Experimental and computational evidence of halogen bonds involving astatine

Abstract

The importance of halogen bonds-highly directional interactions between an electron-deficient σ-hole moiety in a halogenated compound and an acceptor such as a Lewis base-is being increasingly recognized in a wide variety of fields from biomedicinal chemistry to materials science. The heaviest halogens are known to form stronger halogen bonds, implying that if this trend continues down the periodic table, astatine should exhibit the highest halogen-bond donating ability. This may be mitigated, however, by the relativistic effects undergone by heavy elements, as illustrated by the metallic character of astatine. Here, the occurrence of halogen-bonding interactions involving astatine is experimentally evidenced. The complexation constants of astatine monoiodide with a series of organic ligands in cyclohexane solution were derived from distribution coefficient measurements and supported by relativistic quantum mechanical calculations. Taken together, the results show that astatine indeed behaves as a halogen-bond donor-a stronger one than iodine-owing to its much more electrophilic σ-hole.

On the Interplay between Charge‐Shift Bonding and Halogen Bonding

“…[42,44,[48][49][50][51][52][53] For instance, the interaction energy of At-mediated XBs can be affected by SOC effects up to 35 %. Nevertheless, many studies have demonstrated that the quantum calculations should also include the relativistic spin-orbit interaction for At-containing compounds.…”

“…The sum runs over a gaussian expansion (index k) of semi-local short-range radial potentials, which are different for different orbital angular-momentum quantum numbers l, and, for a given l, for the two total oneelectron angular-momentum quantum numbers j = l � 1 = 2 .P lj is the 2c projector onto the complete space of functions with angular symmetry l, j around the core under study. [26,44,46] The small-core pseudo-potentials ECPnMDF with n = 60, 28 and 10 were used for the At, I and Br atoms, respectively. Note that a transcription of such kind of pseudo-potentials into a scalarrelativistic spin-averaged part (averaged relativistic potential b V AREP ) and an effective one-electron spin-orbit operator ( b V SO ) is easily possible.…”

“…Indeed, astatine is to date the heaviest halogen element and the most polarisable one. [44] Since no conventional spectroscopic tools can be used, the conclusions were notably supported by quantum mechanical calculations. Astatine (Z=85) is a radioelement whose second long-lived isotope, At-211, has been recently identified as being of high potential interest for nuclear medicine applications.…”

Rapid Identification of Halogen Bonds in Co‐Crystalline Powders via 127I Nuclear Quadrupole Resonance Spectroscopy

“…[51] The changes represent increases of 0.4 %t o1 .6 %f or 35 Cl, 2.3 % to 3.8 %f or 79/81 Br, and 2.9 %t o4 .0 %f or 127 I. Each nucleus has au nique Sternheimer antishielding factor [57,58] and quadrupole moment (Q[ 127 I] = À617 mb; Q[ 79 Br] = 308.7(2) mb); Q[ 81 Br] = 257.9(2) mb; Q[ 35 Cl] = À81.12 mb), [59] with the latter taken into account using Equation (1) and solving for V 33 .The substantial shifts in the resonance frequencies as ar esult of halogen bonding far exceed the possible variation due to temperature fluctuations.…”

“…Halogen bonding is an on-covalent interaction occurring between the area of elevated electrostatic potential associated with ar egion of depleted electron density on ah alogen (halogen-bond donor), [1,2] named the s-hole, [3][4][5] and an ucleophilic center such as aL ewis base (halogen-bond acceptor). [6] Over the last decades,halogen bonding has emerged as av ersatile non-covalent interaction for use in crystal engineering, [7,8] pharmaceuticals, [9][10][11] material sciences, [12,13] and anion receptors, [14][15][16][17] to name af ew,w ith several reviews available on the subject.…”