Superoxo structure

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

The first step consists of the formation of the dioxygen adduct which can have either a superoxo structure (1) if the metal is a potential one-electron donor, or a peroxo structure (2) if the metal is a potential two-electron donor. These superoxo or peroxo complexes can be considered as the formal, but not chemical, analogs of the superoxide 02 and peroxide 022- anions. The superoxo complex (1) can further react with a second reduced metal atom to give the /x-peroxo species (3), which can cleave itself into the oxo species (4), which may be hydrolyzed to give the hydroxo species (6) or react with a second metal atom to give the p.-oxo species (5). The alkylperoxo (7) and hydroperoxo (8) species can result from the alkylation or protonation of the peroxo species (2), or from anion exchange from metal salts by alkyl hydroperoxides or hydrogen peroxide. 

The catalytic cycle of TauD has been studied extensively 20-22) and is schematically depicted in Fig. 2 starting fix)m the aKG-bound structure (A). When taurine enters the substrate binding pocket the last water ligand of the metal is released (structure B) prior to dioxygen binding (structure C). The terminal oxygen atom of the iron(III)-superoxo structure subsequently attacks the a-keto... 

Molecular weight of repeated unit with superoxo structure... 

It is noteworthy that the same reaction with a copper(I) complex with HB(3,5-iPr2Pz)3 causes an instantaneous formation of a dimeric J,-Ti Ti2-peroxo complex [118-120]. Thus, the more steric bulk of HB(3-tBu-5-iPrpz)3) inhibits the secondary reaction between the putative superoxo complex and a copper(I) counterpart generating a p.- n iti -peroxo dimer. While the employment of a N3 tripodal hydrotris(pyrazolyl)borate substantiates a side-on superoxo structure, the other complex prepared with a N4 ligand adopts a terminal coordination of the superoxide as indicated in Fig. 9. 

Figure 14.17 Structures of (a) the tetraperoxochromate(V) ion [Cr (02)4] , (b) the pyridine oxodiperoxo-chromium(VI) complex [Cr 0(02)2py], and (c) the triamminodiperoxochromium(IV) complex [Cr" (NH3)3(02)2] showing important interatomic distances and angles. (This last compound was originally described as a chromium(II) superoxo complex [Ci (NH3)3(02)2] on the basis of an apparent 0-0 distance of 131 pm/ and is a salutary example of the factual and interpretative errors that can arise even in X-ray diffraction studies. " ...

Collman JP, Hutchison JE, Lopez MA, Tabard A, Guilard R, Seok WK, Ibers JA, L Her M. 1992. Synthesis and characterization of a superoxo complex of the dicobalt cofacial diporphyrin [(/u,-02)Co2(DPB)(l,5-diphenylimidazole)2][PF6], the structure of the parent dicobalt diporphyrin Co2(DPB), and a new synthesis of the free-base cofacial diporphyrin H4(DPB). J AmChemSoc 114 9869. 

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

EPR Labile, rhombic type spectrum corresponding to Ti-superoxo species spectral features sensitive to the type of silicate structure, temperature, solvent and pH... 

It has been postulated on the basis of the crystal structure of [Mn (tpp)(02 )], that the peroxide ligand in Fe(III)-peroxo porphyrin species is coordinated in a side-on bidentate manner (Scheme 2) (8). Interestingly, based on the deuterium NMR studies the [Mn tppXOa )] complex was characterized as the Mn(II)-superoxo species (55). This discrepancy has been explained by an alteration of the normal d orbital ordering, where the highest energy d orbital is a d 2- -02 hybrid not the (8). These examples show that... 

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 structure of the active component, manganese pyrophosphate, has been reported in the literature (24). It is layer like with planes of octahedrally coordinated Hn ions being separated by planes of pyrophosphate anions (P20y ). Examination of models of this compound gave calculated Hn-Hn thru space distances of 3.26 and 3.45 angstroms, a metal-metal distance close to that found for binuclear dibridged peroxo- and superoxo- complexes of cobalt ( ). 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

The crystal structure of the dioxygen complex Tp V(0)(02)(L) has been determined, and revealed a side-on bonded O2 ligand. Based on the O - O distance of 1.379(6) A and vo-o of 960 cm it was formulated as a V(V) peroxo complex, even though these values are on the borderline between the peroxo and superoxo designations. The mechanism of this reaction is curious. Reaction with 02 showed incorporation of solely in the O2 ligand and not in the 0x0 groups. It appears that the O2 binding step must be preceded by a disproportionation (2 V(IV) V(III) + V(V)) followed by reaction of V(III) with O2. 

Fig. 4. Structures of the various spin states of a [Ru2] catalyst intermediate. For the closed-shell species no Ru-oxo structure exists but all structure optimizations converge to the superoxo state.

Crystal structure analysis has been carried out for several hydroperoxo complexes. The 0-0 bond length (1.40-1.48 A) (79,87,90,100,103,105) is significantly longer than that in superoxo complexes and close to the 1.49 A value for hydrogen peroxide (54). 

Marcus theory (15) has been applied to the study of the reductions of the jU,2-superoxo complexes [Co2(NH3)8(/u.2-02)(/i2-NH2)]4+ and [Co2(NH3)10(ju.2-O2)]6+ with the well-characterized outer-sphere reagents [Co(bipy)3]2+, [Co(phen)3]2+, and [Co(terpy)2]2+, where bipy = 2,2 -bipyridine, phen = 1,10-phenanthroline, and terpy = 2,2 6, 2"-terpyridine (16a). The kinetics of these reactions could be adequately described using a simple outer-sphere pathway, as predicted by Marcus theory. However, the differences in reactivity between the mono-bridged and di-bridged systems do not appear to be explicable in purely structural terms. Rather, the reactivity differences appear to be caused by charge-dependent effects during the formation of the precursor complex. Some of the values for reduction potentials reported earlier for these species (16a) have been revised and corrected by later work (16b). 

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