Cobalt, atomic

Introduction of the cobalt atom into the corrin ring is preceeded by conversion of hydrogenobyrinic acid to the diamide (34). The resultant cobalt(II) complex (35) is reduced to the cobalt(I) complex (36) prior to adenosylation to adenosylcobyrinic acid i7,i -diamide (37). Four of the six remaining carboxyhc acids are converted to primary amides (adenosylcobyric acid) (38) and the other amidated with (R)-l-amino-2-propanol to provide adenosylcobinamide (39). Completion of the nucleotide loop involves conversion to the monophosphate followed by reaction with guanosyl triphosphate to give diphosphate (40). Reaction with a-ribazole 5 -phosphate, derived biosyntheticaHy in several steps from riboflavin, and dephosphorylation completes the synthesis. 

In anaerobic (or more correcdy, almost anaerobic or microaerophilic) bacteria such as Propionibacterium shermanii a fundamental difference occurs in that the cobalt atom is introduced at a much earlier stage, possibly to precorrin-2 or precorrin-3 A. 

FIGURE 18.28 The structure of cyanocobalamin (top) and simplified structures showing several coenzyme forms of vitamin Bi2- The Co—C bond of 5 -deoxyadenosylcobalamin is predominantly covalent (note the short bond length of 0.205 nm) but with some ionic character. Note that the convention of writing the cobalt atom as Co" " attributes the electrons of the Co—C and Co—N bonds to carbon and nitrogen, respectively. 

This could account for the paramagnetism, but esr evidence shows that the 2 cobalt atoms are actually equivalent, and X-ray evidence shows the central Co-O-O-Co group to be planar with an 0-0 distance of l3l pm, which is very close to the 128 pm of the superoxide, 02, ion. A more satisfactory formulation therefore is that of 2 Co atoms joined by a superoxide bridge. Molecular orbital theory predicts that the unpaired electron is situated in a rr orbital extending over all 4 atoms. If this is the case, then the jr orbital is evidently concentrated very largely on the bridging oxygen atoms. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

The metal-vapor technique was applied to cobalt atoms and r-BuC = P (01JOM(635)212). The mixture of products that resulted includes the mixed-ligand sandwiches 170 and 171. Further interaction of complex 170 with [W(C0)5(THF)] leads to the coordination of the W(CO)5-group via the phosphorus heteroatom of the four-membered ring to yield 172. 

Vitamin B12 is a coordination compound with cobalt as its central atom. It contains 4.4% cobalt by mass and has a molar mass of 1.3 X 103 g/mol. How many cobalt atoms are in a molecule of vitamin B12 ... 

Scheme 1 outlines the retrosynthetic analysis of the Woodward-Eschenmoser A-B variant of the vitamin B12 (1) synthesis. The analysis begins with cobyric acid (4) because it was demonstrated in 1960 that this compound can be smoothly converted to vitamin B12.5 In two exploratory corrin model syntheses to both approaches to the synthesis of cobyric acid,6 the ability of secocorrinoid structures (e. g. 5) to bind metal atoms was found to be central to the success of the macrocyclization reaction to give intact corrinoid structures. In the Woodward-Eschenmoser synthesis of cobyric acid, the cobalt atom situated in the center of intermediate 5 organizes the structure of the secocorrin, and promotes the cyclization... 

For this calculation, let us assume a simple case in which a suitable monochromatic beam of intensity /x falls perpendicularly upon unit area (1 square centimeter) of a monolayer of cobalt atoms resting upon a transparent substrate. The first term in Equation 4-9, A I, is given by... 

To conclude Chapter 6, let us connect it with Chapter 4 through a simple calculation involving the intensity of cobalt Ka as measured by counting 1 square centimeter over a 1-second interval. The measured intensity for massive cobalt corresponds to about 500,000 counts. By means of Equations 6-7 and 6-9, we calculate from this number a value of 23 counts for AI (Equation 6-9) for a monolayer of cobalt atoms. The measured value from Table 4-4 is 15 counts. The good agreement shows that absorption effects may be calculated with confidence. 

The potential usefulness of x-ray emission spectrography for trace analysis is implicit in the results of approximate calculations presented in Chapter 4. Thus, it was estimated that the intensity of cobalt Ka generated under practicable conditions in a monolayer (area, 1 sq cm) of cobalt atoms might give 133 counts per second (4.16). Such a sample weighs 0.2 pg. 

The rate of isotopic exchange in the solid state, between cobalt in the cation and in the anion of [60Co(H2O)6] [Co(edta)]2 4 H20, was increased [1144] by irradiation (100 Mrad) of the reactant. It was concluded that exchange occurred via vacancies, rather than through motion of a ring of cobalt atoms, one from a cationic site and the other from a neighbouring anionic site. 

New developments in the field of vitamin B12 reactions of the cobalt atom in corrins and vitamin B12 model compounds. G. N. Schrauzer, Angew. Chem., Int. Ed. Engl., 1976,15, 417-426 (67). 

In cobalamin, vitamin Bu, one of the six ligands forming an octahedral structure around a cobalt atom is an organic molecule attached through a carbon-cobalt bond (red). The bond is weak and easily broken. 

Derivation of the Structure.—The observed intensities reported by Ludi et al. for the silver salt have been converted to / -values by dividing by the multiplicity of the form or pair of forms and the Lorentz and polarization factors (Table 1). With these / -values we have calculated the section z = 0 of the Patterson function. Maxima are found at the positions y2 0, 0 1/2, and 1/21/2. These maxima represent the silver-silver vectors, and require that silver atoms lie at or near the positions l/2 0 2,0 y2 z, V2 V2 z. The section z = l/2 of the Patterson function also shows pronounced maxima at l/2 0,0 y2, and y2 x/2, with no maximum in the neighborhood of y6 ys. These maxima are to be attributed to the silver-cobalt vectors, and they require that the cobalt atom lie at the position 0 0 0, if z for the silver atoms is assigned the value /. Thus the Patterson section for z = /2 eliminates the structure proposed by Ludi et al. 

Ozin, Hanlan, and Power, using optical spectroscopy (49,121). In view of the marked temperature-effect observed for the cobalt system, we shall focus on this cluster system here. Evidence for cobalt-atom aggregation at the few-atom extreme first came from a comparison of the optical data for Co Ar — 1 10 mixtures recorded at 4.2 and 12 K (see Fig. 4). A differential of roughly 8 K in this cryogenic-temperature regime was sufficient to cause the dramatic appearance of an entirely new set of optical absorptions in the regions 320-340 and 270-280 nm (see Fig. 4). Matrix variation, from Ar, to Kr, to Xe, helped clarify atom-cluster, band-overlap problems (see Fig. 5). 

Cobalt atom reactions with ethylene were also studied (121). By using techniques similar to those described for Cu (122) and Ni (101), it has proved possible to synthesize a novel series of mononuclear and binuclear cobalt-ethylene complexes, Co(C2H4) , = 1, or 2, and... 

Fig, 37. Far-infrared spectra of chromium, iron, and cobalt atom reactions with benzene, benzene-(f and benzene/benzene-d mixtures in argon matrices at 10-12K 171). 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

Fig, 43. Reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms. Open circles, BH units shaded circles, C-CHj units and dark circles, C-H units. One cyclopentadienyl ring has been omitted on compound III, for clarity (J73). 

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