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)Cl₂]Cl reduced in situ by zinc. Chemical oxidation of [3]⁺ by FcPF₆ 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 ²E state, while [2]⁺ also emits from the ²T₁ 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.