Where Are the tpy Embraces in [Zn{4′-(EtO)2OPC6H4tpy}2][CF3SO3]2?

In this paper, the bromoand phosphonate-ester-functionalized complexes [Zn(1)2][CF3SO3]2 and [Zn(2)2][CF3SO3]2 (1 = 4′-(4-bromophenyl)-2,2′:6′,2′′-terpyridine, 2 = diethyl (4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)phosphonate) are reported. The complexes have been characterized by electrospray mass spectrometry, IR and absorption spectroscopies, and multinuclear NMR spectroscopy. The single-crystal structures of [Zn(1)2][CF3SO3]2.MeCN.1/2Et2O and [Zn(2)2][CF3SO3]2 have been determined and they confirm {Zn(tpy)2}2+ cores (tpy = 2,2′:6′,2′′-terpyridine). Ongoing from X = Br to P(O)(OEt)2, the {Zn(4′-XC6H4tpy)2}2+ unit exhibits significant “bowing” of the backbone, which is associated with changes in packing interactions. The [Zn(1)2]2+ cations engage in head-to-tail 4′-Phtpy...4′-Phtpy embraces with efficient pyridine...phenylene π-stacking interactions. The [Zn(2)2]2+ cations pack with one of the two ligands involved in pyridine...pyridine π-stacking; steric hindrance between one C6H4PO(OEt)2 group and an adjacent pair of π-stacked pyridine rings results in distortion of backbone of the ligand. This report is the first crystallographic determination of a salt of a homoleptic [M{4′-(RO)2OPC6H4tpy}2]n+ cation.


Scheme 1.
Examples of anchoring ligands (cyclometallating H(C^N), bpy N^N, and tpy N^N^N) used in sensitizers in metal complex dyes in dye-sensitized solar cells.
Despite the interest in sensitizers based on polypyridine metal complexes incorporating phosphonic acid anchors, there is remarkably little crystallographic data available to provide insights into structural details, in particular, packing interactions that may influence aggregation of the surface-adsorbed species.A search of the CSD (v.5.40, November 2018 [24] for metal-bonded {4′-O3P-tpy} or {4′-O3P-C6H4tpy} units gave surprisingly few hits [25][26][27][28][29][30][31][32], with only one featuring a 1,4-phenylene spacer [32].It is also interesting to note that all of the reported structures relate to phosphonate esters rather than the parent phosphonic acids, and that the majority of the studies were motivated by application of the complexes as logic gates, photosensitizers, and catalysts.Here, we report the syntheses and single-crystal structures of the two zinc(II) complexes [Zn(1)2][CF3SO3]2 and [Zn(2)2][CF3SO3]2 (where ligands 1 and 2 are defined in Scheme 2) and investigate the effects that the phosphonate ester group has on intermolecular interactions in the solid state.
Compound 1 was prepared by the one-pot method of Wang and Hanan [33], and compound 2 was prepared from 1 by the method reported in [34].

[Zn(1)2][CF3SO3]2
A solution of Zn(CF3SO3)2 (155 mg, 0.426 mmol) in MeCN (10 mL) was added to a solution of the 1 (347 mg, 0.895 mmol) in MeCN (10 mL).The reaction mixture was heated at 55 °C for 15 h and then the product was precipitated from the colourless solution by the addition of Et2O.The precipitate was collected by filtration, washed with cold CH2Cl2 and Et2O, and dried under vacuum.

Synthesis and Solution Characterization of Complexes
Ligands 1 and 2 were prepared as previously reported [33,34] S3 and S4).The base peak in each spectrum corresponded to the [M − 2CF3SO3] 2+ ion with half-mass separations between the peaks in the peak-envelope.The 1 H and 13 C NMR spectra were assigned by 2D methods.Figures 1 and 2 display the aromatic regions of the 1 H NMR spectra, with the full spectra shown in Figures S5 and  S6.COSY and HMQC spectra are shown in Figures S7-S10.The shift to lower frequency for the proton H A6 (see Scheme 2 for labelling) ongoing from free to coordinated ligand was typical of the formation of the {M(tpy)2} unit, with H A6 lying over the π-system of the adjacent ligand.For example, in 2 (in CDCl3), the resonance for H A6 appeared at δ 8.74 ppm [34], while in [Zn(2)2][CF3SO3]2 (in CD3CN), it was at δ 7.88 ppm.In [Zn(1)2][CF3SO3]2, the 1 H NMR signal for H C2 showed a NOESY cross-peak to the signal for H B3 (Figure S11).Assignment in [Zn(2)2][CF3SO3]2 was also aided by characteristic coupling between the 31 P nucleus and protons H C2 and H C3 , respectively (Figure 2).
A comparison of Figures 3 and 4 shows that going from bromo to phosphonate ester functionalities results in a significant "bowing" of the backbone of the {Zn(4′-XC6H4tpy) 2} 2+ unit, as is emphasized in the overlay in Figure 6.We have previously noted this feature in a number of compounds containing central {M(4′-XC6H4tpy)} 2+ motifs.However, detailed analysis of the packing interactions did not reveal a universal explanation for the phenomenon [44][45][46] Pyridine rings containing N4 and N4 i engage in the face-to-face π-stacking interaction shown in Figure 7a with centroid...centroid and interplane separations of 3.63 and 3.24 Å, respectively.Symmetry-related intercation π-stacking contacts result in the formation of double-chains of cations running parallel to the crystallographic b-axis (Figure 7a).The bending of the backbone of the ligand incorporating atoms P1 and O1 appears to be associated with steric hindrance between the C6H4PO(OEt)2 group and an adjacent pair of π-stacked pyridine rings.This also leads to close P-O...H-C contacts as shown in Figure 7b, parameters for which are O1...H271 ii = 2.43 Å, O1...H211 iii = 2.58 Å, and O1...H201 iii = 2.69 Å (symmetry codes are defined in the caption to Figure 7).Chains are further associated through short P-O...H-C contacts between phosphate ester groups of adjacent cations (Figure 8).This involves atom O5 (attached to P2) and H222 iv of the OCH2 group attached to P1 iv (Figure 8b).Comparisons of the packing interactions in [Zn(2)2][CF3SO3]2 with related structures are limited by the lack of structural information (see Section 1: Introduction).However, analysis of the structure of [ReCl{4′(4-(MeO)2OPC6H4tpy}2] (which contains a 7-coordinate Re(III) centre) revealed that the dominant features in the packing involved close OMe...πarene contacts rather than tpy...tpy π-stacking (CSD refcode PACXOI) [32].

Conclusions
We have prepared the bromo-and phosphonate-ester-functionalized bis(tpy) complexes