Ferrocenylhydridoborates: Synthesis, Structural Characterization, and Application to the Preparation of Ferrocenylborane Polymers

Mono- and ditopic lithium ferrocenylhydridoborates Li[FcBH(3)] (2) and Li-2[H3B-fc-BH3] (4) have been synthesized from FcB(OMe)(2)/(MeO)(2)B-fc-B(OMe)(2) and Li[AlH4] (Fc = ferrocenyl; fc = 1,1'-ferrocenylene). X-ray quality crystals were grown from OEt2. Depending on the amount of Li+-coordinated solvent molecules, dimeric (2(OEt2)(2)) or tetrameric (2(OEt2)) aggregates are observed in the solid state. The ditopic derivative 4 crystallizes as two different macrocyclic dimers (4(OEt2)(5) and 4(OEt2)(6)) in the unit cell. Each of the four aggregates is held together mainly by RBH3-eta(2)-Li bonds. Addition of Me3SiCl to 2 or 4 generates the corresponding boranes FcBH(2) (5) and H2B-fc-BH2 (6), which can be trapped by adduct formation with NMe2Et or SMe2. In contrast, when OEt2 is present as the sole Lewis basic donor, no stable ether adducts are obtained, but condensation takes place leading to Fc(2)BH (10) and the novel borane polymer [-fcB(H)-](n) (9) respectively. In situ generation of FcBH(2) (5) in the presence of cyclohexene gives Fc(2)BCy and BCy3 but no FcBCy(2), thereby indicating that 5 undergoes condensation to 10 more quickly than hydroboration of an internal olefin can occur (Cy = cyclohexyl). Fc(2)BH (10) was further studied as a model system for the optimization of modification reactions of polymer [-fcB(H)-](n) (9). Hydroboration of PhCCH or tBuCCH with 10 proceeds smoothly and quantitatively to give the corresponding vinylboranes Fc(2)B(CH=CHR) (11(Ph), R = Ph; 11(tBu), R = tBu), which were fully characterized. In a similar manner, the polymeric borane 9 was successfully transformed into ferrocenylborane polymers [-fcB(CH=CHR)-](n) (12(Ph), R = Ph; 12(tBu), R = tBu) that contain vinyl groups attached to boron. The structures of polymers 12 were confirmed by NMR and IR spectroscopy and mass spectrometry. The MALDI-TOF spectra of 12(Ph) and 12(tBu) showed patterns of equidistant peaks with peak separations that are consistent with the masses of the expected repeating units of each of the polymers. The absorption maxima in the UV-vis spectra of polymers 12 are significantly red-shifted in comparison to the dimeric model systems 11.