Polynuclear macrocyclic complexes

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Much attention is currently devoted to the synthesis and properties of shape-persistent macrocycles[l]. Such compounds are interesting for a variety of reasons including formation of columnar stacks potentially capable of performing as nanopores for incorporation into membranes or for the generation of nanowires[2]. Furthermore, in shape-persistent macrocycles incorporating coordination units, enc/o-cyclic metal-ion coordination may be exploited to generate nanowires[3], whereas e.ro-cyclic coordination can be used to construct large arrays of polynuclear metal complexes[4]. Shape-persistent macrocycles with reactive substituents may also be linked to other units to yield multicomponent, hierarchical structures. 

P. Guerrieio, S. Tamburini, P.A. Vigato, From mononuclear to polynuclear macrocyclic or macroacyclic complexes, Coord. Chem. Rev. 139, 17-243, 1995. 

So far our own interest has been devoted to polyazacycloalkanes of general formula (-CH2CH2NH-)j which contain from 7 to 12 nitrogen atoms separated each other by ethylenic chains (see Figure 1). Variations in structural properties of their metal complexes can be studied across the series of [3k]aneNj receptors as a function of the number, k, of nitrogens. As k increases the size of the macrocyclic cavity increases and also the ligand as a whole becomes more flexible. Consequently, both mononuclear and polynuclear metal complexes can be obtained, with a variety of geometrical structures. ... 

Table 10. IR-, TG/DTA- and room temperature conductivity (powder, 1 kbar) data of mononuclear and polynuclear metal complexes with macrocyclic ligands and bisaxially coordinated isocyanides...

Table 13.9 Room temperature electrical conductivity data for mono- and polynuclear cyanometal complexes of macrocyclic ligands [13,14]...

A number of the above complexes may be alternatively considered as macrocyclic ligands or compartmental ligands, but as the emphasis has been primarily in terms of the local copper(I) stereochemistry and the polynuclear nature of the complexes, they have been included above. As there is no crystallographic data on biological copper(I) systems, this section will have to await the further refinement of the structure of Panutirus Interruptus hemocyanin.353... 

Higher order polynuclear complexes are accessible via polytopic macrocyclic monocycles. The tritopic hexaazamacrocycle [27]N603 was recently shown to bind three copper(II) ions, with two triply bridging p-OH groups (28).95... 

PCMU can be classified by the same scheme which is suitable for low-molecular weight metallochelates (LMWM) [9]. Therefore they are subdivided into molecular, intra-complex, macrocyclic and polynuclear types which in turn are grouped depending on the nature of the donor atoms (0,0- N,N- N,0- P,P-chelates, etc.). 

The Cu ion is classified as a soft acid (see Hard Soft Acids and Bases), which predicts reasonably well the types of ligands that will be most stabilizing and are, thus, commonly observed in Cu complexes. The preference of Cu for softer ligands is quite apparent in the homoleptic complexes, for instance the halides discussed above. Polynuclear compounds are quite commonly seen in the chemistry of Cn. Thus, the solid-state structure cannot be reliably predicted from the reaction stoichiometry or from the empirical formula of the resulting compound. The careful selection of ligands, for instance, an appropriate macrocyclic ligand can ensnre the formation of a mononuclear complex if one is desired. 

The hexaaza [ISJaneNe forms complexes with transition metal ions and with certain alkali and alkaline earth and lanthanide ions. For the higher aza macrocycles with seven or more donor atoms, dinuclear complexes become possible. A systematic investigation of both the structural and thermodynamic aspects of copper complexes formed with the larger polyaza macrocycles from heptaaza to dodecaaza has been published. All of the macrocycles were found to form hydroxo species as well as polynuclear complexes. A number of structures have been determined for the higher polyaza macrocycles, both in complexed and noncomplexed forms, and structures range from highly boat shaped to nearly planar. ... 

Coordination OrganometalUc Chemistry Principles Electron Transfer Reactions Theory Macrocyclic Ligands Metal Ion Toxicity Oxides Solid-state Chemistry Polynuclear Organometallic Cluster Complexes Sulfur Organic Polysulfanes Thallium Organometallic Chemistry. 

Polynuclear iron(II) and cobalt(III) oximehydrazonates have arisen from the template macrocyclization of the initial nonmacrocyclic tris-complexes with polydentate ligands resulting from the condensation of the corresponding diketones and their monooximes with hydrazine [193]. The tris-complexes formed have... 

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