Conformational changes of nickel(II) diamine complexes in the solid state

Abstract

Conformational changes in some nickel(II) diamine complexes have been studied by differential scanning calorimetry. Two tris(ethylenediamine)nickel(II) thiocyanates, (1) and (3), have been synthesised. Complex (1) undergoes a phase transition on heating, transforming into species (2)(at 416 K, ΔH= 1.7 kJ mol^–1) which reverts to (1) upon cooling. Complex (3) does not undergo a phase transition. Conformational forms with a particular configuration about the metal ion, δδλ, δλλ, δδδ, have been suggested for complexes (1)–(3) respectively on the basis of the enthalpy change for (1)→(2) and the interconversion (1)→(3) in solution. The complex [Ni₂(en)₄(NCS)₄](4), having both bridging and chelating ethylenediamine, has also been synthesised. On heating it undergoes a solid-state reaction (406–434 K, ΔH= 9.8 kJ mol^–1) yielding trans-chelated monomeric [Ni(en)₂-(NCS)₂](5). The complex [Ni(pd)₂(NCS)₂](6)(pd = 1,3-propanediamine) has been found to transform into an isomer (7) after two overlapping phase transitions (446–467.5 K, ΔH= 20.1 kJ mol^–1). Isomer (7), on keeping overnight in a humid atmosphere (relative humidity 60–70%), is transformed into another isomer (8) which on heating undergoes phase transitions in two steps (445–462 K, ΔH= 9.7 kJ mol^–1; 465–477 K, ΔH= 3.9 kJ mol^–1). The intermediate species (9) has been isolated. Conformational changes such as trans-chair-chair (6)→ skew boat-skew boat (7)→cis-chair-chair (8)→ skew boat-skew boat (9)→ species (10)[(7)] have been proposed on the basis of enthalpy changes for the interconversions. The complex [Ni(pd)₃]Br₂·H₂O (11) after dehydration shows a phase change yielding an isomer (13)(408–428 K, ΔH= 7.1 kJ mol^–1). Isomer (13) reverts to (11) on addition of one molecule of water. This phenomenon is probably due to a chair ⇌ skew boat transformation of the chelated ligand.