Carbon dioxide fixation by an unprecedented hydroxo lead-chromium carbonyl complex: Synthesis, reactivity, and theoretical calculations

The novel hydroxo-bridged dimeric lead-chromium carbonyl complex [Et ₄N]₂[{PbCr₂(CO)₁₀} ₂(μ-OH)₂] ([Et₄N]₂[1]) was synthesized from the reaction of PbCl₂ and Cr(CO)₆ followed by metathesis with [Et₄N]Br in a KOH/MeOH solution. The X-ray crystallographic structure shows that dianion 1 consists of two Pb{Cr(CO)₅}₂ units bridged by two hydroxo fragments in which the Pb atoms are further coordinated with two Cr(CO)₅ groups, resulting in a distorted tetrahedral geometry. A CO₂ molecule can insert itself into dianion 1 to form two new carbonate complexes, [Et ₄N]₂[{PbCr₂(CO)₁₀}(CO₃)] ([Et₄N]₂[2]) and [Et₄N]₂[{PbCr ₂(CO)₁₀}₂(CO₃)] ([Et ₄N]₂[3]), depending on the reaction conditions. In addition, complex 2 can be transformed into 3 in CH₂Cl₂ solution at an elevated temperature. While the carbonate group in dianion 2 is bonded to one Pb atom, which is coordinated with two Cr(CO)₅ fragments, the carbonate group in 3 bridges the two Pb centers in a μ-1κ²OO′:2κ²OO′ fashion in which each Pb atom is further bonded to two Cr(CO)₅ moieties. Complexes 2 and 3 can be converted back the hydroxo complex 1 under appropriate conditions. All three unprecedented lead-chromium compounds, 1-3, were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analyses. The nature and formation of complexes 1-3 were also examined by molecular orbital calculations using the B3LYP method of the density functional theory.