Copper polynuclear species

A number of complexes of copper with 1,1-dithiolenes are known they are interesting, inasmuch as they form (1) polynuclear species, e.g., [Cu4(i-mnt)3]2 . Recently, a copper(III) complex of 1,1-dicarboeth-oxy-2-ethylenedithiolate (DED ) was prepared (375) by oxidation of aqueous solutions of K2[Cu(DED)2] with a 10-15% excess of Cu(II) or H202, and of (BzPh3P)2[Cu(DED)2] with I2. The possibility of this system as a model for the Cu "/Cu. system in n-galactose oxidase has been pointed out. Lewis and Miller (113) also prepared M[Cu(S2C CHN02)2] (M = Cu, or Zn) and Cu[Cu S2C C(CN)2 2], and found that they are effective insecticides. 

Even this diagram does not give a clear impression of the relative proportions of the various copper compounds present in solution. However, provided no polynuclear species are present, it is a relatively simple matter to use the values of x to evaluate these proportions and to plot them as a function of a single variable. Figure 6 shows a diagram of this kind using the same data as figure 5 calculated for pE = 10 and variable chloride activity under the assumption that all compounds have the same activity coefficient. It would not be difficult to allow for different values of activity coefficients if these were known. 

While the structures so far described in Figure 4.1-4.7 involve distinct molecular and polynuclear copper(I) species, they are characterized by the relatively low coordination numbers of two, three and four. Such low coordination numbers are conducive10 to bridging ligand functions and hence to infinite lattices involving chains (or ribbons), sheets and three-dimensional lattices.199 Figure 4.8 summarizes the types of chain structures characteristic of copper(I). 

In studies with amino acids and similar compounds, the ligand is nearly always monomeric. This situation changes greatly in those cases where the ligand can form dimers and higher condensation n-mers, such as phosphate (20), borate (21, 22, 23), and where polynuclear species are possible, such as with the copper (II) ion (24) and the Fe(III)ion (25). To extend these studies and to consider the analyses of other types of systems that reversibly form complexes, the mannitol-borate system which has previously been studied (26, 27), but without taking all forms of the borate ligands into account was examined. 

Some of these ligands have been shown to coordinate to three different copper atoms in polynuclear species, acting both as a chelate and as a bridging ligand.96... 

The formation of copper(II) complexes with terpy has been investigated fairly intensively. The interaction is pH dependent, and numerous hydroxy, aqua, and polynuclear species are present in aqueous solution 94, 245,278). In general, an Eigen-Wilkins mechanism appears to be operative, although the kinetics are complicated by ligand-protonation equilibria 263,390,391). In acidic solution, 1 1 complexes predominate (567). A number of substituted terpyridine ligands have been evaluated as potential colorimetric reagents for copper 400). The adsorption behavior of copper(II)-terpy complexes at silica surfaces has been studied 499). Such complexes are reasonably active as catalysts for the hydrolysis of fluorophosphate esters 456). 

Copper complexes are known in oxidation states ranging from 0 to +4, although the +2 (cupric) and the +1 (cuprous) oxidation states are by far the most common, with the divalent state predominating. Only a relatively small number of Cu complexes have been characterized and the Cu° and oxidation states are extremely rare. A few mixed valence (see Mixed Valence Compounds) polynuclear species have also been isolated examples include a CuVCu species and a Cu /Cu catenane. The coordination numbers and geometries (see Coordination Numbers Geometries) of copper complexes vary with oxidation state. Thus, the majority of the characterized Cu complexes are square planar and diamagnetic, as is common for late transition metals with d electronic configurations. 

These results indicate that the isolated copper species on ZSM-5 have an activity for the decomposition reaction of NO different from that of the dimeric or polynuclear copper species, probably because there are different reaction mechanisms (237). The results obtained with the Cu(ll)ZSM-5 catalyst also suggest that Cu " ions promote the spontaneous low -tempera-ture dehydroxylation of nearby Brpnsted sites or the elimination of lattice oxygen anions which play a vital role in the decomposition of NO. When the dimeric or polynuclear species of Cu are present, the spontaneous elimination of the lattice oxygen bridging the two Cu"+ sites does not occur at low temperatures however, this reaction occurs at high temperatures. The activity for the decomposition of NO is nearly zero at about 573 K, but in the presence of O2 a different reaction mechanism is initiated and this results in the enhancement of NO conversion. Moreover, the presence of stronger Brpnsted sites in ZSM-5 can explain why only the CuZSM-5 catalyst exhibits much higher activity for the reduction of NO in NO-NH3-O2 reaction systems. 

Fig. 2. Copper(II) precursors for oxamato-bridged polynuclear species.

No structurally characterized discrete Cu complexes are known. A few species have been observed in the gas phase, or at very low temperatures using matrix-isolation techniques, including polynuclear copper species such as Cu2 and Cu3 and several copper-carbon monoxide and copper-ethylene species. There have also been reports of reactive species thought to contain zerovalent copper, and several clusters or other species where, arguably, some of the Cu is present in the Cu oxidation state. ... 

The determination of molecular size in solution is a frequent problem for eoordination compounds e.g., in lithium and copper, as well as in transition-metal earbonyl and hydroxo/oxo chemistry, one finds numerous examples of polynuclear species. Increasingly, use is made of NMR diffusion measurements to directly assess molecular volumes in solution. 

There are few data for the first monomeric hydrolysis constant of copper(II). The reviews of Plyasunova et al. (1997) and Powell et al. (2007) both favoured the data of Paulson and Kester (1980) however, these data are not consistent with other low ionic strength data (Arena et al., 1976 Sylva and Davidson, 1979) that were also able to detect the formation of the dominant polynuclear species of cop-per(II). Data are also available for this stability constant at elevated temperatures from Var yash (1985). This latter data are used from 100 to 350 "C, whereas other available data are preferred at lower temperatures. When these data are assessed, it is clear that the stabihty constant derived for 25 °C and zero ionic strength is more positive than that which would be derived from the fixed ionic strength data of Paulson and Kester (1980). Due to these facts, the data of Paulson and Kester (1980) are not retained in this review. Data for the stability constant of CuOH" are listed in Table 11.54. 

This is a very restricted oxidation state of copper but may be considered to occur in the polynuclear copper species Cu2, Cu3 and Cus, which have been characterized by matrix isolation techniques. Copper(O) also occurs in species formed by the reaction of copper metal vapour and carbon monoxide gas. Matrix isolation techniques have characterized a monomeric [Cu(CO)3] trigonal planar species and a dimeric [(CO)3CuCu(CO)3] species.32... 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

The following sections (53.4.4.2-6) attempt to describe the electronic properties of simple mononuclear complexes of the copper(II) ion,47,48 to show how these are related to the different stereochemistries of the copper(II) ion and how these properties are modified by the formation of polynuclear complexes.17,30 Particular emphasis is placed on the appearance of the different types of electronic property and how they may be used to provide qualitative evidence for the different types of copper-copper interactions, and hence for possible polynuclear structure formation, particularly in the solid state. While the main emphasis will be on the electronic properties in the solid state, where X-ray evidence may be obtained for a single magnetic species,10 the measurement of the electronic properties in solution will also be described, although in solution a mixture of complex species may be present in equilibrium and complicate the interpretation of the electronic properties.584,816,817,824... 

At the end of this section on the relationship between the electronic properties and the stereochemistry of complexes of the copper(II) ion, it is worth summarizing the most useful physical techniques which offer a criterion for the presence of a polynuclear copper(II) complex rather than a mononuclear complex. These are (i) magnetic susceptibility measurements down to near absolute zero, for the determination of O or / values (ii) ESR spectra of magnetically dilute systems, in the solid state or in solution, to obtain hyperfine data and (iii) cyclic voltammetry to show evidence for a one-step reduction process in a Cu2 species. 

Recent research reveals that the oxidative polymerization of phenol with 02 also obeys the multielectron transfer with the polynuclear copper complexes [74], The multielectron oxidation of substrates provides new active species,... 

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. ... 

A plot of p([Cu2 ]/[Cu7ot]) 3S a function of pH for three separate titrations fall on a single curve despite up to fivefold differences in measured dissolved copper concentration at a given pH (Figure 2). This behavior of the ratio [Cu2+]/[Cujot] is indicative of the formation of mononuclear hydrolysis species and excludes the possibility that the observed reduction in free cupric ion may have been caused by precipitation of Cu(0H)2 (solid) or the formation of polynuclear complexes. Analysis of data for p[Cu2+], pECujoj] and pH in the pH range 7.7 to 10.8 indicated the presence of two hydrolysis species (CuOH and Cu(0H)2) whose stability constants are given in Table I. Our value of the stability constant for the monohydroxo complex (106.48) falls... 

Copper(i) forms several kinds of polynuclear complex in which four Cu atoms lie at the vertices of a tetrahedron. The earliest to be recognized were the Cu4I4L4 (L = R3P, R3As) species, in which there is a triply bridging I atom on each face of the Cu4 tetrahedron and one ligand, L, is coordinated to a Cu atom at each vertex (25-H-II). The phosphine complexes can be made... 

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