Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in η1-fashion as an axial ligand. The solid and solution EPR at 120K and 1H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the 1H NMR spectra of the molecule. Also, the ortho- and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho- and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of π spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential 1H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of -40 to +40°C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex. © Indian Academy of Sciences.