Heme carbonyls: Environmental effects on vc-o and Fe-C/C-O bond length correlations

Article abstract of DOI:10.1021/ja053148x

The synthesis and characterization of four low-spin (carbonyl)iron(11) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated), and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of v_c-o in toluene solution (1969-72 cm^-1) but a large range of CO stretching frequencies in the solid-state (1926-1968 cm^-1). The large solid-state variation results from CO interactions in the solid state, as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N _lm-Fe-CO units. The values of v_c-o and the Fe-C and C-O bond distances are strongly correlated and provide a structural, as well as a spectroscopic, correlation of the π back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

Full text of DOI:10.1021/ja053148x

Published on Web 09/27/2005
Heme Carbonyls: Environmental Effects on νC-O and
Fe-C/C-O Bond Length Correlations
Nathan J. Silvernail,† Arne Roth,† Charles E. Schulz,*,‡ Bruce C. Noll,† and
W. Robert Scheidt*,†
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Knox College,
Galesburg, Illinois, 61401
Received May 13, 2005; E-mail: scheidt.1@nd.edu
Abstract: The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates,
[Fe(TPP)(CO)(L)], where L ) 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated),
and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of
νC-O in toluene solution (1969-72 cm-1) but a large range of CO stretching frequencies in the solid-state
(1926-1968 cm-1). The large solid-state variation results from CO interactions in the solid state, as shown
by an examination of the crystal structures of the four complexes. The high precision of the four structures
obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-
C-O and NIm-Fe-CO units. The values of νC-O and the Fe-C and C-O bond distances are strongly
correlated and provide a structural, as well as a spectroscopic, correlation of the π back-bonding model.
The interactions of CO described are closely related to the large range of CO stretching frequencies observed
in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).
Introduction
has been extensively studied by vibrational spectroscopy; the
trends observed in isoelectronic series of carbonyl species are
Studies involving the carbonyl (CO)1 ligand have had a
profound influence in both classical inorganic and organo-
metallic chemistry, as well as bioinorganic chemistry systems,
such as the iron porphyrinate (heme) carbonyls. The carbonyl
ligand has been used to probe the nature of metal-ligand
bonding and to provide information on the nature of the
environment of prosthetic groups in proteins. For inorganic
carbonyl derivatives, it has long been noted that there is a
relationship between the strength of the metal-carbon bond and
that of the carbon-oxygen bond.2 The overall bonding interac-
tion is that any feature that leads to strengthening the C-O
bond concomitantly leads to a weakening of the metal-carbon
bond. This is the basis of the so-called π-back-bonding model
that is widely applied as a general model for bonding in
complexes with π-acceptor (π-acid) ligands. This correlation
a well-known phenomenon.3 In heme carbonyl derivatives,
trends in the Fe-C-O unit vibrations have long been estab-
lished and are known as the “inverse correlation” of the iron-
carbon stretching frequency (νFe-CO) and the carbon-oxygen
stretching frequency (νC-O).4,5 An increase in the Fe-C bond
order is seen to lead to a decrease in the C-O bond order.
Vibrational spectroscopy has been used since the inception
of bioinorganic chemistry to study protein active sites. Vibra-
tional spectroscopic techniques such as infrared (IR),6 resonance
Raman (rR),7 and nuclear resonance vibrational spectroscopy
(NRVS),8 allow the use of diatomic probe ligands to study
bonding and the environment near the binding site in metal-
containing proteins.
The diatomic ligands CO, CN-, NO, and O2 all display ligand
stretches that can be observed with vibrational techniques and
have been extensively employed in the study of hemes and heme
proteins.7-13 The CO ligand has a high affinity for iron(II)
* To whom correspondence should be addressed: E-mail Scheidt.1@nd.edu
(W.R.S.); Fax (574) 631-4044 (W.R.S.).
† University of Notre Dame.
‡ Knox College.
(1) The following abbreviations are used in this paper. Proteins: Mb,
myoglobin; MbCO, myoglobin with coordinated CO; Hb, hemoglobin;
HbCO, hemoglobin with coordinated CO. Porphyrins: Por, generalized
porphyrin dianion; OEP, dianion of octaethylporphyrin; Deut, dianion of
deuteroporphyrin; TpivPP, dianion of meso-R,R,R,R-tetrakis(o-pival-
amidophenyl)porphyrin; TPP, dianion of meso-tetraphenylporphyrin.
Ligands: Im, generalized imidazole; 1-MeIm, 1-methylimidazole; 2-Me-
HIm, 2-methylimidazole; 1,2-Me2Im, 1,2-dimethylimidazole; SEt-, anion
of ethanethiol; THF, tetrahydrofuran; THT, tetrahydrothiophene; Py,
pyridine. Spectroscopic techniques: IR, infrared absorption spectroscopy;
rR, resonance Raman spectroscopy; NRVS, nuclear resonance vibrational
spectroscopy.
(3) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 3rd ed.; Wiley and Sons: New York, 1978; pp 279-281.
(4) Tsubaki, M.; Ichikawa, Y. Biochim. Biophys. Acta 1985, 827, 268.
(5) Uno, T.; Nishimura, Y.; Masamichi, T.; Makino, R.; Iizuka, T.; Ishimura,
Y. J. Biol. Chem. 1987, 262, 4549.
(6) Alben, J. O.; Fiamingo, F. G. In Optical Techniques in Biological Research;
Rousseau, D. L., Ed.; Academic Press: Orlando, 1984; pp 133-179.
(7) (a) Rousseau, D. L.; Ondrias, M. R. In Optical Techniques in Biological
Research; Rousseau, D. L., Ed.; Academic Press: Orlando, 1984; pp 65-
132. (b) Kerr, E. A.; Yu, N. T. Biological Applications of Raman
Spectroscopy; Spiro, T. G., Ed.; Wiley and Sons: New York, 1988; Vol.
3, pp 39-95.
(8) Scheidt, W. R.; Durbin, S. M.; Sage, J. T. J. Inorg. Biochem. 2005, 99, 60.
(9) Wang, J. H.; Nakahara, A.; Fleischer, E. B. J. Am. Chem. Soc. 1958, 80,
1109.
(2) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley and Sons: New York, 1999; pp 636-
639.
9
14422
J. AM. CHEM. SOC. 2005, 127, 14422-14433
10.1021/ja053148x CCC: $30.25 © 2005 American Chemical Society

Heme Carbonyls
A R T I C L E S
hemes,9,14-17 and νC-O is conveniently observed.13 Conse-
quently, CO has been widely used as a probe ligand to detect
vacant coordination sites in reduced heme proteins. Carbon
monoxide is also an interesting biomolecule that is a product
of heme catabolism. Accordingly, the background body burden
of CO is relatively high.18 Since CO has a higher binding
constant than that of dioxygen, this natural CO level is
potentially toxic. Nature has solved the toxicity problem by
designing factors into the heme proteins hemoglobin (Hb) and
myoglobin (Mb) so that their CO affinity is reduced by
approximately 2 orders of magnitude compared to that of
analogous iron(II) porphyrinates.19,20
The origin of the molecular basis of the differences (selectiv-
ity) in the O2 and CO affinities between the heme proteins and
low-molecular-weight hemes has been an intensively studied
problem in bioinorganic chemistry. An early prominent idea
was that the environment of the protein ligand binding pocket
somehow imposes constraints on the Fe-C-O unit but not on
the Fe-O-O unit. There has been much discussion about
possible geometric constraints of the Fe-C-O unit in
MbCO.10,21-25 Although a number of protein crystal structures
have supported a rather bent Fe-C-O unit,26-29 more recent
results do not.30,31 Additionally, polarized IR photoselection
measurements on Mb single crystals and in solution support an
only slightly bent Fe-C-O unit.24,25 The original idea of
geometric constraints imposed by the protein has largely shifted
to investigations of the global (electrostatic) environment near
the CO binding site.23 CO has been widely used to investigate
the specific and global (electrostatic) environment.
frequencies has been attributed to varying conformational, and
subsequently, electrostatic environments in the heme pocket.34-36
A complete understanding of these bands is of interest since
they provide information about the relationship of conforma-
tional substates with functional states of the protein.23-29,69
A
large number of mutation studies have been dedicated to
understanding the multiple-peak phenomenon, differing environ-
(34) Frauenfelder, H.; Sligar, S. G.; Wolynes, P. G. Science 1991, 254, 1598.
Young, R. D.; Frauenfelder, H.; Johnson, J. B.; Lamb, D. C.; Nienhaus,
G. U.; Phillip, R.; Scholl, R. Chem. Phys. 1991, 158, 315.
(35) Alben, J. O.; Beece, D.; Bowne, S. F.; Doster, W.; Eisenstein, L.;
Frauenfelder, H.; Good, D.; McDonald, M. C.; Marden, M. C.; Moh, P.
P.; Reinisch, L.; Reynolds, A. H.; Shyamsunder, E.; Yue, K. T. Proc. Natl.
Acad. Sci. U.S.A. 1982, 79, 3744.
(36) Makinen, M. W.; Houtchens, R. A.; Caughey, W. S. Proc. Natl. Acad.
Sci. U.S.A. 1979, 76, 6042.
(37) Phillips, G. N., Jr.; Teodoro, M. L.; Li, T.; Smith, B.; Olsen, J. S. J. Phys.
Chem. B 1999, 103, 8817.
(38) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.;
Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(39) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
(40) (a) Fleischer, E. B.; Srivastava, T. S. J. Am. Chem. Soc. 1969, 91, 2403.
(b) Hoffman, A. B.; Collins, D. M.; Day, V. W.; Fleischer, E. B.; Srivastava,
T. S.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 3620.
(41) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc. 1981,
103, 4763.
(42) Sheldrick, G. M. Program for empirical absorption correction of area
detector data; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1996.
(43) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(44) Patterson, A. L. Phys. ReV. 1934, 46, 372.
(45) Sheldrick, G. M. SHELXL-97: FORTRAN program for crystal structure
refinement; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(46) A reviewer has raised the question of possible problems with correlation
of the parameters in the disordered system. We have carried out three types
of refinement of the disordered imidazole. The first was the “traditional”
refinement with constrained imidazole ring and equivalent Fe-N bond
distances. We also carried out two refinements with unconstrained axial
Fe-N bonds, one with constrained imidazole ring parameters and the second
with free imidazole ring parameters. These two refinements gave essentially
the same Fe-N distances and occupancies of the two rings. We concluded
that there was a real chance that the two ring orientations and differing
Fe-N distances was the correct choice to be made for this particular
problem, which is clearly a difficult one.
The heme proteins myoglobin and hemoglobin have been
among the most studied CO-ligated proteins. In the CO
derivatives of native Mb and Hb, up to four distinct CO
stretching frequencies can be observed.32,33 These bands have
been designated A0, A1, A2, and A3 in Mb. This range of
(47) Salzmann, R.; Ziegler, C. J.; Godbout, N.; McMahon, M. T.; Suslick, K.
S.; Oldfield, E. J. Am. Chem. Soc. 1998, 120, 11323.
(48) Silvernail, N. J.; Noll, B. C.; Scheidt, W. R. unpublished results.
(49) White, D. K.; Cannon, J. B.; Traylor, T. G. J. Am. Chem. Soc. 1979, 101,
2443.
(10) Spiro, T. G.; Zgierski, M. Z.; Kozlowski, P. W. Coord. Chem. ReV. 2001,
219-221, 923.
(50) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert T. R.; Suslick, K.
S. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 564.
(51) Little. R. G.; Ibers, J. A. J. Am. Chem. Soc. 1974, 96, 4452.
(52) Kim, K.; Ibers, J. A. J. Am. Chem. Soc. 1991, 113, 6077.
(53) Slebodnick, C.; Duval, M. L.; Ibers, J. A. Inorg. Chem. 1996, 35, 3607.
(54) Hashimoto, T.; Dyer, R. L.; Crossley, M. J.; Baldwin, J. E.; Basolo, F. J.
Am. Chem. Soc. 1982, 104, 2101.
(55) Ricard, L.; Weiss, R.; Momenteau, M. J. Chem. Soc., Chem. Commun.
1986, 818.
(56) Collman, J. P.; Sorrell, T. N. J. Am. Chem. Soc. 1975, 97, 4133.
(57) Salzmann, R.; McMahon, M. T.; Godbout, N.; Sanders, L. K.; Wojdelski,
M.; Oldfield, E. J. Am. Chem. Soc. 1999, 121, 3818.
(58) Slebodnick, C.; Fettinger, J. C.; Peterson, H. B.; Ibers, J. A. J. Am. Chem.
Soc. 1996, 118, 3216.
(59) Kim, K.; Fettinger, J. C.; Sessler, J. L.; Cyr, M.; Hugdahl, J.; Collman, J.
P.; Ibers, J. A. J. Am. Chem. Soc. 1989, 111, 403.
(60) Peng, S. M.; Ibers, J. A. J. Am. Chem. Soc. 1976, 98, 8032.
(61) Buchler, J. W.; Kokisch, W.; Smith, P. D. Struct. Bonding (Berlin) 1978,
34, 79.
(62) Scheidt, W. R.; Haller, K. J.; Fons, M.; Mashiko, T.; Reed, C. A.
Biochemistry 1981, 20, 3653.
(63) Collman, J. P.; Sorrel, T. N.; Dawson, J. H.; Trudell, J. R.; Bunnenberg,
E.; Djerassi, C. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 6.
(64) Caron, C.; Mitschler, A.; Rive´re, G.; Richard, L.; Schappacher, M.; Weiss,
R. J. Am. Chem. Soc. 1979, 101, 7401.
(65) Momenteau, M.; Scheidt, W. R.; Eigenbrot, C. W.; Reed, C. A. J. Am.
Chem. Soc. 1988, 110, 1207.
(66) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.; Scheidt,
W. R. J. Am. Chem. Soc. 1995, 117, 935.
(67) Several bis-ligated iron(II) complexes of general formula [Fe(Por)(L)2],
where L is an imidazole derivative, have been structurally characterized.
The values of Fe-N(L) are as follows (a) [Fe(TPP)(1-MeIm)2], 2.014(5)
Å, Hoard, J. L. personal communication to W.R.S.; (b) [Fe(TPP)(1-
VinIm)2], 2.004(2) Å; [Fe(TPP)(1-BzIm)2], 2.017(4), Safo, M. K.; Scheidt,
W. R.; Gupta, G. P. Inorg. Chem. 1990, 29, 626.
(68) Cao, C.; Dahal, S.; Shang, M.; Beatty, A. M.; Hibbs, W.; Schulz, C. E.;
Scheidt, W. R. Inorg. Chem.2003, 42, 5202.
(69) Scheidt, W. R.: Piciulo, P. L. J. Am. Chem. Soc. 1976, 98, 1913.
(11) Kitagawa, T.; Ozaki, Y. Struct. Bonding (Berlin) 1987, 64, 71.
(12) Alben, J. O. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New
York, 1978; Vol. 3, pp 323-345.
(13) Caughey, W. S.; Alben, J. O. Biochemistry 1968, 7, 175.
(14) Li, T.; Quillin, M. L.; Phillips, G. N., Jr.; Olsen, J. S. Biochemistry 1994,
33, 1433.
(15) Rougee, M.; Brault, D. Biochemistry 1975, 14, 4100.
(16) Rougee, M.; Brault, D. Biochem. Biophys. Res. Commun. 1974, 57, 654.
(17) Gibson, Q. H.; Roughton, F. J. Proc. R. Soc., Ser. B, Biol. Sci. 1957, 146,
206.
(18) Landaw, S. A.; Callahan, E. W., Jr.; Schmid, R. J. Clin. InVest. 1970, 49,
914.
(19) Collman, J. P.; Brauman, J. I.; Halbert, T. R.; Suslick, K. S. Proc. Natl.
Acad. Sci. U.S.A. 1976, 73, 3333.
(20) Antonini, E.; Brunori, M. Hemoglobin and Myoglobin in Their Reactions
With Ligands; North-Holland: London, 1971.
(21) Spiro, T. G.; Smulevich, G.; Su, C. Biochemistry 1990, 29, 4497.
(22) (a) Jewsbury, P.; Yamamoto, S.; Minato, T.; Saito, Minoru, S.; Kitagawa,
T. J. Am. Chem. Soc. 1994, 116, 11586. (b) Jewsbury, P.; Yamamoto, S.;
Minato, T.; Saito, Minoru, S.; Kitagawa, T. J. Phys. Chem. 1995, 99, 12677.
(23) Springer, B. A.; Sligar, S. G.; Olsen, J. S.; Phillips, G. N., Jr. Chem. ReV.
1994, 94, 699.
(24) Ivanov, D.; Sage, J. T.; Keim, M.; Powell, J. R.; Asher, S. A.; Champion,
P. M. J. Am. Chem. Soc. 1994, 116, 4139.
(25) Lim, M.; Jackson, T. A.; Anfinrud, P. A. Science. 1995, 269, 962.
(26) Kuriyan, J.; Wilz, S.; Karplus, M.; Petsko, G. A. J. Mol. Biol. 1986, 192,
133.
(27) Teng, T.-Y.; Sˇrajer, V.; Moffat, K. Nat. Struct. Biol. 1994, 1, 701.
(28) Schlichting, I.; Berendzen, J.; Phillips, G. N., Jr.; Sweet, R. M. Nature
1994, 371, 808.
(29) Cheng, X.; Schoenborn, B. P. J. Mol. Biol. 1991, 220, 381.
(30) Vojtechovsky´, J.; Chu, K.; Berendzen, J.; Sweet, R. M.; Schlichting, I.
Biophys. J. 1999, 77, 2153.
(31) Kachalova, G. S.; Popov, A. N.; Bartunik, H. D. Science 1999, 284, 473.
(32) Shimada, H.; Caughey, W. S. J. Biol. Chem. 1982, 257, 11893.
(33) Potter, W. T.; Hazzard, J. H.; Choc, M. G.; Tucker, M. P.; Caughey, W. S.
Biochemistry 1990, 29, 6283.
9
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A R T I C L E S
Silvernail et al.
ments, and substrate selectivity (CO vs O2) in myoglobin.14,37
These investigations have demonstrated that CO frequency
changes can be the result of very specific interactions with ligand
pocket residues or the general electrostatic environment of the
pocket or both.
and stirred overnight under a CO atmosphere. The solution was then
cannula-transferred into 7 mm glass tubes, carefully layered with
hexanes, and the tubes were flame sealed. X-ray quality crystals of
[Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 were isolated after 48 h at 4°C. IR
νCO in KBr: 1953 and 1948 cm-1
.
Synthesis of [Fe(TPP)(CO)(2-MeHIm)]‚C7H8. A solution of 35
mg (0.40 mmol) of 2-methylimidazole, in 6 mL of toluene, was
introduced to dry [Fe(TPP)] (0.05 mmol) via cannula transfer and stirred
for 1 h. The reaction solution was then purged with CO for 30 min
and stirred overnight under a CO atmosphere. The solution was then
cannula-transferred into 7 mm glass tubes, carefully layered with
hexanes, and the tubes were flame sealed. X-ray quality crystals of
[Fe(TPP)(CO)(2-MeHIm)]‚C7H8 were isolated after 48 h at 4°C. IR
In this paper, we present the synthesis, spectroscopic, and
structural studies for four new carbonyl(iron(II)) tetraphenylpor-
phyrinates, each with an imidazole as the sixth ligand. The
complexes are of the form [Fe(TPP)(CO)(L)], where L is
2-methylimidazole, 1,2-dimethylimidazole (two crystalline forms),
or 1-methylimidazole. The crystals of all four compounds are
of extremely high quality and allowed the determination of the
structural parameters at very high levels of accuracy. Although
the species show nearly identical CO stretching frequencies in
solution, the crystalline species show a wide range of νC-O
values. The variation in the solid-state νC-O values have been
examined in light of the differing crystalline environments.
These provide information on the nature of some of the
environmental effects that can lead to shifts in νC-O. These
compounds also provide additional, quantitative data on classical
carbonyl-metal π-bonding. There are differences in both the
Fe-C and C-O bond lengths in the four compounds. Although
the differences are relatively small, the structure determinations
show that there is a strong correlation between the two distances
that follows the indirect correlation, i.e., an increase in the Fe-C
distance leads to a decrease in the C-O distance. The observed
structural correlation appears to be consistent with other carbonyl
species whose structural data was determined with lower
accuracy and precision.
νCO in KBr: 1926 cm-1
.
Synthesis of [Fe(TPP)(CO)(1,2-Me2Im)]. A solution of 52 mg (0.53
mmol) of 1,2-dimethylimidazole in 5 mL of toluene, was introduced
to dry [Fe(TPP)] (0.13 mmol) via cannula transfer and stirred for 1 h.
The reaction solution was then purged with CO for 30 min and stirred
overnight under a CO atmosphere. The solution was then cannula-
transferred into 8 mm glass tubes, carefully layered with hexanes, and
the tubes were flame sealed. X-ray quality crystals of [Fe(TPP)(CO)-
(1,2-Me2Im)] were isolated after 25 days at 25 °C. IR νCO in KBr:
1963 cm-1
.
Synthesis of [Fe(TPP)(CO)(1-MeIm)]‚C6H6. A solution of 0.08
mL (0.94 mmol) of 1-methylimidazole, in 15 mL of benzene, was
introduced to dry [Fe(TPP)] (0.26 mmol) via cannula transfer and stirred
for 1 h. The reaction solution was then purged with CO for 30 min
and stirred overnight under a CO atmosphere. The solution was then
cannula-transferred into 8 mm glass tubes, carefully layered with
hexanes, and the tubes were flame sealed. X-ray quality crystals of
[Fe(TPP)(CO)(1-MeIm)]‚C6H6 were isolated after 25 days at 25 °C.
IR νCO in KBr: 1968 cm-1
.
X-Ray Crystallographic Studies. All crystals were placed in inert
oil, mounted on a glass pin, and transferred to the cold gas stream of
the diffractometer. Crystal data were collected and integrated using a
Bruker Apex system, with graphite monochromated Mo KR (λ )
0.71073 Å) radiation at 100 K (700 Series Oxford Cryostream) for all
complexes. All data were collected to 2θmax g 65° with the exception
Experimental Section
General Information. All reactions were carried out under strictly
anaerobic conditions using standard Schlenk techniques under an argon
atmosphere. Ethanethiol and 1-methylimidazole were used as received
(Acros). 2-Methylimidazole (Aldrich) was recrystallized from acetone
prior to use. 1,2-Dimethylimidazole (Aldrich) was recrystallized from
ethyl ether prior to use. Carbon monoxide gas was used as received
(Mittler Specialty Gases). Benzene and toluene (Fisher) were purified
by distillation in a nitrogen atmosphere over sodium/benzophenone
ketyl. All solvents were freeze/pump/thaw degassed (thrice) prior to
use. Free base [H2TPP] was prepared according to Adler et al.38 [Fe-
(TPP)(Cl)] was prepared according to the metalation procedure of Adler
et al.39 [Fe(TPP)]2O was prepared by washing a solution of [Fe(TPP)-
Cl] in methylene chloride with 2 M aqueous sodium hydroxide solution
(3×) and drying the collected organic layers over magnesium sulfate
followed by recrystallization from methylene chloride/hexanes.40 [Fe-
(TPP)] was prepared by stirring a solution of [Fe(TPP)]2O and excess
ethanethiol in benzene for 2 d.41 After reduction, solvent and excess
ethanethiol were removed under vacuum.
Infrared spectra were recorded on a Nicolet Nexus 870 FT-IR
spectrometer. Solid-state infrared samples were prepared by gently
mulling a suitable crystal between two NaCl plates with a small amount
of Nujol to allow dispersion. Solution infrared samples were prepared
by bubbling CO into a solution of 4 mM Fe(TPP) and 1-2 M imidazole
in toluene. Samples were prepared for Mo¨ssbauer spectroscopy by
weighing approximately 40 mg of selected crystals, grinding them in
a small volume of Apiezon M grease to form a mull, and placing them
into a Mo¨ssbauer cup. Measurements were performed on a constant-
acceleration spectrometer from 15 to 300 K.
of [Fe(TPP)(CO)(1,2-Me2Im)] (2θmax
) 56.5°). The program
SADABS42 was applied for absorption corrections.
The structures of [Fe(TPP)(CO)(1-MeIm)]‚C6H6 and [Fe-
(TPP)(CO)(1,2-Me2Im)] were solved by direct methods in SHELXS-
97,43 while the structures of [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and [Fe-
(TPP)(CO)(2-MeHIm)]‚C7H8 were solved using the Patterson method
in SHELXS-97.44 All structures were refined using SHELXL-97.45 All
nonsolvent atoms were found after successive full-matrix least-squares
refinement cycles on F2 and refined with anisotropic thermal parameters.
Solvent molecule atoms were refined anisotropically when possible.
Hydrogen atom positions were idealized with a riding model and fixed
thermal parameters [Uij ) 1.2Uij(eq) or 1.5Uij(eq)] for the atom to which
they are bonded with the exception of N-H hydrogen in [Fe(TPP)-
(CO)(2-MeHIm)]‚C7H8 and the hydrogens of the 2-methyl group in
[Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8. These four hydrogen atoms were
found in difference Fourier maps, and their positions and temperature
factors were refined in successive full-matrix least-squares refinements.
Complete crystallographic details are given in the Supporting
Information and are summarized in Table S1. The required 2-fold
symmetry imposed on [Fe(TPP)(CO)(1,2-Me2Im)] leads to a linear Fe-
C-O bond angle and two separate orientations (equally occupied) of
the 1,2-dimethylimidazole described with the Fe-NIm bond tilted off
the 2-fold axis. A rigid group approximation was used to fully resolve
the two distinct orientations of the ring. The rigid group orthogonal
coordinates can be found in Table S26 in the Supporting Information.
[Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 also contains a disordered 1,2-di-
methylimidazole over two positions. These were refined as two rigid
imidazole rings, which were found to have relative occupancies of 62%
Synthesis of [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8. A solution of 29
mg (0.30 mmol) of 1,2-dimethylimidazole, in 6 mL of toluene, was
introduced to dry [Fe(TPP)] (0.04 mmol) via cannula transfer and stirred
for 1 h. The reaction solution was then purged with CO for 30 min
9
14424 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

Heme Carbonyls
A R T I C L E S
Figure 2. ORTEP diagram (50% probability ellipsoids) of [Fe(TPP)(CO)-
(1,2-Me2Im)]. Significant hydrogen atoms are displayed, all other hydrogen
atoms are omitted for clarity. This diagram illustrates the linearity of the
Fe-C-O group imposed by the 2-fold symmetry. The 2-fold disordered
1,2-Me2Im ligand is shown.
Figure 1. ORTEP diagrams (50% probability ellipsoids) of (top) [Fe(TPP)-
(CO)(1,2-Me2Im)]‚C7H8 (only the major orientation of imidazole shown)
and (bottom) [Fe(TPP)(CO)(2-MeHIm)]‚C7H8. Significant hydrogen atoms
are displayed, all other hydrogen atoms are omitted for clarity. Note the
tilt of Fe-NIm off the heme normal and the near linearity of the Fe-C-O
group.
Figure 3. ORTEP diagram (50% probability ellipsoids) of [Fe(TPP)(CO)-
(1-MeIm)]‚C6H6. Significant hydrogen atoms are displayed, all other
hydrogen atoms are omitted for clarity.
for the major position and 38% for the minor position and unequal
axial Fe-N distances.46
Results
dihedral angle between the 24-atom plane of the porphyrin and
the plane of the imidazole is 90° (symmetry imposed). The
ORTEP diagram for [Fe(TPP)(CO)(1-MeIm)]‚C6H6 is shown
in Figure 3. This diagram illustrates the near linearity of the
Fe-C-O and C-Fe-NIm angles. The dihedral angle between
the 24-atom plane of the porphyrin and the plane of the
imidazole is 83°.
Formal diagrams of atom displacements from the 24-atom
mean planes of [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and [Fe(TPP)-
(CO)(2-MeHIm)]‚C7H8 are given in Figure S1. In these and
subsequent formal diagrams, the position of the imidazole
methyl group at the 2-carbon position is indicated by the small
circle, and the average Fe-N pyrrole distances are displayed. The
relative orientation of the axial imidazole ligand is shown as a
projection onto the core. Positive displacements are toward the
CO. The angle between the imidazole plane and the nearest
Np-Fe-Np for [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and [Fe(TPP)-
(CO)(2-MeHIm)]‚C7H8 is approximately 45°. The two imidazole
orientations are also shown.
The crystal and molecular structures of four six-coordinate
CO iron(II) porphyrinates have been obtained. The structures
of [Fe(TPP)(CO)(1,2-Me2Im)], [Fe(TPP)(CO)(1,2-Me2Im)]‚
C7H8, and [Fe(TPP)(CO)(2-MeHIm)]‚C7H8, all with sterically
hindered imidazoles, have not previously been reported. We also
report the structure of a benzene-solvated form of [Fe(TPP)-
(CO)(1-MeIm)] that has been previously isolated as a toluene
solvate.47,48
Crystallographic details for all four structures are summarized
in Table S1. ORTEP diagrams of [Fe(TPP)(CO)(1,2-Me2Im)]‚
C7H8 and [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 are illustrated in
Figure 1. The side-on view of these porphyrinates give a distinct
perspective of not only the porphyrin core conformation but
also the tilting of the Fe-NIm bond off the heme normal. The
dihedral angles between the 24-atom porphyrin plane and the
imidazole plane is 85° (for both orientations) and 82°, respec-
tively. The ORTEP diagram for [Fe(TPP)(CO)(1,2-Me2Im)] is
shown in Figure 2. The side-on view illustrates the linear Fe-
C-O group and symmetric disorder of the axial ligand that is
required by crystallographically imposed 2-fold symmetry. The
Formal diagrams of [Fe(TPP)(CO)(1-MeIm)]‚C6H6 and [Fe-
(TPP)(CO)(1,2-Me2Im)] showing atom displacements from the
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14425

A R T I C L E S
Scheme 1
Silvernail et al.
(CO)(2-MeHIm)]‚C7H8, and [Fe(TPP)(CO)(1-MeIm)]‚C6H6,
defined as the angle between the Fe-C vector and the heme
normal, are 0°, 4.2°, 1.1°, and 1.0°, respectively.
The presence of the hindering R-methyl group in the
2-methyl- or 1,2-dimethyl-substituted imidazoles leads to a
modest off-axis tilt of the Fe-N bond from the heme plane
normal. Values are 6.2° for [Fe(TPP)(CO)(1,2-Me2Im)], 7.5°
and 7.6° for [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8, 3.7° for [Fe-
(TPP)(CO)(2-MeHIm)]‚C7H8, and 3.4° for [Fe(TPP)(CO)-
(1-MeIm)]‚C6H6. A larger effect is the unequal Fe-N-C angles;
the Fe-N-C angles for the carbon bearing the methyl group
are ∼10.5° larger than that of the unsubstituted carbon (132.3°;
(average) compared to 121.8°). This pattern is common to all
known iron derivatives coordinated to hindered imidazoles.65,66
Interestingly, the relative orientations of the two axial tilts are
such that the axial NIm-Fe-C angles have values close to 180°.
The hindered imidazole ligand also leads to some elongation
of the Fe-NIm bond compared to analogous derivatives with
unhindered imidazoles. There appears to be some variation in
this axial bond length even with the same ligand, consequently
we can only estimate the elongation effect as g∼0.04 Å.
However, the Fe-N bond distances trans to Fe-C(CO) are
relatively long in all of the carbonyl complexes. The Fe-N(1-
MeIm) distances are found to be ∼2.04 Å for the [Fe(Por)-
(CO)(1-MeIm)] complexes (Table 1), and the analogous distance
for derivatives with hindered imidazoles longer still. Estimates
of the magnitude of the increase in Fe-N(L) can be determined
by comparison with a series of bis-ligated six-coordinate iron-
(II) porphyrinate complexes bonded to imidazole ligands.67 The
bis-imidazole complexes67 display axial Fe-N(L) bonds ranging
from 2.004(2) to 2.017(4) Å; a lengthening of g∼0.03 Å is
thus inferred. The bond-lengthening is somewhat smaller than
that observed for the thiocarbonyliron(II) porphyrinates where
a lengthening of about 0.10 Å was inferred68 or that in iron(II)
nitrosyl porphyrinates where the Fe-N(L) distances of the
ligand trans to nitrosyl is lengthened by >0.2 Å.69-71 The
carbonyl ligand thus has a significant structural trans effect, but
one that is smaller than those seen in a number of other diatomic
ligands coordinated to iron(II) porphyrinates.
24-atom mean plane are illustrated in Figure S3 and Figure S2,
respectively. The formal diagrams illustrate the angle between
the imidazole plane and the nearest Np-Fe-Np for both of these
structures is approximately 30°. The two equally occupied
imidazole orientations can also be observed.
Carbonyl stretching frequencies and notable structural features
for these and related structures have been investigated. The
ν
C-O’s reported for the four new structures vary from 1968 to
1926 cm-1. Mo¨ssbauer spectra were collected at 15-300 K in
zero field for [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and [Fe(TPP)-
(CO)(1-MeIm)]‚C6H6.
Discussion
Synthetic Aspects. The synthetic steps in preparing six-
coordinate carbonyl iron(II) porphyrins are illustrated in Scheme
1. All reactions must be carried out under strict anaerobic
conditions. While the synthesis of carbonyl iron(II) porphyrin
complexes with unhindered imidazoles is straightforward, the
synthesis of iron(II) carbonyl porphyrinates with hindered
imidazoles is surprisingly difficult due to their reduced affinity
for CO. Although the values of K1, as illustrated in Scheme 1,
for hindered and unhindered imidazoles are nearly equal, the
CO binding constants (K2) are substantially different.16 Rougee
and Brault report a 200-fold decrease in the CO binding constant
when the trans ligand is a hindered imidazole compared to
unhindered imidazole. This binding constant difference is such
that rapid crystallization of carbonyl products yields a mixture.
Thus, both the six-coordinate CO-ligated derivative and the five-
coordinate imidazole-ligated species cocrystallize when the trans
ligand is a hindered imidazole. This is not the case with
unhindered imidazole derivatives. Thus, the isolation of the pure
hindered imidazole derivatives, [Fe(TPP)(CO)(L)], is possible
only by preparing and harvesting single crystals.
Molecular Structures. We report the molecular structures
of four six-coordinate iron(II) carbonyl complexes determined
at high resolution and high accuracy that allow us to make strong
structural and spectroscopic correlations. These correlations will
be given in subsequent sections.
Bond lengths and angles with uncertainties for the LIm-Fe-
C-O coordination group for these four new structures and, for
comparison, all known heme carbonyl structures are given in
Table 1. The Fe-C-O angle in these new derivatives deviate
only slightly from precise linearity. Values for [Fe(TPP)(CO)-
(1,2-Me2Im)]‚C7H8, [Fe(TPP)(CO)(2-MeHIm)]‚C7H8, and [Fe-
(TPP)(CO)(1-MeIm)]‚C6H6 are 175.9°, 176.0°, and 177.0°,
respectively, while that for [Fe(TPP)(CO)(1,2-Me2Im)] is 180°,
as required by the imposed 2-fold axis. The Fe-C-O tilt for
[Fe(TPP)(1,2-Me2Im)], [Fe(TPP)(CO)(1,2-Me2Im)], [Fe(TPP)-
The four molecules whose structures are reported here all
have nonplanar porphyrin core conformations. Detailed formal
diagrams of the displacements of each atom from the mean plane
of the 24-atom core are displayed in Figures S1-S3 of the
Supporting Information.
The toluene solvates of [Fe(TPP)(CO)(1,2-Me2Im)] and [Fe-
(TPP)(CO)(2-MeHIm)] have nearly identical (ruffled) core
conformations, as can be seen in Figure S1. Indeed, crystals of
the two molecules are isomorphous and the molecular structures
are nearly indistinguishable with the exception of small differ-
ences in the axial ligands. This is shown in the overlay diagram
of Figure 4. The overlay diagram was constructed as described
in the figure caption. The largest difference is in the position
of the CO oxygen atoms that are 0.384 Å apart.
The ruffled cores in [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and
[Fe(TPP)(CO)(2-MeHIm)]‚C7H8 are the result of the imidazole
orientation. The projection of the imidazole plane on the
porphyrin core is approximately 45° from the nearest Fe-Np
(70) Scheidt, W. R.; Brinegar, A. C.; Ferro, E. B.; Kirner, J. F. J. Am. Chem.
Soc. 1977, 99, 7315.
(71) Wyllie, G. R. A.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2003, 42,
5722.
9
14426 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

Heme Carbonyls
A R T I C L E S
Table 1. Carbonyl Stretching Frequencies and Notable Structural Features for [Fe(TPP)(CO)(1,2-Me2Im)]·C7H8,
[Fe(TPP)(CO)(2-MeHIm)]‚C7H8, [Fe(TPP)(CO)(1,2-Me2Im)], and [Fe(TPP)(CO)(1-MeIm)]‚C6H6 and Related Compounds
a
b
c
c
Fe−
Ca
Fe−C−
Ob
Fe−Lax
C−Fe
−Lax
C−
Oa
νC
νC
-O
ref
-O
[Fe(TPP)(CO)(2-MeHIm)]‚C7H8
[Fe(TPP)(CO)(2-MeHIm)]
[Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8
[Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8
[Fe(TPP)(CO)(1,2-Me2Im)]
[Fe(TPP)(CO)(1,2-Me2Im)]
[Fe(TPP)(CO)(1-MeIm)]‚C6H6
[Fe(TPP)(CO)(1-MeIm)]‚C7H8
[Fe(TPP)(CO)(1-MeIm)]‚2.5C7H8
[Fe(TPP)(CO)(1-MeIm)]
1.7410(14)
1.7537(15)
1.764(2)
175.96(13)
175.95(14)
180g
2.1018(12)
175.42(5)
1.1488(17)
1.1408(19)
1.138(3)
1926d
tw
1972e
tw
tw
tw
tw
54, tw
tw
47
48
tw
52
52
53
53
55
56, 19
57
58
54
54
2.0779(11)
2.1402(18)
2.133(2)
176.77(6)
168.73(8)
173.85(9)
1948, 1953d
1948, 1953d
1963d
f
1972e
1969h
1969i
1.7600(17)
1.793(3)
1.7636(13)
177.03(15)
179.3(3)
178.76(13)
2.0503(14)
2.071(2)
2.0400(11)
176.78(6)
178.3(3)
178.38(5)
1.139(2)
1.061(3)
1.1437(17)
1968d
1967d
[Fe(C2Cap)(CO)(1-MeIm)]
[Fe(C2Cap)(CO)(1-MeIm)]
1.742(7)
1.748(7)
1.748(7)
1.713(8)
1.728(6)
172.9(6)
175.9(6)
173.9(7)
180g
2.043(6)
2.041(5)
2.027(5)
2.102(6)
2.062(5)
174.7(3)
177.8(3)
174.5(3)
173.0(2)
180g
1.161(8)
1.158(8)
1.171(8)
1.161(10)
1.149(6)
2000d
2000d
1978d
1974d
[Fe(OC3OPor)(CO)(1-MeIm)]‚1.5C7H8
[Fe(OC3OPor)(CO)(1,2-Me2Im)]‚2CHCl3
[Fe(Tpiv2C12P)(CO)(1-MeIm)]
[Fe(TpivPP)(CO)(1-MeIm)]
[Fe(OEP)(CO)(1-MeIm)]
[Fe(C3Cap)(CO)(1,2-Me2Im)]
[Fe(C3Cap)(CO)(1,2-Me2Im)]
[Fe(C2Cap)(CO)(1,2-Me2Im)]
[Fe(â-PocPivP)(CO)(1,2-Me2Im)]
[Fe(TPP)(CO)(Py)]
180g
1958i
1965i
1965h
1984e
1984j
1999j
1.744(5)
1.800(13)
175.1(4)
178.0(13)
2.077(3)
2.046(10)
176.8(2)
178.9(5)
1.158(5)
1.107(13)
1.768(7)
1.77(2)
172.5(6)
179(2)
2.079(5)
2.10(1)
176.3(3)
177.5(8)
1.148(7)
1.12(2)
59
60
61
1980k
1967
[Fe(OEP)(CO)(Py)]
[Fe(Deut)(CO)(THF)]
[Fe(TPP)(CO)(THF)]
[Fe(TpivPP)(CO)(THF)]
[Fe(TPP)(CO)(SEt)]
[Fe(TpivPP)(CO)(THT)]
HbCO
1.706(5)
1.78(1)
178.3(14)
2.127(4)
2.352(2)
177.4(9)
1.144(5)
1.17(1)
1955l
1955k
1961k
1920m
1970i
1951n
1927n
1945n
1969n
62
62
63
64
63
13
34
34
MbCO
34
a Å. b deg. c cm-1 d Nujol mull. e Toluene solution saturated with base. f Second conformation of the 1,2-Me2Im is approximately 38% occupied. g Symmetry
.
imposed linearity. h CDCl2 solution. i Benzene solution. j Toluene solution. k Pyridine solution. l Tetrahydrofuran solution. m Chlorobenzene solution. n Aqueous
solution.
lengths of 1.985(8) and 1.988(6) Å, and the shortening observed
for these two derivatives is in general agreement with expecta-
tions. Ruffled-core conformations have been observed for a
number of six-coordinate iron(II) porphyrinates with a single
diatomic ligand. Ruffled cores (Fe-Np distances given in
parentheses) have been observed in [Fe(Deut)(CO)(THF)]
(1.981(3) Å),62 [Fe(TpivPP)(O2)(1-MeIm)] (1.979(13) Å),74 [Fe-
(TpivPP)(O2)(2-MeHIm)] (1.995(4) Å),75 [Fe(OEP)(CS)(CH3-
OH)] (1.993(1) Å),68 and [Fe(TpivPP)(CO)(NO2)]- (1.997(6)
Å).76
The other two derivatives, [Fe(TPP)(CO)(1-MeIm)]‚C6H6 and
[Fe(TPP)(CO)(1,2-Me2Im)], have core conformations that are
primarily saddled and are similar to each other. These two
Figure 4. An overlay diagram showing the structures of [Fe(TPP)(CO)-
complexes contain imidazoles that are oriented approximately
(1,2-Me2Im)]‚C7H8 (-) and [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 (- - -).
30° from the nearest Fe-Np bond (see Figures S2 and S3.) The
average Fe-Np bond distance for [Fe(TPP)(CO)(1-MeIm)]‚C6H6
and [Fe(TPP)(CO)(1,2-Me2Im)] are 2.005(6) and 2.004(1) Å,
respectively, somewhat longer than those observed for the
ruffled derivatives.
Saddling is a commonly observed core conformation of six-
coordinate iron(II) porphyrinates with a single diatomic ligand.
Saddled cores (Fe-Np distances given in parentheses) have been
observed in [Fe(OEP)(CO)(1-MeIm)] (2.000(3) Å),57 [Fe(TPP)-
(CO)(Py)] (2.02(3) Å),60 [Fe(TPP)(NO)(1-MeIm)] (2.009(13)
These structures were fit by superimposing the four pyrrole nitrogens and
orientated so the mean plane of these nitrogens are perpendicular to the
page. The molecules can be seen to have nearly identical core and phenyl
ring conformations. The CO oxygens are separated by 0.384 Å.
bond (Figure S1). This places the 2-methylimidazole group near
a methine carbon atom, and the steric congestion leads to core
ruffling. The effect of the two different hindered imidazoles on
core conformations is nearly identical. Some time ago, Hoard72,73
pointed out that such core ruffling leads to a shortening of the
equatorial Fe-Np bonds. [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and
[Fe(TPP)(CO)(2-MeHIm)]‚C7H8 have average Fe-Np bond
(74) Jameson, G. B.; Rodley, G. A.; Robinson, W. T.; Gagne, R. R.; Reed, C.
A.; Collman, J. A. Inorg. Chem. 1978, 17, 850.
(75) Jameson, G. B.; Molinaro, F. S.; Ibers, J. A.; Collman, J. P.; Brauman, J.
I.; Rose, E.; Suslick, K. S. J. Am. Chem. Soc. 1980, 102, 3224.
(76) Nasri, H.; Ellison, M. K.; Shang, M.; Schulz, C. E.; Scheidt, W. R. Inorg.
Chem. 2004, 43, 2932.
(72) Hoard, J. L. Ann. N. Y. Acad. Sci. 1973, 206, 18.
(73) Collins, D. M.; Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94,
6689.
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14427

A R T I C L E S
Silvernail et al.
Table 2. Mo¨ssbauer Data for [Fe(TPP)(CO)(L)] and Related
Complexes
the temperature range of 15-293 K. The quadrupole splitting
value for [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 with its sterically
hindered imidazole ligand has a higher value than that of any
other derivative. This suggests that the magnitude of the
quadrupole splitting could be sensitive to variation in the
geometry around the iron atom. A systematic investigation of
this issue is currently being planned.
b
complexa
temp, K
∆Eq
δb
ref
[Fe(Por)(CO)(L)]
[Fe(TPP)(CO)(1,2-Me2Im)]C7H8
[Fe(TPP)(CO)(1-MeIm)]C6H6
[Fe(OEP)(CO)(1-MeIm)]C6H6
293
200
100
15
293
200
100
15
293
200
100
15
77
77
295
4.2
4.2
0.71
0.65
0.66
0.64
0.35
0.32
0.32
0.30
0.40
0.37
0.37
0.34
0.35
0.57
0.53
+0.35
+0.36
0.17
0.23
0.29
0.25
0.16
0.24
0.25
0.26
0.18
0.25
0.23
0.24
0.20
0.28
0.18
0.27
0.26
tw
tw
tw
tw
tw
tw
tw
tw
tw
tw
tw
tw
80
80
81
82
83
Environmental Effects on νC-O Values. In heme proteins,
CO stretching frequencies have been found to vary from 1904
to 1984 cm-1 86
These differences in νC-O are believed to be
.
the result of both the interactions with specific residues in the
ligand binding pocket and generalized electrostatic effects. In
early IR studies, the CO stretching frequencies of MbCO were
found to vary between 1927 and 1969 cm-1. These were perhaps
the first studies that demonstrated that the protein environment
had a substantial effect on CO stretching frequencies.87 The
multiple CO stretching frequencies are believed to reflect
differences in ligand binding pocket conformational states.34-36
The relative intensities of these modes are affected by pH,
temperature, and pressure.88 Three CO stretching frequencies
are found in MbCO. These states and CO frequencies are 1969
(A0), 1945 (A1), and 1927 cm-1 (A3).35 Subsequently, CO has
been utilized to examine the electrostatic properties near the
diatomic binding pocket in Mb and Mb mutants. Phillips et al.
have demonstrated that the stretching frequency of CO can act
as a gauge of the electrostatic fields near the ligand binding
site.14 Thus, the vibrational properties of CO gives a useful
experimental measure of environmental effects in heme proteins.
Understanding such effects could assist in determining how
discrimination in binding between NO, CO, and O2 in heme
proteins arises.14,23 In an interesting theoretical study, Franzen
suggests that specific orientations of the ligand binding pocket
groups play an important role in the CO/O2 discrimination.89
[Fe(TPP)(CO)(1-MeIm)]
[Fe(TPP)(CO)(Py)]
[Fe(TPP)(CO)(Pip)]
MbCO
HbCO
[Fe(Por)(CS)(L)] and [Fe(Por)(CS)]
[Fe(OEP)(CS)(1-MeIm)]
293
4.2
293
4.2
0.47
0.42
1.95
1.93
0.03
0.14
-0.03
0.08
68
68
68
68
[Fe(OEP)(CS)]
[Fe(Por)(NO)(L)] and [Fe(Por)(NO)]
[Fe(TPP)(NO)(1-MeIm)]
293
4.2
4.2
100
0.80
0.73
1.24
1.26
0.24
0.35
0.35
0.35
71
71
84
85
[Fe(TPP)(NO)]
[Fe(OEP)(NO)]
a Abbreviations given in the references. b Value in mm/s.
Å),71 [Fe(TPP)(NO)(NMe2Py)] (2.006(11) Å),71 [Fe(TPP)(NO)-
(4-MePip)] (2.009(8) Å),71 [Fe(OEP)(CS)(1-MeIm)] (2.001(4)
Å),68 and [Fe(OEP)(CS)(Py)] (1.999(1) Å).68
The relationship between core conformation and the orienta-
tion of planar axial ligands is clearly shown in these newly
reported species. [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 and [Fe-
(TPP)(CO)(1,2-Me2Im)] (with a 2-fold symmetry axis along the
Fe-C-O) have identical ligands, yet their imidazole orienta-
tions are substantially different and subsequently so are their
core conformations. Although the energies associated with the
crystallization of the two crystalline forms is not well under-
stood, it is interesting to note that the crystal system formed
over the shorter time is the slightly more ordered. These two
sets of molecular structures further illustrate that axial ligand
orientations affect core conformations. Finally, we note that there
continues to be speculation that core perturbations may play a
crucial role in the activity of heme centers in biology.77-79
Mo1ssbauer. Variable-temperature Mo¨ssbauer data for three
new carbonyl derivatives, two of which have been structurally
characterized, have been obtained. These new data are reported
in Table 2 along with previously reported data from the
literature. The data are consistent with other low-spin carbonyls.
The new derivatives display very little temperature dependence
on either the quadrupole splitting or the isomer shift value in
The four iron(II) carbonyl derivatives reported here show a
large range of CO stretching frequencies in the solid state. The
CO absorbances, measured on carefully mulled single crystals,
showed νC-O’s from 1968 to 1926 cm-1. These values show a
larger-than-expected range of frequencies, since the generally
observed value for νC-O for “classical” [Fe(Por)(CO)(L)]
derivatives is ∼1970 cm-1 61
. Indeed, the CO stretching frequen-
cies of these four complexes in toluene solution are all observed
at 1969-1972 cm-1. We expect that the complexes have a
(80) Havlin, R. H.; Godbout, N.; Salzmann, R.; Wojdelski, M.; Arnold, W.;
Schulz, C. E. Oldfield, E. J. Am. Chem. Soc. 1998, 120, 3144.
(81) James, B. R.; Sams, J. R.; Tsin, T. B.; Reimer, K. J. J. Chem. Soc., Chem.
Commun. 1978, 746.
(82) Maeda, Y.; Harami, T.; Morita, Y.; Trautwein, A.; Gonser, U. J. Chem.
Phys., 1981, 75, 36.
(83) Lang, G.: Marshall, W. Proc. Phys. Soc., 1981, 75, 36.
(84) Nasri, H.; Ellison, M. K.; Chen, S.; Hunyh, B. H.; Scheidt, W. R. J. Am.
Chem. Soc. 1997, 119, 6274.
(85) Bohle, D. S.; Debrunner, P. G.; Fitzgerald. J.; Hansert, B.; Hung, C.-H.;
Thompson, A. J. J. Chem. Soc., Chem. Commun. 1997, 91.
(86) Vogel, K. M.; Kozlowski, P. M.; Zgierski, M. Z.; Spiro, T. G. Inorg. Chim.
Acta 2000, 297, 11.
(87) McCoy, S.; Caughey, W. S. In Probes of Structure and Function of
Macromolecules and Membranes, Probes of Enzymes and Hemoproteins;
Chance, B., Yonetani, T., Mildvan, A. S., Eds.; Academic: New York,
1971; Vol. 2, p 289.
(88) Fuchsman, W. H.; Appleby, C. A. Biochemistry 1979, 18, 1309. Ansari,
A.; Berendzen, J.; Braunstein, D.; Cowen, B. R.; Frauenfelder, H.; Hong,
M. K.; Iben, I. E. T.; Johnson, B.; Ormos, P.; Sauke, T. B.; Scholl, R.;
Schulte, A.; Steinbach, P. J.; Vittitow, J.; Young, R. D. Biophys. Chem.
1987, 26, 337. Frauenfelder, H.; Alberding, N. A.; Ansari, A.; Braunstein,
D.; Cowen, B. R.; Hong, M. K.; Iben, I. E. T.; Johnson, J. B.; Luck, S.;
Marden, M. C.; Mourant, J. R. J. Phys. Chem. 1990, 94, 1024.
(89) Franzen, S. J. Am. Chem. Soc. 2002, 124, 13271.
(77) (a) Ma, J. G.; Zhang, J.; Franco, R.; Jia, S. L.; Moura, I.; Moura, J. J.;
Kroneck, P. M.; Shelnutt, J. A. Biochemistry 1998, 37, 12431. (b) Senge,
M. O. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000; Vol. 1, p 240. (c) Kadish, K.
M.; Van Camelbecke, E.: Royal, G. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000;
Vol. 8, p 3.
(78) Roberts, S. A.: Weichsel, A.; Qiu, Y.; Shelnutt, J. A.; Walker, F. A.;
Montfort, W. R. Biochemistry 2001, 40, 11327.
(79) Pellicena, P.; Karow, D. S.; Boon, E. M.; Marletta, M. A.; Kuriyan, J.
Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 12854.
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Heme Carbonyls
A R T I C L E S
Table 3. Listing of the Closest Contacts to CO (All H‚‚‚O e 3.5 Å) and Related Geometrical Features
[Fe(TPP)(CO)-
[Fe(TPP)(CO)-
(1,2-Me2Im)] C7H8
(minor)a
[Fe(TPP)(CO)-
(2-MeHIm)] C7H8
(1,2-Me2Im)]
‚
C7H8
‚
[Fe(TPP)(CO)-
(1,2-Me2Im)]
[Fe(TPP)(CO)-
‚
(major)a
(1-MeIm)]‚C6H6
ν
C-O, cm-1
1926
1953
1948
1963
1972
d
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
2.38 (N-Hc)
2.53 (CH3 )
2.96, 117.5
106.7
2.71 (CH3)
2.96, 79.8
91.9
3.09 (CH3)
2.96, 101.0
73.2
2.60 (Ph-H)
3.29, 155.0
2.86 (ImC-He)
2.62 (âC-Hf)
2.65 (ImC-H)
3.08, 143.4
108.2
2.71 (Ph-H)
3.14, 129.1
108.3
2.84 (Ph-H)
3.33, 118.0
112.8
2.95 (Ph-H)
3.26, 105.1
100.5
2.98 (CH3)
3.30, 112.2
3.16, 114.7
3.66, 96.2
139.3
3.11 (CH3)
3.68, 105.4
136.8
2.60 (Ph-H)
3.29, 155.0
129.7
3.24 (Ph-H)
3.61, 111.7
105.2
-
-
-
-
-
-
3.26, 126.6
125.2
2.62 (âC-H)
3.26, 126.6
125.2
2.79 (Ph-H)
3.17, 104.0
105.3
2.79 (Ph-H)
3.17, 104.0
105.3
3.20 (Ph-H)
3.40, 103.9
166.7
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
2.72 (Ph-Hg)
3.38, 124.7
127.3
3.11 (Ph-H)
3.56, 101.9
112.6
3.09 (CH3)
3.91, 99.7
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
142.2
129.7
3.24 (Ph-H)
3.61, 111.7
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
-
-
-
-
-
-
105.2
93.7
3.20 (Ph-H)
3.40, 103.9
100.2
3.08 (CH3)
3.30, 112.2
O‚‚‚H, (type)b
O‚‚‚X, Fe‚‚‚O‚‚‚X
X-H‚‚‚O
-
-
-
93.7
94.5
a Major orientation and minor orientation of the adjacent imidazole. b Each entry consists of three lines. Line 1 has the O‚‚‚H distance in Å, followed by
H-atom type. Line 2 has the O‚‚‚X distance in Å, where X is the atom to which the H is bonded, followed by the Fe‚‚‚O‚‚‚X angle in degrees, followed by
line 3 where the X-H‚‚‚O angle in degrees is given. c Imidazole N-H. d Imidazole 2-methyl. e â-pyrrole H. f Imidazole 3-carbon H. g Phenyl H.
common environment in the toluene solution. The solution IR
measurement for [Fe(TPP)(CO)(1-MeIm)] is straightforward,
but the solution equilibria with the hindered imidazoles/CO
requires measurements with a CO-saturated solution and high
concentrations of the imidazole. When both of these conditions
are satisfied, a single C-O stretching frequency is observed
for a toluene solution of [Fe(TPP)(CO)(1,2-Me2Im)]; similar
results for [Fe(TPP)(CO)(2-MeHIm)] are observed with the band
for the six-coordinate CO complexes clearly at 1972 cm-1. The
small range of solution CO stretching frequencies clearly
indicates that the effect of the trans imidazole ligand is minimal.
Thus, the available data strongly suggest that the decreased solid-
state CO frequencies in these four CO derivatives are the result
of solid-state environmental effects. Accordingly, we have
examined the crystal structures and the CO environment of the
four complexes in detail.
charge. However, the distance is clearly consistent with a real
hydrogen-bonding interaction. This interaction of the neighbor-
ing imidazole N-H hydrogen with the carbonyl oxygen leads
to a 46 cm-1 shift from the solution IR measurement to 1926
cm-1 in the solid state. This low frequency is reminiscent of
the 1927 cm-1 frequency observed for the A3 conformer of
sperm whale Mb,35 that has been attributed to hydrogen bonding
from the distal histidine (H64).30,32,34
The [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 crystal lattice places
methyl hydrogen atoms from the closest neighboring coordinated
imidazole close to the carbonyl oxygen atom. The crystalline
disorder described earlier leads to two distinct environments
for the carbonyl oxygen atom from the two orientations of the
coordinated imidazole, as shown in Figure 5. For the major
ligand orientation, the O‚‚‚H and O‚‚‚C distances are 2.50 and
2.98 Å, respectively. The major orientation also has a second
hydrogen from the methyl group with an O‚‚‚H distance of 2.69
Å. The Fe‚‚‚O‚‚‚H angle of [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8
is 118.5° and the Fe‚‚‚O‚‚‚C angle is 101.0°. The minor
orientation O‚‚‚H and O‚‚‚C distances from the imidazole methyl
group are 3.11 and 3.68 Å, respectively. Indeed the closest
contacts in the minor orientation are those in common with those
of the major orientation (see Table 3). If the close methyl group
environment is influencing νC-O, two CO bands would be
expected in the solid state. Indeed, there are two bands; the
split in the νC-O band shown in Figure 6. Consistent with the
unequal occupation of the two imidazole sites, the intensities
of the two bands are not equal. The major orientation, at 62%
occupancy, likely accounts for the more intense absorbance at
1953 cm-1, while the minor orientation, at 38% occupancy, has
νC-O at 1948 cm-1. Thus, the two frequencies represent the
difference in moving the methyl group about 0.7 Å closer to
the carbonyl oxygen, as seen in Figure 5. That there are two
We first discuss the crystal structures of [Fe(TPP)(CO)(2-
MeHIm)]‚C7H8 and [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 where the
largest solid-state influence on νC-O is displayed. In these two
isomorphous crystal structures, the molecules pack in the
herringbone-like pattern shown in Figure S4. There are a number
of O‚‚‚H contacts less than 3.5 Å in all four derivatives
(tabulated in Table 3); the closest contacts in these two
derivatives come from contact with atoms of the coordinated
imidazole of an adjacent molecule. The herringbone packing
in the crystal structure of [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 places
the N-H hydrogen from the adjacent imidazole at the proper
distance and orientation for a hydrogen-bonding interaction. The
O‚‚‚H and O‚‚‚N distances are 2.38 and 3.16 Å, respectively.
The Fe‚‚‚O‚‚‚H angle of [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 is
112.0°, and the Fe‚‚‚O‚‚‚N angle is 114.7°. The canonical
hydrogen bond contact distances for O‚‚‚H and O‚‚‚N are 2.0
and 2.9 Å, respectively.90 Although the observed O‚‚‚H distance
of 2.38 Å is not the shortest possible such distance, shorter O‚
‚‚H distances typically involve an oxygen with a negative
ν
C-O values clearly shows that a methyl group close to the CO
oxygen has an effect; that the effect of a closer methyl group
approach apparently leads to a higher value of the stretching
frequency is perhaps unexpected. However, single-site mutations
(90) Hamilton, W. C.; Ibers, J. A. In Hydrogen Bonding in Solids; Benjamin,
W. A.: New York, 1968; p 16.
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A R T I C L E S
Silvernail et al.
The unsolvated form of [Fe(TPP)(CO)(1,2-Me2Im)] is packed
into a lattice with all porphyrin planes parallel. The environment
of the CO oxygen has a pair of (2-fold-equivalent) O‚‚‚H and
O‚‚‚C separations of 2.62 and 3.26 Å, respectively; the Fe‚‚‚
O‚‚‚H angles are 138.8°. The hydrogen atoms are from the
â-carbons of adjacent porphyrins. Aspects of this environment
are illustrated in Figure S7. This environment leads to νC-O
)
1963 cm-1, shifted from the 1972 cm-1 observed in toluene
solution. Even though the oxygen-hydrogen atom contacts are
closer in this derivative than in the solvated form, the difference
in the orientation and symmetry of the close contacts leads to
a smaller shift in νC-O from the solution value.
The solid-state C-O stretching frequency in [Fe(TPP)(CO)-
(1-MeIm)]‚C6H6 is unshifted from its solution value. Although
the carbonyl oxygen contact distances in this solid-state species
are not significantly different from those of other derivatives,
the detailed geometry has some significant differences. The
closest contact has the C-O bond nearly parallel to the H-C
bond of the imidazole of an adjacent molecule; the contacts
are relatively short at O‚‚‚H and O‚‚‚C distances of 2.651 and
3.082 Å, respectively. The unshifted value (1968 cm-1) of νC-O
clearly shows that short contact distances alone will not shift
the vibration. The differences in the orientation of the adjacent
imidazole in the three derivatives with close imidazole ap-
proaches are compared in Figure 7. The observed structural data
and associated vibrational stretching frequencies for bound CO
show that H-interactions with the CO oxygen atom that occur
from the side will be much more significant than those that
occur from the end. This generalization had been suggested by
Franzen89 on the basis of detailed ligand binding pocket
modeling calculations.
Figure 5. Diagram illustrating the approach of methyl group hydrogens
from the two distinct ligand orientations to the C-O of an adjacent molecule
in [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8. The major 1,2-dimethylimidazole
orientation has H‚‚‚OC distances of 2.50 and 2.69 Å, and the C‚‚‚OC
distance is 2.96 Å. In the minor orientation, the C‚‚‚OC distance has
increased to 3.68 Å.
These structural results clearly show that the effects of close
contacts on νC-O in the carbonyl complexes depends on the
distance, hydrogen-atom type, and orientation of the hydrogen-
atom contacts to the CO oxygen atom. All variables are
significant, and dissecting the individual contributions is clearly
a challenge. Phillips et al.37 have approached this from a study
of the distal pocket electrostatic potential of wild type and Mb
mutants. These studies have demonstrated that much of the νC-O
shifts to higher frequencies are due to the loss of hydrogen-
bonding interactions with histidine 64 (H64) in the distal binding
pocket.14 The calculations of Franzen,89 as noted above,
emphasize the importance of the orientation between the
H-group and the oxygen atom in addition to the generalized
electrostatic potential. Indeed, there are other possible contribu-
tions to the value of νC-O including the possibility of core
conformation effects. It is important to note that the detailed
structural information we have provided allows for the pos-
sibility of further detailed calculations.
Figure 6. IR spectrum of a single crystal of [Fe(TPP)(CO)(1,2-Me2Im)]‚
C7H8 mulled in Nujol. Two C-O stretches are observed as a result of distinct
methyl group interactions with two orientations of a 1,2-dimethylimidazole
from an adjacent molecule in the crystal lattice.
in MbCO in which the distal histidine is changed to leucine,
Schlicting and co-workers have carried out a high-resolution
(1.15 Å) structure determination of MbCO.30 In this study, three
different conformers that involve differing orientations of the
distal histidine 64 were found and were associated with νC-O
of conformers A1, A3, and A0. Conformer A3 (1927 cm-1),
which is a minor occupancy conformer, displays a hydrogen
bond from the histidine N-H. Other investigators (Sage91 and
Ray et al.92) have suggested that A1 (1945 cm-1) is the most
isoleucine, or valine are all observed to have νC-O increased
from the A1 value (native) of 1945 cm-1 14
.
Interestingly, there is a small difference in the tilt of the
carbonyl ligand in the two isomorphous species [Fe(TPP)(CO)-
(L)], L ) 2-methyl- and 1,2-dimethylimidazole. This is shown
by the difference (0.38 Å) in the oxygen atom position when
the two cores are overlaid as shown in Figure 4. The differences
are such that the N-H‚‚‚O interaction appears to be attractive
(CO tilts toward imidazole) while that of the CH3‚‚‚O interaction
appears to be less attractive to modestly repulsive (CO tilts away
from imidazole).
(91) Sage, J. T. JBIC 1994, 116, 4139.
(92) Ray, G. B.; Li, X.-Y.; Ibers, J. A,; Sessler, J. L.; Spiro, T. G. J. Am. Chem.
Soc. 1994, 116, 162.
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Heme Carbonyls
A R T I C L E S
Figure 7. Drawing comparing the orientation of the closely approaching imidazole ligand in [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 (left) with those of [Fe-
(TPP)(CO)(1-MeIm)]‚C7H6 (center) and [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 (right). Note the strong similarity in imidazole plane orientation between the left
and right structures.
Figure 9. Plots showing the relationship between the Fe-C and C-O
distances (Å) and νC-O (cm-1). The two panels show data for (1) [Fe-
Figure 8. Plot of Fe-C vs C-O distances (Å). Data illustrated in the
(TPP)(CO)(2-MeHIm)]‚C7H8, (2) [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8, (3) [Fe-
main panel are (1) [Fe(TPP)(CO)(1,2-Me2Im)], (2) [Fe(TPP)(CO)(1-MeIm)]‚
(TPP)(CO)(1,2-Me2Im)]‚C7H8 (second νC-O), (4) [Fe(TPP)(CO)(1,2-Me2-
C6H6, (3) [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8, and (4) [Fe(TPP)(CO)(2-
Im)], and (5) [Fe(TPP)(CO)(1-MeIm)]‚C6H6. Error bars are displayed.
MeHIm)]‚C7H8. Error bars are displayed. The linear fit has a correlation
Correlation coefficients for both panels are R ) 0.96.93 A plot including
coefficient of R ) 0.98.93 The inset shows all iron(II) carbonyl imidazole
structures, with the main panel correlation line overlaid.
all iron(II) carbonyl imidazoles is included in the Supporting Information.
Values for all structurally characterized iron(II) carbonyl imi-
likely hydrogen-bonded conformer. Although the exact interac-
dazole derivatives are shown in the inset of Figure 8. The linear
tions of CO in MbCO cannot be fully elucidated, the results
fit correlation line from the main panel is overlaid on these data;
we have observed for [Fe(TPP)(CO)(2-MeHIm)]‚C7H8 further
suggests that N-H interactions can cause large shifts in νC-O
the general fit to the correlation line is apparent. The larger
range of values in the distribution of the data reflects the
generally lower precision of the other measurements, as well
as the possible presence of small systematic errors.
toward lower frequency akin to shifts observed from free heme
(∼1970 cm-1) to A3 in MbCO.
Fe-C/C-O Bond Length Correlations. Spectroscopic
Not surprisingly, there are correlations between the measured,
solid-state C-O stretching frequencies and the C-O and Fe-C
bond distances in these four new derivatives. Figure 9 shows
plots of νC-O vs the C-O bond distance (left panel) and νC-O
vs the Fe-C bond distance (right panel). The increasing value
of νC-O as the C-O distance decreases and the Fe-C distance
increases can be clearly seen. The correlations for all equivalent
literature data are given in Supporting Information Figure S5.
The correlation fit lines from Figure 9 are imposed; although
the general correlation may be recognized, the scatter clearly
reflects the lower precision of most earlier structures. It should
also be recognized that the correlations of Figure 9 are all based
on tetraphenylporphyrin derivatives, while those of Figure S5
are principally those for tetraarylporphyrin derivatives that
include many substituted-phenyls. There is very likely some
porphyrin dependence (cis effects) on these correlations that
will lead to differing trend lines. This is likely to contribute to
some of the scatter in Figure S5.
studies of a large number of protein and porphyrin systems with
CO bound to iron(II) have shown that there is an inverse
5,10,61
correlation of νC-O and νFe-CO
.
This phenomenon is
attributed to electron donation from the occupied iron dπ orbitals
to the empty π* orbitals on CO thus causing a reduction in the
C-O bond order and a concomitant increase in the Fe-C bond
order.61 This classical π-back-donation effect should, in prin-
ciple, lead to an inverse change in the Fe-C vs C-O bond
distance. However, such bond-distance changes are typically
too small to be reliably observed in single-crystal structure
determinations. For the present structures, which have been
determined at very high precision and accuracy, we find that
the structural equivalent to the spectroscopists π-back-bonding
inverse correlation for iron carbonyls can be observed.
A plot of the observed Fe-C vs C-O bond distances for the
four structures is given in Figure 8; the data are shown in tabular
form in Table 1. The inverse linear correlation is clear; the
correlation coefficient of the fit, given by Pearson’s R,93 is 0.98.
Axial Ligand Distortions: Structural Effects? One early
motivation for this study was the theoretical suggestion put
(93) A linear correlation coefficient, Pearson’s R,94 was used to judge the fit to
the data.
(94) Walpole, R. E.; Meyers, R. H.; Meyers, S. L.; Ye, K. Probability and
Statistics for Engineers and Scientists, 7th ed.; Prentice Hall: Upper Saddle
River, NJ, 2002; pp 391-394.
R ) ∑i(xi - xj)(yi - yj)/ ∑ (x - xj)2 ∑ (y - yj)2
x
x
i
i
i
i
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J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14431

A R T I C L E S
Silvernail et al.
forward by Jewsbury et al.22 that the possible distortions
(bending/tilting) of the Fe-C-O group of myoglobin are the
result of effects on the proximal (trans) side of the porphyrin
plane. In this model, the off-perpendicular distortion of the Fe-
C-O unit is caused by a nonequilibrium orientation of the
proximal residue (histidine in the native protein) and not effects
from the distal side. Our experimental test of this proposal was
the preparation of a carbonyl complex with a sterically hindered
trans imidazole ligand. A hindered imidazole would necessarily
have a substantial off-axis tilt of the Fe-N bond that, if the
Jewsbury hypothesis is valid, should then lead to some distortion
of the iron carbonyl group.
The first such compound characterized was unsolvated [Fe-
(TPP)(CO)(1,2-Me2Im)]. This complex crystallized in a mono-
clinic space group with required 2-fold symmetry along the axial
bond direction. The Fe-N(imidazole) bond is indeed off-axis
with a tilt of 6.2°; the required 2-fold symmetry leads to the
disorder shown in Figure 2. However, the Fe-C-O unit is
linear, as required by the imposed symmetry, with no evidence
of disorder apparent in the thermal parameters. The imposed
symmetry left some uncertainty in the result, and additional data
were sought. Unfortunately, we were unable for some time to
prepare appropriate crystalline materials to further study the
question. Then, two related species were obtained nearly
simultaneously.
Figure 10. Plot of the C-Fe-NIm angle vs the Fe-NIm distance (Å). All
iron(II) carbonyl imidazole structures that do not contain a porphyrin strap
are included (1) [Fe(TPP)(CO)(1-MeIm)]‚C6H6, (2) [Fe(TPP)(CO)(1,2-Me2-
Im)]‚C7H8, (3) [Fe(OEP)(CO)(1-MeIm)], (4) Fe(TPP)(CO)(2-MeHIm)]‚
C7H8, (5) [Fe(TPP)(CO)(1,2-Me2Im)], and (6) [Fe(TPP)(CO)(1,2-Me2Im)]‚
C7H8 (minor orientation of the axial ligand). Error bars are displayed. The
line showing the correlation has a correlation coefficient of R ) 0.84.93
The inset illustrates the values for all reported iron(II) carbonyl imidazole
structures with the same correlation line as given in the main panel.
of the Fe-NIm bond length vs the C-Fe-NIm angle. Figure 10
displays these values for all iron(II) carbonyl imidazole por-
phyrinates that are not “strapped” or “capped.” A decrease in
the value of the C-Fe-NIm angle accompanies a lengthening
of the Fe-NIm bond. These data points are fit linearly with a
correlation coefficient of R ) 0.84.93 The inset of Figure 10
shows the values for all iron(II) carbonyl porphyrinates with
imidazole as a sixth ligand reported to date; the correlation line
from the main panel is superimposed.
This correlation does suggest that proximal ligand pocket
effects could have a role in the overall OC-Fe-histidine
geometry in MbCO or HbCO. Any proximal ligand pocket effect
that leads to modulation of the Fe-NHis distance could be
expected to affect the C-Fe-NHis angle. What physiological
role this effect may play is currently unknown. It is to be noted
that distal pocket substitutions in many heme protein systems
have been shown to affect ligand binding and discrimination;
it is not difficult to imagine both distal and proximal control of
ligand binding.
Structure analysis for extremely high-quality crystals of the
toluene solvates of [Fe(TPP)(CO)(1,2-Me2Im)] and [Fe(TPP)-
(CO)(2-MeHIm)] became available. Crystals of [Fe(TPP)(CO)-
(2-MeHIm)]‚C7H8 have a completely ordered imidazole; both
complexes exhibit an off-axis Fe-N(imidazole) bond that does
not seem to lead to any unusual distortions of the Fe-C-O
unit. Indeed, the structural features associated with these Fe-
C-O units do not display any more anomalous features than
those observed in [Fe(TPP)(CO)(1-MeIm)]‚C6H6, which has an
unhindered imidazole. We conclude that any anomalous CO
orientational effects (tilting and bending) do not follow the
simple correlation pattern as suggested by Jewsbury et al.22
Although there are some small Fe-C-O tilts in the three new
compounds, they do not correlate directly with proximal ligand
orientation and the pattern suggested by Jewsbury.
There does, however, seem to be an interesting relationship
in the structural parameters involving the axial ligands. The
effect was suggested from the [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8
result. As noted earlier, the axial 1,2-dimethylimidazole is
disordered over two positions that are pseudo-2-fold related.
The two ligand positions are slightly asymmetric with different
Fe-N(Im) bond distances and nonequal site occupancies.
Although correlation effects with such closely overlapped groups
are difficult to define, all attempts to force equivalent Fe-N
distances were unsuccessful. We believe that there are some
differences in the two orientations. Final crystallographic results
are shown in Figure S6 of the Supporting Information. Energeti-
cally, the major and minor imidazole orientations are probably
only slightly different from each other. Nonetheless, the less-
occupied orientation site not only has an unusually long Fe-
NIm bond (2.143 Å) but also a C-Fe-NIm angle (168.6°) that
is abnormally small. A search of the C-Fe-NIm angle for all
other iron(II) carbonyl imidazoles in the literature showed that
angles less than 173° had not been previously reported. This
search did suggest that there is a relationship between values
Summary. Four new six-coordinate iron(II) imidazole- and
carbonyl-ligated porphyrinates have been prepared and char-
acterized. Crystallographic data that were both of very high
quality and high resolution were used in their structural
characterization. Although the IR spectra of the four species
showed similar values of νC-O in toluene solution, large
differences (46 cm-1 range) are seen in solid-state νC-O’s. These
frequency differences result from the differing crystalline
environments of the CO. The lowest frequency (1926 cm-1) is
the result of a hydrogen bond to the CO oxygen. The structural
data are of sufficient accuracy to allow the observation of the
correlation of the Fe-C vs C-O distances that is expected from
the classical picture of π back-bonding for metal carbonyls. The
Fe-C and C-O distances are also strongly correlated with the
observed solid-state CO stretching frequencies. The off-axis
tilting of the hindered imidazole ligand trans to CO appears to
have little effect on possible tilts and bends of the Fe-C-O
unit. However, there does appear to be a correlation between a
decreasing C-Fe-NIm angle and an increasing Fe-NIm bond
distance.
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14432 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

Heme Carbonyls
A R T I C L E S
Acknowledgment. We thank the National Institutes of Health
for support of this research under Grant No. GM-38401 (W.R.S).
We thank Prof. Timothy Sage for useful discussions.
Im)]; Table S1, giving a summary of all crystallographic
information; Tables S2-S25, giving complete crystallographic
details, atomic coordinates, bond distances and angles, aniso-
tropic temperature factors, and hydrogen positions for [Fe(TPP)-
(CO)(1,2-Me2Im)]‚C7H8, [Fe(TPP)(CO)(2-MeHIm)]‚C7H8, [Fe-
(TPP)(CO)(1,2-Me2Im)], and [Fe(TPP)(CO)(1-MeIm)]‚C6H6;
Table S26, orthogonal coordinates of the 1,2-dimethyl imidazole
used in rigid body refinement of [Fe(TPP)(CO)(1,2-Me2Im)]
and [Fe(TPP)(1,2-Me2Im)]‚C7H8; and crystallographic data
available as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
Supporting Information Available: Figures S1-S3, formal
diagrams displaying the perpendicular displacement of core
atoms from the 24-atom mean planes; Figure S4, diagram
showing herringbone-like crystal packing pattern of [Fe(TPP)-
(CO)(1,2-Me2Im)]‚C7H8 and [Fe(TPP)(CO)(2-MeHIm)]‚C7H8;
Figure S5, plot illustrating Fe-C/C-O distances (Å) vs νC-O
(cm-1) for all carbonyl iron(II) imidazoles; Figure S6, ORTEP
diagram of [Fe(TPP)(CO)(1,2-Me2Im)]‚C7H8 showing both
orientations of the imidazole; Figure S7, displaying the environ-
ment of the oxygen atom in unsolvated [Fe(TPP)(CO)(1,2-Me2-
JA053148X
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J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14433