The preparation of dimolybdenum(V,V) complexes from molybdenum quadruply bonded metal-metal dimers

Oxidation of quadruply bonded metal-metal dimers in the presence of good π-accepting ligands results in the formation of Mo^V---Mo^V compounds of the type [MO₂(μ-X)₂(Y)(Y′)]²⁺ (X = O or S; Y,Y′ = O,O; S,S; O,S). Reaction of MO₂(O₂CCH₃)₄ with oxygen in the presence of Na₂mnt (mnt = 1,2-dicyanoethylene-2,2-dithiolate) gives [MO₂(μ-S)₂(O)(S)(mnt)₂]²⁻ (1). The compound crystallizes in the monoclinic space group P2₁/c, with cell dimensions a = 19.547(4), b = 15.210(4), c = 18.754(6) Å, β = 101.69(2)°, V= 5460(2) ų, and Z = 4. Similarly, oxidation of o-dichlorobenzene solutions of Mo₂Cl₄(CH₃CN)₄ and 4,4′-dimethyl-2,2′-dipyridyl (dmpby) or, more directly, the reaction of Mo₂Cl₄(dmbpy)₂ with oxygen leads to the formation of a red solid, which was characterized by X-ray crystallography to be Mo₂(μ-O)₂(O)₂(Cl)₂(dmbpy)₂ (2). Red diamond crystals, prepared by slow evaporation of CH₃CN solutions of 2, are trigonal and in the space group P3₁21 with cell dimensions a = 16.135(4), b = 16.135(4), c = 10.709(3) Å, V = 2414.4(13) ų and Z = 3. In both structures, the geometry about each of the molybdenum atoms is a distorted square pyramid with terminal oxygen or sulphur atoms at the apices and in a syn conformation. The molybdenum-molybdenum bond distances of 2.858(1) Å and 2.562(2) Å in structures of 1 and 2, respectively, are typical of other Mo^V---Mo^V dimers and indicative of a single Mo---Mo bond.     

Artificial metalloenzymes: (strept)avidin as host for enantioselective hydrogenation by achiral biotinylated rhodium-diphosphine complexes

We report on the generation of artificial metalloenzymes based on the noncovalent incorporation of biotinylated rhodium-diphosphine complexes in (strept)avidin as host proteins. A chemogenetic optimization procedure allows one to optimize the enantioselectivity for the reduction of acetamidoacrylic acid (up to 96% ee (R) in streptavidin S112G and up to 80% ee (S) in WT avidin). The association constant between a prototypical cationic biotinylated rhodium-diphosphine catalyst precursor and the host proteins was determined at neutral pH: log K(a) = 7.7 for avidin (pI = 10.4) and log K(a) = 7.1 for streptavidin (pI = 6.4). It is shown that the optimal operating conditions for the enantioselective reduction are 5 bar at 30 degrees C with a 1% catalyst loading.     

A priori bounds for approximations of Markov programs

This note determines a priori bounds for B. L. Fox's [J. Math. Anal. Appl., 34 (1971), 665–670] scheme of approximating discounted Markov programs, thus refining bounds recently obtained by D. J. White (Notes in Descision Theory No. 43, University of Manchester, 1977). The approximation scheme focuses careful attention on only a subset of the state space and uses a fixed function to characterize future returns outside the designated subset. The a priori bounds are useful to design the specific approximation, that is, to select the appropriate subset on which the approximation is based.     

A constructive proof by elementary transformations of Roth's removal theorems in the theory of matrix equations

W. E. Roth gave necessary and sufficient conditions for the existence of solution(s) of certain types of linear matrix equations. Proofs were based on invariant factors and were long and complicated. Other shorter but non-constructive proofs have since been provided by later authors. We present here very brief constructive proofs based on the simplest of mathematical techniques, namely row- and column-reduction of a matrix.     

A broad view of queueing theory through one issue

This is an overview and appreciation of the contributions to this special issue.     

Unsupervised cell identification on multidimensional X‐ray fluorescence datasets

A novel approach to locate, identify and refine positions and whole areas of cell structures based on elemental contents measured by X‐ray fluorescence microscopy is introduced. It is shown that, by initializing with only a handful of prototypical cell regions, this approach can obtain consistent identification of whole cells, even when cells are overlapping, without training by explicit annotation. It is robust both to different measurements on the same sample and to different initializations. This effort provides a versatile framework to identify targeted cellular structures from datasets too complex for manual analysis, like most X‐ray fluorescence microscopy data. Possible future extensions are also discussed.     

Backbonding in Thorium(IV) and Uranium(IV) Diarsenido Complexes with tBuNC and CO

The coordination of tBuNC and CO with the diarsenido complexes (C₅Me₅)₂An(η²-As₂Mes₂), An=Th, U, has been investigated. For the first time, a comparison between isostructural complexes of Th^IV and U^IV has been possible with CO; density functional calculations indicated an appreciable amount of π backbonding that originates from charge transfer from an actinide-arsenic sigma bond. The calculated CO stretching frequencies in the Th^IV and U^IV diarsenido complexes are consistent with the experimental measurements, both show large shifts to lower frequency. We demonstrate that the π backbonding is crucial to explaining the red shifts of CO frequency upon An^IV complex formation. Interestingly, this interaction essentially correlates to the parallel orientation of π*(C−O) orbitals relative to the An−As bond.     

Mass separator for radioactive isotopes

A new isotope separator has been designed, constructed, and put into routine operation for separation of ¹³³Xe providing a major advancement and significant cost reduction in preparation of this radioactive isotope. The design features and advantages are discussed that expedite high purity separation of relatively small quantities of this isotope. These advantages could be easily used to expedite separation of other shorter-lived radioactive isotopes.     

1,3-Diene-Containing Phase Transfer Catalysts and Their Removal From Solution by a Diels-Alder Reaction with an Immobilized Triazolinedione Dienophile

8E,10E-Dodecadienyl-tri-n-butylamonium and -tri-n-butylphosphonium methanesulfonates were prepared and shown to be effective phase transfer catalysts. After reaction the catalysts can be removed quantitatively from the organic phase by addition of a silica gelimmobilized triazolinedione dienophile followed by filtration.     

The drawing behavior of linear polyethylene: Effect of electron irradiation on drawing and subsequent mechanical behavior of drawn products

Isotropic monofilaments of linear polyethylene were subjected to electron irradiation in different atmospheres at a range of dose levels. It was found that the gel content remained very small up to a dose of about 2.5 Mrad (the gel point), and thereafter increased with irradiation dose in a manner similar to that found by other workers. Drawing of the irradiated filaments was very dependent on dose level, and high draw ratios of 30:1 could not be obtained above the gel point. Although irradiation of the spun monofilaments increased the initial tensile modulus of the drawn material to a small extent, neither the tensile strength nor the shear strength of the monofilaments (the latter measured with a pull-out adhesion technique) was significantly affected. On the other hand, the tensile strain at failure decreased dramatically as the irradiation dose was increased. The results are discussed in terms of our present state of knowledge regarding the structure and properties of highly drawn linear polyethylene, taking into consideration the cross-linking produced by the irradiation.