Metal-aryloxide bonds

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). ... 

Metal-Oxygen Bond Lengths, M—O—C Angles, and Torsion Angles for Three-Coordinate Aluminum and Gallium Alkoxides and Aryloxides... 

In this section, we will highlight the development in the use of metal alkox-ides for the synthesis of new and interesting organometallic compounds, many of these are either inaccessible or difficult to synthesize by common synthetic procedures. We will not discuss (a) the chemistry of organometallic compounds containing alkoxides as supporting ligands, for which excellent reviews by Chisholm and co-workers (154, 513, 514) are available and (b) intramolecular cyclometalation (i.e., C—H bond activation) reactions of metal aryloxides due to the availability of an excellent account of this topic in a review article by Rothwell (515). Furthermore, a brief mention of the use of a related metal derivative (i.e., metal aryloxide) will be made merely for comparison. 

For this type of transformation, the insolubility of one of the products (i.e., lithium aryloxide) in pentane appears to be the driving force. Another distinct feature of metal aryloxides is their susceptibility to undergo intramolecular C—H bond activation (i.e., cyclometalation) reactions to afford new organometallic systems supported by aryloxide ligands (515). 

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. 

Although the development of metal atom vapour technology over the past three decades has shown tremendous utility for the synthesis of a wide range of organometallic compounds (many of which were inaccessible by conventional techniques), the use of this technique for the synthesis of metal alkoxides and related derivatives does not appear to have been fully exploited. In 1990, Lappert et al. demonstrated the utility of this technique for the synthesis of M—O—C bonded compounds by the isolation of alkaline earth metal aryloxides. 

The existence of aryloxide oxygen to metal r-bonding should be manifested in a variety of ways. The extent of jr-donation would depend on a large number of interrelated factors (formal metal oxidation state, molecular symmetry, coordination number, nature of ancillary ligands, etc.), which ultimately control the electron deficiency of the metal centre and the availability of suitable empty r-acceptor orbitals. How some of these factors influence various parameters is discussed below. 

Table 6.1 Metal-ligand bond lengths for mixed alkyl, aryloxides of niobium and tantalum...

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. 

The addition of 4-methylphenol to the ruthenium species [Ru(>j -Me2P-CH2)(Me) Me3)4] produces the cyclometallated aryloxide [Ru(0- / -C6H4)(PMe3)4]. Reaction with CO and CO2 was found to lead to insertion into the metal-aryl bond to produce five- and six-membered metallacycles respectively. ... 

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