Spin-coupling in dimers of 2,3-dicyano-5,6-dichlorosemiquinone radical anions in the crystalline state.
A crystal engineering approach is used to stabilize a radical anion in the crystalline state and to modulate the separation distance within π-stacks of anion radicals. Alkali metal salts of 2,3-dicyano-5,6-dichlorosemiquinone (C8Cl2N2O2, DDQ∙- radical anions were prepared and their crystal structures determined: LiDDQ·2H2O·(CH3)2CO, RbDDQ·2H2O and CsDDQ·2H2O. In these structures, stacked dimers of radical anions are formed within π-stacked columns. Within the stacked dimers, interplanar separation distances are significantly shorter than the sum of the van der Waals radii for two C atoms; the shortest is 2.812 Å for the Li salt and the longest is 2.925 Å for the Cs salt. Diamagnetic character, observed by electron paramagnetic resonance spectroscopy, indicates spin-coupling of the unpaired electrons within the radical anion dimer. The electron-rich cyano substituents on DDQ∙- influence the electron redistribution within the ring skeleton. The crystalline compounds are also characterized by IR spectroscopy, complemented by quantum-chemical calculations based on both isolated and periodic models.
Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia.
Krešimir
K
Molčanov
Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia.
Darko
D
Babić
Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia.
Biserka
B
Kojić-Prodić
National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia.
Jernej
J
Stare
Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia.
Nadica
N
Maltar-Strmečki
Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia.
Lidija
L
Androš
IR spectroscopy; density functional theory calculations; diamagnetism; semiquinone radical; spin coupling; thermal analysis; π-interaction