Structural characterisation of a diprotonated ligand lanthanide complex—a key intermediate in lanthanide ion association and complex dissociation pathways

Abstract

The X-ray structure is reported of a diprotonated Gd(III) tetra-aqua complex of the ligand 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (DOTAM), a thermodynamic intermediate in metal ion association and dissociation pathways.

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Early in the development of radiolabelled antibody conjugates for immunotherapy, it was recognised that it was the kinetic stability of the radioisotope complex that was of paramount importance in devising suitable conjugates for use in vivo. Pioneering work therefore focused on the use of macrocyclic complexes of ⁹⁰Y, for example, driven by the hypothesis that such systems would tend to resist acid (and cation) catalysed dissociation pathways.^1,2a Furthermore, working with radioisotopes required that the forward rate of complexation was fast i.e. that labelling was complete within a few minutes, stimulating studies measuring the rate of association as a function of pH^1,2 and counterion. Similar themes were considered in the development of macrocyclic complexes of gadolinium as contrast agents for MRI.^2b,2c As the free Gd aqua ion is rather toxic, the complex injected must be kinetically stable over its lifetime in the body. Again, the dominant kinetic pathway for loss of gadolinium involves an acid-catalysed mechanism.

As a consequence of the interest in the rates of complexation and dissociation, several studies have been reported on this topic, usually examining the kinetic behaviour of anionic, neutral and cationic macrocyclic complexes or related open-chain analogues.^3–8 For example, Brucher has examined the kinetics of association and dissociation of lanthanide(III) complexes of DOTA, 1. Evidence from pH metric and NMR studies was put forward supporting the rapid formation of an intermediate diprotonated species [H₂LnDOTA]⁺, which expels two protons leading to generation of the mono-anionic complex.^4b

In the acid-catalysed dissociation pathway, both mono- and diprotonated species were implicated with the latter likely to be the more kinetically labile. Similar arguments had been advanced earlier by Chang⁶ Desreux⁷ and Parker^1,2,8 for a wider range of macrocyclic ligands with Y³⁺ and Gd³⁺ ions. Thus, an [MLH₂] species has been implicated (L = DOTA and its congeners) in both metal ion association and dissociation pathways in aqueous media, in accord with the principle of microscopic reversibility.

Chart 1

During studies directed at exploring the role of the anion in defining water exchange rates and the structure of the second hydration sphere,^9,10 the complexation of DOTAM, 2, with Gd(ClO₄)₃ in water was examined. Large hexagonal crystals grew over a period of 12 h following admixture of 2 with excess Gd(ClO₄)₃·6H₂O in water. Analysis by single crystal X-ray crystallography at 120 K‡ revealed the formation of a tetra-aqua complex with five perchlorate anions (3). The Gd ion was bound (Fig. 1) to the four ligand amide carbonyl oxygens and to four waters in a regular square antiprism, with a twist angle between the two O₄ planes of 42.4(19)°. The ring adopted the common quadrangular [3333] conformation with two trans-disposed nitrogens protonated. Indeed, the ring conformation was very similar to that found in [(H₂DOTA]Cl₂)⁵, in which the ligand was considered as being pre-disposed to metal ion complexation. Two other instances of 1,7-trans-diprotonated tetrasubstituted cyclen ligands exhibit similar behaviour in their ring conformation.^11,12 Analysis of the Cambridge Structural Database (November 2005 release, version 5.27)¹³ revealed no examples of a similar poly(aqua) complex with any single ligand. Although there are thirty-four examples of various tetra-aqua Gd complexes, the only relevant one involves a catenated structure with tetra-carboxyethyl-cyclam in which the Gd is bound to four carboxylate oxygens from four different ligands.¹⁴

Fig. 1 Thermal ellipsoid plot of the tetra-aqua cation in 3, showing the complex from the side and above (alkyl hydrogen atoms and a minor component of disorder are omitted for clarity and thermal ellipsoids are drawn at 50% probability).

The structure of the cation in 3 was compared with the nine-coordinate mono-aqua gadolinium cation defined in Gd[DOTAM](NO₃)₃·3H₂O.^10a The distance across the ring was analysed by comparing the average distance between the trans carbonyl oxygen atoms (O1⋯O3/O2⋯O4 in Fig. 1). In the case of 3, this distance is considerably smaller than in the nitrate (3.99(13) Å cf. 4.51(2) Å), demonstrating how the ligand has to open out to allow it to enclose the metal. Using the four nitrogen atoms of the tetraazadodecane ring as a guide, it was also possible to overlay the two structures to show how they are related and give an indication as to how the gadolinium binds to the ligand (Fig. 2). Although there are other possibilities, notably involving ring deprotonation prior to metal ion association, this analysis suggests that as the ligand is deprotonated and three of the water molecules displaced, the ligand could open out slightly, allowing the gadolinium to enter the ligand. As this occurs, there could be a concerted change in the geometry of the ligand, with the position of the carbonyl arm moving, associated with a cooperative change in the ring conformation. What is of particular interest in the case of 3, is the disorder in one of the amide arms of the ligand, which is of note since all the NH₂ groups participate in hydrogen bonding N–H⋯N/O interactions (D⋯A distances in the range 2.660(3)–3.140(11) Å). This disordered component (C11A–C12A–N6A), may indicate that there are vibrations in the solid state structure that may be linked to the deprotonation event and change in coordination that occurs in solution.

Fig. 2 The overlay of the tetra-aqua ion, 3, with Gd[DOTAM(OH₂)]³⁺ shown from the side and from above, with arrows added to show how the structure changes to accommodate the gadolinium in the centre of the ligand. The tetra-aqua ion is shown with bonds and atoms drawn using solid lines and Gd[DOTAM(OH₂)]³⁺ is shown with broken lines. Selected atoms towards the rear of the structure and hydrogen atoms are omitted for clarity.

In summary, a thermodynamic intermediate in the metal ion association and dissociation pathways of macrocyclic octadentate ligands has been isolated, and its structure verified by single crystal X-ray diffraction. Comparison with the nine-coordinate mono-aqua gadolinium cation has suggested the conformational changes that may occur when the ligand deprotonates and the metal is enclosed by the octadentate ligand.

We thank Durham University and the EPSRC for support, and Prof. J. A. K. Howard for useful discussions.

References

1. D. Parker, Chem. Soc. Rev., 1990, 19, 271; J. P. L. Cox, K. J. Jankowski, R. Kataky, M. A. W. Eaton, N. R. A. Beeley, A. T. Millican, A. Harrison and C. A. Walker, J. Chem. Soc., Chem. Commun., 1989, 979; M. K. Moi, C. F. Meares and S. J. de Nardo, J. Am. Chem. Soc., 1988, 110, 6266.
2. (a) C. J. Broan, J. P. L. Cox, A. S. Craig, R. Kataky, D. Parker, A. Harrison, A. M. Randall and G. Ferguson, J. Chem. Soc., Perkin Trans. 2, 1991, 87; (b) P. Caravan, J. J. Ellison, T. J. McMurry and R. B. Lauffer, Chem. Rev., 1999, 99, 2293; The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging, ed. A. E. Merbach and K. Toth, Wiley, New York, 2001; (c) D. Parker, R. S. Dickins, H. Puschmann, C. Crossland and J. A. K. Howard, Chem. Rev., 2002, 102, 1977.
3. E. Brucher, Top. Curr. Chem., 2002, 221, 103; L. Burai, R. Kiraly, I. Lazar and E. Brucher, Eur. J. Inorg. Chem., 2001, 813.
4. (a) E. Szilagyi, E. Toth, Z. Kovacs, J. Platzek, B. Raduchel and E. Brucher, Inorg. Chim. Acta, 2000, 298, 226; (b) E. Toth, E. Brucher, I. Lazar and I. Toth, Inorg. Chem., 1994, 33, 4070.
5. S. Aime, A. Barge, J. I. Bruce, M. Botta, J. A. K. Howard, J. M. Moloney, D. Parker, A. S. de Sousa and M. Woods, J. Am. Chem. Soc., 1999, 121, 5762.
6. K. Kumar and M. F. Tweedle, Pure Appl. Chem., 1993, 65, 515; C. A. Chang, L. C. Francesconi, M. F. Malley, K. Kumar, J. Z. Gougoutas, M. F. Tweedle, D. W. Lee and L. J. Wilson, Inorg. Chem., 1993, 32, 3501; K. Kumar, M. A. Chang and M. F. Tweedle, Inorg. Chem., 1993, 32, 587.
7. X. Wang, T. Jin, V. Comblin, A. Lopez-Mut, E. Merciny and J. F. Desreux, Inorg. Chem., 1992, 31, 1095.
8. D. Parker, K. Pulukkody, T. J. Norman, L. Royle and C. S. Broan, J. Chem. Soc., Perkin Trans. 2, 1993, 605.
9. D. Parker, Chem. Soc. Rev., 2004, 33, 156.
10. (a) A. Barge, M. Botta, D. Parker and H. Puschmann, Chem. Commun., 2003, 1386; (b) S. Aime, A. Barge, M. Botta, F. Fedeli, A. Mortillaro, D. Parker and H. Puschmann, Chem. Commun., 2002, 1120.
11. K. Kobayashi, S. Tsuboyama, K. Tsuboyama and T. Sakuroii, Acta. Crystallogr., Sect. C: Cryst. Struct. Commun., 1994, 50, 306.
12. M. Woods, S. Aime, M. Botta, J. A. K. Howard, J. M. Moloney, M. Navet, D. Parker, M. Port and O. Rousseaux, J. Am. Chem. Soc., 2000, 122, 9781.
13. F. H. Allen, Acta Crystallogr., Sect. A, 1998, 54, 758; I. J. Bruno, J. C. Cole, P. R. Edgington, M. Kessler, C. F. Macrae, P. McCabe, J. Pearson and R. Taylor, Acta Crystallogr., Sect. B, 2002, 58, 389.
14. W. Liu, T. Jiao, Y. Li, Q. Liu, M. Tan, H. Wang and L. Wang, J. Am. Chem. Soc., 2004, 126, 2280.
15. J. Cosier and A. M. Glazer, J. Appl. Crystallogr., 1986, 19, 105.
16. SMART-NT, Data Collection Software, version 5.0, Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin, USA, 1999.
17. SAINT-NT, Data Reduction Software, version 5.0, Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin, USA, 1999.
18. SHELXTL, version 5.1, Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin, USA, 1999.

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Footnotes

† Electronic supplementary information (ESI) available: Kinetic analysis. See DOI: 10.1039/b605876k

‡ X-Ray structure determination for 3: A typical colourless crystal was selected, cut to a suitable size (0.28 × 0.12 × 0.06 mm), mounted in fluoropolyether oil on a hair and quench cooled to 120 K using an Oxford Cryosystems Cryostream 600 series open flow N₂ cooling device.¹⁵ Using a Bruker SMART-CCD 1 K area detector diffractometer, with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å), several sets of ω-scans (0.3° frames⁻¹) at different ϕ settings were collected. Cell parameters were determined and refined using the SMART software¹⁶ and raw frame data were integrated using the SAINT program.¹⁷ The structure was solved by direct methods and refined by full-matrix least squares on F² using SHELXTL software¹⁸ (crystal data are listed below). Reflection intensities were corrected for absorption effects by numerical integration based on measurements and indexing of the crystal faces (using SHELXTL software).¹⁸ Non-hydrogen atoms were refined with anisotropic thermal parameters, except for the minor component of disorder in one of the amide arms of the DOTAM ligand (N6A, C11A, & C12A, 30%). Although all the hydrogen atoms were visible in the difference map, refinement was unstable, so non-aqueous hydrogen atoms were positioned geometrically and refined using a riding model. Aqueous protons and NH⁺ protons were located in the difference map and refined with the thermal parameters riding on the parent oxygen atom and the distances restrained using “same distance” restraints between atoms in similar environments. All water molecules were refined as ordered, with hydrogen atoms making sensible D–H⋯A interactions (D⋯A distances in the range 2.721(4)–3.671(2) Å). Single crystal X-ray diffraction data: C₁₆H₄₈Cl₅GdN₈O₃₁, M_r = 1183.12, monoclinic (P2₁/n), a = 11.2765(5) Å, b = 18.3391(7) Å, c = 19.7210(8) Å, β = 94.0440(10)°, V = 4068.2(3) ų, Z = 4, µ = 2.072 mm⁻¹D_calc = 1.932 Mg m⁻³, T = 120(2) K, 47754 reflections collected, 10883 independent [R(int) = 0.0462], R₁ = 0.0352, wR₂ = 0.0729 [I > 2σ(I)]. CCDC reference number 605527. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b605876k

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