Axially chiral monomeric and dimeric square planar Pd(ii) complexes and their application to chiral tectonics

Abstract

Mononuclear and dinuclear square planar palladium(II) complexes (denoted by [(hfac)Pd(II)(L-LH)] and [(hfac)Pd(II)(L-L)Pd(II)(hfac)], respectively) were synthesized. Here hfac⁻, HL-L⁻ and L-L²⁻ denote hexafluoroacetylacetonato, monoprotonated and non-protonated bis-β-diketonato ligands, respectively. Three bis-β-diketones were used as HL-LH: 1,2-diacetyl-1,2-dibenzoylethane (denoted by dabeH₂), 1,2-diacetyl-1,2-bis(3-methylbutanoyl)ethane (baetH₂) and 1,2-diacetyl-1,2-propanoylethane (dpeH₂). Both the monomeric and dimeric Pd(II) complexes were chiral due to the orthogonal twisting of the two non-symmetric diketonato moieties in HL-L⁻ and L-L²⁻, respectively. Optical resolution of [(hfac)Pd(II)(dabe)Pd(II)(hfac)] was achieved chromatographically on a chiral column to obtain a pair of optical antipodes which were stable against racemization. As for the other complexes, resolution was possible only after replacing hfac⁻ with a bulky ligand such as dibenzoylmethanato (dbm⁻). Although a dinuclear complex with a symmetric bis-β-diketonato ligand, [(hfac)Pd(II)(taet)Pd(II)(hfac)] (taet²⁻ = 1,1,2,2-tetraacetylethanato), was achiral, the replacement of the terminal ligands with non-symmetric β-diketonates yielded an axially chiral complex such as [(phacac)Pd(II)(taet)Pd(II)(phacac)], wherein phacac⁻ denotes 1-phenyl-1,3-butanedionato. The UV and CD spectra of the Pd(II) complexes were analyzed with the help of the TDDFT calculations. The chiral monomeric species, [(dbm)Pd(II)(R- or S-baetH)], formed a heterometallic tetranuclear complex, [Fe(III){(dbm)Pd(II)(R- or S-baet)}₃], in methanol solution.