Evolution of catalytically active species in paired PdCl2–CuCl2/[BMim]Cl for hydrolysis of β-1,4-glycosidic bonds

Abstract

In this work, we identified the prevailing catalytically active species evolved from paired PdCl₂–CuCl₂ in 1-butyl-3-methylimidazolium chloride ([BMim]Cl) that displayed dramatic synergy between the two metal chlorides in catalyzing the hydrolysis of β-1,4-glycosidic bonds in cellulose. Ultraviolet-visible (UV-vis), far infrared (FIR), and electron paramagnetic resonance (EPR) spectroscopy provide clear evidence for the prominent evolutionary pathways to form the catalytically active species. It is observed that the strong synergy between [Pd^IICl₄]²⁻ and [Cu^IICl₄]²⁻ is initiated by the presence of a saccharide. In the absence of a saccharide, the spectroscopic features of [Pd^IICl₄]²⁻ and [Cu^IICl₄]²⁻ in their paired solution show little change with respect to those of individual metal chloride solutions. The evolution of a highly active heterometallic Pd^II–Cl–Cu^I complex from paired PdCl₂–CuCl₂ in the presence of saccharides is revealed by the transition of the FIR absorption peak of Pd^II–Cl. Glycolaldehyde as the simplest model compound, which is the same as the reducing end group of saccharides, is applied to determine the possible catalytic species. X-ray absorption spectroscopy (XAS) verifies that [Cu^IICl₄]²⁻ is reduced to [Cu^ICl₃]²⁻ by glycolaldehyde. The HCl generated from reduced [Cu^IICl₄]²⁻ is also found to contribute to the hydrolysis of cellobiose, serving as another catalytically active species in paired PdCl₂–CuCl₂/[BMim]Cl. Paired PdCl₂–CuCl₂ pretreated with glycolaldehyde exhibits dramatically higher activity than direct use of paired PdCl₂–CuCl₂ in cellobiose conversion. Strikingly, the Pd^II–Cl–Cu^I complex is found to be substantially more active than HCl in the hydrolysis of β-1,4-glycosidic bonds as evidenced by the sustained high cellobiose conversion after neutralization of HCl with NaOH.