Manuel Schmitt

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.

The 'buckled dimer' defines the key properties of silicon and germanium(100) semiconductor surfaces and governs their behaviour during passivation, functionalization and device fabrication. Each dimer combines a Lewis-acidic 'down' atom with a Lewis-basic 'up' atom, creating a polarized motif that is difficult to isolate or to quantify. Accordingly, their atomically resolved chemistry remains challenging to interrogate. Here we show that structural constraint can translate this surface motif into a molecular system. A calix[4]pyrrolato ligand enforces a unique cis-bent geometry in a dinuclear Ge(II) complex, generating a polarized Ge-Ge unit that emulates the ambiphilic character of a Ge(100) buckled dimer. Structural analysis, quantitative Lewis acidity/basicity measurements and solution-phase reactivity reveal pronounced surface-like features, but clear divergence from conventional digermenes. The rigid geometry creates spatially separated, exceptionally strong Lewis acidic and basic sites that govern distinct reactivity patterns. Accordingly, this work establishes constrained main-group complexes as molecular surrogates for interrogating solid-vacuum interface chemistry in solution.

Quantifying Lewis acidity is essential for understanding and optimizing the performance of Lewis acids in diverse applications. Next to the widely accepted Gutmann‐Beckett (GB) method, using triethyl phosphine oxide (TEPO) as a probe, the Childs method—employing trans‐crotonaldehyde (TCA)—gained attention as an NMR‐based technique for measuring effective Lewis acidity (eLA). Despite its steady use, the robustness of Childs method and its correlation with other measures remain underexplored. Previous comparisons between the GB and Childs scales revealed significant discrepancies, suggesting that hard and soft acid/base (HSAB) characteristics may be operative. In this study, we compare thermodynamic data for TCA binding to 117 Lewis acids (representing global Lewis acidity, gLA) with their corresponding NMR‐induced chemical shifts in TCA. Our findings showcase notable deviations that reinforce key distinctions between eLA and gLA perspectives. Moreover, we identify significant limitations in the Childs method. First, the weak donor strength of TCA limits its applicability to only the strongest Lewis acids. Second, the exposed protons of TCA are prone to secondary interactions, obscuring the measurement of true Lewis acidity. Finally, our analysis reconciles discrepancies, refuting earlier assumptions that these arise from HSAB effects.

Oxidation boosts the potential of bis(catecholato)silanes, transforming them into Lewis superacidic radical cations with remarkable catalytic activity in dimerization, hydrodeoxygenation, and carbonyl-olefin metathesis.

The quantification of Lewis acidity is of fundamental and applied importance in chemistry. While the computed fluoride ion affinity (FIA) is the most widely accepted thermodynamic metric, only sparse experimental values exist. Accordingly, a benchmark of methods for computing Lewis pair formation enthalpies, also with a broader set of Lewis bases against experimental data, is missing. Herein, we evaluate different density functionals against a set of 112 experimentally determined Lewis acid/base binding enthalpies and gauge influences such as solvation correction in structure optimization. From that, we can recommend r²SCAN‐3c for robust quantification of this omnipresent interaction.

Wir stellen die Synthese und vollständige Charakterisierung des Kupfer‐Distickstoff‐Komplexes [(η¹‐N₂)Cu{Al(OR^F)₄}] 2 (R^F=C(CF₃)₃) vor, welcher durch eine Kaskaden‐Metathese‐Reaktion zwischen Ag[Al(OR^F)₄] und einem CuI‐Überschuss in iso‐Perfluorhexan (i‐pfh) unter N₂‐Atmosphäre hergestellt wurde. Die Titelverbindung 2 weist eine außerordentlich hohe N₂‐Streckschwingungsfrequenz von 2313/2314 cm⁻¹ (IR/Raman) auf und wurde durch Einkristall‐ und Pulver‐Röntgendiffraktometrie charakterisiert. Quantenchemische Ladungsverschiebungsanalysen basierend auf natürlichen Orbitalen der chemischen Valenz (CD‐NOCV) legen nahe, dass die Kupfer‐Distickstoff‐Interaktion durch eine schwache π‐Rückbindung charakterisiert wird, welche jedoch im Vergleich zu allen literaturbekannten Übergangsmetall‐Distickstoff‐Komplexen stark abgeschwächt ist.

We report the synthesis and full characterization of the copper dinitrogen complex [(η¹‐N₂)Cu{Al(OR^F)₄}] 2 (R^F=C(CF₃)₃) prepared by a cascade metathesis reaction of Ag[Al(OR^F)₄] with CuI‐excess in iso‐perfluorohexane (i‐pfh) under N₂ atmosphere. Title compound 2 features an extraordinarily high N₂ stretching frequency at 2313/2314 cm⁻¹ (IR/Raman) and was characterized by single‐crystal and powder X‐ray diffractometry. Quantum chemical charge displacement analysis based on natural orbitals of chemical valence (CD‐NOCV) indicates that the copper‐dinitrogen interaction is still governed by weak π‐backdonation, but is significantly reduced compared to all literature‐known transition metal dinitrogen complexes.

The dimerization of nitrogen monoxide (NO) is highly relevant in biochemical and environmental redox processes. Here, it is shown how structral constraint and element-ligand cooperativity can steer this reaction.

An excellent synthon for a ‘naked’ Au⁺ salt, [Au(CO)₂][F{Al(OR^F)₃}₂], was synthesized in 1,2,3,4-tetrafluorobenzene with the synergistic Ag⁺/0.5 I₂ system and used to generate the cations [Au(P₄)₂]⁺ and [Au(P₄S₃)₂]⁺ as well as the novel half-sandwich complex [Au(C₆H₆)(CO)]⁺.

The quantification of Lewis acidity is of fundamental and applied importance in chemistry. However, if neutral and charged Lewis acids are compared, a coherent ranking has been elusive, and severe uncertainties were accepted. With this study, we present a systematic computational analysis of Lewis base affinities of 784 mono‐, di‐ and tricationic Lewis acids and their comparison with 149 representative neutral Lewis acids. Evaluating vacuum fluoride ion affinities (FIA) reveals a charge‐caused clustering that prohibits any meaningful ranking. Instead, solvation‐corrected FIA _solv is identified as a metric that overcomes charge sensitivity in a balanced manner, allowing for a coherent evaluation of Lewis acidity across varying charge states. Analyzing the impact of molecular volume on solvation‐induced FIA damping provides rationales for fundamental trends and guidelines for the choice or design of neutral and cationic Lewis acids in the condensed phase. Exploring alternative scales, explicit counteranion effects, and selected experimental case studies reaffirms the advantages of solvation‐corrected FIA _solv as the most versatile and practical approach for the quantitative ranking of general (thermodynamic) Lewis acidity.