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The halogen-free synthesis of oligosilazanes has been observed upon dehydrocoupling silanes with ammonia at 25 °C using [(2,6-iPr2PhBDI)Mn(µ-H)]2. Extending this methodology to polymethyl-hydrosiloxanes afforded thermally robust polysiloxazane solids, and the dehydrocoupling of 1,3,5,7-tetramethylcyclotetrasiloxane with ammonia afforded a polysiloxazane having a weight-average molecular weight of 4300 g/mol. A representative oligosilazane has been applied to a copper surface and found to afford a 20 μm thick coating that resists corrosion after 24 h under water. Addition of ammonia to [(2,6-iPr2PhBDI)Mn(µ-H)]2 allowed for characterization of the catalyst resting state, [(2,6-iPr2PhBDI)Mn(µ-NH2)]2, which has been found to mediate Si‒N dehydrocoupling. 

Although silane diamine copolymers have captured the attention of the catalysis community, the optimization of their synthesis and end uses have yet to be explored. In this study, a well-defined Earth-abundant metal catalyst, [(2,6-iPr2PhBDI)Mn(µ-H)]2, has been found to couple organosilanes to diamines to prepare networks that feature varied silane substitution and diamine chain lengths. By performing dehydrocoupling in the absence of solvent with 0.01 mol% catalyst loading, substrate utilisation turnover frequencies of up to 300 s-1 have been achieved at early reaction times, the highest Si–N dehydrocoupling activity ever observed. These networks have been employed as absorbents for common organic solvents, a property that had not been studied for this class of materials. By incorporating a long-chain hydrophobic linker, one network has been found to absorb 7.7× its orginal mass in THF and recycling has been demonstrated upon solvent removal. Controlling the degree of dehydrocoupling also offered an opportunity to deposit coatings from freshly-prepared silane diamine polymer solutions and monitor their integrity upon curing in air. While uniform and persistent coatings have been obtained from 1,6-hexanediamine derived polymers, the need to prepare dilute solutions that have a short shelf-life and the tackiness associated with extended dry times have been identified as potential limitations. 

A series of low-valent iron complexes that feature a phosphine-substituted α-diimine (DI) ligand have been synthesized. Reduction of (Ph2PPrDI)FeBr2 with an excess of Na/Hg in the presence of carbon monoxide afforded the corresponding dicarbonyl complex, (Ph2PPrDI)Fe(CO)2. Through multinuclear NMR and single crystal X-ray diffraction analysis, this complex was found to possess a 3-coordinate DI ligand. Upon heating for 10 days at 110 °C while applying intermittent vacuum, (Ph2PPrDI)Fe(CO)2 was successfully converted to the corresponding monocarbonyl complex, (Ph2PPrDI)Fe(CO), which was found to feature a tetradentate chelate. Similar reactivity was explored using the analogous bis(tert-butyl)phosphine-substituted ligand, tBu2PPrDI. Addition of this chelate to FeBr2 afforded (tBu2PPrDI)FeBr2, and subsequent reduction yielded (tBu2PPrDI)FeBr, which was found to possess a tridentate DI ligand by single crystal X-ray diffraction. Performing the reduction of (tBu2PPrDI)FeBr2 in the presence of CO afforded the corresponding dicarbonyl complex, (tBu2PPrDI)Fe(CO)2. Like aryl-substituted (Ph2PPrDI)Fe(CO)2, alkyl-substituted (tBu2PPrDI)Fe(CO)2 was found to feature a pendant phosphine arm. However, heating (tBu2PPrDI)Fe(CO)2 under vacuum did not allow for phosphine substitution and conversion to the corresponding monocarbonyl complex, highlighting the importance of phosphine π-acidity for substitution and the stabilization of low-valent iron. 

The phosphine-substituted aryl diimine cobalt catalyst, (Ph2PPrADI)Co, has been found to mediate the dehydrocoupling of diamines or polyamines to poly(methylhydrosiloxane) (PMHS) to generate hydrogen and crosslinked solids in an atom-efficient fashion. The resulting siloxane diamine and siloxane polyamine networks persist in the presence of air or water at room temperature and can tolerate temperatures of up to 1,600 °C. Upon lowering the catalyst loading to 0.01 mol%, (Ph2PPrADI)Co was found to catalyze the dehydrocoupling of 1,3-propanediamine and PMHS (m = 35) to generate a siloxane diamine foam with a turnover frequency of 157 s-1 relative to diamine consumption, the highest activity ever reported for Si‒N dehydrocoupling. Furthermore, upon systematically reducing the number of potential branch points, the (Ph2PPrADI)Co catalyzed dehydrocoupling of diamines with hydride-terminated poly(dimethylsiloxane) (PDMS) was found to yield linear siloxane diamine polymers with molecular weights of up to 47,300 g/mol. 

This article covers historical and recent efforts to catalyse the dehydrocoupling of amines and silanes, a direct method for Si–N bond formation that offers hydrogen as a byproduct. In some applications, this transformation can be used as a sustainable replacement for traditional aminosilane synthesis, which demands corrosive chlorosilanes while generating one equivalent of ammonium salt waste for each Si–N bond that is formed. These advantages have driven the development of Si–N dehydrocoupling catalysts that span the periodic table, affording mechanistic insight that has led to advances in efficiency and selectivity. Given the divergence in precursors being used, characterization methods being relied on, and applications being targeted, this article highlights the formation of monomeric aminosilanes separately from oligomeric and polymeric aminosilanes. A recent study that allowed for the manganese catalysed synthesis of perhydropolysilazane and commercial chemical vapor deposition precursors is featured, and key opportunities for advancing the field of Si–N dehydrocoupling catalysis are discussed. 

Through the application of a redox-innocent aryl diimine chelate, the discovery and utilization of a cobalt catalyst, ( Ph 2 PPr ADI)Co, that exhibits carbonyl hydrosilylation turnover frequencies of up to 330 s −1 is described. This activity is believed to be the highest ever reported for metal-catalyzed carbonyl hydrosilylation.