Binuclear metal complexes

As a rule, however, the distance between the donor and the acceptor in such binuclear bridge metallocomplexes is not large. Only a few molecules of this type are known in which the electron transfer occurs over considerable distances, comparable with those for electron transfer between randomly arranged centres in vitreous matrices. Consider the results of research on electron tunneling over large distances in bridge systems. 

The electron transfer from L to Co(III) to a distance of the order of 4 A is described well by an ordinary first-order kinetics with the rate constant of the order of kt 104s-1 at T = 295 K. 

A detailed study of the effect of the medium and temperature on the intramolecular electron transfer rate constant kt in various metal complex systems of the bridge structure has been carried out [25]. The values of kt were found to increase and the activation energy to decrease with increasing polarity of the medium. These effects were accounted for in terms of the modern electron transfer theory (see the case Er J in Fig. 5 of Chap. 3) by greater changes in the free energy, AG°, due to a higher redox potential of the L/Lr pairs in a more polar medium. 

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The cyclooctapyrroles shown in Figure 55 appear predestined to form binuclear metal complexes since the loop-shaped conformation of these macrocycles exhibits two structurally identical, helical N4 cavities. Enantiomers of such complexes, which are presumably generally very stable towards racemization owing to the rigidity of the molecule imposed by the incorporation of the metal, are of interest as possible models for binuclear metalloenzymes and as potential catalysts in asymmetric synthesis. The first two ligands as well as their recently obtained palladium complexes601 were... 

Chiorboli C, Indelli MT, Scandola F (2005) Photoinduced Electron/Energy Transfer Across Molecular Bridges in Binuclear Metal Complexes. 257 63-102 Collin J-P, Heitz V, Sauvage J-P (2005) Transition-Metal-Complexed Catenanes and Rotax-anes in Motion Towards Molecular Machines. 262 29-62 Collyer SD, see Davis F (2005) 255 97-124 Commeyras A, see Pascal R (2005) 259 69-122 Correia JDG, see Santos I (2005) 252 45-84 Costanzo G, see Saladino R (2005) 259 29-68 Credi A, see Balzani V (2005) 262 1-27 Crestini C, see Saladino R (2005) 259 29-68... 

Chiorboli C, IndeUi MT, Scandola F (2005) Photoinduced Electron/Energy Transfer Across Molecular Bridges in Binuclear Metal Complexes. 257 63-102 Chiorboli C, see IndelliMT (2007) 280 215-255... 

A review of the stabilities and catalytic properties of binuclear metal complexes of large-ring N,0 macrocycles concentrates on the iron(II) and iron(III) complexes of (184) and (185). ... 

Although most of the binuclear metal complexes appear to undergo sequential steps, i.e. (E ° — E2°) > 0, a few examples are known22 where a concerted two-electron charge transfer occurs, i.e. ( , Ei °) < 0, so that addition of the second electron is easier than the first. 

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

Fig. 3.23 The RS (left) and RR (right) diasteromers of binuclear metal complexes formed by hydrazine molecules and halogen atoms (metal green, nitrogen blue, and halogen yellow)...

With the purpose of further increasing the number of adjustable photochromic parameters of the subunit, a binuclear metal complexe of porphyrazine bearing six bis(trimethylthiophenyl) functionalities at the periphery was synthesized in our laboratory [40] (Scheme 12). The structure of binuclear porphyrazine was rigidly constrained in a coplanar arrangement with extended n-conjugated subunits. 

Werner, A. Michels, M. Zander, L. Lex, J. Vogel, E. Figure eight cyclooc-tapyrroles enantiomeric separation and determination of the absolute configuration of a binuclear metal complex, Angew. Chem. Int. Ed. 1999, 38, 3650-3653. 

An elegant illustration of this principle is found in the binuclear metal complex MM(fsa)2en MeOH, where... 

While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For U022 , the diradical excited state ( U-OO is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re2(CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atom-abstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. 

Binuclear metal complexes often exhibit unusually low ET rates (33, 46, 75). And, at comparable separation distances, the rates of ET in ruthenium-modified proteins are well below those of bridged organic donor-acceptor compounds (90, 118, 126). It is of interest to consider how much of this difference is due to electronic coupling factors. For example, in the complex Os(II)(dioxA)Co(III), A j,(Os to Co) is 7.2 x 10 s (33, 46). Similarly, Ru(BCO)Co has a rate of less than 3.6 x 10 s (4). The Na(Sp)Bi compounds of Closs and Miller having a similar-sized spacer and a smaller driving force have ET rates of approximately 1 x lO s (20,... 

However, the first topological analysis of the experimental electron density in a binuclear metal complex concludes that the metal-metal interaction is unshared. In their high-resolution X-ray diffraction study at 120 of Mn2(CO)io (Figure 15) Bianchi et al.160 quote a positive value of V2p at the... 

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