Magneto-Structural Correlations in Bioinorganic Chemistry

Dinuclear Copper(II) Complexes Incorporating a New Septadentate Polyimidazole Ligand

“…In compound 6 , the distorted square pyramidal geometry of the Cu II ions results in a ground state configuration of the complex dominated by d x 2- y 2 orbitals that orient their lobes toward both the alkoxo and pyrazolate bridging ligands. As noted by others, 1a,b strong antiferromagnetic exchange coupling is anticipated for dinuclear complexes containing d x 2- y 2 magnetic orbitals and in compounds where the energy differences between the symmetric (d s ‘‘) and antisymmetric (d a ‘‘) combinations of magnetic orbitals are large. 1a, In this case, the S = 0 state is lower in energy than the S = 1 state, and the singlet−triplet (S−T) splitting is reflected in the magnitude of the exchange parameter J . [Cu 2 (L−Et)N 3 ] 2+ is an example of a strong antiferromagnetically coupled complex that is essentially diamagnetic at room temperature ( J > −500 cm -1 ) .…”

“…The different magnetic properties of the pyrazolate- and acetate-bridged complexes may be rationalized as resulting from differences in the coordination geometries around the Cu II ions as well as different metal−ligand magnetic orbital interactions associated with both complexes. Using the active electron approximation model, only the magnetic orbitals of the metal and the HOMO's of the bridging ligands were used to rationalize the origin of the magnetic exchange interaction 1d. In the absence of a second bridging ligand (acetate or pyrazolate), μ-alkoxo-bridged dinuclear copper complexes are antiferromagnetically coupled ( J = −316 cm -1 ) …”

“…1 H and 13 C NMR spectra were obtained using either a Varian XL-300 or a Bruker AMX-500 spectrometer. The variable-temperature solid-state magnetic susceptibility data were collected on a Quantum Design MPMS5 SQUID susceptometer equipped with a 55 kG magnet and operating in the range 1.8−400 K. Diamagnetic corrections were estimated using Pascal's constants and were subtracted from the experimental molar susceptibility to obtain the paramagnetic molar susceptibility 1k…”

“…Magnetism has played an important role in the development of our understanding of the structural and electronic factors that govern spin exchange phenomena in inorganic and bioinorganic systems . The detection of magnetic exchange coupling between paramagnetic metal centers has been particularly useful in deducing the structures of polynuclear metal compounds and the active sites of much larger metallobiomolecules …”

Singly and Doubly Excited States of Exchange-Coupled Dimers

“…They also show some intense absorptions above 25,000 cm-1. 24 So far in our discussion we have only considered excitations that are spin forbidden in the single ion. The participation of excitations that are spin allowed in the single ion is not nearly as well understood.…”