Intriguing combinations of p-cymene, chloride ion, phosphines, and amines in ruthenium metal centers: Which ligand decoordinates for ROMP?

Abstract

Cationic complexes of type [RuCl(Cym)(PR₃)(NH₂R)]PF₆ were synthesized from reactions of the precursors [RuCl₂(Cym)(PPh₃)] (1) and [RuCl₂(Cym)(P(tol₃)] (2) with NH₂Bn (1a; 2a) and NH₂Bu (1b; 2b), where p-Cym = η⁶-p-cymene, pH = phenyl, tol = p-tolyl, Bu = butyl, and Bn = benzyl. These four phosphine/amine-complexes were thoroughly characterized using single X-ray crystallography, various spectroscopic techniques (FTIR; UV–Vis; ¹H and ¹³C NMR), and DFT calculations. The impact of different ligand combinations within the coordination spheres on opening metathesis polymerization (ROMP) of norbornene (NBE) was studied as a function of temperature (20–50 °C) and irradiation using a 456 nm lamp. Both gravimetric and NMR analyses in the absence of light showed low NBE conversion, which increased under light irradiation. NMR spectra revealed that irradiation at wavelengths corresponding to the metal-centered absorption band causes the amines to decoordinate, while the p-cymene and phosphine ligands remain coordinated. NMR spectra from non-irradiated solutions, in the absence of monomer, indicated dimerization of ethyl diazoacetate, which was used as a carbene source. This observation suggests that the amine ligand is replaced during metal-carbene formation. Electrochemical measurements of the oxidation potentials for the phosphine/amine complexes indicate low d_π electronic density at the ruthenium centers, with evidence of substantial PR₃←Ru(II) backbonding. These findings were further confirmed by Natural Bond Orbital (NBO) analysis. The substitution of a chloride ion in the precursors was found to weaken the arene←Ru(II) π bond. The combination of electron density from both the phosphine and the amine ligands helps maintains the p-cymene-Ru bond, rendering the complex inert in the absence of light.