Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions.

The five new copper(I) complexes [Cu2(μ-Cl)2(κ1-PCPt-Bu)] (1), [Cu2(μ-Br)2(κ1-PCPt-Bu)] (2), [Cu2(μ-I)2(κ1-PCPt-Bu)] (3), [Cu2(μ-CN)2(κ1-PCPt-Bu)] (4), and [Cu4(μ3-SCN)4(κ1-PCPt-Bu)2]·CH2Cl2 (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) A, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCPt-Bu ligand in a κ1-manner and two bridging (pseudo)halido ligands in a μ2-bonding mode to generate a nonplanar Cu2(μ-X)2 framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCPt-Bu ligands in a κ1-manner and the four bridging thiocyanate groups in a μ3-manner. These complexes were used as catalysts for the hydrogenation of CO2 to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu2(μ-I)2(κ1-PCPt-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.