Site isolation and epoxidation reactivity of a templated ferrous bis(phenanthroline) site in porous silica
Site isolation and epoxidation reactivity of a templated ferrous bis(phenanthroline) site in porous silica
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DOI：
10.1002/anie.200603423
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Stack, T. Daniel P.
中科院分区：
文献类型：
--
作者：
Terry, Tracy J.;Dubois, Geraud;Stack, T. Daniel P.
Considerable attention has been devoted recently to the immobilization of olefin oxidation catalysts onto solid supports to increase catalyst stability and allow for catalyst recycling and product separation.[1–3] While the reactivity of many immobilized olefin oxidation catalysts is less than their homogeneous counterparts, some species show enhanced efficiency and/or greater enantioselectivity.[4] The manganese catalysts reported by Jacobs and co-workers benefit remarkably from the site isolation afforded by immobilization: while the homogeneous analogues are prone to forming Mn dimers that decompose H2O2, site isolation through immobilization yields more oxidant-efficient and substrate-efficient epoxidation catalysts.[2, 5] An additional advantage of such site isolation is the potential to create metal-coordination environments with labile exogenous ligands not readily accessible in a homogeneous medium. Herein, we demonstrate this concept by creating a derivatized FeII–bis (1, 10-phenanthroline) site, referred to hereafter as a FeII–bisphen site, which contains additional labile ligands. This ligand coordination site is created through a metal-template/metal-exchange procedure on mesoporous silica. Spectroscopic and catalytic oxidative reactivity studies support the formation of such a stabilized FeII–bisphen binding site, which is only transiently stable in homogenous solution. Efficient epoxidation of terminal and electron-deficient olefins has been demonstrated recently using catalysts with the related tetra-coordinating nitrogen ligands [FeII (mep)] 2+[6] and [MnII {(R, R)-mcp}] 2+.[7, 8] In the latter case, replacing (R, R)-mcp, a tetradentate nitrogen ligand, with 2 equivalents of phen provides [MnII (phen) 2X2] 2+(where X is a weak-field, labile monodentate ligand), which is the most active MnII catalyst among 20 complexes examined under identical reaction conditions.[9][FeII (phen) 2X2] 2+ should also be a potential catalyst for olefin epoxidation. While [MnII-(phen) 2X2] 2+ complexes form readily upon mixing two equivalents of phen with a variety of MnII salts in solution, the addition of two equivalents of phen to FeII salts, such as FeII (OTf) 2 (OTf= CF3SO3 À), leads rapidly to the highly stable, low-spin [FeII (phen) 3] 2+ complex and free FeII;[FeII-(phen) 2X2] 2+ is unstable thermodynamically and kinetically with respect to ligand disproportionation in water as well as in the solvent conditions used for catalysis (see below).[10] Thus, monomeric ferrous bisphen species with accessible coordination sites, which are necessary for most electrophilic oxidation reactions,[9, 11] remain unexplored as catalysts. For this investigation, the micelle-templated silica SBA-15 was selected as a support because of its facile synthesis, large pore diameter (ca. 60), and superior hydrothermal and chemical stability.[3, 12] The phenanthroline derivative 1 was synthesized on a multigram scale in an overall yield of 60%. The purposeful incorporation of the sulfur moiety provides an elemental tag for ligand quantification by inductively coupled plasma (ICP) spectroscopy and the trialkoxysilyl group enables facile immobilization on a silica surface. Ligand 2, which was synthesized in a similar manner, provides metal complexes for comparative studies in homogeneous solution.[13]Two methods were used for the immobilization of 1 onto SBA-15 silica: metal templating and random ligand grafting. The templating method involves the initial formation of the thermodynamically stabilized CuI–bisphen complex [Cu12] 1+, covalent attachment of this discrete complex to the silica, removal of the copper atom template, and final metalation of the immobilized 1 with Fe (OTf) 2. Exposing …
