fe(O15NNEt2)2]ClO4. IR (NaCl, cm-1): υ
15NO/N15
N
1252 m.
[(TTP)Fe(ONNEt2)2]ClO4. Yield: 66%. IR (KBr, cm-1):
A similar reaction using either of the two monoaryl nitrosa-
mines Ph(Me)NNO or Ph(Et)NNO resulted in the generation of
(TPP)Fe(NO) in lower yields (estimated ∼1/4 of that from the
Ph2NNO reaction, as judged by IR spectroscopy).
υ
NO/NN 1267 m; also υClO 1136 m, 1122 m, 1109 s, 1091 s, 626 m.
4
[(TPP)Fe(ONN(CH2Ph)2)2]ClO4. Yield: 61%. IR (KBr,
cm-1): υNO/NN 1268 m; also υClO 1125 m, 1096 s, 1073 m,
4
Solid-State Structural Determinations. Details of crystal data
and refinement are given in Table 1. Single-crystal X-ray diffrac-
tion data were collected using an instrument with a Bruker
APEX ccd area detector with graphite-monochromated Mo KR
624 m. Slow evaporation of a CH2Cl2/hexane (3:1 ratio) solu-
tion of the product generated suitable crystals of the mixed
salt [(TPP)Fe(ONN(CH2Ph)2)2][(TPP)Fe(ClO4)2] for X-ray dif-
fraction studies.
˚
[(TPP)Fe(O15NN(CH2Ph)2)2]ClO4. IR (KBr, cm-1):υ
radiation (λ = 0.71073 A). The structures were solved by direct
15NO/N15
N
methods using the SHELXTL system and refined by full-matrix
1254 m.
[(TPP)Fe(ONN(cyclo-CH2)4)2]ClO4. Yield: 72%. IR (KBr,
cm-1): υNO/NN 1239 m; also υClO 1093 s br, 1075 m, 622 m.
least-squares methods on F2.
(i). [(OEP)Fe(ONNMe2)]ClO4 CH2Cl2. Cell parameters were
3
4
determined from a non-linear least-squares fit of 5660 peaks in
the range 2.21 < θ < 28.29ꢀ. A total of 28480 data were measured
in the range 1.60 < θ < 28.30ꢀ using ω oscillation frames. The
data were merged to form a set of 10055 independent data with
R(int) = 0.0443 and a coverage of 100.0%. The triclinic space
group P1 was determined by statistical tests and verified by
subsequent refinement. Non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atom positions
were initially determined by geometry and refined by a riding
model; hydrogen atom displacement parameters were set to 1.2ꢀ
(1.5ꢀ for methyl) the displacement parameters of the bonded
atoms. A total of 525 parameters were refined against 36 restraints
and 10055 data to give wR(F2) = 0.1298 and S = 1.001 for
[(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4. Yield: 57%. Suitable
crystals for X-ray diffraction studies were grown at room
temperature by slow evaporation of a solution of the product
in CH2Cl2/hexane (3:1 ratio). IR (KBr, cm-1): υNO/NN 1242 s;
also υClO 1122 w, 1096 s br, 1071 m, 623 m.
4
[(TPP)Fe(O15NN(cyclo-CH2)5)2]ClO4. IR (KBr, cm-1):
υ N 1225 m.
15NO/N15
Preparation of the Five-Coordinate [(OEP)Fe(ONNMe2)]-
ClO4. This compound was generated in a manner similar to
(26) Lunn, G.; Sansone, E. B.; Keefer, L. K. Carcinogenesis 1983, 4, 315–
319.

4408 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Table 1. Crystal Data and Structure Refinement
[(TPP)Fe(ONN(CH2Ph)2)2]-
[(TPP)Fe(ONNMe2)2]ClO4 [(TPP)Fe(ClO4)2] b
a
b
compound
[(OEP)Fe(ONNMe2)]ClO4
[(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4
formula (f.w.)
(C38H50FeN6O5Cl1)
C48H40ClFeN8O6 (915.21)
(C116H84Fe2N12O10Cl2)
(CH2Cl2)2 (2158.40)
97(2)
(C54H48Cl1FeN8O6)
3
3
3
(CH2Cl2) (847.07)
100(2)
Triclinic
(CH2Cl2)2 (1166.16)
113(2)
Triclinic
T (K)
crystal system
103(2)
Monoclinic
C2/c
Triclinic
P1
space group
˚
P1
P1
a (A), R (deg)
12.441(2), 64.645(6)
13.634(3), 66.076(6)
14.814(3), 69.444(7)
2027.2(7), 2/1
1.388
17.535(2), 90
17.625(2), 119.315(2)
15.7926(18), 90
12.3159(14), 100.690(5)
13.7776(16), 107.620(5)
16.972(2), 107.860(5)
12.0211(12), 113.7120(10)
14.7595(14), 96.7340(10)
17.9330(17), 104.3430(10)
2736.7(5), 2
˚
b (A), β (deg)
˚
c (A), γ (deg)
3
3
V, Z/Z0
4255.8(8) A , 4
2486.5(5) A , 1/0.5
˚
˚
D(calcd), g/cm3
abs coeff, mm-1
F(000)
1.430
0.479
1900
1.441
0.524
1114
1.415
0.578
1206
0.620
890
crystal size (mm)
θ range for data collection
index ranges
0.40 ꢀ 0.31 ꢀ 0.17
1.60-28.30ꢀ
-16 e h e 16
-18 e k e 18
-19 e l e 19
28480
0.26 ꢀ 0.16 ꢀ 0.12
1.76-28.29ꢀ
-23 e h e 22
-23 e k e 22
-20 e l e 20
25807
0.74 ꢀ 0.38 ꢀ 0.19
1.84-26.00ꢀ
-15 e h e 15
-16 e k e 16
-20 e l e 20
27426
0.50 ꢀ 0.31 ꢀ 0.17
1.53-27.50ꢀ
-15 e h e 15
-19 e k e 19
-23 e l e 23
33328
reflections collected
independent reflns
max. and min transmission 0.984 and 0.787
10055 [Rint = 0.0443]
5153 [Rint = 0.0281]
0.9448 and 0.8856
5153/0/293
9718 [Rint = 0.0149]
0.9071 and 0.6979
9718/0/670
12423 [Rint = 0.0176]
0.9081 and 0.7610
12423/0/688
1.032
data/restraints/parameters 10055/36/525
goodness-of-fit on F2
final R indices [I>2σ(I)]
R indices (all data)
1.001
1.034
1.004
R1 = 0.0488
wR2 = 0.1298
0.716 and -0.554
R1 = 0.0334
wR2 = 0.0898
0.485 and -0.382
R1 = 0.0299
wR2 = 0.0845
0.342 and -0.502
R1 = 0.0431
wR2 = 0.1189
0.942 and -0.896
largest diff. peak and
-3
˚
hole, e A
a This compound crystallizes as a dichloromethane solvate. b These two complexes crystallize as bis-dichloromethane solvates.
weights of w = 1/[σ2(F2) þ (0.0620P)2 þ 1.4200P], where P =
range 2.6 < θ < 28.3ꢀ. Non-hydrogen atoms were refined
2
2
[Fo þ 2Fc ]/3. The final R(F) was 0.0488 for the 8001 observed,
[F > 4σ(F)], data. The largest shift/s.u. was 0.001 in the final
refinement cycle.
anisotropically, and all hydrogen atoms were included with
idealized parameters.
The crystallographic data for the complexes are summarized
in Table 1.
(ii). [(TPP)Fe(ONNMe2)2]ClO4. Intensity data, which cov-
ered approximately the full sphere of the reciprocal space, were
measured as a series of ω oscillation frames each spanning 0.3ꢀ
for 25 s/frame. Coverage of unique data was 97.3% complete to
56.6ꢀ(2θ). Cell parameters were determined from a non-linear
least-squares fit of 6877 reflections in the range 2.3<θ < 28.3.
All the non-hydrogen atoms were refined anisotropically, and
all the hydrogen atoms were included with idealized parameters.
The asymmetric unit contains half of the C48H40N8O2Fe cation
that sits around the crystallographic inversion center, and a
perchlorate anion that sits on the crystallographic 2-fold center.
Results and Discussion
Appel and co-workers have provided spectroscopic evi-
dence for a direct interaction between carcinogenic nitrosa-
mines and the ferric center of liver microsomal cytochrome
P450.13 This Type II interaction (Figure 1) complements the
typical substrate-distal pocket Type I interaction character-
istic of cytochrome P450-induced R-hydroxylation reactions.
Despite this observation of a Type II interaction between
nitrosamines and cytochrome P450, there is surprisingly little
information regarding the direct interactions of nitrosamines
with the metal center in iron porphyrins.
Several years ago, we reported the first such adduct between
a nitrosamine and an iron porphyrin, namely, the ferric high-
spin complex [(TPP)Fe(ONNEt)2]ClO4.21,22 To date, this
remains the only synthetic iron porphyrin nitrosamine com-
pound in the literature. As part of our work to expand on our
knowledge regarding the structural consequences of such
interactions, we have prepared other ferric porphyrin nitrosa-
mine compounds and determined their crystal structures.
The six-coordinate compounds were prepared by displa-
cing coordinated solvent from the precursor compound.
(iii). [(TPP)Fe(ONN(CH2Ph)2)2][(TPP)Fe(ClO4)2] (CH2Cl2)2.
3
Both the iron porphyrin cation and anion were located on crystal-
lographic centers of symmetry. Cell parameters were determined
from a non-linear least-squares fit of 8770 peaks in the range 2.48 <
θ < 28.27ꢀ. A total of 27426 data were measured in the range 1.84
< θ < 26.00ꢀ using ω oscillation frames. The data were merged to
form a set of 9718 independent data with R(int) = 0.0149 and a
coverage of 99.6%. The triclinic space group P1 was determined
by statistical tests and verified by subsequent refinement. Non-
hydrogen atom were refined with anisotropic displacement para-
meters. Hydrogen atom positions were initially determined by
geometry and refined by a riding model; hydrogen atom displace-
ment parameters were set to 1.2ꢀ the displacement parameters of
the bonded atoms. A total of 670 parameters were refined against
9718 data to give wR(F2) =0.0845 and S = 1.004 for weights of
½ðporÞFeðTHFÞ2ꢁClO4 þ excess R2NNO
f ½ðporÞFeðONNR2Þ2ꢁClO4 þ 2THF
R2 ¼ Me2, Et2, ðCH2PhÞ2, ðcyclo-CH2Þ4, ðcyclo-CH2Þ5
2
2
w = 1/[σ2(F2) þ (0.0500P)2 þ 1.4800P], whereP = [Fo þ 2Fc ]/3.
The final R(F) was 0.0299 for the 9320 observed, [F > 4σ(F)],
data. The largest shift/s.u. was 0.001 in the final refinement cycle.
(iv). [(TPP)Fe((cyclo-CH2)5NNO)2]ClO4 (CH2Cl2)2. Inten-
3
sity data, which approximately covered the full sphere of the
reciprocal space, were measured as a series of two oscillation
frames each 0.3ꢀ for 17 s/frame. Coverage of unique data was
98.6% complete to 55.0ꢀ (2θ). Cell parameters were determined
from a non-linear least-squares fit of 8246 reflections in the
ð1Þ
Thus, the reactions of [(por)Fe(THF)2]ClO4 (por = TPP,
TTP) with five different nitrosamines (N-nitrosodimethyl-
amine (Me2NNO), N-nitrosodiethylamine (Et2NNO),

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4409
Table 2. IR Data for υNO and υNN in Nitrosamine Iron Porphyrin Complexes and
the Corresponding Uncomplexed Nitrosamines
compound
υ
ΝΟ/ΝΝ (cm-1
)
ref
a
[(OEP)Fe(ONNMe2)]ClO4 a
[(TPP)Fe(ONNMe2)2]ClO4a
[(TTP)Fe(ONNMe2)2]ClO4
1239
1256
1255
1271
1252
1267
1239
1242
1225
1268
1254
this work
this work
this work
this work22
this work
this work
this work
this work
this work
this work
this work
b
[(TPP)Fe(ONNEt2)2]ClO4
[(TPP)Fe(O15NNEt2)2]ClO4
[(TTP)Fe(ONNEt2)2]ClO4
b
a
a
[(TPP)Fe(ONN(cyclo-CH2)4)2]ClO4a
[(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4
[(TPP)Fe(O15NN(cyclo-CH2)5)2]ClO4
a
a
[(TPP)Fe(ONN(CH2Ph)2)2]ClO4
a
[(TPP)Fe(O15NN(CH2Ph)2)2]ClO4
free nitrosaminesc
υ
ΝΟ (cm-1
)
υ
ΝΝ (cm-1
)
ref
Me2NNO
Et2NNO
(cyclo-CH2)4NNO
(cyclo-CH2)5NNO
(PhCH2)2NNO
1460
1454
1428
1437
1463
1035
1060
1154
1087
1120
27
27
27
27
27
a IR spectra were recorded as KBr pellets. b IR spectra were recorded
as dried samples on NaCl discs. c IR spectra were recorded as CCl4
solutions.
Figure 3. IR spectra of the unlabeled (top) and nitroso 15N-labeled
(bottom) diethylnitrosamine complex [(TPP)Fe(ONNEt2)2]ClO4 as dried
samples on NaCl plates.
N-nitrosodibenzylamine ((PhCH2)2NNO), N-nitrosopyrrol-
idine ((cyclo-CH2)4NNO), and N-nitrosopiperidine ((cyclo-
CH2)5NNO)) in toluene generate, after workup, the dark
purple products in 57-72% isolated yields. The nitrosamine
ligands dissociate readily from the iron center in these com-
pounds, and we find that the use of excess nitrosamine assures
reasonable yields of the products. In addition, we find that these
products convert in solution to the ferric bis-aquo derivatives
and the ferric μ-oxo dimer (i.e., [(por)Fe]2(μ-O)) in the prese-
nce of moisture and laboratory air/oxygen, respectively. The
mechanism of decomposition of the bis-nitrosamine complexes
to the μ-oxo dimer is unknown. In the solid state, however, these
bis-nitrosamine compounds are fairly air-stable.
The IR spectra of the six-coordinate ferric [(por)Fe-
(ONNR2)2]þ complexes as KBr pellets (or as dried samples
on NaCl disks) show either one or two new bands in the
1200-1300 cm-1 region that could be associated with the
υNO and υNN for the coordinated nitrosamines. However,
nitrosamine 15N-labeling reveals that only one of these is
associated with the coordinated nitrosamines (Table 2 and
Supporting Information), suggesting an overlap of the υNO
and υNN bands in these complexes. For example, the IR
spectrum of [(TPP)Fe(ONNEt2)2]ClO4 shows a new band at
1271 cm-1 (absent in the precursor complex [(TPP)Fe-
(THF)2]ClO4) that shifts to 1252 cm-1 upon 15N-labeling
of the nitrosamine (Figure 3). Similar 15N shifts in the IR
spectra are observed upon isotopic labeling of the nitrosa-
mines in the complexes [(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4
(Δ = 14 cm-1) and [(TPP)Fe(ONN(CH2Ph)2)2]ClO4 (Δ =
17 cm-1) (Supporting Information).
Figure 4. Dipolar resonance contributions of nitrosamines.
nitrosamines are best represented by a resonance hybrid
having a significant contribution from structure B in
Figure 4.
Unlike the case for the free nitrosamines, the υNO and υNN
bands are coincident in the six-coordinate iron porphyrin
nitrosamine compounds (Table 2). For these [(por)Fe-
(ONNR2)2]ClO4 compounds, bands centered at ∼1100 (br)
and ∼622 cm-1 were also observed for the υ3 and υ4 vibra-
tions of the uncoordinated perchlorate anion.28
Because of the weak binding of the nitrosamine ligands to
the ferric center in these six-coordinate compounds, as
evidenced by ready displacement of these ligands by water
(moisture), we reasoned that we might be able to take
advantage of the stabilization energy provided by the π-π
porphyrin stacking observed in some OEP-type compounds
to prepare a five-coordinate nitrosamine compound. This
turned out to be the case, and we successfully prepared the
mononitrosamine compound [(OEP)Fe(ONNMe2)]ClO4
and determined its crystal structure (see later).
Arylnitrosamines and Ferric Porphyrins. We attempted
to prepare the analogous arylnitrosamine adducts by
reacting the precursor compound [(TPP)Fe(THF)2]ClO4
with three aryl nitrosamines Ph(Me)NNO, Ph(Et)NNO,
and Ph2NNO. We were unsuccessful at detecting or
isolating the expected nitrosamine adducts. It is likely
that the electron-withdrawing phenyl group(s) in these
We note that the uncomplexed aliphatic secondary nitros-
amines used in this study display υNO and υNN bands in the
1428-1463 and 1035-1154 cm-1 ranges, respectively, in CCl4.27
Thus, the observed υNOs in the ferric porphyrin nitrosamine
adducts are lower (i.e., weaker N-O bond) than those of the
free nitrosamines, and the corresponding υNNs are higher
(i.e., stronger N-N bond); this suggests that the coordinated
(27) Williams, R. L.; Pace, R. J.; Jeacocke, G. J. Spectrochim. Acta 1964,
20, 225–236.
(28) Gowda, N. M. N.; Naikar, S. B.; Reddy, G. K. N. Adv. Inorg. Chem.
1984, 28, 255–299.

4410 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Figure 6. Proposed N-binding of nitrosamines to ferrous porphyrins.
ðTPPÞFe þ Ph2NNO f ðTPPÞFeðNOÞ
ð2Þ
The denitrosation reactions involving the monoaryl
nitrosamines generally took longer to generate the
(por)Fe(NO) products. The N-N bond in aromatic
N-nitrosamines are more readily cleaved than those in
unconstrained aliphatic N-nitrosamines,29,30,35 and this is
the likely reason that we observe the formation of the
(por)Fe(NO) products during the room temperature re-
actions of the aromatic nitrosamines with the four-
coordinate (por)FeII compounds. In contrast, the (por)-
Fe(NO) compounds do not form readily during the room
temperature reactions using the aliphatic nitrosamines.
This denitrosation process is not inconsistent with the
observation of P450-dependent denitrosations of nitro-
samines,36 although the direct N-binding of nitrosamines
to the iron centers in P450 enzymes has not been firmly
established to date. We note that a variety of organic
nitroso compounds such as alkyl nitrites (RONO), alkyl
thionitrites (RSNO), and other N-nitroso compounds
such as Diazald and NONOates also serve as nitrosylat-
ing agents to metal-containing precursors to generate
metal-NO moieties.37-40
Ferrous Nitrosyl Nitrosamine Derivatives. Adduct for-
mation between ferric porphyrins and NO give the ferric-
NO adducts that are best described as {FeNO}6 species
according to the Enemark-Feltham notation.41-43 Lehnert
and co-workers have recently provided computational
and experimental data that illustrated the interesting
complexity of the {FeNO}6 class of compounds.44 In
some cases, but not in all cases, these ferric-NO moieties
may be described as “ferrous-nitrosonium” FeII-NOþ
species.
Figure 5. N-binding and O-binding modes of nitrosoarenes to iron
porphyrins.
nitrosamines disfavor the dipolar resonance contribution
(B in Figure 4) thus also disfavoring the partially negative
charge on the nitrosamine O atom needed for coordina-
tion to the cationic “hard” ferric ion. We did find,
however, that prolonged exposure of the [(TPP)Fe-
(THF)2]ClO4 compound to these nitrosamines (e.g., for
2 d) resulted in the formation of the known five-coordinate
(TPP)Fe(NO) compound. Arylnitrosamines are known to
act as NO donors.29,30 Subsequent reduction of the cationic
ferric center by NO is likely responsible for this reductive
nitrosylation process to give the (TPP)Fe(NO) product.31
Nitrosamines and Ferrous Porphyrins. Expanding on
the work by Mansuy and co-workers,32 we have shown
that nitrosoalkanes bind to ferrous porphyrins via the N
atoms of the C-nitroso functionalities to give Fe{N-
(=O)R} moieties.33 In the case of nitrosoarenes, we also
demonstrated N-binding of the nitroso moiety to the
ferrous center (A in Figure 5).34 However, the hitherto
unprecedented O-binding mode of the nitrosoarene was
evident in its binding to ferric porphyrins if the ArNO
ligand was appropriately para-substituted to enable a
contribution of a dipolar resonance form (B and C in
Figure 5).34
The O-binding modes in structures B and C in Figure 5
are analogous to those observed in our ferric nitrosamine
complexes. We thus sought to prepare ferrous porphyrin
complexes displaying the analogous N-bonded nitrosa-
mine binding modes (Figure 6) from the reaction of
nitrosamines with ferrous porphyrins.
The room temperature reactions of the aliphatic nitro-
samines with the ferrous (por)FeII complexes did not, at
least in our hands, form isolable adducts or other pro-
ducts. On the other hand, the room temperature reactions
of the three arylnitrosamines with the (por)FeII com-
pounds produced the known five-coordinate nitrosyl
(por)Fe(NO) derivatives. For example, the reaction of
(TPP)FeII with Ph2NNO in CH2Cl2 produced (TPP)Fe-
(NO) in ∼38% isolated yield (eq 2).
We explored the possibility that NO may displace one
of the nitrosamines in the ferric bis-nitrosamine com-
plexes to form the mixed nitrosyl-nitrosamine derivative.
(35) Zhu, X.-Q.; Hao, W.-F.; Tang, H.; Wang, C.-H.; Cheng, J.-P. J. Am.
Chem. Soc. 2005, 127, 2696–2708.
(36) Appel, K. E.; Gorsdorf, S.; Scheper, T.; Spiegelhalder, B.; Wiessler,
M.; Schoepke, M.; Engeholm, C.; Kramer, R. In Relevance to Human Cancer
of N-Nitroso Compounds, Tobacco Smoke and Mycotoxins; O'Neill, I. K.,
Chen, J., Bartsch, H., Eds.; International Agency for Research on Cancer (IARC):
Lyon, France, 1991; Vol. 105, pp 351-357.
(37) Lee, J.; Chen, L.; West, A. H.; Richter-Addo, G. B. Chem. Rev. 2002,
102, 1019–1065.
(38) Richter-Addo, G. B. Acc. Chem. Res. 1999, 32, 529–536.
(39) Wang, P. G.; Xian, M.; Tang, X.; Wu, X.; Wen, Z.; Cai, T.; Janczuk,
A. J. Chem. Rev. 2002, 102, 1091–1134.
(40) Melzer, M. M.; Jarchow-Choy, S.; Kogut, E.; Warren, T. H. Inorg.
Chem. 2008, 47, 10187–10189.
(41) Westcott, B. L.; Enemark, J. H. In Inorganic Electronic Structure and
Spectroscopy; Lever, A. B. P., Solomon, E. I., Eds.; Wiley and Sons: New York,
1999; Vol. 2 (Applications and Case Studies), Chapter 7.
(42) Feltham, R. D.; Enemark, J. H. Top. Stereochem. 1981, 12, 155–215.
(43) Enemark, J. H.; Feltham, R. D. Coord. Chem. Rev. 1974, 13, 339–
406.
(29) Zhu, X.-Q.; He, J.-Q.; Li, Q.; Xian, M.; Lu, J.; Cheng, J.-P. J. Org.
Chem. 2000, 65, 6729–6735.
(30) Miura, M.; Sakamoto, S.; Yamaguchi, K.; Ohwada, T. Tetrahedron
Lett. 2000, 41, 3637–3641.
(31) Cheng, L.; Richter-Addo, G. B. In The Porphyrin Handbook; Guilard,
R., Smith, K., Kadish, K. M., Eds.; Academic Press: New York, 2000; Vol. 4
(Biochemistry and Binding: Activation of Small Molecules), pp 219-291.
(32) Mansuy, D.; Battioni, P.; Chottard, J.-C.; Riche, C.; Chiaroni, A. J.
Am. Chem. Soc. 1983, 105, 455–463.
(33) Sohl, C. D.; Lee, J.; Alguindigue, S. S.; Khan, M. A.; Richter-Addo,
G. B. J. Inorg. Biochem. 2004, 98, 1238–1246.
(34) Wang, L.-S.; Chen, L.; Khan, M. A.; Richter-Addo, G. B. Chem.
Commun. 1996, 323–324.
(44) Praneeth, V. K. K.; Paulat, F.; Berto, T. C.; George, S. D.; Nather,
C.; Sulok, C. D.; Lehnert, N. J. Am. Chem. Soc. 2008, 130, 15288–15303.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4411
Figure 8. Structure of the cation of [(TPP)Fe(ONNMe2)2]ClO4. Hydro-
gen atoms have been omitted for clarity.
Figure 7. IR spectra of the products from the reactions of the unlabeled
and 15N-labeled [(TPP)Fe(ONNEt2)2]ClO4 complexes with NO, as dried
samples on NaCl plates.
Reaction of ferric [(TPP)Fe(ONNEt2)2]ClO4 with NO
gas results in the formation of a compound formulated
as [(TPP)Fe(NO)(ONNEt2)]ClO4 based on its IR spec-
trum (Figure 7).22
Thus, a new band at 1923 cm-1 becomes apparent in
the IR spectrum of the reaction product. This band is
assigned to υNO, whereas the 1267 cm-1 band is assigned
to the υNO/NN bands (overlapped) of the coordinated
nitrosamine and is shifted by -4 cm-1 from the analo-
gous band in its [(TPP)Fe(ONNEt2)2]ClO4 precursor
(Figure 3, top). The assignment of this band to υNO/NN
is confirmed by 15N-isotope labeling of the nitrosamine
(Figure 7, bottom) where the new nitrosamine band at
1248 cm-1 is also shifted by -4 cm-1 from the analogous
band in its [(TPP)Fe(O15NNEt2)2]ClO4 precursor (Figure 3,
bottom).
Similar IR bands are observed in the spectra of the
products from the reactions of NO with [(TPP)Fe-
(ONNMe2)2]ClO4. The product from the reaction of solid
[(TPP)Fe(ONNMe2)2]ClO4 with NO displays a υNO at
1910 cm-1 with a υNO/NN that is -3 cm-1 shifted upon
NO binding. In contrast, the product from the solution
phase reaction displays a υNO at 1922 cm-1 and a υNO/NN
that is -11 cm-1 shifted upon NO binding. We hypothe-
size that the product from the solid-gas reaction is
probably a kinetic product, and that the product from
the solution phase reaction is the thermodynamic pro-
duct. However, we have yet not been able to crystallize
any of these nitrosyl [(por)Fe(NO)(ONNR2)]þ complexes
for a crystal structural determination. Importantly, ni-
trosyl porphyrins of the {FeNO}6 formulation with
trans water, alcohol, imidazole, indazole, and pyrazine
donors display υNO bands above 1900 cm-1, and these
compounds generally contain linear or near-linear
FeNO moieties.31 Thus, the observed υNOs of the
Figure 9. Structure of the cation of [(TPP)Fe(ONN(CH2Ph)2)2]-
[(TPP)Fe(OClO3)2]. Hydrogen atoms have been omitted for clarity.
[(TPP)Fe(NO)(ONNR2)]ClO4 complexes suggest linear
or near-linear FeNO geometries in these nitrosyl-nitro-
samine complexes. In addition, the similar υNO/NN band-
(s) of the precursor [(TPP)Fe(ONNR2)2]ClO4 complexes
to those of the nitrosyl derivatives [(TPP)Fe(NO)-
(ONNR2)]ClO4 suggest that the nitrosamine O-binding
mode is retained in the nitrosyl derivatives.
Crystallography and EPR spectroscopy. Bis-nitrosa-
mine Complexes. We were successful in obtaining X-ray
diffraction quality crystals of three of the new bis-nitro-
samine complexes from their CH2Cl2/hexane solutions.
The corresponding crystal structures of these compounds
€
(46) Rademacher, P.; Stølevik, R.; Luttke, W. Angew. Chem. Int. Ed. Eng.
1968, 7, 806.
(47) Gdaniec, M.; Milewska, M. J.; Polonski, T. Angew. Chem., Int. Ed.
1999, 38, 392–395 (CCDC: JOQHEC).
(45) Rademacher, P.; Stølevik, R. Acta Chem. Scand. 1969, 23, 660–671.

4412 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Figure 10. Structure of one of the two independent cations of [(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4. Hydrogen atoms have been omitted for clarity.
Figure 11. (Top) View of the nitrosamine orientations relative to the porphyrin cores, with the view along the axial O-Fe bonds. Nitrosamine substituents
have been removed for clarity. (Bottom) Perpendicular atom displacements (in units of 0.01 A) of the porphyrin cores from the 24-atom mean porphyrin
planes.
˚
[(TPP)Fe(ONNMe2)2]ClO4, [(TPP)Fe(ONN(CH2Ph)2)2]-
ClO4, and [(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4 are shown
in Figures 8-10, respectively, with a focus on the structures
of the nitrosamine-containing cations.
The orientation of the nitrosamine skeletons relative to the
respective porphyrin planes are shown in Figure 11, and
selected bond lengths and angles are listed in Table 3.
In all three crystal structures, the nitrosamine ligands
are O-bound to the ferric centers in these compounds.
Prior to this work, the only other structurally characteri-
zed iron porphyrin nitrosamine compound was [(TPP)-
Fe(ONNEt2)2]ClO4;21,22 X-ray structural characteriza-
tion of this compound also revealed the O-binding
mode of the diethylnitrosamine ligands. Combined with
this latter compound, these remain the only structu-
rally characterized iron porphyrin nitrosamine com-
pounds reported in the literature. Further, these are also
the only iron nitrosamine complexes (porphyrin- or

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4413
˚
Table 3. Selected Bond Lengths (A) and Angles (deg) for the Iron Porphyrin Nitrosamine Compounds and Organic Nitrosamines
compound
Fe-O
O-N
N-N
N-C
Fe-N(por)
—Fe-O-N —O-N-N —N-N-C —NTot —O-Fe-O
[(OEP)Fe(ONNMe2)]ClO4
2.102(2)
1.251(3)
1.275(3) 1.445(4),
1.465(5)
1.978(2),
1.985(2),
1.985(2),
1.986(2)
2.038(1),
2.040(1)
109.8(1)
111.5(1)
112.7(1)
115.9(2)
114.2(1)
114.5(1)
122.7(3), 360.0 n.a.
116.2(3)
[(TPP)Fe(ONNMe2)2]ClO4
2.136(1)
1.271(1)
1.271(2)
1.284(1) 1.456(2),
1.458(2)
122.9(1), 359.7 180.0
115.6(1)
[(TPP)Fe(ONN(CH2Ph)2)2]-
[(TPP)Fe(OClO3)2]•(CH2Cl2)2
cation
2.124(1)
1.288(2) 1.471(2)
1.469(2)
2.040(1),
2.040(1)
[2.003(1),
1.986(1)]
124.3(1) 360.0 180.0(1)
114.3(1)
anion
[2.237(1)]
[(TPP)Fe(ONN(c-CH2)5)2]-
[ClO4]•(CH2Cl2)2
cation 1
2.086(1)
2.110(1)
1.275(2)
1.271(2)
1.235(2)
1.252c
1.281(2) 1.465(2)
1.471(3)
2.050(1)
2.046(1)
2.050(1)
2.042(1)
115.7(1)
113.3(1)
114.2(2)
114.6(2)
113.6(2)
115.2c
116.7(2) 359.7 180.0(1)
125.6(2)
116.6(2) 359.6 180.0(1)
cation 2
1.282(2) 1.462(3)
1.472(3)
1.344(2) 1.461(2)
126.2(2)
120.3(3)
116.4(3)
117.6c
Me2NNOa
(cyclo-CH2)5NNOb
1.315c
1.455c
1.442c
124.6c
a Determined by electron diffraction.45,46 b In complex with cholic acid.47 c Data obtained from the Cambridge Structural Database.
Figure 12. View of the cation and anion of [(TPP)Fe(ONN(CH2Ph))2][(TPP)Fe(OClO3)2] showing their orientation to each other. Hydrogen atoms have
been omitted for clarity.
nonporphyrin-containing) that display the sole O-bind-
ing mode of the bound nitrosamines (Figure 2c).
The crystal structure of [(TPP)Fe(ONNMe2)2]ClO4
reveals only one cation (Figure 8) and one associated
anion. However, the crystal structure of “[(TPP)Fe-
(ONN(CH2Ph)2)2]ClO4” reveals the presence of a bis-nitro-
samine cation [(TPP)Fe(ONN(CH2Ph)2)2]þ (Figure 9) and
a bis-perchlorate anion [(TPP)Fe(OClO3)2]- (Figures 12
and 13). The two porphyrin planes in this [(TPP)-
Fe(ONN(PhCH2)2)2][(TPP)Fe(OClO3)2] complex are or-
iented 43ꢀ to each other (Figure 12). In the porphyrin-
containing anion (Figure 13), the Fe-N(por) bonds are
That these compounds are best described as having
ferric high-spin centers is supported by the analysis of
the structural data for the Fe-porphyrin cores,21,22 and
unambiguously by EPR spectroscopy (see later). The
equatorial Fe-N(por) and axial Fe-O bond lengths are
˚
˚
2.038(1)-2.050(1) A and 2.086(1)-2.136(1) A, respec-
tively; these are similar to the observed data for other
high-spin ferric porphyrins as documented by Scheidt and
co-workers.48,49
˚
1.986(1)-2.003(1) A, and the axial Fe-O(ClO3) bonds
˚
are 2.237(1) A. For comparison, the Fe-N(por) bond
distance data for the somewhat related and structurally
(48) Scheidt, W. R.; Reed, C. A. Chem. Rev. 1981, 81, 543–555.
(49) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: New York, 2000; Chapter 16.
characterized six-coordinate bis-perchlorate π-cation

4414 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
the Fe(por) unit and the perchlorates in this latter electron-
rich complex.
To the best of our knowledge, the structure of
[(TPP)Fe(ONN(PhCH2)2)2][(TPP)Fe(OClO3)2] contains
the first structural data for any bis-perchlorate fer-
ric porphyrin anion (Figures 12 and 13). As shown in
Figure 13, the perchlorate ligands essentially eclipse and
straddle a porphyrin pyrrole N-atom.
The crystal structure of [(TPP)Fe(ONN(cyclo-CH2)5)2]-
ClO4 reveals the presence of two independent cations, and
their relative orientations with respect to the perchlorate
anion is displayed in Figure 14; the two porphyrin planes
are oriented 50ꢀ to each other.
Nitrosamine Ligands. In the six-coordinate com-
pounds, the O-N and N-N bond distances are 1.271-
˚
˚
(2)-1.275(2) A and 1.281(2)-1.288(2) A, respectively.
The related distances in free Me2NNO are 1.235(2) A
˚
˚
(O-N) and 1.344(2) A (N-N) as determined by electron
diffraction,45 and in cocrystallized N-nitrosopiperidine
are 1.252 A (O-N) and 1.315 A (N-N) (Table 3).47 Thus,
the N-O bond distances are longer and N-N bond
distances are shorter in the nitrosamine porphyrin com-
plexes, suggesting that the complexed nitrosamines are
best represented by a resonance hybrid having a signifi-
cant contribution from the dipolar structure (Figure 4B)
which is also supported by IR spectroscopic data de-
scribed in the previous sections. The crystal structure of
free N-nitrosodibenzylamine has not been reported.
In all the bis-nitrosamine porphyrin crystal structures,
the sums of angles around the amino N-atoms of the
bound nitrosamines are 360ꢀ, consistent with a significant
contribution of the dipolar resonance structure shown
in Figure 4B that necessitate planarity of this amino
N-atom. In addition, in the [(TPP)Fe(ONNMe2)2]þ ca-
tion (Figure 8), the nitrosamine atoms O1, N3, N4, C23,
and C24 are essentially in a planar arrangement, with
O1-N3-N4-C23 and O1-N3-N4-C24 torsion angles
of -175.0ꢀ and -0.8ꢀ, respectively, with a mean deviation
˚
˚
˚
of 0.02 A from the ONNC2 plane. A similar planarity of the
five atoms (ONNC2) is observed in [(TPP)Fe(ONN-
(CH2Ph)2)2]þ and [(TPP)Fe(ONN(cyclo-CH2)5)2]þ which
is consistent with the significant contribution from the
dipolar structures in these coordinated nitrosamines. The
deviations of the nitrosamine N-N bonds from the normal
to the porphyrin planes are 6.0ꢀ (for the Me2NNO complex;
Figure 11a) and 1.8ꢀ (for the (PhCH2)2NNO complex;
Figure 11b), revealing the near perpendicularity of the
nitrosamine planes to the porphyrin planes in these two
complexes. In the (cyclo-CH2)5NNO complex, however,
which contains two independent cations, the corresponding
deviations of the nitrosamine N-N bonds from the normal
to the porphyrin planes are 32.5ꢀ (for cation 1; Figure 11c)
and 4.5ꢀ (cation 2; Figure 11d). The deviation from perpen-
dicularity is evident in the comparison of Figures 11c and
11d. The off-axis tilts (from the porphyrin normal) of the
axial Fe-O bonds are 2.8ꢀ (for the Me2NNO complex) and
3.7ꢀ (for the (PhCH2)2NNO complex). The data for the
two cations of the ((cyclo-CH2)5)2NNO complex are 9.3ꢀ
(cation 1) and 3.0ꢀ (cation 2). Clearly, cation 1 of this latter
complex displays the largest deviations from perpendicular
binding of the nitrosamine ligands to the iron center.
Figure 13. (a) Structure of [(TPP)Fe(OClO3)2]-. H atoms have been
omitted for clarity. (b) View of the ClO4 orientation relative to the
-
porphyrin core, with the view along the O-Fe bond. (c) Perpendicular
atom displacements (in units of 0.01 A) of the porphyrin core from the
24-atom mean porphyrin plane.
˚
þ
˚
radical compounds [(por )Fe(OClO3)2] are 2.045 A (avg)
3
(por=TPP) and 1.999(2) A (por = OEP).50,51 The related
˚
˚
axial Fe-O(ClO3) bond distances are 2.13(1)
(por = TPP) and 2.148(1) A (por = OEP). When comp-
A
˚
ared with the data for the cationic [(por þ)Fe(OClO3)2]
3
compounds, the longer Fe-O(ClO3) bond in the anion
[(TPP)Fe(OClO3)2]- suggests a weaker interaction between
(50) Scheidt, W. R.; Song, H. S.; Haller, K. J.; Safo, M. K.; Orosz, R. D.;
Reed, C. A.; Debrunner, P. G.; Schulz, C. E. Inorg. Chem. 1992, 31, 939–
941.
(51) Gans, P.; Buisson, G.; Duee, E.; Marchon, J. C.; Erler, B. S.; Scholz,
W. F.; Reed, C. A. J. Am. Chem. Soc. 1986, 108, 1223–1234.
Interestingly, the two internal N-N-R angles in the
bound nitrosamines are not equivalent. The internal

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4415
Figure 14. View of the two cations of [(TPP)Fe(ONN(cyclo-CH2)5)2]ClO4 showing their orientation with respect to the perchlorate anion.
Figure 15. (Left) 5 K solution EPR spectrum (black) of 3 mM [(TPP)Fe(ONNMe2)2]ClO4 in toluene in the presence of 200 equiv of N-nitrosodimethy-
lamine, and simulated spectrum (red). Fit parameters: S = 5/2; D = 10 cm-1, E/D = 0.020; gx = 2.050, gy = 1.950, gz = 2.010; 9.429 GHz. (Right) 5 K
solid-state EPR spectrum of [(TPP)Fe(ONNMe2)2]ClO4 (black) and simulated spectrum (red). Fit parameters: S = 5/2; D = 10 cm-1, E/D = 0.025;
gx = 1.950, gy = 1.950, gz = 2.010; 9.424 GHz. (See also Table 4).
angle, labeled as L below, is consistently larger than the
outer angle labeled as S.
Thus, the internal angles range from 114.27(12)-
116.67(16)ꢀ, and the outer angles range from 122.86-
(11)-126.17(17)ꢀ. We are currently not able to provide
an explanation for this variation, as there does not appear
to be any steric reasons (e.g., from steric clashes) for this
difference.
We recorded and analyzed the low-temperature frozen-
solution and solid-state EPR spectra of the complex
[(TPP)Fe(ONNMe2)2]ClO4 to unambiguously determine

4416 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Table 4. Fit Parameters for the EPR Spectra of Ferric Porphyrin Nitrosamine Compounds, Recorded at 5 K
D (cm-1
)
E/D
J (cm-1
)
gx
gy
gz
Δgx
Δgy
Δgz
a
a
a
compound
rel. ratio
S
[(TPP)Fe(ONNMe2)2]ClO4
solution
solid
5/2
5/2
10
10
0.020
0.025
2.050
1.950
1.950
1.950
2.010
2.010
0.080
0.130
0.080
0.110
0.045
0.049
[(OEP)Fe(ONNMe2)]ClO4
blue traceb
1.00
0.002
0.0001
3/2
3/2
5/2
10
10
10
0.030
0.030
0.010
2.0
0.7
2.070
2.000
1.952
2.070
2.150
1.950
2.070
2.150
2.010
0.370
0.300
0.080
0.370
0.070
0.080
0.600
0.070
0.013
red traceb
black traceb
a Δg is the g strain. b The individual components are represented as color traces in Figure 19.
Figure 17. View of the π-π stacking involving the cation of
[(OEP)Fe(ONNMe2)]ClO4, showing (top) the 3.3 A separation of the
˚
porphyrin planes, and (b) the slippage of the porphyrin rings relative to
each other. Hydrogen atoms have been omitted for clarity.
trace in red), and the spectrum of the complex as a solid
(also at 5 K) is shown on the right. Displaying effective g
values of ∼6 and 2, the data clearly point to a six-
coordinate high-spin ferric metal center with a spin of
S = 5/2 in an effectively axial ligand field (E/D ∼ 0.02).
The magnitude of the D value is not well defined in the
simulation, but is clearly >5 cm-1. The high-spin state is
commonly observed for six-coordinate ferric porphyrins
Figure 16. (a) Structure of the cation of [(OEP)Fe(ONNMe2)]ClO4.
Hydrogen atoms have been omitted for clarity. (b) View of the Me2NNO
orientation relative to the porphyrin core, with the view along the O-Fe
˚
bond. (c) Perpendicular atom displacements (in units of 0.01 A) of the
porphyrin core from the 24-atom mean porphyrin plane.
the spin-state of the metal center. The EPR spectrum of
the complex in toluene at 5 K is shown on the left of
Figure 15 (experimental trace in black, and simulated
(52) Hoshino, A.; Ohgo, Y.; Nakamura, M. Inorg. Chem. 2005, 44, 7333–
7344.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4417
bound nitrosamine in this five-coordinate complex re-
veals significant differences when compared with the data
from the six-coordinate analogues. For example, the
˚
O-N bond distance is 1.251(3) A, which is significantly
˚
shorter than the ∼1.27 A distances in the related six-
coordinate complexes. This is probably due to a de-
creased dipolar resonance contribution (B in Figure 4)
in the stabilization of the nitrosamine coordination to the
more electron-rich OEP complex. However, the N-N
Figure 18. Sketch of the geometric parameters for the OEP-OEP π
stacking.
˚
bond distance of 1.275(3) A is not significantly different
from the analogous distances in the six-coordinate deri-
vatives. The Fe-O-N bond angle of 109.8(1)ꢀ in this
five-coordinate compound is smaller than the related
angles in the six-coordinate complexes, and the tilt of
the Fe-O vector from the normal to the porphyrin plane
is 4.5ꢀ.
The [(OEP)Fe(ONNMe2)]ClO4 complex has a “di-
meric” nature involving π-π stacking interactions be-
tween the OEP macrocycles (Figure 17a). This is possible
because the π systems of the porphyrin rings effectively
block the sixth coordination site and provide additional
stabilization of the five-coordinate complex during its
crystallization. The mean plane separation (M.P.S.) bet-
˚
ween the 24-atom porphyrin planes is 3.3 A, and the
˚
Fe-Fe distance is 3.9 A (Figure 18). The lateral shift,
defined here as the “apparent horizontal slippage of one
˚
porphyrin with respect to the other” is ∼1.5 A; this
horizontal slippage is illustrated in Figure 17b. This
π-π stacking interaction, combined with the magnitudes
of the MPS and the lateral shift, places this compound in
Class S with a strong π-π interaction, according to the
nomenclature of Scheidt and Lee54 and in the same group
as the [(OEP)Fe(NO)]þ compound.55 This strong π-π
interaction, when combined with the shorter observed
˚
˚
Fe-N(por) distances of ∼1.98 A (cf. 2.04-2.05 A in the
six-coordinate high-spin derivatives discussed above)
suggests a different spin-state for this five-coordinate
ferric compound.
Figure 19. (Bottom) 5 K solid-state EPR spectrum of [(OEP)Fe-
(ONNMe2)]ClO4 (black line) and the simulated spectrum (red line) based
on the sum of the spectra of three different species as shown on the top.
(Top) Simulation of three individual species present in the solid-state EPR
spectrum of [(OEP)Fe(ONNMe2)]ClO4. The ratio of the three species blue/
red/black is 1:0.002:0.0001. Fit parameters (9.424 GHz): Blue: S = 3/2;
D = 10 cm-1, E/D = 0.030; J = 2.0 cm-1; gx = 2.070, gy = 2.070, gz =
2.070; Red: S=3/2;D=10cm-1, E/D= 0.030; J=0.7cm-1;gx = 2.000,
gy = 2.150, gz = 2.150; Black: S = 5/2; D = 10 cm-1, E/D = 0.010; gx =
1.952, gy = 1.950, gz = 2.010. All fit parameters are listed in Table 4.
The experimental solid-state EPR spectrum of [(OEP)-
Fe(ONNMe2)]ClO4 at 5 K (Figure 19, bottom (black line))
reveals a more complicated situation than the six-coordinate
ferric nitrosoamine compounds described above. In this
case, the main signal is centered around an effective g value
of 3.5 as shown in Figure 19, bottom, which corresponds to
an intermediate-spin (S = 3/2) complex. Simulation of the
experimental spectrum (Figure 19, top) suggests the pre-
sence of three species in a 1.00:0.002:0.0001 ratio. These
species have been identified as
with weak O-bonded ligands, such as pyridine-N-oxide,
DMSO, DMF, and MeOH.52,53 All EPR fit parameters
are listed in Table 4.
Five-Coordinate Complex. We were fortunate, using a
combination of an octaethylporphyrin macrocycle and
dimethylnitrosamine, to achieve the crystallization of a
five-coordinate iron porphyrin nitrosamine complex
[(OEP)Fe(ONNMe2)]ClO4. The structure of the cation
and the axial (perpendicular) orientation of the nitrosa-
mine relative to the porphyrin plane is shown in Figure 16.
There are several points to note about this structure. The
Fe atom is axially displaced from the 24-atom mean
(i) a largely predominant intermediate-spin species
displaying relatively weak exchange coupling of
two Fe centers (S = 3/2; J = 2 cm-1; using the
Spin Hamiltonian H = JAB SA SB) in the face-
3
3
to-face arrangement of two complexes as deter-
mined from the crystal structure. The antiferro-
magnetic exchange coupling with J = 2 cm-1 is
necessary to fit the shape of the g ∼ 3.5 feature.
As before, the value of D is not well-defined, but
˚
porphyrin plane by 0.16 A toward the nitrosamine ligand.
Such axial displacements of Fe from the mean porphyrin
planes in five-coordinate iron porphyrins is not uncom-
mon. Interestingly, however, the geometrical data for the
D is clearly >5 cm-1
;
(54) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1–70.
(55) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2000, 39,
5102–5110.
(53) Mashiko, T.; Kastner, M. E.; Spartalian, K.; Scheidt, W. R.; Reed,
C. A. J. Am. Chem. Soc. 1978, 100, 6354–6362.

4418 Inorganic Chemistry, Vol. 49, No. 10, 2010
Xu et al.
Figure 20. (Left) 5 K frozen-solution EPR spectrum of 3 mM [(OEP)Fe(ONNMe2)]ClO4 in toluene with 1 equiv of Me2NNO present (top,
black trace). The red trace shows the simulated spectrum of the six-coordinate high-spin (S = 5/2) complex, and the final difference spec-
trum (black-red) is shown in blue in the bottom panel. (Right) 5 K frozen-solution EPR spectrum of 3 mM [(OEP)Fe(ONNMe2)]ClO4 in
toluene in the presence of 200 equiv of Me2NNO (black trace, top). The red trace shows the simulated spectrum of the six-coordinate high-spin
(S = 5/2) complex, and the final difference spectrum (black-red) is shown in blue in the bottom panel. In both cases, the signal of the six-
coordinate high-spin (S = 5/2) bis-nitrosamine complex does not completely cancel out in the difference spectrum (blue traces) because the
simulation does not fully reproduce the experimental shape of the frozen-solution EPR spectrum of the high-spin species as evident from
Figure 15, left.
(ii) a second intermediate-spin species (impurity)
with a weaker coupling between two Fe centers
(S = 3/2; J = 0.7 cm-1). Here, the weaker anti-
ferromagnetic coupling of J = 0.7 cm-1 is
necessary to fit the signal at g ∼1.4; and
(iii) a very small amount of an impurity, likely of
the six-coordinate high-spin (S = 5/2) com-
plex with two nitrosamines bound to the ferric
heme.
Summary
We have prepared a number of nitrosamine adducts of
synthetic iron porphyrins from the reactions of the cationic
[(por)Fe(THF)2]ClO4 precursors in toluene with dialkylni-
trosamines. We have structurally characterized, by single-
crystal X-ray crystallography, three six-coordinate [(por)Fe-
(ONNR2)2]ClO4 compounds and a five-coordinate [(OEP)-
Fe(ONNMe2)]ClO4 derivative. We have shown that the
N-O and N-N vibrations of the coordinated nitrosamine
groups in the [(por)Fe(ONNR2)2]ClO4 complexes occur in
the 1239-1271 cm-1 range. EPR spectroscopy at liquid He
temperature reveals a high-spin ferric center in the six-
coordinate bis-nitrosamine [(TPP)Fe(ONNMe2)2]ClO4 com-
plex, but an intermediate-spin ferric center in the five-co-
ordinate mononitrosamine [(OEP)Fe(ONNMe2)]ClO4 deri-
vative. Importantly, all the nitrosamine ligands in the com-
plexes reported here bind to the ferric centers via a sole η1-O
binding mode. This class of compounds represents the only
reported examples of isolable iron porphyrins complexed with
nitrosamine ligands. Given the observation that nitrosamines
can bind directly to the ferric center in P450 enzymes, we can
conclude that the observed O-binding of nitrosamines in our
ferric model systems is possible for ferric P450s as well. Such
binding may, by itself, not lead directly to metabolic reactivity
of the nitrosamines. On the other hand, the ferrous derivatives
are more prone to display chemical reactivity of the bound
nitrosamines.
EPR spectroscopy on frozen solutions of the five-
coordinate complex [(OEP)Fe(ONNMe2)]ClO4 at 5 K
in toluene further suggests that in the presence of excess
ligand, this compound converts to the corresponding
bis-ligated six-coordinate species. The frozen-solution
EPR spectrum of [(OEP)Fe(ONNMe2)]ClO4 in the
presence of an additional small amount (1 equiv) of
ligand is shown in Figure 20, left (black line), and
reveals that the predominant species is the five-coordi-
nate intermediate-spin complex with only a small
amount of the six-coordinate species present. Addi-
tion of excess ligand then leads to the conversion of
the magnitude of the five-coordinate complex to the
six-coordinate high-spin derivative [(OEP)Fe(ONN-
Me2)2]ClO4 as shown in Figure 20, right. As expected,
the strong signal at g ∼ 1.4 observed in the solid state is
missing in the solution data, again emphasizing that
this signal is likely due to weak antiferromagnetic ex-
change coupling between two complexes in the face-to-
face arrangement observed in the crystal structure
(Figure 17a). This signal therefore disappears in the
solution spectra.
Acknowledgment. We are grateful to the National
Institutes of Health (GM-064476; G.B.R.A.) and the
National Science Foundation (CHE-0846235; N.L.) for

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4419
funding of this research. The authors thank the National
Science Foundation (CHE-0130835; G.B.R.A.) and the
University of Oklahoma for funds to purchase of the
X-ray diffractometer and computers.
Internet at http://pubs.acs.org. CCDC 745891-745894 con-
tains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, U.K.; fax: þ44
1223 336033.
Supporting Information Available: Figures S1-S15 contain-
ing IR spectra. This material is available free of charge via the