Structural Features of Cationic Palladium(II) Acetylacetonate Complexes with Dialkylbiarylphosphine Ligands

The ligand exchange reaction between the [Pd(acac)(MeCN)₂]BF₄ with the dicyclohexylbiarylphosphine ligands afforded complexes [Pd(acac)(ξPhos)]BF₄ (acac = acetylacetonate; ξPhos = SPhos (dicyclohexyl (2′,6′-dimethoxy[1,1′-biphenyl]-2-yl)phosphine) (1), XPhos (dicyclohexyl(2′,6′-diisopropyl[1,1′-biphenyl]-2-yl)phosphine) (2), RuPhos (dicyclohexyl(2′,6′-diisopropoxy[1,1′-biphenyl]-2-yl)phosphine) (3), and DavePhos (2′-(dicyclohexylphosphino)-N,N-dimethyl-[1,1′-biphenyl]-2-amine) (4)), which were characterized by a variety of spectroscopic techniques. Nevertheless, for the interaction of [Pd(acac) (MeCN)₂]BF₄ with dicyclohexyl(2′-methyl[1,1′-biphenyl]-2-yl)phosphine (MePhos) the ESI⁽⁺⁾-MS data indicated decomposition of [Pd(acac)(MePhos)]BF₄ (5) in acetonitrile. Single-crystal X-ray diffraction study shows that the complexes 1 and 2 adopt for the Pd(II) center a typical distorted square planar coordination geometry, with a stabilizing interaction between the Pd and the ipso-carbon of the non-phosphorus-containing aryl ring. The metal–ligand and (complex cation)–anion interactions in the complexes have been examined using the quantum theory of atoms in molecules and by an energy decomposition analysis employing natural orbitals for chemical valence. DFT calculations of 1-5 using the r²SCAN-3c methodology together with X-ray diffraction and ¹³C NMR data suggested formation of an arenium ion upon complexation for complexes 1 -4, whereas for 5 η²-π-interaction were identified through calculations. DFT calculations were additionally employed for detailed assignment of all important bands in the IR spectra of 1-4. Furthermore, 1-4 are suitable precatalysts for the allylation of morpholine with allyl alcohol, achieving a turnover number of 200 moles of 4-allylmorpholine per mole of palladium.