Negative thermal expansion and metallophilicity in Cu3[Co(CN)6]
A F Sapnik, X Liu, H L B Boström, C S Coates, A R Overy, E M Reynolds, A Tkatchenko, and A L Goodwin
Journal of Solid State Chemistry 258, 298-306 (2018)

We report the synthesis and structural characterisation of the molecular framework copper(I) hexacyanocobaltate(III), Cu3[Co(CN)6], which we find to be isostructural to H3[Co(CN)6] and the colossal negative thermal expansion material Ag3[Co(CN)6]. Using synchrotron X-ray powder diffraction measurements, we find strong positive and negative thermal expansion behaviour respectively perpendicular and parallel to the trigonal crystal axis: αa = 25.4(5) MK−1 and αc = −43.5(8) MK−1. These opposing effects collectively result in a volume expansivity αV = 7.4(11) MK−1 that is remarkably small for an anisotropic molecular framework. This thermal response is discussed in the context of the behaviour of the analogous H- and Ag-containing systems. We make use of density-functional theory with many-body dispersion interactions (DFT+MBD) to demonstrate that Cu…Cu metallophilic (‘cuprophilic’) interactions are significantly weaker in Cu3[Co(CN)6] than Ag…Ag interactions in Ag3[Co(CN)6], but that this lowering of energy scale counterintuitively translates to a more moderate—rather than enhanced—degree of structural flexibility. The same conclusion is drawn from consideration of a simple lattice dynamical model, which we also present here. Our results demonstrate that strong interactions can actually be exploited in the design of ultra-responsive materials if those interactions are set up to act in tension.