Formation of triniobocene cationic and neutral mononiobocene species as a function of solvent in the reaction of Cp2Nb{(μ-H)2BR2}, (R2 = C4H8, C5H10, C8H14) with B(C6F5)3

Reactions of the niobocene cyclic organohydroborates CP2Nb{(mu -H)(2)BR2} (R-2 = C4H8, C5H10, C8H14), with B(C6F5)(3) in the poorly coordinating solvent toluene and in the coordinating solvent diethyl ether give different products. In toluene, the salt [Cp2Nb(mu -H)(eta (5)-eta (1)-C5H4)-Nb(eta (5)-eta (1)-C5H4)(2)Nb{(mu -H)(eta (5)-C5H4B(C6F5)(2))}](+)[HB(C6F5)(3)](-), 1, which contains a triniobocene cation, is formed. On the other hand in diethyl ether, the reaction of Cp2Nb{(mu -H)(2)BR2} with B(C6F5)(3) produces the covalent complex CpNb(C6F5){(mu -H)(eta (5)-C5H4B(C6F5)(2))}, 2. In the formation of these compounds Nb-III is oxidized to Nb-IV. Upon the basis of ESR spectra, 1 and 2 are paramagnetic, each with one unpaired electron. The structures of 1 and 2 were determined by single-crystal X-ray analysis. Complex 2 forms weak adducts in the solvents THF and pyridine. The Nb-H-B bridge is cleaved at the Nb-H site to form CpNb(C6F5)-(L){eta (5)-C5H4B(H)(C6F5)(2)} (L = THF, pyridine). In pyridine, a second adduct in much smaller yield is also formed, CpNb(C6F5)(H){eta (5)-C5H4B(Py)(C6F5)(2), with the Nb-H-B bridge being cleaved at the H-B site.