Hydrogen-bonded networks through second-sphere coordination

The reaction of 4, 7-phenanthroline (1) with aqueous transitionmetal complexes [Mn(H2O)6][NO3]2, [Co(H2O)6][NO3]2, [Ni(H2O)6[NO3]2, [Mn(H2O)6][ClO4]2, and [Co(H2O)6][ClO4]2 does not produce coordination complexes between these metal cations and the N-donor ligand as expected. Instead, supramolecular hydrogenbonded networks are formed between the nitrogen donor atoms of 4, 7-phenanthroline and the OH groups of coordinated water molecules: M-O-H...N interactions. This motif of second-sphere coordination for 1 can be exploited as a tool for crystal engineering. As a demonstration of the generality of this new interaction as a supramolecular building block, five X-ray crystal structures are reported that utilise this hydrogen bonding scheme; [Co(H2O)4(NO3)2].(1)2 (2a), [Co(MeCN)2(H2O)4][ClO4]2.(1)2 (2b), [Ni(H2O)4(NO3)2].(1)2 (3a), [Mn(H2O)4(NO3)2].(1)2 (4a), and [Mn(H2O)6][ClO4]2.(1)(4).4H2O (4b). Each network involves complete saturation of the hydrogen-bond donor sets between the aqua complex and 1 using primarily M-O-H...N(1) and M-O-H...O(anion), interactions. Thermogravimteric analysis shows these materials to have stabililities similar to coordination polymers involving metal-ligand bonds; this demonstrates that second-sphere hydrogen bonding has potential for the construction of polymeric metal-containing materials.     

Formamidino and carbamoyl-formamidino complexes of manganese and rhenium carbonyls

Lithio-1,3-diarylformamidines undergo reaction with the monomeric and dimeric carbonyl chlorides of manganese and rhenium, [M(CO)₅Cl] and [M(CO)₄Cl₂] to yield respectively the carbamoyl-formamidino [M(CO)₄-CON(R)CHNR] and the formamidino [M(CO)₄RNCHNR] types of complex. The carbamoyl complexes undergo decarbonylation to produce the formamidino complexes in low yield.     

Structure and dynamics of metallomacrocycles: recognition of zinc xylyl-bicyclam by an HIV coreceptor

As platforms for the design of metal-based therapeutic and diagnostic agents, macrocycles are rigid enough to provide strong metal binding sites and orient functional groups stereoselectively, yet flexible enough to accommodate structural changes required for induced-fit recognition of biological targets. We consider the recognition of the Zn(II) complex of the bis-tetraazamacrocycle xylyl-bicyclam, a potent anti-HIV agent, by the coreceptor CXCR4, a G-protein-coupled receptor used by HIV for membrane fusion and cell entry. NMR studies show that the macrocycles of Zn(II)(2)-xylyl-bicyclam perchlorate exist in aqueous solution as two major configurations, trans-I (nitrogen chirality R,S,R,S), and trans-III (S,S,R,R). Acetate addition induced a major structural change. X-ray crystallography shows that the acetate complex contains the unusual cis-V cyclam configuration (R,R,R,R and folded) with bidentate coordination of acetate to Zn(II) plus second-coordination-sphere double H-bond formation between diagonal NH protons on the opposite cyclam face and acetate carboxylate oxygens. Detailed 1D and 2D NMR studies show that the major configuration of Zn(II)(2)-xylyl-bicyclam acetate in aqueous solution is cis-V/trans-I. Molecular modeling shows that an analogous cis-V site can be formed when Zn(II)(2)-xylyl-bicyclam binds to CXCR4, involving the carboxylate groups of Asp262 (Zn(II) coordination) and Glu288 (double H-bonding). The second cyclam can adopt the trans-I (or trans-III) configuration with Zn(II) binding to Asp171. These interactions are consistent with the known structure-activity relationships for bicyclam anti-HIV activity and receptor mutation. Consideration of the anti-HIV activity of xylyl-bicyclam complexes of other metal ions suggests that affinity for carboxylates, configurational flexibility, and kinetic factors may all play roles in receptor recognition. For example, Pd(II) cyclam complexes interact only weakly with axial ligands and are inflexible and inactive, whereas Co(III) cyclams bind carboxylates strongly, are configurationally flexible, and yet have low activity. Our findings should aid the design of new generations of active macrocycles including highly specific chemokine receptor antagonists.     

Treatment of zinc-contaminated water using a multistage ferrihydrite sorption system

Previous studies demonstrated the environmental and economic benefits of treating lead(II)-contaminated water streams with ferrihydrite in multiple equilibrium sorption stages. In this work, multistage ferrihydrite sorption systems were evaluated for their effectiveness in reducing single-solute zinc(II) (Zn(II)) concentrations in contaminated water streams to very low levels. As for lead(II) (Pb(II)), experimental data and modeling results indicate that a multistage sorption system can significantly reduce Zn(II) effluent concentrations for the same total amount of sorbent or, alternatively, dramatically lower total sorbent consumption for the same effluent Zn(II) concentration. Compared to Pb(II), however, Zn(II) removal requires on the order of 10 times more sorbent to achieve the same target effluent concentration for the same pH and number of stages. Model predictions were made using a steady-state, multistage, equilibrium adsorber model that was previously developed for and integrated into OLI Systems' Environmental Simulation Program (ESP). The modified triple-layer model was used to simulate Zn(II) surface-liquid equilibria within the adsorber model. Engineering screening evaluations again indicate that a 2- to 3-stage sorption process can provide significant economic savings when compared to a 1-stage process operating with the same target effluent Zn(II) concentration. Additional equilibrium stages beyond 2 or 3 provide diminishing economic returns. The major economic driver for multiple contacting stages is reduced capital investment and operating costs for sludge handling, dewatering, and disposal.     

Stereoselective total syntheses and reassignment of stereochemistry of the freshwater cyanobacterial hepatotoxins cylindrospermopsin and 7-epicylindrospermopsin

A stereoselective total synthesis of the structure 1 proposed for the freshwater cyanobacterial heptatotoxin cylindrospermopsin has been accomplished in approximately 30 operations starting from commercially available 4-methoxypyridine. Utilizing methodology developed by Comins, the tetrasubstituted piperidine A-ring unit of the hepatotoxin was efficiently constructed. The two remaining stereocenters in the natural product were then set by a stereospecific intramolecular N-sulfinylurea Diels-Alder cyclization/Grignard ring opening/allylic sulfoxide [2,3]-sigmatropic rearrangement sequence previously developed in these laboratories, leading to key intermediate 29. The stereochemical assignment of alcohol 29, which contains all six of the stereogenic centers of the natural product, was confirmed by an X-ray crystal structure determination of a derivative. Installation of the D-ring uracil moiety was effected by using our new methodology developed for this purpose, and construction of the C-ring guanidine completed the total synthesis of racemic structure 1. However, the (1)H NMR data for this compound do not match that of cylindrospermopsin, but instead agree with the data reported for 7-epicylindrospermopsin, a minor toxic metabolite that co-occurs with cylindrospermopsin. Therefore, we propose a revision of the stereochemical assignments of these natural products such that cylindrospermopsin is now represented as structure 2 and 7-epicylindrospermopsin is 1. This reassignment was further confirmed by Mitsunobu inversion of the C-7 alcohol 51 to epimer 52, and conversion of this compound to tetracyclic diol 57, which has previously been transformed to cylindrospermopsin (2).     

Scaling in second-order electron correlation calculations using systematic sequences of even-tempered basis sets

Improved results can often be obtained from second-order Rayleigh-Schrödinger perturbation calculations of electron correlation energies using large basis sets by introducing a scaling factor in the zero-order Hamiltonian. The scaling parameter may be determined from full third-order calculations using a smaller basis set. This scaling procedure can be applied in a systematic fashion by employing a sequence of even-tempered basis sets. Calculations illustrating this approach for the beryllium atom and the neon atom are presented. The scaling procedure is also employed in conjunction with a universal systematic sequence of basis functions. Calculations illustrating this Correlation energy — Mang-body perturbation theory.Work supported in part by S.R.C. Research Grant GR/B/4738.6.S.R.C. Advanced Fellow.     

Fast 3D shape screening of large chemical databases through alignment-recycling

Background  

Large chemical databases require fast, efficient, and simple ways of looking for similar structures. Although such tasks are now fairly well resolved for graph-based similarity queries, they remain an issue for 3D approaches, particularly for those based on 3D shape overlays. Inspired by a recent technique developed to compare molecular shapes, we designed a hybrid methodology, alignment-recycling, that enables efficient retrieval and alignment of structures with similar 3D shapes.     

Direct, stereoselective substitution in [Rh(CO)(2)Cl](2)-catalyzed allylic alkylations of unsymmetrical substrates

[Rh(CO)(2)Cl](2) has been found to possess the unusual property of catalyzing allylic alkylations of unsymmetrical allylic carbonates with high levels of regioselectivity to provide products arising from substitution at the carbon atom bearing the leaving group, irrespective of the structure of the starting carbonate. The substitution reaction occurs with retention of stereochemistry at the reacting center, and the carbon-carbon double-bond stereochemistry of primary (Z)-allylic carbonates is maintained. [reaction: see text]     

Selective syntheses of novel polyether fullerene multiple adducts

We have applied a modified macrocyclic tether approach to control multiple additions to C60. The technique of 3He NMR was used to confirm the selective formation of specific C60 multiple adducts by the macrocyclic tether approach. An oligoglycol was used as a flexible linker to produce macrocyclic polyether-linked malonates 5, 6, 8, and 9 under solid-liquid PTC (phase-transfer-catalysis) conditions. The formation of a single C60 tris-adduct, 3, from macrocyclic malonate 1 and 3He@C60 was proven by 3He NMR. Similarly, multiple additions to C60 of macrocyclic polyether malonate 5 gave C60 bis-adduct 10 selectively, while the reaction of C60 with macrocyclic malonate 8 gave bis-adducts 11 and 12. A similar process with macrocyclic malonate 6 gave tris-adduct 13 with high selectivity as well. Saponification of these C60 multiple adducts gives the corresponding polyacids that are potentially useful in biological applications. Macrocyclic polyether fullerenes are a new class of ionophores, which could be interesting for molecular recognition and for the development of biosensors.