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 (1a), NO2 (1b)
and Br (1c); M = Ni, R = H (2a), NO2 (2b) and Br (2c) and mnt2– = 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 (a) and [Bz,NO2-BzBimy]BF4 (b), respectively, the conformational change of the aryl groups
have been observed in their respective metal-dithiolate compounds 1a, 1b, 2a, and 2b.
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 (c) and that in resulting ion pair compounds 1c and 2c. 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 p-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 1a, 1b, 1c, 2a, 2b, and 2c. In this
context, it is worth mentioning that the nickel containing ion pair
compounds 2a, 2b, and 2c are
isomorphous with corresponding copper analogues 1a, 1b, and 1c. The near-IR absorbance bands at around
1210 nm, observed for the copper compounds (1a–1c), 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 (2a–2c), 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 1a–1c are additionally characterized by electron spin resonance
(ESR) studies.