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), Oft (1d)), ferrocenophanes 1,1'-fc[B(mu-PPh2)R](2) (R = Br (2a), Me (2b), PPh2 (2 d)) and 1,1'-fc[B(mu-PPh2)OEt][B(L mu-PPh2)PPh2] (2c) are obtaine d upon reaction with LiPPha. 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 Forsen-Hoffman spin saturation transfer method. The reaction of 2a with Lipz (pt = pyrazolide) gives the ansa-ferrocene 1,1'-fc[B(mu-PPh2)PPh2)[B(mu-pz)Br] (3a), featuring an interannular (BNNBP) heterocyclic bridge. When Lipz* (pt* = 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(mu-PPh2)Br][B(mu-pz)Br[ (3c), which bears Br substituents at both bridgehead boron centers. 2d and 3e have been characterized by X-ray crystallography. Treatment of 2a with excess gamma-picoline (pic) results in an ansa-bridge opening and bromide substitution with the formation of the ionic species {1,1'-fc[B(PPh2)pic(2)](2)}Br-2 (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.