Metal oxophilicity

Compared with late metals, oxophilic (f metals are less tolerant of functional groups in their reaction with organic compounds. Tliese groups, present in any complex organic compound, either block open sites at the metal needed for catalysis or produce undesired side reactions. This helps explain why organic reactions (Chapters 9 and 14) where most substrate functionality must remain untouched. Early metals can still be good catalysts for hydrocarbons lacking heteroatom functionality, as in alkene polymerization (Chapter 12), typically catalyzed by (fTi and Zr. 

The only reports of directed synthesis of coordination complexes in ionic liquids are from oxo-exchange chemistry. Exposure of chloroaluminate ionic liquids to water results in the formation of a variety of aluminium oxo- and hydroxo-contain-ing species [4]. Dissolution of metals more oxophilic than aluminium will generate metal oxohalide species. FFussey et al. have used phosgene (COCI2) to deoxochlori-nate [NbOa5] - (Scheme 6.1-1) [5]. 

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]. 

Scheme 8 displays reactions where 43 behaves as a source of a Zr(II) derivative.30 They can be formally viewed as oxidative additions to the [p-Bu -calix[4]-(0Me)2(0)2Zr] fragment. The main driving force in the case of ketones is the high oxophilicity of the metal, which induces the reductive coupling of benzophenone leading to 45, or the addition of dibenzoyl causing the formation of the dioxo-metallacycle in 46, which contain a C-C double bond. It has to be mentioned that... 

In the absence of an excessive driving force derived, as in the case of CO, from the oxophilicity of the metal, the reaction with Bu NC allowed one to better single out almost all of the steps of the migratory insertion of the isocyanide into the Zr-C bonds, as shown in Scheme 19.47... 

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... 

The oxophilic character of the early transition metals may provide such stabilization in the form of dihapto binding (I). This unusual donor behavior was first... 

Ytterbium triflate [Yb(OTf)3] combined with TMSG1 or TMSOTf are excellent reagents for the conversion of a-methyl styrene and tosyl-imines into homoallylic amides 32 (Equation (19)) (TMS = trimethylsilyl).29 These conditions produce the first examples of intermolecular imino-ene reactions with less reactive imines. Typically, glyoxalate imines are necessary. A comprehensive examination of the lanthanoid metal triflates was done and the activity was shown to directly correlate with the oxophilicity scale. The first report used preformed imines, and subsequently it was found that a three-component coupling reaction could be effected, bypassing the isolation of the intermediate imine.30 Particularly noteworthy was the successful participation of aliphatic aldehydes to yield homoallylic amines. 

Compounds and complexes of the early transition metals are oxophilic because the low d-electron count invites the stabilization of metal-oxo bonds by 7T-bond formation. To a substantial extent, their reactivity is typical of complexes of metals other than rhenium. That is particularly the case insofar as activation of hydrogen peroxide is concerned. Catalysis by d° metals - not only Revn, but also CrVI, WVI, MoVI, Vv, ZrIV and HfIV - has been noted. The parent forms of these compounds have at least one oxo group. Again the issue is the coordination of the oxygen donating substrate, HOOH, to the metal, usually by condensation ... 

Early transition metal based catalysts react with a variety of polar substrates and impurities, except the molybdenum ones substituted electronically and sterically (Figure 16.12) in such a way that they become less oxophilic . In... 

The organoactinide surface complexes exhibited catalytic activities comparable to Pt supported on sihca [at 100% propylene conversion at —63°C, >0.47s (U) and >0.40 s (Th)], despite there being only a few active sites (circa 4% for Th, as determined by CO poisoning experiments and NMR spectroscopy) [92]. Cationic organoactinide surface complexes [Cp An(CH3 ) ] were proposed as catalytic sites. This hypothesis could be corroborated by the use of alkoxo/hydrido instead of alkyl/hydrido surface ligands, which led to a marked decrease of the catalytic activity, owing to the oxophilic nature of the early actinides [203, 204]. Thermal activation of the immobihzed complexes, support effects, different metal/ligand environments and different olefins were also studied. The initial rate of propylene conversion was increased two-fold when the activation temperature of the surface complexes under H2 was raised from 0 to 150°C (for Th 0.58 0.92 s" ). 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

Titanium alkoxides are also effective and sought-after initiators for the ROP of lactides due to a low toxicity, which minimizes the problems linked to the presence of catalyst residues in commercial PLA products [18, 19]. Despite impressive advancements in the use of Lewis acidic metal initiators in the preparation of PLAs, surprisingly little attention has been paid to the group 4 metal (Ti, Zr, Hf) initiators, probably due to the highly oxophilic nature of M(1V) which has a natural tendency to form aUcoxy-bridged multinuclear complexes. Verkade and coworkers previously demonstrated a series of titanium aUcoxide complexes 118-122 (Fig. 17) that function as moderately efficient initiators in bulk homopolymeization of L-lactide and rac-lactide, some of these initiators displaying a well-controlled polymerization behavior [119]. 

By virtue of the oxophilicity of aluminium, many compounds exist in which at least one of the metal s formal valencies is occupied by oxygen or, alternatively, where the coordination state of the metal is raised to 4, 5 or 6 by the donation of electron density from oxygen. This review presents aluminium oxides first and thereafter aluminium hydroxides and organooxides. Although the discussion concentrates on the sohd-state structural properties of such systems, solution structural, theoretical and reactivity studies are also presented. 

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