Pyrazine-bridged dimer

Taube s recent isolation of a pyrazine-bridged Ru -Ru dimer... 

Mixed valency occurs in minerals (e.g. 63 ), metal-chain compounds, dimers and oligomers and metal complexes, and even in organic and biological systems (Brown, 1980 Day, 1981). Among the dimeric and oligomeric metal complexes exhibiting mixed valency, the pyrazine-bridged Ru (II, III) ammine complex. 

ESCA spectra have been recorded for a series of pyrazine-bridged ruthenium dimers including [(NH3)5Ru(pyz)Ru(NH3)5]s H and [(bipy)2ClRu(pyz)RuCl(bipy)2]3+.50 The data show that both Ru and Ruin are present in those complexes although the metal sites are equivalent. The speed with which ESCA monitors electron distribution (ca. 10"17 s) would seem to make it an excellent spectrosocpic tool for such valence-state assignments. 

The pyrazine-bridged dimer (330) is prepared as shown in Scheme 67. Detailed cyclic vol-tammetric studies on this and related N-heterocyclic ligand bridged dimers indicate a weak cluster-cluster interaction across the bridge at anodic potentials but significant interactions at cathodic potentials as the electron content of the clusters increases. Electronic spectral measurements support... 

Figure 1.5.6 Energy diagram of a light-induced charge-transfer system and two examples the quinhydrone dimer is formed by hydrogen bonding and disproportionates into radicals upon deprotonation a pyrazine-bridged heme dimer shows a 800-nm charge transfer band indicating an electron transfer from one porphyrin to the other. (From Fuhrhop et al., 1980.)...

Fuhrhop, J.-H., Baccouche, M., Biinzel, M. (1980). Bin bei 800 nm absorbierendes Ham-Dimer mit Prazinbriicke, Angew. Chem., 92 320 A pyrazine-bridged heme dimer absorbing at 800 nm, Angew. Chem. Int. Ed. Engl, 19 322. 

Cyclic voltammetry demonstrated that the pyrazine-bridged dimer Ru3(pyz)Ru3 has an extensive oxidation state chemistry, and that the cluster-cluster mixed-valence ions [Ru3(pyz)Ru3] and [Ru3(pyz)Ru3] are discrete species in solution. Ruthenium ESCA data for the perchlorate salt of the +1 ion reveal the presence of distinct cluster units, Ru3 (pyz)Ru3 and therefore indicate that intercluster interactions across pyrazine are weak. However, the electrochemical data reveal that the extent of cluster-cluster interaction increases as the electron content of the system increases. The synthetic chemistry here is very promising, and we should be able to prepare complex two-dimensional systems including polymers and to investigate cluster-cluster and metal ion-cluster interactions. 

It has been often argued that in a mixed valence state of a pyrazine bridged dimer the appearance of a symmetric (pz) band at 1,580-1,590 cm provides evidence for valence localization (class II), while a fully delocalized class III system does not show the z symm(pz) band. In the present case, it was not possible to discuss the degree of delocalization based on the intensity of the symm(pz) band, because the acetate ligand (COO)asym bands obscure the z symm(pz) band. Meyer and co-workers pointed out the difficulty in the discrimination between localization and delocalization from the presence or the absence of symm(pz). ... 

In the present treatment, attention will be focused on localized systems. It is convenient to break the localized classification down even further, where the basis for the distinction lies in experimental observations. In the bpy dimer, where the bridging ligand is pyrazine, the rate of electron oscillation between Ru(II) and Ru(III) sites is slower than the vibrational timescale, at least for those vibrational levels... 

From a solution of the tetranuclear mixed-valence platinum complexes reported above, and in the presence of pyrazine a tetranuclear Ptm complex of formula [(N03)(NH3)2PtIII(pyrl)2PtIII(NH3)(Ju-NH2)]24+ was obtained [28]. It was composed of two a-pyrrolidonate-bridged Pt111 dimers which, in turn, were bridged by two NH2 ligands. In the Pt111 species, the intradimer Pt-Pt distance drops to 2.608 A. 

The process has been established for a series of pentaamminemetal(III) compounds (of Co, Rh, or Ir) with bridging imidazolate (21). The (jl-pyrazine mixed decaammine diruthenium(II)/(III) dimer has also been prepared (180), in addition to a diruthenium compound with a 1,4-dicyanobicyclo[2.2.2]octane bridging ligand and a mixed ruthe-nium(III)/cobalt(III) compound with the same bridging ligand (8). Osmium(III) dimers have also been reported (192). 

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