Role of π-π Stacking and Halogen Bonding by 1,4-Diiodoperchlorobenzene To Organize the Solid State To Achieve a [2+2] Cycloaddition Reaction

The formation of a series of polymeric isostructural solids that utilize 1,4-diiodoperchlorobenzene (C6I2Cl4) as a halogen bond donor has been achieved. These cocrystals are based upon C6I2Cl4 along with one of three isosteric bipyridines, namely, trans-1,2-bis(4-pyridyl)ethylene (BPE), 4,4'-azopyridine (AP), or 1,2-bis(4-pyridyl)acetylene (BPA). In all cases, a one-dimensional chain is formed that is held together by I center dot center dot center dot N halogen bonds in which molecules of C6I2Cl4 stack in a homogeneous face-to-face pi-pi stacking orientation. As a consequence of this type of interaction, the various halogen bond acceptors also engage in a similar homogeneous stacking arrangement. Because of this stacking pattern, the carbon-carbon double bonds (C=C) within (C6I2Cl4)center dot(BPE) are found to be parallel and within the correct distance to undergo a solid-state [2 + 2] cycloaddition reaction. Upon exposure to ultraviolet light, (C6I2Cl4)center dot(BPE) undergoes a stereoselective and high yielding photoreaction (ca. 89%) that produces rctttetrakis(4-pyridyl)cyclobutane (TPCB). A photoreactive material was also achieved via a solvent-free approach where dry vortex grinding of both C(6)I(2)Cl(4 )and BPE resulted in a solid that also undergoes a [2 + 2] cycloaddition reaction with a similar overall yield. Density functional theory calculations demonstrate that the homogeneous pi-pi stacking that is observed in each cocrystal is preferred when compared to the hypothetical heterogeneous stacking pattern of the aromatic rings.