Anion-binding mode in a sulfanylphenyl urea complex: solid state symmetry breaking and solution chelation

Abstract

An anion-bridged, hydrogen bonded coordination polymer apparently adapts its symmetry to form a Z′ = 1 or 2 structure according to the hydrogen bonding demands of the anion, and in solution there is evidence for a discrete anion-binding coordination complex.

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There is a significant interest in anion complexation by coordination complex hosts capable of hydrogen bonding with anions. Beer et al. have reported dithiocarbamate-based thiourea and amide derived hosts,^1–3 while Gale et al.^4–6 and we^7–10 have adopted a more labile self-assembling approach using monodentate pyridyl ligands. Similar species with a crystal engineering focus have been recently reported by Barboiu et al.,¹¹ while packing of related ligands has been analysed by Nangia et al.¹² A crucial aspect is the role of the metal ion, in conjunction with the donor group orientation of the ligand, in the spatial organisation of the anion-binding groups. Thus we have reported a series of complexes of type [Ag(TUP)₂]⁺X⁻ (1; TUP = 3-p-tolylureidopyridine) in which the Ag(TUP)₂⁺ cleft is complementary to nitrate anions, resulting in solid state and solution phase chelation of nitrate by the Ag(I) complex. In contrast, complexes with other anions exhibit divergent orientation of the urea groups.¹⁰ We now report the synthesis and supramolecular chemistry of a new ligand capable of simultaneous coordination to cations and anions and suggest that the hydrogen bonding may be linked to a Z′ = 1 to Z′ = 2 symmetry breaking in the complex with BF₄⁻ compared with the NO₃⁻ analogue. Lowered crystallographic symmetry and hence structures exhibiting Z′ > 1 is a topic of considerable current interest.^13–20

The thioether 1-(3-methylsulfanyl-phenyl)-3-pheny-l-urea (MPUP) was prepared by reaction of aniline with 3-methylsulfanyl-phenyl isocyanate in dichloromethane in 88% yield. The thioether group was chosen to be complementary to soft metal ions, particularly Ag(I). Reaction of MPUP with AgNO₃ and AgBF₄ gave the related products [Ag(MPUP)₂]X (X = NO₃2a; BF₄2b). Complexes 2 were both characterised by X-ray crystallography.

†

The crystal structures of 2a and 2b are very similar. They are based on stacks of [Ag(MPUP)₂]⁺ units with the Ag(I) ion bound to the sulfur atoms and urea oxygen atoms of an adjacent [Ag(MPUP)₂]⁺ compex. The molecular structure of 2a is shown in Fig. 1a. As noted for complexes of type 1 the angle at silver (S–Ag–S) deviates significantly from linearity (142.15(2)°) allowing expansion of the coordination sphere, in this case by long interactions to the carbonyl oxygen atoms from an adjacent complex, Fig. 1b, to give a very approximately tetrahedral geometry. The BF₄⁻ salt 2b exhibits a very similar structure with S–Ag–S = 145.5° (average over two independent molecules) (Fig. 2). However, 2b exhibits monoclinic symmetry (P2₁/c) with Z′ = 2 while 2a is orthorhombic Pbca (Z′ = 1). The deviation from metric orthorhombic symmetry in 2b is significant with β = 91.81° and gives a structure with two symmetry-independent molecules. The structural similarity between 2a and 2b is remarkable given that both compounds exhibit four NH⋯anion hydrogen bonds and that the anions are of very different geometry. This dichotomy may be rationalised as follows: the antiparallel stacks of cations form a solid state anion-binding pocket comprising four urea NH donors arranged in pairs at approximately 90° to one another. This arrangement is not complementary to the trigonal planar geometry of NO₃⁻ but suits the 4̄ symmetry of the tetrahedral BF₄⁻. The nitrate anion in 2a forms two R²₂(8) hydrogen bonded motifs²¹ bridging between ligands on adjacent complexes, as observed previously in several examples.^22,23 The double 8-membered ring is a particularly favourable synthon.²⁴ In contrast to the previous examples, however, the bis(urea)NO₃⁻ unit is not planar and the urea and nitrate H-bond acceptor ‘NO₂’ fragments of the NO₃⁻ anion are mutually inclined at angles of 41.1 and 42.3° to give a urea–urea interplanar angle of 81.9°. Thus the nitrate adopts a ‘twisted’ orientation in between the urea groups to adapt to the symmetry mismatched cavity, Fig. 3a. The result is a high symmetry structure with inter-stack separations of 8.13 Å (half the crystallographic b axis).

Fig. 1 (a) Thermal ellipsoid plot of [Ag(MPUP)₂]NO₃ (2a) showing hydrogen bonding to nitrate. (b) Stacks linked by long range Ag–O interactions. (c) ‘Twisted’ orientation of the nitrate anion in between stacks. Selected bond lengths: Ag⋯S(1) 2.4906(7), Ag⋯S(2) 2.5087(7), Ag⋯O(1) 2.449(2) and Ag⋯O(2) 2.411(2) Å. Hydrogen bond distances for 2a: N(2)⋯O(3) 2.902(4), N(1)⋯O(4) 3.041(4), N(4)⋯O(5) 2.872(4), N(3)⋯O(4) 3.111(4) Å.

Fig. 2 Crystal packing in [Ag(MPUP)₂]BF₄ (2b) showing non-twisted hydrogen bonding to tetrafluoroborate incorporating flat 8-membered hydrogen bonded rings. Hydrogen bond distances: N(1)⋯F(2) 2.97(2), N(2)⋯F(4) 2.95(2), N(3)⋯F(3) 2.93(2), N(4)⋯F(1) 3.05(2), N(5)⋯F(6) 2.89(2), N(6)⋯F(8) 3.00(2), N(7)⋯F(7) 2.96(2), N(8)⋯F(5) 3.00(2) Å.

Fig. 3 Anion environments (a) NO₃⁻ in 2a and (b) BF₄⁻ in 2b.

The BF₄⁻ anion incorporates an intrinsic 90° twist between ‘BF₂’ pairs and hence, in contrast to nitrate, is symmetry-matched to the cavity formed by the two urea groups. As with nitrate, both independent anions in the structure of 2b form pairs of R²₂(8) hydrogen bonded rings but in all cases the O=C(NH)₂⋯F₂B units are essentially planar—no twist is required, Fig. 3b. However it is the BF₄⁻ complex that exhibits lowered crystallographic symmetry with two independent molecules in the asymmetric unit.‡ Superposition of the two unique complexes without introducing any rotation (Fig. 4) shows that they are conformationally almost identical but that the ligands are simply shifted perpendicular to the chain direction by ca. 0.6 Å in one complex relative to the other. The average displacement is manifest as an expansion of the stacked column of cations in the ac plane and hence an opening of the β angle. This alternating displacement of the cations in 2b and consequent symmetry breaking may be regarded as a simple modulation of the 2a structure which is brought about by the larger size and more optimal hydrogen bonding to BF₄⁻; the inter-stack separation in 2b is 8.36 Å, 0.23 Å more than in 2a. Thus it is speculated that the strong, directional, multiple interactions to BF₄⁻ outweigh the drive towards crystal close packing.

Fig. 4 Unrotated superposition of the two crystallographically independent formula units in 2b viewed along crystallographic b axis showing the deviation of the unit cell β angle from 90°.

Analysis of molecular volumes for 2a and 2b using the X-seed molecular volume calculator²⁵ confirms that it is the Z′ = 1 structure 2a that exhibits the more optimal packing (71.5 vs. 69.8 %) with the dominant hydrogen bonding in 2b apparently causing steric compression in the cationic chain (volume of the cationic portion of the asymmetric unit in 2b is 912.0 ų compared to 927.1 ų for the analogous unit in 2a). Thus optimal hydrogen bonding apparently results in a loss of packing efficiency.

The hydrogen bonded polymeric nature of complex 2 in the solid state contrasts to the discrete solution and solid state structures of [Ag(TUP)₂]NO₃.^9,10 In order to gain insight into the solution structure of the system we carried out a series of ¹H NMR spectroscopic experiments. Fig. 5a shows the ¹H NMR spectrum of the MPUP ligand in acetone-d₆. Upon addition of NBu₄NO₃ there is a surprising upfield shift of the two overlapping urea NH resonances at ca. 8 ppm. This result contrasts to the marked downfield shift on interaction of free TUP with nitrate and suggests that MPUP does not interact with nitrate in the same way, nor as strongly. In the presence of half an equivalent of AgCF₃SO₃, however, there is a more usual downfield shift of the urea NH resonances suggesting some interaction of Ag(MPUP)₂⁺ with the poorly hydrogen bond accepting triflate anion. Simultaneous addition of both AgCF₃SO₃ and NBu₄NO₃ results in a further upfield shift suggesting nitrate binding in preference to triflate by Ag(MPUP)₂⁺. Thus Ag(MPUP)₂⁺ is an effective solution host for nitrate anion as with Ag(TUP)₂⁺ even though the free MPUP ligand does not behave in the same way as free TUP. Evidence for the stability of Ag(MPUP)₂⁺ as a stable, discrete solution species comes from the ESI-MS specta of 2a and 2b, both of which show peaks at m/z 623 and 625 corresponding to ¹⁰⁷Ag(MPUP)₂⁺ and ¹⁰⁹Ag(MPUP)₂⁺, respectively.

Fig. 5 ¹H NMR spectra of MPUP in acetone-d₆ (a) 7.7 mM solution; (b) with 3.8 mM NBu₄NO₃; (c) with 3.8 mM AgCF₃SO₃ and (d) with both 3.8 mM NBu₄NO₃ and 3.8 mM AgCF₃SO₃.

In conclusion, we have demonstrated that the geometry of the MPUP ligand results in divergent urea groups in cations of type [Ag(MPUP)₂]⁺ and hence intermolecular anion bridges in the solid state. Hydrogen bonding to anions may be linked to the solid state symmetry breaking and, in this simple comparison, may represent a distortion of molecular shape in response to the requirements of the strong hydrogen bonding interactions. A full systematic study of this system is underway. In solution, however, there is evidence for discrete aggregates analogous to related pyridyl species.¹⁰

Acknowledgements

We thank the EPSRC for funding.

Notes and references

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Footnotes

† Crystal data for 2a: C₂₈H₂₈Ag₁N₅O₅S₂, M = 686.56, colourless cuboid, 0.40 × 0.06 × 0.06 mm³, orthorhombic, space group Pbca (No. 61), a = 18.2449(9), b = 16.2679(8), c = 18.9904(10) Å, V = 5636.5(5) ų, Z = 8, D_c = 1.618 g cm⁻³, F₀₀₀ = 2800, SMART APEX, Mo Kα radiation, λ = 0.71073 Å, T = 120K, 2θ_max = 64.0°, 79432 reflections collected, 9324 unique (R_int = 0.02). Final GoF = 0.9004, R1 = 0.0338, wR2 = 0.0752, R indices based on 5834 reflections with I > 2σ(I) (refinement on F²), 370 parameters, 0 restraints. Lp and absorption corrections applied, μ = 0.912 mm⁻¹. Crystal data for 2b: C₂₈H₂₈AgBF₄N₄O₂S₂, M = 711.34, colourless needle, 0.30 × 0.05 × 0.04 mm³, monoclinic, space group P2₁/c (No. 14), a = 18.309(3), b = 16.716(3), c = 18.722(3) Å, β = 91.812(3)°, V = 5727.1(17) ų, Z = 8, D_c = 1.650 g cm⁻³, F₀₀₀ = 2880, Bruker SMART, Mo Kα radiation, λ = 0.71073 Å, T = 120(2)K, 2θ_max = 50.0°, 32555 reflections collected, 9849 unique (R_int = 0.1762). Final GoF = 1.113, R1 = 0.1685, wR2 = 0.3604, R indices based on 6288 reflections with I > 2σ(I) (refinement on F²), 371 parameters, 0 restraints. Lp and absorption corrections applied, μ = 0.911 mm⁻¹. The crystals of 2b were of very poor quality leading to poor overall precision. However, the connectivity, gross structural features and crystal packing interactions that are of interest for the present study are unambiguous. CCDC reference numbers 291485 and 291486. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b516962c

‡ Checks using PLATON ADSYM indicate no additional symmetry or pseudosymmety in 2b even using generous tolerance settings.

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