Optical and magnetic properties of -indigo&z.rad; radical anions. Magnetic coupling between -indigo&z.rad; ( = 1/2) mediated by intermolecular hydrogen N-H O&z.dbd;C bonds

The radical anion salt {cryptand[2.2.2](K + )} 6 { trans -indigo} 7 ·5.5C 6 H 4 Cl 2 ( 1 ) with a non-integer negative charge on indigo (−0.86 per molecule on average) was obtained. The salt manifests an effective magnetic moment of 1.59 μ B at 300 K and strong enough antiferromagnetic coupling of spins to be mediated by intermolecular hydrogen bonds. Reduction of trans -indigo yields salt {cryptand[2.2.2](K + )} 6 { trans -indigo} 7 ·5.5C 6 H 4 Cl 2 ( 1 ). The trans -indigo&z.rad; − radical anions are bonded by intermolecular hydrogen N-H O&z.dbd;C bonds of 2.11-2.17 Å.