Uniaxial negative thermal expansion and metallophilicity in Cu3[Co(CN)6]

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: α = 25.4(5) MK a
−1 and α = − 43.5(8) MK c
−1. These opposing effects collectively result in a
volume expansivity α = 7.4(11) MK V
−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 GULP 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.