¹J(¹⁹⁹Hg¹⁹⁹H g) values of up to 284 kHz in complexes of [Hg–Hg]²⁺ with crown ethers: the largest indirect coupling constants

Abstract

The values of the ¹J(¹⁹⁹Hg¹⁹⁹Hg) coupling constant in solution of the asymmetric dimercury(2+) complexes with the stoichiometries Hg₂²⁺/18-crown-6 and Hg₂²⁺/18-crown-6/15-crown-5 are 220300 and 284100 Hz, respectively, representing the largest scalar couplings measured.

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Very large scalar coupling constants have been observed between heavy metal nuclei and have been attributed to relativistic effects. The largest value known at present is the one-bond ¹⁹⁹Hg¹⁹⁹Hg coupling in the polycation [Hg–Hg–Hg]²⁺1 with a magnitude of 139600 Hz.¹ The largest coupling constant between two different nuclei is a 71060 Hz ¹J(¹⁹⁵Pt²⁰⁵Tl) coupling observed for the compound [(NC)₅Pt–Tl].²

The use of solid-state ¹⁹⁹Hg NMR for the determination of the ¹⁹⁹Hg¹⁹⁹Hg coupling of [Hg–Hg]²⁺ salts involving magnetically inequivalent mercury atoms, has been proposed,³ but appears not to have been carried out. We report here on the synthesis of asymmetric dimercury(2+)–crown ether complexes which are kinetically stable on the NMR time scale at low temperatures and provide straightforward access to the ¹⁹⁹Hg¹⁹⁹Hg coupling constants.

Treatment of [Hg–Hg]²⁺ with 1 equiv. of 18-crown-6 in CH₂Cl₂ produces the dimercury(2+) complex 2 in which one Hg is complexed to the macrocyclic crown ether.† The mixed ligand complex [Hg₂(18-crown-6)(15-crown-5)(Me₂SO)](O₃SC F₃)₂3a containing one 18-crown-6 and one 15-crown-5 co-ordinated to dimercury(2+) was formed upon reaction of 2 with 1 equiv. of 15-crown-5.†

The ¹⁹⁹Hg NMR spectra of 2 and 3 consist of a superposition of the patterns of the two isotopomers with one ¹⁹⁹Hg nucleus (abundance 14.00% each) and the isotopomer containing two ¹⁹⁹Hg nuclei (abundance 2.84%). The latter arises from an AB spin system. The outer transitions of the AB system could not be observed because of very low probabilities (e.g. 0.3% of the inner transitions as calculated for 3b) and because of linewidths of ca. 60–80 Hz for 2 and 3, which are attributed to chemical shift anisotropy (CSA) relaxation. The coupling constant is readily derived from the distances of the inner lines of the AB system and of the shifts ν_a and ν_b which are obtained from the patterns of the isotopomers containing one ¹⁹⁹Hg nucleus. The value of the ¹J(¹⁹⁹Hg¹⁹⁹Hg) coupling constant in 2 is 220300 Hz. An even larger ¹⁹⁹Hg¹⁹⁹Hg coupling constant (263200 Hz) is observed for the mixed 18-crown-6/15-crown-5 complex 3a in CH₂Cl₂. The ¹⁹⁹Hg NMR spectrum of a solution of 3a in MeOH at 223 K shows the presence of two asymmetric dimercury(2+) complexes. The parameters of one of these species correspond to those of [Hg₂(18-crown-6)(15-crown-5)(Me₂SO)](O₃SC F₃)₂3a in CH₂Cl₂, the other species is assigned to [Hg₂(18-crown-6)(15-crown-5)(MeOH)](O₃SCF₃ )₂3b, which is thought to be formed according to the equilibrium (1).

[Hg₂(18-crown-6)(15-crown-5)(Me₂S O)]²⁺ + MeOH ⇌3a[Hg₂(18-crown-6)(15-crown-5)(MeOH)]²⁺ + Me₂SO3b (1)

The size of the Hg–Hg coupling constant of 3b is 284100 Hz, and we believe that this represents the largest scalar coupling constant recorded so far. The ¹⁹⁹Hg shifts of the mercury atoms complexed to 18-crown-6 macrocycles appear at low frequencies compared with the Hg atoms coordinated by 15-crown-5 or bridging Me₂SO.

The complexes 2 and 3c (all attempts to isolate 3a gave 3c) were characterised by single crystal X-ray diffraction:‡ as can be seen from Fig. 1, 2 exists as a dimer {[Hg₂(18-crown-6)(Me₂SO)(μ-Me₂SO) ](O₃SCF₃)₂}₂ in the solid state. Curiously, the crystals of 3c are built from two different complexes: the asymmetric unit contains one [Hg₂(18-crown-6)(15-crown-5)(Me₂SO)](O₃SC F₃)₂ and one [Hg₂(18-crown-6)(15-crown-5)(H₂O)](O₃SCF ₃)₂ species which is shown in Fig. 2. In 2 and 3c the coordination geometry of the mercury atoms complexed to a 18-crown-6 is hexagonal-bipyramidal with the other Hg atom and one oxygen of Me₂SO or H₂O in axial position and the crown oxygens in the equatorial sites. The 18-crown-6 ligands are located slightly off-centre with respect to the mercury atoms, presumably as a result of the steric demands of the Me₂SO ligand coordinated to the same Hg atom. The mercury atoms reside slightly off the 18-crown-6 plane which is shifted towards the terminal Me₂SO ligands. The 15-crown-5 macrocycle is coordinated in a ‘half-sandwich’ manner.

Fig. 1 Structure of {[Hg₂(18-crown-6)₂(Me₂SO)(μ-Me ₂SO)]}₂⁴⁺ (cation of 2). Thermal ellipsoids are shown at the 20% probability level.

Fig. 2 Structure of {[Hg₂(18-crown-6)(15-crown-5)(H₂O)]²⁺ (part of 3c). Thermal ellipsoids are shown at the 20% probability level.

Notes and references

1. R. J. Gillespie, P. Granger, K. R. Morgan and G. J. Schrobilgen, Inorg. Chem., 1984, 23, 887.
2. M. Maliarik, K. Berg, J. Glaser, M. Sandström and I. Tóth, Inorg. Chem., 1998, 37, 2910.
3. R. A. Santos and G. S. Harbison, J. Am. Chem. Soc., 1994, 116, 3075.
4. P. Peringer, J. Inorg. Nucl. Chem., 1980, 42, 1501.
5. G. M. Sheldrick, SHELXS-86: program for crystal structure solutions. Universität Göttingen, 1986..
6. G. M. Sheldrick, SHELXL-93: program for refinement of crystal structures. Universität Göttingen, 1993..

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Footnotes

† Synthesis and data: for 2: a mixture of [Hg(Me₂SO)₆](O₃SCF₃)₂ ⁴ (0.1 mmol, 0.0968 g) in CH₂Cl₂ (0.5 mL) and elemental mercury (0.1 mL) was stirred vigorously for 2 h and 0.1 mmol (0.0264 g) of 18-crown-6 was then added. Colourless crystals were obtained from a 0.4 M solution in MeOH after 10 days at 277 K (0.060 g, 54%). Anal. Calc. for C₃₆H₇₂F₁₂Hg₄O₂₈S ₈: C, 19.31; H, 3.24. Found C, 19.34; H, 3.26%. ¹⁹⁹Hg NMR (53.63 MHz, shifts relative to 2 mmol HgO in 1 ml 60% HClO₄, sweep width 50 kHz, time domain size 64 K, typical number of scans 20000, CH₂Cl₂, 243 K): δ 435 (Hg atom complexed to 18-crown-6), 944; J(¹⁹⁹Hg¹⁹⁹Hg) 220300 (±790) (corresponding to an uncertainty of the difference of the two inner AB lines of ±4 data points).

For 3a: a solution of 0.1 mmol (0.1120 g) of 2 in 0.5 mL of MeOH was treated with 0.1 mmol of 15-crown-5 (0.0220 g). All attempts to isolate 3a gave 3c: colourless crystals of 3c were obtained upon standing of a solution of 3a in MeOH for 3 weeks in contact with the atmosphere (0.070 g, 56%). Anal. Calc. for C₅₀H₉₆F₁₂Hg₄O₃₆S ₅₂O: C, 24.20; H, 3.98. Found C, 24.08; H, 3.89%. ¹⁹⁹Hg NMR for 3a, (CH₂Cl₂, 223 K): δ 304 (Hg atom complexed to 18-crown-6), 1023; J(¹⁹⁹Hg¹⁹⁹Hg) 263200 (±570); i, (MeOH, 223 K): δ 317, 1032, J(¹⁹⁹Hg¹⁹⁹Hg) 266600 (±590); 3b (MeOH, 223 K): δ 345, 972, J(¹⁹⁹Hg¹⁹⁹Hg) 284100 (±860).

‡ Crystal data: for 2: C₃₆H₇₂F₁₂Hg₄O₂₈S ₈, M = 2239.78, triclinic, space group P1̄ (no. 2), a = 9.547(5), b = 11.836(2), c = 15.548(2) Å, α = 78.39(2), β = 77.21(4), γ = 83.79(2)°, V = 1674.5(9) ų, T = 213(2) K, Z = 1, Mo-Kα radiation (λ = 0.71073 Å), 5809 reflections collected, 4795 independent reflections (R_int 0.0274), 3457 reflections with I > 2σ(I), R1 [I > 2σ(I)] = 0.0373, wR2 (all data) = 0.0726, goodness-of-fit 1.021. The structure was solved by direct methods (SHELXS-86),⁵ and refined by full-matrix least-squares methods on F² (SHELXL-93).⁶

For 3c: C₅₀H₉₆F₁₂Hg₄O₃₆S ₅·H₂O, M = 2481.94, triclinic, space group P1̄ (no. 2), a = 12.289(2), b = 15.902(4), c = 21.254(4) Å, α = 92.25(2), β = 93.06(2), γ = 93.35(2)°, V = 4136.7(15) ų, T = 213(2) K, Z = 2, Mo-Kα radiation (λ = 0.71073 Å), 8498 reflections collected, 8004 independent reflections (R_int = 0.0508), 5405 reflections with I > 2σ(I), R1 [I > 2σ(I)] = 0.0566, wR2 (all data) = 0.1483, goodness-of-fit 1.032. The structure was solved and refined as for 2.^5,6 CCDC 182/1851. See http://www.rsc.org/suppdata/cc/b0/b007581g/ for crystallographic files in .cif format.

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