Silvernail et al.
plex.13,14 Some of the more salient observations were the
off-axis tilting of the Fe-NNO vector from the heme normal
and an equatorial asymmetry observed in the Fe-Np
distances with a short pair of bonds bracketing the projection
of the tilted and bent NO ligand onto the porphyrin core.
These observations were further noted, and the electronic
basis for the tilt and the equatorial asymmetry was subse-
quently recognized in several DFT calculations.15-18
In solid-state vibrational studies utilizing the NRVS
technique,19 a substantial temperature-dependent variation
in the Fe-NNO stretch was noted.20 A low-temperature
crystallographic study of [Fe(TPP)(NO)] showed that there
was an apparent phase change. This led to the detailed
temperature-dependent study of the phase change and the
resulting phenomenon that accompanies the phase change.
The resulting order-disorder phase change, which is com-
pletely reversible, is reported herein. The phase change also
leads to the observation that, at low temperatures, the
structure of [Fe(TPP)(NO)] shows the off-axis tilting of the
Fe-NNO bond seen in a number of other iron nitrosyl species.
from brown to brown-orange, and the final solution may contain
some undissolved material. [Fe(TPP)(NO)] was then precipitated
by the addition of 100 mL of methanol with thorough stirring. The
[Fe(TPP)(NO)] precipitate was isolated by filtration onto sintered
glass. Crystals of [Fe(TPP)(NO)] were prepared by dissolving 25
mg (0.04 mmol) of the isolated solid in 7 mL of chloroform and
carefully layering the solution in 8 mm glass tubes with degassed
methanol followed by flame sealing. X-ray quality crystals of
[Fe(TPP)(NO)] were isolated after 10 days at room temperature.
IR νNO in KBr: (1700 cm-1). Note that, if KBr is not thoroughly
dried, to >130 °C, an additional νNO band is present at 1677 cm-1
.
This band is irreversibly converted on heating of the KBr pellet
(to the 1700 peak).
X-Ray Crystallographic Studies. Four distinct single crystals
were indexed at eight temperatures in ascending and descending
order using a Bruker D8 Apex II system, with graphite-monochro-
mated Mo KR (λ ) 0.71073 Å) radiation from 33 to 293 K (CRYO
Industries or 700 Series Oxford Cryostream).24 Crystals at 273,
290, and 293 K could be indexed as tetragonal (I-centered), whereas
crystals at 33, 90, 100, 130, and 180 K could be indexed in the
j
triclinic crystal system in the space group P1. Cell parameters and
crystal domains were further investigated using cell_now.26 All
examples of the triclinic phase are composed of four domains related
by ∼90 or ∼180° rotations about the primitive triclinic [1, 0, 0]
Experimental Section
27
axis.
General Information. All reactions were carried out under
anaerobic conditions using standard Schlenk techniques under an
argon atmosphere. All solvents were freeze/pump/thaw degassed
(×3) prior to use. Methanol (Acros) and pyridine (Fisher) were
used as received. Nitric oxide (Mittler Specialty Gases) was purified
by passing it through a trap containing 4 Å molecular sieves
immersed in an ethanol/dry ice slurry.21 Free base porphyrin
[H2TPP] was prepared according to the method of Adler et al.22
[Fe(TPP)(Cl)] was prepared according to the metalation procedure
of Adler et al.23 IR measurements were taken on a Nicolet Nexus
670 FT-IR spectrometer. Selected single crystals were ground/mixed
minimally with KBr. An approximately 50:1 (KBr/[Fe(TPP)(NO)])
mixture was formed into a pellet using a hydraulic press.
Synthesis of [Fe(TPP)(NO)]. [Fe(TPP)(NO)] was prepared using
a modification of the previously reported synthesis.12 A solution
of 100 mg of [Fe(TPP)(Cl)], 3 mL of chloroform, 0.2 mL of
methanol, and 0.05 mL of pyridine was placed in a Schlenk flask
and freeze/pump/thaw degassed (×3). The solution was then purged
with argon for 5 min, followed by bubbling of NO for ap-
proximately 5 min. Upon the addition of NO, the solution turned
A total of 11 data sets were collected, and the programs
SADABS28 and TWINABS29 were applied for absorption correc-
tion. All structures were solved using direct methods as imple-
mented in XS30 and refined using XL.30 All atoms were found after
successive full-matrix least-squares refinement cycles on F2 and
refined with anisotropic thermal parameters. Hydrogen atoms were
placed at calculated geometries and allowed to ride on the position
of the parent atom. Hydrogen thermal parameters were set to 1.2
times the equivalent isotropic U of the parent atom.
Crystal structure data collected at 273, 290, and 293 K were
refined in the I4/m space group, where the molecule has required
4/m symmetry (see Table 1). The iron and nitrosyl nitrogen atoms
are disordered over two positions equally occupied as required by
the mirror plane that lies in the porphyrin plane. The nitrosyl oxygen
atom is disordered over eight positions, four positions above and
four positions below, the porphyrin plane, equally occupied as
required by the 4-fold axis along the Fe-NNO vector. The porphyrin
phenyl groups are rotated 90° from the porphyrin plane with
required mirror symmetry.
Crystal structure data collected at 33, 90, 100, 130, and 180 K
j
were refined in P1, Z ) 1. However, alternatively they may be
(13) Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404.
(14) Scheidt, W. R.; Duval, H. F.; Neal, T. J; Ellison, M. K. J. Am. Chem.
Soc. 2000, 122, 4651.
(15) Leu, B. M.; Zgierski, M. Z.; Wyllie, G. R. A.; Scheidt, W. R.; Sturhahn,
W.; Alp, E. E.; Durbin, S. M.; Sage, J. T. J. Am. Chem. Soc. 2004,
126, 4211.
(16) Praneeth, V. K. K.; Na¨ther, C.; Peters, G.; Lehnert, N. Inorg. Chem.
2006, 45, 2795.
(17) (a) Ghosh, A.; Wondimagegn, T. J. Am. Chem. Soc. 2000, 122, 8101.
(b) Ghosh, A. Acc. Chem. Res. 2005, 38, 943.
(18) Cheng, L.; Novozhilova, I.; Kim, C.; Kovalevsky, A.; Bagley, K. A.;
Coppens, P.; Richter-Addo, G. B. J. Am. Chem. Soc. 2000, 122, 7142.
(19) Scheidt, W. R.; Durbin, S. M.; Sage, J. T. J. Inorg. Biochem. 2005,
99, 60.
(20) Silvernail, N. J.; Sage, J. T.; Alp. E. E.; Sturhahn, W.; Zhang, Z;
Scheidt, W. R. Work in progress. There does not appear to be a
relationship in these phenomena.
described in a nonstandard setting, I1, Z ) 2,27 to more conveniently
j
compare with the structures described in I4/m. The Fe-N-O unit
(24) The Oxford control unit is calibrated at the factory using the phase
change of Rochelle’s salt at 109 K.25 The factory calibration was
checked in our laboratory with a well-insulated iron-constantan
thermocouple and an Omega 199 temperature meter. All reported
temperatures are believed accurate to within 2 K.
(25) Tomaszewski, P. E. Phase Trans. 1992, 38, 127.
(26) Sheldrick, G. M. cell_now; Bruker-Nonius AXS: Madison, WI, 2001.
j
j
(27) The transformation matrix (M) converts P1, Z ) 1 to I1, Z ) 2.
0
M ) -1
1
1
1
0
-1
1
0
(21) Dodd, R. E.; Robinson, P. L. Experimental Inorganic Chemistry;
Elsevier: New York, 1957; p 253.
(22) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(23) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
The rotations in the triclinic I-centered cell are about the c axis.
(28) Sheldrick, G. M. SADABS; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
2006.
(29) Sheldrick, G. M. TWINABS; Universita¨t Go¨ttingen, Go¨ttingen, Ger-
many, 2006.
(30) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
972 Inorganic Chemistry, Vol. 48, No. 3, 2009