Significant Role of Supramolecular Interactions on Conformational Modulation of Flexible Organic Cation Receptors in a Metal-Bis(dithiolate) Coordination Complex Matrix

Abstract

A series of new ion-pair complexes [Bz,R-BzBimy]₂[M­(mnt)₂] {[Bz,R-BzBimy]⁺ = 1-benzyl-3-(4-R-benzyl)­benzimidazolium; M = Cu, R = H (<b>1a</b>), NO₂ (<b>1b</b>) and Br (<b>1c</b>); M = Ni, R = H (<b>2a</b>), NO₂ (<b>2b</b>) and Br (<b>2c</b>) and mnt^2– = maleonitriledithiolate} have been prepared and characterized by routine spectral analyses including single crystal X-ray crystallography. Due to the flexible nature of aryl groups (−CH₂–Ar) in benzimidazolium cations, [Bz,H-BzBimy]¹⁺ and [Bz,NO₂-BzBimy]¹⁺ of compounds [Bz,H-BzBimy]­BF₄ (<b>a</b>) and [Bz,NO₂-BzBimy]­BF₄ (<b>b</b>), respectively, the conformational change of the aryl groups have been observed in their respective metal-dithiolate compounds <b>1a</b>, <b>1b</b>, <b>2a</b>, and <b>2b</b>. However, no change in orientation of the associated phenyl groups is observed between the cationic organic receptor [Bz,Br-BzBimy]¹⁺ of compound [Bz,Br-BzBimy]₂BF₄ (<b>c</b>) and that in resulting ion pair compounds <b>1c</b> and <b>2c</b>. Fluxional behavior of the aryl groups in the cationic organic receptor (benzimidazolium moiety, [Bz,R-BzBimy]⁺), when it is ion-paired with different counteranions, e.g., tetrafluoroborate (BF₄^–) and [M­(mnt)₂]^2–, is mainly dependent on the supramolecular interactions (for example, S···H, N···H, O···H, Br···Br, etc., weak contacts) between the relevant cation and anion. The <i>p</i>-substituents (H, NO₂, and Br) of one of the phenyl rings in benzimidazolium moiety (cationic part) are found to be responsible for the structural diversities, observed in the crystal structures of metal-dithiolate ion pair compounds <b>1a</b>, <b>1b</b>, <b>1c</b>, <b>2a</b>, <b>2b</b>, and <b>2c</b>. In this context, it is worth mentioning that the nickel containing ion pair compounds <b>2a</b>, <b>2b</b>, and <b>2c</b> are isomorphous with corresponding copper analogues <b>1a</b>, <b>1b</b>, and <b>1c</b>. The near-IR absorbance bands at around 1210 nm, observed for the copper compounds (<b>1a</b>–<b>1c</b>), have been attributed to the charge transfer from the copper dithiolate anion [Cu­(mnt)₂]^2– to the benzimidazolium cation [Bz,R-BzBimy]⁺. The absorption bands, observed at around 862 nm for nickel compounds (<b>2a</b>–<b>2c</b>), can be assigned to combined transitions consisting of d–d, MLCT, π → π* electronic transitions. DFT calculations have been carried out to determine stability of bare organic molecules, used in this study, in the perspective of their apparent stability (energy consideration) in the matrix of metal dithiolate coordination complex. Hirshfeld surface analyses have been performed to assess additional supramolecular perceptions into crystal structure features. The relevant fingerprint plot areas portray the percentages of different intermolecular interactions in the crystal structures. Copper compounds <b>1a</b>–<b>1c</b> are additionally characterized by electron spin resonance (ESR) studies