Designing Molecular Catalysts with Enhanced Lewis Acidity

Abstract

One of the key challenges in catalysis is the generation of catalytically active metal centers that are highly Lewis acidic so that the metal center can easily bind with a nucleophilic monomer to initiate a catalytic process. With this goal in mind, we pursued the designed synthesis of catalytically active metal centers with enhanced Lewis acidity, adopting two different synthetic strategies. One is the introduction of oxygen between two different metal atoms, and the other is the chemical attachment of highly electronegative fluorine around the catalytically active metal center. The attachment of the oxygen between the two metal centers also brings the metals into close proximity at the molecular level, resulting in a pronounced chemical communication between the metals. The compounds with different metals have often modified the fundamental properties of the individual metal atoms through the well-known “cooperative interaction” that is otherwise difficult to achieve. The synthetic strategy takes advantage of the Brønsted acidic character of the M(Osingle bondH) moiety in building up a new class of heterometallic complexes. Further, the discovery of Me3SnF as one of the most useful fluorinating reagents for organometallic complexes leads to the successful preparation of organometallic fluorides of Group-4 metals. This synthetic development has resulted in the availability of catalysts of a new class bearing enhanced Lewis acidic metal centers resulting either from oxygen bridging or from the attachment of a highly electronegative fluorine to a catalytically active metal center. In many cases, these complexes have proved to be excellent candidates for olefin polymerization, ring-opening polymerization of caprolactone, olefin epoxidation, and olefin hydroformylation. The improvement in the catalytic properties is a result of the presence of a more electrophilic metal center, which is essential for the catalysis.