Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis

Abstract

Cationic amidotitanocene complexes [Cp2Ti(NPhAr)][B(C6F5)4] (Cp=η5‐C5H5; Ar=phenyl (1 a), p‐tolyl (1 b), p‐anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA‐NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti−N bond in 1 a–c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a–c undergo photolytic Ti−N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H3BNHMe2 results in fast homolytic Ti−N cleavage to give [Cp2Ti(H3BNHMe2)][B(C6F5)4] (3). 1 a–c are highly active precatalysts in olefin hydrogenation and silanes/amines cross‐dehydrogenative coupling, whilst 3 efficiently catalyzes amine‐borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H2 activation key step was disclosed using the Activation Strain Model (ASM).

Hydrosilylation and Silane Polymerization Catalyzed by Group 4 Amidometallocene Cations

“…We have previously shown that amidotitanocene cations readily decompose by Ti−N bond homolysis under visible light irradiation (Scheme 3). 9 This behavior is typical of LMCT reactivity at a 3d metal, 39 and it is exacerbated by the well-documented reductibility of d 0 titanocene derivatives on the one hand 40−42 and the strong basicity of amides on the other hand. 43 The resulting Ti(III) complex can be trapped in the presence of THF.…”

“…Complex 1d was characterized by single crystal X-ray diffraction (XRD) analysis, which revealed (as expected) a κ 2 -N,N′ coordination mode for the amide (Figure ). Surprisingly, the Ti–N1 distance (amide) is only slightly shorter (0.046 Å) than the Ti–N2 distance (pyridine); it is also considerably longer (0.087 Å) than the Ti–N1 distance in Ib , which was previously characterized by XRD . Together, these observations indicate that the additional electron donation from the pyridine ring weakens the interaction between the metal and the amide.…”

“…In light of the above, we became interested in exploring the behavior of cationic amidometallocene complexes in catalytic reactions involving silanes, with the hope of gaining access to more efficient Cp 2 MH + manifolds ( d 0 , cationic )­(Chart ). Within this context, it is worth mentioning that complexes I also catalyze the dehydrocoupling of amines and silanes with unprecedented turnover numbers (TON approaching 4000) for a nonprecious metal . Although the mechanism of this transformation has yet to be established, the high activity of Ia – c strongly suggests that it does not involve Cp 2 TiH as the active catalyst.…”

“…For a number of years, we have been investigating the chemistry of Group 4 Cp 2 M­(ERR′) + cations, where E is a group 15 element. − In the case of amidometallocene cations (E = N, Chart ), the spatial proximity of the nitrogen lone pair of electrons with the d 0 metal center provides sufficient stabilization to make these species “bottlable”: thus, [Cp 2 M­(NRR′)]­[X] complexes (X = weakly coordinating borate; I : M = Ti; II : M = Zr) can be stored indefinitely in the freezer of an Ar glovebox. , …”

Silicon–nitrogen bond formation via dealkynative coupling of amines with bis(trimethylsilyl)acetylene mediated by KHMDS

“…The dehydrocoupling between amines and hydrosilanes seems to be the most attractive route, due to the formation of H 2 as the by-product (Figure 1, c). [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Unfortunately, this approach is completely impractical for the preparation of a highly important trimethylsilyl-protecting group, due to the pyrophoricity of gaseous Me 3 SiH. Moreover, the control of the chemoselectivity for Si-H/N-H dehydrocoupling has proved to be challenging due to the possible formation of several products including polymeric silazanes and a mixture of mono-and di-silylated amines.…”