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-Me(2)NPh; 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 (1)H 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 approximately 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.

"Inverse sodium hydride": a crystalline salt that contains H(+) and Na(-).

A crystalline salt has been synthesized that contains H(+) and Na(-) rather than the usual hydride oxidation states of H(-) and Na(+). The key is irreversible encapsulation of H(+) within the cage of 3(6)adamanzane (Adz). The internal proton is kinetically inert to reduction by Na(-) in solution in NH(3)-MeNH(2) mixtures. Synthesis of the sodide is accomplished by a metathesis reaction between Na and AdzH(+)X(-) in which X(-) is a sacrificial anion such as glycolate, isethionate, or nitrate. Reduction or deprotonation of the sacrificial anion forms insoluble byproducts and AdzH(+)Na(-) in solution. After solvent removal, the sodide is dissolved in dimethyl ether and transferred through a frit into a separate chamber for crystallization. The compound was characterized as the sodide by analysis, NMR spectra, and optical absorption spectroscopy.