11004
J. Phys. Chem. 1994, 98, 11004- 11006
Ab Initio Hartree-Fock and Local Density Functional Calculations on Prototype Halogenated Porphyrins. Do Electrochemically Measured Substituent Effects Reflect Gas-Phase Trends? Abhik Ghosh Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, and the Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, Minnesota 5541 5 Received: June 6, 1994; In Final Form: July 28, 1994@
First-principles quantum chemical methods have been employed to simulate and predict the electronic effects of peripheral polyhalogenation in porphyrins. Hartree-Fock theory performs unexpectedly poorly in calculations of valence ionization potentials of polyhalogenated porphyrins. In contrast, the results of local density functional calculations of the lowest ionization potentials of free base porphyrin, meso-tetrafluoroporphyrin, and P-octachloroporphyrin are consistent with existing electrochemical information. In the absence of macrocycle buckling due to steric interactions with other substituents, meso-tetrafluorination and P-octachlorination increase the first ionization potential of free base porphyrin by 50.1 and -0.6 eV, respectively.
Even after many decades of sustained activity, porphyrin chemistry remains a vibrant field of research. An important recent development in this field has been the synthesis of polyhalogenated porphyrins.’ Iron and manganese complexes of certain polyhalogenated porphyrins have been found to be highly active catalysts for various hydrocarbon oxygenation reaction^.^^^ In addition to intense efforts to characterize the chemical reactivity of polyhalogenoporphyrins and their metal c ~ m p l e x e s ,a~ ~small ~ number of studies have sought to characterize the salient structural and electronic properties of these novel ligands, using such techniques as crystallography? electrochemistry? Raman spectroscopy?b X-ray photoelectron spectroscopy,6 and theoretical calc~lations.~A broad goal of our research in this area is to apply modem first-principles quantum chemical methods to extend our understanding of the electronic nature of these materiak8-10 We have been considerably successful in semiquantitative simulations and predictions of photoelectron spectra of a number of free-base porphyrinic molecules, including unsubstituted p ~ r p h y r i n ,octaethylporphyrin, ~.~ lo phthalocyanine,’ octaethylporphyrazine,’ and a series of tetraphenylporphyrins,lO using ab initio Hartree-Fock (HF) theory. Our recent theoretical surveylo of substituent effects in porphyrins also reported ab initio HF calculations on halogenated porphyrins. However, our calculated substituent effects in halogenated porphyrins could not be compared to experiment,1° since the appropriate physical measurements had not been reported at that time. Recent electrochemical data5 on halogenated porphyrins gave us an opportunity to reexamine our HF-based predictions,1° revealing an unexpected failure of HF theory to correctly simulate the influence of peripheral halogen substituents on the porphyrin macrocycle. In view of the success of HF theory in describing electronic differences among a variety of other porphyrinic the present failure in the case of polyhalogenoporphyrinsreflects a hitherto unnoticed, electronic peculiarity of these halogenated molecules. In contrast to HF theory, local density functional (LDF) calculations,12 which account for electron correlation in a computationally expedient manner, yield valence IPS of polyhalogenoporphyrins that are in excellent agreement with the electrochemical data.8 @
Abstract published in Advance ACS Abstracts, October 15, 1994.
0022-3654/94/2098-11004$04.50/0
xJ4x
X
X X
PH,: PF4H2: PCI,H,:
H H CI
Y H
F H
Figure 1. Structures of molecules studied in this work.
We have computed the two lowest ionization potentials (IPS) of unsubstituted porphyrin (PH?), meso-tetrafluoroporphyrin (PF&), and P-octachloroporphyrin (PClgHZ), all in free base form, to compare the performance of HF and LDF theories (Figure 1). We chose P F a 2 and PCl& as simplified models of known compounds, P-octaethyl-meso-tetrafluoroporphyrin and several /3-polychloro-meso-tetraarylporphyrins,respect i ~ e l y .The ~ large calculations reported here have been feasible owing to our use of the direct SCF method,13 as implemented in the program system DISC0.14 We used double-c (DZ)15-18 and numerical double-f plus polarization (“DNP’) basis sets in our HF and LDF19 calculations, respectively. Earlier studies with larger basis sets have shown that, for porphyrinic molecules, our DZ basis sets result in orbital energies that are essentially converged to the HF limit.8J0s1 The three molecules studied were assumed to have partially optimized, D2h-symmetric geometries, which have been obtained from ref 10. The cationic states studied for these molecules were the lowest, closely spaced 2A, and 2Blu (&h notation) states, which are generdy weheparated energeticdy from other excited ~ t a t e s . ~ , ~ ~ 0 1994 American Chemical Society
Letters
J. Phys. Chem., Vol. 98, No. 43, I994 11005
TABLE 1: Computed IPS(eV) of Free Base Porphyrins computed IP (eV) cationic molecule final state IKT IASCFILDF erelax(eV) EA^,,^ (eV) PH2
'A,
PH2
'BI,
6.23 6.44
5.84 5.97
7.38 7.21
0.39 0.47
1.54 1.24
PF4H2 PF4H2
'A, 'Blu
7.15 6.83
6.75 6.40
7.87 7.29
0.40 0.43
1.12 0.89
PClsHz PClsHz
'A, B ' 1,
7.50 7.53
6.98 6.96
7.93 7.83
0.52 0.57
0.95 0.87
TABLE 2: Substituent Effects (eV) on Valence IPS Relative to PH2 2Aufinal state 2Blufinal state molecule IKT IASCF IALDF IKT IASCF IALDF PH2 (zerolevel) 0.00 0.00 0.00 0.00 0.00 0.00 0.91 0.49 0.39 0.43 0.08 PF4H2 0.92 PClsHz 1.27 1.14 0.55 1.09 0.99 0.62 (These 2A, and 2Blu states correspond to the lowest 2Alu and 2A2ustates of a &-symmetric metalloporphyrin cation.) Here we have computed IPS (denoted by Z with different subscripts) using Koopmans' theorem (ZKT) and via HF-ASCF (ZASCF) or ALDF ( Z ~ D F ) calculations. A ASCF or ALDF IP is defined as the difference in total energy between a molecule's un-ionized initial state (Le., the neutral porphyrin) and the ionized final state (i.e., the porphyrin cation radical). In order to compare the performance of HF and LDF theories, it is useful to dissect ZALDF into additive terms. The IPS,according to Koopmans' theorem, refer to a frozen-orbital picture in which the orbitals are assumed to be identical in the neutral initial and final ionized states and thus ignore the effects of orbital relaxation upon removal of the electron. The relaxation energy is given by
The relaxation energy, erelax,is always positive, since orbital relaxation will always result in a lower energy for the ionized final state than in the frozen-orbital approximation. Assuming that the difference between the ASCF and ALDF IPS is due primarily to correlation effects included in the latter type of calculation, the effect of differential correlation (eAcorr)in the initial and final states is given by
In general, one can assume that the correlation energy of a molecule scales with the number of electrons. The neglect of correlation in HF calculations thus leads to a smaller error in the total energy of the ionized final state than for the neutral initial state, and therefore, the differential correlation effect on the IPS, eAcorr,is usually positive. The ALDF IPScan now be expressed as
Table 1 lists the two lowest computed IPS for each of the three porphyrins studied. Table 2 recasts these data into substituent effects relative to unsubstituted PH2. For example, the substituent effect of meso-tetrafluorination is given by the difference Z(PF4H2) - Z(PH2), where the IPS(denoted I) are evaluated using a particular theory and for a particular final state symmetry. Table 1 shows that the relaxation energy, erelax,varies little among the three molecules and the two final state symmetries. In other words, substituent effects are approximately equal on IKTand ZASCF. The differential correlation effect, eAcorr,varies
significantly among different porphyrins. Thus, as Table 2 shows, substituent effects at the LDF level differ markedly from those at the HF level. Below we compare substituent effects on the HF and LDF IPS to trends in electrochemical oxidation potentials. At the Hartree-Fock level (ZKTand ZASCF), meso-tetrafluorination exerts an effect of about 0.9 and 0.4 eV on IPS for the 2A, and 2Blu ionized states, respectively. These significant substituent effects contrast sharply with a difference of merely 0.02 V between the one-electron oxidation potentials of octaethylporphyrin and /?-octaethyl-meso-tetrafluoroporphyrin?a The lowest LDF IPS of PH2 and PF4H2 differ by only 0.08eV (see Table l), in excellent agreement with a near-zero shift of the oxidation potential as a result of meso-tetrafluorination of ~ctaethylporphyrin.~" A comparison of our computed results on PClgH2 with existing electrochemical data on p-polychlorinated porphyrins is slightly more involved. While we have assumed a planar D2h-symmetric geometry for PClgH2, all known P-octahalogenated porphyrins are also meso-tetraaryl-substitutedand have strongly buckled macrocycle geometries as a consequence of steric effect^.^ We can estimate the effect of p-octachlorination alone on the first IP of PH2 from the electrochemical oxidation potential of 7,8,17,18-tetrachloro-5,10,15,20-tetramesitylporphyrin, which has a nearly planar porphyrin 7,8,17,18Tetrachlorination of P-unsubstituted tetramesitylporphyrin increases the one-electron oxidation potential by 0.24 V.5b Assuming that the electronic effects of P-chlorination are additive, one would predict that P-octachlorination should increase the oxidation potential of porphyrin by about 0.48eV, provided there is no macrocycle distortion due to bulky meso substituents. This predicted substituent effect of 0.48V, based on electrochemical data, is in excellent agreement with a difference of 0.55-0.62 eV between the ALDF IPS of PH2 and PClgH2 (see Table 2). At the HF level, the effect of poctachlorination on the two lowest IPS of PH2 is about 1 eV or more, which is far too high compared to the estimate based on electrochemical data. The electrochemical results show that peripheral polyhalogenation alone results in surprisingly modest shifts in the oxidation potentials of porphyrins. This raises the question of whether these modest electrochemical substituent effects really reflect substituent effects on gas-phase IPS. In other words, are the electrochemical results observed by artifacts such as solvent effects that are difficult to quantify? In the absence of gasphase IPS of halogenated porphyrins, we can suggest a plausible answer to this question on the basis of our LDF results. The agreement of the LDF results with electrochemical data suggests that electrochemical oxidation potentials of the halogenated porphyrins in question indeed reflect the substituent effects that would be obtained from gas-phase ultraviolet photoelectron spectroscopic measurements. The modest magnitudes of the electronic effects of peripheral polyhalogenation are, therefore, an intrinsic characteristic of the interaction between the porphyrin core and the halogen substituents and not an artifact of the electrochemical measurements. A synthetic consequence of this result is that the design of the ultimate high-potential porphyrin ligands cannot be based on peripheral polyhalogenation alone, but must utilize other substituents (e.g., carbalkoxy, pentafluorophenyl, polyfluoroalkyl, etc.). In conclusion, ab initio HF calculations, with orbital energies essentially converged to the HF limit, fail to semiquantitatively reproduce the interaction of peripheral halogen substituents with the porphyrin core. The poor performance of HF theory for halogenated porphyrins contrasts with successful simulations,
11006 J. Phys. Chem., Vol. 98, No. 43, 1994 at the HF level, of core and valence photoelectron spectra of several other porphyrinic molecu1es.*J0J1 In contrast to HF results, substituent effects at the LDF level are completely consistent with electrochemical data on halogenated porphyrins. The results obtained here underscore the critical role that electron correlation plays in determining calculated properties of porphyrinic molecules, particularly halogenated porphyrins.
Acknowledgment. This research was supported by the Minnesota Supercomputer Institute and Biosym Technologies, Inc. The author is indebted to Professor Jan Almlof for helpful discussions. References and Notes (1) (a) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987,26, 1338. (b) Wijesekera, T.; Matsumoto, A,; Dolphin, D.; Lexa, D. Angew. Chem., Int. Ed. Engl. 1990,29,1028. (2) (a) Ellis, P. E.; Lyons, J. E. Coord. Chem. Rev. 1990,105, 181. (b) Grinstaff, M. W.; Hill, M. G.;Labinger, J. A.; Gray, H. B. Science 1994,264,1311. (3) For a recent review on metalloporphyrin-catalyzed oxidation reactions, see: Meunier, B. Chem. Rev. 1992,92,1411. (4) (a) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin, R.N.; Gold, A.; White,P. S.; Brigaud, 0.;Battioni, P.; Mansuy, D. Inorg. Chem. 1992,31,2044.(b) Ochsenbein, P.; Mandon, D.; Fischer, J.; Weiss, R.; Austin, R.; Jayaraj, K.; Gold, A,; Temer, J.; Bill, E.; Miither, M.; Trautwein, A. X. Angew. Chem., Inr. Ed. Engl. 1993,32, 1437. (c) Marsh, R. E.; Schaeffer, W. P.; Hodge, J. A,; Hughes, M. E.; Gray, H. B.; Lyons, J. E.; Ellis, P. E., Jr. Acta Crystallogr. C 1993,49, 1339. (d) Schaeffer, W. P.; Hodge, J. A.; Hughes, M. E.; Gray, H. B.; Lyons, J. E.; Ellis, P. E., Jr.; Wagner, R. W. Acra Crystallogr. C 1993,49, 1342. (5) (a) Naruta, Y.; Tani, F.; Maruyama, K. Tetrahedron Lett 1992, 33,1069. (b) Ochsenbein, P.; Ayougou, K.; Mandon, D.; Fischer, J.; Weiss, R.; Austin, R. N.; Jayaraj, K.; Gold, A.; Temer, J.; Fajer, J. Angew. Chem., lnt. Ed. Engl. 1994,33,348. (6) (a) Ghosh, A.; LeGoff, E.; M e n , A.; Schropp, R. J . Org. Chem., submitted. See also: (b) Ghosh, A. J. Org. Chem. 1993,58, 6932. (7) For a semiempirical theoretical study of P-octahalogeno-mesotetraarylporphyrins, see: Brigaud, 0.;Battioni, P.; Mansuy, D.; GiessnerPrettre, C. New J . Chem. 1992,16, 1031.
Letters (8) Ghosh, A.; Almlaf, J.; Gassman, P. G. Chem. Phys. Lett., 1991, 186, 113.
(9) Ghosh, A.; Almlof, J. Chem. Phys. Len. 1993,213,519. (10) Gassman, P. G.; Ghosh, A.; Almlof, J. J. Am. Chem. SOC. 1992, 114,9990. (11) Ghosh, A,; Gassman, P. G.; Almlof, J. J . Am. Chem. SOC. 1994, 116,1932. (12) For a review of chemical applications of density functional theory, see: Ziegler, T. Chem. Rev. 1991,91,651. (13) Almlof, J.; Faegri, K.; Korsell, K. J . Compur. Chem. 1982,3,385. (14) Almlof, J.; Faegri, K.; Feyereisen, M.; Korsell, K. DISCO, a direct electronic structure code. (15) For the HF calculations, the basis sets for C, N, and F were (6s3p)/ [3s2p], employed a general contraction scheme,16 and were obtained from: van Duijneveldt, F. B. IBM Res. Rep. 1971,RJ945. The basis set for hydrogen, (3s)/[2s], was also obtained from this publication. The hydrogen exponents were multiplied by a scaling factor of 1.44. For molecular applications, a scaling factor of 1.2 is optimal for Slater-type orbital basis sets for hydrogen, which corresponds to a scaling factor of 1.44 for Gaussian basis sets. (16) (a) Raffeneti, R. C. J . Chem. Phys. 1973,58, 4452. (b) Schmidt, M. W.; Ruedenberg, K. J . Chem. Phys. 1979,71,3951. (17) The sp basis set for C1 was obtained from: Roos, B.; Siegbahn, P. Theor. Chim. Acta 1970,17,209.The value of the d exponent (0.68) was obtained from: Roos, B.; Siegbahn, P. Theor. Chim. Acta 1970,17, 199. Overall, the basis set used for C1 was (10s6pld)/[5s4pld]. (18) For all elements, the outermost s and p primitives were uncontracted so that they could constitute basis functions on their own. (19) The LDF calculations were canied out with the program system DMOL, using methods described in: Delley, B. J . Chem. Phys. 1992,92, 508. DMOL is distributed by Biosym Technologies, Inc., of San Diego, CA. The exchange-correlation functional used is that due to von Barth and Hedin: yon Barth, U.;Hedin, L. J. Phys. C 1972,5, 1629. (20) This characteristic of porphyrins is embodied in Gouterman’s fourorbital model of the electronic structure of porphyrins: Gouteman, M. In The Porphyrins: Dolphin, D., Ed.; Academic: New York, 1978; Vol. LII, Part A, p 1. (21) This characteristic of porphyrins is supported by ultraviolet photoelectron spectroscopic measurements: (a) Khandelwal, C.; Roebber, J. L. Chem.Phys. Len.1975,34,355. (b) Yip, K. L.; Duke, C. B.; Salaneck, W. R.; Plummer, E. W.; Loubriel, G. Chem. Phys. Lett. 1977,49,530. (c) Kitagawa, S . ; Morishima, I.; Yonezawa, T.; Sato, N. Inorg. Chem. 1979, 18, 1345. (d) Dupuis, P.; Roberge, R.; Sandorfy, C. Chem. Phys. Lett. 1980,75,434.
