Cooperative Jahn−Teller Interactions in Dynamic Copper(II) Complexes. Temperature Dependence of the Crystal Structure and EPR Spectrum of Deuterated Ammonium Copper(II) Sulfate Hexahydrate

Abstract

The crystal structure of the deuterated Tutton salt (ND 4 ) 2 [Cu(D 2 O) 6 ](SO 4 ) 2 determined by X-ray diffraction at several temperatures between 100 and 321 K is reported. The intermediate and longest Cu-O bond lengths of the Jahn-Teller distorted octahedral Cu(D 2 O) 6 2+ ion progressively converge as the temperature is raised above ∼200 K, and this is accompanied by a partial rotation of the ammonium and sulfate groups. The g-values derived from the EPR spectrum of the compound exhibit similar behavior, as do those of ∼1% Cu 2+ doped into the isomorphous zinc(II) compound. The temperature dependence of the g-values of Cu 2+ -doped (ND 4 ) 2 [Zn(D 2 O) 6 ](SO 4 ) 2 may be interpreted satisfactorily by a model which assumes a Boltzmann thermal distribution between two energy states which differ solely in the orientation of the Cu(D 2 O) 6 2+ ion in the lattice. However, such a model does not satisfactorily explain the behavior of pure (ND 4 ) 2 [Cu(D 2 O) 6 ](SO 4 ) 2 , and it is suggested that this is due to cooperative interactions. A new model, in which the probable energy state of each complex is estimated after taking into account the likely orientations of its neighbors in the lattice, is described. Application of this model suggests that the thermal behavior is dominated by the cooperative interactions between complexes, these being transmitted via the hydrogen-bonding network. Comparisons are drawn with the cooperative interactions observed in more strongly coupled Jahn-Teller systems and in compounds for which the structural changes are associated with equilibria between two spin states.

Exchange Interactions between Dissimilar JT Copper Ions in K2Zn0.92Cu0.08(SO4)2 � 6D2O and (ND4)2Mg0.92Cu0.08(SO4)2 � 6D2O Crystals

“…These spectra are well observable up to 35 K for (I) and 80 K for (II). This supposition is supported by the modelling of the averaging process in deuterated (ND 4 ) 2 Cu(SO 4 ) Á 6D 2 O indicating the predominant role of the interactions between the complexes in the inequivalent positions [25]. The above difference in temperatures reflects the difference in the energy separation between the ground and the first excited JT configurations (72 cm --1 in I [21,23] and 108 cm --1 in II [24], respectively).…”

EPR of two Cu2+ complexes showing dynamic Jahn-Teller effects

“…Agreement with experiment is satisfactory showing that the complex present in the doped zinc(II) compound and the pure copper(II) compound have similar potential surfaces. This contrasts with the Tutton salts, where the temperature dependence of the EPR spectra of the doped systems commences at significantly lower temperatures than for the corresponding pure copper(II) compounds [6,10,17]. The energy separation between the lowest vibronic wavefunctions, -81 cm -', obtained with the RHW method is quite similar to that obtained by Simmons et al [13] with the SG approach, -77 cm -', confirming that this model is a good approximation for the compound.…”

“…The fact that the temperature dependence of the bond lengths may be explained with the SG and RHW models suggests that cooperative interactions between the complexes are negligible in [Cu(bipy)2(NO2)]NO 3 . This contrasts with the ammonium copper(II) Tutton salt, where these have a strong influence on the thermal behavior [ 10]. Presumably, the bulky nature of the aromatic amines, and lack of hydrogen bonding interactions, moderates the cooperative interactions in [Cu(bipy)2(NO,)]NO3.…”

“…The RHW model was subsequently used to interpret the temperature dependence of the EPR spectra of various mixed-ligand complexes [7,8] and has been extended to treat the temperature dependence of both the gvalues and crystal structures of pure copper(II) complexes [9]. For these studies it was sometimes found necessary to include cooperative interactions between neighboring complexes [ 10] or to take into account the temperature dependence of the rate of exchange between the vibronic wavefunctions [11].…”

Electron Spin Relaxation of Cu(II) Ions in ZnGeF6·6H2O Crystal with Strong Jahn-Teller Effect

“…Copper(II) ions introduced in the host lattice of ZnGeF 6 · 6H 2 O substitute Zn 2+ ions and produce significant distortion of the nearly perfect O h -symmetry water octahedra. This is due to strong Jahn-Teller effect leading to the three distortions of the water octahedra along Cu-O bonds separated by identical potential energy barriers of the order of 450-600 cm −1 [12]. An influence of the Jahn-Teller effect is visible in temperature dependence of the EPR spectrum as it will be shown in the next section.…”