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Coordination chemistry of multidentate ligands from a multi-component reaction
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posted on 10.01.2017by Almesaker, Ann
The application of a multi-component reaction approach has been explored in the synthesis of intricate ligands incorporating the commonly employed ligand motifs bis(2-pyridylmethyl)amine (bPMA) and tris(2-aminoethyl)amine (tren). The synthesis of these ligands demonstrates that multi-component reactions can be an area with high exploratory power in ligand syntheses as they have the potential to afford ligands with a diverse variety of structures by a simple methodology.
The coordination chemistry of one bPMA based ligand has been examined with a range of transition metal ions (i.e. MnII, CoII and CuII). The structures of several of these products have been determined by X-ray crystallography and their properties examined by various spectroscopic methods. The complexes are discussed in terms of similarities and differences in the solid state between the different metal ions in combination with different counter ions. Mononuclear and dinuclear complexes, and one case of a 1D coordination polymer were isolated.
The spectroscopic properties of copper(II) complexes formed by two analogous tren based ligands were studied. From these studies, it could be concluded that the two ligands formed analogous complexes in both solution and in the solid state. Data on three of the copper(II) tren complexes were collected using synchrotron radiation due to the small size of the isolated crystals. These complexes were found to have trigonal bipyramidal coordination geometry around the mononuclear copper(II) centre. Two isomeric copper(II) nitrate complexes with minor differences in the copper(II) geometries were isolated. In addition, a copper(II) fluoride complex [CuLF]BF4·THF (L = tren based ligand) was studied which interestingly exhibited a very short Cu-F bond, in fact the
shortest reported to present time.
The copper(II) complexes were further investigated in catalytic oxidation reactions and small ion stabilisation studies. The complexes had poor catalytic activity in catechol oxidation and in oxidative coupling of 2,6-di-tert-butylphenol. However, the bPMA based copper(II) complex showed an initially relatively high catalytic activity in the oxidative coupling reaction, but underwent deactivation. The complexes were not worthy of further investigation in these reactions due to the low catalytic activity.
Preliminary results pointed to the fact that the tren based ligands were able to stabilise Cu-OOH species in the solution at low temperatures (-60 and -80 °C). Such complexes are of interest in the elucidation of enzymatic mechanisms of monooxygenases such as peptidylglycine α-hydroxylating monooxygenases and dopamine β-monooxygenases. These Cu-OOH complexes also demonstrated evidence of partial ligand oxidation upon warming the solutions containing the complexes.
Overall a range of ligands structures with intricate structures have been synthesised by a multi-component approach and used to prepare transition metal complexes (CuII, CoII and MnII) which display interesting properties due to the unusual ligand frameworks with large pendant substituents.