Alkoxides as ancillary ligands

Rothwell and Wolczanski have developed considerable titanium organometallic chemistry utilizing sterically hindered alkoxides as ancillary ligands. 

Siloxides, like alkoxides, have been employed as ancillary ligands of transition metal complexes, markedly influencing the reactivity of a metal center by electronic and steric effects of the substituents at the silicon [15, 16]. 

Non-cyclopentadienyl single-site lanthanide alkoxides mostly feature N-donor-based ancillary ligands. Examples include bulky bis(arylamidinate)-yttrium(III) alkoxides, phenoxides and amides such as complexes (307)-(309), which initiate the ROP of LA.892 However, control over molecular weight is poor and polydispersities are broad (typically >1.5), with the exception of (309) in the presence of exogenous benzyl alcohol. 

The polymerization activities in the presence of Cp Ti compounds containing different ancillary ligands, i.e. alkoxide and chloride ligands, with MAO are as follows in order of decreasing catalytic activity [19] Cp Ti(0,Pr)3 and... 

A perusal of the synthetic Schemes 1-6 indicates the vital role played by the steric bulk of an alkoxide and other ancillary ligands in providing access not only to new compounds but to new types of structurally interesting complexes as well (Table III, Figs. 38-46). 

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

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