Solid‐State Photochemical [2+2] Cycloaddition Reaction of Mn II Complexes

Three Mn II complexes have been synthesized under similar experimental conditions. Of these [Mn 2 (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 [Mn 2 (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 P 2 1 / c to C 2/ 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 Mn II complexes are indeed found to be photoreactive. To our surprise, contrary to their Zn II 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.