Synthetic and Spectroscopic Studies of Metal Carboxylate Dimers

Abstract:
Synthetic and spectroscopic studies on several complexes in the metal carboxylate series are described. These complexes are of general formula M 2 (02CR) 4 where M is a transition metal and "0 2 CR is a bridging carboxylate ligand. The metals used in this study are molybdenum, rhodium, and ruthenium. The studies were undertaken to help understand the nature of the metal -metal interaction in these complexes and to see what effect this interaction has on the reactivity of these complexes.
To effect removal of the bridging carboxylate ligands, acetonitrile solutions of Rh2(0 2 CCH 2 CH 2 CH3) 4 and Mo 2 (0 2 CCH 3 ) 4 were reacted with stoichiometric amounts of the strong non-complexing acids CF3SO3H and (CH 3 CH 2 ) 20.HBF4 . This generated Rh2 (02 CH 2 CH 2 CH 3 ) 2 2+ and Mo 2 (02CCH 3 )?
2+ species in solution. The former was not isolated, but characterized in solution by NMR and UV-visible spectroscopy. Two derivatives of Mo 2 (02 CCH 3 ) 2 2+ were isolated: CMo 2 (0 2 CCH3) 2 (CH 3 CN) 4 ](CF 3 S03) 2 and [Mo 2 (0 2 CCH3) 2 (CH3CN) 5 ](BF 3 0H) 2 . The reactivity of the former complex towards oxidative addition was investigated. The complex was found to be quite stable towards oxidation in contrast to other Mo(II) complexes. Rhodium trifluoroacetate was reacted with various Lewis bases to give adducts of general formula Rh2 ( 02^3)482 as had been previously reported for Rh 2 (0 2 CR)4. However, with pyridine and £-butyl Isonitrile, complexes of general formula Rh2(0 2CCF3)4B4 were isolated constituting a new class of adduct. With phosphorus donors, Rh-Rh bond cleavage occurred to give monomeric Rh(I) and Rh(III) complexes. This demonstrates enhanced reactivity for Rh2(02CCF3)4 compared to rhodium alkyl carboxylate dimers. The chemical and electrochemical generation of Rh2(02CCH 2 CH 2CH3)4B2 + is described. These results and EPR spectra of these species are explained using a molecular orbital model. The strength of the rhodium Lewis base interaction determines the chemical and spectroscopic properties of these species. The formally mixed oxidation state complex Ru2(02CCH2CH2CH3)4Cl was studied by powder magnetic susceptibility measurements over the temperature range 5-300 K, by EPR spectroscopy In various glasses at 4 K and by Far IR spectroscopy at room temperature. In agreement with previous reports, the complex has a quartet ground state with unpaired electron spin density delocalized over both Ru atoms. Reactivity studies of this compound with Lewis bases are described. A bispyridine adduct of ruthenium butyrate chloride is reported.
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