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

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journal contribution
posted on 10.06.2021, 22:13 by Jean-Marc Mörsdorf, Hubert Wadepohl, Joachim Ballmann
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 α-elimination or a reductive alkane elimination. In the (overall unproductive) α-elimination step, H2 and the parent alkylidene were regenerated, while the sought-after transient d2-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 η2-alkyne derivatives via vinyl intermediates, which rearrange via a remarkable, hitherto unprecedented, hydrogen shift reaction.