Structural features and electrochemical properties of ansa-ferrocenes with pyrazabole bridges

The stability of ferrocenophanes ansa-1,1'-Fc[B(R)(mu-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/+)(o)' 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. pi donors R and electron accepters 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(+)).