Synthesis and structural characterisation of 1’-(diphenylphosphino)ferrocene-1-phosphonic acid, its ammonium salts and Pd(II) complexes

Abstract

A new polar phosphinoferrocene ligand, viz. 1’-(diphenylphosphino)ferrocene-1-phosphonic acid (H2L), was prepared by hydrolysis of the corresponding phosphonate ethyl ester. However, the compound is relatively unstable, gradually decomposing upon prolonged storage, ultimately affording phosphine oxide H2LO. When the phosphine moiety was protected (e.g., in phosphine oxide H2LO and adduct H2L·BH3), no decomposition was observed. An alternative approach to prepare more stable H2L surrogates by converting the phosphonic acid into ammonium salts (dabcoH)(HL) and [(OHCH2CH2)2NH2](HL) (dabco = 1,4-diazabicyclo[2.2.2]octane) resulted in no significant stabilisation. H2L reacted with [PdCl2(cod)] (cod = cycloocta-1,5-diene), producing the bis(phosphine) complex, trans-[PdCl2(H2L-κP)2]. When mixed with Pd(II)-acetylacetonate (acac) complexes with ortho-metallated auxiliary ligands, [(LCY)Pd(acac)] (LCY = 2-[(dimethylamino-κN)methyl]phenyl-κC1 and 2-[(methylthio-κS)methyl]phenyl-κC1), H2L gave rise to bis-chelate complexes of the [(LCY)Pd(HL-κ2O,P)] type. H2L, the ammonium salts featuring the HL− anion, and all Pd(II) complexes were structurally characterised by single-crystal X-ray diffraction analysis. Variations in phosphonate P-O bond lengths observed in the crystal structures were rationalised by DFT computations.