Anisotropic Thermal Expansion of Structurally Related Lanthanide–Mercury(II) Cyanide Coordination Polymers

Abstract

Three sets of related lanthanide−mercury(II) cyanide coordination polymers were synthesized by the reaction of LnCl 3 •xH 2 O (Ln = Ce, Nd, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu) with Hg(CN) 2 and structurally characterized. [Ce(OH 2 ) 5 ][Hg(CN) 2 Cl] 3 •2H 2 O is a 3-D material with sheet-based architecture; its thermal expansion behavior shows uniaxial negative thermal expansion (−18.3(8), 39(2), and 68.3( 16) ppm K −1 along the a, b, and c axes, respectively). This anisotropic thermal behavior is postulated to be driven elastically by weak Hg•••Cl interactions: large area expansion of the sheets causes negative thermal expansion in the perpendicular direction. Using lanthanides heavier than Ce yielded 2-D sheet-based compounds with the formula [Ln(OH 2 ) x ] 2 [Hg(CN) 2 ] 5 Cl 6 •2H 2 O (Ln = Nd and Eu, x = 7; Ln = Gd, Tb, Dy, Ho, Tm, Yb, and Lu, x = 6). Although there was also evidence for elastic behavior within these materials, both showed uniaxial zero thermal expansion (Ln = Nd: 27.9(17), 22.4(10), and 0.6(12) ppm K −1 along the I, II, and III principal axes, respectively; Ln = Tb: 39.6(12), 1.1(17), and 36.1(7) ppm K −1 along the a, b, and c axes, respectively). Despite their similar structural architecture, this zero thermal expansion was found to occur in different directions�within the plane of the 2-D sheets for [Nd(OH 2 ) 7 ] 2 [Hg(CN) 2 ] 5 Cl 6 •2H 2 O but in the direction perpendicular to the 2-D sheets for [Tb(OH 2 ) 6 ] 2 [Hg(CN) 2 ] 5 Cl 6 •2H 2 O. Overall, this system of compounds reveals the delicate relationship between coordination polymer structure and thermal expansion.