Macrocyclic Chromium(III) Catecholate Complexes.

The synthesis and structural, electrochemical, spectroscopic, and magnetic characterizations of Cr(HMC) catecholate and semiquinonate complexes are reported herein, where HMC is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. [Cr(HMC)(Cat)] complexes (Cat = catecholate, []; tetrachlorocatecholate, []; and 3,5-di--butylcatecholate, []) were prepared from the reaction between appropriate catechol and [Cr(HMC)Cl]Cl reduced by zinc. Chemical oxidation of [] by FcPF resulted in -[Cr(HMC)(SQ)] ([], SQ = 3,5-di--butylsemiquinonate). Single crystal X-ray diffraction studies revealed the chelation of the Cat/SQ ligand around the Cr metal center and confirmed the Cat/SQ nature of the ligands. Reversible oxidations of Cat to SQ were observed in the cyclic voltammograms of []-[], while the Cr center remains redox inactive. The absorption spectrum of the SQ complex [] exhibits an intense spin-forbidden transition in solution. Time-delayed phosphorescence spectra recorded at 77 K revealed that all catecholate complexes emit from the E state, while [] also emits from the T state. Temperature-dependent magnetic susceptibility measurements indicate the Cat complexes exist as = 3/2 systems, while the SQ complex behaves as an = 1 system, resulting from strong antiferromagnetic coupling of the = 3/2 Cr center with the = 1/2 SQ radical. Density functional theory (DFT) shows the similarities between the SOMOs of [] and [] and differences in their LUMOs in the ground state.