Abstract: The [S(2)B(18)H(19)](-) anion 1, from syn-B(18)H(22) 2 with NaH and elemental sulfur, has an unusual arachno-type eleven-vertex {SB(10)} subcluster that has an open hexagonal pyramidal {B(7)} structural feature. This is conjoined, with two boron atoms in common, to a second {SB(10)} subcluster of conventional nido eleven-vertex geometry. Protonation of 1 forms neutral [S(2)B(18)H(20)] 4. Subsequent deprotonation of 4 yields the fluxional [S(2)B(18)H(19)](-) anion 5, which is isomeric with 1. Neutral 4and anion 5 do not have the {B(7)} hexagonal pyramidal feature. Neutral 4 consists of conventional nido eleven-vertex {SB(10)} and arachno ten-vertex {SB(9)} subclusters conjoined with a single spiro boron atom in common. Anion 5 is closely related to 4, but with an additional inter-boron intercluster link. Anion 5 spontaneously reverts to anion 1 over a few hours at room temperature, remarkable in that the open {B(7)} hexagonal pyramid is regenerated. DFT B3LYP/6-31G* calculations suggest definitive structures for 4 and 5 that are substantiated by agreement between observed NMR delta((11)B) values and boron nuclear shielding as calculated by the GIAO approach on the DFT-calculated structures. Extension of this approach additionally defines transition states and intermediates for the fluxionality of 5, and also for the reassembly of the starting anion 1, together with its {B(7)} feature, from fluxional 5. The fluxionality of 5 involves the inter-subcluster transfer of a {BH} unit. The reassembly of 1 from 5 involves a DSD rearrangement and two successive hydrogen-atom hops. Confidence in the application of this method to these large macropolyhedral assemblies is afforded in the first instance by good agreement between delta((11)B)(OBS) and delta((11)B)(CALC) for the structurally characterised original anion 1, the only species amongst these to be crystallographically established.
Abstract: In an attempt to build up borane-based multicluster assemblies, thermolysis of [6,9-(SMe2)2-arachno-B10H12] 1 in inert hydrocarbon solution, followed by chromatographic separation, has resulted in the isolation not only of the previously established single-cluster product from this reaction, [5-(SMe2)-nido-B10H12] 2 (30%), but also the two two-cluster species [6,9-(SMe2)2-arachno-B10H11-1-(6-nido-B10H13)] 3 (20%) and [1,6-(nido-B10H13)2] 6 (ca. 0.5%) and the two three-cluster species [6,9-(SMe2)2-arachno-B10H10-1,5-(6-nido-B10H13)2] 4 (5%), characterized crystallographically, and [6,9-(SMe2)2-arachno-B10H10-1,3-(6-nido-B10H13)2] 5 (<1%), identified by NMR spectroscopy. An improved crystallographic investigation of [5-(SMe2)-nido-B10H12] 2 is also presented. The feasibility of the stability of species resulting from multiple adjacent substitution of nido-decaboranyl units on the [6,9-(SMe2)2-arachno-B10H12] skeleton is tested by DFT calculations. In an extension, to attempt the use of pre--linked two-cluster compounds as starting substrates, two-cluster [5-(SMe2)-4-(2-nido-B10H13)-nido-B10H11] 7 (0.6%) has been isolated from the reaction of SMe2 with [1,5-(nido-B10H13)2], other identified products being compound 1 (39%) and compound 3 (10.5%).
Abstract: Some synthetic and structural systematics for monocarbaboranes, using the C-phenylated motif as the example, are investigated. The 10-vertex [6-Ph-nido-6-CB(9)H(11)](-) anion 1, from reaction of PhCHO with B(10)H(14) in KOH/H(2)O, is a useful entry synthon into C-phenyl monocarbaborane chemistry. Treatment of anion 1 with Na/thf yields the 10-vertex [1-Ph-closo-1-CB(9)H(9)](-) anion 2a, whereas treatment of anion 1 with iodine in alkaline solution yields the isomeric 10-vertex [2-Ph-closo-2-CB(9)H(9)](-) anion 2b, which isomerises quantitatively to 2a on heating under reflux in DME. Thermolysis of anion 1 yields the 9-vertex [4-Ph-closo-4-CB(8)H(8)](-) anion 5, whereas treatment of anion 1 with FeCl(3)/HCl gives neutral 9-vertex [4-Ph-arachno-4-CB(8)H(13)] 3. Compound 3 gives neutral 9-vertex [1-Ph-nido-1-CB(8)H(11)] 4 in refluxing toluene, and gives the 7-vertex [2-Ph-closo-2-CB(6)H(6)](-) anion 7 and the 8-vertex [1-Ph-closo-1-CB(7)H(7)](-) anion 6 in refluxing toluene with NEt(3). Reaction of 1 with [BH(3)(thf)] yields the 11-vertex [7-Ph-nido-7-CB(10)H(12)](-) anion 8 which can be converted to the 12-vertex [1-Ph-closo-1-CB(11)H(11)](-) anion 10 using [BH(3)(SMe(2))]; alternatively, anion 1 yields anion 10 directly on treatment with [BH(3)(NEt(3))]. Treatment of anion 8 with I(2)/KOH yields the 11-vertex [2-Ph-closo-2-CB(10)H(10)](-) anion 9. The structures of anions 1, 2a, 2b, 5, 6, 7, 8, 9 and 10 have been established by single-crystal X-ray diffraction analyses of their [NEt(4)](+) salts, and those of neutral 3 and 4 estimated by DFT calculations at the B3LYP/6-31G* level; similar calculations have also been applied to the new anionic closo species 2a, 2b, 5, 6, 7, 9 and 10. Crystals of the [NEt(4)](+) salt of the [2-Ph-closo-2-CB(6)H(6)](-) anion 7 required synchrotron X-radiation for sufficient diffraction intensity for molecular-structure elucidation. The syntheses are in principle generally applicable to give extensive derivative C-aryl and C-alkyl chemistries.
Abstract: Reduction of 1,2-closo-C2B10H12 followed by treatment with [RuCl2(p-cymene)]2(p-cymene = C6H4MeiPr-1,4) affords the 13-vertex ruthenacarborane 4-(p-cymene)-4,1,6-closo-RuC2B10H12, characterised both spectroscopically and, in two crystalline forms, crystallographically. Although asymmetric in the solid state, having a docosahedral cage architecture with cage C atoms at vertices 1 and 6, this species clearly has Cs symmetry on the NMR timescale at room temperature. However, the fluctional process in operation can be arrested at low temperature, and an activation energy of 43.1 kJ mol(-1) is estimated. A computational study of the related species 4-(eta-C6H6)-4,1,6-closo-RuC2B10H12 reveals that the fluctionality is due to a double diamond-square-diamond process, first suggested by Hawthorne et al for the analogous CpCo species. These calculations yield an activation energy of 40.4 kJ mol(-1), in excellent agreement with that derived from experiment. Reduction of 1,2-Ph(2)-1,2-closo-C2B10H10 followed by treatment with [RuCl2(eta-C6H6)]2 or [RuCl2(p-cymene)]2 yields the analogous species 1,6-Ph2-4-(eta-C6H6)-4,1,6-closo-RuC2B10H10 and 1,6-Ph2-4-(p-cymene)-4,1,6-closo-RuC2B10H10, respectively. These C,C-diphenyl compounds were again studied spectroscopically and crystallographically, the p-cymene species again showing two crystalline modifications. In contrast to their CpCo and Cp*Co analogues all three ruthenacarboranes do not undergo isomerisation in refluxing toluene.
Abstract: Scoring forms a major obstacle to the success of any docking study. In general, fast scoring functions perform poorly when used to determine the relative affinity of ligands for their receptors. In this study, the objective was not to rank compounds with confidence but simply to identify a scoring method which could provide a 4-fold hit enrichment in a screening sample over random selection. To this end, LigandFit, a fast shape matching docking algorithm, was used to dock a variety of known inhibitors of type 4 phosphodiesterase (PDE4B) into its binding site determined crystallographically for a series of pyrazolopyridine inhibitors. The success of identifying good poses with this technique was explored through RMSD comparisons with 19 known inhibitors for which crystallographic structures were available. The effectiveness of five scoring functions (PMF, JAIN, PLP2, LigScore2, and DockScore) was then evaluated through consideration of the success in enriching the top ranked fractions of nine artificial databases, constructed by seeding 1980 inactive ligands (pIC50 < 5) with 20 randomly selected inhibitors (pIC50 > 6.5). PMF and JAIN showed high average enrichment factors (greater than 4 times) in the top 5-10% of the ranked databases. Rank-based consensus scoring was then investigated, and the rational combination of 3 scoring functions resulted in more robust scoring schemes with (cScore)-DPmJ (consensus score of DockScore, PMF, and JAIN) and (cScore)-PPmJ (PLP2, PMF, and JAIN) yielding particularly good results. These cScores are believed to be of greater general application. Finally, the analysis of the behavior of the scoring functions across different chemotypes uncovered the inherent bias of the docking and scoring toward compounds in the same structural family as that employed for the crystal structure, suggesting the need to use multiple versions of the binding site for more successful virtual screening strategies.
Abstract: Single-crystal conventional-tube and synchrotron X-ray diffraction studies of the anions in [NMe(4)][arachno-B(9)H(12)-4,8-Br(2)] 1 and K[arachno-B(9)H(14)] 2, and also of the series of adducts [arachno-B(9)H(13)-4-L], where L is P(CCH)(3) (3), NHEt(2) (4), NC(5)H(5) (5), or NH(2)CH(2)Ph (6), are reported. Structural studies of 1-6, determined at low temperatures, located all atoms, including bridging and endo-terminal hydrogen atoms. The basic boron-hydride clusters of these, and of all the other known species with the arachno nine-vertex i-nonanborane geometry reported in the literature, are isostructural and feature three bridging and two endo-terminal hydrogen atoms on the open face. This arrangement is different from that previously reported for Cs[arachno-B(9)H(14)] 7 and for [arachno-B(9)H(13)-4-(NCMe)] 9. However, a new X-ray diffraction data set and refinement experimentally confirm the [3 x mu-H, 2 x endo] arrangement for 9 also. The experimental results for 1-6 support recently reported calculations for [B(9)H(14)](-), which predict both the structures and the (11)B NMR chemical shifts. These conclusions are also supported by calculations for 3, 4, and 9 and also for the [arachno-B(9)H(13)-4-(NCS)](-) anion in [NMe(4)][B(9)H(13)(NCS)] 8.
Abstract: The new fluoroboranes exo-2-FB4H9 1 and trans-MeCH=CHBF2 2 have been obtained unexpectedly and in good yield from the reaction of tetramethylammonium octahydrotriborate (NMe4B3H8) with boron trifluoride and propyne (MeC identical to CH).
Abstract: The gas-phase reaction of the phosphaalkyne P identical to CtBu with tetraborane(10), B4H10, yields the nido five-vertex phosphacarbaborane cluster compound 2-tBu-1,2-PCB3H5 2 with an unusual 31P NMR peak shift of -500.5 ppm.
Abstract: Thermolysis in the solid state of Cs+[arachno-CB9H14]-, or of Cs+[nido-CB9H12]-, or the oxidation of nido-1-CB8H12 with I2 in THF at -78 degrees C in the presence of NEt3, gives the first nine-vertex closo monocarbaborane, the stable [closo-4-CB8H9]- anion, in yields of 56, 61 and 75%, respectively.
