Cobalt derivatives structural parameters

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

ESR spectroscopy is effective for determining such coordination structures for cobalt(II) and iron(III) porphyrins [25]. For example, the ESR parameters gj. - 2.3 and g// = 2.0 Both signals are split into hyperfine lines by the Co nucleus, and several of the parallel signals are further split into equal intensity super-hyperfine lines. The latter are derived from the nitrogen nucleus of the nitrogenous ligand at the fifth coordination site of a cobalt porphyrin. 

Radical cations can be derived from aromatic hydrocarbons or olefins by reaction with one-electron oxidants. Antimony pentachloride and cobaltic ion are among the oxidants that have been used. Most radical cations have limited stability, but the sensitivity of EPR spectral parameters to structure have permitted many conclusions about the nature of radical cations despite their limited stability. 

The kinetics of oxidation of Pseudomonas aeruginosa azurin, bean plastocyanin, and Rhus v. stellacyanin by the tris-cobalt(iii) complexes of phen and three of its derivatives have been reported (Table 15) the reactivity order for Co(phen)3 + as the oxidant (stellacyanin > plastocyanin > azurin) matches that found previously for the Fe(edta) reduction of the proteins (and for which ionic strength and pH effects have now been reported). It is suggested that the activation parameters for electron transfer from reduced plastocyanin and azurin may be accounted for in terms of oxidant-induced protein structural changes which expose active sites that are, by comparison with stellacyanin, inaccessible to reagent attack. Segal and Sykes have extended the work with plastocyanin and Co(phen)3 + to higher concentrations (up to 4.0 X 10 mol of oxidant and have observed a deviation from linearity in the... 

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