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Discrete Complexes Immobilized onto Click-SBA-15 Silica: Controllable Loadings and the Impact of Surface Coverage on Catalysis

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Jun Nakazawa , Brian J. Smith, and T. Daniel P. Stack*

Department of Chemistry, Stanford University, Stanford, California 94305, United States

J. Am. Chem. Soc., 2012, 134 (5), pp 2750–2759

DOI: 10.1021/ja210400u

Publication Date (Web): January 24, 2012

stack@stanford.edu

Section:

Physical Organic Chemistry

Abstract

Azidopropyl functionalized mesoporous silica SBA-15 were prepared with variable azide loadings of 0.03–0.7 mmol g^–1 (2–50% of maximal surface coverage) through a direct synthesis, co-condensation approach. These materials are functionalized selectively with ethynylated organic moieties through a copper-catalyzed azide alkyne cycloaddition (CuAAC) or “click” reaction. Specific loading within a material can be regulated by either the azide loading or limiting the alkyne reagent relative to the azide loading. The immobilization of ferrocene, pyrene, tris(pyridylmethyl)amine (TPA), and iron porphyrin (FeTPP) demonstrates the robust nature and reproducibility of this two-step synthetic attachment strategy. Loading-sensitive pyrene fluorescence correlates with a theoretically random surface distribution, rather than a uniform one; site-isolation of tethered moieties 15 Å in length occurs at loadings less than 0.02 mmol g^–1. The effect of surface loading on reactivity is observed in the oxygenation of SBA-15-[Cu^I(TPA)]. SBA-15-[Mn^II(TPA)]-catalyzed epoxidation exhibits a systematic dependence on surface loading. A comparison of homogeneous, site-isolated and site-dense complexes provides insight into catalyst speciation and ligand activity.

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History

Published In Issue February 08, 2012
Article ASAPJanuary 24, 2012
Received: November 04, 2011

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Discrete Complexes Immobilized onto Click-SBA-15 Silica: Controllable Loadings and the Impact of Surface Coverage on Catalysis