Electrochemical and spectroscopic studies on dicarboxylato niobocene complexes

Reaction of sodium oxalate or malonate onto the niobocene dichloride Nb(η5-C5H4SiMe3)Cl2 (1) yields the dicarboxylato complexes Nb(η5-C5H4SiMe3)2(O,O′-OCOOCO) (4a) and Cp′2Nb(O,O′-OCOCH2OCO) (5a) where the dicarboxylato ligand is proved to be chelating. An alternative way to yield the dicarboxylato niobium (IV) complexes consists in the electrochemical reduction of 1 in the presence of the corresponding carboxylic acid followed by the oxidation of the electrogenerated complex. In addition to 4a and 5a the latter method has been shown to yield the succinato and orthophtalato analogs Nb(η5-C5H4SiMe3)2(O,O′-OCOCH2CH2OCO) (6a) and Nb(η5-C5H4SiMe3)2(O,O′-OCOC6H4OCO) (7a). The structure of the complexes as well as the mechanism of their formation is discussed above electrochemical and spectroscopic data. A special emphasis is made on the ESR data which strongly depends on the length of the dicarboxylato chain.