Polyfunctional Tetraaza-Macrocyclic Ligands: Zn(II), Cu(II) Binding and Formation of Hybrid Materials with Multiwalled Carbon Nanotubes

The binding properties toward Cu(II) and Zn(II) ions of HL1, HL2 and HL3 ligands, constituted by tetraaza-macrocyclic rings decorated with pyrimidine pendants, were investigated by means of potentiometric and UV spectrophotometric measurements in aqueous solution, with the objective of using the related HL-M(II) (HL = HL1-HL3; M = Cu, Zn) complexes for the preparation of hybrid MWCNT-HL-M(II) materials based on multi walled carbon nanotubes (MWCNTs), through an environmentally friendly non-covalent procedure. As shown by the crystal structure of [Cu(HL1)](ClO4)2, metal coordination takes place into the macrocyclic ring, while the pyrimidine residue remains available for attachment onto the surface of the MWCNTs via π-π stacking interactions. Based on equilibrium data showing the formation of highly stable Cu(II) complexes, the MWCNT-HL1-Cu(II) was prepared and characterized. This compound proved very stable toward lixiviation processes (release of HL1 and/or Cu(II)); thus it was used for the preparation of its reduced MWCNT-HL1-Cu(0) derivatives. XPS spectra and TEM microscopy showed that MWCNT-HL1-Cu(0) contains Cu(0) nanoparticles, of very small (less than 5 nm) and regular size, uniformly distributed over the surface of the MWCNTs. Also the MWCNT-HL1-Cu(0) material proved very resistant to detachment of its components. Accordingly, both MWCNT-HL1-Cu(II) and MWCNT-HL1-Cu(0) are promising candidates for applications in heterogeneous catalysis.