Water Acidity when coordinated

The TEAF system can be used to reduce ketones, certain alkenes and imines. With regard to the latter substrate, during our studies it was realized that 5 2 TEAF in some solvents was sufficiently acidic to protonate the imine (p K, ca. 6 in water). Iminium salts are much more reactive than imines due to inductive effects (cf. the Stacker reaction), and it was thus considered likely that an iminium salt was being reduced to an ammonium salt [54]. This explains why imines are not reduced in the IPA system which is neutral, and not acidic. When an iminium salt was pre-prepared by mixing equal amounts of an imine and acid, and used in the IPA system, the iminium was reduced, albeit with lower rate and moderate enantioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Nevertheless, as other kinetic studies have indicated a pre-equilibrium with imine, it is possible that the proton formally sits on the catalyst and the iminium is formed during the catalytic cycle. It is, of course, possible that the mechanism of imine transfer hydrogenation is different to that of ketone reduction, and a metal-coordinated imine may be involved [55]. 

Metal hexacyanoruthenates possess a lower symmetry. Several compounds have highly disordered structures, especially when no alkali cations are present for charge compensation. Such a complex defect structure has been found for a completely potassium free Prussian blue precipitated very slowly from a solution in concentrated hydrochloric acid [25, 26]. Here, the structure still remains cubic face-centered however, one-third of the [M1 -1(CN)6] is vacant, randomly distributed and that space is filled with water molecules. The coordination sphere of the remaining ions is maintained... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

The oxides and acids of antimony resemble those of arsenic, except that antimony in antimonic acid has coordination number 6, the formula of antimonic acid being HSb(OH)3. A solution of potassium antimonate, K+[Sb(OH)g], finds use as a test reagent for sodium ion sodium antimonate, NaSb(OH)3, one of the very few sodium salts with slight solubility in water (about 0.03 g per 100 g), is precipitated. The. antimonate ion condenses to larger complexes when heated this condensation may ultimately lead to macromolecular structures, such as... 

Picolinic acid forms with TRUE(VI) normal 1 2 chelates and 1 3 complexes [43,63]. The composition of complex formed depends strongly on the conditions of synthesis. Under precipitation from hot solutions, 1 2 complexes are obtained. At the room temperature, low pH and high concentration of picolinate ion, 1 3 complexes are formed. Complexes with An to Pic ratio 1 3 can accept up to 3 water molecules when kept at air. In the 1 2 complexes, picolinate ion is coordinated by central atom through nitrogen and oxygen atoms with the formation of five-membered chelate ring as concluded from IR... 

Strontium is a naturally occurring element found in rocks, soil, dust, coal, and oil. Naturally occurring strontium is not radioactive and is referred to as stable strontium. Stable strontium in the environment exists in four stable isotopes, " Sr (read as strontium 84), Sr, Sr, and Sr. Twelve other unstable isotopes are known to exist. Its radioactive isotopes are Sr and °Sr. Strontium is chemically similar to calcium. It was discovered in 1790. The isotope Sr is a highly radioactive poison, and was present in fallout from atmospheric nuclear explosions and is created in nuclear reactors. Atmospheric tests of nuclear weapons in the 1950s resulted in deposits and contaminations. °Sr has a half-life of 28 years and is a high-energy beta emitter. Its common cationic salts are water soluble it forms chelates with compounds such as ethylenediaminetetraacetic acid strontium coordination compounds are not common. Powdered metallic strontium may constitute an explosion hazard when exposed to flame. 

Metal hexacyanoruthenates possess a lower symmetry. Several compounds have highly disordered structures, especially when no alkali cations are present for charge compensation. Such a complex defect structure has been found for a completely potassium free Prussian blue precipitated very slowly from a solution in concentrated hydrochloric acid [25, 26]. Here, the structure still remains cubic face-centered however, one-third of the [M - (CN)(, is vacant, randomly distributed and that space is filled with water molecules. The coordination sphere of the remaining m1 -1 ions is maintained unchanged however, the mean coordination sphere of the M ions is decreased (mW(nC)4.5(H2O)i.5). No iron ions occupy interstitial positions, that is, only two types of iron environments exist. Since that special kind of Prussian blue has been the first and hitherto only Prussian blue that could be obtained as sufficiently large crystals to perform a single crystal structure analysis, practically all textbooks, and later publications present that defect structure as the real structure of Prussian blue, completely forgetting that this defect structure is an extreme that forms... 

Zinc aqua (Zn-OH2) species are prevalent in biological systems. When coordinated to a Zn(II) center, a water molecule can have a pATa value that varies from to 11, with [Zn(OH2)6]2+ having a pKa — 9.0.5 The position of a specific Zn-OH2 unit in this range depends on the primary and secondary ligand coordination environment of the zinc center. A more Lewis acid Zn(II) center, and hence a lower Zn-OH2 pK value, is produced when the total number of primary ligands is low (e.g. 4) and these ligands are neutral donors. For example, the Zn-OH2 moiety in [(THB)Zn(OH2)]2+ (Fig. la) exhibits a pATa value of 6.2.16 This indicates that a tetrahedral (NHis)3Zn(II)-OH2 moiety, as is found in active site of carbonic anhy-drase (CA), could have a pATa at or below physiological pH for the zinc-bound aqua... 

Another approach to water-soluble phosphines with the emphasis on metal recycling was reported by van Leeuwen and co-workers [30], They have synthesized a number of diphosphines that, when coordinated to rhodium, form complexes having an amphiphilic character. The ligands synthesized are based on BISBI and Structures 20-22, and hydroformylation (for example) can be conducted in a homogeneous (organic) phase [30 a]. After it has been used in the hydroformylation of olefins the catalyst can be removed by acidic extraction. It was established that these novel diphosphines form active and highly selective catalysts. This amphiphilic approach, i.e., rhodium recycling abased on the extraction and re-extraction principle, will be discussed in more detail in Section 7.5. 

An interesting variant of the disproportionation mechanism is provided by the use of water as a ligand. The protons of water become more acidic upon coordination to a metal center, especially when this is in a high oxidation state, thus getting involved in the overall decomposition stoichiometry with the eventual formation of hydroxo or 0x0 species. We have shown that the oxidation of compound CpMoH(PMc3)3 in wet acetonitrile leads to quite different products relative to the same process, described above, in dry acetonitrile [17]. The products and the proposed mechanism are shown in Scheme 13, all boxed spe-... 

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