Pyridine monodentate

Bonati has classified the pyrazole complexes into two groups compounds containing neutral pyrazoles (HPz), called 2-monohaptopyrazoles since it is the N-2 pyridinic nitrogen lone pair which confers on them the ligand properties and compounds containing pyrazole anions (Pz) which can act as monodentate or, more often, as exobidentate ligands (72CRV497). 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Tris(ferf-butoxy)siloxide molecular precursors of V(IV) and V(V) can be prepared via simple silanolysis reactions. For example, OV[OSi(O Bu)3]3 was obtained in 85% yield by reaction of OVCI3 with excess HOSi(O Bu)3 in the presence of pyridine [79]. Although crystals of sufficient quality for an X-ray structural analysis of 0V[0Si(0 Bu)3]3 were not obtained, its identity was confirmed by various spectroscopic and analytical techniques. Additionally, ( BuO)3VOSi(O Bu)3 and ( BuO)2V[OSi(O Bu)3]2 were obtained via reaction of V(0 Bu)4 with 1 and 2 equiv of HOSi(O Bu)3, respectively, in toluene at 80 °C [80] (Eq. 5). Both (fBu0)3V0Si(0 Bu)3 and CBu0)2V[0Si(0 Bu)3]2 are monomeric in the solid state, and possess only monodentate siloxide ligands... 

Ligand self-assembly through coordinative bonding has been used to increase the bulkiness of a monodentate tris-3-pyridyl phosphine ligand employing the zinc porphyrin/pyridine interaction (Scheme 33) [95-97]. The corresponding rhodium catalyst allowed for regioselective hydroformylation of2-octene [95]. 

Mixed O, N donor molecules are truly extensive and structurally diverse, and only a few selected examples will be presented. In line with other 2-substituted pyridine analogs reported in this chapter, it is worthwhile noting the chemistry of 2-pyridone (or 2-hydroxypyridine, Hopy), which can form O-bonded monodentate complexes such as Co(Hopy)4(N03)2, but as the monoanion is an effective chelate ligand, forming Co(opy)2 and Co(bpy)(opy)2 compounds.454 An unusual solid state melt reaction with Co(OAc)2 yields the dodecanuclear cluster Co12 (OH)6(OAc)6(opy )12.455... 

Analogous to the dithiophosphate complexes, cis octahedral complexes are formed with bident-ate ligands like phenanthroline, 2,2 -bipyridine, or tmeda and trans octahedral complexes are formed with monodentate ligands like pyridine and its derivatives. Some examples are mentioned with linear bidentate ligands like 4,4 -bipyridine, which build up ID chains by bridging the [NiL2] molecules.926-933... 

This skeletal isomerization is strongly retarded by the presence of free pyridine and is not observed when the monodentate pyridine ligands are replaced by bidentate ligands such as 2,2 -bipyridyl 07). This behavior is explained by the need to dissociate a pyridine ligand before the skeletal isomerization can occur, and can be useful in mechanistic investigations (vide infra). 

The search for new coordination SCO polymers based on the assembling of iron(II) and bridging molecules other than 1,2,4-triazole- or l-i -tetrazole-based ligands has afforded a series of frameworks closely related to the 2D system [Fe(btr)2(NCX)2]-H20 (X=S, Se). This series of compounds formulated as [FeL2(NCS)2]-nSolv can be considered as derived from the formal substitution of btr by bis-monodentate pyridine-like ligands such as bispyridylethylene (bpe, n=l, Solv=MeOH), fraus-4,4 -azopyridine (azpy,... 

This alkylation reaction can be applied to intramolecular alkylation affording cyclic products, as shown in Equations (19)-(21). The reaction of 2-vinylpyridines with 1,5- or 1,6-dienes results in the formation of five- or six-membered carbocycles with good efficiency.20,20a,20b In addition to pyridine functionality, oxozole and imidazole rings can be applied to this intramolecular cyclization. When the reaction is conducted in the presence of a monodentate chiral ferrocenylphosphine and [RhCl(coe)2]2, enantiomerically enriched carbocycles are obtained. A similar type of intramolecular cyclization is applied to TV-heterocycles. The microwave irradiation strongly... 

With olefins other than ethene two isomeric chlorohydrins can be obtained, one of them being chiral. When pyridine was replaced by monodentate chiral amines in [PdCl3(pyridine)], the enantioselectivities were low (8-12%) (Scheme 8.3) [14]. The mononuclear [PdCl2(L2)] complexes (L2 = sulfonated p-tolyl-BINAP) performed better providing the chird chlorohydrin in 46-76% e.e. Even better activities and... 

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