Syntheses, characterisation and solid state thermal studies of 1-(2-aminoethyl)piperidine (L), 1-(2-aminoethyl)pyrrolidine (L′) and 4-(2-aminoethyl)morpholine (L″) complexes of nickel(II): X-ray single crystal structure analyses of trans-[NiL2(CH3CN)2](ClO4)2, trans-[NiL2(NCS)2] and trans-[NiL″2(NCS)2]

Abstract

Reactions of nickel(II) salts with substituted ethane-1,2-diamine where one of the amine nitrogens is a part of a flexible cyclic ring, e.g. 1-(2-aminoethyl)piperidine (L), 1-(2-aminoethyl)pyrrolidine (L′) and 4-(2-aminoethyl)morpholine (L″) produce a number of complexes of the type: (i) Ni(A-A)2X2 (where X=CF3CO2 −, SCN− and NO2 −; A-A represents L/L′/L″); (ii) Ni(A-A)2(CH3CN)2X2 (X=ClO4 − and NO3 −); (iii) Ni(A-A)2(H2O)2X2 (X=CF3SO3 −, Cl−, Br− and I−); and (iv) Ni(A-A)2(H2O)4X2 (X=0.5SO4 2−, 0.5SeO4 2− and CF3SO3 −). The complexes possess octahedral geometry. The major complexes upon desolvation retain trans-geometry, some of which are cis with respect to the counter-anion and a few of them are square planar. X-ray single crystal structure analyses of trans-[NiL2(CH3CN)2](ClO4)2, trans-[NiL2(NCS)2] (violet) and trans-[NiL″2(NCS)2] (sky-blue) have been done. The violet and sky-blue thiocyanato species have blue and green coloured isomers, respectively, and these pairs of isomers are proposed to be conformational isomers. Solid state thermal investigation of the complexes has been carried out. The complexes show thermochromism due to deaquation–anation/deaquation reaction/change of conformation. Only [NiL2](ClO4)2, [NiL′2(CF3CO2)2] and [NiL″2(NO2)2] undergo thermally induced phase transition. The effect of flexible ring size on diamine has been discussed.