Structural and reactivity investigations using organo-copper(i) and zinc(ii) complexes with hydrogen and carbon dioxide

Abstract

Heterogeneous copper zinc catalysts are widely used in syn-gas or carbon dioxide hydrogenation to methanol, but the structures and reactivity at the catalytic active sites are still not fully understood. Well-defined metal complexes and clusters may help reveal the structures of key intermediates, including for catalytic reactions involving gaseous species. Here, a tri-copper(I) complex, [Cu₃{µ:κ³(PPC)-dppm′}₂{µ:κ²(PP)-dppm′}] 1 (dppm′ = [(Ph₂P)₂CH]⁻) is used as the starting point to investigate reactivity with hydrogen and carbon dioxide in solution and at low temperatures (20–60 °C) and gas pressures (1 bar or lower). Complex 1 reacts with hydrogen to afford a copper-hydride cluster, [Cu₅(µ-H)(µ₃-H)₂(µ:κ³(PPC)-dppm′)₂(µ:κ²(PP)-dppm)₂] 2, which contains five Cu^I centres and three hydrido ligands. The reaction of 2 with carbon dioxide provides a tri-copper(I)-bis(formate)(hydrido) complex, [Cu₃(µ₃-H)(κ¹(O)-O₂CH)(µ:κ²(OO)-O₂CH)(µ:κ²(PP)-dppm)₃] 3, in which two molecules of carbon dioxide react with the hydrides to produce formate ligands. Treating 1 with an organozinc(II) complex, [Zn(C₆F₅)₂], affords a hetero-tetrametallic Cu^I₂Zn^II₂ complex, [Cu₂(µ:κ³(PPC)-dppm′)₂{Zn(C₆F₅)₂}₂] 4. Complex 4 features two cationic Cu(I) and two anionic Zn(II) centres, adopting an unusual copper zincate speciation, and reacts with carbon dioxide to form a bis(carboxylate)-Cu^I₂Zn^II₂ complex, [Cu₂(µ:κ⁴(PPOO)-dppm″)₂{Zn(C₆F₅)₂}₂] 5; carbon dioxide inserts into the Zn–C bond and forms a new C–C bond with the methine carbon of the dppm′ ligand. Overall, the structures and reactivity indicate that Cu^I sites readily form hydrides, on exposure to hydrogen; these cuprous hydrides react with CO₂ to produce formate, and cuprous zincates react with CO₂ to afford carboxylates.