Reductive Hydrogenation under Single-Site Control: Generation and Reactivity of a Transient NHC-Stabilized Tantalum(III) Alkoxide

Abstract

One of the most attractive routes for the preparation of reactive tantalum(III) species relies on the efficient salt-free hydrogenolysis of tantalum(V) alkyls or tantalum(V) alkylidenes, a process known as reductive hydrogenation. For silica-crafted tantalum alkyls and alkylidenes, this process necessarily proceeds at well-separated tantalum centers, while related reductive hydrogenations in homogeneous solution commonly involve dimeric complexes. Herein, an NHC scaffold was coordinated to a novel tri(alkoxido)tantalum(V) alkylidene to circumvent the formation of dimers during reductive hydrogenation. Employing this new model system, a key intermediate of the process, namely a hydrido-tantalum alkyl, was isolated for the first time and shown to exhibit a bidirectional reactivity. Upon being heated, the latter complex was found to undergo either an αelimination or a reductive alkane elimination. In the (overall unproductive) α-elimination step, H 2 and the parent alkylidene were regenerated, while the sought-after transient d 2 -configured tantalum(III) derivative was produced along the reaction coordinate of the reductive alkane elimination. The reactive low-valence metal center was found to rapidly attack one of the NHC substituents via an oxidative C−H activation, which led to the formation of a cyclometalated tantalum(V) hydride. The proposed elemental steps are in line with kinetic data, deuterium labeling experiments, and density functional theory (DFT) modeling studies. DFT calculations also indicated that the S = 0 spin ground state of the Ta(III) center plays a crucial role in the cyclometalation reaction. The cyclometalated Ta(V) hydride was further investigated and reacted with several alkenes and alkynes. In addition to a rich insertion and isomerization chemistry, these studies also revealed that the former hydride may undergo a formal cycloreversion and thus serve as a tantalum(III) synthon, although the original tantalum(III) intermediate is not involved in this process. The latter reactivity was observed upon reaction with internal alkynes and led to the corresponding η 2 -alkyne derivatives via vinyl intermediates, which rearrange via a remarkable, hitherto unprecedented, hydrogen shift reaction.

Tantalum η2-Vinyl Complexes Bearing an [η5:σ-Me2C(C5H4)(C2B10H10)]2– Ligand: Syntheses and Structural Characterizations

“…Coordination of RCCH and reductive elimination offer a hydridotantalum­(III) alkyne complex B that is analogous to 6 . Migratory insertion of the alkyne into the Ta–H bond yields a σ-vinyl species C , leading to the formation of the Ta η 2 -vinyl complexes 2 / 4 . ,, If p- C 6 H 4 CC–D was used as a substrate, the corresponding deuterated product 2 - d 1 was isolated in 81% yield. The presence of D was clearly shown in the D NMR spectrum, supporting the above-proposed mechanism (see the SI for details).…”

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

“…The redox-innocence of a niobium(V) NHC complex is further confirmed by the fact that Ballmann's niobium NHC complex (Figure 1J) is inert under reductive conditions (hydrogen atmosphere) while its tantalum congener is not. 70 However, except for Arnold and Ballmann's work, no general overview of the reactivity, scope, and limitations of niobium NHC complexes has been reported so far and the chemistry of niobium NHC complexes is still in its infancy.…”

Selective Reduction of CO2 to a Tantalum Formate Complex and Release of Methyl Formate from the Tantalum(V) Center

“…Very recently, we reported on the reductive hydrogenation of NHC-coordinated tantalum alkylidenes to afford the cyclometalated tantalum(V) hydride 1 and its deuterated analogue 1-d 14 , respectively (see Scheme 2, OR F = OC-(CF 3 ) 2 CH 3 ). 10 These novel hydrido tantalum complexes were shown to selectively react with sterically undemanding substrates (e.g., ethylene and acetylene), while sterically more encumbered molecules (e.g., stilbene and tolane) failed to react. 10 On the basis of this observation, we reasoned that the binding pocket in 1 may be suitable for hosting a small formate moiety, while the ensuing rather bulky tantalum formate complex should not react with a second equivalent of 1.…”

“…10 These novel hydrido tantalum complexes were shown to selectively react with sterically undemanding substrates (e.g., ethylene and acetylene), while sterically more encumbered molecules (e.g., stilbene and tolane) failed to react. 10 On the basis of this observation, we reasoned that the binding pocket in 1 may be suitable for hosting a small formate moiety, while the ensuing rather bulky tantalum formate complex should not react with a second equivalent of 1. Hence, we hoped to disfavor the formation of a diolatebridged complex and obtain a previously elusive tantalum formate.…”

“…Very recently, we reported on the reductive hydrogenation of NHC-coordinated tantalum alkylidenes to afford the cyclometalated tantalum­(V) hydride 1 and its deuterated analogue 1- d 14 , respectively (see Scheme , OR F = OC­(CF 3 ) 2 CH 3 ) . These novel hydrido tantalum complexes were shown to selectively react with sterically undemanding substrates (e.g., ethylene and acetylene), while sterically more encumbered molecules (e.g., stilbene and tolane) failed to react .…”