Ligands higher acids

The use of an extended arene (tetrahydroanthracene) in [OsCl(en)(ri6-tha)]+ (29) gave rise to a similar potency (112). This is in contrast with the data for ruthenium-arenes, where the same substitution gave rise to a 10-fold increase in activity. Further work therefore needs to determine if the extended Os-arenes can intercalate into DNA in a manner similar to Ru-arenes. Replacement of the iV /V-chelating ligand en for other AyV-bidentates with pyridine, aliphatic amine, or azopyridine donor atoms leads to loss of activity, probably because of slower hydrolysis and higher acidity of the coordinated water (112). 

Thus, complexes of Pa should be formed in much more dilute HC1 solutions, while much higher acid concentrations are needed to form complexes of Ta. The calculations also predicted the following sequence in the formation of various types of complexes as a function of the acid concentration M(0H)2C14 > MOCl4 > MC16 in full agreement with experimental results for Nb, Ta and Pa. The calculations also reproduced the sequence F > Cl > Br in the formation of ML6 (L = F, Cl, and Br) as a function of the ligand L, as the data of Table 20 show. 

The reactivity of the imidazolium cation mainly stems from the relatively higher acidity (piTa = 21-23) of the H2 hydrogen of the imidazolium nueleus, which has been found to be roughly intermediate between the acidities of acetone (pA a = 19.3) and ethyl aeetate (p a = 25.6). In faet, it is well known that deprotonation at the C2 position of the imidazolium salt generates 7V-heterocyclic carbene ligands.Not surprisingly, the formation of metal-carbene eomplexes has been observed in Pd-eatalyzed Heck-type reactions performed in ILs (Seheme 10). In these cases, the side-reaetion has a benefieial effect since the carbenes are most probably stabilizing the catalytically active species. 

Mikami found that the ene reaction of 1,1-di- and trisubstituted olefins with glyoxylate ester can be catalyzed by the titanium complexes prepared from BINOL and Ti(O Pr)2Cl2, Ti(O Pr)2Br2, or Ti(O Pr)4 [128]. The remarkable level of enan-tioselectivity and rate acceleration observed with these BINOL-Ti catalysts stems from the favorable infiuence of the inherent C2 symmetry and the higher acidity of BINOL ligands compared with aliphatic diols. The reaction is applicable to a variety of 1,1-disubstituted olefins and furnishes the ene products with excellent enantiomeric excesses (Scheme 14.48). However, no reaction occurs when mono-and 1,2-disubstituted olefins were adopted as the reactants. 

The organic epoxide reagent (ethylene oxide, propylene oxide, and epichloro-hydrin) serves as proton scavenger and is stoichiometrically consumed in this sol-gel reaction. Given the higher acidity of water ligands of solvated metal cations (salts) in solution, they tend to spontaneously form hydroxide ligands (see Eq. (17.5)). The proton that is released can protonate the epoxide ... 

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