Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

Abstract

Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp− and Tp*− (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]−, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SBF6), [Tp*2Mn]SbF6 (2SBF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the “conventional” high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm−1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

Rational Design of Tetranuclear Complexes Employing N‐Imidoylamidine Based Ligands

“…Further examination into the structural metrics of [Mn III (Py 2 ImAm) 3 ] reveal a trigonal distortion in the octahedral geometry of the Mn III ion along the C 3 axis. Not only is this reminiscent of previous examples of low spin Mn III compounds, but it should also be noted that there are only a small number of complexes that exhibit this spin state for an Mn III ion . This distortion results in slight changes to the energy levels of the d ‐orbitals (Figure S7), such that the orbital contribution to the magnetic moment for an S = 1 system should be quenched.…”

“…Considering the lack of orbital moment from the ground state S = 1 configuration of [Mn III (Py 2 ImAm) 3 ] it is reasonable to assume the primary contributor to the D ‐parameter is a low‐lying singlet excited state (Figure S7). While this is difficult to conclude exactly without electronic structure calculations, it was found that the D ‐value increases as the energy of the singlet excited state decreases in the [Tp 2 Mn III ] + family . With respect to the fitting, any attempts to restrain D < 0, did not accurately reproduce the data.…”

“…While this is difficult to conclude exactly without electronic structure calculations, it was found that the D-value increases as the energy of the singlet excited state decreases in the [Tp 2 Mn III ] + family. [23] With respect to the fitting, any attempts to restrain D < 0, did not accurately reproduce the data. Additionally, the g-factor obtained is only marginally smaller than what is expected for a truly isotropic system (i.e., 2.00), which is reasonable for the S = 1 ground state with minimal population of the excited state.…”

“…Not only is this reminiscent of previous examples of low spin Mn III compounds, but it should also be noted that there are only a small number of complexes that exhibit this spin state for an Mn III ion. [23][24][25][26][27][28][29] This distortion results in slight changes to the energy levels of the d-orbitals ( Figure S7), such that the orbital contribution to the magnetic moment for an S = 1 system should be quenched. However, considerable ZFS parameters have been obtained for the related low spin Mn III ion in [Tp 2 Mn III ] + (Tp = hydridotris-(pyrazolyl)borate anion) and its derivatives (D = +14.60 to +18.07 cm -1 , method dependent), where such findings were supported by MCD, magnetic susceptibility and high-field EPR studies.…”

“…However, considerable ZFS parameters have been obtained for the related low spin Mn III ion in [Tp 2 Mn III ] + (Tp = hydridotris-(pyrazolyl)borate anion) and its derivatives (D = +14.60 to +18.07 cm -1 , method dependent), where such findings were supported by MCD, magnetic susceptibility and high-field EPR studies. [23,30] With respect to [Mn III (Py 2 ImAm) 3 ], the variable temperature susceptibility and isothermal magnetization curves were fit using the PHI [22] program (Eq. S1-S3, S6), with the best fit parameters revealing a g-factor of 1.97 and a D-value of +8.55 cm -1 (Table 1).…”