Resonance raman spectra of high oxidation state iron porphyrin dimers

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)₂^N+ 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)₂^N+ 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.