Nitrogen-Coordinating Ligands

Aryl bromides and iodides can be coupled with terminal aryl- or alkyl-substituted alkynes even in the absence of copper additives. The reactions were performed in NMP as solvent TBAOAc as base at 110°C. The robustness of the catalyst was tested in water with pyrroUdine as base and, despite the small amounts of diyne or enyne formed during the reactions of up to 20%, TONs of up to 970 were observed. By using a structurally related catalyst, however, the reactions could be 

On a similar note, palladium complexes bearing bis-pyrazolyl-derived ligands with pendant imidazolyl-substituents have been used in ionic liquids, and could be recycled up to six times at an initial catalyst loading of lmol% [76]. 

In the same study the authors also described the use of aryl bromides as electrophiles, although in this case a higher reaction temperature of 60 C and a longer reaction time were needed. 

Previously, Buchmeiser and coworkers developed catalysts which were based on bis-pyrimidine building blocks [77] and proved to be competent in cross oupling reactions involving aryl iodides, aryl bromides and, with some limitations, also 

The supported version of the catalyst was obtained via ROMP using the molybdenum-based Schrock catalyst. An inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis of the polymer revealed a palladium to bis (pyrimidine) ratio of 2 1. The authors speculated that this could be explained by coordination of palladium by alternate and repetitive bis (pyrimidine) units of the polymer chains. 

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These few reactions are illustrative of the very extensive chemistry of CoIU complexes with nitrogen-coordinating ligands. 

Experimental confirmation of the metal-nitrogen coordination of thiazole complexes was recently given by Pannell et al. (472), who studied the Cr(0), Mo(0), and W(0) pentacarbonyl complexes of thiazole (Th)M(CO)5. The infrared spectra are quite similar to those of the pyridine analogs the H-NMR resonance associated with 2- and 4-protons are sharper and possess fine structure, in contrast to the broad, featureless resonances of free thiazole ligands. This is expected since removal of electron density from nitrogen upon coordination reduces the N quad-rupole coupling constant that is responsible for the line broadening of the a protons. 

Halide complexes are also well known but complexes with nitrogen-containing ligands are rare. An exception is the blue phthalocyanine complex formed by reaction of Be metal with phthalonitrile, 1,2-C6H4(CN)2, and this affords an unusual example of planar 4-coordinate Be (Fig. 5.5). The complex readily picks up two molecules of H2O to form an extremely stable dihydrate, perhaps by dislodging 2 adjacent Be-N bonds and forming 2 Be-O bonds at the preferred tetrahedral angle above and below the plane of the macrocycle. 

The most common catalysts for ATRP are complexes based on a copper(T) halide and nitrogen based ligand(s). Various ligands have been employed and those most frequently encountered are summarized in Table 9.5. Typically, four nitrogens coordinate to copper. The bidentate bipyridyl (bpy) ligands 132-133 are known to form a 2 1 complex. The tetradentate ligands are expected to form a 1 1 complex. 

Organometallic intramolecular-coordination compounds containing a nitrogen donor ligand. 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

Three-coordinate silver(I) complexes with nitrogen donor ligands are of the form [AgXL2] (L = 3-ferrocenylpyridine, X = OTf L = 3- or 4-NCPy, 2-, 3-, or 4-NCC6H4NH2, X = N03). 

Therefore, sulfur coordination of NCS ligand to ruthenium shows a carbon resonance at around 120-125 ppm, compared to the nitrogen coordination of NCS ligand, where the 13C resonance is in the region of 130-145 ppm.46... 

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