Negative thermal expansion and metallophilicity in Cu${}_3$[Co(CN)${}_6$]

Abstract

We report the synthesis and structural characterisation of the molecular framework copper(I) hexacyanocobaltate(III), Cu${}_3$[Co(CN)${}_6$], which we find to be isostructural to H${}_3$[Co(CN)${}_6$] and the colossal negative thermal expansion material Ag${}_3$[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: ${\alpha }_a$ = 25.4(5)\,MK${}^{-1}$ and ${\alpha }_c$ = $-$43.5(8)\,MK${}^{-1}$. These opposing effects collectively result in a volume expansivity ${\alpha }_V$ = 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$\dots$Cu metallophilic (`cuprophilic') interactions are significantly weaker in Cu${}_3$[Co(CN)${}_6$] than Ag$\dots$Ag interactions in Ag${}_3$[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.

10 pages, 9 figures