Analyses of Structure and Molecular Movement of Tris(N-salicylidenealkylaminato) Co(III) Complexes in Solution Using High Resolution NMR

Abstract

The preparation of tris(N-salicylidenealkylaminato) Co(III) complexes (Co(salalkam)₃) is reported. In order to clarify the molecular structure and dynamic property of the Co(salalkam)₃, magnetic resonance studies of the complex were investigated in solution by ¹H-, ¹³C-, and ⁵⁹Co-NMR spectra and ¹³C, ⁵⁹Co- spin-lattice relaxation time measurements. The introduction of an alkyl group to the N-atom of salicylideneaminato group could induce the formation of the mer-complex due to the steric interaction between the substituent group and the phenyl group of the salicyl group. The coordination geometry of a salicylideneaminato ligand around a Co(III) ion was not affected by the length of N-alkyl substituents of the ligand. It is considered that in such large molecules as those investigated here, the dipole-dipole relaxation is dominant. So, molecular movement may be qualitatively discussed on the basis of the T₁ values. That is to say, ¹³C-spin-lattice relaxation times were interpreted as due to movement of the substituted alkyl groups. The T₁ values for the long alkyl substituents were much larger than those of the short alkyl (CH₃, C₂H₅, C₃H₇) substituents. This fact suggests that the rotation for short alkyl substituents would be inhibited by the coordination and the segmental movement of the alkyl groups would become more brisk along the extended carbon chains. The T₁ values for the ⁵⁹Co atom, in which nuclear quadrupolar relaxation is dominant, became smaller (from 0.64 to 0.19 ms) along the extended alkyl chains in the complexes. This means that the molecular movement of the complex would become more stagnant along the extended carbon chains.