Hydroxide-promoted core conversions of molybdenum-iron-sulfur edge-bridged double cubanes: oxygen-ligated topological PN clusters

The occurrence of a heteroatom X (C, N, or O) in the MoFe7S9X core of the iron-molybdenum cofactor of nitrogenase has encouraged synthetic attempts to prepare high-nuclearity M-Fe-S-X clusters containing such atoms. We have previously shown that reaction of the edge-bridged double cubane [(Tp)2Mo2Fe6S8(PEt3)4] (1) with nucleophiles HQ- affords the clusters [(Tp)2Mo2Fe6S8Q(QH)2](3-) (Q = S, Se) in which HQ- is a terminal ligand and Q(2-) is a mu2-bridging atom in the core. Reactions with OH- used as such or oxygen nucleophiles generated in acetonitrile from (Bu3Sn)2O or Me3SnOH and fluoride were examined. Reaction of 1 with Et4NOH in acetonitrile/water generates [(Tp)2Mo2Fe6S9(OH)2]3- (3), isolated as [(Tp)2Mo2Fe6S9(OH)(OC(=NH)Me)(H2O)](3-) and shown to have the [Mo2Fe6(mu2-S)2(mu3-S)6(mu6-S)] core topology very similar to the P(N) cluster of nitrogenase. The reaction system 1/Et4NOH in acetonitrile/methanol yields the P(N)-type cluster [(Tp)2Mo2Fe6S9(OMe)2(H2O)](3-) (5). The system 1/Me3SnOH/F- affords the oxo-bridged double P(N)-type cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}5- (7), convertible to the oxidized cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}4- (6), which is prepared independently from [(Tp)2Mo2Fe6S9F2(H2O)](3-)/(Bu3Sn)2O. In the preparations of 3-5 and 7, hydroxide liberates sulfide from 1 leading to the formation of P(N)-type clusters. Unlike reactions with HQ-, no oxygen atoms are integrated into the core structures of the products. However, the half-dimer composition [Mo2Fe6S9O] relates to the MoFe7S9 constitution of the putative native cluster with X = O. (Tp = hydrotris(pyrazolyl) borate(1-)).     

Reversal of polarization in amidophosphines: neutral- and anionic-kappaP coordination vs. anionic-kappaP,N coordination and the formation of nickelaazaphosphiranes

Nickel(ii) chloride reacts with the bis(tert-butylamino)diazadiphosphetidine {Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)} to form trans-[{Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)}(2)NiCl(2)]. In solution and the solid-state each heterocyclic ligand coordinates nickel through one phosphorus atom only. For comparison the solid-state structure of the known trans-[NiCl(2)(PEt(3))(2)] was also determined and it was found that the two complexes have almost identical bond parameters about nickel. The nickel-amidophosphine complexes [{Bu(t)OP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], [(PBu(n)(3))ClNi{Bu(t)NP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], and [{Me(2)Si(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))] were synthesized and X-ray structurally characterized. In these mono- and di-nuclear nickel complexes the nickel ions are coordinated in pseudo square-planar fashions, by one trialkylphosphine ligand, one chloride ligand and one kappaP,N-coordinated amidophosphine moiety from tert-butylamido-substituted heterocycles. Attempts to create nickel complexes chelated in a kappa(2)P fashion by the o-phenylenediamine-tethered mono- and di-anionic 1-{Me(2)Si(micro-NBu(t))(2)PN} 2-{Me(2)Si(micro-NBu(t))(2)PNH}C(6)H(4) and 1,2-{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4), respectively, afforded instead [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)NiCl] and [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)Ni{PEt(3)}], each complex having kappaP,N and kappaP coordinated amidophosphine ligands.     

Synthesis of a new class of β-iodo N-alkenyl 2-pyridones

A new method for the synthesis of β-iodo N-alkenyl 2-pyridones from substituted 2-propargyloxypyridines has been discovered . These compounds present a unique complement of orthogonal functionality and structural characteristics that are unavailable via other routes. The ready access to these compounds renders them an important entry point for the preparation of more complex N-alkyl pyridone-containing targets.     

2,2′‐Bi[benzo[b]thiophene]: an unexpected isolation of the benzo[b]thiophene dimer

The crystal structure of 2,2′‐bi[benzo[b]thiophene], C₁₆H₁₀S₂, at 173 K has triclinic (P) symmetry. It is of interest with respect to its apparent mode of synthesis, as it is a by‐product of a Stille cross‐coupling reaction in which it was not explictly detected by spectroscopic methods. It was upon crystal structure analysis of a specimen isolated from the mother liquor that this reaction was determined to give rise to the title compound, which is a dimer arising from the starting material. Two independent half‐molecules of this dimer comprise the asymmetric unit, and the full molecules are generated via inversion centers. Both molecules in the unit cell exhibit ring disorder, and they are essentially identical because of their rigidity and planarity.     

Characterization of [Re2Cl3(μ-dppm)2(NCCH3)2][Cl] and [Re2Cl3(μ-dppm)2(NCC6H5)2][Cl]·2CH2Cl2 by X-ray Crystallographic and Two-Dimensional NMR Spectroscopic Techniques

The complex Re₂Cl₆(P-n-Bu₃)₂ prepared in situ reacts with CH₃CN to form a blue-green solution. Addition of the chelating phosphine bis(diphenylphosphino)methane (dppm) results in the formation of the complex [Re₂Cl₃(-dppm)₂(NCCH₃)₂][Cl] (1) upon heating. The two acetonitrile molecules adopt a trans geometry on the rhenium center with the axially coordinated chlorine. The analogous trans benzonitrile species [Re₂Cl₃(-dppm)₂(NCC₆H₅)₂][Cl]·2CH₂Cl₂ (2) is synthesized under the same reaction conditions. The coupling constants of the AABB ³¹P{¹H} NMR spectra of the compounds were elucidated from ³¹P–³¹P homonuclear J-resolved NMR experiments. Additional characterization methods include ³¹P{¹H} NMR spectroscopy, UV-vis spectroscopy, and X-ray crystallography.     

Surface ordering in dilute dihexadecyl dimethyl ammonium bromide solutions at the air-water interface

At elevated temperatures and in dilute solution, we have observed lamellar surface ordering at the air-water interface of dihexadecyl dimethylammonium bromide, DHDAB, in the presence of electrolyte. With increasing temperature, the onset in ordering is observed between 35 and 40 degrees C. At 40 degrees C, there is an abrupt change in the lamellar spacing, from approximately 33 to approximately 40 A. Furthermore, in the presence of the cosurfactant benzyl alcohol, the ordering occurs at a lower temperature, between 20 and 25 degrees C. The change in lamellar spacing with temperature is attributed to a surface-induced transition, similar to the Lbeta to Lalpha phase transition observed in bulk lamellar dispersions.     

The structure of mixed nonionic surfactant monolayers at the air-water interface: the effects of different alkyl chain lengths

The structure of mixed nonionic surfactant monolayers of monodecyl hexaethylene glycol (C10E6) and monotetradecyl hexaethylene glycol (C14E6) adsorbed at the air-water interface has been determined by specular neutron reflectivity. Using partial isotopic labeling (deuterium for hydrogen) of the alkyl and ethylene oxide chains of each surfactant, the distribution and relative positions of the chains at the interface have been obtained. The packing of the two different alkyl chain lengths results in structural changes compared to the pure surfactant monolayers. This results in changes in the relative positions of the alkyl chains and of the ethylene oxide chains at the interface. The role of the alkyl chain length is contrasted with that of the ethylene oxide chain length, determined from results reported previously on the nonionic surfactant mixture of monododecyl triethylene glycol (C12E3) and monododecyl octaethylene glycol (C12E8).     

Total Synthesis of (−)‐Salinosporamide A via a Late Stage C−H Insertion

The synthesis of (?)‐salinosporamide A, a proteasome inhibitor, is described. The synthesis highlights the assembly of a densely decorated pyrrolidinone core via an aza‐Payne/hydroamination sequence. Central to the success of the synthesis is a late‐stage C?H insertion reaction to functionalize a sterically encumbered secondary carbon. The latter functionalization leads to an enabling transformation where most of the prototypical strategies failed.     

Structural and functional characteristics of rhenium clusters derived from redox chemistry of the triangular [ReIII3(mu-Cl)3] core unit

The present study investigates structural and functional aspects of the redox chemistry of rhenium(III) chloride [Re3Cl9] (1) in aqueous and organic solvents, with emphasis on the dioxygen-activating capabilities of reduced rhenium clusters bearing the Re3(8+) core. Dissolution of 1 in HCl (6 M) generates [Re3(mu-Cl)3Cl9]3- (2a), which can be isolated as the tetraphenylphosphonium salt (2b). Anaerobic one-electron reduction of 1 by Hg in HCl (6-12 M) produces [(C6H5)4P]2[Re3(mu-Cl)3Cl7(H2O)2].H2O (3), the structure of which features a planar [Re3(mu-Cl)3Cl3] framework (Re3(8+) core), involving two water ligands that occupy out-of-plane positions in a trans arrangement. Compound 3 dissociates in the presence of CO, yielding [(C6H5)4P]2[ReIII2Cl8] (4) and an unidentified red carbonyl species. In situ oxidation (O2) of the reduced Re3(8+)-containing cluster in HCl (6 M) produces quantitatively 2a, whereas oxidation of 3 in organic media results in the formation of [(C6H5)4P]4[(Re3(mu-Cl)3Cl7(mu-OH))2].2CH2Cl2 (5). The structure of 5 reveals that two oxygen ligands (hydroxo units) bridge asymmetrically two Re3(9+) triangular clusters. The origin of these hydroxo units derives from the aquo ligands, rather than O2, as shown by 18O2 labeling studies. The hydroxo bridges of 5 can be replaced by chlorides upon treatment with Me3SiCl to afford the analogous [(C6H5)4P]4[(Re3(mu-Cl)3Cl7(mu-Cl))2].10CH2Cl2 (6). The reaction of 5 with Hg in HCl (6 M)/tetrahydrofuran regenerates compound 3. Complexes 1-3 exhibit nitrile hydratase type activity, inducing hydrolysis of CH3CN to acetamide. The reaction of 3 with CH3CN yields [(C6H5)4P]2[Re3(mu-Cl)3Cl6.5(CH3CN)1.5(CH3C(O)NH)0.5] (7), the structure of which is composed of [Re3(mu-Cl)3Cl7(CH3CN)2]2- (7a) and [Re3(mu-Cl)3Cl6(CH3CN)(CH3C(O)NH)]2- (7b) (Re3(8+) cores) as a disordered mixture (1:1). Oxidation of 7 with O2 in CH3CN affords [(C6H5)4P]2[Re3(mu-Cl)3Cl7(CH3C(O)NH)].CH3CN (8) and small amounts of [(C6H5)4P][ReO4] (9). Compound 8 is also independently isolated from the reaction of 2b with wet CH3CN, or by dissolving 5 in CH3CN. In MeOH, 5 dissociates to afford [(C6H5)4P]2[Re3(mu-Cl)3Cl8(MeOH)].MeOH (10).