Immobilized metal complexes in porous organic hosts: Development of a material for the selective and reversible binding of nitric oxide

Delivery of NO to specific targets is important in fundamental studies and therapeutic applications. Various methods have been reported for delivery o f NO in vivo and in vitro; however, there are few examples of systems that reversibly bind NO. Reported herein is the development of a new polymer (P- 1[Co-II]) that reversibly binds NO. P-1[Co-II] has a significantly higher a ffinity for NO compared to O-2, CO2, and CO. The polymer is synthesized by template copolymerization methods and consists of a porous methacrylate net work, containing immobilized four-coordinate Co-II sites. Binding of NO cau ses an immediate color change, indicating coordination of NO to the site-is olated Co-II centers. The formation of P-1[Co(NO)] has been confirmed by EP R, electronic absorbance, and X-ray absorption spectroscopies. Electronic a nd X-ray absorbance results for P-1[Co-II] and P-1[Co(NO)] show that the co ordination geometry of the immobilized cobalt complexes are similar to thos e of their monomeric analogues and that NO binds directly to the cobalt cen ters. EPR spectra show that the binding of NO to P-1[Co-II] is reversible i n the solid state; the axial EPR signal associated with the four-coordinate Co-II sites in P-1[Co-II] is quenched upon NO binding. At room temperature and atmospheric pressure, 40% conversion of P-1[Co(NO)] to P-1[Co-II] is a chieved in 14 days; under vacuum at 120 degreesC this conversion is complet e in similar to1 h. The binding of NO to P-1[Co-II] is also observed when t he polymer is suspended in liquids, including water.