Novel Si(II)⁺ and Ge(II)⁺ Compounds as Efficient Catalysts in Organosilicon Chemistry: Siloxane Coupling Reaction

Abstract

Novel catalytically active cationic Si­(II) and Ge­(II) compounds were synthesized and isolated in pure form. The Ge­(II)+-based compounds proved to be stable against air and moisture and therefore can be handled very easily. All compounds efficiently catalyze the oxidative coupling of hydrosil­(ox)­anes with aldehydes and ketones as oxidation reagents and simultaneously the reductive ether coupling at very low amounts of <0.01 mol %. Because the catalysts also catalyze the reversible cyclotrimerization of aldehydes, paraldehyde can be used as a convenient source for acetaldehyde in siloxane coupling. It is shown that the reaction is especially suitable to make siloxane copolymers. Moreover, a new fluorine-free weakly coordinating boronate anion, B­(SiCl3)4 –, was successfully combined with the Si­(II) and Ge­(II) cations to give the stable catalytically active ion pairs Cp*Si:+B­(SiCl3)4 –, Cp*Ge:+B­(SiCl3)4 –, and [Cp­(SiMe3)3Ge:+]­B­(SiCl3)4 –.

Unexpected Formation of the Highly Symmetric Borate Ion [B(SiCl3)4]−

“…The synthesis of H[B(SiCl 3 ) 4 ] ( 1 ) has been filed recently as patent by two of our co‐authors (S. B. and L. R.) [66] . The application of the [B(SiCl 3 ) 4 ] − ion as new fluorine‐free weakly coordinating anion has been described by one of the patent assignees and her co‐workers [67] . Tetrasilylborate, [B(SiH 3 ) 4 ] − , has been mentioned in two papers dealing with quantum mechanical calculations and simulations on boron doped silicon clusters [68,69] .…”

“…[66] The application of the [B(SiCl 3 ) 4 ] À ion as new fluorinefree weakly coordinating anion has been described by one of the patent assignees and her co-workers. [67] Tetrasilylborate, [B(SiH 3 ) 4 ] À , has been mentioned in two papers dealing with quantum mechanical calculations and simulations on boron doped silicon clusters. [68,69] The only known compounds with a BSi 4 core are tetrakis(trimethylsilyl)borate, [B(SiMe 3 ) 4 ] À (3), and similar compounds which are stabilized by trimethylsilyl groups.…”

Catalytic Reductive Alcohol Etherifications with Carbonyl‐Based Compounds or CO2 and Related Transformations for the Synthesis of Ether Derivatives

“…On the other hand, Fritz‐Langhals et al. synthesized a novel cationic germanium complex [Cp*Ge]B(Ar F ) 4 able to catalyze the reductive coupling between ethoxytrimethylsilyl and hexanal in the presence of triethylsilane as reductor (entry 18) [68] …”

“…Finally, last year Fritz‐Langhals et al. synthesized novel cationic Si II and Ge II compounds and successfully applied them as catalysts in oxidative coupling of silanes to siloxanes and simultaneous reductive self‐etherification of aldehydes/ketones to afford ethers (Scheme 6Q) [68] …”

Main Group Catalysis: Cationic Si(II) and Ge(II) Compounds as Catalysts in Organosilicon Chemistry

“…The cyclopentadienyl (Cp)-coordinated cationic Si(II) and Ge(II) compounds 1 and 2, respectively (Scheme 1), however, should be good candidates for catalysis because of their open coordination sphere [29,30] which facilitates the interaction with various substrates being a good prerequisite for catalysis. In 2011, Jutzi [31] reported the first example of a reaction with Me 5 CpSi + B(Ar F ) 4 − (1a, Ar F = C 6 F 5 ) as a catalyst: the degradation of oligo(ethyleneglycol) ethers with 5-10 mol% of this compound (Scheme 2).…”