Nickel complexes tetraaza macrocycles

Tetraaza macrocycles nickel complexes, 5, 5 synthesis, 2, 903 Tetraazaporphyrin, octaphenyl-metallation, 2, 858 Tetraazaporphyrins synthesis, 2, 857... 

Tetraaza macrocycles nickel complexes, S Tetragonality copper(II) complexes, 603 Theophylline cadmium complexes, 957 Thermolysin zinc, 1006 Thiabendazole metal complexes, 951 Tollen s reagent, 780 Transcription DNA polymerases, 1007 Trans effect... 

Other complexes with tetraaza macrocycles have been prepared by reaction of [Au(en)2]Cl3, ethylenediamine, or nitroethane and formaldehyde, although with nitroethane an acyclic ligand was also obtained (293).1715,1716 A gold(III) complex with a hexaaza macrocycle (1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) has been obtained by a transmetallation reaction from the nickel compound [NiL]2+ by reaction with [AuC14], 1717 The chemistry of tetraazamacrocycles in aqueous solution has been reported.1718... 

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

M(iii) and Cn(m) complexes. In early classic studies the redox chemistry of tetraaza macrocyclic complexes of Ni(n) and Cu(n) (of the Curtis and reduced Curtis type) was investigated in acetonitrile (Olson Vasilevskis, 1969 1971). These authors were the first to report the electrochemical generation of Ni(m) and Cu(m) complexes of such N4-cyclic ligands. Since this time, a considerable number of related studies, involving both nickel and copper macrocyclic species, have been reported. 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

In recent years tetraaza macrocycles have been found to stabilize both Ni1 and NiIn oxidation states in nickel complexes. 

The tetrathia macrocycles containing up to 13-membered chelate rings are too small to encircle the nickel(II) atom in a square-planar chelation like tetraaza macrocycles do, and give rise to dinuclear complexes. In contrast, a planar chelation was found with the 14-membered macrocycle l,4,8,ll-[S4]-14-ane.1967... 

The field of nickel complexes with macrocydic ligands is enormous and continuous interest in this area in recent years has resulted in innumerable publications. A number of books and review articles are also available covering the general argument of the bonding capability of the various macrocydic ligands towards transition and non-transition metals. 22 2627 Synthetic procedures for metal complexes with some tetraaza macrocycles have been reported.2628 Kinetics and mechanism of substitution reactions of six-coordinate macrocydic complexes have also been reviewed.2629... 

A summary of nickel(H) complexes formed by representative saturated polyaza macrocycles is reported in Table 103, together with a concise description of the synthetic procedures and some of their physicochemical properties. Most studies are concerned with nickel(II) complexes with tetraaza macrocycles, but examples of complexes with triaza and pentaaza macrocycles are not rare. 

In contrast to nickel(II) complexes with saturated tetraaza macrocycles, which exhibit a variety of coordination numbers and geometries, the ligands being in either a planar or a folded coordination, the majority of the complexes formed by unsaturated tetraaza macrocycles are square planar. Few six-coordinate complexes have been prepared with 14- and 16-membered macrocycles. It is expected that imino groups present in the chelate rings reduce the possibility of the folded coordination of the macrocycles and the conformational possibilities of the chelate rings. 

Nickel(II) complexes with a variety of tetraaza macrocycles have been found to undergo facile one-electron redox reactions. Such reactions have been accomplished by means of both chemical and electrochemical procedures. The kinetic inertness and thermodynamic stability of the tetraaza macrocyclic complexes of nickel(II) make them particularly suitable systems for the study of redox processes. A very extensive summary of the potentials for the redox reactions of nickel(II) complexes with a variety of macrocycles is given in ref. 2622. 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

Macrocyclic ligands, mainly tetraaza macrocycles, have been widely used in stabilizing nickel(III). No nickel(IV) species has been isolated, although many nickel(III) complexes with dianionic macrocyclic ligands undergo one-electron oxidation. In this section we will use the nomenclature of Section 50.5.9 to indicate the macrocycles. 

An extensive study of the redox properties of tetraaza macrocyclic complexes of nickel has been performed by Busch and co-workers.3056,3133,3134 Electrochemical data for selected Nim/Nin couples are reported in Table 117. From an analysis of the EPR spectra it has been found that acetonitrile, as well as other molecules or ions like Cl and S04, can coordinate in axial position to give six-coordinate complexes.3056,3141 The g values are indicative of a dj... 

Most structural characterization of these complexes has been carried out by EPR methods (64, 73- 76) and, with a few notable exceptions, the nickel(III) species show the expected elongated tetragonal geometry with axial coordination of solvent or counterions confirmed by hyper-fine interactions (Table I). The smallest member of the tetraaza macrocycle series, [12]aneN4, is unusual in that the ligand folds to give cis coordination because the metal ion is too large to allow a planar... 

Like other tetraaza metallo(I) complexes, the nickel(I) macrocyclic ions are powerful and labile reducing agents. A point of some interest in these systems is to design a complex couple for which the nickel(I) state is accessible at reasonable potentials. Provided the tetraaza macrocyclic ligand maintains close to planar microsymmetry, reorganizational barriers for a low-spin d8-d9 system might be expected to be small (194). 

A great variety of aza macrocycle complexes have been formed by condensation reactions in the presence of a metal ion, often termed template reactions . The majority of such reactions have inline formation as the ring-closing step. Fourteen- and, to a lesser extent, sixteen-membered tetraaza macrocycles predominate, and nickel(II) and copper(II) are the most widely active metal ions. Only a selection of the more general types of reaction can be described here, and some closely related, but non metal-ion-promoted, reactions will be included for convenience. The reactions are classified according to the nature of the carbonyl and amine reactants. 

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

Figure 6-18. The condensation of the [Ni(en)3]2+ salts with acetone yields nickel(n) complexes of a new tetraaza macrocyclic ligand.

The formation of macrocyclic ligands by template reactions frequently involves the reaction of two difunctionalised precursors, and we have tacitly assumed that they react in a 1 1 stoichiometry to form cyclic products, or other stoichiometries to yield polymeric open-chain products. This is certainly the case in the reactions that we have presented in Figs 6-8, 6-9, 6-10, 6-12 and 6-13. However, it is also possible for the difunctionalised species to react in other stoichiometries to yield discrete cyclic products, and it is not necessary to limit the cyclisation to the formal reaction of just one or two components. This is represented schematically in Fig. 6-19 and we have already observed chemical examples in Figs 6-4, 6-11 and 6-18. We have already noted the condensation of two molecules of 1,2-diaminoethane with four molecules of acetone in the presence of nickel(n) to give a tetraaza-macrocycle. Why does this particular combination of reagents work Again, why are cyclic products obtained in relatively good yield from these multi-component reactions, rather than the (perhaps) expected acyclic complexes We will try to answer these questions shortly. 

Fujiwara, M., Nakajima, Y., Matsushita, T., and Shono, T. (1985) Preparation and characterization of copper(II) and nickel(II) complexes with novel tetraaza-macrocyclic Schiff bases, Polyhedron, 4(9), 1589-1594. 

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