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"Dehydrocoupling of Phosphine-Boranes: Mechanistic Studies, New Catalysts, and the Development of Novel Polyphosphinoboranes"

Final Report Summary - TJIM (Dehydrocoupling of Phosphine-Boranes: Mechanistic Studies, New Catalysts, and the Development of Novel Polyphosphinoboranes)

The goals of the proposal focused on two main tenets: (a) establishment of the mechanism of phosphine-borane dehydrocoupling, and the subsequent development of new and improved catalyst systems; (b) development of new polyphosphinoborane polymers and oligomers, and determination of potential applications. A novel catalytic system for the polymerization of primary phosphine-boranes has been developed. This novel system, in conjunction with the newly developed phosphine-borane monomers (substrates) has facilitated the synthesis of a range of new polymers. The polymers were then utilized for the soft lithographic fabrication of microstructures.

Highlights include:
1) Development of a novel catalytic system utilizing iron-based species (accessed by facile synthesis), which for the first time operates under substantially less forcing conditions than previous systems. The use of earth-abundant iron as the catalytically active metal represents breakthrough progress over the prior art which relied on expensive rhodium-based catalysts. Moreover, the ability to perform the reactions in solvent at 100oC is a substantial improvement over previous systems which operated at 120oC+ in the melt. This has not only facilitated faster isolation by a simple precipitation into cold non-polar solvent, but has also circumvented a major drawback of the systems operating in the melt; cross-linking leading to polymers with poor polydispersity, or at the extreme insolubility. The resulting polymers from our iron-catalysed systems feature high molecular weight, low polydispersity and are easily manipulated for subsequent fabrication. The resulting polymers were utilized for the soft lithographic fabrication of microstructures, which could be manipulated further by plasma etching. This work is currently in press (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201411957) with follow-up work nearing submission.

2) Delineation of a mechanism for the iron-catalysed system. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism. Moreover, NMR, as well as mass spectrometry studies provide evidence for the on-metal growth of phosphine-borane oligomers. Additionally, in collaboration with the Weller group (University of Oxford), significant strides have been made in the isolation of reactive intermediates in the rhodium catalysed dehydrogenation/dehydrocoupling of phosphine-boranes. Furthermore, through the synthesis of novel phosphine-borane substrates, we have delineated the effects of both steric and electronic profiles of phosphine boranes on their respective reaction rates. This work was recently published (Inorganic Chemistry, 2014, 53, 3716. DOI: 10.1021/ic500032f).

3) Additional activities include the development of new dehydrocoupling catalysts bearing phosphido-borane species as supporting ligands. We have shown paramagnetic titanocene-phosphido-borane species to be efficient precatalysts for the dehydrogenation of amine-boranes (analogues to phosphine-boranes). This work has important implications for the development of low-cost/earth-abundant metal based catalysts for the dehydrocoupling of main group species. This work will be submitted for publication within the coming weeks.

The proposed transfer of knowledge objectives progressed well throughout the duration of the fellowship. Throughout his time in the Manners group, Dr. Jurca has acquired significant experience in catalysis, polymer synthesis, and characterization. In addition to his work with phosphine-boranes, Dr. Jurca has become involved with multiple projects and collaborations, ranging from polyamino-borane depolymerisation studies to aluminium-borate nanowire synthesis. Through the supervision of graduate students, and teaching several graduate courses on inorganic coordination chemistry, Dr. Jurca has been able to transfer knowledge and skill sets to the students.

Dr. Jurca participated in multiple national conferences in his home country of Canada (two Canadian Society for Chemistry National Meetings, and two Inorganic Discussion Weekend meetings). Additionally, he delivered two invited lectures at the University of Bristol, and the University of Ottawa. By means of this fellowship, he has established collaborative contacts within the EU which will be maintained in his future career.

The work completed during the course of this fellowship has made a substantial impact on the field of phosphine-borane dehydrocoupling and polyphosphinoborane polymers. The discovery of iron-based catalysts for the dehydrocoupling of phosphine-boranes presents major progress over the prior art which relied on expensive rhodium-based catalysts. As a measure of the impact of this work, the publication currently in press (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201411957) has been designated as a “hot paper”. Additionally, there are several subsequent publications on this topic which are nearing dissemination and will be completed by Dr. Jurca who has chosen to remain with the Manners group as a postdoctoral researcher to complete the ongoing studies.

Dr. Jurca participated in the inaugural Bristol Bright Night (2014). Bristol Bright Night is part of the annual ‘Researchers’ Night’ programme which is funded by the EU Commission and aims to engage the European public in celebrating the latest and most stimulating research at a local and an international level. Dr. Jurca was based at a public venue, where he, along with other scientists, met with the public to discuss broad ranging topics related to research and answer questions. Additionally, he participated in meetings with the planning committee prior to the event.