The molecular and electronic structure features of silatranes, germatranes, and their carbon analogs

Abstract

Molecular geometries of fifty six metallatranes N(CH 2 CH 2 Y) 3 M-X and fifty six carbon analogs HC(CH 2 CH 2 Y) 3 M-X (M = Si, Ge; X = H, Me, OH, F; Y = CH 2 , O, NH, NMe, NSiMe 3 , PH, S) were optimized by the DFT method. Correlations between changes in the bond orbital populations, electron density ρ(r), electron density laplacian ∇ 2 ρ(r), |λ 1 |/λ 3 ratio, electronic energy density E(r), bond lengths, and displacement of the central atom from the plane of three equatorial substituents and the nature of substituents X and Y were studied. As the number of electronegative substituents at the central atom increases, the M←N, M-X, and M-Y bond lengths decrease, while the M←N bond strength and the electron density at critical points of the M←N, M-X, and M-Y bonds increase. An increase in electronegativity of a substituent (X or Y) is accompanied by a decrease in the ionicities of the other bonds (M-X, M-Y, and M←N) formed by the central atom (Si, Ge). A new molecular orbital diagram for bond formation is proposed, which takes into account the interaction of all five substituents at the central atom (M = Si, Ge) in atrane molecules.Recently, metallatranes N(CH 2 CH 2 Y) 3 M-X (com pounds with intramolecular interaction M←N) have at tracted particular attention of researchers owing to their unique properties. 1-5 For instance, silatranes are less re active in the reactions of nucleophilic substitution at sili con compared to their tetracoordinate analogs. 3 A nitro gen atom involved in intramolecular interaction remains inert to electrophiles, e.g., methyl iodide and tetracyano ethylene. 3 Besides, silatranes and germatranes exhibit a broad spectrum of biological activities, which makes these compounds promising for being used in medicinal chem istry. 6,7 It is believed that the biological activity of silatranes and germatranes is due to a weak transannular interaction; therefore, the emphasis is placed on the stud ies of the nature of the M←N bond in these compounds. 6,7 At present, transannular interaction is mainly studied by X ray analysis. This method was used in studies of a large number of silatranes and germatranes with substituents X and Y of the same type. Because of this, the available X ray analysis data are insufficient for correlating the effects of substituents X and Y and the parameters of transannular interaction. It is significant that the metal-donor intramolecular distance in the solid phase strongly depends on the crystal field effect and on the dipole dipole interactions. 8,9 This is confirmed by, e.g., the existence of three different modifications of 1 phenyl silatrane with transannular distances, Si←N, of 2.132(4), 2.156(4), and 2.193(5) Å. 10-12 Different Ge←N distances were also reported for 1 fluorogermatrane in two inde pendent studies, namely, 2.011(9) Å (see Ref. 13) and 2.104(2) Å (see Ref. 14).Gas phase electron diffraction (GED) data are also of considerable interest for assessing the effects of axial and equatorial substituents on the strength of transannular interaction in metall...

Electrochemical oxidation of benzyl germatranes

“…Driven by the interest for germatranes, a remarkable effort in their synthetic chemistry was accomplished [6][7][8][9][10][11]. Chemical structure, molecular modeling and theoretical studies of these compounds [12][13][14][15][16] at different levels of theory, including correlated post-Hartee-Fock methods [16] allowed better understanding of the specificity of trans-annular N ? Ge bond and revealed the importance of interaction of Ge with all 5 substituents [13][14][15].…”

“…Chemical structure, molecular modeling and theoretical studies of these compounds [12][13][14][15][16] at different levels of theory, including correlated post-Hartee-Fock methods [16] allowed better understanding of the specificity of trans-annular N ? Ge bond and revealed the importance of interaction of Ge with all 5 substituents [13][14][15]. Biological activity of germatranes that is closely bound with the substitution character at Ge was extensively studied [1,[4][5][6]17,18].…”

Si-Fluoro substituted quasisilatranes (N→Si) FYSi(OCH2CH2)2NR

“…Si bonding we estimated the experimental and theoretical k 1 /k 3 ratio values in 9 and 5 which are equal to 0.346, 0.304 and 0.227, respectively. These values can be considered as extent of covalent character of the coordination bonds in hypervalent organosilicon compounds [16,21]. The k 1 /k 3 obtained fall within the range typical for most of silatranes.…”

“…The choice of PW91-PW method instead of standard B3LYP and MP2 calculations of isolated molecules is caused by necessity to take into account the crystal packing. The analysis of literature concerning pentacoordinated silicon species has shown that interatomic Si-N distances in isolated molecules [16][17][18] are in most cases exceed the experimental values (revealed by X-ray diffraction) by 0.2-0.3 Å, that makes difficult direct comparison experimental and theoretical results. Unfortunately, the disorder of C1 and C2 atoms in compound 3 made impossible the experimental study and quantum chemistry calculation of its crystal structure.…”

“…All bonds formed by Si atom are elongated in comparison to experimental values by ca. 0.02 Å, so the precision of PW91-PW calculations is better than that of hybrid functionals [16] and MP2 [17,18] in case of isolated molecules of silatranes.…”