Trinuclear rare earth pyridyl-β-diketonate complexes: developing new methods towards finding lost toroidal spin states

A series of rare earth (RE) 'triangles' have been synthesised to investigate the low energy excitations of toroic molecules using inelastic neutron scattering (INS) experiments. β-Diketonate, 1-(2-pyridinyl)-1,3-butandione (<i>o</i>-pbdH) has been employed to synthesise [{Ho₃(O)₂(<i>o</i>-pbd)₃Cl(H₂O)₅}₂]·8Cl·3H₂O·0.5EtOH (1) and [{Dy₃(O)₂(<i>o</i>-pbd)₃Cl(H₂O)₅}₂]·6Cl·2EtOH·2hexane (2). With further examples, [Ho₃(OH)₂(<i>o</i>-pbd)₃Cl(H₂O)₃(EtOH)₂]·3Cl (3), [{Ho₃(OH)₂(<i>o</i>-pbd)₃Cl(H₂O)₅}₂]·6Cl·2H₂O (4) and [Dy₃(O)(OH)(<i>o</i>-pbd)₃(NO₃)₄(H₂O)₂]·2NO₃·H₂O (5) aimed towards the generation of variation in coordinated ancillary ligands that are capable of reducing symmetry and inhibiting toroic behaviour. A sterically bulky 3-(1-naphthyl)-1-(2-pyridyl)-propane-1,3-dione (<i>o</i>-nppdH) ligand impeded the formation of the RE₃ motif with [Dy₂(O)₂(<i>o</i>-nppd)₅(H₂O)]·6Cl (6) isolated. The RE₃ complexes are structurally similar to known trinuclear based single molecular toroics (SMTs), including RE-β-diketonate analogues prone to mixing between electronic states. Consistent with these, a non-magnetic SMT ground state was not observable in the new complexes utilising conventional magnetometry. Spectroscopic confirmation of toroic properties with INS was attempted for the first time on 1 and the structurally similar SMT [Ho₃(OH)₂(<i>o</i>-dppd-d₁₀)₃Cl(H₂O)₅]. The observed intensity of a broad Lorentzian at 0.69(1) meV for [Ho₃(OH)₂(<i>o</i>-dppd)₃Cl(H₂O)₅] and 0.44(1) meV for 1, is remarkably close to the calculated energy gap ∼5 cm^-1 (or 0.62 meV) and potentially provides a new probe that can validate complexes with a poorly separated toroidic ground state.