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.

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

The role of the ketimide ligand geometry in Ti half-sandwich complexes and its consequent effects on olefin polymerisation catalysis (ethylene, styrene, 1-hexene, and ethylene/1-hexene copolymerization) were investigated under various conditions. [CpTiCl2(N=CtBu2)] was used as a reference compound for comparison with the recently described complex [η5-C5H4CMe2CMe2C(tBu)=N-κNTiCl2] and a new derivative that has a longer linker between Cp and the ketimide, [η5-C5H4CH2CH2CMe2C(tBu)=N-κNTiCl2]. The presence of a distorted intramolecularly tethered ketimide moiety reduces the polymerisation activity significantly in systems that contain Al-based cocatalysts (methylaluminoxane, triisobutylaluminum). However, in Al-free systems, both types of compounds provided active polymerisation catalysts. Notably, the recently reported activation system Et3SiH/B(C6F5)3 was, for the first time, demonstrated to activate Ti complexes for ethylene and 1-hexene (co) polymerisation catalysis by hydride transfer.

Publication:
Varga V.; Večeřa M.; Gyepes R.; Pinkas J.; Horáček M.; Merna J.; Lamač J., ChemCatChem 2017, 9, 3160 – 3172.

Reactions of chromocene [CrCp2] (Cp = η5-C5H5) with cyclopentadienyltitanium trichlorides [Ti(η5-C5H5–nMen)Cl3] (n = 0–5) and [Ti(C5Me4Et)TiCl3] in toluene resulted in precipitation of ion pairs of [CrCp2]+[Ti(η5-C5H5–nMen)Cl3]– (1–6) and [CrCp2]+[Ti(η5-C5Me4Et)Cl3]–. Heating their toluene solutions to 100 °C yielded the titanocene chloride – cyclopentadienylchromium dichloride complexes with the metals linked by bridging chloride ligands. For n = 0–3, the purple-violet complexes [CpCp′Ti(µ-Cl)2Cr(Cp)Cl] (Cp′ – cyclopentadienyl ligand of the titanium component) were stable at 100 °C, and the single-crystal structure of [Cp2Ti(µ-Cl)2Cr(Cp)Cl] was determined. The bridging complexes of the same type for n = 4, 5 and the C5Me4Et ligand were contaminated with titanocene dichlorides containing mixed auxiliary ligands [TiCp(C5HMe4)Cl2], [TiCp(C5Me5)Cl2], and [TiCp(C5Me4Et)Cl2], respectively. In addition, they contained the hitherto unknown (CpCrCl)n. Complexes activated with MMAO polymerised ethylene into toluene to very high molecular weights polyethylene (Mn ≥ 500 kg mol–1); upon activation with B(C6F5)3/Et3SiH in dichloromethane, they produced Mn = 9–23 kg mol–1. The titanium species was the catalytically dominant one in both cases.

Publication:
Varga V.; Pinkas J.; Císařová I.; Kubišta J.; Horáček M.; Mach K.; Gyepes R. , Eur. J. Inorg. Chem. 2018, 2637–2647.

Catalytic hydrodehalogenation (HDH) of aliphatic organohalides such as trifluorotoluenes by Et3SiH proceeds in the presence of readily available group 4 metal compounds: Cp′2MX2 (Cp′ = η5-C5H5 or η5-C5Me5; X = F, Cl, or Me; M = Ti, Zr, or Hf), CpTiCl3 and TiCl4 with a catalytic amount of B(C6F5)3. The use of metallocenes in combination with the borane activator leads to a better selectivity of the reaction, i.e., suppression of Friedel–Crafts alkylations of arenes.

Publication:
Dunlop D.; Pinkas J.; Horáček M.; Žilková N.; Lamač M., Dalton Trans., 2020, 49, 2771-2775.

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).

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