Pendent arms

The presence of N-donors in pendent arms makes Schiff base ligands very effective for the formation of bimetallic complexes with two Ni11 ions. Dinuclear complexes of compartmental Schiff base ligands are covered in Section 6.3.4.12. 

Upon recrystallization, [Ni(tpzlmtacn)]2+ affords [Ni(L)(MeCN)]2+ (L = l,4-bis(pyrazol-l-ylmethyl)-l,4,7-triazacyclononane) via a N-dealkylation reaction and loss of a pendent arm.1420 More rational routes to Ni complexes of tacn ligands with only one or two pendent arms have been developed.1431,1432 In [Ni(L)(X) ]x (e.g., L= l-(3-aminopropyl)-l,4,7-triazacyclononane (n = 2) or l-(l-methylimidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 2) or l,4-bis(l-methyl-imidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 1), the coordination sphere is completed by additional ligands that bind either terminal (X = C1 , H20) or bridge two metal ions (X = N3 , OH, oxalate). The Ni11 complex of l,4-bis(2-pyridylmethyl)-l,4,7-triazacyclononane has been shown to be extremely inert to ligand dissociation in aqueous solution.1433 In (562), the tacn ligand provides a single bidentate arm.1434... 

Unsymmetric compartmental ligands that allow for the controlled synthesis of unsymmetric Ni2 or heterobimetallic NiM complexes have received particular attention.1876,1892 A wide range of such ligands derived particularly from 2-hydroxy-3-hydroxymethyl-5-methylbenzaldehyde and 2-hydroxy-3-hydroxymethyl-bromo-benzaldehyde has now been prepared and used for Ni com-plexation. These ligands have monopodal iminic pendent arms and either mono- or dipodal aminic pendent arms and the terminal donors of the pendent arms can be provided by pyridine, imidazole, and tertiary amino groups.1893-1897 Complexes are usually prepared by reaction of the requisite Ni11 salts with the preformed ligand. 

The binding of phenoxyl radical species to zinc has been observed. The pendent arm macro-cyclic ligand 1-ethyl-4,7-t-bis(3-butyl-5-methoxy-2-hydroxybenzyl)-l,4,7-triazacyclononane... 

Moore and co-workers have also studied the complexation of zinc with other tetraaza macrocycles and pendent arm derivatives. Zinc complexes of 11-(2 -dimethylaminoethyl)-1,4,7-trimethyl-1,4,7,11-tetra-azacyclotetradecane have been characterized.695 A pyridine analog of this... 

Breslow demonstrated the catalytic effect of having not only a Lewis acid zinc center but also an auxiliary catalytic center held in close, but non-binding, proximity. An imidazole or thiophenol (81) demonstrated increase in effectiveness in cyclization of a phosphate derivative.706,707 Pyridine pendents on this macrocycle gave a stability for zinc in line with the Irving Williams series for two pyridine and three pyridine pendents.708 A contrast was noted with other metals (Ni, Cu, Cd, Pb, Fe, and In), all of which increase in stability with three pendent arms as does zinc this was attributed to coordination preferences. 

A number of crown and related ligand systems have been synthesized with pendent arms that contain donors other than nitrogen. Potentiometric titration studies of dihydroxamate derivatives... 

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

Dicompartmental ligands bearing picolyl pendent arms on the amine nitrogen donors were synthesized and the monozinc complex structurally characterized.756... 

In an attempt to establish unequivocally the spectroscopic features of coordinated (vs uncoordinated) phenoxyl radicals a series of phenolato precursor complexes containing a spectroscopically and redox-innocent Ga(III), Sc(III), or Zn(II) central metal ion were synthesized (142-148). In order to avoid metal-ligand bond dissociation in solution, the phenolate or, after one-electron oxidation, phenoxyl moieties were covalently attached to the strongly metal ion binding 1,4,7-triazacy-clononane (149) backbone. Thus a series of phenolate pendent-arm macrocyclic... 

L Ligands containing an 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms. 

R2, R3 Denote the substitution pattern at the phenol (phenoxyl) pendent arm the ortho position is indicated first, then the para position Bu = ferf-butyl, Met = methoxy, Me = methyl, SMe = thiomethyl. 

The first (phenoxyl)zinc(II) complexes have been prepared by using a similar strategy (143-145). Ligands containing a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms form very stable (phenolato)zinc(II) precursor complexes (Table II). Accordingly, their cyclic voltammograms display one, two or three reversible one-electron oxidation waves that in all cases have been shown by spectroelectrochemistry and/or EPR spectroscopy to be ligand based. 

Table IV lists a series of octahedral (phenolato)chromium(III) precursor complexes that contain one or three oxidizable coordinated phenolato pendent arms (146, 154). These complexes display characteristic electrochemistry Each coordinated phenolato ligand can undergo a reversible one-electron oxidation. Thus complexes with one phenolato moiety exhibit in the C V one reversible electron-transfer process, whereas those having three display three closely spaced (AE1/2 250 mV) ligand-centered one-electron transfer processes, Eqs. (7) and (8).

Tolman (144, 202) and Wieghardt (203, 204) and their co-workers used amine macrocycles with a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms (Table VIII). In all cases, square-base pyramidal (phenolate)copper(II) precursor complexes were isolated and in many instances structurally characterized by X-ray crystallography. Depending on the number of coordinated phenolates, these complexes are reversibly one-electron oxidized yielding the (phenoxyl)copper(II) species that were characterized in solution by UV-vis, EPR, and RR spectroscopy. 

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