NMR Spectroscopic Evidence for Magnetically Induced Ring Currents in All-Metal-Aromatic [Bi$_4$]$^{4+}$

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 [Bi$_4$]$^{4+}$ 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 [Li$_4$Br]$^{3+}$-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 $^1$H and $^{13}$C NMR signals observed for the Bi4-containing species provide direct spectroscopic evidence for magnetically induced ring currents in the [Bi$_4$]$^{4+}$ 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 [Bi$_4$]$^{4+}$ ring formation. ... mehrComplementary calculations support the assignment of a magnetically responsive, σ-delocalized [Bi$_4$]$^{4+}$ core, reproduce a rare spectroscopically observed puckering motion of the rhomboidal [Bi$_4$]$^{4+}$, 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.