Spectroscopic and Computational Investigation of Low-Spin Mn(III) Bis(scorpionate) Complexes

Six-coordinate Mn^III 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 Mn^III 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 Mn^IV 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 ³A_2g electronic ground state for the three complexes, and provide information on the energy of the "conventional" high-spin excited state (⁵E_g) 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, ³E_g, 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) Mn^III complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.