Reactivity of [(C₅Me₅)Fe(C₅H₄BBr₂)] and [(OC)₃Mn(C₅H₄BBr₂)] toward Et₃SiH: Facile Access to [(OC)₃Mn(C₅H₄BH₂)]₂ and to Boron-Bridged Dinuclear Organometallics

Abstract

The reaction of Fc#BBr2 with 2 equiv of Et3SiH gives the boron-bridged dinuclear species Fc#2BBr with concomitant liberation of B2H6 (Fc# = (C5Me5)Fe(C5H4)). The transformation is slow (48 h) but nevertheless provides yields of >85%. Fc#2BBr can conveniently be converted into the aminoborane Fc#2BNMe2 by treatment with Me3SiNMe2. For an X-ray crystal structure analysis, Fc#2BNMe2 was hydrolyzed to the borinic acid Fc#2BOH. A cross-coupling experiment using Et3SiH and an equimolar mixture of FcBBr2 and Fc#BBr2 led to the formation of the mixed product Fc(Fc#)BBr with high selectivity (Fc = (C5H5)Fe(C5H4)). Similar to the case for Fc#2BBr, Cym2BBr can be prepared from CymBBr2 and Et3SiH (Cym = (OC)3Mn(C5H4)). In contrast to FcBH2, which exists only as transient intermediate on the way to Fc2BH, CymBH2 is an isolable species. The compound was synthesized from a concentrated toluene solution of CymBBr2 and excess neat Et3SiH at −78 °C and forms B−H−B-bridged dimers (CymBH2)2 in the solid state (X-ray crystallography). Our reactivity studies indicate that the Et3SiH-induced coupling reaction of derivatives LnM(C5H4BBr2) is fastest for electron-rich cyclopentadienyl complexes. Addition of NMe2Et to (CymBH2)2 affords the monomeric amine adduct CymBH2(NMe2Et). The reaction of (CymBH2)2 with HCCtBu in a stoichiometric ratio of 1:4 (or slightly higher) results in a mixture of products from which the dinuclear hydroboration product (CymB(C(H)═C(H)tBu))2C(H)−C(H)2tBu could be isolated and structurally characterized. To obtain the divinylborane CymB(C(H)═C(H)tBu)2 in high yield and pure form, (CymBH2)2 has to be dissolved in excess neat HCCtBu.