Characterization of tetranuclear manganese complexes and thermochromic copper(II) and nickel(II) complexes

Two different projects are described. In the first, the magnetic and electronic properties of four tetranuclear manganese complexes that are potential models for the oxygen evolving complex of photosystem II are examined. The complexes, based upon x-ray crystallography are cubane like structures containing a single manganese ion with a +4 oxidation state and three manganese ions with an oxidation state of +3. Based upon magnetic susceptibility measurements and high magnetization experiments the complexes were found to have an S = 9/1 ground state. This is the result of an antiferromagnetic interaction between Mn(III) and Mn(IV) ions and a ferromagnetic interaction between Mn(III) ions. Further characterization comes from theoretical X-alpha LCAO calculations on one of the complexes. These calculations correctly predicted the ground state and temperature dependence of the effective magnetic moment. Based upon these results, a model describing the effects of spin polarization on ground and excited state stability.; The second project examined the structural, thermodynamic and dynamic properties of a series of thermochromic complexes. The systems studied were Cu(II) and Ni(II) complexes of N,N-diethylethylenediamine with either tetrafloroborate or perchlorate counter anions. Based upon a combination of x-ray, heat capacity, nuclear magnetic resonance and molecular dynamics calculations a mechanism for the thermochromic transition was proposed. In this case, molecules are static at temperature below the transition temperature and then gain access to greater than 700 additional degrees of freedom and are dynamically disordered above the transition temperature.