An Unusual Stable Mononuclear Mn^III Bis‐terpyridine Complex Exhibiting Jahn–Teller Compression: Electrochemical Synthesis, Physical Characterisation and Theoretical Study

The mononuclear manganese bis‐terpyridine complex [Mn(tolyl‐terpy) 2 ](X) 3 ( 1 (X) 3 ; X=BF 4 , ClO 4 , PF 6 ; tolyl‐terpy=4′‐(4‐methylphenyl)‐2,2′:6′,2“‐terpyridine), containing Mn in the unusual +III oxidation state, has been isolated and characterised. The 1 3+ ion is a rare example of a mononuclear Mn III complex stabilised solely by neutral N ligands. Complex 1 3+ is obtained by electrochemical oxidation of the corresponding Mn II compound 1 2+ in anhydrous acetonitrile. Under these conditions the cyclic voltammogram of 1 2+ exhibits not only the well‐known Mn II /Mn III oxidation at E 1/2 =+0.91 V versus Ag/Ag + (+1.21 V vs. SCE) but also a second metal‐based oxidation process corresponding to Mn III /Mn IV at E 1/2 =+1.63 V (+1.93 V vs. SCE). Single crystals of 1 (PF 6 ) 3 ⋅2 CH 3 CN were obtained by an electrocrystallisation procedure. X‐ray analysis unambiguously revealed its tetragonally compressed octahedral geometry and high‐spin character. The electronic properties of 1 3+ 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 (PF 6 ) 3 ⋅2 CH 3 CN. 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.