Studies on substituted thioureas and their Nickel(II) complexes

Abstract

Prior to 1968 there had been several papers in which complexes of nickel(II) with nitrogen-substituted thioureas were shown to have a variety of stereochemistries both in the solid state and in solution. In that year the present author conducted an investigation of the solid-state and solution properties of the nickel complexes with thiourea. In order to explain the wide variety of stereochemistries (octahedral, tetragonal, square planar and tetrahedral) which could be obtained by varying the phase or the anion, a model was proposed in which the structure of the complexes was said to be determined by a delicate balance between crystal packing, hydrogen bonding, steric and electronic effects. In the present work we have used a variety of techniques in an attempt to elucidate the relative importance of these and other factors in determining the stereochemistries of nickel(II) complexes with various nitrogen-substituted thioureas. Our studies on the uncomplexed ligands, using infra-red spectra – and near-infra-red spectra in particular – have shown that extensive hydrogen bonding occurs, both in the solid state, and in solutions of concentrations as low as 10-3M. In the course of this study we have also examined the equilibria between the different conformers of the thioureas, and the effects of solvent, concentration and substituent. In solvents such as carbon tetrachloride and dichloromethane strong hydrogen bonds, N - H···S, result in the formation of cyclic dimers; in acetone solutions of high concentrations, association occurs via the Z N-H to give long-chain polymers. In the near-infra-red region we have assigned bands to the overtones of the N-H and C-H stretching vibrations, to the combination bands of the N-H stretching and deformation modes, and to the combination bands of the C-H stretching and deformation modes. We have used these bands to study the ligands and their complexes. In addition to the strong hydrogen bonds formed between the thioureas, intramolecular hydrogen bonds have been shown to be of great importance in the complexes, as are intermolecular hydrogen bonds between coordinated thiourea and the outer coordination sphere – which plays a considerable part in influencing the stereochemistries. In an endeavour to elucidate the nature of the bonding in the various complexes, we have derived equations in the ZDO approximation from the open-shell equations of Roothaan. The equations for octahedral and tetrahedral Ni(II) complexes have been derived in the ZDO formation, and made invariant to the rotation of the local atomic axes. A program has been written for the solution of these equations and used in a CNDO/2 type calculation on thiourea. Additional information on possible electronic effects has been obtained from a Ligand Field analysis of the d-d bands in the electronic spectra of the complexes. The use of magnetic data (obtained by the NMR method of Evans) in conjunction with ligand field spectra, provides evidence for the stereochemistries in the solid and in solution. Our analysis of the spectra of the various complexes has shown that changing the substituents on the thiourea has only a slight effect on the electronic properties of the system – in accord with other literature evidence. In the course of this study we have prepared and examined complexes with a number of differently substituted thioureas. Thioureas, with one to four branched and straight chain alkyl groups, as well as aromatic groups (as substituents on the nitrogen), have been used as ligands. The electronic, steric and hydrogen bonding effects of the substituents have been investigated. The type of complex formed depends markedly on the phase and the anion. The perchlorate complexes are generally octahedral in the solid state, though square planar complexes can also be isolated. They dissolve in solvents such as acetone and dichloromethane to give square planar species NiL42+ and highly tetragonal species NiL62+, having similar spectra to that of NiL42+. The octahedral species NiL62+ is only formed in exceptional circumstances. The stability of the planar species NiL42+ is ascribed to the formation of a ring M0 involving the nickel pz orbital and the thiocarbonyl Π and Π* orbitals. The tetragonal species NiL62+ is thought to be formed by attack on the fifth and sixth positions of NiL42+, coordination being through the Π and Π* orbitals of the attacking ligands. Steric and hydrogen bonding (both intra- and inter- molecular) effects are though to be important in these complexes. The formation, in the solid state, of the octahedral species NiL62+ is ascribed to crystal packing forces and extensive hydrogen bonding. When the anion is chloride, bromide or iodide the predominant solid state species are tetragonal NiL4X2 complexes having a diamagnetic ground state and a thermally populated triplet excited state. In a number of cases tetrahedral species, NiL2X2, have been isolated; octahedral complexes, NiL6X2, are rare. In solution the stable species are, without exception, tetrahedral NiL2X2. Intramolecular hydrogen bonding, N-H···X, is important, as is a cooperative effect between halide and ligand Π and Π* orbitals. The nitrate complexes are generally octahedral in the solid state, but dissolve to give a species NiL2(NO3)2. This complex is octahedral and thought to contain bidentate nitrate ions. Hydrogen bonding and outer sphere effects are important here. The factors which determine the stereochemistries have been examined and their relative importance assessed. A model has been proposed in which solid state effects, hydrogen bonding, the outer coordination sphere, steric interactions and –bonding have been invoked.