Isolable Silicon-Based Polycations with Lewis Superacidity

Abstract

Molecular silicon polycations of the types R₂ Si²⁺ and RSi³⁺ (R=H, organic groups) are elusive Lewis superacids and currently unknown in the condensed phase. Here, we report the synthesis of a series of isolable terpyridine-stabilized R₂ Si²⁺ and RSi³⁺ complexes, [R₂ Si(terpy)]²⁺ (R=Ph 1²⁺ ; R₂ =C₁₂ H₈ 2²⁺ , (CH₂ )₃ 3²⁺ ) and [RSi(terpy)]³⁺ (R=Ph 4³⁺ , cyclohexyl 5³⁺ , m-xylyl 6³⁺ ), in form of their triflate salts. The stabilization of the latter is achieved through higher coordination and to the expense of reduced fluoride-ion affinities, but a significant level of Lewis superacidity is nonetheless retained as verified by theory and experiment. The complexes activate C(sp³ )-F bonds, as showcased by stoichiometric fluoride abstraction from 1-fluoroadamantane (AdF) and the catalytic hydrodefluorination of AdF. The formation of the crystalline adducts [2(F)]⁺ and [5(H)]²⁺ documents in particular the high reactivity towards fluoride and hydride donors.

Keywords: Coordination compounds; Hydride transfer; Hydrodefluorination; Main group elements; Silanes.

Figure 1

Previously studied silicon‐based monocations and polycations (L=terpyridine; this work) with their fluoride ion affinities (FIA [kJ mol⁻¹]).

Figure 2

Projection of the calculated electrostatic potential (B3LYP‐D3BJ/def2svp) onto the isodensity surface (0.025 e⁻ Bohr⁻³) of the dications R₂Si²⁺ (top) and their terpyridine complexes [R₂Si(terpy)]²⁺ (bottom).

Figure 3

Molecular structures of 1 ²⁺, [2(OTf)]⁺ and [3(OTf)]⁺ (from left to right; thermal ellipsoids at 50 % probability; H atoms omitted for clarity and terpy ligand reduced to wireframe).

Scheme 1

Synthesis of the triflate salts of the dications 1 ²⁺, 2 ²⁺ and 3 ²⁺.

Figure 4

Reaction of [R₂Si(terpy)][OTf]₂ with [PPh₄][SbF₆] or 1‐fluoroadamantane (AdF) and the molecular structure of [2(F)]⁺ (thermal ellipsoids at 50 % probability; Si‐F 1.6693(14) Å).

Figure 5

Projection of the calculated electrostatic potential (B3LYP‐D3BJ/def2svp) onto the isodensity surface (0.025 e⁻ Bohr⁻³) of trications RSi³⁺ (top) and their terpy complexes [RSi(terpy)]³⁺ (bottom). No minimum geometry was found for CySi³⁺.

Figure 6

Molecular structures of [4(OTf)₂]⁺, [6(OTf)]²⁺ (A) and [6(OPEt₃)]³⁺ (B) (from left to right; thermal ellipsoids at 50 % probability; H atoms are omitted for clarity and terpy ligand reduced to wireframe).

Scheme 2

Synthesis of the triflate salts of trications 4 ³⁺, 5 ³⁺ and 6 ³⁺.

Figure 7

Reactivity of 5[OTf]₃ and 6[OTf]₃ towards Et₃SiH with depiction of the molecular structure of [5(H)]²⁺.