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Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021Visible-Light-Driven Anti-Markovnikov Hydrocarboxylation of Acrylates and Styrenes with CO2 He Huang†, Jian-Heng Ye†, Lei Zhu†, Chuan-Kun Ran, Meng Miao, Wei Wang, Hanjiao Chen, Wen-Jun Zhou, Yu Lan, Bo Yu and Da-Gang Yu He Huang† Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Jian-Heng Ye† Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Lei Zhu† School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030 , Chuan-Kun Ran Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Meng Miao Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Wei Wang Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Hanjiao Chen Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Wen-Jun Zhou Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 , Yu Lan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030 College of Chemistry, and Institute of Green Catalysis, Zhengzhou University, Zhengzhou 450001 , Bo Yu Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 and Da-Gang Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Analytical & Testing Center, Sichuan University, Chengdu 610064 https://doi.org/10.31635/ccschem.020.202000374 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Light-driven carbon dioxide (CO2) capture and utilization is one of the most fundamental reactions in Nature. Herein, we report the first visible-light-driven photocatalyst-free hydrocarboxylation of alkenes with CO2. Diverse acrylates and styrenes, including challenging tri- and tetrasubstituted ones, undergo anti-Markovnikov hydrocarboxylation with high selectivities to generate valuable succinic acid derivatives and 3-arylpropionic acids. In addition to the use of stoichiometric aryl thiols, the thiol catalysis is also developed, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. The UV–vis measurements, NMR analyses, and computational investigations support the formation of a novel charge-transfer complex (CTC) between thiolate and acrylate/styrene. Further mechanistic studies and density functional theory (DFT) calculations indicate that both alkene and CO2 radical anions might be generated, illustrating the unusual selectivities and providing a novel strategy for CO2 utilization. Download figure Download PowerPoint Introduction Photosynthesis is one of the most fundamental reactions in nature. For more than one century, chemists have been mimicking nature and searching for highly efficient visible-light photochemistry to realize novel chemical transformations in an environmentally friendly way.1 As diverse organic compounds cannot be excited by visible light, external photocatalysts are indispensable for most cases.2–5 Compared with widely investigated photocatalysis, visible-light-driven charge-transfer complex (CTC) or electron donor and acceptor (EDA) complex in the absence of external photocatalyst is less developed, although it is more economical and sustainable for industry.6 Notably, the light-driven CTC of thiols/disulfides and alkenes has attracted a great deal of attention in organic and polymer chemistry.7–9 For example, Wang reported a novel visible-light-driven CTC of alkenes and disulfides with alkenes as electron donors and disulfides as electron acceptors (Figure 1b).9 While few examples of CTCs of thiolates and (hetero)arenes have been reported,10–13 to the best of our knowledge, the CTCs of thiolates and acrylates/styrenes have not been reported. Figure 1 | (a) Selected examples for valuable succinic acid and 3-arylpropionic acid derivatives. (b–d) Visible-light-driven CTCs between alkenes and disulfides or thiolates and acrylates/styrenes. CTC, charge-transfer complex; 4-PE-HE, 4-phenethyl Hantzsch ester. Download figure Download PowerPoint Carbon dioxide (CO2), a well-known greenhouse gas, has gained considerable attention as an ideal one-carbon building block in chemical transformations due to its abundance, low cost, and sustainability.14–23 Hydrocarboxylations of alkenes with CO2, especially transition-metal-catalyzed hydrocarboxylations, represent efficient strategies to generate bioactive and synthetically useful carboxylic acids.24 Recently, in addition to UV-light photocatalysis,16,25–27 visible-light photocatalysis has emerged as an intriguing strategy to realize novel organic transformations with CO2.17,18,22,28–42 Notably, Iwasawa pioneered this strategy by realizing the hydrocarboxylation of alkenes with Markovnikov regioselectivity via visible-light photoredox/Rh dual catalysis.27,41 Besides Markovnikov selectivity, König et al.35 also realized a novel hydrocarboxylation of styrenes with anti-Markovnikov regioselectivity26,43–45 via photoredox/nickel dual catalysis. In these cases, both photocatalysts and transition-metal catalysts are indispensable. Moreover, CO2 is proposed to undergo reaction with highly reactive organometalic intermediates or a low-valent transition-metal catalyst, both of which are two-electron processes. In contrast, it is still highly challenging for visible-light-driven single-electron activation of CO2 to generate its corresponding radical anion, which is reactive and with selectivity that is hard to control. Anti-Markovnikov hydrocarboxylation of acrylates with CO2,46 which is in high demand to provide valuable succinic acid derivatives for pharmaceuticals (Figure 1a),47 in the organic and polymer chemistry industry,48 remains an unresolved challenge, we wondered whether we could realize this in an economical, efficient, and selective way. We envisioned that the electron-rich thiolates and electron-deficient acrylates/styrenes might form the CTC to facilitate the single-electron reduction of acrylates/styrenes or CO2 to generate radical anions, thus promoting anti-Markovnikov hydrocarboxylation of acrylates/styrenes (Figure 1c). We recognized, however, that such a strategy faced many challenges. For example, the light-driven thiol–ene reaction, oligmerization, and/or polymerization with C–S bond formation are competitively efficient.7,8,49 The thiocarboxylation33 and transesterification of acrylates may also occur, thus further complicating the reaction mixture. Herein, we report our success in realizing the first anti-Markovnikov selective hydrocarboxylation of acrylates and styrenes with CO2 under visible-light-driven photocatalyst-free conditions, enabled by a novel CTC between thiolate and acrylate/styrene (Figure 1d). Experimental Section Experimental methods To an oven-dried Schlenk tube (10 mL) equipped with a magnetic stir bar is added the alkene (0.2 mmol, 1.0 equiv for nonliquid substrates). Then the tube is moved into a glovebox and charged with NaOtBu (0.5 mmol, 2.5 equiv). The tube is sealed, evacuated, and backfilled with CO2 three times. Subsequently, the tube is opened and N-Methyl-2-pyrrolidinone (NMP) (2 mL) is added, followed by thiophenol (0.4 mmol, 2.0 equiv), tBuOH (0.4 mmol, 2.0 equiv), and alkene (0.2 mmol, 1.0 equiv for liquid substrates) via syringe under CO2. Once added, the resulting mixture is degassed by using a freeze-pump-thaw procedure (two times). Then the Schlenk tube is backfilled with CO2 and sealed at atmospheric pressure of CO2 (1 atm). The reaction is stirred and irradiated with a 30 W blue light-emitting diode (LED) lamp (1 cm away, with cooling fan to keep the reaction temperature at 25 °C and the reaction region located in the center of LED lamp) for 24 h. The resulting mixture is diluted with 3 mL EtOAc and quenched by 1.5 mL 2 N HCl, then stirred for 5 min. The reaction mixture is extracted by EtOAc six times, and the combined organic phases are concentrated in vacuo. The residue is purified by silica gel flash column chromatography (petroleum ether/EtOAc/AcOH 10/1/0.4%∼3/1/0.4%) to give the pure desired product. Further details may be found in the Supporting Information. Computational methods All the density functional theory (DFT) calculations are carried out with the Gaussian 0950 series of programs. DFT method ωB97XD51 with a standard 6–31+G(d) basis set is used for geometry optimizations. The solvent effects are considered by an universal solvation model based on solute electron density (SMD)52. Harmonic vibrational frequency calculations are performed for all of the stationary points to confirm them as local minima or transition structures, and to derive the thermochemical corrections for the enthalpies and free energies. The large basis set 6–311+G(d,p) is used to calculate the single-point energies to give more accurate energy information. Results and Discussion At the beginning of the research, we hypothesized that the addition of a proton source and the use of sterically hindered thiols might inhibit the side reactions, including thiocarboxylation and thiol–ene reactions. After systematic screening (Table 1), the selective hydrocarboxylation of methacrylate 1a proceeded in the presence of the easily available and bulky thiol, 2,4,6-triisopropylthiophenol, as well as NaOtBu and tBuOH to give the desired product 2a in 73% yield (Entry 1). The resulting chemo- and regioselectivities were both excellent; only very low yields of 2a′ (8% yield) and 2a″ (<5% yield) were obtained. Neither the Markovnikov-type hydro- nor thiocarboxylation was observed. Control experiments confirmed the essential roles of thiol, light, base, and CO2 (Entries 2–5). In the absence of tBuOH, 2a was obtained in a much lower yield (Entry 6). Using iPrOH instead of tBuOH gave a lower yield (Entry 7). The use of less sterically hindered thiols, such as 4-tert-butylthiophenol, resulted in poorer chemoselectivity (Entries 8–10). Other bases and solvents were tested, but they gave worse results (Entries 11 and 12). Table 1 | Optimization of the Reaction Conditions Entry Alteration Yield/2a Yield/2a′ Yield/2a″ 1 None 76% (73%) 8% <5% 2 Without ArSH N.D. N.D. N.D. 3 Without light N.D. 34% N.D. 4 Without NaOtBu N.D. 33% N.D. 5 Under N2 instead of CO2 N.D. N.D. N.D. 6 Without tBuOH 42% 6% 6% 7 iPrOH instead of tBuOH 65% 12% <5% 8 p-tBuC6H4SH as ArSH 64% 24% <5% 9 p-tBuC6H4SH, FeCl3 (5 mol %) Trace 5% 66% 10 Entry 9, tBuOH (2 equiv) Trace 16% 76% 11 K2CO3 instead of NaOtBu 46% 21% <5% 12 DMSO instead of NMP 33% 16% <5% Reaction conditions: 1a (0.2 mmol), 2,4,6-triisopropylthiophenol (ArSH, 0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h. NMR yields are determined by crude 1H NMR with CH2Br2 as internal standard. Yield of isolated product is provided in parenthesis. DMSO, dimethyl sulfoxide; LED, light-emitting diode; N.D., not detected; RT, room temperature. With satisfactory conditions (Condition A: Table 1, Entry 1) in hand, we aimed to investigate the scope of acrylates (Figure 2). A diversity of 2-methylacrylates bearing tertiary ( 1a– 1e), secondary ( 1f– 1h), and primary ( 1i and 1j) alkyl substituents in the ester moiety all underwent hydrocarboxylation under these conditions with high chemo- and regioselectivity. Besides methyl, other kinds of primary ( 1k– 1o), secondary ( 1p– 1r), and tertiary ( 1s) alkyl groups, as well as aryl groups ( 1t– 1w), were well tolerated at the α-position of tert-butyl acrylates. Excellent chemoselectivity was also observed in the hydrocarboxylation of 2-allylacrylate 1o. Furthermore, β-substituted acrylates also provided the corresponding products 2x– 2ba in good yields. Notably, the challenging tetrasubstituted acrylates with high steric hindrance, such as 1ca and 1da, also gave the desired products in moderate yields accompanied by recovery of the starting material (in 28% for 1ca and 44% yield for 1da, respectively) without any products of reduction, thiol–ene reaction and thiocarboxylation detected. Moreover, some acrylates ( 1ea– 1ia) with bioactive motifs, including isoborneol, L-(–)-menthol, α-terpineol, 4-carvomenthenol, and β-cholesterol, were applicable in this transformation, providing the corresponding acids in moderate to good yields. Figure 2 | Substrate scope of acrylates and styrenes. Condition A: Table 1, Entry 1, isolated yields. Condition B: 3 (0.2 mmol), p-tBuC6H4SH (0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. Condition C: 1 or 3 (0.2 mmol), p-tBuC6H4SH (0.02 mmol), 4-PE-HE (0.6 mmol), NaOtBu (0.4 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. aThe diastereoselectivity (d.r.) values are about 1∶1 by crude NMR. b3 mL of NMP was used. cd.r. = 3∶1. dd.r. = 1.8∶1. eThe Z/E ratio of acrylates 1 is 1∶1. fd.r. = 5∶1. gd.r. > 19∶1, cis-isomer (see Supporting Information for more details). h3 equiv of ArSH and 3.5 equiv of NaOtBu were used, 48 h. iGram scale. j2 equiv of p-tBuC6H4SNa was used instead of p-tBuC6H4SH, NaOtBu, and tBuOH. kThe ratio (95∶5) of 4p and 4a is determined by ultra-performance liquid chromatography (UPLC). LED, light-emitting diode; RT, room temperature; N.D., not detected; 4-PE-HE, 4-phenethyl Hantzsch ester. Download figure Download PowerPoint As 3-arylpropionic acids also are valuable compounds (Figure 1a),35,43–45 we further investigated the visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of styrenes (Figure 2). Under slightly modified reaction conditions (Condition B: p-tBuC6H4SH is used instead of 2,4,6-triisopropylthiophenol in Condition A), diverse styrenes bearing either electron-neutral ( 3a– 3c, 3i, and 3m) or electron-donating ( 3d, 3f, and 3j) groups on the arene moiety all reacted smoothly to afford the desired products 4 in moderate to good yields. A diversity of functional groups and heteroarenes ( 3g and 3ba) were tolerated well. The electron-withdrawing CF3 group ( 3h) could also be compatible to give desired product in 31% yield when employing p-tBuC6H4SNa. Notably, o-allyloxystyrene 3l underwent the reaction smoothly to afford 4l with excellent chemoselectivity. 3-Chlorostyrene 3q provides desired product 4q in good yield with very little dehalogenation (95∶5 ratio 4q: 4a). The styrenes bearing mono-, di-, and trisubstituted arenes as well as α-substitution all were suitable substrates under our protocol. A bioactive product 4ia, an antidiabetic GPR40 agonist, was generated in 74% yield through the hydrocarboxylation of 3ia. More challenging substrates, such as trisubstituted alkene ( 3ma), could also give the desired product in 50% yield. Notably, the gram-scale reaction of 3a went smoothly to give 4a in 78%. Interestingly, a dihydrobenzothiazole could replace the aryl thiol for the generation of 4a in 68% under otherwise identical reaction conditions (see Supporting Information for more details). Given that the aryl thiolates act as the electron donors to form disulfides as main byproducts and are not incorporated into the desired products, we hypothesized whether we could realize a thiol catalysis in the presence of stoichiometric reductant. If successful, to the best of our knowledge, such a strategy would represent the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. However, significant challenges remained due to facile side reactions of aryl thiols with alkenes, including thiol–ene reaction and thiocarboxylation, which would consume the catalyst and thus terminate the catalytic cycle. Moreover, competitive reduction of alkenes, CO2 and disulfides by the external reductant might be difficult to control. Therefore, we strove to realize this organocatalytic strategy (see Supporting Information for more details). Fortunately, we found that 4-phenethyl Hantzsch ester (4-PE-HE)53 was an ideal reductant to promote the thiol catalysis, providing the desired product 3a in 70% yield (Figure 2). Under the catalytic reaction conditions (Condition C), diverse acrylates, such as those bearing different ester moieties ( 1a, 1f, 1h, and 1i), α-alkyl ( 1k, 1r, and 1s), α-phenyl ( 1t), and β-ethyl ( 1y) substitution, all were suitable substrates. Moreover, bioactive derivatives of acrylates, such as 1ha and 1ia, could also undergo such a transformation smoothly. Besides acrylates, styrenes bearing mono- ( 3b, 3d, 3e, 3i, and 3m) and disubstituted ( 3r and 3x) arenes delivered the desired products 4 smoothly. The successful application of α-aryl ( 3ca– 3ea) and α-methyl ( 3ja) styrenes in this catalytic hydrocarboxylation was also realized. To gain more insights into the reaction mechanism, we tested various control experiments (Figure 3). When 2,2,6,6-tetramethyl-1-piperinedinyloxy (TEMPO) was added to the reaction mixture, the desired product 2a was not detected (Figure 3a). "Radical clock" experiments resulted in exclusive formation of the corresponding ring-opening products (Figure 3b). These results suggested the formation of radical intermediates. In addition, some reductive coupling products of 3a, the reduction products of 3aa, and the reductive ring-opening products of 3la were also detected under N2 or CO2 (Figures 3c–3e). Cyclic voltammetry (CV) test demonstrated that the reaction mixture is highly reductive (see Supporting Information for more details). Electron paramagnetic resonance (EPR) spectroscopy further supported the reduction of alkenes to generate alkyl radicals in this system (Figures 3f and 3g). The experiments using 13C-labeled CO2 generated the 2a, sodium formate, and sodium oxalate, all with 13C-labeling (Figure 3h). Moreover, both sodium formate and sodium oxalate were detected under the reaction conditions in the absence of alkenes (Figure 3i), which suggested that CO2 could be also reduced in such highly reductive conditions. Figure 3 | (a–i) Mechanistic studies. EPR, electron paramagnetic resonance; HRMS, high-resolution mass spectra; LED, light-emitting diode; N.D., not detected; RT, room temperature; TEMPO, 2,2,6,6-tetramethyl-1-piperinedinyloxy. Download figure Download PowerPoint To probe our hypothesis of formation of CTC, we conducted the UV–vis absorption spectra tests. A slight bathochromic shift was observed when we tested the mixture of p-tBuC6H4SNa with either 1a or 3a (Figures 4a and 4b). When we used a more electron-deficient 4-(trifluoromethyl)styrene, the bathochromic shift was more significant (Figure 4c). To seek more evidence for the CTC formation, we carried out 19F-NMR experiments, mixing a certain amount of 4-(trifluoromethyl)styrene with increasing amounts of p-tBuC6H4SNa. The chemical shift of the CF3 group distinctly moved downfield (Figure 4d), indicating the increased electron density on the 4-(trifluoromethyl)styrene via electron donation from strongly reductive thiolates (−2.66 V vs Ag/Ag+; see Supporting Information for more details) to styrenes. All these results indicated the formation of CTC between electron-rich thiolates and electron-deficient styrenes. Figure 4 | UV–vis measurements and DFT calculations on the CTC between thiolate and acrylate/styrene. (a) UV–vis measurement of 1a. (b) UV–vis measurement of 3a. (c) UV–vis measurement of 4-(trifluoromethyl)styrene. (d) Correlation between the chemical shift of the CF3 group of 4-(trifluoromethyl)styrene and the relative p-tBuC6H4SNa concentration by 19F-NMR. (e) The energy of HOMO of CTC between thiolate and 1a is −6.45 eV. (f) The energy of LUMO of CTC between thiolate and 1a is +0.86 eV. (g) ESP surface of singlet state of CTC. (h) ESP surface of triplet state of CTC. CTC, charge-transfer complex; DFT, density functional theory; ESP, electrostatic potential; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital. Download figure Download PowerPoint Besides the experimental evidence, we further investigated the formation of CTC between thiolate and acrylate 1a using DFT calculations at the ωB97XD/6–31+G(d) level of theory. The energy gap was reduced as 7.31 eV (Figures 4e and 4f) compared with 1a and thiolates ( Supporting Information Figures S18a–S18d), which might arise from the electron donation from the highest occupied molecular orbital (HOMO) of thiolate to the lowest unoccupied molecular orbital (LUMO) of 1a. The reduced HOMO–LUMO gap explained well the bathochromic shift of absorption of CTC. Moreover, DFT calculations of electrostatic potential (ESP) surface of singlet and triplet states showed that the electron donation from thiolates to 1a take place in the excited state (Figures 4g and 4h). Although CO2 might also act as an electron acceptor, the DFT calculations indicated the formation of CTC between thiolate, and CO2 might be less favored because of its higher energy gap (8.06 eV) and excited energy (65.5 kcal/mol; Supporting Information Figures S21 and S23) than that of thiolate–alkene CTC (7.31 eV; 56.5 kcal/mol; Supporting Information Figure S23) in our reaction system. Based on the experimental and computational results, we proposed possible pathways for the hydrocarboxylation of 3a (Figure 5a). Irradiation of the CTC 5 might generate styrene radical anion 6 (Path a) or CO2 radical anion 7 (Path b) as well as thiol radical 8. reactions of 6 with CO2 and 7 with 3a could generate the 9, which might undergo to would to desired product Besides the desired hydrocarboxylation to give and thiocarboxylation are also possible via the intermediates 11 and which might be generated from the reaction of 8 and 3a or Figure 5 | and corresponding DFT CTC, charge-transfer complex; DFT, density functional theory. Download figure Download PowerPoint To probe the of such pathways (Figure a and we further DFT The CTC at triplet state could the electron to styrene or CO2 resulting styrene radical anion 6 or CO2 radical anion 7 suggested that CO2 reduction might be more (Figure Moreover, the density in styrene 3a and acrylate 1a radical anion was also with higher density in which explained the high regioselectivity (see Supporting Information for more details). We further investigated the of chemoselectivity by DFT calculations (Figure The radical addition of 3a with 7 through transition state the radical 9 with an energy of to the However, the corresponding energy for the radical addition of 3a with thiol which to the was to The generated radical 11 was also more than Therefore, the of hydrocarboxylation was and more than In addition, an radical addition of 3a with 7 through resulted in Markovnikov However, results showed that Markovnikov selectivity was due to its high energy in with anti-Markovnikov from 9, a higher energy of thiocarboxylation ( was which was to a for hydrocarboxylation with lower energy ( Further DFT calculations for different thiol catalysis and were also investigated in our reaction system for (see Supporting Information for more details). We have a novel CTC between thiolate and which is supported by UV–vis measurements, NMR and computational Based on the CTC, the first visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of acrylates and styrenes with CO2 is realized to generate valuable succinic acid derivatives and 3-arylpropionic unusual and excellent and Moreover, this system is also and for diverse acrylates and styrenes with different including the challenging tri- and tetrasubstituted Notably, aryl thiol be used as a catalyst, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. Further mechanistic studies and DFT calculations indicate that both alkene and CO2 radical anions might be generated in the reaction mixture. As this photocatalyst-free and photochemistry great potential for application in organic and the polymer further investigations and of this system are in our Supporting Information Supporting Information is of The the A on this is with the support was provided by the the of and the Sichuan and Technology and and the for the in Chen Yu light-driven organic in in König a in in in 8. Wang Wei Wang Wang of and of an C–S via of via Electron

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We report the first investigation results of the nonlinear optical properties of As2Te3. More specifically, the nonlinear optical absorption properties of the prepared α-As2Te3 were investigated at wavelengths of 1.56 and 1.9 μm using the open-aperture (OA) Z-scan technique. Using the OA Z-scan technique, the nonlinear absorption coefficients (β) of α-As2Te3 were estimated in a range from (− 54.8 ± 3.4) × 104 cm/GW to (− 4.9 ± 0.4) × 104 cm/GW depending on the irradiance of the input beam at 1.56 μm, whereas the values did from (− 19.8 ± 0.8) × 104 cm/GW to (− 3.2 ± 0.1) × 104 cm/GW at 1.9 μm. In particular, the β value at 1.56 μm is an order of magnitude larger than the previously reported values of other group-15 sesquichalcogenides such as Bi2Se3, Bi2Te3, and Bi2TeSe2. Furthermore, this is the first time report on β value of a group-15 sesquichalcogenide at a 1.9-μm wavelength. The density functional theory (DFT) calculations of the electronic band structures of α-As2Te3 were also conducted to obtain a better understanding of their energy band structure. The DFT calculations indicated that α-As2Te3 possess sufficient optical absorption in a wide wavelength region, including 1.5 μm, 1.9 μm, and beyond (up to 3.7 μm). Using both the measured nonlinear absorption coefficients and the theoretically obtained refractive indices from the DFT calculations, the imaginary parts of the third-order optical susceptibilities (Im χ(3)) of As2Te3 were estimated and they were found to vary from (− 39 ± 2.4) × 10–19 m2/V2 to (− 3.5 ± 0.3) × 10–19 m2/V2 at 1.56 μm and (− 16.5 ± 0.7) × 10–19 m2/V2 to (− 2.7 ± 0.1) × 10–19 m2/V2 at 1.9 μm, respectively, depending on the irradiance of the input beam. Finally, the feasibility of using α-As2Te3 for SAs was investigated, and the prepared SAs were thus tested by incorporating them into an erbium (Er)-doped fiber cavity and a thulium–holmium (Tm–Ho) co-doped fiber cavity for both 1.5 and 1.9 μm operation.

Paramagnetic lanthanide(III) complexes stable in aqueous solutions have gained increasing interest in the recent years due to their importance as contrast agents in magnetic resonance imaging (MRI). Lanthanide(III) complexes with macrocyclic ligands derived from 1,4,7,10‐tetraazacyclododecane (cyclen) are widely used for the design of MRI probes because of their high thermodynamic stability and kinetic inertness. The rational design of more efficient contrast agents requires a better understanding of the structure and dynamics of these systems in solution. This contribution reviews the work of the author and his collaborators on the solution structure and dynamics of lanthanide(III) complexes with different cyclen‐based ligands and closely related systems. DFT calculations provide molecular geometries and relative energies of the different stereoisomers of these complexes in good agreement with the experimental data. The conformational analysis performed with the aid of density functional theory (DFT) calculations was validated with the investigation of the YbIII‐induced 1H NMR paramagnetic shifts, which encode information on the position of the observed NMR nuclei with respect to the LnIII ion. Additionally, DFT calculations provide a better understanding of the dynamic processes responsible for the interconversion between the square‐antiprismatic (SAP) and twisted‐square‐antiprismatic (TSAP) isomers of these complexes in solution, which might proceed through the inversion of the cyclen unit or the rotation of the pendant arms. The activation barriers obtained from theoretical calculations show a good agreement with the experimental values obtained from variable‐temperature NMR spectroscopy. The work presented in this paper shows that DFT calculations in combination with NMR spectroscopy provide detailed information on the structure and dynamics of lanthanide(III) complexes at the molecular level and represent a powerful tool for the characterization of lanthanide(III) complexes relevant to the field of MRI contrast agents.

Review: ca. 100 refs.

New palladium(II) complexes of the free-base tetrakis[2,3-(5,6-di-2-pyridylpyrazino)porphyrazine], [Py 8TPyzPzH 2], have been prepared and their physicochemical properties examined. The investigated compounds are the pentanuclear species [(PdCl 2) 4Py 8TPyzPzPd], the monopalladated complex [Py 8TPyzPzPd], and its corresponding octaiodide salt [(2-Mepy) 8TPyzPzPd](I) 8. All three Pd (II) complexes have a common central pyrazinoporphyrazine core and differ only at the periphery of the macrocycle, where the simple dipyridinopyrazine fragments present in [Py 8TPyzPzPd] bear four PdCl 2 units coordinated at the pyridine N atoms in the pentanuclear complex, [(PdCl 2) 4Py 8TPyzPzPd], or carry pyridine-N(CH 3) (+) moieties in the iodide of the octacation [(2-Mepy) 8TPyzPzPd] (8+). The structural features of the pentanuclear complex [(PdCl 2) 4Py 8TPyzPzPd], partly supported by X-ray data and solution (1)H NMR spectra of the [(CN) 2Py 2PyzPdCl 2] precursor, were elucidated through one- and two-dimensional (1)H NMR spectra in solution and density functional theory (DFT) calculations. Structural information on the monopalladated complex [Py 8TPyzPzPd] was also obtained from DFT calculations. It was found that in the complex [(PdCl 2) 4Py 8TPyzPzPd] the peripheral PdCl 2 units adopt a py-py coordination mode and the generated N 2PdCl 2 moieties are directed nearly perpendicular to the plane of the pyrazinoporphyrazine ring, strictly recalling the arrangement found for the palladated precursor [(CN) 2Py 2PyzPdCl 2]. NMR and DFT results consistently indicate that of the four structural isomers predictable for [(PdCl 2) 4Py 8TPyzPzPd], one having all four N 2PdCl 2 moieties pointing on the same side of the macrocyclic framework (i.e., isomer 4:0, plus the 3:1 and the 2:2-cis and 2:2-trans isomers), the 4:0 isomer ( C 4 v symmetry) is the predominant form present. According to cyclic voltammetry and spectroelectrochemical results in pyridine, dimethyl sulfoxide (DMSO), and dimethylformamide (DMF), the monopalladated complex [Py 8TPyzPzPd] undergoes four reversible or quasi-reversible one-electron ligand-centered reductions, similar to the behavior also observed for the pentanuclear complex [(PdCl 2) 4Py 8TPyzPzPd], which shows an additional reduction peak attributable to the presence of PdCl 2. Owing to the electron-withdrawing properties of the PdCl 2 units, the pentanuclear complex is easier to reduce than the mononuclear complex [Py 8TPyzPzPd], some related [Py 8TPyzPzM] complexes, and their porphyrin or porphyrazine analogues, so much so that the corresponding monoanion radical is generated at potentials close to 0.0 V vs SCE in DMSO or DMF. In turn, the monoanion of [(2-Mepy) 8TPyzPzPd](I) 8 is also extremely easy to generate electrochemically. Indeed, because of the eight positively charged N-CH 3 (+) groups in this complex the first reduction occurs at potentials close to +0.10 V in DMSO or DMF. The redox behavior of the mono- and pentapalladated complexes has been rationalized on the basis of a detailed DFT analysis of their ground-state electronic structure.

Molecular Simulations of Nanoscale Transformations in Ionic Semiconductor Nanocrystals

The structure of tert-butylphosphonic acid in the solid, in solution, and in the gas phase was studied by single-crystal X-ray diffraction, (1)H and (31)P NMR spectroscopic studies in solution, solid-state (31)P NMR spectroscopy, and electrospray ionization mass spectrometry. In addition, density functional theory (DFT) calculations at the B3LYP/6-31G*, B3LYP/6-31+G*, and B3LYP/6-311+G* level of theory for a large number of H-bonded aggregates of the type (tBuPO(3)H(2))(n) (C(n), P(n); n=1-7) support the experimental work. Crystallization of tBuPO(3)H(2) from polar solvents such as CH(3)CN or THF gives the H-bonded one-dimensional polymer 2, whereas crystallization from the less polar solvent CDCl(3) favors the formation of the H-bonded cluster (tBuPO(3)H(2))(6).CDCl(3) (1). In CDCl(3) the hexamer (tBuPO(3)H(2))(6) (C(6)) is replaced by smaller aggregates down to the monomer with decreasing concentration. DFT calculations and natural bond orbital (NBO) analyses for the clusters C(1)-C(7) and the linear arrays P(1)-P(7) reveal the hexamer C(6) to be the energetically favored structure resulting from cooperative strengthening of the hydrogen bonds in the H-bonded framework. However, the average hydrogen bond strengths calculated for C(6) and P(2) do not differ significantly (42-43 kJ mol(-1)). The average distances r(O.O), r(Obond;H), r(Pdbond;O), and r(Pbond;OH) in C(1)-C(7) and P(1)-P(7) are closely related to the hydrogen bond strength. Electrospray ionization mass spectrometry shows the presence of different anionic species of the type [(tBuPO(3)H(2))(n)-H](-) (A(1)-A(7), n=1-7) depending on the instrumental conditions. DFT calculations at the B3LYP/6-31G* level of theory were carried out for A(1)-A(6). We suggest the dimer [(tBuPO(3)H(2))(2)-H](-) (A(2)) and the trimer [(tBuPO(3)H(2))(3)-H](-) (A(3)) are the energetically favored anionic structures. A hydrogen bond energy of approximately 83 kJ mol(-1) was calculated for A(2). Electrospray ionization mass spectrometry is not suitable to study the assembling process of neutral H-bonded tert-butylphosphonic acid since the removal of a proton from the neutral aggregates has a large influence on the hydrogen bond strength and the cluster structure.

External Photocatalyst-Free Visible Light-Promoted 1,3-Addition of Perfluoroalkyl Iodides to Vinyldiazoacetates

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Isolation of RuIII-bda (17-electron specie) complex with an aqua ligand (2-electron donor) is challenging due to violation of the 18-electron rule. Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family, the structure and initial formation of the RuIII-bda aqua complex are still controversial. Herein, we challenge this often overlooked step by designing a pocket-shape Ru-based complex 1. The computational studies showed that 1 possesses the crucial hydrophobicity at the RuV(O) state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts. Through characterization of single-crystal structures at the RuII and RuIII states, a pseudo seven-coordinate “ready-to-go” aqua ligand with RuIII...O distance of 3.62 Å was observed. This aqua ligand was also found to be part of a formed hydrogen-bonding network, providing a good indication of how the RuIII-OH2 complex is formed.

Two novel Pt(IV) complexes of aromatic cytokinins with possible antitumor properties were prepared by reaction of selected aminopurines with K(2)PtCl(6). The structures of both complexes, 9-[6-(benzylamino)purine] pentachloroplatinate (IV) and 9-[6-(furfurylamino)purine] pentachloroplatinate (IV), were characterized in detail by using two-dimensional NMR spectroscopy ((1)H, (13)C, (15)N, and (195)Pt) in solution and CP/MAS NMR techniques in the solid state. We report for the first time the X-ray structure of a nucleobase adenine derivative coordinated to Pt(IV) via the N9 atom. The protonation equilibria for the complexes in solution were characterized by using NMR spectroscopy (isotropic chemical shifts and indirect nuclear spin-spin coupling constants) and the structural conclusions drawn from the NMR analysis are supported by relativistic density-functional theory (DFT) calculations. Because of the presence of the Pt atom, hybrid GGA functionals and scalar-relativistic and spin-orbit corrections were employed for both the DFT calculations of the molecular structure and particularly for the NMR chemical shifts. In particular, the populations of the N7-protonated and neutral forms of the complexes in solution were characterized by correlating the experimental and the DFT-calculated NMR chemical shifts. In contrast to the chemical exchange process involving the N7-H group, the hydrogen atom at N3 was determined to be unexpectedly rigid, probably because of the presence of the stabilizing intramolecular interaction N3-H···Cl. The described methodology combining the NMR spectroscopy and relativistic DFT calculations can be employed for characterizing the tautomeric and protonation equilibria in a large family of transition-metal-modified purine bases.

Orbital imaging and assessment of different orbital models for the valence shell of methanol: Comparison of electron momentum spectroscopy measurements with near-Hartree–Fock limit, MRSD-CI, localized valence bond and density functional theory