Mononuclear model complexes

The model compounds exhibit emissions of weak intensity in the range 420—550 nm. A red shift is observed in the solid state. The presence of aur-ophilic interactions in the solid state was demonstrated by X-ray crystallography (dAu-Au = 3.176 A). A red shift of the emission band is observed when going from the mononuclear model complexes 50 and 51 to polymer 48 (Fig. 23). This indicates that conjugation exist in these materials. 

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 supramolecular complex (34a) bridged by a -CH2CH(OH)CH2- chain with two dmb units as a peripheral ligands on the Ru center exhibited a high photocatalytic ability for CO2 reduction to CO ( co = 0.12, TNco = 170). This supramolecule was a much better photocatalyst than the 1 1 mixed system of mononuclear model complexes [Ru(dmb)3] (35) and /ac-Re(dmb)(CO)3Cl (36) (first example of a supramolecular photocatalyst exhibiting a high catalytic activity for this reaction [54]. 

Mononuclear Ni complexes have been investigated as functional models for individual steps of the reactions mediated by the CODH/acetyl coenzyme A synthase.2018-2020 These are mentioned in the respective sections on mononuclear Ni complexes. The dinuclear type (770) complexes are... 

Table 2. Selected Dinuclear Phosphoesterase Model Complexes with Rate Increases by Metal-Metal Cooperation over Related Mononuclear Species...

Table 76 Mononuclear Model Copper(H) Complexes of Biological Copper Systems... 

Mononuclear Mov complexes with sterically hindered monodentate thiolato ligands are of interest as models for special molybdenum enzymes (the Mo oxidases). Here dimer formation via thiolato bridges and sulfide formation by C—S bond cleavage do not occur. Such a ligand is TIPTH. The complex [Mo(CO)2(TIPT)3) (14), with essentially trigonal prismatic coordination about Mo and trans CO groups in the axial sites, can be obtained.91... 

Mononuclear platinum complexes often have been used as models for catalytic intermediates since systematic studies of synthesis, reactivity, and mechanism are often convenient and because metallic platinum is a very important catalyst. However, using binuclear or polynuclear platinum complexes as models for proposed intermediates in heterogeneous catalysis has not been studied, probably because planned routes to such complexes have not been available. This chapter describes our first studies in this area. 

The possibility of formation of an O—O bond at a single Mn site of the OEC, on the other hand, seems remote, but nevertheless, two mononuclear model Mn-peroxo complexes, a peroxo-Mnm porphinato 64 [157] and a peroxo-Mn111 pyrazolylborato complex 65 [158], have been structurally characterized. Both complexes contain side-on peroxo groups. Complex 65 has been crystallized in two forms, one brown and one blue. A peroxo-H—N (pyrazole) hydrogen bond is found only in the blue form (Figure 29). 

Spontaneously adsorbed monolayers of the dimeric complex (Figure 5.11) [(pOp) Os(bpy)2 (4-tet) Os(bpy)2 Cl]3+, where pOp is 4,4 -bipyridyl, bpy is 2,2/-bipyridyl and 4-tet is 3,6-bis(4-pyridyl)-l,2,4,5-tetrazine, have been assembled on platinum microelectrodes in an attempt to address these issues [33]. Significantly, as illustrated in Figure 5.11, the voltammetric response associated with the Osn/m reaction is unusually ideal for both metal centers. Studies using mononuclear model compounds reveal that the redox responses centered at approximately 0.620 and 0.300 V correspond to the inner [(pOp) Os(bpy)2 (4-tet)]2+ and outer [(4-tet) Os(bpy)2 Cl]+ moieties, respectively. The observation of two well-defined voltammetric waves indicates that electron transfer can occur across the [(pOp) Os(bpy)2 (4-tet)]2+ bridge to the outer [Os(bpy)2 Cl]+ moiety, i.e. charge trapping does not occur. 

In this chapter, we discuss mostly the bonding in mononuclear homoleptic complexes ML using two simple models. The first, called crystal field theory (CFT), assumes that the bonding is ionic i.e., it treats the interaction between the metal ion (or atom) and ligands to be purely electrostatic. In contrast, the second model, namely the molecular orbital theory, assumes the bonding to be covalent. A comparison between these models will be made. 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

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