Anisotropic Exchange Effects in Temperature and Pressure Dependences of EPR Zero-Field Splitting in [(C6H5)3(n-propyl)P]2Cu2Cl6

X-band single-crystal and powder EPR data were collected in the temperature range 4.2−300 K and under hydrostatic pressure up to 500 MPa for [(C6H5)3(n-propyl)P]2Cu2Cl6 (C42H44P2Cu2Cl6). The crystal and molecular structure have been determined from X-ray diffraction. The compound crystallizes in the monoclinic space group P21/n (Z = 2) and have unit cell dimensions of a = 9.556(5) Å, b = 17.113(3) Å, c = 13.523(7) Å, and β = 96.10(4)°. The structure consists of two controsymmetric Cu2Cl6 2- dimers well separated by complex anions. EPR spectra are typical for the triplet S = 1 state of Cu2Cl6 2- dimer with parameters g x = 2.114(8), g y = 2.095(8), g z = 2.300(8), and D x = 0.025(1) cm-1, D y = 0.057(1) cm-1, and D z = −0.082(1) cm-1 at room temperature. The D tensor is dominated by a contribution from anisotropic exchange but the dipole−dipole Cu−Cu coupling is not much less. The anisotropic exchange integrals were estimated to be as follows: J xy,x 2 - y 2 an = −45 cm-1, J xy,xy an = +17 cm-1, J xy,yz an = +62 cm-1. The D tensor components are strongly temperature dependent and linearly increase on cooling with an anomalous nonlinear behavior below 100 K. The D values increase linearly with pressure, but the effect is much smaller than the temperature effect. This suggests that the D vs T dependence is dynamical in origin. EPR data, a possible mechanism, and contributions to the observed dependences are discussed and compared to EPR results for similar compounds.