Unexpected nitrosyl-group bending in six-coordinate {M(NO)}6 σ-bonded aryl(iron) and -(ruthenium) porphyrins

Article abstract of DOI:10.1021/ja010276m

The six-coordinate nitrosyl σ-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe-(NO)(p-C₆H₄F) and (OEP)Ru(NO)(p-C₆H₄F) (OEP = octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both {MNO}⁶ species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C₆H₄F) the Fe-N-O angle is 157.4(2)°, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2°, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru-(NO)(p-C₆H₄F) the Ru-N-O angle is 154.9(3)°, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8°, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly σ-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe-(NO)(p-C₆H₄F) and, combined with the results of extended Hueckel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO π-bonding as measured by infrared and ⁵⁷Fe Moessbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm^-1) and the ruthenium complex (1773 cm^-1) are significantly reduced compared to their respective {MNO}⁶ counterparts. The Moessbauer data for (OEP)Fe(NO)(p-C₆H₄F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.

Full text of DOI:10.1021/ja010276m

6314
J. Am. Chem. Soc. 2001, 123, 6314-6326
Unexpected Nitrosyl-Group Bending in Six-Coordinate {M(NO)}6
σ-Bonded Aryl(iron) and -(ruthenium) Porphyrins
George B. Richter-Addo,*,† Ralph A. Wheeler,*,† Christopher Adam Hixson,† Li Chen,†
Masood A. Khan,† Mary K. Ellison,‡ Charles E. Schulz,§ and W. Robert Scheidt*,‡
Contribution from Department of Chemistry and Biochemistry, UniVersity of Oklahoma, 620 Parrington
OVal, Norman, Oklahoma 73019, Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Knox College, Galesburg, Illinois 61401
ReceiVed January 31, 2001
Abstract: The six-coordinate nitrosyl σ-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe-
(NO)(p-C6H4F) and (OEP)Ru(NO)(p-C6H4F) (OEP ) octaethylporphyrinato dianion) have been synthesized
and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting
of the MNO group for both {MNO}6 species as well as significant lengthening of trans axial bond distances.
In (OEP)Fe(NO)(p-C6H4F) the Fe-N-O angle is 157.4(2)°, the nitrosyl nitrogen atom is tilted off of the
normal to the heme plane by 9.2°, Fe-N(NO) ) 1.728(2) Å, and Fe-C(aryl) ) 2.040(3) Å. In (OEP)Ru-
(NO)(p-C6H4F) the Ru-N-O angle is 154.9(3)°, the nitrosyl nitrogen atom is tilted off of the heme normal
by 10.8°, Ru-N(NO) ) 1.807(3) Å, and Ru-C(aryl) ) 2.111(3) Å. We show that these structural features
are intrinsic to the molecules and are imposed by the strongly σ-donating aryl ligand trans to the nitrosyl.
Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe-
(NO)(p-C6H4F) and, combined with the results of extended Hu¨ckel calculations, show that the observed bending
and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital
interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the
Fe-NO π-bonding as measured by infrared and 57Fe Mo¨ssbauer spectroscopies. The solid-state nitrosyl stretching
frequencies for the iron complex (1791 cm-1) and the ruthenium complex (1773 cm-1) are significantly reduced
compared to their respective {MNO}6 counterparts. The Mo¨ssbauer data for (OEP)Fe(NO)(p-C6H4F) yield
the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our
study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.
Introduction
unit (i.e., the NO ligand is not assigned a charge, whereas all
of the other ligands are). Thus, for example, the (por)Fe(NO)
and anionic [(por)Fe(NO)X]- compounds4 (X ) monoanionic
ligand) are classified as {FeNO}7 species, whereas the [(por)-
Fe(NO)]+ and the neutral (por)Fe(NO)X compounds are clas-
sified as {FeNO}6 species.
To date, two types of Fe-N-O linkages in iron nitrosyl
porphyrins have been characterized by single-crystal X-ray
crystallography.5,6 The first involves the linear Fe-N-O linkage
that is typical of {FeNO}6 porphyrin species, trivially known
as the ferric nitrosyl form. The second is the bent Fe-N-O
linkage that is typical of {FeNO}7 porphyrin complexes and
The nature of binding of nitric oxide (NO) to the metal center
in iron porphyrins has received renewed interest since the
discovery that the enzyme that produces NO in the body (NO
synthase) and the enzymatic receptor for NO (soluble guanylyl
cyclase) both contain heme. Bonding descriptions of the metal
nitrosyl linkage (i.e., metal-N-O) generally rely on two types
of formalism.1 The oxidation-state formalism assigns the linear
NO group as NO+ (isoelectronic with CO) and the bent NO
group as NO-. However, this formalism does not take into
account the degree of bending of the MNO fragment and is
often fraught with unrealistic oxidation state assignments. For
example, the tetranitrosyl Cr(NO)4 has four linear NO groups;2
hence, the Cr atom should be assigned a 4- charge based on
this formalism. A much more reliable description for MNO
geometry is based on the now widely accepted Enemark-
Feltham notation. In this notation, a nitrosyl complex is
described as a {M(NO)x}n complex, where x is the number of
NO ligands, and n is the total number of electrons in the metal
d and NO π* orbitals.3 Importantly, NO is assumed to be a
neutral ligand, and the MNO fragment is considered as a single
(3) (a) Feltham, R. D.; Enemark, J. H. Top. Stereochem. 1981, 12, 155-
215. (b) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339-
406.
(4) Abbreviations: por, a generalized porphyrinato dianion; porph,
unsubstituted porphinato dianion; OEP, dianion of 2,3,7,8,12,13,17,18-
octaethylporphyrin; TPP ) dianion of 5,10,15,20-tetraphenylporphyrin; TTP,
dianion of 5,10,15,20-tetra -p-tolylpophyrin; TpivPP, dianion of 5,10,15,-
20-R,R,R,R-tetrakis-o-pivalamidophenylporphyrin; T(p-OCH3)PP, dianion
of 5,10,15,20-tetra-p-methoxyphenylporphyrin; Iz, indazole; Pz, pyrazole;
Py, pyridine; Prz, pyrazine; 1-MeIm, 1-methylimidazole; 4-MePip, 4-me-
thylpiperidine; Neop, neopentyl; Mb, myoglobin; Np, porphinato nitrogen
atom.
† University of Oklahoma.
(5) Cheng, L.; Richter-Addo, G. B. Binding and Activation of Nitric
Oxide by Metalloporphyrins and Heme. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 4 (Biochemistry and Binding: Activation of Small
Molecules), pp 219-291.
‡ University of Notre Dame.
§ Knox College.
(1) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University
Press: New York, 1992.
(2) Hedberg, L.; Hedberg, K.; Satija, S. K.; Swanson, B. I. Inorg. Chem.
1985, 24, 2766-2771.
(6) See Table 6 in ref 5 for a listing of structurally characterized
nitrosylmetalloporphyrins.
10.1021/ja010276m CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/05/2001

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6315
just prior to use: CH2Cl2 (CaH2), benzene (Na), hexane (CaH2 or Na),
toluene (Na), and THF (CaH2).
commonly referred to as the ferrous nitrosyl form. In both types
of species a limited variability of the Fe-N-O angle has been
observed; the angle in {FeNO}6 species ranges from 170° to
exactly linear and from 140 to 150° in {FeNO}7 species.
Prior to our current study, all iron nitrosyl porphyrins fit into
either of these two limiting linear {FeNO}6 or bent {FeNO}7
categories. We now report a very unusual observation: that the
formally {FeNO}6 ferric complex (OEP)Fe(NO)(p-C6H4F)
displays an unexpectedly strongly bent and tilted Fe-N-O
geometry. The synthesis and characterization of the analogous
ruthenium species, (OEP)Ru(NO)(p-C6H4F), confirms these
unexpected structural results. We show that the bending of the
MNO group in these rather unusual molecules is due to intrinsic
electronic factors.
We have substantiated our experimental findings with hybrid
Hartree-Fock density functional and extended Hu¨ckel calcula-
tions. Other semiempirical MO, ab initio MO, and density
functional calculations have been performed to investigate ligand
tilting7 or MNO bending8-14 in transition metal complexes.
Coppens and co-workers have recently reported density func-
tional calculations that approximately reproduce the experimen-
tally observed axial N-atom tilt and Fe-Np asymmetry in the
X-ray structure of the five-coordinate complex, (OEP)Fe(NO).15
Most recently, Ghosh and Wondimagegn have reported density
functional calculations that investigate the specific orbital
interactions implicated in nitrosyl ligand tilting in the five-
coordinate (por)M(NO) compounds of Fe and Co.16 Similar
findings were subsequently reported by Patchkovskii and
Ziegler.17 In related work, a seminal density functional study
of carbon monoxyhemes by Ghosh and Bocian18 showed an
electronic preference for a linear L-Fe-C-O unit but indicated
very soft force constants for combined tilting and bending
motions of the axial ligands. In contrast, our HF/DF calculations
clearly show that the structure of (porph)Fe(NO)(p-C6H4F) with
both axial ligands tilted off-axis and with the FeNO group bent
represents a minimum-energy structure whose general features
are consistent with the high-quality X-ray diffraction structure.
Extended Hu¨ckel molecular orbital calculations show the orbital
mixing that leads to a low barrier for FeNO bending once the
ligands are tilted off the perpendicular.
Chemicals. Octaethylporphyrin, OEPH2, was purchased from Mid-
century Chemicals. (OEP)FeCl was synthesized by a modified literature
method.19,20 (OEP)Ru(CO), NOBF4, and (p-C6H4F)MgBr (1 M in THF
or 2 M in ether) were purchased from Aldrich Chemical Co. NO gas
was purified by passing it through 4 Å molecular sieves immersed in
a dry ice/ethanol slush bath to remove higher oxides of nitrogen.21
Chloroform-d (99.8%) was obtained from Cambridge Isotope Labo-
ratories and subjected to three freeze-pump-thaw cycles and stored
over Linde 4 Å molecular sieves. Elemental analyses were performed
by Atlantic Microlab, Norcross, Georgia.
Instrumentation. Infrared spectra were recorded on a BioRad FT-
1
155 FTIR or Perkin-Elmer 883 spectrometer. H NMR spectra were
obtained on a Varian XL-400 MHz spectrometer and the signals
referenced to the residual signal of the solvent employed. All coupling
constants are in Hz. UV-vis spectra were recorded on a Hewlett-
Packard HP8453 Diode Array instrument. The solid-state Mo¨ssbauer
sample was immobilized in Apiezon grease, and the Mo¨ssbauer
measurements were performed on a constant acceleration spectrometer
from 4.2 to 293 K with optional zero field and in a 4-T superconducting
magnet system (Knox College).
Preparation of (OEP)Fe(NO)(p-C6H4F). The organometallic (OEP)-
Fe(p-C6H4F) was generated from the reaction of (OEP)FeCl and (p-
C6H4F)MgBr, according to literature methods.22,23 The toluene solution
containing (OEP)Fe(p-C6H4F) was dried with MgSO4 and loaded on a
basic alumina column. The volume of the collected fraction was reduced
in vacuo. X-ray quality crystals of (OEP)Fe(NO)(p-C6H4F) (υNO, 1791
cm-1 (Nujol)) were obtained by transferring ∼1 mL of the concentrated
toluene solution of (OEP)Fe(p-C6H4F) to an 8 × 150 mm glass tube
inside an extra-long Schlenk tube. NO gas was then bubbled slowly
into the solution for less than 1 min, and NO-saturated hexanes were
then layered over the solution in the tube. A bulk sample of (OEP)-
Fe(NO)(p-C6H4F) for Mo¨ssbauer measurements was obtained by
bubbling NO gas into the concentrated solution of (OEP)Fe(p-C6H4F)
in a Schlenk tube and layering the solution with NO-saturated hexanes.
All manipulations including crystallizations were done in the dark.
Preparation of (OEP)Ru(NO)(p-C6H4F). Method I. To a CH2Cl2
(20 mL) solution of (OEP)Ru(CO) (0.100 g, 0.151 mmol) was added
ClNO (ca. 0.155 mmol in CH2Cl2 solution). The mixture was left to
stir for 20 min. The color of the solution changed from pink-red to
brown-red. The mixture was taken to dryness, and the residue was
suspended in THF (40 mL). Excess (p-C6H4F)MgBr (0.95 mL, 1.0 M
in THF, 0.95 mmol) was added, the IR spectrum indicated the
completion of the reaction after 20 min (the 1831 cm-1 band due to
the intermediate (OEP)Ru(NO)Cl complex was replaced by a new band
at 1761 cm-1), and the color of the near homogeneous solution turned
from brown-red to bright red. All of the solvent was removed in vacuo,
and the residue was redissolved in CH2Cl2 and filtered through silica
gel. The solvent was allowed to evaporate under inert atmosphere to
generate a purple crystalline solid (OEP)Ru(NO)(p-C6H4F).0.13CH2-
Cl2 (0.091 g, 0.118 mmol, 78% yield).
Experimental Section
All reactions were performed under an atmosphere of argon or
prepurified nitrogen using standard Schlenk techniques or in an
Innovative Technology Labmaster 100 Dry Box unless stated otherwise.
Solvents were distilled from appropriate drying agents under nitrogen
Method II. To a CH2Cl2 (20 mL) solution of (OEP)Ru(CO) (50
mg, 0.076 mmol) was added NOBF4 (9 mg, 0.077 mmol). The mixture
was left to stir for 30 min, during which time the 1921 cm-1 band in
the IR spectrum was replaced by a new band at 1873 cm-1. The color
of the solution changed from pink-red to brown-red. All of the solvent
was removed in vacuo, and THF (20 mL) was added. Excess (p-C6H4F)-
MgBr (0.4 mL, 1.0 M in THF, 0.4 mmol) was added, and the reaction
was monitored by IR spectroscopy. The reaction reached completion
after 10 min (the initial band at 1872 cm-1 was replaced by a band at
(7) Kubacek, P.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4320-
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(20) Buchler, J. W. In Porphyrins and Metalloporphyrins; Smith, K. M.,
Ed.; Elsevier Scientific Publishing: Amsterdam, The Netherlands, 1975;
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(21) Dodd, R. E.; Robinson, P. L. Experimental Inorganic Chemistry;
Elsevier: Amsterdam, 1954.
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Quantum Chem. 1998, 69, 31-35.
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A.; Coppens, P.; Richter-Addo, G. B. J. Am. Chem. Soc. 2000, 122, 7142-
7143.
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6316 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Table 1. Crystal Data and Structure Refinement
Richter-Addo et al.
(OEP)Ru(NO)(p-C6H4F)
(OEP)Ru(NO)(p-C6H4F)
(OEP)Fe(NO)(p-C6H4F)
formula (fw)
T, K
diffractometer
crystal system
space group
unit cell dimensions
C42H48FN5ORu (758.92)
173(2)
Siemens P4
triclinic
P1h
a ) 10.5223(11) Å,
R ) 96.051(11)°
b ) 10.8549(14) Å,
â ) 97.08(1)°
c ) 15.793(2) Å,
γ ) 99.464(10)°
1751.1(4) Å3, 2
1.439
C42H48FN5ORu (758.92)
303(2)
Siemens P4
triclinic
P1h
a ) 10.6209(8) Å,
R ) 95.705(8)°
b ) 10.9386(12) Å,
â ) 97.009(7)°
c ) 15.918(2) Å,
γ ) 99.610(7)°
1796.2(3) Å3, 2
1.403
C42H48FN5OFe (713.70)
130(2)
Nonius FAST
triclinic
P1h
a ) 10.487(3) Å,
R ) 96.045(15)°
b ) 10.833(3) Å,
â ) 97.114(16)°
c ) 15.804(5) Å,
γ ) 99.466(16)°
1742.9(9) Å3, 2
1.360
V, Z
D(calcd), g/cm3
abs coeff, mm-1
final R indices [I > 2σ(I)]
R indices (all data)
0.495
0.483
0.480
R1 ) 0.0394, wR2 ) 0.0878
R1 ) 0.0550, wR2 ) 0.1158
R1 ) 0.0490, wR2 ) 0.0982
R1 ) 0.0814, wR2 ) 0.1313
R1 ) 0.0628, wR2 ) 0.1344
R1 ) 0.0902, wR2 ) 0.1496
1761 cm-1). The color of the solution turned from brown-red to bright
red, and the product appeared to be more soluble than the intermediate
(OEP)Ru(NO)BF4 compound. All of the solvent was removed in vacuo,
and the residue was redissolved in CH2Cl2 and filtered through silica
gel. The CH2Cl2 solvent was allowed to evaporate under inert
atmosphere to generate the purple crystalline product in 82% yield.
Calculations. All Hartree-Fock density functional (HF/DF) calcula-
tions were performed for the lowest-energy singlet state of (porph)-
Fe(NO)(p-C6H4F) using the program GAUSSIAN 94.28,29 The 3-21G
basis set was chosen for preliminary calculations because it is deemed
the smallest basis set capable of giving reasonable geometries.30,31 The
three-parameter, hybrid HF/DF method used, B3LYP, is widely used
and is described in the literature.29,32 The geometry was fully optimized
in C1 symmetry by using internal coordinates. Since the basis set used
for the initial calculations have well-known limitations,28,30,31,33 we are
currently repeating and extending our calculations using larger basis
sets.
Qualitative molecular orbital arguments in this paper are based on
extended Hu¨ckel calculations.34-37 The basis set included single Slater-
type functions for all orbitals except metal 3d orbitals, which were
approximated by double-ú functions. Parameters for the various atoms
were obtained from previous calculations reported in the literature38-40
and are listed in Table S17 (Supporting Information). The geometry
for (porph)Fe(NO)(p-C6H4F) was taken from the crystal structure of
(OEP)Fe(NO)(p-C6H4F) reported here, the ethyl substituents were
replaced by hydrogen atoms, and then the molecule was symmetrized
to the C2ν point group. Symmetrization was accomplished by using
average bond distances and angles from the X-ray structure and giving
the porphyrin ring a planar, D4h symmetry structure. All angular
distortions preserved the original bond lengths. Qualitative conclusions
were confirmed by repeating our extended Hu¨ckel calculations using
the exact X-ray diffraction core structure of (OEP)Fe(NO)(p-C6H4F).
.
Anal. Calcd for C42H48O1N5Ru1F1 0.13CH2Cl2: C, 65.72; H, 6.32;
N, 9.10; Cl, 1.20. Found: C, 65.30; H, 6.35; N, 8.81; Cl, 1.20. IR
(CH2Cl2, cm-1): υNO ) 1770. IR (THF, cm-1): υNO ) 1761. IR (KBr,
cm-1): υNO ) 1759 s; also 2965 m, 2929 w, 2869 w, 1571 w, 1470 m,
1446 m, 1372 m, 1317 w, 1269 s, 1222 m, 1151 s, 1110 w, 1055 m,
1019 s, 991 w, 960 m, 926 w, 839 m, 810 s, 743 s, 723 w, 714 m, 560
w. 1H NMR (CDCl3, δ): 10.18 (s, 4H, meso-H of OEP), 5.28 (s, CH2-
Cl2), 4.17 (t, J ) 9, 2H, m-H of p-C6H4F, overlapping with CH3CH2
of OEP), 4.10 (m, 16H, CH3CH2 of OEP), 1.94 (t, J ) 8, 24H, CH3-
CH2 of OEP), -0.46 (d, 1H, J ) 9, o-H of p-C6H4F), -0.48 (d, 1H, J
) 9, o′-H of p-C6H4F). UV-vis spectrum (λ (ꢀ, mM-1 cm-1), 4.74 ×
10-6 M in CH2Cl2): 360 (82), 404 (144), 555 (17) nm.
X-ray Structure Determinations. Complete crystallographic details,
atomic coordinates, anisotropic thermal parameters, and fixed hydrogen
atom coordinates are included in the Supporting Information. Details
of the crystal data and refinement are given in Table 1.
(i) (OEP)Fe(NO)(p-C6H4F). The structure determination was carried
out on a Nonius FAST area-detector diffractometer (at the University
of Notre Dame) with a Mo KR rotating anode source (λ ) 0.71073
Å). Detailed methods and procedures for small-molecule X-ray data
collection with the FAST system have been described previously.24 Data
collection was performed at 130(2) K. A dark purple crystal of (OEP)-
Fe(NO)(p-C6H4F) was used for the structure determination. The
structure was solved using the direct methods program SHELXS-86;25
subsequent difference Fourier syntheses led to the location of the
remaining atoms. The structure was refined against F2 with the program
SHELXL-97, in which all data collected were used including negative
intensities. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were idealized with the standard SHELXL-97
idealization methods.
(ii) (OEP)Ru(NO)(p-C6H4F). The data were collected both at
303(2) K and 173(2) K on a Siemens (Bruker) P4 diffractometer
(University of Oklahoma) using Mo KR radiation (λ ) 0.71073 Å).
The data were corrected for Lorentz and polarization effects, and an
empirical absorption correction based on ψ scans was applied.26 The
structure was solved by the heavy-atom method using the SHELXTL
system,27 and refined by full-matrix least-squares on F2 using all
reflections. All of the non-hydrogen atoms were refined anisotropically,
and the hydrogen atoms were included in the refinement with idealized
parameters.
Results
Syntheses of Complexes. (OEP)Fe(NO)(p-C6H4F) was pre-
pared by the sequential reaction of (OEP)FeCl with (p-C6H4F)-
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(24) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314-
1318.
(25) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(26) North, A. T. C.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351-359.
(27) Siemens SHELXTL, Release 5.03 ed.; Siemens, Ed.; Siemens
Analytical X-ray Instruments Inc.: Madison, WI, 1994.
(36) Jordan, T.; Smith, H. W.; Lohr, L. L., Jr.; Lipscomb, W. N. J. Am.
Chem. Soc. 1963, 85, 846-851.
(37) Wolfsberg, M.; Helmholtz, L. J. Chem. Phys. 1952, 20, 837-843.
(38) Anderson, A. B.; Hoffmann, R. J. Chem. Phys. 1974, 60, 4271.
(39) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397.
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7240.

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6317
Table 2. Representative IR Stretching Frequencies for Nitrosyl
Porphyrins of Fe and Ru
Thus, the subsequent addition (eq 4) of excess (p-C6H4F)MgBr
to (OEP)Ru(NO)Cl in THF at room-temperature results in the
replacement of the starting υNO band at 1831 cm-1 with a new
band at 1761 cm-1 attributed to υNO of the (OEP)Ru(NO)(p-
C6H4F) product over a 20 min period. This product is isolated,
after workup, analytically pure in 78% yield.
The second method involves the sequential reaction of (OEP)-
Ru(CO) with NOBF4 (in CH2Cl2 solvent, eq 5) to produce the
intermediate [(OEP)Ru(NO)]BF4,44 followed by reaction of this
intermediate with (p-C6H4F)MgBr (in THF solvent, eq 6).45
metal
coord. {MNO}n υNO
a
compound
no.
n )
(cm-1
)
ref
Iron
(TPP)Fe(NO)(1-MeIm)
(OEP)Fe(NO) triclinic
(OEP)Fe(NO)(CH3)
(OEP)Fe(NO)(p-C6H4Me)
(OEP)Fe(NO)(p-C6H4OMe)
(OEP)Fe(NO)(p-C6H4F)
(OEP)Fe(NO)(C6H5)
(OEP)Fe(NO)(p-C6F4H)
[(OEP)Fe(NO)]+
6
5
6
6
6
6
6
6
5
6
6
7
7
6
6
6
6
6
6
6
6
6
1625b 47
1673 48
1766 41
1785 41
1790 41
1791 this work
1795 41
CH2Cl2
1839 41
(OEP)Ru(CO) + NOBF4
8 [(OEP)Ru(NO)]BF4 + CO
1862b 49
(5)
(OEP)Fe(NO)(NO2)
1883 50
1914 51
[(OEP)Fe(NO)(Iz)]+
[(OEP)Ru(NO)]BF4 + (p-C6H4F)MgBr THF8
(OEP)Ru(NO)(p-C6H4F) (6)
Ruthenium
(TTP)Ru(NO)(CH3)
(OEP)Ru(NO)(p-C6H4F)
(TTP)Ru(NO)(p-C6H4F)
(OEP)Ru(NO)(OMe)
(OEP)Ru(NO)(SC(Me)2CH2-
NHC(O)Me)
(OEP)Ru(NO)(SC6F4H)
(OEP)Ru(NO)(OH)
(OEP)Ru(NO)Cl
6
6
6
6
6
6
6
6
6
6
1743b 42
1759b this work
1773b 42
1780 52
The product of eq 6 was isolated analytically pure in 82% yield
after workup. The IR and 1H NMR spectroscopic properties of
(OEP)Ru(NO)(p-C6H4F) are consistent with this formulation.
Molecular Structures. The molecular structure of (OEP)-
Fe(NO)(p-C6H4F) was determined at 130(2) K and is shown in
Figure 1a. The relative orientation of the axial ligands is shown
in Figure 1b, and the displacements of the porphyrin core atoms
from the 24-atom porphyrin plane are shown in Figure 1c.
Single-crystal X-ray diffraction data for the (OEP)Ru(NO)(p-
C6H4F) analogue were collected at low temperatures (173 and
243 K) and at 303 K. No significant differences in the structure
were observed at these three temperatures, and the axial C-Ru-
NO geometries were virtually identical. The results from the
173 K determination are cited here. The molecular structure of
(OEP)Ru(NO)(p-C6H4F) is shown in Figure 2a. The relative
orientation of the axial ligands is shown in Figure 2b, and the
displacements of the porphyrin core atoms from the 24-atom
porphyrin plane are shown in Figure 2c. Selected bond lengths
and angles for both structures are collected in Table 3.
1789b 53
6
6
6
6
6
6
6
6
6
6
6
1798b 54
1806b 43
1827b 55
1835b 43
1853b 44
1856b 53
(OEP)Ru(NO)(ONO)
[(OEP)Ru(NO)(H2O)]+
[(OEP)Ru(NO)(OdC(Me)NHCH2-
6
C(Me)2SH)]+
a Nujol mull. b KBr pellet.
MgBr (eq 1) and then NO gas (eq 2).
(OEP)FeCl + (p-C6H4F)MgBr toluene8 (OEP)Fe(p-C6H4F)
(1)
(OEP)Fe(p-C6H4F) + NO toluene8 (OEP)Fe(NO)(p-C6H4F)
(2)
Because of the air- and light-sensitive nature of (OEP)Fe(NO)-
(p-C6H4F), we obtained X-ray quality crystals directly from the
reaction solution in the dark. This {FeNO}6 species, (OEP)Fe-
(NO)(p-C6H4F), was characterized by IR and Mo¨ssbauer spec-
troscopies. The υNO of 1791 cm-1 (Nujol) is within the 1766-
1839 cm-1 (Nujol) range observed for related (OEP)Fe(NO)R
complexes reported by Guilard and Kadish (Table 2).41
Two methods were used to prepare the new (OEP)Ru(NO)-
(p-C6H4F) complex needed for this study. The first is based on
the synthesis of the related species, (TTP)Ru(NO)(p-C6H4F),42
and involves the reaction of the previously reported (OEP)Ru-
(NO)Cl43 with the (p-C6H4F)MgBr Grignard reagent. We have
found that the precursor (OEP)Ru(NO)Cl can be conveniently
prepared by nitrosyl chloride addition to (OEP)Ru(CO) (eq 3),
and the crude product is sufficiently pure for the next step
without any significant loss in yield of the final organometallic
product.
Discussion
Molecular Structures. There are a number of very interesting
structural features for the metal-NO linkages in the iron and
ruthenium organometallic porphyrinates described in this paper.
These structural features appear to be intrinsic to the complexes
and due to the presence of a strongly σ-bonded aryl group.
Important structural features include the unexpectedly strong
bending of the M-N-O group, the off-axis tilt of the nitrosyl
nitrogen atom, and the long M-N(NO) bond distance.
The unexpectedly strong bending of the Fe-N-O group in
(OEP)Fe(NO)(p-C6H4F) complex is clearly visible in Figure 1a.
This is the first time that such a strongly bent FeNO geometry
in a formally ferric {FeNO}6 complex has been observed.56 The
nonbonded interactions described subsequently make evident
that crystal packing effects are not responsible for the observed
(43) Miranda, K. M.; Bu, X.; Lorkovic, I.; Ford, P. C. Inorg. Chem.
1997, 36, 4838-4848.
(44) 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.
(45) This method has been used to prepare related organometallic osmium
nitrosyl porphyrins (ref 46).
(46) Cheng, L.; Chen, L.; Chung, H.-S.; Khan, M. A.; Richter-Addo, G.
B.; Young, V. G., Jr. Organometallics 1998, 17, 3853-3864.
(47) Scheidt, W. R.; Piciulo, P. L. J. Am. Chem. Soc. 1976, 98, 1913-
1919.
CH2Cl2
(OEP)Ru(CO) + ClNO
8 (OEP)Ru(NO)Cl + CO (3)
(OEP)Ru(NO)Cl + (p-C6H4F)MgBr THF8
(OEP)Ru(NO)(p-C6H4F) (4)
(41) Guilard, R.; Lagrange, G.; Tabard, A.; Lanc¸on, D.; Kadish, K. M.
Inorg. Chem. 1985, 24, 3649-3656.
(42) Hodge, S. J.; Wang, L.-S.; Khan, M. A.; Young, V. G., Jr.; Richter-
Addo, G. B. Chem. Commun. 1996, 2283-2284.
(48) Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404-
7405.
(49) Scheidt, W. R.; Lee, Y. J.; Hatano, K. J. Am. Chem. Soc. 1984,
106, 3191-3198.

6318 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Richter-Addo et al.
Figure 1. (a) Molecular structure of (OEP)Fe(NO)(p-C6H4F). Hydrogen
atoms have been omitted for clarity. (b) Top view (perpendicular to
the porphyrin plane) showing the orientation of the axial groups with
respect to the porphyrin skeleton. (c) Perpendicular atom displacements
from the 24-atom porphyrin plane (in 0.01 Å units).
Figure 2. (a) Molecular structure of (OEP)Ru(NO)(p-C6H4F). Hydro-
gen atoms have been omitted for clarity. (b) Top view (perpendicular
to the porphyrin plane) showing the orientation of the axial groups
with respect to the porphyrin skeleton. (c) Perpendicular atom displace-
ments from the 24-atom porphyrin plane (in 0.01 Å units).
bending. The Fe-N-O angle in {FeNO}6 and {FeNO}7
porphyrin species are clustered at near linearity and bent
( ∼145°), respectively. Thus the value of 157.4(2)° in (OEP)-
(50) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999,
38, 100-108.
(51) Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1999, 121, 5210-
5219.
Fe(NO)(p-C6H4F) lies between the range of angles found in
{FeNO}6 and {FeNO}7 porphyrin species. The Ru-N-O group
in (OEP)Ru(NO)(p-C6H4F) and (TTP)Ru(NO)(p-C6H4F) are also
significantly bent. At 154.9° and 152° respectively, these Ru-
N-O groups are clearly the most bent of the {RuNO}6 species
listed in Table 4.
(52) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. J. Am.
Chem. Soc. 1978, 100, 3015-3024.
(56) Photoreduction of the (por)M(NO)(R) complexes by the X-ray beam
is unlikely, since the bent MNO geometry was observed after very short
data collection times (for the (OEP)Fe- and (TTP)Ru- complexes, using
area detectors) or longer data collection times (for the (OEP)Ru- complex,
using a single-point detector). Other {FeNO}6 complexes under the same
conditions do not get reduced. We do note that a probable photoreduction
of the ferric nitrophorin-NO complex to the ferrous derivative has been
reported (ref 57).
(53) Yi, G.-B.; Khan, M. A.; Powell, D. R.; Richter-Addo, G. B. Inorg.
Chem. 1998, 37, 208-214.
(54) Yi, G.-B.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem. 1996,
35, 3453-3454.
(55) Lorkovic, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.; Schoonover,
J. R.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 11674-11683.

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6319
Table 3. Selected Bond Lengths and Angles for
(OEP)M(NO)(p-C6H4F)
Table 4 (1.717(7)-1.740(7) Å). It is thus almost 0.1 Å longer
than expected. Similarly, the Ru-N(NO) distance in (OEP)-
Ru(NO)(p-C6H4F) is very long at 1.807(3) Å. In comparison
with those of other {RuNO}6 complexes, where the bond lengths
have been determined with sufficient accuracy, the Ru-N(NO)
bond length in (OEP)Ru(NO)(p-C6H4F) is on average 0.07 Å
longer (Table 4). The observed M-N(NO) lengthening and
M-N-O bending in these {MNO}6 species results from the
addition of a strongly σ-bonding axial ligand trans to the nitrosyl
in both the iron and ruthenium derivatives. Furthermore, the
axial Fe-C(aryl) bond length of 2.040(3) Å in (OEP)Fe(NO)-
(p-C6H4F) is 0.085 Å longer than in the related species, (TPP)-
Fe(C6H5) (Table 6). A similar lengthening of the axial Ru-
C(aryl) bond by 0.1 Å is observed in (OEP)Ru(NO)(p-C6H4F)
versus (OEP)Ru(C6H5) (Table 6). We note that although the
axial Ru-C(aryl) bond length of 2.111(3) Å in (OEP)Ru(NO)-
(p-C6H4F) is within the 1.943(9) Å (in Ru(o-tol)4)74 to 2.250-
(4) Å (in (dmpe)2RuH(C6F5))75 range observed for other Ru-
C(aryl) bonds in coordination compounds, it is well outside the
1.999(4)-2.005(7) Å range previously observed for (por)Ru-
C(aryl) bonds (Table 6). Clearly, the nitrosyl ligand exerts a
structural trans effect reflected in the elongated M-C bond
distances in both derivatives.
Bond Lengths (Å)
(OEP)Fe(NO)(p-C6H4F)
(OEP)Ru(NO)(p-C6H4F)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(5)
N(5)-O(1)
Fe(1)-C(1)
2.001(2)
Ru(1)-N(3)
Ru(1)-N(4)
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-N(5)
N(5)-O(1)
Ru(1)-C(37)
2.052(3)
2.024(2)
2.023(2)
2.015(2)
1.728(2)
1.153(3)
2.040(3)
2.059(3)
2.068(3)
2.056(3)
1.807(3)
1.146(4)
2.111(3)
Bond Angles (deg)
(OEP)Ru(NO)(p-C6H4F)
Ru(1)-N(5)-O(1) 154.9(3)
(OEP)Fe(NO)(p-C6H4F)
Fe(1)-N(5)-O(1) 157.4(2)
C(1)-Fe(1)-N(5) 168.85(11) C(37)-Ru(1)-N(5) 166.59(13)
N(5)-Fe(1)-N(1)
N(5)-Fe(1)-N(2)
N(5)-Fe(1)-N(3)
N(5)-Fe(1)-N(4)
C(1)-Fe(1)-N(1)
C(1)-Fe(1)-N(2)
C(1)-Fe(1)-N(3)
C(1)-Fe(1)-N(4)
98.56(10) N(5)-Ru(1)-N(3)
84.43(10) N(5)-Ru(1)-N(4)
87.09(10) N(5)-Ru(1)-N(1)
97.24(10) N(5)-Ru(1)-N(2)
87.83(10) C(37)-Ru(1)-N(3)
86.43(10) C(37)-Ru(1)-N(4)
99.07(12)
83.47(13)
86.67(12)
99.74(13)
87.92(11)
85.09(12)
86.35(11)
91.70(12)
86.51(9)
C(37)-Ru(1)-N(1)
91.89(10) C(37)-Ru(1)-N(2)
The average Fe-Np bond length in (OEP)Fe(NO)(p-C6H4F)
is 2.016(11) Å, which is almost 0.02 Å longer than the average
Fe-Np bond length for all of the other {FeNO}6 derivatives
(Table 4). Indeed, the Fe-Np bond length is slightly longer than
that of the most accurately determined five-coordinate {FeNO}7
structure. In contrast, the average Ru-Np bond length of
2.059(7) Å in (OEP)Ru(NO)(p-C6H4F) is not longer than those
of other {RuNO}6 derivatives (Table 4). However, the average
metal-porphyrinato nitrogen distances in both (OEP)Fe(NO)-
(p-C6H4F) and (OEP)Ru(NO)(p-C6H4F) are longer (by ca. 0.02
and 0.03 Å respectively) than those found in other six-
coordinate, low-spin d5 species, although in the ruthenium case
there are very few examples with which to compare. The reason
for this M-Np elongation is not clear. However, it could be
due to the presence of the strongly σ-donating anionic ligand
trans to the nitrosyl.
In both (OEP)Fe(NO)(p-C6H4F) and (OEP)Ru(NO)(p-C6H4F)
the metal atom is displaced slightly toward the nitrosyl ligand
(by 0.09 Å in Fe and 0.12 Å in Ru). Similar small out-of-plane
displacements toward the nitrosyl ligand are seen for all of the
six-coordinate {MNO}6 species where the accuracy of the X-ray
structural results are not limited by axial ligand disorder. The
porphyrin plane is unremarkably planar as can be seen from
Figures 1c and 2c. Figures 1b and 2b show the orientation of
the axial ligands with respect to the porphyrin plane. The
dihedral angle between the Fe-N-O plane and the plane
defined as N(5)-Fe-N(2) is 38.8°. The similarly defined
dihedral angle for the phenyl ligand is 19°. This angle is smaller
than the phenyl dihedral angle found in (OEP)Ru(C6H5) and
In addition, the Fe-N(O) vector in (OEP)Fe(NO)(p-C6H4F)
is tilted off the normal to the mean porphyrin plane by 9.2° (R
in Table 5). Other nitrosyl porphyrin derivatives have also been
shown to display off-axis tilting of the nitrosyl nitrogen
atom.42,48,65-67 The NO group is further tilted 22.6° (â) from
the Fe-N(O) vector in the direction away from the porphyrin
normal. The C(1) atom of the axial phenyl group is also tilted
off the normal to the 24-atom porphyrin plane. The tilt of 3.1°
(γ) is toward the NO group; the C-Fe-N angle is 168.85 (11)°.
Distortions from axial symmetry for (OEP)Ru(NO)(p-C6H4F)
are also shown in Table 5. The axial N(5) atom of the NO group
is tilted 10.8° (R) from the normal to the 24-atom porphyrin
plane (10.2° from the normal to the four-nitrogen plane). The
axial C(37) atom of the aryl ligand is tilted by 3.5° (γ) in the
general direction of the NO tilt. The angles for these off-axis
tilts for the related (TTP)Ru(NO)(p-C6H4F) complex42 are shown
for comparison in Table 5.
The axial bond distances in the formally ferric {FeNO}6
complex (OEP)Fe(NO)(p-C6H4F) are also unusual. The Fe-
N(O) distance of 1.728(2) Å is very long in comparison to those
of other {FeNO}6 complexes where the range of Fe-N(O)
distances is 1.627(2)-1.671(2) Å. Indeed, the Fe-N(O) distance
in (OEP)Fe(NO)(p-C6H4F) is within the range of values
observed for the ferrous {FeNO}7 porphyrin species given in
(57) Ding, X. D.; Weichsel, A.; Andersen, J. F.; Shokhireva, T. K.;
Balfour, C.; Pierik, A. J.; Averill, B. A.; Montfort, W. R.; Walker, F. A. J.
Am. Chem. Soc. 1999, 121, 128-138.
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1997, 36, 3876-3885.
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S. Inorg. Chem. 1997, 36, 1992-1993.
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Chem. Soc. 2000, 122, 4651-4659 and references therein.
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Chem. 1994, 33, 2815-2822.
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Chem. Soc., Dalton Trans. 1988, 669-671.
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6320 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Richter-Addo et al.
Table 4. Selected Comparisons for Axial Bonding Parameters in {MNO}6 and {MNO}7 Metalloporphyrins
M-Np a
M-NNO
MNOb
M-La
N-Oa
∆
ref
a
a,c
complex
A. {FeNO}6 Metalloporphyrin Derivatives
[(OEP)Fe(1-MeIm)(NO)]+
[(OEP)Fe(Pz)(NO)]+
2.003(5)
2.004(5)
1.996(4)
1.995(8)
1.999(6)
2.013(3)
1.996(4)
1.994(1)
1.994(5)
2.016(11)
1.6465(17)
1.627(2)
1.632(3)
1.632(3)
1.652(5)
1.776(5)
1.671(2)
1.644(3)
1.6528(13)
1.728(2)
177.28(17)
176.9(3)
177.6(3)
176.5(3)
174.4(10)
177.1(7)
169.3(2)
176.9(3)
173.19(13)
157.4(2)
1.9889(16)
1.988(2)
2.010(3)
2.039(2)
2.001(5)
2.063(3)
1.998(2)
1.135(2)
1.141(3)
1.136(4)
1.131(4)
1.150
0.925(6)
1.144(3)
1.112(4)
1.140(2)
1.153(3)
0.02
0.01
0.04
0.06
0.0d
51
51
51
51
49
58
50
49
59
[(OEP)Fe(Iz)(NO)]+
{[(OEP)Fe(NO)]2Prz}2+
[(TPP)Fe(H2O)(NO)]+
[(TPP)Fe(HO-i-C5H11)(NO)]+e
(TpivPP)Fe(NO2)(NO)
[(OEP)Fe(NO)]ClO4•CHCl3
[(OEP)Fe(NO)]ClO4
0.05f
0.09
0.29
0.32
0.09
(OEP)Fe(NO)(p-C6H4F)
2.040(3)
this work
B. {FeNO}7 Metalloporphyrin Derivatives
(TPP)Fe(NO)
(OEP)Fe(NO)
(TPP)Fe(NO)(1-MeIm)
(TPP)Fe(NO)(4-MePip)(1)
(TPP)Fe(NO)(4-MePip)(2)
2.001(3)
2.010(13)
2.008(12)
2.004(9)
1.998(10)
1.717(7)
1.7307(7)
1.743(4)
1.721(10)
1.740(7)
149.2(6)
142.74(8)
142.1(6)
138.5(11)
143.7(6)
1.122(12)
1.1677(11)
1.121(8)
1.141(13)
1.112(9)
0.21
0.27
0.07
0.09
0.11
60
48
47
61
61
2.180(4)
2.328(10)
2.463(7)
C. {RuNO}6 Metalloporphyrin Derivatives
(TPP)Ru(NO)(ONO)
(OEP)Ru(NO)(ONO)
(TTP)Ru(NO)(ONO)
(TTP)Ru(NO)(OMe)
(TTP)Ru(NO)(OH)
(TPP)Ru(NO)(OH)
2.047(5)
2.060(5)
2.053(6)
2.050(3)
2.055(5)
2.050(4)
2.060(3)
2.041(4)
2.055(4)
2.060(2)
2.050(8)
2.05(2)
1.72(2)
180.0
1.90(2)
1.12(2)
0.0d
NRg
0.13
0.0d
0.05
0.0d
NRg
0.0d
NRg
0.07
0.10
NRg
0.12
43
43
62
63
62
43
43
44
64
53
53
42
1.758(7)
1.752(6)
1.84(4)h
1.751(5)
1.726(9)
1.723(11)
1.888(5)h
1.737(5)
1.769(3)
1.708(6)
1.807
174.0(8)
173.3(6)
180.0
167.4(6)
180.0
173.7(12)
171.0(7)
170.2(5)
172.8(3)
177.8(5)
152
1.984(6)
1.998(6)
1.80(5)h
1.943(5)
1.873(11)
1.956(11)
1.888(5)h
2.022(5)
2.3901(10)
2.049(4)
2.095(6)
2.111(3)
1.177(9)
1.152(9)
NRg
1.142(8)
1.179(9)
1.155(14)
1.138(12)
1.160(6)
1.114(4)
1.141(7)
1.159
(OEP)Ru(NO)(OH)
[(OEP)Ru(NO)(H2O)]+
(TTP)Ru(NO)(NSO)
(OEP)Ru(NO)(SC(Me)2CH2NHC(O)Me)
[(OEP)Ru(NO)(OdC(Me)NHCH2C(Me)2SH)]+
(TTP)Ru(NO)(p-C6H4F)
(OEP)Ru(NO)(p-C6H4F)
2.059(7)
1.807(3)
154.9(3)
1.146(4)
this work
a Values given in Å. b Values given in degrees. c Displacement of metal from 24-atom porphyrin plane toward NO. d Metal is exactly in-plane
due to symmetry. e This complex displayed extensive disorder. f Displacement of the metal from the four-nitrogen mean plane. g Not reported.h The
axial ligands are disordered over two positions due to crystallographic symmetry.
Table 5. Axial Ligand Tilts (deg) in Six-Coordinate
Table 6. Iron and Ruthenium Porphyrin Compounds with Axial
Alkyl/Aryl (Non-Carbonyl) Ligands
(Por)M(NO)(Aryl) Compoundsa
compound
(OEP)Ru(C6H5)
(OEP)Ru(Neop)
M-C (Å)
reference
2.005(7)
2.069(7)
2.12(1)
68
69
[(OEP)Ru(Neop)]2(µ-Li)2
[(OEP-N-Ph)Ru(C6H5)]BF4
(TPP)Ru{)C(CO2Et)2}(MeOH)
(TTP)Ru(NO)(p-C6H4F)
(OEP)Ru(NO)(p-C6H4F)
(TPP)Fe(C6H5)
(T(p-OCH3)PP)Fe(C(O)(n-Bu))
(T(p-OCH3)PP)Fe(CH3)
(OEP)Fe(NO)(p-C6H4F)
2.100(3)
1.999(4)
1.829(9)
2.095(6)
2.111(3)
1.955(3)
1.965(12)
1.979(9)
2.040(3)
69
70
71
42
this work
72
73
73
this work
angle between Ph
and MNO planes reference
compound
R
â
γ
(OEP)Fe(NO)(p-C6H4F)b 9.2 22.6 3.1
(OEP)Ru(NO)(p-C6H4F)c 10.8 25.1 3.5
(TTP)Ru(NO)(p-C6H4F)c 12.0 27.8 4.3
19.8
16.1
14.0
this work
this work
42
a The R and γ angles are with respect to the normal to the 24-atom
c
mean porphyrin plane. b Data collected at 130(2) K. Data collected
The crystal packing diagram of the ruthenium complex, (OEP)-
Ru(NO)(p-C6H4F), is given in the Supporting Information
(Figure S1). The intermolecular contacts are similar to those of
the isomorphous Fe analogue: the NO groups of two adjacent
molecules also point toward each other, with an O(1)‚‚‚O(1A)
distance of 2.88 Å; the next closest intermolecular contacts are
over 3.56 Å and are listed in the caption of Figure S1. The
crystal packing diagram of (TTP)Ru(NO)(p-C6H4F) is also given
in the Supporting Information (Figure S2). The packing in the
tetraaryl derivative is clearly different from the OEP derivatives.
The closest intermolecular distance involving the NO group in
this derivative is between the oxygen atom and a nearby tolyl
group at an O(1)‚‚‚C(18A) distance of 3.23 Å. Other pertinent
intermolecular distances are greater than 3.35 Å and are listed
in the figure caption. There is thus no evidence that the
M-N-O group in any of the three structures is significantly
affected by crystal packing.
at 173(2) K.
(TPP)Fe(C6H5) where the phenyl ligands more nearly bisect the
Np-M-Np angle.68,72 As in the iron case, the axial aryl and
RuNO groups are not coplanar, displaying an angle of 16.1°
between the six-carbon aryl plane and the RuNO plane (Table
5).
The crystal packing of (OEP)Fe(NO)(p-C6H4F) is shown in
Figure 3. The closest intermolecular distance involving the NO
group is between two nitrosyl oxygens, with the O(1)‚‚‚O(1A)
distance of 2.92 Å. The nonbonded distance between two
oxygen atoms of (hypothetical) linear NO groups on adjacent
molecules was calculated to be 2.88 Å. The next closest
intermolecular distance (3.58 Å) is that between the nitrosyl
N(5) and an ethyl substituent C(72B) atom on a neighboring
molecule (more distances are listed in the caption of Figure 3).

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6321
Figure 3. Crystal packing diagram for (OEP)Fe(NO)(p-C6H4F) showing symmetry-related units. The axial NO and aryl groups, as well as most
of the ethyl substituents, have been omitted for three of the molecules for the sake of clarity. Closest intermolecular contacts in Å: O(1)‚‚‚O(1A)
) 2.92, N(5)‚‚‚C(72B) ) 3.58, O(1)‚‚‚C(51C) ) 3.62, O(1)‚‚‚C(52C) ) 3.64, O(1)‚‚‚C(72B) ) 3.68, N(5)‚‚‚C(52D) ) 3.78. Symmetry operators
used to generate equivalent molecules: A ) -x, -y+1, -z+1; B ) -x, -y, -z+1; C ) -x+1, -y+1, -z+1; D ) x-1, y, z.
The unusual geometry of the M-N-O group in these
{MNO}6 species is also reflected in the observed nitrosyl
stretching frequencies which should be quite sensitive to the
electronic structure at the metal center. (OEP)Fe(NO)(p-C6H4F)
has an unusually low nitrosyl stretching frequency of 1791 cm-1
compared to all of the structurally characterized five- or six-
coordinate {FeNO}6 porphyrin complexes which have a υNO
range of 1862-1937 cm-1 and which have linear or nearly linear
Fe-N-O groups (angles range from 169.3 to 180.0°).66 Some
representative nitrosyl stretching frequencies are listed in Table
2 for iron and ruthenium porphyrin species. The nitrosyl
stretching frequencies in five- or six-coordinate {FeNO}7
porphyrin complexes are much lower (1616-1690 cm-1). The
Fe-N-O groups in these species are bent with angles ranging
from 138.1 to 149.2°. The structurally characterized (OEP)Fe-
(NO)(p-C6H4F) species has an Fe-N-O angle of 157.4(2)°
which is intermediate between the {FeNO}6 and {FeNO}7
porphyrin complexes. It is also evident from the values given
in this table that the σ-bonded alkyl or aryl(iron) derivatives
have nitrosyl stretching frequencies intermediate between the
{FeNO}7 and {FeNO}6 derivatives. Although the other σ-bond-
ed alkyl or aryl(iron) derivatives given in the table have not
been structurally characterized, it is tempting to suggest a
continuum of bent Fe-N-O groups with the most strongly
donating methyl derivative having the most bent Fe-N-O
group and the least donating tetrafluorophenyl derivative having
a near linear Fe-N-O group. Indeed such a suggestion was
made earlier,41 without any structural data. Attempts to structur-
ally characterize these species are in progress. The ruthenium
derivatives should show similar trends. (OEP)Ru(NO)(p-C6H4F)
has the lowest nitrosyl stretching frequency (for the OEP
derivatives) of 1759 cm-1 and the most bent angle of
154.9(3)°. This observation is consistent with the presence of
the strongly σ-interacting trans R group and subsequent
increased back-donation of electron density onto the NO ligand.
All of the other structurally characterized nonorganometallic
{RuNO}6 species have linear or near linear Ru-N-O angles.
The TTP derivative (TTP)Ru(NO)(CH3), which has not been
structurally characterized, has an even lower υNO as a result of
the more strongly donating methyl group trans to the nitrosyl.
No ruthenium porphyrin compounds of the {RuNO}7 formula-
tion have been reported; hence, a comparison between the
{RuNO}6 and {RuNO}7 systems cannot be made at this time.
Quantum Chemical Studies of (porph)Fe(NO)(p-C6H4F).
Extended Hu¨ckel molecular orbital (MO)34-37 and preliminary
hybrid Hartree-Fock (HF/DF) density functional calcula-
tions33,76-78 were done to investigate orbital energy changes
upon (i) tilting the p-C6H4F and NO ligands off the molecular
axis and (ii) bending the NO ligand at the nitrogen.
A complete B3LYP hybrid HF/DF geometry optimization was
performed for the (porph)Fe(NO)(p-C6H4F) molecule starting
from the experimental, X-ray diffraction geometry. The quan-
titative accuracy of the HF/DF calculation is limited by the small
basis set size, but calculated bond distances for the optimized
structure are qualitatively similar to those determined by X-ray
diffraction (compared in Figure 4). In the calculated minimum-
energy structure, the Fe-N (NO) bond is tilted 11.3° (R) from
the perpendicular to the four-nitrogen porphyrin plane, whereas
the Fe-C bond is tilted 4.4° (γ). The calculated tilt angles both
agree well with the experimentally determined angles, and as
in the X-ray structure, both ligands are tilted in the same general
direction. In the calculated structure, projections of the Fe-
(76) Ghosh, A. Acc. Chem. Res. 1998, 31, 189-198.
(77) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(78) Ziegler, T. Chem. ReV. 1991, 91, 651-667.

6322 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Richter-Addo et al.
Figure 5. Molecular orbital energy level diagram of (porph)Fe(NO)-
(p-C6H4F). The B2 symmetry LUMO and the A2 symmetry HOMO
are also shown.
Figure 4. Selected geometrical parameters (in Å and degrees) for
(porph)Fe(NO)(p-C6H4F), calculated by using the B3LYP hybrid
Hartree-Fock/density functional method, compared with corresponding
parameters for (OEP)Fe(NO)(p-C6H4F), determined by X-ray diffraction
(in parentheses). The tilting angles and atom displacement (∆) are with
respect to the four-nitrogen porphyrin plane.
in detail in the following paragraphs. The ligands were then
tilted off the molecular symmetry axis by reducing the C-Fe-
N(O) angle, and finally, the Fe-N-O angle was bent. Other
exploratory calculations were performed to determine whether
FeNO bending could occur independently of ligand tilting off
the symmetry axis and to determine whether the orientation of
the p-C6H4F ligand exerts a substantial electronic effect on the
direction of ligand tilting and FeNO bending. Although some
of the calculations described below refer to an idealized structure
of (porph)Fe(NO)(p-C6H4F), qualitative conclusions were con-
firmed by repeating our calculations using the X-ray diffraction
core structure of (OEP)Fe(NO)(p-C6H4F) (with the Fe atom
placed in the plane of the porphyrin and the ethyl groups
replaced with hydrogens).
N-O and phenyl planes exactly bisect an Np-Fe-Np angle.
The calculated structure also shows a significant bending of
the FeNO group (150.4°, δ in Figure 4). The iron atom was
found to be 0.08 Å out of the four pyrrole nitrogen atom plane,
toward the NO ligand (0.06 Å experimental). Although the
calculated conformation of the porphyrin ring is slightly ruffled,
several different conformations probably have similar energies.
The calculated NO distance was 1.193 Å (1.153(3) Å experi-
mental). The calculated Fe-N(NO) distance of 1.666 Å is 0.062
Å shorter than the experimental distance of 1.728(2) Å, and
the calculated Fe-C distance (1.968 Å) is 0.072 Å shorter than
the experimentally determined Fe-C distance (2.040(3) Å).
Calculated Fe-Np distances show differences of similar mag-
nitudes, but the asymmetry of the Fe-N4(porph) core is
consistent with that determined experimentally for (OEP)Fe-
(NO)(p-C6H4F). Specifically, the Fe-Np bonds farthest from
the aryl and NO ligands are shorter than the Fe-Np bonds closer
to the axial ligands. The shorter calculated distances are 1.964
Å, compared to experimental distances of 2.001(2) Å and 2.015-
(2) Å, whereas the longer calculated distances are 2.002 Å
compared with experimental distances of 2.024(2) Å and
2.023(2) Å. We note that this experimentally observed asym-
metry in the Fe-N4(por) core (reproduced by calculations) is
opposite to that observed in the five-coordinate (OEP)Fe(NO)
compound.48
Figure 5 shows the frontier molecular orbital energy level
diagram of (porph)Fe(NO)(p-C6H4F), derived from extended
Hu¨ckel calculations. Orbitals with the correct symmetry to form
σ bonds have A1 symmetry, whereas those with appropriate
symmetry to form π bonds have B1 or B2 symmetry. The
approximately nonbonding orbitals are labeled 1A1 and 3A1.
The porphyrin contribution to the 1A1 orbital has the nodal
structure of A2u in Gouterman’s four-orbital model of porphyrin
electronic structure (different symmetry labels arise because of
the lower symmetry of the nitrosyl complex and because of our
choice of orientation for the x and y axes). The 2A1 orbital of
2
2
(porph)Fe(NO)(p-C6H4F) is composed mainly of the metal dx -y
atomic orbital which has the wrong nodal structure to overlap
with the NO nitrogen lone pair. The Fe-NO π-bonding
molecular orbitals (1B1 and 1B2) consist of a mixture of iron
d
xz or dyz orbitals, respectively, with the NO π* orbitals to give
Initial extended Hu¨ckel calculations for (porph)Fe(NO)(p-
C6H4F) were performed for model structures with the C-Fe-N
and Fe-N-O angles fixed at 180° (as described in the
Experimental Section and Supporting Information Table S17).
The frontier molecular orbitals for this structure are described
a strong π-bonding interaction. The molecule’s HOMO is the
Fe-NO nonbonding orbital labeled A2, illustrated in Figure 5,
and concentrated on the porphyrin ring. A2 has no contribution
from the iron and corresponds to the A1u orbital in the four-
orbital model of the porphyrin. The HOMO’s composition

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6323
Figure 6. Orbitals 1B1 and 2A1 in Figure 5 mix upon tilting axial ligands to give the orbital shown on the right.
Figure 7. Bending the nitrosyl ligand mixes nitrogen (and oxygen) pz character into the orbital shown on the left to better align the nodes on the
NO π* and metal d orbitals, as shown on the right.
implies that oxidation of the molecule will result in loss of
electron density primarily from the porphyrin ring, rather than
the metal.79 The HOMO is well separated from a LUMO (2B2)
and second LUMO (2B1) composed primarily of NO π* orbitals
interacting in an antibonding way with some metal dxz or dyz
contributions. The sets 2B1/2B2 and 3B1/3B2 are both antibond-
ing between iron and NO, but differ from each other because
the metal-porphyrin π interaction is bonding in the lower-
energy 2B1/2B2 set, and antibonding in the 3B1/3B2 pair. The
LUMO’s composition implies that reduction would result in a
slight weakening and lengthening of the Fe-NO bond, but a
slight strengthening and shortening of the Fe-N(porphyrin)
bonds.
Axial Ligand Tilting and/or Bending. The change in total
energy and changes in individual orbital energies upon tilting
the p-C6H4F and NO ligands off the normal to the porphyrin
ring are very small. The total energy surface is relatively flat
and shows an increase of only 5 kcal/mol upon tilting the N
atom of the NO ligand by 5° (while keeping the FeNO group
linear). No large changes in MO energies appear upon tilting
the NO ligand off-axis by as much as 10°. Changes in total
energy and individual orbital energies, observed upon bending
the NO ligand at nitrogen, imply that once the NO ligand tilts
off-axis, FeNO prefers to bend away from a perfectly linear
geometry. The total energy surface also suggests that increased
tilting favors increased bending. Figure S4 shows the change
in total energy and changes in individual orbital energies
observed upon bending the NO ligand at the nitrogen, once the
NO ligand is tilted 6° off-axis.
detail,7 and thus we will not repeat the analysis here. The most
important feature for our subsequent analysis of ligand bending
is the mixing between the orbitals labeled 1B1 and 2A1 in Figure
5, as shown in Figure 6. The mixing hybridizes the metal
contribution to maintain a strong bonding interaction between
the metal orbital and the NO π* contribution to the resulting
MO.
Hoffmann, Chen, and Thorn have analyzed orbital energy
changes and orbital mixing for ligand bending, but only for
geometries in which the bent axial ligands eclipse the equatorial
ML bonds.9 Despite the difference between the geometry
analyzed previously and that described here, the analysis of
Hoffmann et al. offers key clues to the reasons why FeNO
bending between the equatorial ligands is energetically favorable,
especially after tilting the axial ligands. First, a molecular orbital
2
2
analogous to the metal dx -y orbital calculated here increases
sharply in energy if the axial FeNO group bends to eclipse an
equatorial ML bond. This orbital’s energy increases because
bending the ligand allows an antibonding interaction to develop
9
2
2
between the metal dx -y and NO π* orbitals. In contrast, when
the axial ligand is bent to bisect the Np-Fe-Np angle, the nodal
plane of the NO π* orbital coincides with the maximum
2
2
amplitude of the dx -y orbital and the two have zero overlap.
A second determinant of the large energy barrier for bending
an axial ligand along an equatorial M-L bond is the loss of
π-bonding between the metal dxz and NO π* orbitals.9 If the
axial NO ligand is also tilted and is instead bent between the
equatorial ligands, the primary effect of NO bending is a mixing
of nitrogen (and oxygen) pz atomic orbitals into the orbital
illustrated in Figure 6. The resulting orbital is sketched on the
right side of Figure 7 and shows that axial ligand tilting and
NO bending effectively conspire to align the nodal surfaces of
the metal and nitrosyl contributions to the orbital 3A′ and
increase their bonding interaction. Moreover, the energy change
of the 3A′ orbital is modulated because its metal dxz component
Although no large orbital energy changes are evident upon
tilting the axial ligands, significant orbital mixing occurs. Orbital
mixing for axial ligand tilting has already been analyzed in some
(79) Electrochemical data for the related (por)Fe(NO)(Ph) (por ) TPP,
OEP) compounds in PhCN have been reported, but the sites of redox activity
were not investigated in detail (ref 41).

6324 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Richter-Addo et al.
Table 7. Mo¨ssbauer Parameters for Nitrosyl Derivatives at
Zero-Applied Field
∆EQ, δFe,
mm/s mm/s T, K reference
A. {FeNO}6 Complexes
(OEP)Fe(NO)(p-C6H4F)
0.56
0.57
1.55
1.64
1.63
1.65
0.05 293
0.14
this work
4.2 this work
[(OEP)Fe(NO)]ClO4
0.13 293
0.20 4.2 59
0.12 293 59
0.20 4.2 59
59
0.02
0.02
59
[(OEP)Fe(NO)]ClO4•CHCl3
[(OEP)Fe(Iz)(NO)]ClO4
1.99 -0.07 293
1.92
1.64
4.2 59
4.2 97
[(OEP)Fe(1-MeIm)(NO)]+
(CH3C(O)N(CH3)2 solution)
(TPP)Fe(NO2)(NO)
1.37
1.36
1.36
1.43
1.48
1.43
0.02 293
0.13 77
50
80
4.2 50
50
50
4.2 50
0.13
0.04 293
0.01 293
Figure 8. Mo¨ssbauer spectrum of (OEP)Fe(NO)(p-C6H4F) in zero
applied field at 4.2 K. The solid line is a fit to the data which yields
the following parameters: quadrupole splitting ∆EQ ) + 0.57 mm/s,
isomer shift δ ) 0.14 mm/s, and Lorentzian width γ(fwhm) ) 0.27
mm/s.
(T(p-OCH3)PP)Fe(NO2)(NO)
(TpivPP)Fe(NO2)(NO)
0.09
B. {FeNO}7 Complexes
[(TpivPP)Fe(NO2)(NO)]- (form 2) 1.20
0.35
0.35
4.2 81
4.2 81
(TPP)Fe(NO)
(OEP)Fe(NO)
1.2
Fe(NO)(p-C6H4F) is within the range of values previously
observed for {FeNO}6 porphyrinates. All of these {FeNO}6
species have isomer shifts smaller than those observed for low-
spin iron(III) porphyrinates with two neutral nitrogen donors
as the axial ligands.88-94 We have previously ascribed the lower
values of isomer shift in the formally ferric nitrosyl derivatives
as the result of strong π-donation from iron to NO rather than
an assignment of +4 as the oxidation state of iron.59 σ-Donation
by a sixth ligand is expected to increase the s-density at iron
and to lead to a decrease in isomer shift compared to the value
observed in the base five-coordinate species. This is indeed
observed. The isomer shifts for the species with nitrite, indazole,
and imidazole as the sixth ligand are all smaller than those
observed for [(OEP)Fe(NO)]+. That the species with nitrite as
a sixth ligand has a larger isomer shift (less s-density) than those
with imidazole or indazole is unexpected and must represent a
net decreased π-donation from iron to NO in the nitrite complex
or a net increase in π-accepting at NO in the imidazole and
indazole complexes. We believe that the latter is more probable.
1.26
0.35 100
82
C. Fe(II) CO Complexes
(TPP)Fe(1-MeIm)(CO)
(TPP)Fe(Py)(CO)
(TpivPP)Fe(1-MeIm)(CO)
MbCO
0.35
0.57
0.27
0.35
0.20 293
0.28 293
0.27
0.27
83
83
4.2 84
4.2 85
is distributed over several MOs due to its interaction with the
porphyrin π system.
Mo1ssbauer Studies. We have carried out a detailed Mo¨ss-
bauer investigation of (OEP)Fe(NO)(p-C6H4F) in order to study
the electronic structure of this highly unusual complex. The
Mo¨ssbauer spectra demonstrate the purity of the preparations.
Importantly, the spectra obtained in an applied magnetic field
(Figure S3) also demonstrates the diamagnetic ground state of
the complex. The possible nature of the coupling between the
iron center and the nitric oxide ligand that leads to an effective
diamagnetic state has been discussed recently.59 The Mo¨ssbauer
study in applied magnetic field yields the sign (positive) of the
quadrupole splitting constant, the value of the asymmetry
parameter, the isomer shift δ, and the quadrupole splitting
constant ∆EQ. The underlying diamagnetism unfortunately limits
any magnetic information on electronic structure.
The Mo¨ssbauer spectrum for (OEP)Fe(NO)(p-C6H4F) is
shown in Figure 8. The isomer shift and quadrupole splitting,
obtained in zero field, are reported in Table 7 along with all
available Mo¨ssbauer data for other {FeNO}6 porphyrinate
derivatives. The data given show that there is wide variation in
the Mo¨ssbauer parameters of the relatively few {FeNO}6
porphyrinate derivatives that have been characterized. Clearly,
there is variation in electronic structure as measured by
Mo¨ssbauer spectroscopy among the group. The species char-
acterized include two crystalline forms of the five-coordinate
cation, [(OEP)Fe(NO)]+, which can be considered as the base
complex for comparison with all of the other derivatives. Even
though the two crystalline forms of the cation have significantly
different nitrosyl stretching frequencies, (∆ ≈ 30 cm-1), the
two forms have very similar Mo¨ssbauer parameters in both
applied and zero field.
(80) Settin, M. F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431-1435.
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Chem. Soc. 1991, 113, 5497-5510.
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Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
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Soc. 1988, 110, 5644-5650.
As is well recognized, the iron isomer shift is a measure of
the s-electron density at the nucleus and within a closely related
set of complexes is also an excellent indicator of the formal
charge (oxidation state) of iron.86,87 The isomer shift for (OEP)-
(93) Scheidt, W. R.; Osvath, S. R.; Lee, Y. J.; Reed, C. A.; Shaevitz,
B.; Gupta, G. P. Inorg. Chem. 1989, 28, 1591-1595.
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395.

Bent Fe-N-O Linkages in {M(NO)}6 Porphyrins
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6325
The strongly σ-donating aryl ligand in (OEP)Fe(NO)(p-C6H4F)
would be expected to lead to the smallest value of isomer shift.
That this difference is not observed must result from decreased
π-donation from iron to NO. In this σ-aryl complex the
decreased π-bonding is clearly reflected in the structure of the
bent Fe-N-O group.
The quadrupole splitting constant for (OEP)Fe(NO)(p-C6H4F)
of +0.57 mm/s is much smaller than any of the other nitrosyl
species and indeed is much smaller than any low-spin iron(III)
porphyrinate.86 The very low value of the quadrupole splitting
suggests that the anisotropy in the iron 3d shell is small. Given
the ground-state diamagnetism of these {FeNO}6 complexes,
an analysis of the quadrupole splitting constant variation in terms
of a low-spin d6 electronic configuration appears to be war-
ranted. The quadrupole splitting constant (∆EQ) is related to
the electric field gradient tensor (EFG) at the nucleus by the
following equation:
least σ-basic ligand, indazole, leads to the strongest Fe-N-O
π-bonding, while the most σ-basic ligand, p-fluorophenyl, leads
to the weakest Fe-N-O π-bonding.
Interestingly, this variation in Fe-N-O π-bonding is not
exactly suggested by the pattern of the nitrosyl stretching
frequency changes, which are commonly thought to be due to
differences in Fe to NO π-back-donation and where the indazole
species has the highest υNO at 1914 cm-1. The observation that
υ
NO of the five-coordinate {FeNO}6 species is in the middle of
the range of six-coordinate {FeNO}6 species is unexpected and
suggests subtle effects on the nitrosyl stretching frequency.
However, the differences in υNO follow the observed differences
in Fe-N-O angle, although the experimental X-ray data are
somewhat hampered by crystallographic disorder problems for
many of the nitro(nitrosyl) derivatives. The indazole and related
derivatives have effectively linear Fe-N-O groups; the nitro-
(nitrosyls) appear to have Fe-N-O groups bent by ap-
proximately 10°, while the aryl species has the strongly bent
Fe-N-O group. The sequence of decreasing υNO follows the
observed variation in the bending of Fe-N-O. A theoretical
evaluation of possible contributions to the Mo¨ssbauer spectra
and a comparison of calculated values of the asymmetry
parameter, η, and quadrupole splitting parameter, ∆EQ, would
be desirable but are beyond the scope of this investigation. See
for example Havlin et al.83 for an example of such detailed
theoretical calculations.
1/2
1
2
η2
3
∆EQ ) eQVzz 1 +
(
)
where e is the electron charge, Q is the quadrupole moment,
and Vzz is the largest component of the EFG tensor and
Vxx - Vyy
η )
Vzz
The principal components of the EFG satisfy |Vzz| > |Vyy| >
|Vxx|. The experimentally determined asymmetry parameter, η,
in these complexes is zero within the experimental uncertainty.
When the asymmetry parameter is zero, the EFG can be
restricted to the Vzz component. The contributions to Vzz can be
restricted to valence, covalence, and ligand contributions, with
the valence and covalence contributions expected to dominate.
The anisotropy of the iron 3d shell can be expressed by the
following:95
Concluding Remarks
We have prepared and structurally characterized the very
unusual Fe and Ru compounds (OEP)M(NO)(p-C6H4F). The
observation of significantly bent M-N-O groups in these
{MNO}6 species is unprecedented. The current dogma that
assigns a linear MNO linkage in ferric nitrosyl hemes has to be
modified to include the possibility of low-energy bending
coupled with axial tilting of the FeNO group. Bent Fe-N-O
linkages are now seen to be possible for both formally ferrous
and ferric nitrosyl porphyrins. The bending of the M-N-O
group seen in (OEP)M(NO)(p-C6H4F) appears intrinsic to
species of the type (por)M(NO)X when X is a strong σ-donor
ligand. Indeed, Mo¨ssbauer data for several {FeNO}6 species
show that the sixth ligand has a significant effect on the Fe-
NO π-bonding. Our results also suggest that related classes of
compounds of the type (por)M(NO)X could also display similar
axial NO tilting/bending features.96,97 Further studies to inves-
tigate the effect of the trans axial ligand on this tilting/bending
feature of NO ligands are in progress. Quantum calculations
show that the unexpected bending of the FeNO group coupled
with axial FeNO tilting is due to electronic factors. Specifically,
hybrid Hartree-Fock/density functional calculations for (porph)-
Fe(NO)(p-C6H4F) show that a structure with both the p-C6H4F
and NO ligands tilted off-axis and the FeNO group bent is a
minimum-energy structure. These calculations approximately
reproduce the observed tilt angles, extent of FeNO bending,
and the N-O, Fe-NO, and Fe-Np bond distances in the parent
(OEP)Fe(NO)(p-C6H4F) compound. Extended Hu¨ckel calcula-
tions show that bending the FeNO group to bisect a Np-Fe-
Np angle gives a lower energy barrier than bending in a direction
eclipsing an equatorial Fe-Np bond because overlap between
n(dxz) + n(dyz)
∆nd ) n(dx2-y2) + n(dxy) - n(dz2) -
2
Differences in the observed quadrupole splitting parameters in
the {FeNO}6 species should be reflected by the differing
anisotropy of the valence shell occupation ∆nd. Changes in the
anisotropy of the valence shell will be determined by the σ-donor
and π-accepting capabilities of the two axial ligands. From our
data for {FeNO}6 species, this must include variation in the
π-bonding interaction between iron and NO. The low quadrupole
splitting constant in the present aryl complex is consistent with
the strong σ-donating ability of the aryl anion. This, and the
weakened π-accepting capability of NO, both serve to decrease
the value of ∆EQ. The difference in ∆EQ for the nitrite complex
(decreased) and the indazole complex (increased) compared to
the five-coordinate reference species must be due to differences
in the Fe-NO π-bonding; increased π-acceptance in the
indazole derivative and decreased π-acceptance in the nitrite
species. That the imidazole complex has a quadrupole splitting
constant similar to that of the five-coordinate species must reflect
the combination of the σ-donation and modest π-donor ability
of imidazole along with increased π-donation from iron to NO.
These Fe-N-O π-bonding differences are totally consistent
with the isomer shift data considered earlier. The Mo¨ssbauer
data thus strongly support the idea that the sixth ligand, trans
to NO, has a significant effect on the Fe-N-O π-bonding. The
2
2
the NO π* orbital and the metal dx -y orbital is zero.
(96) We have also very recently observed axial NO tilting and bending
in the crystal structure of the related (OEP)Os(NO)(OEt) complex: Cheng,
L.; Powell, D. R.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem. 2001,
40, 125-133
(97) Schu¨nemann, V.; Benda, R.; Trautwein, A. X.; Walker, F. A. Isr.
J. Chem. 2000, 40, 9.
(95) Paulsen, H.; Kro¨ckel, M.; Grodzicki, M.; Bill, E.; Trautwein, A.
X.; Leigh, G. J.; Silver, J. Inorg. Chem. 1995, 34, 6244-6249.

6326 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Richter-Addo et al.
Acknowledgment. We thank the National Institutes of
Health for support of this work (GM38401, W.R.S.; GM53586,
GBR-A). Funding from the National Science Foundation (CHE-
9625065, GBR-A) is also acknowledged. Funds for the purchase
of the FAST area-detector diffractometer was provided through
NIH Grant RR-06709 to the University of Notre Dame. We
are grateful to the University of Oklahoma Graduate College
for an Alumni Graduate Fellowship for C.A.H.
factors, and fixed hydrogen atom positions for (OEP)Ru(NO)-
(p-C6H4F) at the two temperatures; Table S17 giving extended
Hu¨ckel parameters used in this work; Figures S1 and S2 showing
the closest intermolecular contacts with the nitrosyl group in
(OEP)Ru(NO)(p-C6H4F) and (TTP)Ru(NO)(p-C6H4F), respec-
tively; Figure S3 showing the Mo¨ssbauer spectrum of (OEP)-
Fe(NO)(p-C6H4F) in an applied magnetic field at 4.2 K; Figure
S4 presenting a Walsh diagram indicating changes in individual
orbital energies for (porph)Fe(NO)(p-C6H4F) upon bending the
NO ligand (PDF). X-ray crystallographic files, in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Supporting Information Available: Tables S1-S6, giving
complete crystallographic details, atomic coordinates, bond
distances and angles, anisotropic temperature factors, and fixed
hydrogen atom positions for (OEP)Fe(NO)(p-C6H4F); Tables
S7-S16, giving complete crystallographic details, atomic
coordinates, bond distances and angles, anisotropic temperature
JA010276M