Electrooxidation of Hypercoordinated Derivatives of Silicon and Reactivity of Their Electrogenerated Cation Radicals: 1-Substituted Silatranes.

Electrochemical oxidation of 1-R-substituted silatranes 1 (R = Me, vinyl, (CH2)2CN, CH2Ph, CH2(C10H7), Ph, C6H4Me, p-Cl-C6H4, Cl)-classical representatives of pentacoordinated silicon compounds-and the formation of their short living cation radicals upon reversible or quasi-reversible one-electron withdrawal were studied by means of cyclic and square-wave voltammetry, faradaic impedance spectroscopy and real-time temperature-dependent EPR spectroelectrochemistry supported by DFT B3PW91/6-311++G(d,p) (C-PCM, acetonitrile) calculations. The main reaction responsible for the decay of 1+• is shown to be their deprotonation, and ways of increasing the stability of these species are proposed.

Molecular Design of Silicon-Containing Diazenes: Absorbance of E and Z Isomers in the Near-Infrared Region.

The effective use of photochromic systems based on azo compounds in a number of applications, especially biomedical and pharmacological ones, is impeded by the unresolved problem of their E⇆Z isomerization in the near-IR region, NIR (780-1400 nm). We have demonstrated at the TD-DFT, STEOM-DLPNO-CCSD and CASSCF-NEVPT2 levels of theory that the presence of a silylated diazene core -Si-N=N-Si- with three-, tetra- or five-coordinated silicon atoms practically guarantees the absorption of the E and Z forms of such derivatives in NIR and the amazing (185-400 nm) separation of their first absorption bands. In particular, the maximum λ1 of the first n→π* band of the E isomer of azosilabenzene ASiB is at ∼1030 nm, while for the Z isomer λ1 ≅1340 nm. Based on the found bistable azo compounds (ASiB, bis(silyl)- SiD and bis(silatranyl)- SaD diazenes) and their derivatives with E and Z absorption in NIR, unique photoswitches can be created for a number of applications, in particular, for photothermal therapy.

The hierarchy of ab initio and DFT methods for describing an intramolecular non-covalent SiN contact in the silicon compounds using electron diffraction geometries.

In the series of silatranes XSi(OCH2CH2)3N, 1 (X = Me, 1a; H, 1b; F, 1c) with the known gas electron diffraction (GED) structures, the problematic geometry of 1-methylsilatrane 1a has been revised. In particular, the new value of the SiN distance (dSiN) in 1a turned out to be ∼0.06 Å longer than the generally accepted one. This dSiN resolves the long-standing contradiction between the data of the structural and spectral experiments regarding the sensitivity of 1 to the medium effect. We also performed the ab initio and DFT study of the combined series of silatranes 1a-c, silylalkylamines H3Si(CH2)3NMe2 (2a) and F3SiCH2NMe2 (2b), silylhydrazines F3SiN(Me)NMe2 (2c) and F3SiN(SiMe3)NMe2 (2d), and silyloxyamines ClH2SiONMe2 (2e,f), (F3C)F2SiONMe2 (2g,h) and F3SiONMe2 (2i), in which the GED dSiN values are in a wide range of 2-3 Å. None of the involved quantum chemical methods has succeeded in reproducing all the experimental gas-phase dSiN values in 1a-c, 2a-i with an acceptable accuracy (0.01-0.03 Å). The problems of the used methods, primarily CCSD with the Pople basis sets, are caused by four molecules with the geminal SiNN and SiON fragments (2d,f-i) and dSiN < 2.3 Å. A reasonable hierarchy of computationally accessible theory levels for studying the physicochemical manifestation of the non-covalent intramolecular SiN interactions can be constructed only at dSiN > 2.3 Å: MP2 < PBE0 ∼ B3PW91 ∼ SCS-MP2 < CCSD < CCSD(T).

"Outlaw" Dipole-Bound Anions of Intra-Molecular Complexes.

Using the example of silatranes XSi(OCH2CH2)3N (X = Me, H, F, Cl), XS, it was found that the effect of the dipole-bound (DB) electron on the cage intramolecular complexes does not fit into the standard views. Upon the transition from XS to the DB anions XS-, the unusual shortening of the internuclear Si···N distance is always observed. For X = Cl, it is equal to 0.15 Å, which is a record length for all DB anions known from the literature. The formation of DB anions with the cage structure has principal features, controlled not only by the "critical" value of the dipole moment (µ > 2.5 D), but also by a geometric factor, such as the degree of pyramidality of the N(CH2)3 moiety-the positive end of the molecular dipole of XS. It was a surprise that the effect of the substituent X on the extent of the structural rearrangement in the process XS → XS- cannot be explained using the values of the electron detachment energy of XS- or the initial strength of the coordination Si ← N bond in XS. The unique sensitivity of the silatrane geometry to the addition of an excess electron is governed by the rate of increase of their dipole moment with the shortening of the dative Si ← N contact. The conclusions drawn are supported by the high-accuracy CCSD and CCSD(T) calculations and the experimental (RET-PES) data. There is no real reason to doubt that the peculiarities of the formation of DB anions of XS- can also be characteristic of many hundreds of their structural analogues XM(YCH2CH2)3N (M = Si, Ge, Sn, Pb, Ti, Al, Cr, Fe, Ni...; Y = O, NR, CH2, S), i.e., substituted 5-azabicyclo[3.3.3]undecans.

Assignment of photoelectron spectra of intramolecular silicon complexes: 1-vinyl- and 1-phenylsilatranes.

The problematic experimental photoelectron spectra of silatranes XSi[OCH2CH2]3N (X = vinyl and Ph) were theoretically studied. The ab initio electron propagator theory, many-body methods, and model vibrational Hamiltonian were employed to establish the nature of bands in the energetically lowest part of the photoelectron spectrum of these intramolecular silicon complexes. The first vertical ionization energy corresponds to a nitrogen atom lone pair, nN, in 1-vinylsilatrane, and the essentially doubly degenerate π-orbitals πPh and πPh' of the aromatic ring in 1-phenylsilatrane. Peaks at 9.6 and 9.9 eV in the spectra of both compounds, previously associated with the removal of an electron from the lone pair level of the equatorial oxygens, should actually be assigned to the bonding orbital HV1 of their 3c-4e axial CSi←N moiety. Taking silatranes with X = H, Me, OEt, F, vinyl, and Ph as examples, it was found that the length of the dative Si←N contact and the ionization energy of HV2 (formally nN) were in a good linear relationship. Regardless of the nature of the X substituent, this relationship could be used for the identification of orbitals that participate in the formation of the coordination Si←N bond in the wide series of XSi[OCH2CH2]3N and their structural analogs.

Dipole-Bound Anions of Intramolecular Complexes.

Dipole-bound molecular anions are often envisioned as unperturbed neutral, polar molecules with single excess electrons. We report the observation of intramolecular structural distortions within silatrane molecules due to the formation of their dipole-bound anions. The combination of Rydberg electron transfer-anion photoelectron spectroscopy (RET-PES) and ab initio computational methodologies (CCSD and MP2) was used to study 1-hydro- (HS) and 1-fluoro- (FS) silatranes and their dipole bound anions, HS- and FS-. The vertical detachment energies (VDEs) of HS- and FS- were measured to be 48 and 93 meV, respectively. Ab initio calculations accurately reproduced these VDE values as well as their photoelectron spectral profiles. This work revealed significant shortening (by ∼0.1 Å) of dative Si ← N bond lengths when HS and FS formed dipole-bound anions, HS- and FS-. Detailed computational (Franck-Condon) analyses explained the absence of vibrational features in the photoelectron spectra of HS- and FS-.

Electrochemical Oxidation and Radical Cations of Structurally Non-rigid Hypervalent Silatranes: Theoretical and Experimental Studies.

Using 18 silatranes XSi(OCH2 CH2 )3 N (1) as examples, the potentials of electrochemical oxidation E0 of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials Ep (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E0 (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (dSiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1+.(s) , whereas the second one, 1+.(l) , has a longer (l) Si⋅⋅⋅N distance (dSiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom Ns . The vertical model of adiabatic ionization (1→1+.(s) or 1→1+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E0 (1) with the strength characteristics of the dative contact N→Si were revealed.

Assignment of photoelectron spectra of silatranes: first ionization energies and the nature of the dative Si←N contact.

The problematic experimental photoelectron spectra of fluoro- and ethoxy-silatranes, XSi[OCH2CH2]3N (X = F and OEt), were assigned using theoretical spectra obtained by combining the OVGF//CCSD vertical ionization energies with the vibrational widths of the electronic transitions (linear vibronic coupling formalism, LVC). Taking into account the overlapping of the silatrane bands with the bands of probable impurities, bicyclic amines, (OH)XSi(OCH2CH2)2NCH2CH2OH, allowed us to reliably determine the position of the low-energy bands (at ∼9.7 eV for F- and at ∼9.2 eV for EtO-silatrane) associated with the ionization from a nitrogen lone pair level. For XSi[OCH2CH2]3N (X = F, H, OEt, Me), the correlation between the first vertical ionization energies, VIEs1, and the geometrical, electronic and orbital characteristics of the Si←N bonding was found. Its analysis suggests that the Si←N coordination in silatranes is orbital-controlled rather than charge-controlled.

Radical anions of hypervalent silicon compounds: 1-substituted silatranes.

The first representatives of the radical anions of silatranes XSi(OCH2CH2)3N ­ organic derivatives of the pentacoordinate silicon atom (X = Ph, 1; p-NO2PhO, 2a; m-NO2PhO, 2b; o-NO2PhO, 2c) ­ were obtained and characterized by EPR spectroscopy. The structure of 1(−˙), 2a(−˙), 2b(−˙), and 2c(−˙) in polar solvents (C-PCM and COSMO models) was studied at the UMP2 and UB3PW91 levels of theory. The variation of structural characteristics and pentacoordinate character of the silicon atom in 1, 2 upon the attachment of an additional electron to them is discussed. The experimental hyperfine coupling constants aN, aH and those calculated with the UTPSSh/IGLOIII and UB3LYP/N07D methods using the UB3PW91 geometry (taking into account an effect of the potassium cation in the case of 1(−˙)) are in good mutual agreement.

Analysis of the hypersensitivity of the (29)Si NMR chemical shift of the pentacoordinate silicon compounds to the temperature effect. N-(Silylmethyl)acetamides.

Theoretical investigation of the phenomenon of hypersensitivity of the (29)Si NMR chemical shift, δ, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of N-(silylmethyl)acetamides MeC(O)NMeCH2SiX3 (X = Me, 1; OMe, 2; F, 3) and MeC(O)NMeCH2SiMe2F (4) with the use of experimental dynamic NMR (DNMR) (29)Si data. It is based on the following: (i) the analysis of the potential energy surface of molecules 1-4 in polar solvents and the energetics of interconversion between their possible isomeric forms; (ii) the calculations of δ at different temperatures taking into account the dependence of the dielectric constant (ε) of the medium on T, and (iii) the isolation of dynamic, geometrical, and polar contributions to the temperature drift of δ. The results obtained allowed us to give a consistent explanation of the DNMR (29)Si spectra of acetamides 1-4 and to elucidate the nature of an unusual effect of T on δ.

Correlation among the gas-phase, solution, and solid-phase geometrical and NMR parameters of dative bonds in the pentacoordinate silicon compounds. 1-Substituted silatranes.

Silatranes XSi(OCH2CH2)3N exhibit a good linear relationship between their experimental and calculated (IGLO and GIAO) values of the NMR chemical shifts of (15)N, δN, and the lengths of dative bonds Si←N, dSiN, determined in the gas phase (ED, CCSD), solutions (COSMO PBE0, B3PW91), and crystals (X-ray). An aggregate of the obtained data provides strong evidence that the gas-phase value of dSiN in MeSi(OCH2CH2)3N should be greater by ∼0.05 Å than that determined in the electron diffraction (ED) experiment (2.45 Å). Given this condition, a long-standing contradiction between the data of the structural (X-ray, ED) and NMR (15)N experiments for the molecules of 1-methyl- and 1-fluorosilatrane regarding the sensitivity of their coordination contact Si←N to the medium effect is resolved.

A new approach to the design of neutral 10-C-5 trigonal-bipyramidal carbon compounds: a "π-electron cap" effect.

DFT (B3LYP, M06-2X) and MP2 methods are applied to the design of a wide series of the potentially 10-C-5 neutral compounds based on 6-azabicyclotetradecanes: XC(1)(YCH2CH2CH2)3N 1-3, XC(1)(YC6H4CH2)3N 4-6, XC(1)[Y(tBuC6 H3)CH2]3N 7-9 and carbatranophanes C6H3[XC(1)(YC6H3CH2)3N] 10-25 (X = Me, F, Cl; Y = O, NH, CH2, SiH2; Z = O, CH2, (CH2)2, (CH2)3). Carbatranophanes 10-25 are characterized by a sterical compression of their axial 3c-4e XC(1)←N fragment with respect to that in the parent molecules 4-6. A magnitude of the revealed effect depends on a valence surrounding of the central carbon atom C(1), the size and the nature of the side chains (Z) that link the "π-electron cap" with a tetradecane backbone. This circumstance allowed us to obtain 10-C-5 structures with the configuration of the bonds around the C(1) atom, which corresponds to practically an ideal trigonal bipyramid. In these compounds, the values of the covalence ratio χ (C1←N) of approximately 0.6 for the coordination C(1)←N contacts with a covalent contribution (atoms in molecules (AIM) and natural bond orbital (NBO)) are record in magnitude. These values lie close to a low limit of the interval of the χ(Si←D) change (0.6-0.9) being characteristic of the dative and ionic-covalent (by nature) Si←D bond (D = N, O) in the known 10-Si-5 silicon compounds.

C(Ar)-H···O hydrogen bonds in substituted isobenzofuranone derivatives: geometric, topological, and NMR characterization.

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).

Bifurcate hydrogen bonds. Interaction of intramolecularly H-bonded systems with Lewis bases.

The structure and the hydrogen bonding in the systems formed by the intramolecularly H-bonded systems, namely, maltol (3-hydroxy-2-methyl-4-pyrone), 5, 2,4,6-trinitrophenol, 6, acetylacetone enol, 7, with Lewis bases, phosgene, 8, dioxane, 9, and DMSO, 10, have been studied by density functional theory (B3LYP) and MP2 using the 6-311G* basis set. The continuum solvent effect was simulated by IEF-PCM model. The hydrogen bond analysis using the atoms in molecules (AIM) method was applied by using the MP2(full)/6-311++G** electron density to establish the nature of the bifurcate hydrogen bond (BHB) in these systems as well as contributory factors for its stabilization. The nature of interaction in the intermolecular H-complexes formed by compounds 5- 7 with the Lewis bases 8- 10 was shown to depend on the strength of the intramolecular hydrogen bond O...H and the strength of the base. The critical values of the CO...H and NO...H angles for which the formation of BHB is possible, have been determined.

Molecular design of neutral intramolecular complexes bearing two silicon atoms anchored by a carbonyl oxygen atom: N,N'-bis(silylmethyl)propylene ureas.

B3LYP and MP2 computations have been performed on a variety of Si,Si'-substituted N,N'-bis(silylmethyl)propylene ureas. According to electron-density atoms-in-molecules (AIM) and electron localization function (ELF) quantum-topological analyses, a transition from the unstable non-chelate forms of these compounds to mono- and bis-chelate forms results in the successive interaction of one and two tetracoordinate silicon atoms with the carbonyl oxygen and the formation, respectively, of one and two covalent, polar Si...O bonds. This previously unknown X-Si<--O-->Si-X type of bonding in isomers possessing an anchor structure may be classified as a five-center, six-electron (5c-6e) bond. The factors that favor the existence of Si,Si'-substituted N,N'-bis(silylmethyl)propylene ureas exclusively in the form of stable, bridged complexes (the size of equatorial ligands and the electronegativity of axial substituents at the silicon atom, change in the donor capability of the carbonyl group, and effect of the polar solvent) are discussed.