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 wi de variety of applications where site-specific analyte binding is desired. The structure of the binding site is often crucial to the efficient functio n 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 diffractio n and X-ray absorption spectroscopy (XAS) studies for a series of monomers {[Co(III)1(dmap)(2)][PF6] and [Co(III)2(1-MeIm)][PF6], where 1 is the ligan d bis[2-hydroxy-4-(4-vinylbenzylmethoxy)benzaldehyde]ethylenediimine and 2 is bis[2-hydroxy-4-(4-vinylbenzylmethoxy)benzyliminopropyl]methyl amine} an d related copolymers. Copolymerization of the 6-coordinate complex [Co(III) 1(dmap)(2)][PF6] with an organic cross-linker forms immobilized 6-coordinat e 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)]. Demetalat ion of P-1[Co-III(dmap)(2)] and P-2[Co-III(1-MeIm)] affords immobilized sit es that contain tetra- or pentadentate ligands, respectively. Rebinding of Coo ions affords P-1[Co-II] with sites containing cobalt complexes having s quare planar coordination geometry, whereas P-2[Co-II] has immobilized squa re pyramidal cobalt complexes. XAS studies support these coordination geome try assignments and show that the rigidity of the porous host maintains sit e architecture even after chemical modification.