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

The binding properties of HL1, HL2, and HL3 ligands toward Cu(II) and Zn(II) ions, 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 multiwalled carbon nanotubes (MWCNTs), through an environmentally friendly noncovalent procedure. As shown by the crystal structure of [Cu(HL1)](ClO 4 ) 2 , metal coordination takes place in the macrocyclic ring, whereas the pyrimidine residue remains available for attachment onto the surface of the MWCNTs via π-π stacking interactions. On the basis of equilibrium data showing the formation of highly stable Cu(II) complexes, the MWCNT-HL1-Cu(II) material 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. X-ray photoelectron spectroscopy and transmission electron microscopy images 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.