Dinitrogen Activation by Titanium Sandwich Complexes

Abstract

The activation of dinitrogen by titanium sandwich complexes of the general form (eta5-C5Me4R)2Ti (R = CHMe2, CMe3, SiMe3) has been systematically investigated. Low-temperature, in situ, solution infrared spectroscopy has allowed detection of monomeric bis-dinitrogen complexes of titanium that are isostructural with more familiar dicarbonyl derivatives. One example, (eta5-C5Me4CHMe2)2Ti(N2)2, has also been characterized by X-ray diffraction and reveals weakly activated dinitrogen ligands. From the solution IR data, the relative azophilicity of the titanium sandwich complexes has been established and increases with smaller cyclopentadienyl substituents.

Crystal structure of an isomeric bis[(η 5:η 1-6,6-di-p-tolylpentafulvene)(η 5-pentamethylcyclopentadienyl)titanium(III)]-μ 2,η 1:η 1-dinitrogen complex, C60H66N2Ti2

“…Besides many early transition metal complexes which are capable to activate and bind molecular nitrogen [5][6][7][8][9], our group reported the easy and smooth formation of dinuclear nitrogen-bridged low-valent titanium complexes [1]. These complexes appear to serve as synthons for di erent titanocenes, due to the weakly activated and displaceable dinitrogen.…”

N–H Group Transfer and Oxidative Addition Chemistry Promoted by Isolable Bis(cyclopentadienyl)titanium Sandwich Complexes

“…Inverting the order of addition whereby 1 was added to a saturated [D 6 ]benzene solution of ammonia increased the amount of 1-(NH 2 Based on the experimental data, a mechanism for the formation of 1-NH 2 was proposed and is presented in Figure 6. Addition of ammonia to the sandwich forms the transient ammonia complex 1-NH 3 , which in the presence of unreacted 1 undergoes formal deprotonation to yield 1-NH 2 and the titanium(III) hydride 1-H. Increased amounts of the N-H oxidative addition product 1-(NH 2 )H from the inverse addition experiment support this initial pathway.…”

“…To determine whether bis(cyclopentadienyl)titanium imido complexes bearing larger nitrogen substituents would exhibit improved thermal stability, (η 5 [13] Allowing [D 6 ]benzene solutions of 2=NSiMe 3 to stand at 23°C for days resulted in cyclometalation of the cyclopentadienyl methyl groups both adjacent and distal to the [SiMe 3 ] substituent. All four diastereomers were detected by NMR spectroscopy, but only two formed in appreciable quantities.…”

“…Oxidative addition of the N-H bonds of amines has also been observed. Addition of NH 3 to either (η 5 -C 5 Me 4 SiMe 3 ) 2 Ti or (η 5 -C 5 Me 4 iPr) 2 Ti yielded the corresponding titanocene amido hydrides along with the titanocene(III) amides, (η 5 -C 5 Me 4 R) 2 TiNH 2 , arising from formal deprotonation by unreacted sandwich from the putative titanocene ammonia complex.…”

“…[1,2] Our laboratory has discovered that several complexes of the general formula, (η 5 -C 5 Me 4 R) 2 Ti (R = iPr, tBu, SiMe 3 , SiMe 2 Ph) coordinate dinitrogen upon cooling in solution ( Figure 1). [3] Both the maximum temperature of dinitrogen coordination [3] and the number of N 2 ligands [4] are sensitive to the nature of the ring substituents, where smaller, electron-withdrawing groups increase the azophilicity of the titanium. Unusual examples of titanocene monocarbonyl complexes have also been prepared by careful addition of CO to the corresponding sandwich (Figure 1).…”