Supramolecular aggregation by means of charge-assisted hydrogen bonds in acid-base adducts containing amidinium cations

Abstract

Amidinium cations are molecular building blocks widely used in crystal engineering studies in view of their capacity to form strong and directional charge-assisted N-H...O bonds. The X-ray crystallographic structures of three acid-base complexes, amidinium-acetate and amidinium-(2-hydroxyethoxy)acetate in two polymorphic forms, are reported, together with a systematic analysis of acid-base adducts containing amidinium cations found in the Cambridge Crystallographic Database. A detailed discussion on the preferred packing patterns adopted by these adducts shows that the supramolecular synthon characterized by a dimeric coupling of RNþ-H and O functionalities via an O H bond is highly conserved and dominates the crystal ’s\ud three-dimensional arrangement. Calculations of the two polymorphs’ crystal energies revealed that the total energy difference between the polymorphs is less than 1 kcal/mol, H-bonding contributions being strictly comparable and the main difference being imputed to van der Waals and angular and torsional intramolecular terms, in agreement with the fact that in the two crystals the flexible anion adopts completely different\ud conformations. This is in keeping with the idea that, for an ‘a priori ’ predictability of crystal packing motifs, it is important not only to perfectly match the number of H-bond donors and acceptors but also to have a relative rigidity of the molecular fragments constituting the co-crystal

Supramolecular synthons in fluorinated and nitrogen-rich ortho-diaminotriazoles

“…The amino N5 atom is trigonal (the sum of valence angles at N5 is 359°), while N4 is pyramidal (sum is 325°) and their mutual arrangement is B-type (vide supra). An R 2 2 ð8Þ motif involving N2 and N5 is present also in this structure, but as the result of the cation and the acetate anion facing to each other through charge aided hydrogen bonds [46,47] which is a very efficient way to satisfy the ''bumps in hollows'' golden rule of close packing of layers [48].…”

Hydrogen bonds and ionic interactions in Guanidine/Guanidinium complexes: a computational case study

“…Thus, it is usually said that HBs between amino acids within a protein or those HBs established between ligands and receptors are enhanced or supplemented by electrostatic forces. This type of charge-assisted HB (CAHB) has been widely discussed in the literature by Gilli and Gilli [1][2][3][4][5][6][7] who described them as very strong HB interactions with an important covalent component. This concept has been used by many authors to discuss molecular structures.…”

“…How much are ionic forces really enhancing the strength of HB interactions? As mentioned before many authors searched for an answer to these questions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. For example, in the recently published review of Meot-Ner [18] on ionic HB (IHB), B-H ?…”

Co-crystal/salt crystal structure disorder of trichloroacetic acid–N-methylurea complex with double system of homo- and heteronuclear O–H···O/N–H···O hydrogen bonds: X-ray investigation, ab initio and DFT studies

“…Their unusual stabilization energy makes them to be distinguished from ordinary hydrogen bonds. All cases of strong and very strong hydrogen bonds have been classified into charge assisted hydrogen bond (CAHB) and resonance assisted hydrogen bond (RAHB) [13] which have been widely studied [14][15][16][17][18][19][20][21][22][23]. The enhancement of hydrogen bonding energy by additional factors, as the polarity of the donor and acceptor groups, can lead to proton transfer between molecules with formation of a salt.…”