Rational outer‐sphere design can be used to optimize heterogeneous catalysts. Thus, imprinting of bulk silica was used to prepare tethered active sites with hydrophilic (1) and hydrophobic environments (2). In the Knoevenagel condensation of isophthalaldehyde with malonitrile, for example, 1 gave rate enhancements of about 50 and 30 relative to 2 and a commercial catalyst consisting of a monolayer of 3‐aminopropyl groups on silica, respectively.
Rationales Design der äußeren Oberfläche kann zur Optimierung von heterogenen Katalysatoren genutzt werden. Durch molekulares Prägen von Siliciumdioxid wurden so aktive Stellen mit hydrophilen (1) und hydrophoben Umgebungen (2) erzeugt. Die Knoevenagel‐Kondensation von Isophthalaldehyd mit Malonitril verlief mit 1 etwa 50‐ oder 30‐mal schneller als mit 2 bzw. einem kommerziellen Katalysator aus einer Monoschicht von 3‐Aminopropylgruppen auf Silliciumdioxid.
The effect of chemical environment surrounding a synthetic heterogeneous catalyst active site is investigated using the hydrophilic imprinting of silica. Two model reaction systems have been used for this study: (i) Knoevenagel condensation of 3-nitrobenzaldehyde and malononitrile and (ii) Suzuki coupling of bromobenzene and phenylboronic acid. Using a catalyst in which isolated imprinted amines are surrounded by an acidic silanol-rich environment led to rate accelerations of over 120-fold relative to catalysts in which the amines are surrounded by a hydrophobic environment consisting of trimethylsilyl functional groups for system (i). This result parallels our previous study on the effect of the outer sphere composition on rate acceleration of Knoevenagel reactions using isophthalaldehyde as the aldehyde reactant. We also extended our method for the hydrophilic imprinting of bulk silica to organometallic systems, by successfully synthesizing a tethered palladium complex within the imprinted pocket. This material was used as an active catalyst for (ii). Our results show that a hydrophobic framework environment results in higher initial turnover frequencies than an acidic silanol-rich framework for the Suzuki coupling reaction of bromobenzene and phenylboronic acid, albeit with a lower overall effect than observed in the Knoevenagel system (i). Altogether, these results demonstrate the control of chemical reactivity via the rational design of the outer sphere using an imprinting approach.
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