2 Homometallic Alkoxides

626 Alkoxo and Aryloxo Derivatives of Metals
Hydrido, phenoxides have been isolated by treatment of trans-[Ni(H)Cl(PR3)2](RD
Pri ,Cy,CH2Ph) and trans-[Pt(H)(NO3)(PEt3)2] with NaOPh. 243,244
Bis(aryloxides) of nickel have been obtained by metathetical exchange reactions
as well as addition of phenols to [Ni(PMe3)4]. 496,497 In the case of palladium, treatment
of [Pd(O2CMe)2] with two equivalents of NaOAr in the presence of chelating
diamine ligands yields [Pd(OAr)2(L–L)] (L–L D bipy, TMEDA, etc.). 498 The platinum
compounds [Pt(OAr)2(dppe)] react with CO to form intermediate bis(aryloxycarbonyl)
complexes which finally produce ArOCO2Ar, CO2, and[Pt(CO)2(dppe)]. 238 Mechanistic
studies show that the overall deoxygenation of one of the aryloxide ligands
proceeds via a benzyne intermediate.
Treatment of [Pd2Cl2(L–L)2] (L–LD dppm, dmpm) with Na or KOAr leads to the
corresponding Pd(I)–Pd(I) bis(aryloxides). 499 Reaction with CO led to insertion into
the Pd–Pd bond.
The allyl compounds [( 3-allyl)M( 2-OAr)2M( 3-allyl)] (M D Ni, 500 Pd501 )have
been obtained by treating [( 3-allyl)Ni( 2-Br)2Ni( 3-allyl)] and [( 3-allyl)Pd( 2-
Cl)2Pd( 3-allyl)] with Li/NaOAr and [NBun 4 ][OH]/phenol respectively. The nickel
compounds were found to exist as a mixture of cis/trans isomers. The equilibration
of isomers, either by allyl rotation or opening up of an aryloxide bridge, can occur on
the NMR time scale depending on the nature of the aryloxide ligand. 500 The OC6F5,
OC6H3(CF3)2-3,5 and OC6H2(CF3)3-2,4,6 derivatives initiate the rapid polymerization
of 1,3-cyclohexadiene and 1,3-butadiene to high-molecular-weight 1,4-linked polymers.
The palladium complexes, OAr D OC6H4X-4 (X D H, Me, Cl, Br, NO2) react with
PPh3 to form [( 3-allyl)Pd(OAr)(PPh3)]. 501
6.2.12 Group 11 Metal Aryloxides
An important aspect of copper aryloxide chemistry is the homogeneous copper/amine
catalysed oxidative coupling of phenols. For example the repeated C–O coupling of 2,6dimethylphenol
leads to a poly(phenylene-ether) which is an extensively used plastic
owing to its high chemical and thermal stability. 502,503 Typically the reactions are
carried out using either Cu(II) salts (e.g. chloride or nitrate) or Cu(I) compounds in
the presence of base, amine (e.g. N-methylimidazole) and oxygen as the oxidant. The
mechanism of the reaction is complex, but has received both theoretical 504,505 and
experimental 506 study. Kinetics show the reaction obeys simple saturation kinetics
with respect to phenol, with phenol oxidation possibly being the rate-determining
step. 507 The theoretical and experimental data support a mechanism in which dinuclear
phenolate-bridged copper(II) species act as intermediates affording phenoxonium cations
after a double one-electron transfer. A number of simple coordination compounds of
Cu(II) aryloxides have been isolated (Table 6.42) typically with electron-withdrawing
substituents, e.g. [Cu(OC6H3Cl2-2,6)2(N-methylimidazole)2], 508 or chelating aryloxides,
e.g. [Cu(OC6H3Ph-4-CH2NEt2-2)2] 509 and [Cu( 2-OAr)(OAr)]2 (OAr D OC6H3(2 0 -py-
Bu t -4)-2). 510
Copper(I) aryloxides can be formed by treating CuCl with NaOAr 511 or by adding
phenols to organo-copper(I) compounds such as mesityl-copper or [MeCu(PPh3)]. 512
In the absence of donor ligands the [Cu(OAr)]n derivatives are sparingly soluble
and adopted structures were assumed on the basis of known Cu(I) alkyls and alkoxides.
However, since 1996 the homoleptic compounds [Cu( 2-OC6H3Ph2-2,6)]4 and