ic8b00352_si_001.pdf (1.97 MB)
One-Dimensional Chains of Paddlewheel-Type Dichromium(II,II) Tetraacetate Complexes: Study of Electronic Structure Influenced by σ- and π‑Donation of Axial Linkers
journal contribution
posted on 2018-04-11, 16:48 authored by Po-Jung Huang, Yoshiki Natori, Yasutaka Kitagawa, Yoshihiro Sekine, Wataru Kosaka, Hitoshi MiyasakaPaddlewheel-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
([Cr2II,II(OAc)4] = [Cr2II,II]) with pyridine/pyrazine-type organic linkers
(μ2-Lax) having different σ- and
π-donating abilities to clarify the electronic structure of
[Cr2II,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 (pKb) of Lax, which
quantitatively correlates with the σ-donor strength of Lax, modulates the Cr–Cr and Cr–Lax distances and the energy separation (ES–T) between the diamagnetic (singlet) and thermally populated paramagnetic
(triplet) states. The Cr–Cr and Cr–Lax distances
are strongly influenced by σ- and π-donation from Lax, 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
[Cr2II,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 [Cr2II,II(RCO2)4(Lax)2].