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

Abstract

Mono- and ditopic lithium ferrocenylhydridoborates Li[FcBH(3)] (2) and Li(2)[H(3)B-fc-BH(3)] (4) have been synthesized from FcB(OMe)(2)/(MeO)(2)B-fc-B(OMe)(2) and Li[AlH(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)-eta(2)-Li bonds. Addition of Me(3)SiCl to 2 or 4 generates the corresponding boranes FcBH(2) (5) and H(2)B-fc-BH(2) (6), which can be trapped by adduct formation with NMe(2)Et 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(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 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(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 horizontal lineCHR) (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 horizontal lineCHR)-](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.

Synthesis, Coupling, and Condensation Reactions of 1,2‐Diborylated Benzenes: An Experimental and Quantum‐Chemical Study

“…The crystal lattice of compound 3⋅ OEt 2 (Figure 1; Table 1) contains parallel columns {Li[1,2‐C 6 H 4 (BH 3 ) 2 ]} ∞ , which consist of alternating Li + ions and [1,2‐C 6 H 4 (BH 3 ) 2 ] 2− ligands: each Li1 atom is coordinated by four trihydroborate moieties in a double‐chelating manner58, 59 and each [1,2‐C 6 H 4 (BH 3 ) 2 ] 2− ligand bridges two Li1 atoms. All {Li[1,2‐C 6 H 4 (BH 3 ) 2 ]} ∞ stacks contain a 2‐fold screw axis.…”

“…Furthermore, the related condensation reaction of 1,1′‐fc(BH 2 ) 2 in SMe 2 provides the BH‐bridged poly(ferrocenylene) [fcB(H)] n with the liberation of BH 3 ⋅ SMe 2 (fc=(C 5 H 4 ) 2 Fe) 58. 70, 71 Despite numerous efforts, we have so far not been able to provide evidence for the formation of 8 ‐type cyclic dimer 11 , even though [1.1]diborataferrocenophanes ( 12 ) are known (Figure 4).…”

“…each Li1 atom is coordinated by four trihydroborate moieties in a double-chelating manner [58,59] and each [1,2- (4) . According to Edelstein's [60] correlation of the metal-boron distances as a measure of the denticity of a trihydroborate group, B···Li distances of about 2.50 are typical of an h 2 -RBH 3 -Li coordination mode.…”

A Quest for Ligand‐Unsupported Li+–π Interactions in Mono‐, Di‐, and Tritopic Lithium Arylborates

“…[35,36] Given this background, we next chose the ditopic monohydridoborate Li 2 [8] (Scheme 2) as the new target system because (i) the moderately coordinating character of the hydridoborate moiety should exert a directing effect that increases the probability for π interaction between Li + and the phenylene ring and (ii) the cyclic structure of the 9-borabicyclo[3.3.1]-nonane (9-BBN) fragment should help to avoid substituent scrambling, and its lipophilicity should increase the solubility of the lithium salt in less polar solvents. The synthesis of Li 2 [8] started from p-bis(trimethylstannyl)benzene (6), [39] which is transformed into the triorganylborane 7 by the reaction with 9-BBN(Cl) [40] in toluene at 120°C (see SI for more details on the synthesis, NMR spectroscopic data, and the crystal structure of 7).…”

“…[34] [36] but had to recognize that the Li + ions tended to coordinate to the peripheral hydride substituents rather than to the benzene or cyclopentadienyl π faces.…”