High-frequency and field EPR investigation of (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III)

High-field and frequency electron paramagnetic resonance (HFEPR) of solid ( 8,12-diethyl-2,3,7, 13,17,18-hexamethylcorrolato)manganese(III), 1, shows t hat in the solid state it is well described as an S = 2 (high-spin) Mn(III) complex of a trianionic ligand, [(MnC3-)-C-III], just as Mn(III) porphyrin s are described as [(MnP2-)-P-III](+). Comparison among the structural data and spin Hamiltonian parameters reported for 1, Mn(III) porphyrins, and a different Mn(III) corrole, [(tpfc)Mn(OPPh3)], previously studied by HFEPR ( Bendix, J.; Gray, H. B.; Golubkov. G.; Gross, Z. J. Chem. Soc., Chem. Commu n. 2000, 1957-1958), shows that despite the molecular asymmetry of the corr ole macrocycle, the electronic structure of the Mn(HI) ion is roughly axial . However, in corroles, the S = 1 (intermediate-spin) state is much lower i n energy than in porphyrins, regardless of axial ligand. HFEPR of 1 measure d at 4.2 K in pyridine solution shows that the S = 2 [(MnC3-)-C-III] system is maintained. with slight changes in electronic parameters that are likel y the consequence of axial pyridine ligand coordination. The present result is the first example of the detection by HFEPR of a Mn(III) complex in sol ution. Over a period of hours in pyridine solution at ambient temperature, however, the S = 2 Mn(III) spectrum gradually disappears leaving a signal w ith g = 2 and Mn-55. hyperfine splitting. Analysis of this signal, also obs ervable by conventional EPR. leads to its assignment to a manganese species that could arise from decomposition of the original complex. The low-tempe rature S = 2 [(MnC3-)-C-III] state is in contrast to that at room temperatu re. which is described as a S = 1 system deriving from antiferromagnetic co upling between an S = (3)/(2) Mn(II) ion and a corrole-centered radical cat ion: [(MnC.2-)-C-II] (Licoccia, S.; Morgante. E.; Paolesse. R., Polizio, F. ; Senge, M. O., Tondello. E.; Boschi, T, Inorg. Chem. 1997, 36, 1564-1570). This temperature-dependent valence state isomerization has been observed f or other metallotetrapyrroles.