Transition Metal Activation for Catalysis
SiHB System (silane-borane adduct)
The basic concept of the transition metal/hydrosilane/borane adduct (SiHB) system, introduced in 2016, represents a method of transition metal catalyst activation based on a sequential reaction of hydrosilane with a Lewis acidic borane, typically B(C6F5)3, and a transition metal complex. SiHB system could be a valuable alternative to MAO or alkyl aluminium/borate salt activators in catalytic olefin polymerization, where the transition metal complex structure plays a key role in the catalytic reaction, determining the activity and selectivity towards a particular polymer structure.
Generally, the proposed hydride transfer from hydrosilane to B(C6F5)3 and finally to the transition metal can directly activate readily available halide complexes forming a metal–hydride bond as a prerequisite for their catalytic performance. Until now, we have studied the utilization of the system for processes such as (co)polymerization of olefins, hydrodehalogenation using silanes, and hydrogenation with molecular hydrogen. The range of transition metals was also extended from group 4 to late transition metal elements.
However, the story of this system began earlier and comes from the study of titanocene complexes and their functionalisation.
Hydrosilane-B(C6F5)(3) adducts were found to activate zirconocene dihalides and generate ternary catalytic systems possessing moderate to high activity in ethylene polymerization to high density polyethylene (HDPE). The activation efficacy of the adducts increased with increasing hydride donor ability and decreased with steric crowding of the particular hydrosilane used. NMR investigation revealed the formation of a stable intermediate whereas a crucial role of the [HB(C6F5)(3)](-) anion as a hydride donor for generation of an active cationic zirconium hydride center was elucidated.
Brookhart’s nickel alpha-diimine complex activated with a hydrosilane/B(C6F5)(3) (SiHB) adduct forms a highly active catalytic system for ethylene polymerization. Under optimal conditions, the activity of the system depends on the nature of hydrosilane and decreases in the order R3SiH > Ph2SiH2 > PhSiH3. The decrease in system activity within the hydrosilane series is correlated with increasing formation of Ni(I) species. In addition to their activation effect, hydrosilanes act as efficient chain termination/chain transfer agents, with the Si/Ni ratio controlling the molecular weight of the resulting polyethylene (PE). The use of Et3SiH generated elastomeric, highly branched polymers with a saturated chain-end, while systems using Ph2SiH2 and PhSiH3 led to branched end-functionalized PEs terminated with the hydrosilyl functionality (i.e. br-PE-SiPh2H or br-PE-SiPhH2).
Boron-based Lewis Acids in Catalysis
In collaboration with the Institute of Inorganic Chemistry CAS we systematically investigate catalytic properties of novel solid materials based on boron clusters – activated boranes (ActBs). These amorphous polymeric composites are synthesized by co-thermolysis of nido-B10H14 clusters with various aromatic or aliphatic hydrocarbons to give microporous materials with pronounced Lewis acidity, which reaches comparable strength as the well-established molecular boron Lewis acid B(C6F5)3 (BCF). The nature of these Lewis sites is still unknown, but we expect the presence of tricoordinated boron atoms within the crosslinked structure.
We have demonstrated the potential of these novel materials as heterogeneous catalysts for reductive deoxygenation/hydrosilylation reactions of carbonyl substrates as well as for dehydration of alcohols to olefins, and for dehalogenation of aliphatic chlorides and fluorides using triethylsilane as the reductant. Notably, the catalysts displayed tuneable activity and selectivity depending on their structure and they exhibited sufficient robustness allowing their use under continuous flow conditions in selected cases.
Borane cluster-based porous covalent networks, named activated borane (ActB), were prepared by cothermolysis of decaborane(14) (nido-B10H14) and selected hydrocarbons (toluene, ActB-Tol; cyclohexane, ActB-cyHx; and n-hexane, ActB-nHx) under anaerobic conditions. These amorphous solid powders exhibit different textural and Lewis acid (LA) properties that vary depending on the nature of the constituent organic linker. For ActB-Tol, its LA strength even approaches that of the commonly used molecular LA, B(C6F5)3. Most notably, ActBs can act as heterogeneous LA catalysts in hydrosilylation/deoxygenation reactions with various carbonyl substrates as well as in the gas-phase dehydration of ethanol. These studies reveal the potential of ActBs in catalytic applications, showing (a) the possibility for tuning catalytic reaction outcomes (selectivity) in hydrosilylation/deoxygenation reactions by changing the material’s composition and (b) the very high activity toward ethanol dehydration that exceeds the commonly used γ-Al2O3 by achieving a stable conversion of ∼93% with a selectivity for ethylene production of ∼78% during a 17 h continuous period on stream at 240 °C.
Lamač, M. ; Urbán, B.; Horáček, M.; Bůžek, D.; Leonová, L.; Stýskalík, A.; Vykydalová, A.; Škoch, K.; Kloda, M.; Mahun, A.; Kobera, L.; Lang, K.; Londesborough, M.G.S.; Demel, J. ACS Catal. 2023, 13, 14614−14626.
https://doi.org/10.1021/acscatal.3c04011
Activated borane (ActB), a metal- and halogen-free porous borane cluster polymer with a significant Lewis acidity, has been successfully used as a heterogeneous catalyst for hydrodehalogenation reactions of aliphatic fluorides and chlorides using triethylsilane as a reductant. In analogy to known homogeneous systems, full dehalogenation of organohalides is achieved with a predominant formation of Friedel–Crafts alkylation products in aromatic reaction media. Importantly, the herein described material is robust, tolerates elevated reaction temperatures and can be re-used, while reaching a turnover number (TON) of up to 5190. These features make it an attractive candidate for a sustainable disposal of halogenated pollutants by heterogeneous catalysis.
Udnoor, A.; Urbán, B.; Škoch, K.; Hynek, J.; Horáček, M.; Lamač, M.; Demel, J. Catal. Sci. Technol. 2024, 14, 4458–4465.
https://doi.org/10.1039/D4CY00732H