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

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)(O2CH)(NH3)(6)](n+) and [Mn-2(mu -O)(O2CH)(2) (NH3)(6)](n+) in the (MnMnIV)-Mn-IV, (MnMnIV)-Mn-III, and (MnMnIII)-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 particu lar, change in the bridging structure has a dramatic effect on the nature o f 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)( O2CH)(NH3)(6)](n+) involve the J(xz/xz) and J(yz/yz) pathways but due to th e tilt of the Mn2O2 core, they are less efficient than in the planar di-mu -oxo structure and, consequently, the calculated, exchange coupling constan ts 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 el ongated Mn-III geometries with the distortion axis along the Mn-Oc bonds. I n the III/IV dimer, the tilt of the Mn2O2 core enhances the crossed exchang e J(x)(-y)(/z)(2)(2)(2) pathway relative to the planar di-mu -oxo counterpa rt; leading to significant delocalization of the odd electron. Since this d elocalization pathway pastially converts the-Mn-IV ion into low-spin Mn-III . the magnetic exchange in the ground state can be considered to a;ise 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)(O2CH) (2)(NH3)(6)](n+), both the III/III dimer and the lowest energy structure fo r the III/IV dimer involve high-spin Mn-III, but the Jahn-Teller axis is no w orientated along the Mn-oxo bond, giving rise to axially compressed. Mn-I II geometries with long Mn-O-c equatorial bonds. In the IV/IV dimer, the fe rromagnetic 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) p athway largely negates J(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high spin a nd low-spin Mn-III are:almost degenerate. In the high-spin case, the odd el ectron is localized on the Mn-III center, and the resulting antiferromagnet ic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly de localized due to the crossed J(yz/z)(2) pathway, and cancellation between f erromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling; The delocalization partially converts the Mn-IV ion into high-spi n 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 configuration ally mixed. Thus, in principle, the ground-state magnetic coupling in the m ixed-valence dimer will involve contributions from three spin-ladders two a ssociated with the delocalized low-spin structure nd the third arising from the localized high-spin structure.