Resonance raman spectra of high oxidation state iron porphyrin dimers

Mark A. Crisanti, Thomas G. Spiro, Daniel R. English, David N. Hendrickson, Kenneth S. Suslick

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Resonance Raman spectra are reported and discussed for a series of single-atom-bridged iron porphyrin dimers, including (FeTPP)2C and [(FeTPP)2N]ClO4 (where TPP = 5, 10, 15, 20-tetraphenylporphyrinato), with the use of B-band excitation (4067 Å). The symmetric Fe-X-Fe stretching frequency has been identified by using 54Fe substitution. This band is at 440 and 465 cm-1, respectively, for (FeTPP)2C and [(FeTPP)2N]C1O4 in methylene chloride. The Fe-Xstretching force constants are estimated to be 4.62 and 5.42 mdyn/Å, respectively, with a stretch-stretch interaction constant of 1.75 mdyn/Å. The large size of these constants indicates substantial x bonding between the iron and the bridging carbon or nitrogen atoms. Using Badger's rule, we estimate that the Fe-N bond length in (FeTPP)2N+ is 0.05 A shorter than that for Fe-C in (FeTPP)2C. The core-size marker bands for these two complexes (1564 and 1568 cm-1, respectively) indicate an expansion of 0.015 A for the former, reflecting the increased nonbonding interaction caused by the shortened bond distances. The porphyrin C-N breathing mode increases for (FeTPP)2N+ compared to (FeTPP)2C (1370 vs. 1365 cm-1); this may reflect increased x donation from the bridging carbon to the iron atom. Coordination of pyridine lowers the frequencies associated with the Fe-X-Fe stretch to 424 and 445 cm-1, respectively, and decreases the Fe-X force constants. Pyridine coordination also lowers the core-size marker frequencies (by 8 and 2 cm-1 for (FeTPP)2 and [(FeTPP)2N]C104, respectively), presumably as a result of nonbonding interactions between the pyridine and the porphyrin. The C-N brea hing frequency of the porphyrin is also lowered (4 and 3 cm-1, respectively), as expected from the decreased effective charge on the iron atom. Changes also occur in the resonant enhancement patterns and are discussed in terms of the electronic structure of these complexes.

Original languageEnglish (US)
Pages (from-to)3897-3901
Number of pages5
JournalInorganic Chemistry
Issue number24
StatePublished - Nov 1984

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry


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