Exploring the impact of alignment media on RDC analysis of phosphorus-containing compounds: a molecular docking approach.

Residual dipolar couplings (RDCs) are employed in NMR analysis when conventional methods, such as J-couplings and nuclear Overhauser effects (NOEs) fail. Low-energy (optimized) conformers are often used as input structures in RDC analysis programs. However, these low-energy structures do not necessarily resemble conformations found in anisotropic environments due to interactions with the alignment medium, especially if the analyte molecules are flexible. Considering interactions with alignment media in RDC analysis, we developed and evaluated a molecular docking-based approach to generate more accurate conformer ensembles for compounds in the presence of the poly-γ-benzyl-L-glutamate alignment medium. We designed chiral phosphorus-containing compounds that enabled us to utilize 31P NMR parameters for the stereochemical analysis. Using P3D/PALES software to evaluate diastereomer discrimination, we found that our conformer ensembles outperform moderately the standard, low-energy conformers in RDC analysis. To further improve our results, we (i) averaged the experimental values of the molecular docking-based conformers by applying the Boltzmann distribution and (ii) optimized the structures through normal mode relaxation, thereby enhancing the Pearson correlation factor R and even diastereomer discrimination in some cases. Nevertheless, we presume that significant differences between J-couplings in isotropic and in anisotropic environments may preclude RDC measurements for flexible molecules. Therefore, generating conformer ensembles based on molecular docking enhances RDC analysis for mildly flexible systems while flexible molecules may require applying more advanced approaches, in particular approaches including dynamical effects.

Synthesis, photophysics and two-photon absorption of imidazole-centred tripodal chromophores.

Tripodal push-pull chromophores with D-(π-A)3 arrangement were synthesized using 1-methyl-2,4,5-triphenyl-1H-imidazole as a central electron donor, and their thermal, electrochemical, photophysical and non-linear optical properties were studied and corroborated with quantum-chemical calculations. Their facile synthesis involved Suzuki-Miyaura and Knoevenagel reactions, allowing the installation of various peripheral electron acceptors such as formyl, cyano, ester, trifluoromethyl and more complex moieties such as malonic/acetic acid derivatives, indan-1,3-dione and rhodanine. All phenyl rings appended at the central imidazole core were more or less twisted depending on the peripheral substitution. Although imidazole undergoes reversible one-electron oxidation, peripheral acceptors are reduced irreversibly in a multi-electron process. This behaviour is further seen as a variation of the LUMO, while the HOMO remained almost unaltered across the whole series. TD-DFT calculations revealed centrifugal charge transfer from the central imidazole to all C2, C4 and C5 branches occupied by the LUMO, LUMO+1 and LUMO+2. The HOMO-LUMO gap is tuneable within the range of 3.55-2.31 eV, while the longest-wavelength absorption/emission maxima were found within the broad range of 304-448/393-612 nm. Although the absorption spectra are solvent-independent, the emission depends strongly on the solvent polarity and the electron-withdrawing power of the peripheral acceptors. Extended chromophores with complex electron acceptors were investigated as two-photon absorbers, revealing relatively good cross-section values of up to 521 GM and a figure-of-merit (ΦF × Î´2PA) of around 190 GM.

Can Copper(I) and Silver(I) be Hydrogen Bond Acceptors?

Gold(I) centers can form moderately strong (Au⋅⋅⋅H) hydrogen bonds with tertiary ammonium groups, as has been demonstrated in the 3AuCl+ (3+ =1-(tert-butyl)-3-phenyl-4-(2-((dimethylammonio)methyl)phenyl)-1,2,4-triazol-5-ylidene) complex. However, similar hydrogen bonding interactions with isoelectronic silver(I) or copper(I) centers are unknown. Herein, we first explored whether the Au⋅⋅⋅H bond originally observed in 3AuCl+ can be strengthened by replacing Cl with Br or I. Experimental gas-phase IR spectra in the ∼3000 cm-1 region showed only a small effect of the halogen on the Au⋅⋅⋅H bond. Next, we measured the spectra of 3AgCl+ , which exhibited significant differences compared to its 3AuX+ counterparts. The difference has been explained by DFT calculations which indicated that the Ag⋅⋅⋅H interaction is only marginal in this complex, and a Cl⋅⋅⋅H hydrogen bond is formed instead. Calculations predicted the same for the 3CuCl+ complex. However, we noticed that for Ag and Cu complexes with less flexible ligands, such as dimethyl(2-(dimethylammonio)phenyl)phosphine (L7 H+ ), the computations predict the presence of the respective Ag⋅⋅⋅H and Cu⋅⋅⋅H hydrogen bonds, with a strength similar to the Au⋅⋅⋅H bond in 3AuCl+ . We, therefore, propose possible complexes where the presence of (Ag/Cu)⋅⋅⋅H bonds could be experimentally verified to broaden our understanding of these unusual interactions.

Reactivity of a N-Coordinated Germylene-Borane Complex: From Ge→B to Ge→Ga Coordination.

Reactivity studies of the GeII →B complex L(Cl)Ge⋅BH3 (1; L=2-Et2 NCH2 -4,6-tBu2 -C6 H2 ) were performed to determine the effect on the GeII →B donation. N-coordinated compounds L(OtBu)Ge⋅BH3 (2) and [LGe⋅BH3 ]2 (3) were prepared. The possible tuning of the GeII →B interaction was proved experimentally, yielding compounds 1-PPh2 -8-(LGe)-C10 H6 (4) and L(Cl)Ge⋅GaCl3 (5) without a GeII →B interaction. In 5, an unprecedented GeII →Ga coordination was revealed. The experimental results were complemented by a theoretical study focusing on the bonding in 1-5. The different strength of the GeII →E (E=B, Ga) donation was evaluated by using energy decomposition analysis. The basicity of different L(X)Ge groups through proton affinity is also assessed.

Lithium, Magnesium, and Zinc Centers N,N'-Chelated by an Amine-Amide Hybrid Ligand.

The synthesis and structure of lithium, magnesium, and zinc complexes N,N'-chelated by a hybrid amine-amido ligand ([2-(Me2NCH2)C6H4NR]-, abbreviated as LNR, where R = H, SiMe3, or Bn) are reported. The reaction of the least sterically demanding LNH with various magnesium sources gives the hexameric imide [LNMg]6 (4) by the elimination of n-butane from LNHMgnBu (2) or by the reaction of LNHLi (1) with MeMgBr. [LNH]2Mg (3) is obtained through the addition of 0.5 equiv of nBu2Mg or Mg[N(SiMe3)2]2 to LNH2 and with 1 equiv of nBu2Mg reacting to 2. Both LNHMgN(SiMe3)2 (6) and isostructural LNHZnN(SiMe3)2 (16) have been prepared using two different approaches: monodeprotonation of LNH2 by Zn/Mg[N(SiMe3)2]2 in a 1:1 ratio or ligand substitution of 2 or LNHZnEt (12) by 0.5 equiv of Sn[N(SiMe3)2]2. The reactions of 2 or 3 with 1 provide the heterotrimetallic complex [LNH]4Li2Mg (5). Benzyl- or trimethylsilyl-substituted anilines [LN(SiMe3)H (7) and LN(Bn)H (8)] with 0.5 equiv of nBu2Mg allow the formation of homoleptic bis(amides) of the [LN(R)]2Mg type (10 and 11). Nevertheless, only the silylated secondary amine 7 is able to provide the heteroleptic n-butylmagnesium amide LN(SiMe3)MgnBu (9) upon reaction with an equimolar amount of nBu2Mg. Similarly, 12, [LNH]2Zn (13), LN(R)ZnEt (17 and 18), and [LN(R)]2Zn [R = SiMe3 (19) and Bn (20)] were prepared by the monodeprotonation of LNH2 or LN(R)H using Et2Zn in the corresponding stoichiometric ratio. LNHZnI was prepared by the nucleophilic substitution of an ethyl chain in 12 by molecular iodine. A heterometallic complex, [LNH]4Li2Zn (14), analogous to 5 was prepared from 12 or 13 with 1 or 2 equiv of 1, respectively.

Molecular Rearrangement of Pyrazino[2,3-c]quinolin-5(6H)-ones during Their Reaction with Isocyanic Acid.

New tetrahydropyrazino[2,3-c]quinolin-5(6H)-ones were prepared from 3-chloroquinoline-2,4(1H,3H)-diones and ethylene diamine. In their reaction with HNCO, an unprecedented molecular rearrangement produced new types of hydantoin derivatives. All prepared compounds were characterized on the basis of their 1H, 13C, and 15N NMR and ESI mass spectra and some were authenticated by X-ray analysis of single crystalline material. A proposed mechanism for rearrangement is discussed in this essay. The CDK and ABL inhibition activity as well as in vitro cytotoxicity of the prepared compounds was also tested.

New Types of Ge2 and Ge4 Assemblies Stabilized by a Carbanionic Dicarborandiyl-Silylene Ligand.

The first Ge(0)-Ge(II) germylone-germylene-paired Ge2 complex (LSi)2Ge2 (4) and the molecular Ge4 cluster (LSi)2Ge4 (5) supported by the chelating carbanionic ortho-C,C'-dicarborandiyl-silylene ligand LSi [L = C,C'-C2B10H10, Si = PhC(tBuN)2Si] have been synthesized and isolated via reduction of the corresponding precursors chlorogermyl-germyliumylidene chloride (2), [(LSi)2Ge(Cl)Ge]+Cl-, and (LSi)2Ge4Cl4 (3) with C8K, respectively. The latter precursors were obtained from the unexpected outcome of the reaction of the ortho-C,C'-dicarborandiyl phosphine-silylene ligand PLSi (1) {P = P[N(tBu)CH2]2} and GeCl2·dioxane. Compound 2 is formed in higher yields (65% yields) by the salt metathesis reaction of the C-lithium dicarborandiyl-C'-silylene salt LiLSi (6) [Li = Li(OEt2)2] with GeCl2·dioxane. The molecular structures of all these species (1-6) have been established and confirmed spectroscopically and crystallographically. The electronic structures of 4 and 5 were elucidated by density functional theory calculations. While 4 possesses a localized dative Ge(0)→Ge(II) bond, the Ge-Ge σ bonds in 5 are delocalized in the Ge4 cluster core. Featuring a donor-acceptor interaction between two chelating silylenes and the Ge4 core, compound 5 represents a unique molecular model for a Ge4 cluster.

Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

The titled 2,1-benzazaphosphole (1) (i. e. ArP, where Ar=2-(DippN=CH)C6 H4 , Dipp=2,6-iPr2 C6 H3 ) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC≡CR (R=CO2 Me, C5 F4 N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3, that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5. Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH)2 (R=Me, tBu, Ph) resulting in the addition products 7-9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [µ-ArP(AuCl)2 ] (10), [(µ-ArP)4 Ag4 ][X]4 (X=BF4 (11), OTf (12)) and [µ-ArP(Co2 (CO)6 )] (13). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.

Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

Invited for the cover of this issue are Zoltán Benko, Libor Dostál and co-workers at the University of Pardubice and the Budapest University of Technology and Economics. The image depicts signs for the two different pathways representing the two differing reaction types which were clearly observed for 2,1-benzazaphosphole. Read the full text of the article at 10.1002/chem.202101686.

Green-, Red-, and Infrared-Emitting Polymorphs of Sterically Hindered Push-Pull Substituted Stilbenes.

The synthesis, XRD single-crystal structure, powder XRD, and solid-state fluorescence of two new DPA-DPS-EWG derivatives (DPA=diphenylamino, DPS=2,5-diphenyl-stilbene, EWG=electron-withdrawing group, that is, carbaldehyde or dicyanovinylene, DCV) are described. Absorption and fluorescence maxima in solvents of various polarity show bathochromic shifts with respect to the parent DPA-stilbene-EWGs. The electronic coupling in dimers and potential twist elasticity of monomers were studied by density functional theory. Both polymorphs of the CHO derivative emit green fluorescence (527 and 550 nm) of moderate intensity (10 % and 5 %) in polycrystalline powder form. Moderate (5 %) red (672 nm) monomer-like emission was also observed for the first polymorph of the DCV derivative, whereas more intense (32 %) infrared (733 nm) emission of the second polymorph was ascribed to the excimer fluorescence.

Reaction Outcome Critically Dependent on the Method of Workup: An Example from the Synthesis of 1-Isoquinolones.

A striking dependence on the method of workup has been found for annulation of benzonitriles ArC≡N to N-methyl 2-toluamide (1), facilitated by n-BuLi (2 equiv): quenching the reaction by a slow addition of water produced the expected 1-isoquinolones 2; by contrast, slow pouring of the reaction mixture into water afforded the cyclic aminals 5 (retaining the NMe group of the original toluamide). The mechanism of the two processes is discussed in terms of the actual H+ concentration in the workup. Both 2 and 5 were then converted into the corresponding 1-chloroisoquinolines 3, coupling of which, mediated by (Ph3P)2NiCl2/Zn, afforded bis-isoquinolines 4.

Thiaborane Icosahedral Barrier Increased by the Functionalization of all Terminal Hydrogens in closo-1-SB11H11.

The electrophilic substitution of icosahedral closo-1-SB11H11 with methyl iodide has resulted in two B-functionalized thiaboranes, 7,12-I2-2,3,4,5,6,8,9,10,11-(CH3)9-1-closo-SB11 and 7,8,12-I3-2,3,4,5,6,9,10,11-(CH3)8-closo-1-SB11, with the former being significantly predominant. These two icosahedral thiaboranes are the first cases of polysubstituted polyhedral boron clusters with another vertex that differs from B and C. Such polyfunctionalizations have increased the earlier observed thiaborane icosahedral barrier, not exhibiting any reactivity toward bases, unlike the parent thiaborane. The search for methylation pathways has revealed that the complete B11-methylation is impossible, like in the case of decaborane(14), where this seems to be a result of the positively charged upper parts of these two molecules.

Tetrazene-Characterization of Its Polymorphs.

The reinvestigation of tetrazene single crystalline material by means of X-ray methods resulted in a slightly different structure when compared to previously published data. Reaction conditions responsible for different crystalline modification formation were investigated. Newly described C form was found to be the primary reaction product and the combined action of temperature and the presence of water over time is required for the transition to the A form. Both forms were described by X-ray powder diffraction. Tetrazene was also subjected to infrared and Raman spectroscopy, which allowed differentiating between the forms. The molecule was isotopically labeled with 15N atoms at two different locations (ring and nitrogen sidechain) and employed in assigning vibrational modes to the resulting bands. Differences between sensitivities to mechanical stimuli of the two modifications were investigated and found industrially insignificant. In the same vein, the performance of either modification in primer composition and primer was identical.

Changing the Reactivity of Zero- and Mono-Valent Germanium with a Redox Non-Innocent Bis(silylenyl)carborane Ligand.

Using the chelating C,C'-bis(silylenyl)-ortho-dicarborane ligand, 1,2-(RSi)2 -1,2-C2 B10 H10 [R=PhC(NtBu)2 ], leads to the monoatomic zero-valent Ge complex ("germylone") 3. The redox non-innocent character of the carborane scaffold has a drastic influence on the reactivity of 3 towards reductants and oxidants. Reduction of 3 with one molar equivalent of potassium naphthalenide (KC10 H8 ) causes facile oxidation of Ge0 to GeI along with a two-electron reduction of the C2 B10 cluster core and subsequent GeI -GeI coupling to form the dianionic bis(silylene)-supported Ge2 complex 4. In contrast, oxidation of 3 with one molar equivalent of [Cp2 Fe][B{C6 H3 (CF3 )2 }4 ] as a one-electron oxidant furnishes the dicationic bis(silylene)-supported Ge2 complex 5. The Ge0 atom in 3 acts as donor towards GeCl2 to form the trinuclear mixed-valent Ge0 →GeII ←Ge0 complex 6, from which dechlorination with KC10 H8 affords the neutral Ge2 complex 7 as a diradical species.

Redox Noninnocent Monoatomic Silicon(0) Complex ("Silylone"): Its One-Electron-Reduction Induces an Intramolecular One-Electron-Oxidation of Silicon(0) to Silicon(I).

A monatomic zerovalent silicon(0) complex ("silylone") stabilized by the chelating bis(silylenyl)-ortho-carborane ligand, 1,2-(LSi)2-1,2-C2B10H10 [L = PhC(NtBu)2], has been synthesized from the redox reaction of the dipotassium bis(silylenyl)-nido-carboranate salt, 1,2-(LSi)2-1,2-C2B10H10K2, and NHC-SiCl2 (NHC = {[HCN(2,6-iPr2C6H3)]2C:}). Markedly different from previous examples, this silylone undergoes reduction due to the closo-C2B10 cluster backbone, which is prone to accept up to two electrons to form the cage-opened dianionic nido-C2B10 cluster core. Surprisingly, the closo-C2B10 core of the silylone consumes only one molar equiv of potassium naphthalenide, in addition, one electron is intramolecularly transferred from the Si0 atom to the C2B10 core to form an elusive bis(silylene)-stabilized SiI radical cation which undergoes homocoupling to the corresponding isolable dicationic SiI-SiI complex.

Organogermanium(II) Hydrides as a Source of Highly Soluble LiH.

The reactions of monomeric C,N-chelated organogermanium(II) hydride L(H)Ge⋅BH3 with organolithium salts RLi yielded lithium hydrogermanatoborates (Li(THF)2 {BH3 [L(H)GeR]})2 . Compound (Li(THF)2 {BH3 [L(H)GePh]})2 was used as a source of LiH for the reduction of organic C=O or C=N bonds in nonpolar solvents accompanied by the elimination of a neutral complex L(Ph)Ge⋅BH3 . The interaction of (Li(THF)2 {BH3 [L(H)GePh]})2 with the polar C=O bond was further investigated by computational studies revealing a plausible geometry of a pre-reactive intermediate. The experimental and theoretical studies suggest that, although the Li atom of (Li(THF)2 {BH3 [L(H)GePh]})2 coordinates the C=O bond, the GeH fragment is the active species in the reduction reaction. Finally, benzaldehyde was reduced by a mixture of L(H)Ge⋅BH3 with PhLi in nonpolar solvents.

Hetero Diels-Alder Reactions of Masked Dienes Containing Heavy Group†15 Elements.

Treatment of N,C,N-chelated organopnictogen(I) compounds ArE (1-3) (Ar=2,6-(RN=CH)2 C6 H3 , E/R=As/Dmp (1), Sb/tBu (2), and Bi/tBu (3), where Dmp=2,6-Me2 C6 H3 ) with various electron-deficient alkynes RC≡CR' (R/R'=CO2 Me (DMAD), R/R'=H/CO2 Me, or R/R'=NC5 F4 ) affords different types of heterocyclic compounds. In these reactions, 1-3 act as hidden dienes and participate in hetero-Diels-Alder (DA) reactions, a feature that has been only rarely reported for heavier pnictogen compounds. In this way, reactions of 1 furnished the set of 1-arsanaphthalenes 4-6. The most likely mechanism involves two steps, that is, an arsa-DA reaction giving a 1-arsa-1,4-dihydro-iminonaphthalene, followed by CH→NH proton migration. In contrast, this reaction sequence terminates at the first step in the case of the antimony analogue 2, thus giving the 1-stiba-1,4-dihydro-iminonaphthalenes 7 and 8. Reactions of the bismuth congener 3 gave one of two products depending on the alkyne used. Reaction of 3 with DMAD gave 1-bisma-1,4-dihydro-iminonaphthalene 9, whilst its reaction with two equivalents of HC≡C(CO2 Me) gave bismacyclohexadiene (10). All compounds have been characterized by NMR, IR, and Raman spectroscopies and X-ray diffraction analysis. The experimental data were complemented by a computational study, including a description of the reaction profiles of the hetero-DA reactions and an assessment of the aromaticities of the heterocyclic naphthalene derivatives.

Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H†NMR Chemical Shifts Induced through H⋠⋠⋠I- Hydrogen Bond.

Spin-orbit (SO) heavy-atom on the light-atom (SO-HALA) effect is the largest relativistic effect caused by a heavy atom on its light-atom neighbors, leading, for example, to unexpected NMR chemical shifts of 1 H, 13 C, and 15 N nuclei. In this study, a combined experimental and theoretical evidence for the SO-HALA effect transmitted through hydrogen bond is presented. Solid-state NMR data for a series of 4-dimethylaminopyridine salts containing I- , Br- and Cl- counter ions were obtained experimentally and by theoretical calculations. A comparison of the experimental chemical shifts with those calculated by a standard DFT methodology without the SO contribution to the chemical shifts revealed a remarkable error of the calculated proton chemical shift of a hydrogen atom that is in close contact with the iodide anion. The addition of the relativistic SO correction in the calculations significantly improves overall agreement with the experiment and confirms the propagation of the SO-HALA effect through hydrogen bonds.

Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H†NMR Chemical Shifts Induced through H⋠⋠⋠I- Hydrogen Bond.

Invited for the cover of this issue is the group of Michal Straka and Martin Dracínský (IOCB Prague, Czech Academy of Sciences). The image depicts a neutron star, which is used to represent the relativistic effects between a heavy element and a hydrogen atom reported in this work. Read the full text of the article at 10.1002/chem.202001532.

Undiscovered Potential: Ge Catalysts for Lactide Polymerization.

Polylactide (PLA) is a high potential bioplastic that can replace oil-based plastics in a number of applications. To date, in spite of its known toxicity, a tin catalyst is used on industrial scale which should be replaced by a benign catalyst in the long run. Germanium is known to be unharmful while having similar properties as tin. Only few germylene catalysts are known so far and none has shown the potential for industrial application. We herein present Ge complexes in combination with zinc and copper, which show amazingly high polymerization activities for lactide in bulk at 150 °C. By systematical variation of the complex structure, proven by single-crystal XRD and DFT calculations, structure-property relationships are found regarding the polymerization activity. Even in the presence of zinc and copper, germanium acts as the active site for polymerizing probably through the coordination-insertion mechanism to high molar mass polymers.