Pyramidalization of the Carboxamide sp2-Center in Peptide Structures.

A CSD search in the Cambridge Crystallographic Database for the substructure N-CαH-C'(═O)-N gave 24,180 peptide structures for analysis of the pyramidalization of the sp2-hybridized carboxamide group C'(═O)NCα, which had not been investigated before. The dependence of the pyramidalization θ = O-N-C'-Cα on the rotation angle ψ = O═C'-Cα-N about bond C'-Cα resulted in a curve with three maxima, three minima, and six zero-crossings. Surprisingly, the ψ/θ analysis of the individual amino acid building blocks showed that all of them exhibited similar curves, irrespective of their different R substituents. This unusual behavior is explained by a 3-fold short-range potential set up by the three covalent bonds, emanating from Cα. The tie-up of the rotation angle ψ and the pyramidalization θ in a rigid coupling is remarkable. In the 24,180 peptide structures, subjected to X-ray crystallography, there is no dynamics. For peptides in solution, the rotation/pyramidalization curve ψ/θav determines the degree of pyramidalization θ, when the rotation angle ψ runs through a full 360° circle. Density functional theory (DFT) calculations of alaninamide supported the analysis.

Luminescent solvent-free liquids based on Schiff-base boron difluoride complexes with polyethylene glycol chains.

A series of Schiff-base boron difluoride complexes with polyethylene glycol chains were synthesized and their photophysical properties were examined. These complexes maintained the solvent-free liquid state even at room temperature and their glass transition temperatures (Tg) were determined to be around -40 °C. The complexes showed blue to yellow luminescence under UV irradiation in the solvent-free liquid state with good emission quantum yields (Φ) of up to 0.26. The luminescence colour could also be tuned by dissolving organic dyes in the blue luminescent liquid sample. Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed to further understand the photophysical properties.

Circularly polarized phosphorescence with a large dissymmetry factor from a helical platinum(II) complex.

A chiral platinum(II) complex with a helical Schiff-base [4]helicene ligand exhibits intense red circularly polarized phosphorescence (CPP) with a glum of 0.010 in the dilute solution state. The intense CPP was caused by a change in the electronic transition character based on the induction of the helical structure.

Circularly Polarized Luminescence from Schiff-base [4]Helicene Boron Complexes.

Boron complexes with Schiff-base [4]helicene ligands were synthesized. These complexes were characterized by NMR spectroscopy and their helical molecular structures were unequivocally established by X-ray diffraction (XRD) analysis. The helical boron complexes exhibited efficient photoluminescence under UV irradiation, and the circularly polarized luminescence (CPL) properties were investigated for optically pure samples. Density functional theory (DFT) calculations were conducted to further understand their photophysical properties including chiroptical responses.

External environment sensitive circularly polarized luminescence properties of a chiral boron difluoride complex.

A chiral Schiff-base boron difluoride complex bearing a diethylamino group was synthesized. Its photophysical properties were investigated and compared with those of its non-substituted analogue. The complex was found to exhibit solvatofluorochromism with bluish-white emission in moderately polar solvents and intense blue emission in nonpolar solvent. Circularly polarized luminescence (CPL) properties were also examined and it was found that the absolute value of the luminescence dissymmetry factor (g lum) increases significantly in the KBr-dispersed pellet state compared to the solution state. Notably, CPL intensity of the complex enhanced approximately three times upon addition of CH3SO3H in CH2Cl2. Density functional theory (DFT) calculations were conducted to further understand the photophysical properties.

Twofold and Threefold Sinusoidal Patterns in Coupled Molecular Motions of 184,025 Structures of Phenylethane, Nitroethane, and Carboxylate Derivatives.

Scatter plot analyses for 14,169 phenylethanes of the substructure Cß-CαH2-Ph with three open coordination positions at Cß and 150,568 phenylethanes of Cß-CαHX-Ph with an additional open coordination position X at Cα have been performed, based on searches in the Cambridge Structural Database. The correlation of rotation angle ψ = Cß-Cα-Ci-Co with a pyramidalization angle θ = Co-Co'-Ci-Cα in a 360° rotation about the bond Cα-Ci reveals a sinusoidal pattern with three maxima and minima, whereas the correlation of rotation angle ψ with bond angle ω = Cß-Cα-Ci and bond length d = Cß-Cα results in sinusoidal patterns with two maxima and minima. A total of 3993 nitro derivatives of the substructure Cß-CαHX-NO2 confirm the results and show that atoms Ci/Co/Co' in the phenyl compounds can be replaced by atoms N/O/O' without any change in the two- and threefold patterns. In 15,295 methyl acetates of the substructure Cß-CαHX-C'(═O)OMe, pyramidalization of the group CαC'(═O)OMe results in a chiral flat tetrahedron with four different corners. (Rθ)/(Sθ) selectivity in the configuration of the tetrahedron is induced by the bonds Cα-Cß, Cα-H, and Cα-X, emanating from the tetrahedral center Cα. It is surprising that bonds as different as Cα-Cß, Cα-H, and Cα-X (X = H, C, N, O, S, etc.) give almost the same induction intensities.

Multi-colour circularly polarized luminescence properties of chiral Schiff-base boron difluoride complexes.

A series of chiral Schiff-base boron difluoride complexes was synthesized and their photophysical properties were examined. These complexes showed multi-colour (blue, yellow and red) photoluminescence in solution and in the solid state with good emission quantum yield (Φ) depending on the π-systems of the ligands. The chiral complexes exhibited circularly polarized luminescence (CPL) with an absolute luminescence dissymmetry factor (glum) of up to the 1.3 × 10-3 in solution and 1.9 × 10-2 in the drop-cast film state. Density functional theory (DFT) and time-dependent (TD) DFT calculations were conducted to further understand the photophysical properties.

Sign control of circularly polarized luminescence of chiral Schiff-base Zn(II) complexes through coordination geometry changes.

The handedness of circularly polarized luminescence (CPL) of Zn(II) complexes with a chiral Schiff-base ligand can be controlled by coordination geometry changes in solution and in the solid state. The relationships between molecular structure and chiroptical properties were discussed with the results of X-ray diffraction analysis and DFT calculations.

Enhancement of Chiroptical Responses of trans-Bis[(ß-iminomethyl)naphthoxy]platinum(II) Complexes with Distorted Square Planar Coordination Geometry.

Invited for this month's cover picture are the groups of Masahiro Ikeshita and Takashi Tsuno at Nihon University and Yoshitane Imai at Kindai University (Japan). The cover picture shows the comparison of circularly polarized luminescence (CPL) properties of square planar platinum(II) complexes with different coordination geometry. Computational studies have been carried out to investigate these structure-dependencies, and revealed that the distortion of the coordination geometry results into an enhancement the chiroptical responses of these compounds. Read the full text of their Research Article at 10.1002/open.202100277.

Enhancement of Chiroptical Responses of trans-Bis[(ß-iminomethyl)naphthoxy]platinum(II) Complexes with Distorted Square Planar Coordination Geometry.

The relationship between the coordination geometry and photophysical properties of trans-bis[(ß-iminomethyl)naphthoxy]platinum(II) was investigated both experimentally and theoretically. A series of platinum(II) complexes with differently substituted iminomethyl groups were synthesized, and their photophysical properties were examined in solution, in the crystalline, and in the PMMA film-dispersed state, respectively (PMMA=poly(methyl methacrylate)). These complexes showed structure-dependent emission spectra, in which the color of the luminescence in the crystalline state varied over a range of about 40 nm depending on the specific bowl-shaped molecular structure. The chiral complexes with (R,R)- and (S,S)-configurations were found to have structure-dependent chiroptical properties both in solution and the PMMA film-dispersed state such that the intensity of circular dichroism (CD) and circularly polarized luminescence (CPL) were enhanced with bulky cyclic substituents at the nitrogen atoms. A theoretical study using density functional theory (DFT) and time-dependent (TD)-DFT calculations revealed that the enhancement of chiroptical responses is due to the amplification of the magnetic dipole moment caused by the distortion of the square planar geometry.

Rotation about a Covalent Bond and Pyramidalization of an Adjacent sp2 Center are a Synchronized Molecular Motion.

The correlation of the rotation about the Cα-C' bond and the pyramidalization of the sp2-hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron in the substructure Cß-CαH-C'(═O)-OMe of substituted methyl acetates revealed that the two processes are not independent of each other but parts of a common molecular motion, as outlined in the preceding back-to-back paper. In the present study, we generalized the substructure to Xß-CαH-C'(═Y)-Z with X, Y, and Z = O, N, C, and S, extending the analysis to several hundred thousand structures of the type carboxylates, carboxamides, ketones, imines, olefins, peptides, lactates, carbothioates, and phenyl derivatives, retrieved from the Cambridge Structural Database. ψ/θ Scatter plots of the individual structure points and their averaging in ψ/θav curves result in wavelike patterns with three maxima and minima and inversion symmetry at ψ = 0° and ±180 for a 360° rotation of Cß about the Cα-C' bond. The pyramidalization of the sp2-hybridized group CαCiCoCo', which is part of the aromatic system, even disturbs the planarity of phenyl rings. Density functional theory calculations confirm the results of the CSD search.

Chirality of the Conformation Attacks the Planarity of the sp2 Carbon Atom in a Covalent Bond.

The correlation of the rotation about the bond C'-Cα and the pyramidalization of the sp2 hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron was investigated for the substructure Cß-CαH-C'(═O)-OMe. A search in the Cambridge Structural Database (CSD) gave 15,295 structures with a substituted methyl acetate group at the end of the molecules. The scatter plot of the rotation angle ψ = O═C'-Cα-Cß versus the pyramidalization angle θ = O(MeO)C'Cα and the ψ/θav curve show an unusual undulating pattern with three maxima and minima for a 360° rotation about the bond C'-Cα. There is no net chiral induction from the (Mψ)/(Pψ) conformations of the bond C'-Cα to the (Rθ)/(Sθ) configurations of the flattened tetrahedron because the contributions of the three maxima and minima cancel each other. The wavelike ψ/θav curve demonstrates that the rotation about the bond C'-Cα and the pyramidalization of the group C'(═O)(OMe)Cα are not independent processes. They are coupled with each other in one common molecular motion. The ψ/θav curve appears as the third harmonic of a sinusoidal fundamental frequency. DFT calculations of the propanoate anion, methyl propanoate, and methyl 2-methylpropanoate confirm the results of the CSD search.

Chiral Selectivity in the Achiral Amino Acid Glycine.

A Cambridge Structural Database (CSD) search for the zwitterion of glycine, the only natural amino acid which does not have a chiral center at Cα, and its derivatives H3N-CαH2-C'(Ocis)Otrans(H/R/M) gave 425 hits, 420 of which are chiral because of the ψ conformation with regard to the central C'-Cα bond (ψ = OcisC'CαN ≠ 0 or 180°). Chiral ψ conformations distort the planar carboxylic group CαC'(Ocis)Otrans group stereoselectively to an asymmetric flat tetrahedron with four different corners, specified by (Rθ)/(Sθ). Negative rotation angles ψ predominantly induce pyramidalization angles θ = OtransC'CαOcis below -180°, resulting in a high diastereoselectivity for (Mψ,Sθ)-glycine. Positive rotation angles ψ preferentially give (Pψ,Rθ)-glycine. Density functional theory calculations confirm the results of the data analysis, indicating an increase of pyramidalization θ for increasing rotation angles ψ. This is corroborated by the analysis of section averages of the structural data, showing an increase of the ψ/θ selectivity in the series 1.5, 2.8, 3.5, and 6.0 for increasing rotation angles ψ. Without any chiral bias, 419 glycine derivatives crystallize in the ratio racemate/conglomerate 359:65.

Selective distortion of the planar group Cα C'(O)O to a chiral flat tetrahedron in the amino acid alanine.

Based on a Cambridge Structural Database (CSD) search, a meta-analysis of 116 structures of alanine H3 NCα H(CH3 )C'(O)O and its derivatives H3 NCα H(CH3 )C'(O)O(H/R/M), protonated, esterified, or coordinated at the carboxylic group, shows that in the first step of a chirality chain, the L configuration at Cα induces (M) and (P) conformations with respect to rotation around the central C'─Cα bond. In the second step, the (M) and (P) conformations selectively distort the planar carboxylic group Cα C'(Ocis )Otrans to asymmetric flat (R) and (S) tetrahedra. High diastereoselectivities are caused by the two players attraction N…Ocis and repulsion Otrans …CMe , which work together in (L,M,R) configurations but against each other in (L,P,S) configurations.

Chirality in amino acids beyond the Cα configuration.

Based on a CSD search, a meta-analysis of 1179 structures of 19 natural amino acids H3 NCα H(R)C'(O)O and their derivatives H3 NCα H(R)C'(O)O(H/R/M), protonated, esterified, or coordinated at the carboxylic group, shows that the chirality chain with its two steps, established in the preceding paper for alanine, can be extended to natural amino acids. High diastereoselectivities are observed in the induction from the L configuration at Cα to the -ψ and +ψ conformations, which in turn distort the planar carboxylic group Cα C'(Ocis )Otrans to asymmetric flat tetrahedra, showing that the chirality chain is an integral part of natural amino acids.

The Chirality Chain in Valine: How the Configuration at the Cα Position through the O cis C'CαN Torsional System Leads to Distortion of the Planar Group CαC'(O cis )O trans to a Flat Tetrahedron.

Solid-state structures, based on a Cambridge Structural Database (CSD) search, show that there is a CαN/C'O cis attraction in the torsional system O cis C'CαN of valine, causing a chirality chain. The Cα configuration controls the chirality of the rotation around the C'-Cα bond, which in turn induces a distortion of the planar unit CαC'(O)O to a flat asymmetric tetrahedron. Conformational "reactions" take place in an energy profile with respect to clockwise and counterclockwise rotation around the C'-Cα bond as well as stretching and flattening of the tetrahedron. The molecular property CαN/C'O cis attraction of valine is maintained in its di- and tripeptides.

Trend-Analysis of Solid-State Structures: Low-Energy Conformational 'Reactions' Involving Directed and Coupled Movements in Half-Sandwich Compounds [CpFe(CO){C(=O)R}PPh3].

Invited for this month's cover picture are Prof. Dr. Henri Brunner from the University of Regensburg (Germany) and Prof. Dr. Takashi Tsuno from Nihon University (Japan). The cover picture shows the conformational reaction of JIDLUD→FIHTUL. The order of sample points of solid-state structures reveals information concerning low-energy, directed, and coupled movements in molecules. Read the full text of their Communication at https://doi.org/10.1002/open.201800007.

Trend-Analysis of Solid-State Structures: Low-Energy Conformational 'Reactions' Involving Directed and Coupled Movements in Half-Sandwich Compounds [CpFe(CO){C(=O)R}PPh3].

Trends in solid-state structures were used to identify preferred intramolecular movements in half-sandwich compounds [CpFe(CO){C(=O)R}PPh3]. Three weak interactions were analyzed: 1) the CH/π donor-acceptor interaction of phenyl rings in the PPh3 ligand, 2) the PhPPh3 face-on Cp stabilization, and 3) the hydrogen bond between the oxygen atom of the acyl group and an ortho-C-H bond of one of the PPh3 phenyl rings. Clockwise and counter-clockwise rotations established directed and coupled movements of the PPh3 ligand, the acyl group, and the phenyl rings within the PPh3 ligand.

PPh3 Propeller Diastereomers: Bonding Motif PhPPh3 Face-On π-Ar in Half-Sandwich Compounds [(π-Ar)LL'MPPh3].

Chiral-at-metal compounds (R Ru,S C)/(S Ru,S C)-[CyRu(1O-2N)PPh3]PF6 and (R Ru,S C)/(S Ru,S C)-[CyRu(2O-1N)PPh3]PF6 were prepared using anions 1O-2N- and 2O-1N- of the Schiff bases, derived from the hydroxynaphthaldehydes and (S)-1-phenylethylamine. The pure (R Ru,S C)-diastereomers were obtained by crystallization. In the unit cell of (R Ru,S C)-[CyRu(1O-2N)PPh3]PF6, there are three independent molecules, which differ in the propeller sense of the PPh3 ligand. Molecules [1] and [2] have (M PPh3 )-configuration and molecule [3] has (P PPh3 )-PPh3 configuration. PPh3 diastereoisomerism is discussed including other pairs of compounds, differing only in the PPh3 configuration. A conformational analysis reveals an internal stabilization inside the PPh3 ligand by a system of attractive CH/π interactions and a new bonding motif PhPPh3 face-on π-Ar, both characteristic features of [(π-Ar)LL'MPPh3] compounds. The propeller diastereomers interconvert via a low-energy pathway and a high-energy pathway, corroborated by density functional theory calculations.

Comment on "Conformational analysis of triphenylphosphine ligands in stereogenic monometallic complexes: tools for predicting the preferred configuration of the triphenylphosphine rotor" by J. F. Costello, S. G. Davies, E. T. F. Gould and J. E. Thomson, Dalton Trans., 2015, 44, 5451.

In half-sandwich compounds of the type [Cp*ML1L2PPh3] the PPh3 propeller is stabilized by attractive CH/π interactions in which Co-H bonds specifically interact with the Ci and Co atoms of neighbouring phenyl rings, as in the T-shaped benzene dimer (i/o = ipso/ortho). This stabilization was not taken into account in a recent conformational analysis based on van der Waals energy calculations and minimization of steric compression (Dalton Trans., 2015, 44, 5451-5466). It is shown that in all 116 structures discussed in this analysis the CoH-Ci/o distances fall below the sum of the van der Waals radii, establishing attractive CH/π interactions, although the short contacts could easily be avoided by phenyl rotation to relieve steric strain. In 53 of the described structures there are acyl substituents which form conformation-determining Co-HO(acyl) hydrogen bonds that are not taken into account in the recent analysis. The steric-only model is not an adequate description of M-PPh3 complexes.