Binuclear Systems

Digestive Ripening of Binuclear Systems, Such As CdSe... 

Besides (166), other binucleating cryptands have been synthesized (Lehn, 1980). The availability of systems such as these once again provides a means for fixing a pair of metal ions in close proximity in a well-defined environment. Such species are ideal for the types of studies involving binuclear systems outlined in Section 3.4. 

The transitions of type (c) are defined by two transition temperatures, one for decreasing (Ti/2j), and one for increasing temperature (Ti/2 ). Two-step transitions (Fig. Id), first reported in 1981 for an iron(III) complex of 2-bromo-salicylaldehyde-thiosemicarbazone [21], are relatively rare and have their origins in several sources. The most obvious is the presence of two lattice sites for the complex molecules. There are several examples of this [22]. In addition, binuclear systems can give rise to this effect, even when the environment of each metal atom is the same—in this instance the... 

DR. ALBERT HAIM (State University of New York at Stony Brook) Dr. Meyer considered plots of optical transition energy versus 1/D(optical) minus 1/D(static) for various types of systems, some of which were binuclear and clearly delocalized. If instead, one considers a ruthenium(II) pentaamine bound to N-methyl-4,4,-bipyridinium, is this in any way different from the bridging situation In some instances there was a similar dependence for both the mononuclear systems and the binuclear systems. But some of these mononuclear systems did not seem to behave similarly. Is there any connection between whether that simple linear relationship works or not and whether the system is localized or delocalized ... 

In contrast Shilov and his group propose a very different mechanism (Scheme 20), which involves a bridging dinitrogen and four vanadium atoms, each acting as a one-electron reductant (contrasting to Schrau-zer s proposal that vanadium acts as a two-electron reductant). The hydrazine may then be reduced to ammonia. On theoretical grounds Shilov considers that dA metal ions would make the best dinitrogen reducers in these binuclear systems (44), and there is ample evidence to support the formulation of vanadium(II) dinuclear species in aqueous solution. However, Shilov postulates and finds vanadium(III) products, which Schrauzer ascribed to a conproportionation (289) as shown in Eq. (78). 

Di- and polynuclear metal carbonyl compounds have a general tendency to engage in a type of fiuxional behavior called carbonyl scrambling. This type of behavior arises because of some of the inherent properties of metal to CO bonding. As shown in Fig. 1-13, the energy of a binuclear system consisting of two metal atoms... 

The electron hopping frequency may be estimated from time-dependent perturbation theory. If Hab is treated as a constant perturbation, the system will start to oscillate between the two diabatic states once the perturbation is turned on. In a bimolecular reaction, for example, the perturbation is turned on upon formation of the precursor complex, while in a covalently attached (bridged) binuclear system it can be turned on upon reduction (oxidation) of one end of the fully oxidized (reduced) system by an external reagent or by photoexcitation. If the system is in the diabatic reactant state at / = 0, then the probability of it being in the product state at some later time t is given by the Rabi formula [27]. 

Using variations of this procedure with the salts C0X2 (X = Cl, Br), six products have been isolated and characterized of which four (obtained as major products) are binuclear systems and two (minor, but extremely interesting, products) are mononuclear. Thus, if the reactant ratios of Co dppm and NaBH4 are 1 2 2 and the addition time of the NaBH4 is 30 minutes, the ion pair [Co(ti -dppm)2(CO)][Co(CO)4] (8 P -6.1 Vco- 2010, 1950, 1898, 1855, 1835 cm b, 1, is formed. This product was origindly formulated (6) as Co2(Ti -dppm)2(CO)4 containing an unsupported Co-Co bond but the structure has since been corrected... 

An important conclusion from these comparative studies is that mononuclear Zn(II) complexes can be as efficient for nitrocefin hydrolysis as binuclear systems, thus providing evidence that the second zinc center (Zn2) in metallo- 3-lactamases is not required for nucleophile activation or catalytic activity. 

As shown in the above examples, several attractive situations, related to some typical combinations of the metal ions, may be investigated in particular from the viewpoint of the spin dimensionality and the nature of the exchange pathways [5-11]. Obviously, the binuclear systems present the simplest examples for studying exchange interactions, as reported in several review papers. They are commonly characterized by an antiferromagnetic coupling between... 

Some of the binuclear systems reported so far derive from the 2H hexagonal-like structure described above, in which only two-thirds of the octahedral sites are occupied by metal ions. 

Although there are a comparatively large number of binuclear systems which contain a II —CO, a limited number of polynuclear species have been characterized. The only example of a trimetallic cluster with a n —CO is Cp3Nb3(CO)7 (182,183). Equation (30) gives the synthesis for this species,... 

For the case of S = 5 — 1 a different treatment is necessary. Analogously to the case of binuclear systems, the Zeeman operator couples states of different spin. The product, involving the molecular-state g-tensor, can be rewritten as follows... 

Three types of monophosphido derivatives are involved in the synthesis of binuclear systems. 

Equally strong interactions do not necessarily have to exist in square planar binuclear complexes, a problem that is of great concern to us. There is very little information in the literature about the synthesis or properties of such materials. Possibly planar binuclear systems are the ethanetetraaldehyde-bridged nickel aminotroponeiminato complexes, 2, reported by Trofimenko (26). The preparation of the bisimidazole complexes 3 was described (27) of which the tetracarbonyl derivatives might well be the very first example of completely planar binuclear transition metal compounds that are capable of extended interactions. 

A great deal more is known for binuclear systems, which are better for comparison to O2 adducts within the trinuclear T2/T3 multicopper oxidase active site (see Figure 11). Mononuclear ligand-Cu(I) or dicopper(I) complexes with a binucleating ligand react with O2 to yield peroxo or hydroperoxo-dicopper(II) complexes. These products have been characterized, often via low-temperature spectroscopic studies (see Chapter 8.15). Kitajima s -peroxo (side-on peroxo) dicopper(II) adduct ... 

The previous sections have documented the fact that binuclear complexes can be obtained with a variety of novel structural features and that these undergo, in some cases, unprecedented chemical reactions. We can expect that work along these lines will continue to point out new structural types and new classes of reactions. Progress so far can be said to have laid a foundation for the basic coordination chemistry of the most readily obtained binuclear systems. It remains to be seen whether these reaction types can be clearly utilized in the design of new catalysts or whether unusual forms of catalytic activity for binuclear complexes will be discovered serendipitously. 

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