Monocyclopentadienyltitanium(III) Complexes with Hydridoborato Ligands

Abstract

The synthesis, properties and reactivity of monocyclopentadienyltitanium(III) complexes stabilized by hydridoborato (BH3R)‒ ligands are reported. The reactions of the titanium(IV) trichlorides [Ti(h5-C5H5-nMen)Cl3] with excess LiBH3R (R = H, Me) afford dimeric [{Ti(h5-C5H5-nMen)(BH4)(μ-BH4)}2] (n = 5 (1), 4 (2), 0 (3)) or monomeric [Ti(h5-C5Me5)(BH3Me)2] (4) titanium(III) derivatives. Compound 3 is a thermally unstable oil and decomposes at room temperature forming a mixed-valence TiII/TiIII tetrametallic species [{Ti(h5-C5H5)(BH4)}2{(μ3-B2H6)Ti(h5-C5H5)}2] (5) with dianionic diborane(6) ligands in a rare μ3-k2:k2:k2-B2H6 coordination mode. DFT calculations for 5 agree with a triplet ground state with two inner titanium(II) atoms bonded by a single bond and two outer titanium(III) centers. In contrast to 3, the monomeric tetrahydrofuran adduct derivative [Ti(h5-C5H5)(BH4)2(thf)] (6) is a stable solid. Dimeric 1-3 react with 2,6-lutidinium salts (LutH)X (X = BPh4, OSO2CF3) to afford ion pairs, [Ti(h5-C5H5-nMen)(BH4)L2](BPh4) (L = thf, n = 5 (7), 4 (8), 0 (9); L = py, n = 5 (10)), or molecular aggregates, [{Ti(h5-C5Me5)(BH4)(μ-O2SOCF3)}2] (11) and [{Ti(h5-C5H5-nMen)(μ-O2SOCF3)2}x] (n = 5, x = 3 (12); n = 4, x = 4 (13)), via formation of H2 and (Lut)BH3. Diamagnetic titanium(II) derivatives [{Ti(h5-C5Me5)(μ-H)}2{(μ-H)2Al(BH3R)(thf)}2] (R = H (14), Me (15)) were isolated in reactions of complexes 1 and 4 with LiAlH4 in tetrahydrofuran.