Exploring Pnictogen Zintl Clusters for Small Molecule Activation and Catalysis

Abstract

Zintl clusters are anionic polyatomic p-block element clusters and can be regarded as intermediaries between discrete molecules and bulk solids. For instance, the heptapnictogen trianions, [Pn7]3– where Pn = P or As (which are the focus of study for this thesis), can be structurally related to elemental phosphorus or arsenic. Thus, these solution-soluble pnictogen clusters can be used as molecular models to better understand and inform future reactivity with their heterogenous counterparts. Despite being known for over a century, studies into the reactivity of these clusters with small molecules are in their infancy and applications as catalysts largely unknown. With initial efforts focused on small molecule activation chemistry, a library of tris(silyl) functionalised heptapnictogen clusters (P7(SiR3)3) were prepared and employed in iso(thio)cyanate capture. The heteroallene was found to react via either C=N or C=O bond activation, and the selectivity of which bond is activated could be controlled by tuning either the heteroallene or silyl-unit on the cluster. A key finding from this work is that one activated heteroallene could be exchanged for another. Next, a novel boron functionalised Zintl cluster, [(BBN)P7]2– (BBN: 9-borabicyclo[3.3.1]nonane) was prepared by an elementary reaction type (dehydrocoupling) previous unknown in Zintl functionalisation chemistry. Next, [(BBN)P7]2– was employed in the hydroborative reduction of the C=O bonds of aldehydes and ketones, as well as carbon dioxide (CO2). In the case of CO2 reduction, complete selectivity towards the formation of the methanol surrogate methoxyborane was observed. This work was the first example of transition metal-free Zintl cluster catalysis and where the cluster is an active participant in the catalysis. Using the same [(BBN)P7]2– catalyst, the reduction of pyridines, imines and nitriles was also achieved. Finally, a family of Group 13 functionalised clusters with the general formula [(R2E)Pn7]2– (E = B, Al, In; Pn = P, As) were prepared and found to be competent catalysts in the hydroboration of CO2. Of these clusters, [(Ph2In)P7]2– and [(BBN)P7]2– were previously reported, while [(iBu2Al)P7]2– and [(iBu2Al)As7]2– were developed. This family of clusters was compared in its CO2 reduction catalytic competency by analysis of the turnover numbers (TONs) and turnover frequencies (TOFs). Kinetic studies provided further insights into the mechanism.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Meera Mehta (Supervisor), Richard Winpenny (Supervisor) & David Mills (Supervisor)