Macrocyclic ligands, electrochemistry

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX2L] (with L = (203), R,R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.551 The redox behaviour of these compounds was of interest because it was considered that the potentially -acceptor macrocycle (203 R = R = Me) may promote the formation of Mn° or Mn1 species or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation it was also of interest to study whether this was the case for the present systems. 

The synthesis, X-ray structure, NMR, and UV-visible spectroscopy, and electrochemistry of a macrocyclic platinum(II) complex containing the tetradentate 1,4,7,10-tetrathiacyclodecane ligand, [12]aneS4 (144) have been reported.350 Related complexes including [Pt([13]aneS4)]2+ and [Pt([16]aneS4)]2+ have also been prepared, and molecular mechanics calculations complemented... 

The 1,4,7-trithiacyclononane ligand, [9]aneS3, zinc complex was synthesized to compare with the electrochemistry of related complexes and showed an irreversible oxidation and an irreversible reduction at +1.30 V and —1.77 V vs. ferrocene/ferrocenium, and the X-ray crystal structure of the bis macrocycle zinc complex was reported.5 0,720... 

Electrochemistry-EPR. The redox potentials of the porphyrazines, 69a, 69b, 70a, and 70b were measured using cyclic voltammetry (Table XX). The redox potentials of the molybdocene appended porphyrazines 70a and 70b showed marked changes from that observed for the parent ligands 69a or 69b suggesting that the peripheral metalation by molybdocene profoundly alters the physiochemical properties of the macrocycle by more than just the sum of the two parts (6). 

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

The electrochemistry of dioxoruthenium(VI) and dioxoosmium(VI) complexes with polypyridyl and macrocyclic tertiary amine ligands has been extensively studied by cyclic voltammetric techniques. In general, cA-dioxo species have higher reduction potentials than the corresponding trans-Aiaxo species. " " For the trans-Aioxo species, the d, orbital ordering... 

The electrochemistry of dioxoosmium(VI) complexes has also been extensively studied. The tra 5-dioxoosmium(VI) complexes of polypyridyl and macrocyclic tertiary amine ligands display very similar proton-coupled electron transfer couples. In aqueous solutions at pH < 5-7 the cyclic voltammograms of n-a i-[0s (0)2(bpy)2] show a remarkable reversible three-electron couple and a one-electron Os coimle. In the Pourbaix diagram two break points are observed in the pH dependence of the Os couple, which correspond to the pAa values of Os —OH2 and Os —(OHXOH2) (Figure 10). The redox reactions are shown in Equations (41)-(43). At pH >8 the 3e Os wave splits into a pH-independent le Os wave and a 2e/2H" Os wave (Equations (44) and (45)). 

There have been two books that contain compilations of the electrochemistry of Os (572,573). There have also been reviews that cover the electrochemistry of certain classes of complexes with ligands such as porphyrins (142), dithiocarbamates (463), and macrocyclic complexes (39, 93). The purpose of this section is not to provide a comprehensive review of electrochemical studies over recent years, but rather to give some insight into the factors that affect the redox potentials and their use in obtaining information about 7r bonding and backbonding. Particular emphasis is placed on the similarities and differences between analogous Os and Ru complexes. 

This section includes selected examples of electrochemistry of gold and silver complexes with other ligands and structural types that were not included in previous sections. We have also included a list of references divided into the following areas that can be consulted for additional information sulfur- and nitrogen-containing macrocycles , porphyrins " and clusters . 

Despite a large variety of known compounds, or perhaps because of it, previous reviews on metalloporphyrin electrochemistry have concentrated for the most part on describing the behavior of simple model compounds with octaethyl-porphyrin (OEP) or tetraphenylporphyrin (TPP) [2-10] macrocycles, and were most often arranged according to a specific element (Fe, for example), group of elements, the Periodic Table of the elements or the nature of the metal-ligand bond examples in the latter case include porphyrins with metal-carbon bonds [3, 4, 11, 12] and those with metal-metal bonds [4, 11, 13]. Unfortunately, these approaches and perspectives are truly useful only if the reader is made aware of how changes in structure... 

The first synthetic porphyrins whose electrochemistry was studied in nonaqueous solvents were largely those with TPP or OEP macrocycles. The electrochemical behavior of the TPP and OEP complexes are generally similar to each other, but the more basic OEP derivatives are almost always easier to oxidize and harder to reduce than the TPP complexes containing the same central metal ion and the same set of axial ligands as shown in Fig. 2(a) for (TPP)Zn and (OEP)Zn. 

A comprehensive review of o-bonded iron porphyrin electrochemistry has recently been published [12], and results on these compounds will not be discussed in the present paper. Several reviews have been published on the redox tuning of iron porphyrins over the last 20 years [2, 7, 10, 192] and this topic will also not be covered in the present paper. The exact potential for the Fe(III)/Fe(II) reaction will depend on the type of axial ligand coordinated to the Fe(III) or Fe(II) forms of the porphyrin. Axial ligands such as NO, C6H5 and 0104 will change drastically the potential at which the Fe(III)/Fe(II) redox couple is observed, but shifts of E /i for this redox reaction will occur upon solvent binding to the Fe(III) and/or Fe(II) form of the compound. The basicity of the porphyrin macrocycle will also influence E ji for the Fe(II)/Fe(III) electrode process. 

The CT-bonded porphyrins of the type (P)M (R) are characterized by two reversible macrocycle-centered reductions for compounds with M = Al, Ga, In, or T1 central ions, but as shown in Sch. 12, the oxidation pathway and stability of the electrogenerated species will depend on both the specific metal ion and the type of a-bonded axial ligand. Further details for the electrochemistry of these compounds... 

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