2 Homometallic Alkoxides

Metal Aryloxides 631
The two-coordinate, linear anion [Cu(OC6H3Me2-2,6)2] contains very short Cu–OAr
distances of 1.806 (6) ˚A and 1.798 (8) ˚A. This terminal aryloxide distance elongates
dramatically to 2.023 (5) ˚A infive-coordinate [Cu(OAr)(dppe)]2( -dppe). 511
Besides their insertion chemistry (Section 5.2) Cu(I) aryloxides have been shown to
form tetra(aryl) ortho-carbonates with CCl4, 520 and (ArO)2CS from CS2. 521
The chemistry of silver(I) and gold(I) aryloxides is not as important as that of copper.
The reaction of the gold(III) iodide cis-[AuMe2(I)(PPh3)] with KOAr produces the
corresponding Au(III) aryloxides which form hydrogen-bonded adducts with phenols. 522
The bis(aryloxide) Na[Au(OPh)2(C6H4NO2-2)2] is formed from the corresponding
chloride and NaOPh. 523
6.2.13 Group 12 Metal Aryloxides
Interest in the structures of monomeric aryloxides of zinc and cadmium was initially
aroused by the observation that [Zn(OC6H2But 3-2,4,6)2(THF)2] formed a distorted
tetrahedral geometry524,525 while [Cd(OC6H3But 2-2,6)2(THF)2] formedatrans, squareplanar
geometry in the solid state. More recently this interest has been heightened by
the observation that discrete zinc aryloxides containing bulky ortho-substituents act as
homogeneous catalysts for the copolymerization of epoxides with carbon dioxide, 250 a
reaction that can be achieved with other zinc catalysts526–528 and offers potential for
the utilization of CO2. Mechanistic studies of the reactivity indicate that epoxide ring
opening requires a vacant site at the metal centre whereas attack at the electrophilic
carbon of CO2 occurs directly from the nucleophilic oxygen bound to zinc. Hence
aryloxides containing bulky ortho-substituents will initiate epoxide ring opening but
not attack CO2. The initially formed Zn–O–CHR–CH2–OAr (from an ˛-olefin epoxide)
now has an exposed nucleophilic Zn–O bond which can attack CO2. 250 The details of
the insertion chemistry of zinc and cadmium aryloxides is discussed in Section 5.2.2.
The bis(aryloxides) [M(OAr)2(L)n] (MDZn, Cd) can be readily synthesized by
addition of phenols to [MfN⊲SiMe3⊳2g2] in the presence of ligands L. With the
bulky 2,6-di-tert-butylphenoxide, phosphine ligands form trigonal planar species
such as [Zn(OC6H3But 2-2,6)2(PR3)] (PR3 D PMe2Ph, PCy3). 529 A large number of
four-coordinate adducts [M(OAr)2(L)2] (L D OEt2, THF, THT) have also been
characterized. The zinc compounds are invariably distorted tetrahedral, 250 whereas
both tetrahedral and trans, square planar geometries are possible for cadmium
derivatives (Tables 6.45 and 6.46). 251,530 Although [Zn(OC6H2Me3-2,4,6)2(py)2] is
four-coordinate, cadmium forms trigonal bipyramidal [Cd(OC6H3But 2-2,6)2(py)3] with equatorial aryloxides. The addition of KOC6H3But 2-2,6 to THF solutions of
[M(OC6H3But 2-2,6)2(THF)2] produces the three-coordinate anions [M(OC6H3But 2- 2,6)3] (MD Zn, Cd). 531 More complex anionic zinc species are formed with less bulky
sodium aryloxides. 532
A variety of donor ligand-free, dimeric zinc aryloxides of general formula [(X)Zn( 2-
OAr)2Zn(X)] (ArO D bulky aryloxide; X D alkyl 533,534 or amide535 ) are known.
6.2.14 Group 13 Metal Aryloxides
Aryloxides are very important ancillary ligands in the chemistry of the group 13 metals.
Particular interest in the structure and bonding (see Section 4.2) of aluminium aryloxides