A series of six novel salts of 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radical anions with aromatic cations is studied by structural characterization, magnetization, and conductivity measurements. Different types of stacking of DDQ radicals, which determine electrical and magnetic properties, are observed: regular stacks of equidistant radicals (resulting in semiconductivity along the direction of stacking), stacks of pancake-bonded dimers, isolated pancake-bonded dimers, and mixed stacks comprising DDQ radicals and aromatic cations. General features of pancake bonding of DDQ radicals are defined: parallel ring planes, offset along the carbonyl•••carbonyl axis by 2.05−2.15 Å, and interplanar separations of 2.90−2.95 Å (for pancake-bonded dimers) or 3.08−3.24 Å (for regular stacks of equidistant radicals).
Five charge transfer complexes of electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine
(TMPD or
Wurster’s blue) with quinoid electron acceptors tetrafluoro-
(F4Q), tetrachloro- (Cl4Q), tetrabromoquinone
(Br4Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ) were prepared and studied by X-ray crystallography,
impedance spectroscopy, and DFT computations. Mixed donor–acceptor
π-stacks were observed in all compounds; in TMPD·F4TCNQ, the moieties formed 2D stacked arrays. All moieties
had a partial radical character due to charge transfer. The presence
of two-electron multicenter bonding (pancake bonding) was confirmed
by quantum chemical computation. In TMPD·F4Q, alternating
longer and shorter interplanar distances were consistent with formation
of pancake-bonded dimers; in complexes of TMPD with Cl4Q, Br4Q, and TCNQ, the stacks were equidistant implying
existence of pancake-bonded polymers.
The effects of temperature (80−400 K) and pressure (up to 4.67 GPa) variations on stacks of equidistant semiquinone radicals were studied by single-crystal X-ray diffraction using a salt of 2-amino-5-methyl-N-methylpyridinium (MMA) and 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ). A temperature-driven phase transformation occurs in the temperature range 150−240 K. The nature of stacking changes upon cooling: equidistant radicals in the room-temperature phase are re-arranged into two-electron/multicenter (2 e/mc) bonded dimers of radicals at low temperature. The phase transition proceeds via a pair-exchange dynamic mechanism. By application of pressure, the interplanar separation in the stacks is reduced by 0.37 Å, from 3.25 Å at ambient pressure to 2.88 Å at 4.67(3) GPa. This may indicate that the covalent component of the interaction considerably increases, consistent with previously performed density functional theory (DFT) calculations.
Pancake bonding in dimers of semiquinoid radical cations in a novel compound, TMPD chloride co-crystal with 2,5-dichlorohydroquinone, is studied by a combination of X-ray charge density and quantum chemical computations. The rings in a pancake-bonded dimer are parallel with a transversal offset, the interplanar separation distance is 3.1191(1) Å, and the centroid distance is 3.2022(2) Å. AIM analysis of experimental charge density revealed four symmetry-independent bonding critical points and a cage critical point between the TMPD cations in a dimer, with a maximum electron density of 0.055 e Å −3 . A highest occupied molecular orbital extends between both rings of a dimer. The estimated energy of the covalent component is −3.24 kcal mol −1 .
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