Metal-free synthesis and characterization of functionalized polyaminoboranes Carlos Antônio de Albuquerque Pinheiro
To cite this version:
Carlos Antônio de Albuquerque Pinheiro. Metal-free synthesis and characterization of functionalized polyaminoboranes. Polymers. Université Rennes 1, 2017. English. NNT : 2017REN1S092. tel- 01743830
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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. ANNÉE 2017
THÈSE / UNIVERSITÉ DE RENNES 1 sous le sceau de l’Université Bretagne Loire
pour le grade de DOCTEUR DE L’UNIVERSITÉ DE RENNES 1 Mention : Chimie
Ecole doctorale 3M Matière, Molécules et Matériaux Carlos Antônio de Albuquerque Pinheiro Préparée à l’unité de recherche UMR-CNRS 6226 Institut des Sciences Chimiques de Rennes UFR Sciences et Propriétés de la Matière
Thèse soutenue à l'Université de Metal-free synthesis Rennes 1 and characterization le 20 octobre 2017 devant le jury composé de : of functionalized Ghenwa Bouhadir Maître de conférences, Université de Toulouse 3 polyaminoboranes Rapporteure Umit Demirci Professeur, Université de Montpellier Rapporteur Loïc Le Pluart Professeur, Université de Caen Examinateur Sophie Guillaume Directrice de Recherches, Université de Rennes 1 Examinatrice Gilles Alcaraz Directeur de Recherches, Université de Rennes 1 Directeur de thèse
Metal-free synthesis and characterization of functionalized polyaminoboranes
Carlos Antônio de Albuquerque Pinheiro
Empty your mind, be formless, shapeless
— like water.
Now you put water in a cup, it becomes the cup;
You put water into a bottle it becomes the bottle;
You put it in a teapot it becomes the teapot.
Now water can flow or it can crash.
Be water, my friend.
Bruce Lee, 2000
Acknowledgment
Desde pequeno, sempre quis viajar e conhecer os lugares mencionados nos livros que lia. A oportunidade de fazer um doutorado na França me permitiu conciliar um sonho de infância com meu desenvolvimento acadêmico. Hoje, me sinto realizado tanto pessoalmente quanto profissionalmente. Muitas pessoas foram imprescindíveis para concluir essa etapa de minha vida com sucesso.
Antes de mais nada, gostaria de agradecer a minha família, que sempre me apoiou, nunca desacreditando do meu potencial, em especial minha mãe Selma, meu irmão do coração Eduardo e minhas tias Celi e Stella.
Me gustaría también agradecer a mi novia Patricia, por su cariño, amor y paciencia, incluso los momentos de mucho estrés en el fin de la tesis. Gracias mi amor.
I’d also like to thank my friends, made during the first period in Toulouse, specially Marion and Roberto, for the several hours talking about science and/or nothing, around some coffee and/or beer.
Je souhaite aussi remercier mes amis de la deuxième époque que si passait à
Rennes, surtout Lucas, Anabella, Eleonora, Cyril, Rachid, Camila et tous les autres membres du bâtiment 10 C, sans lesquelles le séjour dans la Bretagne serait impossible.
Um grande abraço para minha família gaúcha, Lili, Betão e Pepê, que me acolheu pela terceira vez com a casa e o coração abertos, transformando uma simples
época de trabalho em um período de felicidade e aprendizado.
Un grand merci à Gilles, qui m’a accepté comment thésard et m’a guidé par les moments les plus ténèbres comme un ange et par les moments des faiblesses avec force, ce que m’a permis de passer au-delà de mes limites.
Um grande obrigado a Katia, que também me acompanha desde o mestrado de maneira tão carinhosa que as vezes a vejo como tia ao invés de professora.
Thanks to the O group and the other members of the LCC in Toulouse, with whom we shared good moments and helped me so much to adapt to the new country.
Thank you also members of the OMC group in Rennes, very important to me in the final period of this journey.
Je souhaite remercier tous les membres du jury qui ont accepté de participer à ma soutenance de thèse et de juger mon travail. Merci à Umit Demirci et à Ghenwa
Bouhadir pour fournir une excellente discussion et proportionner une vue plus approfondie de mon sujet de thèse. Merci à Loic Le Pluart et Sophie Guillaume pour enrichir la discussion avec votre expertise. Je remercie encore à Gilles Alcaraz, sans toi, je serai encore en train de rédiger.
Table of contents
List of compounds ...... xii
Introduction to inorganic polymers ...... 3
I.1 Carbon-based polymers ...... 3
I.2 p-block-containing inorganic polymers ...... 6
I.2.1 Group XIV-containing inorganic polymers ...... 7
I.2.2 Group XV-containing inorganic polymers ...... 23
I.2.3 Group XVI-containing polymers ...... 30
I.3 Boron-based polymers ...... 32
I.3.1 Boron-Carbon hybrid polymers ...... 33
I.3.2 Boron-Oxygen hybrid polymers ...... 34
I.3.3 Boron-Nitrogen hybrid and inorganic polymers ...... 35
I.4 Project presentation ...... 43
Dehydrocoupling versus boron transfer reaction ...... 47
II.1 Reactivity of diisopropylamine-borane and diethylamine in the
presence of the [RuH2(η2-H2)2(PCy3)2] ...... 52
II.2 Reactivity of diisopropylaminoborane with amines: insight in the
mechanism of the metal-free boron transfer ...... 65
II.2.1 Influence of the substitution pattern of the amine...... 75
II.2.2 Reactions using primary amines ...... 76
II.2.3 Reaction with methylamine ...... 85
II.3 Experimental section ...... 93
Investigation of the polymerization reaction ...... 123
III.1 The role of the temperature ...... 123 ix
III.2 Evidence of polymer formation ...... 125
III.3 The role of the substitution pattern ...... 130
III.4 Proof of polymer formation ...... 136
III.5 Mechanistic considerations ...... 137
III.6 Overview of the linear poly(alkylamino)boranes ...... 140
III.7 Conclusion ...... 145
III.8 Experimental section ...... 147
III.8.2 Poly(methylamino)borane ...... 151
III.8.3 Poly(ethylamino)borane ...... 158
III.8.4 Poly(propylamino)borane ...... 168
III.8.5 Poly(butylamino)borane ...... 173
III.8.6 Poly[(ethylamino)borane-ran-(propylamino)borane] (1:1) ...... 178
III.8.7 Poly[(ethylamino)borane-ran-(propylamino)borane] (1:2) ...... 182
III.8.8 Poly[(ethylamino)borane-ran-(butylamino)borane] (1:1) ...... 185
III.8.9 Poly[(ethylamino)borane-ran-(butylamino)borane] (1:2) ...... 190
Enhancing the substitution pattern ...... 197
IV.1 Formation of poly(alkylamino)boranes bearing unsaturated carbon
side chains ...... 197
IV.2 Formation of silicon-containing polyaminoboranes ...... 205
IV.3 Post-polymerization functionalization approaches ...... 206
IV.4 Perspectives ...... 209
IV.5 Conclusion ...... 209
IV.6 Experimental section ...... 211
x
IV.6.1 Poly(trimethylsilylmethylamino)borane ...... 211
IV.6.2 Poly(dimethylethoxysilylpropylamino)borane ...... 213
IV.6.3 Poly(methyldiethoxysilylpropylamino)borane ...... 215
IV.6.4 Poly(triethoxysilylpropylamino)borane ...... 217
IV.6.5 Poly[tris(trimethylsiloxy)(silylpropylamino)]borane ...... 219
IV.6.6 Poly(dimethylsiloxaneamino)borane (1:1) ...... 221
IV.6.7 Poly(dimethylsiloxaneamino)borane) (1:2) ...... 223
General conclusion ...... 227
Bibliography ...... 229
xi
List of compounds
1. diisopropylaminoborane
2. diisopropylamine-borane
3. (diethylamino)(diisopropylamino)borane
4. N,N,N',N'—tetraethylcyclodiborazane or cyclodiborazane
5. diethylamine-borane
xii
6. diethylaminoborane
7. bis(diethylamino)borane
8. N,N’-dimethyl-ethylenediamine
9. N,N'-dimethyl-1,3,2-diazaborolidine
10. N,N’-dimethyl-ethyleneamine-monoborane
11. (diisopropylamino)(penthylamino)borane
xiii
12. bis(penthylamino)borane
xiv
Résumé
Les polymères sont un domaine de recherche particulièrement actif en raison de leur vaste utilisation dans notre vie courante. Depuis leur découverte à la fin du dix-neuvième siècle jusqu’à nos jours, l'état de l’art dans ce domaine s'est considérablement développé et l'émergence de polymères hybrides et inorganiques sont venus compléter la gamme.
Chapitre 1:
Dans ce contexte, le premier chapitre retrace brièvement leur évolution depuis les premiers polymères synthétiques "tout carbone" obtenus par le procédé Ziegler-
Natta (Figure 1 a) aux polymères hybrides (Figure 1 b) et inorganiques (Figure 1 c) qui intègrent dans leur squelette des éléments du bloc p.
Figure 1. Illustration de différents types des polymères: (a) Polymère organique; (b) Polymère hybride; (c) Polymère inorganique.
Dans ce chapitre, nous nous sommes plus particulièrement intéressés aux polymères inorganiques et à leur mode de synthèse. Une compilation non-exhaustive des principales familles de polymères inorganiques dérivés des éléments du bloc p (Si,
Ge, Sn, S, P, As, B) sont répertoriés par groupe d'éléments (groupe XIII, XIV, XV et
XVI). Leur obtention fait intervenir différentes méthodologies de synthèse qui sont directement reliées à la nature du squelette du polymère inorganique envisagé.
A titre d'exemple et dans le cas des polymères dérivés du groupe XIV, on peut retenir le couplage réducteur de Würtz à partir de dérivés halogénés ou encore les
xv
Résumé polymérisations par ouverture de cycle comme les voies de synthèse privilégiées couramment mises en œuvre pour leur synthèse (Figure 2).
Figure 2. Synthèse de polysilanes par a) couplage de Würtz (haut) b) ouverture de cycle (bas).
En ce qui concerne les polymères hybrides et/ou inorganiques du groupe XV et
XVI, des approches similaires ont été détaillées dans la littérature. Mais on retiendra surtout les réactions de polycondensation à partir de précurseurs halogénés comme cela est illustré dans la Figure 3.
Figure 3. Synthèse de polymères dérivés d'éléments des groupes XV et XVI par polycondensation.
Cependant, notre intérêt s'est plus particulièrement porté sur les polymères inorganiques à base de bore qui intègrent un squelette possédant une unité de répétition "BN". Ces polymères font depuis quelques années l'objet d'une intense xvi
Metal-free synthesis and characterization of functionalized polyaminoboranes recherche de par leur connexion avec le stockage chimique et réversible de l'hydrogène, et d'autre part comme précurseur privilégié de céramiques à base de bore.
Depuis 2006 et les travaux précurseurs de K. Goldberg et M. Heinekey, les premiers polyaminoboranes ont pu être obtenus à partir d'adduits amine-boranes, par voie catalytique, en faisant intervenir des complexes organométalliques de type pincer
à base d'iridium.
D'autres études ont été menées dans ce domaine, et il a été montré que ces polymères résultent d'une réaction de couplage déshydrogénant qui peut être catalysée par un grand nombre de complexes organométalliques, principalement à base d'iridium et de ruthénium (Figure 4).
Figure 4. Couplage déshydrogénant d'amine-boranes catalysé à l'iridium.
La formation maitrisée de liaisons B-N est dans ce contexte une problématique majeure qui a jusqu'à maintenant été abordée par le biais de la déshydrogénation d'adduits amine-boranes pour la conception de ce type de polymères. Au cours de ce travail de thèse, l'utilisation de diisopropylaminoborane (1) comme source simple, facile et efficace de bore a été développée.
Chapitre 2:
Dans le cadre d'études concernant l'activation de liaisons BH dans la première sphère de coordination du ruthénium, il a été montré que des adduits amine-boranes pouvaient être déshydrogénés en présence du complexe [RuH2(η2-H2)2(PCy3)2] et les aminoboranes résultant, stabilisés par coordination, pour conduire aux complexes bis(-BH) correspondants [RuH2(η2:η2-H2B-NR2)].
xvii
Résumé
Par la suite, cette stratégie a été appliquée à des adduits diamine-monoboranes
(RR'N~NHR-BH3) pour évaluer le rôle du groupe amino périphérique au cours de la réaction. Il a ainsi pu être démontré que celui-ci joue un rôle essentiel et dans le cas d'un groupement protique (RHN~NHR-BH3) un second équivalent de dihydrogène est produit avec formation d'un hétérocycle boré par création de liaison B-N intramoléculaire.
Sur cette base, une version intermoléculaire a tout d'abord été envisagée par une réaction analogue entre un adduit amine-borane (R2NH-BH3) et une amine secondaire (R'2NH) en présence de [RuH2(η2-H2)2(PCy3)2] comme catalyseur. Nous avons pu montrer que la réaction procède en deux temps avec 1) réaction de transfert de bore et 2) déshydrogénation catalysée par le ruthénium.
L'étude de cette réactivité a également montré que le même produit de réaction peut être obtenu sans métal, à partir de l'amine secondaire et du diisopropylaminoborane (1), ce dernier étant la version déshydrogénée de l'adduit diisopropylamine-borane (2) (Figure 5).
Figure 5. Déshydrogénation versus tansfert de bore.
La réaction inattendue de transfert de bore a mis en lumière le potentiel du diisopropylaminoborane (1) comme source de bore, sans l'intervention d'un catalyseur
à base de métal de transition. xviii
Metal-free synthesis and characterization of functionalized polyaminoboranes
Cette réaction a été étudiée plus en profondeur d'un point de vue mécanistique par le biais de la réactivité du diisopropylaminoborane (1) avec la diéthylamine mais
également avec la N,N'-diméthyléthylènediamine (8). Cette première étude a
également été complétée par une approche théorique du mécanisme en collaboration avec le Laboratoire de Physique et Chimie des Nano-objets (LCPNO) à Toulouse.
Au cours de cette étude, la formation de produits secondaires où le groupement boré semble avoir été rehydrogéné ont pu être détectés. Cela nous a mené à essayer de comprendre leur nature ainsi que leur mode de formation. Pour ce faire, une étude mettant en jeu plusieurs amines primaires a été conduite. Dans ce cas, on observe une réaction similaire qu'avec la diéthylamine mais avec des rapports en produits secondaires très différents dont la quantité peut être modulée par la stœchiométrie
(Figure 6).
Figure 6. Réaction du diisopropylaminoborane avec la pentylamine.
Là encore, une étude mécanistique a été conduite afin d'expliquer la formation de l'ensemble des produits de la réaction, complétée par une étude théorique effectuée par des théoriciens du LCPNO à Toulouse. Nous avons pu montrer également que l'issue de la réaction pouvait également être modifiée par d'autres paramètres tels que la température. Cela a été clairement mis en évidence par l'utilisation de la méthylamine à -40°C.
xix
Résumé
Chapitre 3:
L'utilisation de la méthylamine à -40°C nous a permis d'obtenir le poly(methylamino)borane avec un excellent rendement. Dans ce chapitre, nous avons tiré profit de cette réactivité à plus basse température pour démontrer que le diisopropylaminoborane (1) pouvait être utilisé comme pourvoyeur de "BH2" pour la formation d'une large gamme de poly(alkylamino)boranes de haute masse molaire.
Les études qui ont été menées ont principalement consisté à synthétiser, isoler mais également mettre au point les techniques d'analyses capables de rendre compte des résultats obtenus (spectrométrie de masse, chromatographie par exclusion stérique, RMN à l'état solide, spectrométrie infrarouge…).
Figure 7. Poly(alkylamino)boranes.
L'étude théorique a dans ce cadre corroboré nos résultats et a démontré que l'obtention des polymères résulte d'un contrôle cinétique de la réaction. L'obtention par cette nouvelle approche a également permis la préparation de copolymères par simple mélange d'amines primaires (Figure 8). xx
Metal-free synthesis and characterization of functionalized polyaminoboranes
Figure 8. Synthèse de copolymères poly(alkylamino)boranes.
Chapitre 4:
L'approche synthétique mise en œuvre pour la préparation de polyaminoboranes s'est avérée compatible avec des fonctions organiques insaturées
(allyle, propargyle) ce qui a constitué le point de départ de l'étude envisagée dans ce quatrième et dernier chapitre.
En effet, avoir à disposition des polymères possédant des groupements fonctionnels insaturés nous a conduit dans un premier temps à entreprendre une stratégie de fonctionnalisation suivant la polymérisation.
Cette stratégie a été abordée par une approche métathétique à partir du poly(allylamino)borane en présence de catalyseur de Grubbs, mais également par une approche "click" à partir du poly(propargylamino)borane et d'azoture de benzyle en présence d'un catalyseur de cuivre (Figure 9).
Ces approches sont encore en cours d'études mais les résultats sont prometteurs et devrait offrir de nouvelles perspectives pour ce type de polymères.
Pour finir, nous avons également abordé la synthèse de polyaminoboranes fonctionnels possédant des groupements siliciés pouvant servir dans un second temps dans des processus de réticulation selon des stratégies de type sol-gel.
Une série de polyaminoboranes a ainsi pu être préparée avec d'excellents rendements (Figure 10). Les caractérisations de ces polymères sont actuellement complétées.
xxi
Résumé
Figure 9. Stratégies de fonctionnalisation post- polymérisation.
Figure 10. Synthèse de polyaminoboranes à groupement silicié.
Conclusion :
Au cours de ce travail de thèse, j'ai étudié la réactivité du diisopropylaminoborane (1) en présence d'amines secondaires et primaires. J'ai montré que la réactivité dépendait des conditions opératoires mises en jeu et que l'issue de la réaction pouvait être contrôlée par la stœchiométrie aussi bien que par la température.
xxii
Metal-free synthesis and characterization of functionalized polyaminoboranes
Le diisopropylaminoborane (1) s'est avéré un réactif particulièrement efficace comme agent de transfert de groupe "BH2". Le mode de transfert a été rationalisé par une étude mécanistique complète et corroboré par une étude théorique.
Le pilotage de la réaction par la température a permis l'obtention inattendue de poly(alkylamino)boranes. Les polymères obtenus par cette voie ont fait l'objet d'une
étude plus approfondie tant pour leur synthèse que pour leur caractérisation ou leur potentiel synthétique.
Ce travail, qui est loin d'être achevé, ouvre des perspectives nouvelles quant à la chimie des polyaminoboranes fonctionnels.
xxiii
Chapter I: Introduction to inorganic polymers
Introduction to inorganic polymers
I.1 Carbon-based polymers
The term polymer was first used in 1833, by Jöns Jacob Berzelius. It comes from the Greek polus, meaning “many, much” and meros, meaning “parts”.1 The actual definition of the word polymer was done by Hermann Staudinger, in 1920: “a polymer is a substance composed of molecules of high relative molecular mass, having multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass”.2
Polymers are indispensable to our existence. The information to our body foundation is written with polymers, more specifically, polynucleic acids, also known as deoxyribonucleic and ribonucleic acids (DNA and RNA), present in all livings things.
Nonetheless, the concept of linking together small molecules to form bigger ones connected by covalent bonds, in other words, the formation of a polymer, is recent, dating from the 1920’s, when Hermann Staudinger made his great breakthrough proposing this view, awarding him a Nobel prize in 1953.3
The implementation of this knowledge brought us human-made polymers. This type of polymers saw a great development in the 1950’s,4 specially due to the Ziegler-
Natta catalysts. They can be classified according to their solubility and are all used for olefin polymerization. The discovery of their reactivity was made by Ziegler, while
Natta performed the studies about the stereoselectivity of the polymerization.5
The homogenous version is based on metallocene complexes, whereas the heterogeneous variant is often titanium or zirconium-based (Figure I-1). Nonetheless, both need alkylaluminium compounds to perform the polymerization reaction.6
3
Introduction to inorganic polymers
Figure I-1. Examples of Ziegler-Natta homogeneous catalysts.
Concerning the heterogeneous catalysis, the type of the metal drastically controls the mechanism of the reaction. Whereas traces of nickel salts hinder the formation of long carbon chains, titanium tetrachloride strongly favor the chain growth instead of the elimination step, even at mild conditions (Scheme I-1).
Scheme I-1. Influence of the metal salt in the ethene reactions.
These new catalysts represented a revolution in the polymer industry, because the process used at that time to obtain analogous polyethylene required harsh conditions (with temperatures and pressures up to 200 °C and 2000 atm), while
Ziegler-Natta process works at atmospheric pressure and room temperature.6
More than that, the polymers produced by the Ziegler-Natta catalysts also present many advantages over their natural counterparts such as the capability of tuning their properties depending on the desired application by just modifying or changing the reaction parameters or the feedstock.7
If the Ziegler-Natta process enabled to prepare a variety of polymers by carbon- carbon single bond formation, a further step in polymer chemistry consisted in reshaping or modifying the existing carbon-carbon bonds. One major achievement in
4
Metal-free synthesis and characterization of functionalized polyaminoboranes this field involved the olefin metathesis7 that allows the exchange of substituents between two different olefins.9
This reaction developed by Yves Chauvin, Robert H. Grubbs and Richard R.
Schrock, for which they received the Nobel prize in chemistry in 2005, has proved to be a particularly efficient tool in polymer chemistry and materials science.8
Indeed, the control of the reaction parameters allows the production of different products from the same feedstock (Scheme I-2).3 and the reaction can be used in the post-polymerization and functionalization of diverse alkene containing polymers.
Scheme I-2. Metathesis reactions: Ring-opening Methathesis Polymerization (ROMP), Acyclic Diene Metathesis Polymerization (ADMET) and Ring Closing Metathesis (RCM).
Polymer engineering is a broad and fertile field in which the carbon-carbon bond holds a central place. In less than a century, organic polymers have become ubiquitous in every aspect of modern life, from electronics to medicine, and daily life materials.
Conversely, polymers with a skeletal structure that does not include carbon atoms in their backbone, also called inorganic polymers, have been investigated.
Attention is being increasingly focused toward this polymer family due to their new and attractive properties compared to organic polymers.
Nowadays, there is a need for further studies and better understanding of their properties as well as developing new synthetic strategies and processes.9
5
Introduction to inorganic polymers
This work falls within this context and focusses on the synthesis of original boron based inorganic polymers through mild reaction conditions involving an environmentally friendly solvent-free process.
These novel polymers incorporate new lateral functions and display specific properties derived from the presence of p-block- elements in the backbone or/and in the lateral chains (Scheme I-3), that are going to be briefly discussed in the following section.
Scheme I-3. p-block E and A elements backbone containg polymers with a) a R carbon side chain or b) a p-block element E’ containing side chain.
I.2 p-block-containing inorganic polymers
When the word “polymer” is mentioned, the ubiquitous carbon-based backbone polymers come to mind, especially those derived from petroleum, because they have plenty of well-established commercial products. Adding main group elements to the composition of the polymer greatly changes its properties.10
One well known example is the addition of sulfur as a cross-linking element of carbon-based rubber.11 This process called vulcanization notably increases their mechanical properties12 and completely changed the tire industry.13
6
Metal-free synthesis and characterization of functionalized polyaminoboranes
Figure I-2. Vulcanization of carbon-backbone chains.11
Polymers containing p-block elements can be classified in three different types:
• Polymers with such elements in the side-chains are still called organic polymers.
• Polymers with carbon and p-block elements in the backbone are known as hybrid polymers.
• In the absence of carbon in their backbone, these polymers are known as
the inorganic polymers.14
Although all three are briefly commented, the latter polymer is the focus of this work. To further illustrate this point and without claiming to cover all the main-group- containing polymers and their relative properties, a non-exhaustive list of relevant p- block-containing polymers will be presented.
I.2.1 Group XIV-containing inorganic polymers
Extending the chemistry of carbon-based polymers to heavier elements and taking advantage of a similar valence shell led to major developments.
I.2.1.1 Silicon-containing polymers
Silicon is a group XIV element just below the carbon in the periodic table. It is one of the most abundant elements on Earth and is usually found in the form of "SiO2". 7
Introduction to inorganic polymers
There are four major families of silicon-containing inorganic polymers.15 They are polysilanes [SiRR’]n (a) , polysiloxanes [SiRR’O]n (b), polysilsesquioxanes R-[SiO1.5]n (c) and polysilazanes [SiRR’NR’’]n (d) (Figure I-3).16
Figure I-3. Different types of silicon-containing inorganic polymers: (a) polysilanes, (b) polysiloxanes, (c) polysilsesquioxanes and (d) polysilazanes.
The polysilanes are polymers with a backbone composed exclusively of silicon.
Their applications are based on their reactivity and polysilanes are often used as additives in coating formulations as adhesion promoters of metal or inorganic materials on inert surfaces.17
The -conjugated polysilanes also display intrinsic properties characterized by a strong absorption and an efficient emission in the UV region and also a high photoconductivity that are of prime interest for optical applications.18,19
Although known since the early 1920’s, all commons methods of synthesis still struggle to yield high molecular weights and narrow dispersity, adding the fact that
Si-Cl and Si-H containing monomers are moisture and oxygen sensitive. The preparation of such polymers to an industrial scale and application suffers from a lack of efficient synthetic methods and remains challenging.19
The most common method for the preparation of polysilanes was developed by
Kipping and Sands20 and involves a Würtz-type reductive dehalogenative coupling reaction (Scheme I-4). In this method, dichlorodiorganosilanes are reacted with a stoichiometric amount of sodium in a high-boiling solvent, under an inert atmosphere.
8
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-4. Würtz-type coupling of chlorosilanes
These harsh conditions severely reduce the scope of this route preventing the use of delicate functionalities in the silicon-containing precursors and making new routes necessary to obtain novel properties. One successful example of improvement in the preparation of polysilanes is the work of Jones et al.,21 with the room- temperature and high-yield synthesis of the poly(methylphenyl)silane by using polar solvent such as THF instead of the generally used toluene (Scheme I-5).
Scheme I-5. Influence of the solvent in the formation of poly(methylphenyl)silane.
Jones and Holder rationalized these results22 that were attributed to the ability of THF to stabilize the anionic chain carriers and favor the chain growth, suppressing the formation broad dispersity.
An alternative way for the synthesis of such polymers involves the anionic ring- opening polymerization, performed by Sakurai et al. in 1989 from masked disilenes.23
In this method, alkyllithiums are used as initiators to open cyclic organosilicon
9
Introduction to inorganic polymers compounds that undergo anionic polymerization by slowly warming a frozen solution of THF containing the silicon source and the alkyllithium solubilized in toluene
(Scheme I-6).
Scheme I-6. General case for the synthesis of polysilanes by anionic ring-opening polymerization.
As first showed by Matyjaszewski K. et al.24 and later on by Suzuki et al.25, a similar approach using strained silacycles can be involved as a route towards polysilanes.
The third most used method to synthesize polysilanes is the dehydrocoupling of mono or disubstituted silanes (RnSiH4-n with n=1,2) (Scheme I-7).26
10
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-7. Dehydroucoupling of primary (top) and secondary (bottom) silanes.
One advantage of this method is the good control of the polymerization reaction provided by the ligands of the metallic complexes used as catalysts. The principal catalysts used are group IV derived metallocenes (Cp2MCl2/nBuLi; M = Ti, Zr, Hf),27 although late transition metals such as rhodium,28 nickel29 and iridium30 have proved to be also efficient for such a reaction.
Putting aside the particularities of each metallic complex, Tilley proposed the general mechanism involving σ-bond metathesis exchange reaction enabling the dehydrocoupling process of silanes.31 This mechanism is based on the dehydrogenation reaction from a silane and a metal hydride followed by the coupling of the resulting silyl metal complex with a hydrosilane to form the Si-Si bond and regenerating the active metal hydride species (Scheme I-8).
Alongside polysilanes that have only silicon in their backbone, polymers displaying a Si-O repeating unit are ubiquitous in our environment and represent since the 1940's probably one of the most strongly developed family of polymers nowadays.
Known as silicones, these polymers that have one silicon-carbon bond and a siloxane linkage belong to the family of polysiloxanes [SiRR’O]n (Figure I-4 a) or polysilsesquioxanes R-[SiO1.5]n (Figure I-4 b) depending on the Si/O ratio.
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Introduction to inorganic polymers
Scheme I-8. Tilley's -bond metathesis mechanism for the dehydrocoupling polymerization of silanes.
Figure I-4. Structures of generic polysiloxane (a) and (b) ladder type polysilsesquioxane.
The formation of polysiloxanes and related polymers are generally driven by the formation of strong silicon-oxygen bond, that has a bond dissociation energy (BDE) of 513 kJ/mol (in Me3Si-OMe), stronger than the carbon-oxygen BDE of 348 kJ/mol (in
MeOMe).32
This specific bond confers the polysiloxanes high thermal stability, retaining their properties from -100 to 250 °C, allowing them to be used as heat-transfer agents
12
Metal-free synthesis and characterization of functionalized polyaminoboranes as well as high-performance elastomers33 but also giving them a high flexibility, compressibility and permeability.34
The polysilsesquioxane, on the other hand, can be seen as a crosslinked polysiloxanes forming rigid units as random, cage, partial cage or ladder type structures. This change in the structuration results in a decrease of the flexibility of the chains and in an increase of the glass-transition temperature as well as their resistance to chain scission.34
The common method for synthesizing this class of polymer consists in the hydrolysis of chlorosilanes, giving a mixture of chlorosilanol and silanediol that finally stabilizes by condensation via dehydration and dehydrochlorination reactions, yielding to cyclic oligomers and linear polysiloxanes (Scheme I-9).35
Scheme I-9. Synthesis of polysiloxanes via hydrolysis/condensation reactions.
Although this reaction can occur under either acidic or basic conditions, the latter usually gives higher molecular weights. In addition, crosslinking reactions can be achieved by the use of methyltrichlorosilanes.36
Another way to the formation of such polymers is the ring-opening polymerization (ROP) of cyclic siloxanes. This reaction is a chain-growth process that can be performed by using both cationic37 or anionic initiators.38
The anionic polymerization is initiated by a nucleophile, breaking the cycle and adding a charge to the terminal oxygen which, in turn, performs a nucleophilic attack at another ring, breaking another cyclic precursor and propagating the reaction. The termination is performed with the addition of a monochlorinated monomer (Scheme
I-10).36
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Introduction to inorganic polymers
Scheme I-10. Anionic ring-opening polymerization.
Concerning the cationic polymerization, the initiation is based on the protonation of the monomer that reacts with another monomer forming an oxonium ion, which , in turn, reacts in a similar way and allowing the propagation via the nucleophilic attack of other monomer to the oxonium (Scheme I-11).36
Scheme I-11. Cationic ring-opening polymerization
The third main family of silicon-containing inorganic polymers is the polysilazanes. Contrarily to the previous class of silicones, their backbone is composed of a Si-N repeating unit [SiRR’NR’’]n.
14
Metal-free synthesis and characterization of functionalized polyaminoboranes
Their major use is as precursors for ceramic materials,39 producing silicon nitride upon pyrolysis, that present a very high resistance to oxidation and good mechanical properties even at high temperature.40
The synthesis of this type of polymers is usually performed by ammonolysis or aminolysis reaction, treating chlorosilanes with ammonia or amines, respectively. The reaction results in the substitution of the chlorines by an amino group followed by the polymerization of the resulting diaminosilanes by condensation (Scheme I-12).41
Scheme I-12. Aminolysis/condensation sequence for the polysilazanes synthesis.
Polysilazanes can be also obtained by catalyzed dehydrocondensation reaction between a diaminosilane and a primary amine (Scheme I-13). This method is performed under transition metal catalysis conditions and offers the advantage of avoiding the contamination by chlorine.42
Scheme I-13. Polysilazane synthesis by catalyzed dehydrocoupling reaction.
The use of metal complexes allows another interesting pathway consisting in the ring-opening polymerization of cyclic silazanes (Scheme I-14). The major advantage of this method is the use of cyclic oligomers byproducts from other reactions to produce more desirable linear products.43
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Introduction to inorganic polymers
Scheme I-14. Ring opening polymerization of cyclic polysilazanes.
I.2.1.2 Heavier group XIV-containing inorganic polymers
Heavier elements of the group XIV present close characteristics related to those of the silicon and carbon, because of their equivalent valence shell. Accordingly, the broad scale application of silicon-containing polymers brought attention to the possibility of making polymers using the elements of the same group.44
Although they have not yet reached the stage for industrial applications, there is significant research in this field due their unique feature mostly connected to the σ- conjugation properties along the backbone of the polymers containing such elements.45,46 This σ-delocalization grants them electronic properties similar to unsaturated conjugated polyenes.
They can be seen as the structural analogues of polymeric hydrocarbons of general formula R(MR2)nR (R = alkyl or aryl).47
The current biggest challenge for this type of polymers came first from the scale- up production, since it is hard to find cheap and readily available precursors.47 This started to change in 1985, when Trefonas and West prepared the first high molecular weight polygermane by treating n-Bu2GeCl2 with sodium metal.49
Along with the germanium-based homopolymer, copolymers including silicon were also obtained (Scheme I-15). The control of the molecular mass was performed by Size-Exclusion Chromatography, showing a bimodal distribution with one polymer of high mass and dispersity, and one other of lower mass. Nonetheless, all the polymers were obtained in quite low yield, not exceeding 22%.49
16
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-15. Polymerization reaction of germanium and silicon alkylchlorides yielding soluble polymers.
An alternative synthetic pathway was developed by Kobayashi and Cao in
1993.50 Their method involved 1,4-dioxane complex of germanium dichloride as source of germanium with organolithium compounds (Scheme I-16). Although the molecular mass was lower than the ones obtained by Trefonas and West, the dispersity was in that case significantly better (around 1 for most of the polymers) and the polymers were produced in moderated to high yields, up to 98 % when using nBuLi.
Scheme I-16. Synthesis of polygermanes with organolithiums reagents.
Soon after, Mochida and Chiba,51 published the preparation of a series of polygermanes inspired by the Würtz-type coupling of dialkyldichlorogermanes at high temperatures, using alkaline metals (Scheme I-17).
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Introduction to inorganic polymers
Scheme I-17. Synthesis of several polygermanes using the Würtz-coupling method.
In addition, they also disclosed the preparation of poly(germane)s from diiodogermylene with phenyl Grignard reagents,51 The resulting polymers are obtained in higher yields, with a lower molecular mass but a narrower dispersity than the Würtz-coupling polymerization (Scheme I-18).
Scheme I-18. Synthesis of several polygermanes from diiodogermylene.
More recently, the synthesis of even higher molecular weight poly(germane)s was unlocked using efficient catalytic process. Reichl et al. performed the demethanative catalytic coupling of HGeMe3 in the presence of 1 mol % of
Ru(PMe3)4(GeMe3)2 (Scheme I-19), affording poly(dimethyl)germanes at room temperature, in high yields (81-97%) and molecular masses (2~7 × 104).52
Scheme I-19. Synthesis of polygermanes by demethanative catalytic coupling.
Choi and Tanaka also disclosed a metal-catalyzed approach, synthesizing the poly(phenyl)Germane by dehydrogenative coupling of phenylgermane in ca 60 %
18
Metal-free synthesis and characterization of functionalized polyaminoboranes yield and low molecular mass (3 × 103 g/mol) using zirconocene-based catalysts
(Scheme I-20).53
Scheme I-20. Zirconium catalyzed dehydrogenative coupling of phenylgermane.
The σ-conjugation properties of these polymers were explored even further very recently by Ohshita et al. that performed the synthesis of poly(dithieno)germoles, putting together the electron delocalization of the germanium backbone with the π- conjugation of its substituents.54
They used the Würtz-coupling reaction to perform the polymerization of dichlorodithienogermole (Scheme I-21) and studied the influence of such electron-rich substituent showing interesting properties from σ- and σ-π conjugation and π- stacking.
Scheme I-21. Synthesis of polydithienogermoles using the Würtz-coupling reaction.
Contrarily to poly(siloxane)s that are widely developed and used, polygermoxanes are scarce. Nonetheless, Oshita et al. managed to synthesize them by oxidizing poly(dithienogermole-4,4-diyl)s with trimethylamine N-oxide in toluene under reflux (Scheme I-22) affording the corresponding polygermoxanes in 37% and
55% yield with molecular weights of 5200 (Ðn = 1.6) and 2200 (Ðn = 2.1) for the ethyl and butyl derivatives, respectively. The interest in such synthesis, however was
19
Introduction to inorganic polymers merely the comparison with the σ-conjugation properties with their parent polygermanes.54
Scheme I-22. Synthesis of polygermoxanes by oxidation of polygermanoles.
The bonding orbitals in inorganic tin-polymers display more diffused orbitals than their lighter group XIV-containing polymers analogues inducing less stable Sn-
Sn bonds (187 kJ/mol) but a stronger σ-conjugation.47
Although the formation of polystannanes is not facilitated because of the aforementioned characteristics, their study started with the seminal work of Neuman et al.55 In 1963 they reported the synthesis low molecular weight polymeric diethyltin, from diethyltin dihydride using tin catalyst at 80-100 °C (Scheme I-23).
Scheme I-23. Synthesis of low molecular weight polydiethyltin.
The synthesis of high molecular weight polystannanes free from of cyclic oligomers was achieved only recently.56 Caseri et al. used a rhodium-based complex to dehydrogenate dibutyltin dihydride affording linear poly(dibutyl)stannane in 72 % isolated yield with a relatively low molecular weight (2 × 104 g/mol) (Scheme I-24).
20
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-24. Synthesis of linear polydibutyltin by dehydrocoupling.
Later on, the same group successfully managed to extend the substitution pattern of the inorganic polymer (Scheme I-25) starting from a larger range of dialkylstannanes in good to high yields 64-89 %) with molecular weight in the range of 1 × 104 to 1 x 105.57
Scheme I-25. Synthesis of linear polydialkyltin by dehydrocoupling.
Nonetheless, the polymers remained alkyl substituted until in 2009 when the same group developed the synthesis of poly(alkylphenyl)stannanes as well as copolymers of di(3-propylphenyl)stannane and dibutylstannane (Scheme I-26).58
Later on, Lechner et al. investigated three different synthetic route of polystannanes, using the Wilkinson’s catalyst [RhCl(PPh3)2], dehydrogenative coupling using tetramethylethylenediamine (TMEDA) and Würtz coupling using sodium in liquid ammonia (Figure I-5).59
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Introduction to inorganic polymers
Scheme I-26. Synthesis of poly(dialkylphenyl)stannanes (top) and polystannanes copolymers (bottom).
Figure I-5. Overview of the reactions investigated
The Wilkinson’s catalyst showed the best results for the synthesis of poly(dialkyl)stannanes, affording (n-Bu2Sn)n of 5.7 × 104 Da with a Ðn of 2.2, while the
TMEDA and NH3/Na routes were best suited for aromatic-containing substituents, affording polymers such as the [(4-n-BuPh)2Sn]n with Mw of 4.6 × 104 Da (Ðn=3.2) and
8 × 103 Da (Ðn=1.5), respectively.
A simpler and straightforward approach for the synthesis of polystannanes was developed by Foucher and Harrypersad in 2015.60 They showed a novel route under mild conditions by polycondensation of tin hydrides with tin amides, affording homo-
22
Metal-free synthesis and characterization of functionalized polyaminoboranes and copolymers of average molecular weight ranging from 9.1 × 104 until 2.42 × 105 Da
(Scheme I-27).
Scheme I-27. Synthesis of polystannanes by polycondensation.
Similarly to the other elements of the group XIV-based polymers, polystannoxanes incorporating a Sn-O repeating unit have been also reported but only in tin/oxygen clusters. This feature results from the ability of tin to extend its coordination and formed bridged polymers by developing extra Sn-O interactions.
In this field, one can quote as an example the study published by Jousseaume et al. in 199861 concerning the synthesis in two steps of tin oxo-polymers by hydrolysis of alkyl(trialkynyl)stannanes derived from trichloroalkylstannanes (Scheme I-28).
Scheme I-28. Synthesis of Sn-O oxo-polymers from alkyltrichlorostannanes.
I.2.2 Group XV-containing inorganic polymers
Main group-containing polymers also include polymers from the group XV.
With five electrons in their outer valence shell, group XV elements are more electron rich than their group XIV counterpart. They are often used in -conjugated materials,62 because of their conjugation ability by virtue of their available lone pair that can participate in -conjugation by interacting with a neighboring system.
Nitrogen only-based polymers are not stable enough be formed due to the high stability of nitrogen triple bound (BDE: 946 kJ/mol) in comparison to single (BDE: 297
23
Introduction to inorganic polymers kJ/mol) and double bound (BDE: 456 kJ/mol). Polymers with "-N=N-" patterns tends to stabilize by loss of dinitrogen molecules (N≡N).32 However, polymers containing nitrogen in combination with other p-block element do exist in plenty and they will be commented accordingly.
I.2.2.1 Phosphorus-containing polymers
Polyphosphazenes are polymers that consist in a backbone of phosphorus and nitrogen with a "PNRR’" repeating unit. They were the first synthetic phosphorus- containing polymer made in 1895 by Stokes by reaction of phosphorus pentachloride with ammonium chloride. The backbone only had phosphorus and nitrogen with an empirical formula proposed as (PNCl2)n for this polymer described as an oily chloride
(Scheme I-29).63
Scheme I-29. Synthesis of the first polyphosphazene polymer.
Observing that the (PNCl2)n polymer reacted with water, the reactivity of this soluble chlorinated polymer towards different organic nucleophiles was evaluated, affording stable poly(organo)phosphazenes (Scheme I-30).64
Scheme I-30. Generic synthesis of poly(organo)phosphazenes.
Starting from the poly(dichloro)phosphazene, the synthetic approach for this class of polymer largely remains the most widely used and several different pendant
24
Metal-free synthesis and characterization of functionalized polyaminoboranes groups could have been introduced. Crosslinked matrix could be formed as well by simple heating of the starting poly(dichloro)phosphazene (Scheme I-31).65
Scheme I-31. Common routes to poly(organo)phosphazenes via macromolecular substitution.
Unfortunately, its reactivity towards water and other nucleophiles requires significant concern related to its storage and handling motivating the development of new synthetic pathways via different precursors.66
One example of a direct synthetic route to poly(organo)phosphazenes is the anionic polymerization of N-silylphosphoranimines (RO)3P=N-SiMe3 performed by
Matyjaszewski et al in 1993.67
They showed that the silyl group could be removed from the nitrogen by a catalytic amount of a fluoride derivative generating, as the initiation reaction, the corresponding imino anion that can perform, in its turn, a nucleophilic substitution of an alkoxy group at the phosphorus. The alkoxy anion reacts with the silyl group of the precursor and enables the reaction to propagate (Scheme I-32).
The main importance of these polymers comes from the intrinsic properties of the -P=N- inorganic backbone.68
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Introduction to inorganic polymers
Scheme I-32. Direct synthesis of polyphosphazenes from phosphoranimines.
Polyphosphazenes display very interesting properties due to the π-overlapping between the nitrogen and phosphorus atoms in adjacent position. This specific connectivity also confers these polymers high chain flexibility that is translated into low glass transition temperature.69 The PN bond is also very resistant to thermal treatment and hemolytic scission,70,71 leading to fire-retardant applications that can be improved by the addition of others p-block elements.72
Together with the polyphosphazenes, polyphosphonates are also known for their properties as flame-retardant materials.73 Their usual synthetic route is via polycondensation, first explored by Kim et al. in 1983, when the phenylphosphonic dichloride was treated at low temperature by phenol-containing reagents, leading to a polycondensation (Scheme I-33).74
Scheme I-33. Synthesis of polyphosphonates by polycondensation.
Polythiophosphazenes constitute another type of phosphorus containing polymer. This class of polymer possesses sulfur in their backbone in addition to phosphorus and nitrogen. These polymers were first synthesized via thermal ring-
26
Metal-free synthesis and characterization of functionalized polyaminoboranes opening polymerization of a cyclothiophosphazene, that went through nucleophilic attack as the others chlorinated polymers (Scheme I-34).75
Scheme I-34. Synthesis of polythiophosphazanes
Polyphosphinoboranes are a more recent class of phosphorus-containing polymer. They contain a P-B repeating unit and are usually obtained by a rhodium complex via dehydropolymerization.76 Among their applications, these inorganic polymers attract a lot of attention as pre-ceramic materials, due to their high ceramic yield of until 80%.77
The first non-oligomeric and well-defined polyphosphinoborane was obtained by Manners et al. in 1999 by catalytic dehydrocoupling reaction of phosphine- boranes,77 from phenylphosphine-borane in the presence of a rhodium-catalyst.
Phenylphosphine-borane leads to the formation of the corresponding polyphosphinoborane with a molecular weight of 5600 g/mol whereas diphenylphosphine-borane simply dimerized linearly (Scheme I-35).
Scheme I-35. Synthesis of linear polyphosphinoboranes.
27
Introduction to inorganic polymers
Four years later, the same group expanded the scope of this approach to a variety of monosubstituted phosphine-boranes. Using the same rhodium-based catalyzed reaction, they were able to synthesize four types of high molecular weight
(~19×103 Da) polyphosphinoboranes (Scheme I-36).77
Scheme I-36. Synthesis of a polyphosphinoboranes
A reinvestigation of this reaction performed by Clark et al. in 200679 showed that by controlling the reaction parameters, in the presence of rhodium catalysts, the phosphine-borane would afford the cyclic trimer [{Ph2P-BH2}3] and the cyclic tetramer
[{Ph2P-BH2}4] in a 2:1 ratio (Scheme I-37).
Scheme I-37. Effect of the temperature on the formation of linear or cyclic phosphinoboranes.
More recently, Scheer, Manners et al. disclosed a new and metal-free route to polyphosphinoboranes.80 The reaction consists in the thermolysis of trimethylamine- phosphinoborane adducts under relatively mild conditions leading to the loss of a trimethylamine and formation of high molar weight inorganic polymers (Scheme I-38).
28
Metal-free synthesis and characterization of functionalized polyaminoboranes
The authors claimed that the polymerization occurs via the formation of the free monomer that further reacts by self-condensation (polymerization/oligomerization) in a head-to-tail fashion.
Scheme I-38. Polymerization/Oligomerization of Lewis base stabilized phosphanylboranes.
I.2.2.2 Heavier group XV-containing polymers
In sharp contrast with phosphorus inorganic polymers, the examples with heavier group XV elements are scarce and only very few examples are described in the literature.
Among the different possible strategies, the ring-opening polymerization of small cyclic molecules remains one of the most efficient strategies for polymer synthesis. In this respect, cyclic compounds displaying only element-element bonds have appeared to be good candidates for this type of approach.
In the case of arsenic cyclic compounds, their synthesis was achieved in 1970 by
Wast et al. by the reduction of the alkylarsonic acid with hypophosphorus acid, affording pentamer or hexamer according to the substitution pattern (Scheme I-39).81
One way to perform the synthesis of arsenic-based polymer was published in
1973 by Bellama et al.82 The polymer was formed by the treatment of the pentamer cyclo-(MeAs)5 with I2 and subsequent heating in the presence of 2,2’- azobisisobutyronitrile or by irradiation with an incandescent lamp to obtain the linear red-brown solid poly(methylarsine), that turns into a ladder type structure rather than a linear one (Scheme I-40).
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Introduction to inorganic polymers
Scheme I-39. Synthesis of five- and six-membered cyclooligoarsine.
Scheme I-40. Synthesis a As-based ladder polymer.
Most of the polymers incorporating heavy group XV elements such as arsenic or antimony are hybrid inorganic polymers incorporating carbon chains in their backbone and would deserve a separate discussion.
I.2.3 Group XVI-containing polymers
This family of inorganic polymers is mainly developed around the sulfur and nitrogen. Among them, polythionylphosphazenes,75 polyoxothiazene83 and polysulfurnitride84 (Scheme I-41) have attracted great attention due to their applications in photonics and electronic conductivity.
30
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-41. Sulfur-containing polymers.
Polythionylphosphazenes were already described in combination with phosphorus (vide supra).
Polymers having sulfur and nitrogen in their backbone were reported since
1960, in low molecular weights and contaminated by impurities. This changed with
Roy's work who published a novel route to this class of polymer based on the condensation of sulfonimidates.83 His method was efficient enough to synthesize homo- and copolymers of high molecular weights up to 4 × 105 g/mol (Scheme I-42).
Scheme I-42. Synthesis of poly(alkyl/aryl)oxothiazenes.
Polysulfurnitride also called polythiazyl is a polymer only containing sulfur and nitrogen (SN)x. The main route synthesizing this polymer is through the solid-
31
Introduction to inorganic polymers
state polymerization of S2N2 produced by the cracking of S4N485 on silver wool (Scheme
I-43).86
Scheme I-43. Synthesis of polysulfurnitride.
This synthesis, however, takes around 8 weeks to be completed and the S4N4 is explosive. Considering the interest in this compound due to its metallic conductivity properties, other synthetic pathways are being studied (Scheme I-44).84,87
Scheme I-44. Alternative synthesis of polysulfurnitride.
I.3 Boron-based polymers
As mentioned earlier, many inorganic polymers have one or several p-block elements in their backbone. Among the elements of group XIII and in connection with polymers, boron containing polymers are the focus of an intense research activity.
A myriad of examples has been developed for a broad spectrum of applications, mostly in the case of organic polymers with pendant boron containing groups.
However, hybrid inorganic and inorganic boron-containing polymers exist in a more limited number.
The main feature of boron compounds comes from its electron-deficient nature.
Trisubstituted boron containing compounds are stable and the boron remains with one free p-orbital. Consequently, they are Lewis acids and can react at boron, more or less reversibly, with a wide range of Lewis bases. When incorporated in a conjugated system, the boron can participate to the conjugation of the system via its vacant p orbital and one can take advantage of this feature especially for sensing88 or optoelectronics.89
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Metal-free synthesis and characterization of functionalized polyaminoboranes
I.3.1 Boron-Carbon hybrid polymers
One of the first relevant work about the synthesis of conjugated organoboron polymers came from Chujo et al in 1990.90 They were able to perform a polymerization reaction by haloboration of terminal diynes with boron tribromide (Scheme I-45).
Scheme I-45. Synthesis of boron-containing hybrid polymers by haloboration.
A related synthetic method to incorporate a boron atom and a boron-containing backbone is the hydroboration reaction. This method involves either one monosubstituted borane (R-BH2) or two disubstituted boranes R-B(H)-X-B(H)R to perform the reaction to the diyne. One example of this route is illustrated in the Scheme
I-46, that shows the hydroboration of aromatic diynes by mesitylborane.91
Scheme I-46. Synthesis of a boron-containing hybrid polymer by hydroboration.
The main advantage of these methods is the high selectivity of the reaction only takes place in the terminal alkyne position. More than that, the obtained polymers incorporate trisubstituted boron atoms having a free π orbital providing them interesting electronic and optical properties due to the extended π-conjugation along the backbone.92
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Introduction to inorganic polymers
Motivated by the extended electron delocalization along the π-systems aforementioned, several studies were carried to transpose this chemistry to the boron and other main group elements.
I.3.2 Boron-Oxygen hybrid polymers
Polymers containing boron-oxygen started being studied in the 1970’s, via the polymeric condensation of phenylboronic acid with aliphatic diols, and their further distribution of the alkoxy groups under thermal conditions to afford oligomers
(Scheme I-47 a).93
The synthesis of this type of polymer with a high molecular mass was only possible years later by Wolfe and Wagener. They used a ruthenium-based catalysts to perform a acyclic diene metathesis in boronate with unsaturated alkoxy groups
(Scheme I-47 b).94
Scheme I-47. Synthesis of (top) oligoboronate and (bottom) polyboronate.
Along with the optical properties showed for trisubstituted boron-containing polymers, polyboronates are appealing precursors for the formations of new materials, such as covalent organic frameworks95 and complex nanostructures.96
A similar strategy to the addition of boron to diynes polymers was also used to make copolymers having not only boron-carbon bonds but also boron-nitrogen connectivity in the backbone.97
34
Metal-free synthesis and characterization of functionalized polyaminoboranes
I.3.3 Boron-Nitrogen hybrid and inorganic polymers
In 1992, Chujo et al. performed the hydroboration-polymerization of dicyano compounds leading to head-to-tail dimerized iminoboranes moiety in the repeating unit of the polymer. They were obtained as moisture stable polymers in more than 83% yield, with a molecular weight up to 30300 g/mol (Scheme I-48).98
Scheme I-48. Synthesis of boron-containing by hydroboration-polymerization.
Since then, the formation of hybrid organic-inorganic polymers has been considerably investigated. An interesting pathway was developed by Lacôte et al. in
2015 using the formation of acid-base Lewis adducts as the driving force to the polycondensation resulting from the reaction between borohydrides and ammonium salts.99
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Introduction to inorganic polymers
Scheme I-49. Synthesis of hybrid organic-inorganic polymers by polycondensation.
One year later, Helten et al. proposed a novel pathway to obtain a para- phenylene and diaminoborane groups in the main chain, by using mesityldichloroborane and N,N'-bis(trimethylsilyl)-para-phenylenediamines in a 2:1 ratio.
The reaction results in a polycondensation process with elimination of trimethylsilylchloride and 2 × 104 g/mol molecular weight soluble poly[N-(para- phenylene)diiminoboranes] displaying a "NBN-phenylene" repeating unit were obtained accordingly. Their study revealed a moderate -conjugation across the NBN unit (Scheme I-50).100
One year later, Helten et al. described a novel synthetic route to poly[N-(para- phenylene) diminoboranes] via a similar strategy from N,N'-bis(trimethylsilyl)-para- phenylenediamine and [(para-phenylene)bis(arylbromoborane)]. A polymer with the
"BN-phenylene-NB-phenylene" repeating unit was obtained with a 4.6 × 104 g/mol molecular weight and a broad dispersity (Ðn=4.1).101
Their study revealed a -conjugation across the B=N units along the polymer backbone and extension of the conjugation path with increasing chain length (Scheme
I-51).
36
Metal-free synthesis and characterization of functionalized polyaminoboranes
Scheme I-50. Synthesis of poly[N-(para-phenylene)diimidoborane
Scheme I-51. Synthesis of organic-inorganic hybrid polymer by polycondensation.
Although hybrid polymer presents interesting properties, the synthesis of a pure inorganic polymer remains an actual challenge that relies on efficient synthetic strategies enabling the implementation of the desired repeating units of the polymer backbone. In this aim, the boron-nitrogen bond was proved to be effective and has become a valuable tool for the construction of inorganic polymers.
Manners et al. published a work in this direction (Scheme I-52). Starting from amine-boranes, they performed dehydrocoupling reactions with rhodium catalyst, in
37
Introduction to inorganic polymers parallel to the synthesis of linear phosphinoboranes,77 unveiling the formation cyclic aminoboranes and borazines.102
Scheme I-52. Dehydrocoupling reactions of amine-boranes catalysed by rhodium catalyst.
Since amine-boranes are isoelectronic with alkanes, Goldberg et al. used a iridium-based metal complex (POCOP)Ir(H)2 (POCOP = [η3-1,3-(OPtBu2)2C6H3]), known for alkane dehydrogenation,103 to undergo the same reaction with the inorganic moiety, proving that this complex was able to undergo the dehydrogenation of the ammoniaborane, addressing the question of the reversible chemical hydrogen storage.104
In the course of the reaction, a precipitate formed that was insoluble in most common solvents preventing to collect information for in-depth analysis. However, on the base of solid-state 11B NMR spectroscopy, a [H2N-BH2]n polyaminoborane structure was assigned to the precipitate.
Further work with ammoniaborane and methylamine-borane (Scheme I-53) revealed the Ir(POCOP) catalyzed reaction enthalpy was found to be -6.7 ± 0.6 kcal/mol for ammoniaborane, -6.8 ± 0.6 kcal/mol for methylamineborane and -6.7 ± 0.1 kcal/mol for the 1:1 ammoniaborane/methylaminoborane mixture, suggesting that reversible
38
Metal-free synthesis and characterization of functionalized polyaminoboranes hydrogen absorption and release under mild conditions using amine-boranes may not be feasible.105
Concerning the structure of the resulting solids that could have been isolated, further characterization by mass spectrometry confirmed the methylaminoborane repeating unit when starting from methylamine-borane. The case of ammoniaborane remained non-conclusive due to the insolubility of the precipitate.
The mass spectroscopy also revealed the soluble species that could be analyzed resulted in a mixture of cyclic poly(methylamino)borane oligomers of different size
(MeNHBH2)n (2 [MeNH2BH2][MeNHBH2]n[MeNH2]) (4 Scheme I-53. Iridium-POCOP catalyzed dehydrogenation of ammoniaborane and amine- borane. The same year, Manners et al. showed that the Ir(POCOP) catalyzed dehydrogenation of methylamine-borane and butylamine-borane led to high molecular weight polyaminoboranes (Scheme I-54) analyzed by SEC.106 Scheme I-54. Synthesis of homo- and random copolymers 39 Introduction to inorganic polymers Table I-1 Synthesis and characterization data for polyaminoboranes. Adapted from reference [106]. Monomer precursor Polyaminoborane Yield Mw (SEC) Ðn (SEC) (ratio) (%) 5 MeNH2•BH3 (MeNH•BH2)n 60 1.6×10 2.9 (MeNH•BH2)3n/ 5 MeNH2•BH3/NH3•BH3 (3:1) 72 1.6×10 11.0 (NH2•BH2)n (MeNH•BH2)n/ 4 MeNH2•BH3/NH3•BH3 (1:1) 91 4.7×10 3.9 (NH2•BH2)n 5 nBuNH2•BH3 (nBuNH•BH2)n 82 4.1×10 1.6 nBuNH2•BH3/MeNH2•BH3 (nBuNH•BH2)3n/ 18 1.8×105 1.9 (3:1) (MeNH•BH2)n nBuNH2•BH3/MeNH2•BH3 (nBuNH•BH2)n/ 48 2.4×105 2.2 (1:1) (MeNH•BH2)n Two years later, and since the principle of dehydrocoupling of adducts was unlocked, a detailed study of this reaction was done107 clearly showing the dehydrocoupling reaction of amine-boranes could be catalyzed by a wide range of late transition metal precatalyst affording polyaminoboranes with variable molecular weights and dispersity between 7 × 103 to 4.6 × 105 g/mol and 1.2 to 13, respectively.107 The mechanism of such reaction is still unclear, especially regarding the role of the metal.108,109 The same authors showed the same polymers could be produced upon Ir-POCOP catalysis from either amine-borane or the linear diborazane110 (Scheme I-55). They also brought out the existence of redistribution reactions during the process. 40 Metal-free synthesis and characterization of functionalized polyaminoboranes Table I-2. Overview of the synthesis of polyaminoboranes, reproduced from reference [107]. 41 Introduction to inorganic polymers Scheme I-55. Proposed mechanism of polymerization of (a) MeNH2-BH3 and (b) MeNH2- BH2-NHMe-BH3 A hydrogen transfer reaction was highlighted in the absence of any metal between monomeric amine-boranes and aminoborane and demonstrated by using monomeric diisopropylaminoborane (1) with ammoniaborane and methylamine- borane (Scheme I-56).110 Scheme I-56. Reaction of ammonia- and methylborane with diisopropylaminoborane (1). Recently, a strong contribution to the field of boron-containing inorganic polymers was provided by Helten et al. who managed to synthesize oligo(iminoboranes) of 12-14 BN units on average.111 The developed approach is in the continuity of they work about poly[N-(para- phenylene)diiminoboranes] and involves the reactivity bis(silylamino)boranes with dichloroboranes leading to polyiminoboranes in 83% yield by polycondensation reaction and elimination of trimethylsilylchloride (Scheme I-57). The formed 42 Metal-free synthesis and characterization of functionalized polyaminoboranes oligomers remain soluble due to the octyl substituent at the boron enabling SEC analysis that revealed a molecular weight of 1900 g/mol.111 Scheme I-57. Synthesis of oligo(iminoborane)inorganic polymers I.4 Project presentation Since the Staudinger’s seminal works on polymeric compounds, this field of research saw great development in terms of bond engineering somewhat mimicking the strategies involved around the carbon-carbon bonds to efficiently build new backbone and polymers. The organic polymer chemistry gained from introducing main group elements into their composition, the backbone included, forming hybrid polymers. In the last decades, a wide range of new polymers were prepared and new routes were unlocked upon complete change of the backbone from carbon to p-block elements. The growth of this exciting field is tied with the good mastering and understanding of the bond formation involving main group elements. In the case of boron-nitrogen based inorganic polymers, their formation usually takes place upon catalytic dehydrocoupling via the activation of both BH and NH bonds, and preserving the integrity of the existing BN linkage. This aspect will be studied in the next chapters with the chemistry of diisopropylaminoborane (1) at the heart. 43 Introduction to inorganic polymers 44 Chapter II: Dehydrocoupling versus boron transfer reaction Dehydrocoupling versus boron transfer reaction Carbon-based polymers such as polyethylene are usually made from stable monomers, such as ethene and propene.112 However, polyaminoboranes are a family of polymers constituted of aminoboranes repeating units that are particularly reactive towards oxygen and moisture mostly unstable in their monomeric form.113 In 1970, McGee and Kwon were able to generate and characterize the prototypical monomeric aminoborane (H2NBH2), the inorganic analogue of ethene, under cryogenic conditions.114 They worked at -196 °C and used radiofrequency discharge in borazine vapor to produce the monomeric aminoborane and trap it. Aminoborane was claimed to be identified by infrared spectroscopy (Figure II-1) with characteristic NH and B-H vibration bands at 3200 and 2300 cm-1, respectively, at 4.2K and 77K (Figure II-1 a and Figure II-1 b) that disappear at higher temperature (Figure II-1 c) or upon exposure to air (Figure II-1 d). This result indicates that the temperature plays a crucial role in the stabilization of this molecule114 has been considered until that time as an elusive species. In 2010, Alcaraz et al. disclosed the stabilization of monomeric aminoboranes in the first coordination sphere of a metal center.115 The reaction of amine-boranes (H3B- NHRR') with the [RuH2(η2-H2)2(PCy3)2] ([Ru]) under stoichiometric conditions results in the dehydrogenation of the starting amine-borane followed by its coordination to the ruthenium center to give the corresponding bis(-BH) ruthenium complex (Scheme II-1 and Figure II-2).116,117 In collaboration with the group of A. Weller in Oxford and in the case of the diisopropylaminoborane (1), this coordination mode has been extended to different metal complexes.118 47 Dehydrocoupling versus boron transfer reaction Figure II-1. Infrared spectra of aminoborane at various temperatures. (a) at 4.2 K; (b) at 77 K; (c) at 118 K; (d) at room temperature. Figure reproduced from reference [3]. Scheme II-1. Synthesis of the bis(-BH) ruthenium complex. 48 Metal-free synthesis and characterization of functionalized polyaminoboranes 2 2 Figure II-2. X-ray crystal structure of [RuH2(η :η -H2BNH2)(PCy3)2]. Figure reproduced from reference [117]. The scope of this ligation mode could have been illustrated through the stabilization of elusive aminoborane intermediates once generated by dehydrogenation of the corresponding amine-borane at the metal center, as exemplified later on by the groups of Turculet119 and Weller120 (Scheme II-2). Scheme II-2. (a) Formation of bis--BH metal complexes from four-coordinate (Cy- 119 F PSiP)RuX complexes; (b) Synthesis of [Rh2(L)2(-H)(-H2B=NH2)][BAr 4] i 120 (L=R2P(CH2)3PR2; (R =Ph, Pr)) 49 Dehydrocoupling versus boron transfer reaction To better understand the involved dehydrogenation process, further studies were undertaken and conducted with amine-boranes displaying a remote amino group in the molecule. Dehydrogenation of a series of diamine-monoborane compounds mostly derived from ethylenediamines was studied in the presence of the ruthenium complex [RuH2(η2-H2)2(PCy3)2].121,122 The outcome of the reactions strongly depends on the nature of the remote amino function (Scheme II-3). In the case of a tertiary amino pendant group, the reaction is stoichiometric in nature, resulting in the formation of the corresponding bis(-BH) ruthenium complex with no interaction between the remote nitrogen and the boron. In the case of a secondary amino pendant group, the reaction is catalytic in ruthenium, leading to the formation of the corresponding diazoborolidine by intramolecular BN bond formation.121,122 Scheme II-3. Dehydrogenation of diamine-monoboranes in the presence of 2 [RuH2(η -H2)2(PCy3)2]. 50 Metal-free synthesis and characterization of functionalized polyaminoboranes A similar strategy was presented by M. Hill et al. in 2013 with group II-based catalysts. They performed dehydrocoupling reactions of amine boranes in the presence of protic amines,123 showing that similar reactions could also proceed intermolecularly and lead to the corresponding diaminoboranes. It was postulated that unsaturated H2B=NR2 species, shortly after being generated, would proceed into coupling reactions. Interestingly, the substituent pattern played a crucial role orientating the reaction toward the formation of the symmetrical or asymmetrical derivatives in the dehydrocoupling reactions (Scheme II-4). Scheme II-4. Group II catalyzed synthesis of asymmetrical and symmetrical diaminoboranes. Another similar approach was conducted by Conejero et al. in 2016.124 They used a highly reactive cationic NHC-platinum (II) complex to synthesize diaminoboranes through dehydrocoupling of amine-boranes and amines (Scheme II-5). Their method allows the access to symmetrical diaminoboranes in very good yields (76~99 %) and selectivity (up to 96% of asymmetrical diaminoborane). Nonetheless, the synthesis of most asymmetrical compounds requires high reaction times (24-92 h) and mild heating. Once again, the substitution pattern has a high influence on the reaction time, from days to few minutes, and on the selectivity of the reaction, from 25 % to 96% of the asymmetrical diaminoborane. 51 Dehydrocoupling versus boron transfer reaction Scheme II-5. Platinum catalyzed synthesis of symmetrical and asymmetrical diaminoboranes. In order to expand the scope of our system also connected to the strategy of BN bond formation, the first objective for my PhD work was to transpose our initial intramolecular conditions (Scheme II-3) to intermolecular dehydrocoupling reactions by involving mixtures of amine-boranes and secondary amines and under mild conditions (Figure II-3). Figure II-3. Approach to intermolecular Ruthenium catalyzed dehydrocoupling reaction of amineboranes in the presence of a secondary amine. II.1 Reactivity of diisopropylamine-borane and diethylamine in the presence of the [RuH2(η2-H2)2(PCy3)2] In line with our previous works and especially given that its reactivity at ruthenium is well known, diisopropylamine-borane (2) appeared particularly well suited as a precursor to start this study. The high steric hindrance of the diisopropyl groups at nitrogen prevents any dimerization or self-coupling reaction once dehydrogenated.125,126 In addition, the resulting bis(-BH) ruthenium complex has been also well studied118 and a good knowledge of the system diisopropylamine-borane (2)/diisopropylamino-borane 52 Metal-free synthesis and characterization of functionalized polyaminoboranes (1)/diisopropylaminoborane bis(-BH) Ru complex should greatly facilitate the reactivity studies (Scheme II-6). Scheme II-6. Comparison between one less hindered amine (top) and the 2 2 i [RuH2(η :η -H2B-N Pr2) (bottom). The first reaction performed consisted in transposing the conditions for the catalyzed dehydrogenation cyclization (CDC) of diamine-monoboranes121,122 to diisopropylamine-borane (2) and diethylamine in the presence of [RuH2(η2- H2)2(PCy3)2] expecting to produce the unsymmetrical (diethylamino)(diisopropylamino) borane (3) (Scheme II-7). Monitoring the reaction by 11B NMR spectroscopy showed that the entire starting amine-borane precursor was consumed after 27 h, at room temperature. A set of two signals could be observed in the spectrum in 1:4 integration ratio. The same reaction repeated in toluene, pentane or neat always gave the same results. In the 11B 53 Dehydrocoupling versus boron transfer reaction NMR spectrum, the major product resonates as a relatively shielded triplet at 2.5 ppm and collapses into a singlet upon proton decoupling. Scheme II-7. Model reaction with the expected product for the intermolecular dehydrocoupling. The value of the coupling constant JBH (112 Hz) is indicative of a direct 1JB-H. The second observable deshielded signal in the spectrum at 29 ppm appears as a doublet that collapses upon proton decoupling with also a direct coupling constant (1JBH = 141 Hz). These data are in agreement with the presence in the mixture of a major compound displaying a B(sp3)H2 group and a B(sp2)H-containing minor compound that could be the desired unsymmetrical diaminoborane (Figure II-4). Concerning the major compound, a first hypothesis would have been to propose the presence of diisopropylaminoborane (1) in the reaction mixture, resulting from the dehydrogenation of the diisopropylamine-borane (2). Since 1 is stable as a monomer and resonates at 35 ppm as a triplet in the 11B NMR,126 this hypothesis could not be retained. Given the nature of the starting reagents, one possibility likely to rationalize these results consisted in considering the presence of N,N,N',N'—tetraethylcyclodiborazane (4) as the major compound. This product was prepared as an authentic sample from diethylamine-borane (5) via thermally induced dehydrocoupling reaction and further purified by distillation (Scheme II-8). 54 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-4. 11B NMR (top) and 11B{1H} (bottom) NMR spectra of the crude reaction. Scheme II-8. Synthesis of the N,N,N',N'-tetraethylcyclodiborazane (4). Its 11B NMR spectrum fits perfectly with the one of the major compound of the reaction, ascertaining its identity as the cyclodiborazane (4). Concerning the second compound present in a smaller quantity in the mixture, the 11B NMR data are clearly in favor of a diaminoborane compound. These data were then compared to those of a pure sample of the desired (diethylamino) (diisopropylamino)borane (3) prepared in 79% isolated yield via an alternative synthetic pathway described in Scheme II-9. 55 Dehydrocoupling versus boron transfer reaction Figure II-5. 11B NMR spectrum of the authentic samples: diisopropylaminoborane (1) (top); cyclodiborazane(4) (middle) crude reaction (bottom) Scheme II-9. Synthesis of the (diisopropylamino)(diethylamino)borane (3). This difference is evidenced in the 11B spectra of both compounds, and one can see that they do not perfectly fit (Figure II-6), preventing at that stage to ascribe any definitive structure to the diaminoborane formed in minor quantity during the reaction. 56 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-6. Comparison of the enlarged 11B spectrum of the crude reaction (bottom) with the authentic sample of (diethylamino)(diisopropylamino)borane (3) (top) Fractional trap-to-trap distillation was performed at different controlled temperatures enabling the separation of several fractions. The 1H and 13C{1H} NMR analyses of the fraction containing the more volatile compounds showed clearly the presence of a large quantity of free diisopropylamine not connected anylonger to boron. This feature, in addition to the 11B NMR data of the crude reaction mixture, prompted us to propose the bis(diethylamino)borane (7) for the identity of the diaminoborane. An authentic sample of compound 7 was prepared in almost quantitative yield from cyclodiborazane (4) and diethylamine (Scheme II-10) and its NMR (Figure II-7) data are fully in agreement with those of the minor compound present in the crude reaction mixture. At that stage, the outcome of the reaction between diisopropylamine-borane (2) and diethylamine treated with [RuH2(η2-H2)2(PCy3)2] can be summarized in the Scheme II-11. 57 Dehydrocoupling versus boron transfer reaction Scheme II-10. Synthesis of bis(diethylamino)borane (7). Figure II-7. Comparison of the enlarged 11B spectra of the crude reaction (top) with the authentic bis(diethylamino)borane (7). Scheme II-11. Reaction of diisopropylamine-borane (2) with diethylamine in the presence of 2 [RuH2(η -H2)2(PCy3)2]. 58 Metal-free synthesis and characterization of functionalized polyaminoboranes These unexpected results motivated us to further investigate the role of the metal complex and better understand this transformation. Considering that no amine-borane was present at the end of the reaction, that [RuH2(η2-H2)2(PCy3)2] is known to perform dehydrogenation reaction of amine- boranes and form the corresponding bis(-BH) ruthenium complex [RuH2(η2:η2- H2BNiPr2)(PCy3)2] ([Ru-σBH2]), we checked the role of this latter complex as a possible catalytic active species in the reaction. It was synthesized from diisopropylaminoborane (1) and [RuH2(η2-H2)2(PCy3)2] according to a reported procedure (Scheme II-12).122 2 2 i Scheme II-12. Synthesis of [RuH2(η :η -H2BN Pr2)(PCy3)2]. The same reaction was performed using complex [RuH2(η2:η2-H2BNiPr2)(PCy3)2] in catalytic amount (Scheme II-13). Surprisingly and whatever the conditions involved, no dehydrocoupling could be observed. What happened instead was the B(sp3)H3 transfer from the diisopropylamine-borane (2) to the diethylamine giving the diethylamine-borane (5) as the major compound. Scheme II-13. Catalytic test of the reaction between diisopropylamine-borane (2) and the 2 2 i diethylamine with [RuH2(η :η -H2BN Pr2)(PCy3)2]. 59 Dehydrocoupling versus boron transfer reaction To clarify the reactivity, two reactions involving diisopropylamine-borane (2) and diethylamine were performed in parallel. One using 2.5 mol% of the [RuH2(η2:η2- H2BNiPr2)(PCy3)2]) and the other without metal complex, as the blank reaction. After 27 h at room temperature, the major product was the diethylamine-borane (5) for both reactions. Only traces of bis(diethylamino)borane (7) and N,N'- diethylcyclodiborazane (4) were detected when using the [RuH2(η2:η2- H2BNiPr2)(PCy3)2]). Traces of the starting diisopropylamine-borane (2) in the blank could be also detected (Figure II-8). Figure II-8. 11B NMR spectra of the reaction of diisopropylamine-borane (2) and 2 2 i 2 2 diethylamine: a) without [RuH2(η :η -H2BN Pr2)(PCy3)2]) (top) and b) with [RuH2(η :η - i H2BN Pr2)(PCy3)2] (bottom). After 4 days, no evolution was seen. Then, a second solution containing 2.5 mol% catalytic amount of the [RuH2(2:H2)2(PCy3)2] was added to both Schlenks and the reaction monitored by 11B NMR. Monitoring the reaction revealed the partial, 3h after the addition of the [RuH2(H2)2(PCy3)2], and the total transformation of the initially 60 Metal-free synthesis and characterization of functionalized polyaminoboranes formed diethylamine-borane (5), into the corresponding cyclodiborazane (4) along with a minor amount of bis(diethylamino)borane (7) after 27 h (Figure II-9). Figure II-9. 11B NMR spectra of the evolution of the reaction before and after addition of a catalytic amount of [RuH2(H2)2(PCy3)2] to the 27h stirred solution. a) Before (top); b) 3h after (middle) and c) 24h after addition (bottom). With these results in hand, one can make the following assumptions: 1. The first step of this transformation and also the quickest results is the B(sp3)H3 borane transfer from the diisopropylamine to the diethylamine to give diethylamine-borane (5). 2. Intermolecular dehydrocoupling likely to produce an unsymmetrical diaminoborane is no observed upon catalytic conditions under these conditions neither with [RuH2(η2-H2)2(PCy3)2] nor with [RuH2(η2:η2-H2BNiPr2)(PCy3)2]. 3. [RuH2(η2:η2-H2BNiPr2)(PCy3)2] do not catalyze the dehydrogenation of the diethylamine-borane (5) even after 96 h. 4. [RuH2(H2)2(PCy3)2] catalyzes the dehydrogenation of the diethylamine-borane adduct (5). 61 Dehydrocoupling versus boron transfer reaction Scheme II-14. Role of the metal center in the dehydrocoupling reaction of the diisopropylamine-borane (2) and the diethylamine. To get more insight into the role of the ruthenium complex in this transformation, a test reaction was conducted by mixing a strict stoichiometric amount of diisopropylaminoborane (1) and diethylamine at room temperature without ruthenium complex. After 20 h, the 11B NMR spectrum of the reaction mixture showed a similar distribution pattern of boron compounds than the one observed with the diisopropylamine-borane (2) and diethylamine in the presence of a catalytic amount of [RuH2(η2-H2)2(PCy3)2], along with traces of additional compounds. This time, no dihydrogen formation could be detected during the reaction. Worthnoting is the distortion of the doublet at δ 30 ppm and traces of a new compound displaying a shielded quartet at ca δ -20 ppm and assigned to (diethylamino)(diisopropylamino)borane (3) and diisopropylamine-borane (2), respectively, by comparison with authentic samples (Figure II-10). 62 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-10. 11B NMR spectra of the reaction between the diisopropylaminoborane (1) and diethylamine. Figure II-11. 11B spectra of the (diethylamino)(diisopropylamino)borane (3)(top), bis(diethylamino)borane (7) (middle) and the crude reaction between the diisopropylaminoborane (1) and diethylamine (bottom). 63 Dehydrocoupling versus boron transfer reaction Since the cyclodiborazane (4) is formed in the absence of any transition metal catalyst, one cannot invoke any dehydrogenation process likely to justify its formation in the medium. Further analysis of the spectra seems to indicate that bis(diethylamino)borane (7) and diethylamine-borane (5) as well as (diethylamino)(diisopropylamino)borane (3) and diisopropylamine-borane (2) are respectively produced in a similar ratio during the reaction (Scheme II-15). Scheme II-15. Reaction between the diisopropylaminoborane (1) and the diethylamine. These results clearly show that a B(sp2)H2 boron transfer here takes place in the absence of any metal center. This reaction is accompanied by the formation of diaminoboranes and BH3-containing side-products in similar ratio. In line to the very first experiments of this study, these results reinforce the hypothesis that diethylamine-borane (5) is produced in a first stage of the reaction and that [RuH2(η2-H2)2(PCy3)2] catalyzes its dehydrogenation more efficiently than [RuH2(η2:η2-H2BNiPr2)(PCy3)2] does in similar conditions. 64 Metal-free synthesis and characterization of functionalized polyaminoboranes II.2 Reactivity of diisopropylaminoborane with amines: insight in the mechanism of the metal-free boron transfer The reaction involving the diisopropylaminoborane (1) and the diethylamine starts from BH2-containing monoaminoborane species and finishes up with a mixture of BH2, BH and BH3-containing compounds in various proportions. Even if all these compounds have been clearly identified, their formation remains unclear at this stage, and more particularly with regard to the BH3-containing species whose formation seems correlated to the BH-containing ones. This feature is closely related to the regeneration step of dehydrogenated ammoniaborane and related amine-boranes (Scheme II-16) and this transformation has been extensively studied in the field of reversible chemical dihydrogen storage.127,128 Scheme II-16. Representation of the favoured dihydrogen release from ammoniaborane and the unfavoured regeneration process. Adding a hydrogen and transforming a “B(sp2)H” group into its “B(sp3)H2” counterpart generally involves hydrides reagents as exemplified by Liu et al. (Scheme II-17).129,130 Scheme II-17. Addition of hydrogen to B(sp2)H. 65 Dehydrocoupling versus boron transfer reaction Recently, Manners et al. showed such a transformation could originate from the redistribution reaction between an aminoborane and an amine-borane (Figure II-12).110,131 Figure II-12. Proton/hydride transfer between the dimethylamine-borane and the diisopropylaminoborane. By putting together the current studies done so far in this field, Paul et al. presented in 2015 a theoretical work displaying a plausible mechanistic picture involving the different scenarios of metal-free proton/hydride transfer between amine- boranes and aminoboranes.132 Since in our case, diisopropylaminoborane (1) is the only boron containing species initially introduced, we suspect a more complicated mechanism takes place during the reaction The stability of the dimer was evaluated in the presence of one equivalent of diethylamine. After 65 h of stirring at room temperature, no reaction occurs. (Figure II-13). The bis(diethylamino)borane (7) is exclusively formed upon heating at 130 °C (Scheme II-18). The triplet shows that the dimer remained without reacting. The doublet shows the formation the BH compound. 66 Metal-free synthesis and characterization of functionalized polyaminoboranes Scheme II-18. Stability test of the N,N'-diethylcyclodiborazane (4). Figure II-13. 11B NMR spectra of reaction between the cyclodiborazane (4) and the diethylamine a) after 65 h at RT (bottom) b) after heating (top). In line with our experimental results, a mechanism can be proposed to rationalize the formation of all the compounds observed at the end of the reaction. The reaction results in the formation of diisopropylamine and cyclodiborazane (4) as the major boron-containing compound. Although diisopropylaminoborane (1) showed no detectable reaction with amines such Et3N, pyridine or diisopropylamine,126 here the reaction must involve the formation of Lewis acid-base adduct between the diisopropylaminoborane (1) and diethylamine followed by a Cope-type elimination affording in a 1:1 ratio the diisopropylamine and diethylaminoborane (6) that dimerizes into cyclodiborazane (4). 67 Dehydrocoupling versus boron transfer reaction Scheme II-19. Formation of the cyclodiborazane (4). Although stable in the solid state, cyclodiborazane (4) undergoes a dissociative equilibrium in solution and its diethylaminoborane (6) monomeric form133 can be observed via the presence of a triplet at ca 36 ppm in the 11B NMR spectrum. Following the findings of Manners et al. for the hydride/proton intermolecular transfer, the minor formation of bis(diethylamino)borane (7) and diethylamine-borane (5) in the same ratio could result from a similar hydrogens exchange process involving diethylaminoborane (6) and its Lewis acid-base adduct with diethylamine during the reaction. The same process involving diisopropylaminoborane (1) would also explain the formation of traces of diisopropylamine-borane (2) and (diethylamino)(diisopropylamino)borane (3) detected at the end of the reaction in equal quantity. 68 Metal-free synthesis and characterization of functionalized polyaminoboranes Scheme II-20. Proposed mechanism for the reaction between the diamine and the diisopropylaminoborane (1). DFT theoretical calculations (B3LYP) were performed by Dr. C. Dinoi and Dr. I. Del Rosal on the basis of these assumptions. The energy profile of the reaction is presented in Figure II-14 (vide infra). One can note that the Lewis acid-base adduct formation takes place with almost no change in the enthalpy (-0.2 kcal/mol) via a rather low activation barrier (1.9 kcal/mol). The N-to-N hydrogen shift in the Lewis acid-base adduct is slightly endothermic (1.6 kcal/mol) and operates via an activation barrier of 13.9 kcal/mol. 69 Dehydrocoupling versus boron transfer reaction Figure II-14. Computed (DFT-B3LYP level) energy profile of the reaction between Et2NH i and Pr2NBH2 (1). 70 Metal-free synthesis and characterization of functionalized polyaminoboranes The dimerization of diethylaminoborane (6) into the cyclodiborazane (4) is an exothermic process (-13.3 kJ/mol) displaying the highest activation barrier (14.2 kcal/mol) and constituting the rate determining step of this exothermic process. Cyclodiborazane (4) is the thermodynamic compound of the reaction. With a lower barrier, but being higher in energy, bis(diethylamino)borane (7) and diethylamine-borane (5) are the kinetic compounds of the reaction, which is in agreement with our experimental observations. Given the cyclodimerization energy barrier is the highest of the all reactions taking place in the process, one would expect the formation of the bis(diethylamino)borane (7) and diethylamine-borane (5) in a larger quantity whereas only produced as traces experimentally. One can explain this result by the stoichiometry of the reaction and the absence of available diethylamine in the reaction mixture to enable this reaction pathway. As mentioned above, cyclodiborazane (4) does not react with diethylamine at room temperature and lead to the formation of bis(diethylamino)borane (7) upon heating. In that case, the Lewis acid-base adduct formed between diethylamine and diethylaminoborane (6) after retrodimerization would undergo a dehydrogenation as reported by Bertrand et al. in the case of pinacolborane.134 One possibility to check this hypothesis would be to perform the same reaction in the presence of a second reactive amine that could in turn react with the aminoborane once formed and before its dimerization (Figure II-15). Figure II-15.Reaction strategy between the diisopropylaminoborane (1) and a diamine. To check our hypothesis, we decided to investigate the reactivity of the N,N’- dimethylethylenediamine (8) with diisopropylaminoborane (1) with different 71 Dehydrocoupling versus boron transfer reaction stoichiometric ratio and conditions. We expected that the second amine function would react as the diethylamine, but intramolecularly. The stoichiometric reaction of N,N'-dimethylethylenediamine (8) with diisopropylaminoborane (1) was performed at room temperature, in toluene or solventless and the reaction was monitored by multinuclear NMR spectroscopy. The 11B NMR spectrum (Figure II-16) shows the presence of the two major compounds in a 1:1 ratio. The first one giving a doublet at 29 ppm (1JBH = 140 Hz) is a BH-containing molecule identified as the N,N'-dimethyl-1,3,2-diazaborolidine (9) also produced in the CDC reaction.122 The second molecule displaying a quartet at -14 ppm was identified as the N,N’-dimethylethylenediamine-monoborane (10), corresponding to the BH3-adduct of the initial amine. The diisopropylamine was also identified in the reaction mixture and the minor compounds are going to be discussed later. Figure II-16. 11B NMR spectrum of the reaction between the diamine (8) and the diisopropylaminoborane (1). 72 Metal-free synthesis and characterization of functionalized polyaminoboranes These results are in agreement with our mechanism hypothesis and confirmed by the use of a substoichiometric amount of diamine. Scheme II-21. Reaction bewteen the diisopropylaminoborane (1) and the N,N'-dimethyl- ethylenediamine (8) In the presence of 1.5 equiv. of the diisopropylaminoborane (1), the reaction lead to the formation of the diazaborolidine (9) and the diamine-monoborane adduct (10) reacts with the excess of diisopropylaminoborane (1) via the pendant secondary amine function as described above with diethylamine, leading to the formation of several others boron containing molecules. The newly produced compounds (Scheme II-22) could be identified either by comparison with authentic samples prepared separately or on the basis of their 11B NMR typical signature. A similar result is obtained when adding 0.5 equiv. of diisopropylaminoborane (1) to a 1/1 mixture of N,N'-1,3,2-diazaborolidine (9) and N,N'-dimethylamine-monoborane (10). Scheme II-22. Influence of the stoichiometry in the reaction between the diisopropylaminoborane (1) and the dimethyl-ethylenediamine (8). 73 Dehydrocoupling versus boron transfer reaction The results showed so far allowed us to propose a mechanism for the N,N’- dimethylethylenediamine (8), illustrated at the Scheme II-23, similar to the one showed for the diethylamine. Figure II-17. Reaction of diaminemonoborane (10) with diisopropylaminoborane (1). Scheme II-23. Proposed mechanism of the reaction between the diisopropylaminoborane (1) and the N,N'-dimethylamine (8). 74 Metal-free synthesis and characterization of functionalized polyaminoboranes Interestingly, this system starts from a B(sp2)H2-containing species (comparable to a "spent fuel") and we finish with a B(sp3)H3-containing molecule (comparable to a "regenerated fuel") in 50 % yield, without using any metal-based complex added and at room temperature. This type of reaction can be seen as what would be needed to finish the cycle of the reversible chemical hydrogen storage from dehydrogenated products derived from ammoniaborane.127 II.2.1 Influence of the substitution pattern of the amine. In 1968, Brown et al. showed that primary amine-boranes, when dehydrogenated, would lead to trimerization.135 More recently, Framery and Vaultier studied further the influence of the substitution pattern in the thermolysis of primary aminoborane.136 In 2009, Ogino et al. showed a more in depth study of the comparison of the dehydrocoupling reactions of primary- and secondary-amine borane adducts.137 Although his study was mainly focused at the group 6 carbonyl complexes, it was shown that dehydrocoupling of secondary amine-boranes leads to dimers, whereas primary amine-boranes generally form borazine (Scheme II-24). Scheme II-24. Dehydrogenation reactions of a) dimethylamine-borane forming the dimethylaminoborane dimer (top) and b) methyl- or ethylamineborane mainly forming the corresponding borazine. 75 Dehydrocoupling versus boron transfer reaction In light of the results with secondary amines, and since no hydrogen is produced during the reaction, the scope of the reaction was naturally extended to primary amines. II.2.2 Reactions using primary amines For practical reasons, n-pentylamine was chosen to start this study for its moderate boiling point that enable heating conditions if required compared to lighter analogues that are more volatile compounds. Table II-1. Boiling point of several primary amines Amine Boiling Point (°C) Methylamine 6.7 Ethylamine 16.6 n-Propylamine 49.5 n-Butylamine 77.8 n-Pentylamine 104 The reaction with pentylamine and the diisopropylaminoborane (1) was similarly performed and monitored as described with diethylamine. The 11B NMR spectrum of the crude reaction mixture is presented in Figure II-18. The two shield signals at -18 and -20 ppm were assigned to pentylamine- borane and diisopropylamine-borane (2), respectively, by comparing their NMR data with authentic samples prepared separately (Figure II-19). The broad signals at ca -5 ppm could be attributed to cyclotriborazanes compounds [nPentNH2-BH2]3 on the basis of the chemical shift136 and compared with an authentic sample prepared by the partial dehydrogenation of pentylamine-borane upon thermal conditions (Figure II-20). 76 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-18. 11B NMR (top) and 11B{1H}(bottom) spectra of the reaction between the diisopropylaminoborane (1) and the pentylamine. 11 1 i Figure II-19. B{ H} spectra of nPentNH2-BH3 (top) , Pr2NH-BH3 (2) (middle) and of the crude reaction mixture (bottom). 77 Dehydrocoupling versus boron transfer reaction 11 Figure II-20. B NMR spectra of an authentic sample of [nPentNH2-BH2]3 (bottom) and the crude of the reaction between the diisopropylaminoborane (1) and the pentylamine (top). The deshielded signal at 27 ppm with a moderate shoulder shows the presence of diaminoborane compounds. Careful examination of the 1H{11B} spectrum confirms this observation and reveals the presence of two diaminoboranes displaying a doublet at 4.22 ppm (3JNH-BH = 9.3 Hz) and a triplet at 4.08 ppm (3JNH-BH = 6.8 Hz) attributed to a R2N-BH-NHR and a RNH-BH-NHR grouping, respectively (Figure II-21). In line of our observations with the reactivity with diethylamine, these signals were attributed to (diisopropylamino)(pentylamino)borane (11) and bis(pentylamino)borane (12) respectively, the second one been produced in larger amount. Our hypothesis was confirmed by performing the reaction under superstoichiometric conditions using 1.5 equiv. of pentylamine. These conditions lead to the complete disappearance of the diisopropylamine-borane (2) and the (diisopropylamino)(pentylamino)borane (11) in the mixture (Figure II-22), and 78 Metal-free synthesis and characterization of functionalized polyaminoboranes maximize the formation of pentylamine-borane and bis(pentylamino)borane (12) (in a 1:1 ratio) at the expense of the compound at -5 ppm. Figure II-21. 1H NMR (bottom) and 1H{11B} (top) NMR enlarged spectra of the crude reaction mixture between diisopropylaminoborane (1) and pentylamine. Scheme II-25. Reaction between the diisopropyalminoborane (1) and the pentylamine. 79 Dehydrocoupling versus boron transfer reaction Figure II-22. 11B NMR (bottom) and 11B{1H} NMR (top) spectra of the reaction between the pentylamine and the diisopropylaminoborane (1) in a 1.5:1 ratio. Figure II-23. Comparison of the 1H{11B} spectra of the reaction between the pentylamine and the diisopropylaminoborane (1) in 1:1 (top) and 1:1.5 ratio (bottom). 80 Metal-free synthesis and characterization of functionalized polyaminoboranes Scheme II-26. Study of the stoichiometric control of the reaction between the diisopropylaminoborane (1) and the pentylamine in a 1:1.5 ratio. The (diisopropylamino)(pentylamino)borane (11) was synthesized following the same reaction than the (diethylamino)(diisopropylamino)borane (3) and could be fully characterized by multinuclear NMR spectroscopy. (Scheme II-27) and (Figure II-24). Scheme II-27. Synthesis of an authentic sample of (diisopropylamino)(pentylamino)borane (11). Upon closer investigation, we realized that the broad signal at δ -6 ppm in 11B with an important shoulder could result from a mixture of different sets of conformers (Figure II-25). 81 Dehydrocoupling versus boron transfer reaction Figure II-24. Comparison between the 1H{11B} NMR of a) bis(pentylamino)borane (12) (bottom), b) (diisopropylamino)(pentylamino)borane (11) (middle) and c) the crude of the reaction between the diisopropylaminoborane (1) and the penthylamine in a 1:1 ratio (top). Figure II-25. Illustration of the four possible cyclotriborazanes isomers; (a) stands for axial and (e) stands for equatorial positions. Based on the results of this study and on the reactivity of secondary amines, we were able to propose the following mechanism involving similar transformations (Scheme II-28). 82 Metal-free synthesis and characterization of functionalized polyaminoboranes Scheme II-28. Proposed mechanism for the reaction between the pentylamine and the diisopropylaminoborane (1). The proposition of this mechanism was studied using a similar theoretical approach, confirming our hypothesis as illustrated in Figure II-26. 83 Dehydrocoupling versus boron transfer reaction Figure II-26. Energy profile calculated (B3PW91) for the reaction at room temperature of the pentylamine and the diisopropylaminoborane (7). 84 Metal-free synthesis and characterization of functionalized polyaminoboranes II.2.3 Reaction with methylamine At room temperature and 1 atm, methylamine is a gas (Teb = 6.7°C). For this reason, the reaction was conducted in a Fisher-Porter bottle containing neat diisopropylaminoborane (1) under a methylamine atmosphere (1 atm) at 0 °C to favor its condensation in the reaction medium. Soon after the beginning of the reaction, a biphasic system formed with a white solid at the surface preventing the remaining diisopropylaminoborane (1) in the bottom phase to react further with the amine as evidenced by 11B NMR spectroscopy with the presence of a characteristic triplet at 35 ppm ( Figure II-27). Figure II-27. 11B NMR spectrum of the reaciton between the diisopropylaminoborane (1) and methylamine. The reaction was repeated at a higher stirring rate, in order to make a vortex and favor the reaction. In addition to the expected products, a white solid of poor solubility in most organic solvents was again formed, preventing any direct characterization by standard in solution techniques. 85 Dehydrocoupling versus boron transfer reaction A two steps separation was performed including first a trap-to-trap distillation of the crude, and second the separation of the solid in the remaining mixture. NMR analysis of the volatiles and of the filtrate led us to identify of the species present at the end of the reaction (Figure II-28) by comparison with authentic samples prepared separately (Figure II-29 and Figure II-30). Figure II-28. 11B NMR spectrum of the volatiles. The deshielded doublet at δ(11B) 28 ppm excludes the presence of N,N,N- trimethylborazine (δ(11B) 35 ppm) (Figure II-29) and is in agreement with the presence of bis(dimethylamino)borane (MeNH)2BH as reported by Manners et al.138 In their study of the depolymerization of [MeNH-BH2]n with tributylphosphine or in amine- exchange reactions from trimethylamine-alkylthioborane adducts.139 The shielded quartet at -18 ppm is in agreement with the presence of methylamine-borane in the volatiles since it sublimes easily (35°C/1.10-3 mbar) (Figure II-30).140 86 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-29. 11B NMR enlarged spectra of a) volatiles from the reaction between diisopropylaminoborane (1) and methylamine (bottom), b) N,N’,N’’-trimethylborazine (top). Figure II-30. 11B NMR spectra of the methylamine-borane (top) and the doublet found in the volatiles of the reaction bewteen diisopropylaminoborane (1) and methylamine (bottom). 87 Dehydrocoupling versus boron transfer reaction The broad signal at ca -5 ppm is attributed to a mixture of N,N',N"- trimethylcyclotriborazane by comparison with authentic samples prepared separately (Figure II-31). However, the poorly resolved signal at the 11B NMR of the crude would suggest the presence of additional homologous species or oligomers of higher mass.138 Figure II-31. 11B NMR spectrum of a) the filtrate (bottom), b) Mixture of synthesized e-e-e and e-e-a N,N',N"-trimethylcyclotriborazane isomers (middle), and c) Pure e-e-e N,N',N"- trimethylcyclotriborazane isomer (top). Since the remaining solid presented very low solubility, its characterization was performed using infrared spectroscopy. It showed the presence of vibration bands for the BH2 at 2300 cm-1, CH at 2950 cm-1 and NH at 3271 cm-1 (Figure II-30). By comparison with reported data,33,107,140 the structure of the solid can be assigned in first instance to poly(methylamino)borane. 88 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure II-32. Infrared spectrum of the insoluble solid found in the reaction between diisoproplaminoborane and methylamine Figure II-33. Infrared spectra of a) ammonia-borane (top), b) poly(ammonia)borane (middle) and c) poly(methylamino)borane (bottom), reproduced from reference [33]. 89 Dehydrocoupling versus boron transfer reaction Figure II-34. Infrared spectrum of ammonia-borane (top), poly(ammonia)borane (middle) and polymethylaminoborane (bottom), reproduced from reference [140]. As a conclusion, one can say that the reaction between the diisopropylaminoborane (1) and the methylamine at 0 °C leads to similar distribution of boron-containing compounds except diisopropylamine-borane (2) and the unsymmetrical (diisopropylamino)(methylamino)borane compared the reaction with penthylamine. The formation of cyclotriborazane has been clearly demonstrated legitimizing the proposed reaction mechanism. Scheme II-29. Reaction between the diisopropylaminoborane (1) and methylamine. These interesting results, especially the metal-free formation of the inorganic polymethylaminoborane polymer from the gaseous methylamine and the diisopropylaminoborane (1) raised the question of the scope and limitation of the 90 Metal-free synthesis and characterization of functionalized polyaminoboranes method that could lead to unprecedented compounds without the use of expensive metal-based catalysts. This particular aspect of the reactivity will be presented and discussed in the following chapters. 91 Dehydrocoupling versus boron transfer reaction II.3 Experimental section Unless otherwise mentioned, the experiments were performed using either standard Schlenk tube or glovebox techniques. The solvents used were dry and oxygen-free collected from a solvent purification system MBraun SPS-800 Series. THF was dried using sodium and benzophenone under reflux. The complexes [Ru(COD)(COT)]141, [RuH2(η2-H2)2(PCy3)2]142, [RuH2(η2:η2-H2BNiPr2)]117, iPr2NH-BH3,126 iPr2N-BH2,126 Et2NH-BH3,133 MeNH2-BH3,102 MeNHCH2CH2NH(BH3)Me,122 N,N'- dimethyl-1,3,2-diazaborolidine,122 were synthesized according to literature procedures. NMR samples of sensitive compounds were all prepared in the glovebox, using NMR tubes fitted with Teflon speta, J. Young or pressurizable NMR tubes. NMR spectra were obtained on Brucker AV 300 (1H 300.13 MHz, 13C 75.48 MHz, 31P 121.50 MHz), AV 400 (1H 400.13 MHz, 13C 100.61 MHz, 11B 128.38 MHz, 31P 161.98 MHz) or AV 500 (1H 500.33 MHz, 13C 125.81 MHz, 11B 160.53 MHz, 31P 202.55 MHz) spectrometers. References were taken as described in the literature. 1H and 13C chemical shifts are referenced internally to residual protio-solvents. 31P and 11B are relative to 85 % H3PO4 and BF3·OEt2 external reference respectively. All chemical shifts are given in ppm and coupling constants, in Hertz (Hz). Due to complexity of the spectra and/or overlaps, and because of the quadrupolar moment of boron, several signals might not be observable. Solid state NMR experiments were performed on a Bruker AVANCE I 300 MHz (7T) or Avance III 600 MHz (14T). IR spectra were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer equipped with a universal ATR accessory for measures of pure compounds in the solid state or on a IRAffinity-1 SHIMADZU spectrometer equipped with a PIKE technologies GladiaATR device. Mass spectrometry experiment was performed at the CRMPO in Rennes on a ThermoFisher Q-Exactive for ESI experiments on polymers, and by the Mass service of the University of Toulouse (Université Paul Sabatier) for (diethylamino)(diisopropylamino)borane. Elemental analyses were performed at the CRMPO in Rennes on a Thermo Fisher Flash 1112 or a Thermo Fisher Icap 6500 Duo analyzer. High moisture- and air- sensitivity of all the complexes and compounds avoid any elemental analysis. 93 Dehydrocoupling versus boron transfer reaction SEC chromatograms were performed at the Ecole National Supérieure de Chimie de Rennes on a Viscotek VE1122 delivery system, using a flow rate of 1 mL/min in THF with 0.1 w/w % n-BuNBr, calibrated with PolyCamTM for polystyrene standards (1160 < Mn < 102000 Daltons), using a VE3580 RI Detector at 40 °C with an injection concentration of 1 mg/mL. The columns used were ChromTeck 40 °C; T Guard, Org. Guard column 10x4.6mm followed by LT5000L, mixed medium, Org. 300x7.8mm; Diethylamine (commercially available): NMR data: 1. 1H (Tol-d8; 298 K; 400.16 MHz): δ 2.50 (q, 4H, CH2), 0.98 (t, 6H, CH3), 0.33 (s, 1 H, NH); 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 44.76 (s, 2C, CH2), 16.08 (s, 2C, CH3); Diethylamine-borane: 1. 1H (Tol-d8; 298 K; 400.18 MHz): δ 3.28 (s broad, 1H, NH), 2.45 (m, 2H, CH2), 2.33 (m, 2H, CH2), 1.85 (q, 3H, BH3), 0.91 (m, 6H, CH3); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ -16.03 (q, 1JHB = 97 Hz); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 48.99 (s, CH2), 11.62 (s) (CH3). N,N-diethylcyclodiborazane: 94 Metal-free synthesis and characterization of functionalized polyaminoboranes Diethylamine-borane (16.814 g, 193 mmol) is weighted and left under stirring at 140 ºC until evolution of gas (H2) has ceased. The crude product was purified by trap-to- trap distillation affording N,N-diethylcyclodiborazane in almost quantitative yield. NMR data: 1. 1H (Tol-d8; 298 K; 400.16 MHz): δ 2.69 (q, 8H, CH2), 0.80 (t, 12H, CH3) 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 2.45 (t, 1JHB = 112 Hz) 3. 13C{1H}dep (Tol-d8; 298 K; 100.63 MHz): δ 51.28 (s, 4C, CH2), 9.87 (s, 4C, CH3). Infrared spectrum 95 Dehydrocoupling versus boron transfer reaction Bis(diethylamino)borane: A mixture of N,N-diethylaminoborane (0.414 g, 2.432 mmol) and diethylamine (0.5 mL, 4.833 mmol) was heated neat in a Fisher-Porter bottle at 130°C for 24h. Bis(diethylamino)borane was purified by trap-to-trap distillation and obtained in almost quantitative yield. NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.18 MHz): δ 4.02(s, BH), 2.96 (q, 8H, CH2), 1.02 (t, 12H, CH3); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 28.87 (d, 1JHB = 125.82 Hz, BH); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 44.8 (s, CH2), 17.37 (s, CH3); Diisopropylamine (commercially available): NMR data: 1. 1H (Tol-d8; 298 K; 400.16 MHz): δ 2.79 (sep, 1H, CH), 0.95 (d, 6H, CH3); NH missing 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 44.93 (s, 2C, CH2), 23.30 (s, 4C, CH3). 96 Metal-free synthesis and characterization of functionalized polyaminoboranes Diisopropylamine-borane: NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.18 MHz): δ 2.70 (m, 2H, CH), 2.61 (s broad, 1H, NH), 1.87 (s, 3H, BH3), 0.97 (d, 6H, CH3), 0.86 (d, 6H, CH3); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ -20.17 (q, 1JHB = 97.58, BH3); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 52.13 (s, CH), 21.01 (s, CH3), 19.20 (s, CH3); Infrared 97 Dehydrocoupling versus boron transfer reaction Diisopropylaminoborane NMR data: 1 11 1. H{ B} (Tol-d8; 298 K; 400.16 MHz): δ 4.58 (s, 2H, BH2), 3.20 (sep, 2H, CH), 1.05 (d, 6H, CH3); 2. 11B (Tol-d8; 298 K ; 128.39 MHz): δ 35.48 (t, 1JHB = 127.11 Hz); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 53.02 (s, 2C, CH), 25.89 (s, 4C, CH3); (Diisopropylamino)(diethylamino)borane : In a Schlenk tube, a cold (-8°C) solution of diethylamine (1.5 mL) in anhydrous diethyl ether (15 mL) was prepared and 9 mL of butyllithium (1.6 M in hexanes) were added. After 3 h, removal of the volatiles under vacuum afforded lithium diethylamide as a white pyrophoric solid. After its solubilization in 10 mL of anhydrous diethyl ether and cooling at -78°C, (diisopropylamino)(chloro)borane (2g, 13.56 mmol) diluted in 15 mL of diethylether were added and the Schlenk washed with an additional volume of diethyl ether (5 mL). After 30 min, the cold bath was removed. The mixture was left under stirring overnight at room temperature. After removal of the volatiles under vacuum, the compound was purified by trap-to-trap distillation as a colorless liquid (1.474 g, 79%), that turned green after some weeks. 98 Metal-free synthesis and characterization of functionalized polyaminoboranes NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.18 MHz): δ 4.17, (s, 1H, BH), 3.40 (m, 2H, CH), 2.98 (q, 4H, CH2), 1.13 (d, 12H, CH3 - isopropyl), 1.03 (t, 6H, CH3 – ethyl). 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 27.93 (d, 1JHB = 122 Hz, BH). 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 45.77 (s, CH), 45.65 (s, CH2), 25.26 (s, CH3 – isopropyl), 17.35 (s, CH3 – ethyl). HRMS DCI-CH4 [M-H]- calcd. For C10H24BN2 : m/z 183.2038. Found 183.2040 (1.1 ppm). Infrared spectrum 99 Dehydrocoupling versus boron transfer reaction N,N’-dimethylethylenediamine (commercially available) NMR data: 1. 1H (Tol-d8; 298 K; 400.16 MHz): δ 2.48 (s, 4H, CH2), 2.25 (s, 6H, CH3), 0.91 (s, 2H, NH); 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 51.51 (s, CH2); 36.16 (s, CH3); N,N’-dimethylethylenediamine borane adduct NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.18 MHz): δ 4.54 (s, 1H, NHBH3), 4.54 (s, 1H, NHBH3), 2.56 (m, 2H, CH2), 2.20 (m, 2H, CH2), 2.12 (d, 3H, CH3), 2.07 (s, 3H, CH3), 1.90 (q, ?, BH3), 0.52 (s, 1H, NH); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ -14.33 (q, 1JHB = 96.29 Hz, BH3); 3. 13C{1H} (Tol-d8; 298 K; 100.64 MHz): δ 56.03 (s, C( 2)H2), 47.38 (s, C(3)H2, 42.20 (s, C( 1)H3, 36.28 (s, C(4)H3). 100 Metal-free synthesis and characterization of functionalized polyaminoboranes Infrared spectrum 101 Dehydrocoupling versus boron transfer reaction N,N’-dimethylethylenediamine diborane adduct In a schlenk tube containing a cold (-20°C) diethyl ether (15 mL) solution of purified diamine (2 mL, 18.58 mmol), 3.6 mL (37.96 mmol) of borane dimethylsulfide were added drop by drop. The solution became first solid, but turned into a liquid upon warming at room temperature. After 4 h of stirring and removal of the volatiles under vacuum, the title compound was isolated as a light white powder. The powder was washed with toluene and pentane affording a dry powder (2 g, 93%) of poor solubility, after drying. NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.16 MHz): δ 2.11 (s, 2H, NH), 1.84 (s, BH3), 1.83 (d, BH3) , 1.81 (d, 6H, CH3), 1.76 (d, 4H, CH2); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ -15.18 (q, JHB = 100.14 Hz, BH3); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 54.09 (s, CH2), 52.51 (s, CH2), 43.62 (s, CH3), 43.18 (s, CH3) – [Signals from the diastereomers]. 102 Metal-free synthesis and characterization of functionalized polyaminoboranes Infrared spectrum: N,N'-dimethyl-1,-3,-2-Diazaborolidine : NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.18 MHz): δ 3.98 (s, BH), 2.97 (s, 4H, CH2), 2.59 (s, 6H, CH3); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 29.23 (d, JHB = 141.23 Hz, BH); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 52.86 (s, CH2), 35.25 (s, CH3). 103 Dehydrocoupling versus boron transfer reaction Infrared spectrum N,N’-dimethylethylenediaminoborane dimer Synthesis: Product in situ characterized from the stoichiometric reaction by addition of the N,N'-dimethylethylenediamine to diisopropylaminoborane in toluene at room temperature. NMR data: 1. 11B (Tol-d8; 298 K; 128.39 MHz): δ 0.01 (t, BH2 center), -11.65 (q, BH3 tail) 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 58.73 (s, CH2), 53.91 (s, CH2), 53.18 (s, CH2), 50.91 (s, CH2), 48.83 (s, CH3), 43.45 (s, CH3), 43.16 (s, CH3), 38.57 (s, CH3); 104 Metal-free synthesis and characterization of functionalized polyaminoboranes n-Pentylamine (commercially available) NMR data: 1. 1H (Tol-d8; 298 K; 400.16 MHz): δ 2.49 (t, 2H, CH(1)2), 1.23 (m, 6H, CH2), 0.85 (t, 3H, CH3), 0.63 (s, 2H, NH2); 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 43.10 (s, C(1)H2), 34.67 (s, C(2)H2), 29.98 (s, (3) (4) C H2), 23.46 (s, C H2), 14.68 (s, CH3). Infrared Spectrum 105 Dehydrocoupling versus boron transfer reaction n-Pentylamine borane adduct Synthesis: Pentylamine-borane was obtained following the same procedure than for diisopropylamine-borane126 NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.16 MHz): δ 2.84 (s, 2H, NH2), 2.27 (m), 1.94 (s, BH3), 1.21 (m), 1.04 (m), 0.91 (m), 0.77 (m) 2. 1B (Tol-d8; 298 K; 128.39 MHz): δ -18.75 (br q, 1JHB = 84.74 Hz, BH3); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 49.23 (s, C(1)H2), 29.35 (s, C(2)H2), 29.25 (s, C(3)H2), 22.95 (s, C(4)H2), 14.39 (s, CH3); Infrared spectrum 106 Metal-free synthesis and characterization of functionalized polyaminoboranes (Pentylamino)(diisopropylamino)borane: Synthesis: To a 50 mL schlenk flask, 10 mL of diethyl ether and 1 mL of pentylamine were transferred. The flask was cooled down until -50 °C and left under high stirring, when 5.4 mL of a 1.6 M solution of butyllithium was added, turning the solution into a viscous white liquid. After 45 min, the cold bath was removed and the reaction was left under stirring for more 3 h. Within a second cold bath (-78 °C), 1.722g of the (diisopropylamino)(chloro)borane were added drop by. After 1h, the suspension was filtered to remove the LiCl and the volatiles removed under vacuum, keeping the filtrate at low temperature with a cold bath to prevent removal of the desired product. This strategy was repeated until the compound was isolated pure as a volatile oil. NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.16 MHz): δ 4.20 (d, 1JHB = 9.2 Hz, 1H, BH), 3.14 (hept, 2H, CH iPr), 2.95 (dt, 2H, C(1)H2), 2.31 (br s,1H, NH), 1.18-1.37 (m, 6H, CH2 C(2)~C(4)), 1.07 (d, 12H, CH3 iPr), 0.87 (t, 3H, CH(5)3); 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 27.82 (d, 1JHB = 122 Hz) 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 47.11 (s), 46.23 (CH iPr), 35.42, 29.76 (C(3)), 24.70 (broad, CH3 iPr) 23.42 (C(4)), 14.78 (CH3). 107 Dehydrocoupling versus boron transfer reaction Infrared spectrum N,N',N"-tri-N-pentylborazine Synthesis: In a Fischer-Porter bottle, the pentylamine-borane (0.451 g, 4.465 mmol)) was heated at 135 ºC for 20 h leading to the corresponding borazine (0.345g, 1.186 mmol) in 79.7% isolated yield without further purification. 108 Metal-free synthesis and characterization of functionalized polyaminoboranes NMR data: 1. 1H{11B} (Tol-d8; 298 K; 400.16 MHz): δ 4.72 (s) (B)3.32 (t) (CH2-N), 1.53 (quintuplet), 1.28 (m), 0.86 (t) (CH3) ; 2. 11B (Tol-d8; 298 K; 128.39 MHz): δ 33.57 (s broad); 3. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 52.16, 36.00, 29.66, 23.38, 14.72 (CH3). Infrared spectrum Ammonia Borane (commercially available): NMR data: In solution: 1. 1H (CDCl3; 298 K; 400.16 MHz): δ 3.36 (t, NH, 3H), 1.61 (q, BH, 3H); 2. 1H{11B} (CDCl3; 298 K; 400.16 MHz): δ 3.36 (t, NH), 1.61 (t broad); 3. 11B (CDCl3; 298 K; 128.39 MHz): δ -21.50 (q, BH); 4. 11B{1H} (CDCl3; 298 K; 128.39 MHz): δ -21.52 (s, BH); 109 Dehydrocoupling versus boron transfer reaction Solid state NMR: 1. 1H MAS (303 K; 399.6 MHz): δ 6.19, 1.02; 2. 1H/15N (303 K; 40.49 MHz): δ 16.15; 3. 11B{1H} (298 K; 128.21 MHz): δ 0.36, -24.93, -28.37; Infrared 110 Metal-free synthesis and characterization of functionalized polyaminoboranes Poly(aminoborane) Synthesis: To a Fischer-porter bottle in the glovebox, 10 g of diisopropylaminoborane was added. Then, the bottle was connected to a three-way line, cool to -40 °C, and the system first purged with a flux of ammonia. Under high stirring, the reaction was then performed under an ammonia pressure of 5 bars for 5 minutes. After warming to room temperature and removal of the volatiles under vacuum, a white solid was isolated, weighting more than the theoretical 100 % yield. The solid was then washed with diethylether and then with dry acetontrile affording a white solid in 74% yield (1.712g), identified as the polyaminoborane. Analysis of the oil resulting from the filtrate after removal of the volatile reveals the presence of diisopropylamine-borane as the main byproduct. Solid state NMR: 1. 1H MAS (303 K; 399.6 MHz): δ 2.69, 0.54; 2. 1H/15N (303 K; 40.49 MHz): δ 28.15, 16.86; 3. 11B{1H} (Tol-d8; 298 K; 128.21 MHz): δ -13.99, -21.98; 111 Dehydrocoupling versus boron transfer reaction Infrared before washing: superimposition of 3 batches prepared separately. Infra-red after washing with diethylether and acetonitrile: no vibration band in the 2950 cm-1 region. 112 Metal-free synthesis and characterization of functionalized polyaminoboranes Elemental analysis: Element Theoretical 1° Analyses 2° Analyses 3° Analyses C 0 3.8 3.76 3.78 H 13.98 13.28 13.15 13.22 N 48.55 42.94 42.29 42.62 B 37.47 Methylamine Borane Synthesis: Methylamine-borane was synthetized according to published procedure (C. A. Jaska, K. Temple, A. J. Lough and Ian Manners, J. Am. Chem. Soc., 2003, 125, 9424). NMR data: 1. 1H (CDCl3; 298 K; 400.18 MHz): δ 3.56 (t, NH, 2H), 2.58 (t, CH, 3H), 1.54 (q, BH, 3H); 2. 1H{11B} (CDCl3; 298 K; 400.16 MHz): δ 3.56 (t, NH), 2.58 (t, CH), 1.54 (s, BH); 3. 11B (CDCl3; 298 K; 128.39 MHz): δ -18.03 (q, BH); 4. 11B{1H} (CDCl3; 298 K; 128.39 MHz): δ -18.02 (s, BH); 5. 13C{1H} (CDCl3; 298 K; 100.64 MHz): δ 35.00 (s). 113 Dehydrocoupling versus boron transfer reaction Infrared 1(e),3(e),5(e)-N,N',N"-Trimethylcyclotriborazane (Isomer 1) Synthesis: Methylammonium chloride (44.632g, 661 mmol) was gradually added to a THF solution (150 mL) of sodium borohydride (25g, 660.8 mmol) under argon, at room temperature. The mixture was left under stirring for 72h, until the evolution of H2 has ceased. The solution was then filtered and the solid washed with dry THF (3 x 50 mL) of dry THF. Drying under vacuum resulted in a pasty oil, which was washed with pentane (3 x 50 mL) and dry again to give a gel-like solid. This gel-like solid was in turn heated at 70 °C for 30h (6h per day and the evolution of the reaction monitored by 11B NMR). The products were finally filtered and washed pentane (8 x 50 mL) concentrating Isomer I in the solid (less soluble), the filtrate containing a mixture of both isomers. 114 Metal-free synthesis and characterization of functionalized polyaminoboranes The solid containing a major amount of isomer I was purified by sublimation under vacuum (Isomer 2 : Tsub = 50°C – Isomer I : Tsub = 70°C). Isomer I could be obtained pure enough for NMR identification. Selected NMR data: 1. 11B (Tol-d8; 298 K; 128.39 MHz): δ 4.76 (t, 1JBH = 115 Hz, BH2) 2. 13C{1H} (Tol-d8; 298 K; 100.63 MHz): δ 35.66 (s, CH). Infrared: 1(e),3(e),5(a)-N,N',N"-Trimethylcyclotriborazane (Isomer 2) 115 Dehydrocoupling versus boron transfer reaction Selected NMR data: 1. 11B (CDCl3; 298 K; 128.39 MHz): δ -4.72 (t, 1JBH = 107 Hz, BH2), -5.40 (t, 1JBH = 95 Hz, BH2) 2. 13C{1H} (CDCl3; 298 K; 100.63 MHz): δ 37.50 (s, CH3), 37.12 (s, CH3) Infrared 116 Metal-free synthesis and characterization of functionalized polyaminoboranes Poly(methylaminoborane) Synthesis: A Fischer-porter bottle containing 1 mL of diisopropylaminoborane was connected to a three-way line and cooled down with ice (0 °C). The system was first purged with a flux methylamine. Under high stirring, the reaction was then performed under a methylamine pressure for 5 minutes. After warming to room temperature and removal of the volatiles under vacuum, the resulting white solid was washed with toluene and pentane affording a white-off solid (0.1995 g) in 75.2% yield. The obtained polymer remains contaminated by traces of methylamine-borane detected by in solution NMR performed in DMSO-d6. Solid state NMR: 1. 1H (303 K; 399.60): δ 0.53 2. 13C CPMAS Vr = 10KHz (303.0 K; 100.49 MHz): δ 35.5 3. 13C REESE MAS Vr = 10KHz (303.0 K; 100.49 MHz): δ 35.5 4. CP 1H/15N (303.0 K; 40.49 MHz): δ 15.3 5. 11B{1H} MAS Vr = 10 kHz (303.0 K; 128.21 MHz): δ -9.7, -20.17 (minor impurity). 117 Dehydrocoupling versus boron transfer reaction Infrared Elemental Analysis: Element Theoretical 1° Analyses 2° Analyses 3° Analyses C 28.01 28.48 28.69 28.59 H 14.11 13.97 13.95 13.96 N 32.67 31.92 31.55 31.74 B 25.21 - - - 118 Metal-free synthesis and characterization of functionalized polyaminoboranes Trimethylborazine Synthesis: A Fischer-Porter bottle containing methylamine-borane (0.8342 g, 18.58 mmol) was heated (100 °C) for 3h while connected to a bubbler. The paste turned into a white solid upon heating (probably the mixture of the cyclotriborazane isomers) with loss of dihydrogen. Then, the system was closed and heated to 160 °C for 3h. Trimethylborazine was purified by trap-to-trap distillation (Eb = 130°C) as an colorless oil (0.278g, 37%). NMR data: 1. 1H{11B} (CDCl3; 298 K; 400.18 MHz): δ 4.47 (s, BH), 3.05 (s, CH3); 2. 11B (CDCl3; 298 K; 128.39 MHz): δ 33.5 (d, BH, 1JBH = 135 Hz) 13 1 3. C{ H} (CDCl3; 298 K; 100.63 MHz): δ 38.29. 119 Dehydrocoupling versus boron transfer reaction Infrared 120 Chapter III: Investigation of the polymerization reaction Investigation of the polymerization reaction The reaction between the methylamine and diisopropylaminoborane (1) at low temperature produces poly(methylamino)borane. In comparison with the reactions performed before and involving primary or secondary amines, this is the first time the formation of a polymer is clearly observed. Further investigation of the reaction was undertaken to explain these results. The influence of the temperature on the outcome of the reaction was first studied. III.1 The role of the temperature Among the primary amines, butylamine was selected because of its boiling point of 78 °C and its low melting point of -49 °C. Its physical properties allow it to remain liquid both at room and at relatively low temperature under solventless conditions. The outcome of the solventless reaction at room temperature (Figure III-1) followed the pattern already shown for the primary amines with formation of B(sp2)H- , B(sp3)H2- and B(sp3)H3- containing species at ca 30 ppm, -5 ppm and -20 ppm, respectively. Figure III-1. 11B (left) and 11B{1H} (right) NMR spectra of the solventless reaction between the diisopropylaminoborane (1) and butylamine at room temperature and at different ratios: a) 1:1 (top); b) 1:2 (middle) and c) 1:4 (bottom). 123 Investigation of the polymerization reaction Performing the same reaction neat at low temperature led to a different reaction outcome. With a stoichiometric amount of n-butylamine at -40 °C, the formation of a white precipitate was initially observed to end up with a gel-like solution. The system was kept at low temperature for 2 h, then slowly warmed and left at room temperature for 20 h. An aliquot was taken and the crude reaction analyzed by 11B NMR spectroscopy in solution in CDCl3. Boron-containing species similar to the ones obtained in the reaction with the diethylamine could be detected in the solution (Figure III-2). Figure III-2. 11B (bottom) and 11B{1H} NMR (top) spectra of the reaction between the diisopropylaminoborane (1) and butylamine at low temperature. After evaporation of the volatiles under vacuum and workup, a white plastic like solid was obtained in 78% yield. Insoluble in most common organic solvents, it showed a poor solubility in diethylether and THF preventing any in solution NMR analysis for which blind spectra are obtained. Infrared analysis (Figure III-3) was carried and showed vibration bands at the same zones as for poly(methylamino)borane but with vibration bands in the CH (CH 124 Metal-free synthesis and characterization of functionalized polyaminoboranes = 2960 cm-1) region proportionally stronger when compared to the NH region (NH = 3250 cm-1). This feature is in agreement with the presence of the butyl substituent displaying a greater number of CH bonds than a methyl. Figure III-3. Infrared of poly(butylamino)borane III.2 Evidence of polymer formation The evidence of the polymeric nature of the isolated solid material was given by the SEC analysis of the sample. Although this technique requires soluble samples, this was circumvented by the addition of the tetrabutyl ammonium bromide that increases the solubility of the polymer. These conditions were already used by Manners et al. for the SEC analysis of poly(aminoborane)s obtained under catalytic conditions from amine-boranes.107 The polystyrene was used as a standard and the solubilization of the samples were performed by stirring 1 mg of the polymer in 1 mL of THF containing 0.1 w/w % of nBu4NBr for 1 day. The stability of the poly(butylamino)borane was also evaluated 125 Investigation of the polymerization reaction by performing the same analysis after stirring for 7 days of at room temperature (Figure III-4). Figure III-4. SEC analysis of poly(butylamino)borane after stirring for 1 day (black) and 7 days (blue). The SEC analysis shows the formation of two sets of polymers: one with a retention time of 5.7 minutes that results from high molecular weight polymers (2.990× 106 Da) of broad dispersity (Ðn = 5.8), and a second one at 8.8 minutes corresponding to oligomers of lower molecular weight (1289 Da) and moderate dispersity (Ðn = 1.1). One can see the very good stability of the polymer since no change could be observed in the analysis after a stirring of 7 days. In comparison with data from the literature for the only reported example of polybutylaminoborane,107 a ten times greater molecular weight is obtained in our case (3×106 DA vs 4×105 Da).106 To complement the study on the polymeric aspect of the poly(butylamino)borane, the purification of the sample from the oligomeric part could be achieved by dialysis through a semipermeable cellulose membrane. 126 Metal-free synthesis and characterization of functionalized polyaminoboranes After one day of stirring, SEC analysis of the dialysate shows only the presence of oligomers that have diffused through the membrane, while the SEC analysis of the solution contained in the dialysis bag showed a lesser quantity of oligomers (Figure III-5). Figure III-5. SEC graph of the poly(butylamino)borane at short retention time before (blue) and of the dialysate after (black) dialysis. The analysis of the sample was also performed by solid state NMR spectroscopy. The results turned to be also in agreement with the polymeric nature of the isolated material. Best evidence was given by the 13C solid state NMR (Figure III-6). A set of four different broad signals at δ 13.5, 20.2, 29.75 and 49.6 ppm in perfect agreement with the presence of the four different carbons constituting the nButyl group bounded to nitrogen. 127 Investigation of the polymerization reaction Figure III-6. 13C NMR spectrum of the poly(butylamino)borane Electrospray-ionization mass spectroscopy (EI-MS) showed two different polymer distributions for the poly(butylamino)borane (Figure III-7). At low mass/charge, one can see plethora of peaks due to presence of oligomers. At higher mass/charge, one can see two different distribution patterns of different intensity (Figure III-8). The difference of 85.107 between two major distribution peaks is fully in agreement with the theoretical mass of 85.11 for the repeating unit nBuNH-BH2. The difference in mass of 73.092 between a distribution pattern of lesser intensity and the next pattern of higher intensity corresponds to a BuNH2 unit and is indicative of nbutylamine end-groups in the polymer. 128 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-7. EI-MS of poly(butylamino)borane. Figure III-8 Enlarged electrospray-ionization mass spectrum of poly(butylamino)borane. 129 Investigation of the polymerization reaction At this stage of the analyses, one can be confident in the polymeric nature of the isolated material which structure is fully in agreement with poly(butylamino)borane. III.3 The role of the substitution pattern The scope and the limitations of the involved synthesis were evaluated by using different primary amines. The reaction was performed with ammonia (5 bar) at -40°C following the same experimental protocol. A white solid formed instantaneously. After 2 h, the tube was manually shaken to help the homogenization of the medium. Then, a control was made by in solution 11B NMR spectroscopy (Figure III-9) revealing a similar outcome of the reaction. with soluble B(sp2)H-, B(sp3)H2- and B(sp3)H3- containing species at ca 25, -12 and -22 ppm, respectively. Figure III-9. 11B NMR (bottom) and 11B{1H} NMR (top) spectra of the reaction between the diisopropylaminoborane and ammonia. 130 Metal-free synthesis and characterization of functionalized polyaminoboranes After evaporation of the volatiles a white solid was obtained in more than 100 % yield respect to the expected theoretical mass. The infrared spectrum showed the expected BH ( ≈ 2300 cm-1) and NH ( ≈ 3300 cm-1) vibration bands. The two NH bands at 3295 and 3247 cm-1 are characteristic of the poly(aminoborane) NH2 vibrators. Surprisingly, the infrared spectrum also showed CH vibration bands (2900 cm-1) indicating remaining impurities in the polymer (Figure III-10). Figure III-10. Non-volatiles products from the reaction between diisopropylaminoborane and ammonia After several washings with diethylether and drying under vacuum, a second control was performed by in solution 11B NMR spectroscopy. A featureless spectrum was obtained with no peak except the broad signal given by the glass of the tube and the NMR probe. This time, no CH vibration bands could be detected in the infrared spectrum (Figure III-11). Polyaminoborane was finally obtained in 74 % isolated yield. 131 Investigation of the polymerization reaction Figure III-11. Infrared spectrum of the polyaminoborane In order to remove ambiguity for the nature of the obtained polymers, a comparison with the polymers obtained via catalyzed dehydrogenation of amine- boranes was undertaken. The same purification method was repeated for the poly(methylamino)borane and the recorded infrared spectra compared with the infrared of the linear polyaminoborane and poly(methylamino)borane obtained by the use of the Iridium POCOP metal complex.107 A perfect match of the IR spectra was obtained ascertaining the identical nature of the polymers synthetized via the two different methods (Figure III-12). Analogously, thermogravimetric analyses were also conducted and compared with the literature (Figure III-13), more particularly with the polyaminoborane obtained by Manners et al in 2010 (Figure III-14).107 132 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-12. Infrared of the polymers obtained in this work (left) and the BN compounds 107 obtained by Manners et al. : a) ammoniaborane (NH3BH3); b) polyaminoborane; c) pyrolyzed; d) poly(methylamino)borane. While both only display one event with a close inflection point at 190 °C and 205 °C, respectively, our polymer showed 35% weight loss, although more than the 15% theoretic mass of hydrogen, in a close resemblance to the polyaminoboranes obtained by Baumann et al.143 and Geanangel and Wendlandt,144 the polymer synthetized by the iridium complex had 60 % weight loss at a 10 °C/min scan rate. The poly(methylamino)borane was also compared (Figure III-15 and Figure III-16). Again, both have one major event with an inflection around 170 °C, but this time the weight loss was in a closer resemblance, around 70 %. 133 Investigation of the polymerization reaction Figure III-13. Thermogravimetric analysis of the polyaminoboranes produced by metal-free systems. Figure III-14. Thermogravimetric analysis of the polyaminoboranes produced by iridium catalyzed systems. Reproduced from reference [2]. 134 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-15. Thermogravimetric analysis of the poly(methylamino)boranes produced by metal-free systems. Figure III-16. Thermogravimetric analysis of the poly(methylamino)boranes produced by iridium catalyzed systems. Reproduced from reference [106]. 135 Investigation of the polymerization reaction III.4 Proof of polymer formation The Table III-1 shows a good match for elemental analyses between the theoretical and the experimental mean values for polymers containing NH2, CH3NH and nBuNHBH2 repeating units. Table III-1. Theoretical and mean values for the polymers containing the specific repeating motive. NH2BH2 MeNHBH2 nBuNHBH2 Element Theoretical Mean Theoretical Mean Theoretical Mean C 0.00 3.78 28.01 28.59 56.55 54.75 H 13.98 13.22 14.11 13.96 14.24 14.05 N 48.55 42.62 32.67 31.74 16.49 16.20 The synthesis was further extended to ethyl and propyl amines, affording the corresponding poly(ethylamino)- and poly(propylamino)boranes in similar yield (around 75 %). The formation of copolymers was also undertaken by preliminarily mixing the desired amines in given ratio before the addition into the cold diisopropylaminoborane (1). The reaction generalization is showed at the Scheme III-1. Scheme III-1.Generic method for the copolymerization of aminoboranes 136 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-17. 13C NMR spectra of poly(alkylamino)boranes: methyl (red), ethyl (green), propyl (blue), butyl (purple). III.5 Mechanistic considerations The computational calculations presented in the former chapter in the case of the pentylamine are repeated at Figure III-18 for clarity. 137 Investigation of the polymerization reaction Figure III-18 Energy profile of the reaction at room temperature of the pentylamine and the diisopropylaminoborane At low temperature, the monomers formed in situ by BH2-transfer reaction from the diisopropylaminoborane (1) to the introduced pentylamine do not have enough energy to cross for the trimerization energy barrier. The formation of the pentylamine- pentylaminoborane adduct is kinetically favored and this adduct can react in turn with another pentylaminoborane monomer to give a linear dimer. This iterative Lewis acid-base association with a new pentylaminoborane monomer generated in the medium is favored at low temperature and constitutes the chain growth mechanism producing a pentylamine terminated polymer which is in agreement with the EI-MS analyses. 138 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-19. Energy profile of the reaction at low temperature of the pentylamine and the diisopropylaminoborane (1). 139 Investigation of the polymerization reaction Interestingly, this kind of reaction was proposed in 1970, by McGee and Kwon, when they submitted borazine to radiofrequency in a low-pressure -196 °C system.114 In this work, the reactive monomer can be easily generated providing a general synthesis of poly(alkylamino)boranes with the diispropylaminoborane (1) as the BH2 source. III.6 Overview of the linear poly(alkylamino)boranes In this section, a summary of the characterization methods used at the polymers will be presented. Figure III-20 and Figure III-21 show the infrared spectra of the homopoly(alkylamino)boranes and copoly(alkylamino)boranes, respectively. One can see the NH, CH and BH vibrations bands at the 3200 cm-1, 2950 cm-1 and 2300 cm-1 zones, respectively. Contrarily to the primary amine polymer derivatives [RNH-BH2]n that contains only one NH, the polyaminoborane [NH2-BH2]n shows two NH vibrations bands due to the presence of the second hydrogen at the nitrogen, in accordance with the literature data.145 One can also note the expected absence of CH vibration bands contrarily to the others poly(alkylamino)boranes. 140 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-20. Infrared spectra of the homopoly(alkylamino)boranes Figure III-21. Infrared spectra of the copoly(alkylamino)boranes and polyaminoborane. 141 Investigation of the polymerization reaction The gel permeation technique (Figure III-22 and Figure III-23) shows an increase of the average molecular weight of the samples in a direct relation to the substitution pattern at the nitrogen following the increase of length of the hydrocarbon chain. This can be attributed a) to the higher contribution of the carbon to the weight of the repeating unit and b) because longer carbon chains induce an increased solubility enabling the detection and the analysis of higher mass species that are generally retained during the membrane filtration preceding the injection. Surprisingly, the poly(butylamino)borane showed an average molecular weight 10 times higher than the others poly(alkylamino)boranes. The stability of this polymer and of its related copolymers was also evaluated. They showed to be quite stable even after 7 days of stirring in HPLC grade THF, at room temperature, under air conditions. Mw = 198 131 Mw = 218 000 Mw = 351 000 Mw = 2 999 000 Figure III-22. Size-Exclusion Chromatography of the poly(alkylamino)boranes. 142 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure III-23. Size-Exclusion Chromatography of the poly(alkylamino)boranes. 13C solid state NMR of the copolymers was performed and the data gathered in the Figure III-24. The copolymers formed from mixtures of amines display signals in the same region then their related homopolymers. Elemental analyses of the poly(alkylamino)boranes were performed and gathered in the Figure III-25. One can see a very good match between theoretical and experimental results in all cases. However, a slight difference in the quantity of carbon is observed for the copolymers. My hypothesis is that the shorter chains amines are also less hindered favoring the nucleophilic attack at the boron and, resulting in a discrepancy between experimental and theoretical values. 143 Investigation of the polymerization reaction R = Et R = But R = Et; R’ = Pr R = Et; R’ = Bu R = Et; 2R’ = Bu Figure III-24. 13C NMR spectra of the poly(alkylamino)boranes and their related homopolymer. 60 50 40 C theor. 30 C exp. H theor. 20 H exp. N theor. 10 N exp. 0 Figure III-25. Elemental analyses of the poly(alkylamino)boranes and of the copolymers 144 Metal-free synthesis and characterization of functionalized polyaminoboranes III.7 Conclusion With the results obtained and compiled in this chapter, we have clearly showed that the polymerization of alkylaminoboranes is favored at low temperature at the expense of the trimerization and concerted hydrogen transfer processes mainly observed at room temperature. This feature have been ascertained by theoretical calculations that showed the formation of linear products is a lower thermodynamic barrier process and polyaminoboranes are kinetically favored. This synthetic method is efficient (~80%) and can be performed in solventless conditions, without any expensive metal complex, reducing the environment impact and the cost of a possible large-scale application. When considering the use of metal complexes based on ruthenium and iridium for polyaminoboranes synthesis via dehydrocoupling strategies, the syntheses are accompanied of dihydrogen production. Late transition metals are known for their ability to react with dihydrogen and for their use as hydrogenation catalysts. This property hampers the use of unsaturated chain attached to the nitrogen in dehydrooupling of unsaturated amine-boranes, the unsaturated chains being likely to be hydrogenated. As mentioned in the first chapter, unsaturated functions in polymers are of great synthetic interest for post-polymerization functionalization sequences, such as cross and ring-closing metathesis, “click” chemistry and cross-linking condensation. This specific aspect will be further developed and discussed in the following chapter. 145 III.8 Experimental section To a Schlenk tube containing neat diisopropylaminoborane (1) at -40 °C under argon atmosphere, the chosen amine was slowly added under high stirring. Then, homogenization of the medium was achieved with the help of a spatula and the mixture left at low temperature for 2 h, warmed up to room temperature and stirred for 20 h. After that, solvent is added to facilitate the transfer of the mixture (diethylether or acetonitrile, depending on the solubility of the formed polymer), and the products are filtered. The solid is washed usually three times with 15 mL of solvent, and the purity of the solid controlled by solution 11B{1H} NMR. The absence of signal is indicative of a pure solid that is dried under vacuum and weighted for yield calculation. The reactions can be performed under solventless conditions but the presence of diethylether does not modify the outcome of the reaction. 147 Investigation of the polymerization reaction III.8.1.1 Polyaminoborane Infrared Elemental analysis Element Theoretical Experimental C 0 3.78 H 13.98 13.22 N 48.55 42.62 148 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 15N (303.0 K; 40.49 Mhz): 20.77 1N NMR 149 Investigation of the polymerization reaction Size-Exclusion Chromatography Not soluble enough to perform this characterization Mass Spectroscopy: EI-MS Not soluble enough to perform this characterization 150 Metal-free synthesis and characterization of functionalized polyaminoboranes III.8.2 Poly(methylamino)borane Infrared Elemental analysis Element Theorical Experimental C 28.01 28.59 H 14.11 13.96 N 32.67 31.74 151 Investigation of the polymerization reaction Thermogravimetric analysis Solid state NMR Nucleus Peaks: 13C (303.0 K; 100.49 MHz): 31.52 15N (Vr= 7kHz; 303.0 K; 30.42 Mhz): 11.20 152 Metal-free synthesis and characterization of functionalized polyaminoboranes 13C NMR 15N NMR 153 Investigation of the polymerization reaction Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,55E+00 7,43E+04 1,98E+05 5,00E+05 1,35E+05 2,67E+00 8,93E+00 1,07E+03 1,36E+03 1,67E+03 8,19E+02 1,27E+00 154 Metal-free synthesis and characterization of functionalized polyaminoboranes Mass Spectroscopy: EI-MS 155 Investigation of the polymerization reaction 156 Metal-free synthesis and characterization of functionalized polyaminoboranes 157 Investigation of the polymerization reaction III.8.3 Poly(ethylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 42.22 41.73 H 14.17 14.30 N 24.62 24.13 158 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Solid state NMR Nucleus Peaks: 1H (Vr= 12 kHz; 303.0 K; 399.60 MHz): δ -0.77, -2.29, -2.50, -3.83, -5.00; 13C (Vr= 12 kHz; 303.0 K; 100.49 MHz): δ 44.65, 13.19 15N (Vr= 12 kHz; 303.0 K; 30.42 Mhz): δ 27.87; 159 Investigation of the polymerization reaction 1H NMR 13C NMR 160 Metal-free synthesis and characterization of functionalized polyaminoboranes 15N NMR Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,73E+00 8,02E+04 2,18E+05 8,37E+05 8,56E+04 2,72E+00 8,97E+00 9,36E+02 1,25E+03 1,73E+03 7,24E+02 1,34E+00 161 Investigation of the polymerization reaction Mass Spectroscopy: EI-MS 162 Metal-free synthesis and characterization of functionalized polyaminoboranes 163 Investigation of the polymerization reaction 164 Metal-free synthesis and characterization of functionalized polyaminoboranes 165 Investigation of the polymerization reaction 166 Metal-free synthesis and characterization of functionalized polyaminoboranes 167 Investigation of the polymerization reaction III.8.4 Poly(propylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 42.22 41.73 H 14.17 14.30 N 24.62 24.13 168 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 1H (Vr= 20 kHz; 303 K; 399.60 MHz): δ -0.84, -2.40, -2.63, -3.43, -4.15 ; 13C (Vr= 12 kHz; 303.0 K; 100.49 MHz): δ 50.08, 20.69, 11.27 169 Investigation of the polymerization reaction 1H NMR 13C NMR 170 Metal-free synthesis and characterization of functionalized polyaminoboranes Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,58E+00 1,00E+05 3,51E+05 1,48E+06 1,23E+05 3,51E+00 8,85E+00 1,04E+03 1,35E+03 1,81E+03 1,02E+03 1,31E+00 171 Investigation of the polymerization reaction Mass Spectroscopy: EI-MS 172 Metal-free synthesis and characterization of functionalized polyaminoboranes III.8.5 Poly(butylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 56.55 54.75 H 14.24 14.05 N 16.49 16.20 173 Investigation of the polymerization reaction Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 1H (Vr= 12 kHz; 303 K; 399.60 MHz): δ -0.79, -2.36, -2.61, -3.45, -3.75, -4.15, -5.03; 13C (Vr= 12 kHz; 303.0 K; 100.49 MHz): δ 49.72, 29.75, 20.20, 13.50; 174 Metal-free synthesis and characterization of functionalized polyaminoboranes 1H NMR 13C NMR 175 Investigation of the polymerization reaction Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 5,72E+00 5,17E+05 2,99E+06 1,72E+07 1,44E+06 5,78E+00 8,83E+00 1,19E+03 1,29E+03 1,43E+03 1,06E+03 1,08E+00 176 Metal-free synthesis and characterization of functionalized polyaminoboranes Mass Spectroscopy: EI-MS 177 Investigation of the polymerization reaction III.8.6 Poly[(ethylamino)borane-ran-(propylamino)borane] (1:1) Infrared Elemental analysis Element Theorical Experimental C 46.98 41.54 H 14.19 12.97 N 21.91 19.50 178 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 1H (Vr= 7 kHz; 303 K; 600.27 MHz): δ -0.83, -2.40, -3.49, -3.89, -4.19, -5.08 ; 13C (Vr= 7 kHz; 303.0 K; 150.94 δ 56.44, 50.34, 43.41, 21.34, 13.38, 11.58 ; MHz): 179 Investigation of the polymerization reaction 1H NMR 13C NMR 180 Metal-free synthesis and characterization of functionalized polyaminoboranes Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,65E+00 2,47E+04 6,86E+04 1,28E+05 1,03E+05 2,78E+00 8,88E+00 9,65E+02 1,10E+03 1,25E+03 9,14E+02 1,14E+00 181 Investigation of the polymerization reaction III.8.7 Poly[(ethylamino)borane-ran-(propylamino)borane] (1:2) Infrared Elemental analysis Element Theorical Experimental C 48.34 44.20 H 14.20 13.53 N 21.14 19.52 182 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,52E+00 1,85E+05 1,03E+06 4,18E+06 1,41E+05 5,57E+00 8,88E+00 9,45E+02 1,10E+03 1,31E+03 9,23E+02 1,16E+00 183 Investigation of the polymerization reaction 184 Metal-free synthesis and characterization of functionalized polyaminoboranes III.8.8 Poly[(ethylamino)borane-ran-(butylamino)borane] (1:1) Infrared Elemental analysis Element Theorical Experimental C 50.80 48.68 H 14.10 13.73 N 19.43 19.12 185 Investigation of the polymerization reaction Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 1H (Vr= 12 kHz; 303 K; 600.27 MHz): δ -0.86, -2.36, -3.48, -3.74, -3.83, -4.14, -5.03, - 5.10; 13C (Vr= 12 kHz; 303.0 K; 150.94 MHz): δ 49.24, 44.01, 29.45, 20.28, 13.53; 15N (Vr= 7 kHz; 303.0 K; 30.42 Mhz): δ 26.15 186 Metal-free synthesis and characterization of functionalized polyaminoboranes 1H NMR 13C NMR 187 Investigation of the polymerization reaction 15N NMR Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 5,98E+00 2,68E+05 1,04E+06 4,06E+06 6,09E+05 3,86E+00 8,90E+00 9,32E+02 1,04E+03 1,18E+03 8,97E+02 1,11E+03 188 Metal-free synthesis and characterization of functionalized polyaminoboranes 189 Investigation of the polymerization reaction III.8.9 Poly[(ethylamino)borane-ran-(butylamino)borane] (1:2) Infrared Elemental analysis Element Theorical Experimental C 52.95 51.48 H 14.22 13.98 N 18.53 18.99 190 Metal-free synthesis and characterization of functionalized polyaminoboranes Thermogravimetric analysis Solid state NMR: Nucleus Peaks: 1H (Vr= 12 kHz; 303 K; 600.27 MHz): δ -1.09, -2.38, -2.64, -3.52, -3.80, -4.19, -5.05; 13C (Vr= 12 kHz; 303.0 K; 150.94 MHz): δ 49.97, 44.01, 29.59, 20.32, 13.51; 191 Investigation of the polymerization reaction 1H NMR 13C NMR 192 Metal-free synthesis and characterization of functionalized polyaminoboranes Size-Exclusion Chromatography Start RT Mn Mw Mz Mp Ðn (min) 6,08E+00 2,57E+05 7,05E+05 2,07E+06 4,66E+05 2,74E+00 8,92E+00 9,26E+02 1,07E+03 1,31E+03 8,30E+02 1,16E+03 193 Chapter IV: Enhancing the substitution pattern Enhancing the substitution pattern The preparation of functionalized polymers via reliable and reproducible synthetic methods is an actual challenge. In this context, the field of polyaminoboranes remains an area of investigation in which practically no research has been conducted so far, probably because of the lack of adapted syntheses. This chapter is more particularly dedicated to this aspect with the aim of increasing the range organic functions attached to the nitrogen in this family of polymers. This last chapter can be considered as a research perspective for highly functionalized polyaminoboranes. Although this part of the project is on-going and far from being completed, preliminary results and strategies will be presented. IV.1 Formation of poly(alkylamino)boranes bearing unsaturated carbon side chains A first direction concerned the preparation of polyaminoboranes bearing unsaturated carbon chains. In this aim, we first evaluated to possibility of using unsaturated amine (Figure IV-1). Figure IV-1. Proposition of the synthesis of unsaturated polyaminoboranes The allylamine (CH2=CHCH2NH2) was chosen as the prototypical reagent since the only requirement for our reaction to work was the use of a primary amine at low temperature. 197 Enhancing the substitution pattern The reaction followed the same procedure by slowly adding the amine to cold diisopropylaminoborane (1) under stirring, for 2h, warming to room temperature and additional stirring for 20 h followed by the filtration using a polar solvent. Monitoring the crude reaction by in solution 11B NMR spectroscopy showed, as expected, a similar spectroscopic profile (Figure IV-2). After work-up, the average isolated yield of this reaction turned to be slightly lower (ca 60%) compared to other inorganic polyaminoboranes synthesized polymers. Figure IV-2. 11B and 11B{1H} spectra of the reaction between the diisopropylaminoborane (1) and the allylamine at low temperature The formation of the polyaminoborane polymer bearing allyl side chains was ascertained by solid state 13C NMR (Figure IV-3). A comparison between the poly(propylamino)borane and the poly(allylamino)borane clearly shows the presence of the two C(sp2) carbon atoms of the allyl group that resonate in the 120-135 ppm zone, while the poly(propylamino)borane displays more shielded signal around 10-20 ppm. Both have a signal at 50 ppm corresponding to the C(sp3) carbon atom bound to the nitrogen. The thermogravimetric analyses are particularity interesting. Whereas the other linear polyaminoboranes present a slow mass loss in an endothermic event, the poly(allylamino)borane displays a narrow exothermic peak corresponding to a fast energy release. (Figure IV-4). 198 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure IV-3. 13C solid state NMR spectra of the poly(propylamino)borane (top) and the poly(allylamino)borane (bottom). Figure IV-4. DSC (top) and TGA (bottom) analysis of the poly(allylamino)borane The Size-Exclusion Chromatography showed a poorly defined multimodal distribution (Figure IV-4) with the higher molecular mass of 8.26 × 108 Da with 45 % 199 Enhancing the substitution pattern and a dispersity of 3.02. This high molar mass is probably due to reticulation reactions via hydroboration. Figure IV-5. SEC of the poly(allylamino)borane. The presence of the unsaturation can be also ascertained by infrared spectroscopy (Figure IV-6). One can observe the presence of the C=C vibration band at 1645 cm-1 in similar intensity than the one of the =C-H band of the unsaturation at 3080 cm-1, along with the expected NH, C(sp3)H and BH at 3260 cm-1, 2950 cm-1 and at 2392 cm-1, respectively. The formation of poly(allylamino)borane was also investigated by mass spectroscopy showing an experimental mass of 69.0750 for the CH2=CH-CH2-NH-BH2 repeating unit in agreement with the expected theoretical mass. 200 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure IV-6. Infrared spectrum of the poly(allylamino)borane Figure IV-7. EI-MS of poly(allyamino)borane. 201 Enhancing the substitution pattern Enlarging the spectrum (Figure IV-8), one can observe two additional distribution patterns of lower intensity and corresponding to an allyl-NH2 unit and to a H2B-NH(allyl)-BH2 unit that are indicative of the end-groups in the polymer. Figure IV-8. Enlarged EI-MS of poly(allyamino)borane The success of the formation of the poly(allylamino)borane led us to apply the polymerization using the propargylamine displaying a similar structure with a triple bond instead of the double bond. In the infrared spectrum (Figure IV-9), the C≡C bond vibration band are generally observed around 2200 cm-1, with a low intensity. The strong band of the BH at 2300 cm-1 and the atmospheric noise at 2100 cm-1 prevent any clear identification. Nonetheless, one can see one hidden band at 3293 cm-1 and another one at 646 cm-1, both related to the ≡C-H of the propargyl group. 202 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure IV-9. Infrared spectrum of the poly(propargylamino)borane Following the trend of the polyallylamine, the SEC (Figure IV-10) revealed a multimodal distribution with the Mw higher mass detected for the astonishing value of 3.9 × 109 Da and a dispersity of 2.9. The peak a higher quantity of product also presents a very high Mw mass of 1.6 107 Da with a higher Ðn of 7.9. At this stage, these preliminary results need to be confirmed by reproducing the synthesis of this polymer in order to perform complementary SEC analyses. The thermogravimetric analyses (Figure IV-11) showed a behavior similar to the one of poly(allylamino)borane with a fast an intense energy release that happens at 140°C instead of 120 °C for the poly(allylamino)borane. 203 Enhancing the substitution pattern Figure IV-10. SEC of poly(propargylamino)borane Figure IV-11. TGA of poly(propargylamino)borane (red) and poly(allylamino)borane (black) 204 Metal-free synthesis and characterization of functionalized polyaminoboranes Unfortunately, the EI-MS analysis was not conclusive and need to be repeated for ascertaining the structure of the polymer. IV.2 Formation of silicon-containing polyaminoboranes The interesting properties derived from the presence of silicon group in polymers constituted also an attractive direction for project. In this aim, a series of silicon-containing primary amines (Figure IV-12) were also investigated following the same experimental protocol. The main requirement was in the first instance to be liquid at -40 °C. Figure IV-12. List of silicon-containing amines 205 Enhancing the substitution pattern The preliminary results are summarized in (Table IV-1). Table IV-1 Synthesis and characterization of polyaminoboranes derived from silicon- containing amines. Amine entry Ratio Yield Higher Mw Ðn (SEC) (%) (SEC) 1 1:1 75 5.8×109 3.8 2 1:1 56 1.4×107 5.3 3 1:1 54 1.3×107 6.8 4 1:1 34 1.3×107 12.8 5 1:1 75.7 1.6×107 4.6 6 1:1 61.7 1.6×107 13.9 6 2:1 54.5 1.8×107 22.3 At that stage, the results achieved to date are particularly encouraging and the on-going analyses need to be completed for an in-depth discussion of the polymers properties. IV.3 Post-polymerization functionalization approaches In cooperation with the Laboratorio de Reactividade e Catalise da Univerdade do Rio Grande do Sul (Brazil), the polymers containing unsaturated chains were involved in post-polymerization functionalization strategies in order to evaluate their potential. Starting from the poly(allylamino)borane, a metathetic post- functionalization strategy was first envisaged (Figure IV-13). Grubbs CatalystTM 2nd generation146 and Hoveyda-GrubbsTM 2nd generation147 were used for their good capability to perform ring-closing metathesis reactions and/or cross metathesis in biphasic media, envisaged here in solid/liquid conditions due to the low solubility of the inorganic polymer (Figure IV-14).148,149 206 Metal-free synthesis and characterization of functionalized polyaminoboranes Figure IV-13. Representation of the reactions: (a) ring-closing metathesis; (b) cross metathesis Figure IV-14. Involved ruthenium-based catalysts: (a) (1,3-Bis(2,4,6-trimethylphenyl)-2- imidazolidinylidene)dichloro(phenylmethylene)(tricyclohexylphosphine)ruthenium (G2); (b) (1,3-Bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(o- isopropoxyphenylmethylene)ruthenium (HG2). The targeted transformation was performed with different catalyst loadings, solvent and stirring methods. Although the definitive characterization is pending, initial results can be already disclosed such as the formation of ethene, identified by CG-MAS in both reactions from the reaction Schlenk atmosphere and a change at the infrared profile when comparing with the starting polymer (Figure IV-15 and Figure IV-16). 207 Enhancing the substitution pattern Figure IV-15. Infrared spectra of the ring-closing metathesis using the (G2) as catalyst at different loadings: (top) 1 mol%; (middle) 3 mol %; (bottom) poly(allylamino)borane before reaction Figure IV-16. Infrared spectra of the cross metathesis using G2 as catalyst with different conditions: (top) mechanic stirring and 1-decene as olefin; (top-middle) 1-decene as olefin; (bottom-middle) 1-hexen as olefin; (bottom) poly(allylamino)borane before the reaction 208 Metal-free synthesis and characterization of functionalized polyaminoboranes Further analyses are needed to definitely explain and rationalize these preliminary results but beginning of study clearly open the way to the preparation of functionalized polyaminoboranes as a novel family of polymers never studied so far. IV.4 Perspectives In the direction of the post-polymerization functionalization reactions, the future work will be towards the full characterization of the polymers obtained by the reactions of olefin metathesis. The poly(propargylamino)borane will be also investigated in “click” chemistry150 post-polymerization functionalization strategies following the approach depicted in Figure IV-17. Figure IV-17. "Click" chemistry post-polymerization functionalization of poly(propargylamino)borane IV.5 Conclusion The results showed in this chapter showed that the metal-free syntheses of polyaminoboranes can be widely used with a variety of functional amines. The novel polymers presented in this chapter could not be probably obtained by a metal catalyzed dehydrocoupling strategy. This approach enabled us to collect primary results towards post-polymerization functionalization. We also showed that the synthesis of silicon-containing polyaminoboranes are feasible affording ultra-high molecular weight polymers up to 5.8×109 Da. This class of 209 Enhancing the substitution pattern polymers should be further modified by performing sol-gel like condensation reactions. The synthesis of functionalized polyaminoboranes is now unlocked in mild conditions, high yields and without the need of expensive metal-complexes opening the route of promising new materials. 210 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6 Experimental section IV.6.1 Poly(trimethylsilylmethylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 41.76 37.37 H 12.27 11.23 N 12.17 10.51 211 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 212 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.2 Poly(dimethylethoxysilylpropylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 48.56 47.03 H 11.64 11.39 N 8.09 8.13 213 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 214 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.3 Poly(methyldiethoxysilylpropylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 47.30 45.78 H 10.92 10.70 N 6.89 6.87 215 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 216 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.4 Poly(triethoxysilylpropylamino)borane Infrared Elemental analysis Element Theoretical Experimental C 46.36 42.00 H 10.37 9.23 N 6.01 5.74 217 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 218 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.5 Poly[tris(trimethylsiloxy)(silylpropylamino)]borane Infrared Elemental analysis Element Theoretical Experimental C 39.43 39.45 H 9.93 9.62 N 3.83 3.91 219 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 220 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.6 Poly(dimethylsiloxaneamino)borane (1:1) Infrared Elemental analysis Element Theoretical Experimental C 35.91 35.44 H 8.93 8.80 N 3.22 2.69 221 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 222 Metal-free synthesis and characterization of functionalized polyaminoboranes IV.6.7 Poly(dimethylsiloxaneamino)borane) (1:2) Infrared Elemental analysis Element Theoretical Experimental C 35.43 35.35 H 8.92 8.7 N 3.18 3.12 223 Enhancing the substitution pattern Thermogravimetric analysis Size-Exclusion Chromatography 224 General conclusion General conclusion During my PhD research work, the reactivity of diisopropylaminoborane (1) with amines was addressed. The studies carried out led us to demonstrate that diisopropylaminoborane (1) can behave as an excellent "BH2" transfer reagent in the presence of primary amines. An in-depth study of this reactivity showed that the involved process is not straightforward and concerted hydrogen transfer side reactions have also been highlighted. The mechanism was proposed with the help of theoretical calculations and it is in common agreement with the modification of the outcome of the reaction by varying the experimental conditions. At low temperature under solventless conditions, the "BH2" transfer reaction takes place but functionalized poly(alkylamino)boranes are obtained. Prior to my work, such compounds could only be prepared from amine-boranes by metal- catalyzed dehydrocoupling reactions. With our method, the average molecular mass of the polymers is up to 10 times higher than the ones obtained under metal catalyzed conditions. The substitution pattern of the polymer can be easily tuned and has been studied by increasing the carbon chain length at nitrogen, ranging from the simplest polyaminoborane to poly(butylamino)borane. This method also allowed the preparation of random copolymers by simply mixing the desired primary amines before the addition to the diisopropylaminoborane. Mechanistic investigations were performed with the support of theoretical calculations in cooperation with the LCPNO in Toulouse. The results allowed us to rationalize the overall mechanism highlighting the formation of linear polymers kinetically favorized, in perfect accordance to the experimental observations. 227 General conclusion The mild conditions used in the formation of the poly(alkylamino)boranes enabled for the first time the introduction of a variety of functions and substitution patterns including unsaturated chains, such as allyl and propargyl. 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