Syntheses, structures, magnetic and spectroscopic properties of bis(pentane-2,4-dionato)chromium(III) complexes incorporating 3- or 4-pyridyl-substituted nitronyl and imino nitroxides

Preparation and characterization by X-ray analysis and ²H NMR spectra have been described for twelve chromium(III)-nitroxide complexes of cis-[Cr(acac)₂Cl(L)] and cis- and trans-[Cr(acac)₂(L)₂]PF₆ with monodentate coordination through the pyridyl-N atom in the nitroxides, where acac = pentane-2,4-dionate and L = 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl 3-oxide (NIT3py or NIT4py) and 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl (IM3py or IM4py). An intramolecular magnetic interaction between Cr^III and the nitroxide was found to be weakly antiferromagnetic and ferromagnetic, respectively, in the NIT3py or IM3py complexes and the NIT4py or IM4py complexes. The difference in sign of these magnetic interaction constants between the 3- and 4-pyridyl-substituted nitroxides can be interpreted by a spin polarization mechanism for the π-electron system including the nitroxide SOMO π* and Cr^III d_π orbitals. The inherently weak magnetic interactions and absence of an intensity enhancement of the spin-forbidden d-d transition band in these nitroxides complexes are in contrast to our previous results for the chelated NIT2py [4,4,5,5-tetramethyl-2-(2-pyridyl)imidazolin-1-oxyl 3-oxide] and IM2py [4,4,5,5-tetramethyl-2-(2-pyridyl)imidazolin-1-oxyl] complexes. This may be accounted for by the difference in the mechanism of magnetic interactions; spin-polarization mechanism and exchange mechanism, respectively.