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 = eta(2):eta(2)-cycloocta-1,5-diene) produced the respective mono- and dipalladium complexes, trans[ PdCl2(L-kappa P)(2)] and trans-[PdCl(mu-Cl)(L-kappa P)](2). Compound [PdCl(mu-Cl)(3-kappa P)](2) was found to dehydrate readily, giving rise to [PdCl(mu-Cl)(Ph(2)PfcC(Me)=CH2-kappa P)](2).
Furthermore, ligands 1-3 cleaved [(L-NC)Pd(mu-Cl)](2) (L-NC = 2-((dimethylamino-kappa N)methyl)phenyl-kappa C-1), yielding [(L-NC)PdCl(L-kappa P)], 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-kappa O)(3-kP)][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.