Macrocyclic Chromium(III) Catecholate Complexes

The synthesis and structural, electrochemical, spectroscopic, and magnetic characterizations of Cr III (HMC) catecholate and semiquinonate complexes are reported herein, where HMC is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. cis- [Cr(HMC)(Cat)] + complexes (Cat = catecholate, [ 1 ] +; tetrachlorocatecholate, [ 2 ] +; and 3,5-di- tert -butylcatecholate, [ 3 ] + ) were prepared from the reaction between appropriate catechol and [Cr III (HMC)Cl 2 ]Cl reduced in situ by zinc. Chemical oxidation of [ 3 ] + by FcPF 6 resulted in cis -[Cr(HMC)(SQ)] 2+ ([ 3 ] 2+, SQ = 3,5-di- tert -butylsemiquinonate). Single crystal X-ray diffraction studies revealed the cis- 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 [ 1 ] + –[ 3 ] +, while the Cr III center remains redox inactive. The absorption spectrum of the SQ complex [ 3 ] 2+ exhibits an intense spin-forbidden transition in solution. Time-delayed phosphorescence spectra recorded at 77 K revealed that all catecholate complexes emit from the 2 E state, while [ 2 ] + also emits from the 2 T 1 state. Temperature-dependent magnetic susceptibility measurements indicate the Cat complexes exist as S = 3/2 systems, while the SQ complex behaves as an S = 1 system, resulting from strong antiferromagnetic coupling of the S = 3/2 Cr center with the S = 1/2 SQ radical. Density functional theory (DFT) shows the similarities between the SOMOs of [ 1 ] + and [ 2 ] + and differences in their LUMOs in the ground state.