Syntheses and reactivity of transition metal complexes of macrocycles containing sulfur and nitrogen ligating atoms

Date

2018-06-26

Authors

Chandrasekhar, Savitri

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Abstract

The ligands of the ten-membered series, [10]-aneS3, [10]- aneS2N, [10]-aneSN2 and the macrobicyclic ligand - 1,4-bis(1-aza- 4,8-dithia-4-cyclodecyl) ethane, and their transition metal complexes were successfully synthesised. Chromium (III) complexes of the homoleptic hexaaza ligands [18]-aneN6 and [20]-aneN6 were synthesised, structurally characterised and their spectral properties studied. Bis complexes of Ni(II) with [10]-aneS3, [10]-aneS2N and [10]-aneSN2 were octahedral as is evident from their crystal structures. The esr spectra of the corresponding Ni(III) complexes are characteristic of a low spin d7 ion in a compressed octahedral coordination in the complex based on [10]-aneS2N and an elongated octahedral coordination in the complexes based on [10]-aneSN2 and [10]-aneS3. The spectral and electrochemical properties of the various Ni(II) complexes are compared with each other. The redox reactivity of the Ni(II) complex based on [10]-aneS3 was studied. The synthesis of a macrobicyclic ligand is described. The Ni(II)complex of the macrobicyclic ligand is a distorted octahedron and the esr spectrum of the Ni(III) complex is characteristic of a low spin d7 ion in a compressed geometry. The covalency parameter K, and the energy separation between the low spin ground state and the first excited high spin state were determined from the esr and the electronic spectra of the Ni(III) complex. Two isomers for the Pd(II) bis complexes of [10]-aneS2N were obtained and characterised by X-ray methods and nmr spectroscopy. Evidence for the formation of a high spin Pd(II) octahedral species has been provided for the first time. Oxidation of the Pd(II) complex is metal centered and the esr spectra of the Pd(III) complexes are characteristic of a low spin d7 PdS4N2 core. Fe(II), Fe (III), Co(II), Co(III), Ni (II) and Pd(ll) bis complexes of the ligand [10]-aneS3 were synthesised and characterised by elemental analysis, nmr and esr spectroscopies, where appropriate, and their spectral and electrochemical properties studied. The crystal structures of Fe(II), Co (II) and Ni(II) bis complexes of [10]-aneS3 were octahedral with three S atoms from each of the thioether ligands coordinated to the central metal ion. The esr spectra of the Fe(III) bis complexes of [9]-aneS3 and [10]-aneS3 were characteristic of a low spin d5 complex ion. The ligand field distortion parameters were obtained from the electronic and esr spectra and the energies of the Jahn-Teller splitting were estimated. The Co(II, bis complex of [10]-aneS3 is low spin. The half-wave potentials due to the [special characters omitted] and [special characters omitted] couples were obtained by cyclic voltammetry. The electron self exchange rate constant for the [special characters omitted] and the [special characters omitted] couples were determined by the 59Co nmr line broadening technique for the first time. The self exchange rate constant for the [special characters omitted] couple was determined by the 'H nmr line broadening technique. The crystal structure of the Pd(II) bis complex of [10]—aneS3 is essentially square planar with significant interactions from the axial S atoms. This complex is fluxional as is evidenced in the variable temperature nmr spectra.

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Keywords

Transition metal complexes, Synthesis, Reactivity

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