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 [Et4N]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,
[Et4N]2[{PbCr2(CO)10}(CO3)] ([Et4N]2[2]) and [Et4N]2[{PbCr2(CO)10}2(CO3)] ([Et4N]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.