Oxygenation of cobalt Porphyrinates: Coordination or oxidation?

Article abstract of DOI:10.1021/ic902309f

The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHlm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N_lm = 2.145(3) and (Co-Np)_av = 1.979(3) . This five-coordinate complex, in the presence of dioxygen and excess 2methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(ll), and the formation of the neutral cobalt(lll) complex [Co(α,α,β,β-TpivPP)(2-MeHlm)(2-MelirT]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N_Im distances ranging from 1.952 to 1.972 , but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2MeHIm)(O₂)] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies. 2010 American Chemical Society.

Full text of DOI:10.1021/ic902309f

2398 Inorg. Chem. 2010, 49, 2398–2406
DOI: 10.1021/ic902309f
Oxygenation of Cobalt Porphyrinates: Coordination or Oxidation?
Jianfeng Li, Bruce C. Noll, Allen G. Oliver, Guillermo Ferraudi, A. Graham Lappin,* and W. Robert Scheidt*
The Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
Received November 21, 2009
The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is
˚
reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-NIm = 2.145(3)
˚
and (Co-Np)av = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-
methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of
cobalt(II), and the formation of the neutral cobalt(III) complex [Co(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-]. Two
distinct examples of this complex have been structurally characterized, and both have structural parameters consistent
˚
with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-NIm distances ranging from 1.952 to 1.972 A,
but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding
network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-
MeHIm)(O2)] is an intermediate state for the oxidation reaction and that the atropisomerization process is
photocatalyzed. A reaction route is proposed based on the spectroscopic studies.
Introduction
reactions required for understanding the O2 interactions with
cobalt porphyrinates, [Co(Por)(L)],5 are shown below.3
The interaction of cobalt complexes with dioxygen to form
species with coordinated dioxygen have beenknown for some
time; both reversible and irreversible dioxygen binding have
been reported.1,2 Similarly, five-coordinate cobalt(II) por-
phyrinates show reversible binding of dioxygen.3 Cobalt
myoglobin and cobalt hemoglobin, “coboglobin”, in which
the heme prosthetic group (iron protoporphyrin IX) is
replaced by cobalt protoporphyrin IX display reversible
oxygen binding similar to those of their native iron counter-
parts.4 These species have been the subject of interest for
many years in regard to the nature of the bond between
cobalt and oxygen in the monomeric oxygen adducts.2 The
K1
CoðPorÞ þ L
s
½CoðPorÞðLÞꢀ
ð1Þ
F
s
R
K2
½CoðPorÞðLÞꢀ þ L
s
½CoðPorÞðLÞ ꢀ
ð2Þ
ð3Þ
F
s
R
2
½CoðPorÞðLÞꢀ þ O2h½CoðPorÞðLÞðO2Þꢀ
½CoðPorÞðLÞꢀ þ ½CoðPorÞðLÞðO2Þꢀh
½ðLÞðPorÞCoðO2ÞCoðPorÞðLÞꢀ
ð4Þ
*To whom correspondence should be addressed. E-mail, scheidt.1@
nd.edu (W.R.S.), alexander.g.lappin.1@nd.edu (A.G.L.). Fax: (574) 631-
6652.
(1) Niederhoffer, E. C.; Timmons, J. H.; Martell, A. E. Chem. Rev. 1984,
84, 137.
(2) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139.
(3) (a) Walker, F. A. J. Am. Chem. Soc. 1970, 92, 4235. (b) Walker, F. A.
J. Am. Chem. Soc. 1973, 95, 1154.
(4) (a) Hoffman, B. M.; Petering, D. H. Proc. Natl. Acad. Sci. U.S.A.
1970, 67, 637. (b) Chien, J. C. W.; Dickinson, L. C. Proc. Natl. Acad. Sci. U.S.A.
1972, 69, 2783.
(5) Abbreviations: Por, generalized porphyrin dianion; Np, porphyrinato
nitrogen; NAx, nitrogen of axial ligands; Nim, nitrogen of imidazole ligands;
Lsixth, the sixth ligand of the porphyrin complex; Δ24: displacement of metal
atom from the 24-atom mean plane; EPR, electron paramagnetic resonance.
Mb, Myoglobin; TpivPP, dianion of R,R,R,R-tetrakis(o-pivalamidophenyl)-
porphyrin; TpivPP0, dianion of R,R,β,β-tetrakis(o- pivalamidophenyl)-
porphyrin; RIm, generalized imidazole; 1-MeIm, 1-methylimidazole; 1-EtIm,
1-ethylimidazole; 2-MeHIm, 2-methylimidazole; 2-MeIm-, 2-methyl-
imidazolate.
Some features of these reactions are noteworthy. First, the
binding of nitrogen ligands (L) to form six-coordinate
[Co(II)(Por)(L)2] is extremely unfavorable at room tempera-
ture (reaction 2), which is the result of the destabilization of
the six-coordinate complex by the singly populated dz2
orbital.6,7 For example, in toluene this binding takes place
only at very high ligand concentrations (with piperidine, a
∼105-fold ligand excess is required).6,7 It is perhaps not
surprising that no bis(imidazole)-ligated Co(II) porphyrinate
has ever been isolated. Second, a number of Co(II) porphyrins
(6) Scheidt, W. R. J. Am. Chem. Soc. 1974, 96, 84.
(7) (a) Stynes, D. V.; Stynes, H. C.; James, B. R.; Ibers, J. A. J. Am. Chem.
Soc. 1973, 95, 1796. (b) Stynes, H. C.; Ibers, J. A. J. Am. Chem. Soc. 1972, 94,
1559.
r
pubs.acs.org/IC
Published on Web 01/27/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2399
can form 1:1 mononuclear oxygen adducts that are generally
reversible (reaction 3).3,8 Third, both reversible and irrever-
sible reactions have been reported for forming the 1:2 peroxo-
bridged binuclear complexes [(L)(Por)Co(O2)Co(Por)(L)]
(reaction 4). For example, in a carefully purified toluene
solution the decreased EPR signal due to formation of
[(L)(Por)Co(O2)Co(Por)(L)] could be restored by heating
or pumping off dioxygen to decompose the diamagnetic
binuclear products.3 However, under some conditions an
irreversible reaction takes place. Yamamoto and co-workers
concluded that oxygen-bridged complexes were formed in an
irreversible manner from cobalt tetraphenylporphyrin when
L was imidazole or benzimidazole after oxygenation at room
temperature; reactions in the presence of other amines were
reversible.9
Schlenkware, and cannula techniques. Benzene, tetrahydrofur-
an (THF), and heptane were distilled over sodium/benzophe-
none and ethanol over magnesium. Research grade oxygen
(99.999%) was purchased from PRAXAIR and used as re-
ceived. [H2(TpivPP)] and [Co(TpivPP)] were prepared accord-
ing to a local modification of the reported syntheses.8b,13
UV-vis spectra were recorded on a Perkin-Elmer Lambda 19
UV/vis/near-IR spectrometer.
Synthesis of [Co(II)(TpivPP)(2-MeHIm)]. [Co(II)(TpivPP)]
(10.1 mg, 9.5 mmol) was dried in vacuum for 30 min and
dissolved in 2.5 mL benzene to give a clear dark red solution.
This solution was added to 2-methylimidazole (4.1 mg, 49.9
mmol) and 8 drops of ethanol by cannula. The mixture was
stirred for 20 min and transferred into 8 mm ꢁ 250 mm glass
tubes which were layered with heptane as nonsolvent. The tubes
were kept in a 5 °C refrigerator, and X-ray quality crystals were
obtained after 2 weeks.
It is also important to note that the oxygen reduction
catalyzed by porphyrins has been of great interest in fields as
diverse as biology, photosynthesis, and electrocatalysis.10
For instance, cobalt porphyrin complexes have been shown
to catalyze the four-electron reduction of O2 to H2O (reaction 5)
when adsorbed on graphite electrodes,11 and to mainly
catalyze the two-electron reduction of O2 to yield hydrogen
peroxide (H2O2) (reaction 6) when deposited onto gold
electrodes.12
[Co(III)(TpivPP0)(2-MeHIm)(2-MeIm-)]. [Co(II)(TpivPP)]
(8.6 mg, 8.1 mmol) was dried in vacuum for 30 min and dissolved
in 2 mL benzene. This solution was transferred to 2-methylimi-
dazole (9.7 mg, 118.2 mmol) and 8 drops of ethanol by cannula.
Oxygen was then bubbled into the mixture for 3 min. X-ray
quality crystals were obtained in 8 mm ꢁ 250 mm sealed glass
tubes by liquid diffusion using heptane as nonsolvent. Two
crystalline forms have been isolated that crystallize in the
triclinic or monoclinic crystal systems. The two differ only in
the benzene solvent content.
Equilibrium Constant Determinations. UV-vis spectra were
recorded on a Perkin-Elmer Lambda 19 UV/vis/near-IR spec-
trometer in a specially designed combined 1 and 10 mm inert
atmosphere cell. A solution of the four-coordinate Co(II)
porphyrin was prepared using benzene as solvent. The ligand
solution was prepared by dissolving 2-methylimidazole in
EtOH, and the concentration for UV-vis measurements is
0.06 M. The 2-MeHIm solution was titrated into [Co(II)-
(TpivPP)] solution. UV-vis spectra of [Co(II)(TpivPP)] in
different concentrations of 2-MeHIm were measured. The
equilibrium constant for the equilibria was calculated with the
nonlinear least-squares program SQUAD.14 SQUAD calculates
best values for the equilibrium constants by employing multiple-
wavelength absorption data for varying concentrations of the
reactants. Absorbance data, at 10 nm increments spanning the
visible region, were input into SQUAD. The calculated results
O2 þ 4Hþ þ 4e- f 2H2O
ð5Þ
O2 þ 2Hþ þ 2e- f H2O2
ð6Þ
As part of our study of the vibrational and dynamical
characterization of dioxygen complexes of cobalt and iron
porphyrinates we report in this paper the reactions of cobalt
picket fence porphyrin with 2-methylimidazole and dioxy-
gen. Although the reaction with 2-methylimidazole and the
four-coordinate cobalt derivative proceeded as expected to
yield the five-coordinate species, the further reactions of this
product and dioxygen led to interesting new six-coordinate
cobalt(III) products. We report the molecular structures of
both the five-coordinate complex, [Co(TpivPP)(2-MeHIm)]
and the six-coordinate species, [Co(TpivPP0)(2-MeHIm)(2-
MeIm-)]. Interestingly, the picket fence porphyrin is atropi-
somerized during the oxidation reaction to produce new six-
coordinate species. The atropisomerization of the porphyrin
ligand occurs even though all reactions were carried out at
ambient temperature. The atropisomerization process ap-
pears to be photocatalyzed, and we have investigated the
reaction via flash photolysis experiments.
are K1 = 3.5 ( 0.5 ꢁ 104 M-1
.
K1
CoðTpivPPÞ þ 2-MeHIm
X-ray Structure Determinations. Single-crystal experiments
s
CoðTpivPPÞð2-MeHImÞ ð7Þ
F
s
R
were carried out on a Bruker Apex system with graphite-
˚
monochromated Mo KR radiation (λ = 0.71073 A). The
structures were solved by direct methods using SHELXS-9715
and refined against F2 using SHELXL-97;16,17 subsequent dif-
ference Fourier syntheses led to the location of most of the
remaining nonhydrogen atoms. For the structure refinement all
data were used including negative intensities. All nonhydrogen
atoms were refined anisotropically if not remarked otherwise
below. Except as noted below, hydrogen atoms were idealized
Experimental Section
General Information. All reactions and manipulations were
carried out under argon using a double-manifold vacuum line,
(8) (a) Collman, J. P.; Gagne, R. R.; Kouba, J.; Ljusberg-Wahren, H.
J. Am. Chem. Soc. 1974, 96, 6800. (b) 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. (c) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.; Hayes,
S. E.; Suslick, K. S. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 564.
(9) Yamamoto, K.; Kwan, T. J. Catal. 1970, 18, 354.
(10) Boulatov, R. In N4-Macrocyclic Metal Complexes; Zagal, J. H.,
Bedioui, F., Dodelet, J.-P., Eds.; Springer: New York, 2006; Vol. 1, pp 1-36.
(11) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,
4294.
(13) Collman, J. P.; Gagne, R. R.; Halbert, T. R.; Lang, G.; Robinson,
W. T. J. Am. Chem. Soc. 1975, 97, 1427.
(14) Leggett, D. J. In Computational Methods for the Determination of
Formation Constants; Leggett, D. J., Ed.; Plenum: New York, 1985; Chapter 6.
(15) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(16) Sheldrick, G. M.Program for the Refinement of Crystal Structures;
€
€
€
Universitat Gottingen: Gottingen, Germany, 1997.
P
P
P
P
(17) R1 = ||Fo| - |Fc||/ |Fo| and wR2 = { w(Fo2 - Fc2)2/ wFo
} .
4
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.
(12) Yoshimoto, S.; Inukai, J.; Tada, A.; Abe, T.; Morimoto, T.; Osuka,
A.; Furuta, H.; Itaya, K. J. Phys. Chem. B 2004, 108, 1948.

2400 Inorganic Chemistry, Vol. 49, No. 5, 2010
Li et al.
Table 1. Complete Crystallographic Details for [Co(TpivPP)(2-MeHIm)] 2C2H5OH, [Co(R,R,β,β-TpivPP)(2-MeHIm), (2-MeIm-)](monoclinic), and [Co(R,R,β,β-
3
TpivPP)(2-MeHIm)(2-MeIm-)](triclinic)
[Co(R,R,β,β-TpivPP)(2-MeHIm)
[Co(R,R,β,β-TpivPP)(2-MeHIm)
[Co(TpivPP) (2-MeHIm)] 2C2H5OH
(2-MeIm-)] (monoclinic)
(2-MeIm-)](triclinic)
3
chemical formula
FW
C72H82CoN10O6
1242.41
18.5616(7)
19.6540(7)
17.8515(6)
90
90.834(1)
90
6511.7(4)
C2/c
C87.9H90.9CoN12O4
1438.36
24.4864(15)
14.8242(9)
21.1191(11)
90
104.159(2)
90
7433.2(7)
P21/c
C79.05H82.05CoN12O4
1323.15
˚
a, A
13.6800(5)
21.1364(7)
26.2946(9)
95.239(2)
101.426(2)
102.866(1)
7191.9(4)
P1
4
100
1.222
0.297
˚
b, A
˚
c, A
R, deg
β deg
γ, deg
3
˚
V, A
space group
Z
temp, K
4
100
1.267
0.324
4
100
1.285
0.293
Dcalcd, g cm-3
μ, mm-1
final R indices
[I > 2σ(I)]
final R indices
(all data)
R1 = 0.0459
wR2 = 0.1292
R1= 0.0593
wR2 = 0.1400
R1 = 0.0769
wR2 = 0.1968
R1 = 0.1015
wR2 = 0.2146
R1 = 0.0807
wR2 = 0.2282
R1 = 0.1192
wR2 = 0.2597
with the standard SHELXL-97 idealization methods. The pro-
gram SADABS18 was used to apply an absorption correction. A
brief summary of the crystallographic data is given in Table 1.
Complete crystallographic details, atomic coordinates, aniso-
tropic thermal parameters, and fixed hydrogen atom coordi-
nates are given in the Supporting Information for the three
structures.
of the “pickets”, the pivalamido methyl carbon atoms are found
to disorder into two sets of positions. An ideal tetrahedral
group19 is applied as rigid group to constrain each tert-butyl
group. The occupancies of two pairs of tert-butyl groups are
refined to be 0.732(9) and 0.268(9). The remaining three “pick-
ets” are ordered. The second porphyrin complex (Co2) is more
disordered. For the first “picket”, the whole pivalamido group
and the attached phenyl group are disordered into two sets of
positions. For the second “picket”, both the pivalamido methyl
carbons and the carbonyl group are disordered into two sets of
positions. For the third “picket”, the pivalamido methyl car-
bons are disordered into two sets of positions. For each pair of
these disordered moieties, their occupancy factors are refined by
means of “free variables”. The fourth “picket” is ordered. The
two axial ligands (2-methylimidazole and 2-methylimidazolate)
on each of the porphyrin complex are found to be completely
ordered. The hydrogen atoms are located and refined in two
steps. In the first step, all the hydrogen atoms, except those on
the five-membered imidazole (or imidazolate) ring of the four
axial ligands, are located by calculations. In the second step, the
hydrogen atoms on the five-membered imidazole (or imi-
dazolate) ring of the four axial ligands are carefully found from
the Fourier maps. It was found that all the hydrogen atoms were
located except those for N16 and N28. Thus, the axial ligands of
N15-C1-N16-C2-C3-C4 and N27-C13-N28-C14-C15
-C16 are determined as the 2-methylimidazolate ligands. To
verify the hydrogen atom assignment of the imidazole and
imidazolate ligands, these hydrogen atoms were refined in the
least-squares processes allowing both coordinates and isotropic
temperature factors to vary.
[Co(TpivPP)(2-MeIm)] 2C2H5OH. A black crystal with the
3
dimensions 0.50 ꢁ 0.22 ꢁ 0.21 mm3 was used for the structure
determination. In this structure, the asymmetric unit contains a
half 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.
[Co(TpivPP0)(2-MeHIm)(2-MeIm-)](monoclinic).
A black
crystal with the dimensions 0.49 ꢁ 0.20 ꢁ 0.12 mm3 was used
for the structure determination. The asymmetric unit contains
one six-coordinate picket fence porphyrin complex and 2.65
benzene solvent molecules. The two axial ligands are found to be
completely ordered. The hydrogen atoms were located and
refined in two steps. In the first step all the hydrogen atoms,
except those on the five-membered imidazole (or imidazolate)
ring of the two axial ligands, are located either in the Fourier
maps or by calculation. In the second step, the hydrogen atoms
on the five-membered imidazole (or imidazolate) ring of the two
axial ligands are carefully located from the Fourier maps.
Hydrogen atoms for all imidazole or imidazolate ligands were
located except that for N8. Thus the axial ligand of N7-
C5-N8-C6-C7-C8 is determined as the 2-methylimidazolate
ligand. To verify the hydrogen atom assignment of the imidazole
and imidazolate ligands, these hydrogen atoms were refined in
the least-squares processes allowing both coordinates and iso-
tropic temperature factors to vary. Refined C-H distances
Photochemical Procedures. Absorbance changes, ΔA, occur-
ring in a time scale longer than 10 ns were investigated at room
temperature with a flash photolysis apparatus described else-
where.20 In these experiments, 10 ns flashes of 351 nm light were
generated with a Lambda Physik SLL-200 excimer laser. The
energy of the laser flash was attenuated to values equal to or less
than 20 mJ/pulse. A right angle configuration was used for the
pump and the probe beams. Concentrations of the chromo-
phores in the solution were adjusted to provide homogeneous
concentrations of photogenerated intermediates over the 1 cm
optical path of the probe beam. Both carefully degassed solu-
tions and solutions saturated with O2 or air were used in the
photochemical experiments.
˚
range from 0.84 to 1.13 A, N-H distances range from 0.73 to
0.94, and temperature factors for all hydrogen atoms range from
0.02 to 0.09.
[Co(TpivPP0)(2-MeHIm)(2-MeIm-)](triclinic). A black crys-
tal with the dimensions 0.40 ꢁ 0.32 ꢁ 0.12 mm3 was used for the
structure determination. The asymmetric unit contains two six-
coordinate picket fence porphyrin complexes, and 2.35 benzene
solvent molecules. In the first porphyrin complex (Co1), on one
(18) Sheldrick, G. M.: Program for empirical Absorption Correction of
€
€
€
Area Detector Data. Universitat Gottingen: Gottingen, Germany, 1996.
(19) Kitazawa, T.; Nishikiori, S.-I.; Kuroda, R.; Iwamoto, T. J. Chem.
Soc., Dalton Trans. 1994, 1029.
(20) Guerrero, J.; Piro, O. E.; Wolcan, E.; Feliz, M. R.; Ferraudi, G.;
Moya, S. A. Organometallics 2001, 20, 2842.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2401
Table 2. Selected Structural Parameters for Five-Coordinate [Co(II)(Por) (RIm)]
and Related Iron(II) Speciesa
b
Δ24
c
d
φim
complex
(M-Np)av M-Nim
ref
Co(II) Complexes
0.15 1.979(3)
[Co(II) (TpivPP)
(2-MeHIm)]
[Co(II) (TPP)
(1,2-Me2Im)]
[Co(II)(TPP)-
(1-MeIm)]
[Co(II)(OEP)-
(1-MeIm)]
[Co(II)(OC3OP)-
(1-MeIm)]
2.145(3) 21.6 tw
2.216(2) 20.0 23
0.18 1.985(3)
0.13 1.977(6)
0.16 1.96(1)
0.13 1.985(6)
2.157(3) 4.1
2.15(1) 10
24
25
2.132(3) 15.6 26
Figure 1. Thermal ellipsoid plot of [Co(II)(TpivPP)(2-MeHIm)] dis-
playing a partial atom labeling scheme. The molecule has a required 2-
foldaxis ofsymmetryperpendiculartothe porphyrin plane. Thustheaxial
imidazole is disordered over two positions related by the required
symmetry axis, but only one orientation is shown. Thermal ellipsoids of
all atoms are contoured at the 50% probability level. Hydrogen atoms
have been omitted for clarity.
Fe(II) Complexes
0.43 2.072(6)
[Fe(II) (TpivPP)
(2-MeHIm)]
[Fe(II) (TPP)
(1,2-Me2Im)]
[Fe(II) (TTP)
(2-MeHIm)]
[Fe(II)(Tp-OCH3PP)
(2-MeHIm)]
[Fe(II) (Tp-OCH3PP)
(1,2-Me2Im)]
2.095(6) 22.8 27
2.158(2) 20.9 28
2.144(1) 35.8 28
2.155(2) 44.5 28
2.137(4) 20.7 28
2.127(3) 24.0 29
0.42 2.079(8)
0.39 2.076(3)
0.51 2.087(7)
0.38 2.077(6)
0.38 2.073(9)
The reaction kinetics was investigated by following the
absorbance change at given wavelengths of the spectrum and
incorporating those changes in
21
F ¼ ðΔAinf -ΔAtÞ=ðΔAinf -ΔA0Þ
[Fe(II)(TPP)(2-MeHIm)]
1.5C6H5Cl
3
In the expression of the dimensionless parameter F, ΔA0 is the
absorbance change at the beginning of the reaction, ΔAt is
determined at an instant t of the reaction and ΔAinf is deter-
mined at the end of the reaction. Values of F were fitted to the
integrated rate law by a nonlinear least-squares method.
[Fe(II)(OEP)(1,2-Me2Im)] 0.45 2.080(6)
[Fe(II)(OEP) (2-MeHIm)] 0.46 2.077(7))
2.171(3) 10.5 30
2.135(3) 19.5 30
a
Distances in A, angles in deg, tw = this work. b Displacement of the
metal atom from the 24-atom mean plane; a positive number indicates a
displacement toward the imidazole ligand. c Bond distance between iron
atom and imidazole nitrogen atom. d Dihedral angle between the planes
defined by the closest Np-M-Nim and the imidazole plane.
˚
Results and Discussion
Structures. We have been investigating the vibrational
spectroscopy of dioxygen complexes of iron porphyri-
nates with the technique of nuclear resonance vibration
spectroscopy (NRVS).22 Interesting crystallographic dif-
ficulties for several six-coordinate dioxygen complexes of
iron in those studies have led us to the examination of the
interaction of dioxygen with cobalt(II) porphyrinates.
One of the systems we have studied is that of the cobalt(II)
picket fence porphyrin derivative with the hindered imi-
dazole 2-methylimidazole. The use of hindered imida-
zoles is a necessary condition to prepare five-coordinate
iron(II) species, but this is not apparently necessary for
cobalt(II) species. Indeed, to our knowledge only five-
coordinate derivatives result from the reaction of four-
coordinate cobalt(II) porphyrin derivatives with any
imidazole derivative. Thus the preparation, crystallization,
andstructuredeterminationof [Co(II)(TpivPP)(2-MeHIm)]
proceeded smoothly; the structure is displayed in Figure 1.
The structural parameters of [Co(II)(TpivPP)(2-
MeHIm)] can be compared with those previously ob-
served on five-coordinate imidazole-ligated cobalt(II)
and iron(II) derivatives given in Table 2. The significant
differences between the cobalt(II) and iron(II) derivatives
are the consequence of the differing spin states. All
cobalt(II) derivatives have a low-spin d electron config-
uration of (dxy,dxz,dyz)6(dz2)1 whereas iron(II) derivatives
have a high-spin d electron configuration of (dxz)2(dyz,
dxy)2(dz2)1(dx2-y2)1. These electronic state properties lead
to the large differences in the metal out-of-plane displace-
ments and the much larger in-plane M-Np bond distance
values for iron.
The structural parameters of [Co(II)(TpivPP)(2-
MeHIm)] have very similar values to those of the other
cobalt(II) derivatives given in Table 2. The axial Co-NIm
(21) Frost, A. A.; Pearson, R. G. Kinetics and Mechanism; John Wiley &
Sons: New York, 1953; Chapter 3, pp 28-31.
(22) An introduction to NRVS is given in: Scheidt, W. R.; Durbin, S. M.;
Sage, J. T. J. Inorg. Biochem. 2005, 99, 60.
(23) Dwyer, P. N.; Madura, P.; Scheidt, W. R. J. Am. Chem. Soc. 1974, 96,
4815.
˚
bond distance of 2.145 (3) A to the hindered imidazole is
similar to those found in the unhindered derivatives,
rather than somewhat larger owing to the increased steric
˚
hindrance (cf. the 2.216-A value found for [Co(TPP)(1,2-
(24) Scheidt, W. R. J. Am. Chem. Soc. 1974, 96, 90.
(25) Little, R. G.; Ibers, J. A. J. Am. Chem. Soc. 1974, 96, 4452.
(26) Jene, P. G.; Ibers, J. A. Inorg. Chem. 2000, 39, 5796.
(27) Jameson, G. B.; Molinaro, F. S.; Ibers, J. A.; Collman, J. P.;
Brauman, J. I.; Rose, E.; Suslick, K. S. J. Am. Chem. Soc. 1980, 102, 3224.
(28) 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.
(29) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2002, 41,
2173.
Me2Im)]). Hoard and Scheidt had suggested that the axial
Co-Nim bond in the five-coordinate porphinato com-
plexes should be readily stretched by reason of the pre-
sence of the odd electron in the dz2 orbital of the metal
atom.31 However, we note that a similar, shorter than
(30) Hu, C.; An, J.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem.
2006, 45, 4177.
(31) Hoard, J. L.; Scheidt, W. R. Proc. Natl. Acad. Sci. U.S.A. 1973, 70,
3919.

2402 Inorganic Chemistry, Vol. 49, No. 5, 2010
Li et al.
picket fence porphyrin have atropisomerized from the R,
R,R,R conformation (all up) to an R,R,β,β conformation.
This atropisomerization has occurred even though the
reaction solutions were never heated above ambient
temperature. Illustrations of the other two characterized
cobalt centers in the triclinic crystalline form are given as
Supporting Information, Figures S2 and S3. These cen-
ters have also undergone atropisomerization to the R,
R,β,β conformer.
The other striking feature of all three cobalt centers is
the six-coordinate structure with two imidazole ligands.
The two axial Co-N bonds at all three centers are both
˚
very short at 1.95-1.97 A. As shown in Table 3, such
short bonds are inconsistent with the cobalt center having
a þ2 oxidation state, but are consistent with a þ3 oxida-
tion state. We can thus conclude the cobalt must have
been oxidized upon oxygenation. Other structural fea-
tures are also consistent with oxidation of the cobalt
centers. The coordination of two imidazole ligands to a
cobalt(II) center would be unprecedented. As is clearly
evident in the illustration of Figure 2, the porphyrin core
is severely ruffled. This severe ruffling, quantitatively
illustrated in Supporting Information, Figures S4 and
S5, leads to very short equatorial bonds. The two new
Co(III) structures have the shortest (Co-Np)av distances
Figure 2. Thermal ellipsoid diagram of [Co(R,R,β,β-TpivPP)(2-MeHIm)-
(2-MeIm-)](monoclinic) displaying the atom labels. The hydrogen atoms of
the axial ligands are shown to display the hydrogen bond with the next
˚
molecule (N6 N8: 2.662(5)A; N6 H1 N8: 171(6)°). Thermal ellip-
3 3 3
3 3 3
3 3 3
soids of all atoms are contoured at the 50% probability level. Hydrogen
atoms, except those of the imidazole ligands, have been omitted for clarity.
expected, axial distance to the hindered imidazole is also
seen in the analogous iron(II) picket fence derivative
(Table 2). We are unable to say whether this is a specific
effect of picket fence porphyrin. The porphyrin core of
[Co(II)(TpivPP)(2-MeHIm)] shows a modest saddling
behavior consistent with the required 2-fold symmetry.
A formal diagram along with averaged values of the bond
distances and angles in the core is given in Supporting
Information, Figure S1.
˚
(1.934(5), 1.943(5), and 1.951(4) A) among the complexes
of Table 3. As is well-known, core ruffling and short
M-Np bonds are tightly coupled parameters, with core
ruffling leading to shortened M-Np bonds.38,39 The
ruffling also accommodates the two sterically hindered
axial ligands with their demanding 2-methyl group in
relative perpendicular orientations; the dihedral angles
are 71.2°, 80.2°, and 85°. (Table 3).
Five-coordinate [Co(II)(TpivPP)(2-MeHIm)] had been
prepared to prepare and characterize the six-coordinate
dioxygen complex [Co(II)(TpivPP)(2-MeHIm)O2]. The
reaction of O2 with the crystalline five-coordinate cobalt-
(II) complex under the same conditions as that used
previously for the five-coordinate iron(II)27 leads only
to a modestly (partially) oxygenated complex. This result
is comparable to the solid state oxygenation of the B12
derivative Co(II)alamin, the product of which reaction
An oxidation of the cobalt ion would require the
presence of an anion. In both crystalline complexes this
anion is provided by the deprotonation of one of the two
axial imidazoles so that the axial ligands to the cobalt in
both crystalline forms consists of one imidazole and one
imidazolate ligand. Moreover, a careful examination of
the crystal packing and final difference Fourier syntheses
are strongly consistent with the identification and com-
plete ordering of the axial ligands. The imidazolate ligand
of one cobalt center is hydrogen bonded to the imidazole
ligand of the next molecule so as to form an infinite one-
dimensional chain and which can be formulated as
contains about 30% of the unoxygenated speices.32
A
completely oxygenated porphyrin species can be pre-
pared, but the conditions and results are still under study.
They will be reported along with the characterization of
other (more traditional) oxygenated cobalt species.33
The direct reaction of a benzene solution [Co(II)-
(TpivPP)(2-MeHIm)] with dioxygen yielded an unantici-
pated product rather than a crystalline dioxygen complex.
In the course of carrying out these reactions we have
obtained the same compound in two different forms. The
two crystalline systems contained a total of three unique
cobalt centers, all three are essentially equivalent. Figure 2
displays a thermal ellipsoid plot of one of the centers.
There are several striking features. First, the pickets of the
Co-2-MeIm- H-2-MeIm-Co
. A packing
3 3 3
3 3 3
3 3 3
diagram of a portion of the cell of the monoclinic form
of [Co(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] is shown
in Figure 3; an analogous diagram for the triclinic form is
given in Supporting Information, Figure S6. In all cases,
the ligand identified by the X-ray analysis as the imida-
zolate has the shorter axial Co-N bond (see Table 3);
however, the difference in one case is not statistically
significant.
The hydrogen bonds found in the two new structures
are relatively strong with N N distances of 2.662(5),
3 3 3
˚
2.636(5), and 2.667(5) A. These studies would suggest that
an atropisomerization for a metal complex will have a
(32) Hohenester, E.; Kratky, C.; Krautler, B. J. Am. Chem. Soc. 1991,
113, 4523.
(33) Li, J.-F.; Oliver, A.; Noll, B. C.; Scheidt, W. R., manuscript in
preparation.
(34) Scheidt, W. R. J. Am. Chem. Soc. 1974, 96, 84.
(35) Little, R. G.; Ibers, J. A. J. Am. Chem. Soc. 1974, 96, 4440.
(36) Lauher, J. W.; Ibers, J. A. Inorg. Chem. 1974, 96, 4447.
(37) Bang, H.; Edwards, J. O.; Kim, J.; Lawler, R. G.; Reynolds, K.;
Ryan, W. J.; Sweigart, D. A. Inorg. Chem. 1992, 114, 2843.
(38) Walker, F. A.; Nasri, H.; Turowska-Tyrk, H.; Mohanrao, K.;
Watson, C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109.
(39) Hoard, J. L. Ann. N.Y. Acad. Sci. 1973, 206, 18.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2403
Table 3. Selected Structural Parameters of Six-coordinate [Co(Por)(L)2] Complexesa
θ f
ref
b
c
d
e
e
complex
Δ24
(Co-Np)av
Co-Lfifth
Co-Lsixth
φfifth
φsixth
Co(II) Complexes
[Co(II)(TPP)(pip)2] (2-fold)
[Co(II)(OEP)(3-MePy)2] (2-fold)
0.00
0.00
1.987(2)
1.992(1)
2.436(2)
2.386(2)
2.436(2)
2.386(2)
34
35
12.03
12.03
0.00
Co(III) Complexes
1.934(5)
[Co(III)(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)]
[Co(III)(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] (mol 1)
[Co(III)(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] (mol 2)
[Co(III)(TPP)(HIm)2]OAc g
0.00
1.972(3)
1.972(3)
1.957(3)
1.945(15)
1.942(5)
1.953(3)
1.957(3)
1.952(3)
1.945(15)
1.941(6)
43.4
36.6
37.7
43
34.3
33.1
44.4
43
80.2
71.2
85.7
0
tw
tw
tw
36
37
-0.03
0.05
0.00
1.943(5)
1.951(4)
1.982(11)
1.977(5)
[Co(III)(TDCPP)(1-MeIm)2]BF4
0.01
11.2
10.0
89.8
a
Distances in A, angles in deg, tw = this work. b Displacement of the metal atom from the 24-atom mean plane, a positive number indicates a
˚
displacement toward the imidazole ligand. c Bond distance between iron atom and imidazole (pip, Py) nitrogen atom. d Bond distance between iron atom
and the coordinate atom of sixth ligand. e Dihedral angle between the planes defined by the closest Np-Co-N
im/im- and the imidazole or imidazolate
plane. f Dihedral angle between the planes defined by two axial ligand planes. g The data is cited for one of the two half-porphyrins in the asymmetric unit.
for the imidazolate ring. In [Co(R,R,β,β-TpivPP)(2-
MeHIm)(2-MeIm-)](monoclinic), for example, the an-
gles around the two nitrogen atoms are105.5(3) and
105.2(3)° for 2-MeIm- and 105.4(3) and 108.4(4)° for 2-
MeHIm. Second, the imidazolate ligand is known to be a
stronger field ligand than imidazole and is also expected
to be a better σ donor;42 these effects lead to an ∼0.02
axial Co-N bond distance shortening in [Co(R,R,β,β-
TpivPP)(2-MeHIm)(2-MeIm-)](monoclinic).
It is to be presumed that formation of the hydrogen-
bonded chain is much more favorable with the atropi-
somerized porphyrin ligand compared to the R,R,R,R
conformer. The issue of which particular atropisomer is
formed is an interesting question. The perpendicular
orientation of the two axial ligands would, at first glance
suggest that the most likely atropisomer would be the R,β,
R,β form. However, the R,R,β,β conformer would appear
to better allow the close contacts the hydrogen-bonded
five-membered rings require. The hydrogen-bonded
chains are helical; the space group requirements do how-
ever require that both helical hands are present. We had
originally thought that the R,R,β,β conformer resulted
from a concerted process in which a binuclear intermedi-
ate was important, but the photolysis experiments (vide
supra) do not suggest such an intermediate.
Figure 3. Partial packing diagram of complex [Co(TpivPP0)(2-MeHIm)-
A few studies on the atropisomerization of picket fence
(2-MeIm-)](monoclinic) showing the unit cell and the hydrogen bonds
between the axial ligands. A second chain of opposite helicity exists in
the complete crystal structure.
porphyrin complexes have been reported.43,44 The pro-
cess would seem to have a relatively high activation
barrier. Atropisomerism of any metal complex of a picket
fairly high activation barrier. These distances are much
˚
shorter than the mean value of 2.900 A, found for a total
fence porphyrin will almost certainly require ruffling
of the porphyrin core to increase the distance between
the ortho substituent and the pyrrole hydrogens, there-
by reducing the interaction responsible for restricted
phenyl ring rotation.43 Thus, a cobalt(III) complex with
the generally observed strong ruffling is a reasonable
system for atropisomerization. However, no atropisome-
rized structures of a picket fence complex has been
reported. Marchon and co-workers have reported the
occurrence of atropisomerism in another “pocketed”
porphyrin system, the chiroporphyrins, that responded
of 391 instances from the Cambridge Structural Data-
base40 of a hydrogen bond formed by an imidazole
derivative to a nitrogen atom acceptor in a solid-state
structure.41 Moreover, some structural differences de-
rived from the two different axial ligands (2-MeHIm
and 2-MeIm-) are expected and readily observed. First,
the bond distances and angles involving the two nitrogen
atoms are expected to be more similar in imidazolate than
in imidazole because of the increased bond delocalization
(40) Allen, F. H. Acta Crystallogr. 2002, B58, 380. Bruno, I. J.; Cole, J. C.;
Edgington, P. R.; Kessler, M. C.; Macrae, F.; McCabe, P.; Pearson, J.; Taylor, R.
Acta Crystallogr. 2002, B58, 389.
(41) Hu, C.; Noll, B. C.; Piccoli, P. M. B.; Schultz, A. J.; Schulz, C. E.;
Scheidt, W. R. J. Am. Chem. Soc. 2008, 130, 3127.
(42) Landrum, J. T.; Hatano, K.; Scheidt, W. R.; Reed, C. A. J. Am.
Chem. Soc. 1980, 102, 6729.
(43) Freltag, R. A.; Whitten, D. G. J. Phys. Chem. 1983, 87, 3918.
(44) Walker, F. A.; Buehler, J.; West, J. T.; Hinds, J. L. J. Am. Chem. Soc.
1983, 105, 6923.

2404 Inorganic Chemistry, Vol. 49, No. 5, 2010
Li et al.
Figure 5. Selected UV-vis spectra taken in benzene. (a) [Co(II)-
(TpivPP)] (3.53 ꢁ 10-5 M) in 1.22 ꢁ 10-1 M 2-MeHIm solution. (b)
The solution was exposed to air for 15 min. (c) The solution was exposed
to air for 30 min. (d) The solution was exposed to air for 40 min. (e) The
solution was exposed to air for 5 h. (f) The solution was exposed to air
overnight.
Figure 4. Selected UV-vis spectra taken in benzene under argon. The
enlarged spectra from 500 to 700 nm are measured in the 10 mm UV cell.
(a) [Co(II)(TpivPP)] (3.59 ꢁ 10-5 M). (b) [Co(II)(TpivPP)] in 2.40 ꢁ 10-5
M 2-MeHIm solution. (c) [Co(II)(TpivPP)] in 6.00 ꢁ 10-5 M 2-MeHIm
solution. (d) [Co(II)(TpivPP)] in 1.20 ꢁ 10-4 M 2-MeHIm solution. (e)
[Co(II)(TpivPP)] in 2.40 ꢁ 10-4 M 2-MeHIm solution. (f) [Co(II)-
(TpivPP)] in 4.76 ꢁ 10-4 M 2-MeHIm solution. (g) [Co(II)(TpivPP)] in
1.06 ꢁ 10-3 M 2-MeHIm solution.
to changes in complexation.45 The apparently facile
atropisomerization of the cobalt picket fence derivative
suggests that this feature is deeply involved in the me-
chanism of the reaction of the cobalt(II) complex with
dioxygen. In particular this suggests the possibility of a
binuclear O2 bridged intermediate. This possibility led us
to carefully study the solution reaction chemistry of these
systems.
Reactions. We have monitored the reaction of [Co(II)-
(TpivPP)] with 2-methylimidazole in both the presence
and the absence of oxygen. In the absence of oxygen, the
titration of a benzene solution of [Co(II)(TpivPP)] with
ethanol solutions of 2-MeHIm gave the results shown in
Figure 4. The Soret and R bands decrease in intensity and
shift very slightly to longer wavelength as the concentra-
tion of ligand is increased from 2.40 ꢁ 10-5 M to 1.06 ꢁ
10-3 M. The results are consistent with previously re-
ported results46 and suggests only the five-coordinate
complex is formed (reaction 1). No evidence for the
formation of a bis(imidazole) complex (reaction 2), even
at higher concentrations of imidazole than those shown in
Figure 4, was observed.
When the reaction system was exposed to air, a de-
crease of the Soret band at 411 nm and the simultaneous
appearance of a new Soret band at ∼445 nm was ob-
served. In the visible region, the band at 529 nm was
gradually replaced by one at 561 nm (Figure 5). The
appearance of the new, 34 nm, blue-shifted Soret band
Figure 6. Selected UV-vis spectra taken in benzene. The spectra from
480 to 800 nm are measured in the 10 mm UV cell. (a) [Co(II)(TpivPP)]
(3.59ꢁ 10-5 M) in 5.97 ꢁ 10-3 M2-MeHImsolution. (b) Oxygen bubbled
for 3 min. (c) Argon purge for 5 min.
strongly indicates the oxidation of Co(II) to Co(III).47
These initial reactions were carried out with an ethanolic
solution of 2-MeHIm. Similar spectral changes are
observed when the reactions were run in the absence of
ethanol. These spectral changes are consistent with forma-
tion of [Co(III)(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] we
described above.
Further insight into the processes were given by addi-
tional spectral measurements. In a carefully purified
benzene solution of [Co(II)(TpivPP)] (ethanol-free), ex-
cess solid 2-MeHIm was added and dissolved. Then
oxygen gas was introduced for 3 min. The immediately
recorded UV-vis spectra showed a decrease of the Soret
band at 411 nm and the appearance of a shoulder at ∼439
nm (Figure 6). Simultaneously in the visible, the band at
529 nm moved to 549 nm. The reported spectral observa-
tion of oxygen bonding for [Co(TpivPP)(1-MeIm)] and
[Co(TpivPP)(1,2-Me2Im)] are the shifts of λmax from 530
to 547 nm and from 529 to 546 nm, respectively.8b This is
(45) (a) Mazzanti, M.; Marchon, J.-C.; Shang, M.; Scheidt, W. R.; Jia, S.;
Shelnutt, J. A. J. Am. Chem. Soc. 1997, 119, 12400. (b) Veyrat, M.; Ramasseul,
R.; Turowska-Tyrk, I.; Scheidt, W. R.; Autret, M.; Kadish, K. M.; Marchon, J.-C.
Inorg. Chem. 1999, 38, 1772. (c) Song, Y.; Haddad, R. E.; Jia, S.-L.; Hok, S.;
Olmstead, M. M.; Nurco, D. J.; Schore, N. E.; Zhang, J.; Ma, J.-G.; Smith, K. M.;
Gazeau, S.; Pecaut, J.; Marchon, J.-C.; Medforth, C. J.; Shelnutt, J. A. J. Am.
Chem. Soc. 2005, 127, 1179.
(46) Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1150.
(47) Stynes, D. V.; Stynes, H. C.; Ibers, J. A.; James, B. R. J. Am. Chem.
Soc. 1973, 95, 1142.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2405
Scheme 1. Proposed Reaction Route to the Formation of [Co(III)(R,
R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] from [Co(II)(TpivPP)(2-MeHIm)]
Figure 7. Transient spectra recorded when an O2 saturated solution of
[Co(TpivPP)(2-MeHIm)] ([Co(TpivPP)] (3.59 ꢁ 10-5 M) in 2.80 ꢁ 10-3
M 2-MeHIm benzene solution) was irradiated at 351 nm. The delays from
the laser flash are indicated in the figure.
The photochemical behavior of [Co(TpivPP)] was in-
vestigated by nanosecond flash photolysis in benzene
solution, irradiating at 351 nm. The complex in the
presence and absence of 2-MeHIm showed no significant
evidence for the formation of any transient species.
Admission of O2 to the system results in the formation
of the O2 bound complex, [Co(TpivPP)(2-MeHIm)(O2)],
that ultimately yields the product [Co(TpivPP)(2-
MeHIm)(2-MeIm-)]. The initial photochemical process
shows formation of a transient with a lifetime in the
microsecond range, Figure 7. When observed at wave-
lengths where the absorption of the ultimate reaction
product does not interfere, the transient decays back to
the starting complex by second-order kinetics indica-
ting that the transient is most likely Coδþ(TpivPP)(2-
consistent with our observations and indicates formation
of the oxygen adduct of [Co(TpivPP)(2-MeHIm)(O2)].
This reversible reaction (Reaction 3) was confirmed by an
argon purge of the reaction solution that led to an
immediate diminishing of the shoulder at ∼ 439 nm and
the return of λmax from 549 to 529 nm. Importantly, the
best results were obtained when the processes were car-
ried out in subdued light. Conversely, if the argon purge
was not done and the solution was then stirred continu-
ously under ambient light, the shoulder at ∼439 nm
gradually shifts to 445 nm with a decrease of the band
at 411 nm. This behavior, consistent with the formation of
Co(III), is irreversible and unaffected by an argon purge.
If the solution of the cobalt complex, imidazole, and O2 is
stirred overnight in the dark, only slight changes in the
spectra are observed.
Two conclusions from these spectral observations can
be drawn: (1) the first product of the oxygenation reaction
is the oxygen adduct [Co(TpivPP)(2-MeHIm)(O2)] and
(2) the oxidation reaction (Co(II) f Co(III)) can proceed
without a proton source other than 2-methylimidazole.
There are a number of possibilities (and questions) in the
reaction sequence between the formation of the mono-
meric oxygen complex and the final cobalt(III) product.
Clearly the porphyrin ligand must be atropisomerized,
but what is the driving force for the atropisomerization of
picket fence porphyrin when all the reactions are at room
temperature? Normally picket fence porphyrin is stable to
atropisomerization at room temperature.13 Protons are
required if O2 is the oxidant and must be provided by the
2-methylimidazole, which then becomes the second li-
gand of the final oxidized product. Does this imply that a
binuclear oxygen-bridged species, (L)(Por)Co(O2)Co-
(Por)(L), is an important species as suggested by James
et al.47 or is only a monomeric reactive O2 complex
involved as suggested by Asperger et al.48 The importance
of a light-induced process in taking the reaction further
led us to explore the photochemical behavior of this
system.
MeHIm)(O2)δ-
H
2-MeIm. At 450 nm, a wave-
3 3 3 3 3 3
length where the product formation reaction is observed,
the process is first order and independent of 2-MeHIm,
with a rate constant of 5 ꢁ 106 s-1. (See Supporting
Information, Figure S8.) Experimental difficulties pre-
vent the determination of the Soret maximum of this
initial product, but it has absorption characteristics simi-
lar to the final reaction product. It is likely that the rate
limiting/determining step in this instance corresponds to
the inversion/rotation of the ortho-substituted “picket
fence” groups. Although thermally induced atropisome-
rization of picket fence porphyrins is slow,43 the values
obtained herein are consistent with the photocatalyzed
process reported for the free ligand by Whitten.43 Indeed,
when coordinated to a small cation such as cobalt(III),
further acceleration can be expected as the porphyrin is
subject to out-of-plane distortions.
The photochemical study supports a photoinitiated
transient state that involves the atropisomerization of
picket fence porphyrin. On the basis of the experimental
observations and analysis, we propose the following
reaction scheme for the formation of [Co(III)(R,R,β,
β-TpivPP)(2-MeHIm)(2-MeIm-)] starting from five-
coordinate [Co(TpivPP)(2-MeHIm)] (Scheme 1). [Co-
(TpivPP)(2-MeHIm)(O2)] is the first reaction product,
and whose formation is reversible. In the presence of
excess 2-MeHIm and O2, a transient state is initiated by
photolysis. In this transient state, both the steric effect
from bulky 2-MeHIm ligand and the photo energy are
(48) Dokuzovic, Z.; Ahmeti, X.; Pavlovic, D.; Murati, I.; Asperger, S.
Inorg. Chem. 1982, 21, 1576.

2406 Inorganic Chemistry, Vol. 49, No. 5, 2010
Li et al.
presumed to be the driving force for the atropisomeriza-
tion of picket fence porphyrins. With the release of HO2 ,
It is thus possible that a preorganized hydrogen-bonded
•
(TpivPP)Co-O2 H-2-MeHIm ensemble is ready for
3 3 3
the final product of [Co(III)(R,R,β,β-TpivPP)(2-MeHIm)-
the photoinitiated atropisomerism followed by oxidation
of cobalt and coordination of the resulting imidazolate
ligand.
(2-MeIm-)] is produced.
Finally, we emphasize the importance of the hydrogen
bond between Coδþ(Por)(L)(O2)δ-and the hydrogen bond
donor (2-methylimidazole in this study) in the oxygenation/
oxidation reactions. The presence of a hydrogen bond donor
to the coordinated dioxygen ligand appears crucial to the
question of whether reversible oxygenation or oxidation
occurs. As noted in the Introduction, both reversible and
irreversible reactions had been claimed for forming the
binuclear complex (L)(Por)Co(O2)Co(Por)(L) (reaction 4).9
In this report irreversible reactions were reported for two
imidazole ligands with an acidic hydrogen (e.g., imidazole
and the 1-position NH for benzimidazole); none of the
ligands reported as having reversible oxygenation behavior
had acidic protons. James and co-workers had suggested
the importance of imidazole hydrogen bonding in Co(Por)-
Summary. The synthesis and structural characterization
of the five-coordinate complex [Co(TpivPP)(2-MeHIm)] is
reported. The reaction of this species in the presence of
dioxygen and light to yield a novel cobalt(III) complex
coordinated by imidazole and imidazolate has been studied
by structural analysis and flash photolysis. Among other
surprising features, these reaction conditions lead to the
atropisomerization of the picket fence porphyrin from the
usual R,R,R,R conformer to the R,R,β,β conformer with the
two axial ligands having relative perpendicular orientation.
The atropisomerism occurs under very mild conditions and
appears to be photoinitiated.
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant GM-
38401 and the NSF for X-ray instrumentation support
under Grant CHE-0443233.
(L) O2
H
Im, which was believed to lower the
3 3 3 3 3 3 3 3 3
transition state by ∼4 kcal mol-1, giving products of (L)-
(Por)Co(O2)Co(Por)(L) or Co(Por)(L)(OH).47 Definitive
characterization of products in these reactions is not avail-
able.
Supporting Information Available: Figures S1, S4, and S5
giving 24-atom mean plane drawings for [Co(TpivPP)(2-MeHIm)],
[Co(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] (monoclinic) and
[Co(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)] (triclinic). Figures
S2 and S3 giving ORTEP diagrams for [Co(R,R,β,β-TpivPP)(2-
MeHIm)(2-MeIm-)] (triclinic). Figure S6 giving packing dia-
gram of complex [Co(R,R,β,β-TpivPP)(2-MeHIm)(2-MeIm-)]
(triclinic) showing the unit cell and the hydrogen bonds between
the axial ligands. Figure S7 giving selected UV-vis spectra
taken in benzene. Figure S8 giving oscillographic traces re-
corded at 450 and 525 nm. Tables S1-S18 giving complete
crystallographic details, atomic coordinates, bond distances and
angles, anisotropic temperature factors, and fixed hydrogen
atom positions for [Co(TpivPP)(2-MeIm)], [Co(R,R,β,β-TpivPP)-
(2-MeHIm)(2-MeIm-)] (monoclinic) and [Co(R,R,ββ,β-TpivPP)-
(2-MeHIm)(2-MeIm-)] (triclinic). X-ray crystallographic informa-
tion (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
In other reported cases, the solvent provides the hydro-
gen bonding donor. The formation of L5CoO2CoL5 (L is
nitrogenous bases) in water was reported to be very rapid
and difficult to reverse.49 In alcoholic media as the proton
source, the oxidation of [Co(II)(Por)(L)] to [Co(III)-
(Por)(L2)]þ is reported.48 In the reversible case reported
by Walker, the carefully purified toluene solution ex-
cluded water, and the EPR signal can be restored by
pumping off the dioxygen.3 In our system, the combina-
tion of the protecting “picket fence” and the bulky 2-
methylimidazole would appear to preclude the formation
of the peroxo-bridged binuclear complex but could allow
hydrogen bonding to the O2 by excess 2-methylimidazole.
(49) Simplicio, J.; Wilkins, R. G. J. Am. Chem. Soc. 1967, 89, 6092.