Geminal Charge-Assisted Tetrel Bonds in Bis-Pyridinium Methylene Salts

C(sp3) atoms are known to act as electrophilic sites in self-assembly processes, and in all cases reported till now, they form only one interaction with nucleophiles; that is, they function as monodentate tetrel bond donors. This manuscript reports experimental (X-ray structural analysis) and theoretical evidence (DFT calculations), proving that the methylene carbon in bis-pyridinium methylene salts establishes two short and directional C(sp3)···anion interactions; that is, they function as bidentate tetrel bond donors.


S.1.1 Materials
All the compounds were acquired from commercial suppliers (Sigma-Aldirch, TCI America) and used without further purification.

S.1.2 Characterization of the compounds
General Remarks. 1 H and 13 C NMR spectra were recorded at ambient temperature on Nuclear Magnetic Resonance Spectrometer AVANCE III, Bruker-BioSpin. All the chemical shifts are given in ppm and the Js in Hz. D 2 O was used as solvent in NMR spectra. FT-IR spectra were obtained using a Nicolet Nexus FT-IR spectrometer equipped with UATR unit.

S.2.1. General Remarks:
For compound 1: Data collections were performed at the XRD2 beamline of the Elettra Synchrotron, Trieste (Italy). 1 The crystals were dipped in NHV oil (Jena Bioscience, Jena, Germany) and mounted on the goniometer head with kapton loops (MiTeGen, Ithaca, USA). Complete datasets were collected at 100 K (nitrogen stream supplied through an Oxford Cryostream 700) through the rotating crystal method. Data were acquired using monochromatic wavelength of 0.620 Å on Pilatus hybrid-pixel area detectors (DECTRIS Ltd., Baden-Daettwil, Switzerland). The diffraction data were indexed, integrated and scaled using XDS. 2 The structure was solved by the dual space algorithm implemented in the SHELXT code. 3 Fourier analysis and refinement were performed by the fullmatrix least-squares methods based on F 2 implemented in SHELXL (Version 2018/3). 4 The Coot program was used for modeling. 5 Anisotropic thermal motion refinement have been used for all atoms. Hydrogen atoms were included at calculated positions with isotropic U factors = 1.2•U eq or Uf actors = 1.5•U eq for solvent hydrogens (U eq being the equivalent isotropic thermal factor of the bonded non hydrogen atom). Pictures were prepared using Ortep-3 6 and CCDC Mercury 7 software.

For compound 2:
The single crystal data were collected at 100 K (nitrogen stream supplied through an Oxford Cryostream 1000) using a Bruker D8 VENTURE diffractometer, equipped with a PHOTON III photon-counting detector. Unit cell refinement and data reduction were performed using SHELXL-

2018/3. 4
For compound 3: The single crystal data were collected at 100 K (nitrogen stream supplied through an Oxford Cryostream 1000) using a XtaLAB Synergy diffractometer, equipped with a HyPix detector. Unit cell refinement and data reduction were performed using CrysAlisPro 1.171.41.98a.

For compound 4:
The single crystal data of the compounds were collected at room temperature using a Bruker SMART APEX II CCD area detector diffractometer. Data collection, unit cell refinement and data reduction were performed using Bruker SAINT. Structures were solved by direct methods using SHELXT 3 and refined by full-matrix least-squares on F 2 with anisotropic displacement parameters for the non-H atoms using SHELXL-2016/6 4 . Absorption correction was performed based on multi-scan procedure using SADABS. Structure analysis was aided by use of the programs PLATON. 8 The hydrogen atoms were calculated in ideal positions with isotropic displacement parameters set to 1.2•U eq of the attached atom.        Table S. 5 Hits in the CSD containing the 1,1'-methylene bis(pyridin-1-ium) moiety. The search is set up with the structure of the 1,1'-methylene bis(pyridin-1-ium) and a positive charge to each nitrogen.

S3. Cambridge Structural Database (CSD) Surveys
In blue, hits displaying a contact between the carbon of the -CH 2 -and a nucleophile (N, P, O, S, Se, F, Cl, Br, I considered) shorter than the sum of the respective van der Waals radii and the angle N + -C···nucleophile between 160 and 180°. According to Batsanov's 9 suggestion, the crystallographic vdW radius of carbon and iodine were set to 170 pm and 210 pm, respectively.

Computational methods
The energetic features of the adducts analyzed in this work were calculated at the PBE0 [10] -D3 [11] /def2-TZVP [12] level of theory using either the crystallographic coordinates or fully optimized geometries. This level of theory has been used before [13][14][15] to analyze similar interactions and it has been proved to provide results similar to high level ab initio methods. [16] The GAUSSIAN-16 program has been used for the energetic calculations and NBO analysis. [17] The basis set superposition error for the calculation of interaction energies has been corrected using the counterpoise method. [18] Molecular electrostatic potential (MEP) surfaces have been computed at the same level of theory and represented using several isovalues of electron density to map the electrostatic potential. The QTAIM analysis [19] has been performed using the AIMAll program at the same level of theory. [20] In order to assess the nature of interactions in terms of being attractive or repulsive and revealed them in real space, we have used NCIPLOT index, which is a method for plotting non-covalent interaction regions, [21] based on the NCI (Non-Covalent Interaction) visualization index derived from the electronic density. [22]