Ruthenium biimidazole complexes as anion receptors

Abstract

The behavior of the compound [RuCl(cym)(H₂biim)][BAr′₄] (cym = η⁶-para-isopropylmethylbenzene, Ar′ = 3,5-bis(trifluoromethyl)phenyl), synthesized from [{RuCl(cym)}₂(µ-Cl)₂], H₂biim and NaBAr′₄, has been studied as a receptor of anions both in solution and in the solid state.

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The occurrence of two N–H bonds able to simultaneously bind a hydrogen bond acceptor in molecules as simple as ureas¹ has prompted the study of their behavior as receptors of anions.² The same feature occurs in the syn conformation of 2,2′-biimidazole (H₂biim). However, the strong self-association of H₂biim molecules results in a low solubility and competes against anion binding. Thus, we have found that the low solubility of H₂biim in CD₃CN makes it NMR-invisible in this solvent, whereas in DMSO-d₆, in which H₂biim is sparingly soluble, δ (N–H) does not change appreciably upon addition of, for instance, an equimolar amount of tetrabutylammonium chloride. This behavior can be modified in two ways. The first one is the functionalization of biimidazole with appropriate organic substituents. This strategy has been recently used by Sessler et al.^3a to change the solid state structural pattern, and by Causey and Allen to study the behavior towards anions in solution.^3b The second way would be to coordinate H₂biim to a metal fragment, which would: (a) enforce the syn conformation and therefore preorganize H₂biim optimally for anion binding, (b) suppress self-association, (c) enhance the polarization of the biimidazole N–H bonds, making them better hydrogen bond donors, and, for cationic metal fragments, (d) add coulombic attraction to the hydrogen bond interaction. Biimidazole has been extensively used as a ligand and, in several instances, X-ray diffraction studies have showed the presence of hydrogen bonding between its N–H groups and an external anion.⁴ Extended structures based on this motif have been exploited for crystal engineering.^3a,4d However, the solution behavior of H₂biim complexes as receptors of anions has never been quantitatively studied.⁵ Our first results in this area are the subject of the present paper.

Carmona et al. reported the high yield preparation of the compound [RuCl(cym)(H₂biim)][BF₄] (cym = η⁶-para-isopropylmethylbenzene) by means of the reaction of [{RuCl(cym)}₂(μ-Cl)₂] with H₂biim followed by anion metathesis with NaBF₄.⁶ The [RuCl(cym)(H₂biim)]⁺ cation seemed a good candidate for our studies because of the ease of preparation, stability towards air and moisture, and the relative substitutional inertness usually attributed to [RuCl(η⁶-arene)(L–L)]⁺ complexes. In order to minimize the competition with external anions we preferred the BAr′₄ (Ar′ = 3,5-bis(trifluoromethyl)phenyl) counteranion.⁷ Thus, the [RuCl(cym)(H₂biim)][BAr′₄] salt (1) was prepared in virtually quantitative yield by the reaction of [{RuCl(cym)}₂(μ-Cl)₂], H₂biim⁸ and NaBAr′₄⁹ (see Scheme 1), and spectroscopically characterized.¹⁰‡ A band at 3207 cm⁻¹ in the IR spectrum of 1 (KBr pellet) was assigned to ν(N–H). In the 1 ∶ 1 adducts obtained, on addition of tetrabutylammonium chloride or nitrate (see below), these bands shifted to 3099 and 3113 cm⁻¹, respectively, suggesting N–H⋯X interactions. Compound 1 was found to be very soluble, even in moderately polar dichloromethane, a feature common to most BAr′₄ salts.

Scheme 1 Synthesis of [RuCl(cym)(H₂biim)][BAr′₄] (1).

The two N–H groups of coordinated H₂biim appeared as a singlet at δ 11.43 in the ¹H NMR of 1 in CD₃CN. The addition of tetrabutylammonium bromide, nitrate, hydrogensulfate, iodide or perrhenate shifted this signal to higher frequencies. Fast anion exchange was found in each case (titration curves are shown in Fig. 1 and in the Electronic Supplementary Information†), and binding constants were determined using the WinEQNMR program (see Table 1).¹¹

Fig. 1 ¹H NMR titration plot of receptor [RuCl(cym)(H₂biim)][BAr′₄] (1) in CD₃CN with Br⁻.

Table 1 Binding constants values for 1 in CD₃CN and DMSO-d₆^a

Anion	K in CD3CN/M^−1	K in DMSO-d6/M^−1
a Errors are given in parentheses
Br^−	4527 (±841)	579 (±92)
NO3^−	4828 (±587)	451 (±20)
HSO4^−	5920 (±370)	651 (±88)
I^−	1114 (±207)	—
ReO4^−	145 (±28)	—
Cl^−	—	970.3 (±21)

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The N–H ¹H NMR signal broadened and finally vanished when less than an equimolar amount of Bu₄NCl was added to the CD₃CN solution of 1, indicating a strong 1⋯Cl⁻ interaction in this solvent. The same was found for the more basic fluoride, dihydrogenphosphate and cyanide anions. In the more competitive DMSO-d₆ solvent, the N–H ¹H NMR signal remained visible when 1 was titrated with Cl⁻, but not with F⁻, H₂PO₄⁻ or CN⁻. To obtain values of the binding constants, for a comparison to be made between the different anions, the titrations of 1 with Br⁻, NO₃⁻ and HSO₄⁻ were repeated in DMSO-d₆, the results of which are showed in Table 1. In this solvent, the interaction between 1 and the anions I⁻ and ReO₄⁻ was found to be too weak, resulting in no observable change in the chemical shift of the N–H signals of 1.

The supramolecular adducts [RuCl(cym)(H₂biim)][Cl] and [RuCl(cym)(H₂biim)][NO₃] were structurally characterized by means of single crystal X-ray diffraction, and graphical representations of the results are displayed in Fig. 2.§ Interestingly, these adducts crystallized separately from [Bu₄N][BAr′₄], which remained in solution.¶ In the structure of the chloride adduct there are two molecules per asymmetric unit. In one of them (shown in Fig. 2a), the chloride anion interacts with the two N–H groups of only one of the Ru complexes. In the second, the chloride anion is disordered and is located over two sites in a 1 ∶ 1 ratio. Each of these 50% occupancy chlorides interacts with the two N–H groups from one metal complex and with one N–H group from a neighbouring metal complex.

Fig. 2 (a) View of the crystalline structure of the adduct [RuCl(cym)(H₂biim)][Cl]. (b) View of the crystalline structure of the [RuCl(cym)(H₂biim)][NO₃] adduct.

In the structure of the nitrate adduct, shown in Fig. 2b, each N–H group interacts with one nitrate oxygen, there being no additional contacts. Strong hydrogen bond interactions are indicated by the N⋯Cl (3.097(9) and 3.107(8) Å) and N⋯O (2.719(6) and 2.867(6) Å) distances.

The results shown in Table 1 indicate that 1 is a non-selective receptor. It is attractive to speculate that the complementarity in shape between host and guest found for [RuCl(cym)(H₂biim)][NO₃] could partly compensate for the higher basicity of chloride and bromide with respect to nitrate, resulting in binding constants of the same order.

To conclude, we have investigated for the first time the solution behavior of metal biimidazole complexes as anion receptors. The compound [RuCl(cym)(H₂biim)][BAr′₄] has been found to establish strong, non-selective interactions with several simple inorganic anions. The lack of selectivity can, at least in part, be attributed to ion pairing. In this context, the behavior of neutral metal biimidazole complexes is currently being investigated in our lab, and will be reported in a future publication.

We thank Ministerio de Ciencia y Tecnología (grant BQU2003-08649) and European Union (ERG to L. R.) for their support of this work. L. I. thanks Ministerio de Educación y Ciencia for a FPU pre-doctoral fellowship. D. M. and L. R. thank Ministerio de Educación y Ciencia for Ramón y Cajal fellowships. R. A. K. would like to acknowledge the States of Guernsey and the Cambridge University Domestic and Millennium funds.

Notes and references

1. (a) P. J. Smith, M. V. Reddington and C. S. Wilcox, Tetrahedron Lett., 1992, 41, 6085–6088; (b) T. R. Kelley and M. H. Kim, J. Am. Chem. Soc., 1994, 116, 7072–7080; (c) J. Scheele, P. Timmerman and D. N. Rheinhoudt, Chem. Commun., 1998, 2613–2614.
2. For general overviews of the supramolecular chemistry of anions, see: (a) Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-James and E. García-España, Wiley, New York, 1997; (b) F. P. Schmidtchen and M. Berger, Chem. Rev., 1997, 97, 1609–1646; (c) Coord. Chem. Rev., 2003, 240, ed. P. A. Gale (this volume is specially dedicated to this area); (d) For metal-containing receptors of anions, see: P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486–516.
3. (a) W. E. Allen, C. J. Fowler, V. M. Lynch and J. L. Sessler, Chem.–Eur. J., 2001, 7, 721–729; (b) C. P. Causey and W. E. Allen, J. Org. Chem., 2002, 67, 5963–5968.
4. Some recent examples: (a) M. G. B. Drew, V. Félix, I. S. Gonçalves, F. E. Kühn, A. D. Lopes and C. C. Romão, Polyhedron, 1998, 17, 1091–1102; (b) S. Fortin and A. L. Beauchamp, Inorg. Chem., 2001, 40, 105–112; (c) S. Fortin, P. Fabre, M. Dartiguenave and A. L. Beauchamp, J. Chem. Soc., Dalton Trans., 2001, 3520–3527; (d) R. Atencio, M. Chacón, T. González, A. Briceño, G. Agrifoglio and A. Sierraalta, Dalton Trans., 2004, 505–513; (e) R. Atencio, K. Ramírez, J. A. Reyes, T. González and P. Silva, Inorg. Chim. Acta, 2005, 358, 520–526.
5. Beauchamp and co-workers have recently reported that hydrogen bond association, structurally characterized in the solid state, persists in CH₂Cl₂ solution for Re(III) H₂biim complexes and chloride and carboxylate anions, see ref. 4b,c.
6. D. Carmona, J. Ferrer, A. Mendoza, F. J. Lahoz and L. A. Oro, Organometallics, 1995, 14, 2066–2080.
7. S. Nieto, J. Pérez, V. Riera, D. Miguel and C. Alvarez, Chem. Commun., 2005, 546–548.
8. E. E. Bernarducci, P. K. Bharadwaj, R. A. Lalancette, K. Krogh-Jespersen, J. A. Potenza and H. J. Schugar, Inorg. Chem., 1983, 22, 3911–3920.
9. M. Brookhart, B. Grant and A. F. Volpe, Jr., Organometallics, 1992, 11, 3920–3922.
10. We have recently prepared the analogous phenanthroline compound [RuCl(cym)(phen)][BAr′₄] using the same procedure: C. Menéndez, D. Morales, J. Pérez, V. Riera and D. Miguel, Organometallics, 2001, 20, 2775–2781.
11. M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.

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Footnotes

† Electronic Supplementary Information (ESI) available: General X-ray information and ¹H NMR titration plots of 1 in DMSO-d₆ and CD₃CN. See DOI: 10.1039/b510016j

‡ Synthesis of [RuCl(cym)(H₂biim)][BAr′₄] (1): To a solution of [{RuCl(cym)}₂(μ-Cl)₂] (0.100 g, 0.163 mmol) in CH₂Cl₂ (20 mL) and MeCN (0.25 mL), NaBAr′₄ (0.290 g, 0.326 mmol) and H₂-biim (0.048 g, 0.359 mmol) were added. After stirring for 24 h, filtration and evaporation of the filtrate gave an orange solid that was washed with hexane (2 × 20 mL) and dried in vacuo. Yield: 0.380 g, 87%. ¹H NMR (CD₃CN): δ 11.43 (s br, 2 H, NH of H₂biim), 7.69 (m, 14 H, BAr′₄ and H₂biim), 7.38 (d, ³J_HH = 1.56 Hz, 2 H, H₂biim), 5.86, 5.65 (AA′BB′ system, J_AB = J_A′B′ = 6.2 Hz, 4 H, cym), 2.71 (m, 1 H, CH of ⁱPr), 2.16 (s, 3 H, CH₃ ) and 1.11 (d, ⁴J_HH = 7.2 Hz, 6H, CH₃ of ⁱPr). ¹³C {¹H}NMR (CD₃CN): δ 161.6 (q, ¹J_CB = 49.9 Hz, Cⁱ of BAr′₄), 138.2 (s, H₂biim), 134.7 (s, C^o of BAr′₄), 131.0 (s, H₂biim), 129.0 (q, ²J_CF = 31.7 Hz, C^m of BAr′₄), 124.5 (q, ¹J_CF = 271.2 Hz, CF₃ of BAr′₄), 121.0 (s, C-ⁱPr, cym), 120.3 (s, H₂biim), 117.7 (s, C^p of BAr′₄), 103.2 (s, C-Me, cym), 83.1 (s, C_A of cym), 81.5 (s, C_B of cym), 30.9 (s, CH of ⁱPr), 21.2 (s, CH₃ of ⁱPr) and 18.0 (s, CH₃ of cym). Anal. calc. for C₄₈H₃₂BClF₂₄RuN₄: C, 45.46; H, 2.54; N, 4.42. Found: C, 45.81; H, 2.39; N, 4.12%.

§ Crystal data for adduct [RuCl(cym)(H₂biim)][Cl] (dichloromethane and water co-crystallized, see ESI†): C_16.50H₂₃Cl₃N₄ORu, crystal dimensions 0.23 × 0.18 × 0.10 mm, triclinic, space group P-1, a = 11.094(2), b = 12.523(3), c = 17.560(4) Å, α = 104.49(3), β = 92.84(3), γ = 110.07(3)°, V = 2193.9(8) ų, Z = 4, T = 293(2) K, D_c = 1.516 g cm⁻³, Mo-K_α radiation (λ = 0.71073 Å), 13548 reflections collected, 4506 independent reflections (3.61 ≤ θ ≤ 21.00°), R₁ = 0.0610, wR2 = 0.1430, GOF on F² = 1.107, CCDC 279201. Crystal data for adduct [RuCl(cym)(H₂biim)][NO₃]: C₁₆H₂₀ClN₅O₃Ru, crystal dimensions 0.21 × 0.12 × 0.05 mm, orthorhombic, space group Pbca, a = 16.048(3), b = 13.798(3), c = 16.470(4) Å V = 3646.9(13) ų, Z = 8, T = 180(2) K, D_c = 1.701 g cm⁻³, Mo-K_α radiation (λ = 0.71073 Å), 23164 reflections collected, 4170 independent reflections (3.55 ≤ θ ≤ 27.48°), R₁ = 0.0503, wR2 = 0.1040, GOF on F² = 1.014, CCDC 279202. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b510016j

¶ Typically, the four ions are present in the crystals obtained from mixtures of a cationic receptor (added as its salt with a given counteranion) and tetrabutylammonium salt of the target anionic guest.

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