Plasticity in [(R_4-xR¹_x)₄N]₄[Cu₄{S₂C₂(CN)₂}₄] (x = 0-4) is molded by a guest cation on an elastic anionic host

Abstract

The elastic anion [Cu₄(mnt)₄]^4- {mnt = maleonitriledithiolato, [S₂C₂(CN)₂]^2-} serves as a host to a series of guest cations [(R_4-xR¹_x)₄N]⁺ (R = Et, R¹ = Me; x = 0-4) to form the complexes [(R_4-xR¹_x)₄N]₄[Cu₄{S₂C₂(CN)₂}₄]. The molecular structural design information stored in the cation is molded to the elastic anionic host-framework through H-bonding to create a plastic supramolecular entity. The reaction of CuCl with Na₂(mnt) in a 1:1 metal/ligand ratio, followed by interaction with the respective cation, gives the cation-dependenttetranuclear host-guest assemblies [Me₄N]₄[Cu₄(mnt)₄] (1), [Me₃EtN]₄[Cu₄(mnt)₄] (2), [Me₂Et₂N]₄[Cu₄(mnt)₄] (3), [MeEt₃N]₄[Cu₄(mnt)₄] (4) and [Et₄N]⁴[Cu₄(mnt)₄] (5), which have been characterized by X-ray structural studies in the solid state. The host cavity shrinks and expands, with varying Cu···Cu separations ranging from 2.754 to 3.952 Å, to accommodate the relevant guest ion during the crystallization process. All these complexes show identical ¹³C NMR and UV/Vis spectra in solution and their identical cyclic voltammetric redox behaviors also imply the presence of a common entity. The ESI mass spectra in acetonitrile confirm the presence of this common {Cu₄(mnt)₄} core. The elastic {Cu₄(mnt)₄} core participates in extensive hydrogen bonds in the presence of different quaternary cations in solution during the crystallization process and the cation determines the final shape of the anion in the solid state. Depending on the length and nature (symmetric or non-symmetric) of the alkyl groups attached to the quaternary cation, the overall geometry of the anion assumes different degrees of openness in a partially opened umbrella shape. The {Cu₄(mnt)₄} moiety is cleaved by treatment with PPh₃ to give a monomer, which has been isolated as [Et₄N][Cu(mnt)PPh₃] (6).