Oxophilic metal complex

For instance, insertion of CO in the M-H, M-COR and M-CF3 bonds is usually thermodynamically unfavorable (on the other hand, extrusion of CO from M-CHO, M-COCOR and M-COCF3 bonds is favorable). Oxophilic metal complexes, however, play this role of Lewis acids. The additional resulting driving force can then make the CO insertion possible into a thorium-hydride bond (see Chap. 12), whereas this was impossible with transition-metal-hydrides ... 

In contrast, Fe-Hg-X complexes show little tendency to form halide bridged species and less is known about complexes containing Zn. We first reported the formation of Fe-Si-O-M four membered ring systems with soft metals M = Ag, Rh, Pd, and Pt, and then prepared bimetallic complexes with more oxophilic metals in order to better understand the conditions for the occurrence of this unusual (t-alkoxy-silyl bridging mode. We have expanded our studies on Cd-containing complexes [3b-d] to Group 13 elements and we report here about the synthesis and reactivity of new, stable heterometallic Fe-M (M =... 

The reactivity of (C, Me-)pSm(THF)2 with CO can also be compared with that of (C Me Ti. Both organometallic reagents are soluble, strongly reducirrg complexes of oxophilic metals. As shown in reaction 7, decamethyltitanocene forms a carbonyl complex rather than... 

In the beginning, the major focus was on the early transition metals such as Ti, Zr and Hf, but today the potential of the late transition metals complexes of Ni, Pd, Co and Fe is well recognised. As the late transition metals are characteristically less oxophilic than the early metals, they are more tolerant towards polar groups. Therefore, it was assumed that with late transition metals catalysts one could produce a wide range of different polymers. 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

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. 

The conversion of the dithiocarbonates into alkenedithiolates involves base hydrolysis, which is usually effected with sodium alkoxides in alcohol. With the dianion in hand, the synthesis of complexes follows the usual course, as described above. Obviously, oxophilic metal centers, for example, Ti(IV) and Nb(V) (62), are incompatible with the usual alcohol solutions of in situ generated alkenedithiolates. In such cases, the anhydrous salts Na2S2C2R2 are employed in nonhydroxylic solvents, although after complex formation protic solvents are typically employed for cation exchange. 

There are a number of reactions of C02 with metal complexes in which prior coordination is likely. For example, the anion W(CO)5OH reacts with C02, COS, or CS2 to give the corresponding bicarbonate or thiocarbonate complexes.104 With complexes of oxophilic metals like Ti or Zr, deoxygenation to CO may occur while in others disproportionation to give CO and C03 occurs an example of the latter reaction is... 

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. 

More recently, a Pd(II) salt was shown to catalyze the 1,2-insertion polymerization of a 7-oxanorbornadiene derivative (Fig. 10-16) [50]. The resulting saturated polymer, when heated, gives polyacetylene via a retro-Diels-Alder reaction. (This reaction is reminiscent of the Durham route to polyacetylene discussed below). One advantage of this technique over other routes is that it employs a late transition metal polymerization catalyst. Catalysts using later transition metals tend to be less oxophilic than the d° early transition metal complexes typically used for alkene and alkyne polymerizations [109,110]. Whereas tungsten alkylidene catalysts must be handled under dry anaerobic conditions, the Pd(II)-catalyzed reaction of water-insoluble monomers may be run as an aqueous emulson polymerization. 

The coordination of an oxophilic metal to the oxycarbene side chain offers the possibility to tune the properties and the reactivity of the carbene complex. A review on metalloxycarbene complexes has been published recently [88] among them boroxycarbene complexes have received the widest attention and have been applied to organic synthesis. Their reactivity strongly depends on the boron coordination sphere as demonstrated for dialkyl- and difluoroboroxycar-bene complexes. 

For Class B (substitution labile) metal complexes, reequilibration to more thermodynamically favorable coordination modes will be very rapid relative to immobilization. Such behavior is typical of first-row TM complexes. In addition, these ions are usually very oxophilic, so the metal complexes are typically subject to ICC interactions with oxide materials. Since these metal ions are generally immobilized under conditions of thermodynamic control, all pertinent speciation equilibria, including ICC reactions (Section III.B), must be considered in order to understand or predict the outcome of immobilization reactions. It is essential to understand the relevant equilibria if direct imprinting of active site structures is to be successful. The studies of Klonkowski et al. (210-213), for example, underscore this point Sol-gel immobilization of copper complexes bearing silylated amine and ethylenediamine ligands were shown by EPR to result in multiple copper environments, suggesting competition between immobilization and ICC reactions. 

Most of the olefin complexes examined in this study exhibit an unspectacular reactivity towards molecular oxygen, i. e. either ligand exchange reactions, 0-0-bond activation by highly oxophilic metals Sc, Ti, and V, or even complete absence of any reaction are observed (eg. even Cu(C2H4) is unreactive). However, in the case of the iron complexes extensive oxidation reactions are observed. Indeed, not only olefins attached to an iron cation react effectively with molecular oxygen, even stable molecules like benzene and acetone are rapidly oxidized in the presence of Fe+. 

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