Building Blocks for Molecule-Based Magnets: Radical Anions and Dianions of Substituted 3,6-Dimethylenecyclohexane-1,2,4,5-tetrones as Paramagnetic Bridging Ligands

We have prepared four tetraaryl derivatives of 3,6-dimethylene-1,2,4,5-tetraoxocyclohexane (aryl = Ph; 4-MeOPh; 4-Me2NPh; and 3,5-(t-Bu)2-4-MeOPh) with guidance from an earlier reported ab initio analysis (Misiolek, A. W.; Jackson, J. E. J. Am. Chem. Soc. 2001, 123, 4774−4780). These electron acceptors may be chemically or electrochemically reduced to the mono- and dianions desired as building blocks for the assembly of molecule-based magnets. Cyclic voltammetry shows that the potential of the first reduction wave depends on the electron donor ability of the aryl ring substituents, ranging from −0.28 V for the tetraphenyl derivative to −0.78 V for the p-dimethylamino substituted analogue (vs ferrocene/ferrocinium+ at 0.46 V). Spin density distributions in the semiquinone moieties were elucidated by electron paramagnetic resonance (EPR) and electron−nuclear double resonance (ENDOR) observations of hyperfine couplings to internal 1H sites and bound alkali metal cations. X-ray diffraction studies of the sodium and potassium salts of the octa-t-butyltetramethoxy derivative reveal the structure of the monoanion and its tendency to self-assemble with metal cations into one-dimensional chains in the solid state. Within the chains the anions display the expected bridging and chelating mode of coordination; SQUID magnetometry revealed weak intermolecular spin−spin couplings of 2J = −0.2 and ∼0 K for the sodium and potassium salts, respectively. NIR transitions in the electronic spectra of the monoanions in solution are consistent with the expected low energy gap between frontier orbitals and its tunability by substituent variations. EPR studies of the free dianions and monoradical analogues indicate diradical localization into separate triphenylmethyl-like monoradicals via twisting of the diarylmethylene termini.