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ab initio calculations; density functional calculations; electrochemistry; Jahn-Teller distortion; manganese
Abstract:
The mononuclear manganese bis‐terpyridine complex [Mn(tolyl‐terpy)2](X)3 (1(X)3; X=BF4, ClO4, PF6; tolyl‐terpy=4′‐(4‐methylphenyl)‐2,2′:6′,2“‐terpyridine), containing Mn in the unusual +III oxidation state, has been isolated and characterised. The 13+ ion is a rare example of a mononuclear MnIII complex stabilised solely by neutral N ligands. Complex 13+ is obtained by electrochemical oxidation of the corresponding MnII compound 12+ in anhydrous acetonitrile. Under these conditions the cyclic voltammogram of 12+ exhibits not only the well‐known MnII/MnIII oxidation at E1/2=+0.91 V versus Ag/Ag+ (+1.21 V vs. SCE) but also a second metal‐based oxidation process corresponding to MnIII/MnIV at E1/2=+1.63 V (+1.93 V vs. SCE). Single crystals of 1(PF6)3⋅2 CH3CN were obtained by an electrocrystallisation procedure. X‐ray analysis unambiguously revealed its tetragonally compressed octahedral geometry and high‐spin character. The electronic properties of 13+ were investigated in detail by magnetic measurements and theoretical calculations, from which a D value of +4.82 cm−1 was precisely determined. Density functional and complete active space self consistent field ab initio calculations both correctly predict a positive sign of D, in agreement with the compressed tetragonal distortion observed in the X‐ray structure of 1(PF6)3⋅2 CH3CN. The different contributions to D were calculated, and the results show that 1) the spin–orbit coupling part (+2.593 cm−1) is predominant compared to the spin–spin interaction (+1.075 cm−1) and 2) the excited triplet states make the dominant contribution to the total D value.
