Peroxo complexes structure

Because there exist a number of reviews which deals with the structural and mechanistic aspects of high-valent iron-oxo and peroxo complexes [6,7], we focus in this report on the application and catalysis of iron complexes in selected important oxidation reactions. When appropriate we will discuss the involvement and characterization of Fe-oxo intermediates in these reactions. 

Unfortunately, there is no Ti-hydroperoxo compound of known structure to be used as a model. Conversely, the structure of several Ti-peroxo complexes are known by diffraction studies [111-113], all of them showing the side on r] geometry. None of these compounds is known to be active in partial oxidation reactions [114,115]. Similar considerations can be addressed... 

Dioxygen and its ions can bind in mononuclear and dinuclear structures in a number of ways,962 as illustrated in Scheme 1. The typical reaction of dioxygen with Co compounds involves a number of these binding forms, outlined in Scheme 2. Mononuclear Co111—peroxo complexes are relatively rare, but yellow trigonal bipyramidal complexes [Co(02)L2]+ (L = chelating phosphines dppe or dppp) have been characterized structurally where the 022 is bonded to the Co in the side-on r]2 form (Co—O 1.858(7) 1.881(4) A), with O—O stretching frequencies ( 870 cm-1) consistent with Coin-peroxo speciation.963... 

In 2000 Meyer and co-workers reported a novel example of unusual 4-peroxo coordination (complex (146)), as well as of a structurally analogous complex in which the 0-0 linkage is formally cleaved and replaced by two OH units (147), while at the same time the overall tetra-nuclear framework is fully conserved.145... 

Using 1,4,8,11-tetraazacyclotetradecane, the structure of complex (800) (distorted trigonal planar Cu-Cu 6.739 A) was determined. Reactivity with 02 was investigated to demonstrate the formation of trans-l,2-peroxo species.585 As part of their work with copper(I) complexes with 02, the structure of a dicopper(I) complex ((801) distorted tetrahedral 7.04 A), supported by macrocyclic ligand environment, was reported by Comba and co-workers. Tolman and co-workers structurally characterized a three-coordinate copper(I)-phenoxide complex (802) (planar T-shaped) that models the reduced form of GO.587 The copper(I) analogue [Cu(L)][CF3-SO3]-0.43MeOI I (803) of a copper(II) complex (534) was also reported to demonstrate the role of ligand framework conformability in CV /Cu1 redox potentials.434 Wilson and co-workers... 

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... 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

For the sake of simplicity the aqua ligand is added in cis position to the methyl group, as suggested by the X-ray structure of the di(peroxo) complex [19]. Agreement between calculated and experimental geometries of 5b is very satisfactory bond lengths deviate, in general, at most by 0.04 A. The... 

Figure 11. Transition structures S-6 to S-9b for the epoxidation of propenol by the Re di(peroxo) complex 5. Selected distances in A.

While numerous peroxo complexes of Ti, V, Cr/Mo/W and Re have been experimentally isolated and well characterized, there are only few examples of alkyl peroxo complexes whose X-ray structures are known, namely (di(picolinato)VO(OOtBu)(H20) [116] and [((T 2-ter -butylperoxo) titanatrane)2 3 dichloromethane] [117], Evidence for the existence of other... 

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. 

We used different equatorial base ligands L, = NH3, ONH3 and L = NH3 or none as axial ligand. There is striking structural similarity with the di(peroxo) complexes of Re that arise in the MT0/H202 catalytic system (La = H20 or none) ... 

The computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

The groups of Philipsborn [84], Heaton [85-88] and Mann [89-91] have used ° Rh NMR extensively to elucidate structural and mechanistic aspects of a wide variety of metal carbonyl and metal cluster complexes. Further, Zamaraevl [92] has shown that NMR studies on several quadrupolar nuclei, e. g. Mo, help with the characterization of the alkyl peroxo-complexes, which are thought to be inter-... 

The reactivity of the peroxo metal complexes indicated in Scheme 1 is much higher than that of the peroxidic precursor, either H2O2 or ROOH. This fundamental feature is the direct link between the chemistry of metal catalyzed oxidations and that of peroxo metal complexes These are in fact, in the majority of the systems, well defined species in solutions and in numerous cases they can be isolated and fully characterized also in the solid state. As far as the structure of the active complexes indicated in Scheme 1 is concerned, both in the solid state and/or in solution, several pieces of evidence indicate that most of them share a triangular arrangement of the peroxo moiety around the metal center (see below). In principle, all the /z-peroxo complexes structures, named following... 

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