Controlling the Self‐Assembly of a Mixed‐Metal Mo/V–Selenite Family of Polyoxometalates

Five mixed‐metal mixed‐valence Mo/V polyoxoanions, templated by the pyramidal SeO₃^2? heteroanion have been isolated: K₁₀[Mo^VI₁₂V^V₁₀O₅₈(SeO₃)₈]?18 H₂O ( 1 ), K₇[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)]?31 H₂O ( 2 ), (NH₄)₇K₃[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)(Mo^V₆V^V‐ O₂₂)]?40 H₂O ( 3 ), (NH₄)₁₉K₃[Mo^VI₂₀V^V₁₂V^IV₄O₉₉(SeO₃)₁₀]?36 H₂O ( 4 ) and [Na₃(H₂O)₅{Mo_18?xV_xO₅₂(SeO₃)} {Mo_9?yV_yO₂₄(SeO₃)₄}] ( 5 ). All five compounds were characterised by single‐crystal X‐ray structure analysis, TGA, UV/Vis and FT‐IR spectroscopy, redox titrations, and elemental and flame atomic absorption spectroscopy (FAAS) analysis. X‐ray studies revealed two novel coordination modes for the selenite anion in compounds 1 and 4 showing η,μ and μ,μ coordination motifs. Compounds 1 and 2 were characterised in solution by using high‐resolution ESI‐MS. The ESI‐MS spectra of these compounds revealed characteristic patterns showing distribution envelopes corresponding to 2? and 3? anionic charge states. Also, the isolation of these compounds shows that it may be possible to direct the self‐assembly process of the mixed‐metal systems by controlling the interplay between the cation “shrink‐wrapping” effect, the non‐conventional geometry of the selenite anion and fine adjustment of the experimental variables. Also a detailed IR spectroscopic analysis unveiled a simple way to identify the type of coordination mode of the selenite anions present in POM‐based architectures.     

Influence of the macro and micro-porous structure on the mechanical behavior of poly(l-lactic acid) scaffolds

The development of macroporous biodegradable polymeric materials with three-dimensional pore structure is an important research field in tissue engineering. Structural scaffolds not only provide the cells with a mechanical support, but also perform an interactive physico-chemical role in tissue regeneration, thus it becomes important to be able to tune their mechanical properties to deliver appropriate mechanical signals to adhered cells for proper tissue regeneration. This work presents two series of poly(l-lactic acid) (PLLA) scaffolds in which we modulated the mechanical properties by systematically changing two synthesis parameters: polymer/solvent ratio and polymer-solution/porogen percentage. The peculiarity of the constructs is the presence of a double porosity: micropores generated by dioxane solvent using a freeze extraction technique and macropores produced by the leaching of macroporogen spheres. An increase in the PLLA/dioxane ratio decreases the micropores size and also influences to some extent the macropores size, due to the ability of dioxane to swell macroporogen particles. On the other hand, an increase in the amount of macroporogen increases the porosity by increasing the dimension of pore the throats connecting the macropores. Consequently, the increase in the PLLA/dioxane ratio produces a significant decrease in the permeability, and an increase in the apparent compression Young's modulus and aggregate modulus. When increasing the amount of macroporogen the permeability significantly increases and a decrease in the mechanical properties of the scaffolds is observed. Summarizing, with a systematic change of two fabrication parameters (amount of dioxane and macroporogen) the structural characteristics of the scaffolds were modulated and thereby their mechanical and transport properties were controlled.     

Hyper-heuristics: a survey of the state of the art

Hyper-heuristics comprise a set of approaches that are motivated (at least in part) by the goal of automating the design of heuristic methods to solve hard computational search problems. An underlying strategic research challenge is to develop more generally applicable search methodologies. The term hyper-heuristic is relatively new; it was first used in 2000 to describe heuristics to choose heuristics in the context of combinatorial optimisation. However, the idea of automating the design of heuristics is not new; it can be traced back to the 1960s. The definition of hyper-heuristics has been recently extended to refer to a search method or learning mechanism for selecting or generating heuristics to solve computational search problems. Two main hyper-heuristic categories can be considered: heuristic selection and heuristic generation. The distinguishing feature of hyper-heuristics is that they operate on a search space of heuristics (or heuristic components) rather than directly on the search space of solutions to the underlying problem that is being addressed. This paper presents a critical discussion of the scientific literature on hyper-heuristics including their origin and intellectual roots, a detailed account of the main types of approaches, and an overview of some related areas. Current research trends and directions for future research are also discussed.     

Synthesis, characterization, and X-ray analysis of new N,N′-disubstituted-1,4-diazepanes

The synthesis and structural characterization of six 1,4-diazepanes prepared by condensation of 2,2′-(1,3-propanediyldiimino)diphenols with glyoxal or 2,3-butanedione to give bisbenzoxazolidines and subsequent reduction with BH₃-DMS is reported. The new 1,4-diazepanes were obtained in yields between 75% and 83% and characterized by NMR, IR, and HRMS. In addition, the crystal structure analysis showed that the seven-membered ring in 1,4-diazepanes (12a, 12e, and 12d) and bisbenzoxazolidines (11a, 11c, 11f) adopt in all cases twisted chair conformation. Compounds 12a and 12e show disorder in the seven member ring, as a result of the conformational flexibility of the seven-membered ring.     

On the automatic discovery of variants of the NEH procedure for flow shop scheduling using genetic programming

We use genetic programming to find variants of the well-known Nawaz, En-score and Ham (NEH) heuristic for the permutation flow shop problem. Each variant uses a different ranking function to prioritize operations during schedule construction. We have tested our ideas on problems where jobs have release times, due dates, and weights and have considered five objective functions: makespan, sum of tardiness, sum of weighted tardiness, sum of completion times and sum of weighted completion times. The implemented genetic programming system has been carefully tuned and used to generate one variant of NEH for each objective function. The new NEHs, obtained with genetic programming, have been compared with the original NEH and randomized NEH versions on a large set of benchmark problems. Our results indicate that the NEH variants discovered by genetic programming are superior to the original NEH and its stochastic version on most of the problems investigated.     

Controlling transformations in the assembly of polyoxometalate clusters: {Mo11V7}, {Mo17V8} and {Mo72V30}

The reaction of molybdate with vanadium(V) in the presence of sulfite anions is explored showing how, via cation control, stepwise assembly through the {Mo(11)V(7)} cluster yields a {M(25)} cluster-based compound, [Mo(VI)(11)V(V)(5)V(IV)(2)O(52)(μ(9)-SO(3))(Mo(VI)(6)V(V)O(22))](10-) (1a), which was first discovered using cryospray mass spectrometry, whereas switching the cation away from ammonium allows the direct formation of the spherical 'Keplerate' {Mo(72)V(30)} cluster.