pH-Responsive Switching Properties of a Water-Soluble Metallamacrocyclic Phenylalaninehydroximate La(III)-Cu(II) Complex: Insight into Tuning Protonation Ligand States

The water-soluble copper(II)-lanthanum(III) complex based on phenylalaninehydroximate ligands La(H2O)(4)[15-MCCuPhalaha-5](Cl)(3) (1) has been synthesized and characterized structurally and spectroscopically. The X-ray structure of 1 demonstrated that the ring copper ions are placed in distorted square pyramidal geometries with various axial coordination ligands. UV/Vis absorption spectra reveal halochromic behavior of 1 in the aqueous solution which represents reversible color changes from blue to colorless in response to pH variations. This effect has been described as pH-induced "on-off-on" switch through protonation and deprotonation of the phenylalaninehydroximate ligands in the metallamacrocyclic complex 1. To investigate the electronic structures of the complexes as well as the plausible mechanism of the halochromism, DFT calculations on the La(H2O)(4)[15-MCCu(II)Phalaha-5](3+) ion and the protonated La[15-MCCu(II)Phalaha(H)-5](4+) species were carried out. The calculations demonstrate that the protonation of the phenylalaninehydroximate ligands in the metallacrown is accompanied by variations in the electron density distribution and HOMO-LUMO energy gap, which correlate with the spectroscopic changes observed.