Synthesis and characterisation of palladium(ii) complexes with hybrid phosphinoferrocene ligands bearing additional O-donor substituents

Abstract

While 1,1′-bis(diphenylphosphino)ferrocene (dppf) is widely used as a ligand in coordination chemistry and catalysis, its congeners with oxygen-containing functional groups have long been overlooked. Accordingly, we studied the coordination behaviour in Pd(II) complexes of three phosphinoferrocene ligands bearing secondary O-donor groups, Ph₂PfcR, wherein R = CHO (1), Ac (2) and CMe₂(OH) (3), and fc = ferrocene-1,1′-diyl. Depending on the stoichiometry, reactions of 1–3 (L) with [PdCl₂(cod)] (cod = η²:η²-cycloocta-1,5-diene) produced the respective mono- and dipalladium complexes, trans-[PdCl₂(L-κP)₂] and trans-[PdCl(μ-Cl)(L-κP)]₂. Compound [PdCl(μ-Cl)(3-κP)]₂ was found to dehydrate readily, giving rise to [PdCl(μ-Cl)(Ph₂PfcC(Me)CH₂-κP)]₂. Furthermore, ligands 1–3 cleaved [(L^NC)Pd(μ-Cl)]₂ (L^NC = 2-((dimethylamino-κN)methyl)phenyl-κC¹), yielding [(L^NC)PdCl(L-κP)], which were converted into the cationic complexes [(L^NC)PdCl(L)]X (L/X = 1/PF₆, 2/SbF₆, 3/PF₆). Compounds with ligands 1 and 2 were structurally authenticated as stable bis-chelate complexes. In contrast, the product featuring ligand 3 was rather unstable and converted into [(L^NC)Pd(AcOEt-κO)(3-κP)][PF₆] upon recrystallisation. Weak oxygen coordination was confirmed via reactions of [(L^NC)PdCl(L)]X with (PhCH₂NEt₃)Cl in which the parent chloride complexes were regenerated, and this was further corroborated by DFT computations. Our findings, pointing to hemilabile coordination of 1–3, are relevant for catalysis because de-coordination of the weaker binding O-donor moiety may open a vacant site for a substrate, thereby enhancing the catalytic properties of metal complexes with ligands of this type.