Structural and Magnetic Properties of [M(C₅H₅NO)₆]L₂ (M = Cu, Zn; L = ClO4 - , BF4 -)

The perchlorate and fluoborate salts of the hexakis(pyridine N-oxide) complexes of copper(II) and zinc(II) have been studied at 25°C by single-crystal x-ray diffraction and EPR and in the 1-20 K region by magnetic susceptibility and heat capacity measurements. The room temperature EPR shows site splitting consistent with a static Jahn-Teller distortion while the bulk susceptibilities are isotropic (g = 2.3) and exhibit a large Weiss constant (θ = -2.3 K). The magnetic heat capacity is analyzed in terms of the Heisenberg linear chain model. The complexes crystallize in the space group R3 with the metal atom at the center of the unit cell, which imposes a center of inversion, and a threefold rotation axis at the metal atom. In the perchlorate complexes, the anions have one Cl-O bond along the threefold axis, pointing away from the anion. The BF4 - anions in the other complexes are positionally disordered, the predominant position being the same as in the perchlorates while a minor position has a B-F bond pointing toward the anion along the threefold axis. The cations have near-octahedral coordination about the metal atom, with metal-ligand bond lengths of 2.102 and 2.087 Å for zinc and copper, respectively. Although the Jahn-Teller effect is dynamic in the crystal lattice, space group R3, the distortions from trigonal symmetry are static on the EPR time scale up to ambient temperature, the first observation of this phenomenon. Crystal data: [Cu(C5H5NO)6](ClO4)2, space group R3, Z = 1, a = 9.620 (2) Å, α = 81.21 (2)°, R = 3.6% for 1076 reflections; [Cu(C5H5NO)6](BF4)2, space group R3, Z = 1, a = 9.621 (2) Å, α = 81.46 (2)°, R = 3.8% for 1221 reflections; [Zn(C5H5NO)6](ClO4)2, space group R3, Z = 1, a = 9.632 (2) A, α = 81.07 (2)°, R = 3.8% for 1234 reflections; [Zn(C5H5NO)6](BF4)2, space group R3, Z = 1, a = 9.621 (1) Å, α = 81.25 (4)°, R = 3.9% for 1241 reflections