Ellison et al.
and Collman in 19736 and used the sterically hindered
2-methylimidazole ligand. It was expected that this ligand
would lead to a significantly distorted molecule only if six-
coordinate species were formed. However, stereochemical
issues concerning five-coordinate species remain. The crys-
talline complex of [Fe(TPP)(2-MeHIm)]7 prepared by Reed
and Collman was structurally characterized in the laboratory
of the late Prof. J. L. Hoard. A preliminary report of the
structure was given at an ACS meeting,8 and results were
additionally cited and used by Hoard and Scheidt,9 but
complete structural details were never published. One
crystallographic feature that marred the metrical usefulness
of the structure was the presence of crystallographically
required 2-fold disorder normal to the porphyrin plane. This
2-fold axis leads to positional disorder in the axial imidazole
and significantly limits the accuracy of some features
involving the axial ligand and possibly that of the porphyrin
ligand as well. A related species, [Fe(TpivPP)(2-MeHIm)],
also displayed this type of disorder10 and suffers the same
limitations. As described below, we report the structure of a
new, more ordered variant of the five-coordinate species [Fe-
(TPP)(2-MeHIm)] that reveals a number of stereochemically
important features for iron(II) porphyrinates that are possibly
functionally significant.
The preparation of [Fe(TPP)(2-MeHIm)] is an outgrowth
of an attempt to obtain a definitive resonance Raman study
of the only spin-state/coordination number combination still
wanting.11,12 The missing combination is that of the (probably
rare) six-coordinate high-spin iron(II) derivative. Since
resonance Raman spectroscopy has proven to be invaluable
in the assignment of spin-state/coordination number com-
binations in a variety of hemoprotein derivatives, we thought
this issue deserved to be re-explored. The choice of a
molecule for Raman study was the species [Fe(TPP)(THF)2],
which was structurally characterized in 198013 and was the
further subject of an experimental electron density study in
1986.14 Crystalline [Fe(TPP)(THF)2] was resynthesized for
study by Raman spectroscopy. As part of this investigation
of [Fe(TPP)(THF)2], we also prepared crystalline, five-
coordinate, high-spin [Fe(TPP)(2-MeHIm)] for parallel study
by Raman spectroscopy.
The crystalline [Fe(TPP)(2-MeHIm)] species that we
prepared was found to crystallize in a new crystal system,
different from that of the earlier study of [Fe(TPP)(2-
MeHIm)].8 Most importantly, this new crystalline system
does not demand any disorder associated with the axial or
porphyrin ligand. Accordingly, we carried out a structure
determination of this new crystalline form of [Fe(TPP)(2-
MeHIm)]. Although we found a small disorder problem
involving the axial ligand, we have obtained a structure that
does not have structural features obscured by 2-fold disorder
of the axial ligand. Most interestingly, the steric bulk of the
2-methylimidazole ligand leads not only to to an asym-
metrical orientation of the axial ligand, which had been
previously noted, but also to effects on the porphyrin ligand
conformation as well. The sterically demanded tilting of the
axial ligand in binding to iron, which leads to an orientation
of the Fe-NIm that is 8.3° from the heme normal, is
apparently transmitted into an asymmetric interaction be-
tween iron and the porphyrin ligand. This is manifested in
an asd asymmetrically distorted nonplanar porphyrin core
and in apparent nonequivalent equatorial Fe-Np bond
distances.
Experimental Section
General Information. Chlorobenzene was washed with con-
centrated sulfuric acid and then with water until the aqueous layer
was neutral, dried with MgSO4, and distilled twice over P2O5 under
argon. Benzene and hexanes were distilled over sodium benzophe-
none. Ethanethiol (Aldrich) was used as received. 2-Methylimida-
zole (Aldrich) was recrystallized from toluene/methanol and dried
under vacuum. All other chemicals were used as received from
Aldrich or Fisher. meso-Tetraphenylporphyrin (H2TPP) was pre-
pared according to Adler et al.15 [Fe(TPP)Cl] was prepared
according to a modified Adler preparation;16 [Fe(TPP)]2O was
prepared from [Fe(TPP)Cl].17 Mo¨ssbauer measurements were
performed on a constant acceleration spectrometer at 4.2-293 K
in zero field and at 4.2 K in 6 and 9 T fields using a superconducting
magnet system (Knox College). The solid-state Mo¨ssbauer sample
was immobilized in Apiezon grease.
Synthesis of [Fe(TPP)(2-MeHIm)]‚1.5C6H5Cl. The following
reactions were done using standard Schlenk techniques. [Fe(II)-
(TPP)] was prepared by reduction of [Fe(TPP)]2O (65 mg, 0.04
mmol) with ethanethiol (∼ 1 mL) in ∼ 15 mL benzene according
to Stolzenberg et al.18 The benzene solution was stirred overnight
followed by solvent removal under vacuum. The solid [Fe(II)(TPP)]
was never exposed to air to avoid the easily formed [Fe(TPP)]2O.
Excess 2-methylimidazole (32 mg, 0.40 mmol) in 15 mL of dry
(6) Collman, J. P.; Reed, C. A. J. Am. Chem. Soc. 1973, 95, 2048.
(7) Abbreviations: OEP, dianion of 2,3,7,8,12,13,17,18-octaethylporphy-
rin; Piv2C8P, dianion of R,R,5,15-[2,2′-(octanediamido)diphenyl]-R,R,-
10,20-bis(o-pivalamidophenyl)porphyrin; TMP, dianion of 5,10,15,20-
tetramesitylporphyrin; TPP, dianion of 5,10,15,20-tetraphenylporphyrin;
TpivPP, dianion of R,R,R,R-tetrakis(o-pivalamidophenyl)porphyrin or
picket fence porphyrin; THF, tetrahydrofuran; 1-MeIm, 1-methylimi-
dazole; 1,2-Me2Im, 1,2-dimethylimidazole; 2-MeHIm, 2-methylimi-
dazole; 2-MeIm-, 2-methylimidazolate.
(8) (a) Collman, J. P.; Kim, N.; Hoard, J. L.; Lang, G.; Radonovich, L.
J.; Reed, C. A. Abstracts of Papers; 167th National Meeting of the
American Chemical Society; Los Angeles, CA, April 1974; American
Chemical Society: Washington, DC; INOR 29. (b) Hoard, J. L.,
personal communication to W.R.S. In particular, Prof. Hoard provided
a complete set of atomic coordinates for the molecule.
(9) Hoard, J. L.; Scheidt, W. R. Proc. Natl. Acad. Sci. U.S.A. 1973, 70,
3919.
(10) 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.
(11) Mylrajan, M.; Andersson, L. A.; Sun, J.; Loehr, T. M.; Thomas, C.
S.; Sullivan, E. P., Jr.; Thomson, M. A.; Long, K. M.; Anderson, O.
P.; Strauss, S. H. Inorg. Chem. 1995, 34, 3953.
(12) Kincaid, J. R. In The Porphyrin Handbook; Kadish, K. M.; Smith, K.
M.; Guilard, R.; Academic Press: San Diego, CA, 2001; Chapter 51.
(13) Reed, C. A.; Mashiko, T.; Scheidt, W. R.; Spartalian, K.; Lang, G. J.
Am. Chem. Soc. 1980, 102, 2302.
(15) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(16) (a) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl.
Chem. 1970, 32, 2443. (b) Buchler, J. W. In Porphyrins and
Metalloporphyrins; Smith, K. M., Ed.; Elsevier Scientific Publishing:
Amsterdam, The Netherlands, 1975; Chapter 5.
(17) (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.
(18) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc.
1981, 103, 4763.
(14) Lecomte, C.; Blessing, R. H.; Coppens, P.; Tabard, A. J. Am. Chem.
Soc. 1986, 108, 6942.
2174 Inorganic Chemistry, Vol. 41, No. 8, 2002