Transition metal oxides, spectroscopic characterization

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

The intermediate formation of alkyl peroxide complexes has been postulated, and in several cases observed with spectroscopic and spectrometric techniques in several selective procedures based on metal catalyzed oxidation with hydroperoxides, Ti and V ions being among the transition metals most widely used for this purpose. However, to date the few examples of alkyl peroxide complexes isolated and characterized in the solid state refer to (dipic)V0(00Bu-f)(H20) 8, synthesized by Mimoun and coworkers in 1983, and to a dimeric Ti complex [((/7 -OOBu-f)titanatrane)2(CH2Cl2)3] 9, synthesized by Boche and coworkers. ... 

Through the above series of examples, it is clear that EPR offers many advantages for the characterization of paramagnetic species on oxide surfaces. The obvious Umitation of the technique is of course that it only detects paramagnetic centers. However, if paramagnetic centers, such as defects, radicals or transition metal ions, are involved in a heterogeneous process, then EPR is the ideal spectroscopic technique. To date most of the studies applied to oxides have used the traditional cw-EPR method. Modern pulsed techniques offer far more sensitivity and resolution than cw-EPR, and it is certainly hoped that these pulsed techniques will be more widely used as commercial spectrometers become more numerous in research laboratories. Compared to cw-EPR, the numerous hyperfine techniques... 

U V- Vis. Spectroscopic measurements in the ultraviolet and visible range of the electronic spectrum (UV-Vis) can be used to probe electronic transitions in certain metal atoms and ion complexes. The energy of an electronic transition can depend upon the symmetry of the metal ion being different for transitions in a metal complex displaying tetrahedral (Td) symmetry from the same metal showing an octahedral (Oh) symmetry. Thus, it is possible to use UV-Vis spectroscopy to interrogate the symmetry of certain metal ions bound to oxide surfaces. We show here a few examples of the use of UV-Vis spectroscopy to characterized supported metal oxides. 

Abstract The transition metal complexes of the non-innocent, electron-rich corrole macrocycle are discussed. A detailed summary of the investigations to determine the physical oxidation states of formally iron(IV) and cobalt(IV) corroles as well as formally copper(III) corroles is presented. Electronic structures and reactivity of other metallocorroles are also discussed, and comparisons between corrole and porphyrin complexes are made where data are available. The growing assortment of second-row corrole complexes is discussed and compared to first-row analogs, and work describing the synthesis and characterization of third-row corroles is summarized. Emphasis is placed on the role of spectroscopic and computational studies in elucidating oxidation states and electronic configurations. 

III.B.2), complexes with manganese, chromium, as well as second- and third-row transition metal ions (e.g., ruthenium) oxidation reactions with dioxygen alone or with other peroxides (e.g., ferf-butyl-peroxide) the stabilization and spectroscopic characterization of mononuclear superoxo, peroxo, and oxo complexes other catalytic processes (e.g., the iron-catalyzed aziridination), enantioselective reactions with chiral bispidine ligands and the iron oxidation chemistry continues to produce novel and exciting results. 

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