Stepwise assembly and structural characterization of oligonuclear ferrocene aggregates with boron–nitrogen backbone

The reaction of FcBBr2 [1; Fc=(C5H5)Fe(C5H4)] with either 4-bromo-N,N-bis(trimethylsilyl)aniline or lithium N-tert-butyltrimethylsilylamide or sodium N,N-bis(trimethylsilyl)amide or 4-methoxyaniline in the appropriate stoichiometric ratio yields mononuclear {FcB(Br)N(SiMe3)(C6H4Br) (3); FcB(Br)N(SiMe3)(CMe3) (4)}, dinuclear {[FcB(Br)]2NSiMe3 (5); [FcBN(H)(C6H4OMe)]2NC6H4OMe (6)}, trinuclear {[FcBN(C6H4OMe)]3 (7)} and tetranuclear {[FcB(Br)N(SiMe3)FcB]2NH (8)} ferrocene complexes, which are held together by covalent boronnitrogen bonds. All compounds 3–8 have been characterized by X-ray crystallography. Compound 7 features a puckered borazine core, while the BN backbone of 8 adopts a helical conformation. Treatment of 1,1′-fc(BBr2)2 [10; fc=(C5H4)2Fe] with 4-bromo-N,N-bis(trimethylsilyl)aniline produces the 1,3-dibora-[3]ferrocenophane 13, in which a B(Br)N(C6H4Br)B(Br) bridge spans both cyclopentadienyl rings. Polymeric byproducts have not been observed. These results indicate a strong preference for the formation of intramolecular BNB ansa-bridges over intermolecular BNB crosslinks.