Octahedral Adducts of Dichlorosilane with Substituted Pyridines: Synthesis, Reactivity and a Comparison of Their Structures and ²⁹Si NMR Chemical Shifts

Abstract

H(2)SiCl(2) and substituted pyridines (Rpy) form adducts of the type all-trans-SiH(2*)Cl(2)2 Rpy. Pyridines with substituents in the 4- (CH(3), C(2)H(5), H(2)C=CH, (CH(3))(3)C, (CH(3))(2)N) and 3-positions (Br) give the colourless solids 1 a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H(2)SiCl(2) with an electron-deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl(3) yields the ionic complexes [SiH(2)(Rpy)(4)]Cl(2*)6 CHCl(3) (Rpy=4-methylpyridine (3 d) and 4-ethylpyridine (3 e)). All products are investigated by single-crystal X-ray diffraction and (29)Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si-N distances vary between 193.3 pm for 1 c (4-(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH(2) units, except for the ethyl-substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol(-1) and a maximum barrier of 12.5 kJ mol(-1). The (29)Si NMR chemical shifts (delta(iso)) range from -145.2 to -152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the (29)Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si-H bonds and very polar Si-Cl bonds, with the highest bond polarisation being observed for the Si-N interaction, which must be considered a donor-acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.

Photo‐Driven Si‐C Bond Cleavage in Hexacoordinate Silicon Complexes

“…In previous studies we have also recognized this coordinative behavior of bond lengthening upon Cl vs. F substitution [19]. It can be interpreted as enhanced O and N donor action in chlorosilicon complexes owing to the longer Si−Cl bond with pronounced ionic contributions [20].…”

Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand

“…This ligand, H 2 L, has already been reported in the literature [85] and was synthesized by condensation of o-phenylenediamine with two equivalents of pyrrole-2-carbaldehyde (Scheme 2). Prior to reacting with the chlorosilanes to be chelated upon substitution of two Cl atoms [SiCl 4 , PhSiCl 3 , Ph 2 SiCl 2 , (Anis) 2 SiCl 2 and (4-Me 2 N-C 6 H 4 )PhSiCl 2 ], H 2 L was converted into its bis(trimethylsilyl) derivative (Me 3 Si) 2 L, as its conversion with further chlorosilanes (in a transsilylation reaction) produces chlorotrimethylsilane as a liquid and volatile byproduct, which allows for easy separation from the target complex. Whereas SiCl 4 , PhSiCl 3 , and Ph 2 SiCl 2 were commercially available, (Anis) 2 SiCl 2 and (4-Me 2 N-C 6 H 4 )PhSiCl 2 were prepared from SiCl 4 and PhSiCl 3 , respectively, and suitable Grignard reagents (see Experimental Section).…”

“…Prior to reacting with the chlorosilanes to be chelated upon substitution of two Cl atoms [SiCl 4 , PhSiCl 3 , Ph 2 SiCl 2 , (Anis) 2 SiCl 2 and (4-Me 2 N-C 6 H 4 )PhSiCl 2 ], H 2 L was converted into its bis(trimethylsilyl) derivative (Me 3 Si) 2 L, as its conversion with further chlorosilanes (in a transsilylation reaction) produces chlorotrimethylsilane as a liquid and volatile byproduct, which allows for easy separation from the target complex. Whereas SiCl 4 , PhSiCl 3 , and Ph 2 SiCl 2 were commercially available, (Anis) 2 SiCl 2 and (4-Me 2 N-C 6 H 4 )PhSiCl 2 were prepared from SiCl 4 and PhSiCl 3 , respectively, and suitable Grignard reagents (see Experimental Section). [7][8][9][10][11]18,19,[73][74][75][76][77] chelators, we have now explored silicon coordination chemistry with a dianionic tetradentate (N,N′,N′,N) chelator.…”

“…Whereas SiCl4, PhSiCl3, and Ph2SiCl2 were commercially available, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2 were prepared from SiCl4 and PhSiCl3, respectively, and suitable Grignard reagents (see Experimental Section). Complexes LSiCl 2 , LSiPhCl, LSiPh 2 , LSi(Anis) 2 and LSiPh(4-Me 2 N-C 6 H 4 ) were obtained as orange solids that exhibited poor solubility in various organic solvents. Therefore, the emphasis of characterization is put on solid-state methods ( 29 Si CP/MAS NMR spectroscopy, single-crystal X-ray diffraction) and computational methods.…”

“…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 SiC 5 coordination sphere, B [4][5][6]; and hexacoordinate silicon with a tetradentate chelator and two Si-CH 3 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 Lewis bases in their ligand sphere [30][31][32][33][34][35][36][37][38], silicon coordination chemistry continues to be an attractive research field, reflected by frequently published research articles [39][40]…”