Utilising Boron containing Lewis Acid Functions as "Stores" for Hydride and Other functional groups and its impact on Catalytic Reactions

Student thesis: Doctoral Thesis

Abstract

Transition metal mediated reactions play a vital role in the catalysis of many chemical transformations. Whilst most of these transformations occur at the metal centre, with ligands acting as spectators, new systems incorporating the direct involvement of coordinated ligands within catalytic cycles are starting to emerge. These so called “ligand cooperation” systems are beginning to incur ever increasing interest as their potential in a multitude of activations and transformations are demonstrated. This project aims to build on and add to the work already undertaken through the development of new ligand systems and complexes for the storage and manipulation of hydrogen and other functionalities within ligand cooperative catalytic cycles. Focusing, in particular, on the use of 2-mercaptopyridyl base boron ligands.The thesis first builds on work previously undertaken by Owen on the synthesis and reactivity of platinum and palladium pincer type complexes of the bis-2-mercaptopyridyl ligand, [HB(mp)2]. A range of complexes containing different phosphines are synthesised and their reactivity explored. New insights are gained into the formation of the complexes through the in situ characterisation of an intermediate species. The complexes are also applied to the catalytic allyl substitution of allyl acetates with conversions and reactivities that are comparable to similar complexes.
The thesis then moves to the synthesis and characterisation of rhodium and iridium complexes containing a new ligand, [H3B(mp)]-, system in which the boron (BH3 unit) is tethered to the metal centres via a single 2-mercaptopyridly supporting unit. These borohydride complexes do not undergo spontaneous hydride migration to form metal-borane complexes, however, upon
addition of other ligands hydride migration and further reactivity can be promoted. The addition of carbon monoxide and isocyanides to a NBD complex results in the sequential migration of a hydride to the metal centre followed by the migration of a NBE moiety from the metal to the boron. This novel transformation results in the formation of a intramolecular hydroboration product. Substituted analogues of this ligand system, that possess an R group (where R = Ph or Me) bound to the boron, have also been prepared and coordinated to rhodium and iridium centres. These complexes also fail to undergo spontaneous hydride migration.
Despite their similarities with the complexes of [H3B(mp)]-, those of [H2B(R)(mp)]- II demonstrate rather different reactivity. Addition of PPh3 to these complexes results in double hydride migration and a previously unprecedented ring opening of NBD to a 4-ethylcyclopentenyl moiety. The resulting complexes feature a metal borane interaction that was investigated for its propensity towards small molecule activation. Indeed the complexes were able to activate hydrogen across the M−B interaction and were active in the catalytic hydrogenation of olefins and towards the catalytic dehydrocoupling of amino-boranes.
Date of Award18 Jun 2020
Original languageEnglish

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