Abstract
Quantum chemical calculations on model copper paddlewheel (CPW) complexes of general formula [Cu-2(mu(2)-O2CR)(4)L-2] establish two local coordination geometries at the metal centers depending on the balance between equatorial and axial ligand fields. When the equatorial field is stronger than the axial field (large ligand field asymmetry), dx2-y2 dominates the stereochemical activity of the d(9) shell resulting in a relatively rigid, "orbitally directed" planar or square pyramidal structure. However, if the axial field is significantly increased, or the equatorial field moderately weakened, a small ligand field asymmetry results and both dx2-y2 and dz2 are involved in the stereochemical activity. This results in a "plastic," distorted trigonal bipyramidal geometry where the former axial ligand moves into one of the original four equatorial positions. Linkers already used to synthesize zinc-dabco MOFs (dabco = 1,4-diazabicyclo[2.2.2]octane) are shown to generate plastic CPW secondary building unit analogs with potential implications for conferring breathing behavior for MOFs which would currently be assumed to be rigid. (c) 2019 Wiley Periodicals, Inc.