Title: 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$\ldots$Cu metallophilic (`cuprophilic')
interactions are significantly weaker in Cu$_3$[Co(CN)$_6$] than Ag$\ldots$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.