Single-crystal EPR study of [Cu2(tert-Bupy)4(N3)2](ClO4)2 (tert-Bupy = 4-tert-Butylpyridine): anisotropic exchange in a ferromagnetically coupled copper(II) binuclear complex

The compound [C~~(r-Bupy),(N~)~](Clis0 .m,a)d~e up of binuclear copper(I1) cations and noncoordinated perchlorate anions. Within the binuclear unit, the copper(I1) ions are bridged by two azido groups in an end-on fashion. The magnetic behavior had shown that the metal centers were ferromagnetically coupled. This paper presents a single-crystal EPR investigation carried out at X-band frequency at 4.2 K. The principal values and the orientations of the g and D tensors associated with the triplet ground state were determined. Since two magnetically nonequivalent sites are present in the unit cell, the observed principal directions of g and D can be oriented in the molecular frame in two different ways. The solution in which one of the principal directions of D is as close as possible to the CwCu direction is physically the most likely. D is composed of a dipolar contribution DdlP and an anisotropic exchange contribution Dcx. This latter contribution is predominant, and its magnitude is related to the value of the two-electron-exchange integralpxf-~.*ys.tY2.Tyin)v olving the overlap density p(xf-Y2JY) between the xy-type orbital of an ion and the x2 - y2 type orbital of the other. A rough estimation of 150 cm-' is obtained for j(x2-Y2,xYs2-Y2sY), leading to around 300 cm-I for the 3Bzg-1B3ua nd 3B3,-1B2g splittings within the states arising from the interaction between the 2B, (xy) ground state of an ion and the 2AI(x z - y2) excited state of the other.