Solid-State Photochemical [2+2] Cycloaddition Reaction of MnII Complexes.

Abstract

Three MnII complexes have been synthesized under similar experimental conditions. Of these [Mn2 (benzoate)4 (L)2 ] (where L=4-styrylpyridine or 4spy, 1 and 2-fluoro-4'-styrylpyridine or 2F-4spy, 3) are paddlewheel complexes, but crystallized in different space groups. Whereas [Mn2 (benzoate)4 (3F-4spy)4 ] (3F-4spy=3-fluoro-4'-styrylpyridine), 4 is a dinuclear complex having different stoichiometry from 1 and 3 with two pairs of 3F-4spy ligands aligned in face-to-face manner. An irreversible phase transition occurs from the space group P21 /c to C2/c when 1 was heated up to 125 °C to 2 in a single-crystal-to-single-crystal fashion or when ground 1 to powder. 2 is isomorphous and isostructural to 3. Complimentary π-π interactions in head-to-tail alignment of the styrylpyridine ligands furnishes 1D aggregates in 1-3 which are congenial to undergo [2+2] cycloaddition reaction under UV light. Whereas, face-to-face alignment of the 4spy pairs in 4 is expected to provide a head-to-head photoproduct. All the MnII complexes are indeed found to be photoreactive. To our surprise, contrary to their ZnII analogues, 2 and 3 were not found to be photosalient. The percentage volume expansion during the photoreaction as determined from the density measurements, was found to be too low (3.2 and 4.6 % respectively for 2 and 3) to have this behavior.