Sacrificial ligand

Two process variants have been described. A type n process [91] resembles the Union Carbide process for butenes hydroformylation (see 8.6.1). Rapid oxidation of the diphosphite ligands by air ingress in vacuum distillation columns is mitigated by addition of an excess of sacrificial ligand (tri-orthotolylphosphine). In a type IVA process, very much like the Kuraray process (see 8.6.4.1), apolar and polar solvents are applied to effect the desired combinahon of one-phase reachon fohowed by phase separation, with most Rh/diphosphite catalyst in the apolar layer [92,93]. 

Another popular method for the generation of palladium(O) catalysts in situ from palladium(ii) pre-catalysts is the sacrificial elimination of a carbon-based ligand for example, palladacycle catalysts often eliminate a ben-zylamine by-product, releasing the palladium(O) complex. There are some examples in which NHCs became the sacrificial ligand in the generation... 

H2 or O2 from water in the presence of a sacrificial reductant or oxidant employ a mthenium complex, typically [Ru(bipy)2], as the photon absorber (96,97). A series of mixed binuclear mthenium complexes having a variety of bridging ligands have been the subject of numerous studies into the nature of bimolecular electron-transfer reactions and have been extensively reviewed (99—102). The first example of this system, reported in 1969 (103), is the Creutz-Taube complex [35599-57-6] [Ru2(pyz)(NH3. 

Electrocatalysis with nickel-bpy complexes has been shown useful for synthetic applications,202,211 especially when used in combination with the sacrificial anode process in an undivided cell (Equation (45)).207,211 Under these very simple experimental conditions, efficient nickel catalysts can be also generated in the presence of the cheap pyridine ligand.212... 

Figure 12.9 Repair of damaged DNA by sacrificial alkylation of one of the zinc cysteine thiolate ligands of the Ada DNA repair protein. (Reprinted with permission from Parkin, 2004. Copyright (2004) American Chemical Society.)...

Furthermore, the best ligands L are p-diketones with an electron-donating substituent at position 2 (40). As for the sacrificial reducer, only primary or secondary alcohols are effective i-propanol does a good job, and was used in the standard procedure that was developed. 

A study involving the photocatalytic properties of two series of complexes of the type Re(diimine)(CO)3X was conducted, where the diimine was substituted a bpy or phen or other related ligands, and X was Cl, CN and other common pseudohalides. It demonstrated that all the complexes that exhibited luminescence were capable of CO2 reduction under nonaqueous conditions using sacrificial quenching [99]. No clear correlation of activity with structure was discerned however it was clearly demonstrated that this extended class of materials has a great potential for photocatalytic CO2 reduction. Of note is that Tc(bpy)(CO)3Cl also is emissive, and is a photocatalyst. 

Direct electrochemical synthesis is carried out according to the next procedure. Sheets of copper, nickel, or zinc are used as sacrificial anodes, and platinum is used as the cathode. Methanol is used as a solvent and LiC104 as a supporting electrolyte. The ligand (0.5 g) is dissolved in methanol (30 mL) by heating and then the obtained solution is cooled to room temperature. The electrolysis is carried out for 1 hr (current 20 mA applied voltage 20-30 V). The formed solid is filtered, washed with hot methanol (3x5 mL), and dried in air. 

Zn, Cd, Co, Ni, and Cu sheets (2x2 cm2) were used as sacrificial anodes and platinum wire was used as inert cathode. Methanol (50 mL) was used as solvent and Me4NC104 (10 mg) as a supporting electrolyte. The electrolysis was carried out at 7 = 30mA and initial K = 20V for 1.5 hr at 25°C in an argon stream. 2-Tosylaminoaniline, pyridine-2-carbaldehyde, or 2-(N-2-tosylaminophenyl)aldiminopyridine (1 mmol) were used as ligands. 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

A material with nitrogen-coordinated Ru was obtained from a silica-linked 2-(phenylazo)pyridine ligand. Results for cyclobutanol oxidation with 02 and the sacrificial oxidant isobutyraldehyde indicate that one- and two-electron oxidations occur simultaneously. The stability of the catalyst is not always guaranteed, probably because acids may be formed in oxidations of alcohols (284). Leaching problems are also encountered with a polymer-bound Ru Schiff base complex, used in oxidation of benzyl alcohol (285). 

Ru(bpy)3]3+ and [Ru(bpy)3]+ are commonly produced as photoproducts in the photochemistry of [Ru(bpy)3]2+ and, as noted, are potent oxidants and reductants, respectively. Barring a redox reaction, these complexes show good stability towards both substitution and racemization,63 however Ru(III) complex is not indefinitely stable in aqueous solutions. Over time spontaneous reduction of [Ru(bpy)3]3+ to [Ru(bpy)3]2+ is observed with some sacrificial degradation of the bpy ligands... 

Fig. 16. Structures of different types of ligands acting as two-electron redox relays in natural and artificial systems Flavins such as (11) are the essential constituents of flavodoxines and flavoproteins 137). The a-ketoglutarate anion (a-KG, 12) is a typical example of a sacrificial redox mediator which decomposes during catalysis (72). Synthetic chelates such as bis-arylimino-acenaphthene (BIAN, 13) have been proposed for the development of bio-inspired multielectron transfer photosensitizers 138).

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