The novel 1 : 1 donor–acceptor complex, 3,4-dimethoxycinnamic acid–2,4-dinitrocinnamic acid. Crystal engineering, structure, and anomalous lack of solid-state topochemical reactivity

Abstract

Donor–acceptor interactions have been exploited to engineer a crystal structure of a 1 : 1 molecular complex (2a) of the title acids (1a) and (1d), where solid-state topochemical 2 + 2 photoaddition to yield pseudo-symmetrical cyclobutanes is permitted. Orange crystals of (2a) are triclinic, P, a= 8.857(5), b= 13.872(8), c= 8.578(4)Å, α= 101.41(4), β= 100.98(4), γ= 95.98(4)°, Z= 2, R= 0.054 on 1 315 non-zero reflections. The crystal structure consists of interleaving donor and acceptor molecules in the stack with two unequal separations of 3.80 and 4.97 Å with the former overlap relating ‘potentially reactive’ double bonds. Contrary to expectation, complex (2a) is photostable. This inertness is shared by several other aromatic nitro compounds and some mechanistic possibilities are discussed. A classification of solid-state 2 + 2 cycloaddition reactions is attempted on the basis of how far topochemical predictions match with observed photochemical behaviour.