Spectroscopic Comparison of Dinuclear Ti+ and Ti2+ μ‐η1:η1 Dinitrogen Complexes with Cp*/Pentafulvene and Amine/Amide Ligation: Moderate versus Strong Activation of N2

Abstract

AbstractThe two dinuclear, end‐on dinitrogen‐bridged titanium complexes with cp*/pentafulvene ligation, [{(η5‐C5Me5)Ti(η6‐C5H4CR2)}2(μ‐η1:η1‐N2)], R = p‐tolyl (1) and >CR2 = adamantylidene (2), are compared to an analogous titanium dinitrogen complex with a mixed amine/amide/chloride ligand set, [{(Me3Si)2NTiCl(TMEDA)}2(μ‐η1:η1‐N2)] (3; TMEDA = tetramethylethylenediamine). The N–N stretching vibrations of complexes 1 and 2 are observed at 1749 and 1755 cm–1, respectively, indicative of a moderate activation of the dinitrogen ligand. In contrast, the N–N stretch of 3 is located at 1284 cm–1, reflecting a much more highly activated N2 ligand. These findings are in qualitative agreement with N–N bond lengths that have been observed experimentally. DFT calculations reveal that the major difference in the electronic structures of 1, 2, and 3 is the fact that only one of the N2 π* orbitals is (doubly) occupied in the case of 1 and 2, whereas both π* orbitals are (doubly) occupied in the case of 3. The reason for this finding is that in 1 and 2 two of the three electrons of each Ti+ are involved in back‐bonding interactions with the terminal fulvene ligands, whereas no such interactions exist between the Ti2+ centers of 3 and its terminal set of donor ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)