D. K.
854
Lavallee and W. S. Hamilton
The Resonance Energy of Tetraphenylporphyrin David K. Lavallee" Department of Chemistw, Colorado State University, Fort Collins, Colorado 80523
and Walter S. Hamilton Department of Chemistry, Texas Woman's University, Denton, Texas 76204 (Received October 20, 1976) Publication costs assisted by the Petroleum Research Fund
The synthetic macrocyclic ligand tetraphenylporphyrin has been used in a large number of studies intended to model properties of natural porphyrins. The resonance energy of the delocalized macrocyclic ring system of porphyrins is a most important aspect in determining many of their spectral, electrochemical, and reactive properties. The previous reported value for the resonance energy of tetraphenylporphyrin was over twice as great as the resonance energies of natural porphyrins (560 kcal/mol vs. 236 f 9 kcal/mol) but physical and chemical properties of these species are generally very similar. In redetermining the resonance energy calorimetrically, a resonance energy for tetraphenylporphyrin of 244 kcal/mol, in close agreement with values for natural porphyrins, has been obtained.
Introduction The synthetic porphyrin meso-a,&y,&tetraphenylporphyrin (hereafter tetraphenylporphyrin) has been studied extensively in order to understand properties of natural porphyrins. The facile synthetic method developed by Adler et al.' has made this material readily obtainable. Tetraphenylporphyrin is easily purified and readily modified synthetically. Recent work has involved model systems for oxygen binding by hemoglobin and myoglobin using the cobalt,' iron: and manganese4 complexes of tetraphenylporphyrin and para-substituted tetraphenylporphyrins as well as the iron complexes of significantly modified tetraphenylporphyrins such as the "picket fence" porphyrin of Collman et al.5 and the "capped" porphyrin made by Baldwin et al.6 Tetraphenylporphyrin complexes have also been employed in model studies with the object of mimicking and understanding properties of the cytochromes, notably the P-450cytochromes. Our group has used modified tetraphenylporphyrin complexes in studies of porphyrin metallation and metal ion insertion.8-1' In such studies, the properties of the meso substituted porphyrins have generally been compared with those of the pyrrole substituted natural porphyrins (Figure 1) and found to be remarkably similar. Extensive kinetic studies of metal complexation by a large number of synthetic and natural porphyrins have also demonstrated the similar reactivities of these ~pecies.'~-l~ Absorption spectra of meso-substituted and pyrrole substituted porphyrins also show very similar patterns with the exception of the spectra of protonated porphyrins which are influenced by the participation of the phen 1rings in the conjugated ?r system of the Considering the importance of tetraphenylporphyrin in current research and the general similarities of the properties of these compounds, the extreme variation between results for the resonance energy of the porphine ring system (not including the phenyl ring resonance energy) of these species is indeed disturbing. The value given for tetraphenylporphyrin," based on resulta of Long0 et a1.,20is 560 kcal/mol vs. values generally in the range of 200-250 kcal/mol for natural porphyrinslg (Table I). While undertaking an investigation of the effect of distortion upon porphyrin resonance energy, we decided to The Journal of Physical Chemistry, Vol. 81, No. 9 , 1977
obtain combustion data for tetraphenylporphyrin, which we report herein. Experimental Section Apparatus and Procedures. The apparatus and experimental procedures have been described previously.21 The temperature rise was measured by quartz thermometry. The internal volume of the bomb was 0.342dm3. For every experiment approximately 1 g of water was added to the bomb, and the bomb was flushed and charged to 30 atm with pure oxygen. The samples were weighted to an accuracy of 0.01mg, and correctionsfor air buoyance were applied. Ignition was accomplished by fusing a 10 cm length of no. 44 s.w.g. platinum wire in contact with 4 of Whatman No. 1 filter paper. The a small piece ( ~mg) value of hE "/Mfor the filter paper was taken as -4118 f 10 cal !g'The extent of combustion was based on the mass of sample. All calculations, including conversion of time and temperature measurements to initial and final temperature, correction for heat exchange between calorimeter and jacket, and reduction to the standard state, were carried out on a digital computer.23 The computer program followed the procedure of Hubbard et al.24The calorimeter was calibrated with benzoic acid, NBS sample 39i, which had a heat of combustion of 26.434 f 0.0003 absolute kJ 8-l under certificate conditions. Materials. Tetraphenylporphyrin was synthesized by the method of Adler et al.' using freshly distilled pyrrole and benzaldehyde in propionic acid. The resulting purple crystals were thoroughly washed and chromatographed on basic alumina with chloroform as eluent to remove small amounts of tetraphenylchlorin. The center of the tetraphenylporphyrin fraction was carefully filtered and the visible absorption spectrum showed no evidence for any tetraphenylchlorin or protonated tetraphenylporphyrin. The chromatographed material was crystallized from dichloromethane, washed with copious amounts of water and cold methanol, dried, and recrystallized from 1:l dichloromethane/toluene. The resulting crystals were washed and dried and observed by visible absorption spectroscopy after dissolution in dichloromethane. Material from separate syntheses and different recrystallizations were used in this study.
Resonance Energy of Tetraphenylporphyrin
855
TABLE I: Illustrative Resonance Energies of Porphyrinsa Compounds
REIb RE1rb
a. Naturally derived porphyrins Protoporphyrin I X dimethyl ester Mesoporphyrin I X dimethyl ester Coproporphyrin I tetramethyl ester Isouroporphyrin I1 octamethyl ester Phylloerythrin monomethyl ester Chloroporphyrin e4 dimethyl ester Pheoporphyrin a, dimethyl ester Chlorin e4 dimethyl ester b. Synthetic non-meso-substituted porphyrins Etioporphyrin I Octaethylporphyrin PorphinC c. Meso-substituted synthetic porphyrind Te traphenylporphyrine Te traphenylporphyrinf
259 265 243 164 221 225 211 215
163 179 157 78 135 139 126 136
240 257 582
155 171 497
560 244
452 158
a Based on the combustion data of A. Stern and G. Klebs, Justus Liebigs Ann. Chem., 505, 295 (1933), unless otherwise specified. These results have been presented by See text for explanation. Reference George, ref 19. 20. d After allowing the contribution of 1217.67 kcal for each phenyl group. e Reference 19, using data of Longo et al. f This work.
TABLE 11: Physical Properties at 298.15 K C, cal p , g/mL deg-' g-'
Tetraphenylporphyrin Benzoic acid Fuse
1.27 1.320 1.50
0.200 0.289 0.400
(aE/aP)T,
cal atm-' g"
(- 0.0030) -0.00278 (0.0028)
r
The density of the crystals was determined b suspension in aqueous ZnClzand found to be 1.27 g/cm . The space group (triclinic, was determined by use of an Enraf-Nonius precision camera and copper x-radiation.
Results Units of Measure and Auxiliary Quantities. All data reported are based on the 1961 atomic weights26 for purposes of comparison to previously reported data on porphyrins. For reducing weights in air to weights in vacuo and correcting to standard states, the values summarized in Table I, all for 298.15 K, were used for density, p , specific heat, C,, and (aE/aP)Tfor the substances. For benzoic acid and the filter paper fuse, the values were taken from the literature. The specific heat of tetraphenylporphyrin was measured with a Perkin-Elmer DSC-1B differential
Figure 1. The porphine ring system. The methine bridge or meso positions are designated by greek letters. rnesea,P,y,&Tetraphenylporphyrin has a phenyl group substituent at each of these positions. The natural porphyrins such as protoporphyrin have substituents on the pyrrole &carbon atoms with hydrogen atoms on the bridge positions.
scanning ~ a l o r i m e t e r and , ~ ~ the energy coefficient was estimated. CalorimetricResults. The apparent energy equivalent of the calorimeter, e(calor), was determined from nine calibration runs. The average value was 2391.14 f 0.22 cal deg-' where the uncertainty is expressed as the standard deviation of the mean. Five satisfactory combustion experiments were obtained for tetraphenylporphyrin. Data for the combustion experiments are summarized in Table 11. The values for AE,O/M at 298.15 K for tetraphenylporphyrin in Table I1 refer to the equation: C,,H3,N4, t
IO3/,
O,,
-+
44CO,, t 15H,01
(1)
Derived Results. Values of enthalpy of combustion derived from mass of sample and current best values28of the enthalpies of formation of gaseous carbon dioxide and liquid water were combined to derive a value for the enthalpy of formation in the condensed state. This value is listed in Table 111. The enthalpy of sublimation2' was combined with the enthalpy of formation in the condensed state to derive the standard enthalpy of formation in the gaseous state also shown in Table 111. Discussion Resonance energies for aromatic systems may be calculated by a variety of methods. A survey and critical account of the approaches to these calculations for porphyrins has been written by George." We have chosen to
TABLE 111: Combustion Experimentsa for Tetraphenylporphyrin 0.40059 0.40460 0.37062 0.51 166 0.00413 0.00413 0.00398 0.00421 1.35311 1.46226 1.47662 1.86508 0.05484 0.05995 0.05629 0.05923 - 3496.47 - 3530.79 - 3235.48 - 4459.66 -5.85 - 5.81 - 5.74 -7.49 1.89 1.90 2.00 2.45 5.25 5.18 5.55 6.73 17.02 17.02 16.40 17.35 0.21 0.21 0.21 0.21 -8680.21 -8682.04 - 8680.89 - 8678.44 -8680.49 i: 0.59 Derived results at 298.15 K, kcal mol-' AE," = -5336.39 i: 1.8 AHsub= 35 f 3.0' ~ H , " = - 5 3 3 9 . 6 5 i: 1.8 A H P ( g ) = 211.7 f 3.8 AH;= 176.7 f 1.9
m' (compd), g m"' (fuse), g A.t,, deg n1(H,O), mol €(calor) (-A t c ) , cal e(cont) ( - A t , ) , calb A E , corr to std states, cal AEf(dec) (HNO,), cal - m"' A EcolM (fuse), cal AE~,,,,cal AE,"/M (compd), cal g-l Mean value and std dev of the mean
0.4561 3 0.00420 1.66387 0.05939 - 3978.55 - 6.60 2.17 5.86 17.31 0.21 - 8680.86
a The uncertainty interval is taken as twice the final overall standard d e ~ i a t i o n . ~Reaction temperature is 298.15 K. Symbols and terminology are those of ref 4. ei(cont) (ti- 25 "C - tf + tcorr). Reference 28. In this reference, the crystalline form is not specified. In our work, triclinic crystals were used. Since tetraphenylporphyrin crystallizes in at least three space groups (ref 30), the sublimation energy in our case may be slightly different from the reference value.
The Journal of Physical Chemlstty, Vol. 81, No. 9. 1977
D. K. Lavallee and W. S. Hamilton
856
TABLE IV: Calculation of t h e Resonance Energy of T e t r a p h e n y l p o r p h yrina
AH,’, porphin r i n g system
I Porphinb
- 4(C-H)
+ 4( Cb-C)
I1
AH,”,
p h e n y l groups
4255.94 4321.22 3E(C-C) t -402.12 -402.12 5E(C-H) 356.64 355.64 3E(C=C) ___.-
A l t e r e d porphin 4209.46 4294.74 P h e n y l group 4 p h e n y l groups 4870.64 _ _ _4870.64
316.01 502.65 399.00 1217.66
9080.10 9165.38
AH,” found Resonance energy
-9322.9 _ _ _ _ 9322.9
244.3
157.5
a All values in k c a l mol-’, see t e x t for explanation of choice of values. Reference 19.
calculate the resonance energy for tetraphenylporphyrin by two methods. The Cox and Pilcher method2’ assigns the enthalpy of the C-C bond without resonance contribution in an aromatic system from the butadiene/butane redistribution reaction, resulting in a value of 94.66 kcal/mol. The resonance energy calculated from this value is referred to here and in George’s work as RE11 and is likely to be a lower limit of the resonance energy.lg Another calculation is based on the energy of the ethylene/ethane reduction reaction and results in E(C-C) = 89.12 kcal/mol. Resonance energies calculated on this basis are given as REI and are likely to be upper limits. Attempts to better define resonance energies rely on determining the proper model for comparison and have not been universally accepted. For our purposes, comparisons of values for various porphyrins are essential but of tetraabsolute values are not. The data for Mao phenylporphyrin which are presented allow one to recalculate resonance energies according to any preferred model. Table IV presents the calculation of the resonance energy of tetraphenylporphyrin. The “normal” resonance energy of the phenyl ring fragments has been subtracted. The value for porphine is that given by George.lg The value for E(C-H) which is used to subtract the contribution of the methine hydrogen atoms which are present in porphine but not meso-substituted por hyrins is the normal value for an aromatic C-H bond?‘Several conjugation schemes for the porphine moiety have been presented,lg but all involve conjugation at the methine bridge position. The value for E(C-C) is that of a single C-C bond in toluene3’ since the methine carbon-phenyl carbon bonds in tetraphenylporphyrin structures are found to be of the bond length appropriate to single b~nding.’~ The value for the phenyl ring contribution is independent of the method of resonance energy calculation since 3E(C-C) RE is the same by either method. The value for AHfosof tetraphenylporphyrin given in Table J Y is arrived of 211.7 f 3.8 given in Table I11 and the a t from MfoS AHf” values for the gaseous element^.^' It is evident that the results presented herein support the previously obtained results of comparisons of properties of tetraphenylporphyrin and the natural porphyrins.
+
The Journal of Physlcal Chemistry, Vol. 81, No. 9, 1977
It appears that the porphine ring system resonance energy is very similar for these species. Our value for REI of 244 kcal/mol compares well with the values of 236 f 9 kcal/ mol for 11porphyrins cited by George.” The phenyl rings of tetraphenylporphyrin are not coplanar in the crystal structure31and appear to contribute little to the resonance energy of the compound since the assumption of additivity of the phenyl resonance energy which has been employed in the calculation gives a value of the porphine resonance energy which is within the range found for non-mesosubstituted porphyrins. It would also appear that the slight ruffling of the porphine nucleus found in the crystal structure of tetraphenylp~rphyrin~l does not result in a major change in resonance ene;p from the planar system found in crystalline porphine. Further efforts are being directed toward an evaluation of the effect on the resonance energy of porphyrin caused by distortions of the porphine system that result from N-substitution. Acknowledgment. We express gratitude to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
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