Synthesis, stability, and (de)hydrogenation catalysis by normal and abnormal alkene- and picolyl-tethered NHC ruthenium complexes

TL;DR: A series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes were synthesized from 2-methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7) as mentioned in this paper.

About: This article is published in Organometallics.The article was published on 2019-06-21 and is currently open access. It has received 20 citations till now. The article focuses on the topics: Cyclopentadienyl complex & Ruthenium.

Summary

Introduction

• N-heterocyclic carbenes (NHCs) have the ability to exhibit both innocent and non-innocent behavior in metal-mediated transformation reactions.
• 4,5(a) However, the formation of C(4)-bound aNHCs via transmetallation of the corresponding Ag-aNHC intermediate is generally limited because of redox reactions of the imidazolium salt with the strong oxidant Ag2O.3.
• The need for more facile routes to access these desirable aNHCs remains relevant, as the rational preparation of aNHC metal complexes continues to be a synthetic challenge.
• Abnormal coordination selectivity has also been related to steric control imparted by the tether length and the bite angle, as well as to the nature of the anion of the aNHC precursor.
• Here the authors report the synthesis of eight new abnormally bound NHC half-sandwich Ru(II) complexes and demonstrate the strong binding of these aNHC ligands through acid stability studies.

Results and Discussion

• Formation of the C(2)-isopropyl functionalized Ru(II)-aNHC complexes 9 and 10, also known as 10   Scheme 4.
• Nonetheless, complexes 1 and 3 performed considerably better than the precursor salts (entries 9,10), revealing a direct impact of the tethered aNHC ligand on the catalytic activity.
• Conversions were lower with substrates containing electron-withdrawing groups such as 4'-chloroand 4'-nitro-acetophenone (entries 2, 3).

Conclusions

• Variation of the arene ligand (p-cymene vs. cyclopentadienyl) and of the chelating tether of aNHC ligands (alkenyl vs. picolyl) provided access to six unique half-sandwich aNHC Ru(II) complexes.
• In addition, Ag-mediated C(2)-demethylation resulted in the identification of two normally-bound NHC Ru(II) side-products.
• Symmetrization of the N-alkene substituents of the aNHC ligand, as well as employing an iPr-group on the C(2)-position of the imidazolium precursor, prevented C(2)dealkylation and allowed for the selective C(4)-ruthenation for both the p-cymene and cyclopentadienyl Ru(II) precursors.
• Preliminary catalytic studies involving transfer hydrogenation suggest a greater impact of vacant coordination sites available via halide substitution (p-cymene Ru(II) complexes) than via reversible alkene and/or phosphine dissociation (cyclopentadienyl Ru(II) complexes).
• The transfer hydrogenation results indicate that chelating aNHC ligand systems provide a dynamic platform for the development of active, selective, and long-lived catalysts.

Figures

Figure 1: Order of donor strength of imidazolylidene and pyrazolylidene-based NHCs.

Figure 4: ORTEP plot of compound 10. Thermal ellipsoids are drawn at 50% level. For clarity, the phenyl moieties of PPh3 are shown as wireframe presentations, and non-coordinating anions and hydrogens are omitted.

Table 2: Transfer hydrogenation of benzophenone using complexes 1–10 a

Figure 5: 1H-NMR spectra of complex 1 upon titration with DCl showing resonances of complex 1 (a–d), free p-cymene (▲) and an aNHC Ru complex with non-coordinating N-alkenyl tether (●).

Figure 3: ORTEP plots of complexes 5–8. Thermal ellipsoids are drawn at 50% level. For clarity, the phenyl moieties of PPh3 are shown as wireframe presentations, and non-coordinating anions and hydrogens are omitted (structures of 7 and 8 have been obtained from a co-crystal).

Figure 2: ORTEP plots of compounds 1 and 3. Thermal ellipsoids are drawn at 50% level. For clarity, the phenyl moieties of PPh3 are shown as wireframe presentations, and non-coordinating anions and hydrogens are omitted.

Table 4: Oxidation of ±1-phenylethanol using 1-10 a

Table 1: Selected bond lengths (Å) and angles (º) of complexes 1, 3, 5–8, and 10

Table 3: Substrate screening in the transfer hydrogenation reaction using 1 as catalyst precursor a

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Synthesis, stability, and (de)hydrogenation catalysis by normal and abnormal alkene- and picolyl-tethered nhc ruthenium complexes" ?

In this paper, a series of p-cymene and cyclopentadienyl Ru ( II ) -aNHC derivatives have been synthesized from 2methylimidazolium salts with either an N-bound alkenyl ( 1, 3 ) or picolyl tether ( 6, 7 ).