Host-guest chemistry of copper(II)-histidine complexes encaged in zeolite Y

Structural analysis has been carried out on copper(II)-histidine (Cu2+/His) complexes after immobilization in the pore system of the zeolites NaY and de-aluminated NaY (DAY). The aim of this study was to determine the geometrical structure of Cu2+/His complexes after encaging, to obtain insight into both the effect of the zeolite matrix on the molecular structure and redox properties of the immobilized complexes. In addition to N-2 physisorption and X-ray fluorescence (XRF) analyses, a combination of UV/Vis/NIR, ESR, X-ray absorption (EXAFS and XANES), IR, and Raman spectroscopy was used to obtain complementary information on both the first coordination shell of the copper ion and the orientation of the coordinating His ligands. It was demonstrated that two complexes (A and B) are formed, of which the absolute and relative abundance depends on the Cu2+/His concentration in the ion-exchange solution and on the Si/Al ratio of the zeolite material. In complex A, one His ligand coordinates in a tridentate facial-like manner through N-am, N-im, and O-c, a fourth position being occupied by an oxygen atom from a zeolite Bronsted site. In complex B, two His ligands coordinate as bidentate ligands; one histamine-like (N-am, N-im) and the other one glycine-like (N-am, O-c). In particular the geometrical structure of complex A differs from the preferred structure of Cu2+/His complexes in aqueous solutions; this fact implies that the zeolite host material actively participates in the coordination and orientation of the guest molecules. The tendency for complex A to undergo reduction in inert atmosphere to Cu1+ (as revealed by dynamic XANES studies) suggests activation of complex A by the interaction with the zeolite material. EXAFS analysis confirms the formation of a distorted four coordinate geometry of complex A, suggesting that the combination of zeolite and one His ligand force the Cu2+ complex into an activated, entactic state.