Towards spin-crossover in pseudo-octahedral iron(III) complexes of strongly absorbing phenanthridine-based N^N^O- ligands

Abstract

Paramagnetic complexes which can undergo spin crossover between different magnetic states that also strongly absorb light are highly attractive for the pursuit of optical magnetic technologies. Against this backdrop, we prepared a library of Fe(III) complexes ligated by monoanionic, tridentate N^N^O- ligands (RL: (2-R1-(4-imino-methyl)-phenanthridinyl)]-5-R2-phenolate where R1 = tBu, F, OMe, R2 = H or R = tBu, R2 = tBu) bearing strongly absorbing π-extended phenanthridine (benzo[c]quinoline) donors: [Fe(tBuL)2]PF6, [Fe(tBuLtBu)2]PF6, [Fe(FL)2]PF6, and [Fe(OMeL)2]PF6. The solid-state structures of three of the series were determined using single-crystal X-ray diffraction, confirming pseudo-octahedral coordination geometries. Electronic absorption spectra shows strong absorbances near 340 nm and 415 nm for the series due to π-π* and ligand-to-metal-charge-transfer (LMCT) transitions, respectively, along with a long absorptive tail that extends well into the visible region, only attenuating close to 850 nm. Magnetic measurements in the solid-state (SQUID magnetometry) and in solution (Evans’ Method) revealed the influence of ligand substituents. For example, the magnetic moment of [Fe(FL)2]PF6 in the solid-state remains more or less constant at a χmT ∼ 2.3 over a large temperature range (25–300 K) indicative of a mixed spin configuration that is maintained in solution at ambient temperature, while [Fe(tBuL)2]PF6 and [Fe(tBuLtBu)2]PF6 only reach values of ∼1.5 at higher temperature. In comparison, the χmT of [Fe(OMeL)2]PF6 undergoes a much more abrupt drop upon cooling, from ∼2.5 to below 0.5 at 100 K, consistent with spin-crossover type behavior.