C–H...X (X = F, Cl, Br, I) Versus π -Stacking in the Crystal Packing of Compounds Containing the {M(tpy)X 3 } Motif

: Analysis of the data in the Cambridge Structural Database (CSD) for compounds containing an {M(tpy)X 3 } motif (tpy = 2,2’:6’,2”-terpyridine, M = any metal, X = F, Cl, Br, I) reveals 17 isostructural mononuclear [M(tpy)X 3 ] compounds crystallizing without lattice solvent; both face-to-face π -stacking of pyridine rings and C–H3/H3’...X hydrogen bonding appear to be equally important. Regardless of coordination number (CN = 6, 7 or 8) and nuclearity (mono-or dinuclear), a recurring packing feature in other compounds containing an {M(tpy)X 3 } unit is the presence of bifurcated Cl...H3/H3’ interactions, complemented in some cases by Cl...H5’/H3” interactions, consistent with the acidic H3, H3’, H5’, and H3” atoms of a coordinated tpy ligand. Octahedral [M(tpy)F 3 ] complexes crystallize as hydrates with strong F...H–OH hydrogen bonding dominating the crystal packing.


Introduction
The ligand 2,2':6',2"-terpyridine (tpy) typically coordinates to metal ions in a terdentate, bis-chelating mode with a concomitant conformational change from s-trans,s-trans to s-cis,s-cis (Scheme 1).Mononuclear six-coordinate complexes of the type [M(tpy) 2 ] q+ (q ≥ 0) and [M(tpy)X 3 ] q± (q ≥ 0) are common [1][2][3][4], and complexes with higher coordination numbers are known with larger metal ions, such as those from the f -block.Solid-state packing interactions between {M(tpy) 2 } cations have been the focus of detailed investigations by McMurtie and Dance [5][6][7].Offset-face-to-face (OFF) and edge-to-face (EF) interactions between pyridine rings are a recurring feature, and the introduction of a 4'-aryl substituent introduces an additional π-stacking motif leading to the embraces shown in Scheme 2 [5].The offset arene...arene contacts displayed in Scheme 2 are typical packing arrangements, not only in tpy-containing compounds but also in the solid-state structures of many metal coordination compounds containing aromatic nitrogen-donor ligands [8]; in 2009, Dance and Scudder highlighted their role in molecular crystals [9].These authors recognized and defined a quadruple aryl embrace (QAE) as the fundamental packing interaction present in the chiral, hexagonal motif that is a recurring feature in salts of [M(bpy) 3 ] q+ (bpy = 2,2'-bipyridine) complexes.More recently, using data from the Cambridge Structural Database (CSD) [10], we demonstrated the significance of short contacts between atoms H3 and H3 of the bpy ligands in [M(bpy) 3 ] q+ and the counterion (see Scheme 3 for atom numbering) which may be augmented by interactions of the anion with bpy atoms H4 and H5.This combination of CH...X contacts results in anions residing either in the center or lying above and/or below the centroid of the hexagonal {M(bpy) 3 } 6 motif [11,12].We extended these investigations to octahedral cis-[M(bpy) 2 X 2 ] coordination compounds and showed that a recurrent packing motif is a dimeric unit containing CH bpy ...X contacts and intermolecular face-to-face π-stacking of bpy ligands; in some cases, CH bpy ...X contacts are the dominant features of the crystal-packing [13].With this latter observation in mind, we turned our attention to [M(tpy)X 3 ] q± (q ≥ 0) complexes in which X = F, Cl, Br, or I.As with bpy, the H3 and H3' atoms of a coordinated tpy ligand are acidic [14], and we were interested in exploring whether the solid-state structures of compounds containing {M(tpy)X 3 } domains showed intermolecular CH...X interactions with these atoms.Herein, we describe the patterns in crystal packing that are found in complexes of the type [M(tpy)X 3 ] q± (q ≥ 0).Structural data for this article have been retrieved from the CSD [10] (see the Methods Section).
Crystals 2023, 13, x FOR PEER REVIEW 2 of 13 cases, CHbpy...X contacts are the dominant features of the crystal-packing [13].With this latter observation in mind, we turned our attention to [M(tpy)X3] q± (q ≥ 0) complexes in which X = F, Cl, Br, or I.As with bpy, the H3 and H3' atoms of a coordinated tpy ligand are acidic [14], and we were interested in exploring whether the solid-state structures of compounds containing {M(tpy)X3} domains showed intermolecular CH...X interactions with these atoms.Herein, we describe the patterns in crystal packing that are found in complexes of the type [M(tpy)X3] q± (q ≥ 0).Structural data for this article have been retrieved from the CSD [10] (see the Methods Section).
Scheme 1. Representations of tpy in the s-trans,s-trans conformation found in the free ligand, the change to an s-cis,s-cis conformation upon metal coordination, and in an octahedral [M(tpy)2] q+ complex.

Scheme 3.
Structures of the free ligands bpy and tpy with atom numbering and a schematic representation of the structure of mer-[M(tpy)X3] showing the axial (ax) and equatorial (eq) halogenido ligands, X; the three nitrogen donor atoms together with Xeq define a mean plane.In complexes containing unsubstituted terdentate tpy ligands, H3 and H3'' are equivalent, as are H3' and H5'.

Methods
Conquest (version 2022.3.0 including November 2022 updates) [15] was used to search the CSD [10] for compounds containing {M(tpy)X3} motifs (M = any metal = '4M' in Conquest; X = any halogen).The H atoms of the tpy ligand were explicitly defined, thereby excluding functionalized tpy ligands.The inter-ring C-C bonds in the tpy ligand and the M-N and M-X bonds were specified as being of "any type" in Conquest [15].The connectivity of the M and X atoms were not constrained.In the text, the abbreviation CN is used for coordination number.Analysis of the structures was carried out using the program Mercury (version 2022.3.0)[16].In structures in which the H atom coordinates were not Scheme 1. Representations of tpy in the s-trans,s-trans conformation found in the free ligand, the change to an s-cis,s-cis conformation upon metal coordination, and in an octahedral [M(tpy) 2 ] q+ complex.
Crystals 2023, 13, x FOR PEER REVIEW 2 of 13 cases, CHbpy...X contacts are the dominant features of the crystal-packing [13].With this latter observation in mind, we turned our attention to [M(tpy)X3] q± (q ≥ 0) complexes in which X = F, Cl, Br, or I.As with bpy, the H3 and H3' atoms of a coordinated tpy ligand are acidic [14], and we were interested in exploring whether the solid-state structures of compounds containing {M(tpy)X3} domains showed intermolecular CH...X interactions with these atoms.Herein, we describe the patterns in crystal packing that are found in complexes of the type [M(tpy)X3] q± (q ≥ 0).Structural data for this article have been retrieved from the CSD [10] (see the Methods Section).
Scheme 1. Representations of tpy in the s-trans,s-trans conformation found in the free ligand, the change to an s-cis,s-cis conformation upon metal coordination, and in an octahedral [M(tpy)2] q+ complex.

Scheme 3.
Structures of the free ligands bpy and tpy with atom numbering and a schematic representation of the structure of mer-[M(tpy)X3] showing the axial (ax) and equatorial (eq) halogenido ligands, X; the three nitrogen donor atoms together with Xeq define a mean plane.In complexes containing unsubstituted terdentate tpy ligands, H3 and H3'' are equivalent, as are H3' and H5'.

Methods
Conquest (version 2022.3.0 including November 2022 updates) [15] was used to search the CSD [10] for compounds containing {M(tpy)X3} motifs (M = any metal = '4M' in Conquest; X = any halogen).The H atoms of the tpy ligand were explicitly defined, thereby excluding functionalized tpy ligands.The inter-ring C-C bonds in the tpy ligand and the M-N and M-X bonds were specified as being of "any type" in Conquest [15].The connectivity of the M and X atoms were not constrained.In the text, the abbreviation CN is used for coordination number.Analysis of the structures was carried out using the program Mercury (version 2022.3.0)[16].In structures in which the H atom coordinates were not Scheme 2. Three possible embraces of two {M(4'-Phtpy)} units (4'-Phtpy = 4'-phenyl-2,2':6',2"terpyridine); these are described in reference [5].
Crystals 2023, 13, x FOR PEER REVIEW 2 of 13 cases, CHbpy...X contacts are the dominant features of the crystal-packing [13].With this latter observation in mind, we turned our attention to [M(tpy)X3] q± (q ≥ 0) complexes in which X = F, Cl, Br, or I.As with bpy, the H3 and H3' atoms of a coordinated tpy ligand are acidic [14], and we were interested in exploring whether the solid-state structures of compounds containing {M(tpy)X3} domains showed intermolecular CH...X interactions with these atoms.Herein, we describe the patterns in crystal packing that are found in complexes of the type [M(tpy)X3] q± (q ≥ 0).Structural data for this article have been retrieved from the CSD [10] (see the Methods Section).
Scheme 1. Representations of tpy in the s-trans,s-trans conformation found in the free ligand, the change to an s-cis,s-cis conformation upon metal coordination, and in an octahedral [M(tpy)2] q+ complex.
Scheme 3. Structures of the free ligands bpy and tpy with atom numbering and a schematic representation of the structure of mer-[M(tpy)X3] showing the axial (ax) and equatorial (eq) halogenido ligands, X; the three nitrogen donor atoms together with Xeq define a mean plane.In complexes containing unsubstituted terdentate tpy ligands, H3 and H3'' are equivalent, as are H3' and H5'.

Methods
Conquest (version 2022.3.0 including November 2022 updates) [15] was used to search the CSD [10] for compounds containing {M(tpy)X3} motifs (M = any metal = '4M' in Conquest; X = any halogen).The H atoms of the tpy ligand were explicitly defined, thereby excluding functionalized tpy ligands.The inter-ring C-C bonds in the tpy ligand and the M-N and M-X bonds were specified as being of "any type" in Conquest [15].The connectivity of the M and X atoms were not constrained.In the text, the abbreviation CN is used for coordination number.Analysis of the structures was carried out using the program Mercury (version 2022.3.0)[16].In structures in which the H atom coordinates were not Scheme 3. Structures of the free ligands bpy and tpy with atom numbering and a schematic representation of the structure of mer-[M(tpy)X 3 ] showing the axial (ax) and equatorial (eq) halogenido ligands, X; the three nitrogen donor atoms together with X eq define a mean plane.In complexes containing unsubstituted terdentate tpy ligands, H3 and H3" are equivalent, as are H3' and H5'.

Methods
Conquest (version 2022.3.0 including November 2022 updates) [15] was used to search the CSD [10] for compounds containing {M(tpy)X 3 } motifs (M = any metal = '4M' in Conquest; X = any halogen).The H atoms of the tpy ligand were explicitly defined, thereby excluding functionalized tpy ligands.The inter-ring C-C bonds in the tpy ligand and the M-N and M-X bonds were specified as being of "any type" in Conquest [15].The connectivity of the M and X atoms were not constrained.In the text, the abbreviation CN is used for coordination number.Analysis of the structures was carried out using the program Mercury (version 2022.3.0)[16].In structures in which the H atom coordinates were not available, H positions were added in Mercury [16].All H positions were normalized in Mercury, with C-H = 1.089Å.The settings in Mercury for a 'short contact' (sum of the van der Waals radii + 0.1 Å) were applied to identify H...X interactions.Disordered structures were included in the analysis.
The remaining 44 solid-state structures fell into categories of mononuclear (33) and dinuclear (11) species, including several redeterminations at either the same or different temperatures.We consider these groups of structures separately in the following discussion.In the absence of additional ligands, the {M(tpy)X 3 } moiety (in both mononuclear and dinuclear complexes) is typically octahedral (or, more correctly, pseudo-octahedral) and constrained to a meridional arrangement of the three nitrogen donors of the tpy.The three halogenido ligands are concomitantly in an orthogonal meridional arrangement (Scheme 3).Accordingly, there are two chemically and structurally distinct types of halogen atomstwo axial ligands (ax) and one equatorial ligand (eq), with the axis orthogonal to the mean plane of the tpy ligand (Scheme 3).We note that there is no trend in the relative lengths of the M-X ax and M-X eq bonds, with equal numbers of structures exhibiting longer axial or longer equatorial distances, notwithstanding the shorter M-N distances to the central as opposed to the terminal pyridine rings of the tpy donor.

Mononuclear [M(tpy)X 3 ], CN = 6
Of the 33 mononuclear structures in the CSD (see Section 3) containing an {M(tpy)X 3 } motif, 30 exhibit a CN of 6 and, with one exception, are octahedral.The CSD entry with refcode SORSAT, [Sb(tpy)F 3 ], possesses a pseudo-pentagonal bipyramidal structure in which one axial site is occupied by a stereochemically active lone pair of the Sb atom [23].Of the remaining 29 octahedral [M(tpy)X 3 ] or [M(tpy)X 3 ] + complexes, 17 are isostructural.These CSD entries are given in Table S1 in the Supplementary Materials, and we note that this family consists only of chlorido and bromido complexes.No coordinates are available for the entries with refcodes QQQCBD [Al(tpy)Cl 3 ] [24], TPYGAC [Ga(tpy)Cl 3 ] [25], TERPIN [26], and TERPTL [26].However, the space groups and cell dimensions (Table S1) confirm the structural relationship of these compounds to the others in Table S1.For refcodes TPYGAC and TERPIN, coordinates are available for redetermined structures (TPYGAC01, TERPIN01, and TERPIN02).Compounds in this isostructural group for which atomic coordinates are available are given in Table 1.We have omitted the redetermination of the structure of [Fe(tpy)Cl 3 ] at 296 K (refcode HUJKIG01) from detailed discussion because of large esds in the metrics [27].a As defined in Figures 1 and 2. b Close tpy...tpy contacts between red and blue chains as defined in Figure 2c.
Figure 1 illustrates the packing of molecules in the solid-state structure of [Fe(tpy)Cl 3 ] (refcode HUJKIG [33]), which is representative of the compounds listed in Table 1.The original reports of these structures gave little description of the crystal packing.Reid and coworkers described the packing in [Sc(tpy)Cl 3 ] (refcode KEZZOH) in terms of "π-stacking interactions (3.82 Å) between the aromatic ring of the terpy ligand of the adjacent molecule, connecting them into 1D zig-zag chains" [31].This interaction can be identified in the chains that follow the crystallographic b-axis with the cited value of 3.82 Å referring to the centroid...centroid distance (Figures 1a and 2a).The diagram in Figure 2b illustrates how these π-stacking interactions relate to the packing as viewed down the b-axis (compare Figure 2b with 1b).The centroid...centroid distance of 3.82 Å is typical for π-interactions between pairs of coordinated pyridine ligands [8].Centroid...centroid distances and interring plane angles corresponding to those shown for [Sc(tpy)Cl 3 ] in Figure 2 were measured using Mercury [16] for all the compounds in Table 1.The centroid...centroid distances range from 3.79 to 4.01 Å (Table 1, column 4), with the longest distances observed in the bromido derivatives.Figure 2c illustrates the packing of adjacent chains along the crystallographic a-axis.Although the chains lie parallel to one another, the slippage of the tpy units between red and blue chains in Figure 2c results in there being significantly weaker face-to-face π-interactions between these chains.This is quantified in the metrics presented in Table 1 (column 5) for the inter-chain close contacts between tpy domains.
Crystals 2023, 13, x FOR PEER REVIEW 5 of 13 observed in the bromido derivatives.Figure 2c illustrates the packing of adjacent chains along the crystallographic a-axis.Although the chains lie parallel to one another, the slippage of the tpy units between red and blue chains in Figure 2c results in there being significantly weaker face-to-face π-interactions between these chains.This is quantified in the metrics presented in Table 1 (column 5) for the inter-chain close contacts between tpy domains.We now turn our attention to the role that C-H...X interactions play in the compounds presented in Table 1.The presence of "extensive intermolecular hydrogen bonding" in the crystal packing of [Ru(tpy)Cl3] (refcode CUBQEV [35]) and [In(tpy)Cl3] (refcode TERPIN01 [29]) was noted by the original authors.However, the metrics of these CH...X contacts have not, to our knowledge, been discussed, and there have been no comparisons made across the series of isostructural complexes in Table 1.If we return to the crystal packing in [Fe(tpy)Cl3] (Figure 1) and focus on C-H...Cl close contacts, we observe observed in the bromido derivatives.Figure 2c illustrates the packing of adjacent chains along the crystallographic a-axis.Although the chains lie parallel to one another, the slippage of the tpy units between red and blue chains in Figure 2c results in there being significantly weaker face-to-face π-interactions between these chains.This is quantified in the metrics presented in Table 1 (column 5) for the inter-chain close contacts between tpy domains.We now turn our attention to the role that C-H...X interactions play in the compounds presented in Table 1.The presence of "extensive intermolecular hydrogen bonding" in the crystal packing of [Ru(tpy)Cl3] (refcode CUBQEV [35]) and [In(tpy)Cl3] (refcode TERPIN01 [29]) was noted by the original authors.However, the metrics of these CH...X contacts have not, to our knowledge, been discussed, and there have been no comparisons made across the series of isostructural complexes in Table 1.If we return to the crystal packing in [Fe(tpy)Cl3] (Figure 1) and focus on C-H...Cl close contacts, we observe We now turn our attention to the role that C-H...X interactions play in the compounds presented in Table 1.The presence of "extensive intermolecular hydrogen bonding" in the crystal packing of [Ru(tpy)Cl 3 ] (refcode CUBQEV [35]) and [In(tpy)Cl 3 ] (refcode TER-PIN01 [29]) was noted by the original authors.However, the metrics of these CH...X contacts have not, to our knowledge, been discussed, and there have been no comparisons made across the series of isostructural complexes in Table 1.If we return to the crystal packing in [Fe(tpy)Cl 3 ] (Figure 1) and focus on C-H...Cl close contacts, we observe that the equatorial Cl atom (Cl eq , defined in Scheme 3) forms a bifurcated contact with atoms H3 and H3' of an adjacent molecule (Figure 3a and Table 1) allowing the crystal packing to be described in terms of chains supported by non-classical C-H3/H3'...Cl eq hydrogen bonds.
Corresponding C-H3/H3'...X eq contacts for all compounds in this octahedral [M(tpy)X 3 ] family are given in Table 1 and exhibit similar metrics.A comparison of Figure 3a with Figure 1a shows that the hydrogen-bonded chains are orthogonal to the chains defined by the tpy...tpy π-stacking interactions.
Crystals 2023, 13, x FOR PEER REVIEW 6 of 13 that the equatorial Cl atom (Cleq, defined in Scheme 3) forms a bifurcated contact with atoms H3 and H3' of an adjacent molecule (Figure 3a and Table 1) allowing the crystal packing to be described in terms of chains supported by non-classical C-H3/H3'...Cleq hydrogen bonds.Corresponding C-H3/H3'...Xeq contacts for all compounds in this octahedral [M(tpy)X3] family are given in Table 1 and exhibit similar metrics.A comparison of Figure 3a with Figure 1a shows that the hydrogen-bonded chains are orthogonal to the chains defined by the tpy...tpy π-stacking interactions.Further analysis of the structure of [Fe(tpy)Cl3] (Figure 3) shows that the axial Cl atoms (Clax) are involved in intermolecular C-H3/H4/H5...Clax interactions within and between π-stacked chains (the latter defined as in Figure 2c).These contacts are depicted by the pale-green and red hashed lines in Figure 3b,c, respectively.There are additional C-H3/H4...Clax contacts (shown in blue in Figure 3b,c) to the adjacent set of chains, and this interaction is also supported by a face-to-face π-stacking contact between the central pyridine rings of two tpy ligands (centroid...centroid = 3.98 Å ).The metrics for the C-H3/H4/H5...Clax interactions are given in Table S2 (see Supplementary Materials) for all the [M(tpy)X3] compounds.
The compounds [Cr(tpy)Cl3] .DMSO (refcode LEWKEE [42]) and [Rh(tpy)Cl3] .DMSO (refcode IGAWIW [43]) are isostructural and exhibit a centrosymmetric dinuclear motif supported by C-H3/H3'...Clax hydrogen bonds (Figure 4a) and a face-to-face tpy...tpy πinteraction (Figure 4b).Metrics for the interaction are given in Table 2.The motif in Figure Further analysis of the structure of [Fe(tpy)Cl 3 ] (Figure 3) shows that the axial Cl atoms (Cl ax ) are involved in intermolecular C-H3/H4/H5...Cl ax interactions within and between π-stacked chains (the latter defined as in Figure 2c).These contacts are depicted by the pale-green and red hashed lines in Figure 3b,c, respectively.There are additional C-H3/H4...Cl ax contacts (shown in blue in Figure 3b,c) to the adjacent set of chains, and this interaction is also supported by a face-to-face π-stacking contact between the central pyridine rings of two tpy ligands (centroid...centroid = 3.98 Å).The metrics for the C-H3/H4/H5...Cl ax interactions are given in Table S2 (see Supplementary Materials) for all the [M(tpy)X 3 ] compounds.

Figure 2 .
Figure 2. (a) Arrangement of [Sc(tpy)Cl3] molecules into chains through π-stacking as described by Reid and coworkers, and (b) a view down the b-axis showing the same π-stacking in part (a) for comparison with Figure 1b (refcode KEZZOH) [31].(c) Arrangement of adjacent chains (blue and red) containing the π-stacking defined in diagrams (a,b).Note that in the discussion, the term "chain" refers to the π-stacked chains defined in this figure.

Figure 2 .
Figure 2. (a) Arrangement of [Sc(tpy)Cl3] molecules into chains through π-stacking as described by Reid and coworkers, and (b) a view down the b-axis showing the same π-stacking in part (a) for comparison with Figure 1b (refcode KEZZOH) [31].(c) Arrangement of adjacent chains (blue and red) containing the π-stacking defined in diagrams (a,b).Note that in the discussion, the term "chain" refers to the π-stacked chains defined in this figure.

Figure 2 .
Figure 2. (a) Arrangement of [Sc(tpy)Cl 3 ] molecules into chains through π-stacking as described by Reid and coworkers, and (b) a view down the b-axis showing the same π-stacking in part (a) for comparison with Figure 1b (refcode KEZZOH) [31].(c) Arrangement of adjacent chains (blue and red) containing the π-stacking defined in diagrams (a,b).Note that in the discussion, the term "chain" refers to the π-stacked chains defined in this figure.