Hydrocarbyl complexes oxidation

The Tpx ligands can mimic the coordination environment created by three imidazolyl groups from histidine residues, which is frequently found in the active sites of metalloenzymes. Higher valent bis(ix-oxo) species, [(Tpx)M( i-0)2M(Tpx)] via 0-0 cleavage of [(Tpx)M( x-r 2 r 2-02)M(Tpx)] intermediates, but also peroxo, hydroperoxo, and alkylperoxo species, active species undergoing oxidative C-C cleavage reaction, stable hydrocarbyl complexes, and dinuclear xenophilic complexes, [(Tpx)M-M L71], are all relevant to chemical and biological processes, most of which are associated with transition metal catalytic species. 

N-heterocyclic carbenes (NHC) are considered extremely effective hgands for homogeneous catalysis (Figure 6.2). These specific carbenes often lead to high efficiencies in metal-catalyzed reactions compared to traditional phosphines [49, 50). NHC complexes are usually considered to be very stable, due to their electronic properties and the unusually high bond dissociation energies (BDE) associated with NHCs [51]. Previous work has shown that Ni-hydrocarbyl complexes of NHCs readily decompose by reductive elimination to yield the 2-substituted imidazohum salts [52, 53). Later studies have shown that the reverse reaction, oxidative addition of imidazohum salts to zerovalent Group 10 metals, is feasible [53]. 

Some of hydrocarbyl complexes mentioned above undergo skeletal tearrangement of the ligand upon thermolysis or oxidation. As shown in Scheme 2, such sequential skeletal rearrangement on the triruthenium cluster provides mechanistic insight into the reaction performed on a metal surface. In this regard, many studies have been performed using carbonyl clusters. 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

Olefin epoxidation is an important industrial domain. The general approach of SOMC in this large area was to understand better the elementary steps of this reaction catalyzed by silica-supported titanium complexes, to identify precisely reaction intermediates and to explain catalyst deachvahon and titanium lixiviation that take place in the industrial Shell SMPO (styrene monomer propylene oxide) process [73]. (=SiO) Ti(OCap)4 (OCap=OR, OSiRs, OR R = hydrocarbyl) supported on MCM-41 have been evaluated as catalysts for 1-octene epoxidation by tert-butyl hydroperoxide (TBHP). Initial activity, selechvity and chemical evolution have been followed. In all cases the major product is 1,2-epoxyoctane, the diol corresponding to hydrolysis never being detected. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

Double oxidative additions occur to generate hydrocarbyl-bridged complexes ... 

As stated earlier, (11.3.1), the multiple insertion of carbon monoxide into the same metal-hydrocarbyl bond is a rather elusive reaction. On the other hand, multiple insertion of isocyanide has been reported for nickel(II). For example, when the nickelfO) derivative Ni(t-BuNC)4 was treated with Mel in hexane at RT, consecutive insertion of three RNC groups was observed to give the product of reaction (e), as a consequence of a primary oxidative addition of the alkyl iodide to the nickel(O) complex. It is interesting that one of the two terminal fragments of the five-membered metallacycle is reminiscent of an arrangement of the first insertion product. 

One important property of CF3-transition metal complexes became apparent almost immediately when all of the low-valent, late transition metal trifluoromethylated compounds then known were found to be significantly more thermally and oxidatively stable than the analogous methylated species. Tetracarbonyl(trifluoromethyl)cobalt(I), for example, was isolated by distillation at 91°C, whereas the hydrocarbyl Co(CO)4(CH3) decomposes at subambient temperatures (72). Additionally, while the reverse of the decarbonylation reaction, CO insertion, is commonly observed in methylated transition metal species, these reactions are essentially unknown for trifluoromethyl metal complexes (13). Prior to 1980, evidence for CO insertion into an M—CF3 bond had been reported in only one case. That reaction employed the photolysis of Mn(C05)(CF3) in an argon matrix at 17 K, and the identity of the product was not determined (14). The clear implication of the above results is that MCF3 metal—carbon bonds are significantly less reactive and thus presumably stronger than MCH3 metal—carbon bonds. 

The electrochemical properties of a number of isostructural rhe-nium(V) oxo, rhenium(V) imido, osmium(VI) nitrido, of formula [M(E)(X)(Y)(Tp )] (Tp = Tp or Tp, E = O, N-tolyl, N X, Y = hydrocarbyl, halide, triflate), have been described. The reactivity of these complexes as inner-sphere oxidants does not correlate with their peak reduction potentials, whereas the ease of the oxidation of these compounds well parallels their reactivity as oxidants.206... 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

Whereas all hydrocarbyl [Pt (R)2(L)2] species (R = hydrocarbyl, L2 = bidentate N-donor ligands) reveal irreversible oxidation waves in cyclic voltammetry, mesityl [Pt°mes2(L)2] complexes reveal reversible oxidation waves. In these cases, oxidation to stable, paramagnetic, but EPR silent [Pt mes2(L2)] species is possible. 

Ahn and Marks have employed solid-state CP/MAS NMR to investigate the chemistry of several organotanta-lum hydrocarbyl/alkylidene complexes supported on high surface area oxides.In some cases, chemisorption resulted in the formation of cation-like species, for example, Cp Ta( CH3)3. ... 

Some interesting rearrangements of the hydrocarbyl ligands have been reported in the oxidation of p-i-Tf tf tf -benzene complex 48. Treatment of 48 with Z equiv. of ferrocenium salt resulted in rotation of the face-capping benzene ligand, and the dicationic complex 63, in which the C6 cycle was shown to be coordinated in a bis(77 -allyl) fashion, was obtained (Equation (21)). ... 

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