Macrocyclic coordination chemistry

Abstract

This chapter reviews the literature published during 2006 on macrocyclic coordination chemistry. Porphyrin ligands and supramolecular chemistry are not covered in this review as there is insufficient space and adequate coverage elsewhere, although a few examples may be included where particularly relevant. In general, the focus is on the coordination properties and reactivity of the compounds, although some applications will also be discussed, particularly in a brief section at the end of the chapter. Some representations of X-ray structures have been included to show particularly interesting coordination geometries, ion encapsulation or ligand orientation. HighlightsHighlight achievements for 2006 include characterisation of some unusual species of biological relevance that have been stabilised using the unique coordination properties of certain macrocyclic ligands, including a study of non-haem oxo-and non-oxo iron(IV) complexes and isolation of an 'end-on' nickel(II) superoxide complex. 1,2 The conversion of nitrite to nitrate is also of biological importance; a novel ruthenium(VI) oxo macrocyclic species was shown to react with nitrite to produce a coordinated nitrate anion, using the macrocycle as a stable chelator resistant to oxidative degradation. 3 Some interesting examples of solid state materials were reported involving simple macrocyclic species including a pressure sensitive compound where the coordination geometry around a palladium(II) ion was influenced by pressure due to the orientation of macrocyclic donor atoms, and a robust redox active coordination polymer built around nickel(II) cyclam units. 4,5

trans-Disubstituted diamido/diamine cyclam zirconium complexes

“…Despite the level of sophistication ligand design has reached, and though various combinations of hard and soft donors have been employed, early transition metal chemistry of saturated azamacrocycles still remains relatively unexplored. This type of ligand set is attractive to transition metal chemistry due to (i) the enhanced stability that a cyclic array presents, (ii) the flexibility to adopt different conformations and (iii) a macrocycle cavity size that forces the metal centres to sit above the plane of the donors leaving two adjacent coordination positions available at the metal coordination sphere [3]. In addition, and contrasting with azamacrocycles such as porphyrins or tetraazannulenes, saturated macrocycles are less prone to undesired secondary reactions [4].…”

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

“…Macrocyclic ligands are excellent hosts for transition metal ions, as evident from numerous examples in biology [3, 4] as well as synthetic coordination chemistry [113, 114]. Initial efforts to prepare a dinucleating macrocycle led to synthesis of a bis(terphenylcarboxylate) compound linked by 1,3-bis(aminomethyl)-4,6-diisopropylbenzene (MAr Tol CO 2 2− , Table 1) [115].…”

Synthesis and Characterization of Phosphorus-Containing Isocyclam Macrocycles and Their Nickel Complexes

“…Macrocycles are molecules with a ring structure of at least 12 atoms that exhibit increasingly strong structural preorganization as the ring size increases while at the same time retaining a high flexibility. , Although naturally occurring macrocycles with more than 50 ring members are known, most natural macrocycles have a size of 14-, 16-, or 18-membered rings . Due to their drug-like properties, their good solubility, lipophilicity, metabolic stability, and bioavailability, sophisticated macrocycles were shown to possess high therapeutic potential. ,, Along this line, multidentate macrocycles are predominantly used as chelating ligands in transition metal complexes, which in turn find main application in the fields of biomedical imaging, biomimetics, and catalysis. , Here, in particular, tetradentate N-macrocyclic systems, with the 14-membered ring cyclam 1 (=1,4,8,11-tetraazacyclotetradecane) as one of the most common representatives, proved to be versatile applicable (Figure ). − While the nickel complex [Ni­(cyclam)] 2+ 2 with the tetraaza ligand 1 is known as an efficient and selective electrocatalyst for the reduction of CO 2 to CO, the iron complex [Fe­(TMC)] 2+ 3 (TMC = 1,4,8,11-tetramethylcyclam) comprising a fully methylated version of the macrocycle 1 is known for the activation of O 2 (Figure ).…”

“…Due to their drug-like properties, their good solubility, lipophilicity, metabolic stability, and bioavailability, sophisticated macrocycles were shown to possess high therapeutic potential. ,, Along this line, multidentate macrocycles are predominantly used as chelating ligands in transition metal complexes, which in turn find main application in the fields of biomedical imaging, biomimetics, and catalysis. , Here, in particular, tetradentate N-macrocyclic systems, with the 14-membered ring cyclam 1 (=1,4,8,11-tetraazacyclotetradecane) as one of the most common representatives, proved to be versatile applicable (Figure ). − While the nickel complex [Ni­(cyclam)] 2+ 2 with the tetraaza ligand 1 is known as an efficient and selective electrocatalyst for the reduction of CO 2 to CO, the iron complex [Fe­(TMC)] 2+ 3 (TMC = 1,4,8,11-tetramethylcyclam) comprising a fully methylated version of the macrocycle 1 is known for the activation of O 2 (Figure ). , In these systems, the macrocyclic ligand is the crucial component as it stabilizes the unusual intermediate Ni I and Fe IV oxidation states. − …”

“… − While the nickel complex [Ni­(cyclam)] 2+ 2 with the tetraaza ligand 1 is known as an efficient and selective electrocatalyst for the reduction of CO 2 to CO, the iron complex [Fe­(TMC)] 2+ 3 (TMC = 1,4,8,11-tetramethylcyclam) comprising a fully methylated version of the macrocycle 1 is known for the activation of O 2 (Figure ). , In these systems, the macrocyclic ligand is the crucial component as it stabilizes the unusual intermediate Ni I and Fe IV oxidation states. − …”