Magnetic Interactions in Transition Metals
The project uses computational methods (namely density functional theory) to investigate the structures and magnetic interactions between the metal centers in polynuclear manganese clusters which model the metal sites in a number of oxomanganese enzymes. The calculations on the model systems allow the principal magnetic pathways, and their magnitudes, to be identified, which can then be used to rationalize the observed magnetic behavior in the biological systems.
Significant Achievements, Anticipated Outcomes and Future Work
Broken-symmetry, density functional calculations on the mixed oxo- and carboxylato-bridged complexes [(NH3)4Mn(m -O)2(m -O2CH)Mn(NH3)4]n+ and [(NH3)4Mn(m -O)(m -O2CH)2Mn(NH3)4]n+ which serve as model systems for several oxomanganese enzymes, have been completed for the MnIVMnIV, MnIIIMnIV and MnIIIMnIII oxidations states. A comparison with our earlier work on the planar di-oxo system [(NH3)4MnO2Mn(NH3)4]n+ has yielded valuable insights into how the interactions in these magnetically-coupled manganese clusters depend on the number and type of bridging ligands, and the oxidation states of the metal centres. The principal magnetic interactions in [Mn2(m-O)2(HCO2)(NH3)6]n+ were found to involve the Jxz/xz and Jyz/yz pathways but due to the tilt of the Mn2O2 core, they are less efficient than in the planar di-m-oxo structure. In both the MnIIIMnIV and MnIIIMnIII dimers, the MnIII centres are high spin and the Jahn-Teller effect gives rise to an axially elongated MnIII geometry. In the mixed-valence MnIIIMnIV dimer, the crossed exchange Jx2-y2/z2 pathway is quite efficient, leading to significant delocalisation of the odd electron. Since this delocalisation pathway partially converts the MnIV ion into low-spin MnIII, the coupling can be considered to arise from two interacting spin ladders, one the result of coupling between MnIV (S=3/2) and high-spin MnIII (S=2), the other between MnIV (S=3/2) and low-spin MnIII (S=1). In [Mn2(m-O)(HCO2)2(NH3)6]n+, the MnIIIMnIII dimer and the lowest energy structure for the MnIIIMnIV dimer both involve high-spin MnIII but the Jahn-Teller effect gives rise to an axially compressed MnIII geometry. In the MnIVMnIV dimer, the ferromagnetic Jyz/z2 pathway partially cancels Jyz/yz and as a consequence, the antiferromagnetic Jxz/xz pathway dominates the magnetic coupling. In the MnIIIMnIII dimer, the competition between the ferromagnetic Jyz/z2 and antiferromagnetic Jyz/yz and Jxz/xz pathways results in relatively weak overall antiferromagnetic coupling. In the MnIIIMnIV dimer, the structures involving high-spin and low-spin MnIII are almost degenerate. In the high-spin case, the odd electron is localised on the MnIII centre and the resulting antiferromagnetic coupling is similar to the MnIVMnIV dimer. In the alternative low-spin structure, the odd electron is significantly delocalised due to the crossed Jyz/z2 pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalisation partially converts the MnIV ion into high-spin MnIII and consequently the spin ladders arising from coupling of MnIV(S=3/2) with high-spin (S=2) and low-spin (S=1) MnIII are configurationally mixed. Thus, in principle the ground state magnetic coupling in the mixed valence dimer involves contributions from three spin-ladders, two associated with the delocalised low-spin structure and the third arising from the localised high-spin structure.
We have also undertaken density functional calculations on the m-oxo, m-peroxo bridged model complexes [Mn2III(m-O)(m-O2)(NH3)8]2+ and [Mn2III(m-O)(m-O2)(NH3)6(H2O)2]2+ in order to rationalize the unusual stability of the m-peroxo bridge in the structurally characterised trimer [Mn3III(m3-O)(m-O2)(AcO)2(dien)3]2+. The calculations reveal that the stability of the peroxide bridge can be attributed to a Jahn-Teller elongation of the axial Mn-N bonds perpendicular to the Mn2(m-O)(m-O2) plane which results in a stabilization of the high-spin MnIII oxidation state. However, the difference between the relative energies of the bridged and cleaved peroxide structures is small (ca 0.5 eV), the lowest energy structure depending on the nature of the terminal ligands. Calculations on the model trimer [Mn3III(m3-O)(m-O2)(HCO2)2(NH3)9]2+ indicate that the energetic differences between the cleaved and uncleaved structures is even smaller (ca 0.2 eV) and although the peroxo-bridge still remains intact, it is likely to be quite fascile.
Computational Techniques Used
The Amsterdam Density Functional (ADF) computer package was used for all calculations.
Publications, Awards and External Funding
ARC Large Grant (2000-2002): A density functional study of oxomanganese enzymes.
ARC Large Grant (2001-2003): Periodic and redox-induced trends in metal-metal bonding.
McGrady, J.E. and Stranger, R. Redox-Induced Formation and Cleavage of O-O Bonds in a Peroxo-Bridged Manganese Dimer: a Density Functional Study. Inorganic Chemistry, 38, 1999, 550-558.
Delfs, C.D. and Stranger, R. Magnetic Exchange in [Mn2(m -O)3(tmtacn)2]2+: Metal-Metal Bonding or Superexchange? Inorganic Chemistry, 39, 2000, 491-495.
Delfs, C.D. and Stranger, R. Oxidation State Dependence of the Geometry, Electronic Structure, and Magnetic Coupling in Mixed Oxo- and Carboxylato-Bridged Manganese Dimers. Inorganic Chemistry, 40, 2001, 3061-3076.
Delfs, C.D. and Stranger, R. Investigating the Stability of the Peroxide Bridge in m -oxo and Di-m -oxo Manganese Clusters. Inorganic Chemistry, 2002, submitted for publication.