Wheel and Axle Topology Driven Halogen Bonds for the Construction of Molecular Arrays in Hexacoordinated Sn(IV)Porphyrins: A Structural and Theoretical Investigation

The present study illustrates how halogen bonds (XB) in conjunction with judiciously selected molecular scaffolds can be used for the construction of molecular arrays (ladder, one-dimensional (1D), and two-dimensional (2D) frameworks) in a series of Sn(IV)-porphyrin derivatives, which topologically resemble a “wheel–axle” duo. In all the complexes investigated here, the wheel is constructed with Sn(IV)-5,10,15,20-meso-tetrakis(4-bromophenyl)porphyrin [Sn(L)2-TBrPP], which is relatively rigid, and the two pairs of diametrically opposite Br atoms can get involved in various kind of halogen bond interactions, depending upon the complementary atom(s) present at the axle. Detailed single crystal X-ray structural studies of these complexes reveal the diverse occurrence of Br···O, Br···Br, Br···π halogen bonds, and these XBs are not only restricted between the wheel···axle alone, but also can occur among themselves (i.e., wheel···wheel and axle···axle). Different types of XB directed molecular associations are observed; for example, ladder type supramolecular associations occur in 1 and 2, interlinked 1D framework in 4, molecular chains in 7, 2D-framework in 8, etc. Complementary theoretical studies with Hirshfeld surface analysis show the definite role of Br···Br interactions in the overall stability and mapping of electrostatic potential isosurfaces with the aid of density functional theory in 8 definitely shows the presence of a σ-hole, a requisite feature to show XBs in the crystalline state. The detailed structural and theoretical studies presented here clearly vouch for the use of wheel and axle topology driven halogen bonds for the construction of molecular arrays in hexa-coordinated Sn(IV)-porphyrin derivatives. Photophysical studies show that the variation of axial ligands has a minimal effect on fluorescence as well as the excited state lifetime in these complexes.