§ Hydrogen peroxide disproportionation reactions: dismutation reactions
were performed at room temperature in a sealed (PTFE septum) 10 mL
reaction vial equipped with a magnetic stirbar and a capillary gas delivery
tube linked to a graduated burette filled with water. The reaction vial was
charged with 1 mmol of the iron porphyrin catalyst, 25 mmol of 1,5-
dicyclohexylimidazole, 1.5 mL of CH2Cl2, and 0.5 mL of CH3OH. The
solution was stirred for 5–10 minutes to ensure gas pressure equilibration.
An aliquot of 10.4 M (30%) aqueous H2O2 (0.11 mL) was added to the
reaction mixture via syringe, and the reaction mixture was stirred
vigorously. The time was set to zero immediately after addition of H2O2.
The conversion of the reaction was monitored volumetrically, and the
TON for produced O2 (n) was calculated through the perfect gas equation
pV = nRT, assuming that p = 1 atm. The identity of the oxygen gas was
confirmed independently by the alkaline pyrogallol test.
motif is not sufficient in and of itself to stabilize a monomeric
iron(III) hydroxide porphyrin.6 Indeed, in addition to a hydrogen-
bonding manifold, decoration of the porphyrin periphery with aryl
groups with the appropriate stereoelectronic properties is required
for the development of highly active and robust systems for O–O
bond activation and oxidation catalysis.
This work was supported by the National Institutes of Health
(GM 47274). J. R. thanks the Fannie and John Hertz Foundation
for a doctoral fellowship. The authors thank Dr D. R. Manke for
assistance with crystallography and D. Wright for uplifting
achievements.
" All oxidation potentials recorded in CH2Cl2 vs. SCE using tetrabuty-
lammonium perchlorate (TBAP) or tetrabutylammonium hexafluoropho-
sphate (TBAPF6) as the electrolyte.
Notes and references
I Control H2O2 disproportionation experiments employing iron(III)
chloride 5,10,15,20-(2,6-dimethoxylphenyl)porphyrin and iron(III) chloride
5,10,15,20-(pentafluorophenyl)porphyrin both result in the production of
negligible amounts of O2 under the conditions described above.§
{ The synthesis of Hangman porphyrins 1 and 2 and synthetic precursor 7
have been reported previously.3,6 The synthesis of Hangman porphyrin 3
was accomplished as follows: a mixture of xanthene aldehyde 7 (0.10 g,
0.25 mmol), pentafluorobenzaldehyde (0.735 g, 3.75 mmol) and pyrrole
(0.275 mL, 4.0 mmol) in chloroform (425 mL) was purged with nitrogen
for 45 min, after which a portion of BF3?OEt2 (0.168 mL, 1.32 mmol) was
added via syringe. The solution was stirred at room temperature under
nitrogen in the dark for 1 h and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(0.68 g, 3.0 mmol) was added to the reaction. After stirring for an
additional hour under nitrogen, the solvent was removed under reduced
pressure. The dark residue was redissolved in dichloromethane (100 mL)
containing 2% triethylamine and filtered through celite. The filtrate was
loaded directly onto a silica gel column and eluted with dichloromethane
until no more porphyrinic product was detected by analytical TLC. Further
purification by column chromatography on silica was accomplished using
hexanes and dichloromethane (5 : 2) as the eluent to give the porphyrin
ester as a violet microcrystalline solid (87 mg, 31% yield based on starting
aldehyde). The purified porphyrin ester (80 mg, 0.0676 mmol) was
dissolved in a mixture of acetic acid (20 mL) and sulfuric acid (5 mL).
Water (6.0 mL) was added to the green solution and the reaction was
refluxed under N2 in the dark for 7 days. The reaction was cooled to room
temperature and extracted with 50 mL of dichloromethane. The organic
layer was washed with water, dried over Na2SO4, and the solvent was
removed by rotary evaporation. Purification by column chromatography
on silica was accomplished using dichloromethane as the eluent to provide
the freebase precursor to Hangman porphyrin 3 as a purple solid in near
quantitative yield. 1H NMR (500 MHz, CDCl3, 25 uC): d = 8.91 (m, 5H),
8.79 (d, J = 7 Hz, 2H), 8.09 (d, J = 4 Hz, 1H), 7.98 (d, J = 4 Hz, 1H), 7.72
(d, J = 4 Hz, 1H), 7.62 (d, J = 4 Hz, 1H), 1.99 (s, 6H), 1.57 (s, 9H), 1.25 (s,
9H), 22.76 (s, 2H). HRESIMS (MH+) calcd for C62H39F15N4O3 m/z =
1173.2855; found, 1173.2854.
Iron insertion into the freebase Hangman described above was
accomplished as follows: a combination of the freebase Hangman (60 mg,
0.51 mmol), FeBr2 (350 mg), and CH3CN (30 mL) was refluxed under
nitrogen for 8 h, opened to air, and brought to dryness under vacuum. The
solids were redissolved in dichloromethane (100 mL) and washed with
water (4 6 75 mL). The organic layer was stirred with 20% HCl (50 mL)
for 75 min, washed with water (5 6 100 mL), and taken to dryness. The
resulting residue was purified by column chromatography on silica gel,
eluting first with dichloromethane to remove less polar impurities and then
with 5% methanol in dichloromethane to elute the product. Following
concentration of the product under reduced pressure, the dark brown
material was re-treated with HCl as described above to furnish 3 as a
brown powder (48 mg, 84% yield). HRFABMS ([M 2 Cl]+) calcd for
C71H70N4O3Fe m/z = 1082.4797; found, 1082.4773. Anal. calcd for
C71H70ClN4O3Fe: C, 76.23; H, 6.31; N, 5.01. Found: C, 76.44; H, 6.19; N,
4.82%.
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Crystal data: freebase precursor to 3: C62H39F15N4O3, M = 1172.97,
orthorhombic, space group Pca21, a = 23.784(3), b = 14.5290(16), c =
3
16.258(2) A, U = 5618.1(12) A , Z = 4, Dc = 1.387 Mg m23, T = 183(2) K,
m = 0.120 mm21, wR2 = 0.1842 (5035 independent reflections), R1 = 0.0708
[I . 2s(I)]. CCDC 628103. For crystallographic data in CIF or other
electronic format, see DOI: 10.1039/b616884a
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2644 | Chem. Commun., 2007, 2642–2644
This journal is ß The Royal Society of Chemistry 2007