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The intracellular concentration of diadenosine tetraphospate (Ap4A) increases upon exposure to stress conditions. Despite being discovered over 50 years ago, the cellular functions of Ap4A are still enigmatic. If and how the varied Ap4A is a signal and involved in the signaling pathways leading to an appropriate cellular response remain to be discovered. Because the turnover of Ap4A by Ap4A cleaving enzymes is rapid, small molecule inhibitors for these enzymes would provide tools for the more detailed study of the role of Ap4A. Here, we describe the development of a high-throughput screening assay based on a fluorogenic Ap4A substrate for the identification and optimization of small molecule inhibitors for Ap4A cleaving enzymes. As proof-of-concept we screened a library of over 42 000 compounds toward their inhibitory activity against the Ap4A phosphorylase (Rv2613c) of Mycobacterium tuberculosis(Mtb). A sulfanylacrylonitril derivative with an IC50of 260 ± 50 nM in vitrowas identified. Multiple derivatives were synthesized to further optimize their properties with respect to their in vitroIC50values and their cytotoxicity against human cells (HeLa). In addition, we selected two hits to study their antimycobacterial activity against virulent Mtbto show that they might be candidates for further development of antimycobacterial agents against multidrug-resistant Mtb.

The neutral hexacoordinate silicon(IV) complex 6(SiO2N4skeleton) and the neutral pentacoordinate silicon(IV) complexes 7–11(SiO2N2C skeletons) were synthesized from Si(NCO)4and RSi(NCO)3(R=Me, Ph), respectively. The compounds were structurally characterized by solid‐state NMR spectroscopy (6–11), solution NMR spectroscopy (6and 10), and single‐crystal X‐ray diffraction (8and 11were studied as the solvates 8⋅CH3CN and 11⋅C5H12⋅0.5 CH3CN, respectively). The silicon(IV) complexes 6(octahedral Si‐coordination polyhedron) and 7–11(trigonal‐bipyramidal Si‐coordination polyhedra) each contain two bidentate ligands derived from an α‐amino acid: (S)‐alanine, (S)‐phenylalanine, or (S)‐tert‐leucine. The deprotonated amino acids act as monoanionic (6) or as mono‐ and dianionic ligands (7–11). The experimental investigations were complemented by computational studies of the stereoisomers of 6and 7.

Herein we report for the first time full details on the synthesis and structural characterization of novel homodinuclear bridging cobalt and nickel borylene complexes containing bridging carbonyl ligands, an unusual coordination motif rarely before observed for homodinuclear borylene complexes. Furthermore, the homodinuclear nickel complex represents the first instance of a nickel borylene complex. Quantum chemical analyses of charge‐density topology, electron localization function (ELF) and natural charges indicate the absence of direct metal–metal bonds in both the cobalt and nickel systems, in contradiction with electron counting. The topology of the Laplacian of the electron density and of the ELF around the bridging boron atom is consistent with a bis‐metallo‐substituted borane situation for the dicobalt system, but with a three‐center‐bonding borylene for the dinickel complex.

Bonding in borylene‐, carbene‐, and vinylidene‐bridged dinuclear manganese complexes [MnCp(CO)2]2X (X=B‐tBu, B=NMe2, CH2, CCH2) has been compared by analyses based on quantum theory of atoms in molecules (QTAIM), on the electron‐localization function (ELF), and by natural‐population analyses. All of the density functional theory based analyses agree on the absence of a significant direct MnMn bond in these complexes and confirm a dominance of delocalized bonding via the bridging ligand. Interestingly, however, the topology of both charge density and ELF related to the Mn‐bridge‐Mn bonding depend qualitatively on the chosen density functional (except for the methylene‐bridged complex, which exhibits only one three‐center‐bonding attractor both in −∇2ρand in ELF). While gradient‐corrected functionals provide a picture with localized two‐center XMn bonding, increasing exact‐exchange admixture in hybrid functionals concentrates charge below the bridging atom and suggests a three‐center bonding situation. For example, the bridging boron ligands may be described either as substituted boranes (e.g., at BLYP or BP86 levels) or as true bridging borylenes (e.g., at BHLYP level). This dependence on the theoretical level appears to derive from a bifurcation between two different bonding situations and is discussed in terms of charge transfer between X and Mn, and in the context of self‐interaction errors exhibited by popular functionals.

The λ6Si‐silicate [K(18‐crown‐6)]2[Si(NCO)6] (10) was synthesized by treatment of Si(NCO)4with KNCO in the presence of 18‐crown‐6. Compound 10(SiN6skeleton) is the first example of a hexa(cyanato‐N)silicate. It was characterized by solid‐state and solution NMR spectroscopy, and the acetonitrile solvate 10·2CH3CN was studied by single‐crystal X‐ray diffraction. To differentiate between the two isomeric [Si(NCO)6]2−and [Si(OCN)6]2−dianions, computational studies were performed.