Supramolecular mononuclear

In conclusion, the electrochemical data offer a fingerprint of the chemical and topological structure of these dendrimers. Furthermore, the knowledge of the electrochemical properties of the mononuclear components and the synthetic control of the supramolecular structure allow the design of dendrimers with predetermined redox patterns. 

Strong intermolecular interactions between active SCO mononuclear building blocks stem from the presence of efficient hydrogen-bonding networks or 7i-7i stacking interactions and have led to abrupt spin transitions [1], sometimes with associated hysteresis [2-4]. Despite the important efforts made by crystal engineers in establishing reliable connections between molecular and supramolecular structures on the basis of intermolecular interactions, the control of such forces is, however, difficult and becomes even more complicated when uncoordinated counter-ions and/or solvent molecules are present in the crystal lattice. 

The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

In this section, I will comment on selected examples of luminescent supramolecular architectures built through Au-Au interactions, both in the solid state and/or, when there is enough evidence, in solution. This topic will be divided into four parts based on the unit that by repetition gives rise to the supramolecular network, that is mononuclear, binuclear, trinuclear and higher nuclearity systems. 

One of the most studied mononuclear systems that usually leads to supramolecular networks and that also exhibits very rich photophysics and photochemistry is the [Au (CN)2] anion. This complex is among the most stable two-coordinate complexes of the transition ions, with a stability constant of 1037 [9], being reasonably stable to air, moisture, temperature and light, which could make it appropriate for practical applications. 

The low rate-enhancement brought about by the dinuclear complex in the reaction of 14 suggests a very modest formation, in the very dilute solution, of the productive intermediate 17 [SrOEt] [SrO2CR], a supramolecular complex composed of one molecule of ditopic ligand 17, two Sr ions, one EtO ion, and one substrate molecule. In contrast, the absence of any difference between mononuclear and dinuclear catalyst in the cleavage of 20 demonstrates that only one metal... 

Electroswitching of structure takes place when a redox change induces a reversible structural or conformational process in a molecule, such as an electrochemically activated intramolecular rearrangement [8.259]. On the supramolecular level it consists of the electroinduced interconversion between two states of different superstructure. A case in point is the reversible interconversion of a double-helical dinuclear Cu(l) complex M2L22+ [8.260] and of a mononuclear Cu(ll) complex ML2+ in a sequential electrochemical-chemical process [8.261] ... 

Thus, in the next examples, we comment on supramolecular entities, including linear chains, two-dimensional sheets, and even three-dimensional networks. From them, perhaps the most common structural arrangement is that in which the metals form extended linear systems, usually built from mononuclear units, dinuclear, using polydentate donor ligands and polymetallic units. It is worth mentioning that in this part and also in the following we consider only structures built by gold-heterometal interactions, not bonds thus formal clusters will not be considered. 

This concept may also be extended to polynuclear helicates [38]. When 2-amino-quinoline and 4-chloroaniline were mixed with the phenanthroline dialdehyde shown in Scheme 1.10, a dynamic library of potential ligands was observed to form. The addition of copper(I) causes this library to collapse, generating only dicopper and tricopper helicates. As in the mononuclear case of Scheme 1.9, the driving force behind this selectivity appeared to be the formation of structures in which all ligand and metal valences are satisfied. The use of supramolecular (coordination) chemistry to drive the covalent reconfiguration of intraligand bonds thus... 

For supramolecular assemblies, intramolecular processes may quench the emission of A to a degree which depends on the relative efficiency of the process when compared with emission. It is often useful to compare the photophysical and chemical behavior of the supramolecular species, e.g. A -L-B, with an appropriate model compound, for instance, AH, which contains the photochemically active component, A, in the absence of any units capable of interacting with A. For example, from luminescence lifetime measurements, the rate of electron transfer may be estimated by comparing the excited-state lifetime of the mononuclear model complex, tModei, with that of the supramolecular species, rsupra, by using the following equation ... 

The compound [Cu (aetabd)(H20)]6[Cr (CN)6] [Cu°(aetabd)] [Cr (CN)6] le (C104)3 (68) represents an interesting example of the supramolecular organization of dinuclear and mononuclear units. The anionic components of this salt are three (C104) anions and six crystaUographically equivalent dinuclear [Cu (aetabd)J [Cr (CN)g] anions [Fig. 13(a)]. The latter species contains Cu(II) centers in a SBP coordination environment. The basal plane of the SP is formed by the tetradentate ligand aetabd and the axial site is occupied by the bridging cyanide. 

The last section describes rmique and high photocatal5rtic activities of the rheniiun(I) diimine carbonyl complexes, especially for CO2 reduction. The photocatalyses of mononuclear rhenium complexes, multicomponent systems, supramolecular systems with a Ru(II) complex as a photosensitizer, and a rhenium complex with periodic mesoporous organosilica as a light-harvesting system. 

The amount of research performed and literature published on electron-transfer reactions of metal-polypyridine complexes is enormous. Several excellent reviews [42, 74, 93-97] and books [38, 62, 98, 99] deal with polypyridine eomplexes, their redox chemistry, photochemistry, and applications. Hereinafter, the most prominent aspects of electron transfer reactivity of mononuclear metal-polypyridine eomplexes will be surveyed without attempting to cover exhaustively the vast original literature. Instead, the main purpose of this chapter is to single out the structural, thermodynamic, and kinetic factors which enable and eontrol the special and diverse electron-transfer behavior of metal-polypyridine complexes in their electronic ground and excited states. Although supramolecular eleetron-transfer chemistry of metal-polypyridines is not discussed here in detail, beeause it is covered in Volume 3 of this monograph, links connecting the redox behavior of mononuclear polypyridine eomplexes and their supramolecular counterparts will be briefly outlined. 

Also other active metal complexes (i.e., several Co SchifF base [232] and Mn diimine complexes [233]) have been supported in this way. The high dispersion of the complexes in the cages of the molecular sieves allows to study the redox properties of mononuclear complexes that are imstable in solution [234]. The increased stability of the obtained materials, the easier handling of heterogeneous catalysts and the high yields achieved make these supramolecular systems a very promising candidate for further catalyst development in fine chemical synthesis. 

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