Solvent-dependent rotational phenomena in μ-nitrido-[2,3,9,10,16,17,23,24-octa(n-pentoxy)phthalocyaninato]diiron complex

Abstract

The syntheses and spectroscopic properties of new μ-nitrido-[2,3,9,10,16,17,23,24-octa(n-pentoxy)phthalocyaninato]diiron complexes in Fe(IV)Fe(IV) and Fe(+3.5)Fe(+3.5) oxidation states are reported. UV-vis spectra of both dimers demonstrate a strong solvatochromic behavior. UV-vis and 1H NMR data reveal the presence of two different forms for both oxidation states in apolar and polar solvents. Fe K-edge X-ray absorption near-edge structures (XANES) spectra of two Fe(IV)Fe(IV) forms also show a significant difference: pre-edge feature in toluene is shifted to higher energy compared to that in tetrahydrofuran. The difference of properties between two forms is explained by solvent-dependent mutual orientation of two phthalocyanine rings. A strong sharp Q-band in UV-vis spectrum at 642 nm and small splitting of aromatic protons (0.18 ppm) in 1H NMR spectrum observed in apolar solvents (toluene, benzene, cyclohexane) imply that the green form is in more symmetric D4d configuration. In turn, a broad multipreaked Q-band and relatively high splitting of aromatic protons (0.56 ppm) observed in THF and CH2Cl2 for the blue form are in agreement with less symmetric D4 conformation. The Fe(μ-N)Fe structure is relatively rigid showing no free rotation at room temperature in NMR time scale. Tuning of the structure of N-bridged diiron macrocyclic complexes by introduction of substituents and modification of electronic properties of the complexes via conformation changes might be useful for optimization of their catalytic properties as well as for their potential application in sensors.