Structural Features and Electrochemical Properties of ansa-Ferrocenes with Pyrazabole Bridges
摘要:
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(+)).
各种立体刚性柄-ferrocenes 2与ö桥已经通过1,1'- diborylferrocenes(1,1'-的Fc(BBRR)的反应获得的亚苯基型2 ; FC =二茂铁; R = CH 3,溴,OEt,NC 4 H 8)与选定的吡唑衍生物。随后的分子内BN加合物的形成建立了环间桥。合成方法是基于通过等电子,自组装,固定的硼氮键取代某些关键的碳-碳键,因此遵循非共价合成的原理。吡唑硼酸酯桥似乎不会引起二茂铁芯的任何明显变形。大多数取代的硼或吡唑导致袢二茂铁结构。然而,在硼处同时存在强电子给体(即吡咯烷)和强吸电子吡唑取代基(即酯基)都会导致具有三配位硼原子的开链结构2i。四价硼中心在安莎-1,1'--Fc的[B(BR)(μ-PZ)] 2,图2a,示出了异常高的倾向经历亲核取代反应,即使它们位于桥头位置。NMR光谱学以及化学证据都反对在这些反应过程中未桥连的开链中间体。