Salen-silicon

Hypervalent silicon compounds attract interest from both the structural and reactivity point of view [1]. The azomethine AJV -ethylene-bis(2-hydroxyacetophenoneimine) (salen H2 1), was formed by condensation of ethylenediamine with 2-hydroxyacetophenone. We set out to synthesize hexacoordinate silicon complexes containing the salen ligand. The anion salen is able to chelate the silicon atom through four donor atoms. There are some rare examples of salen-silicon compounds known from the literature [2], but characterization of these compounds seems to be doubtful [3]. Structural aspects are uncertain due to the lack of crystal structure data. 

We were able to prepare salen silicon complexes by reaction of the salen ligand as free acid (Scheme 2) or sodium salt (Scheme 1) with the corresponding chloro silicon compound. It is also possible to use Si2Cl6 as starting compound to get (salen )SiCl2 (Scheme 2). 

To obtain more soluble compounds we used the ligand system Af,(V -ethylene-bis(3,5-di-/erf-butylsalicylideneimine) (salen H2, 9). Analytical data are shown In Table 1. Salen -silicon complexes are available by the same procedures as the salen complexes. 

Salen Silicon Complexes — a New Type of Hexacoordinate Silicon.500... 

Scheme 1. Formation of penta- and hexacoordinate salen-silicon complexes by elimination and addition of acids.

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

Hexacoordinated salen-silicon complexes can undergo Wurtz-type coupling reactions to produce the first examples of oligosilanes and polysilanes containing hexacoordinated silicon backbones [149]. This principle has been utilized to couple salen-type complexes containing thiocyanato ligands [Si(salen )(NCS)2l to a mixture of linear oligosilanes with a hexacoordinated silicon backbone SCN-[Si(salen )] -NCS with n = 2-8 [147]. 

In 2000, Kagan and Holmes reported that the mono-lithium salt 10 of (R)- or (S)-BINOL catalyzes the addition of TMS-CN to aldehydes (Scheme 6.8) [52]. The mechanism of this reaction is believed to involve addition of the BI NO Late anion to TMS-CN to yield an activated hypervalent silicon intermediate. With aromatic aldehydes the corresponding cyanohydrin-TMS ethers were obtained with up to 59% ee at a loading of only 1 mol% of the remarkably simple and readily available catalyst. Among the aliphatic aldehydes tested cyclohexane carbaldehyde gave the best ee (30%). In a subsequent publication the same authors reported that the salen mono-lithium salt 11 catalyzes the same transformation with even higher enantioselectivity (up to 97% Scheme 6.8) [53], The only disadvantage of this remarkably simple and efficient system for asymmetric hydrocyanation of aromatic aldehydes seems to be the very pronounced (and hardly predictable) dependence of enantioselectivity on substitution pattern. Furthermore, aliphatic aldehydes seem not to be favorable substrates. 

Similar experimental conditions allowed the hydroxylation of 1,2-dihydronaphtha-lene in high yield with an ee > 90% [137]. In the latter case, the addition of ferricyanide ion was used also to electrochemically maintain Os concentration in the anolyte. Such reactions were achieved in two-phase systems. Metal complexes can be used for indirect oxidation such as the oxidation of phosphines by Nb species [138] as well as Ni salen used in the oxidation of silicon compounds [139] (Scheme 20). 

Recently we reported an easy route obtaining enamine-functionalized silicon compounds with a pentacoordinate silicon atom from organotrichlorosilanes and ethyIene-iV,iV -bis(2-hydroxyaceto-phenoneimine) (salen H2) [1]. 

Cationic salen germanium complexes seem to be more stable than analogous ones of silicon. Therefore, even picrate coordinates with the silicon atom of 3b, although an unusually long Si-O bond results. 

Summary Our investigations on the coordination behavior of ethylene-ACV -bis(2-oxy-4-methoxybenzophenoneiminate) 1, a tetradentate chelating ligand of the salen type, toward diorgano-substituted silicon atoms led to the syntheses and X-ray structure analyses of novel hexacoordinate silicon complexes. The first X-ray structures of a metal-free hexacoordinate dimethyl silane and of a hexacoordinate cw-configurated silicon complex with a salen type ligand are presented. 

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

The X-ray structure analysis of (salen )SiF2 (8) clearly demonstrates the octahedral coordination of the silicon atom (Fig. 1) [4], There are a number of crystal stractures of bis-chelate compounds with hexacoordinate silicon. Most of these had essentially a tetrahedral arrangement around silicon with the coordinated nitrogen donor atoms capping the tetrahedra at relatively large distances (N-Si between 2.5 and 3.0 A) [5]. 

The Si-F distance in 8 corresponds well with bond lengths found in other hypervalent silicon compounds (1.60-1.73 A) [Id, 6]. The distances Si-O and Si N are remarkably short [6]. The distortion of the octahedral coordination environment around silicon probably originates from the conformation of the chelating salen ligand. 

Wurtz condensation of (salen )SiCl2 by alkaline metals gives polysilanes containing main chain hexacoordinate silicon. Coupling with acetylendiyls affords polycarbosilanes with a Si-C -Si backbone (Scheme 3). 

A New Type of Silicon Complex with Salen-Type Ligands... 

Summary Our investigations on silicon compounds of etfaylene-MiV"-bi (2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex — hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This stmctural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Brpnsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms. 

The synthesis of pentacoordinate silicon complexes with enamine-functionalized salen ligands is easily performed by the addition of MeSiCls and diethylamine to a salen ligand (eq 22). ... 

Scheme 5.126 Enantioselective oxidation of silicon enolates 512 mediated by salen complex 511b.

The structure of the salen ligand system has been modified by using a dipyrrin -instead of the ethylenediamine-unit. The novel pentacoordinated dipyrrin-silicon complexes (31, see Scheme 4, and some of its derivatives) showed efficient red or near-IR fluorescence, and the structural interconversion between silanol and siloxane derivatives resulted in significant changes in the optical properties [150]. 

In principle, the formation of the porous matrix around a preformed molecular catalyst (bottle-around-ship) and the construction of the molecular catalyst in a preformed porous material (ship-in-a-bottle) have become two popular strategies for the encapsulation of molecular catalysts since the 1980s. Complexes such as [(BINAP)Ru(p-cymene)Cl)Clj, [(MeDuphos)Rh(cod)]OTf, (Salen)Mn, PrPybox-RuClj, and so on, have been entrapped in a silicon membrane, poly (vinyl alcohol) film, microcapsules, or silica matrix via the in situ formation of the network around the complexes (polymerization and sol-gel process were involved in the network formation) [53-55], However, the catalysts prepared are generally poor in activity, selectivity, and stability. The swelling of the polymer host material and the inhomogeneous cavity formed around the metal complexes may be the main reasons for the low activity and stabihty. In addition, the in situ formation of the... 

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