Pyridine based complexes

Stage why the acridine-based complex 16 catalyzes acetal fomiation in the absence of base, while no reaction was observed under the same conditions with the pyridine-based complexes [50] vide supra). Perhaps the much longer Ru-N bond and bent middle acridine ring result in hemilability of the acridine moiety, affording a potential vacant site and a localized internal base . Alcohol coordination, followed by its deprotonation by the adjacent acridinyl nitrogen, can lead after p-H elimination to a hemiacetal (via aldehyde), which tmdergoes dehydration. 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

Complexing extraction of pyridine bases from coal coking products with organic solvents 97KGS3. 

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. 

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

Table 5 Stereochemistries of copper(I) complexes of pyridine-based ligands.3...

Thermodynamics of complex formation of silver with several ligands such amines,368 hindered pyridine bases,369 nitrogen donor solvents,370 and azoles371 have been carried out. Other studies include the secondary-ion mass spectra of nonvolatile silver complexes,372 the relationship between Lewis acid-base behavior in the gas phase and the aqueous solution,373 or the rates of hydride abstraction from amines via reactions with ground-state Ag+.374... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Catalyst 58, in which the oxazoline ring has been replaced with an imidazoline, gave ee-values in the low 90% region for substrates 36 and 38-40 [42]. However, for certain substrates (see Section 30.5), replacement of the oxazoline by an imidazoline has resulted in significantly higher enantioselectivity. Recently, a number of pyridine- and quinoline-derived iridium complexes 59-62 have been developed, which gave promising enantioselectivities with substrates 36-39 [43, 44]. However, these catalysts cannot yet compete with the most efficient oxazoline-based complexes and complex 14. 

Y. Liu, J. Guo, J. Feng, H. Zhang, Y. Li, and Y. Wang, High-performance blue electroluminescent devices based on hydroxyphenyl-pyridine beryllium complex, Appl. Phys. Lett., 78 2300-2302 (2001). 

Intramolecular Lewis base complexation of the germanium atom also did not appear to be a valuable alternative to overcome the lability of these elusive intermediates, since the germanones expected from oxidation of the base-stabilized germylenes 140131 (with Me3NO) and 141132 (with 02, DMSO, or pyridine oxide) either rearrange as already mentioned [Section VI,A,4, Eq. (33)]131 or dimerize to digermadioxetane 160132 [Eq. (35)]. 

The next group of [Ru(bpy)2L2] complexes to be considered involves pyridine-based ligands. The structural and spectroscopic properties of cM-[Ru(bpy)2(py)2] have been reported." Hydrolysis of ci5-[Ru(bpy)2(py)Cl] gives cw-[Ru(bpy)2(py)(H20)] with 80% retention of configuration. Retention of configuration also accompanies the oxidation of the latter to cis-[Ru(bpy)2(py)(0)] " ", racemization of which is slow the use of this Ru complex as a chiral oxidant has been examined." The kinetics of the comproportionation reaction between [Ru (bpy)2(py)(0)] " " and [Ru(bpy)2(py)(H20)] + in MeCN have been studied using stopped-fiow methods. The Ru product is unstable, and in MeCN solution undergoes disproportionation... 

Some important reactions of chromium hexacarbonyl involve partial or total replacements of CO ligands by organic moieties. For example, with pyridine (py) and other organic bases, in the presence of UV hght or heat, it forms various pyridine-carbonyl complexes, such as (py)Cr(CO)5, (py)2Cr(CO)4, (py)3Cr(CO)3, etc. With aromatics (ar), it forms complexes of the type, (ar)Cr(CO)3. Reaction with potassium iodide in diglyme produces a potassium diglyme salt of chromium tetracarbonyl iodide anion. The probable structure of this salt is [K(diglyme)3][Cr(CO)4lj. 

The pyridine-containing ruthenium-based complex XXVI developed by Nolan [59] from the indenylidene complex DC, promoted the RCM of various dienes (Equation 8.6). Kinetic studies were carried out and showed that, despite a rapid initiation, the presence of pyridine in the reaction mixture has a negative effect on the stability of the active species and only moderate catalytic conversions were obtained [59] (Scheme 8.21). 

Pyridine-based ligands which have been used for dendrimers are 2,2-bipyridine (bpy) 17,2,3-bis(2-pyridyl)pyrazine (2,3-dpp) 18 and its monomethylated salt 19, and 2,2 6, 2"-terpyridine 20. Their transition metal complexes possessing dendritic structures were first reported in the collaborative work of Denti, Campagne, and Balzani whose divergent synthetic strategy has led to systems containing 22 ruthenium centers. - The core unit is [Ru(2,3-dpp)3] 21 which contains three... 

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