DISORDERED CUN6 JAHN-TELLER CENTERS IN AKIS(1-METHYLTETRAZOLE)COPPER(II)TETRAFLUOROBORATE - A TEMPERATURE-DEPENDENT X-RAY-DIFFRACTION AND EPR STUDY

The influence of the molecular crystalline arrangement upon the state of a Jahn-Teller-active center has been investigated in crystals of th e complex Cu(mtz)(6)(BF4)(2), where mtz = 1-methyltetrazole. Crystal s tructures at 293, 123, and 93 K were determined by X-ray diffraction f or the copper complex and at 293 and 100 K also for the analogous zinc complex, Zn(mtz)(6)(BF4)(2). The respective lattice parameters for th e copper complex at 293, 123, and 93 K are as follows: a = 18.137(4), 17.597(4), 17.575(4) Angstrom; b = 10.237(4), 10.131(4), 10.133(4); c = 18.446(5), 18.531(4), 18.535(4) Angstrom; beta = 112.62(2), 113.55(2 ), 113.61(2)degrees. Those for the zinc complexes at 293 and 100 K, re spectively, are as follows: a = 18.153(2), 17.663(2) Angstrom; b = 10. 289(1), 10.159(2) Angstrom; c = 18.506(3), 18.578(3) Angstrom; beta = 113.21(1), 114.15(2)degrees. The crystal system is monoclinic, space g roup P2(1)/n (Z = 4), for all crystals with two crystallographically i nequivalent pairs of centrosymmetric molecules, M(mtz)(6)(BF4)(2), in the unit cell. The two inequivalent Cu(mtz)6(2+) complexes, Cu(A) and Cu(B), both exhibit Jahn-Teller distortions, but in different ways, th e Cu-N distances for the unit on site A being 2.015(4), 2.031(5), and 2.384(5) A at 93 K, while those for the unit on site B are 2.053(5), 2 .126(5), and 2.204(5) Angstrom. However, the Jahn-Teller radii of the two complexes, as calculated from the metal-ligand distances and the U tensors of the two CuN6 units, were both found to be 0.41(3) Angstrom . EPR experiments at room temperature on polycrystalline samples of th e pure copper compound and of the copper-doped zinc compound confirm t he presence of two different Jahn-Teller centers; both complexes are r apidly pulsating, but the CuN6 units on site A are confined predominan tly to one potential well of the warped Mexican hat potential, whereas the CuN6 units on site B have density in all three wells. At 78 K, ho wever, the spectrum of the polycrystalline material is consistent with a single site having an axial g tensor with maximum anisotropy (g(par allel to) = 2.300(5), g(perpendicular to) = 2.068(5)). While the low-t emperature X-ray results also indicate a structure in which the Cu(A) center is exclusively populated in one potential well, the U tensor an d potential well population data for the Cu(B) centers indicate that a t 93 K a nonpulsating averaged structure based on tetragonally elongat ed CuN6 units is being observed. The more pronounced preference for th e CuN6 octahedron on site A to show elongation in one specific directi on, in contrast to that on the B site, appears to be due to the differ ing impacts of the local-site strains at the two distinct centers of s ymmetry, and a simple model for evaluating a crystal ''packing'' strai n from the bond length data for the isomorphous zinc complex is descri bed.