Thermodynamic and Activation Parameters for Dissociation of [CpCr(CO)₃]₂and [Cp*Cr(CO)₃]₂into Paramagnetic Monomers from¹H NMR Shift and Line Width Measurements

Abstract

1H NMR spectra for solutions prepared by dissolution of [CpCr(CO)3]2 and [Cp*Cr(CO)3]2 in toluene in the temperature range 190−390 K are interpreted in terms of thermodynamic and kinetic parameters for dissociation of the diamagnetic dimers into the paramagnetic monomers CpCr(CO)3 and Cp*Cr(CO)3. There is no evidence in this temperature range for thermally populated excited states or non-Curie magnetic behavior of the monomers making a significant contribution to the NMR. An expression for the temperature dependence of the NMR chemical shift at limiting fast interchange of monomer and dimer in terms of the ΔH° and ΔS° for dimer dissociation is applied in determining the thermodynamic parameters for Cr−Cr bond homolysis of [CpCr(CO)3]2 ( = 15.3 ± 0.6 kcal mol-1; = 39 ± 2 cal K-1 mol-1) and [Cp* Cr(CO)3]2 ( = 14.2 ± 0.4 kcal mol-1; = 47 ± 2 cal K-1 mol-1). Rate constants and activation parameters have been evaluated from 1H NMR line broadening in the region of slow dimer−monomer interchange for dissociation of [CpCr(CO)3]2 (k1 (240 K) ≈ 59 s-1; = 17 ± 2 kcal mol-1; = 21 ± 6 cal K-1 mol-1) and [Cp*Cr(CO)3]2 (k2 (240K) ≈ 1.4 × 104 s-1; = 16 ± 1 kcal mol-1; = 30 ± 6 cal K-1 mol-1). Paramagnetic shifts also were used in deriving electron−proton coupling constants (AH) for CpCr(CO)3 (8.22 × 105 Hz) and Cp*Cr(CO)3 (1.33 × 106 Hz).