Synthesis and Characterisation of Novel PNP Pincer Ligands for C-H and CO2 Activation with Iridium

Abstract

Functionalisation of CO2 and C-H bonds are important challenges in sustainable chemistry. Selective C-H functionalisation allows the use of a wider variety of chemical building blocks and the activation of CO2 promises access to a cheap, renewable and non-toxic C1 source in synthetic chemistry. Activation of these molecules is difficult however and often requires catalysts to facilitate reactivity, and although many catalysts and complexes have been developed for these transformations both remain significant challenges.

PNP pincer complexes have previously been shown to activate both C-H bonds and CO2 in separate applications. Although PNP pincer ligands are a particularly useful class of ligands the methods by which they can be synthesised relies on the use of expensive and pyrophoric phosphines. Here we have expanded on an inexpensive and versatile procedure for the synthesis of a range of PNP pincer ligands with various substituents that may be explored for sequential C-H and CO2 activation.

In addition to the synthesis of PNP pincer ligands, sulphur, borane and selenium have been explored as protection methods for ease of purification and storage of these valuable ligands. Structural analysis of PNP-selenides and Ir PNP complexes revealed that phosphorus substituents significantly influence the electronic properties of the ligands, with more electron-donating alkyl substituents enhancing donor strength. A combination of XRD data, DFT geometry optimisations and SambVca buried volume calculations demonstrated that steric profiles and catalytic pocket accessibility in a series of cationic iridium PNP pincer complexes are highly tunable within the PNP framework, with smaller less bulky substituents (Me and Furyl) granting more access to the iridium centre.

SambVca and hydricity calculations were used to identify cationic and neutral iridium PNP hydride complexes suitable for catalytic CO2 insertion into C-H bonds to produce carboxylic acids from abundant and cheap starting materials. Some representative complexes have been synthesised and tested for the desired reactivity, though were unsuccessful. A combination of DFT and SambVca calculations revealed that neutral PNP iridium(i) complexes with strongly electron-donating alkylphosphines and small substituents such as Me are likely to be more adequate candidates for sequential C-H and CO2 activation and CO2 hydrogenation reactions than the bulky tBu variations currently used for separate C-H and CO2 activation reactions.

Date of Award	7 May 2025
Original language	English
Awarding Institution	University of Bath
Sponsors	Royal Society
Supervisor	Uli Hintermair (Supervisor) & Matthew Jones (Supervisor)