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

The 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)

On the Coordination of Dinitrogen to Group 4 Metallocenes

“…For titanium complexes, the calculated stretching frequencies show the existence of an N–N multiple bond close to a double bond in the side‐on isomer and strongest in the end‐on mode. It can be seen that the experimental frequencies agree with an end‐on arrangement, as previously reported by Tuczek et al15 Similar results were obtained for zirconium models with an end‐on form that display the highest frequency and the greatest multiple‐bond character for the dinitrogen fragment. However, the zirconium side‐on complex clearly shows an N–N bond that is weaker than the Ti one, which is reflected in the larger bond length and lower vibrational frequency than the hydrazine molecule.…”

“…Among the six orbitals of dinitrogen unit, three bonding (σ + 2π) and only one antibonding (π*) orbitals can be considered to be fully occupied in the model complex. As a result of this electron configuration, one can formally suggest a value equal to two for the bond order of the dinitrogen fragment, which acquires a diazenide character, N 2 2– 15…”

“…This study has also been completed with a structural analysis from the available experimental data. At present, only a theoretical study of the electronic structure and vibrational characterization of an end‐on titanium complex has been published by Tuczek et al15 Consequently, the goal of this article is to systematically investigate the coordination mode of dinitrogen to metallocene fragments in [M 2 Cp 4 N 2 ] (M = Ti, Zr, and Hf) complexes.…”

Theoretical, spectroscopic, and mechanistic studies on transition‐metal dinitrogen complexes: Implications to reactivity and relevance to the nitrogenase problem

“…The end‐on bridging mode of N 2 is considered on the basis of the three dinuclear titanium complexes 4a, 4b , and 5 (Scheme 1) 87. They contain dinitrogen in a linear μ‐1,2 geometry but have different sets of terminal ligands.…”

Can [M(H)2(H2)(PXP)] Pincer Complexes (M=Fe, Ru, Os; X=N, O, S) Serve as Catalyst Lead Structures for NH3 Synthesis from N2 and H2?

“…[2] Significant efforts, many of them driven by an attempt to understand biological nitrogen reduction, have been made to synthesize and characterize transition-metal complexes that bond with and activate the N 2 molecule. A wealth of metal complexes (for example, titanium, [3] zirconium, [4, 9c] vanadium, [5] molybdenum, [6] and iron complexes [7] ) were shown to bond with and activate the N 2 molecule to varying extents, some of them even breaking the N À N structure motif completely. Many of the theoretical and spectroscopic data available have been analyzed.…”