The Rose Tech Explorer - November 16, 1962

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Isomerisation of nido-[C₂B₁₀H₁₂]²⁻ dianions::Unprecedented rearrangements and new structural motifs in carborane cluster chemistry

We thank the EPSRC for support (DMcK funded by project EP/E02971X/1).Dianionic nido-[C2B10]2- species are key intermediates in the polyhedral expansion from 12- to 13-vertex carboranes and metallacarboranes, and the isomer adopted by these nido intermediates dictates the isomeric form of the 13-vertex product. Upon reduction and metallation of para-carborane up to five MC2B10 metallacarboranes can be produced (Angew. Chem., Int. Ed., 2007, 46, 6706), the structures of which imply the intermediacy of 1,7-, 3,7-, 4,7-, 7,9- and 7,10-isomers of the nido-[C2B10]2- species. In this paper we use density functional theory (DFT) calculations to characterise the reduction of closo-C2B10H12 carboranes and the subsequent isomerisations of the nido-[C2B10H12]2- dianions. Upon reduction para-carborane initially opens to [1,7-nido-C2B10H12]2- (abbreviated to 1,7) and [4,7-nido-C2B10H12]2- (4,7) and isomerisation pathways connecting 1,7 to 7,9, 4,7 to 7,10 and 1,7 to 3,7 have been characterised. For ortho- and meta-carborane the experimental reduction produces 7,9 in both cases and computed pathways for both processes are also defined; with ortho-carborane rearrangement occurs via 7,8, whereas with meta-carborane 7,9 is formed directly. The 7,9 isomer is the global minimum nido-structure. The characterisation of these isomerisation processes uncovers intermediates that adopt new structural motifs that we term basket and inverted nido. Basket intermediates feature a two-vertex basket handle bridging the remaining 10 vertices; inverted nido intermediates are related to known nido species, in that they have 5- and 6-membered belts, but where the latter, rather than the former, is capped, leaving a 5-membered open face. These new intermediates exhibit similar stability to the nido species, which is attributed to their relation to the 13-vertex docosahedron through the removal of 5-connected vertices. Isomerisation pathways starting from nido geometries are most often initiated by destabilisation of the cluster through a DSD process causing the 3-connected C7 vertex to move into a 4-connected site and a neighbouring B vertex to become 3-connected. The ensuing rearrangement of the cluster involves processes such as the pivoting of a 4-vertex diamond about its long diagonal, the pivoting of two 3-vertex triangles about a shared vertex and DSD processes. These processes are all ultimately driven by the preference for carbon to occupy low-connected vertices on the open 6-membered face of the resulting nido species.Publisher PDFPeer reviewe

The exopolyhedral ligand orientation (ELO) in 3-(nitrato-κO)-3,3-bis(triphenylphosphane-κP)-3-rhoda-1,2-dicarba-closo-dodecaborane(11) dichloromethane 2.2-solvate

In the title compound, [Rh(C2H11B9)(NO3)(C18H15P)2]·2.2CH2Cl2, studied as a 2.2-solvate of what was assumed to be dichloromethane, the nitrate ligand liesciswith respect to both cage C atoms. Accordingly, the compound displays a pronounced preferred exopolyhedral ligand orientation (ELO) which is traced to both the greatertransinfluence of the cage B over the cage C atoms and the greatertransinfluence of the triphenylphosphane ligands over the nitrate ligand. The overall molecular architecture therefore agrees with that of a number of similar 3-L-3,3-L′2-3,1,2-closo-MC2B9H11species in the literature.</jats:p

Reduction-induced facile isomerisation of metallacarboranes:synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC₂B₉H₁₁

One-electron reduction of 3-Cp-3,1,2-closo-CoC2B9H11 followed by heating to reflux in DME (b.p. 85 °C) induces isomerisation to 4-Cp-4,1,2-closo-CoC2B9H11.</p

Icosahedral metallacarborane/carborane species derived from 1,1′-bis(o-carborane)

We thank ORSAS (GS) and the EPSRC (DE and DMcK supported by project EP/E02971X/1, WYM supported by project EP/I031545/1) for funding.Examples of singly-metallated derivatives of 1,1[prime or minute]-bis(o-carborane) have been prepared and spectroscopically and structurally characterised. Metallation of [7-(1[prime or minute]-1[prime or minute],2[prime or minute]-closo-C2B10H11)-7,8-nido-C2B9H10]2- with a {Ru(p-cymene)}2+ fragment affords both the unisomerised species [1-(1[prime or minute]-1[prime or minute],2[prime or minute]-closo-C2B10H11)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (2) and the isomerised [8-(1[prime or minute]-1[prime or minute],2[prime or minute]-closo-C2B10H11)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (3), and 2 is easily transformed into 3 with mild heating. Metallation with a preformed {CoCp}2+ fragment also affords a 3,1,2-MC2B9-1[prime or minute],2[prime or minute]-C2B10 product [1-(1[prime or minute]-1[prime or minute],2[prime or minute]-closo-C2B10H11)-3-Cp-3,1,2-closo-CoC2B9H10] (4), but if CoCl2/NaCp is used followed by oxidation the result is the 2,1,8-CoC2B9-1[prime or minute],2[prime or minute]-C2B10 species [8-(1[prime or minute]-1[prime or minute],2[prime or minute]-closo-C2B10H11)-2-Cp-2,1,8-closo-CoC2B9H10] (5). Compound 4 does not convert into 5 in refluxing toluene, but does do so if it is reduced and then reoxidised, perhaps highlighting the importance of the basicity of the metal fragment in the isomerisation of metallacarboranes. A computational study of 1,1[prime or minute]-bis(o-carborane) is in excellent agreement with a recently-determined precise crystallographic study and establishes that the {1[prime or minute],2[prime or minute]-closo-C2B10H11} fragment is electron-withdrawing compared to H.Publisher PDFPeer reviewe

Structural correlates of antimicrobial efficacy in IL-8 and related human kinocidins

AbstractChemokines are small (8–12 kDa) effector proteins that potentiate leukocyte chemonavigation. Beyond this role, certain chemokines have direct antimicrobial activity against human pathogenic organisms; such molecules are termed kinocidins. The current investigation was designed to explore the structure–activity basis for direct microbicidal activity of kinocidins. Amino acid sequence and 3-dimensional analyses demonstrated these molecules to contain iterations of the conserved γ-core motif found in broad classes of classical antimicrobial peptides. Representative CXC, CC and C cysteine-motif-group kinocidins were tested for antimicrobial activity versus human pathogenic bacteria and fungi. Results demonstrate that these molecules exert direct antimicrobial activity in vitro, including antibacterial activity of native IL-8 and MCP-1, and microbicidal activity of native IL-8. To define molecular determinants governing its antimicrobial activities, the IL-8 γ-core (IL-8γ) and α-helical (IL-8α) motifs were compared to native IL-8 for antimicrobial efficacy in vitro. Microbicidal activity recapitulating that of native IL-8 localized to the autonomous IL-8α motif in vitro, and demonstrated durable microbicidal activity in human blood and blood matrices ex vivo. These results offer new insights into the modular architecture, context-related deployment and function, and evolution of host defense molecules containing γ-core motifs and microbicidal helices associated with antimicrobial activity