Solid state photodimerization in an organic salt of 1,2-bis(4-pyridyl)ethylene and trifluoromethane sulphonic acid via pedal-like motion

Abstract: A novel organic salt, [bpeH].TFMS (1), of trans-1,2-bis(4-pyridyl)ethylene (bpe) was synthesized using trifluoromethane sulphonic acid (CF ₃SO ₃H , TFMS) by self-assembly and its crystal structure was determined by X-ray crystallography. Compound 1 forms hydrogen-bonded assembly of [bpeH]⁺ cations and CF3SO3- anions which are held together by N–H⋯ N, C–H⋯ O and C–H⋯ F hydrogen-bonding interactions. An attempt has been made to orient the olefinic C=C bonds of bpe in the organic salt via weak hydrogen-bonding interaction for photodimerization reaction in the solid state. The [ bpeH ] ⁺ cations are in parallel orientation in the infinite one dimensional (1D) chain through N–H⋯ N hydrogen bonding. The olefinic C=C bonds of bpe are aligned in criss-cross orientation with a center-to-center distance of 4.97 Å. Even though the distance and the orientation of olefinic C=C bonds do not conform with the Schmidt topochemical criteria, compound 1 undergoes photodimerization reaction in solid state to produce stereo-specific rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) (2) in 100% yield. The photoreactivity is expected to take place through cooperative movement of 1D chains and pedal-like motion of the pyridyl rings over the double bonds. The photodimerized product 2 undergoes acid-catalyzed isomerization to produce other cyclobutane isomers such as rcct-tpcb and rtct-tpcb. This strategy for synthesizing organic salts is a facile step for the synthesis of cyclobutane derivatives. Graphical Abstract: SYNOPSIS A novel organic salt of trans-1,2-bis(4-pyridyl)ethylene and trifluoromethane sulphonic acid was characerized by X-ray crystallography. The salt shows unusual photoreactivity via pedal-like motion of C=C double bonds which change from criss-cross to parallel orientation. The photodimerized product undergoes acid-catalyzed isomerization in solution and produces all the isomers of tetrakis(4-pyridyl)cyclobutane. [Figure not available: see fulltext.].