Salicylaldimine Ruthenium Alkylidene Complexes: Metathesis Catalysts Tuned for Protic Solvents

Abstract

Tuning the electronic and steric environment of olefin metathesis catalysts with specialized ligands can adapt them to broader applications, including metathesis in aqueous solvents. Bidentate salicylaldimine ligands are known to stabilize ruthenium alkylidene complexes, as well as allow ring-closing metathesis in protic media. Here, we report the synthesis and characterization of exceptionally robust olefin metathesis catalysts bearing both bidentate salicylaldimine and N-heterocyclic carbene ligands, including a trimethylammonium-functionalized complex adapted for polar solvents. NMR spectroscopy and X-ray crystallographic analysis confirm the structures of the complexes. Although the N-heterocyclic carbene-salicylaldimine ligand combination limits the activity of these catalysts in nonpolar solvents, this pairing enables efficient ring-closing metathesis of both dienes and enynes in methanol and methanol-water mixtures under air.

Figure 1

Olefin metathesis catalysts.

Figure 2

Solid-state molecular structure of complex 7a. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability. Selected bond lengths (Å) and angles (°): Ru–C37 1.838(2), Ru–C16 2.032(2), Ru–Cl 2.0530(15), Ru–O 2.1080(18), Ru–N1 2.3976(6), C16–N2 1.347(3), C16–N3 1.346(3), C7–N1 1.301(3), C37–H37 0.9500, C37–C38 1.476(3); C37–Ru–C16 98.28(9), C37–Ru–O 98.70(9), C16–Ru–N1 158.34(8), O–Ru–N1 89.40(6), C37–Ru–Cl 88.79(8), C16–Ru–O 83.79(7), C37–Ru–N1 103.07(8), C16–Ru–Cl 94.73(6), O–Ru–Cl 172.50(4), N1–Ru–Cl 89.35(5).

Scheme 1

Synthesis of NHC–imine complexes 7a–c by the reported route from 6a [17] and our route from 8.