Ligand ENDOR of Metal Complexes in Powders

Abstract

Magnetically dilute crystalline powders of the following metal complex systems have been prepared by coprecipitation: copper salicylaldoxime in palladium salicylaldoxime, copper dimethylglyoxime in nickel dimethylglyoxime, copper picolinate in phthalic acid and also in zinc picolinate, copper 2-methylpicolinate in zinc 2-methylpicolinate, copper quinaldinate in zinc quinaldinate, copper 8-hydroxyquinolinate in phthalimide and also in zinc 8-hydroxyquinolinate, and silver picolinate in phthalic acid and also in zinc picolinate. These samples have been investigated with electron–nuclear double-resonance (ENDOR) spectroscopy. At some settings of the magnetic field, single-crystal-type ENDOR spectra are obtained which arise substantially from molecules in a single orientation with respect to the magnetic field. At other settings of the field, powder-type ENDOR spectra are found which arise from molecules in many orientations. In either case the spectra are simpler and easier to interpret than the EPR powder spectra of the samples. Hyperfine and quadrupole couplings of the ligand nuclei have been measured. In all cases it has been established that the unpaired spin density in the ligand molecules resides in σ orbitals, and that the spin densities at the ligand nuclei have the same sign as the spin density at the metal ion. The nitrogen hyperfine couplings are larger and the quadrupole couplings smaller in the silver complexes, indicating stronger metal–ligand interaction. In samples containing water molecules which coordinate with the metal ion, the in-plane metal–ligand interaction is weaker. The hyperfine couplings in copper salicylaldoxime are unusually large, compared with the other copper systems with mixed oxygen–nitrogen ligands.