Peroxo complexes structural properties

Catalytic Activity of Some Vanadium Peroxo Complexes Structural Properties of Vanadium Peroxo Complexes Vanadium in Mushrooms Vanadium in Coal... 

It is instructive to compare the properties of metal peroxo and alkyl (or hydro) peroxo groups for the case of Ti because experimental structures of both types are known [117, 119-121] and Ti compounds are catalysts for such important processes as Sharpless epoxidation [22] and epoxidation over Ti-silicalites [122], where alkyl and hydro peroxo intermediates, respectively, are assumed to act as oxygen donors. Actually, the known Ti(t 2-02) complexes are not active in epoxidation. [121-124] However, there is evidence [123] that (TPP)Ti(02) (TPP = tetraphenylporphyrin) becomes active in epoxidation of cyclohexene when transformed to the cis-hydroxo(alkyl peroxo) complex (TPP)Ti(OH)(OOR) although the latter has never been isolated. 

The above /(-hydroxo /(-peroxo complex represents an important class of complexes, and a series of L4Co(0H)(02)CoL43+ complexes has been prepared with other amines such as NH3, tn, tren, and trien (108, 231-244) the structures of these have been established by X-ray crystal-structure analysis (108,109,242,243). The kinetics of formation and the chemical properties of these dinuclear species have been studied in detail, as discussed in a recent review by Fallab and Mitchell (119). 

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by... 

W. A. Herrmann, J. D. G. Correia, G. R. J. Artus, R. W. Fischer, C. C. Romao, Multiple bonds between main group elements and transition metals, 155. (Hexamethylphosphoramide) methyl (oxo) bis(Ti -peroxo)rhenium(Vll), the first example of an anhydrous rhenium peroxo complex Crystal structure and catalytic properties, J. Organomet. Chem. 520 (1996) 139. 

Vanadium. Vanadyl complexes of the three ligands 14, 24, and 28 were prepared from vanadyl sulfate and the metal-free ligands. They have the expected spectroscopic properties, and this is confirmed by the X-ray crystal structure of the vanadyl complex of 24, which is shown in Fig. 23(a) and has metal-donor bond distances in the expected range. In terms of reactivity, these complexes are rather unspectacular. No haloperoxidase reactivity was observed, and oxidation of alkanes and alkenes is rather sluggish (138). This is not unexpected since with the pentadentate ligands there is no free site at the vanadium(IV) center for metal-substrate interaction. The same is true for the vanadium(V) oxo-peroxo complexes with tetradentate coordination of the pentadentate ligand. 

Girerd, J. J. Banse, F. Simaan, A. J. Characterization and properties of non-heme iron peroxo complexes. In Metal-Oxo and Metal-Peroxo Species in Catalytic Oxidations Structure Bonding wo. 97, Springer-Verlag, 2000, p. 145. 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

Secondly, there is an indication that metal(III)-peroxo side-on complexes are in general in equilibrium with corresponding metal(II)-superoxo end-on species. The position of such equilibrium could depend on various factors as structural and electronic properties of the porphyrin ligand, coordination of an axial ligand trans to peroxide/superoxide, solvent medium, temperature and involvement of coordinated peroxide/superoxide in possible hydrogen bonding or electrostatic interactions. These are interesting questions which should be addressed in future studies. 

The properties of Kitajima s p-p2 Ti2-pcroxo dicopper(II) complex lead to the conclusion that this is the likely structure in oxyhemocyanin and oxytyrosi-nase this is perhaps the most important contribution from this type of model chemistry. A distorted or closely related peroxo-dicopper(II) species appears to be involved in aromatic hydroxylation proceeding in a well-characterized tyrosinase model system. 

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