2 Homometallic Alkoxides

616 Alkoxo and Aryloxo Derivatives of Metals
The structurally characterized compound fac,cis-[Re(OC6H4Me-4)(CO)3(PPh3)2] is
produced by addition of 4-methylphenol to the corresponding methyl compound. 466
The insertion chemistry of this and related alkoxides has been discussed (Section 5.2).
6.2.9 Group 8 Metal Aryloxides
As with other first row d-block metals, the aryloxide bond is an important component
of many coordination compounds of iron formed with polydentate ligands. Simple
aryloxide ligands have been applied mainly to the synthesis of binary compounds
and adducts, as supporting ligands in (typically sulfur) cluster compounds and as
the axial group in iron porphyrin compounds (Table 6.33). Treatment of the diethyl
[FeEt2(bipy)2] with phenols HOC6H4X-4 was found to yield the corresponding
bis(aryloxides). 467 Kinetic studies supported a pathway involving initial dissociation of
bipy, coordination of phenol, and then elimination of ethane. The homoleptic iron(II)
aryloxide [(ArO)Fe( 2-OAr)2Fe(OAr)] (OAr D OC6H2But 3-2,4,6) was obtained from
the corresponding bis(trimethylsilyl)amide. 457 Treatment of FeCl3 with LiOAr in EtOH
was found to produce the iron(III) anions [Fe(OAr)4] (Eq. 6.31, OAr D OC6HMe4-
2,3,5,6; OC6H2Cl3-2,4,6). 109 The bonding in these anions was of interest as a model for
tyrosine binding. Interestingly the anions undergo reversible one-electron reductions
at 1.32 V and 0.45 V (vs SCE) respectively, showing how substituents on the
phenoxide ring can influence metal-based redox processes.
A large number of high-spin, five-coordinate Fe(III) complexes [(porphyrin)Fe(OAr)]
have been isolated and studied. 468,469 Cyclic voltammetric studies show that the
Fe(II)/Fe(III) couple lies at a significantly negative potential. 470 A reasonable correlation
between the reduction potential and Hammett -values for p-substituents on the
aryloxide ligand was found. The high-spin Fe(II) anion [(porphyrin)Fe(OPh)] has been
isolated and structurally characterized. 471 At low temperatures formation of low-spin
adducts occurs with ligands such as pyridine and 1-methylimidazole. 472 The reaction
of [(porphyrin)Fe(Ar)] compounds with O2 has been investigated. Initial insertion into
the Fe–Ar bond was believed to be followed by rapid homolysis. 142 The aryloxide
chemistry of ruthenium473 and osmium474 porphyrin compounds has also been investigated.
Aryloxide ligands also play an important role in the area of biologically relevant
iron–sulfur cluster chemistry. 475–477 Thecubiccluster[Fe4( 2-S)4(OAr)4] 2 shows
significant spin delocalization onto the phenoxide nucleus. 478 The first and second
reduction potentials for the [Fe4( 2-S)4] 2C core were found to undergo significant
negative shifts (more difficult to reduce) upon replacing arenethiolates by aryloxide
ligands. An important class of rhombic dodecahedral clusters [Fe6( 4-S)4(OAr)4] 3
has been isolated and extensively studied. 479,480
Ruthenium chemistry typically finds aryloxide ligands attached to the metal in low
oxidation states. The extensive number of carbonyl aryloxide compounds reported for
this metal exemplifies this (Table 6.34). The cluster compound [(H)2Ru6(CO)16( 2-O-
2-C6H4)] and derivatives contain the metallated phenoxide bridging two metal centres
6 as well as -bound to another.