Selective Ring‐Opening Reactions of [1]Ferrocenophanes with Boron Halides: A Novel Route to Functionalized Ferrocenylboranes and Boron‐Containing Oligo‐ and Poly(ferrocene)s

The reactivity of strained [1]ferrocenophanes, [Fe( η ‐C 5 H 4 ) 2 ER x ] (ER x =SiMe 2 , 1 a ; SiMePh, 1 b ; SnR 2 , 1 c ), towards boron halides has been investigated and has been shown to provide a facile pathway to ferrocene derivatives functionalized with Lewis acidic boron centers. The boron halides RBX 2 (R=Cl, Ph, fc; X=Cl, Br) (fc=Fe( η ‐C 5 H 4 ) 2 ) lead to selective cleavage of the Si−Cp bonds in 1 a and 1 b to give, depending on the reaction stoichiometry, functionalized mono‐ or diferrocenylboranes R n B[( η ‐C 5 H 4 )Fe( η ‐C 5 H 4 SiMe 2 Cl)] 3− n ( 2 a : RCl, n =2; 2 b : RCl, n =1; 2 c : RPh, n =1) and R n B[( η ‐C 5 H 4 )Fe( η ‐C 5 H 4 SiMePhCl)] 3− n ( 2 d : RCl, n =2) in high yields. Compounds 2 a ‐ d were characterized by multinuclear NMR spectroscopy, mass spectrometry, and by single‐crystal X‐ray diffraction (for 2 b ). Most likely due to steric constraints, a triferrocenylborane was not obtained even from the reaction of BCl 3 with an excess of 1 a , whereas facile formation of the diferrocenylphenylborane 2 c from PhBCl 2 and two equivalents of 1 a was observed. Selective hydrolysis of the B−Cl bonds of chlorodiferrocenylborane 2 b in the presence of trace amounts of water led to the silylated tetranuclear ferrocene complex [(ClMe 2 Sifc) 2 B‐O‐B(fcSiMe 2 Cl) 2 ] ( 3 ) without cleavage of the Si−Cl bonds. The structure of 3 was confirmed by an X‐ray diffraction study. Studies of the reactivity of the higher Group 14 homologue of 1 a and 1 b , the tin‐bridged [1]ferrocenophane 1 c , revealed that facile addition of B−Cl bonds occurs across the Sn−Cp bonds to yield the 1‐stannyl–1′‐borylferrocenes [(ClMes 2 Sn)fc(BClR)] ( 4 a : RCl; 4 b : RPh; Mes=2, 4, 6‐Me 3 C 6 H 2 ). The new synthetic methodology can be extended to bifunctional Lewis acids such as the bis(boryl)ferrocene 1,1′‐fc(BBr 2 ) 2 , which affords the linear boron‐bridged ferrocene trimer 1,1′‐[fc{B(Br)fcSiMe 2 Br} 2 ] 5 in 54 % isolated yield. In order to incorporate the functionalized ferrocenylboranes into polymer structures, compound 2 c was reduced with Li[BEt 3 H] to give the silicon‐hydride functionalized species [PhB{( η ‐C 5 H 4 )Fe( η ‐C 5 H 4 SiMe 2 H)} 2 ] ( 6 ), which was then used as a capping reagent in the transition metal catalyzed polymerization of 1 a . This process leads to the incorporation of the ferrocenylborane unit into the main chain of a poly(ferrocenylsilane) to afford [PhB{(fcSiMe 2 ) n −1 fcSiMe 2 H} 2 ] ( 7 ).