Ferrocene-Based Pincer Complexes of Palladium:  Synthesis, Structures, and Spectroscopic and Electrochemical Properties

Palladium P,C,P pincer complexes based on ferrocene were synthesized for the first time. The reaction of the 1,3-bis((dialkylphosphino)methyl)ferrocenes {1,3-(R2PCH2)2C5H3}Fe(C5H5) (2a, R = Pri; 2b, R = But) with PdCl2(NCPh)2 in refluxing 2-methoxyethanol leads to the corresponding pincer complexes PdCl[{2,5-(R2PCH2)2C5H2}Fe(C5H5)] (3a,b). The ferrocene-based binuclear compound 3b reacts with [Cp2Fe]PF6 to form the ferrocenium-based pincer complex {PdCl[{2,5-(But 2PCH2)2C5H2}Fe(C5H5)]}PF6 (4b), and this is the first example of ferrocene-based phosphine chelate oxidation centered on the iron atom. Treatment of complexes 3a,b with NaBH4 in refluxing ethanol affords the complexes Pd(BH4)[{2,5-(R2PCH2)2C5H2}Fe(C5H5)] (5a, R = Pri; 5b, R = But), containing a BH4 − group coordinated to the metal in the rarely occurring unidentate mode. The structures of 3a,b and 5b were confirmed by X-ray analyses. Cyclic voltammetric investigations of complexes 2b and 3a,b are also reported. The structural and spectroscopic features of the palladium ferrocene-based pincer complexes are discussed and compared with those of related compounds.