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{(μ-H)2BR2} (R2 = 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(μ-H)(η5-η1-C5H4)Nb(η5-η1-C5H4)2Nb{(μ-H)(η5-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{(μ-H)2BR2} with B(C6F5)3 produces the covalent complex CpNb(C6F5){(μ-H)(η5-C5H4B(C6F5)2)}, 2. In the formation of these compounds NbIII is oxidized to NbIV. 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){η5-C5H4B(H)(C6F5)2} (L = THF, pyridine). In pyridine, a second adduct in much smaller yield is also formed, CpNb(C6F5)(H){η5-C5H4B...