Solid state reactions offer a unique opportunity for synthesizing complex molecules with amazing regio- and stereo-specificity otherwise difficult to synthesize by conventional organic synthetic methodologies. In particular, the solid state [2 + 2] reaction is a well studied reaction in terms of a crystal engineering perspective. The main challenge in performing solid state reactions is bringing the molecules into reactive orientations such that the reactive groups are within a certain proximal distance. The out surge in organic and inorganic crystal engineering studies offers several strategies for bringing the molecules together in the crystal lattice. These strategies include from weak interactions (halogen bonds, halogen···halogen, π···π, cation···π) to strong hydrogen bonds (O-H···O, N-H···O, O-H···N and N-H···N) to coordination complexes/polymers. To date many studies are available which use such strategies for conducting single, double, triple or multiple [2 + 2] reactions, [4 + 4] reactions, Diels-Alder reactions and polymerization reactions of acetylene molecules. Some of the crystal engineering strategies include the use of external templates which can be removed after the reaction and some of the other strategies deals with the modification of reactant molecules. In this review, the current status of various strategies will be outlined with respect to the nature of the interactions involved.

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Ag 3 3 3 Ag interactions were shown to template [2 þ 2] photochemical reactions of unsymmetrically substituted olefin molecules, namely 4-pyridyl acrylic acid (1), 4-pyridyl acryl amide (2), and methyl ester of 4-pyridyl acrylic acid (3) in their Ag(I) complexes [Ag(1) 2 (NO 3 )] 3 MeOH (4); [Ag(2) 2 (NO 3 )] (5); and [Ag(3) 2 (NO 3 )] 3 MeOH (6). Nitrate ions were found to bridge these Ag 3 3 3 Ag interactions in two ways: as a discrete dimer in 4 and 5 and as a zigzag 1D-polymer in the case of 6. Irradiation of these complexes in UV/sunlight produced respective head-to-head dimers in nearly 100% yields, as supported by 1 H NMR spectroscopy. In the case of 5, the transformation was shown to occur in single crystal to single crystal fashion by determining the crystal structure of partially irradiated materials of 5. Further, we have also found that the irradiation of mechanically grounded samples of AgNO 3 with 2 or 3 produces a similar [2 þ 2] reaction, to result in respective head-to-head dimers in good yields.The wealth of knowledge on intermolecular interactions in the solid state is of immense help in engineering solid state reactions, 1 in particular [2 þ 2] photodimerization of olefins containing different substitutions. [2][3][4] In this regard, the use of small organic molecules which can clip the two olefins in the required geometry via hydrogen bonds is an elegant and very well explored strategy. 3 Further, coordination bonds 4 and exotic interactions such as halogen bonding 2d were also shown to template these reactions. However, in most cases, the olefin is bis-pyridyl ethylene or containing symmetrical substitution on both ends of the olefin. The reactions of olefins containing unsymmetrical substitutions are rarely studied via template reactions. 2g,h The resultant cyclobutane products from such hetero functional olefins (pyridine and some other functionality) have a potential to act as useful multidentate ligands for the construction of MOFs. 5 In this communication we would like to report [2þ2] reactions of such unsymmetrical olefins, namely 4-pyridyl acrylic acid (1), 4-pyridyl acryl amide (2), and the methyl ester of 4-pyridyl acrylic acid (3) using Ag 3 3 3 Ag interactions as a template.The crystal structures of silver nitrate complexes of iso-nicotinamide suggested that Ag 3 3 3 Ag interactions could be a better template for the [2 þ 2] reactions in these compounds. 6 Ag 3 3 3 Ag interactions, either ligand/anion supported or unsupported, are very well-known in the literature, 7-12 but to date only two examples were reported in the literature for their utility in templating of [2 þ 2] photochemical reactions of olefins. 10 Silver ions can exert argentophilic interactions even at distances little more than the sum of their van der Wall radii (3.44 Å ), maintaining the angular specificities. 11 Thus, even a very weak argentophilic interaction can be expected to bring the double bonds within a reactive distance (<4.2 Å ). 1a The strongly bridging counteranions such as carboxylate ions bridge Ag(...

Synthesis of a stereospecific four-twelve-four fused tricyclic compound containing a tetraamide macrocycle has been achieved by the solid state [2+2] reaction through utilization of Ag···Ag interaction. The influence of anions on crystal packing has been utilized for the synthesis of a regioselective mono cyclobutane compound via the Ag···π interactions.

The compound trimesic acid (H 3 TMA) with 2,6-bis(4-pyridylmethylene)cyclohexanone (1) was shown to form two-/three-/four-component cocrystals or salts depending on the reaction conditions. Crystals of salt were obtained when crystallized H 3 TMA and 1 from MeOH alone, whereas the crystals of neutral components were obtained when crystallized from MeOH and phenol mixture. Both the structures (salt and cocrystal) exhibited huge cavities which are occupied by the self-interpenetration of the networks. This self-interpenetration of the networks found to be eschewed in favor of guest inclusion when the crystallization reactions are conducted in the presence of suitable aromatic guest molecules such as biphenyl, naphthalene, anthracene, phenanthrene, pyrene, triphenylene, and 1,4-hydroquinone. The crystal structures of cocrystals contain hydrogen-bonded layers with huge cavities, the self-interpenetrated layers have a honeycomb geometry with the cavity dimensions of 23 Â 42 A ˚2 and the guest included layers have a brick wall geometry with cavity sizes of 8 Â 23 A ˚2. The guest occupied volume of these materials varies from 12 to 38% of crystal volume. In the four-component systems, the adducts of guest molecules were found between pyrene-phenol, pyrene-1,4-hydroquinone, and phenanthrene-o-cresol. In three-component systems, the packing of guest biphenyls is similar to that of the biphenyl crystal structure itself.

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