New preparative routes to polyphosphazene homopolymers and block copolymers with applications in nanoscience
Final Report Abstract
Fundamental aspects regarding processes involving two classes of important inorganic materials have been elucidated, namely a living polymerization route to polyphosphazenes and dehydrocoupling/ dehydrogenation processes of amine-borane adducts. In 1995 Manners and Allcock reported the first example of a living polymerization route to polyphosphazenes. This process involves the treatment of Cl3P=NSiMe3 with PCl5. In contrast to other routes, this method provides control over molecular weight and polydispersity. It has enabled the preparation of block copolymers and provided access to well-defined materials suitable for self-assembly studies as well as to biodegradable macromolecular anti-cancer drug carriers. Although the discovery of this route has permitted a range of synthetic advances, the polymerization mechanism has not been fully elucidated. The currently accepted mechanism involves initiation by the reaction of the monomer with PCl5 to form the cationic species [Cl3p=N=PCl3]*, which was believed to be the true initiator of the polymerization. Unexpectedly, our recent studies revealed that the cation [Cl3p=N=PCl3]*, when combined with a weakly nucleophilic anion, is completely inactive towards the polymerization of Cl3p=NSiMe3. We found that Cl- anions, which are generated at an early stage of the PCl5-initiated polymerization, are not innocent as has previously been implied, but in fact play a key cooperative rote with the living cations in both the initiation and chain propagation steps. Although the active cation is not even required for the polymerization to proceed, its presence dramatically increases the rate of propagation and it provides molecular weight control. These results have led us to reassess the mechanism of the PCls-initiated living polymerization of Cl3P=NSiMe3. Amine-borane adducts attract considerable current interest from the perspectives of chemical hydrogen storage, transfer hydrogenation of organic substrates, and the preparation of new inorganic polymeric materials. In striking contrast to aliphatic amine-boranes, knowledge of the chemistry of aniline-borane, and arylamine-borane adducts in general, was very limited. Within the scope of this project, the structural chemistry and dehydrocoupling behavior of aniline-borane and related primary arylamine-borane adducts both in solution and in the solid state has been explored in detail for the first time. These labile donor-acceptor species have been found to undergo "spontaneous" loss of hydrogen without the need for an externally added catalyst. The reaction proceeds via a series of intermediates to yield cyclotriborazanes, borazines, and unexpected oligomeric products via mechanistic pathways of surprising complexity. Recently, Manners and co-workers have reported that the first-row metallocene complex [Cp2Ti], generated in situ from relatively cheap starting materials, CpzTiCl2 and nBuLi, functions as a very efficient homogeneous catalyst for the dehydrocoupling/dehydrogenation of amine-borane adducts. Subsequently, a range of [Cp2Ti] and related [Cp2Zr] precatalysts were presented, which also showed high activity. Mechanistic proposals for the dehydrocoupling process came from different groups. They have differed in terms of specific details but for all a series of diamagnetic M(ll) and M(IV) intermediates have been invoked. Our in-depth mechanistic investigations have provided strong evidence for a key role of paramagnetic M(lll) intermediates in these processes.
Publications
- "Spontaneous" Ambient Temperature Dehydrocoupling of Aromatic Amine-Boranes, Chem. Eur. J. 2012, 18, 4665-4680
H. Helten, A. P. M. Robertson, A. Staubitz, J. R. Vance, M. F. Haddow, I. Manners
- A Cooperative Role for the Counteranion in the PCl5-Initiated Living, Cationic Chain Growth Polycondensation of the Phosphoranimine Cl3P=NSiMe3.
J. Am. Chem. Soc., 2012, 134 (37), pp 15293–15296
V. Blackstone, S. Pfirrmann, H. Helten, A. Staubitz, A. Presa Soto, G. R. Whittell, I. Manners
(See online at https://doi.org/10.1021/ja307703h) - Mechanism of Metal-Free Hydrogen Transfer between Amine-Boranes and Aminoboranes. J. Am. Chem. Soc., 2012, 134 (40), pp 16805–16816
E. M. Leitao, N. E. Stubbs, A. P. M. Robertson, H. Helten, R. J. Cox, G. C. Lloyd-Jones, I. Manners
(See online at https://doi.org/10.1021/ja307247g) - Paramagnetic Titanium(III) and Zirconium(III) Metallocene Complexes as Precatalysts for the Dehydrocoupling/Dehydrogenation of Amine-Boranes.
Angewandte Chemie, Vol. 125. 2013, Issue 1, pp 455–458.
Angewandte Chemie International Edition, Vol. 52.2013, Issue 1, pp 437–440.
H. Helten, B. Dutta, J. R. Vance, M. E. Sloan, M. F. Haddow, S. Sproules, D. Collison, G. R. Whittell, G. C. Lloyd-Jones, I. Manners
(See online at https://doi.org/10.1002/anie.201207903)
