The enrichment ratio of atomic contacts in the crystal structure of isomeric, triply protonated, 4′-functionalized terpyridine cations with [ZnCl4]2− as counter-ion

We report herein the synthesis, crystallographic analysis and a study of the non-covalent interactions observed in the new 4′-substituted terpyridine-based derivative bis[4′-(isoquinolin-2-ium-4-yl)-4,2′:6′,4′′-terpyridine-1,1′′-diium] tris-[tetrachloridozincate(II)]. The compound is similar in its formulation to the recently reported 2,2′:6′,2′′ terpyridinium analogue, although rather different and much simpler in its structural features, mainly in the number and type of non-covalent interactions present, as well as in the supramolecular structure they define.


Chemical context
We have recently reported the use of the 4 0 -pyridyl-substituted terpyridine 4 0 -(isoquinolin-4-yl)-2,2 0 :6 0 ,2 00 -terpyridine (22TP) in the synthesis of the tetrachloridozincate salt (22TPH 3 ) 2 [ZnCl 4 ] 3 ÁH 2 O (II) containing the triply protonated cation (22TPH 3 ) 3+ (Granifo et al., 2017). The structural study of (II) demonstrates the concerted way in which a series of non-covalent interactions, viz. hydrogen bonding, anion-and stacking, participate in the crystal packing. The repulsive nature of theinteraction between the triply protonated (22TPH 3 ) 3+ cations is counteracted by the [ZnCl 4 ] 2À anions through abundant peripheral hydrogen bonding and anioninteractions to the aromatic rings. A useful tool to highlight those contacts, which are statistically favored in a given structure, is the enrichment ratios approach (Jelsch et al., 2014) based on the Hirshfeld surface, and whose application in the present case showed unexpectedly large enrichment ratios for the cationic CÁ Á ÁN contacts in (II) as compared to those in the unprotonated base 22TP. This result was rationalized through the atomic and ring natural bond order charges (NBO), calculated by Maclagan and co-workers (Maclagan et al., 2015) for a series of aromatic N-heterocyclic compounds. Concisely, in a protonated species, the hydrogen and nitrogen in the N-H group carry an almost constant charge q, with an average of q(H) = 0.43 AE 0.01 and q(N) = À0.46 AE 0.1. The other atoms in the aromatic rings, C and H, receive the remaining positive charge, i.e. 0.57 AE 0.01 unit charge. A further remarkable result is that the q(N) values appear almost invariant when going from the neutral to the protonated base. Now, given that protonation leads to an increase on the positive charge in the C atoms and that the negative charge of the N atoms is almost invariant, a natural conclusion is that this ought to enrich the cationic CÁ Á ÁN interactions. In an attempt to explore the effect of the position of the protonated N atoms on this type of interaction, we decided to protonate the already known isomeric base 4 0 -(isoquinolin-4-yl)-4,2 0 :6 0 ,4 00 -terpyridine (44TP) (Granifo et al., 2015) and to study the crystal structure of the new related compound (44TPH 3 ) 2 [ZnCl 4 ] 3 (I). Fig. 1 shows the molecular geometry as well as atom and ring labelling for (I). There is one (44TPH 3 ) 3+ independent cationic moiety, protonated at N1 and N3 in the lateral pyridine rings (hereinafter py) and at N4 in the isoquinoline group (hereinafter, isq). The three negative charges required for charge balance are provided by one full independent [ZnCl 4 ] 2À (tcz) anion in general position and a second one sitting on a twofold axis (thus providing only half of the charge). The general formulation is then (44TPH 3 ) 2 [ZnCl 4 ] 3 , similar to the 2,2 0 :6 0 ,2 00 analogue (II) but without water as solvent. In this respect, the analogy goes a bit further: the pseudosymmetry observed in (II), which linked both (otherwise independent) (44TPH 3 ) 3+ cations becomes genuine symmetry in (I), expressed through the crystallographic twofold operation through the tcz group at Zn2.

Structural commentary
Bond distances and angles are unremarkable in the (44TPH 3 ) 3+ moiety, with only minor departures from commonly accepted values in general, and from those in (II) in particular. The most relevant features come from the dihedral angles involving the internal planar groups, and it is here where the molecular differences with (II) are more apparent. The terpyridine nucleus presents significant out-of-plane rotations of the lateral pyridinium groups with regard to the central py one, and similarly with the pendant isq rings [dihedral angles: 2, 1 = 15.87 (16) ; 2, 3 = 25.80 (16) ; 2, 4 = 48.49 (15) , plane labels taken from their N heteroatoms]. This large rotation of the isq group is required to avoid 'bumping' between the otherwise colliding atoms H7 and H23. The experimental d(H7Á Á ÁH23) distance is 2.36 Å , while in a perfectly planar disposition this value would collapse down to ' 0.80 Å . This 'anti-bumping' argument appears to be reinforced by the difference between the angles C16, [C24-C16-C8 = 124.7 (3) > C17-C16-C8 = 116.2 (3) ], suggesting some kind of an H7Á Á ÁH23 repulsion.

Supramolecular features
As in (II), the most conspicuous aspect of the structure of (I) is its packing scheme, derived from a number of different intermolecular interactions, presented in Table 1 (N/C-HÁ Á ÁCl), Table 2 (-) and Table 3 (Zn-ClÁ Á Á/ + ), which for convenience of description have been assigned an individual 'code' or sequence number (from #1 to #17). Among these, hydrogen bonds are the most abundant and are clearly divided into two groups: stronger N-HÁ Á ÁCl (#1 to #5) and weaker Molecular view of the asymmetric unit in (I), with displacement ellipsoids drawn at the 50% probability level. Atom Zn2 lays onto a twofold symmetry axis. Symmetry code: (i) Àx + 1, y, Àz + 3 2:

Hirshfeld surface and enrichment ratio
Calculations of the recently introduced enrichment ratio (ER) approach using the Hirshfeld surface methodology (Jelsch et al., 2014) were performed with MoProViewer (Guillot et al., 2014). Considering that the ER is the ratio between the actual contacts and those that should result from random ones, values larger than unity for any pair of elements mean they have a high tendency to form contacts in the crystal structure, the opposite happening for pairs with values lower than unity.
d is the ClÁ Á ÁX distance where X is the atom in the ring nearest the Cl anion; is the angle subtended by the Cl-Cg vector to the ring normal; is the angle subtended by the X-Cg and X-Cg vectors (for < 90 , the anion projects within the ring and for 90 < , the anion projects outside the ring; n (in n ) is the number of interacting atoms. NB according to standard requirements for anionÁ Á Á interactions (Giese et al. 2015(Giese et al. , 2016, should be < 100 . (2) 3.748 23.9 94.3 1 Symmetry codes: (x) x, Ày + 1, z À 1 2 ; (xi) x, Ày + 2, z À 1 2 ; (xii) x, Ày + 1, z À 1 2 .

Figure 3
Same as Fig. 2 Table 6 show that the CÁ Á ÁN contacts are significantly enriched (ER = 2.15). When these results are compared with those obtained in (II), a very similar behavior is observed, i.e., in spite of changing the position of the protonated pyridyl N atoms, the system reorients itself as to favour the CÁ Á ÁN interactions, evidencing the validity of the application of the criterion based on atomic charges.  Table 6 Hirshfeld contact surfaces and ERs for (44TPH 3 ) 3+ .

Database survey
The H atoms bound to carbon (H C ) and nitrogen (H N ) are differentiated. The first column corresponds to 'interior' atoms and the remaining columns to 'exterior' ones.   Table 5 Hirshfeld contact surfaces and ERs for 44TP.

Figure 5
Right and left: Hirshfeld surfaces of the independent entities of (I) shown in (c) (conveniently set apart as to avoid overlapping) and colored in accordance with the species contributing most to the electron density on the surface; (a) surfaces coloured according to the interior atoms (b) surfaces coloured according to the exterior atoms. Colour key: H C : grey, H N : light blue, C: dark brown, N: blue, Cl: green, Zn: purple.  A characteristic found in these structures, in common with the case reported herein, is that only the N atoms of the three outermost pyridyl groups are protonated and that the lateral rings of the terpyridine portion adopt a syn-syn conformation with respect to the central pyridine ring. In most of the reported cases it was found that, in spite of the repulsive electrostatic nature between positively charged (LH 3 ) 3+ cations, thestacking interactions appear enhanced when the -system is charged. Due to lack of reported information, quantitative comparison of the ERs could only be made with the (already discussed) structure (II).

Synthesis and crystallization
The tetrachloridozincate salt (44TPH 3 ) 2 [ZnCl 4 ] 3 was prepared by the reaction of 4 0 -(isoquinolin-4-yl)-4,2 0 :6 0 ,4 00 -terpyridine (44TP; Granifo et al., 2015), ZnCl 2 and concentrated HCl (37%). 44TP (4.8 mg) was placed in a small beaker and dissolved with concentrated HCl (0.5 ml) and then 0.5 ml of water was added. To this solution was added an excess of ZnCl 2 (48.0 mg) and the resulting solution was stirred for 1.5 min. The clear solution was allowed to stand at room temperature for a few days to give colourless block-shaped crystals, which were washed with methanol (3 Â 1 ml) and then dried with hot air.

Bis[4′-(isoquinolin-2-ium-4-yl)-4,2′:6′,4′′-terpyridine-1,1′′-diium] tris(tetrachloridozincate)
Crystal data (C 24  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.