Long-Range Exchange Interactions and Integer-SpinSt= 2 EPR Spectra of a CrIIIZnIICrIIISpecies with Multiplet Mixing

Synthesis, structural, and spectroscopic characterization of the linear cationic complex [LCr(III)(mu-[dmg)(3)Zn(II))Cr(III)L](2+) (1) in which L = 1,4,7-trimethyl-1,4,7-triazacyclononane and dmg is the dimethylglyoximato anion are reported. The Cr...Cr distance of 1 is 7 A. SQUID magnetic susceptibility measurements reveal the presence of long-range exchange interaction of the Cr(III) terminal ions, mediated by the diamagnetic Zn(II)(dmg)(3) "bridging ligand" (J(0) = -4.4 cm(-1), H(ex) = -2J(0)S(1)S(2), S(i) = 3/2). Multifrequency EPR measurements (S-, X-, Q-band) on frozen solutions were used to establish the intramolecular nature of the exchange coupling and to determine the zero-field splitting (ZFS) parameters and the anisotropic contributions of spin coupling. An effective spin Hamiltonian description was applied for interpretation of the spectra originating from the S(t) = 2 total spin manifold which included up to fourth-order terms for the ZFS. By the help of alternative simulations with the full coupling matrix for two spins S(i) = 3/2 (16 x 16) it could be shown that the higher-order terms in the effective description owe their origin to multiplet mixing due to competing single-ion ZFS (the absolute value of D(i) = 0.2 cm(-1)) and isotropic exchange interaction. The magnetic anisotropy related to dimer properties could be readily explained by dipolar coupling (j(z) = -0.012 cm(-1)). Implications for the interpretation of other integer-spin EPR spectra are discussed.