Halide Ion Embraces in Tris(2,2′-bipyridine)metal Complexes

An analysis of the [M(bpy)3]n+ (bpy = 2,2′-bipyridine) complexes with halide counterions in the Cambridge Structural Database reveals a common structural motif in two thirds of the compounds. This interaction involves the formation of 12 short C–H…X contacts between halide ions lying within sheets of the cations and H-3 and H-3′ of six [M(bpy)3]n+ complex cations. A second motif, also involving 12 short contacts, but with H-6 and H-5, is identified between halide ions lying between sheets of the [M(bpy)3]n+ cations.


Introduction
The established crystallographic databases, including the Inorganic Crystal Structure Database [1], the NIST Inorganic Crystal Structure Database [2], the Protein Data Bank [3][4][5] and the Cambridge Structural Database (CSD) [6,7], have created unprecedented opportunities for data mining and investigating the intramolecular, intermolecular and supramolecular interactions, which are important in determining the spatial arrangement, orientation and packing of molecules and ions in crystal lattices. For the molecular scientist, the visualization program Mercury [8] and the search software Conquest [9], both incorporated within the CSD, are of particular utility. The opportunities presented for the probing and understanding of the intimate details of crystal packing sometimes result in losing the relationship with general chemical properties and reactivity. In many cases, "weak" interactions, such as van der Waals forces, are typically unimportant in solution behaviour. Nevertheless, the field of crystal engineering has emerged predicated upon the importance of weak interactions, including π-π and C-H-π interactions, as well as halide, pnictide and tetrel bonds in the solid state. However, when "moderate" or "strong" interactions are present in the solid-state structure of a compound, it is appropriate to consider their relevance in its more general chemistry.
A few years ago we noticed dramatic variations in the performance of (apparently) chemically similar batches of [Ir(bpy)(ppy) 2 ](PF 6 ) (Hppy = 2-phenylpyridine, bpy = 2,2 -bipyridine) in light-emitting electrochemical cells [10]. This was shown to be due to the carrying through of small amounts of chloride ions from the synthesis and was demonstrated as a significant solution interaction by 1 Figure 1a) and we described the interaction as a bifurcated hydrogen bond (Figure 1b). The combination of a crystallographically significant solid-state interaction with related solution behaviour allowed us to interpret the phenomenon in terms of the known acidity of H-3 and H-3 in the bpy complexes [11][12][13][14][15][16][17][18][19]. Although we are well aware of the dangers of using distance criteria In this article, we show that the interaction of the halide or halogen atoms with the H-3 and H-3' atoms of the oligopyridine ligands in metal complexes is a recurrent phenomenon in [M(bpy)3] n+ complexes and generates a common structural motif.

The Oligopyridines
We selected the oligopyridines as the metal-binding domains for this study. There is a large body of crystallographic data for metal complexes of these ligands in the Cambridge Structural Database (using version 5.41) and we have, in general, restricted our coverage to complexes containing the parent compound 2,2'-bipyridine in order to eliminate the influence of other interactions between the halogen and substituents on the ligand. Furthermore, the chemistry of these ligands and their complexes is exceptionally well documented [23][24][25][26][27][28][29][30][31][32][33][34][35][36], facilitating correlations between the solid-state features and chemical behaviour.

Supramolecular Interactions in Oligopyridine Complexes
Oligopyridine metal-binding domains played a key role in the development of supramolecular and metallosupramolecular [37][38][39][40] chemistry. The oligopyridines provide an ideal scaffold to investigate supramolecular interactions as they interact with most of the elements in the periodic table. Dance has identified a variety of types of supramolecular embraces between the cations in "simple" oligopyridine complexes as well as a number of cation-anion interactions in these species [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]. In many cases, these interactions, described as "embraces" by Dance, are responsible for the assembly of the cationic lattice, which acts as the host for the anionic guests that are the focus of this article.

Outer-Sphere Complexes and Ion-Pairing
Interactions at the periphery of metal complexes have been known to be of importance for many years, although they have not always been associated with the term "supramolecular". The structural studies of Nassimbeni have been particularly important in identifying the commonest types of interactions and provide a solid body of evidence for the hydrogen-bonding interactions with In this article, we show that the interaction of the halide or halogen atoms with the H-3 and H-3 atoms of the oligopyridine ligands in metal complexes is a recurrent phenomenon in [M(bpy) 3 ] n+ complexes and generates a common structural motif.

The Oligopyridines
We selected the oligopyridines as the metal-binding domains for this study. There is a large body of crystallographic data for metal complexes of these ligands in the Cambridge Structural Database (using version 5.41) and we have, in general, restricted our coverage to complexes containing the parent compound 2,2 -bipyridine in order to eliminate the influence of other interactions between the halogen and substituents on the ligand. Furthermore, the chemistry of these ligands and their complexes is exceptionally well documented [23][24][25][26][27][28][29][30][31][32][33][34][35][36], facilitating correlations between the solid-state features and chemical behaviour.

Supramolecular Interactions in Oligopyridine Complexes
Oligopyridine metal-binding domains played a key role in the development of supramolecular and metallosupramolecular [37][38][39][40] chemistry. The oligopyridines provide an ideal scaffold to investigate supramolecular interactions as they interact with most of the elements in the periodic table. Dance has identified a variety of types of supramolecular embraces between the cations in "simple" oligopyridine complexes as well as a number of cation-anion interactions in these species [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]. In many cases, these interactions, described as "embraces" by Dance, are responsible for the assembly of the cationic lattice, which acts as the host for the anionic guests that are the focus of this article.

Outer-Sphere Complexes and Ion-Pairing
Interactions at the periphery of metal complexes have been known to be of importance for many years, although they have not always been associated with the term "supramolecular". The structural Crystals 2020, 10, 671 3 of 15 studies of Nassimbeni have been particularly important in identifying the commonest types of interactions and provide a solid body of evidence for the hydrogen-bonding interactions with Werner-type coordination compounds [59][60][61][62][63][64][65]. Studies such as these relate directly to the "outer-sphere" complexes implicated in studies of substitution mechanisms, in which the incoming ligands are non-covalently associated with the starting complexes (A-type mechanisms) or intermediates (D-type mechanisms) [66,67]. Of particular importance are kinetic and thermodynamic studies, establishing both the nature and strength of the outer-sphere interactions between the oligopyridine complexes and halides and their relevance to the reaction chemistry of these species [68][69][70][71][72][73][74][75][76][77][78].

Methodology
Conquest (version 2020.1) [9] was used for the primary triage of structures in the database. For the complexes with simple halide anions, the halogen atom was constrained to have the number of bonded atoms = 0 and one of the two nitrogens of a 2,2 -bipyridine was bonded with the bond-type "Any" to any metal (atom type 4M). Hydrogen atoms were added explicitly to the 3 and 3 -positions of the 2,2 -bipyridine ligands. This allowed the identification of complexes containing coordinated oligopyridines (but not 1,10-phenanthrolines) and at least one "free" halide ion in the lattice, and the results for the four halide ions are summarized in Table 1. In all searches, normalized C-H distances of 1.089 Å were specified. A preliminary analysis of these data indicated that the presence of multiple ligand types in heteroleptic complexes made the interpretation and identification of the structural paradigms very difficult and we made the decision to consider only complexes containing at least one 2,2 -bipyridine ligand and subsequently restricted the searches to homoleptic 2,2 -bipyridine complexes for the in-depth analysis. For the subsequent analysis of all data, normalized hydrogen positions were used for the C-H bonds (C-H = 1.089 Å). All graphical representations were generated using Mercury 2020.1 [8] and saved as POV-Ray files and subsequently rendered using POV-Ray v. 3.7.0.8 unofficial [79]. All hydrogen atoms, other than those involved in the interactions with halide or halogen, are generally omitted from the representations whilst the halogen atoms involved in the H . . . X interactions are represented in a space-filling mode.
Of the 64 hits for [M(bpy) 3 ] n+ complexes in the CSD (  [87], leaving a total of 48 entries for detailed analysis.

The Structural Motifs
Of the 48 compounds investigated, 20 (41.7%) exhibit an interaction between the halide ion and 12 hydrogen atoms of six 2,2 -bipyridine ligands from six different [M(bpy) 3 ] n+ cations within a sheet, as presented in Figure 2a-c and listed in Table 2. We denote this as Structure Type 1. Apparently, [Ru(bpy) 3 ]Cl 2 ·6H 2 O (Refcode HAXHIZ) [88] adopts Structure Type 1, but unfortunately the disordered water and chloride were removed from the data deposited and no metrical analysis can be made. The halogen and metal centres in the sheet can be co-planar ( Figure 3a) or may be ruffled about a mean plane ( Figure 3b). An extreme case of ruffling occurs in ∆-[Co(bpy) 3 ]Cl 2 ·2H 2 O·EtOH (Refcode CAMHED) in which the six Co atoms deviate by 0.81 to 3.52 Å from the mean plane through them. The metal-metal distances within the six-sided polygon are remarkably similar in the entries listed in Table 2, with a mean value of 7.70(20) Å. One special case deserves mention at this point: in [Ni(bpy) 3 ] 2 (Hbpy)[Ag 3 I 5 ]I (Refcode DUYBIK) [89], the host motif of six [M(bpy) 3 ] n+ cations is maintained, but the iodide is too large to fit in the cavity as a guest and lies above and below the centre. As a result, there are six "normal" H-3 and six longer H-3 interactions with the halide, as well as six additional H-4 interactions (Figure 4a).
The Structure Type 1 arrangement can be disrupted by the presence of other anions, solvent molecules or additional species capable of hydrogen bonding. However, even in such cases, there is a tendency to retain a significant part of the Structure Type 1 motif and the four entries exhibiting this phenomenon are listed in Table 3. The commonest motif is shown in Figure 4b,c and retains three adjacent [M(bpy) 3 ] n+ cations, interacting through atoms H-3 and H-3 of a bpy from each cation.  [87], leaving a total of 48 entries for detailed analysis.

The Structural Motifs
Of the 48 compounds investigated, 20 (41.7%) exhibit an interaction between the halide ion and 12 hydrogen atoms of six 2,2'-bipyridine ligands from six different [M(bpy)3] n+ cations within a sheet, as presented in Figure 2a-c and listed in Table 2. We denote this as Structure Type 1. Apparently, [Ru(bpy)3]Cl2·6H2O (Refcode HAXHIZ) [88] adopts Structure Type 1, but unfortunately the disordered water and chloride were removed from the data deposited and no metrical analysis can be made. The halogen and metal centres in the sheet can be co-planar (Figure 3a) or may be ruffled about a mean plane (Figure 3b). An extreme case of ruffling occurs in Δ-[Co(bpy)3]Cl2·2H2O·EtOH (Refcode CAMHED) in which the six Co atoms deviate by 0.81 to 3.52 Å from the mean plane through them. The metal-metal distances within the six-sided polygon are remarkably similar in the entries listed in Table 2    Another common motif has been identified in nine entries (18.8%) that involve halide ions that are located between sheets containing [M(bpy) 3 ] n+ cations. In this case, the interactions are with only two cations and with three bpy ligands from each cation. However, in contrast to Structure Type 1, the interactions are not with H-3 and H-3 but rather with an H-5 and H-6 from each ligand (Figure 5a). The distances are typically longer than those observed in Structure Type 1. Nevertheless, the result is once again 12 C-H . . . halogen interactions but involving only two cations (Figure 5b). Typically, the halide ion also exhibits stronger hydrogen-bonding interactions with water molecules lying in the sheet between the cations. This motif will now be described as Structure Type 2 ( Table 4). The interactions with H-6 are typically shorter than those with H-5, but the distances and C-H . . . X angles all indicate that these interactions only represent the weakest of hydrogen bonds and are better described as between a halide guest in a C-H bond-lined host cavity. disordered water and chloride were removed from the data deposited and no metrical analysis can be made. The halogen and metal centres in the sheet can be co-planar (Figure 3a) or may be ruffled about a mean plane (Figure 3b). An extreme case of ruffling occurs in Δ-[Co(bpy)3]Cl2·2H2O·EtOH (Refcode CAMHED) in which the six Co atoms deviate by 0.81 to 3.52 Å from the mean plane through them. The metal-metal distances within the six-sided polygon are remarkably similar in the entries listed in Table 2 [89], the host motif of six [M(bpy)3] n+ cations is maintained, but the iodide is too large to fit in the cavity as a guest and lies above and below the centre. As a result, there are six "normal" H-3 and six longer H-3 interactions with the halide, as well as six additional H-4 interactions (Figure 4a).

A Closer Look at Structure Type 1
In this section, we look briefly at complexes exhibiting Structure Type 1 and in particular their metrical properties. The detailed analyses were made for the complexes with coordinates in the CSD. In some cases, missing hydrogen atoms were added in normalized positions. All analyses used normalized hydrogen positions. In Figure 2, the archetypical motifs looking down onto the chloride, bromide and iodide ions hosted in the cation sheets were presented, and Figure 6a,b show how the motifs attenuate within the sheet.

A Closer Look at Structure Type 1
In this section, we look briefly at complexes exhibiting Structure Type 1 and in particular their metrical properties. The detailed analyses were made for the complexes with coordinates in the CSD. In some cases, missing hydrogen atoms were added in normalized positions. All analyses used normalized hydrogen positions. In Figure 2, the archetypical motifs looking down onto the chloride, bromide and iodide ions hosted in the cation sheets were presented, and Figure 6a,b show how the motifs attenuate within the sheet. * Hydrogen atoms added in normalized positions; † Redetermination and correction of space group for HIRDOB.

A Closer Look at Structure Type 1
In this section, we look briefly at complexes exhibiting Structure Type 1 and in particular their metrical properties. The detailed analyses were made for the complexes with coordinates in the CSD. In some cases, missing hydrogen atoms were added in normalized positions. All analyses used normalized hydrogen positions. In Figure 2, the archetypical motifs looking down onto the chloride, bromide and iodide ions hosted in the cation sheets were presented, and Figure 6a,b show how the motifs attenuate within the sheet. Following the approach of Steiner [114], we present data characterizing the interactions of the halide ions with the cations, hosting them in Tables 2 and 3 We note that closely related motifs containing functionalized [M(bpy) 3 ] n+ scaffolds are observed in a variety of metallosupramolecular structures [115][116][117][118].

An Aside on Chirality
Tris-chelate complexes such as [M(bpy) 3 ] n+ cations are chiral with the metal acting as a stereogenic centre. The absolute configurations of the two enantiomeric forms are denoted as ∆ and Λ (Figure 7) [80,94]. These compounds, therefore, contain only cations with the ∆ or Λ absolute configurations around the halide.
other elements in CPK colours. Hydrogen atoms are omitted for clarity.
Following the approach of Steiner [114], we present data characterizing the interactions of the halide ions with the cations, hosting them in Tables 2 and 3. The mean H…Cl and C…Cl distances are 2.80 Å and 3.87 Å, respectively, and the mean ∠C-H…Cl is 168.7°; these are slightly longer than the mean distances reported by Steiner for (NN)C(sp 2 )-H…Cl (2.54, 3.57 Å) and (NC)C(sp 2 )-H…Cl (2.64, 3.66 Å) [114], although if the increase in mean distance were constant, the expectation values for (CC)C(sp 2 )-H…Cl would be 2.74 and 3.75 Å. The mean H…Br and C…Br distances are 2.87 Å and 3.95 Å and the mean ∠C-H…Br is 171.9°; Steiner reports 2.73 and 2.74 Å for H…Br and 3.72 and 3.74 Å for C…Br distances in (NN)C(sp 2 )-H…Br and (NC)C(sp 2 )-H…Br, respectively. In this case, the expectation values for (CC)C(sp 2 )-H…Br of H…Br 2.75 Å and C…Br 3.78 Å correspond reasonably well to our values. The mean H…I and C…I distances are 3.10 Å and 4.16 Å and the mean ∠C-H…I 167.7°; Steiner reports 2.90 and 2.99 Å for H…I and 3.85 and 4.00 Å for C…I distances in (NN)C(sp 2 )-H…I and (NC)C(sp 2 )-H…I, respectively. In this case, the expectation values for (CC)C(sp 2 )-H…I of H…I 3.08 Å and C…I 4.15 Å correspond well. The interactions are close to linear and it is reasonable to interpret them in terms of weak hydrogen bonds.

An Aside on Chirality
Tris-chelate complexes such as [M(bpy)3] n+ cations are chiral with the metal acting as a stereogenic centre. The absolute configurations of the two enantiomeric forms are denoted as Δ and Λ (Figure 7) [80,94]. These compounds, therefore, contain only cations with the Δ or Λ absolute configurations around the halide. Of the remaining Structure Type 1 compounds, four are found in the Sohncke Space Groups [119] with  [96] all being homochiral. In all of the compounds in non-Sohncke space groups, the cations form homochiral sheets with alternating ∆ and Λ chirality (Figure 8). As a result, every halide ion in the Structure Type 1 motif is surrounded by six homochiral [M(bpy) 3 ] n+ cations.

From 2,2'-bipyridine to 1,10-phenanthroline
The chemistry of bpy complexes usually very closely resembles that of the analogous compounds with phen ligands. We wondered whether the packing motifs would be repeated in [M(phen)3] n+ complexes and therefore made a scouting survey of the [M(phen)3] n+ compounds in the CSD. The typical layer structures of the cations were observed in the majority of cases. However, in no case did we find structures analogous to the Type 1 with [M(bpy)3] n+ , involving the phen H-5 and H-6 protons. Rather, the halide ions are located between the sheets, with typically Type 2 interactions involving phen H-2(9) and H-3 (8). This observation also confirms our belief that Structure Type 1 involves direct C-H…X interactions rather than a simple host-guest binding in a cavity.

Conclusions
We have carried out a detailed analysis of the interactions between homoleptic 2,2'-bipyridine

From 2,2 -bipyridine to 1,10-phenanthroline
The chemistry of bpy complexes usually very closely resembles that of the analogous compounds with phen ligands. We wondered whether the packing motifs would be repeated in [M(phen) 3 ] n+ complexes and therefore made a scouting survey of the [M(phen) 3 ] n+ compounds in the CSD. The typical layer structures of the cations were observed in the majority of cases. However, in no case did we find structures analogous to the Type 1 with [M(bpy) 3 ] n+ , involving the phen H-5 and H-6 protons. Rather, the halide ions are located between the sheets, with typically Type 2 interactions involving phen H-2(9) and H-3 (8). This observation also confirms our belief that Structure Type 1 involves direct C-H . . . X interactions rather than a simple host-guest binding in a cavity.

Conclusions
We have carried out a detailed analysis of the interactions between homoleptic 2,2 -bipyridine metal complexes [M(bpy) 3