EPR and ENDOR Study of Trigonal Bipyramidal Copper Complexes with a Nitrogen Donor Tripodal Ligand

The characteristic features of the electronic structure of some trigonal bipyramidal (tbp) copper complexes of the quadridentate tripodal nitrogen donor ligands, tris(2-aminoethyl)amine (tren) and tris(2-dimethylaminoethyl)amine (trenMe₆), with different ligands at the fifth coordination position of tbp were studied in a powder form doped in diamagnetic zinc complexes by EPR and ENDOR. EPR shows that the copper complexes of tren and trenMe₆ have a ground state with a d_z2 unpaired electron orbital, and that some of the complexes are distorted from a trigonal symmetry to rhombic, to the contrary to expectations from the molecular formula. It is shown that a small structural or ligand-field distortion would give a recognizable spectral deviation from trigonal symmetry. It should be noticed that the hyperfine couplings of all the coordinating nitrogen atoms in each complex were observed by ENDOR, and that they were useful for calrifying the characteristic features of the electronic structures of the tbp complexes. The ligand fields at the fifth coordination position of tbp affects all of the coordination bonds as well as the spin distribution among the ligands. These ligand field effects on the electronic structures are discussed in reference to the ligand field parameters obtained by an EPR analysis. It is also shown that the ENDOR analysis well predicts the sp ratios of the coordinating orbitals of nitrogens, consistent with those obtained from the X-ray structural data on the complexes.