Oxidation state dependence of the geometry, electronic structure, and magnetic coupling in mixed oxo- and carboxylato-bridged manganese dimers.

Abstract

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)and [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)in the Mn(IV)Mn(IV), Mn(III)Mn(IV), and Mn(III)Mn(III) oxidation states. The magnetic coupling in the dimer is profoundly affected by changes in both the bridging ligands and Mn oxidation state. In particular, change in the bridging structure has a dramatic effect on the nature of the Jahn-Teller distortion observed for the Mn(III) centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)() involve the J(xz/xz)and J(yz/yz) pathways but due to the tilt of the Mn(2)O(2) core, they are less efficient than in the planar di-mu-oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the Mn(III) centers are high-spin, and the Jahn-Teller effect gives rise to axially elongated Mn(III) geometries with the distortion axis along the Mn-O(c) bonds. In the III/IV dimer, the tilt of the Mn(2)O(2) core enhances the crossed exchange J(x)()()2(-)(y)()()2(/)(z)()()2 pathway relative to the planar di-mu-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the Mn(IV) ion into low-spin Mn(III), the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between Mn(IV) (S = 3/2) and high-spin Mn(III) (S = 2), the other is the result of coupling between Mn(IV) (S = 3/2) and low-spin Mn(III) (S = 1). In [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)(), both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin Mn(III), but the Jahn-Teller axis is now orientated along the Mn-oxo bond, giving rise to axially compressed Mn(III) geometries with long Mn-O(c) equatorial bonds. In the IV/IV dimer, the ferromagnetic crossed exchange J(yz)()(/)(z)()()2 pathway partially cancels J(yz/yz) and, as a consequence, the antiferromagnetic J(xz/xz) pathway dominates the magnetic coupling. In the III/III dimer, the J(yz/yz) pathway is minimized due to the smaller Mn-O-Mn angle, and since the ferromagnetic J(yz)()(/)(z)()()2 pathway largely negates J(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high-spin and low-spin Mn(III) are almost degenerate. In the high-spin case, the odd electron is localized on the Mn(III) center, and the resulting antiferromagnetic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly delocalized due to the crossed J(yz)()(/)(z)()()2 pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalization partially converts the Mn(IV) ion into high-spin Mn(III), and consequently, the spin ladders arising from coupling of Mn(IV) (S = 3/2) with high-spin (S = 2) and low-spin (S = 1) Mn(III) are configurationally mixed. Thus, in principle, the ground-state magnetic coupling in the mixed-valence dimer will involve contributions from three spin-ladders, two associated with the delocalized low-spin structure and the third arising from the localized high-spin structure.