Diphenoquinones Redux

Abstract

Benzoquinones can undergo reversible reductions and are attractive candidates for use as active materials in green carbon-based batteries. Related compounds of potential utility include 4,4′-diphenoquinones, which have extended quinonoid structures with two carbonyl groups in different rings. Diphenoquinones are a poorly explored class of compounds, but a wide variety can be synthesized, isolated, crystallized, and fully characterized. Experimental and computational approaches have established that typical 4,4′-diphenoquinones have nearly planar cores in which two cyclohexadienone rings are joined by an unusually long interannular CC bond. Derivatives unsubstituted at the 3,3′,5,5′-positions react readily by hydration, dimerization, and other processes. Association of diphenoquinones in the solid state normally produces chains or sheets held together by multiple C–H···O interactions, giving structures that differ markedly from those of the corresponding 4,4′-dihydroxybiphenyls. Electrochemical studies in solution and in the solid state show that diphenoquinones are reduced rapidly and reversibly at potentials higher than those of analogous benzoquinones. Together, these results help bring diphenoquinones into the mainstream of modern chemistry and provide a foundation for developing redox-active derivatives for use in carbon-based electrochemical devices.

Diphenoquinhydrones and Related Hydrogen-Bonded Charge-Transfer Complexes

“…Preparing a diphenoquinhydrone in the form of single crystals was achieved by layering solutions of 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone ( 3c ) in toluene on solutions containing an equimolar amount of 4,4′-dihydroxy-3,3′,5,5′-tetramethylbiphenyl ( 4c ) dissolved in Et 2 O, followed by slow mixing and evaporation. These experiments yielded crystals of the corresponding 1:1 diphenoquinhydrone as dark blades, which proved to belong to the triclinic space group P 1̅.…”

“…The structure of diphenoquinhydrone 3c · 4c is markedly different from those of the individual components. In crystals of 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone ( 3c ) grown from CH 2 Cl 2 , molecules are joined by multiple C–H···O interactions to form corrugated sheets (Figure a), which stack to produce the observed structure . Corrugated sheets are also present in crystals of the corresponding dihydroxybiphenyl 4c grown from EtOH/CHCl 3 , but they are held together by phenolic O–H···O hydrogen bonds (Figure b) .…”

“…In crystals of 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone ( 3c ) grown from CH 2 Cl 2 , molecules are joined by multiple C–H···O interactions to form corrugated sheets (Figure a), which stack to produce the observed structure . Corrugated sheets are also present in crystals of the corresponding dihydroxybiphenyl 4c grown from EtOH/CHCl 3 , but they are held together by phenolic O–H···O hydrogen bonds (Figure b) . Analogous structures are formed by many related compounds, including 4,4′-dihydroxybiphenyl ( 4a ) itself, , the γ-polymorph of hydroquinone, and 2,3,5,6-tetramethylhydroquinone .…”

“…(a) Representation of the structure of crystals of 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone ( 3c ) grown from CH 2 Cl 2 . The image shows part of a corrugated sheet held together by C–H···O interactions (broken lines with O···H distances given in Å).…”

“…The image shows part of a corrugated sheet held together by C–H···O interactions (broken lines with O···H distances given in Å). (b) Representation of the structure of crystals of 4,4′-dihydroxy-3,3′,5,5′-tetramethylbiphenyl ( 4c ) grown from EtOH/CHCl 3 . The image shows part of a corrugated sheet held together by phenolic O–H···O hydrogen bonds (broken lines with O···O distances given in Å).…”