Uniaxial negative thermal expansion and metallophilicity in Cu3[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: α = 25.4(5) MK . 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 Cu 3 [Co(CN) 6 ] than Ag + …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 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.

Low‐Barrier Hydrogen Bonds in Negative Thermal Expansion Material H3[Co(CN)6]

“…The qualitative agreement between theoretical and experimental charges is in general good, but there is a tendency for experiment to give higher positive and negative charges than the theoretical approach. Furthermore, they are in good agreement with a charge of −1.5 e for the anionic complex [Co(CN) 6 ] and +0.5 e for hydrogen used to simulate the thermal response of this material and the isostructural copper and silver analogues by Sapnik et al …”

“…Furthermore, they are in good agreement with ac hargeo fÀ1.5e for the anionic complex [Co(CN) 6 ] and + 0.5e for hydrogen used to simulatet he thermalr esponse of this material and the isostructural coppera nd silver analogues by Sapnike tal. [22] Source function and hydrogen bonding…”

Anomalous Lattice Dynamics in AgC4N3: Insights From Inelastic Neutron Scattering and Density Functional Calculations

“…Therefore, there is a close relation between the nature and magnitude of anisotropic thermal expansion and compressibility. Compounds which exhibit negative linear or area compressibilities are also found to show negative linear or area thermal expansion behavior along that particular directions (Conterio et al, 2008 ; Goodwin et al, 2008a , b , 2009 ; Gupta et al, 2017 ; Sapnik et al, 2018 ). However, the opposite is not true.…”

“…Many cyanide based metal organic flexible framework structures (Conterio et al, 2008 ; Goodwin et al, 2008a ; David et al, 2010 ; Mittal et al, 2012 ; Cairns et al, 2013 ; Gupta et al, 2017 ; Ovens and Leznoff, 2017 ; Sapnik et al, 2018 ) like ZnAu 2 (CN) 4 , M 3 Co(CN) 6 and MAuX 2 (CN) 2 where M = H, Au, Ag, Cu, Fe and X = CN, Cl, Br etc., show large NLC and NTE behaviors. These stem from their special structure and bonding.…”

“…In case of ZnAu 2 (CN) 4 , the NLC, and NTE along hexagonal c-axis arise from the anharmonic nature of the low energy optic phonon modes involving bending of the –Zn–NC–Au–CN–Zn– linkage, mimicking the effect of a spring in terms of compression upon heating and elongation (Gupta et al, 2017 ; Wang et al, 2017 ) under a hydrostatic pressure. The mechanism of the deformation of a wine rack structure produces NLC and NTE behavior in many compounds (Goodwin et al, 2008a ; David et al, 2010 ; Cairns et al, 2012 ; Zeng et al, 2017a ; Sapnik et al, 2018 ). Negative area compressibility and negative area thermal expansion are found to arise from the sliding of atomic layers as a function of pressure or temperature (Zeng et al, 2017b ).…”