Enhancing NMR Sensitivity of Natural‐Abundance Low‐γ Nuclei by Ultrafast Magic‐Angle‐Spinning Solid‐State NMR Spectroscopy

Although magic‐angle‐spinning (MAS) solid‐state NMR spectroscopy has been able to provide piercing atomic‐level insights into the structure and dynamics of various solids, the poor sensitivity has limited its widespread application, especially when the sample amount is limited. Herein, we demonstrate the feasibility of acquiring high S/N ratio natural‐abundance ¹³C NMR spectrum of a small amount of sample (≈2.0 mg) by using multiple‐contact cross polarization (MCP) under ultrafast MAS. As shown by our data from pharmaceutical compounds, the signal enhancement achieved depends on the number of CP contacts employed within a single scan, which depends on the T_1ρ of protons. The use of MCP for fast 2D ¹H/¹³C heteronuclear correlation experiments is also demonstrated. The significant signal enhancement can be greatly beneficial for the atomic‐resolution characterization of many types of crystalline solids including polymorphic drugs and nanomaterials.     

Energetic Materials Based on 5,5′‐Diamino‐4,4′‐dinitramino‐3,3′‐bi‐1,2,4‐triazole

A simple and straightforward synthesis of 5,5′‐diamino‐4,4′‐dinitramino‐3,3′‐bi‐1,2,4‐triazole by the selective nitration of 4,4′,5,5′‐tetraamino‐3,3′‐bi‐1,2,4‐triazole is presented. The interaction of the amino and nitramino groups improves the energetic properties of this functionalized bitriazole. For a deeper investigation of these properties, various nitrogen‐rich derivatives were synthesized. The new compounds were investigated and characterized by spectroscopy (¹H and ¹³C NMR, IR, Raman), elemental analysis, mass spectrometry, differential thermal analysis (DTA), X‐ray analysis, and impact and friction sensitivities (IS, FS). X‐ray analyses were performed and deliver insight into structural characteristics with which the stability of the compounds can be explained. The standard enthalpies of formation were calculated for all compounds at the CBS‐4M level of theory, revealing highly positive heats of formation. The energetic performance of the new molecules was predicted with the EXPLO5 V6.02 computer. A small‐scale shock reactivity test (SSRT) and a toxicity test gave a first impression of the performance and toxicity of selective compounds.     

Pentacoordination at antimony in dibenzostibocines via D–Sb transannular interactions (D = O,S): A structural study

Addition of M₂SCN(CH₂R)₂ to D(C₆H₄S)₂SbCl in methylene chloride solution leaded to the formation of the stable compounds D(C₆H₄S)₂SbS₂CN(CH₂R)₂ (D = O, R = Me, 1; D = S, R = Me, 2; R = Ph, 3). The new dibenzostibocines derivatives containing two different types of dithioligands were characterized by elemental analysis and spectroscopic studies (IR, ¹H and ¹³C NMR). Single crystal X-ray diffraction determinations of complexes 1–3 revealed that the antimony atom acts as an acceptor atom exhibiting an intramolecular transannular interaction with the donor D atom, expanding its coordination from three to five and displaying a skew trapezoidal local geometry.     

1H and 13C NMR assignments for two anthraquinones and two xanthones from the mangrove fungus (ZSUH-36)

We report the unambiguous assignments of the (1)H and (13)C NMR spectra of one new natural product, namely, 6,8-di-O-methyl versiconol (1) together with one known anthraquinone aversin (2) and two xanthones 5-methoxysterigmatocystin (3) and sterigmatocystin (4). These compounds were all isolated from the mangrove endophytic fungus ZSUH-36 from the South China Sea. 1D and 2D NMR experiments including COSY, HMQC and HMBC were used to elucidate the structures. Variations in the (1)H NMR spectrum of 6,8-di-O-methyl versiconol (1) were also observed in the temperature range 25-75 degrees C. In addition, the plausible biogenetic path from 1 to 2 is discussed.     

(1)H and (13)C NMR assignments for five anthraquinones from the mangrove endophytic fungus Halorosellinia sp. (No. 1403)

We report the unambiguous assignments of the (1)H and (13)C NMR spectra of two new natural products, namely, 1,4,5,6,7,9-hexahydroxy-2-methoxy-7-methyl-5beta,9beta,8abeta, 6alpha,10aalpha-hexahydroanthracen-10 (10aH)-one (1) and 1,4,6-trihydroxy-2-methoxy-7-methylanthracene-9, 10-dione (2), together with three known anthraquinones. These compounds were all isolated from the marine endophytic fungus No. 1403 collected from the South China Sea. Compounds 3 and 4 were isolated from the marine fungus for the first time. The structures were elucidated by the spectroscopic methods 1D and 2D NMR including COSY, HMQC, HMBC and NOE, and HREIMS. In our cytotoxicity assays, compound 5 showed cytotoxicity toward KB and KBv-200 cells with IC(50) of 1.40 and 2.58 microg/ml, respectively. In addition, the plausible biogenic relationship of compounds 1, 2, 3 and 4 is discussed.     

Reactions of Sn(NMe2)2 with MPHCy: the effects of alkali metal phosphide coupling (Cy=cyclohexyl; M=Li, Na, K, Rb)

The reactions of the SnII base Sn(NMe2)2 with CyPHM (Cy=cyclohexyl) produce a range of products, depending primarily on the alkali metal (M) involved. The 1:3 stoichiometric reaction of Sn(NMe2)2 with CyPHNa in the presence of the Lewis base donor PMDETA (PMDETA=(Me2NCH2CH2)2NMe) gives [(NaPMDETA)2{Sn(mu-PCy)}3] (3), containing the electron-deficient [{Sn(mu-PCy)}3]2- dianion. Natural bond order (NBO) and electron localisation function (ELF) calculations show that this species is described most appropriately by a two-electron, three-centre Sn3 bonding model. Evidence that 3 results from phosphide coupling is provided by the 1:1 reaction of Sn(NMe2)2 with CyPHNa in the presence of PMDETA, which gives 3 and trace amounts of (NaPMDETA)2[{Sn(mu-PCy)}2(mu-PCyPCy)] (4) (containing one PCyPCy2- dianion). Greater extents of phosphide coupling are observed as the size of the Group 1 metal is increased. Thus, the 1:3 reaction of Sn(NMe2)2 with CyPHK in THF gives the co-crystalline product {(K2 THF)2[{Sn(mu-PCyPCy)}2(mu-PCy)]}0.9{(K2 THF)2[{Sn(mu-PCy)}2(mu-PCyPCy)]}0.1 (5) (containing [{Sn(mu-PCyPCy)}2(mu-PCy)]2- and [{Sn(mu-PCy)}2(mu-PCyPCy)]2- dianions), whereas the analogous reaction of Sn(NMe2)2 with RbPHCy gives [RbPMDETA{(CyP)3SnP(H)Cy}] (6) (containing a cyclic {(CyP)3Sn} unit).     

Reductive Coupling of (Fluoro)pyridines by Linear 3d-Metal(I) Silylamides of Cr–Co: A Tale of C−C Bond Formation,C−F Bond Cleavage and a Pyridyl Radical Anion

Herein, we disclose the facile reduction of pyridine (and its derivatives) by linear 3d-metal(I) silylamides (M=Cr–Co). This reaction resulted in intermolecular C−C coupling to give dinuclear metal(II) complexes bearing a bridging 4,4′-dihydrobipyridyl ligand. For iron, we demonstrated that the C−C coupling is reversible in solution, either directly or by reaction with substrates, via a presumed monomeric metal(II) complex bearing a pyridyl radical anion. In the course of this investigation, we also observed that the dinuclear metal(II) complex incorporating iron facilitated the isomerisation of 1,4-cyclohexadiene to 1,3-cyclohexadiene as well as equimolar amounts of benzene and cyclohexene. Furthermore, we synthesised and structurally characterised a non-3d-metal-bound pyridyl radical anion. The reactions of the silylamides with perfluoropyridine led to C−F bond cleavage with the formation of metal(II) fluoride complexes of manganese, iron and cobalt along with the homocoupling or reductive degradation of the substrate. In the case of cobalt, the use of lesser fluorinated pyridines led to C−F bond cleavage but no homocoupling. Overall, in this paper we provide insights into the multifaceted behaviour of simple (fluoro)pyridines in the presence of moderately to highly reducing metal complexes.     

Chlorination‐promoted Transformation of Isolated Pentagon Rule C78 into Fused‐pentagons‐ and Heptagons‐containing Fullerenes

Cage transformations in fullerenes are rare phenomena which are still not fully understood. We report the first skeletal transformation of an Isolated‐Pentagon‐Rule (IPR) isomer of C₇₈ fullerene upon high‐temperature chlorination which proceeds by six‐step Stone–Wales rearrangements affording non‐IPR, non‐classical (NC ) C₇₈(NC 2)Cl₂₄ with two cage heptagons, six pairs of fused pentagons, and an unprecedented loop‐like chlorination pattern. The following loss of a C₂ unit results in C₇₆(NC 3)Cl₂₄ containing three cage heptagons.     

Trichiliasinenoids A–C,three 6,7-secomexicanolide limonoids with a 7,29-linkage from Trichilia sinensis

Trichiliasinenoids A–C (1–3), three new limonoids with an unprecedented C-29-C-7 connecting carbon skeleton formed by migration of C-7 from C-6 to C-29 of a mexicanolide-type limonoid precursor were isolated from the leaves and twigs of Trichilia sinensis. Their structures were assigned by spectroscopic analysis, and the absolute configurations were determined by X-ray crystallography (1) and ECD calculation (2 and 3). A possible biosynthetic pathway of 1 was also proposed. Compound 2 exhibited moderate cytotoxicity against HL-60 cells and weak cytotoxicity against SMMC-7721cells.     

Investigation of the Crystal Structure,Characterization, Hirshfeld Surface,and Theoretical Study of (C6H14N)5[HP2Mo5O23]·4H2O

The synthesis of an organoammonium diphosphopentamolybdates(VI), (C₆H₁₄N)₅[HP₂Mo₅O₂₃] · 4H₂O ( 1 ), is reportet. The molecular structure and spectroscopic analysis were performed using experimental techniques like X‐ray diffraction, FT‐IR, and UV/Vis. The single crystal analysis of the title compound shows that the compound crystallizes in the monoclinic crystal system with space group P2₁/c and cell constants of a = 12.7934(2) Å, b = 14.8145(2) Å, c = 27.2637(5) Å, and β = 92.9298(14)°. The Hirshfeld surfaces and the associated 2D fingerprint plots were investigated to study the intermolecular interactions in the crystal packing and this study has confirmed that the hydrogen bonding interactions play a dominating role in the stability of crystal structure. The theoretical calculations were done using PM3 semi‐empirical model and several properties were studied.