Structural Features and Electrochemical Properties of ansa -Ferrocenes with Pyrazabole Bridges

The stability of ferrocenophanes ansa -1,1‘-Fc[B(R)(μ-pzR # )] 2 ( 2 ) with pyrazabole bridges depends to a large extent on the substitution pattern of boron (R) and pyrazole (R # ). Cyclic voltammetry measurments on 17 derivatives of 2, with R and R # covering a wide spectrum of electronic features, revealed a pronounced influence of these substituents on the E °‘ 0/+ values of the Fe(II)/Fe(III) redox couple. The electronic effects of R and R # thus appear to be transmitted to a great degree along the pyrazabole framework. π donors R and electron acceptors R # lead to a gradual ansa -bridge weakening and finally opening, if R = pyrrolidinyl and R # = 3,5-CF 3 or 3,4,5-COOEt (NMR spectroscopy; X-ray crystal structure analyses of 2c (R = Me; R # = 3,4,5-H), 2d (R = Me; pzR # = indazolyl), 2n (R = pyrrolidinyl; R # = 3,4,5-H), 2p (R = pyrrolidinyl; pzR # = triazolyl)). Cyclic voltammetry indicates ansa -bridge opening to occur without major alterations of the charge density at boron. In the case of R = pyrrolidinyl there is evidence for negative hyperconjugation into the highly polar B−N(pyrazole) bonds. Oxidation of the central iron atom results in a contraction of the pyrazabole dimer (X-ray crystal structure analysis of 2c + ).