Self-Assembly of Supramolecular Architectures Driven by σ-Hole Interactions: A Halogen-Bonded 2D Network Based on a Diiminedibromido Gold(III) Complex and Tribromide Building Blocks

The reaction of the complex [Au(phen)Br2](PF6) (phen = 1,10-phenanthroline) with molecular dibromine afforded {[Au(phen)Br2](Br3)}∞ (1). Single crystal diffraction analysis showed that the [Au(phen)Br2]+ complex cations were bridged by asymmetric tribromide anions to form infinite zig-zag chains featuring the motif ···Au–Br···Br–Br–Br···Au–Br···Br–Br–Br···. The complex cation played an unprecedented halogen bonding (XB) donor role engaging type-I and type-II XB noncovalent interactions of comparable strength with symmetry related [Br3]− anions. A network of hydrogen bonds connects parallel chains in an infinite 2D network, contributing to the layered supramolecular architecture. DFT calculations allowed clarification of the nature of the XB interactions, showing the interplay between orbital mixing, analyzed at the NBO level, and electrostatic contribution, explored based on the molecular potential energy (MEP) maps of the interacting synthons.


Introduction
The assembly of solid-state architectures by exploiting both covalent and noncovalent interactions between the desired synthons is a challenging goal of supramolecular chemistry [1,2]. The reagents involved can be prepared by using conventional synthetic methods and subsequently reacted, thus triggering a sequence of multiple recognition steps: an initial supramolecular event or chemical reaction generates small units that can subsequently act as building blocks in a second supramolecular event and assemble to form aggregates displaying higher order [3]. This approach is typically followed when polyhalides are involved in the rational design and subsequent construction of supramolecular networks. As far as polyiodides are involved, extended and discrete species, with the general formula [I 2m+n ] n− (n, m > 0), such as [I 4 ] 2− , [I 5 ] − , and [I 7 ] − , can be identified [4]. The largest discrete polyiodide has been reported in ferrocenium salt of [I 29 ] 3− [5], showing a three-dimensional (3D) network built of [I 5 ] − , [I 12 ] 2− , and I 2 units. All polyiodides can be considered as formed by secondary bonds involving I − , I 2 , and [I 3 ] − building blocks, with d I···I distances shorter than 3.6-3.7 Å [6,7]; polyiodides further interact through solid-state contacts with distances up to the sum of van der Waals (vdW) radii (2 · r vdW I = 4.20 Å), leading to the formation of extended or even infinite networks.
The nature of the interaction between the building blocks in polyhalides has been the subject of a vivid debate as a part of the wider discussion on Halogen Bonding (XB), defined as the attractive interaction between an XB donor R-X (R = heteroatom, metal ion, organic group; X = halogen) and an XB acceptor A in R-X···A systems [17,18]. Along with a "covalent" interpretation (A→X-R) based on orbital mixing resulting in a Charge-Transfer (CT) interaction [19], and showing an often-relevant π-contribution [20], the electrostatic σ-hole approach has been largely adopted [21]. According to this view, the interaction would be driven by the anisotropy of the electrostatic potential at the interacting atoms, the depletion of which, called σ-hole and representing an electrophilic region on the X atom, is typically disposed opposite to the covalent R-X bond of the donor group. Theoretical investigations indeed revealed that XB and other sister interactions, such as a hydrogen bond (HB) and chalcogen bond (ChB) [22], can be decomposed into an electrostatic and an orbital-mixing term [23], and that dispersion [24] also often plays a fundamental role [25,26].
Countercations play a fundamental role in templating the architectures of supramolecular polyhalide networks [16,27,28], and cationic metal complexes have been occasionally reported as counterions of extended polyhalide architectures [29,30], with some examples of halido gold complexes forming supramolecular networks based on halogen···halogen interactions [31,32]. Although the double salts of diimine-dichlorido gold(III) complexes show interactions based on aurophilic Au···Au [33,34] and Au···Cl interactions [35], due to the scarce tendency of Cl to originate Cl···Cl interactions, these complexes are unsuitable synthons for extended XB interactions. On the contrary, the less common bromido gold(III) complexes are, in principle, promising building blocks for supramolecular networks, due to their stability accompanied by the tendency of bromine to form soft···soft interactions [11,14,27]. As a proof of concept, we report here on the first example of a tribromide salt of a diimine dibromido gold(III) complex originating an infinite 2D network based exclusively on σ-hole interactions.
A network of weak [42] C-H···Br HB contacts (d C···Br distances < 4.0 Å; d H···Br < 3.0 Å; sum of H and Br vdW radii 3.05 Å) [13] joined the halogen-bonded chains to each other to form an infinite 2D network laying on the ac plane ( Figure S2). Finally, weak contacts between the 1,10-phenanthroline ring and the central Br4 atom of tribromide anions of adjacent 2D layers contributed to building up a 3D supramolecular network made up of parallel layers spaced by b/2 ( Figure S3).
A Potential Energy Map calculated for the [Au(phen)Br 2 ] + cation, optimized at the DFT level, showed σ-holes located on the negatively charged bromido ligands (Q Br1 = Q Br2 = -0.118 |e|) opposite to the Au1-Br1 and Au1-Br2 bonds ( Figure 2). These regions represented the electrophilic ends of the molecule, acting therefore as XB donor sites. A natural bond orbital (NBO) [43,44] analysis showed that the antibonding NBO (BD*) with respect to the Au-Br bonds was located along the same directions (Figures 3 and 4). The [Br 3 ] − ion can act either as an electrophile when interacting via the terminal σ-holes (blue region in Figure S4) along the molecular axis, or as a nucleophile when interacting through its belt of negative electrostatic potential in a bent geometry (red region in Figure S4) [4]. A network of weak [42] C-H···Br HB contacts (dC···Br distances < 4.0 Å; dH···Br < 3.0 Å; sum of H and Br vdW radii 3.05 Å) [13] joined the halogen-bonded chains to each other to form an infinite 2D network laying on the ac plane ( Figure S2). Finally, weak contacts between the 1,10-phenanthroline ring and the central Br4 atom of tribromide anions of adjacent 2D layers contributed to building up a 3D supramolecular network made up of parallel layers spaced by b/2 ( Figure S3).
A Potential Energy Map calculated for the [Au(phen)Br2] + cation, optimized at the DFT level, showed σ-holes located on the negatively charged bromido ligands (QBr1 = QBr2 = -0.118 |e|) opposite to the Au1-Br1 and Au1-Br2 bonds (Figure 2). These regions represented the electrophilic ends of the molecule, acting therefore as XB donor sites. A natural bond orbital (NBO) [43,44] analysis showed that the antibonding NBO (BD*) with respect to the Au-Br bonds was located along the same directions (Figures 3-4). The [Br3]ion can act either as an electrophile when interacting via the terminal σ-holes (blue region in Figure S4) along the molecular axis, or as a nucleophile when interacting through its belt of negative electrostatic potential in a bent geometry (red region in Figure S4) [4].   A network of weak [42] C-H···Br HB contacts (dC···Br distances < 4.0 Å; dH···Br < 3.0 Å; sum of H and Br vdW radii 3.05 Å) [13] joined the halogen-bonded chains to each other to form an infinite 2D network laying on the ac plane ( Figure S2). Finally, weak contacts between the 1,10-phenanthroline ring and the central Br4 atom of tribromide anions of adjacent 2D layers contributed to building up a 3D supramolecular network made up of parallel layers spaced by b/2 ( Figure S3).
The comparison of the Br-Br structural bond lengths within the [Br 3 ] − anion (Br3-Br4 larger than Br4-Br5) and the Br···Br contacts (Br1···Br3 slightly shorter than Br2···Br5 i ) confirmed that the two types of interactions were of comparable strength, the type-I interaction being slightly stronger than the type-II one. The interplay of the structural effects of type-I and type-II XB Br···Br interactions determined the geometry of the polymeric chain in {[Au(phen)Br 2 ](Br 3 )} ∞ and hence the resulting 2D supramolecular network.

Materials and Methods
Commercial solvents (reagent-grade) and reagents were used without further purification. Melting points were determined on a FALC mod. C (up to 290 • C) apparatus. Elemental analyses were carried out with a CHNS/O PE 2400 series II CHNS/O elemental analyzer (T = 925 • C). FTIR spectra were recorded on a Thermo-Nicolet 5700 spectrometer at room temperature. KBr pellets with a KBr beam splitter and KBr windows (4000−400 cm −1 , resolution 4 cm −1 ) were used. 1 H-NMR measurements were carried out in CD 3 CN (stored under molecular sieves prior to use) at 25 • C, using a Bruker Advance 300 MHz (7.05 T) spectrometer operating at 300.13 MHz. Chemical shifts were reported in ppm (δ) and were calibrated to the solvent residue. X-ray single-crystal diffraction data for compound 1 (Tables S1-S3) were collected using a Rigaku XtaLAB P200 diffractometer equipped with a MoKα radiation and operating at 93 K. The data were indexed and processed using CrystalClear-SM Expert 2.1 b45. A multi-scan absorption correction was performed using REQAB. The structure was solved with the ShelXT Version 2018/2 [53] solution program using direct methods and using CrystalStructure 4.3 as the graphical interface. The model was refined with the ShelXL Version 2018/3 [54] using the full matrix least squares minimization on F 2 . The CCSD was accessed by means of Conquest 2022.1.0 (CSD version 5.43). The computational investigation on the complex cation [Au(phen)Br 2 ] + , the tribromide anion [Br 3 ] − , and the [Au(phen)Br 2 ](Br 3 ) system was carried out at the DFT level by adopting the Gaussian 16 [55] suite of programs. Following the results of previously reported calculations on related systems [56][57][58], the PBE0 [59] hybrid functional was adopted, along with the full-electron split valence basis sets (BSs) def2-TZVP [60] for light atomic species (C, H, N, Cl, and Br) and the CRENBL basis sets [61] with RECPs for heavier gold species. BS data were extracted from the EMSL BS Library [62]. Harmonic frequency calculations were carried out to verify the nature of the minima of the optimized geometry. Charge distributions were evaluated at the NBO level (Tables S5 and S8) [43,44,51]. The programs GaussView 6.0.16 [63] and Chemissian 4.53 [64] were used to investigate the optimized structures and molecular orbital shapes.

Conclusions
In conclusion, the rearrangement, anion exchange, and subsequent solid-state selfassembly of the [Au(phen)Br 2 ] + , Br 2 , and Br − building blocks led to the formation of the infinite network {[Au(phen)Br 2 ](Br 3 )} ∞ (1), driven exclusively by noncovalent XB and HB interactions. The bromido ligands of the complex cation [Au(phen)Br 2 ] + acted as donors in type-I and type-II XB interactions with symmetry related [Br 3 ] − anions, thus leading to a rare example of a supramolecular architecture based on interacting halido gold(III) complex and polyhalide building blocks. The isolation of compound 1 clearly showed that bromido gold(III) complexes can behave as promising XB building blocks for the design of fascinating 2D and 3D supramolecular architectures. From a theoretical point of view, both the σ-hole approach and the NBO analysis applied to the [Au(phen)Br 2 ] + and [Br 3 ] − building blocks represent tools capable of rationalizing the resulting supramolecular architectures and to account for subtle structural effects. Further studies are ongoing in our laboratory aimed at isolating different related supramolecular systems based on diimine-dihalido complexes.