Ferrocenophanes with interannular boron-phosphorus bridges: Synthesis, structure, and reactivity toward nitrogen bases

Starting from readily available diborylferrocenes 1,1′-fc[B(Br)R]₂ (fc = (C₅H₄)Fe(C₅H₄); R: Br (1a), Me (1b), OEt (1d)), ferrocenophanes 1,1′-fc[B(μ-PPh₂)R]₂ (R = Br (2a), Me (2b), PPh₂ (2d)) and 1,1′-fc[B(μ-PPh₂) OEt][B(μ-PPh₂)PPh₂] (2c) are obtained upon reaction with LiPPh₂. In contrast, 1,1′-fc[B(Br)pyr]₂ (pyr = pyrrolidinyl, 1e) and 2 equiv of LiPPh₂ give an open-chain ferrocene 1,1′-fc[B(PPh₂)pyr]₂ (2e). The dynamic behavior of the B₂P₂-bridged species has been investigated using variable-temperature NMR spectroscopy and the Forsén-Huffman spin saturation transfer method. The reaction of 2a with Lipz (pz = pyrazolide) gives the ansa-ferrocene 1,1′-fc[B(μ-PPh₂)PPh₂][B(μ-pz)Br] (3a), featuring an interannular (BNNBP) heterocyclic bridge. When Lipz* (pz* = 4-bromo-3,5-dimethylpyrazolide) was used rather than Lipz, two products could be isolated: 3b, which is analogous to 3a, and 1,1′-fc[B(μ-PPh₂)Br][B(μ-pz)Br] (3c), which bears Br substituents at both bridgehead boron centers. 2d and 3c have been characterized by X-ray crystallography. Treatment of 2a with excess γ-picoline (pic) results in an ansa-bridge opening and bromide substitution with the formation of the ionic species {1,1′-fc[B(PPh₂)pic₂]₂}Br₂ (5a). The presence of three-coordinate phosphorus centers in 5a was confirmed by showing their ability to bind Cr(CO)₅ fragments, which afforded the trimetallic complex 5b.