Mixed metal dimer

We have already shown how simultaneous codeposition of two metals in inert-gas matrices can lead to the formation of mixed-metal dimers. As in the case of silver, it was found that irradiation into the atomic absorptions of Cr or Mo results in formation of their respective dimers and trimers (114). In addition to this, however, irradiation into the atomic resonances of the two metals in the presence of each other results (114) in formation of the mixed-metal species CrMo, CrjMo, and CrMo2. It would seem that selective irradiation into the 300-400-nm bands of atomic Cr or Mo excites the 3d 4p, 3dMs 4p , or 4d 5p, ... 

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

All three of the mixed-metal dimers react with CO to give the respective dicarbonyl compounds, CpM(CO)2 (M = Co, Rh, RuH), and (tj-C5Me5)2Zr(CO)2 (30). It is presumed that this reaction is initiated by the breaking of the zirconium-/x-i71,T)2-carbonyl bond. 

The mixed-metal dimer [CrMo(02CMe)4] has been prepared in 30% yield by addition of [Mo(CO)6] in acetic acid, acetic anhydride and CH2C12, to a refluxing solution of [Cr2(02CMe)4(H20)2] in acetic acid and acetic anhydride.13 The yellow product is volatile and gives the expected parent ion peak in the mass spectrum. It has a Raman active v(Cr—Mo) mode at 394 cm-1 consistent with quadruple bonding. The crystal structure gives a metal-metal... 

Mixed metal dimers have also been prepared from frms,-[MCl2L2l species (see Scheme... 

Several dimers containing two different metal atoms joined by a bond of order four have been isolated. Addition of Mo(CO)6 to a refluxing solution of Cr2(02 CCH3)4(H2 0)2 yields the mixed-metal dimer CrMo(02CCH3)4, which was characterized by spectroscopic and analytical data including a set of parent ion peaks in the mass spectrum (134). 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

The mechanisms of these reactions are probably complex but radical processes seem likely. The final products of insertion into the metal metal bond are actually trinuclear species. Insertion into terminal metal hydride or metal alkyl bonds could, in principle, generate mixed-metal dimers. In practice, aside from insertion into the metal halogen bond, such as that shown in equation (65), few such reactions have been reported. 

Their H NMR spectra are characterized by resonances due to the PFe in the positions found for the symmetrical PFe-O-FeP dimer, and a resonance at 38 ppm that is due to the pyrrole-H of the chromium porphyrin unit. The temperature dependence of the magnetic susceptibility of the mixed-metal dimer leads to its formulation as antiferromagnetically coupled Fe S = 5/2) and Cr S = 3/2), yielding S =... 

Oxygen ligands form very stable complexes with hard Lewis-acid met-alloporphyrins such as Sn(IV), Zr(IV), Mn(III), Mo(V), but also with P(V)-porphyrins. Mixed-metal dimers were synthesized from Al(Me)OEP and phosphorus, arsenic or antimony porphyrins in the form of fi-oxo dimers [73]. hi most cases, polymeric structures are obtained because Sn(IV)-, Mn(III)- and Fe(III)-porphyrins can bind two axial ligands on either side of the porphyrin referred to as trans-binding. This is schematically represented in Fig. 25a. An example includes the linear trinuclear or polymers /r-frans-dioxo-MoTPP [74,75]. 

Li, M.P. and Drago, R.S. (1976) An acid-base model for mixed metal dimer formation enthalpies of dimer cleavage and adduct formation of l,5-cyclooctadiene-rhodium(I) chloride dimer. J. Am. Chem. Soc., 98, 5129-5135. 

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