Proc. Nati. Acad. Sci. USA Vol. 74, No. 1, pp. 18-22, January 1977
Chemistry
Synthesis and characterization of "face-to-face" porphyrins (bimetallic ligands/metal-metal interaction/electron spin resonance/dioxygen reduction/dinitrogen reduction)
JAMES P. COLLMAN, C. MICHAEL ELLIOTT, THOMAS R. HALBERT, AND BENJAMIN S. TOVROG* Department of Chemistry, Stanford University, Stanford, Calif. 94305.
Contributed by James P. Collman, November 8,1976
is carried out at 250, the measured rates are such that Greenwood and Gibson (4) conclude that any intermediates must have half-lives of less than 10 Msec. Several mono- and dioxygenases, which reduce oxygen with concomitant oxidation of organic substrates, also contain more than one functional metal ion. These include the monooxygenases laccase (mono-phenol monooxygenase, 4 Cu), and ascorbate oxidase (8 Cu) as well as the dioxygenase L-tryptophan oxygenase (2 hemes and 2 Cu). In contrast to the large number of biological systems that reduce 02, evolutionary processes appear to have developed only a single dinitrogen fixing system, nitrogenase (5). Although the molecular mechanism has not yet been elucidated, a binuclear metal site for dinitrogen reduction has been proposed.t Abiological support for the idea of binuclear 02 reduction by Fe2+ comes from Hammond and Wu's study (7) ofthe Fe2+ / 02 reaction in nonaqueous solution. From the rate law and stoichiometry, it was proposed that the rate-limiting step is direct 4e- reduction of 02 by 2 Fe2+ as shown in Fig. 2 (I II). Although the postulated intermediates I and II in Fig. 2 were not directly observed, each has precedent in porphyrin chemistry. The "picket fence" dioxygen complex (8) is a model for I. "Compounds II" in the peroxidase family (9) are heme analogues of the ferryl intermediate postulated, (II), in Fig. 2. Analogues of the 1A-oxo ferric dimer, III, Fig. 2, are well known in porphyrin chemistry (10). We speculate that a binuclear "face-to-face" Fe2+ macrocyclic complex like IV in Fig. 3 will strongly bind dioxygen, giving V and immediately effecting a 4e- reduction to the bis-ferryl complex VI in a process requiring only a slight conformational change so that the overall reduction should occur without a substantial intervening energy barrier. Protonation of the bis-ferryl complex VI and rereduction of Fe by an electrode surface could result in the overall catalytic reduction of 02 to H20. Similar schemes involving Ru for 02 reduction and Mo or V for N2 reduction are also envisioned. The length of the groups linking the two macrocycles for an 02 reducing system is dictated by geometric estimates of the probable transition states and intermediates in the overall 4echange as is illustrated in Fig. 4. Thus, in an ideal binuclear catalyst the Fe-Fe gap should span the minimum range, about 4.8 A for the prior complex VIII to about 5.8 A for the bis-ferryl product IX, in Fig. 4. Similar estimates can be made for N2 reduction. Schwarz et al. (11) have reported studies on binary porphyrins joined through a single linkage, which is insufficient to enforce a "face-to-face" geometry. Recently, Ogoshi et al. (12) at a symposium mentioned the synthesis of a binary por-
ABSTRACT The syntheses of four binary porphyrins, two of which are constrained to a "face-to-face" conformation, and their Co2+ and Cu2+ derivatives are described. Electron spin resonance indicates that the intermetallic separation in the binuclear "face-to-face" porphyrins is about 6.5-6.8 A. Electronic spectra and proton magnetic resonance spectra support the postulated "face-to-face" conformations. A hypothesis that related compounds may serve as multielectron redox catalysts for 02 and N2 is presented. The development of efficient catalysts for the reversible multielectron reduction of 02 and N2 would have great significance. Such catalysts are essential to the oxygen cathode of an air-powered fuel cell and to electrochemical nitrogen fixation. The major problem in catalyzing four- or six-electron reductions of 02 or N2 is that stepwise one- or two-electron paths must pass through relatively unstable intermediates (Fig. 1). The high energy content of these intermediates precludes rapid stepwise electron transfer operating near the reversible potential of the overall four- or six-electron reduction. The rapid reduction of oxygen, which has a triplet ground state, also requires a mechanism to overcome the conservation of spin and orbital angular momentum. The latter problem might be circumvented by using a metal catalyst that has low-lying paramagnetic states. These arguments have been developed by H. Taube (personal communication). Many monometallic macrocyclic complexes adsorbed on graphite have been examined as catalysts for oxygen reduction (3). The most effective macrocycles have four nitrogen donor atoms. In the phthalocyanine series, the order of reactivity is Fe > Co > Ni > Cu > Mn. However, such studies have failed to reveal any catalyst that is capable of reversible reduction of 02 to water, possibly because with a single metal center, initial 2e- reduction to H202 is always dominant. We have approached this problem by constructing a new class of so-called "face-to-face porphyrins" in which two porphyrin rings are held in parallel conformation. Thus, two metal atoms might act in concert to bind and reduce dioxygen (or dinitrogen) in the gap between the porphyrin rings. Eventually these binuclear, cyclophane porphyrin complexes are to be attached to graphite to be tested as electrode catalysts. The idea of providing reducing equivalents rapidly from more than one metal site has precedent in the chemistry of certain metalloenzymes. The terminal enzyme of cellular respiration, cytochrome oxidase, which carries out the reaction, 02 + 4H+ + 4e -- 2H20, contains two copper ions and two heme groups (a and a3) per subunit. When the reaction with 02 Abbreviations: DAP, diaminoporphine; MDAP, mesoporphine IX diamide porphine; DUBP, diurea binary porphyrin; DTUP, ditolylurea porphyrin; TPP, meso-tetraphenylporphyrin; PMR, proton magnetic resonance; ESR, electron spin resonance; 1-MeImid, 1-methylimidazole. * Present address- Corporate Research, Allied Chemical Co., Morristown, N.J. 07960.
proposed binuclear mechanisms are compared and discussed by Hardy et al. (6).
t Several
18
Proc. Natl. Acad. Sci. USA 74 (1977)
Chemistry: Collman et al. B F2
B
19 B
B
Fe
Fe
+
4H+ P
A 50 HO2 +
'
a %
H0 22
a
'%
'
C
T
+1-78 > H N H +0.64NC 3Z e HO0 2 N2H2 -~.---e 2H5
FIG. 1. Standard reduction potentials of 02 and N2, in V. a Taken from the compilation of deBethune and Loud (1). b Approximated by Shilov (2). c Calculated by application of Hess's Law.
phyrin, but no details were reported. Herein we report the synthesis and physical properties of four binuclear porphyrins having various geometries and degrees of conformational freedom (see Figs. 5 and 6). The unusual spectroscopic properties of these ligands and their Co and Cu derivatives are enumerated, along with estimates of the intermetallic separations. Studies testing the 02 and N2 reducing capabilities of these ligands have not been carried out and are awaiting the synthesis of the Fe, Ru, V, and Mo complexes. MATERIALS AND METHODS An abbreviated nomenclature will be used for simplicity throughout this paper. The symbols a and # refer to the opposite faces of the porphyrin plane, while the terms cis and trans refer to adjacent and opposite meso positions of tetraphenyl porphyrins. In this manner, XI in Fig. 5, properly named 5a,10a-di(2-aminophenyl)-15,20-diphenylporphine becomes aa-cis-diaminoporphine or a,a-cis DAP. Synthesis: XI, a,a-cil DAP and XII, a,a-trans DAP. Condensation of pyrrole (2 eq) with benzaldehyde (1 eq) and 2nitrobenzaldehyde (1 eq) in acetic acid produced a mixture of meso-tetraphenylporphyrin (TPP) and mono-, di-, tri-, and tetra-nitrophenyl porphyrin positional isomers and atropisomers. Similar "mixed aldehyde" condensations have been reported by Little et al. (13). Column chromatographic separation with 1:1 benzene/cyclohexane eluent on silica gel (W. R. Grace type 62) gave several bands, the third and fifth of which were dinitroporphyrins. Reduction of this mixture of dinitroporphyrins by the established SnCI2/HCI method (8) gave a mixture of diamino porphyrins, which were separated by column chromatography with 19:1 dichloromethane/ethyl acetate eluent on silica gel (Merck PF254) into three bands, denoted A, B, and C. Thermal equilibration (2 hr in boiling benzene) of B gave equal parts A and B, while C gave equal parts A and C. The greater retention of B and C on silica gel suggested that they were the a,a atropisomers of trans and cis DAP. Band A therefore contains both a,jB atropisomers. The identities of B as a,a-trans DAP (XII) and C as a,a-cis DAP (XI) were confirmed by the characteristic splitting of the pyrrole C-Hs in the proton magnetic resonance (PMR) spectra, Fe2 +0+ 2
2[Fe= II
K
[FO FeO]21 2
Fe2
31 ke k2
2
2+ 2 2F 012 2+ 2[Fe-0-Fel4
-d 021 dt
III
]2[o02 k2K[Fe2 2
FIG. 2. Mechanism of reaction of Fe2+ with 02 in a nonaqueous environment.
(t) (a) F2FeFe
B IV
(t)
B V
2t ~~~~~~~~4eFe2
B VI
B VII
FIG. 3. Proposed reaction of 02 with a "face-to-face" binary Fe2+ porphyrin. B is an axial base too large to fit in the cavity. Ovals represent porphyrin rings.
and by the derivatization of C with isophthaloyldichloride which is known (8) to span o-amino groups on adjacent phenyl rings in TPP derivatives. Repeated thermal equilibration of A and chromatography led to an overall yield, based on pyrrole, of 1.1% XI and 0.4% XII. XIII, Mesoporphine IX Diamide-cis-NN-di(2-phenyl) diphenyl Porphine (MDAP) Mesoporphyrin IX diacid chloride (X) was generated by SOC12 treatment of a 1-methyl-2-pyrrolidinone solution of mesoporphyrin IX. Addition of one equivalent of a,a-cis DAP (XI), stirring at 25° for 1 hr, and chromatography on silica gel gave MDAP (XIII) in good yield as a violet powder. The product is homogeneous to high-pressure liquid chromatography, and its PMR spectrum is consistent with the proposed structure. It has the expected mass spectral parent ion at 1174 atomic mass units, and a single broad amide carbonyl band in the infrared, centered at 1688 cm-1. The electronic spectrum of XIII appears as a superposition of the spectra of the two free halves. XIV, exo-cis-Diurea Binary Porphyrin (exo-cis DUBP); XV, endo-cis-Diurea Binary Porphyrin (endo-cis DUBP); and XVI, trans-Diurea Binary Porphyrin (trans DUBP) The synthetic schemes for these ligands are outlined in Fig. 5, and are identical in detail, depending only on the starting material. The appropriate diaminoporphyrin isomer was dissolved in dry, oxygen-free benzene containing 1% pyridine and treated with excess phosgene, affording the dicarbamoylchloride (or diisocyanate). Excess phosgene was removed under vacuum, and a benzene solution containing an equimolar amount of diaminoporphyrin was added. After stirring under N2 at 25° overnight, the product was purified by silica gel chromatography. Starting with a,a-cis DAP (XI), there are two plausible binary porphyrin products, exo-cis DUBP (XIV) and endo-cis DUBP (XV). In the exo product (XIV) the porphyrin rings are geometrically disposed away from one another, while in the endo product (XV) they are above one another. The two isomers form in approximately equal amounts. With a,a-trans DAP as the starting material, the only binary porphyrin formed is trans DUBP (XVI). High-pressure liquid chromatography indicates that XIV, XV, and XVI are each homogeneous. Molecular weights obtained by osmometry for XV and XVI are 1308 and 1327 (calculated, 1341). Field desorption mass spectra show the ex1.8A
1.5A
Fe-0=0-Fe
Fe=0
1.2A
2. 8A
O=Fe
44.8A
5.8A
Vill
Ix
FIG. 4. Estimation of Fe-Fe distances for bimetallic 02 reduction.
20
Chemistry:
Collman et al.
Proc. Natl. Acad. Sci. USA 74 (1977)
H2N N
H2N
NH
HN
N
a,a-cis DAP
a,a-trans DAP
Xl' 1 COC 12, excess
,2. second equivalent Xii
Xi
endo-cis DUBP
exo-cis DUBP
XV
XIV
trans-DUBP XVI
FIG. 5. Synthetic scheme for the urea linked binary porphyrins. Geometries are idealized and double bonds are left out of porphyrins and phenyl rings for clarity.
pected molecular ions. Electronic spectral and PMR results support the proposed structures (vide infra). a,a-cis-Di-p-tolylurea Porphyrin (a,a-cis-DTUP) (XVII). This di-aryl urea monomeric porphyrin was made by treating the dicarbamoyl chloride derived from a,a-cis DAP with excess p-toluidine. Introduction of Cu2+ was carried out by heating the porphyrins with excess Cu(OAc)2 in CH2Cl2 containing 1-2% MeOH and excess 2,6-lutidine followed by chromatography on neutral alumina. Introduction of Co2+ was carried out under N2 by heating the porphyrins with excess anhydrous CoC12 in 1:1 tetrahydrofuran/benzene with excess 2,6-lutidine, followed by chromatography on alumina. 0
cl-c Cl-C=;-
Mesoporphyrin IX diacidchloride X I equivalent Xl
NH HN
A
~N
I'd~~~XI MDAP
\
FIG. 6. Synthetic scheme for the amide linked binary porphyrin MDAP. Geometry is idealized and double bonds in porphyrin and phenyl rings are left out for clarity.
Electron Spin Resonance Spectra (ESR). Spectra were collected on a Varian X-band E-12 spectrometer at about -1400. Cobalt samples were prepared under inert atmosphere. All spectra exhibit approximate axial symmetry. In spectra that exhibited splitting due to M-M interaction, the zero field splitting parameter D was determined as follows: in the parallel regions of both Co2+ and Cu2+ complexes, D is measured as the difference in position between the MI = 0 transition and gal taken from an appropriate monomeric porphyrin model. In the perpendicular region of the Co2+ spectra, D is alternatively determined by measuring the difference in line position where dI/dH = 0 (see Fig. 7). RESULTS AND DISCUSSION Electron Spin Resonance. Simple Cu2+ and low spin Co2+ porphyrins are S = '2 systems with' well known ESR characteristics (14). If two such porphyrins are close enough that the metals interact, the bimetallic system will have singlet (S = 0) and triplet (S = 1) states separated by the energy J. If the rate of electron exchange between the metals is fast compared to the resonance frequency (1010 sec'), then each electron experiences nuclear spin equal to the total nuclear spin of the two metals (I = 3 for Cu22+ and I = 7 for Co22+). The resulting hyperfine splitting, A, is half that for a related mononuclear S = 'A system. The ESR transitions will be further split by the zero field splitting, D. D is composed of two terms, Dpseudo (from spin orbit coupling) and Ddd (from the electron-electron dipolar interaction). Ddd is related to the metal-metal (M-M) separation, r, by the equation r = (0.65 gI2/Ddd)'/. It has been shown that Dpeudo can be ignored, and r can be calculated from D = Ddd without having significant effect on the calculated r (15). A more complete' description of the method of determining r is contained in ref. 15. Several examples of ESR involving two interacting S = 1 metalslhave appeared in the literature (15-22). Calculated distances have been in agreement with x-ray crystal structures where available (15, 16). Tables 1 and 2 list pertinent ESR data for several Co2+ and
Chemistry: Collman et al.
Proc. Natl. Acad. Sci. USA 74 (1977)
21
Table 2. ESR parameters and M-M distances from parallel region of spectra of Cu22, derivatives
VERTICAL EXPANSION 1 15x 40x 60x
.
Parameter
Cu22+porphyrin
g11
Alla
DIa rII
exo-cis DUBP (XIV)C
2.19 2.21
210 197 103 94 104 95
-e
-e
-e
-e
94 90 99 94
6.5 6.7 6.4 6.5
+
1-MeImidd
endo-cis DUBP (XV)C +
1-MeImidd
trans-DUBP (XVI)C +
1-MeImidd
a Units of i0-4 cm-l,
B
o.2A.
±1 X 10-4 cm-'.
b c Solvent 2:1 toluene/CH2Cl2. d Solvent 2:1 with
toluene/CH2Cl2
e
3500 G
FIG. 7. ESR spectra of Co2+ TPP-1-MeImid (A) and the Co2+ derivative of endo-cis DUBP (XV) in the presence of excess 1-Melmid
(B).
Cu2+ derivatives of the binary porphyrins along with calculated M-M distances in those cases where M-M interaction was observed. In these cases, the expected halving of A values was apparent. The ESR spectra of Co22+ MDAP and Co22+ exo-cis DUBP with and without axial ligands are typical of those for the corresponding monomeric porphyrins, which show no M-M interactions. The spectra of base adducts of Co2+ end-cis DUBP and Co22+ trans DUBP exhibit clear M-M interaction, while Table 1. ESR parameters and M-M distances from spectra of Co22+derivatives Parameter
Co22 porphyrin
g1
A11 a
D a
MDAP (XIII)C + 1-MeImidd exo-cis-DUBP (XIV)C
2.44 2.31 2.44 2.30
104 76 108 78
-e
e
-e -e
e
-e
-e -e
_f
_f
f
-f
929
+
1-MeImidd
endo-cis-DUBP (XV)c
1-MeImidd
2.30
-f
-f
-f
6.5g -f
2.30
-f
105
6.3
+
a
Units of 10-4 cm , ±1 X
A.
b ffo.1 c Solvent d Solvent
39
Table 3. Soret transitions of porphyrins and metal derivativesa
r1 (A)b
trans-DUBP (XVI)C + 1-MeImidd
10-4 cm-'.
3:2 toluene/o-dichlorobenzene. 3:2 toluene/o-dichlorobenzene with 20-fold molar excess axial base. 1-Melmid is 1-methylimidazole. e No M-M interaction observed. f Spectrum too broad for determination of parameters. g = 97 x i0- cm1 and rni = 6.5 A.
about 1 M base.
No M-M interaction observed.
in the absence of base, the spectra are broadened to the point that parameters cannot be estimated. Fig. 7 shows spectra of Co2+ TPP-1-Melmid (A) and Co22+ endo-cis DUBP.(1MeImid)2 (B), illustrating both the zero field splitting of the parallel region of the binary porphyrin (16 of the 30 theoretical parallel lines are observable) and the determination of the D value (see Materials and Methods ESR section). Examination of molecular models, in conjunction with the ESR information, suggests that 1-alkyl imidazole bases coordinate on the "exterior" of Co22+ endo-cis DUBP and Co22+ trans DUBP. The ESR spectrum of Cu22+ MDAP, shows a weak, concentration-dependent M-M interaction, which arises from inter-molecular dimerization of mesoporphyrin IX halves of two CU22+ MDAP molecules. Such dimerization has been observed for Cu2+ biological porphyrins (21, 22). Tetraphenylporphyrin derivatives show little tendency to dimerize. Correspondingly, the ESR spectra of Co22+ or Cu22+ exo-cis DUBP, endo-cssDUBP, or trans DUBP exhibit no concentration dependence. The spectrum of Cu22+ exo-cis DUBP is similar to that of Cu2+ TPP and gives no evidence of any M-M interaction while, as in the Co2+ systems, Cu22+ end-cis DUBP and Cu22+ trans DUBP show clear M-M interaction. Visible Spectra. Soret peaks are known to arise from ir-7r* transitions (23). In a configuration where two porphyrins are held in close proximity, interaction between the two ir systems can cause Soret shifts. Table 3 illustrates this fact. While the position of the Soret bands for MDAP (XIII) and exo-cis DUBP (XIV) are unshifted from those for the correspondihg mono-
A
3100 G
(A)b
Transition, nm Porphyrin
Free base
M = Cu2+
M= Co2"
a,a-cis DAP (XI) aca-trans DAP (XII) a,a-cis DTUP (XVII) meso-PIXDME b TPP MDAP (XIII) exo-cis DUBP (XIV) endo-cis DUBP (XV) trans DUBP (XVI)
422 422 422 402 422 403,422 417 405 407
-
-
a
b
Solvent for all spectra is benzene. Mesoporphyrin IX dimethyl ester.
399 417 399,417 417 412 412
395 412
394,413 411 408 408
Chemistry: Collman et al.
22
Proc. Natl. Acad. Sci. USA 74 (1977)
Table 4. NMR chemical shifts for internal pyrrole protons of porphyrinsa Porphyrin
Solvent
6 N-H
a,oi-cis DAP (XI) CDC13 cia-cis DTUP (XVII) CDC13 meso-PIXDMEb CDC13 MDAP (XIII) CDC13 exo-cis DUBP (XIV) CDC13/CD30D, 80/20
-2.67 -2.90 -3.89 -2.92, -4.24 -3.25
endo-cis DUBP (XV) CDC13 trans DUBP (XVI) CDC13
-4.22 -4.22
a
At 250.
b
Mesoporphyrin IX dimethyl ester.
The authors wish to thank Henry Taube, Howard Tennent, Michel Boudart, Fred Anson, Robert Gagne, and Henry Ledon for many helpful discussions. This work was supported in part by National Science Foundation Grant CHE75-17018 and Advanced Research Projects Administration Grant N00014-75-C-1171. B.S.T. was supported by a National Science Foundation Postdoctoral Fellowship (1975-76). Conventional low resolution mass spectra were determined at Stanford under the DENDRAL project, National Institutes of Health Grant RR00612-OSA1. We also thank John Shapley, Carter Cook, and the University of Illinois analytical facilities for providing field desorption mass spectra. 1. deBethune, A. J. & Loud, N. A. S. (1964) Standard Aqueous Potentials and Temperature Coefficients at 25° (C. A. Hampel,
Skokie, Ill.).
meric porphyrin halves, the Soret bands of endo-cs DUBP (XV) and trans DUBP (XVI) have been shifted by 15 to 17 nm to shorter wavelength. The Co2+ and Cu2+ derivatives of endo-cis DUBP (XV) and trans DUBP (XVI) also exhibit Soret shifts to shorter wavelength. However, possible M-M interactions cannot be assessed by visible spectra because the metal d-d transitions are obscured by intense porphyrin transitions. Proton Magnetic Resonance (PMR). In a true "face-to-face porphyrin", the chemical shifts of protons on one porphyrin ring should be affected by the ring current of the second porphyrin.
Because of the geometrical dependence of ring-current-induced shifts (24), negligible shifts are expected for pyrrole C-Hs and the phenyl ring protons in all of the binary porphyrins. However, as Table 4 indicates, the internal pyrrole N-Hs are shifted upfield by about 1.4 ppm for endo-cis DUBP and for trans DUBP, in comparison to corresponding monomeric porphyrins. This suggests a "face-to-face" geometry for these two binary porphyrins. Especially noteworthy is the fact that the N-Hs of endo-cis DUBP are shifted about 1 ppm upfield from those of exo-cis DUBP, clearly demonstrating the difference in geometry of these two binary porphyrins, which otherwise have identical chemical structure.
CONCLUSION A series of binary porphyrins having a range of interporphyrin separations has been prepared. The PMR, electronic, and ESR spectra of these porphyrins and/or their metal complexes provide evidence for various degrees of inter-porphyrin interaction, consistent with the intramolecular separations estimated from models. Since the MDAP porphyrins fail to show any physical evidence indicative of inter-porphyrin interaction, we conclude that an "open" conformation of this system is dominant. Similarly, the porphyrin rings in the exo-cis DUBP complexes are not expected to interact and there is no physical evidence for such interaction. In contrast, the isomeric endo-cis DUBP and the rigidly constrained trans DUBP compounds exhibit a range of physical properties characteristic of strong "face-to-face" interactions. Neither the coordination chemistry nor the possibility of electrode catalysis by these "face-to-face" porphyrin complexes has been studied. Thus the veracity of our hypotheses concerning multielectron reduction and electrode catalysis remains to be tested.
2. Shilov, A. E. (1974) Russian Chem. Rev. Engl. Transl. 43, 378-398. 3. Jahnke, H., Schonborn, M. & Zimmerman, G. (1976) Top. Curr. Chem. 61, 133-181. 4. Greenwood, C. & Gibson, Q. H. (1967) J. Biol. Chem. 242, 1782-1787. 5. Zumft, W. G. & Mortenson, L. E. (1975) Biochim. Biophys. Acta 416, 1-52. 6. Hardy, R. W. F., Burns, R. C. & Parshall, G. W. (1973) in Inorganic Biochemistry, ed. Eichorn, G. L. (Elsevier Publishing, New York), Vol. 2, pp. 745-793. 7. Hammond, G. S. & Wu, C.-H. S. (1968) Adv. Chem. Ser. 77, 186-206. 8. Collman, J. P., Gagne, R. R., Reed, C. A., Halbert, T. R., Lang, G. & Robinson, W. T. (1975) J. Am. Chem. Soc. 97, 14271433. 9. Moss, T. H., Ehrenberg, A. & Bearden, A. J. (1969) Biochemistry 8,4159-4162. 10. Fleischer, E. B. & Srivastava, T. S. (1969) J. Am. Chem. Soc. 91,
2403-2405. 11. Schwarz, F. P., Gouterman, M., Muljiani, Z. & Dolphin, D. H. (1972) Bioinorg. Chem. 2,1-32. 12. Ogoshi, H., Sugimoto, H. & Yoshida, Z.-I. (1975) Hukusokan Kagaku Toronkai Koen Yoshishu, 8th, 239-243, Chem. Abe., 84, 82897u. 13. Little, R. G., Anton, J. A., Loach, P. A. & Ibers, J. A. (1975) J.
Heterocycl. Chem. 12,343-349.
14. Subramanian, J. (1975) in Porphyrins and Metalloporphyrins, ed. Smith, K. M. (Elsevier Co., New York), pp. 560-571. 15. Chasteen, N. D. & Belford, R. L. (1970) Inorg. Chem. 9, 169175. 16. Belford, R. L., Chasteen, N. D., So, H. & Tapscott, R. E. (1969) J. Am. Chem. Soc. 91, 4675-4680. 17. Boas, J. F., Pilbrow, J. R. & Smith, T. D. (1969) J. Chem. Soc. A, 723-725. 18. Kokoszka, G. F., Linzer, M. & Gordon, G. (1968) Inorg. Chem.
7, 1730-1735. 19. Wasson, J. R., Shyr, C.-I. & Trapp, C. (1968) Inorg. Chem. 7, 469-473. 20. Blumberg, W. E. & Peisach, J. (1965) J. Biol. Chem. 240; 870876. 21. Boas, J. F., Pilbrow, J. R. & Smith, T. D. (1969) J. Chem. Soc. A, 721-723. 22. Boyd, P. D. W., Smith, T. D., Price, J. H. & Pilbrow, J. R. (1972) J. Chem. Phys. 56, 1253-1263. 23. Gouterman, M. (1961) J. Mol. Spectrosc. 6, 138-163. 24. Jackman, L. M. & Sternhell, S. (1969) Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry (Pergamon Press, New York), pp. 61-113.
Chemistry
Synthesis and characterization of "face-to-face" porphyrins (bimetallic ligands/metal-metal interaction/electron spin resonance/dioxygen reduction/dinitrogen reduction)
JAMES P. COLLMAN, C. MICHAEL ELLIOTT, THOMAS R. HALBERT, AND BENJAMIN S. TOVROG* Department of Chemistry, Stanford University, Stanford, Calif. 94305.
Contributed by James P. Collman, November 8,1976
is carried out at 250, the measured rates are such that Greenwood and Gibson (4) conclude that any intermediates must have half-lives of less than 10 Msec. Several mono- and dioxygenases, which reduce oxygen with concomitant oxidation of organic substrates, also contain more than one functional metal ion. These include the monooxygenases laccase (mono-phenol monooxygenase, 4 Cu), and ascorbate oxidase (8 Cu) as well as the dioxygenase L-tryptophan oxygenase (2 hemes and 2 Cu). In contrast to the large number of biological systems that reduce 02, evolutionary processes appear to have developed only a single dinitrogen fixing system, nitrogenase (5). Although the molecular mechanism has not yet been elucidated, a binuclear metal site for dinitrogen reduction has been proposed.t Abiological support for the idea of binuclear 02 reduction by Fe2+ comes from Hammond and Wu's study (7) ofthe Fe2+ / 02 reaction in nonaqueous solution. From the rate law and stoichiometry, it was proposed that the rate-limiting step is direct 4e- reduction of 02 by 2 Fe2+ as shown in Fig. 2 (I II). Although the postulated intermediates I and II in Fig. 2 were not directly observed, each has precedent in porphyrin chemistry. The "picket fence" dioxygen complex (8) is a model for I. "Compounds II" in the peroxidase family (9) are heme analogues of the ferryl intermediate postulated, (II), in Fig. 2. Analogues of the 1A-oxo ferric dimer, III, Fig. 2, are well known in porphyrin chemistry (10). We speculate that a binuclear "face-to-face" Fe2+ macrocyclic complex like IV in Fig. 3 will strongly bind dioxygen, giving V and immediately effecting a 4e- reduction to the bis-ferryl complex VI in a process requiring only a slight conformational change so that the overall reduction should occur without a substantial intervening energy barrier. Protonation of the bis-ferryl complex VI and rereduction of Fe by an electrode surface could result in the overall catalytic reduction of 02 to H20. Similar schemes involving Ru for 02 reduction and Mo or V for N2 reduction are also envisioned. The length of the groups linking the two macrocycles for an 02 reducing system is dictated by geometric estimates of the probable transition states and intermediates in the overall 4echange as is illustrated in Fig. 4. Thus, in an ideal binuclear catalyst the Fe-Fe gap should span the minimum range, about 4.8 A for the prior complex VIII to about 5.8 A for the bis-ferryl product IX, in Fig. 4. Similar estimates can be made for N2 reduction. Schwarz et al. (11) have reported studies on binary porphyrins joined through a single linkage, which is insufficient to enforce a "face-to-face" geometry. Recently, Ogoshi et al. (12) at a symposium mentioned the synthesis of a binary por-
ABSTRACT The syntheses of four binary porphyrins, two of which are constrained to a "face-to-face" conformation, and their Co2+ and Cu2+ derivatives are described. Electron spin resonance indicates that the intermetallic separation in the binuclear "face-to-face" porphyrins is about 6.5-6.8 A. Electronic spectra and proton magnetic resonance spectra support the postulated "face-to-face" conformations. A hypothesis that related compounds may serve as multielectron redox catalysts for 02 and N2 is presented. The development of efficient catalysts for the reversible multielectron reduction of 02 and N2 would have great significance. Such catalysts are essential to the oxygen cathode of an air-powered fuel cell and to electrochemical nitrogen fixation. The major problem in catalyzing four- or six-electron reductions of 02 or N2 is that stepwise one- or two-electron paths must pass through relatively unstable intermediates (Fig. 1). The high energy content of these intermediates precludes rapid stepwise electron transfer operating near the reversible potential of the overall four- or six-electron reduction. The rapid reduction of oxygen, which has a triplet ground state, also requires a mechanism to overcome the conservation of spin and orbital angular momentum. The latter problem might be circumvented by using a metal catalyst that has low-lying paramagnetic states. These arguments have been developed by H. Taube (personal communication). Many monometallic macrocyclic complexes adsorbed on graphite have been examined as catalysts for oxygen reduction (3). The most effective macrocycles have four nitrogen donor atoms. In the phthalocyanine series, the order of reactivity is Fe > Co > Ni > Cu > Mn. However, such studies have failed to reveal any catalyst that is capable of reversible reduction of 02 to water, possibly because with a single metal center, initial 2e- reduction to H202 is always dominant. We have approached this problem by constructing a new class of so-called "face-to-face porphyrins" in which two porphyrin rings are held in parallel conformation. Thus, two metal atoms might act in concert to bind and reduce dioxygen (or dinitrogen) in the gap between the porphyrin rings. Eventually these binuclear, cyclophane porphyrin complexes are to be attached to graphite to be tested as electrode catalysts. The idea of providing reducing equivalents rapidly from more than one metal site has precedent in the chemistry of certain metalloenzymes. The terminal enzyme of cellular respiration, cytochrome oxidase, which carries out the reaction, 02 + 4H+ + 4e -- 2H20, contains two copper ions and two heme groups (a and a3) per subunit. When the reaction with 02 Abbreviations: DAP, diaminoporphine; MDAP, mesoporphine IX diamide porphine; DUBP, diurea binary porphyrin; DTUP, ditolylurea porphyrin; TPP, meso-tetraphenylporphyrin; PMR, proton magnetic resonance; ESR, electron spin resonance; 1-MeImid, 1-methylimidazole. * Present address- Corporate Research, Allied Chemical Co., Morristown, N.J. 07960.
proposed binuclear mechanisms are compared and discussed by Hardy et al. (6).
t Several
18
Proc. Natl. Acad. Sci. USA 74 (1977)
Chemistry: Collman et al. B F2
B
19 B
B
Fe
Fe
+
4H+ P
A 50 HO2 +
'
a %
H0 22
a
'%
'
C
T
+1-78 > H N H +0.64NC 3Z e HO0 2 N2H2 -~.---e 2H5
FIG. 1. Standard reduction potentials of 02 and N2, in V. a Taken from the compilation of deBethune and Loud (1). b Approximated by Shilov (2). c Calculated by application of Hess's Law.
phyrin, but no details were reported. Herein we report the synthesis and physical properties of four binuclear porphyrins having various geometries and degrees of conformational freedom (see Figs. 5 and 6). The unusual spectroscopic properties of these ligands and their Co and Cu derivatives are enumerated, along with estimates of the intermetallic separations. Studies testing the 02 and N2 reducing capabilities of these ligands have not been carried out and are awaiting the synthesis of the Fe, Ru, V, and Mo complexes. MATERIALS AND METHODS An abbreviated nomenclature will be used for simplicity throughout this paper. The symbols a and # refer to the opposite faces of the porphyrin plane, while the terms cis and trans refer to adjacent and opposite meso positions of tetraphenyl porphyrins. In this manner, XI in Fig. 5, properly named 5a,10a-di(2-aminophenyl)-15,20-diphenylporphine becomes aa-cis-diaminoporphine or a,a-cis DAP. Synthesis: XI, a,a-cil DAP and XII, a,a-trans DAP. Condensation of pyrrole (2 eq) with benzaldehyde (1 eq) and 2nitrobenzaldehyde (1 eq) in acetic acid produced a mixture of meso-tetraphenylporphyrin (TPP) and mono-, di-, tri-, and tetra-nitrophenyl porphyrin positional isomers and atropisomers. Similar "mixed aldehyde" condensations have been reported by Little et al. (13). Column chromatographic separation with 1:1 benzene/cyclohexane eluent on silica gel (W. R. Grace type 62) gave several bands, the third and fifth of which were dinitroporphyrins. Reduction of this mixture of dinitroporphyrins by the established SnCI2/HCI method (8) gave a mixture of diamino porphyrins, which were separated by column chromatography with 19:1 dichloromethane/ethyl acetate eluent on silica gel (Merck PF254) into three bands, denoted A, B, and C. Thermal equilibration (2 hr in boiling benzene) of B gave equal parts A and B, while C gave equal parts A and C. The greater retention of B and C on silica gel suggested that they were the a,a atropisomers of trans and cis DAP. Band A therefore contains both a,jB atropisomers. The identities of B as a,a-trans DAP (XII) and C as a,a-cis DAP (XI) were confirmed by the characteristic splitting of the pyrrole C-Hs in the proton magnetic resonance (PMR) spectra, Fe2 +0+ 2
2[Fe= II
K
[FO FeO]21 2
Fe2
31 ke k2
2
2+ 2 2F 012 2+ 2[Fe-0-Fel4
-d 021 dt
III
]2[o02 k2K[Fe2 2
FIG. 2. Mechanism of reaction of Fe2+ with 02 in a nonaqueous environment.
(t) (a) F2FeFe
B IV
(t)
B V
2t ~~~~~~~~4eFe2
B VI
B VII
FIG. 3. Proposed reaction of 02 with a "face-to-face" binary Fe2+ porphyrin. B is an axial base too large to fit in the cavity. Ovals represent porphyrin rings.
and by the derivatization of C with isophthaloyldichloride which is known (8) to span o-amino groups on adjacent phenyl rings in TPP derivatives. Repeated thermal equilibration of A and chromatography led to an overall yield, based on pyrrole, of 1.1% XI and 0.4% XII. XIII, Mesoporphine IX Diamide-cis-NN-di(2-phenyl) diphenyl Porphine (MDAP) Mesoporphyrin IX diacid chloride (X) was generated by SOC12 treatment of a 1-methyl-2-pyrrolidinone solution of mesoporphyrin IX. Addition of one equivalent of a,a-cis DAP (XI), stirring at 25° for 1 hr, and chromatography on silica gel gave MDAP (XIII) in good yield as a violet powder. The product is homogeneous to high-pressure liquid chromatography, and its PMR spectrum is consistent with the proposed structure. It has the expected mass spectral parent ion at 1174 atomic mass units, and a single broad amide carbonyl band in the infrared, centered at 1688 cm-1. The electronic spectrum of XIII appears as a superposition of the spectra of the two free halves. XIV, exo-cis-Diurea Binary Porphyrin (exo-cis DUBP); XV, endo-cis-Diurea Binary Porphyrin (endo-cis DUBP); and XVI, trans-Diurea Binary Porphyrin (trans DUBP) The synthetic schemes for these ligands are outlined in Fig. 5, and are identical in detail, depending only on the starting material. The appropriate diaminoporphyrin isomer was dissolved in dry, oxygen-free benzene containing 1% pyridine and treated with excess phosgene, affording the dicarbamoylchloride (or diisocyanate). Excess phosgene was removed under vacuum, and a benzene solution containing an equimolar amount of diaminoporphyrin was added. After stirring under N2 at 25° overnight, the product was purified by silica gel chromatography. Starting with a,a-cis DAP (XI), there are two plausible binary porphyrin products, exo-cis DUBP (XIV) and endo-cis DUBP (XV). In the exo product (XIV) the porphyrin rings are geometrically disposed away from one another, while in the endo product (XV) they are above one another. The two isomers form in approximately equal amounts. With a,a-trans DAP as the starting material, the only binary porphyrin formed is trans DUBP (XVI). High-pressure liquid chromatography indicates that XIV, XV, and XVI are each homogeneous. Molecular weights obtained by osmometry for XV and XVI are 1308 and 1327 (calculated, 1341). Field desorption mass spectra show the ex1.8A
1.5A
Fe-0=0-Fe
Fe=0
1.2A
2. 8A
O=Fe
44.8A
5.8A
Vill
Ix
FIG. 4. Estimation of Fe-Fe distances for bimetallic 02 reduction.
20
Chemistry:
Collman et al.
Proc. Natl. Acad. Sci. USA 74 (1977)
H2N N
H2N
NH
HN
N
a,a-cis DAP
a,a-trans DAP
Xl' 1 COC 12, excess
,2. second equivalent Xii
Xi
endo-cis DUBP
exo-cis DUBP
XV
XIV
trans-DUBP XVI
FIG. 5. Synthetic scheme for the urea linked binary porphyrins. Geometries are idealized and double bonds are left out of porphyrins and phenyl rings for clarity.
pected molecular ions. Electronic spectral and PMR results support the proposed structures (vide infra). a,a-cis-Di-p-tolylurea Porphyrin (a,a-cis-DTUP) (XVII). This di-aryl urea monomeric porphyrin was made by treating the dicarbamoyl chloride derived from a,a-cis DAP with excess p-toluidine. Introduction of Cu2+ was carried out by heating the porphyrins with excess Cu(OAc)2 in CH2Cl2 containing 1-2% MeOH and excess 2,6-lutidine followed by chromatography on neutral alumina. Introduction of Co2+ was carried out under N2 by heating the porphyrins with excess anhydrous CoC12 in 1:1 tetrahydrofuran/benzene with excess 2,6-lutidine, followed by chromatography on alumina. 0
cl-c Cl-C=;-
Mesoporphyrin IX diacidchloride X I equivalent Xl
NH HN
A
~N
I'd~~~XI MDAP
\
FIG. 6. Synthetic scheme for the amide linked binary porphyrin MDAP. Geometry is idealized and double bonds in porphyrin and phenyl rings are left out for clarity.
Electron Spin Resonance Spectra (ESR). Spectra were collected on a Varian X-band E-12 spectrometer at about -1400. Cobalt samples were prepared under inert atmosphere. All spectra exhibit approximate axial symmetry. In spectra that exhibited splitting due to M-M interaction, the zero field splitting parameter D was determined as follows: in the parallel regions of both Co2+ and Cu2+ complexes, D is measured as the difference in position between the MI = 0 transition and gal taken from an appropriate monomeric porphyrin model. In the perpendicular region of the Co2+ spectra, D is alternatively determined by measuring the difference in line position where dI/dH = 0 (see Fig. 7). RESULTS AND DISCUSSION Electron Spin Resonance. Simple Cu2+ and low spin Co2+ porphyrins are S = '2 systems with' well known ESR characteristics (14). If two such porphyrins are close enough that the metals interact, the bimetallic system will have singlet (S = 0) and triplet (S = 1) states separated by the energy J. If the rate of electron exchange between the metals is fast compared to the resonance frequency (1010 sec'), then each electron experiences nuclear spin equal to the total nuclear spin of the two metals (I = 3 for Cu22+ and I = 7 for Co22+). The resulting hyperfine splitting, A, is half that for a related mononuclear S = 'A system. The ESR transitions will be further split by the zero field splitting, D. D is composed of two terms, Dpseudo (from spin orbit coupling) and Ddd (from the electron-electron dipolar interaction). Ddd is related to the metal-metal (M-M) separation, r, by the equation r = (0.65 gI2/Ddd)'/. It has been shown that Dpeudo can be ignored, and r can be calculated from D = Ddd without having significant effect on the calculated r (15). A more complete' description of the method of determining r is contained in ref. 15. Several examples of ESR involving two interacting S = 1 metalslhave appeared in the literature (15-22). Calculated distances have been in agreement with x-ray crystal structures where available (15, 16). Tables 1 and 2 list pertinent ESR data for several Co2+ and
Chemistry: Collman et al.
Proc. Natl. Acad. Sci. USA 74 (1977)
21
Table 2. ESR parameters and M-M distances from parallel region of spectra of Cu22, derivatives
VERTICAL EXPANSION 1 15x 40x 60x
.
Parameter
Cu22+porphyrin
g11
Alla
DIa rII
exo-cis DUBP (XIV)C
2.19 2.21
210 197 103 94 104 95
-e
-e
-e
-e
94 90 99 94
6.5 6.7 6.4 6.5
+
1-MeImidd
endo-cis DUBP (XV)C +
1-MeImidd
trans-DUBP (XVI)C +
1-MeImidd
a Units of i0-4 cm-l,
B
o.2A.
±1 X 10-4 cm-'.
b c Solvent 2:1 toluene/CH2Cl2. d Solvent 2:1 with
toluene/CH2Cl2
e
3500 G
FIG. 7. ESR spectra of Co2+ TPP-1-MeImid (A) and the Co2+ derivative of endo-cis DUBP (XV) in the presence of excess 1-Melmid
(B).
Cu2+ derivatives of the binary porphyrins along with calculated M-M distances in those cases where M-M interaction was observed. In these cases, the expected halving of A values was apparent. The ESR spectra of Co22+ MDAP and Co22+ exo-cis DUBP with and without axial ligands are typical of those for the corresponding monomeric porphyrins, which show no M-M interactions. The spectra of base adducts of Co2+ end-cis DUBP and Co22+ trans DUBP exhibit clear M-M interaction, while Table 1. ESR parameters and M-M distances from spectra of Co22+derivatives Parameter
Co22 porphyrin
g1
A11 a
D a
MDAP (XIII)C + 1-MeImidd exo-cis-DUBP (XIV)C
2.44 2.31 2.44 2.30
104 76 108 78
-e
e
-e -e
e
-e
-e -e
_f
_f
f
-f
929
+
1-MeImidd
endo-cis-DUBP (XV)c
1-MeImidd
2.30
-f
-f
-f
6.5g -f
2.30
-f
105
6.3
+
a
Units of 10-4 cm , ±1 X
A.
b ffo.1 c Solvent d Solvent
39
Table 3. Soret transitions of porphyrins and metal derivativesa
r1 (A)b
trans-DUBP (XVI)C + 1-MeImidd
10-4 cm-'.
3:2 toluene/o-dichlorobenzene. 3:2 toluene/o-dichlorobenzene with 20-fold molar excess axial base. 1-Melmid is 1-methylimidazole. e No M-M interaction observed. f Spectrum too broad for determination of parameters. g = 97 x i0- cm1 and rni = 6.5 A.
about 1 M base.
No M-M interaction observed.
in the absence of base, the spectra are broadened to the point that parameters cannot be estimated. Fig. 7 shows spectra of Co2+ TPP-1-Melmid (A) and Co22+ endo-cis DUBP.(1MeImid)2 (B), illustrating both the zero field splitting of the parallel region of the binary porphyrin (16 of the 30 theoretical parallel lines are observable) and the determination of the D value (see Materials and Methods ESR section). Examination of molecular models, in conjunction with the ESR information, suggests that 1-alkyl imidazole bases coordinate on the "exterior" of Co22+ endo-cis DUBP and Co22+ trans DUBP. The ESR spectrum of Cu22+ MDAP, shows a weak, concentration-dependent M-M interaction, which arises from inter-molecular dimerization of mesoporphyrin IX halves of two CU22+ MDAP molecules. Such dimerization has been observed for Cu2+ biological porphyrins (21, 22). Tetraphenylporphyrin derivatives show little tendency to dimerize. Correspondingly, the ESR spectra of Co22+ or Cu22+ exo-cis DUBP, endo-cssDUBP, or trans DUBP exhibit no concentration dependence. The spectrum of Cu22+ exo-cis DUBP is similar to that of Cu2+ TPP and gives no evidence of any M-M interaction while, as in the Co2+ systems, Cu22+ end-cis DUBP and Cu22+ trans DUBP show clear M-M interaction. Visible Spectra. Soret peaks are known to arise from ir-7r* transitions (23). In a configuration where two porphyrins are held in close proximity, interaction between the two ir systems can cause Soret shifts. Table 3 illustrates this fact. While the position of the Soret bands for MDAP (XIII) and exo-cis DUBP (XIV) are unshifted from those for the correspondihg mono-
A
3100 G
(A)b
Transition, nm Porphyrin
Free base
M = Cu2+
M= Co2"
a,a-cis DAP (XI) aca-trans DAP (XII) a,a-cis DTUP (XVII) meso-PIXDME b TPP MDAP (XIII) exo-cis DUBP (XIV) endo-cis DUBP (XV) trans DUBP (XVI)
422 422 422 402 422 403,422 417 405 407
-
-
a
b
Solvent for all spectra is benzene. Mesoporphyrin IX dimethyl ester.
399 417 399,417 417 412 412
395 412
394,413 411 408 408
Chemistry: Collman et al.
22
Proc. Natl. Acad. Sci. USA 74 (1977)
Table 4. NMR chemical shifts for internal pyrrole protons of porphyrinsa Porphyrin
Solvent
6 N-H
a,oi-cis DAP (XI) CDC13 cia-cis DTUP (XVII) CDC13 meso-PIXDMEb CDC13 MDAP (XIII) CDC13 exo-cis DUBP (XIV) CDC13/CD30D, 80/20
-2.67 -2.90 -3.89 -2.92, -4.24 -3.25
endo-cis DUBP (XV) CDC13 trans DUBP (XVI) CDC13
-4.22 -4.22
a
At 250.
b
Mesoporphyrin IX dimethyl ester.
The authors wish to thank Henry Taube, Howard Tennent, Michel Boudart, Fred Anson, Robert Gagne, and Henry Ledon for many helpful discussions. This work was supported in part by National Science Foundation Grant CHE75-17018 and Advanced Research Projects Administration Grant N00014-75-C-1171. B.S.T. was supported by a National Science Foundation Postdoctoral Fellowship (1975-76). Conventional low resolution mass spectra were determined at Stanford under the DENDRAL project, National Institutes of Health Grant RR00612-OSA1. We also thank John Shapley, Carter Cook, and the University of Illinois analytical facilities for providing field desorption mass spectra. 1. deBethune, A. J. & Loud, N. A. S. (1964) Standard Aqueous Potentials and Temperature Coefficients at 25° (C. A. Hampel,
Skokie, Ill.).
meric porphyrin halves, the Soret bands of endo-cs DUBP (XV) and trans DUBP (XVI) have been shifted by 15 to 17 nm to shorter wavelength. The Co2+ and Cu2+ derivatives of endo-cis DUBP (XV) and trans DUBP (XVI) also exhibit Soret shifts to shorter wavelength. However, possible M-M interactions cannot be assessed by visible spectra because the metal d-d transitions are obscured by intense porphyrin transitions. Proton Magnetic Resonance (PMR). In a true "face-to-face porphyrin", the chemical shifts of protons on one porphyrin ring should be affected by the ring current of the second porphyrin.
Because of the geometrical dependence of ring-current-induced shifts (24), negligible shifts are expected for pyrrole C-Hs and the phenyl ring protons in all of the binary porphyrins. However, as Table 4 indicates, the internal pyrrole N-Hs are shifted upfield by about 1.4 ppm for endo-cis DUBP and for trans DUBP, in comparison to corresponding monomeric porphyrins. This suggests a "face-to-face" geometry for these two binary porphyrins. Especially noteworthy is the fact that the N-Hs of endo-cis DUBP are shifted about 1 ppm upfield from those of exo-cis DUBP, clearly demonstrating the difference in geometry of these two binary porphyrins, which otherwise have identical chemical structure.
CONCLUSION A series of binary porphyrins having a range of interporphyrin separations has been prepared. The PMR, electronic, and ESR spectra of these porphyrins and/or their metal complexes provide evidence for various degrees of inter-porphyrin interaction, consistent with the intramolecular separations estimated from models. Since the MDAP porphyrins fail to show any physical evidence indicative of inter-porphyrin interaction, we conclude that an "open" conformation of this system is dominant. Similarly, the porphyrin rings in the exo-cis DUBP complexes are not expected to interact and there is no physical evidence for such interaction. In contrast, the isomeric endo-cis DUBP and the rigidly constrained trans DUBP compounds exhibit a range of physical properties characteristic of strong "face-to-face" interactions. Neither the coordination chemistry nor the possibility of electrode catalysis by these "face-to-face" porphyrin complexes has been studied. Thus the veracity of our hypotheses concerning multielectron reduction and electrode catalysis remains to be tested.
2. Shilov, A. E. (1974) Russian Chem. Rev. Engl. Transl. 43, 378-398. 3. Jahnke, H., Schonborn, M. & Zimmerman, G. (1976) Top. Curr. Chem. 61, 133-181. 4. Greenwood, C. & Gibson, Q. H. (1967) J. Biol. Chem. 242, 1782-1787. 5. Zumft, W. G. & Mortenson, L. E. (1975) Biochim. Biophys. Acta 416, 1-52. 6. Hardy, R. W. F., Burns, R. C. & Parshall, G. W. (1973) in Inorganic Biochemistry, ed. Eichorn, G. L. (Elsevier Publishing, New York), Vol. 2, pp. 745-793. 7. Hammond, G. S. & Wu, C.-H. S. (1968) Adv. Chem. Ser. 77, 186-206. 8. Collman, J. P., Gagne, R. R., Reed, C. A., Halbert, T. R., Lang, G. & Robinson, W. T. (1975) J. Am. Chem. Soc. 97, 14271433. 9. Moss, T. H., Ehrenberg, A. & Bearden, A. J. (1969) Biochemistry 8,4159-4162. 10. Fleischer, E. B. & Srivastava, T. S. (1969) J. Am. Chem. Soc. 91,
2403-2405. 11. Schwarz, F. P., Gouterman, M., Muljiani, Z. & Dolphin, D. H. (1972) Bioinorg. Chem. 2,1-32. 12. Ogoshi, H., Sugimoto, H. & Yoshida, Z.-I. (1975) Hukusokan Kagaku Toronkai Koen Yoshishu, 8th, 239-243, Chem. Abe., 84, 82897u. 13. Little, R. G., Anton, J. A., Loach, P. A. & Ibers, J. A. (1975) J.
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14. Subramanian, J. (1975) in Porphyrins and Metalloporphyrins, ed. Smith, K. M. (Elsevier Co., New York), pp. 560-571. 15. Chasteen, N. D. & Belford, R. L. (1970) Inorg. Chem. 9, 169175. 16. Belford, R. L., Chasteen, N. D., So, H. & Tapscott, R. E. (1969) J. Am. Chem. Soc. 91, 4675-4680. 17. Boas, J. F., Pilbrow, J. R. & Smith, T. D. (1969) J. Chem. Soc. A, 723-725. 18. Kokoszka, G. F., Linzer, M. & Gordon, G. (1968) Inorg. Chem.
7, 1730-1735. 19. Wasson, J. R., Shyr, C.-I. & Trapp, C. (1968) Inorg. Chem. 7, 469-473. 20. Blumberg, W. E. & Peisach, J. (1965) J. Biol. Chem. 240; 870876. 21. Boas, J. F., Pilbrow, J. R. & Smith, T. D. (1969) J. Chem. Soc. A, 721-723. 22. Boyd, P. D. W., Smith, T. D., Price, J. H. & Pilbrow, J. R. (1972) J. Chem. Phys. 56, 1253-1263. 23. Gouterman, M. (1961) J. Mol. Spectrosc. 6, 138-163. 24. Jackman, L. M. & Sternhell, S. (1969) Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry (Pergamon Press, New York), pp. 61-113.
