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Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes
Author(s) -
Colmer Hannah E.,
Margarit Charles G.,
Smith Jeremy M.,
Jackson Timothy A.,
Telser Joshua
Publication year - 2016
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201501250
Subject(s) - chemistry , carbene , excited state , pyrazole , ground state , ligand (biochemistry) , ligand field theory , spin states , triplet state , ab initio , spectrochemical series , magnetic circular dichroism , photochemistry , spectroscopy , boron , crystallography , computational chemistry , stereochemistry , inorganic chemistry , catalysis , molecule , atomic physics , spectral line , ion , organic chemistry , biochemistry , physics , receptor , quantum mechanics , astronomy
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) 3 B] – , (MeIm = 3‐methylimidazole) lead to formation of bis(scorpionate) complexes of Mn III with spin triplet ground states; three of which were investigated herein: [Tp 2 Mn]SbF 6 ( 1 SbF 6 ), [Tp* 2 Mn]SbF 6 ( 2 SbF 6 ), and [{Ph(MeIm) 3 B} 2 Mn]CF 3 SO 3 ( 3 CF 3 SO 3 ). 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 3 A 2g electronic ground state for the three complexes, and provide information on the energy of the “conventional” high‐spin excited state ( 5 E 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, 3 E 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.