Coordination chemistry of methylmercury(II). Synthesis, hydrogen-1 NMR, and crystallographic studies of cationic complexes of MeHg(II) with ambidentate and polydentate ligands containing pyridyl and N-substituted imidazolyl donors and involving unusual coordination geometries
TL;DR: In this article, a linear relation is obtained between 2J(1H-199Hg) and protonation constants for N, where L = N-alkylimidazoles, and the relation is similar to that obtained previously for pyridines.
Abstract: Complexes [MeHgL]N03 (L = 4,4’,4’’-triethyL2,2':6’,2”-terpyridyl (Et3terpy), terpy, bis(2-pyridy1)methanes ((py)CH2's), (py)2CMe2, (py)2CEt2, tris(2-pyridyl)methane, several N-alkylimidazoles, N-(2-pyridyl)imidazole, and N-methyl-2-(2-pyridy1)imidazoles) are obtained from addition reactions in acetone. Proton magnetic resonance spectroscopy is used to deduce coordination behavior of potential uni- or polydentate ligands in methanol, indicating that Et3terpy, terpy, (py)2CH2's, and N-methyl-2-(2-pyridyl)imidazoles act as bidentates to give three-coordinate mercury, but (py)2CR2 (R = Me, Et) are present as unidentates. A linear relation is obtained between 2J(1H-199Hg) for [MeHgL]+ and protonation constants for L, where L = N-alkylimidazoles, and the relation is similar to that obtained previously for L = pyridines. The ambidentate ligand N-(2-pyridyl)imidazole binds to mercury via the imidazole ring. Crystalline [MeHg((py)2CH2)]N03 has (py)CH2 present as a bidentate ligand with “T-shape” coordination geometry based on a dominant C-Hg-N moiety [Hg-N = 2.16(1) A, C-Hg-N = 172 (1) and a weaker Hg-N’ bond [2.75(2) A]. Crystalline [MeHg(Et3terpy)]N03 has Et3terpy present as a tridentate ligand with MeHg(II) bonded strongly to the central nitrogen [2.26(2) A, C-Hg-N = 171(1) and weakly to the terminal nitrogens [2.51(2), 2.61(2) A]. Syntheses of new ligands containing pyridyl and N-methylimidazolyl donor groups are described.
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TL;DR: Tris(2-benzimidazolylmethyl)amines have been found to be superior accelerating ligands for the copper(I)-catalyzed azide-alkyne cycloaddition reaction and the water-soluble ligand (BimC4A)3 was found to been especially convenient for the rapid and high-yielding synthesis of several functionalized triazoles.
Abstract: Tris(2-benzimidazolylmethyl)amines have been found to be superior accelerating ligands for the copper(I)-catalyzed azide−alkyne cycloaddition reaction. Candidates bearing different benzimidazole N-substituents as well as benzothiazole and pyridyl ligand arms were evaluated by absolute rate measurements under relatively dilute conditions by aliquot quenching kinetics and by relative rate measurements under concentrated conditions by reaction calorimetry. Benzimidazole-based ligands with pendant alkylcarboxylate arms proved to be advantageous in the latter case. The catalyst system was shown to involve more than one active species, providing a complex response to changes in pH and buffer salts and the persistence of high catalytic rate in the presence of high concentrations of coordinating ligands. The water-soluble ligand (BimC4A)3 was found to be especially convenient for the rapid and high-yielding synthesis of several functionalized triazoles with 0.01−0.5 mol % Cu.
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TL;DR: In this paper, a discussion on the coordination chemistry of 2,2': 6', 2'-terpyridine and higher oligopyridines is presented.
Abstract: Publisher Summary This chapter presents a discussion on the coordination chemistry of 2,2’: 6’, 2”-terpyridine and higher oligopyridines. The potentially terdentate ligand 2,2’: 6’, 2”-terpyridine presented, was first isolated by Morgan and Burstall as one of the numerous products from the reaction of pyridine with iron (III) chloride. There has been a considerable increase in the use of these ligands in recent years, prompted in part by the attractive photochemical and photophysical properties exhibited by complexes of the related ligands 2,2’-bipyridine and 1,10-phenanthroline. The chapter discusses stabilization of unusual complexes and states that low oxidation states are characterized by an excess of electron density at the metal atom; stabilization may be achieved by the use of ligands capable of reducing that electron density. One of the simplest ways to reduce the electron density is to design ligands with low-lying vacant orbitals of suitable symmetry for overlap with filled metal orbitals. This results in the transfer of electron density from the metal to the ligand (back-donation). In general, ligand nonbonding or π antibonding orbitals are of the correct symmetry for such overlap. The oligopyridines are ideally suited to such roles; they possess a filled highest occupied molecular orbital (HOMO) and a vacant lowest unoccupied molecular orbital (LUMO) of suitable energies for interaction with metal d orbitals. The chapter also discusses monodentate, bidentate, and bridging; terdentate and higher polydentate; cyclometallated, and others.
309 citations
TL;DR: A broad overview of the physical methods used for structural elucidation of ambidentate ligands can be found in this article, with major emphasis being placed on structures emanating from single crystal X-ray diffraction studies.
142 citations
TL;DR: A review of the recent literature involving tris(2-pyridyl) tripod ligands which use nitrogen, phosphorus, arsenic, or carbon as central bridging atoms can be found in this article.
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09 Mar 2007
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