STRUCTURAL, ELECTROCHEMICAL, AND MAGNETIC-PROPERTIES OF A 4-MEMBERED REDOX SERIES ([CR(L3)]N-, N=0-3) IN CATECHOL-BENZOQUINONE COMPLEXES OF CHROMIUM

The complexes tris(3,5-di-tert-butyl-o-benzoquinone)chromium(0) and potassium tris(3,5-di-;tert-butylcatecholato)chromate(III) have been isolated. Electrochemical techniques have demonstrated these complexes to be members of a reversible three-electron redox series with formal reduction potentials of +0.03, -0.47, and -0.89 V vs. SCE in acetonitrile. The four members of this series have been characterized by visible, infrared, and EPR spectroscopies as well as by cyclic voltammetry and magnetic susceptibility. From 4.3 to 298 K the neutral complex is diamagnetic. The mono-, di-, and trianion have, respectively, 1, 2, and 3 unpaired electrons. The EPR spectrum of the monoanion consists of a signal centered at g = 1.98 with 8-G line width and coupling to the metal of 24.3 G. The neutral complex has spontaneously resolved upon crystallization to the A-cis isomer in space group R3 with a = 16.385 (3) A,c= 13.874 (4) A, rf(exptl) = 1.08 g cm-3, and rf(calcd) = 1.11 g cm3 for Z = 3. The crystallographic symmetry of the molecule is C3 while the symmetry of the coordination polyhedron is D}. The ligand structure in the neutral complex retains significant quinone character; the C-0 bond length is 1.285 (8) A and there is an alternation in the C-C bond lengths of the ring which range from 1.370 (10) to 1.433 (9) A. The Cr-0 bond length is 1.933 (5) A. The structural results are compared to those of the trianion, tris(catecholato)chromate(III), in which the ligand retains an undistorted catechol geometry. © 1979, American Chemical Society. All rights reserved.