DFT Analysis of Spin Crossover in Mn(III) Complexes: Is a Two-Electron <i>S</i> = 2 to <i>S</i> = 0 Spin Transition Feasible?

Six-coordinate, rigorously octahedral d⁴ Mn­(III) spin crossover (SCO) complexes are limited by symmetry to an <i>S</i> = 1 (intermediate spin, IS) to <i>S</i> = 2 (high spin, HS) transition. In order to realize the potential <i>S</i> = 0 to <i>S</i> = 2 transition, a lower symmetry and/or change in coordination number is needed, which we explore here computationally. First, a number of complexes are analyzed to develop a reliable and relatively fast DFT protocol for reproducing known Mn­(III) spin state energetics. The hybrid meta-GGA functional TPSSh with a modest split valence plus polarization basis set and an empirical dispersion correction is found to predict correctly the ground spin state of Mn­(III) complexes, including true low-spin (LS) <i>S</i> = 0 systems, with a range of donor sets including the hexadentate [N₄O₂] Schiff base ligands. The electronic structure design criteria necessary for realizing a Δ<i>S</i> = 2 SCO transition are described, and a number of model complexes are screened for potential SCO behavior. Five-coordinate trigonal-bipyramidal symmetry fails to yield any suitable systems. Seven-coordinate, approximately pentagonal bipyramidal symmetry is more favorable, and when a known pentadentate macrocyclic donor is combined with π-acceptor axial ligands, a novel Mn­(III) complex, [Mn­(PABODP)­(PF₃)₂]³⁺ (PABODP = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]­octadeca-1(18),2,12,14,16-pentaene), is predicted to have the right spin state energetics for an <i>S</i> = 0 to <i>S</i> = 2 transition. Successful synthesis of such a complex could provide the first example of a Δ<i>S</i> = 2 SCO transition for d⁴ Mn­(III). However, the combination of a rigid macrocycle and a high coordination number dilutes the stereochemical activity of the d electrons, leading to relatively small structural changes between HS and LS systems. It may therefore remain a challenge to realize strong cooperative effects in Mn­(III) systems