Variation of oxo-transfer reactivity of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands

Article abstract of DOI:10.1021/ic048701a

Derivatives of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands have been prepared in solution and in the solid state. Crystal structures of two of these derivatives, (H₂O)CoTpivPP(NO₂) and (CH₃OH)CoTpivPP

Full text of DOI:10.1021/ic048701a

Inorg. Chem. 2005, 44, 2215−2223
Variation of Oxo-Transfer Reactivity of (Nitro)Cobalt Picket Fence
Porphyrin with Oxygen-Donating Ligands
John Goodwin,*,† Tigran Kurtikyan,‡ Jean Standard,§ Rosa Walsh,| Bin Zheng,† Diedre Parmley,†
James Howard,† Shaun Green,† Arthur Mardyukov,‡ and David E. Przybyla§
Department of Chemistry and Physics, Coastal Carolina UniVersity, P.O. Box 261954,
Conway, South Carolina 29526-6054, Department of Chemistry, UniVersity of South Florida,
Tampa, Florida, Department of Chemistry, Illinois State UniVersity, Normal, Illinois, and
Molecular Structure Research Centre, National Academy of Science, YereVan, Armenia
Received September 15, 2004
Derivatives of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands have been prepared in solution and
in the solid state. Crystal structures of two of these derivatives, (H2O)CoTpivPP(NO2) and (CH3OH)CoTpivPP-
(NO2), have been determined. The ethanol complex (C2H5OH)Co(TPP)(NO2) has been obtained and spectrally
characterized using sublimed layers methodology. The formation constant and the
∆H° value of the association
reaction with ethanol have been determined by FTIR measurements in CCl4 solution. Catalytic oxygen activation
and oxo-transfer reactions of these derivatives have been assessed in solution. Correlations between the oxo-
transfer reactivity, thermodynamics, and characteristics of the nitro ligand show that although calculated and observed
ONO vibrational spectra and bond lengths suggest activation of the NO2 ligand and enhanced oxo-transfer reactions
as seen in the analogous five-coordinate complexes, density functional theory calculations support that
thermodynamics limits oxo-atom transfer reactions in these six-coordinate systems.
Introduction
coordinate (nitro)iron porphyrins was recognized in the mid-
1990s by Castro3 and Scheidt.4 With nitrogenous bases, the
five-coordinate species was generated1 under two sets of
conditions: (1) micromolar porphyrin concentrations in
dichloromethane (DCM) solution using 3,5-dichloropyridine
as the ligand and (2) millimolar porphyrin concentration in
DCM solution but with the Lewis acid lithium ion added as
the perchlorate salt (saturated in DCM). The five-coordinate
species were observed with distinct visible and IR spectra
due to the manipulation of the coordination equilibrium. In
these systems, oxidation of alkenes was found to be possible
without added cocatalysts, but the oxidation reactions were
found to lack specificity. These catalytic reactions generated
a wide range of oxygen-containing products including
Greatly enhanced reactivity of pentacoordinate (nitro)-
cobalt porphyrins CoP(NO2) in oxo-transfer reactions utiliz-
ing molecular oxygen as the oxidant has been studied in
dichloromethane solution.1 These solution-phase systems
were prepared by using hexacoordinate derivatives of cobalt
tetraphenylporphyrin CoTPP and cobalt picket fence por-
phyrin CoTpivPP, in which a pyridine ligand was coordinated
in the sixth coordination site. The original pyridine complex
of (py)CoTPP(NO2) was studied by Tovrog in the late 1970s
and the phthalocyanine analogue later by Ercolani et al. for
their potential use as dioxygen-activation/oxo-transfer cata-
lysts,2 but the five-coordinate species was not recognized as
having enhanced oxo-transfer reactivity until much later.
Similar oxidation reactivity of some six-coordinate and five-
(2) (a) Tovrog, B. S.; Diamond, S. E.; Mares, F. J. Am Chem. Soc. 1979,
101, 270-272. (b) Tovrog, B. S.; Mares, F.; Diamond, S. E. J. Am
Chem. Soc. 1980, 102, 6616-6618. (c) Tovrog, B. S.; Diamond, S.
E.; Mares, F.; Stalkiewicz, A. J. Am Chem. Soc. 1981, 103, 3522-
3526. (d) Diamond, S. E.; Mares, F.; Szalkiewicz, A.; Muccigrosso,
D. A.; Solar, J. P. J. Am Chem. Soc. 1982, 104, 4266-4268. (e)
Ercolani, C.; Paoletti, A. M.; Pennesi, G.; Rossi, G. J. Chem. Soc.,
Dalton Trans. 1991, 1317-1321.
* To whom correspondence should be addressed. E-mail:
jgoodwin@coastal.edu.
† Coastal Carolina University.
‡ National Academy of Science.
§ Illinois State University.
| University of South Florida.
(1) Goodwin, J. A.; Bailey, R.; Pennington, W. T.; Rasberry, R.; Green,
T.; Shasho, S.; Vongsavanh, M.; Echevarria, E.; Tiedeken, J.; Brown,
C.; Fromm, G.; Lyerly, S.; Watson, N.; Long, A.; De Nitto, N. Inorg.
Chem. 2001, 40, 4217-4225.
(3) O’Shea, S. K.; Wang, W.; Wade, R. S.; Castro, C. E. J. Org. Chem.
1996, 61 (18), 6388-6395.
(4) Munro, O. Q.; Scheidt, W. R. Inorg. Chem. 1998, 37, 2308-2316.
10.1021/ic048701a CCC: $30.25
Published on Web 02/25/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 7, 2005 2215

Goodwin et al.
according to the method of Andrews.6 Unlike the previously studied
CoTPP derivatives, the CoTpivPP(NO2) preparation did not require
the presence of an added nitrogenous base such as pyridine for the
formation of the stable oxidized nitro complex and used the
noncoordinating solvent DCM instead of CH3CN. Silver nitrite is
marginally soluble in DCM, but the suspension reacted with mild
heating over several hours. The solid Ag byproduct was filtered
from the solution before the solvent was removed under vacuum.
Completion of the reaction was indicated by a single strong Soret
band at 418 nm. The presence of coordinating impurities was
detectable by broadening and red-shifts of this peak. All materials,
except for dichloromethane solvent and alcohols used for synthesis,
equilibrium, and reaction studies in these syntheses, were used as
received. It was important to remove the amylene or cyclohexene
stabilizer from this solvent prior to its use by washing with sulfuric
acid, sodium hydroxide, and water before drying with calcium
chloride and distilling from phosphorus pentoxide under nitrogen.7
Absolute ethanol and methanol were distilled from molecular sieves
under nitrogen before transfer and use in the glovebox.
epoxides, aldehydes, alcohols, and ketones, but the catalytic
behavior persisted even in the presence of these potential
oxygen-donating ligands at high concentration in solution.
Minor spectral shifts were observed for the porphyrin
catalysts during these reactions, but these were not explained
in the initial studies. The proposed catalytic cycle provides
for two distinct O-transfer steps.1
Crystals of (Aqua)(nitro)cobalt(III) Picket Fence Porphyrin,
(H2O)Co(III)TpivPP(NO2). Serendipitous growth of this crystalline
product took place in a CDCl3 solution used for an NMR sample,
exposed to air.
Crystals of (Methanol)(nitro)cobalt(III) Picket Fence Por-
phyrin, (CH3OH)Co(III)TpivPP(NO2). Growth of this crystalline
product took place in attempts to isolate the pentacoordinate
derivative, Co(III)TpivPP(NO2), under nitrogen atmosphere in the
glovebox using the vapor diffusion method with a CH2Cl2 solution
and hexane. Methanol contamination presumably arose from the
glovebox atmosphere, since this solvent was present but was not
intentionally added.
X-ray Crystallography. Data were collected using a Rigaku
AFC8/Mercury CCD using graphite monochromated Mo KR
radiation (λ ) 0.71073 Å) at room temperature of 22 ( 1 °C. The
structures were solved by Direct Methods and refined using full-
matrix least-squares on F2 (SHELXTL PLUS).
Computational Modeling. Semiempirical PM3 and density
functional theory (B3LYP 6-31G*) calculations were carried out
using PC Spartan Pro or Spartan ’04 (Wavefunction, Inc.) using
cobalt porphine analogues in some cases to reduce computational
time. Equilibrium geometries, heats of formation, and vibrational
frequencies of the nitro ligands were determined for all of the six-
coordinate (L)CoP(NO2) derivatives, where L ) H2O, ROH. No
corrections were made to the calculated vibrational frequencies.
Reaction Studies. (CH3OH)CoTpivPP(NO2) and other alcohol
complexes were generated under nitrogen atmosphere by dissolving
solid (H2O)CoTpivPP(NO2) in methanol or other alcohols and
evaporating the solution. The alcohol complexes could be prepared
directly in solution by addition of alcohols to CoTpivPP(NO2) in
dichloromethane. Visible spectra were recorded in glass cuvettes
with a 1-cm path length. Simple oxo-transfer reactions of the nitro
complexes with oxygen acceptors were carried out under nitrogen
atmosphere to avoid reoxidation of any CoTpivPP(NO) product
with O2. Oxidation catalysis reactions were carried out under ∼1
atm O2 in vessels capped with oxygen-filled balloons. Organic
product analysis was obtained by use of GC-MS analysis with a
Hewlett-Packard model 6890 GC-MS instrument.
The continued catalytic solution phase reactivity of the
“five-coordinate” CoP(NO2) (where P ) porphyrin) in the
presence of oxygen-donating ligands suggested two pos-
sibilities: (1) that the hexacoordinate species formed from
coordination also have enhanced catalytic activity or (2) that
the kinetic lability and small stability constants of these
oxygen-donating ligands with CoP(NO2) results in relatively
high concentrations of the reactive five-coordinate complex.
The present work focuses on the structure and reactivity
of hexacoordinate (nitro)cobalt picket fence porphyrins with
oxygen-donating ligands including water and a selection of
alcohols. Crystal structures of two new derivatives have been
obtained, the formation of aqua, methanol, and ethanol
complexes in solution has been studied spectroscopically,
and the solution-phase stability and oxo-transfer reactivity
of these complexes have been assessed. Further insight into
the variation of oxo-transfer reactivity has been carried out
with computational modeling using semiempirical PM3 and
density functional theory (DFT).
Experimental Section
General Methods. Spectroscopic Methods. Routine infrared
spectra were recorded using a Bomem Michaelson FTIR system
with an attenuated total reflection (ATR) attachment. Solution UV-
visible spectra were recorded under nitrogen atmosphere using
screw-capped 1-cm path length cells with a Hewlett-Packard 8453
diode array spectrometer or in a Vacuum-Atmospheres glovebox
with an Ocean Optics PC1000 fiber optics spectrometer controlled
by OOIBASE32 operating software. Evaluation of equilibrium
constants from absorbance data was achieved using the SPECDEC
program.5
Synthesis of Porphyrin Derivatives. The (nitro)cobalt(III) picket
fence porphyrin, LCoTpivPP(NO2), derivatives were all prepared
by the stoichiometric reaction of Co(II)TpivPP (Midcentury Chemi-
cals) with AgNO2 (Acros Organics, 99%) in refluxing dichlo-
romethane (Sigma) under nitrogen atmosphere in the glovebox
Infrared and UV-Vis Spectroscopy of Sublimed Layers. The
five-coordinate nitro complex of Co(TPP) has been obtained by
(6) Andrews, M. A.; Chang, C.-T. T.; Cheng, C.-W. Organometallics
1985, 4, 268-274.
(7) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Boston, 1996.
(5) Atkins, C. E.; Park, S. E.; Blaszek, J. A.; McMillin, D. R. Inorg. Chem.
1984, 23, 569-572.
2216 Inorganic Chemistry, Vol. 44, No. 7, 2005

ReactiWity of (Nitro)Cobalt Picket Fence Porphyrin
Table 1. Crystal Data and Structure Refinement for
(H2O)CoTpivPP(NO2)
empirical formula
formula weight
temperature
C68H70Cl12Co N9O7
1609.66
293(2) K
wavelength
0.71073 Å
crystal system
space group
rhombohedral
R3hc
unit cell dimensions
a ) 29.9186(10) Å
b ) 29.9186(10) Å
c ) 44.562(3) Å
34544(3) Å3
18
R ) 90°
â ) 90°
γ ) 120°
volume
Z
density (calculated)
absorption coefficient
F(000)
1.393 Mg/m3
0.697 mm-1
14904
crystal size
0.40 × 0.40 × 0.20 mm3
1.82-24.76°
-35 e h e 24,
-33 e k e 35,
-52 e l e 49
57315
θ range for data collection
index ranges
Figure 1. ORTEP diagram of (H2O)CoTpivPP(NO2).
reflections collected
independent reflections
6582 [R(int) ) 0.0680]
completeness to θ ) 24.76° 99.6%
interaction of NO2 gas with a sublimed layer of Co(TPP) according
to the procedure described earlier.8 Then the layer of Co(TPP)-
(NO2) was exposed to an atmosphere of alcohol vapor, the pressure
of which was monitored by the mercury manometer connected with
the optical cryostat. The layer was maintained at these conditions
overnight, and then the excess of gaseous and adsorbed alcohols
was eliminated by prolonged pumping out by the high vacuum
system. The IR and UV-vis spectra of layers in vacuo have been
measured by Specord M-80, Nicolet Nexus, and Specord M-40
spectrometers correspondingly. KBr (IR) and CaF2 (UV-vis) were
used as materials for optical windows and substrates.
Quantitative Measurements on (C2H5OH)CoTPP(NO2) in
CCl4 Solutions. For the measurements of the ethanol complex,
(C2H5OH)CoTPP(NO2) obtained in thin films was quantitatively
dissolved in CCl4 and various quantities of rigorously dried ethanol
were added to this solution. FTIR measurements have been done
in the 0.06 cm width thermostated cell provided with KBr windows.
absorption correction
max and min transmission
refinement method
none
1.000 and 0.826
full-matrix least-squares on F2
6582/0/459
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.071
R1 ) 0.0848, wR2 ) 0.2509
R1 ) 0.1086, wR2 ) 0.2727
1.322 and -0.799 e‚Å-3
largest diff peak and hole
Results and Discussion
Crystal Structures. Figure 1 illustrates the ORTEP
diagram of (H2O)CoTpivPP(NO2). Table 1 lists its crystal
data and structure refinement, and Table S1 in the Supporting
Information lists the crystallographic bond lengths and
angles. Figure 2 illustrates the ORTEP diagram of (CH3-
OH)CoTpivPP(NO2). Table 2 lists its crystal data and
structure refinement, and Table S2 in the Supporting
Information lists the crystallographic bond lengths and
angles. Two structures previously reported by Ohba et al.
for (H2O)CoTPP(NO2) as the CH2Cl2 solvate9 and the
dimethylformamide, DMF, disolvate10 demonstrate orienta-
tional disorder in the nitro and aqua ligand that results in an
averaged structure in which the cobalt center has an inversion
center. As in other (nitro) derivatives of iron11 and cobalt
picket fence porphyrins,12 the solid state crystal structures
presented here have the nitro ligand exclusively in the
sterically hindered “pocket”, with no evidence of geometrical
Figure 2. ORTEP diagram of (CH3OH)CoTpivPP(NO2).
isomers. The crystal structure of (EtOH)CoTPP(NO2)13 in
contrast showed distinct coordination sites for the nitro ligand
and the ethanol ligand. The method of crystal growth
employed in that work combined ethanol and benzene
solvents that depended on long-term stability of the complex
in contact with ethanol at high concentration.
(11) (a) Nasri, H.; Ellison, M. K.; Shang, M.; Schulz, C. E.; Scheidt, W.
R. Inorg. Chem. 2004, 43 (9), 2932-2942. (b) Nasri, H.; Ellison, M.
K.; Krebs, C.; Huynh, B. H.; Scheidt, W. R. J. Am Chem. Soc. 2000,
122 (44), 10795-10804. (c) Ellison, M. K.; Schulz, C. E.; Scheidt,
W. R. Inorg. Chem. 1999, 38 (1), 100-108. (d) The product nitrato
ligand observed from the reaction of coordinated nitrite in: Munro,
O. Q.; Scheidt, W. R. Inorg. Chem. 1998, 37 (9), 2308-2316. (e)
Nasri, H.; Ellison, M. K.; Chen, S.; Huynh, B. H.; Scheidt, W. R. J.
Am. Chem. Soc. 1997, 119 (27), 6274-6283. (f) Nasri, H.; Haller, K.
J.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. Inorg. Chem. 1992, 31
(16), 3459-3467. (g) Nasri, H.; Goodwin, J. A.; Scheidt, W. R. Inorg.
Chem. 1990, 29 (2), 185-191.
(8) (a) Kurtikyan, T. S.; Stepanyan, T. G.; Gasparyan, A. V. Russ. J.
Coord. Chem. 1997, 23, 563-567. (b) Kurtikyan, T. S.; Stepanyan,
T. G.; Gasparyan, A. V.; Zhamkochyan, G. H. Russ. Chem. Bull. 1998,
47, 695-698.
(9) Ohba, S.; Seki, H. Acta Crystallogr. 2002, E58, m162-164.
(10) Ohba, S.; Eishima, M.; Seki, H. Acta Crystallogr. 2000, C56, e555-
e556.
(12) Jene, P. G.; Ibers, J. A. Inorg. Chem. 2000, 39, 3823-3827.
(13) Ohba, S.; Seki, H. Acta Crystallogr. 2002, E58, m183-m185.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2217

Goodwin et al.
Table 2. Crystal Data and Structure Refinement for
(CH3OH)CoTpivPP(NO2)
Table 3. (A) Measured and Calculated Infrared Frequencies and ONO
Bond Angles of (Nitro)cobalt Porphyrin Derivatives. (B) Infrared and
Visible Spectral Characteristics for CoTPP(NO2) Complexes with
Alcohols Obtained in Sublimed Thin Filmsa
empirical formula
formula weight
temperature
C71H79CoN9O7
1229.36
Part A
100(2) K
NO2 sym, cm-1 NO2 asym, cm-1
measd PM3 measd PM3 PM3 measd PM3
ONO angle
wavelength
0.71073 Å
crystal system
space group
unit cell dimensions
monoclinic
ligand
P2(1)/n
a ) 14.9540(12) Å
b ) 19.3065(14) Å
c ) 22.0388(18) Å
6355.2(9) Å3
4
R ) 90°
â ) 92.809(2)°
γ ) 90°
none (five-
1283
1674 1468
1853 none NA
124.41
coordinate)
EtOH
i-PrOH
MeOH
H2O
piperidine
NH3
Cl2py
Py
Me2Npy
Im
2MeIm
1298
1301
1313
1309
1308
1309
1307
1309
1310
NA
1689 1463
1690 1462
1688 1463
1693 1445
1701 1437
1700 1431
1701 1425
1702 1425
1702 1427
1701 NA
1704 NA
1841 1827 NA
1840 1825 NA
1845 1824 120.9 122.84
1842 1827 122.6 122.45
1837 1821 115.4 120.9
1835 1817 NA
1835 1811 119.7 120.7
1835 1810 120.6 120.4
1835 1810 NA
1835 1813 119.8 120.9
1834 1811 120.4 120
122.67
122.52
volume
Z
density (calculated)
absorption coefficient
F(000)
1.285 Mg/m3
0.332 mm-1
2604
121.12
crystal size
0.10 × 0.10 × 0.02 mm3
1.40-28.30°.
-19 e h e 18,
-9 e k e 22,
-9 e l e 28
16170
θ range for data collection
index ranges
120.5
NA
reflections collected
independent reflections
completeness to θ ) 28.30° 80.4%
absorption correction
refinement method
12716 [R(int) ) 0.0598]
Part B
νs(NO2)
νas(NO2)
δ(NO2)
λ (nm)
none
full-matrix least-squares
CoTPP‚NO2
1468 (∼1440) 1282 (1264) 805 (796) 533
on F2
R‚CoTPP‚NO2
R′‚CoTPP‚NO2
1463 (1433)
1463 (1431)
1301 (1282) 809 (803) 541
1298 (1278) 804 (798) 544
1301 (1281) 804 (798) 545
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
12716/0/816
0.894
R′′‚CoTPP‚NO2 1462 (1428)
R1 ) 0.0717,
wR2 ) 0.1444
R1 ) 0.1471,
wR2 ) 0.1734
0.868 and -0.391 e‚Å-3
a R ) CH3OH, R′ ) C2H5OH, R′′ ) i-C3H7OH. In parentheses the data
for the 15NO2 isotopomer are given.
R indices (all data)
largest diff peak and hole
has been investigated previously and it has been shown that
this process can be interrupted at the stage of five-coordinated
nitro-complex formation.
Crystal growth of the pentacoordinate derivative of (nitro)-
cobalt picket fence porphyrin, CoTpivPP(NO2), has eluded
us, but a structure of pentacoordinate (nitro)cobalt tetraphen-
ylporphyrin, CoTPP(NO2), has also been reported by Ohba
et al.14 as the benzene solvate. Its nitro ligand ONO bond
angle was found to be 123.3(4)°,15 consistent with the
relatively large angle predicted earlier by semiempirical
methods.1
Spectroscopy and Computational Modeling. Table 3
lists calculated equilibrium, bond angles, and vibrational
frequencies for the nitro ligand determined by semiempirical
calculations and the corresponding experimental results. No
corrections were made for calculated vibrational frequencies.
The corresponding measured IR values for the (ROH)-
CoTPP(NO2) analogues are also listed in Table 3B. Equi-
librium geometries determined by semiempirical calculations
showed good agreement with results from density functional
theory for representative structures.
It was predicted by Byrn and Strouse that five-coordinate
axial complexes of metal-meso-tetraarylporphyrins conserve
the microporosity of their structure and can serve as a
template for designing of the metalloporphyrin axial com-
plexes with the mixed ligands. This prediction has been
confirmed by the study in which a number of six-coordinate
nitro amino complexes of CoTPP were obtained by interac-
tion of gaseous amines with the layers of CoTPP‚NO2.17 The
same procedure has been used in this work with three
alcohols, methanol, ethanol, and 2-propanol, and thereby,
the formation of the six-coordinate nitro complexes of these
species has been spectrally confirmed.
Figure 3 demonstrates the changes in the FTIR spectra of
the CoTPP‚NO2 sublimed layer (solid line) after exposure
in the atmosphere of ethanol (dashed line). This process
resulted in shifting of the stretching modes of the coordinated
nitro-group from their initial values. The νas(NO2) CoTPP‚
NO2 at 1468 cm-1 undergoes a low-frequency shift and
appears at 1463 cm-1, while νs(NO2) undergoes a high-
frequency shift from 1283 to 1298 cm-1. The same charac-
teristic band-shifting is observed for 2-propanol with small
deviations in frequencies listed in Table 3B. The fact that
these bands and the deformational mode of the nitro-group
δ(NO2) undergo expected isotopic shifts in the experiments
with 15NO2 confirms the validity of this assignment. (Figure
4 and Table 3B). Additionally new bands denoted by
asterisks appear in the spectra in the regions where the intense
bands of alcohols are disposed. It is the bands at 3554 and
The layers of meso-tetraarylporphyrin metallo-complexes
obtained by sublimation onto a low-temperature surface have
a microporous structure16 similar to that of their bulk samples.
Potential reagents can easily diffuse through the layer’s bulk,
and adducts thus formed can be spectrally characterized
without masking effects of the solvent. Using this technique,
the interaction of NO2 gas with sublimed layers of CoTPP
(14) Ohba, S.; Seki, H. Acta Crystallogr. 2002, E58, m169-m171.
(15) Ohba, S., Department of Chemistry, Keio University, Hiyoshi 4-1-1,
Kohoku-ku, Yokohama 223-8521, Japan, personal communication,
July, 2004.
(16) (a) Kurtikyan, T. S.; Gasparyan, A. V.; Martirosyan, G. G.; Zhamkochy-
an, G. H. J. Appl. Spectrosc. 1995, 62, 62-66 (Russ.). (b) Byrn, M.
P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.;
Terzis, A.; Strouse, C. E. J. Am. Chem. Soc. 1993, 115, 9480-9497.
(c) Byrn, M. P.; Strouse, C. E. J. Am. Chem. Soc. 1991, 113, 2501.
(17) Stepanyan, T. G.; Akopyan, M. E.; Kurtikyan, T. S. Russ. J. Coord.
Chem. 2000, 26, 453-457.
2218 Inorganic Chemistry, Vol. 44, No. 7, 2005

ReactiWity of (Nitro)Cobalt Picket Fence Porphyrin
Figure 3. IR spectra of a sublimed layer of Co(TPP) after short time exposure under low pressures (10-1 mmHg) NO2 (solid line) and additional exposure
in an atmosphere of C2H5OH (10 mmHg) overnight (dashed line).
1034 cm-1 that certainly belong to ν(OH) and (C-O) of
coordinated alcohol. Our gas-phase measurements show that
for monomeric alcohols these bands are disposed in the range
of 3660 and 1054 cm-1 correspondingly. Hence the coor-
dination of alcohol through the oxygen leads to decreasing
of ν(OH) and ν(C-O).
The results obtained in these thin-film experiments can
be interpreted as evidence of formation of six-coordinate nitro
alcohol complexes of Co(TPP). The UV-visible spectra give
additional evidence for this conclusion. The band at 533 nm
of Co(TPP)(NO2) shifts to higher wavelengths in Table 3B
and gains in intensity after interaction with alcohol in Figure
5. This behavior is also observed in solution where the visible
spectra are quite similar as shown in Figure 6C. The
directions of shifts in UV-visible and IR spectra upon
additional coordination of alcohols are similar to those
obtained in six-coordinate nitro complexes with amines.
However, the magnitudes of shifts are larger for the latter
that correlated with basicities of the sixth ligands. Notably
this behavior qualitatively coincides with those predicted by
calculations (Table 3A).
Reactions. The unchanging visible spectrum of crystals
of (H2O)CoTpivPP(NO2) dissolved in DCM under nitrogen
atmosphere is shown in Figure 7. Addition of allyl bromide
to this solution showed no change in the spectrum. Figure
6A shows the initial spectrum of (CH3OH)CoTpivPP(NO2)
prepared by dissolving (H2O)CoTpivPP(NO2) in methanol
after 2 min. Apparent in this figure is a shoulder at about
412 nm that grows over time to give the spectrum in Figure
6B. This resulting spectrum is identical to that previously
observed for CoTpivPP in DCM with the Soret position at
412 nm, and its formation is attributable to methanolysis of
the nitro ligand to NO. Similar behavior was observed in
thin-film IR studies of the methanol complexes over time,
with formation of CoTPP(NO) evident. The oxidized organic
products of this reaction have not been isolated since the
reaction was not catalytic and capable of producing large
quantities of products even under oxygen-rich conditions,
so it is not clear whether the oxidation of methanol itself is
occurring in this system or what the reactive porphyrin
species is.
In contrast, when the ethanol complexes were formed in
the same fashion, the solutions retained the visible spectra
represented in Figure 6C indefinitely, indicating no reaction
of the (nitro)cobalt porphyrin ethanol complex with solvent
ethanol. Addition of allyl bromide or cyclohexene to these
ethanol solutions under nitrogen atmosphere also did not
result in detectable change in the visible spectra. In contrast,
addition of triphenylphosphine to the ethanol solutions under
nitrogen resulted in a spectral shift of the porphyrin complex
Lesser shifts of vibrational frequencies of the coordinated
nitro-group in Co(TPP)(NO2) upon additional coordination
of alcohols compared with amines suggest that the electronic
structure of the nitro-group in (ROH)Co(TPP)(NO2) is closer
to those in five-coordinate complexes. This fact also suggests
that six-coordinated nitro-alcohol complexes can also be
active in oxo-transfer reactions performed by nitro complexes
of Co(TPP).
Inorganic Chemistry, Vol. 44, No. 7, 2005 2219

Goodwin et al.
Figure 5. UV-visible spectra of a Co(TPP)(NO2) sublimed layer (solid
line) after exposure under an atmosphere of ethanol (5 mmHg) within 2 h
(dashed line).
proposed mixture of geometrical isomers, we were only able
to obtain an estimate of the average stability constant for
association of ethanol with CoTpivPP(NO2) with the SP-
ECDEC program optimizing the results for a single ethanol
coordination reaction. This value is approximately 350 in
DCM. With stability constants this small, very large con-
centrations of an alcohol or other oxygen-donating ligand
would be required to prevent significant formation of the
reactive five-coordinate species. The lack of reactivity of
ethanol solutions of CoTpivPP(NO2) with alkenes suggests
that the six-coordinate alcohol complex predominates under
these conditions and that this derivative is unreactive. A
simplified system was used to better assess the equilibrium
binding of ethanol spectroscopically in solution. Carbon
tetrachloride solutions of (C2H5OH)CoTPP(NO2) with vari-
ous quantities of added ethanol are almost free from the IR
bands of solvent and alcohol in the 1320-1270 cm-1 spectral
region, in which the νs(NO2) of coordinated nitro groups in
five- and six-coordinate species are disposed. This allows
measurement of concentration and temperature dependences
of FTIR spectra to estimate the equilibrium constant K and
∆H° value of the association reaction. The symmetrical
CoTPP derivative, of course, eliminates the problems
encountered with the picket fence derivative described
previously.
Figure 4. IR spectra of a sublimed layer of Co(TPP)(15NO2) (solid line),
(C2H5OH)Co(TPP)(15NO2) (dashed line), and (C2H5OH)Co(TPP)(NO2)
(dotted line).
Soret band from 429 to 412 nm, consistent with reduction
of the porphyrin to CoTpivPP(NO). Carrying out this reaction
under 1 atm O2 in ethanol solutions resulted in nearly
quantitative catalytic production of triphenylphosphine oxide
confirmed by IR analysis and GC-MS.
Equilibrium measurements for the association of pyridine
derivatives with CoTPP(NO2) were evaluated previously in
DCM solution. The simplicity of the TPP system provided
clear experimental results with isosbestic behavior that were
used to calculate stability constants of 7.0 × 106 and 1.1 ×
108 for 3,5-dichloropyridine and pyridine, respectively.1
Preparation of CoTpivPP(NO2) and (EtOH)CoTpivPP(NO2)
by the methods used in this work did not provide a series of
visible spectra with clear isosbestic points as the concentra-
tion of EtOH increased. This suggests that in the preparation
of CoTpivPP(NO2) without the addition of pyridine or other
bulky ligand, some fraction of product has the nitrite ligand
in the tetrapivalamido pocket as expected, but due to the
lack of selective coordination by the bulky pyridine on the
exposed face, some fraction of the product has nitrite
coordinated on the exposed face instead of within the pocket.
While the crystal structures show only pocket-bound nitrite,
it is likely that the crystalline packing favors this isomer in
the crystallization or that separate crystals are indeed formed.
Using the visible absorbance data collected with this
Figure 8 demonstrates the spectral changes observed upon
addition of ethanol to a CCl4 solution of (C2H5OH)CoTPP-
(NO2). A well-defined isosbestic point at 1293 cm-1 is seen
as evidence of the presence of two species, CoTPP(NO2)
and (C2H5OH)CoTPP(NO2) with maximums of IR bands at
1283 and 1300 cm-1, respectively.
2220 Inorganic Chemistry, Vol. 44, No. 7, 2005

ReactiWity of (Nitro)Cobalt Picket Fence Porphyrin
of formation of CoP(NO2), L, and LCoP(NO2) obtained in
hartrees and converted to kJ/mol were used to calculate ∆H°
values for the association reactions. Using these as estimates
of ∆G° values for these reactions, by ignoring entropy and
solvation effects, allowed comparison to measured stability
constants. The calculated ∆H° values for formation of 3,5-
dichloropyridine, pyridine, ethanol, and water complexes of
the simple Co(porphine)(NO2) complex were as follows, in
order (values approximated from measured K by ignoring
entropy effects are in parentheses, except where indicated):
Cl2py, -46.0 kJ/mol (-39.1 kJ/mol); py, -56.3 kJ/mol
(-45.9 kJ/mol); EtOH, -38.2 kJ/mol (measured values of
∆H° ) -28 kJ/mol and K ) 9.8 implies ∆G° ) -5.65 kJ/
mol and ∆S° ) -75 J/K‚mol); and MeOH, -41.2 kJ/mol
(measurement not available due to decomposition).
The six-coordinate alcohol complexes (ROH)CoTpivPP-
(NO2) in alcohol solutions fail to oxidize alkenes at room
temperature, even though spectroscopic and structural evi-
dence suggests that activation of the NO2 ligand takes place
to a significant degree. Our earlier measurement of the free
energy for the oxo-loss half-reaction for the FeTpivPP(NO2)2/
FeTpivPP(NO) (+50 kJ/mol) complex caused us to conclude
that the transfer of an oxygen atom to triphenylphosphine
from iron-bound NO2 is driven thermodynamically by the
large oxo-gain half-reaction for triphenylphosphine in forma-
tion of triphenylphosphine oxide.18 A value of ∆H° ) -309
kJ/mol is reported for this gas phase oxo-gain half-reaction.
The entropy term was ignored for this approximation.
Oxidation of alkenes to epoxides and other oxygen-contain-
ing species proceeds exothermically, but with relatively small
enthalpy changes. A representative group of oxo-gain half-
reactions have ∆H° values in the range of -80 to -120 kJ/
mol as shown in Table 4A. Clearly, for these standard oxo-
transfer half-reactions from the (nitro)cobalt porphyrin
complex to occur, the standard oxo-loss enthalpies of the
porphyrin system cannot be more than +80 to +120 kJ, and
ideally far less. The results of DFT calculations of standard
heats of formation for representative LCoTPP(NO2) porphy-
rin complexes are listed in Table 4B. The resulting oxo-loss
enthalpies are listed also. The standard oxo-loss enthalpy
calculated for the five-coordinate species (+103.5 kJ/mol)
is significantly smaller than any of the values for six-
coordinate derivatives. The standard oxo-loss enthalpy values
for the alcohol complexes (+143.1 and +144.5 kJ/mol for
the EtOH and MeOH complexes, respectively) are very
similar to those of the water complex (+140.23 kJ/mol) and
nearly as large as that of the pyridine complex (+154.42
kJ/mol). This similarity suggests that even though there may
be differences in activation of the nitro ligand toward oxo-
transfer in different complexes, all these six-coordinate
derivatives lack oxo-transfer reactivity because of the
unfavorable thermochemistry.
Figure 6. (A) Visible spectrum of unstable (CH3OH)CoTpivPP(NO2)
prepared under nitrogen atmosphere from (H2O)CoTpivPP(NO2) crystals.
The spectrum obtained 2 min after preparation shows initial decomposition
with a shoulder at 412 nm. (B) Visible spectrum resulting from decomposi-
tion of (CH3OH)CoTpivPP(NO2) in methanol solution A. The formation
of the peak at 412 nm is consistent with reduction to CoTpivPP(NO) by
reaction with the solvent. (C) Stable visible spectrum of (CH3CH2OH)-
CoTpivPP(NO2) in 3.5 M ethanol in CH2Cl2, under nitrogen atmosphere.
Figure 7. Visible spectrum of stable (H2O)CoTpivPP(NO2) in DCM
solution prepared under nitrogen from (H2O)CoTpivPP(NO2) crystals.
The equilibrium constant of the reaction
CoTPP(NO2) + C2H5OH T (C2H5OH)CoTPP(NO2)
K
at 25 °C derived from these spectra was found as K ) 9.8.
The ∆H° value of the association reaction calculated from
the temperature dependence (Figure S1 in the Supporting
Information) of equilibrium constants was -28 kJ/mol. The
low value of this equilibrium constant indicates that in
solution there is a significant quantity of five-coordinate
species even at significantly higher molar equivalents of
alcohol with respect to porphyrin nitro complex.
As already mentioned, the observed trends in spectroscopic
and structural data suggested that activation of the nitro
ligand may vary with the coordination environment of the
Computational modeling of the energies of formation of
the six-coordinate products was pursued by use of DFT
calculations, and the results are shown as standard enthalpies
of formation at 298.15 K for the porphine or picket fence
porphyrin models. No correction was made for solvation
effects in the thermodynamics calculations. Standard heats
(18) Frangione, M.; Frangione, M.; Port, J.; Baldiwala, M.; Judd, A.; Galley,
J.; DaVega, M.; Linna, K.; Caron, L.; Anderson, E.; Goodwin, J. Inorg.
Chem. 1997, 36, 1904-1911.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2221

Goodwin et al.
Figure 8. FTIR spectral changes observed in the νs(NO2) range for a 5.17 × 10-3 M carbon tetrachloride solution of (C2H5OH)CoTPP(NO2) in the
presence of various concentrations of ethanol: c1 ) 5.17 × 10-3 M; c2 ) 4.9 × 10-2 M; c3 ) 9.8 × 10-2 M; c4 ) 1.7 × 10-1 M; c5 ) 2.5 × 10-1 M.
Table 4. (A) Standard Oxo-Gain Enthalpies for Formation of Epoxides
from Alkenes. (B) Standard Enthalpies of Oxo-Loss Predicted by DFT
porphine complex undergoing bimolecular oxo-transfer to
trimethylphosphine was explored. These results are listed in
Table 4B. While the reliability of the semiempircal calcula-
tions is limited for energy calculations, the values are rather
close. Our attempts to improve these calculations with DFT
models were not successful.
Calculations Using Co(porphine) Core. (C) Standard Enthalpies of
Oxo-Transfer Calculated from Data in Sections A and Ba
Part A
oxo-gain
enthalpy,
kJ/mol
enthalpy of
formation, X,
kJ/molb
enthalpy of
formation, XO,
kJ/mol
oxo-acceptor, X
A similar semiempirical analysis of the thermochemistry
has been undertaken for the peroxynitro intermediate of the
catalytic cycle proposed earlier. In this approach the proposed
peroxynitro intermediate is formed from reaction of molec-
ular oxygen with the air-sensitive nitrosyl cobalt(II) porphy-
rin. The driving force for oxo-loss from this nitrogen-bound
peroxynitrite ligand is predicted to be significantly more
exothermic. Our semiempirical PM3 calculations yield
standard oxo-loss enthalpies for five-coordinate CoP(NO2O)
as -270 kJ/mol and for (py)CoP(NO2O) as -151 kJ/mol.
In this case intermediate dioxygen activation is predicted to
be very strong, but catalysis limited by the thermochemistry
of the second oxo-transfer step. While there is no direct
evidence that it is a pathway in this system, the loss of an
oxygen atom from peroxynitrite to oxidize dioxygen to form
ozone is thermochemically possible with the five-coordinate
system, as was suggested by Castro for (nitro)iron com-
plexes.3 The oxo-gain enthalpy of oxygen is +142 kJ/mol.19
It is clear, however, that with such large predicted differences
in oxo-loss enthalpies for the peroxynitro and nitro com-
plexes, the mechanisms and products of these distinct oxo-
transfer steps are likely to be different and would account
for the loss of specificity in the observed catalytic oxidation
reaction.
cyclohexene
allyl bromide
propylene
-117
-88
-4
48
21
52
-121
-40
-98
-53
-119
-105
-309
ethylene
triphenylphosphinec
Part B
oxo-transfer activation
energy with Me3P by
PM3, kJ/mol
L ligand of oxo-donor
complex LCoP(NO2)
oxo-loss enthalpy
by DFT, kJ/mol
none
+104
+145
+143
+140
+154
+255
+251
+251
+243
+201
CH3OH
CH3CH2OH
H2O
pyridine
Part C
predicted oxo-transfer reaction energies, kJ/mol
cyclo- allyl triphenyl-
hexene bromide propylene ethylene phosphine
-117
-13
28
-88
16
-119
-15
26
-105
-1
40
-309
-205
-164
-162
-159
-155
none
+104
+145
CH3OH
57
CH3CH2OH +143
26
55
24
38
H2O
+140
+154
23
37
52
21
35
49
pyridine
66
35
a All values are at 298.15 K. b Reference 19. c Reference 20.
cobalt center. Applying semiempirical methods to transition
state energies combines the assessment of oxo-loss from the
perspectives of ligand activation and oxo-transfer enthalpies.
For this analysis a model system involving a (nitro)cobalt
(19) NIST Thermochemical Data. http://webbook.nist.gov/chemistry/name-
ser.html.
(20) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449.
2222 Inorganic Chemistry, Vol. 44, No. 7, 2005

ReactiWity of (Nitro)Cobalt Picket Fence Porphyrin
Conclusions
Advanced Technologies (NFSAT, CH 053-02) for financial
support of this work. This research was also funded in part
by the Horry County Higher Education commission’s support
of Coastal Carolina University’s Academic Enhancement
Grants Program. Michael Zaworotko at USF assisted with
the X-ray diffraction studies and NSF-DMR for the use of
the X-ray machine.
Oxygenated products of catalytic oxidation, specifically
alcohols that may be prepared from alkenes, can function as
ligands for (nitro)cobalt(III) porphyrins. Their presence at
low concentration allows oxidation catalysis to continue in
solution due to their weak coordination that allows the
reactive pentacoordinate nitro complex to remain in equi-
librium. Although the nitro ligand in these water and
methanol complexes appears to have a structure more similar
to that in the five-coordinate (nitro)cobalt porphyrin system
than to that in the corresponding pyridine complex, its
reactivity is governed by the overall thermodynamics of the
oxygen-transfer reaction.
Supporting Information Available: Figure S1: Temperature-
dependent FTIR spectral changes in the range of νs(NO2) observed
for carbontetrachloride solution of 1.1 × 10-2 M (C2H5OH)CoTPP-
(NO2) and 5.14 × 10-1 M ethanol (1, 18 °C; 2, 25 °C; 3, 35 °C;
4, 45 °C; 5, 55 °C; 6, 65 °C). Table S1: Bond lengths [Å] and
angles [deg] for (H2O)CoTpivPP(NO2). Table S2: Bond lengths
[Å] and angles [deg] for (CH3OH)CoTpivPP(NO2). This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We gratefully acknowledge the U.S.
Civilian Research and Development Foundation (CRDF,
12001) and the National Foundation for Science and
IC048701A
Inorganic Chemistry, Vol. 44, No. 7, 2005 2223