Cobalt macrocyclic complexes

For the purposes of this review, it is important to recognize the dual nature of cobalt macrocycle complexes. Both species, LCoinH and LCo1, can add to double and triple bonds forming alkyl and alkenyl cobalt chelates but the products are different. The Co111 hydride reaction occurs in a Markovnikov addition while LCo1 provide anti-Markovnikov products. It is believed that eqs 21 and 22 explain the difference. 

Table 1 Reactions of cobalt ) macrocycle complexes with H2O2, 7=0.1M, [H+] = 0.05-0.1M ...

Electrocatalytic ORR carries out in three pathways the 1-electron transfer pathway, producing superoxide ion the 2-electron transfer pathway, producing hydrogen peroxide and the 4-electron transfer pathway, producing water. In a non-aqueous aprotic solvent system, a room-temperature ionic liquid system, and on specific transition-metal, macrocyclic-compounds-coated graphite electrodes in alkaline solutions, 1-electron reduction can be observed. Carbon materials, quinone and derivatives, mono-nuclear cobalt macrocyclic compounds, and some chalcogenides can only catalyze 2-electron ORR. Noble metal, noble metal alloy materials, iron-macrocyclic complexes, di-nuclear cobalt macrocyclic complexes, some chalcogenides, and transition-metal carbide-promoted Pt catalysts can catalyze 4-electron reduction. 

Some cobalt macrocycle complexes such as rra j -RCo(H20) (5,7,7,12,14,14-hexamethyl-l,4,8,ll-tertraazacyclo-tetradeca-4,ll-diene) may exist as different isomers. The isomerization has been studied, and the kinetic and thermodynamic parameters were investigated. " ... 

Compounds 139 are tris(oximehydrazone) derivatives with an iron(ll) ion in the center of the cavity [230]. Compound 140 (Fig. 38) has been known for 30 years [231, 232] and was prepared from a tris(2-aldoximo-6-pyridyl)phos-phine that is capped by a BF unit to encapsulate cobalt(ll), zinc(ll), nickel(ll), and iron(II). All four macrocyclic complexes were characterized later by a comparative X-ray crystallographic study [233-236]. 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

In comparison to the porphyrins, the corrin nucleus contains one less atom in its innermost ring (that is, it contains a 15-membered ring) and, on coordination, only one NH proton is lost to give the macrocycle a single negative charge. A cobalt corrin complex occurs as part of the structure of vitamin B12. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Substitution in 5- and 4-coordinated cobalt(ll) complexes is associative. The macrocycle Me4cyclam imposes a 5-coordinated structure in the complex Co(Me4cyclam)CH3CN which exchanges with solvent CHjCN by an mechanism (AK = —9.6 cm mol ). 

Macrocyclic Fi-donor ligands and vitamin Bj, analogues. The free amine [(97) tet] can be prepared from the previously reported nickel complex. Cobalt-fin) complexes have been prepared with both planar (bcde octahedral) and folded (abed octahedral) co-ordination. Derivatives of the three ligand configurations arising from restricted inversion at the four chiral co-ordinated secondary amino-groups have been prepared (see Scheme 2) and their stabilities and configurations discussed. ... 

Cyclic silylphosphanes, see Silylphosphanes, phosphorus-rich, cyclic Cyclic sulfur-nitrogen compounds, see Sulfur-nitrogen compounds, cyclic Cyclic trithiolate ligand, 38 8-9 Cyclic voltammetry A. chroococcum Fd 1, 38 130-131 fullerene adducts, 44 19 nickel(ll) macrocyclic complexes, 44 112 Rieske proteins, 47 138, 139 Cyclidenes, as cobalt complex ligands, 44 282-284... 

Table 1 lists some of the binding constants and rate constants measured for the reaction of CO2 with redox-active molecules. Various techniques have been used to measure these constants including cyclic voltammetry, pulsed radiolysis, and bulk electrolysis followed by UV-visible spectral measurements. The binding constants span an enormous range from less than 1 to 10 M [13-17]. Co(I) and Ni(I) macrocyclic complexes have been studied in some detail [13-16]. For the cobalt complexes, the CO2 binding constants K) and second-order rate constants for CO2 binding (kf) are largely determined by the Co(II/I) reduction potentials... 

The second type of reaction of a monocarbonyl compound to yield a macrocyclic product is represented by the condensation of [Ni(en)2]2+ or [Cu(en)2]2+ with formaldehyde in the presence of a suitable nucleophile (Scheme 4).15-16 This reaction is related to the condensations of [Co(en)3]3+ with formaldehyde plus nucleophiles to form clathrochelate compounds (Chapter 21.3), and also to the formation of the Coin[14]aneN4(02) complex (8) by reaction of a bis(ethanediamine)cobalt(III) complex with formaldehyde.17... 

Bis(l,2-diaminoethane)copper(II) perchlorate undergoes reaction with acetone to give the quinquedentate complex (43), which is converted to the macrocyclic complex (44) on addition of base, usually excess 1,2-diaminoethane (Scheme 8).86 Similar cobalt(I3) complexes cannot be prepared by metal template methods. 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Propano-linked macrocyclic complexes (69) have been shown to undergo oxidative dehydrogenation with ease under mild conditions (equation 27).157>162 This reaction occurs for nickel(II), copper(II) and cobalt(II) complexes and leads to cobalt(III) dibenzocorromins , which are simple models of the vitamin B12 coenzyme nucleus.163 Macrocyclic complexes containing this ligand chromophore had already been prepared by a direct metal template method (Scheme 29).164 165 However, the oxidative dehydrogenation route offers greater experimental certainty and variety. 

Fisher and Eisenberg (107) have reported on the electrocatalytic reduction of carbon dioxide using macrocycle complexes of nickel and cobalt (e.g., complex 27). An indirect electrochemical reduction of C02 was ac-... 

Matsuoka used a different photosensitizer, p-terphenyl, with a cobalt(III) cyclam as the catalyst [37-39], In a C02-saturated acetonitrile/methanol solution with either TEOA or TEA as the sacrificial reductant, the quantum efficiencies for CO and formic acid production were 15% and 10%, respectively, under 313 nm illumination. Again, however, the TONs and production rates for macrocyclic complexes were low. 

A typical example is seen in the addition of hydrogen cyanide to an imine to yield a cyanoamine (Fig. 4-29). Many of these reactions have been used to best advantage in the synthesis of macrocyclic ligands and complexes, and as such are considered in Chapter 6. A simple example of such a reaction is seen in the addition of HCN to the cobalt(m) complex indicated in Fig. 4-30. The starting complex is also readily prepared by a metal-directed reaction. 

Figure 6-39. The macrocyclic complex 6.40 is too large for a cobalt(n) ion to bind effectively. The imine bonds are activated towards attack by a methanol molecule. This generates a slightly smaller cavity in the complex 6.41.

It is also possible to make rather more dramatic changes in the structure of the preformed capping group, and 7.11, the cobalt(n) complex of a functionalised triaza macrocyclic ligand, reacts with formaldehyde and nitromethane to give 7.12. 

Other Co(II)-complexes that were applied in the photosensitized reduction of C02 to CO (and concomitant H2-evolution) include Co(II)-ethylene glycol dimethyl ether complexes [178], and different tetraaza-macrocyclic Co(II)-complexes such as 27,28. A closely related system, where Ni(II)-tetraaza macrocycle (29) substitutes the cobalt homogeneous complexes in the photosystem including Ru(bpy) + as photosensitizer and ascorbic acid as electron donor, has been reported by Tinnemans [181] and Calvin [182],... 

Macrocyclic N-Donors. Glick et al." have proposed that the greater difference in Co—X axial bond length between the cobalt(n) and cobalt(m) complexes of (16) compared with the corresponding complexes of (17) accounts for the unusually slow self-exchange rate of the former. The electronic spectra of the five-co-ordinate cobalt(n) complexes of the macrocycles (18) and (19) have been reported.100... 

A novel light-reversible redox system has been discovered381 with glycylglycine it is summarized in Scheme 7. Preparations of mixed cobalt(m) complexes of macrocycles and amino-acids, trans-[Co [ 14]aneN4 (amino-acid)2]3 + and trans-[Co Me4-[14]tetraeneN4](amino-acid)2]3+, have been reported using glycine, S-alanine, S-phenylalanine, and S-leucine.382... 

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