Isolation and characterization of a lithiated pyridine - aggregation in the solid state and in solution

Pyridyllithiums are ubiquitous intermediates used for the electrophilic functionalization of pyridines. In this work, the isolation of a pyridyllithium is reported. Single crystal XRD revealed a dimer in the solid state. In THF solution it was identified as a monomer using DOSY NMR spectroscopy. Calculations showed that the pyridyllithium is a very electron rich pyridine superbase, which may account for its high reactivity.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

Evaluating the role of acidic, basic, and polar amino acids and dipeptides on a molecular electrocatalyst for H2 oxidation

Outer coordination sphere interactions reduce the overpotential for H₂ oxidation catalysts (brown ellipse) compared to those that have –COOH groups but don't have stabilizing interactions (blue ellipse).

Structure of Amido Pyridinium Betaines: Persistent Intermolecular C-H⋅⋅⋅N Hydrogen Bonding in Solution

A hydrogen bond of the type C-H⋅⋅⋅X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C-H⋅⋅⋅O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C-H⋅⋅⋅N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C-H⋅⋅⋅N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C-H⋅⋅⋅N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol^-1 per hydrogen bond, which is similar to chemically and biologically relevant C-H⋅⋅⋅O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts.

Organomonophosphines in PtP2Cl2 derivatives: structural aspects

In this review, 260 monomeric Pt(II) complexes are summarized and analyzed in which the inner coordination sphere is built up by a pair of organomonophosphines and two chlorine atoms (PtP2Cl2). These complexes are crystallized in four crystal systems: tetragonal (x17)<orthorhombic (x31)<triclinic (x90)<monoclinic (x122). The square-planar environments have cis- and trans-configurations. The former by far prevails. The mean Pt-L bond distances (cis- vs. trans-configurations) are 2.347 Å (Cl, trans to P) and 2.230 Å (P, trans to Cl) vs. 2.307 Å (Cl, trans to Cl) and 2.320 Å (P, trans to P). There are two types of isomerisms: cis-, trans- and distortion. In general, the derivatives with a cis-configuration are somewhat more distorted than that with a trans-configuration.

Synthetic turn mimetics and hairpin nucleators: Quo Vadimus?

Structural mimicry of peptides has witnessed perceptible progress in the last three decades. Reverse turn and β-hairpin units are the smallest secondary structural motifs that are some of the most scrutinized functional cores of peptides and proteins. The practice of mimicking, without altering the function of the bioactive core, ranges from conformational locking of the basic skeleton to total replacement of structural architecture using synthetic analogues. Development of heterogeneous backbones--using unnatural residues in place of natural ones--has broadened further opportunities for efficient structural rigidification. This feature article endeavours to trail the path of progress achieved hitherto and envisage the possibilities that lie ahead in the development of synthetic turn mimetics and hairpin nucleators.

Mechanistic insight into the ruthenium-catalysed anti-Markovnikov hydration of alkynes using a self-assembled complex: a crucial role for ligand-assisted proton shuttle processes

A combined computational and experimental study into the mechanism of the anti-Markovnikov hydration of phenylacetylene by a self-assembled ligand complex.

The N-H functional group in organometallic catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst-substrate, ligand-ligand, ligand-catalyst, and catalyst-catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.