Metal carbonyl derivatives, oxidative

Reactions with dioxygen generally afford the corresponding metal carbonyl derivatives, with loss of organic aldehyde or acid. This reaction can be expressed as an analog of multiple bond metathesis and corresponds to oxidation of vinylidene to CO [46]. Oxidation of OsHCl(=C=CHPh)(L)2 affords the styryl complex 0sC102( -CH=CHPh)(L)2 [243]. 

A variety of unusual cationic metal carbonyl derivatives displaying uncommon oxidation states, geometries, and magnetic properties have been obtained by the controlled oxidation of some metal carbonyl compounds. 

The oxidative coupling of other organometallic derivatives of alkynes has been reported. For example, when metal carbonyl derivatives 30, formed by mixing... 

The reductions of metal oxides and metal chlorides to metal carbonyl derivatives [see equations (e)-(g) of 14.6.1.1] are further examples. 

The preparation of metal carbonyl anions from Lewis bases and certain neutral metal carbonyl derivatives was described above. In general, such reactions involve disproportionation of the zero-valent metal atom in the metal carbonyl to a cation, with coordination of the base to the metal atom, and to the metal carbonyl anion. A different t3rpe of reaction between Lewis bases and metal carbonyls involves displacement of carbonyl groups in the metal carbonyl by the Lewis base without change in the oxidation state of the central metal atom, e.g.,... 

No anionic metal carbonyl derivatives without an inert gas configuration for the central metal atom are known. A possible reason for this is the very low formal oxidation state of the metal atom. 

Information on the carbonyl chemistry of niobium and tantalum is, to date, very meager. The main difficulty appears to be the reduction of the usual pentavalent derivatives of these metals to the very low formal oxidation states of metal carbonyl derivatives. Nevertheless, the yellow anions [M(C0)6] (M = Nb, Ta) have been obtained by a method analogous to, but more difficult than, one of the preparations of the [VCCO) ]" anion. The method involves reduction of the pentachlorides with sodium metal in diglyme in the presence of high pressures of carbon monoxide (63). The niobium and tantalum derivatives are much more air-sensitive than the analogous vanadium derivative. The niobium derivative has not yet been obtained analytically pure (63). No chemistry of the [Nb(CO)J and the [Ta(CO)6] ions has been reported, even conversion to the neutral carbonyl derivatives [M(CO) (M = Nb or Ta = 1 or 2) or to the carbonyl hydride derivatives HM(CO)6 (M = Nb, Ta) still presenting unsolved problems. 

Despite the formally zerovalent state, the homoleptic metal cabonyls are relatively mild donors, consistent with their ionization potentials of 8.5 0.5 V (Table V). [In solution, these complexes are oxidized irreversibly.] Successive introduction of stronger donor ligands (e.g. phosphines, sulfides, and/or isonitriles) results in a decrease in IP and Eox, consistent with increasing electron density on the metal center. [Compare the IP of Mo(CO)6 (8.50 V) [53] with Mo(CO)2 (Tl2-dppe)2 (6.00 V)] [54]. These metal carbonyl derivatives, especially those derived from Cr(CO)6, Mo(CO)6 and Fe(CO)s, form sets of graded electron donors with potentials that can be varied by selection of the number and donor strength of the noncarbonyl ligands [55]. 

Electrocatalytic substitution initiated by oxidation occurs with Group VI metal carbonyl derivatives. Several of the reactions studied, and ones which give little or no product when uncatalyzed, are shown in equations... 

Exobidentate Coordination Dienic, Carbonyl, and Thiocarbonyl Derivatives of Metals in Higher Oxidation States... 

Depolarization data on trimethylphosphine oxide are now available and the relationship between the symmetric and asymmetric POP vibrations has been equated for diphosphates, and some halogen and metal salt derivatives. The polarization of a carbonyl group produced by its conjugation with an ylide causes a large decrease in vco- This shift to lower frequency is increased further when a double bond is interposed, thus increasing the extent of conjugation. -... 

In earlier studies the in vitro transition metal-catalyzed oxidation of proteins and the interaction of proteins with free radicals have been studied. In 1983, Levine [1] showed that the oxidative inactivation of enzymes and the oxidative modification of proteins resulted in the formation of protein carbonyl derivatives. These derivatives easily react with dinitrophenyl-hydrazine (DNPH) to form protein hydrazones, which were used for the detection of protein carbonyl content. Using this method and spin-trapping with PBN, it has been demonstrated [2,3] that protein oxidation and inactivation of glutamine synthetase (a key enzyme in the regulation of amino acid metabolism and the brain L-glutamate and y-aminobutyric acid levels) were sharply enhanced during ischemia- and reperfusion-induced injury in gerbil brain. 

In many instances it is not necessary to isolate the acetonitrile complex or to carry out the reaction in acetonitrile. The use of amine oxide as a means of displacing carbonyl groups in metal carbonyls is well documented, and reaction proceeds smoothly with the carbonyl in the presence of a variety of ligands—e.g., ethylene or pyridine—to yield the monosubstituted derivatives. The advantage of the acetonitrile adducts is the stability of the compounds and the reactivity of the amine oxide toward acidic ligands. 

We wish to report here on a new and highly efficient catalyst composition for the aerobic oxidation of alcohols to carbonyl derivatives (Scheme 1). The catalyst system is based on 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Mg(N03)2 (MNT) and N-Bromosuccinimide (NBS), utilizes ecologically friendly solvents and does not require any transition metal co-catalyst. It has been shown, that the described process represents a highly effective catalytic oxidation protocol that can easily and safely be scaled up and transferred to technical scale. 

Compounds containing niobium or tantalum in negative formal oxidation states -I and -III are mainly metal carbonyl anions. Although these are organometallic derivatives, the report of efficient procedures for the synthesis of [M(CO)6] since the review of Labinger8 merits mention, as it can be anticipated that these highly reduced and reactive species will be important precursors of a large variety of new coordination compounds and metal clusters. 

Oxidation of ris-[W(CO)2(diphos)2] with I2 leads to the unexpected one electron oxidation product [W(CO)2(diphos)2]+. This paramagnetic complex exhibits only one carbonyl stretching frequency, indicating a trans stereochemistry.307 Reaction of [W(CO)4diars] with excess bromine also results in the seven-coordinate complex [W(CO)3(diars)Br2]+, which appears to be a Win derivative.308 It is unusual that three carbonyls are retained by the metal in this oxidation state. 

Disilacyclobutenes, variously substituted, can be readily oxidized, notably with bromine, to yield the isomeric mixture of hindered tetrasilylethylenes through rotation about the carbon-carbon double bond. They also give a variety of derivatives with silylenes, metal carbonyls and alkynes (Schemes 128 to 132) (80AG(E)620, 78JOM(162)C43, 74CC1013, 81IC3456). 

It is possible that nitric oxide may be displaced from the metal centre prior to oxidation of the phosphine. Indeed, such a nitrosyl displacement in the complex CrCl(7t-Cp)(NO)2, leading to derivatives of the type CrCl(jr-Cp)NO(L), has been reported.118 However, such behaviour is rare (compared with CO displacements in metal carbonyls) and it seems likely that the above reduction takes place by an alternative process. 

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