Low-valent metal complexes

A variety of polysulfido complexes of transition metals has also been synthesized by the reactions of low-valent metal complexes with elemental sulfur. 

This exemplified that the oxidative addition of S-S bond to a low-valent metal complex is one prototype to initiate a reaction using a disulfide. In 1987, Uemura et al. reported an analogous transformation using (PhSe)2 instead of (PhS)2 to afford the phenyl selenobenzoate 58 in up to 78% yield under 100 atm of CO in benzene at 200°C (Eq. 7.44) [50]. 

Before turning to epoxide opening with low valent metal complexes, the reduction of epoxides under Birch conditions [10-13] will be discussed very briefly for historical reasons. The initially formed radical is reduced further to give carbanionic species, that do not display the reactivity of radicals. No C - C bond-forming reactions have initially been reported. 

Scheme 2 Opening of the cydopropylcarbinyl radical and epoxide opening by low valent metal complexes...

Interest in the synthesis and reactivity of coordinatively unsaturated low-valent metal complexes has led to the use of an o-carboranedithiolato ligand in the formation of metalladithiolene ring complexes. Recently, we69 70 and Wrackmeyer et al.1 72 have reported on the synthesis of the 16e cobalt, rhodium, and iridium... 

Ru, Os, and Ir carbene complexes have been prepared from reactions of anionic or low-valent metal complexes with some organic salts or neutral compounds with highly ionic bonds. Oxidative addition of halothiazole and -oxazole species to IrCl(CO)(PMe2Ph)2 affords Ir(III) complexes which on protonation yield cationic carbenes (69), e.g.,... 

Scheme 12.1. Opening of epoxides with low-valent metal complexes.

II. Synthesis and Characterization of Ligand Precursors and Low-Valent Metal Complexes for Small-Molecule Activation... 

Copper(i).—Halides and Cyanides.cation, [Me2N=CH2] , which has been reported previously by Eschenmoser (Symposium on Stereochemistry, Sheffield, 1970) is potentially useful in stabilizing low-valent metal complexes, and its reaction with Cu(CO)Cl has been examined. The results are outlined in Scheme 9. 

These are typically prepared from low concentrations of chemically or photochemically generated low-valent metal complex (Cr2q, L(H20)2Co2 +, or L(H20)Rh2 +) and a large excess of 02 in slightly acidic aqueous solutions according to the chemistry in Eq. (1), where L = N4-macrocycle, (H20)4 or (NH3)4. The rate of formation of the superoxo complexes is mostly limited by the rate of water substitution at the metal centers, except in the case of L(H20)Rh2+ ions, which are pentacoordinate in solution (44). Selected kinetic data are shown in Table I. 

A major breakthrough in the synthesis of transition metal methylene and methylidyne complexes has been achieved by Stone and his group it originates from the simple idea that M=C double bonds in Fischer-type carbenes and M=C triple bonds in carbyne systems should add to low valent metal complexes as do C=C and C=C linkages, respec-... 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

In addition to interacting with the Lewis acid center of the C02 molecule, these same low-valent metal complexes may also interact with the carbon-oxygen 7t-bonds in C02, in much the same way as olefins interact with electron-rich complexes. Finally, the oxygen atoms in C02 may be expected to show weak electron donating ability, possibly coordinating to a very electron-poor metal, although this mode of coordination of C02 is not presently known. 

Facile oxidative addition of o-quinones to low-valent metal complexes containing Pd°, Pt°, Rh1, Ir Ru° and Ru" have been reported561"565 as well as the enthalpy changes associated with such reactions.566... 

Oxidative addition of Cl2CNPh with the low-valent metal complexes Fe(CO)5, [Fe(CO)4]2-, Pt(PPh3)4, and RhCl(PPh3)3 produced Fe(CO)4 CNPh, PtCl2(CNPh)PPh3, and RhCl3(CNPh)(PPh3)2 (107, 108). 

Each of these synthetic methods is limited to low valent metal complexes in each case at most two CF3 groups have been substituted onto a metal ion. In Section III some of the contemporary approaches that have been designed to curtail the enforced reliance upon only two precursors are indicated and some of the early results from these preliminary studies into the chemistry of trifluoromethyl-containing transition metal compounds are presented. 

The concept has been extended to the C-H activation of nitriles, which coordinate strongly with metals. As shown in Scheme 3, coordination of a nitrile to low-valent metal complex (M) would increase both the basicity of the metal complex and the acidity of the C-H bond adjacent to the nitrile, and hence oxidative addition of the metal into the a-C-H bond of the nitrile would occur readily to afford an a-cya-noalkyl metal hydride complex (7), which undergoes isomerization to a N -bonded nitrile complex (8). The reaction of 8 with an C-electrophile forms a carbon-carbon... 

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