Probing the Structure of Immobilized Metal Sites in Porous Organic Hosts by X-ray Absorption Spectroscopy

Abstract

Template copolymerization methods have been used to make materials for a wide variety of applications where site-specific analyte binding is desired. The structure of the binding site is often crucial to the efficient function of the material. We have immobilized cobaltcontaining template complexes in porous organic hosts, allowing the use of spectroscopy to conveniently probe the site structure. Reported herein are results from X-ray diffraction and X-ray absorption spectroscopy (XAS) studies for a series of monomerswhere 1 is the ligand bis[2-hydroxy-4-(4-vinylbenzylmethoxy)benzaldehyde]ethylenediimine and 2 is bis[2-hydroxy-4-(4-vinylbenzylmethoxy)benzyliminopropyl]methyl amine} and related copolymers. Copolymerization of the 6-coordinate complex [Co III 1(dmap) 2 ][PF 6 ] with an organic cross-linker forms immobilized 6-coordinate cobalt sites in P-1[Co III (dmap) 2 ]. Similar findings were obtained for the formation of immobilized cobalt sites in P-2[Co III (1-MeIm)]. Demetalation of P-1[Co III (dmap) 2 ] and P-2[Co III -(1-MeIm)] affords immobilized sites that contain tetra-or pentadentate ligands, respectively. Rebinding of Co II ions affords P-1[Co II ] with sites containing cobalt complexes having square planar coordination geometry, whereas P-2[Co II ] has immobilized square pyramidal cobalt complexes. XAS studies support these coordination geometry assignments and show that the rigidity of the porous host maintains site architecture even after chemical modification.

Preparation and characterization of metal-substituted carotenoid cleavage oxygenases

“…7709 eV that can be fit to a single Gaussian-type function with integrated peak areas of 11.6 for Co-CAO1 and ~ 13 for Co-ACO. These pre-edge areas are on the high side of reported Co(II) pre-edge peak areas, which typically range between 3–6 for six-coordinate sites and > 10 for lower symmetries [42–44]. Our pre-edge analysis, therefore, supports a five-coordinate geometry for the cobalt site in both CAO1 and ACO.…”

Development of porous materials for heterogeneous catalysis: kinetic resolution of epoxides

“…1). 3,8 For P-1*(py)[Co III (dmap)], XAS measurements are consistent with sixcoordinate Co III complexes immobilized within the porous EGDMA host. Both the X-ray absorption near edge spectrum (XANES) and extended X-ray absorption fine structure (EXAFS) data are superimposable with the results for [Co III 1*(vpy)(dmap)] 1 .…”

Proton-Induced Reactivity of NO– from a {CoNO}8 Complex

“…The first inflection energy in the XANES spectrum occurs at 7721.3 eV, as in other XAS-characterized Co III compounds. 17 Simulations of the extended X-ray absorption fine structure spectral region are consistent with Co III in a five-coordinate ligand environment constructed of five O/N ligands at an average distance of 1.88 Å ( Figure S4 ). These results compare favorably to distances obtained from X-ray diffraction (avg: 1.89 Å).…”