Close contacts involving germanium and tin in crystal structures: experimental evidence of tetrel bonds

Modeling indicates the presence of a region of low electronic density (a "σ-hole") on group 14 elements, and this offers an explanation for the ability of these elements to act as electrophilic sites and to form attractive interactions with nucleophiles. While many papers have described theoretical investigations of interactions involving carbon and silicon, such investigations of the heavier group 14 elements are relatively scarce. The purpose of this review is to rectify, to some extent, the current lack of experimental data on interactions formed by germanium and tin with nucleophiles. A survey of crystal structures in the Cambridge Structural Database is reported. This survey reveals that close contacts between Ge or Sn and lone-pair-possessing atoms are quite common, they can be either intra- or intermolecular contacts, and they are usually oriented along the extension of the covalent bond formed by the tetrel with the most electron-withdrawing substituent. Several examples are discussed in which germanium and tin atoms bear four carbon residues or in which halogen, oxygen, sulfur, or nitrogen substituents replace one, two, or three of those carbon residues. These close contacts are assumed to be the result of attractive interactions between the involved atoms and afford experimental evidence of the ability of germanium and tin to act as electrophilic sites, namely tetrel bond (TB) donors. This ability can govern the conformations and the packing of organic derivatives in the solid state. TBs can therefore be considered a promising and robust tool for crystal engineering. Graphical abstract Intra- and intermolecular tetrel bonds involving organogermanium and -tin derivatives in crystalline solids.

Novel Stannatrane N(CH2CMe2O)2(CMe2CH2O)SnO-t-Bu and Related Oligonuclear Tin(IV) Oxoclusters. Two Isomers in One Crystal

The syntheses of the alkanolamine N(CH₂CMe₂OH)₂(CMe₂CH₂OH) (1), of the stannatrane N(CH₂CMe₂O)₂(CMe₂CH₂O)SnO-t-Bu (2), and of the trinuclear tin oxocluster 3 consisting of the two isomers [(μ₃-O)(O-t-Bu){Sn(OCH₂CMe₂)(OCMe₂CH₂)₂N}₃] (3a) and [(μ₃-O)(μ₃-O-t-Bu){Sn(OCH₂CMe₂)(OCMe₂CH₂)₂N}₃] (3b) as well as the isolation of a few crystals of the hexanuclear tin oxocluster [LSnOSn(OH)₃LSnOH]₂ [L = N(CH₂CMe₂O)₂(CMe₂CH₂O)] (4) are reported. The compounds were characterized by ¹H, ¹³C, ¹⁵N, and ¹¹⁹Sn (1-3) nuclear magnetic resonance and infrared spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction analysis (1-4). A graph set analysis was performed for compound 1. The relative energies of 3a and 3b were estimated by density functional theory calculations that show that the energy differences are small.