The Pnictogen Bond: The Covalently Bound Arsenic Atom in Molecular Entities in Crystals as a Pnictogen Bond Donor

Abstract

In chemical systems, the arsenic-centered pnictogen bond, or simply the arsenic bond, occurs when there is evidence of a net attractive interaction between the electrophilic region associated with a covalently or coordinately bound arsenic atom in a molecular entity and a nucleophile in another or the same molecular entity. It is the third member of the family of pnictogen bonds formed by the third atom of the pnictogen family, Group 15 of the periodic table, and is an inter- or intramolecular noncovalent interaction. In this overview, we present several illustrative crystal structures deposited into the Cambridge Structure Database (CSD) and the Inorganic Chemistry Structural Database (ICSD) during the last and current centuries to demonstrate that the arsenic atom in molecular entities has a significant ability to act as an electrophilic agent to make an attractive engagement with nucleophiles when in close vicinity, thereby forming σ-hole or π-hole interactions, and hence driving (in part, at least) the overall stability of the system’s crystalline phase. This overview does not include results from theoretical simulations reported by others as none of them address the signatory details of As-centered pnictogen bonds. Rather, we aimed at highlighting the interaction modes of arsenic-centered σ- and π-holes in the rationale design of crystal lattices to demonstrate that such interactions are abundant in crystalline materials, but care has to be taken to identify them as is usually done with the much more widely known noncovalent interactions in chemical systems, halogen bonding and hydrogen bonding. We also demonstrate that As-centered pnictogen bonds are usually accompanied by other primary and secondary interactions, which reinforce their occurrence and strength in most of the crystal structures illustrated. A statistical analysis of structures deposited into the CSD was performed for each interaction type As···D (D = N, O, S, Se, Te, F, Cl, Br, I, arene’s π system), thus providing insight into the typical nature of As···D interaction distances and ∠R–As···D bond angles of these interactions in crystals, where R is the remainder of the molecular entity.

The Pnictogen Bond, Together with Other Non-Covalent Interactions, in the Rational Design of One-, Two- and Three-Dimensional Organic-Inorganic Hybrid Metal Halide Perovskite Semiconducting Materials, and Beyond

“…The sign and magnitude enable us to gain insight into the electrophilic and nucleophilic character and strength of specific regions on the surface of a molecular entity, respectively [ 54 , 55 ]. In particular, when V S , min or V S , max is positive (i.e., V S , min > 0 or V S , max > 0), they each represent regions that are electrophilic [ 10 , 34 , 35 , 36 , 42 ]. Similarly, when both are negative (i.e., V S , min < 0 or V S , max < 0), they represent regions that are nucleophilic.…”

“…A σ-hole on atom A can be positive, negative or neutral, and is an electron density deficient region, along the extension of an R–A covalent, or coordinate bond, that is more positive relative to the lateral portions of A [ 32 , 55 ]. However, when a V S , min > 0 is found on an R–A covalent, or coordinate, bond extension, it may represent an electron deficient region that could be positive [ 34 ]; it would not be regarded as a π-hole since its location is not perpendicular to the bond axis. A V S , min > 0 occurs, for instance, along the extension of the bond in a P≡P molecule, as well as in an As 2 , Sb 2 or Bi 2 molecule [ 34 , 35 , 36 ].…”

“…A π-hole can be recognized on an atom (for example, N in NO 3 − ), or on a molecular fragment (for example, the central portion of the C≡C bond in acetylene), or at the center of a delocalized system (such as an arene moiety) that has the ability to accept electron density from a lone-pair on a Lewis base (such as O in H 2 O and N in NH 3 ). Examples of this have been discussed elsewhere [ 10 , 34 , 35 , 36 ]. Our survey included only single crystals in the CSD and ICSD that were free of errors and distortions, and that had an R -factor ≤ 0.1.…”

“…It is the first member of the family of pnictogen bonds formed by the first atom of the pnictogen family, Group 15, of the periodic table, and is an inter- or intra-molecular non-covalent interaction [ 10 ]. The possible occurrence of pnictogen bonds in many crystal lattices, formed by covalently bound nitrogen [ 10 , 33 ], phosphorous [ 33 , 35 ], arsenic [ 33 , 34 ], antimony [ 36 ] and bismuth [ 37 ], has already been discussed recently.…”

“…While our discussion is largely focused on the utilization of 1 and 2 in identifying pnictogen bonding in the illustrative crystal systems, the latter two properties were computed for some chosen systems to confirm the occurrence of such interactions between molecular entities that play any appreciable role in the overall stability of the crystal lattice, and hence in the functionality of these materials. Application of the four features above has been informative in rationalizing inter- and intra-molecular interactions of various kinds [ 10 , 34 , 35 , 36 , 38 , 42 ] in a variety of chemical systems, and hence further demonstration is unnecessary. We note that the appearance of N-centered pnictogen bonding in the illustrative crystals is usually accompanied by hydrogen bonds.…”