CO chemistry

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

These rod-shaped ligands share a sterically efficient terminal N-donor and their divalent Co chemistry is well established. They will be discussed here only with selected examples. [Co (NCMe)6](TFPB)2 (TFPB- = tetrakis(3,5-bis(trifhioromethyl)phenyl)borate)) has been synthesized and characterized in the solid state along with a number of other divalent transition metal analogs.357 As a result of the extremely poor coordinating ability of the anion and facile loss of MeCN ligands from the cation, the salt is an excellent source of naked Co2+ ions. Thermolysis up to 100 °C leads to the loss of one MeCN and formation of a r -bound nitrile, whereas above 130 °C decomposition occurs with loss of MeCN and abstraction of fluoride from the anion to form CoF2. 

Halides are common coligands, partly because Co11 halides feature as a common entry point into Co chemistry. As such, examples are scattered throughout the chapter. 

CO Chemistry, The alcohol/base chemistry observed here led logically to a system including C0/H20/K0H, and accordingly, a series of experiments was performed at 400°C. The COSTEAM Process is similar in nature, but without the purposeful addition of base. Also, the process is applied primarily to lignite, though the COSTEAM chemistry has been applied, less successfully, to bituminous coal also. The results we obtained with the basic system, along with the pertinent citations to earlier work by others, were recently presented (4 )... 

For a more detailed analysis of Co chemistry and B12 in particular, see Frausto da Silva and Williams (1991) and Lippard and Berg (1994), and for a recent review of Bi2-dependent enzymes, see Ludwig and Mathews (1997). 

Another intriguing aspect of dien and 4-Me-dien chemistry is the preparation of a trans spanning Pt" complex involving an eight-membered ring (45). The synthetic strategy used to prepare this unusual situation is outlined in Scheme l.673 The use of 4-Me-dien in Co chemistry also results in the formation of an unusual byproduct (46), as well as the expected Co(4-Me-dien)2+ isomers. 4 605... 

The key of this success lies in the opening of the synthesis in which Roush and Wada utilize the various possibilities offered by the buta-diene-Fe(CO), chemistry for acyclic stereocontrol. They thus impressively demonstrate that the use of transition metal 7r-complexes as synthetic building blocks opens new and powerful strategies for the synthesis of complex target molecules. 

One remarkable group of homogeneous CO reactions 1 that of the conversions with unsaturated hydrocarbons using alkynes. alkcnes or dienes as the adducts, lactones, acids or esters are produced. The two most efficient catalyst metals are rhodium and palladium. This part of CO chemistry is a very fascinating one, because carbon dioxide is fixed in the organic compounds, through the formation of a new carbon-carbon bond. 

CO insertion works well for both alkyl and alkenyl derivatives. The resulting acyls may subsequently be converted to aldehydes, acids or esters as shown in Scheme 10 yields (based on zrR) typically range from 50-99%. Two such applications have appeared in the patent literature. CO insertion proceeds with stereochemical retention, as in all metal systems studied. The Cp analogs show quite different CO chemistry. Although insertion to form an acyl (e.g. 41) is the first step, facile rearrangement to alkenoxides (42) ensues (equations 44 and 45 ). 

Dow. 1972. Toxicity studies of n-butyl oxitol and Dowanol EB glycol ether. Dow Chemical Co., Chemistry and Biology Research, Midland, MI. EPA/OTS No. 86-890001223. 

Another large-scale process involving CO chemistry is the carbonylation of methanol to give acetic acid catalyzed by a Rh complex, which requires a cocatalyst (promoter), CH3I (see 14.6.5) ... 

The greater basicity of bridging CO than terminal CO leads to a strikingly richer Z —CO— chemistry for polynuclear carbonyls than for their mononuclear counterparts. For example, simple neutral carbonyls such as Fe(CO)5 and Cr(CO)6 do not form stable adducts with boron or aluminum halides, whereas neutral polynuclear carbonyls containing bridging CO ligands readily form such adducts. 

Another interesting aspect of n —CO chemistry is the bonding. As will be described in this section, the most common situation is to regard II —CO as a formal four-electron donor. However, some CO ligands that ostensibly have the II —CO geometry are best described as two-electron donors and others as six-electron donors. The SI parameter ranges from approximately 2.2 to 3.3 for n —CO compounds (Fig. 4). 

This section has had to be speculative because there is little solid information on the role of CO as an electron mediator. However there are many indications that the CO ligand may be an effective mediator of redox reactions, and hopefully this aspect of CO chemistry will receive closer scrutiny. 

A. Werner Neuere Anschauungen auf dem Gebiete der anorganischen Chemie , 4th edn., Freidrich Vieweg und Sohn, Brunswick, 1920 Earliest account of classical Co " chemistry... 

The behavior of HO2 and OH in the upper troposphere is dominated by CO chemistry. (Because of its 1 to 3 month lifetime, CO is more or less uniformly mixed up to the tropopause. Above the tropopause, CO falls off with increasing altitude. Because of the much slower vertical transport rate in the stratosphere, the rate of the CO-OH reaction competes with the rate of vertical mixing.) Tropospheric CO oxidation proceeds according to reactions 5.24 and 5.25, coupled to reactions 5.1 to 5.3. (Note that 4.36 and 5.25 are the same reaction.) From Section 5.2 we can obtain an expression for the HO2/OH ratio in the upper troposphere. Based on the steady-state relation for HO2, we obtain... 

Chien, R.-L. and Helmer, J. C., Electroosmotic properties and peak broadening in field-amplified capillary electrophoresis, Ana/yf/co/ Chemistry, 63,1354—1361,1991. 

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