Vibronic behaviour of octahedral Cu(II) complexes in Cu(2-benzoylpyridine)(2)(NO3)(2)center dot 2H(2)O crystal with a weak superexchange coupling. Temperature EPR studies

Octahedral [Cu(2-benzoylpyridille)(2)(H2O)(2)] complexes in the Cu(2-Bzpy)(2)-(NO3)(2). 2H(2)O are very dynamical and Cu(II) ions are very weakly coupled as it is shown by the single crystal EPR measurements in the temperature range 4.2-300 Ii. Rigid lattice limit (9 K) of spin-Hamiltonian parameters are: g(x) = 2.137, (g)y = 2.01-1, g(z) = 3.306 as determined by decoupling of exchange merged EPR lines. The ground state is predominantly /x(2) - y(2)] with 8% of the /z(2)] state, This mixing is produced by zero-point motions in the ground vibrational state and results in g(x) > g(y) with strong lowering of the gy-value, The decoupling procedure gave superexchange coupling parameter J = 0.0040 cm(-1), which unexpectedly is temperature independent in contrast to other weakly coupled Cu(II) paramagnets. The g-factors show relatively strong temperature variations with gy practically not affected by temperature and a tendency to averaging of the g(x) and g(z) with room temperature parameters: g(x) = 2.161, g(y) = 2.013, g(z) = 2.385. This is explained as the pseudo Jahn-Teller effect with dynamical transitions between two elongated octahedral axes, i.e. O-Cu-O (x-axis) and H2O-Cu-H2O (n-axis) with N-Cu-N (y-axis) direction in the in-plane coordination with pyridine nitrogen not affected by vibronic dynamics. The vibronic effect can be described by two-well model in the temperature range 80-150 Ii only with energy difference between the ground vibronic state in the well being 220 cm(-1) At lower temperatures Cu(II) complexes are strongly localized in the deepest potential well, whereas at higher temperatures, the vibronic g-factor averaging is dominated by the other mechanism.