Complexes with Organic Substrates

In this chapter, unlike the treatment for inorganic main-group substrates, data are sub-divided into metal-ion oxidants or reductants. In this way the format of previous Reports is maintained. A short section has also been added on systems where intramolecular redox has been shown to take place. 

The hydrolysis of manganese(iii) has been re-examined and equilibrium data have been derived for the reactions 

Further oxidation to the (iv)/(iv) complex is also observed. The latter ion, [MnaOa(phen)4] +, is reduced by a variety of substrates including OH, Cl , and catechols. Addition of one equivalent of OH or Cl yields [Mn202(phen)J + while excess of these reagents induces breakdown of the //-dioxo-bridge. Reaction with catechols results in the formation of quinones and manganese(ii) complexes [equation (1)]. 

In the interaction of manganese(iii) pyrophosphate with cydta, two processes are observed. Slow substitution by cydta results in an inner-sphere redox, while a more rapid outer-sphere oxidation of the ligand by [MnCHgP207)3] also takes place. 

Evidence has been provided for an even-electron process in the oxidation of maleatopenta-amminecobalt(iii) by Mn04. The reaction is rapid in acid media up to a mole ratio of 1 1, but with in excess it takes place more 

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TTie introduction of nitrogen-containing functions (e.g., NMe) into the crown ether constitution has a profound effect (73) on both the structures and strengths of molecular complexes with organic substrates. 

Fig. 4 Interaction of low-valent metal complexes with organic substrates...

The existence of numerous oxygenases that catalyze the direct oxygenation of organic substrates continues to stimulate the search for atom transfer oxidations of hydrocarbons by simple metal-dioxygen complexes. (For a further discussion of reactions of metal-dioxygen complexes with organic substrates via heterolytic pathways, see Section III.C). 

The ease of interaction of Ni(0) complexes with organic substrates has been shown to depend upon both the ligands on nickel and the solvent. The presence of a,a-bipyridyl with the Ni(0) complex and the alkyne led to the isolation of a nickelacyclopropene, an observation in accord with the recently proposed metallocyclic pathway for the Ni(0)-cata-lyzed trimerization of alkynes. Allylic and benzylic ethers and epoxides have been observed to undergo oxidative insertion of Ni(0) into their C-0 bonds with solvent (TMEDA > THE > Et O > CeHe) and ligand (EtsP > PhsP a,a-bipy > COD) effects consistent with an electron-transfer attack by Ni(0). With such sulfur heterocycles as dibenzothiophene, phenoxathiin, phenothiazine, and thian-threne, a 1 1 admixture of (COD)2Ni with a,a-bipyridyl gave as the principal product the desulfurized, ring-contracted cyclic product. 

Metal Complexes with Organic Substrates [(NH3),Co—OCOCH=CHCOjH] + MnVH... 

In addition to complex-formation, the interaction of transition-metal atoms with organic substrates at low temperatures can result in rearrangement of the organic moiety without complexation. Two such reactions have already been briefly mentioned, namely, the polymerization of hexafluoro-2-butyne by Ge and Sn atoms (72) and the polymerization of styrene by Cr atoms (i 1). In this section we shall briefly summarize some of these transition-metal-atom-promoted, organic rearrangements. 

Basically, there are three ways to tune enzyme enantioselectivity by means of additives (i) the additives are placed in the reaction medium together with the organic solvent, the enzyme, and the reagents (ii) the additives are co-lyophilized with the biocatalyst before use in the organic solvent (iii) the additives are complexed with the substrates before their transformation in the organic medium. 

The transition metal nature is not essential for this redox reaction. However, one of the reaction products, namely, the anion-radical 8O4 , can be complexed by a transition metal in a higher oxidation state. This leads to some stabilization of 8O4 and increases its reactive concentration. In other words, further reactions with organic substrates are facilitated (Fristad and Peterson 1984). 

The reactivities of [Ru "(0)(14-TMC)(X)]"+ and its related 15-TMC, 16-TMC, and CRMes coi lexes with organic substrates have also been examined. " " In contrast to polypyridyl Ru =0 species, these macrocyclic Ru =0 complexes are weak oxidants. They oxidize benzyl alcohol to benzaldehyde but do not react with alkenes at room temperature. The lower oxidizing ability of these systems than the polypyridyl systems is due to their lower values. However, [Ru (0)(H20)(N202)](C104)2, which has a higher H value, is able to catalyze the oxidation of norbornylene, styrene, and cyclooctene by PhlO. " ... 

The facile activation of dioxygen by these simple organometalHc complexes generates high-valent (V(V), Cr(V)) metal 0x0 complexes, which may undergo oxygen atom transfer reaction with organic substrates, and thus serve as catalysts for aerobic oxidations (Sect. 3.3). 

In contrast to the biological CO2 fixation during the dark reaction of photosynthesis where very low concentrations of CO2 from air can be fixed at room temperature, most reactions with transition-metal complexes or with organic substrates either require high partial pressure of CO2 or high temperatures. An exception was published quite recently, the rapid fixation of CO2 and O2 from air at a palladium(O) complex 10 (Scheme 11) [77]. 

Complexes with Metal-carbon a-Bonds Formed in Redox Processes Between Transition Metal Complexes and Organic Substrates... 

The identity and distribution of the products varies with organic substrates and with the ratio of substrate nickel complex employed. 

Guests with polar N—H bonds include amides, ureas and related sulfur compounds, substituted hydrazines, and aromatic amines.21-264,265 Complexes with these substrates exhibit a wide variety of stoichiometric and nonstoichiometric associations. A particularly popular association is the 1 2 guest host relationship as typified by the structure of the benzenesulfonamide complex with [18]crown-6 (84).266-267 The weak complexes formed between crown ethers and cryptands with certain proteins allow for solubilizing these species in organic solvents.268... 

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