Alkoxo groups

We define "oxo-lron aggregates" as a collection of three or more iron ions linked continuously by bridging oxo-, hydroxo- or alkoxo groups. The aggregates are labelled according to their nuclearlty. Undoubtedly the molecular structures of the aggregates represent the key to understanding their properties and therefore they will be discussed first. 

The electro-attractive alkoxo groups render metal alkoxides very sensitive to nucleophilic attack.6 They are extremely susceptible to hydrolysis, which under controlled conditions yields oxo alkoxides they require strictly anhydrous conditions of handling. 

A more versatile behavior was observed with dianionic H(T 6H ligands (Table 9) diols act as bidentates,186,187 while cv-hydroxycarboxylic acids behave as monodentate monoanionic ligands, e.g. in cinnamates,188 or as bidentate dianionic ligands189,190 in alkoxycarboxylates. In the case of catechol, ionic species were shown to exist in solution for both metals, but were isolated for tantalum only.191 The bidentate monoanionic diphenylphosphinate ligand can replace from one to three alkoxo groups in Ta(OMe)5 to yield low molecular weight polymers (Table 9).192... 

In addition to the linear oligomers, cyclic derivatives (V33, V44, V/ ) are also known (see Section 2.2). The latter two are readily formed and are ubiquitous components of aqueous solutions. These compounds are relatively inert to complex-ation reactions that preserve their cyclic structure rather, the equilibria shift towards products of other vanadium stoichiometry. Presumably, one way to generate complexes of these oligomers would be to protonate one or more oxygens so that the reactivity is increased. However, neither protonated V4 nor V5 has been identified in solution, nor has there yet been any indication from studies in alcoholic solution that an alkoxo group can replace an oxo ligand, as might be expected if protonation occurs. 

Guram and Buchwald showed that the chemistry could be extended beyond just electron-neutral aryl halides [78]. Using in situ-derived tin amides, this chemistry encompassed aryl halides bearing esters, amino, and alkoxo groups (Eq. (4)). However, reactions that gave 80% yield or greater were still limited to tin amides derived from secondary amines. 

The proposed catalytic cycle is shown in Fig. 8.5. A variety of molybdenum compounds may be used as the precatalyst, and Mo(CO)6 is shown as a representative one. Under the strong oxidizing conditions the precatalyst is oxidized to 8.21, a species that has molybdenum in a 6+ oxidation state and a di-MoOf unit. The other ligands are two solvent molecules and hydroxo and/ or alkoxo groups. In the absence of a solvent, positions occupied by S are occupied by /-butanol, the decomposition product of /-butyl hydroperoxide. The important points to note are that molybdenum is in its highest oxidation state (6+), and there are weakly bound solvent molecules. 

Alkoxo groups not engaged in bridging may, of course act as donors to other metal species. Thus U(OPr% gives adducts with Li, Mg, and A1 alkyls such as... 

In addition to the normal covalent bond, the oxygen atom of the alkoxo group can form single or double pit-pit or pit-dit donor bonds with the metal atoms ... 

Furthermore, the alkoxo group can bind the metals (similar or different) in p2 °r Mi fashion ... 

OR) bridges (VIII and IX) rather than doubly bridged structures (see below). It is also apparent in the structures of MX+ complexes (X), where M is a metal or metalloid and X = Cl or an alkyl/alkoxo group (236). Such compact structures are favored by the ability of the metal atoms to utilize more atomic orbitals for bonding purposes (237). 

Hexamers (239, 240) and tetramers (34, 238) are common in alkali metal alkoxide chemistry, although with more sterically demanding alkoxo groups, the oligomeric nature becomes lower in the solid state (cf. Section IV.B). Interestingly, [NaO-/-Bu] , crystallizes as two independent molecules in the asymmetric unit as a hexamer (n = 6) and a nonamer (n = 9). 

The alkynyltitanium complexes TiCI((C=CR) (R typically = alkyl groups) are prepared in situ by reacting 1-alkynes with TiCU in the presence of Et3N. The synthetic potential of these compounds has been illustrated in reactions with a number of electrophiles. In the presence of terminal alkynes, symmetrical 1,3-diynes are formed. The reaction with chlorodiphenylphosphine produces the corresponding alkynyl phosphine, while treatment with triethylphosphite affords tris(phenylethynyl) phosphine by substitution of the three alkoxo groups. The reaction with trimethyl orthoformate gives acetals.6-9... 

The amido ligand is isoelectronic with the alkyl and alkoxo groups and has the possibility to exhibit N-M ir-interactions in order to stabilize the systems they form. Different synthetic reactions are used to prepare mono-Cp titanium derivatives containing amido ligands. These compounds are normally synthesized by (i) the action of the corresponding amide salt on Cp TiCl3, (ii) displacement of amine from a homoleptic amido titanium compound by a Cp reagent, (iii) dehalosilylation reactions, and (iv) elimination of alkane by reaction of an alkyltitanium compound with amine. 

Several series of mono-aryloxo and bis-aryloxo titanium derivatives have been prepared by the routes outlined in Scheme 349. Activated by MAO the compounds are used as efficient catalysts for the syndiospecific polymerization of styrene and the co-polymerization of ethylene and styrene. The molecular structures of some of these compounds have been determined by X-ray diffraction. The Ti-O-C bond angle in the structure of Cp TiCl2(0-2,6-Pr12C6H3) differs significantly from those that are observed for the other structures. The effect of the substituents, both on the Cp ring and the alkoxo group, plays an essential role for the catalytic activity and the properties of the polymer obtained.578,779,837-847... 

They exchange the alkoxo group with the alcohol in the mixed solvent ... 

Electronegativity of the metal and polarity of the M-O-C bond Nature of alkoxo group R Modifies the molecular complexity Rate increases with chain lengthening Sensitive to hydrolysis Tertiary R > secondary R > primary R OR > OSiRs... 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

Reactions between MClj and hydrogen phosphonates (HL = (RO)2P(0)H R = Et, Pr", Bu, Ph) afforded [MCls L ] (jc = 1-3), wWch were reported to be polymeric through —O— P—O bridges. NbXs when allowed to react with Na[S2P(OR)2] (R = Me, Et, cyclohexyl) gave [Nb(OMe)2Cl S2P(OR)2 2] recrystallization from the appropropriate alcohol afforded complexes having other alkoxo groups. The reaction of dithiophosphates with TaXs did not afford the analogous tantalum derivatives. 

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