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

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 cobalt-containing 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 monomers {[Co III 1 (dmap) 2 ][PF 6 ] and [Co III 2 (1-MeIm)][PF 6 ], where 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.