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  1. Abstract

    The calculation of magnetic transition dipole moments and rotatory strengths was implemented at the zeroth‐order regular approximation (ZORA) two‐component relativistic time‐dependent density functional theory (TDDFT) level. The circular dichroism of the spin‐forbidden ligand‐field transitions of tris(ethylenediamine)cobalt(III) computed in this way agrees very well with available measurements. Phosphorescence dissymmetry factorsand the corresponding lifetimes are evaluated for three N‐heterocyclic‐carbene‐based iridium complexes, two of which contain helicene moieties, and for two platinahelicenes. The agreement with experimental data is satisfactory. The calculations reproduce the signs and order of magnitude of, and the large variations of phosphorescence lifetimes among the systems. The electron spin contribution to the magnetic transition dipole moment is shown to be important in all of the computations.

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  2. Abstract

    This work details the synthesis, characterization, and catalytic activity of reactive low‐coordinate organozinc complexes. The complexes activate hydrogen and they appear to be more active in hydrogenation of ketones and imines than their tridentate pincer analogs. This is thought, in part, to be due to the lack of trailing third phosphorus arm present in previous work. DFT computations reveal a sigma‐bond metathesis mechanism is comparable to an alternative aromatization/dearomatization metal‐ligand cooperative mechanism.

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  3. Abstract

    The first chiral helicene‐NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L‐shaped chiral ligand is composed of an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]‐helicenoid unit. The chiral information was introduced in a key post‐functionalization step of a NHC‐gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X‐ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC‐gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N‐tethered 1,6‐enynes with up to 95 : 5 er.

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  4. Abstract

    Vibrational Raman optical activity (ROA) spectra were calculated under off‐resonance, near‐resonance, and at‐resonance conditions for(A) and under off‐resonance conditions for(B) using a new driver software for calculating the ROA intensities from complex (damped) time‐dependent linear response Kohn‐Sham theory. The off‐resonance spectra ofAandBshow many similarities. At an incident laser wavelength of 532 nm, used in commercial ROA spectrometers, the spectrum ofAis enhanced by near‐resonance with the ligand‐field transitions of the complex. The near‐resonance spectrum exhibits many qualitative differences compared with the off‐resonance case, but it remains bi‐signate. Even under full resonance with the ligand‐field electronic transitions, the ROA spectrum ofAremains bi‐signate when the electronic transitions are broadened such as to yield absorption line widths that are comparable with those in the experimental UV‐vis absorption and electronic circular dichroism spectra.

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  5. Abstract

    The performance of optimally tuned range‐separated hybrid (OT‐RSH) functional calculations in predicting accurate isotropic nuclear magnetic shielding (σ) and chemical shift values is examined. To that end, the results of OT‐RSH and other approximate density functional theory calculations are assessed against recently published benchmark CCSD(T) calculations for a test set consisting of several molecules and bond types. It is found that for atoms in single bonds with a large paramagnetic contribution to σ, OT‐RSH offers a significant improvement in prediction of shielding constants over popular semi‐local and hybrid density functionals, yielding non‐empirical results that are as accurate as those of semi‐empirical density functionals often used for prediction of shielding constants. This success is attributed to the improved fundamental gap prediction of the OT‐RSH approach. For atoms in multiple bonds, however, larger errors often persist. By comparing OT‐RSH and recently reported double‐hybrid functional results, the remaining difficulties are traced to significant non‐local correlation.

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  6. Abstract

    The industrial importance of the CC double bond difunctionalization in vegetable oils/fatty acid chains motivates computational studies aimed at helping to improve experimental protocols. The CC double bond epoxidation is studied with hydrogen peroxide, peracetic acid (CH3CO3H), and performic acid (HCO3H) oxidizing agents. The epoxide ring‐opening mechanism is calculated in the presence of ZnCl2, NiCl2, and FeCl2Lewis acidic catalysts. Computations show that H2O2(∆G= 39 kcal/mol,TS1HP) is not an effective oxidizing agent compared to CH3CO3H (∆G= 29.8 kcal/mol,TS1PA) and HCO3H (∆G= 26.7 kcal/mol,TS1PF). The FeCl2(∆G= 14.7 kcal/mol,TS1FC) coordination to the epoxide oxygen facilitates the ring‐opening via lower energy barriers compared to the ZnCl2(∆G= 19.5 kcal/mol,TS1ZC) and NiCl2(∆G= 29.4 kcal/mol,TS1NC) coordination. ZnCl2was frequently utilized as a catalyst in laboratory‐scale procedures. The energetic span model identifies the FeCl2(FC) catalytic cycle as the best option for the epoxide ring‐opening.

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  7. Abstract

    By attaching pyridine groups to a diaza[6]helicene, a helical, bis‐ditopic, bis‐NN‐coordinating ligand can be accessed. Dinuclear rhenium complexes featuring this bridging ligand, of the form [{Re(CO)3Cl}2(NNNN)], have been prepared and resolved to give enantiopure complexes. These complexes are phosphorescent in solution at room temperature under one‐ and two‐photon excitation. Their experimental chiroptical properties (optical rotation, electronic circular dichroism and circularly polarized emission) have been measured. They show, for instance, emission dissymmetry factors of c.a. ±3x10−3. Quantum‐chemical calculations indicate the importance of stereochemistry on the optical activity, pointing towards further design improvements in such types of complexes.

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  8. Abstract

    The first enantiopure chiral‐at‐rhenium complexes of the formfac‐ReX(CO)3(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems.

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