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.

Varga, V.; Lamac, M.; Horacek, M.; Gyepes, R.; Pinkas, J. Dalton Transactions 2016, 45 (25), 10146-10150.

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).

Varga, V.; Pokorna, K.; Lamac, M.; Horacek, M.; Pinkas, J. Dalton Transactions 2024, 53 (11), 5249-5257.