Investigation of a neat versus magnetically diluted powdered mononuclear MnII 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(tButerpy)(N-3)(2)] (1; tButerpy = 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). (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.