Sonja Herres-Pawlis

Lactide polymerisation with air-stable and highly active zinc complexes with guanidine-pyridine hybrid ligands.

The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl(2)] (C1), [Zn(TMGpy)Cl(2)] (C2), [Zn(DMEGqu)Cl(2)] (C3), [Zn(TMGqu)Cl(2)] (C4), [Zn(DMEGpy)(CH(3)COO)(2)] (C5), [Zn(TMGpy)(CH(3)COO)(2)] (C6), [Zn(DMEGqu)(CH(3)COO)(2)] (C7), [Zn(TMGqu)(CH(3)COO)(2)] (C8), [Zn(DMEGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C9) and [Zn(TMGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M(w)) up to 176,000 g mol(-1) could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

Mechanism of the Living Lactide Polymerization Mediated by Robust Zinc Guanidine Complexes

Zinc bis(chelate) guanidine complexes promote living lactide polymerization at elevated temperatures. By means of kinetic and spectroscopic analyses the mechanism has been elucidated for these special initiators that make use of neutral N-donor ligands. The neutral guanidine function initiates the polymerization by a nucleophilic ring-opening attack on the lactide molecule. DFT calculations on the first ring-opening step show that the guanidine is able to act as a nucleophile. Three transition states were located for ligand rearrangement, nucleophilic attack, and ring-opening. The second ring-opening step was modeled as a representation for the chain growth because here, the lactate alcoholate opens the second lactide molecule via two transition states (nucleophilic attack and ring-opening). Additionally, the resulting reaction profile proceeds overall exothermically, which is the driving force for the reaction. The experimental and calculated data are in good agreement and the presented mechanism explains why the polymerization proceeds without co-initiators.

Palladium(II)-catalyzed cycloisomerization of substituted 1,5-hexadienes: a combined experimental and computational study on an open and an interrupted hydropalladation/carbopalladation/β-hydride elimination (HCHe) catalytic cycle.

The Pd(II)-catalyzed cycloisomerization of 3-alkoxycarbonyl-3-hydroxy-substituted 1,5-hexadienes has been studied experimentally and computationally. Experimentally, the reaction is characterized by a rapid room temperature formation of monomeric as well as dimeric cycloisomerization products using the commercially available precatalyst [(CH(3)CN)(4)Pd](BF(4))(2). In situ NMR measurements indicate the initial kinetic advantage of the desired cycloisomerization pathway to methylene cyclopentanes; however, double bond isomerization, elimination, and dimer formation are competitive undesired pathways. Evaluation of the obtained product structures by NMR spectroscopy and X-ray crystallography indicates that the sole determinant for the monomer/dimer ratio is the regioselectivity of the initial hydropalladation in favor of the allylic (monomer formation) or the homoallylic double bond (dimer formation). In order to account for the experimental results, we propose the coexistence of two product-forming catalytic cycles, an open, monomer generating, as well as an interrupted and redirected, dimer generating, hydropalladation/carbopalladation/β-hydride elimination (HCHe) process. Results from computational studies of the proposed competing catalytic cycles are supportive to our mechanistic hypothesis and pinpoint the pivotal importance of Pd(II)-hydroxo-chelate complexes for the reactivity-stability interplay of on- and off-pathway intermediates.

MoSGrid – a molecular simulation grid as a new tool in computational chemistry, biology and material science

The MoSGrid (Molecular Simulation Grid, http://www. mosgrid.de) project aims to provide remote computational chemistry services within the German Grid Initiative (D-Grid). Submission and monitoring of compute jobs, as well as the retrieval of postprocessed results are realized through a web based portal. The use of standardized portlets and a generally modular approach allows for the simultaneous and independent implementation of frontends for different molecular simulation codes. To date, functional prototypes of portlets for applications from the quantum chemical and the molecular dynamics domain are available, being represented by Gaussian and Gromacs, respectively. The implementation of other quantum chemical codes, as requested by the community, and of codes for docking simulations is in preparation. MoSGrid will furthermore foster efficient and collaborative work by providing secure but shareable repositories for validated data, as well as for reusable recipes and workflows [1-3].

Rare examples of base pairing via a protonated pyridine N atom in two salts of N2,N6-bis(1,3-dimethylimidazolin-2-ylidene)pyridine-2,6-diamine.

The title compounds, namely 2,6-bis[(1,3-dimethylimidazolin-2-ylidene)amino]pyridinium perchlorate, C(15)H(24)N(7)(+) x ClO(4)(-), (I), and bis{2,6-bis[(1,3-dimethylimidazolin-2-ylidene)amino]pyridinium} mu-oxido-bis[trichloridoiron(III)], (C(15)H(24)N(7))(2)[Fe(2)Cl(6)O], (II), are structurally unusual examples of the organization of molecular units via base pairing. The cations in salts (I) and (II) are derived from the bisguanidine N(2),N(6)-bis(1,3-dimethylimidazolin-2-ylidene)pyridine-2,6-diamine, which associates in centrosymmetric pairs via two N-H...N hydrogen-bond interactions. N-H...N bridges are formed between the protonated pyridine N atom and one of the nonprotonated guanidine N atoms, with N...H distances of 2.01 (1)-2.10 (1) A. Compound (I) contains two crystallographically independent cations and anions per asymmetric unit. One of the perchlorate anions is disordered, while the [Fe(2)Cl(6)O](2-) anion lies on an inversion centre.

MoSGrid: efficient data management and a standardized data exchange format for molecular simulations in a grid environment

The MoSGrid (Molecular Simulation Grid) project is currently establishing a platform that aims to be used by both experienced and inexperienced researchers to submit molecular simulation calculations, monitor their progress, and retrieve the results. It provides a web-based portal to easily set up, run, and evaluate molecular simulations carried out on D-Grid resources. The range of applications available encompasses quantum chemistry, molecular dynamics, and protein-ligand docking codes. n nIn addition, data repositories were developed, which contain the results of calculations as well as “recipes” or workflows. These can be used, improved, and distributed by the users. A distributed high-throughput file system allows efficient access to large amounts of data in the repositories. For storing both the input and output of the calculations, we have developed MSML (Molecular Simulation Markup Language), a CML derivative (Chemical Markup Language). MSML has been designed to store structural information on small as well as large molecules and results from various molecular simulation tools and docking tools. It ensures interoperability of different tools through a consistent data representation. n nAt http://www.mosgrid.de the new platform is already available to the scientific community in a beta test phase. Currently, portlets for generic workflows, Gaussian, and Gromacs applications are publicly accessible [1,2].

catena-Poly­[[μ-cyano-[1,3-bis­(tetra­methyl­guanidino)­propane]­dicopper(I)]-μ-cyano]

The structure of the title compound, catena-polyxad[[μ-cyano-1:2C:N-[1,3-bisxad(tetraxadmethylxadguanidino)xadpropane-1κ2N,N′]xaddixadcopper(I)]-μ-cyano-2:1′C:N], [Cu2(CN)2(C13H30N4)]n, shows one-dimensional zigzag {Cu(CN)}∞ chains with copper centres trigonally coordinated either by a chelating guanidine and a cyano ligand or by three cyano ligands.

The diprotonated 2,2-(propane-1,3-diyl)bis(1,1,3,3-tetramethylguanidinium) cation: packing and conformational changes.

Subject to packing with different anions, the title cation undergoes various conformational changes with significantly different N-C-C-C torsion angles, as well as different angles between the NCN2 guanidine planes. The 2,2-(propane-1,3-diyl)bis(1,1,3,3-tetramethylguanidinium) salts reported here, viz. the dibromide, C13H32N6(2+).2Br-, the tetraphenylborate chloride, C13H32N62+.C24H20B-.Cl-, the tetrachloromercurate, (C13H32N6)[HgCl4], and the bis(trifluoromethanesulfonate), C13H32N6(2+).2CF3SO3-, are dominated by strong intermolecular N-H...X hydrogen bonds, which form different packing patterns.

CCDC 887675: Experimental Crystal Structure Determination

Related Article: Michael Wagner, Vajk Deaky, Christina Dietz, Jana Martincova, Bernard Mahieu, Roman Jambor, Sonja Herres-Pawlis and Klaus Jurkschat|2013|Chem.-Eur.J.|19|6695|doi:10.1002/chem.201203511

CCDC 819814: Experimental Crystal Structure Determination

Related Article: J.Borner, I.dos S.Vieira, M.D.Jones, A.Doring, D.Kuckling, U.Florke, S.Herres-Pawlis|2011|Eur.J.Inorg.Chem.||4441|doi:10.1002/ejic.201100540