Transition metal aryloxides

The method can also be applied to the synthesis of transition metal aryloxides, with mixed hydrido, aryloxides sometimes being observed and isolated (Eqs 6.51 and... 

Transition metal aryloxides undergo a number of insertion reactions with small molecules. Some of this reactivity is of fundamental importance, and in a number of cases important mechanistic studies have been carried out. 

Tripodal Carbene and Aryloxide Ligands for Small-Molecule Activation at Electron-Rich Uranium and Transition Metal Centers Karsten Meyer and Suzanne C. Bart... 

TRIPODAL CARBENE AND ARYLOXIDE LIGANDS FOR SMALL-MOLECULE ACTIVATION AT ELECTRON-RICH URANIUM AND TRANSITION METAL CENTERS... 

In the concept of hemilabile functionalised NHC ligands, typical anchor groups for hemilability at the carbene end are amides, alkoxides (aryloxides) and the Cp ligand family. In particular, the Cp functional group is an excellent anchor for early transition metals, whereas the amide and alkoxide functionalised carbenes are often used for lanthanide metals. 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

A number of models for the cyclooligomerization of alkynes by early transition metal catalysts have been prepared by reducing sterically hindered aryloxide... 

Many transition metal complexes contain anionic oxygen donor ligands, and many of these complexes display structures and reactivity that resemble that of more conventional organometallic compounds containing metal-carbon bonds. In some cases the alkoxo ligand is the site of reaction, and in other cases the alkoxide is an ancillary ligand. This section will focus on three main classes of compounds alkoxides (including aryloxides and the parent hydroxides), carboxylates, and p-diketonates. 

A common structural feature of transition metal hydroxide, alkoxide, and aryloxide ligands is their tendency to bridge two or more metals. The extent of such aggregation is reduced when the complexes contain more sterically hindered alkoxides, such as OBu, OCBu y bulky trialkylsiloxides, and 2,6-di-lerl-butylphenoxide. ... 

More recently, the first direct observation of C-O oxidative addition of an aromatic elher to a transition metal complex has been reported by Kakiuchi (Equation 4.76). This reaction occurs by initial chelate-assisted C-H bond cleavage, followed by isomerization to form the final Ru(II) aryloxide complex. 

Another method for the replacement of chloride by aryloxide at transition metal centres is utilizing aryl-ethers. In this case the formation of the metal aryloxide is accompanied by the elimination of either alkyl or trialkylsilyl chloride (Eq. 6.35 ). 

When one begins to consider structural parameters (and indeed chemistry) of later d-block metal aryloxides one is confronted with a different situation. This is highlighted by the values of Ao,c calculated for later transition metal alkyl, aryloxide compounds (Table 6.4). It can be seen that values range from close to zero to positive numbers, Le. in some compounds the M-OAr bond lengths exceed M-C(alkyl) bond lengths This is a dramatically different picture from that seen for early transition metal compounds. 

Table 6.4 Metal-ligand bond lengths for mixed alkyl, aryloxides of later transition metals...

A variety of low valent aryloxide derivatives of the early transition metals undergo intramolecular CH bond activation. Attempts to isolate the d -species [M(OAr)3] or [M(OAr)2Cl] (OAr = 2,6-di-rcrt-butylphenoxide or 2,6-di-phenylphenoxide M = Nb, Ta) by reduction of the corresponding d -chloride leads instead to bis-cyclometallated compounds (Eq. 6.60). ... 

The insertion of carbon monoxide into metal aryloxide bonds appears to be restricted to later transition metal complexes. The initial products of these reactions are aryloxy-carbonyls, which may be stable or undergo further reaction. Three examples of this type of reaction are shown in Eqs (6.66)-(6.68). ... 

The addition of CS2 to the thallium aryloxides [Tl(OAr)] (Ar = 4-methyl, 4-bulyl, 4-rerr-butyl, 3,5-dimethylphenyl) has been shown to be a good synthetic route to the corresponding [Tl(S2COAr)] salts, which can be used to generate transition metal derivatives. ... 

The chemistry of rhenium aryloxides is significant. The homoleptic rhenium aryloxides [Re(OAr)4] (OAr = OC6H3Pr -2,6, OC6H3Me2-2,6) have been obtained from [ReCl4(THF)2] and Ihe corresponding lithium aryloxide." " Cyclic voltammetric studies show these square planar d -species can undergo one-electron reduction. As with other transition metal systems (Section 4.2.3) these aryloxides are more difficult to reduce (more negative potential) than their halide counterparts. The 2,6-dimethylphenoxide compound reacts with alkynes to form adducts [(RCCR)Re(OAr)4]. 

Arene hydrogenation catalysts based on other metals than late transition ones are less numerous. Of particular relevance are the results reported by Rothwell, who found that Nbv and Tav hydride complexes containing bulky aryloxide ligands (Fig. 16.2) are active for the homogeneous hydrogenation of arenes [30]. 

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