One-Dimensional Chains of Paddlewheel-Type Dichromium(II,II) Tetraacetate Complexes: Study of Electronic Structure Influenced by σ- And π-Donation of Axial Linkers

Paddlewheel-type carboxylate-bridged dichromium(II,II) complexes possess intriguing properties such as high redox activity and thermally assisted paramagnetism. However, the relationship of their structures with electronic states and physical properties has not been extensively studied. In this work, we investigated a series of one-dimensional chain complexes based on the paddlewheel-type dichromium(II,II) tetraacetate complex ([Cr₂^II,II(OAc)₄] = [Cr₂^II,II]) with pyridine/pyrazine-type organic linkers (μ₂-L_ax) having different σ- and π-donating abilities to clarify the electronic structure of [Cr₂^II,II] assemblies. The chain compounds are stable in air, probably owing to their robust polymerized forms. X-ray crystallographic studies and magnetic measurements revealed that the basicity (pK_b) of L_ax, which quantitatively correlates with the σ-donor strength of L_ax, modulates the Cr-Cr and Cr-L_ax distances and the energy separation (E_S-T) between the diamagnetic (singlet) and thermally populated paramagnetic (triplet) states. The Cr-Cr and Cr-L_ax distances are strongly influenced by σ- and π-donation from L_ax, while the frontier δ orbital makes only a small contribution to the structural features. Density functional theory calculations were conducted to clarify this issue. The calculations produced the following unanticipated results against the long-known model: (i) the σ bonding orbital is the HOMO and dominates bonding in the [Cr₂^II,II] unit, (ii) the total Cr-Cr bond order is less than 1.0, and (iii) the δ orbital electron density is almost completely localized on the chromium sites. The computational results accurately predict the magnetic behavior and provide evidence for a new configuration of frontier orbitals in [Cr₂^II,II(RCO₂)₄(L_ax)₂].