Publisher Summary This chapter discusses the effect of structural properties of cobalt (III) hexa-amines on their other properties. The chapter discusses complexes ranging from cobalt hexa-ammine, to complexes composed of three secondary and three tertiary amine donors. The chapter discusses only those coordination compounds, having a complete CoN 6 core, thus excluding the enormous number of CoN 6 - x L x complexes. Ammonia and organic amines with an aliphatic skeleton are described in the chapter. The chapter discusses complexes, containing just one kind of ligand—that is, homoleptic complexes. Structural properties and their relationship to kinetic stability (toward hydrolysis and racemization), electronic structure, reduction potentials, and electron transfer processes are explained in this chapter. The standard preparative procedure for the Co(III)–amine complexes consist of the reaction of an aqueous or alcoholic solution of a Co(II) salt with the free base of the amine. Oxidation in the solution is achieved by air, oxygen, H 2 0 2 , or other oxidants. In this chapter, the term molecular mechanics describes a method used frequently to predict structures and relative energies of related complexes (usually isomers). The method involves summing the potential energy of all the interatomic interactions in terms of bond deformation, valence angle deformation, torsion angle deformation, and nonbonded interactions. The chapter discusses the effects of structure on the other properties of the amine complexes predominantly in terms of the steric effects.

Structure, Reactivity, Spectra, and Redox Properties of Cobalt(III) Hexaamines

The four-membered azaheterocycles, azetidines and azetidin-2-ones are thermally stable and easy to handle. Structures of many compounds have been established by X-ray crystallography.

Azetidines react with electrophiles and nucleophiles. The ring opening affords useful amides, alkenes, and amines. Cyclic products such as piperidines, pyrrolidines, and pyrroles have been synthesized from azetidines. Oxidation of 3-hydroxyazetidines forms azetidin-3-ones. Azetidines are synthesized from acyclic precursors like γ-haloamines, γ-aminoalcohols, and β-aminoallenes. Reactions of 1,3-dielectrophiles with primary amines and cycloadditions are also used in the synthesis of azetidines. Reduction of β-lactams affords azetidines. Heterocycles like aziridines, oxetanes, and azabicyclobutanes are transformed into azetidines. Azetidin-2-ones (β-lactams) are precursors of β-amino acids, amides, 3,3-dichloroazetidines, and many other heterocyclic compounds. The syntheses of β-lactams involve cyclizations of β-amino acids, β-amino esters, β-functionalized amides, and cyclocondensations of isocyanates and alkenes, imines and enolates, imines and ketenes, and nitrones and alkynes. The application of chiral auxiliaries and catalysts has made considerable progress in asymmetric synthesis. Applications of microwaves and polymer supports are reported. Although β-lactams are mainly antibacterials, many compounds have been tested and shown promising results as cholesterol absorption inhibitors (CAIs), enzyme inhibitors, anticancer agents, and hypoglycemic agents. A monocyclic azetidin-2-one (ezetimibe) is now in clinical use as CAI.

Azetines and azetes comprise a rare class of constrained azaheterocycles. Only a few stable compounds belonging to these classes are known. Azetines undergo ring opening and cycloaddition reactions. Azetines are synthesized by photochemical cyclization of amides and by partial reduction of azetes. Tri-t-butylazete is a kinetically stabilized compound, which undergoes reduction, cycloaddition, and cycloreversion. It is synthesized by photolysis of pyridazines, and by thermolysis or photolysis of Dewar pyridazines.

Azetidines, Azetines and Azetes: Monocyclic

1.18 – Azetidines, Azetines, and Azetes: Monocyclic

Stereospecific sequential condensation of paraformaldehyde and propionaldehyde with a tripodal bis(triamine)cobalt(III) template rapidly encapsulates the metal, and a subsequent facile reduction of imine functions gives an expanded cavity hexaazabicyclic cage system with unusual structural and chromophore electron properties, complemented by an exceptional stability and relatively fast CoII/III electron exchange rates.

A new template encapsulation strategy for larger cavity metal ion cages

The ability of available molecular mechanics programs to calculate structures and relative energies of metal complexes is examined via a comparative study of five different force fields:  Molmec, Momec91(H), Momec91(C), Xnviron, and Spartan. The method used for assessing the validity of the force fields showed that four of the force fields were able to reproduce successfully the structures of various Co(III) hexaamine cations determined by X-ray analysis, even when these structures were considerably distorted. In certain cases, the calculated relative steric energies were not reliable. Small variations in force fields parameters sometimes led to large changes in the calculated steric energies, and in some instances, in the order of steric strain for different isomers. The most notable changes occurred when metal-dependent parameters were altered.

Critical Evaluation of Metal Complex Molecular Mechanics. Part 1. Cobalt(III) Hexaamines.

Synthesis and properties of novel cobalt(III) and nickel(II) complexes with coordinated azetidine nitrogens: crystal structure of mer-tris(3-(aminomethyl)-3-methylazetidine)cobalt(III) chloride

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Synthesis and properties of novel cobalt(III) and nickel(II) complexes with coordinated azetidine nitrogens: crystal structure of mer-tris(3-(aminomethyl)-3-methylazetidine)cobalt(III) chloride