Tuning of thermal expansion properties of a mixed-ligand MOF by ligand variation

Tuning of thermal expansion properties is highly challenging due to the involvement of different mechanisms that contribute to thermal expansion in different materials. There are some reports that attribute to the tuning and modulation of thermal expansion properties mostly through guest interchange. Herein, we report for the first time a change in thermal expansion properties by changing a ligand without affecting the overall framework. A previously studied MOF, [Co(Pz)(TPA)] ( 1 ), has shown uniaxial colossal linear NTE and PTE with coefficients of thermal expansion (CTE) of −123 and 184 MK −1 along the principal axes X 1 and X 3 , respectively, and a small PTE along X 2 with a CTE of 21 MK −1 from 100 K to 450 K. Replacing pyrazine ( Pz ) ligand in 1 with 4,4′-bipyridine ( Bpy ), the thermal expansion property was changed significantly in the MOF Co[(Bpy)(TPA)] ( 2 ), although the crystal structures of 1 and 2 are analogous to each other. Compound 2 exhibits uniaxial NTE, PTE and ZTE with CTEs of −56.9, 114.26 and −0.38 MK −1 along the principal axes X 1 , X 3 and X 2 , respectively, within a temperature range from 100 K to 450 K. Thermal expansion property of a mixed-ligand MOF was tuned by replacing the pyrazine ( Pz ) ligand with bipyridine ( Bpy ).