Stable Heterometallic Cluster Ions based on Werner's Hexol.

Large aqueous ions are interesting because they are useful in materials science (for example to generate thin films) but also because they serve as molecular models for the oxide-aqueous mineral interface where spectroscopy is difficult. Here we show that new clusters of the type M[(µ-OH)2 Co(NH3 )4 ]3 (NO3 )6 (M=Al, Ga) can be synthesized using Werner's century-old cluster as a substitutable framework. We substituted Group 13 metals into the hexol Co[(µ-OH)2 Co(NH3 )4 ]36+ ion to make diamagnetic heterometallic ions. The solid-state structure of the hexol-type derivatives were determined by single-crystal XRD and NMR spectroscopy and confirmed that the solid-state structure persists in solution after dissolution into either D2 O or [D6 ]DMSO. Other compositions besides these diamagnetic ions can undoubtedly be made using a similar approach, which considerably expands the number of stable aqueous heteronuclear ions.

A facile route to old and new cyclophanes via self-assembly and capture.

Cyclophanes are a venerable class of macrocyclic and/or cage compounds that often feature high strain, unusual conformations and quite surprising properties, many of which are legendary in physical organic chemistry. However, the discovery of new, diverse cyclophanes and derivatives has been hindered by syntheses that are traditionally low-yielding, requiring long reaction times, laborious purification steps and often extreme conditions. Herein, we demonstrate a new self-assembly route to a variety of discrete cyclic and caged disulfide structures, which can then be kinetically captured upon sulfur extrusion at room temperature to give a diversity of new thioether (hetera)cyclophanes in high yield. In addition to the synthesis of novel macrocycles (dimers through hexamers), this process provides an improved route to a known macrobicyclic trithiacyclophane. This technique also enables the facile isolation of a tetrahedral macrotricyclic tetrathiacyclophane in two steps at an ambient temperature.

Solution structural characterization of an array of nanoscale aqueous inorganic Ga13-x In x (0 ≤ x ≤ 6) clusters by 1H-NMR and QM computations.

NMR spectroscopy is the go-to technique for determining the solution structures of organic, organometallic, and even macromolecular species. However, structure determination of nanoscale aqueous inorganic clusters by NMR spectroscopy remains an unexplored territory. The few hydroxo-bridged inorganic species well characterized by 1H Nuclear Magnetic Resonance spectroscopy (1H-NMR) do not provide enough information for signal assignment and prediction of new samples. 1H-NMR and quantum mechanical (QM) computations were used to characterize the NMR spectra of the entire array of inorganic flat-Ga13-x In x (0 ≤ x ≤ 6) nanoscale clusters in solution. A brief review of the known signals for µ2-OH and µ3-OH bridges gives expected ranges for certain types of protons, but does not give enough information for exact peak assignment. Integration values and NOESY data were used to assign the peaks of several cluster species with simple 1H-NMR spectra. Computations agree with these hydroxide signal assignments and allow for assignment of the complex spectra arising from the remaining cluster species. This work shows that 1H-NMR spectroscopy provides a variety of information about the solution behavior of inorganic species previously thought to be inaccessible by NMR due to fast ligand and/or proton exchange in wet solvents.

Exploring Anion-Induced Conformational Flexibility and Molecular Switching in a Series of Heteroaryl-Urea Receptors.

Anion binding studies of 1,10-phenanthroline- and 2-pyridyl-substituted urea-based receptors reveal that guest-dependent conformations exist in structural variants related to a previously investigated bipyridyl-based receptor. Dynamic conformational switching persists in a monofunctional pyridyl-urea receptor, and the preorganization provided by a phenanthroline-based analogue promotes convergence of anion coordinating groups to a single guest. Despite this predisposition for anion coordination, the conformational flexibility of the bipyridyl-based receptor provides the most selective motif for H2PO4- coordination. Furthermore, the two new phenanthrolyl- and pyridyl-receptors serve as models of the bipyridyl-based receptor, elucidating accurate stepwise association constants for 1:2 host/guest binding by this receptor, and suggest that oxoanions prefer the embrace of a "U" conformation in 1:1 complexes.

Chloride-catalyzed, multicomponent self-assembly of arsenic thiolates.

We present the observation that chloride serves as a simple catalyst for the acceleration of a self-assembly reaction between AsCl3 and dithiolate ligands (H2L) to form As2L3 assemblies. Studies on a model monomeric arsenic complex suggest that chloride may accelerate ligand exchange dynamics in pnictogen thiolates in general.

Transmetalation of self-assembled, supramolecular complexes.

Substituting one metal for another in inorganic and organometallic systems is a proven strategy for synthesizing complex molecules, and in some cases, provides the only route to a particular system. The multivalent nature of the coordination in metal-ligand assemblies lends itself more readily to some types of transmetalation. For instance, a binding site can open up for exchange without greatly effecting the many other interactions holding the structure together. In addition to exchanging the metal and altering the local binding environment, transmetalation in supramolecular systems can also lead to substantial changes in the nature of the secondary and tertiary structure of a larger assembly. In this tutorial review we will cover discrete supramolecular assemblies in which metals are exchanged. First we will address fully formed structures where direct substitution replaces one type of metal for another without changing the overall supramolecular assembly. We will then address systems where the disruptive exchange of one metal for another leads to a larger change in the supramolecular assembly. When possible we have tried to highlight systems that use supramolecular self-assembly in tandem with transmetalation to synthesize new structures not accessible through a more direct approach. At the end of this review, we highlight the use of transmetalation in self-assembled aqueous inorganic clusters and discuss the consequences for material science applications.

Pnictogen-directed synthesis of discrete disulfide macrocycles.

Cyclic disulfide macrocycles were rapidly synthesized cleanly and selectively from rigid dithiols via oxidation with iodine when activated by pnictogen additives (As and Sb). Macrocycles were confirmed by (1)H-NMR spectroscopy and X-ray crystallography. A p-xylyl-based disulfide trimer and tetramer crystallized in hollow, stacked columns stabilized by intermolecular, sulfur···sulfur close contacts.

Identifying nanoscale M13 clusters in the solid state and aqueous solution: vibrational spectroscopy and theoretical studies.

Raman spectroscopy, infrared spectroscopy, and quantum mechanical computations were used to characterize and assign observed spectral features, highlight structural characteristics, and investigate the bonding environments of [M13(µ3-OH)6(µ2-OH)18(H2O)24](NO3)15 (M = Al or Ga) nanoscale clusters in the solid phase and aqueous solution. Solid-phase Raman spectroscopy was used to reveal that the metal-oxygen (M-O) symmetric stretch (breathing mode) for the Al13 cluster is observed at 478 cm(-1), whereas this same mode is seen at 464 cm(-1) in the Ga13 cluster. The hydroxide bridges in each cluster are weakly Raman active but show slightly stronger infrared activity. The breathing modes associated with the clusters in the solid state are not clearly visible in aqueous solution. This change in behavior in the solution phase may indicate a symmetry breaking of the cluster or exchange events between protons on the ligands and the protic solvent. Overall, each cluster has several unique vibrational modes in the low wavenumber region (<1500 cm(-1)) that are distinct from the parent nitrate salt and other polymeric species with similar structure, which allows for unambiguous identification of the cluster in solution and solid phases.

Elucidating inorganic nanoscale species in solution: complementary and corroborative approaches.

The challenge of defining a length on the nanoscale is non-trivial. For a well-defined inorganic nanoscale species, a size measurement can describe a number of different dimensions (core, shell, solvation sphere). Often size is reported out of context or even inadvertently misrepresented. Since many of the techniques used to measure size depend on significant and sometimes destructive sample preparation, an additional challenge is defining "what size means" for a nanoscale species in solution. In this Concept, the distinction is made between complementary techniques that can be used together to unveil more information about the material in question, and corroborative techniques, which are used to make multiple measurements of the same property. Additionally, corroborative techniques can be used to measure the same property in and out-of solution so as to reveal details about solution behaviour. We highlight various approaches to this characterization challenge in the context of three case studies that demonstrate the use of both complementary and corroborative techniques to elucidate the various functional dimensions of different types of inorganic nanoscale species in solution.

Single nanoscale cluster species revealed by 1H†NMR diffusion-ordered spectroscopy and small-angle X-ray scattering.

A solved structure: The hydrated Ga(13) cluster, [Ga(13)(µ(3)-OH)(6)(µ-OH)(18)(H(2)O)(24)](NO(3))(15)], persists as a discrete nanoscale structure in an aqueous polar solvent at millimolar concentration. SAXS data confirm the presence of Ga(13) in dimethyl sulfoxide (DMSO). In aqueous [D(6)]DMSO (1)H NMR signals for the hydroxo and aquo ligands of Ga(13) were detected, thus showing a cluster with a hydrodynamic radius of (11.2±0.8) Š(see picture).

Reactions of strained hydrocarbons with alkene and alkyne metathesis catalysts.

Here we describe the metathesis reactions of a strained eight-membered ring that contains both alkene and alkyne functionality. We find that the alkyne metathesis catalyst produces polymer through a ring-opening alkyne metathesis reaction that is driven by the strain release from the monomer. The strained monomer provides unusual reactivity with ruthenium-based alkene metathesis catalysts. We isolate a discrete trimeric species a Dewar benzene derivative that is locked in this form through an unsaturated cyclophane strap.

Miss rate of pancreas divisum by magnetic resonance cholangiopancreatography in clinical practice.

OBJECTIVES: Pancreas divisum is the most common congenital abnormality of the pancreatic anatomy. Magnetic resonance cholangiopancreatography has emerged as a noninvasive method of examining the pancreatic ductal anatomy. We aim to assess the sensitivity of MRCP for pancreas divisum. METHODS: Patients with pancreas divisum at endoscopic retrograde cholangiopancreatography (ERCP) and who had prior MRCP between January 2001 and February 2006 were identified. Sensitivities were calculated for relevant subgroups with binomial 95% confidence intervals. Fisher exact P values were calculated. RESULTS: Four hundred five patients had divisum at ERCP; of these, 111 (27%) had undergone MRCP before ERCP. Seventy-two (65%) patients had MRCP at outside centers. Twenty-three of the 72 MRCPs at referring centers correctly reported divisum. In contrast, the sensitivity was higher for the 24 MRCPs without secretin at our institution: 16 (67%). The sensitivity in the secretin-stimulated group was 67% (10/15). However, this was not different from that of MRCP without secretin. Of note, 9 (18%) of the 49 negative outside MRCPs had divisum suspected by the gastroenterologist reviewing the MRCP images before ERCP. CONCLUSIONS: Pancreas divisum seems to be often missed on MRCP, even when secretin is used. The absence of secretin, use of suboptimal magnetic resonance techniques, and inexperienced pancreatic MRCP examiners are all possible contributing factors.

Does the presence of esophagitis prior to PEG placement increase the risk for aspiration pneumonia?

The aim of this study is to determine if the endoscopic presence of esophagitis predicts aspiration pneumonia after the initiation of enteral feedings in a newly placed PEG tube. A retrospective analysis of 278 patients who received a PEG tube from November 1999 to June 2002 was performed. All PEG procedures performed by a single endoscopist were reviewed from the GI Trac database at the Medical University of South Carolina. Eleven of the procedures were aborted due to technical difficulties. Nine patients received the PEG for gastric decompression only. Seven patients died within 14 days of PEG placement from non-PEG-related complications and were excluded. The resulting 251 patients included for our analysis successfully had PEG tube placement and had at least 14 days of enteral feeding. Esophagitis was defined macroscopically by the endoscopic presence of mucosal edema, friability, or obscurity of the normal vascular pattern in the distal esophagus. Aspiration was defined as the witnessed regurgitation of or tracheal suctioning of PEG feedings. Pneumonia as a consequence of aspiration was defined by development of fever and new infiltrate on chest radiograph within 14 days of PEG placement. Two hundred fifty-one patients had PEG placement (M, 127; F, 124; average age, 62.4 year; age range, 18-95 years) performed by a single endoscopist over a 32-month period. Fourteen (5.6%) of these patients had clinically evident pulmonary aspiration, with seven of them developing pneumonia. Thirteen (93%) of these patients had normal esophageal mucosa. One of the 24 patients (4%) with esophagitis or esophageal ulceration present endoscopically had an aspiration event with subsequent pneumonia. None of the 20 patients found to have some other form of esophageal pathology had an aspiration event. The overall incidence of aspiration pneumonia after the initiation of PEG feedings was 2.7% (7/251). The odds ratio that the presence of esophagitis would predict the development of aspiration pneumonia was 1.60, with a 95% confidence interval of 0.18 to 13.89. This study argues that the presence of esophagitis alone does not increase the risk of aspiration pneumonia from PEG feedings. Other factors apart from esophagitis play an important role in the incidence of aspiration pneumonia with PEG feeding