Aqua complexes dehydration

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

This procedure has been described for inert aqua complexes, and is applicable to many other solvent species. For aqua complexes, [ML,(0H2)3,](CF3S03) (1 g) is placed in a vacuum oven that has been preheated to 100-180° (depending oq the lability of the complex). The solid is kept under vacuum for from 8 to 24 hr depending on the complex, at which stage the substitution is complete. Once dehydrated, the triflato complexes are relatively stable to atmospheric moisture and generally may be manipulated without precautions to exclude air, unless otherwise stated. 

Reactions Hke (1) comprise the initial step in the catalytic cycles of some cdl NIRs, implying N-coordination of N02 into the labile (or vacant) Fe(II)-site, followed by the very fast proton-assisted dehydration of N02 (generation of bound NO ) and reduction/release of NO with formation of the Fe(III)-aqua complex. The latter is reduced by the vicinal... 

Dehydration reactions of aqua-complexes and formation of solvento-compIexes... 

Dehydration of aqua-complexes is commonly performed with zeolites or molecular sieves. [Co(NH3)5(H20)][C104l3, for example, dehydrates in the presence of zeolites and the vacant coordination site is occupied by a solvent molecule (e.g. RCN). When the same cobalt(III) aqua-complex is dehydrated OS in dimethylacetamide, MeCONMc2, the solvento-complex, [Co(NH3)5(MeCONMe2)][C104]3, is formed. If this product is further reacted... 

Synthesis of solvento-complexes by dehydration of aqua-complexes and hydrated salts... 

Molecular sieves are used to remove water from hydrated salts or aqua-complexes. For example, [Mn(MeCN)6][C104]2 and [Fe(MeCN)6l[C104]2 are prepared from the corresponding aqua-complexes using molecular sieves.51/52 when zeolites are used in such dehydration processes, the reaction may proceed only slowly. Thus, hydrated aluminum perchlorate is transformed into [Al(Me0H)6][C104]3 using a zeolite in methanoP but the reaction takes six days. 

Studies of solid state conversions of chloride and bromide complexes of the types [Ni(dl-Bua)2(H20)2] X2 (Bua = 2,3-butanediamine)47 and [Ni(rf/-Stien)2-(H20)2]X2 (Stien = l,3-diphenyl-l,2-diaminoethane X = Cl, Br, NO2) have revealed similar reactions. Dehydration is the initial step in the process and results in conversion of the octahedral aqua-complexes into the square-planar complexes, [Ni(d/-Bua)2]X2 and [Ni(Stien)2]X2. Further heating results in rearrangement to form new octahedral complexes, Eq. 11.3 ... 

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

Much more recently, Kaizaki and Azuma synthesized AA-[Cr2( X-OH)2(I ,I -chxn= l,2-tra x-cyclohexanediamine)4]4+ and AA-[(/ ,/ -chxn)2Cr(ja-OH)2Cr(A,A-chxn)2]4 + in the solid state by dehydrating the hydroxo(aqua) R,R-chxn complex and... 

Examination of the rate of bicarbonate dehydration reactivity of [([12]aneN3)Zn-(0H2)](C104)2 as a function of zinc complex concentration and pH revealed that this reaction is only slightly catalyzed by the aqua (Zn-OH2) form of the complex. A proposed mechanism for bicarbonate dehydration is shown in Scheme 3. Once again, the kinetically determined pKa value matched well with that determined for [([12]aneN3)Zn(0H2)](C104)2, indicating the validity of the proposed mechanism. 

---