Two Electron Multicenter Bonding (pancake Bonding) Between Electron Donors and Acceptors in Charge-Transfer Compounds of N,N,N,N-Tetramethyl-p-phenylenediam Ine and (semi)quinoid Electron Acceptors

Fivecharge transfer complexes of electron donor N,N,N ',N '-tetramethyl-p-phenylenediaminewith quinoid electron acceptors were prepared and studied by X-raycrystallography, impedance spectroscopy, and DFT computations. Mixeddonor-acceptor pi-stacks were observed in all compounds.All moieties had a partial radical character due to charge transfer Five charge transfer complexes of electron donor N,N,N ',N '-tetramethyl-p-phenylenediamine(TMPD orWurster's blue) with quinoid electron acceptors tetrafluoro-(F(4)Q), tetrachloro- (Cl(4)Q), tetrabromoquinone(Br(4)Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F(4)TCNQ) were prepared and studied by X-ray crystallography,impedance spectroscopy, and DFT computations. Mixed donor-acceptor pi-stacks were observed in all compounds; in TMPD center dot F(4)TCNQ, the moieties formed 2D stacked arrays. All moietieshad a partial radical character due to charge transfer. The presenceof two-electron multicenter bonding (pancake bonding) was confirmedby quantum chemical computation. In TMPD center dot F(4)Q, alternatinglonger and shorter interplanar distances were consistent with formationof pancake-bonded dimers; in complexes of TMPD with Cl(4)Q, Br(4)Q, and TCNQ, the stacks were equidistant implyingexistence of pancake-bonded polymers.