Racemization kinetics and spin states of four-coordinate nickel(II) complexes with bidentate NS ligands, [pyrR 2R 3(NR 1S)] 2Ni. A comparative study

Abstract

Nine complexes of the type [pyrR 2R 3(NR 1S)] 2Ni (R 1iPr) were synthesized in order to investigate possible relations between the spin-spin equilibrium and racemization of four-coordinate nickel(II) complexes with bidentate NS ligands. An X-ray structure analysis of bis(1,3-dimethyl-5-thiolato-pyrazol-4-isopropylaldimino- N,S)nickel(II) shows it to be monoclinic, space group I2/ a, with 4 molecules in a cell of dimensions a=19.091(4), b=7.564(2), c=15.760(4) Å and β=101.818(14)°. The structure refined to a final R value of 0.040 for 1144 reflections. The molecule has two-fold symmetry, the coordination at Ni is close to tetrahedral with NiS=2.249(1) Å and NiN=1.989(3) Å. Every complex was almost totally high spin, but small differences in spin states were found by comparisons of isotropic shifts and extinction coefficients for a pure high spin transition. The racemization was studied for three complexes by temperature dependent 1H NMR spectroscopy. Rate constants, resulting in approximate values of Δ H # (11.1–34.6 kJ/mol) and Δ S # (−56.7 to −144 J/(K×mol)), were determined from the iPrMe shifts and linewidths. Solvent effects of the kinetics were examined for one compound: the simultaneous promotion of the low spin form and the racemization rate at low temperature in the most polar solvent indicated that the low spin form may be the racemizing species.