Forshaw, A.P.; Smith, J.M.; Ozarowski, A.; Krzystek, J.; Smirnov, D.; Zvyagin, S.A.; Harris, T.D.; Karunadasa, H.I.; Zadrozny, J.M.; Schnegg, A.; Holldack, K.; Jackson, T.A.; Alamiri, A.; Barnes, D.M.; Telser, J.: Low-Spin Hexa-Coordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes. Inorganic Chemistry 52 (2013), p. 144-159
10.1021/ic301630d

Abstract:
Three complexes of Mn(III) with “scorpionate” type ligands have been investigated by a variety of physical techniques. The complexes are: [Tp2Mn]SbF6 (1), [Tp2*Mn]SbF6 (2), and [{PhB(MeIm)3}2Mn](CF3SO3) (3-OTf), where Tp = hydrotris(pyrazolyl)borate anion, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)3 = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3-OTf is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)3}2Mn]Cl (3-Cl) is also described. These complexes are of interest in that, in contrast to many hexa-coordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solution- and solid-state electronic absorption spectroscopy, solid-state SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied to these complexes. HFEPR in particular was useful in characterizing the S = 1 spin Hamiltonian parameters for 1 and 2, which respectively gave D = +19.97(1), E = 0.42(2) cm-1 and D = +15.89(2), E = 0.04(1) cm-1. In addition, frequency domain Fourier-transform THz EPR (FD-FT THz-EPR) spectroscopy, using coherent synchrotron radiation (CSR), was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over four times larger than in comparable hexa-coordinate Mn(III) systems with S = 2 ground states. Complex 3 (regardless of counteranion) has a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum, so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory (QCT), both density functional theory (DFT) and unrestricted Hartee-Fock (UHF) methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests the tris(carbene)borate is a strong -donor with little or no -bonding.