Atomic nitrogen, N, the first member of the pnictogen family, is one of the major components of Earth’s atmosphere. Its molecular analogue, N₂, is a crucial constituent in feedstocks leading to the production of fertilizers and mining explosives, and high-density materials ^{[1][2][3][4][5]}[1,2,3,4,5]. It is likely to play an ever more important role in the economy of the future as the era of the ammonia economy and “green ammonia” approaches ^{[4][6][7][8][9]}[4,6,7,8,9]. Nitrogen in numerous molecules has been recognized widely as a Lewis base, and thus can serve as an electron density donor D for the formation of various types of intermolecular interactions, including hydrogen bonds (HBs), tetrel bonds (TrBs), pnictogen bonds (PnBs), chalcogen bonds (ChBs) and halogen bonds (XBs), among others (Scheme 1a).

In the crystalline phase, N₂ molecules bond to each other through π···N(lone-pair) interactions, forming a variety of polymorphs referred to as the α-, β-, γ-, δ-, and ε-phases. The occurrence of these non-covalent interactions is unsurprising, given that the bonding region in N₂ carries a positive potential, and the electron density accumulates along and around the extension of the N≡N triple bond ^[11][10].

N forms single, double and triple bonds, and, given its significant electronegativity (χ_Pauling = 3.04; χ_Allen = 3.066), often carries a negative charge and its ability to act as a Lewis base is well known. Ammines can act both as hydrogen bond donors and acceptors. Scheme 1b, for instance, shows the way the nitrogen in dodecane-1,12-diamine bonds non-covalently with its neighbors through hydrogen bonds (r(H···N) = 2.233 Å, ∠N–H···N = 170.9°), resulting in the formation of a crystal lattice ^[10][11]. N in NH₃ and ammines is a ligand in thousands of transition metal complexes, and the coordination of NH₃ and its derivatives by metal ions has been extensively studied for many years, and often used to explore trends in stability constants of the transition metal ions (for example ^{[12][13][14][15][16][17][18][19][20][21]}[12,13,14,15,16,17,18,19,20,21]).

There are many ammonium, diammonium and their derivatives found in solid state structures, including those that may be suitable in the development of photovoltaic and optoelectronic materials (Scheme 2). The presence of cations such as these, often referred to as spacers, or additives, stabilizes the inorganic frameworks, and hence plays a structure-determining role during the synthesis of organic-inorganic hybrid materials and is responsible for their functionality. Lower-dimensional organic-inorganic hybrid metal trihalide perovskite semiconductors are prominent examples. In these, bifunctional organoammonium cations X(CH₂)₂NH₃⁺ (X = OH, Cl, Br, I, CN) control the in- and out-of-plane distortions of X(CH₂)₂NH₃)₂PbI₄ perovskites [22]. The family of ⟨100⟩-oriented layered perovskites with general formula (RNH₃)₂A_n–1B_nX_3n+1 are obtained by taking n layers along the ⟨100⟩ direction of the parent structure [22]. A search for the fragment “–CH₂–CH₂–CH₂–NH₃” in the Cambridge Structural Database (CSD ^[23][24][23,24]) led to 3656 hits; that for the fragment “–X–C–NH₃” or “X–CH₂–NH₃” (X = any atom) led to 16,834 and 9509 hits, respectively, illustrating the wide occurrence of structures featuring an RNH₃⁺ entity. Although a few studies have discussed the importance of pnictogen bonding in the metal trihalide perovskite semiconductors (for example ^[25][26][25,26]), in addition to other non-covalent interactions such as hydrogen bonding and tetrel bonding ^[27][28][29][27,28,29], theour exploration of the geometry of a variety of crystal structures suggests that the occurrence of nitrogen-centered pnictogen bonding in these systems is very common and that it plays an important role in their structural integrity.

This overview is therefore focused on exploring representative crystal structures deposited in the CSD and the Inorganic Chemistry Structural Database (ICSD) ^[30][31][30,31] in which covalently bound positively-charged nitrogen contributes in part to their structural stability through inter- or intra-molecular non-covalent interactions. In many cases, rwesearchers attribute the attraction between N and the negative site as a charge-assisted nitrogen-centered pnictogen bond (or simply, nitrogen bond ^[11][10]). This could be either a σ-hole centered or a π-hole centered nitrogen bonding interaction. A σ-hole on an atom A is recognized along the extension of the R–A covalent bond, and is deficient in electron density, where R is the remainder part of the molecular entity ^[32][33][32,33]. A π-hole can be recognized on an atom (for example, N in NO₃⁻), 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 H₂O and N in NH₃). Examples of this have been discussed elsewhere ^{[11][34][35][36]}[10,34,35,36]. TheOur 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. A statistical analysis was performed using the geometric data obtained from this survey, which is presented before the conclusions section.

The nitrogen bond, or a covalently bound nitrogen-centered pnictogen bond, in chemical systems occurs when there is evidence of a net attractive interaction between the electrophilic region associated with a covalently or coordinately bound nitrogen atom in a molecular entity and a nucleophile in another, or the same, molecular entity. 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 ^[11][10]. The possible occurrence of pnictogen bonds in many crystal lattices, formed by covalently bound nitrogen ^[11][33][10,33], phosphorous ^[33][35][33,35], arsenic ^[33][34][33,34], antimony [36] and bismuth [37], has already been discussed recently.

Four specific features were considered when identifying nitrogen bonding in the illustrative crystal systems chosen in this overview: (1) application of the concept of “less than the sum of the van der Waals (vdW) radii” ^{[38][39][40][41]}[38,39,40,41]; (2) the directionality of the putative interaction ^[42][43][42,43]; (3) the positive nature of the electrostatic potential on the surface of N in molecular entities and its engagement with a negative site ^[11][10]; and (4) the existence of promolecular charge density-based isosurface volumes between the bonded atomic basins ^[44][45][44,45]. While theour 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 ^{[11][34][35][36][38][42]}[10,34,35,36,38,42] in a variety of chemical systems, and hence further demonstration is unnecessary. RWesearchers note that the appearance of N-centered pnictogen bonding in the illustrative crystals is usually accompanied by hydrogen bonds. In the majority of the crystals explored, researcherswe found that these are stronger than the nitrogen bonds; it is therefore reasonable to conclude that they are usually the cause, and that the emergence of nitrogen bonding is probably an effect.

2. Pnictogen Bonding in 1D (One-Dimensional) Perovskite Systems

Many low-dimensional halide-based organic-inorganic hybrid perovskite compounds are known ^{[46][47][48][49]}[149,150,151,152]. They sometimes adopt a step-like (SL-type) structure which is effectively a 1D quantum wire with chains of corner-sharing octahedra “insulated” by blocks of face-sharing octahedra ^[47][150]. For instance, among others, Hoffman et al. ^[47][150] have reported a new series of 1D SL-type structures with the general formula (PA)_2m+4(MA)_m−2Pb_2m+1I_7m+4 (m = 2, 3, 4), where the PbI₆ octahedra connect in a corner- and face-sharing motif that exhibit resistivity trends that are dominated by ionic transport and no photoresponse. Yuan and coworkers ^[50][153] have reported the synthesis, crystal structure and photophysical properties of a 1D organic lead bromide perovskite, C₄N₂H₁₄PbBr₄. It consists of edge-sharing octahedral lead bromide chains [PbBr₄²⁻]_n that are surrounded by the C₄N₂H₁₄²⁺ organic cations to form the bulk assembly of core-shell quantum wires. The 1D structure has enabled the authors to demonstrate the presence of strong quantum confinement with the formation of self-trapped excited states that give efficient bluish white-light emissions with photoluminescence quantum efficiencies of approximately 20% for the bulk single crystals and 12% for the microscale crystals. Shown in Figure 1 are a few illustrative 1D crystal structures that are stabilized by the joint involvement of hydrogen- and pnictogen-bonding interactions between the organic and inorganic frameworks. The monoclinic hybrid compound [C₆H₁₈N₂]₂Sb₂Br₁₀ is monoclinically stable at room temperature, Figure 1a ^[51][154]. The photoluminescence measurements showed a strong emission line at 3.64 eV. The unaided-eye detectable blue luminescence emission came from the excitonic transition in the Sb₂Br₁₀ anions. The stability of the crystal structure arises from the intermolecular interaction between the bioctahedral [Sb₂Br₁₀]⁴⁻ dimers composed of two equivalent distorted octahedrons sharing one edge and the organic N,N-diethylethylendiammonium cation. Although the N–H⋯Br hydrogen bonds were shown to be the sole synthon for holding the crystal network, researchwers found that C–N⋯Br pnictogen bonds in the crystal should not be overlooked. The bond distance associated with the latter are less than the sum of the vdW radii of Br and N and are directional (r(N···Br) = 3.400 Å; ∠C–N···Br = 176.0°). The crystal structure of CH₃NH₂–CH₃NH₃PbI₃, Figure 1b, was also considered to involve H···N hydrogen bonds between the organic moieties CH₃NH₂ and CH₃NH₃⁺) ^[52][155]. The CH₃NH₂···CH₃NH₃⁺ dimers in the CH₃NH₂–CH₃NH₃PbI₃ intermediates were accompanied by 1D-PbI₃⁻ chains (δ-phase). The weakly hydrogen-bonded CH₃NH₂ molecules could be easily released from the CH₃NH₂–CH₃NH₃PbI₃ intermediates, probably causing rapid, spontaneous phase transition from 1D-PbI₃⁻ (δ-phase) to 3D-PbI₃⁻ (α-phase). From the crystal structure shown in Figure 1b reswearchers found that, in addition to H···N hydrogen bonds formed by the methylamine and methylammonium moieties, there are also weak C–N⋯I pnictogen bonds in the crystal that are directional. The weak aspect of these bonds is evident from the N···I intermolecular distances (r(N···I) = 3.655 Å; ∠C–N···I = 176.5°) that are less than the vdW radii sum of N and I atoms, 3.70 Å, and are longer than N···Br pnictogen bonds found in the [C₆H₁₈N₂]₂Sb₂Br₁₀ crystal (Figure 1a). Senior and coworkers ^[54][157] have reported the crystal structures of six halobismuth(III) salts of a variety of substituted aminopyridinium cations. These crystals feature discrete mononuclear [BiCl₆]³⁻ and binuclear [Bi₂X₁₀]⁴⁻ anions (X = Cl or Br), and polymeric cis-double-halo-bridged [Bi_nX_4n]ⁿ⁻ anionic chains (X = Br or I). One of these, bis(4-amino-3-ammoniopyridinium) di-μ-chlorido-bis[tetrachloridobismuth(III)] dihydrate, (C₅H₉N₃)₂[Bi₂Cl₁₀]·2H₂O, incorporates discrete [Bi₂Cl₁₀]⁴⁻ anions. The Bi(III) centers in this and other crystals adopted a slightly distorted octahedral geometry and there was a correlation between the Bi—X bond lengths and the number of classic N—H···X hydrogen bonds that the X ligand accepts, with a greater number of interactions corresponding with slightly longer Bi—X distances. The supramolecular networks formed by classic N—H···X hydrogen bonds include ladders, bilayers and three-dimensional frameworks. As shown in Figure 1c, the geometric stability of (C₅H₉N₃)₂[Bi₂Cl₁₀]·2H₂O not only emerges from the hydrogen bonds, but also C–N···Cl pnictogen bonds that are somewhat stronger than the C–N···Br and C–N···I found in the crystals shown Figure 1a,b, respectively. The structure of catena-[methylammonium bis(μ-iodo)-iodo-lead monohydrate (CH₆N⁺)(I₃Pb⁻)_n,(H₂O) ^[55][158] is shown in Figure 1d. The crystal is a needle-shaped solvation intermediate similar to CH₃NH3PbI₃·DMF that has been recognized as the main cause for the incomplete coverage of the resultant thin film. By avoiding these intermediates, the films crystallized at the gas–solid interface offer several beneficial features for device performance including high surface coverage, small surface roughness, as well as controllable grain size. The stability of the intermediate is organized by N—H···I and N—H···O hydrogen bonds, and directional C—N···I pnictogen bonds. Another illustrative 1D perovskite system in 1D, [(C₈H₁₂N)₂(PbI₄)]_n, that crystallizes as an organic-inorganic hybrid is shown in Figure 2 ^[56][159]. The structure consists of chains of [PbI₂]⁻ units extending along the crystallographic b axis. In the asymmetric unit Pb is on a twofold axis and the two I atoms are in general positions. Each Pb atom is octahedrally coordinated to six I atoms, arranged as chains of edge-sharing octahedra. The N—H···I hydrogen-bonding interactions between the three H atoms of the ammonium group of the organic cation and the metal coordinate I atoms (r(H···I) 2.566, 2.835 and 3.147 Å); these are stronger than the C—H···I formed by a methyl group of the same cation and the coordinate I atoms (viz. r(H···I) 3.312 and 3.393 Å). There are also C—H···π and N—H···π hydrogen bonds between the ammonium and methyl groups of one organic cation and the arene ring system of another cation, as well as the formation of arene C– H···I hydrogen bonds and weakly bound Type-I I···I close contacts. Apart from these, the N site of the ammonium group is involved in forming pnictogen bonds with the nearest I sites of the inorganic framework, Figure 2 (right). These pnictogen bonds are quasi-linear, with (r(N···I) = 3.634 Å; ∠C–N···I = 177.7°.