Magnetic and Optical Studies on an S = 6 Ground-State Cluster [Cr12O9(OH)3(O2CCMe3)15]: Determination of, and the Relationship Between, Single-Ion and Cluster Spin Hamiltonian Parameters

The dodecametallic Cr(III) cluster [Cr12O9(OH)3(O2CCMe3)15] has a ground spin state of S = 6 characterized by the spin Hamiltonian parameters gZZ = 1.965, gXX = gYY = 1.960, DS=6 = +0.088 cm^-1, and ES=6 = 0 (where D and E are the axial and rhombic zero-field splitting parameters, respectively) as determined by multifrequency EPR spectroscopy and magnetization studies. Micro-SQUID magnetization studies reveal steps due to the fine structure of the ground state, with the spacing between the steps in excellent agreement with the DS=6 value determined by EPR. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion g values and D value (= -1.035 cm^-1) of the constituent Cr(III) ions directly. A vector coupling analysis demonstrates that the cluster ZFS is almost entirely due to the single-ion component. Thus, the relative orientations of the local and cluster magnetic axes can lead to a cluster ZFS of opposite sign to the single-ion value, even when this is the only significant contribution.