Abstract
Tetrel bonding is the noncovalent interaction of group IV elements with electron donors. It is a weak, directional interaction that resembles hydrogen and halogen bonding yet remains barely explored. Herein, we present an experimental investigation of the carbon-centered, three-center, four-electron tetrel bond, [N-C-N]+, formed by capturing a carbenium ion with a bidentate Lewis base. NMR-spectroscopic, titration-calorimetric, and reaction-kinetic evidence for the existence and structure of this species is reported. The studied interaction is by far the strongest tetrel bond reported so far and is discussed in comparison with the analogous halogen bond. The necessity of the involvement of a bidentate Lewis base in its formation is demonstrated by providing spectroscopic and crystallographic evidence that a monodentate Lewis base induces a reaction rather than stabilizing the tetrel bond complex. A vastly decreased Lewis basicity of the bidentate ligand or reduced Lewis acidity of the carbenium ion weakens-or even prohibits-the formation of the tetrel bond complex, whereas synthetic modifications facilitating attractive orbital overlaps promote it. As the geometry of the complex resembles the SN2 transition state, it provides a model system for the investigation of fundamental reaction mechanisms and chemical bonding theories.
Highlights
Noncovalent interactions are receiving vastly increasing interest
Due to its exceptional strength, the three-center N−I−N halogen bond is applicable for stabilization of intricate complex supramolecular complexes, for instance.[24−26] The noncovalent interactions in which tetrel elements, i.e., those belonging to group IV of the periodic system, act as electrophiles are typically very weak, Received: September 4, 2018
The resulting complex (3a−f, Figure 2) ought to have a three-center, four-electron [N···C···N]+ tetrel bond and a trigonal bipyramidal geometry, analogous to that of the transition state of SN2.39 Here, we present the synthesis, solution NMR-spectroscopic, calorimetric, and kinetic investigations of the intermolecular tetrel bond complexes 3
Summary
Noncovalent interactions are receiving vastly increasing interest. Over the past decade, the hydrogen bond has been redefined,[1] and the analogous halogen,[2] pnictogen,[3] chalcogen,[4,5] aearogen,[6] and coinage-metal[7] bonds have been categorized[8,9] as either σ-hole[10] or E-bond interactions.[7]. Tetrel bond complex, whereas for a dynamic mixture of asymmetric structures, two sets of signals, or at least significant signal broadening, is expected.[38] We observed a single sharp set of 1H, 13C, and 15N NMR signals at all temperatures, including at −40 °C (Figures S3−S5, Supporting Information) This observation is best compatible with a static and symmetric geometry, even if it cannot fully exclude a very low barrier [N−C···N]+ ⇄ [N···C−N]+ interconversion, with an energy barrier ΔG‡ ≪ 38 kJ/mol and half-life of the possible interconverting states t1/2 ≪ 6.4 μs, when estimated from the chemical shift difference of pyridine (4c, δ(15N) −67 ppm) and 1-tritylpyridinium ion (5c, δ(15N) −153 ppm) as model compounds for the populated, not exchanging free versus N-alkylated end states, the 15N NMR observation frequency 50.67 MHz, and a coalescence temperature < −40 °C.53. The observation is in agreement with the outcome of the NMR and isothermal titration calorimetric studies, and with formation of a stable intermolecular complex
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