Molecular motion of a nickel-bis(dithiolato) complex in solution

Abstract

The molecular motion of the planar bis(maleonitriledithiolato)nickel anion, Ni(mnt)(2)(-), has been studied as a function of temperature using electron spin resonance (ESR) in several polar solvents; they are ethyl alcohol, eugenol, dimethyl phthalate, tri-n-butyl phosphate, tris(2-ethyl-hexyl)phosphate, diglyme, and a dimethylformamide-chloroform mixed solvent. Calculated spectra in agreement with the experimental X-band spectra are obtained using axially symmetric reorientation when the long in-plane axis is the unique (parallel) axis of the rotational diffusion tensor with D parallel/D perpendicular = 3.0-4.0; D parallel and D perpendicular are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively. The reorientational model required for the simulations is either in or close to the Brownian rotational diffusion limit. In the slow motional (low temperature) region, the spectra can be simulated using the glassy g values. As the temperature increases, however, agreement is obtained only if the intermediate g factor, g(y), for the non-axially symmetric Zeeman interaction increases while g(x), g(z), and the motional model remain unchanged; this scheme and others for which gx and g(z) are possibly temperature-dependent are discussed. The values of D perpendicular from the simulations are in general agreement with those from earlier analyses of the width of the central spectral feature. The simulations and width analyses indicate (as do electrochemical, conductivity, and vapor-phase osmometry data) that the paramagnetic species reorienting in solution has a shape similar to that of the Ni(mnt)(2)(-) ion.

Figure 1

Molecular structure of Ni(mnt)₂⁻ (Ni[S₂C₂(CN)₂]₂⁻) and the right-handed principal axes of the anisotropic Zeeman interaction. Values of g_x > g_y > g_z for Ni(mnt)₂⁻ in eugenol and diglyme are given in the text.

Figure 2

Experimental and calculated ESR spectra for Ni(mnt)₂⁻ in DMF-CHCl₃. The calculated spectra were obtained using BRD, D_∥ = D_y, D_∥/D_⊥ = 4.0, T₂⁻ = 0.050F₀, the indicated values of τ₂(0) = (6D_⊥)⁻¹, and g_y = 2.0449 (−80 °C) and 2.0437 (−86 °C); the glassy values is g_y = 2.0429. The magnetic field increases left to right.

Figure 3

Experimental and calculated ESR spectra for Ni(mnt)₂⁻ in eugenol at −9 °C. The calculated spectrum was obtained using the glassy g_i (see the Results section) BRD, D_∥ = D_y, D_∥/D_⊥ = 3.0, T₂⁻ = 0, and D_⊥ = 3.90 × 10⁷ s⁻¹.

Figure 4

Experimental and calculated ESR spectra for Ni(mnt)₂⁻ in eugenol at −9 °C. The calculated spectrum was obtained using the glassy g_i, FD with D_⊥τ = 0.06, D_∥ = D_y, D_∥/D_⊥ = 4.0, T₂⁻¹ = 0.050F₀, and D_⊥ = 3.98 × 10⁷ s⁻.

Figure 5

Experimental and calculated ESR spectra for Ni(mnt)₂⁻ in eugenol at −4 °C (A) and −13.5 °C (B). The calculated spectra were obtained using the same parameters as in Figure 4 with the exception of (A) g_y = 2.0444, D_⊥ = 5.64 × 10⁷ s⁻¹, and (B) g_y = 2.0430, D_⊥ = 2.42 × 10⁷ s⁻¹.

Figure 6

Plots of Δg_y = g_y(T) – g_y(glass) vs τ₂(0) = (6D_⊥)⁻¹ in eugenol (circle), TBP (triangle), EtOH (diamond), DMF-CHCl₃ (x), DMPT (inverted triangle), TEHP (square), and diglyme (+).

Figure 7

Plots of Δg_y = g_y(T) – g_y(glass) vs T in eugenol (circle), TBP (triangle), EtOH (diamond), DMF-CHCl₃ (x), DMPT (inverted triangle), TEHP (square), and diglyme (+).

Figure 8

Comparison of τ₂(0) for the simulations and the principal line width analyses using the VTF equation in eugenol (circle), TBP (triangle), EtOH (diamond), DMF-CHCl₃ (x), DMPT (inverted triangle), TEHP (square), and diglyme (+).