Mechanisms of the polyol reduction of copper(ii) salts depending on the anion type and diol chain length

Abstract

The mechanisms of the polyol reduction of copper(ii) compounds were investigated by various systematic experiments. The course of the reduction in ethylene glycol strongly depends on the anion present in solution. Elemental copper can only be obtained in high yields starting from Cu(NO3)2, Cu(OAc)2, or Cu(OH)2; but not from CuCl2 or CuSO4. Intermediate compounds were observed, namely Cu2O, and the alkoxide compounds Cu(C2H4O2) and Cu3(OAc)2(C2H4O2)2. Cu3(OAc)2(C2H4O2)2 was characterised by single-crystal X-ray diffraction. It is a 2D coordination polymer with bridging bidentate acetate and bidentate deprotonated EG ligands leading to vertex- and edge-linked CuO5 polyhedra. Alkoxide compounds were also detected for higher glycol ethers. With growing glycol ether chain length the stability of alkoxide intermediates decreases, enhancing the stability range of Cu2O. Because the majority of copper species is precipitated as copper(ii) alkoxide compounds or Cu2O before the reduction to elemental copper occurs, only few nuclei are formed in solution leading to microparticles instead of nanoparticles. Much smaller but strongly agglomerated particles were observed under alkaline reaction conditions.

Influence of Common Anions on the Coordination of Metal Cations in Polyalcohols

“…Based on systematic reduction experiments, we recently proposed a sequence of decreasing affinity (hydroxide > acetate > chloride > nitrate/sulfate) towards Cu 2+ ions in EG solutions . The solid‐state structures determined in this work support this sequence.…”

“…The deprotonation of EG is, however, also possible by acetate ions, disregarding acid dissociation constants. For example, the alkoxide compound Cu 3 (OAc) 2 (C 2 H 4 O 2 ) 2 was obtained from a solution of copper(II) acetate in EG, without additional base . Because of the polyol deprotonation, acetates and hydroxides are easier to reduce than sulfates and nitrates, despite their strong coordination ability towards metal cations .…”

“…Thereby, the polyol molecules compete against other ligands, especially the anions of the metal salts used as starting materials. For example, using chloride salts of cobalt(II), nickel(II), and copper(II) as starting materials, does not lead to the formation of metallic particles. This might be rationalized by the chloride anion being a strong ligand that resides in the first coordination shell of the cation in solution and therefore prevents the reduction reaction.…”

“…The formation of complex network structures is probably due to the similar coordination ability of acetate ions (OAc) and EG, and the ability to chelate or link metal cations. Crystal structures of compounds with neutral EG ligands [Ce 0.8 Sm 0.2 (OAc) 3 (EG)] as well as with mono‐deprotonated [Zn 2 (OAc) 3 (C 2 H 5 O 2 )], and double‐deprotonated EG ligands [Cu 3 (OAc) 2 (C 2 H 4 O 2 ) 2 , Sb(OAc)(C 2 H 4 O 2 )], were described. Cu 3 (OAc) 2 (C 2 H 4 O 2 ) 2 occurs as an intermediate during the reduction of copper acetate in EG .…”

“…Crystal structures of compounds with neutral EG ligands [Ce 0.8 Sm 0.2 (OAc) 3 (EG)] as well as with mono‐deprotonated [Zn 2 (OAc) 3 (C 2 H 5 O 2 )], and double‐deprotonated EG ligands [Cu 3 (OAc) 2 (C 2 H 4 O 2 ) 2 , Sb(OAc)(C 2 H 4 O 2 )], were described. Cu 3 (OAc) 2 (C 2 H 4 O 2 ) 2 occurs as an intermediate during the reduction of copper acetate in EG . Although syntheses of metal‐nitrate‐EG compounds were described,, no crystal structures have been determined up to now.…”