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

摘要

Mono- and ditopic lithium ferrocenylhydridoborates Li[FcBH_3] (2) and Li_2[H_3B-fc-BH_3] (4) have been synthesized from FcB(OMe)_2/(MeO)_2B-fc-B(OMe)_2 and Li[AIH_4] (Fc = ferrocenyl; fc = 1,1'-ferrocenylene). X-ray quality crystals were grown from OEt_2. Depending on the amount of Li~+-coordinated solvent molecules, dimeric (2(OEt_2)_2) or tetrameric (2(OEt_2)) aggregates are observed in the solid state. The ditopic derivative 4 crystallizes as two different macrocyclic dimers (4(OEt_2)_5 and 4(OEt_2)_6) in the unit cell. Each of the four aggregates is held together mainly by RBH_3-η~2-Li bonds. Addition of Me_3SiCI to 2 or 4 generates the corresponding boranes FcBH_2 (5) and H_2B-fc-BH_2 (6), which can be trapped by adduct formation with NMe_2Et or SMe_2. In contrast, when OEt_2 is present as the sole Lewis basic donor, no stable ether adducts are obtained, but condensation takes place leading to Fc_2BH (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_2BCy and BCy_3 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_2BH (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_2B(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.