Structural insights into ligand dynamics: Correlated oxygen and picket motion in oxycobalt picket fence porphyrins

Article abstract of DOI:10.1021/ja303475a

Two different oxygen-ligated cobalt porphyrins have been synthesized and the solid-state structures have been determined at several temperatures. The solid-state structures provide insight into the dynamics of Co-O₂ rotation and correlation with protecting group disorder. [Co(TpivPP)(1-EtIm) (O₂)] (TpivPP = picket fence porphyrin) is prepared by oxygenation of [Co(TpivPP)(1-EtIm)₂] in benzene solution. The structure at room temperature has the oxygen ligand within the ligand binding pocket and disordered over four sites and the trans imidazole is disordered over two sites. The structure at 100 K, after the crystal has been carefully annealed to yield a reversible phase change, is almost completely ordered. The phase change is reversed upon warming the crystal to 200 K, whereupon the oxygen ligand is again disordered but with quite unequal populations. Further warming to 300 K leads to greater disorder of the oxygen ligands with nearly equal O₂ occupancies at all four positions. The disorder of the tert-butyl groups of the protecting pickets is correlated with rotation of the O₂ around the Co-O(O₂) bond. [Co(TpivPP)(2-MeHIm)(O₂)] is synthesized by a solid-state oxygenation reaction from the five-coordinate precursor [Co(TpivPP)(2-MeHIm)]. Exposure to 1 atm of O₂ leads to incomplete oxygenation, however, exposure at 5 atm yields complete oxygenation. Complete oxygenation leads to picket disorder whereas partial (40%) oxygenation does not. Crystallinity is retained on complete degassing of oxygen in the solid, and complete ordering of the pickets is restored. The results should provide basic information needed to better model M-O₂ dynamics in protein environments.

Full text of DOI:10.1021/ja303475a

Article
pubs.acs.org/JACS
Structural Insights into Ligand Dynamics: Correlated Oxygen and
Picket Motion in Oxycobalt Picket Fence Porphyrins
Jianfeng Li,†,‡ Bruce C. Noll,‡ Allen G. Oliver,‡ and W. Robert Scheidt*,‡
†College of Materials Science and Opto-electronic Technology, Graduate University of Chinese Academy of Sciences, 19A Yuquan
Road, Beijing 100049, China
‡The Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States
S
* Supporting Information
ABSTRACT: Two different oxygen-ligated cobalt porphyrins
have been synthesized and the solid-state structures have been
determined at several temperatures. The solid-state structures
provide insight into the dynamics of Co−O2 rotation and
correlation with protecting group disorder. [Co(TpivPP)(1-
EtIm)(O2)] (TpivPP = picket fence porphyrin) is prepared by
oxygenation of [Co(TpivPP)(1-EtIm)2] in benzene solution.
The structure at room temperature has the oxygen ligand
within the ligand binding pocket and disordered over four sites
and the trans imidazole is disordered over two sites. The structure at 100 K, after the crystal has been carefully annealed to yield a
reversible phase change, is almost completely ordered. The phase change is reversed upon warming the crystal to 200 K,
whereupon the oxygen ligand is again disordered but with quite unequal populations. Further warming to 300 K leads to greater
disorder of the oxygen ligands with nearly equal O2 occupancies at all four positions. The disorder of the tert-butyl groups of the
protecting pickets is correlated with rotation of the O2 around the Co−O(O2) bond. [Co(TpivPP)(2-MeHIm)(O2)] is
synthesized by a solid-state oxygenation reaction from the five-coordinate precursor [Co(TpivPP)(2-MeHIm)]. Exposure to 1
atm of O2 leads to incomplete oxygenation, however, exposure at 5 atm yields complete oxygenation. Complete oxygenation
leads to picket disorder whereas partial (40%) oxygenation does not. Crystallinity is retained on complete degassing of oxygen in
the solid, and complete ordering of the pickets is restored. The results should provide basic information needed to better model
M−O2 dynamics in protein environments.
these cases, the lower temperature structures resolved disorder,
even though the rotation about the axial Fe−N bond is a very
low barrier process.
Accordingly, we carried out temperature-dependent struc-
INTRODUCTION
■
An understanding of the dynamics of the interactions of
diatomics with hemes and heme proteins has long been
considered fundamental to understanding function.1 Mossbauer
̈
spectroscopy results have shown that the dynamics of the
dioxygen complexes of hemes and heme proteins are especially
rich. The temperature dependence of the quadruple splitting of
oxyhemoglobin is an unusual and distinctive feature.2 The
picket fence derivative [Fe(TpivPP)(1-MeIm)(O2)]3 also
displayed anomalous line widths and quadrupole splittings in
tural investigations for both iron and cobalt dioxygen-ligated
derivatives of picket fence porphyrin. These have included
analogues for both R- and T-state hemoglobin and
cobaltohemoglobin. Although the low-temperature structure
determinations have substantially improved the available
structural details of oxygen complexes, the studies have only
partially resolved the disorder/dynamic structural issues. The
results from the cobalt derivatives are the most illuminating and
provide new information on the interaction between the axial
oxygen ligand and the protecting pickets of the porphyrin. We
present results from two series, [Co(TpivPP)(1-EtIm)(O2)]
and [Co(TpivPP)(2-MeHIm)(O2)], both of which illustrate
distinct features of the interactions between O2 and the pickets.
We will report results for three iron series members
subsequently.10
its Mossbauer spectra that were interpreted by Lang and co-
̈
workers as resulting from multiple configurations of the Fe−O2
unit because of rotation around the Fe−O bond.4 Later,
Oldfield proposed a different model to explain the anomalous
temperature-dependent line width behavior for the Mossbauer
̈
spectra; the model also required rotations around the Fe−O
bond.5 The Co−O2 group in analogous cobalt complexes has
similar dynamics that have been investigated by EPR.6a
Rotation about the Co−O bond was observed both in small
molecule as well as in hemoglobin derivatives.7 Given these
interesting ligand dynamics, we thought that a series of
multiple-temperature X-ray studies could provide information
on the ligand rotations, much like those we were able to map
out for several six-coordinate nitrosyl iron porphyrinates.8,9 In
Received: April 11, 2012
Published: May 30, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Article
C25B, C26B, and C27B). The terminal oxygen atom (O2) has a single
crystallographically independent position and thus exhibits 2-fold
positional disorder. The solvent benzene molecule has a 2-fold axis
with an occupancy of a half molecule per asymmetric unit.
The crystal was then slowly warmed, in the N2 stream, to 188 K at a
speed of 120 K/h and was allowed to stand for 30 min at 188 K. The
crystal was then cooled back to 100 K at a rate of 120 K/h (ramp
cooling). The crystal system was then observed to have changed to the
lower symmetry of primitive triclinic. There are two twin domains with
a rotation of 179.9° with rotation around the 110 reciprocal axis with a
twin ratio of 64:36, and TWINABS16 was used for the absorption
correction. After data collection at 100 K, the structural analysis was
EXPERIMENTAL SECTION
■
General Information. All reactions and manipulations were
carried out under argon using a double-manifold vacuum line,
Schlenkware, and cannula techniques. Benzene, THF, and heptane
were distilled over sodium/benzophenone and ethanol over
magnesium. Research grade oxygen (99.999%) was purchased from
PRAXAIR and used as received. [H2(TpivPP)] and [Co(TpivPP)]
were prepared according to a local modification of the reported
syntheses;11 additional information is in the Supporting Information.
UV−vis spectra were recorded on a Perkin-Elmer Lambda 19 UV/vis/
near-IR spectrometer.
Synthesis of [Co(TpivPP)(1-EtIm)(O2)]. A chemical structure
drawing is given in the Supporting Information. [Co(TpivPP)(1-
EtIm)] was prepared by the reaction of Co(TpivPP) (80 mg, 0.075
mmol) with excess 1-EtIm (0.06 mL, 0.60 mmol) in benzene.12 A red
crystalline powder was obtained by slow introduction of heptane into
the benzene solution over 2−3 h. [Co(TpivPP)(1-EtIm)] (21.3 mg,
0.018 mmol) was dried under vacuum in a 50-mL Schlenk apparatus
for 30 min. Drops of 1-ethylimidazole and benzene (∼5 mL) were
then transferred into the Schlenk apparatus by cannula. This mixture
was gently heated with stirring and cooled to room temperature to
give a clear red solution. Oxygen was then bubbled into this solution
for 5 min. X-ray quality crystals were obtained in 8 mm × 250 mm
sealed glass tubes by liquid diffusion using heptane as nonsolvent.
Synthesis of [Co(TpivPP)(2-MeHIm)(O2)]. [Co(TpivPP)(2-
MeHIm)] was prepared as described previously.12 Moderate-size
single crystals of [Co(TpivPP)(2-MeHIm)] were exposed at room
temperature to pure dioxygen gas saturated with ethanol vapor.
Crystals were obtained from exposure to (a) a ∼1 atm dioxygen
atmosphere for 3 days or (b) to a ∼5 atm dioxygen atmosphere for 5
days in a high pressure reaction cylinder (Parr Instrument; Model
4635). After the oxygenation, the adducts were rapidly moved into the
low temperature (100 K) nitrogen stream for structural character-
ization.
X-ray Structure Determinations. Single-crystal experiments
were carried out on a Bruker Apex II system with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) and a 700 Series
Oxford Cryostream. The structures were solved by direct methods
using SHELXS13 and refined against F2 using SHELXL;13,14
subsequent difference Fourier syntheses led to the location of most
of the remaining non-hydrogen atoms. For the structure refinement,
all data were used, including negative intensities. All non-hydrogen
atoms were refined anisotropically if not remarked otherwise below.
Hydrogen atoms were idealized with the standard SHELXL ideal-
ization methods. The programs SADABS15 and TWINABS16 were
used to apply absorption corrections. Solid-state analysis of crystal
packing distances made use of the program MERCURY17 from the
Cambridge Crystallographic Data Centre. Complete crystallographic
details, atomic coordinates, anisotropic thermal parameters, and fixed
hydrogen atom coordinates are given in the Supporting Information.
[Co(TpivPP)(1-EtIm)(O2)]·C6H6. The complexes displayed differ-
ing structural features depending on the thermal history and are thus
described in some detail. A freshly isolated crystal with dimensions
0.39 × 0.26 × 0.19 mm3 was picked for the temperature-dependent
structure analysis. The crystal was glued to the tip of a thin glass fiber
and quickly moved into the 100 K N2 stream (flash cooling). The
crystal was found to be in the monoclinic crystal system with C2/c as
the space group. The asymmetric unit contains half a picket fence
porphyrin complex, and half a benzene solvent molecule. There is a
crystallographic 2-fold axis passing through the coordinated oxygen
(O1) and the cobalt (Co1) atoms and the axial 1-ethylimidazole
ligand. The disordered, symmetry-related imidazoles were defined with
a rigid group refinement with an ideal 1-ethylimidazole group.18 For
each “picket”, the pivalamido methyl carbon atoms were found to be
disordered into two sets of positions. An ideal tetrahedral rigid group19
was applied to constrain each tert-butyl group. The occupancies of the
two independent pairs of tert-butyl groups are refined to 0.543(13)
(C20, C21, C22, and C23) and 0.457(13) (C20B, C21B, C22B, and
C23B); 0.555(5) (C24, C25, C26, and C27) and 0.445(5) (C24B,
carried out with the space group P1. The asymmetric unit of structure
̅
now contains one complete picket fence porphyrin complex and one
benzene solvent molecule. These molecules are ordered, including
completely ordered picket tert-butyl groups. The only disorder
observed was that of the terminal oxygen atom, which occupies two
crystallographically independent positions with very different refined
occupancies of 0.816(6) (O2A) and 0.184(6) (O2B).
The (same) crystal was then warmed to 200 K, X-ray data for the
structure determination were collected, and then the crystal was
warmed to 300 K and diffraction data were again collected. The same
heating speed of 120 K/h was used. At both temperatures, the
monoclinic crystal system and the C2/c space group were found.
Similar crystal structure models and refinement procedures as those
used originally (C2/c, 100 K) were applied. Disorder models for the
tert-butyl pickets and axial imidazole ligand (2-fold symmetry) were
applied as before. There is one significant difference, the terminal
oxygen atom now occupies two crystallographically independent
positions and thus exhibits approximate 4-fold positional disorder. The
occupancy factors of the two unique oxygen positions were refined by
means of “free variables”, and the final site occupancy factors were
found to be 0.41(1) (O2A) and 0.09(1) (O2B) (200 K) and 0.27(1)
(O2A) and 0.23(1) (O2B) (300 K).
This unusual thermal behavior was verified with a second crystal
with dimensions 0.34 × 0.26 × 0.18 mm3. When flash cooled to 100 K,
the crystal again showed space group C2/c. After several annealing
processes, the structure displayed the P1 space group. The detailed
̅
experimental process with crystallographic information and the
thermal ellipsoid plot of [Co(TpivPP)(1-EtIm)(O2)]·C6H6 (P1,
̅
measured at 80 K) are given in the Supporting Information.
[Co(TpivPP)(2-MeHIm)(O2)]. The first experiment used a crystal
(A) with dimensions 0.50 × 0.22 × 0.21 mm3 that had been reacted
under a ∼1 atm dioxygen atmosphere. After the oxygenation, the
crystal was rapidly moved to a diffractometer under a 100 K nitrogen
stream, and X-ray data were collected. This crystal sample was found
to be incompletely oxygenated.
A second crystal (B) with the dimensions 0.37 × 0.21 × 0.16 mm3
was oxygenated under a ∼5 atm dioxygen atmosphere. After
oxygenation, which was complete, the crystal was rapidly moved to
the diffractometer under a 100 K nitrogen stream and structural data
collected. After the 100 K data collection, the crystal was slowly
warmed to 300 K at 120 K/h under the N2 stream. Then the structural
data were collected at 300 K.
The crystallographic data for all data sets was consistent with that of
the monoclinic crystal system and space group C2/c; this is the same
crystal system and space group as that of the five-coordinate precursor
[Co(TpivPP)(2-MeHIm)]. There is a crystallographic 2-fold axis
passing through the coordinated oxygen (O1) and the cobalt (Co1)
atoms. The 2-methylimidazole ligand is disordered between two
symmetry related sites. The terminal oxygen atom (O2) occupies one
crystallographically independent position, and thus, it exhibits 2-fold
positional disorder. The occupancy of the dioxygen ligand was
carefully judged from the crystallographic results. First, the applied
occupancy factor should give reasonable anisotropic displacement
parameters (ADPs) compared to those of other atoms in the same
molecule. Second, the command of “FMAP-2” was applied to examine
the resulting Fourier difference maps, especially the holes and peaks
around the positions of dioxygen atoms. After careful refinement, the
occupancies of dioxygen were set at 0.4 (crystal A), 1.0 (crystal B, 100
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Journal of the American Chemical Society
Article
Figure 1. Thermal ellipsoid diagrams of the [Co(TpivPP)(1-EtIm)(O2)] structure at 100, 200, and 300 K, showing the thermally dependent history.
The temperature and corresponding space group are given in the highlighted boxes. Both orientations are shown for the axial imidazole when
disordered over two positions. The terminal oxygen populations are also indicated. Thermal ellipsoids are contoured at the 50% probability level for
the 100 K result, 40% for the 200 K result, and 30% for the 300 K result. Hydrogens are omitted for clarity. Two views are given for the 200 and 300
K structures to allow better views of the O2 disorder.
K), and 0.3 (crystal B, 300 K). Two ethanol solvent molecules are
found, and each is disordered around a 2-fold axis. In each structure,
the first ethanol molecule is fully occupied; the second ethanol
molecule occupancy was found to be 1, 0.8, and 0.8, respectively.
Hence, the three structures are [Co(TpivPP)(2-MeHIm)0.4-
(O2)]·2C2H5OH (100 K, crystal A), [Co(TpivPP)(2-MeHIm)-
(O2)]·1.8C2H5OH (100 K, crystal B), [Co(TpivPP)(2-MeHIm)0.3-
(O2)]·1.8C2H5OH (300 K, crystal B), respectively.
After the 300 K data collection, crystal B (characterized as
[Co(TpivPP)(2-MeHIm)0.3(O2)]) was moved into an argon
atmosphere that was saturated with ethanol vapor. Every 12 h, the
sample cylinder was pumped slightly and refilled for several cycles to
keep a pure argon atmosphere. Two days later, the crystal was used for
a final structural characterization. The crystal was rapidly moved to a
100 K nitrogen stream and structural data collected. In this structure,
the asymmetric unit contains half a picket fence porphyrin complex
and two half-ethanol solvent molecules. There is a crystallographic 2-
fold axis passing through the cobalt atom. The 2-methylimidazole
ligand was found to disorder between two 2-fold related sites. Each
solvent ethanol molecule also possesses a 2-fold axis. The crystalline
molecule has thus reverted to [Co(TpivPP)(2-MeHIm)]·1.8C2H5OH.
RESULTS
■
The synthesis and molecular structure characterizations of two
oxycobalt(II) picket fence porphyrin complexes are reported.
The molecular structure determinations have been carried out
at a variety of temperatures between 100 and 300 K. The first is
six-coordinate [Co(TpivPP)(1-EtIm)(O2)], which was synthe-
sized and crystallized from solution. The second, [Co(TpivPP)-
(2-MeHIm)(O2)], utilizing sterically hindered 2-methylimida-
zole, was synthesized from the heterogeneous reaction of
gaseous dioxygen and crystalline, five-coordinate [Co(TpivPP)-
(2-MeHIm)]. Difficulties with incomplete oxygenation are
noted.
[Co(TpivPP)(1-EtIm)(O2)]. The synthesis of [Co(TpivPP)-
(1-EtIm)(O2)] is similar to that of [Fe(TpivPP)(1-MeIm)-
(O2)], but the precursor is five-coordinate [Co(TpivPP)(1-
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Journal of the American Chemical Society
Article
Table 1. Complete Crystallographic Details for [Co(TpivPP)(1-EtIm)(O2)]·C6H6 at 100, 200, and 300 K (All Measurements
Were Conducted on the Same Crystal)
sample treatment
flash cooled
after annealing
warm to 200 K
warm to 300 K
chemical formula
FW
C75H78CoN10O6
1274.40
18.586(4)
19.170(4)
18.316(4)
90
C75H78CoN10O6
1274.40
C75H78CoN10O6
1274.40
18.5681(7)
19.2564(8)
18.4497(7)
90
C75H78CoN10O6
1274.40
18.711(2)
19.566(3)
18.725(3)
90
a, Å
13.0971(9)
13.5557(9)
18.2431(12)
89.216(4)
88.444(3)
88.190(3)
3235.8(4)
1617.9
b, Å
c, Å
α, deg
β, deg
90.25(3)
90
90.169(2)
90
90.824(5)
90
γ, deg
V, Å3
V, Å3/molecule
space group
Z
6526(2)
1632
6596.7(4)
1649.2
6854.6(17)
1713.8
C2/c
P1
C2/c
C2/c
̅
4
2
4
4
temp, K
100
100
200
300
D
calcd, g cm−3
1.297
1.308
1.283
1.235
μ, mm−1
0.326
0.328
0.322
0.310
final R indices
[I > 2σ(I)]
final R indices
(all data)
R1 = 0.0477
wR2 = 0.1139
R1 = 0.0783
wR2 = 0.1226
R1 = 0.0496
wR2 = 0.1276
R1 = 0.0744
wR2 = 0.1466
R1 = 0.0486
wR2 = 0.1266
R1 = 0.0911
wR2 =0.1498
R1 = 0.0606
wR2 = 0.1755
R1 = 0.1091
wR2 =0.2052
Table 2. Complete Crystallographic Details for [Co(TpivPP)(2-MeHIm)] and Its Oxygenated Derivatives
[Co(TpivPP)(2-MeHIm)
0.4(O2)]·2C2H5OH
[Co(TpivPP)(2-MeHIm)
[Co(TpivPP)(2-MeHIm)
0.3(O2)]·1.8C2H5OH
[Co(TpivPP)(2-
MeHIm)·1.8C2H5OH
(O2)]·1.8C2H5OH
chemical
formula
C72H82CoN10O6.8
C71.6H80.8CoN10O7.8
C71.6H80.8CoN10O6.4
C71.6H82CoN10O5.8
FW
1255.21
18.5678(7)
19.5594(8)
17.8219(6)
90
1265.19
18.7514(3)
19.5309(3)
17.8719(3)
90
1242.79
18.9063(5)
19.6633(5)
18.3473(5)
90
1234.40
18.6326(3)
19.6158(3)
17.9050(3)
90
a, Å
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
space group
Z
90.954(2)
90
91.065(1)
90
91.666(1)
90
90.879(1)
90
6471.6(4)
C2/c
6544.13(18)
C2/c
6817.9(3)
C2/c
6543.39(18)
C2/c
4
4
4
4
temp, K
100
100
300
100
D
calcd, g cm−3
1.288
1.284
1.211
1.253
μ, mm−1
0.328
0.326
0.310
0.322
final R indices
[I > 2σ(I)]
final R indices
(all data)
R1 = 0.0509
wR2 = 0.1399
R1 = 0.0593
wR2 = 0.1512
R1 = 0.0559
wR2 = 0.1591
R1 = 0.0676
wR2 = 0.1703
R1 = 0.0758
wR2 = 0.2301
R1 = 0.1219
wR2 = 0.2646
R1 = 0.0572
wR2 = 0.1633
R1 = 0.0733
wR2 = 0.1776
EtIm)]12 instead of six-coordinate [Fe(TpivPP)(1-MeIm)2].
Crystals of [Co(TpivPP)(1-EtIm)(O2)] containing the six-
coordinate porphyrinate and benzene solvate have been
characterized at various temperatures and cooling regimes.
Thermal ellipsoid diagrams of the molecular structure at
different temperatures and thermal history are given in Figure
1. A brief summary of the crystallographic data is given in Table
1, and complete crystallographic details, atomic coordinates,
bond distances, and bond angles of the structure are given in
Tables S1−S24 of the Supporting Information.
In the process of the structural characterization of [Co-
(TpivPP)(1-EtIm)(O2)], we observed that the low-temper-
ature structure (100 K) is different depending on the cooling
process for the specimen. When a fresh crystal is quickly cooled
to 100 K in the N2 stream (flash cooling), the crystals are found
in the monoclinic crystal system, whereas when a crystal is
annealed and then slowly cooled to 100 K, the crystal displays
triclinic diffraction symmetry (Figure 1). This behavior appears
to be both reversible and reproducible. The monoclinic crystal
form is found in the centrosymmetric space group C2/c with
required 2-fold symmetry for the molecule, and as will be
discussed subsequently, the molecule has disordered axial
ligands and disordered pickets. In contrast, the triclinic crystal
form has no required crystallographic symmetry, completely
ordered pickets and axial imidazole, and only minor disorder of
the axial O2 ligand. The terminal oxygen atom occupies two
crystallographically independent positions with occupancies
refined to be 0.816(6) (O2A) and 0.184 (O2B).
When the same crystal was warmed to 200 K and then to 300
K (120 K/h), the crystal was again found in the monoclinic
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Journal of the American Chemical Society
Article
Table 3. Coordination Group and Picket Parameters for [Co(TpivPP)(1-EtIm)(O2)] and [Co(TpivPP)(2-MeHIm)(O2)] at
Differing Temperatures and Conditions
cryst
form T, K
picket
d
no. imid
e
no. O2
f
a
a,
b
a
a
c
c
compd
Co−O
Co−Np
Co−NIm
O−O
Co−O−O
O−Co−NIm
config
orient.
orient.
A. Related to [Co(TpivPP)(1-EtIm)(O2)]
[Co(TPP)
295
1.977(6)
2.157(3)
2.017(2)
1
1
g
(1‑MeIm)]
[Co(TpivPP)
(1‑EtIm)(O2)]
P1
̅
100
1.9029(19)
1.978(3)
1.239(3)
120.3(2)
178.20(11)
4Ord
2
1.048(12)
1.116(4)
131.7(8)
127.5(2)
h
[Co(TpivPP)
C2/c
C2/c
100
200
1.899(2)
1.899(2)
1.9895(6)
1.9739(2)
2.032(2)
173.44(11)
4Dis
4Dis
2
2
2
4
(1‑EtIm)(O2)]
h
[Co(TpivPP)
(1‑EtIm)(O2)]
2.0293(18)
1.131(5)
127.4(3)
173.71(8)
1.13(2)
128.7(14)
128.0(6)
h,i
[Co(TpivPP)
C2/c
300
80
1.942(4)
1.929(6)
1.980(2)
1.982(9)
2.046(2)
2.029(6)
1.129(9)
180
4Dis
2
1
4
2
(1‑EtIm)(O2)]
1.16(2)
135.3(17)
122.0(8)
[Co(TpivPP)
(1‑EtIm)(O2)]#2
P1
̅
1.233(13)
179.2(4)
4Ord
1.02(4)
131(2)
B. Related to [Co(TpivPP)(2-MeHIm)(O2)]
[Co(TpivPP)
C2/c
C2/c
100
100
1.979(3)
2.145(3)
4Ord
4Ord
2
2
j
(2‑MeHIm)]
h
[Co(TpivPP)
(2‑MeHIm)
0.4(O2)]
1.934(6)
1.9751(12)
2.108(3)
1.100(9)
128.8(6)
173.81(15)
2
h
[Co(TpivPP)
(2‑MeHIm)(O2)]
C2/c
C2/c
100
300
1.902(2)
1.993(9)
1.9850(16)
1.986(3)
2.078(2)
2.153(5)
1.110(4)
0.98(2)
128.8(2)
173.93(8)
4Dis
2
2
2
2
h
[Co(TpivPP)
(2‑MeHIm)
0.3(O2)]
139.3(16)
173.10(16)
4Ord?
[Co(TpivPP)
C2/c
100
295
1.983(4)
1.985(3)
2.158(3)
2.216(2)
4Ord
2
1
(2‑MeHIm)]
[Co(TPP)
(1,2‑Me2Im)]
k
a
b
c
d
e
Values in angstroms. Averaged value. Values in degrees. Number of pickets ordered (Ord) and/or disordered (Dis). Number of imidazole
f
g
h
orientations. Number of O2 orientations. Values from ref 54. The differences between 180° and the observed angle represent the off-axis tilt of the
imidazole. Not resolved. Values from ref 12. Values from ref 55.
i
j
k
crystal system in space group C2/c. The axial imidazole, the
dioxygen ligand, and the pickets display increasingly serious
disorder as the temperature is increased, as displayed in Figure
1; the occupancy ratios of the 4-fold disordered dioxygen
ligands are 0.41:0.09 (200 K) and 0.27:0.23 (300 K).
second crystallographically independent position of the
dioxygen is found. Thus, the coordinated dioxygen displays 4-
fold disorder and the trans imidazole 2-fold disorder.
Interestingly, the disorder of the pickets, found in the dioxygen
picket fence complexes, is not a common phenomenon with
pocket ligands other than dioxygen, suggesting that the pickets
are not “inert” with respect to the dioxygen ligand.
Even though an individual single-crystal X-ray analysis
cannot discriminate between crystalline disorder caused by
static phenomena or by dynamic processes occurring in the
solid state, the use of multiple-temperature structure determi-
nations on the same single-crystal specimen can yield
substantial insight into the nature of possible dynamic
processes. A series of iron(II) nitrosyl complexes showed
how multiple-temperature X-ray structures could provide
significant information into the pathways of NO ligand
motion.8,9 The striking success of the NO studies led us to
carry out multiple-temperature studies of picket fence dioxygen
complexes at a variety of temperatures from ambient down to
80−100 K. We have examined the structures of the cobalt(II)
species on the expectation that useful information concerning
the solid-state dynamics of the O2 complexes would be
obtained.
[Co(TpivPP)(2-MeHIm)(O2)]. When crystalline [Co-
(TpivPP)(2-MeHIm)] is exposed to oxygen at 1 atm, the
five-coordinate complex becomes partially oxygenated. The
crystal structure analysis suggests that the oxygenation reaction
is about 40% complete. Oxygenation at 5 atm leads to complete
oxygenation, [Co(TpivPP)(2-MeHIm)(O2)], that retains all
oxygen at 100 K, but molecular oxygen is slowly lost from the
single crystal at temperatures above ∼200 K. The structure
determined at 300 K showed about 30% occupancy of the
remaining coordinated O2. The complex can be completely
deoxygenated, and the structure of such a crystal shows no
apparent degradation. A brief summary of the crystallographic
data of the 2-MeHIm structures is given in Table 2, and
complete crystallographic details, atomic coordinates, bond
distances, and bond angles of these four structures are given in
Tables S25−S48 of the Supporting Information.
DISCUSSION
■
Structure of [Co(TpivPP)(1-EtIm)(O2)]. The structure of
[Co(TpivPP)(1-EtIm)(O2)] has been independently deter-
mined four times with differing temperatures and thermal
The picket fence dioxygen complexes have crystallographically
imposed 2-fold symmetry leading to disorder of the bent
dioxygen ligand and the trans imidazole ligand. In addition, a
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Figure 2. Top view thermal ellipsoid plots of two 100 K [Co(TpivPP)(1-EtIm)(O )] structures (left, C2/c form; right, P1 form) displaying the atom
̅
2
labeling scheme. The terminal oxygen populations are also indicated. In the 100 K P1 structure (right), the short contacts (dashed lines) and values
̅
between terminal oxygen (O2A and O2B) and picket atoms are shown. Thermal ellipsoids of all atoms in both structures are contoured at the 50%
probability level. Hydrogen atoms have been omitted for clarity.
treatments, but all on the same crystal specimen. As shown in
Table 3, the coordination group geometry parameters do not
vary largely. Most of the differences are likely the consequence
of the disorder issues described below, which do vary widely;
values are given in Table 3. As will be discussed subsequently,
some aspects of the thermal behavior were further studied on a
second crystalline specimen that confirmed the general
behavior.
When a freshly prepared crystal of [Co(TpivPP)(1-EtIm)-
(O2)] is subjected to X-ray structure analysis using what are
now routine, modern-day techniques, i.e., flash cooling the
crystal to low temperature (typically 100 K) and then
commencing the diffraction study, the crystals are observed in
the space group C2/c. This space group leads to crystallo-
graphically imposed 2-fold symmetry and to 2-fold disorder in
both axial ligands. However, unlike prior iron results,20,21 the
terminal oxygen is found only in the pair of positions related by
the molecular 2-fold axis and not in another set of positions
that led to effectively 4-fold disorder. Apparently, the relatively
small differences between the M−O2 bonding of the cobalt
dioxygen species and the analogous iron species lead to a
significant difference in the disorder (dynamics) of the
complexes.
A top down view of the cobalt complex is given in the left
panel of Figure 2; additional thermal ellipsoid plots are given in
Figures S2 and S3 of the Supporting Information. As can be
seen in the left panel, even though the O2 only exhibits 2-fold
disorder, all pickets are disordered, with each having two
distinct orientations of the tert-butyl group; none of the picket
disorder is a consequence of the crystallographic symmetry.
Moreover, although there is no necessary crystallographic
correlation between the orientational disorder of the pickets
and the coordinated O2, calculations suggested that picket
behavior is intrinsically related to the oxygen ligand dynamics.22
A close examination of Figure 2, or better Figure S2, shows
details of the orientations of the tert-butyl groups. The
disordered tert-butyl groups pointing toward the oxygen have
two methyl groups oriented inward to the pocket cavity
whereas the picket at the other site has only one methyl group
inward to the pocket cavity. Each of the independent tert-butyl
groups has two distinct but similar orientations, but the two
pickets show distinct patterns. We will represent the orientation
of the tert-butyl groups with the symbol “<o” for those with the
two methyl groups inward and bracketing the oxygen and those
with the single inward methyl group as “>.”
The two sets of orientations of the two inward methyl groups
(<o) interact almost equally with the terminal oxygen atom,
effectively on each side of the oxygen. The contacts are
O2···C27 = 3.26 Å and O2···C25 = 4.41 Å for the first picket
orientation and O2···C25b = 3.33 Å and O2···C27b = 4.02 Å
for the second picket orientation. There is an equivalent set of
such interactions 180° away, as required by the 2-fold
symmetry. The short O···C distances are very near the van
der Waals radii sum of 3.22 Å.23 The second set of disordered
pickets have a single inward methyl group (>), and their
positions are not directly affected by the presence of the oxygen
ligand. Note that additional details of the picket interactions are
given in the Supporting Information figures as appropriate.
The <o orientation has been commonly observed when the
pocket ligand is not linear, for example, N-coordinated nitrite.
In a number of iron(II) and -(III) nitrite picket fence
derivatives,24 the terminal nitrite oxygen atoms are bracketed
by two methyl groups, but the pickets are completely ordered
with a single orientation. One possible explanation for the two
orientations of the picket bracketing the terminal oxygen of the
coordinated O2 is that any rotation of O2 around the Co−O
bond must be correlated with picket motion. That such an O2
rotation must be occurring in the solid state is shown by the
experiments described below.
The disorder of the pickets, the O2, and even the trans
imidazole ligand is shown to be variable by the structures
obtained at different temperatures and cooling conditions. As
described in Figure 1 and the Experimental Section, when the
crystal was annealed after cooling to 100 K, warming to 188 K,
and then slowly recooling to 100 K, substantial structural
change occurred. The crystallographic space group changed to
P1, leading to no required symmetry on the molecule, and the
̅
molecule itself is almost completely ordered. The four pickets
all have a single orientation; the imidazole has a single
orientation and not the two previously required. The
coordinated oxygen molecule still has two orientations, but
the terminal atom occupancies are decidedly asymmetric with
occupancies of the major (O2a) atom at 0.816 (7) and the
minor (O2b) at 0.184. The two Co−O−O planes are not quite
coplanar. These features are all clearly seen in Figure 2 (right
panel) and Figures S4 and S5 of the Supporting Information.
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The picket orientation follows the same pattern as noted before
(<o toward oxygen and > for the other pair), with comparable
short C···O contacts (for O2a = 3.34 Å and O2b = 3.21 Å). In
both 100 K structures, the Co−O−O planes have a near-
perpendicular conformation with respect to the imidazole
plane, suggesting that this orientation is the most favored one.
Clearly, both the O2 and imidazole groups have rotated around
their coordinating bond during the annealing process. The
rotation around the Co−O bond (in either direction) requires
that the picket methyl groups must also move to lower the
barrier. Thus, the dynamics and energetics of O2 rotation
depend on two distinct molecular features: the barrier to
rotation around the Co−O bond and the energy required for
the necessary movement of the pickets. It is likely that the
picket disorder, which is seen as alternate orientations of the
tert-butyl groups, is the likely “residue” from the Co−O2
rotation. Correlated motion of the pickets and O2 in a geared
fashion are required to avoid impossible atom···atom contacts;
the differing positions of the tert-butyl groups must be nearly
energetically equivalent. However, the energetics of the picket
orientation will depend on whether or not there is an O2 ligand
in proximity.
Additional interesting structural features are observed when
the crystal is warmed to 200 K, which was accomplished
without removal from the diffractometer. The space group
reverts back to C2/c, with required 2-fold symmetry for the
molecule, and the complex is again disordered. Indeed,
additional disorder is observed with four terminal oxygen
positions with the unique occupancy factors of 0.412 (11) and
0.088. The major occupancy Co−O−O planes remain nearly
perpendicular to the imidazole plane. The features are
illustrated in Figures 1, S6, and S7. The pickets associated
with the major occupancy terminal oxygen again display a pair
of <o conformations with the closest O···C contacts of 3.31 and
3.26 Å. Picket conformations associated with the minor
occupancy oxygen atom are more complex, with both a <o
and a > conformation. The nonbonded contact with the
terminal oxygen and the closest carbon of the <o conformation
is 3.14 Å, whereas the apparent O···C contact with the > picket
conformation is an impossible 2.50 Å. It is to be noted that the
apparent thermal motion of the minor occupancy oxygen atom
is much more diffuse than that of the major occupancy oxygen
atom. It is not clear whether this is a real phenomenon or an
artifact of the least-squares refinement. We prefer the first
explanation.
Further warming to 300 K continues the trend to increasing
disorder. Figures 1, S8, and S9 display two views of the
molecule. The terminal oxygen atoms again occupy four sites,
but they now have nearly equal populations of 0.27
(2):0.23:0.27:0.23. The orientation of the pickets is also more
diffuse. The orientations of the pickets for the (marginally)
major orientation O2a are now less clear: one is clearly <o, but
the other orientation verges on >; the close O···C contacts are
3.58 and 3.79 Å for the first picket orientation and 2.70 Å for
the second. The picket orientations associated with the second
oxygen atom (O2b) are both close to the > conformation with
calculated shortest contacts of 2.59 and 2.98 Å. Although the
thermal parameters (or more precisely the anisotropic
displacement parameters) for all atoms have increased over
the temperature range, those of the picket methyl groups have
especially increased. As can be seen in Figure S8, the
appearance of these thermal parameters suggests a near
continuum of varying methyl group positions about the
Ctert‑butyl−Camide bond and effectively appears close to free
rotation at 300 K. The apparent easy movement of the pickets
may be the reason that the oxygen atom occupancies are now
near a statistical 0.25.
We have studied the reproducibility and other variations of
the crystal annealing process on a second, independently
prepared crystal of [Co(TpivPP)(1-EtIm)(O2)]. The sequence
of conditions used to explore this temperature variability is
given in Figure S10. The basic observations that were described
above were found again. Flash cooling gave a crystal in space
group C2/c with, as before, two orientations of the terminal
oxygen atom. Annealing again gave a phase change and space
group P1, with two asymmetrically occupied oxygen positions;
̅
however, disorder in all the pickets and a disordered imidazole
were observed. Further annealing cycles, as shown in Figure
S10, were followed by a final structure determination at 80 K.
This result was nearly identical to the 100 K structure noted
earlier, with ordered pickets and imidazole and asymmetric
oxygen atom occupancies of 0.77(2) and 0.23; the O2
occupancies are within two standard deviations of the first
determination. An illustration of this independently determined
structure is given in Figure S11.
We conclude that, in the absence of external constraints, the
bent Co−O−O group would be equally populated in all four of
the quadrants of the porphyrin core with jumps between each
quadrant.25 The temperature-dependent crystallographic stud-
ies of the picket fence derivatives clearly demonstrate that the
location of the dioxygen ligand is the result of the constraints of
the intramolecular interactions between the dioxygen and
pickets. These constraints must control the oxygen motion and
populations, with the constraints relaxing as the temperature
increases. Although the lowest temperature structure after
annealing is almost completely ordered, it is possible that more
elaborate temperature cycling than we have been able to
employ would lead to a single ordered Co−O2 orientation.
Despite the variation of differing interactions in the ligand
binding pocket in the four structures determined, the complex
displays the same core conformation that indeed is almost
quantitatively identical for all four structures. This is true even
for the pair at 100 K that has undergone the phase change.
Formal diagrams illustrating the atomic displacements, in units
of 0.01 Å, from the mean 24-atom plane of the porphyrin are
given in Figure S12. One interesting feature is that the cobalt
atom is always slightly displaced from the mean plane toward
the imidazole ligand and not the dioxygen ligand. Also shown
are the averaged values of bond distances and angles in each of
the four observed structures along with the value of the
estimated standard deviation, calculated on the assumption that
all averaged values are drawn from the same population.
Values of coordination group parameters, those for
coordinated O2, and picket configuration are summarized in
Table 3. Figure S12 gives values for core distances and angles
and also displays the relative orientation of the axial ligands.
The coordination group values compare well with those from
earlier structure determinations of dioxygen cobalt complexes.
The first X-ray characterized monomeric oxygen-carrying cobalt
complex is the six-coordinate [Co(beacen)(Py)(O2)], reported
at 1972 as a model for oxygenated hemoglobin.26 The Co−O2
distance and Co−O2 angle were determined as 1.86 Å and
126°. Since then a number of cobalt−O2 (1:1) structures have
been published, most of which are also ligated to a Schiff
base.27−32 In these cobalt dioxygen structures, the O2 group is
frequently disordered.29 In the relatively accurate low-temper-
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a
Scheme 1. Formal Diagrams of Structures of [Co(TpivPP)(1-EtIm)(O2)] at Different Temperatures and Space Groups
a
The upper panels are edge-on views of the porphyrin plane and ligands with key parameters (distances in Å, angle in deg) shown in the sequence
O−O, O−O−M, M−O, Δ24 (positive to imidazole ligand side), M−Np, and M−Nax. The lower panels are top views of the porphyrin plane showing
the projections of the dioxygen (red line) and imidazole planes (dashed line) and their angles between the related M−Np vectors. Room temperature
values for [Fe(TpivPP)(1-MeIm)(O2)]20 are also shown.
ature structures by Schaefer et al., the Co−O2 distance and
Co−O2 angle are in the ranges 1.870(6)−1.889(2) Å and
116.4(5)−120.0(2)°.29−32 The dioxygen group is positioned
between the Co−O and Co−N bonds30 or bisects the N−Co−
N angle of the Schiff base ligand.26 Either arrangement allows
for good overlap between π* orbitals on the dioxygen group
and dπ orbitals on the cobalt atom. There is a single oxycobalt
porphyrin structure, also based on a picket fence, but with a
trans thiolate ligand it has limited precision with a 2-fold
disordered oxygen ligand.33 Agreement between the differing
oxycobalt structures is satisfactory.
The nearly complete ordering of the dioxygen ligand allows,
for the first time to our knowledge, the possibility of looking for
subtle asymmetry, caused by the O2 ligand, in the structure of
[Co(TpivPP)(1-EtIm)(O2)]. Such asymmetry (an off-axis tilt
of the diatomic ligand and unequal equatorial bond distances)
had been observed in both five-coordinate34,35 and six-
coordinate iron nitrosyls8,9 and five-coordinate cobalt nitro-
syls.36 An off-axis tilt of 8° in oxyCoMb has been deduced from
EPR measurements,37 but possibly in a different sense than that
seen here. The Co−O2 bond is tilted off the normal to the
porphyrin plane by 2.7°. The two Co−Np bonds in the
direction of the tilt are 1.9752 (20) and 1.9756 (21) Å (average
1.9754 (3) Å), whereas the two Co−Np bonds opposite the tilt
are 1.9784 (20) and 1.9814 (21) Å (average = 1.9802 (18) Å).
The differences, while only marginally statistically significant,
are strongly suggestive, given the clear evidence of the
phenomenon observed in the NO complexes. It is to be
recognized that the similarities of O2 and NO as ligands have
long been noted.38,39
porphyrin core. Bisecting oxygen orientations are found for
both cobalt and iron, but significant ordering of the O2 position
can be observed only in the cobalt systems. It can be noted that
the orientation of the axial ligands shows small systematic shifts
as this cobalt system is warmed (left to right, first four panels).
The favored orientation of the axial O2 ligand appears to
approach that of exactly bisecting the bracketing Co−Np bonds,
whereas the imidazole orientation tends toward eclipsing a
Co−Np bond. The orientation of the major Co−O−O plane is
approximately perpendicular to the imidazole plane with
dihedral angles between 71 and 80°. This favored orientation
may reflect modest π donation from the trans imidazole. The
iron system shows a similar preference for the relative
orientation of the Fe−O−O plane and the imidazole plane.
The bonding between the metal and the dioxygen ligand is
expected to be qualitatively similar in the iron and cobalt
derivatives. With the bent, end on (Pauling model) structure,
2
bonding involves the two π* orbitals on O2 with the dz and
2
one of the metal dπ orbitals. Overlap of dz and one π* orbital
leads to the σ bond, and overlap of the other π* orbital with a
2
metal dπ leads to the M−O2 π bond. The singly occupied dz
orbital in cobalt leads to significant charge donation to O2 and
the formulation of a Co(III)−(O−2 ) superoxide species. This is
clearly supported by the EPR spectra41,42 that indicates that the
unpaired electron resides in a π* antibonding molecular orbital
predominantly localized on dioxygen.43 The possible formula-
tion of the oxyiron complexes as Fe(III)−(O−2 ) has been more
contentious starting from the Pauling/Weiss debate44,45 and
continuing to the present time.43b,46,47
The bond between iron and oxygen is a stronger bond than
that in the cobalt system (M−O2 is 0.15 Å shorter in iron) and
reflects a significantly stronger π bond in the iron systems. This
is clearly shown not only by the distance but by the M−O2
stretching frequencies. The Co−O stretch (ν(Co−O))
observed in both oxyCoMb (538 cm−1) and [Co(TpivPP)(1-
MeIm)(O2)] (516 cm−1) can be compared to ν(Fe−O) in
oxyFeMb (570 cm−1) and [Fe(TpivPP)(1-MeIm)(O2)] (568
cm−1).46,48 Both the lower ν(M−O) and ν(O−O)49 in
The top portion of Scheme 1 pictorially summarizes the
coordination group geometry of [Co(TpivPP)(1-EtIm)(O2)]
determined at differing conditions. The scheme also provides a
comparison of the cobalt oxygen structures with that reported
previously for the analogous iron derivative (determined at 297
K).40 The lower portion of Scheme 1 illustrates the relative
orientations of the imidazole and the bent M−O2 ligand with
respect to the orientation of the M−Np directions of the
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Figure 3. Thermal ellipsoid diagrams of [Co(TpivPP)(2-MeHIm)] and its oxygen adducts, showing reaction conditions and routes. The
temperature and corresponding oxygen occupancy (or coordinate conditions) are given in highlighted boxes. In all species, the axial imidazole is
disordered over two positions related by the required 2-fold symmetry axis, but only one orientation is shown. All thermal ellipsoids are contoured at
the 50% probability level. The reaction transitions shown by arrows give the differing apparent thermal motions for the same crystalline specimen.
The two diagrams for five-coordinate [Co(TpivPP)(2-MeHIm)] represent independent determinations of two distinct crystals. Hydrogens are
omitted for clarity.
oxyCoMb and small molecule analogues compared to iron
suggest that the net negative charge on the coordinated
dioxygen is smaller in the iron species than in the cobalt
species.50 In addition, DFT calculation indicated that the
interaction between the distal histidine and the oxygen ligand in
oxyCoMb and oxyMb is stronger for the cobalt complex than
for the iron one, consistent with larger superoxide ion character
of the bound O2 in cobalt.25 Thus, it can be expected that the
Solid-state oxygenation reactions with single-crystal [Co-
(TpivPP)(2-MeHIm)] were then carried out. The reaction
with ethanol-saturated O2 at 1 atm, conditions similar to the
reaction conditions for the preparation of [Fe(TpivPP)(2-
MeHIm)(O2)],21 gave a crystalline product, but only with a
partial O2 occupancy. The structure determination suggested
that the occupancy of coordinated dioxygen in the product was
about 0.4. This is similar to the solid-state oxygenation of the
B12 derivative cob(II)alamin, which was about 70% oxygenated
under similar conditions.52 These results are consistent with the
known, lower, oxygen affinity of CoMb and the cobalt picket
fence derivative compared to the iron analogues.11,53 A fully
occupied oxygen ligand and a satisfactory crystallographic
refinement for [Co(TpivPP)(2-MeHIm)(O2)] can be obtained
by exposure of the crystals to 5 atm of oxygen for 5 days.
Further information on the dynamics of the system is
provided by warming the crystal to 300 K under an N2 stream
after the 100 K data collection. A significant amount of the
dioxygen is lost as the oxygen occupancy decreases to 0.3 as the
temperature increased. Large thermal motion for the pickets
and the dioxygen ligand is seen for the refined crystal structure.
The large apparent thermal motion of the dioxygen ligands is
almost certainly hiding 4-fold disorder. The large thermal
motion of picket atoms is also probably obscuring significant
disorder of the pickets. The same crystal was then kept in a
pure argon atmosphere with EtOH vapor preventing the loss of
solvent. After several cycles of moderate pump and refill with
solvent saturated argon, the crystal characterization showed that
oxygen was totally removed and the five-coordinate [Co-
(TpivPP)(2-MeHIm)] precursor was recovered. The packing
diagrams of Figure S13 illustrate the observations.
−
higher negative charge on “Co(III)−O2 ” would induce larger
dipole−dipole interactions between the terminal oxygen atom
and the picket amides, and result in less ligand oscillation and
picket orientational disorder. This has been observed in a series
of iron and cobalt oxygen basket handle derivatives where the
the cobalt derivatives are more sensitive to changes in the
basket sizes.51 It is most clearly observed in a porphyrin system
where EPR showed much reduced mobility of the Co−O2
when a single amide group, similar to those in a picket fence
porphyrin, was present.6a Although hydrogen bonding was
suggested, the geometry is much more in line with a dipole−
dipole interaction.6a
In Situ Reversible Oxygenation Reactions of [Co-
(TpivPP)(2-MeHIm)]. We have previously reported the
synthesis and structural characterization of five-coordinate
[Co(TpivPP)(2-MeHIm)],12 the precursor to the six-coor-
dinate oxygen derivative. However, the reaction of [Co-
(TpivPP)(2-MeHIm)] with O2 in benzene in the presence of
a small amount of ethanol unexpectedly gave two different
crystalline forms of a cobalt(III) product and not the desired
dioxygen adduct. The photocatalyzed reaction leads to
[Co(TpivPP)(2-MeHIm)(2-MeIm−)] with the deprotonation
of one imidazole providing charge balance. More surprising is
the atropisomerization of the α,α,α,α isomer to yield
[Co(III)(α,α,β,β-TpivPP)(2-MeHIm)(2-MeIm−)]. This
change in conformation of the porphyrin allows for a hydrogen
bonding chain in the axial ligands.12
There are interesting differences in the pickets in these
crystalline species. The five-coordinate precursor, in two
independent determinations, has completely ordered pickets;
this ordering is retained in the first partially oxygenated species.
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Full oxygenation, however, leads to all pickets now disordered
with all pickets displaying two differing orientations. When part
of the O2 is lost at 300 K, the situation is ambiguous. Large
thermal motion is probably hiding disordered pickets and
almost certainly a 4-fold disordered dioxygen. Complete loss of
the dioxygen leads to a fully ordered five-coordinate derivative.
This can be clearly seen in the thermal ellipsoid plots given in
Figure 3, where the reaction sequences are shown and all
thermal ellipsoids are scaled to the 50% probability level.
Additional thermal ellipsoid plots are given in Figures S14−
S21.
Scheme 2. Formal Diagrams of Structures of Five-
Coordinate [Co(TpivPP)(2-MeHIm)] and Its Oxygen
a
Adduct [Co(TpivPP)(2-MeHIm)(O2)]
Structure of [Co(TpivPP)(2-MeHIm)] and Its Oxygen
Adducts. A summary of structural results for [Co(TpivPP)(2-
MeHIm)] and its oxygen adducts under various conditions is
given in Table 3 (part B). All of these species are low spin S =
1/2. The first entry in part B gives the structural parameters for
the five-coordinate complex before any oxygenation reaction.
The next to final entry gives the structural parameters for a
second five-coordinate crystalline sample after it had been
oxygenated and then deoxygenated. We conclude that [Co-
(TpivPP)(2-MeHIm)] can be oxygenated/deoxygenated with-
out significant change in crystallinity, circumstances that are not
always the case.
a
The left two columns of the scheme are edge-on view of the
porphyrin plane and ligands with the key parameters (distances in Å,
angle in degrees) of O−O−M, M−O, Δ24, M−Npav and M−Nax. The
right column of the scheme are top view of the porphyrin plane
showing the projections of dioxygen (red line) and imidazole plane
(dashed line) and their angles between the closest M−Np vectors.
The structure of the five-coordinate precursor is typical of
previously characterized five-coordinate imidazole-ligated co-
balt porphyrinates.54−57 The sterically hindered 2-methylimi-
dazole ligand had been expected to cause a modest increase in
the value of the axial bond, as had been observed and
discussed55 in [Co(TPP)(1,2-Me2Im)] (observed 2.216 (2)Å).
However, as noted previously,12 in the picket fence derivative,
the axial bond length does not show such an elongation. The
observed distances of 2.145(3) and 2.1593(3) Å fall in the
range observed for sterically nonhindered imidazoles; the out-
of-plane displacement is also within the range previously seen.
The changes in structure upon oxygenation are shown in the
top line of Scheme 2 and in Table 3. The out-of-plane cobalt
displacement decreases but does not decrease to zero; the
cobalt remains on the imidazole side of the porphyrin plane.
The axial Co−N(Im) bond decreases to 2.078 Å, consistent
directly pointing to the terminal oxygen atom as well as the pair
of pickets that are orthogonal to the Co−O2 plane. See Figures
S14−S17 for detailed views. This clearly suggests that the
influence of the oxygen ligand on the pickets is, at least in part,
nondirectional. The interaction must be dipole−dipole in
character, as the distances to the picket amides are beyond that
expected for hydrogen bonding. The O···N distances are in the
range 4.0−4.1 Å, and the O···H(N) distances are in the range
> ∼3.2 Å. Calculations by Rovira and Parrinello22 have
indicated that the pickets provide substantial energetic
stabilization for the binding of O2. However, the energetics of
the varying Co−O−O orientations and the picket orientations
must be very similar.
Scheme 2 also illustrates the probable effect on the
experimentally derived structural parameters when the
crystalline sample is a mixture of the six-coordinate species
and the five-coordinate precursor. The apparent metal ion
displacement is between the two limiting values, and the
apparent values of both axial bond distances are longer than the
true values. The effects are substantial, and the observations for
the cobalt system can be expected to be the general result.
Comparison of R- and T-State Oxy Models. The
movement of iron toward the porphyrin plane upon oxygen-
ation, along with the motion of proximal histidine, has been
cited as the driving force in the mechanism of cooperative
oxygen binding in the human hemoglobin.59,60 The movements
of iron and the proximal histidine lead to changes in the protein
quaternary structure of hemoglobin that lead to the
cooperativity of oxygen binding. Cooperativity is retained, but
diminished, in human hemoglobin when the iron is replaced
with cobalt; the Hill coefficient decreases from 2.8 to 2.2.61 The
difference is attributed to the fact that the Co out-of-plane
displacement and subsequent movement upon oxygenation will
be much smaller than that for iron, since cobalt does not
−
with expectations of a Co(III)−(O2 ) formulation, but it does
not decrease to the 2.02−2.03 Å value seen in the 1-EtIm
derivatives (Table 3, part A). The values of the Co−O and
other O2 parameters are seen to be very similar to those
observed for the 1-EtIm derivative and, as discussed previously,
other oxycobalt complexes as well. The limiting structures of
the 1-EtIm and 2-MeHIm complexes both show one Co−O2
orientation which is aligned nearly perpendicular to the
imidazole plane. The differences in structure between [Co-
(TpivPP)(2-MeHIm)] and its oxy derivative are very similar to
the differences found for deoxyCoMb and its oxy form.58 Both
the differences in the out-of-plane displacement and the change
in the axial bond distance are slightly smaller in the picket fence
derivatives.
The second line of Scheme 2 gives the structural parameters
for the oxygenation product when the oxygenation was carried
out at 1 atm of O2 and only partial oxygenation resulted. Two
significant features resulted. At this level of oxygenation, the 2-
fold axis of symmetry remains and requires two positions for
the terminal oxygen and two orientations of the axial 2-MeHIm
ligand. Moreover, the relative orientations of the axial ligands
are the same as in the fully oxygenated derivative. However, the
pickets remain completely ordered. Further oxygenation does
lead to disorder in all pickets. These include the pickets that are
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Journal of the American Chemical Society
Article
undergo a spin state change.62 However, it is essential to note
that the binding of dioxygen does lead to significant difference
changes in model R- and T-state cobalt systems on the change
from a five-coordinate to a six-coordinate species. In both
[Co(TpivPP)(1-EtIm)(O2)] and [Co(TpivPP)(2-MeHIm)-
(O2)] the change in the out-of-plane displacement of cobalt
is about 0.08 Å, with the cobalt remaining further out-of-plane
in the 2-MeHIm system. But, the change in the axial Co−
N(Im) bond distance is almost twice as large in that in
[Co(TpivPP)(1-EtIm)(O2)]. The total change leads to half
again as much motion of the axial imidazole relative to the
porphyrin plane in this R-state model (0.23 Å change in the R-
state system vs 0.16 Å in the T-state system).
(3) Abbreviations: Np, porphyrinato nitrogen; NAx, nitrogen of axial
ligands; M, center metal atom; Nim, nitrogen of imidazole ligands; Δ24,
displacement of metal atom from the 24-atom mean plane; EPR,
electron paramagnetic resonance; Mb, myoglobin; TpivPP, dianion of
α,α,α,α-tetrakis(o-pivalamidophenyl)porphyrin; 1-MeIm, 1-methylimi-
dazole; 1-EtIm, 1-ethylimidazole; 2-MeHIm, 2-methylimidazole; R-Im,
generalized imidazole; Py, pyridine; Mb, myoglobin; Hb, hemoglobin;
CoMb, cobalt substituted myoglobin; CoHb, cobalt substituted
hemoglobin.
(4) Spartalian, K.; Lang, G.; Collman, J. P.; Gagne, R. R.; Reed, C. A.
J. Chem. Phys. 1975, 63, 5375.
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Manuscripts in preparation. Structural data for three [Fe(TpivPP)(R-
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temperatures between 100 and 300 K have been obtained. At 100 K,
all structures are in C2/c and show four-fold positional disorder at the
SUMMARY
■
Two different O2 complexes of picket fence cobalt porphyrin,
[Co(TpivPP)(R-Im)(O2)], where R-Im is either 1-ethyl-
imidazole or the sterically hindered 2-methylimidazole, have
been synthesized. The solid-state structures of the two species
have been determined at a number of temperatures. Under
most conditions, both axial ligands and the pickets of the
porphyrin are disordered. However, crystal annealing leads to
an unusual reversible phase change for [Co(TpivPP)(1-
EtIm)(O2)] that provides an ordered structure and excellent
structural parameters. The studies have provided distinct
information on the interactions between the coordinated O2
and pickets that lead to apparent mutual disorder in the solid
state. The disordered pickets apparently result from the
movement required to allow rotation of the O2 ligand about
the Co−O bond. The results also reveal the subtle effects on
structural parameters from an incomplete oxygenation reaction
in the single crystal.
terminal oxygen (O2). Systematically temperature-dependent Moss-
̈
bauer spectroscopy has been conducted on [Fe(TpivPP)(R-Im)(O2)]
(R-Im = 1-MeIm, 1-EtIm, and 2-MeHIm).
(11) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.;
Hayes, S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761.
(12) Li, J.; Noll, B. C.; Oliver, A. G.; Ferraudi, G.; Lappin, A. G.;
Scheidt, W. R. Inorg. Chem. 2010, 49, 2398.
(13) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
2
(14) R1 = ∑∥Fo| − | Fc∥/∑|Fo| and wR2 = {∑[w(Fo − Fc2)2]/
ASSOCIATED CONTENT
* Supporting Information
4
∑[wFo ]}1/2. The conventional R-factors R1 are based on F, with F set
■
to zero for negative F2. The criterion of F2 > 2σ(F2) was used only for
calculating R1. R-factors based on F2 (wR2) are statistically about twice
as large as those based on F, and R-factors based on ALL data will be
even larger.
S
Line drawing of [Co(TpivPP)(1-EtIm)(O2)], thermal ellipsoid
plots and mean plane diagrams for all structures, brief
description of synthesis of PF porphyrin, complete structural
details for [Co(TpivPP)(1-EtIm)(O2)], complete structural
details for [Co(TpivPP)(2-MeHIm)(O2)], complete structural
details for [Co(TpivPP)(2-MeHIm)], and crystallographic
information files (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(15) Sheldrick, G. M. Program for Empirical Absorption Correction of
Area Detector Data; Universitat Gottingen: Germany, 1996.
̈
̈
(16) Sheldrick, G. M. twinabs; Universitat Gottingen: Germany,
2005.
̈
̈
(17) (a) Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.;
Shields, G. P.; Taylor, R.; Towler, M.; van de Streek, J. J. Appl.
Crystallogr. 2006, 39, 453. (b) Bruno, I. J.; Cole, J. C.; Edgington, P.
R.; Kessler, M. K.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R.
Acta Crystallogr. 2002, B58, 389. (c) Taylor, R.; Macrae, C. F. Acta
Crystallogr. 2001, B57, 815.
(18) Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schulz, C.
E.; Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675.
(19) Kitazawa, T.; Nishikiori, S.-I.; Kuroda, R.; Iwamoto, T. J. Chem.
Soc., Dalton Trans. 1994, 1029.
AUTHOR INFORMATION
Corresponding Author
Scheidt.1@nd.edu
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We thank the National Institutes of Health for support of this
research under Grant GM-38401 to W.R.S.
■
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