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-CF3 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+).