5 research outputs found

    Conformational Control in [22]- and [24]Pentaphyrins( by Meso Substituents and their N‑Fusion Reaction

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    <i>meso</i>-Substituted pentaphyrins( were unexpectedly isolated as N-fused species under Rothemund-type conditions. The reaction mechanism is unknown at present, but the first example of a nonfused [22]­pentaphyrin was reported in 2012. Here, the conformational preferences and N-fusion reaction of [22]- and [24]­pentaphyrins have been investigated using density functional calculations, together with their aromaticity-molecular topology relationships. Two global minima are found for the unsubstituted [22]­pentaphyrin corresponding to <i>T0</i> and <i>T0</i><sup>4,D</sup> Hückel structures. Möbius transition states are located in the interconversion pathways with activation barriers of 27 kcal mol<sup>–1</sup>. Conversely, [24]­pentaphyrin is able to switch between Hückel and Möbius conformers with very low activation barriers. However, nonfused [24]­pentaphyrins are unstable and spontaneously undergo an N-fusion reaction driven by the strain release. On the contrary, nonfused [22]­pentaphyrins could be isolated if a <i>T0</i><sup>4,D</sup> conformation is adopted. Importantly, conformational control of pentaphyrins can be achieved by <i>meso</i>-substituents. Two stable conformations (<i>T0</i><sup>4,D</sup> and <i>T0</i><sup>A,D</sup>) are found for the nonfused [22]­pentaphyrin, which are delicately balanced by the number of substituents. The <i>T0</i><sup>A,D</sup> conformation is preferred by fully <i>meso</i>-aryl pentaphyrins, which is converted to the N-fused species. Interestingly, the removal of one aryl group prevents the N-fusion reaction, providing stable aromatic nonfused [22]­pentaphyrins in excellent agreement with the experimental results

    Qualitative Insights into the Transport Properties of Hückel/Möbius (Anti)Aromatic Compounds: Application to Expanded Porphyrins

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    Expanded porphyrins have been recently identified as promising candidates for conductance switching based on aromaticity and molecular topology changes. However, the factors that control electron transport switching across the metal–molecule–metal junction still need to be elucidated. For this reason, the transport properties of Hückel/Möbius (anti)­aromatic compounds are investigated thoroughly in this work to gain qualitative understanding into the conductivity of these unique macrocycles. Starting from a polyene model, a simple counting rule is developed to predict the occurrence of quantum interference around the Fermi level at the Hückel level of theory. Next, the different approximations of Hückel theory are lifted, enabling the exploration of the influence of each of these approximations on the transport properties of expanded porphyrins. Along the way, a detailed study on the relationship between the conductance and aromaticity/topology has been undertaken. Even though it has been proposed that the π-conjugated systems of expanded porphyrins can be approximated as polyene macrocycles based on the “annulene model”, it turns out that the distortion induced by the pyrrole rings to the electronic structure of the expanded porphyrins causes the simple counting rule for the prediction of quantum interference developed for polyenes to fail in some specific situations. Nevertheless, our back-of-the-envelope approach enables an intuitive rationalization of most of the transport properties of expanded porphyrins. Our conclusions cast further doubt on the proposed negative relationship between conductance and aromaticity and highlight the importance of the connectivity on determining the shape of the transmission functions of the different states. We hope that the new insights provided here will offer experimentalists a road map toward the design of functional, multidimensional electronic switches based on expanded porphyrins

    Reactivity of Tin(II) Guanidinate with 1,2- and 1,3-Diones: Oxidative Cycloaddition or Ligand Substitution ?

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    A series of tin­(IV) guanidinates were prepared by a (4+1) oxidative cycloaddition of four 1,2-diones (3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone, 3,4,5,6-tetrachloro-1,2-benzoquinone, 9,10-phenanthrenedione, 1,2-diphenylethanedione) or by an oxidative addition of a C–Br bond (from 2-bromo-1,3-diphenylpropane-1,3-dione followed by rearrangement) and a Cl–Cl bond (Cl<sub>2</sub> generated from (dichloro-λ<sup>3</sup>-iodanyl)­benzene) with {<i>p</i>Tol-NC­[N­(SiMe<sub>3</sub>)<sub>2</sub>]­N-<i>p</i>Tol}<sub>2</sub>Sn (<b>1</b>). The reactivity of five pentane-1,3-diones and dimethyl malonate with compound <b>1</b> was assessed on the basis of the effect of 1,3-diones on the reaction mechanism in comparison with 1,2-diones. In contrast with oxidation reactions observed for compounds containing conjugated CO bonds, the reactions of the tin­(II) guanidinate with 1,3-diones revealed a high ability for ligand substitution. All the tin compounds prepared were characterized, and ligand substitution reactions were monitored using <sup>1</sup>H, <sup>13</sup>C, and <sup>119</sup>Sn NMR spectroscopy. The molecular structures of one tin­(II) and five tin­(IV) guanidinato complexes investigated were determined by X-ray diffraction. All tin­(IV) compounds display six- or seven-coordination. The UV–vis absorption spectra were recorded and simulated by TDDFT methods in order to get insight into the origin of the nontypical colors of the target tin­(IV) diolato-guanidinates and their keto-functionalized precursors

    Oxidative Additions of Homoleptic Tin(II) Amidinate

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    Seven tin­(IV) amidinates were prepared and isolated from the reactions of tin­(II) bisamidinate [Cy–NC­(<i>n</i>Bu)­N–Cy]<sub>2</sub>Sn with a series of various 1,2-diones ((4 + 1) oxidative cycloaddition mechanism) and chlorine/oxygen molecules, respectively. The ligand substitution effect of (non)­symmetric 1,3-diones to starting stannylene as well as intermolecular CO<sub>2</sub> activation via prepared dimeric stannoxane is also reported. All the prepared tin containing compounds as well as ligand substitution reactions were investigated by the multinuclear NMR (<sup>1</sup>H, <sup>13</sup>C, and <sup>119</sup>Sn) spectroscopic techniques. Molecular structures of one tin­(II) and seven tin­(IV) amidinates investigated were determined by X-ray diffractions and also evaluated by DFT methods. The UV–vis absorption spectra of all desired colored tin­(IV) diolates and its diketo precursors were recorded and simulated by TD-DFT methods

    Synthesis and Structural Characterization of Heteroboroxines with MB<sub>2</sub>O<sub>3</sub> Core (M = Sb, Bi, Sn)

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    Reaction of organoantimony and organobismuth oxides (LSbO)<sub>2</sub> and (LBiO)<sub>2</sub> (where L is [2,6-bis­(dimethylamino)­methyl]­phenyl) with four equivalents of the organoboronic acids gave new heteroboroxines LM­[(OBR)<sub>2</sub>O] <b>1a–2c</b> (for M = Sb: R = Ph (<b>1a</b>), 4-CF<sub>3</sub>C<sub>6</sub>H<sub>4</sub> (<b>1b</b>), ferrocenyl (<b>1c</b>); for M = Bi: R = Ph (<b>2a</b>), 4-CF<sub>3</sub>C<sub>6</sub>H<sub>4</sub> (<b>2b</b>), and ferrocenyl (<b>2c</b>)). Analogously, reaction between organotin carbonate L­(Ph)­Sn­(CO<sub>3</sub>) and two equivalents of organoboronic acids yielded compounds L­(Ph)­Sn­[(OBR)<sub>2</sub>O] (where R = Ph (<b>3a</b>), 4-CF<sub>3</sub>C<sub>6</sub>H<sub>4</sub> (<b>3b</b>), and ferrocenyl (<b>3c</b>)). All compounds were characterized by elemental analysis and NMR spectroscopy. Their structure was described both in solution (NMR studies) and in the solid state (X-ray diffraction analyses <b>1a</b>, <b>1c</b>, <b>2b</b>, <b>3b</b>, and <b>3c</b>). All compounds contain a central MB<sub>2</sub>O<sub>3</sub> core (M = Sb, Bi, Sn), and the bonding situation within these rings and their potential aromaticity was investigated by the help of computational methods