Abstract
In the context of our systematic investigations of penta- and hexacoordinate silicon compounds, which included dianionic tri- (O,N,O′; O,N,N′) and tetradentate (O,N,N,O; O,N,N′,O′) chelators, we have now explored silicon coordination chemistry with a dianionic tetradentate (N,N′,N′,N) chelator. The ligand [o-phenylene-bis(pyrrole-2-carbaldimine), H2L] was obtained by condensation of o-phenylenediamine and pyrrole-2-carbaldehyde and subsequently silylated with chlorotrimethylsilane/triethylamine. Transsilylation of this ligand precursor (Me3Si)2L with chlorosilanes SiCl4, PhSiCl3, Ph2SiCl2, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2 afforded the hexacoordinate Si complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), respectively (Anis = anisyl = 4-methoxyphenyl). 29Si NMR spectroscopy and, for LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), single-crystal X-ray diffraction confirm hexacoordination of the Si atoms. The molecular structures of LSiCl2 and LSiPhCl were elucidated by computational methods. Despite the two different N donor sites (pyrrole N, X-type donor; imine N, L-type donor), charge delocalization within the ligand backbone results in compounds with four similar Si–N bonds. Charge distribution within the whole molecules was analyzed by calculating the Natural Charges (NCs). Although these five compounds carry electronically different monodentate substituents, their constituents reveal rather narrow ranges of their charges (Si atoms: +2.10–+2.22; monodentate substituents: −0.54–−0.56; L2−: −1.02–−1.11).
Highlights
The coordination number of tetravalent silicon can be enhanced with the aid of monodentate or chelating ligands
Hexacoordinate silicon compounds with dianionic tri-(O,N,O′; O,N,N′) [20,64,65,66,67,68,69,70,71,72] and tetradentate chelators, we have explored siliconchelators, coordination chemistry a dianionic tetradentate (O,N,N,O; O,N,N′,O′) [7,8,9,10,11,18,19,73,74,75,76,77]
[20,64,82,83,84], we studied coordination the syntheses and molecular structures of silicon coordination chemistry [20,64,82,83,84], we studied the syntheses and molecular structures of silicon compounds with a pyrrole-2-carbaldimine functionalized (N,N′,N′,N)-chelating dianionic compounds with a pyrrole-2-carbaldimine functionalized (N,N1,N1,N)-chelating dianionic tetradentate compounds with a pyrrole-2-carbaldimine functionalized (N,N′,N′,N)-chelating dianionic tetradentate
Summary
The coordination number of tetravalent silicon can be enhanced (up to five or six) with the aid of monodentate or chelating ligands. Whereas the former preferentially bind to Si atoms that carry strongly electron withdrawing groups (e.g., formation of pyridine adducts of halosilanes, Scheme 1, A, [1,2,3]), the latter offer greater opportunities of creating five- and six-coordinate silicon compounds even in case of the absence of halides from the silicon coordination sphere (e.g., pentacoordinate silicon with SiC5 coordination sphere, B [4,5,6]; and hexacoordinate silicon with a tetradentate chelator and two Si–CH3 groups, C [7]) For various reasons, such as activation of Si–X bonds by silicon hypercoordination [8,9,10,11,12,13,14,15,16], exploring special electronic/optical properties arising from the higher coordination number of silicon in combination with selected ligands [17,18,19,20] or the aim of creating and exploring hitherto unusual Si coordination compounds, e.g., with transition metals [21,22,23,24,25,26,27,28,29] or very soft. (N,N,N,N)-chelators with essentially chemically equivalent donor atoms and “open chain” type chelators such as salen-type (O,N,N,O)-ligands (like the tetradentate ligand in C)
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