Silicon electron donor-acceptor complexes

Unlike carbon, the silicon atom may utilise vacant orbitals to expand its valence beyond four, to five or six, forming additional bonds with electron donors. This is shown by isolated amine complexes. The stabiUty of the organosHane amine complexes varies over a wide range and depends on the nature of the donor and acceptor (2). 

Dihydro-lH-l,5,2-azasilaboroles derive from the 2,5-dihydro-lH-l,2-aza-boroles ( 6.5.3.3) by substitution of the carbon neighboring N by a silicon atom. They may act as four-electron donors using electron density from the C=C double bond and the N atom. The B atom behaves as an acceptor center. Two pathways are known for complex synthesis reaction with a generated transition-metal complex fragment and reaction with metal atoms by the metal-vapor synthesis method. 

While complete X-ray analysis will establish the structure in the solid state, it is useful to have NMR data on the solution state that illustrate the increase of the coordination number of silicon. It would seem that NMR spectroscopy of nuclei participating directly in donor-acceptor interaction is especially important in investigating silicon compounds with an expanded coordination sphere. This requires the use of Si NMR spectroscopy since the electron shell of the silicon atom, the bond angles and lenghts are strongly affected upon complexation. Valuable information could also be obtained with by " N, N, 0, F NMR data since these elements act as donors. Chemical shifts of nuclei other than hydrogen are determined by various factors and not yet understood well anough to provide easily applied correlations of other physical properties of the molecules. 

Silatranes 1 are meanwhile classical cage compounds with donor-acceptor interactions and represent examples of hypercoordinated silicon [2], The donor-acceptor contact in 1 is formed by an interaction of the Lewis-basic amino group with the Lewis-acidic silicon center favored by the chelate effect. Numerous examples show that electron-rich transition metal complexes also possess Lewis-basic properties [3, 4]. Isolobal replacement of the NR3-unit in 1 by a d ML4-unit [5] leads to compounds of type 2 [1,6, 7]. These Si/Ni-cages 2 can be regarded as metallosilatranes. Here we report on the s5mthesis, structure and bonding of 2. 

In complex 155, the NHC ring was nearly orthogonal to the Si" heterocyle plane, consistent with electron donation from the carbene into an orbital of primarily p-character on the silicon center. Cui and co-workers were able to prepare the donor-acceptor stabilized silylones 157 and 158 by treating 154 with aldehydes. These reactions were reported to proceed through silicon heterocyclic ring expansion and insertion leading to 0x0 transfer from the aldehyde to silicon. Unlike Driess complexes 152 and 153, the formation of silylones 157 and 158 required the addition of the Lewis acid AICI3 to stabilize the Si=0 moiety (Scheme 5.26). " ... 

Roewer G, Herzog U, Trommer K, Muller E, Friihauf S (2002) Silicon Carbide - A Survey of Synthetic Approaches, Properties and Applications 101 59-136 Rosa A, Ricciardi G, Gritsenko O, Baerends EJ (2004) Excitation Energies of Metal Complexes with Time-dependent Density Functional Theory 112 49-116 Rosokha SV, Kochi JK (2007) X-ray Structures and Electronic Spectra of the n-Halogen Complexes between Halogen Donors and Acceptors with jc-Receptors. 126 137-160 Rudolf P, see Golden MS (2004) 109 201-229... 

Triorganostannates such as LiSnRs are sufficiently electron rich to be potential candidates for electron transfer to halides [52,126]. As for the silicon analogues MSiRa [115], the electron transfer mechanisms for the reduction by such at-complexes may be quite complicated because of the solvent-induced separation between the SiRJ donor anion and the a acceptor cation M. ... 

---