NMR Spectroscopic Evidence for Magnetically Induced Ring Currents in All-Metal-Aromatic [Bi4]4.

Experimental assignment of aromaticity in all-metal systems remains challenging, particularly for heavy-element clusters, where local magnetic effects can obscure signatures of circular electron delocalization. Here, we investigate a supramolecularly stabilized, formally tetracationic [Bi4]4+ ring by combining structurally meaningful reference compounds, solid- and solution-state NMR spectroscopy, SQUID magnetometry, and exact two-component relativistic calculations. Comparison with an isostructural [Li4Br]3+-encapsulated analogue and a binuclear Bi(III) reference compound disentangles global ring-current effects from local heavy-atom deshielding and ligand-specific conformational effects. On this basis, the unusually downfield-shifted 1H and 13C NMR signals observed for the Bi4-containing species provide direct spectroscopic evidence for magnetically induced ring currents in the [Bi4]4+ core. By contrast, bulk magnetic measurements do not reveal significant diamagnetic exaltation, indicating that this quantity is not a resolved descriptor of aromaticity in these systems. Stoichiometric studies further identify chemically competent intermediates that provide mechanistic snapshots of [Bi4]4+ ring formation. Complementary calculations support the assignment of a magnetically responsive, σ-delocalized [Bi4]4+ core, reproduce a rare spectroscopically observed puckering motion of the rhomboidal [Bi4]4+, and underscore the critical role of spin-orbit coupling on electronic structure and dynamics. Overall, this work moves the discussion of aromaticity from predominantly computational assignment toward experimentally benchmarked analysis in the heavy-element and all-metal regime.