Square-planar nickel macrocyclic complexes reactions

The reader is referred elsewhere for discussions of substitution reactions with macrocyclic ligands (2-6), and in micelles (36) like the effects of bound ligands, these reactions are of importance in reactions of biological interest. Complex-formation reactions involving a change of covalency, such as reactions of square-planar nickel (ll) complexes with nucleophiles, are also omitted (37). All these reactions offer interesting applications of fast-reaction methods. 

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). 

The square-planar Ni(II) complex of an anionic macrocyclic ligand, 21, was prepared from the template reaction of 3,3 -(ethylenebis(imino-methylidene)bis(2,4-pentanedionato)nickel(II) with 1,3-diamino-2-pro-panol [Eq. (10)]. The uncoordinated -OH group reacted smoothly with acylating agents, resulting in -COPh and -COCH3 (28). 

The impressive sulfur-based reactivity of square planar nickel complexes containing tetradentate N2S2 ligands has been known for many years. Interest has recently resurfaced because of the discovery of similar donor sites in metalloproteins that bind nickel, iron, and cobalt.The iV,iV -bis(mercaptoethyl)-l,5-diazacyclooc-tane ligand H2(BME-D ACO) and its nickel complex have been particularly useful in establishing the scope of S-based reactivity with electrophiles as displayed in the reaction summary shown in Scheme 1." The fundamental features of this reactivity include templated macrocycle production, S-oxygenation as contrasted to oxidation, Lewis acid/base adduct formation, metal-ion capture, and the synthesis of heterodi- and polymetallic complexes. ... 

Macrocyclic N-donor ligands. Nickel complexes of macrocyclic ligands have been studied by cyclic voltammetry, and the irreversible or quasi-reversible couples Ni" L Ni L Ni L have been established. The structure of (124) has been reported and the co-ordination is essentially square-planar with a slight tetrahedral distortion. The reaction of [Ni(pn)3] with... 

Macrocyclic complexes (continued) nickel(II), 44 93-94 eatalysis, 44 119-125 configurational isomerization, 44 126 electrochemical properties, 44 112-113 electronic absorption spectra, 44 108-112 reactions, 44 118-119 square-planar and octahedral species, 44 116-118... 

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

Nickel(I) complexes of N4 macrocycles can be prepared by reduction of the corresponding Ni11 complexes with sodium amalgam. They possess more or less distorted square-planar structures.19 By contrast, the one-electron reduction of Ni porphyrin complexes may result in Ni1 porphyrins or Nin jr-anion radicals, depending on the reaction conditions.20 Complexes of this kind are useful models for the Ni sites in certain metalloenzymes (see below). 

The thermodynamics and kinetics of H+ binding to cobalt(I) and nickel(I) macrocycles have been determined. The pAia of Ni(cyclam)(H), / / 5 5 -NiHTIM(H) + and A-rac-CoHMD(H) + are 1.8, 1.9 and 11.7, respectively [14, 24, 27]. As seen from Table 3, protonation rate constants for A-rac-CoHMD depend on acid strength. The results are consistent with an associative reaction of the square-planar complex with an acid, HA. Whereas the spectrum of 7V-rac-CoHMD(H) + suggests the formation of a [Co (H )] + species with an absorption band at 440 nm (520 M cm ), Ni(cyclam)(H) + shows no significant absorbance in the 300-700 nm region [14, 24]. 

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

In summary, a variety of square planar macrocyclic nickel complexes are capable of catalysis of olefin oxidation using strong terminal oxidants. With knowledge of the reaction mechanism and suitable functionalization of the periphery of the macrocycle, new catalysts might be developed which show high substrate selectivity as well as interesting regio- and stereochemistry. 

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