Tuning Ruthenium Carbene Complexes for Selective P−H Activation through Metal‐Ligand Cooperation

Abstract

On the occasion of the 60th birthday of Prof. Holger Braunschweig.

Reversible Dihydrogen Activation and Catalytic Transfer Hydrogenation with Iminophosphinoyl‐Tethered Ruthenium Carbene Complexes

“…The Ru1À N1 bond length of 2.127(2) Å is slightly shorter than in the p-nitroaniline substituted congener (2.148(2) Å), which is well in line with the increased electron donating ability of the aniline moiety. [18] The formation of the desired carbene complex 2 c was finally accomplished by dehydrohalogenation of 7 with sodium tert-butoxide, allowing for the isolation of 2 c as blue solid in 94 % yield (absorption maximum in the UV-VIS spectrum at 607.0 nm; see Figure S42 in the supporting information). 2 c is characterized by a singlet in the 31 P{ 1 H} NMR spectrum at δ P = 60.1 ppm and the absence of a signal for any proton at the carbon bridge in the 1 H NMR spectrum.…”

“…[15] Our group has focused on the use of methandiide-derived late transition metal carbene complexes such as 1 and 2, which feature a highly polarized M=C carbon linkage due to the strong electron withdrawing groups at the carbenic carbon center. [16,17,18] This enabled the facile activation of numerous element hydrogen bonds and small molecules. First investigations on transfer hydrogenation with methandiide-derived carbene complexes focused on the use of the thiophosphinoyltethered carbene complex 1, [16,19] which however showed only low activity in the transfer hydrogenation of ketones with 2propanol as hydrogen source.…”

“…[20] In the past, iminophosphinoylfunctionalized carbene complexes of type 2 (Figure 1) have been shown to enable more selective and also reversible activation reactions compared to carbene complex 1 due to an overall less polarized and therefore more stable Ru=C bond. [18,21,22] For example, complex 2 b showed selective PÀ H bond activation with secondary phosphines by addition across the M=C bond, while compound 1 only gave rise to complex reaction mixtures. We envisioned that this stabilization of the Ru=C linkage would also influence the performance in hydrogen activations and transfer hydrogenation reactions, as the release or transfer of dihydrogen should be facilitated by a more stable Ru=C double bond.…”

“…Our group has focused on the use of methandiide‐derived late transition metal carbene complexes such as 1 and 2 , which feature a highly polarized M=C carbon linkage due to the strong electron withdrawing groups at the carbenic carbon center [16,17,18] . This enabled the facile activation of numerous element hydrogen bonds and small molecules.…”

“…This could be explained by the limited stability of the complex at elevated temperatures and the high polarity of the M=C bond, which results in highly exergonic activations limiting reversibility and the transfer of the activated substrates [20] . In the past, iminophosphinoyl‐functionalized carbene complexes of type 2 (Figure 1) have been shown to enable more selective and also reversible activation reactions compared to carbene complex 1 due to an overall less polarized and therefore more stable Ru=C bond [18,21,22] . For example, complex 2 b showed selective P−H bond activation with secondary phosphines by addition across the M=C bond, while compound 1 only gave rise to complex reaction mixtures.…”