An electron spin resonance study of dimer formation of copper(II) protoporphyrin IX

Boas, John F.; Pilbrow, J. R.; Smith, T. D.

doi:10.1039/j19690000721

Cited by 24 publications

(7 citation statements)

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“…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.…”

Section: Resultsmentioning

confidence: 93%

See 1 more Smart Citation

Synthesis and characterization of "face-to-face" porphyrins.

Collman¹,

Elliott²,

Halbert³

et al. 1977

Proc. Natl. Acad. Sci. U.S.A.

136

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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) 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.tAbiological support for the idea of binuclear 02 reduction by Fe2+ comesfrom Hammondand Wu'sstudy (7) ofthe ...

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Section: Resultsmentioning

confidence: 93%

“…15. Several examples of ESR involving two interacting S = 1 metalslhave appeared in the literature (15)(16)(17)(18)(19)(20)(21)(22). Calculated distances have been in agreement with x-ray crystal structures where available (15,16).…”

Section: Resultsmentioning

confidence: 99%

Synthesis and characterization of "face-to-face" porphyrins.

Collman¹,

Elliott²,

Halbert³

et al. 1977

Proc. Natl. Acad. Sci. U.S.A.

136

View full text Add to dashboard Cite

show abstract

“…The implications of the dimerization process remain obscure. The appropriate ESR spectral data have not been treated quantitatively but merely compared with similar observations on other copper porphyrins (Boas et at., 1969d;Boyd et at., 1972;De Bolfo et at., 1975).…”

Section: Copper II Cytochrome Cmentioning

confidence: 97%

ESR of Copper in Biological Systems

Boas

Pilbrow

Smith

1978

Biological Magnetic Resonance

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“…The polymeric structure of 2 was expected to be retained in solution, as suggested for similar complexes. 46,47 KI-KI 3 is a redox couple, and may react with 2 in two ways: (a) twoelectron oxidation by the I 3 À ion to form the [Cu III Cu III (R) 4 (bpy)] x 2+ Fig. 9 Photoluminescence spectra of (a) 1 and (b) 2 upon excitation at 302 nm (red), 390 nm (brown) and 700 nm (blue).…”

Section: Photoluminescence and Thermoelectric Studiesmentioning

confidence: 99%