Ligand cobalt complexes with

It is possible to observe spin-allowed, d d bands in the visible region of the. spectra of low-spin cobalt(lll) complexes because of the small value of 0Dq, (A), which is required to induce spin-pairing in the cobalt(lll) ion. This means that the low-spin configuration occurs in complexes with ligands which do not cause the low -energy charge transfer bands whieh so often dominate the spectra of low-spin complexes. 

The intermolecular Pauson-Khand reaction of the resulting S/P-cobalt complexes with norbornadiene was studied under thermal and A -oxide activation conditions. Thus, heating the diastereomerically pure complex (R = Ph, R = Cy) with ten equivalents of norbornadiene at 50 °C in toluene afforded the corresponding exo-cyclopentenone in a quantitative yield and with an enantio-selectivity of 99% ee. Under similar conditions, the analogous trimethylsilyl complex (R = TMS, R = Cy) afforded the expected product in a high yield but with a lower enantioselectivity of 57% ee. In order to increase this enantio-selectivity, these authors performed this reaction at room temperature in dichloromethane as the solvent and in the presence of NMO, which allowed an enantioselectivity of 97% ee to be reached. These authors assumed that the thermal activation promoted the isomerisation of the S/P ligand leading to a nonstereoselective process. 

A ligand which links the cobalt complexes with those of chromium and iron is the scorpiand diammac in its isomeric trans and cis conformations (see Scheme 5). 

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

Many polarographic studies of the reduction of cobalt(II) to form an amalgam at a dropping mercury electrode have been reported, but most of the work has focused on systems involving complexes with ligands other than water [1, 2]. In one of the few investigations of the behavior of Co(H20)6 + (in aqueous 0.1 M potassium nitrate) [3], the following information was deduced ... 

The final product is ferrocyanide and cobaltic EDTA, but this goes through an intermediate which can be isolated, and which is an adduct of these twro. Dr. Wilkins tried this system out in his rapid flow rate system and found a rate of association which was about right for substitution rates on a cobaltous ion. So this seemed to be a case where perhaps the nitrogen end of a cyanide was able to coordinate into a cobaltous complex, with either concomitant cr subsequent charge transfer. Yet no transfer of ligand occurs in the overall reaction. 

Pyridine ligands 186 are monodentate, forming complexes of the kind presented by 189. Pyridine is coordinated in the same way in copper acetate 194 [314] and iron rhodanide complexes [315], adducts of cobalt complex with fe(salicylidene)ethyle-nediamine 195 [316] and nickel chelate, formed by tridentate N,S-donor azomethine ligand 196 [317] ... 

Dreos R, Siega P (2006) Kinetics and mechanism of metallacyclization in a chloromethyl-cobalt complex with a salen-type ligand. Organomet 25 5180-5183... 

Fig. 43 Possible intramolecular electron transfer processes present in a cobalt complex with two semiquinone anion ligands...

Cobalt complexes with 14-membered tetraazamacrocyclic ligands have been investigated and successfully employed as catalysts for electrochemical and photochemical reduction of C02 and H20.230 One such complex low-spin [Co(I)HMD] + (HMD = 5,7,7,12,14,14,-hexamethyl-l,4,8,ll-tetraazacyclotetradeca-4,ll-diene (hex-amethylcyclam)) reacts with C02 in H20 and in CH3CN to form the N-rac-[CoHMD(C02)]+ species which is sufficiently stable in dry CH3CN and in a C02... 

The chiral iron(III) Lewis acid 3, derived from an oxazoline ligand, catalyzes Diels-Alder reactions of A -acryloyl-l,3-oxazolidinone (1) and cyclopentadiene (2) with good enantiomeric excess30. Nickel complexes of chiral phosphanes also catalyze Diels-Alder reactions albeit with low enantiomeric excess, not exceeding 15% cc31. Much better results are achieved for cobalt complexes with chiral phosphanes in the presence of a Lewis acid31,32. 

Figure 17.1.4 is a display of spectra obtained for the cobalt complex with the Schiff base ligand bis(salicylaldehyde)ethylenediimine (19) ... 

Figure 17.1.4 Spectra of the cobalt complex with the ligand bis(salicylaldehyde)ethylene-diimine, obtained at an OTTLE. Applied potentials (a) -0.900, b) -1.120, (c) —1.140,...

PURPOSE OP EXPERIMENT Prepare an unknown cobalt complex with ammine and/or chloro ligands, and analyze the complex for anionic chloride and for charge to determine its identity. 

Cobalt complexes with diphosphine ligands have shown features consistent with the above proposals. With an equimolar or excess amount of... 

In order to model the extraordinary functions of B12 proteins cobalt complexes with varying equatorial ligands were proposed. Besides cobalt-corrin complexes, cobalt porphyrin models and other eobalt eomplexes were suggested. The comparison between eobalt-corrin and porphyrin complexes is essential to unraveling nature s ehoice of the complex corronoid in coenzyme B12. 

Cobalt complexes with pyridine ligands, for example, catalyzed the oxidation of neat ethylbenzene to acetophenone in 70% conversion and 90% selectivity [35]. Mn porphyrin complex catalyzes the ethylbenzene oxidation with dioxygen to 3 14 mixture of methylphenylcarbinole and acetophenone in the presence of acetaldehyde [36]. The system CUCI2-crown ether in the presence of acetaldehyde is efficient as catalyst of oxidation of ethylbenzene, indane, and tetralin by dioxygen (70°C) into the corresponding alcohols and ketones with high TON [37]. The oxidations were established to occur via a radical pathway and not by a metal-oxo... 

Rates of reduction by U + of a series of cobalt(iii) ammine complexes, with ligands such as to ensure an outer-sphere mechanism (see Table 3), correlate with rates of reduction by [Ru(NH3)6] + according to the equation... 

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