X-ray Magnetic Circular Dichroism (XMCD) Study of a Methoxide-Bridged Dy^III-Cr^III Cluster Obtained by Fluoride Abstraction from cis-[Cr^III F_2 (phen)_2]^+

An isostructural series of dinuclear chromium(III)-lanthanide(III) clusters is formed by fluoride abstraction of cis-CrF2(phen)(2) by Ln(3+) resulting in LnF(3) and methoxide-bridged Cr-Ln clusters (Ln = Nd (1), Tb (2), Dy (3)) of formula [Cr-III(phen)(2)(mu-MeO)(2)Ln-(NO3)(4)].xMeOH (x = 2-2.73). In contrast to fluoride, methoxide bridges in a nonlinear fashion, which facilitates chelation. For 3, X-ray magnetic circular dichroism (XMCD) provides element-specific magnetization curves that are compared to cluster magnetization and susceptibility data acquired by SQUID magnetometry. The combination of XMCD and SQUID is able to resolve very small magnetic coupling values and reveals a weak Cr-III-Dy-III coupling of j = -0.04(3) cm(-1). The Dy-III ion has a ground-state Kramers doublet of m(J) = +/- 13/2, and the first excited doublet is found to be m(J) = +/- 11/2 at an energy of delta = 57(21) cm(-1). The Cr-III ion exhibits a uniaxial anisotropy of D-Cr = 1.7(1.0) cm(-1). Further, we observe that a weak anisotropic coupling of dipolar origin is sufficient to model the data, suggesting that methoxide bridges do not play a significant role in the magnetic coupling for the present systems.