Homometallic dimers

Porphyrazines have also been used to prepare metal-metal bonded dimers with ruthenium and osmium, specifically homometallic dimers comprised of two metallic octaethylporphyrazine and heterometallic dimers comprised of one metallic oc-taethylporhyrazine ligand and one metallic octaethylporphyrin ligand have been reported (189-191). The major bonding interactions that occur in these systems are directly between the two metal centers. 

Synthesis. Homometallic pz dimers of Ru (227a) and Os (227b) were prepared, in almost quantitative yield, by vacuum pyrolysis of the monomeric Ru(py)2 or Os(py)2 octaethylporphyrazines (226a and 226b). Heterometallic pz dimers, composed of octaethylporphyrazine and octaethylporphyrin were prepared by the same route that initially produces a mixture of two homometallic (16% each) and the heterometallic dimer (68%), Ru (228a) or Os (229b). The redox properties of the three products are significantly different, by 0.4-0.6 V, so the heterometallic dimer can be separated from the homometallic dimers by redox titrations. 

While the Co corrole-Fe/Mn porphyrin dyads were active electrocatalysts, the homobimetallic Co porphyrin-corrole dyads operate at lower overpotentials and favor 4-electron reduction to a greater extent. This was attributed to the locus of reduction in each complex apparently, the porphyrin is reduced first in the homometallic dimer, whereas the corrole is the site of the first reduction in the heterometallic dimers. A later article presented EPR and spectroelectrochemical evidence supporting the assignment of a Co(III) Ji-cation electronic configuration for the oxidized derivatives of monomeric Co triarylcorrolates, suggesting that the dimeric five-coordinate Co corrole catalysts are also Co(III) 7i-cation radicals [147]. Further study is needed to elucidate this point. 

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

The X-ray structure of a interesting homometallic five-coordinate neodymium alkoxide complex [(r-Bu3-CO)2Nd(/t-Cl)(thf)]2 (290) is shown in Fig. 40, and consists of a chloride-bridged dimer with an Nd2Cl2 planar ring. 

One major problem in producing gels containing homogeneous mixtures of a variety of oxides is that the precursors may not all hydrolyze at the same rate. In particular, transition metal alkoxides hydrolyze much more rapidly than silicon alkoxides. The controlled hydrolysis of low-molecular-weight homometallic species described in the previous section can be adapted to prepare mixed alkoxides. For example, pre-hydrolysis of metal alkoxide followed by reaction with the silicon alkoxide gives a mixed dimeric species such as ... 

Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic. Partial hydrolysis of [LiNb(OEt)6]< leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in which both the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1 1 molar ratio) offers Ba4Tii30i8(0C2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron of BaOs units surimposed on a Ti06(Ti03)i2 core this Tii3042 core is related to that of the aluminum salt [A1i304(0H)24(H20)]2+ [83]. The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur. 

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