Investigation of a Neat versus Magnetically Diluted Powdered Mononuclear Mn II Complex by High‐Field and High‐Frequency EPR Spectroscopy

The isolation, structural characterization and electronic properties of a new mononuclear, pentacoordinate Mn II complex [Mn( t Buterpy)(N 3 ) 2 ] ( 1 ; t Buterpy = 4,4′,4′′‐tri‐ tert ‐butyl‐2,2′:6′,2′′‐terpyridine) is reported. The X‐ray structure of 1 reveals that the manganese ion lies in the center of a distorted trigonal bipyramid. The electronic properties of 1 were investigated by high‐field and high‐frequency EPR (HF‐EPR) spectroscopy, allowing the determination of the spin‐Hamiltonian parameters. HF‐EPR spectra were recorded on a neat powder of 1 and also on 1 magnetically diluted in the corresponding zinc complex. From the neat powder HF‐EPR spectra, the following g and zero‐field‐splitting terms, as well as the sign of D , were obtained: D = −0.250(5) cm −1 , E = 0.044(5) cm −1 and g x = g y = g z = 2.000(5). The magnetically diluted sample allows the determination of the hyperfine coupling constant A and confirms the values of the parameters determined with the neat powder: | D | = 0.260(1) cm −1 , E = 0.043(1) cm −1 , g x = g y = 2.001(1) g z = 2.0005(5) and A xx = A yy = A zz = 77.5(5) G. A comparison between these two sets of spin‐Hamiltonian parameters is discussed, as well as the correlation between the D parameter and the coordination number around the manganese ion for a coordination shell characterized by O and/or N atoms (i.e. Mn II ‐containing proteins). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)