Stereoisomerism in Tetrametallic Propeller-like Complexes: a Solid-state and Solution NMR Study on a Tetragallium(III) Derivative

Tetragallium(III) complex in [Ga4(L4-Py)2(dpm)6]•EtOH, with H3L4-Py = 2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol and Hdpm = dipivaloylmethane, was investigated as a diamagnetic analogue of tetrametallic, propeller-like single-molecule magnets (SMMs). The chiral molecular structure partitions the six CH2 protons of each (L4-Py)3- tripodal ligand into two diastereotopic sets. The two signals were clearly detected by 1H NMR spectroscopy in C6D6, proving that Λ and Δ enantiomers interconvert slowly over NMR timescale. Density functional theory calculations provided quantitative agreement with the observed values of chemical shifts and scalar coupling constants across both geminal and long-range interaction pathways. The solid-state structure suggests the occurrence of a lower symmetry stereoisomer (27 mol%), which was clearly identified in the NMR spectra. Since Fe3+ forms distinctly more inert complexes than Ga3+, comparable or greater configurational stability is expected for the isostructural FeIII4, FeIII3CrIII, and FeIII3VIII SMMs, which are difficult to investigate by solution NMR because of the strong paramagnetism.