Oxophilic metal atoms

When a supported metal on an oxide is prepared from an adsorbed precursor incorporating a noble metal bonded to an oxophilic metal, the result may be small noble metal clusters, each more-or-less nested in a cluster of atoms of the oxophilic metal, which is cationic and anchored to the support through metal-oxygen bonds [44,45]. The simplest such structure is modeled on the basis of EXAFS data as Re4Pt2, made from Re2Pt(CO)i2 (Fig. 6) [45]. 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

An obvious question is whether patterned arrays of metal complexes can be formed on supports, and an approach to the preparation of such materials has been made by use of precursors containing more than one metal atom. Thus, attempts have been made to prepare supported metals with pair (and triplet) sites from dimeric (and trimeric) complexes of oxophilic metals, including Mo, W, and Re, which bond strongly to oxide surfaces. 

A distinctive feature, unprecedented in heterometallic alkoxide chemistry, is the exchange of the central metal atoms between two chelating ligands this rearrangement is possibly favored by the greater oxophilicity of barium and its tendency to attain higher coordination states (39a). 

An efficient ethanol electrooxidation catalyst should combine at least two features (i) high tolerance to CO and other intermediate species generated over the surface of the electrocatalyst during alcohol electrooxidation and (ii) ability to break the C-C bond of the ethanol molecule under mild conditions. The most relevant features for the designing of CO tolerant electrocatalysts have been described above namely, Pt modification with more oxophilic metals such as Ru, Mo or Sn renders the best electrocatalysts. This is because such oxophilic atoms promote the formation of -OfT. species (involved in the CO j oxidation reaction) at potentials that are more negative than that on pure Pt (Eq. 9.17). Among those, Sn-modified Pt electrocatalysts are the most active formulations. There is also widespread consensus that the PtsSn phase is the most active one in the CO reaction and early stages of the ethanol electrooxidation process. ... 

The choice of oxophilic metal ions was based on the rationale of minimal interference with the construction of the MOC because of the weak competition of those ions toward coordination to nitrogen atoms of the ligand, thus permitting their utilization in the initial reaction... 

A purely bifunctional mechanism was assumed to be operative in the CO oxidation on Pt3Sn surfaces as well as on other Pt alloys with oxophilic transition metals [157-159]. Here, the oxophilic Sn surface atoms are believed to provide nucleation sites for water and, following stepwise hydrogen abstraction, for its subsequent oxygenated surface products OH and O. CO oxidation on Sn atoms is unlikely [131,160] such that no competition for Sn sites occurs between water and CO molecules. All Pt atoms are covered with CO. 

In contrast to nitrosyls, the absence of a transferable oxygen atom in N2R ligands allows the preparation of stable diazenido complexes of oxophilic, early transition metals see for example Cp2TiCl(N2Ph). Furthermore, there are as yet no diazotate (RN=NO-) forming reactions anolo-gous to the nitrite forming reactions in nitrosyl chemistry (see equations 112 and 113). 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

Niobium (formerly called columbium) and tantalum are Transition Metals having a considerable affinity for oxygen donor groups they are thus called oxophilic see Oxophilic Character). They occur as mixed-metal oxides such as columbites (Fe/Mn)(Nb/Ta)206 and pyrochlore NaCaNb206p. Their discovery in minerals extends back to the beginning of the nineteenth century, when they were believed to be identical and called tantalum. Rose showed that at least two different elements were involved in the minerals, and named the second one niobium. Their separation was resolved around 1866, especially by Marignac. These metals often display similar chemical behavior as a result of nearly identical atomic radii (1.47 A) due to the lanthanide contraction see Periodic Table Trends in the Properties of the Elements)... 

Similar behavior is observed for calix[4]arene (see Calixarenes) complexes of the type (89). Moreover, the oxygen donor atoms are particularly appropriate for the oxophilic early transition metals. A rich chemistry has been developed with complexes containing zirconium - carbon bonds. In this area, aryne zirconocene compounds can be thermally generated and isolated withont trimethylphosphane additional coordination. Zirconium-bntadiene chemistry has been explored as well and constitutes a source of zircoiuum complexes. 

Ligands bearing O, N, or S atoms as donors are used to incorporate both lanthanides and transition metals into the complexes, as lanthanides are hard Lewis acids and oxophilic. 

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