Temperature effects on structure: Five-coordinate (nitrosyl)(tetratolylporphinato)iron(II)

Article abstract of DOI:10.1142/S1088424619501517

We have prepared crystals of [Fe(TTP)(NO)] (TTP = tetratolylporphyrin), a five-coordinate nitrosyl complex and determined its crystal and molecular structure at two temperatures. The crystal structure at 100 K reveals two independent molecules in the asymmetric unit of the structure. One molecule is completely ordered and the second molecule has a moderately disordered nitrosyl ligand. Both molecules show similar structural features: a substantial off-axis tilt of the Fe-N(NO) bond and an asymmetry of the equatorial Fe-Np bonds that is correlated with the tilt. The axial Fe-N(NO) bond distances are 1.7230 (9) and 1.7210 (10) ?; the Fe-N-O bond angles are 141.62 (8) and 140.04 (10). Determination of the structure at ambient temperature (293 K) showed an unexpected phase change, a crystal structure with one molecule per asymmetric unit containing the superposition of the two molecules at lower temperature. However, there was an increase in the NO disorder.

Full text of DOI:10.1142/S1088424619501517

This is a JAM - Just Accepted Manuscript
Journal of Porphyrins and Phthalocyanines
Article Title:
Temperature Effects on Structure: Five-Coordinate
Nitrosyl)(tetratolylporphinato)iron(II)
(
Author(s):
DOI:
Nathan J. Silvernail, Allen G. Oliver, W. Robert Scheidt
10.1142/S1088424619501517
To be cited as:
Nathan J. Silvernail, Allen G. Oliver, W. Robert Scheidt, Temperature Effects on Structure: Five-
Coordinate (Nitrosyl)(tetratolylporphinato)iron(II), Journal of Porphyrins and Phthalocyanines,
doi: 10.1142/S1088424619501517
Received: 20 August, 2019
Accepted: 4 October, 2019
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Contribution from The Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556 USA
Temperature Effects on Structure: Five-Coordinate
Nitrosyl)(tetratolylporphinato)iron(II)
(
†
†
†∗
Nathan J. Silvernail, Allen G. Oliver, and W. Robert Scheidt
Dedicated to Professor Roberto Paolese on the occasion of his 60th birthday
October 2, 2019
E-mail: WRS: scheidt.1@nd.edu
†
University of Notre Dame
∗
To whom correspondence should be addressed

Accepted manuscript to appear in JPP
ABSTRACT: We have prepared crystals of [Fe(TTP)(NO)] (TTP = tetratolylporphyrin),
a five-coordinate nitrosyl complex and determined its crystal and molecular structure at two
temperatures. The crystal structure at 100 K reveals two independent molecules in the asym-
metric unit of structure. One molecule is completely ordered and the second molecule has a
moderately disordered nitrosyl ligand. Both molecules show similar structural features: a sub-
stantial off-axis tilt of the Fe–N(NO) bond and an asymmetry of the equatorial Fe–N bonds
p
that is correlated with the tilt. The axial Fe–N(NO) bond distances are 1.7230 (9) and 1.7210
◦
˚
10) A; the Fe–N–O bond angles are 141.62 (8) and 140.04 (10) . Determination of the structure
(
at ambient temperature (293 K) showed an unexpected phase change, a crystal structure with
one molecule per asymmetric unit containing the superposition of the two molecules at lower
temperature. However, there was an increase in the NO disorder.
KEYWORDS: Crystal structure, tetratolylporphyrin, iron(II), nitric oxide coordination,
crystalline disorder, temperature effects.
2

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Introduction
Recent advances in X-ray instrumentation have led to substantially improved crystal struc-
ture determinations. Area detector technology leads to both rapid and precise x-ray intensity
measurements. This data collection enhancement also led to an enhanced capability of easy
data measurements at temperatures other than ambient and possibly at a number of different
temperatures. Intensity data collection at lowered temperatures leads to molecular structures
with reduced thermal motion, frequently reduced crystalline disorder and, in addition, generally
improved structural data.
In this paper, we report the molecular structure of a five-coordinate nitrosyl derivative,
(
nitrosyl)(tetratolylporphinato)iron(II), a member of a class of species that has suffered from
disorder involving the axial nitric oxide ligand. In general, there are two distinct geometries of
the M–N–O group: linear and bent. In the systems with bent nitrosyls, for iron derivatives,
◦
the Fe–N–O angle is in the range of 140–150 . Apparent rotation of the nitrosyl group around
the Fe–N(NO) bond leads to two or more positions of the nitrosyl oxygen atom. An early
calculation by Hoffman et al. concluded that the rotation around the Fe–N(NO) bond was a
relatively low energy process [1].
The early crystal structure determinations of iron porphyrinate nitrosyls all exhibited disor-
der with multiple positions of the coordinated NO. The first example was that of five-coordinate
[
Fe(TPP)(NO)] [2] with disorder that reflected both mirror and rotational disorder leading to
eight equivalent oxygen atom positions. This disorder was so extreme that we called it the “hy-
dra” [3]. Other five-coordinate systems also displayed substantial disordered FeNO fragments,
i.e., [Fe(TPPBr )(NO)] [4], [Fe(TpivPP)(NO)] [5] and [Fe(TF PP)(NO)] [6].
8
2
The first characterized six-coordinate derivative, [Fe(TPP(1-MeIm)(NO)], also displayed a
substantially disordered NO with two or three distinct oxygen atom locations [7, 8]. Subse-
quently, two crystalline forms of [Fe(TPP)(4-MePip)(NO)] were characterized; only one form
had an ordered FeNO group [9]. Unfortunately, all of foregoing structures were limited in the
detailed description of the FeNO group that could be provided. However, the six-coordinate
3

Accepted manuscript to appear in JPP
derivatives clearly showed that the NO group had a strong structural trans effect that led to
long Fe–N bonds trans to the NO.
The impasse of limited and disordered NO structures was broken by the highly precise
structures of [Fe(OEP)(NO)] [10]. Fortuitously, two crystalline polymorphs of the complex
were obtained; both illustrated the same two striking features. The Fe–N(NO) axial bond
was found to be tilted off-axis along with a concomitant asymmetry in the equatorial Fe–
N bonds that tracked the off-axis tilt. These structural features were confirmed in other
p
well-ordered structures: [Fe(oxoOEC)(NO)] [11] and three forms of [Fe(TPPBr )(NO)] [11]
4
and [Fe(OETAP)(NO)] [12]. The off-axis tilt appears to be an intrinsic feature of nitrosyl
hemes. Similar structural features were seen for two naturally occurring nitrosyl porphyrinate
derivatives: [Fe(DPIX-DME)(NO)] [13] and [Fe(PPIX-DME)(NO)] [14], although the latter
species had a disordered NO group.
Later structural studies included variable temperature structure determinations that showed
controlling crystal temperature provided much insight into NO disorder issues. Multiple tem-
perature studies for [Fe(TPP)(NO)] showed that the crystals underwent a phase change that
decreased, but did not eliminate, NO disorder [15]. More informative still, were temperature
dependent studies on several six-coordinate derivatives Fe(Porph)(1-MeIm)(NO)] [16, 17].
Two crystalline forms of [Fe(TpFPP)(1-MeIm)(NO)] and [Fe(TpOCH PP)(1-MeIm)(NO)]
3
were found to have a single orientation of the FeNO group at 100 K, [Fe(TpOCH PP)(1-
3
MeIm)(NO)] was almost completely ordered at 100 K, whereas [Fe(TPP)(1-MeIm)(NO)] and
[
Fe(TpNO PP)(1-MeIm)(NO)] were found to have substantial disordered FeNO groups even at
2
100 K. ALL however became more disordered when the temperatures were raised to ambient.
Some remained ordered up to a critical temperature below ambient. For systems where the
structure was determined at several temperatures, the disorder increase was linear in tempera-
ture after a certain critical temperature was reached. The papers cited [16, 17] concluded that
the temperature dependent NO orientation effects were the result of temperature-dependent
crystal packing. Analysis of the structural data suggested that in, at least some derivatives,
4

Accepted manuscript to appear in JPP
there was a controlled precise pathway for NO movement. Finally, a van’t Hoff analysis of the
temperature-dependent data allowed for an approximation to the energy difference between the
most stable state (orientation) and a less energetically stable minor orientation; the free energy
differences were between 2 and 4 kJ/mol.
A study of the M ¨o ssbauer spectral differences between five- and six-coordinate iron nitrosyls
[
18] led us to measure temperature-dependent M o¨ ssbauer spectra for several five-coordinate ni-
trosyl derivatives. The area under the M ¨o ssbauer absorption curve was found to have, unexpect-
edly, a very strong temperature dependence for the three different tetraarylporphyrin derivatives
measured: [Fe(TPP)(NO)], [Fe(TTP)(NO)], and [Fe(TMP)(NO)] [19]. These results led to our
initial investigation of the nuclear resonance vibrational spectra (NRVS) of [Fe(TPP)(NO)]
[
20, 21] and to a subsequent study of the combined use of temperature-dependent M ¨o ssbauer
and NRVS to examine the dynamics of disorder of the [Fe(TPP)(NO)] and [Fe(OEP)(NO)]
systems [22].
To further assist in these studies, we have attempted to determine the molecular structures
of additional five-coordinate nitrosyls, especially those with tetraaryl peripheral substituents in
order to better understand any disorder and its possible relation to the dynamics of the systems.
Obtaining adequate crystalline material has turned out to be more challenging than might have
been expected. In this paper, we report the molecular structure of the one such species we
have obtained, that of [Fe(TTP)(NO)], at 100 K. The system was found to crystallize in the
¯
triclinic crystal system (P1) with two molecules in the asymmetric unit, one molecule exhibited
nitrosyl disorder while the other was ordered. We explored a possible increase in disorder by
studying the crystalline derivative at ambient temperature that revealed an unexpected result.
There is an apparent increase in crystalline disorder with the two independent low temperature
molecules merged into an apparent single entity. The disorder in the nitrosyl ligand is indeed
increased at the higher temperature.
5

Accepted manuscript to appear in JPP
EXPERIMENTAL SECTION
Synthesis and Crystallization of [Fe(TTP)(NO]. All reactions and manipulations were
carried out under argon using a double manifold vacuum line, Schlenkware, and cannula tech-
niques. [Fe(TTP)Cl] was prepared using the metallation method described by Adler [23]. A
mixture of H TTP (1.51 g, 2.4 mmol) and FeCl (0.7g, 5.5 mmol) in DMF (75 mL) was refluxed,
2
2
the reaction was monitored using UV-visible spectroscopy. After the reaction was finished, the
solution was cooled to room temperature and methanol was added to the residue upon re-
moval of the solvent under vacuum and then filtered. The obtained solid was washed with
warm methanol until the filtrate was colorless and dried under vacuum. [Fe(TTP)(NO] was
then prepared using the reported reductive nitrosylation procedure of Scheidt and Frisse for
[
Fe(TPP)(NO)] [3]. The reaction solution was purged with NO for 30 min. The solution was
then cannula-transferred into 8 mm glass tubes, carefully layered with methanol, and the tubes
were flame sealed. X-ray quality crystals of [Fe(TTP)(NO)] were isolated after several days. IR
Nujol) ν(NO) 1670 cm 1.
−
(
Structure Determinations. A crystal of [Fe(TTP)(NO)] was glued to the end of a glass
¯
fiber and intensity data collected at 100 K. The structure was solved in the space group P1
by direct methods with the SHELXS program [24]. There are two independent molecules in
the asymmetric unit of structure. One molecule was completely ordered whereas the second
molecule was disordered, displaying two positions of the nitrosyl oxygen atom, separated by
˚
about 0.8 A. We then determined the structure at ambient temperature, in order to explore
issues of further disorder of the nitrosyl group. Quite unexpectedly, the crystals were found
to undergo a phase change with a single molecule in the asymmetric unit. All structures were
2
refined against F by the SHELXL program [24]. A summary of crystal data and data collection
parameters are shown in Table 1.
6

Accepted manuscript to appear in JPP
Table 1. Brief Crystallographic Data and Data Collection Parameters
[
Fe(TPP)(NO)]-100K
[Fe(TPP)(NO)]-293K
formula
FW
48 5
C H36FeN O
754.67
754.67
˚
a, A
10.6302(3)
13.0675(4)
27.3903(8)
80.473(2)
80.3610(10)
81.629(2)
3672.06(19)
4
10.7259(3)
13.1609(4)
14.2790(4)
107.197(2)
94.252(2)
96.362(2)
1901.52(10)
2
˚
b, A
˚
c, A
α
β, deg
γ
3
˚
V , A
Z
¯
P1
¯
P1
space group
3
D
c
, g/cm
1.365
1572
0.457
1.318
786
F(000)
−
1
µ, mm
0.441
crystal dimens, mm
absorption correction
¯
radiation, MoKα, λ
0.466 × 0.302 × 0.299
multiscan
˚
0.71073 A
T, K
100(2)
293(2)
total data collected
unique data
159016
157104
35954 (Rint = 0.0310)
13186 (Rint = 0.0232)
10689
2
on F (SHELXL)
unique obsd data[I > 2 σ(I)] 26371
refinement method
final R indices [I > 2 σ(I)]
final R indices [for all data]
R
R
1
1
= 0.0409, wR
2
= 0.0994
= 0.1148
R
R
1
1
= 0.0389, wR
= 0.0565, wR
2
2
= 0.1004
= 0.1205
= 0.0.0674, wR
2
7

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RESULTS
The crystal and molecular structure of the nitrosyl complex [Fe(TTP)(NO] is reported at low
100 K) and ambient temperatures. The two independent molecules characterized at 100 K are
(
shown in Figures 1 and 2. The disorder in the NO group in molecule 2 is clearly apparent. The
refined values for the occupancies of the two oxygen atoms are 0.823 (4) and 0.177 (4), so that
˚
one oxygen atom position is dominant; the two oxygen atoms are separated by ∼0.8 A.
Cb8
C41
Cb7
Ca8
Ca7
C31
Cb1
Cb2
Cm4
O1
N4
Ca1
N1
N5
Cm3
Ca6
Ca2
Fe1
N3
Cm1
Cb6
C11
Ca5
Ca3
N2
Cb5
Cm2
Ca4
Cb3
C21
Cb4
Figure 1. Thermal ellipsoid plot of [Fe(TTP)(NO] (molecule 1) at 100 K showing atomic labels. Ellipsoids are
drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.
8

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Cd6
C71
Cd5
Cc6
N8
Cm7
Cc5
Cm6
C61
Cd7
Cc7
N9
Cd8
O2a N10
Cc4
Fe2
Cc8
Cd4
Cd3
N7
Cc3
O2
Cm8
N6
Cc1
Cc2
C81
Cm5
C51
Cd1
Cd2
Figure 2. Thermal ellipsoid plot of [Fe(TTP)(NO] (molecule 2) at 100 K showing atomic labels. Ellipsoids are
drawn at the the 50% probability level. Hydrogen atoms have been omitted for clarity.
9

Accepted manuscript to appear in JPP
DISCUSSION
We first consider the coordination features of the iron and the FeNO group for the structural
results obtained at 100 K for [Fe(TPP)(NO)]. The two molecules clearly show the structural
features of a low-spin state. Although the second molecule has a disordered nitrosyl ligand,
the occupancy of the minor oxygen atom is relatively small at 0.177 (4). Accordingly, we will
use the major oxygen atom position for describing the geometrical parameters of molecule 2.
Table 2 summarizes a number of stereochemical parameters of five-coordinate nitrosyls. These
include bond angle (Fe–N–O), bond distances (Fe–N(NO) and N–O), iron displacements from
2
4-atom plane (∆Fe(Ct)), and four nitrogen plane (∆Fe(N )), the orientation of NO (dihedral
4
angle between FeNO and the closest Fe–N vector) and the tilt of the Fe–N(NO) bond vector
p
from the normal to the porphyrin plane. The values for the 100 K structure of [Fe(TPP)(NO)]
are seen to be similar to all others in Table 2 with one possible significant difference. The iron
displacements in [Fe(TPP)(NO)] from the 24-atom plane are slightly larger and suggest that
this is a feature of the tetraaryl derivatives.
Table 2. Bonding parameters for [Fe(Porph)(NO)] complexes.
Complex
Fe–N–Oa
141.62(8)
140.04(10)
141.6(5)
144.4(2)
142.74(8)
147.9(8)
146.9(9)
145(1)
∆Fe(Ct)b,c
0.33
∆Fe(N4)b,d
0.29
0.29
0.29
0.28
0.28
0.31
0.30
0.28
–
Fe–N(NO)b
1.7247(9)
1.7210(10)
1.7160(13)
1.722(2)
N–Ob
orientationa,e
43.8
45.0
28.9
37.9
40.2
0.0
tilta,f
9.6
9.1
8.3
6.5
8.2
5.6
7.1
−
∆N(NO)b,g
ref.
tw
tw
tw
11
11
11
11
11
15
13
14
25
25
11
12
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
Fe(TTP)(NO)](mol 1)h
Fe(TTP)(NO)](mol 2)h
Fe(TTP)(NO)]i
1.1730(13)
1.1817(16)
1.158(8)
1.167(3)
1.1677(11)
1.119(11)
1.144(12)
1.145(16)
1.153(4)
1.187(4)
1.140(8)
1.149(5)
1.142(5)
1.1696(19)
1.155(5)
0.34
0.33
0.25
0.19
0.25
0.17
0.21
−
0.35
0.35
Fe(OEP)(NO)] (A)
Fe(OEP)(NO)] (B)
0.29
0.27
1.7307(7)
1.734(8)
0
Fe(TPPBr4)(NO)] (A )
0.37
0
0
Fe(TPPBr4)(NO)] (A )
Fe(TPPBr4)(NO)] (B)
Fe(TPP)(NO)]
0.32
1.726(9)
0.0
0.29
1.691(11)
1.740(5)
18.4
–k
145.6(4)
143.1(3)
143.4(6)
146.3(4)
146.6(5)
143.11(15)
143.7(4)
0.20
5.8
6.2
–k
–
Fe(DPIX-DME)(NO)]
Fe(PPIX-DME)(NO)]
Fe(3,5-MeBAFP)(NO)] (A)
Fe(3,5-MeBAFP)(NO)] (B)
Fe(oxoOEC)(NO)]
0.26
0.26
0.26
–
1.723(3)
35.0
36
–
0.29
1.719(4)
–
0.35
1.713(4)
∼0
∼0
40.9
–k
0.37
–
1.714(4)
–k
0.26
0.26
–
1.7320(13)
1.721(4)
7.1
7.6
0.22
0.23
Fe(OETAP)(NO)]
0.31
39.6
aValue in degrees. bValue in A. cIron atom displacement from 24-atom mean plane. dIron atom displacement from four nitrogen atom
plane. eMinimum dihedral angle between Fe–N–O and Np–Fe–N(NO) planes. f Deviation from normal to 24-atom mean plane. gTranslation
of the nitrosyl nitrogen atom off the heme normal. h100 K. i293 K. j90 K. kNot reported.
˚
10

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Earlier studies had shown that the nitric oxide ligand led to intrinsic structural distortions
in nitrosyl iron porphyrinates [10, 11]. Typically these features include a substantial off-axis
Fe–N(NO) tilt (tabulated in Table 2) and an asymmetry in the equatorial Fe–N bond distances
p
that is correlated with the Fe–N–O orientation. The difference in the bond distances are statis-
tically significant in all of the structures tabulated in Table 3. The pattern of correlated “long”
and “short” Fe-N bond distances is seen to hold even though there is substantial variation in
p
bond distances amongst the several complexes of Table 3.
a
˚
Table 3. Equatorial Fe–N Bond Distances (A)
p
Complex
Fe(TTP)(NO)](mol 1)b 1.9647(8)
Fe(TTP)(NO)](mol 2)b 1.9683(8)
Fe–N(1)
Fe–N(2)
Fe–N(3)
Fe–N(4)
ref.
tw
[
[
[
[
[
[
[
[
[
[
[
1.9580(9)
1.9650(8)
1.9885(9)
1.9881(9)
1.9871(9)
1.9878(9)
tw
Fe(TTP)(NO)]c
1.9669(11) 1.9654(11)
2.016(2) 1.989(2)
2.0226(6) 2.0000(6)
1.9872(12) 1.9820(11) tw
Fe(OEP)(NO)] (A)
Fe(OEP)(NO)] (B)
Fe(oxoOEC)(NO)]
1.993(2)
2.017(2)
11
11
11
11
11
11
13
12
1.9985(6)
2.0167(6)
2.0174(13) 2.0141(12) 2.0082(13) 1.9974(12)
0
–d
Fe(TPPBr )(NO)] (A ) 2.031(8)
1.976(5)
1.977(6)
1.926(7)
2.005(3)
1.922
2.041(9)
2.027(7)
–e
4
0
0
–d
Fe(TPPBr )(NO)] (A ) 2.004(7)
4
Fe(TPPBr )(NO)] (B)
4
1.976(6)
1.991(3)
1.938
–d
Fe(DPIX-DME)(NO)]
Fe(OETAP)(NO)]
2.034(3)
1.925
1.987(3)
1.941
aEstimated standard deviations of least significant digits are given in parentheses. The effects
of NO tilt on the Fe–N bond distances are indicated as follows: those expected to be “long”
p
are indicated in bold type, those expected to be “short” are given in italic type and those
b
c
that are not expected to be affected are given in Roman type. 100 K. 293 K.
d
e
Equal to Fe–N(2) by symmetry. Equal to Fe–N(1) by symmetry.
11

Accepted manuscript to appear in JPP
The two independent porphyrin planes are nearly coplanar; the dihedral angle between the
◦
mean planes of molecule 1 and molecule 2 is 3.44(1) . Strikingly, the two cores are both strongly
ruffled. Even more striking is the quantitative similarity. The displacements of the core atoms
from the mean plane of the 24-atom core in the two molecules are shown in Figures 3 and 4.
The two diagrams are oriented with respect to the position of the bent axial FeNO group and
the quantitative similarity is apparent. The largest difference between the two molecules is the
nitrosyl ligands, one ordered and the other disordered.
1
1
9
–61
.
3
–
42
5
4
(3
)
–
8
–38
5
2
–27
9
1
1.37
6
(2
)
11
4
2
1
.
)
(9
9
6
4
1
7
7
8
9
(8
)
.
1
1(7)
4
5
122
.
33
4
0
1
24
.
6(8
)
911
41
13 1
0
.
–5
1
0
5
(1
)
.
6
(1
)
–
26
4
9
–
40
–4
1
0
7
.0
(1)
–
39
16
–
59
Figure 3. Formal diagram of the porphinato core in molecule 1 of [Fe(TTP)(NO)] displaying the displacement
˚
of each atom, in units of 0.01 A, from the 24-atom mean plane. The position of the NO ligand is shown by the
solid line. Averaged bond distances and angles for the core atoms are also displayed, the numbers in parentheses
are the estimated standard uncertainties calculated on the assumption that all values are drawn from the same
population.
12

Accepted manuscript to appear in JPP
–
65
2
4
–37
–
0
2
–37
5
2
–35
1
.37
7(3)
1
1
4
0
6
12
2
.2(4)
4
0
36
39
24
.
4(4)
6
1
43
1
1
0
16
–5
–
.
2
(
1
)
–
32
53
40
–2
1
07
.
2(2
)
–
63
–38
21
Figure 4. Formal diagram of the porphinato core in molecule 2 of [Fe(TTP)(NO)] displaying the displacement
˚
of each atom, in units of 0.01 A. from the 24-atom mean plane. The position of the NO ligand is shown by the
solid line. Averaged bond distances and angles for the core atoms are also displayed, the numbers in parentheses
are the estimated standard uncertainties calculated on the assumption that all values are drawn from the same
population.
The determination of the structure at 293 K led to an additional surprise. Increasing the
temperature for crystalline [Fe(TPP)(NO) might have been expected to simply increase the
nitrosyl disorder. This was not observed. Rather, the crystalline system undergoes a phase
change, and the unit cell volume has decreased by slightly less than half that observed at 100
K. The new asymmetric unit contains a single molecule with two oxygen atom positions, one
with a fairly large thermal parameter. Unexpectedly, the structure observed in the asymmetric
unit is the superimposed pair of independent molecules observed at 100 K as illustrated in
Figure 5. Although the issue is somewhat clouded by the new single-molecule asymmetric unit,
the disorder of the NO group has increased. Previous iron porphyrinate nitrosyls studies show
that a NO orientation approximately midway between two adjacent Fe–N bonds is the most
p
favorable. At 100 K, one [Fe(TTP)(NO)] molecule has this orientation at 100% occupancy and
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the second at 82% occupancy. For the merged molecule at 293 K, the preferred orientation has
only a 43% occupancy.
Figure 5. Thermal ellipsoid plot of the molecule in the asymmetric unit of structure at 293 K. (50% probability
ellipsoid s are shown.) Hydrogen atoms are omitted for clarity.
What has led to these differences? We note that the difference in crystal density at the two
temperatures is larger than typical in our experience (see Table 1). This suggests that there is
an increase in crystal packing efficiency at lower temperature, but without substantial change
in the molecular structure. This may be borne out by examining the detailed differences in
conformation at the two temperatures. Figure 6 displays the overlay of the two molecules at
100 K with the ambient structure. The very close overlap is clear, but with one peripheral
phenyl group showing differences. The effects of the differing NO orientation are not expected
to influence the packing.
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Figure 6. An overlay of the two independent molecules at 100 K (light and dark blue) with the ambient
temperature structure displayed in red. Illustration produced with MERCURY.
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SUMMARY and CONCLUSIONS
The crystal and molecular structure of five-coordinate [Fe(TTP)(NO)] at two temperatures
100 and 293 K) reveals that the species show the unsupported off-axis tilt of the NO ligand
(
and the concomitant asymmetry in the Fe–N bond distances of the equatorial plane. The two
p
independent molecules observed at 100 K merge into a single molecule at 293 K with a phase
change and increased NO disorder.
Acknowledgments
We thank the National Institutes of Health for support of this research under Grant GM–
38401.
Supporting Information
Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre
(
CCDC) under numbers CCDC 1947566 and 1947567. Copies can be obtained on request, free
of charge, via www.ccdc.cam.ac.uk/data request/cif or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223-336-033 or email:
deposit@ccdc.cam.ac.uk).
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References and Notes
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2
. The following abbreviations are used in this paper: Porphyrins: Porph, generalized por-
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8
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Tables
Table 1. Crystallographic data.
Table 2. BD
Table 3. asymmetry
Captions for Figures
Figure 1. ORTEP Complete figure caption given in the text.
Figure 2. ORTEP Complete figure caption given in the text.
Figure 3. Complete figure caption given in the text.
Figure 4. Complete figure caption given in the text.
Figure 5. Packing figure caption given in the text.
Figure 6. ocwelay diagram
Captions for Supporting Information Figures
Figure S1. Figure of Alt ORTEP
20