Solution structure of a pentachromium(ii) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy

Abstract

The structure of pentachromium(II) extended metal atom chain [Cr₅(tpda)₄Cl₂] (2), which behaves as a single molecule magnet at low temperature, was investigated by Density Functional Theory (DFT) calculations and spectroscopic studies without the constraints of a crystal lattice (H₂tpda = N²,N⁶-bis(pyridin-2-yl)pyridine-2,6-diamine). DFT studies both in the gas phase and including CH₂Cl₂ solvent effects indicate that an unsymmetric structure (C₄ point group), with pairs of formally quadruply-bonded metal ions and one terminal metal center, is slightly more stable (2.9 and 3.9 kcal mol⁻¹) than a symmetric structure (D₄ point group). Isotopically-labelled samples (2-d₈ and 2-d₁₆) have then been prepared to aid in molecular symmetry determination by combined ¹H and ²H NMR studies in dichloromethane solution. The spectra are strongly suggestive of a symmetric (D₄) framework, indicating fast shuttling between the two unsymmetric forms over the timescale of NMR experiments. Procedures for a high-yield Pd-free synthesis of H₂tpda and for site-selective post-synthetic H/D exchange of aromatic H₂tpda hydrogens are also reported.