Versatile approach toward the self-assembly of heteromultimetallic salen structures

A general route is presented toward the template-directed preparation of self-assembled heteromultimetallic salen structures using noncovalent coordinative metal-ligand interactions. Various higher order assemblies have been studied in detail using a combination of NMR spectroscopy and X-ray crystallography.     

Rigid bis-zinc(II) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.     

Expedient method for the transmetalation of Zn(II)-centered salphen complexes

Various mono- and bis-ZnII complexes derived from salphen ligands have been conveniently transmetalated with a number of transition metal (TM) acetates (M=Ni, Pd, Mn) in THF to afford their respective TM-salphen counterparts in excellent isolated yields (80-100%). This new transmetalation procedure allows in situ switching between supramolecular and catalytic functions of the metallosalphen complex.     

Ruthenium-catalyzed cyclization of 2-alkyl-1-ethynylbenzenes via a 1,5-hydrogen shift of ruthenium-vinylidene intermediates

Catalytic cyclization of 2-alkyl-1-ethynylbenzene derivatives was implemented by TpRuPPh3(CH3CN)2PF6 (10 mol %) in hot toluene (105 degrees C, 36-100 h) to form 1-substituted-1H-indene and 1-indanone products; such cyclizations proceeded more efficiently for substrates bearing electron-rich benzenes. We propose that the cyclization mechanism involves a 1,5-hydrogen shift of initial metal-vinylidene intermediate.     

Basis- and partition identification for quadratic programming and linear complementarity problems

Optimal solutions of interior point algorithms for linear and quadratic programming and linear complementarity problems provide maximally complementary solutions. Maximally complementary solutions can be characterized by optimal partitions. On the other hand, the solutions provided by simplex–based pivot algorithms are given in terms of complementary bases. A basis identification algorithm is an algorithm which generates a complementary basis, starting from any complementary solution. A partition identification algorithm is an algorithm which generates a maximally complementary solution (and its corresponding partition), starting from any complementary solution. In linear programming such algorithms were respectively proposed by Megiddo in 1991 and Balinski and Tucker in 1969. In this paper we will present identification algorithms for quadratic programming and linear complementarity problems with sufficient matrices. The presented algorithms are based on the principal pivot transform and the orthogonality property of basis tableaus. Received April 9, 1996 / Revised version received April 27, 1998? Published online May 12, 1999     

Ligand-template directed assembly: an efficient approach for the supramolecular encapsulation of transition-metal catalysts

Supramolecular encapsulation of small guest molecules inside well-defined cavities of molecular capsules has witnessed broad attention because of the unusual behaviour of these systems. The molecular capsules generally consist of rigid complementary building blocks that are held together by multiple, complementary non-covalent interactions. Interestingly, it has been shown that chemical transformations can take place inside these capsules and in some examples the reaction is accelerated, while in other cases otherwise instable intermediates could be isolated in the capsulated form. Many reactions of interest require a transition-metal (TM) catalyst, and the creation of new capsules in which such catalysts are implemented within the structure is thus required for the development of resourceful type of catalyst systems for these processes. In this concept article we will discuss new strategies to arrive at such systems, with a focus on a ligand-templated approach. In this approach, multifunctional ligands are used as templates for the encapsulation process by supramolecular building blocks and concomitantly for the formation of TM complexes that are active in catalytic processes. The obtained encapsulated transition-metal catalysts show unusual reactivity and selectivity behaviour that will be discussed in detail.     

Resolving stoichiometries and oligomeric states of glutamate synthase protein complexes with curve fitting and simulation of electrospray mass spectra

A complicating factor in analyzing electrospray ionization mass spectra of intact macromolecular heterogeneous protein complexes is the potential overlap of ions from different species present in solution. Therefore, it is often not possible to assign all ion signals. With the aim of allowing the more efficient and comprehensive analysis of very complex mass spectra of intact heterogeneous protein complexes we developed a software program: SOMMS. The program uses simple user input parameters together with Gaussian curve fitting to simulate putative mass spectra of protein (sub)complexes within a specified charge state window. In addition, the program can simulate spectra for heterogeneous protein complexes using bi- and multinomial distributions and it can calculate zero-charge spectra and relatively quantify the abundance of each component in a mixture. As a proof of concept we analyzed the complex mass spectra of alpha-glutamate synthase and alphabeta-glutamate synthase from Azosprillum brasilense. Using our program we could determine that alpha-glutamate synthase is in equilibrium between its dimeric, tetrameric, hexameric and dodecameric conformation, whereas alphabeta-glutamate synthase forms up to 15 different heterooligomeric assemblies composed of alpha- and beta-subunits. Thus, SOMMS allows resolving stoichiometries and oligomeric states of protein complexes even from very complicated mass spectra. These complexes could not be assigned by using maximum entropy calculations. We compared our mass spectrometry data on glutamate synthases with available X-ray, small-angle X-ray scattering and size-exclusion chromatography data.     

Side chain mediated electronic contact between a tetrahydro-4H-thiopyran-4-ylidene-appended polythiophene and CdTe quantum dots

The properties of a mixed CdTe quantum dot/tetrahydro-4H-thiopyran-4-ylidene-functionalized polythiophene system are reported. This system was prepared by exposing trioctylphosphine (TOP)-capped CdTe quantum dots to the polythiophene in solution. Strong fluorescence emission quenching and shortening of the fluorescence emission lifetimes of both the polythiophene and the quantum dots occur when they are mixed, indicating the occurrence of photoinduced charge separation. Photoinduced absorption spectroscopy reveals a considerable decrease in the population of the polythiophene triplet excited state in the mixed system. These results demonstrate that between the quantum dots and the polythiophene there is both physical and electronic contact, which is mediated by the tetrahydro-4H-thiopyran-4-ylidene side chains.     

3D Anisotropic Diffusion on GPUs by Closed-Form Local Tensor Computations

We present an efficient implementation of volumetric anisotropic image diffusion filters on modern programmable graphics processing units (GPUs), where the mathematics behind volumetric diffusion is effectively reduced to the diffusion in 2D images. We hereby avoid the computational bottleneck of a time consuming eigenvalue decomposition in $\mathbb{R}^3$. Instead, we use a projection of the Hessian matrix along the surface normal onto the tangent plane of the local isodensity surface and solve for the remaining two tangent space eigenvectors. We derive closed formulas to achieve this and prevent the GPU code from branching. We show that our most complex volumetric anisotropic diffusion filters gain a speed up of more than 600 compared to a CPU solution.     

Metal-directed assembly of chiral bis-Zn(II) Schiff base structures

Tetra-Schiff bases derived from (chiral) bis-salphen ligand scaffolds furnish, upon metalation with appropriate metal reagents, their multinuclear structures with associated Zn(OAc)(2) or Zn(OH)(2) fragments. The tendency of retaining these salts was investigated using four different (chiral) bis-salphen scaffolds. The presence of the additional Zn ions was supported by NMR studies, mass determinations and X-ray crystallography showing in two cases the possible mode of coordination within these multinuclear structures. In one case, dimerization of the Zn(3) complex leads to a unique hexanuclear Zn(6) complex being a mixture of diastereoisomeric complexes as revealed by NMR spectroscopy.