Crystal structures and electron paramagnetic resonance spectra of [Cu{P(C5H4N)3}2]Br2·8H2O and Cu2+-doped [Zn{P(C5H4N)3}2]Br2·8H2O, examples of a dynamic Jahn–Teller effect in two dimensions

Abstract

Crystals of [Cu{P(C₅H₄N)₃}₂]Br₂·8H₂O, studied at 173 K are triclinic, space group P, with unit-cell dimensions a= 9.082(4), b= 11.340(1), c= 9.084(2)Å, α= 98.40(4), β= 94.78(3), γ= 98.27(7)° and Z= 1; those of [Zn{P(C₅H₄N)₃}₂]Br₂·8H₂O studied at 293 K are monoclinic, space group C2/m with a= 12.506(6), b= 13.588(7), c= 11.593(7)Å, β= 101.62(4)° and Z= 2. The structures were refined to final R= 0.050 for 3192 reflections with I 3.0σ(I), and R= 0.033 for 1737 reflections with I 3.0σ(I), respectively. The centrosymmetric copper complex has two Cu–N bonds [2.002(4)Å] considerably shorter than the other four [2.189(5)× 2, 2.191(5)Å× 2]. However, the temperature dependence of the EPR spectrum suggests that in fact the complex has a tetragonally elongated octahedral geometry with two possible orientations in the crystal lattice, these differing by interchange of the directions of the long and intermediate Cu–N bond directions. These forms are in dynamic equilibrium, with an activation energy of ≈600 cm^–1 for the interchange. Analysis of the relative intensities of the EPR signals observed at ≈ 10 K suggests an energy difference between the structural isomers of ≈4 cm^–1. The zinc complex has crystallographic 2/m symmetry with two independent Zn–N distances [2.150(3)× 2; 2.187(3)Å× 4]. The temperature dependence of the EPR spectrum of this compound doped with ≈1% Cu²⁺ is similar to that of the pure copper(II) compound, but with a lower activation energy for interchange of the structural isomers. The dynamic behaviour of the copper(II) complex in the two compounds is discussed in terms of a potential surface obtained by considering the effects of Jahn–Teller coupling and lattice strain interactions. Bonding parameters derived from the electronic spectrum are consistent with the tetragonally elongated octahedral co-ordination geometry proposed.