Condensation by olation

FIGURE 8.15 Charge vs. critical electronegativity diagram showing five classes of behavior for the zero-charge M[0 2r-J° species of the metal cation In domains 1 and V, the species remain monomeric and solnble. In domain 11 the species condense by olation only, and in domain IV solely by oxolation. Both latter condensation processes may be simnltaneons in domain 111. (From Johvet, J.-R, Metal Oxide Chemistry and Synthesis, John Wiley Sons, Chichester, U.K., 2000, 35. With permission.)... 

The four trimers coordinated to the central aluminum atom may subsequently undergo intramolecular condensation by olation, with elimination of water... 

Zr(IV) is larger than Ti(IV) and its coordination number is higher. In solution it acquires the square-antiprismatic 8-coordination. Zirconium is consequently less acidic than titanium. (Zr(OH2)s] " (pK —0.3) is nonetheless a strong acid that hydrolyzes spontaneously in water to form [Zr(OH)(OH2)7]. In this complex, the charge on the hydroxo ligand is almost zero (5(OH) = —0.007). The complex is acid and forms the dihydroxo complex [Zr(OH)2(OH2)6], which condenses by olation (6(OH) = -0.007] and forms, in the absence of complexing ions, a cyclical tetramer with double liydroxo bridges ... 

The behavior of titanium may be explained by taking into account that the first stage of condensation (by olation) of the zero-charge precursor [Ti(OH)4(OH2.)2] is the formation of the dimer [Ti2(OH)s(OH2)2]° in which both octahedra share an edge (double hydroxo bridge). The growth of this embryo by olation with monomers... 

Around pH 4-5, the amorphous solid could be the product of flocculation or of rapid condensation by olation of the neutralized tetramer [Zr4(OH)i6(OH2)g]° ... 

In the following section, we describe a few compounds formed with the phosphate and elements of increasing formal charge [Zr(IV), Sb(V), W(VI)], AH these elements can be strongly complexed by the phosphate, but the formation of extended networks requires at least one step of condensation by olation (see Section 2.2). As in the case of oxo-hydroxo forms of cations, oxolation alone between complexes leads to heteropolyanions. This is observed in the case of high formal charge elements such as Sb(V) and W(VI). 

We may then consider a reaction mechanism involving octahedral dimers [Al2(OH)(5(OH2)4] whose further condensation by olation leads simply and directly to the planes characteristic of the structure of the hydroxide (Figure 3.10). At pH < 6, the soluble species are octahedral, so that the dimer [Al2(OH)6(OH2)4] may form directly to allow formation of the hydroxide from the gel along the same reaction path. 

It has been established (see Chapter 3) that, for cationic species, condensation by olation is always limited. The same applies to condensation by oxolation between anionic or neutral forms. 

The formation of V20s may be explained by considering the zero-charge monomers [VO(OH)3(OH2)2] and [VP2(OH)(OH2)3]° in equilibrium with the decavanadate. As soon as they appear in solution, these entities condense by olation since both hydroxo and aquo ligands are present. The formation of chains is the result of the structure of the precursors short V =0 bonds prevent condensation... 

For higher valent oxides such as vanadium and tungsten, the charge on the compact polyanions can prevent condensation. Acidification via ion-exchange chromatography is then required to sufficiently neutralize the species involved to allow the formation of a solid phase. Vanadium precursors possess bound water molecules allowing condensation by both olation and... 

This reaction mechanism is oversimplified because gelation of vanadic acid requires the presence of traces of V(1V), which appear spontaneously during acidification with an ion-exchange resin or with addition of alcohol [58,62]. The role of V(1V) as a catalyst is not quite clear yet [63]. It may act as an initiator of the initial condensation, either by olation of vanadic complexes or in the oxolation of the chains. Indeed, the nucleophilic power of the hydroxo ligands in the. species formed by V(IV), ]VO(OH)2(OH2)3l , is probably greater than that of the hydroxo... 

This does not occur in the case of titanium or zirconium, which maintain their ability to condense, at least by olation, in spite of the complexation. In the case of antimony, the formation of basic salts means that condensation must be forced by heat treatment. This is also the case for polyphosphates, which can only be obtained by heat treatment of the acid forms, since polyphosphates are metastable in aqueous solution [57],... 

With the monodentate fluoride ion, it is difficult to have two H2O ligands in trails allowing condensation of opposed coplanar edges. This mode of condensation is possible only with a bidentate ligand ([HSO4]) which leaves only one water molecule in the coordination sphere. As a result, only anatase can form. In both cases, this mechanism may only take place if the complexes are sufficiently stable. Equilibria between various species are probably involved. Under these conditions, it is difficult to know what is the precursor of the solid. Since the oxide always contains some amount of sulfate difficult to remove, it is reasonable to speculate that the complexes are rather stable and that the formation of the solid takes place by incorporation of the sulfated titanium complexes by olation. This is also probably the case with the fluoride. Therefore, the complexing ions of titanium... 

When Xm such that xn, < Xm < Xol.z condensation takes place by olation only, leading to the stable hydroxide M(OH)j. This is the case for manganese(Il), for which xilin = 1-63 < xol,2 = 1-766. 

Some of these elements (Section 2.2, Figure 2.5), such as Sn(IV) and Sb(V), exist as monomeric hydroxo forms in an alkaline medium [SnfOH) ] ", [Sb(OH)6] [2]. After acidification, the aquo ligand appears in the coordination sphere of the cation and the initial condensation steps of [Sn(OH)4(OH2)l, [Sn(OH)4(OH2)2l° and fSb(OH)5(OH2)]° take place by olation. However, because the coordination sphere contains one or two aquo ligands only, most of the condensation process in fact occurs by oxolation. 

Elements such as Si(IV), Sn(IV), Ge(IV), Sb(V) and TefVI), for example, have sizes comparable with Mo(Vl) and W(V1). However, they cannot form tt bonds with oxygen (no terminal oxygen) since they lack accessible d orbitals. Some polyoxoanions formed under specific acid conditions may protonate upon increased acidification, and may increase their condensation by aggregation or oxolation between particles. This leads to large chains or planar structures, or even networks of tetrahedra (Si) or octahedra (Sn, Sb, Te). There is no short M-0 bond in the coordination polyhedron to hinder protonafion and condensation. This might be an opportune time to point out the case of P(V) the presence of the P=0 double bond decreases the electrophilic character of the cation and prevents its condensation in solution. In addition, if the oxolation reaction is the only one to occur in solution, condensation is always limited and causes the formation of polyacids. The formation of solid phases is usually the result of a double condensation process, oxolation and olation. 

Triazinoindole 146 was obtained (74T3997) in a mixture with quinoline 148 on thermolysis or of 3-(alkylthio)-6,7-dihydro[l,2,4]triazino[l,6-c]quinazolin-5-ium-l-olate 144 acid hydrolysis. The reaction presumably took place via the decomposition of 144 to a ketone and 142, which then cyclized. Compound 144 was prepared by the condensation of 142 with aldehydes, ketones, or their equivalents. Reaction of 142 with 3-amino propanol gave 143, which cyclized to 145 and then to 147 with base [80ACH(104)107] (Scheme 32). 

The condensation reactions occur only through the mediation of sylanol groups generated by hydrolysis. There are two main opportunities. The condensation can be carried out by oxolation or olation mechanisms. The oxolation occurs with the participation of two sylonol groups. In the course of the reaction a hydroxo ligand is exchanged for an oxo one [8] ... 

The triaryl compounds 290 (R = Ar) are prepared by condensation of 2-aminophenol with triarylpyrylium salts followed by treatment with alkali. The triphenyl betaine (290 R = Ph) is obtained as a purple solid, mp 165 C (decomp), which shows large thermo/solvatochromic effects. Oxidation of the betaine 290 (R = Ph) with hydrogen peroxide gives the triphenyl-pyridinium-3-olate 291 (R = Ph) (see Section III,A,2) and the pyrrole 292 (R = Ph). The mechanism of this unusual reaction has not yet been Established. 

Mesoionic systems may be readily substituted by electrophiles. Thus the thiazolo mesoion (342) will couple with diazonium salts despite their relatively weak electrophilicity (80KGS621). Substitution in a fused heteroaromatic betaine azine ring, e.g. (343), also takes place with ease. The resonance form (344) of the mesoion (343) shows that the electrophile will attack at C-6. The substitution in this position is also predicted by MO calculations (73JHC487). Similarly the pyridine ring in pyridinium olates is active towards electrophiles and is substituted in the positions ortho and para to the olate function. Bromination of the 5-methyl derivative (321 R = Me) occurs exclusively in the 7-position which is rationalized via the intermediate (345). In the absence of a 5-substituent, attack in either the 5- or 7-position occurs the dibromide is readily formed. No bromination in the thiazole ring is observed. The 2-bromo derivative (346) has been made, however, by condensation between the appropriate mercaptopyridine and 1,1,2,2-tetrabromoethane. 

Oxo-cyclopentene and 3-oxo-cyclohexene react with diisobutylaluminum benzene-tellurolate to produce diisobutylaluminum 3-phenyltelluro-l-cycloalken-l-olates that condensed with butanal and benzaldehyde. The resulting 3-oxo-2-[(organo)hydroxymeth-yl]-l-phenyltcllurocycloalkanes were converted by 3-chloroperoxybenzoic acid to 3-oxo-2-[(organo)hydroxymethyl]-cycloalkenes2. 

At low temperatures arylcyclopropenones condense with enamines by 1,2-addition to give the 2-azoniabicyclo[3.1.0]hex-3-en-3-olates 344, which isomerize on heating to penta-2,4-dienamides 348. In an overall sense these products arise from insertion of the cyclopropenone three carbon unit into the C-N bond of the enamine and at elevated temperatures compounds 344 are not isolated. In certain instances, -aminoenones 345, co-aminocyclopentenones 347 and cyclopentenones 346 (next page) also ensue . Ketene acetals behave in a similar manner to enamines . 

---