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

International audienceThe 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 λ(1) of the first n→π* band of the E isomer of azosilabenzene ASiB is at ∼1030 nm, while for the Z isomer λ(1) ≅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

Isomer-selective dative bond O -> M (M = Si, Ge) for designing new photochromic hemi-indigo systems

International audienceNew photochromic systems MY2X-HI (M = Si, Ge) were designed (at the DFT level of theory) by replacing the hydrogen atom in the nitrogen of the pyrrole ring of hemi-indigo, HI, with silyl- or germyl-substituents. The E isomers of MY2X-HI contain a dative bond O -> M (pentacoordinated atom M) both in the ground S-0 and in the excited S-1 states. This leads to a significant difference in the absorption and fluorescence maxima (lambda(fl) - lambda(abs)) of the Z and E forms of hypervalent hemi-indigos; this difference is more than twice that of the parent HI. There is a trend to increasing separation of first pi ->pi* absorption bands of the E and Z isomers of MY2X-HI with the increase in their relative stability. (C) 2021 Published by Elsevier B.V

Hypervalent benzophenones

International audienceBenzophenones (BPs) of the type 2-R-BP and 2,2'-R-BP, carrying silicon-or germanium-containing substituents R (R = EY2X; E = Si, Ge; Y = H, Me, F, Cl; X = H, F, Cl, OTf) at one or at two ortho-positions of the arene rings, were theoretically designed. Among 26 compounds considered, only 10 monosubstituted 2-R-BP and 5 bis-substituted 2,2'-R-BP (in hexane and DMSO solutions) belong to the first examples of hypervalent benzophenones. In these "true" chelates, the length of the O -> E dative contact (which has a covalent component, according to the AIM analysis) is in the range of 2.0-2.4 angstrom, and the degree of pentacoordination of E is higher than 40%. In these compounds, a significant planarization of the benzophenone part occurs, compared to the initial BP. The TD-DFT calculations suggest the UV spectral properties of the hypervalent BPs to differ fundamentally from those of BP. For the "true" chelates 2-R-BP and 2,2'-R-BP, there is a well pronounced trend of a change of the coordination O -> E contact strength along with the difference between the wavelengths of their first high-intensity band and the pi pi* band of the unsubstituted BP. (c) 2018 Elsevier B.V. All rights reserved

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

International audienceUsing 18 silatranes XSi(OCH CH ) N (1) as examples, the potentials of electrochemical oxidation E of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials E (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E (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 (d 3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom N . The vertical model of adiabatic ionization (1→1 or 1→1 ) 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 E (1) with the strength characteristics of the dative contact N→Si were revealed

Analysis of the Hypersensitivity of the ²⁹Si NMR Chemical Shift of the Pentacoordinate Silicon Compounds to the Temperature Effect. <i>N</i>‑(Silylmethyl)acetamides

Theoretical investigation of the phenomenon of hypersensitivity of the ²⁹Si NMR chemical shift, δ, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of <i>N</i>-(silylmethyl)­acetamides MeC­(O)­NMeCH₂SiX₃ (X = Me, <b>1</b>; OMe, <b>2</b>; F, <b>3</b>) and MeC­(O)­NMeCH₂SiMe₂F (<b>4</b>) with the use of experimental dynamic NMR (DNMR) ²⁹Si data. It is based on the following: (i) the analysis of the potential energy surface of molecules <b>1</b>–<b>4</b> 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 <i>T</i>, 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 ²⁹Si spectra of acetamides <b>1</b>–<b>4</b> and to elucidate the nature of an unusual effect of <i>T</i> on δ

C_Ar–H···O Hydrogen Bonds in Substituted Isobenzofuranone Derivatives: Geometric, Topological, and NMR Characterization

Substituted isobenzofuranone derivatives <b>1a</b>–<b>3a</b> and bindone <b>4</b> are characterized by the presence of an intramolecular C_Ar–H···O hydrogen bond in the crystal (X-ray), solution (¹H 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 <b>1a</b>–<b>3a</b> (independent of substituent nature) and in <b>4</b> with the change in media in the following order: gas phase > CHCl₃ solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for <b>1a</b>–<b>3a</b> and 5.6 kcal/mol for <b>4</b>. Both in crystals and in solutions, hydrogen bond strength increases in the order <b>1a</b> < <b>2a</b> < <b>3a</b> with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating ¹H NMR chemical shifts (δ^calcd – δ^expl < 0.7 ppm) of hydrogen bonded and nonbonded protons in <b>1a</b>–<b>3a</b> and <b>1b</b>–<b>3b</b> (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<i>J</i>(¹³C–¹H) 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)

C_Ar–H···O Hydrogen Bonds in Substituted Isobenzofuranone Derivatives: Geometric, Topological, and NMR Characterization

Substituted isobenzofuranone derivatives <b>1a</b>–<b>3a</b> and bindone <b>4</b> are characterized by the presence of an intramolecular C_Ar–H···O hydrogen bond in the crystal (X-ray), solution (¹H 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 <b>1a</b>–<b>3a</b> (independent of substituent nature) and in <b>4</b> with the change in media in the following order: gas phase > CHCl₃ solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for <b>1a</b>–<b>3a</b> and 5.6 kcal/mol for <b>4</b>. Both in crystals and in solutions, hydrogen bond strength increases in the order <b>1a</b> < <b>2a</b> < <b>3a</b> with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating ¹H NMR chemical shifts (δ^calcd – δ^expl < 0.7 ppm) of hydrogen bonded and nonbonded protons in <b>1a</b>–<b>3a</b> and <b>1b</b>–<b>3b</b> (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<i>J</i>(¹³C–¹H) 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)

C_Ar–H···O Hydrogen Bonds in Substituted Isobenzofuranone Derivatives: Geometric, Topological, and NMR Characterization

Substituted isobenzofuranone derivatives <b>1a</b>–<b>3a</b> and bindone <b>4</b> are characterized by the presence of an intramolecular C_Ar–H···O hydrogen bond in the crystal (X-ray), solution (¹H 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 <b>1a</b>–<b>3a</b> (independent of substituent nature) and in <b>4</b> with the change in media in the following order: gas phase > CHCl₃ solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for <b>1a</b>–<b>3a</b> and 5.6 kcal/mol for <b>4</b>. Both in crystals and in solutions, hydrogen bond strength increases in the order <b>1a</b> < <b>2a</b> < <b>3a</b> with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating ¹H NMR chemical shifts (δ^calcd – δ^expl < 0.7 ppm) of hydrogen bonded and nonbonded protons in <b>1a</b>–<b>3a</b> and <b>1b</b>–<b>3b</b> (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<i>J</i>(¹³C–¹H) 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)

Cage Complexes of Carbenium and Silylium Cations with an Aromatic Base. Is the η⁶ Coordination Type Realizable?

The cage cations [E­((CH₂)₃)₃C₆R₃]⁺ (E = C (<b>1</b>), Si (<b>2</b>); R = H (<b>a</b>), Li (<b>b</b>), F (<b>c</b>)) were studied theoretically using the MP2/6-311++G­(d,p) approach. The migration of the electrophilic center E and a proton around the perimeter of the arene ring in the cations [E­((CH₂)₃)₃C₆H₃]⁺ was considered. Our results are indicative of the π type of E←arene coordination in the intramolecular complexes [E­((CH₂)₃)₃C₆R₃]⁺. The cation [Si­((CH₂)₃)₃C₆H₃]⁺ (<b>2a</b>), which corresponds to a peak (M – CH₃)⁺ at <i>m</i>/<i>z</i> 229 in the mass spectrum of methylsilacyclophane, exists in a firmly established η¹π form, as opposed to the case for the known complexes of the trivalent silicon atom with aromatic bases. The species [C­((CH₂)₃)₃C₆F₃]⁺ (<b>1c</b>), as well as <b>4a</b>, obtained by connecting the equatorial carbon atoms in [Si­((CH₂)₃)₃C₆H₃]⁺ by a methylene bridge, are the first representatives of stable “face” complexes of carbenium and silylium cations with a benzene ring. The process of deprotonation of the complexes [C­((CH₂)₃)₃C₆H₃]⁺, [Si­((CH₂)₃)₃C₆H₃]⁺, and <b>4a</b> was found to be energetically unfavorable, even in the presence of a strong base such as NEt₃. The effect of the counterions BX₄^– (X = F, C₆F₅) and the polarity of solvents on the structure of the above cations was investigated using the examples of toluene and DMSO