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Charge Density Study of a Halogen Bonded Salt of 2-chloropyridine and Hexacyanocobaltate(III) Acid

Cyanometallates are a family of transition metal coordination compounds with the most well-known representative being Prussian blue. Due to them being good Lewis bases cyanometallate anions, most notably hexacyanoferrates, have been used as hydrogen [1] and halogen bond acceptors [2]. It has also been shown that protonated hexacyanoferrates are good hydrogen bond donors, with the resulting compounds exhibiting a wide range of supramolecular motifs[3]. More recently, they have also been used as halogen bond acceptors [4]. Protonated hexacyanocobaltates however, remain largely unexplored in this regard. In this work we have synthesized and analyzed a halogen bonded 2-chloropyridinium hexacyanocobaltate(III) salt ; (H5O2)(2-ClpyH)2[Co(CN)6]. Analysis of the crystalline structure of the compound revealed the presence of OZundel-H···N and Npy-H···N hydrogen as well as Clpy···N halogen bonds which were further studied by experimental charge density methods. Deformation electron density of the 2-ClpyH+ cation reveals the presence of a σ-hole on the chlorine atom which is necessary for the establishment of a halogen bond. The energy of the Cl···N halogen bond (15.8 kJ/mol) is much lower than the N-H···N hydrogen bond (44.5 kJ/mol) making the latter the prevalent intermolecular interaction between the 2-ClpyH+ cation and the [Co(CN)6]3- anion. However, the dominant intermolecular interaction is the O-H···N hydrogen between the anion and the Zundel (H5O2+) cation (81.8 kJ/mol).

Cyanides ; Charge density ; Halogen bond

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