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Macrocyclic Chromium(III) Catecholate Complexes
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DOI：10.1021/acs.inorgchem.0c03224.png)
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Abstract
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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-butyl-semiquinonate). 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 E-2 state, while [2](+) also emits from the T-2(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.
Keywords:
TRANSITION-METAL-COMPLEXES
COORDINATION-COMPOUNDS
ELECTRON-TRANSFER
RADICAL LIGANDS
O-AMINOPHENOL
PHOTOPHYSICS
ADDUCTS
VALENCE
PHOTOCHEMISTRY
REACTIVITY
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