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

Abstract

Six-coordinate, rigorously octahedral d Mn(III) spin crossover (SCO) complexes are limited by symmetry to an S = 1 (intermediate spin, IS) to S = 2 (high spin, HS) transition. In order to realize the potential S = 0 to S = 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) S = 0 systems, with a range of donor sets including the hexadentate [NO] Schiff base ligands. The electronic structure design criteria necessary for realizing a ΔS = 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)(PF)] (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 S = 0 to S = 2 transition. Successful synthesis of such a complex could provide the first example of a ΔS = 2 SCO transition for d 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.

Assessment of Double-Hybrid Density Functional Theory for Magnetic Exchange Coupling in Manganese Complexes

“…TPSSh, with 10% HF exchange, shows the best overall performance among all methods: it achieves deviations from experiment of merely 3-6 cm −1 for complexes 1, 3, and 4, with the largest deviation being 25 cm −1 for the strongest antiferromagnetic coupling in the test set (complex 5). The good performance of TPSSh documented here is in agreement with previous studies on synthetic manganese complexes and bioinorganic model systems [34][35][36]38,[40][41][42][43][80][81][82][83][84][85]. The increase in the percentage of HF exchange to 20% in the B3LYP functional leads to a slight overestimation of the stability of high-spin states and larger deviations from experiment.…”

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

“…When  and P are sufficiently similar a compound can undergo a thermal spin-crossover Scheme 1 High-spin and low-spin configurations of octahedral metal ions (*the S = 1 configuration for d 4 ions is an intermediate spin state for that electron count). 6 equilibrium between high-spin and low-spin forms. 4 Several compounds cited as examples in this article have that property, and lie on the cusp between two spin states.…”

“…21,22 An important example may be the oxy-haem centre in haemoglobin and myoglobin, which recent data have formulated as an intermediate-spin iron(II)/O2 centre. 23 Intermediate-spin states are better-known in octahedral d 4 complexes, where an S = 0 low-spin form is similarly impossible in a degenerate t2g subshell (Scheme 1); 6 and in some tetrahedral, 10 square-planar, 24,25 and five-coordinate 22,25,26 iron complexes.…”

Recent developments in penta-, hexa- and heptadentate Schiff base ligands and their metal complexes

“…Hexadentate Schiff base manganese(III) complexes have attracted wide attention due to their highspin (HS) state and elongated octahedral geometry. They are also widely used for building molecular magnets[284][285][286][287][288], or in extended assemblies[289], since the lengthened Mn(III) units show favorable magnetic anisotropy, with a D < 0 parameter for the zero-field-splitting Hamiltonian. For example, Four six-coordinate mononuclear manganese(III) Schiff base complex [Mn(5-MeO-sal-N-1, 5,8,12)]Y (Y =ClO4 -, 83a; BF4 -, 83b;NO3 -, 83c; CF3SO3 -, 83d), where 5-MeO-sal-N-1,5,8,12 stands for the doubly deprotonated N4O2-hexadentate Schiff base ligand obtained by 2:1 condensation of 5-methoxysalicylaldehyde and N,N'-bis(3-aminopropyl)ethylenediamine, have been reported and investigated to determine the intramolecular ligand distortion, impact of counter anion effects and intermolecular supramolecular structures on the spin crossover behavior by Wang et al(Fig.…”