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The crystallized solvent could influence the lanthanide water bonding?
Inorganica Chimica Acta 359 (2006) 3405–3411 www.elsevier.com/locate/ica
Note
The crystallized solvent could influence the lanthanide water bonding? G. Bombieri a
a,*
, N. Marchini a, S. Ciattini b, A. Mortillaro c, S. Aime
c
Istituto di Chimica Farmaceutica e Tossicologica, Universita` di Milano, Viale Abruzzi 42, I-20131 Milano, Italy b Centro di Cristallografia, Universita` di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Italy c Dipartimento di Chimica, I.F.M. Universita` di Torino, Via P.Giuria 8, I-10125 Torino, Italy Received 24 February 2006; accepted 16 March 2006 Available online 27 March 2006
Abstract Two gadonilium DOTAM complexes [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1) and [Gd(DOTAM)H2O](CF3SO3)3 Æ 0.5H2O Æ CH3CN (2) have the structure of the M isomer, with coordination geometry around the Gd ion capped square antiprismatic (SA). They differ for the ˚ in (2) where two independent molecules are present in the crystal cell. The factors influGd–Owater bond lengths of 2.396(6) and 2.474(7) A encing the Gd–Owater bond distance, important for the water exchange rate in MRI experiments, have been correlated to the different network of hydrogen bonds involving the Gd coordinated water molecule. The X-ray structure of the parent compound [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) reveals an unexpected twisted square antiprismatic (TSA) coordination geometry characteristic of the m isomer. 2006 Elsevier B.V. All rights reserved. Keywords: X-ray crystal structures; Lanthanide complexes; Contrast agents
1. Introduction Gadolinium complexes with polyaminocarboxylate ligands are of common use as a contrast agent for MRI imaging. The preeminence of Gd(III) complexes is due to the long electronic relaxation time and high paramagnetism of the Gd3+ ion. The efficient dipolar magnetic coupling between the proton nuclear spins and the unpaired electrons of the gadolinium ion is the basis of the relaxation enhancement. Important terms for this mechanism are the dissociative exchange of the coordinated water molecule (inner sphere term) to the bulk, associate to the non-coordinate water or waters (outer sphere term) and the prototropic proton exchange of the labile protons either in the inner sphere water or in the macrocyclic ligand. In particular, the macrocyclic DOTA ligand (DOTA = 1,4,7,10-tetraaza-1, 4,7,10-tetrakis(carboxymethyl)cyclododecane) skeleton is *
Corresponding author. Tel.: +39 02 503 17516; fax: +39 02 503 17565. E-mail address: [email protected] (G. Bombieri).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.03.023
the prototype for the development of new contrast agents with higher relaxivity and improved specificity. Important aspects in this field are the understanding of the relationship present in the Gd complexes for MRI, between structure, rate and water exchange mechanism at the metal centre that is generally considered as independent from the nature of the ligand in the second coordination sphere [1]. However, recently the nature of the second hydration sphere resulted important for the water exchange kinetics at the metal ion centres [2]. Factors like the lipophilicity of the ligand substituents, determining the surface accessible area, hinder the access of water of the second hydration sphere with a consequent longer water exchange lifetimes as shown in a study on Europium complexes with modified DOTAM ligands (DOTAM contains neutral amide instead of the acetate groups of DOTA) [3]. The nature also of the counter anion plays a role in the rate of water exchange consequent to the ordering effect imposed by the anion on the structure of the second hydration sphere as shown by GdDOTAM complexes with different counterions [4]. In the halide series, the iodide exchanges
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water more rapidly than bromide and chloride and the relaxivity follows the same trend, while nitrate trifluoroacetate, trifluoromethanesulphonate and chloride exchange water more slowly. We have had the opportunity to investigate by X-ray diffraction two samples of [Gd(DOTAM)H2O] triflate salts with different solvent contents depending on the crystallization conditions [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O and [Gd(DOTAM)H2O](CF3SO3)30.5H2O Æ CH3CN and to clarify the role of the solvent for their structural parameters. The X-ray structures of both together to that of [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN are reported here and discussed. 2. Experimental 2.1. Preparation of the compounds The compounds 1, 2 and 3 were obtained according to the literature [5] by adding to a methanol solution of the ligand DOTAM (1,4,7,10-tetrakis[carbamoylmethyl]1,4,7,10-tetraazacyclodecane), an acetonitrile solution of gadolinium(or praseodymium) trifluoromethane sulfonate then heating under reflux for several hours. To the cooled solution, tetrahydrofuran was added until the complexes precipitated. The resulting solids in powder form were filtered and redissolved in acetonitrile and water added (for 2 and 3). After few days, white prismatic crystals suitable for X-ray analysis were separated, while crystal of 1 was grown as a pale lilac prism by slow evaporation from water. 2.2. X-ray analysis An Enraf Nonius CAD-4 diffractometer was used for data collection (compounds 1 and 3) at room temperature with Mo Ka radiation, while for 2 low temperature data (100 K) were collected on a Nonius Kappa-CCD detector diffractometer with Cu Ka radiation.
The lattice parameters were determined by least-squares refinements of 25 high angle reflections. The structures were solved by direct methods (SIR-92 [6]) and the refinements were carried out by full-matrix least-squares with anisotropic thermal factors for the non-hydrogen atoms. The H-atoms positions were introduced in calculated positions in their described geometries and allowed to ride on the attached carbon atom with fixed isotropic thermal parameters. Refinements were carried out with SHELX-97 [7]. 3. Results and discussion The Gadolinium and Praseodymium complexes were obtained by reacting their respective triflate salt in acetonitrile with the DOTAM ligand in methanol. The single crystals of [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1) were grown from slow evaporation of a water solution. It crystallizes with one [Gd(DOTAM)H2O]3+ complex cation, three triflate counter-ions and three water molecules of crystallization. Its X-ray structure presents a nine coordinate Gd3+ ion (Fig. 1). In the cation, the four nitrogen and four oxygen atoms form two square planes (deviations of the nitrogens and of the oxygens from ˚ ) that their respective square planes are 0.001 and 0.003 A are parallel to each other (interplanar angle 1). The gado˚ out of the oxygen plane and linium ion is 0.716(1) A ˚ from the nitrogen plane. Considering also 1.614(1) A the coordinated water molecule in the capping position, the coordination geometry of the metal ion is a capped square antiprism (SA) with the twist angle between the two planes of the nitrogen and oxygen of 38 (comparable to the values found in Ln-DOTA complexes [8]). ˚ somewhat The Gd–Ow bond distance is 2.394(3) A shorter than that reported for [Gd(DTPA)H2O]2+ ˚ [10], but com2.507(10) [9] [Gd(DOTA)H2O]3+ 2.458(3) A ˚ [11] with the parable with [Gd(DO3A-L2)H2O]3+ 2.429 A same CSAP geometry (angle 39.1). The Gd–O carboxylate ˚ and the bond distances are in the range 2.372(3)–2.388(4) A ˚ comparable to those found in Gd–N 2.638(4)–2.671(4) A
Fig. 1. ORTEP [12] representation of [Gd(DOTAM)H2O]3+ cation (1a) and a view of the coordination polyhedron (1b).
G. Bombieri et al. / Inorganica Chimica Acta 359 (2006) 3405–3411
the previously quoted [Gd(DO3A-L2)H2O]3+. The numerous hydrogen acceptor–donor atoms determine the molecular packing (Fig. 2). In particular the hydrogen bond between the N(3)H2 amine group and the amide oxygen O(4) of the centrosymmetrically related molecule (N(3)– ˚ , angle 160 and vice versa) determine a H O(4) 2.21 A kind of dimer, that is directly or indirectly linked to the triflate oxygens, and to the crystallized water molecules. The coordinated water is turn is linked to two adjacent water molecules that are also connected to the triflate oxygens as shown in Figs. 2 and 3. The crystals of compound [Gd(DOTAM)H2O](CF3SO3)3 Æ 0.5H2O Æ CH3CN (2) were obtained by slow diffusion of water in acetonitrile. It crystallizes with two independent Gd complex molecules as asymmetric units. The geometry around Gd3+ ions is a typical monocapped square antiprism (SA) with a twist angle between the two square planes of the four coordinated nitrogen and the four oxygens of about 39 as in compound 1. The nitrogen atoms are essentially coplanar (rms devia˚ ) as well as the four oxygen atoms (rms devition of 0.015 A
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˚ ), interplanar angle 0.5 for the first ation of 0.020 A molecule, the respective values for second labelled A are ˚ ), oxygens (0.013 A ˚ ) and interplanar nitrogens (0.012 A angle 2.2. A comparison of the bond distances with 1 (Table 2) shows a good agreement for that concerning Gd–O carboxylic, Gd–N and Gd–O1w water for one of the two molecules, while in the second the Gd–O1w is significantly ˚ . This latter value agrees larger 2.474(7) versus 2.396(6) A with that found in compound 1 while the largest value is ˚ found in the analocomparable to the value of 2.466(2) A gous derivative [Gd(DOTAM)H2O](NO3)3 Æ 3H2O [4] with the nitrato anion instead of the triflate even though the coordinated water is in the latter connected via hydrogen bond to two second sphere water molecules and to a nitrate oxygen while an acetonitrile and a triflate are the partners of 2A. The two molecules (2 and 2A) differ in the torsion angle O2–C1–C2–N5 (Table 3). This could be ascribed to a strain due to the different intermolecular contacts with the associate amine group N(1)H2 that is connected (via hydrogen bond interactions) to two adjacent triflate oxy-
Fig. 2. Crystal packing of 1 (dotted lines show H-bond interactions with the relative distances).
Fig. 3. Different H-bond interactions of the coordinated water molecule in 1, 2 and 2A.
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Table 1 Crystal data and structure refinement for compounds 1, 2 and 3 Compound
[Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 1 2H2 O CH3 CN ð2Þ
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3)
Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Calculated density (Mg m3) Absorption coefficient (mm1) Crystal size (mm) h Range () Limiting indices
C19H32F9GdN8O17S3 1068.96 293(2) 0.71073 triclinic P 1
C21H40F9GdN9O14,5S3 1075.05 100(2) 1.54178 triclinic P 1
C21H39F9PrN9O15S3 1065.70 293(2) 0.71073 triclinic P 1
12.709(5) 13.041(4) 13.261(7) 64.83(3) 83.70(4) 83.19(4) 1970.8(14) 2 1.801 1.959 0.28 · 0.20 · 0.19 2.27–24.97 15 6 h 6 15 14 6 k 6 15 0 6 l 6 15 7252/6928 full-matrix least-squares on F2 6928/0/514 1.116 R1 = 0.0395, wR2 = 0.1077 R1 = 0.0424, wR2 = 0.1101
13.1165(8) 13.7321(9) 21.481(9) 88.620(4) 87.041(4) 89.969(3) 3862.9(4) 4 1.849 13.697 0.15 · 0.12 · 0.10 2.06–56.94 14 6 h 6 14 14 6 k 6 13 0 6 l6 22 8012/8012
12.976(4) 13.510(1) 13.544(3) 79.27(2) 70.61(3) 63.26(3) 1998.2(8) 2 1.771 1.490 0.27 · 0.22 · 0.15 2.06–24.98 14 6 h 6 15 15 6 k 6 16 1 6 l 6 16 7946/6989
8012/0/703 1.145 R1 = 0.0891, wR2 = 0.2361 R1 = 0.0914, wR2 = 0.2390
6989/0/636 1.053 R1 = 0.0578, wR2 = 0.1415 R1 = 0.0685, wR2 = 0.1507
Reflection collected/unique Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data)
Table 2 ˚ ) for 1, 2 and 3 Selected bond lengths (A Compound
[Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 12H2 O CH3 CN ð2Þ
Ln(1)–O(1)w Ln(1)–O(2) Ln(1)–O(3) Ln(1)–O(4) Ln(1)–O(5) Ln(1)–N(5) Ln(1)–N(6) Ln(1)–N(7) Ln(1)–N(8)
2.394(3) 2.372(3) 2.388(3) 2.382(3) 2.373(3) 2.645(4) 2.671(4) 2.638(4) 2.652(4)
2.396(6) 2.368(6) 2.388(7) 2.362(6) 2.351(6) 2.648(7) 2.630(7) 2.647(7) 2.670(9)
a
2.474(7)a 2.379(6)a 2.366(6)a 2.345(6)a 2.332(8)a 2.649(9)a 2.630(9)a 2.643(7)a 2.653(7)a
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) 2.516(5) 2.449(4) 2.456(4) 2.410(4) 2.459(4) 2.709(5) 2.757(5) 2.745(5) 2.702(5)
The asymmetric unit is formed by two independent molecules.
Table 3 Selected torsion angles () for 1, 2 and 3 Compound
[Gd(DOTAM)H2O)](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 12H2 O CH3 CN ð2Þ
N5–C9–C10–N6 N6–C11–C12–N7 N7–C13–C14–N8 N8–C15–C16–N5 O2–C1–C2–N5 O3–C3–C4–N6 O4–C5–C6–N7 O5–C7–C8–N8
59.8(6) 58.0(6) 58.6(6) 59.6(5) 26.0(6) 14.4(6) 29.5(6) 25.2(7)
58(1) 58(1) 60(1) 58(1) 31(1) 27(1) 30(1) 27(1)
a
The asymmetric unit is formed by two independent molecules.
57(1)a 59(1)a 59(1)a 63(1)a 14(1)a 36(1)a 31(1)a 29(1)a
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) 59.7(8) 59.9(8) 59.7(8) 58.9(7) 25.2(8) 16.5(9) 29.5(9) 26.6(9)
G. Bombieri et al. / Inorganica Chimica Acta 359 (2006) 3405–3411
˚ and N(1A) O(12) 2.84(1)) gens (N(1A) O(8) 3.19(1) A ˚ ). while N(1) is in contact only with O(24) (2.91 A It is interesting to note that a deviation from the averaged value for the O–C–C–N torsion angle is also in 1, with an analogous value of 14, as in one of the two independent molecules (2A), where a rather strong interaction N(2)– ˚ (triflate) is present. O(9) 2.41 A The two independent molecules have essentially the same geometrical parameters (see Table 2) but they differ for the coordinated water molecule interactions, being in one (A labelled) with the largest Nd–O(1) bond distance ˚ ) the water in contact with the solvate acetoni(2.473(7) A trile and one triflate while in the not labelled with the short˚ ) the water forms est Nd–O(1) bond distance (2.398(6) A hydrogen bonds with a uncoordinated water molecule and with a triflate counter-ion (in compound 1 the H-bond interactions were with two uncoordinated water molecules). It is worthwhile to notice that the largest Gd–water bond is characterized by a shorter value of the Gd–O (mac˚ versus 2.367(3) A ˚ rocycle) distances on average 2.356(3) A in the other. A similar behaviour is shown in the crystal structure of Li[Nd(Hdo3aP)(H2O)] Æ 11.5(H2O) [13] where two different Nd–Owater bond distances(2.499(4) and ˚ ) for the two independent molecules, present in 2.591(4) A the same crystal, are related, respectively, to a lengthening or a shortening of Nd–O phosphate and in both molecules different intermolecular interactions of the coordinated water with the bulk are present. Again in the modified DOTAM (having a lipophilic CH(Me)Ph moiety instead of an amide hydrogen) 6 in the complex [Er6(H2O)](CF3SO3)3 two different Er–Owater distances depending on the outer sphere composition are ˚ ) and with different counter present (2.433(2) and 2.377(3) A ions in the analogous Gd derivatives the Gd–Owater bond ˚ . The differdistances varies from 2.351(3) to 2.415(5) A ences are attributed to different systems of hydrogen bonds [3].
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The molecular packing, differently from 1, is characterized by chain structures developing along the crystallographic axis b, where the chain of the molecules not labelled are running parallel to that A labelled; between them are located the triflate anions connecting both via hydrogen bond interactions (Fig. 4). Crystals having the chemical composition [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) were grown from slow diffusion of water in acetonitrile as pale lilac prisms. The coordination geometry around the metal ion is surprisingly different from the gadolinium derivative, being in this case of TSA type. Fig. 5 shows an ORTEP [12] view of the [Pr(DOTAM)H2O]+3 cation, while in
Fig. 4. Crystal packing of 2 (dotted lines show H-bond interactions).
Fig. 5. ORTEP [12] representation of [Pr(DOTAM)H2O]+3 cation (1a), coordination polyhedron (1b) and coordinated water interactions with the triflate anions (1c).
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Fig. 6. Crystal packing of 3 (dotted lines show H-bond interactions).
Fig. 6 the unit cell content down the crystallographic c-axis is shown. Again the coordination geometry involves four oxygens and four nitrogen atoms from the DOTAM ligand with a water molecule in capping position. The Pr ion is ˚ apart from the oxygen plane and 0.7922(5) A ˚ from the nitrogen plane, that are nearly paral1.7355(5) A lel to each other. The twist angle between the two planes is 24.2, according to the values reported for TSA type of structure [8]. ˚ agrees with the The Pr–Ow bond distance of 2.516(5) A ˚ reported for Na[Pr(DOTA)value of 2.529(3) A H2O] Æ 4H2O [8] as well as the Pr–N and Pr–O bond distances of the macrocycle despite the different types of geometry, being in the DOTA complex of SA type. In addition the interaction of the coordinated water with the solvent molecules is totally different in the two complexes due to the different solvent contents. In [Pr(DOTA)H2O]+3 the coordinated water interacts with two uncoordinated oxygens of the ligand of adiacent molecules, while in [Pr(DOTAM)H2O]+3 two triflate oxygens of two different anions are acceptor of the hydrogen of the coordinated water as shown in Fig. 5. The unique crystallized water molecule is in contact only with triflate anions. The crystal packing is characterized by a chain structure when we consider the hydrogen bond interactions between the symmetry related molecules as shown in Fig. 6. 4. Comments Two gadolinium complexes with the DOTAM ligand and the same triflate counter-ion have different solvent compositions depending on the crystallization conditions. The coordination geometry at the metal centre is the same capped square antiprismatic (SA) for both but different gadolinium coordinated water bond distances are present in compound 2 having two independent molecules crystallized in the same crystal cell with different outer sphere hydrogen bonding systems involving the coordinated water. Other examples in this respect have been presented in the section result and discussion. This is an important tool when considering the factors that could influence the
rate of water exchange in the gadolinium complexes in water solution. It has already been discussed that factors of different lipophilicity of the substituted macrocycle, can hinder water exchange as well as the nature of the counter-ion. The hydrogen bond system around the coordinated water, supported by the nature of the anion is important, but a non-secondary role is played by the solvent that is implied in this system like water–water, water–anion, water–macrocycle, anion macrocycle interactions. The effect on the water exchange comes out as depending from synergy of all these factors, that in many case contribute to a variation of the metal–water bond distance whose concomitant further influences the water exchange. Concerning the praseodimium structure it was surprising to find in the solid state a twist square antiprismatic geometry (TSA) in a complex having the same chemical composition of the parent gadolinium and also for that concerning the outer sphere region, having instead a square antiprismatic geometry (SA). But if we consider that the crystal structure of [Eu(DOTA)H2O](CF3SO3)3 Æ 2CH3OH [14] is constituted by two different stereo-isomers having, respectively, SA and TSA geometries even though consequent to a disorder between two positions of the 1,4,7,10tetraazocyclododecane, the energy difference between the two configurations must be rather low at least for this family of lanthanides, and subtle difference in the synthesis can address the metal coordination geometry towards one or the other form or both as shown by the previously described europium derivative [14]. The outer sphere composition in this case is the same as the dependent amino groups and the coordinate water positions are coincident. 5. Supplementary data A summary of the crystal data, data collection, and structure refinement is presented in Table 1; CCDC Nos. 296826, 296827 and 296828 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic
G. Bombieri et al. / Inorganica Chimica Acta 359 (2006) 3405–3411
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or [email protected]). References [1] C. Cossy, L. Helm, A.E. Merbach, Inorg. Chem. 28 (1989) 2699. [2] D. Parker, R.S. Dickins, H. Puschmann, C. Crossland, J.A.K. Howard, Chem. Rev. 102 (2002) 1977. [3] S. Aime, A. Barge, A.S. Batsanov, M. Botta, D. Delli Castelli, F. Fedeli, A. Mortillaro, D. Parker, H. Puschmann, Chem. Commun. (2002) 1120. [4] A. Barge, M. Botta, D. Parker, H. Puschmann, Chem. Commun. (2003) 1386. [5] S. Aime, A. Barge, J. Bruce, M. Botta, J.A.K. Howard, J.M. Moloney, D. Parker, A.S. de Sousa, M. Woods, J. Am. Chem. Soc. 121 (1999) 5762.
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[6] SIR-92: A. Altomare, M.C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo, A. Cagliardi, G. Polidori, J. Appl. Crystallogr. 27 (1994) 435. [7] G.M. Sheldrick, SHELXL97, Program for Refinement of Crystal Structure, University of Go¨ttingen, Germany, 1997. [8] F. Benetollo, G. Bombieri, L. Calabi, S. Aime, M. Botta, Inorg. Chem. 42 (2003) 148. [9] K. Kumar, C.A. Chang, L.C. Francesconi, D.D. Dischino, M.F. Malley, J.Z. Gougoutas, M.F. Tweedle, Inorg. Chem. 33 (1994) 3567. [10] J.P. Dubost, J.M. Leger, M.H. Langlois, D. Meyer, M. Schaefer, M.C.R.I. Academie Sci., Ser. II Univers. 312 (1991) 349. [11] S. Aime, P.L. Anelli, M. Botta, F. Fedeli, M. Grandi, P. Paoli, F. Uggeri, Inorg. Chem. 31 (1992) 2422. [12] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565. [13] J. Rudovsky, P. Cigler, J. Kotek, P. Hermann, P. Vojtisek, I. Lukes, J.A. Peters, L.V. Elst, R.N. Muller, Chem. Eur. J. 11 (2005) 2373. [14] S. Amin, D.A. Voss Jr., W. De W. Horrocks Jr., C.H. Lake, M.R. Churchill, J.R. Morrow, Inorg. Chem. 34 (1995) 3294.
Note
The crystallized solvent could influence the lanthanide water bonding? G. Bombieri a
a,*
, N. Marchini a, S. Ciattini b, A. Mortillaro c, S. Aime
c
Istituto di Chimica Farmaceutica e Tossicologica, Universita` di Milano, Viale Abruzzi 42, I-20131 Milano, Italy b Centro di Cristallografia, Universita` di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Italy c Dipartimento di Chimica, I.F.M. Universita` di Torino, Via P.Giuria 8, I-10125 Torino, Italy Received 24 February 2006; accepted 16 March 2006 Available online 27 March 2006
Abstract Two gadonilium DOTAM complexes [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1) and [Gd(DOTAM)H2O](CF3SO3)3 Æ 0.5H2O Æ CH3CN (2) have the structure of the M isomer, with coordination geometry around the Gd ion capped square antiprismatic (SA). They differ for the ˚ in (2) where two independent molecules are present in the crystal cell. The factors influGd–Owater bond lengths of 2.396(6) and 2.474(7) A encing the Gd–Owater bond distance, important for the water exchange rate in MRI experiments, have been correlated to the different network of hydrogen bonds involving the Gd coordinated water molecule. The X-ray structure of the parent compound [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) reveals an unexpected twisted square antiprismatic (TSA) coordination geometry characteristic of the m isomer. 2006 Elsevier B.V. All rights reserved. Keywords: X-ray crystal structures; Lanthanide complexes; Contrast agents
1. Introduction Gadolinium complexes with polyaminocarboxylate ligands are of common use as a contrast agent for MRI imaging. The preeminence of Gd(III) complexes is due to the long electronic relaxation time and high paramagnetism of the Gd3+ ion. The efficient dipolar magnetic coupling between the proton nuclear spins and the unpaired electrons of the gadolinium ion is the basis of the relaxation enhancement. Important terms for this mechanism are the dissociative exchange of the coordinated water molecule (inner sphere term) to the bulk, associate to the non-coordinate water or waters (outer sphere term) and the prototropic proton exchange of the labile protons either in the inner sphere water or in the macrocyclic ligand. In particular, the macrocyclic DOTA ligand (DOTA = 1,4,7,10-tetraaza-1, 4,7,10-tetrakis(carboxymethyl)cyclododecane) skeleton is *
Corresponding author. Tel.: +39 02 503 17516; fax: +39 02 503 17565. E-mail address: [email protected] (G. Bombieri).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.03.023
the prototype for the development of new contrast agents with higher relaxivity and improved specificity. Important aspects in this field are the understanding of the relationship present in the Gd complexes for MRI, between structure, rate and water exchange mechanism at the metal centre that is generally considered as independent from the nature of the ligand in the second coordination sphere [1]. However, recently the nature of the second hydration sphere resulted important for the water exchange kinetics at the metal ion centres [2]. Factors like the lipophilicity of the ligand substituents, determining the surface accessible area, hinder the access of water of the second hydration sphere with a consequent longer water exchange lifetimes as shown in a study on Europium complexes with modified DOTAM ligands (DOTAM contains neutral amide instead of the acetate groups of DOTA) [3]. The nature also of the counter anion plays a role in the rate of water exchange consequent to the ordering effect imposed by the anion on the structure of the second hydration sphere as shown by GdDOTAM complexes with different counterions [4]. In the halide series, the iodide exchanges
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water more rapidly than bromide and chloride and the relaxivity follows the same trend, while nitrate trifluoroacetate, trifluoromethanesulphonate and chloride exchange water more slowly. We have had the opportunity to investigate by X-ray diffraction two samples of [Gd(DOTAM)H2O] triflate salts with different solvent contents depending on the crystallization conditions [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O and [Gd(DOTAM)H2O](CF3SO3)30.5H2O Æ CH3CN and to clarify the role of the solvent for their structural parameters. The X-ray structures of both together to that of [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN are reported here and discussed. 2. Experimental 2.1. Preparation of the compounds The compounds 1, 2 and 3 were obtained according to the literature [5] by adding to a methanol solution of the ligand DOTAM (1,4,7,10-tetrakis[carbamoylmethyl]1,4,7,10-tetraazacyclodecane), an acetonitrile solution of gadolinium(or praseodymium) trifluoromethane sulfonate then heating under reflux for several hours. To the cooled solution, tetrahydrofuran was added until the complexes precipitated. The resulting solids in powder form were filtered and redissolved in acetonitrile and water added (for 2 and 3). After few days, white prismatic crystals suitable for X-ray analysis were separated, while crystal of 1 was grown as a pale lilac prism by slow evaporation from water. 2.2. X-ray analysis An Enraf Nonius CAD-4 diffractometer was used for data collection (compounds 1 and 3) at room temperature with Mo Ka radiation, while for 2 low temperature data (100 K) were collected on a Nonius Kappa-CCD detector diffractometer with Cu Ka radiation.
The lattice parameters were determined by least-squares refinements of 25 high angle reflections. The structures were solved by direct methods (SIR-92 [6]) and the refinements were carried out by full-matrix least-squares with anisotropic thermal factors for the non-hydrogen atoms. The H-atoms positions were introduced in calculated positions in their described geometries and allowed to ride on the attached carbon atom with fixed isotropic thermal parameters. Refinements were carried out with SHELX-97 [7]. 3. Results and discussion The Gadolinium and Praseodymium complexes were obtained by reacting their respective triflate salt in acetonitrile with the DOTAM ligand in methanol. The single crystals of [Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1) were grown from slow evaporation of a water solution. It crystallizes with one [Gd(DOTAM)H2O]3+ complex cation, three triflate counter-ions and three water molecules of crystallization. Its X-ray structure presents a nine coordinate Gd3+ ion (Fig. 1). In the cation, the four nitrogen and four oxygen atoms form two square planes (deviations of the nitrogens and of the oxygens from ˚ ) that their respective square planes are 0.001 and 0.003 A are parallel to each other (interplanar angle 1). The gado˚ out of the oxygen plane and linium ion is 0.716(1) A ˚ from the nitrogen plane. Considering also 1.614(1) A the coordinated water molecule in the capping position, the coordination geometry of the metal ion is a capped square antiprism (SA) with the twist angle between the two planes of the nitrogen and oxygen of 38 (comparable to the values found in Ln-DOTA complexes [8]). ˚ somewhat The Gd–Ow bond distance is 2.394(3) A shorter than that reported for [Gd(DTPA)H2O]2+ ˚ [10], but com2.507(10) [9] [Gd(DOTA)H2O]3+ 2.458(3) A ˚ [11] with the parable with [Gd(DO3A-L2)H2O]3+ 2.429 A same CSAP geometry (angle 39.1). The Gd–O carboxylate ˚ and the bond distances are in the range 2.372(3)–2.388(4) A ˚ comparable to those found in Gd–N 2.638(4)–2.671(4) A
Fig. 1. ORTEP [12] representation of [Gd(DOTAM)H2O]3+ cation (1a) and a view of the coordination polyhedron (1b).
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the previously quoted [Gd(DO3A-L2)H2O]3+. The numerous hydrogen acceptor–donor atoms determine the molecular packing (Fig. 2). In particular the hydrogen bond between the N(3)H2 amine group and the amide oxygen O(4) of the centrosymmetrically related molecule (N(3)– ˚ , angle 160 and vice versa) determine a H O(4) 2.21 A kind of dimer, that is directly or indirectly linked to the triflate oxygens, and to the crystallized water molecules. The coordinated water is turn is linked to two adjacent water molecules that are also connected to the triflate oxygens as shown in Figs. 2 and 3. The crystals of compound [Gd(DOTAM)H2O](CF3SO3)3 Æ 0.5H2O Æ CH3CN (2) were obtained by slow diffusion of water in acetonitrile. It crystallizes with two independent Gd complex molecules as asymmetric units. The geometry around Gd3+ ions is a typical monocapped square antiprism (SA) with a twist angle between the two square planes of the four coordinated nitrogen and the four oxygens of about 39 as in compound 1. The nitrogen atoms are essentially coplanar (rms devia˚ ) as well as the four oxygen atoms (rms devition of 0.015 A
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˚ ), interplanar angle 0.5 for the first ation of 0.020 A molecule, the respective values for second labelled A are ˚ ), oxygens (0.013 A ˚ ) and interplanar nitrogens (0.012 A angle 2.2. A comparison of the bond distances with 1 (Table 2) shows a good agreement for that concerning Gd–O carboxylic, Gd–N and Gd–O1w water for one of the two molecules, while in the second the Gd–O1w is significantly ˚ . This latter value agrees larger 2.474(7) versus 2.396(6) A with that found in compound 1 while the largest value is ˚ found in the analocomparable to the value of 2.466(2) A gous derivative [Gd(DOTAM)H2O](NO3)3 Æ 3H2O [4] with the nitrato anion instead of the triflate even though the coordinated water is in the latter connected via hydrogen bond to two second sphere water molecules and to a nitrate oxygen while an acetonitrile and a triflate are the partners of 2A. The two molecules (2 and 2A) differ in the torsion angle O2–C1–C2–N5 (Table 3). This could be ascribed to a strain due to the different intermolecular contacts with the associate amine group N(1)H2 that is connected (via hydrogen bond interactions) to two adjacent triflate oxy-
Fig. 2. Crystal packing of 1 (dotted lines show H-bond interactions with the relative distances).
Fig. 3. Different H-bond interactions of the coordinated water molecule in 1, 2 and 2A.
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Table 1 Crystal data and structure refinement for compounds 1, 2 and 3 Compound
[Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 1 2H2 O CH3 CN ð2Þ
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3)
Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Calculated density (Mg m3) Absorption coefficient (mm1) Crystal size (mm) h Range () Limiting indices
C19H32F9GdN8O17S3 1068.96 293(2) 0.71073 triclinic P 1
C21H40F9GdN9O14,5S3 1075.05 100(2) 1.54178 triclinic P 1
C21H39F9PrN9O15S3 1065.70 293(2) 0.71073 triclinic P 1
12.709(5) 13.041(4) 13.261(7) 64.83(3) 83.70(4) 83.19(4) 1970.8(14) 2 1.801 1.959 0.28 · 0.20 · 0.19 2.27–24.97 15 6 h 6 15 14 6 k 6 15 0 6 l 6 15 7252/6928 full-matrix least-squares on F2 6928/0/514 1.116 R1 = 0.0395, wR2 = 0.1077 R1 = 0.0424, wR2 = 0.1101
13.1165(8) 13.7321(9) 21.481(9) 88.620(4) 87.041(4) 89.969(3) 3862.9(4) 4 1.849 13.697 0.15 · 0.12 · 0.10 2.06–56.94 14 6 h 6 14 14 6 k 6 13 0 6 l6 22 8012/8012
12.976(4) 13.510(1) 13.544(3) 79.27(2) 70.61(3) 63.26(3) 1998.2(8) 2 1.771 1.490 0.27 · 0.22 · 0.15 2.06–24.98 14 6 h 6 15 15 6 k 6 16 1 6 l 6 16 7946/6989
8012/0/703 1.145 R1 = 0.0891, wR2 = 0.2361 R1 = 0.0914, wR2 = 0.2390
6989/0/636 1.053 R1 = 0.0578, wR2 = 0.1415 R1 = 0.0685, wR2 = 0.1507
Reflection collected/unique Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data)
Table 2 ˚ ) for 1, 2 and 3 Selected bond lengths (A Compound
[Gd(DOTAM)H2O](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 12H2 O CH3 CN ð2Þ
Ln(1)–O(1)w Ln(1)–O(2) Ln(1)–O(3) Ln(1)–O(4) Ln(1)–O(5) Ln(1)–N(5) Ln(1)–N(6) Ln(1)–N(7) Ln(1)–N(8)
2.394(3) 2.372(3) 2.388(3) 2.382(3) 2.373(3) 2.645(4) 2.671(4) 2.638(4) 2.652(4)
2.396(6) 2.368(6) 2.388(7) 2.362(6) 2.351(6) 2.648(7) 2.630(7) 2.647(7) 2.670(9)
a
2.474(7)a 2.379(6)a 2.366(6)a 2.345(6)a 2.332(8)a 2.649(9)a 2.630(9)a 2.643(7)a 2.653(7)a
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) 2.516(5) 2.449(4) 2.456(4) 2.410(4) 2.459(4) 2.709(5) 2.757(5) 2.745(5) 2.702(5)
The asymmetric unit is formed by two independent molecules.
Table 3 Selected torsion angles () for 1, 2 and 3 Compound
[Gd(DOTAM)H2O)](CF3SO3)3 Æ 3H2O (1)
½GdðDOTAMÞH2 OðCF3 SO3 Þ3 12H2 O CH3 CN ð2Þ
N5–C9–C10–N6 N6–C11–C12–N7 N7–C13–C14–N8 N8–C15–C16–N5 O2–C1–C2–N5 O3–C3–C4–N6 O4–C5–C6–N7 O5–C7–C8–N8
59.8(6) 58.0(6) 58.6(6) 59.6(5) 26.0(6) 14.4(6) 29.5(6) 25.2(7)
58(1) 58(1) 60(1) 58(1) 31(1) 27(1) 30(1) 27(1)
a
The asymmetric unit is formed by two independent molecules.
57(1)a 59(1)a 59(1)a 63(1)a 14(1)a 36(1)a 31(1)a 29(1)a
[Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) 59.7(8) 59.9(8) 59.7(8) 58.9(7) 25.2(8) 16.5(9) 29.5(9) 26.6(9)
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˚ and N(1A) O(12) 2.84(1)) gens (N(1A) O(8) 3.19(1) A ˚ ). while N(1) is in contact only with O(24) (2.91 A It is interesting to note that a deviation from the averaged value for the O–C–C–N torsion angle is also in 1, with an analogous value of 14, as in one of the two independent molecules (2A), where a rather strong interaction N(2)– ˚ (triflate) is present. O(9) 2.41 A The two independent molecules have essentially the same geometrical parameters (see Table 2) but they differ for the coordinated water molecule interactions, being in one (A labelled) with the largest Nd–O(1) bond distance ˚ ) the water in contact with the solvate acetoni(2.473(7) A trile and one triflate while in the not labelled with the short˚ ) the water forms est Nd–O(1) bond distance (2.398(6) A hydrogen bonds with a uncoordinated water molecule and with a triflate counter-ion (in compound 1 the H-bond interactions were with two uncoordinated water molecules). It is worthwhile to notice that the largest Gd–water bond is characterized by a shorter value of the Gd–O (mac˚ versus 2.367(3) A ˚ rocycle) distances on average 2.356(3) A in the other. A similar behaviour is shown in the crystal structure of Li[Nd(Hdo3aP)(H2O)] Æ 11.5(H2O) [13] where two different Nd–Owater bond distances(2.499(4) and ˚ ) for the two independent molecules, present in 2.591(4) A the same crystal, are related, respectively, to a lengthening or a shortening of Nd–O phosphate and in both molecules different intermolecular interactions of the coordinated water with the bulk are present. Again in the modified DOTAM (having a lipophilic CH(Me)Ph moiety instead of an amide hydrogen) 6 in the complex [Er6(H2O)](CF3SO3)3 two different Er–Owater distances depending on the outer sphere composition are ˚ ) and with different counter present (2.433(2) and 2.377(3) A ions in the analogous Gd derivatives the Gd–Owater bond ˚ . The differdistances varies from 2.351(3) to 2.415(5) A ences are attributed to different systems of hydrogen bonds [3].
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The molecular packing, differently from 1, is characterized by chain structures developing along the crystallographic axis b, where the chain of the molecules not labelled are running parallel to that A labelled; between them are located the triflate anions connecting both via hydrogen bond interactions (Fig. 4). Crystals having the chemical composition [Pr(DOTAM)H2O](CF3SO3)3 Æ H2O Æ CH3CN (3) were grown from slow diffusion of water in acetonitrile as pale lilac prisms. The coordination geometry around the metal ion is surprisingly different from the gadolinium derivative, being in this case of TSA type. Fig. 5 shows an ORTEP [12] view of the [Pr(DOTAM)H2O]+3 cation, while in
Fig. 4. Crystal packing of 2 (dotted lines show H-bond interactions).
Fig. 5. ORTEP [12] representation of [Pr(DOTAM)H2O]+3 cation (1a), coordination polyhedron (1b) and coordinated water interactions with the triflate anions (1c).
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Fig. 6. Crystal packing of 3 (dotted lines show H-bond interactions).
Fig. 6 the unit cell content down the crystallographic c-axis is shown. Again the coordination geometry involves four oxygens and four nitrogen atoms from the DOTAM ligand with a water molecule in capping position. The Pr ion is ˚ apart from the oxygen plane and 0.7922(5) A ˚ from the nitrogen plane, that are nearly paral1.7355(5) A lel to each other. The twist angle between the two planes is 24.2, according to the values reported for TSA type of structure [8]. ˚ agrees with the The Pr–Ow bond distance of 2.516(5) A ˚ reported for Na[Pr(DOTA)value of 2.529(3) A H2O] Æ 4H2O [8] as well as the Pr–N and Pr–O bond distances of the macrocycle despite the different types of geometry, being in the DOTA complex of SA type. In addition the interaction of the coordinated water with the solvent molecules is totally different in the two complexes due to the different solvent contents. In [Pr(DOTA)H2O]+3 the coordinated water interacts with two uncoordinated oxygens of the ligand of adiacent molecules, while in [Pr(DOTAM)H2O]+3 two triflate oxygens of two different anions are acceptor of the hydrogen of the coordinated water as shown in Fig. 5. The unique crystallized water molecule is in contact only with triflate anions. The crystal packing is characterized by a chain structure when we consider the hydrogen bond interactions between the symmetry related molecules as shown in Fig. 6. 4. Comments Two gadolinium complexes with the DOTAM ligand and the same triflate counter-ion have different solvent compositions depending on the crystallization conditions. The coordination geometry at the metal centre is the same capped square antiprismatic (SA) for both but different gadolinium coordinated water bond distances are present in compound 2 having two independent molecules crystallized in the same crystal cell with different outer sphere hydrogen bonding systems involving the coordinated water. Other examples in this respect have been presented in the section result and discussion. This is an important tool when considering the factors that could influence the
rate of water exchange in the gadolinium complexes in water solution. It has already been discussed that factors of different lipophilicity of the substituted macrocycle, can hinder water exchange as well as the nature of the counter-ion. The hydrogen bond system around the coordinated water, supported by the nature of the anion is important, but a non-secondary role is played by the solvent that is implied in this system like water–water, water–anion, water–macrocycle, anion macrocycle interactions. The effect on the water exchange comes out as depending from synergy of all these factors, that in many case contribute to a variation of the metal–water bond distance whose concomitant further influences the water exchange. Concerning the praseodimium structure it was surprising to find in the solid state a twist square antiprismatic geometry (TSA) in a complex having the same chemical composition of the parent gadolinium and also for that concerning the outer sphere region, having instead a square antiprismatic geometry (SA). But if we consider that the crystal structure of [Eu(DOTA)H2O](CF3SO3)3 Æ 2CH3OH [14] is constituted by two different stereo-isomers having, respectively, SA and TSA geometries even though consequent to a disorder between two positions of the 1,4,7,10tetraazocyclododecane, the energy difference between the two configurations must be rather low at least for this family of lanthanides, and subtle difference in the synthesis can address the metal coordination geometry towards one or the other form or both as shown by the previously described europium derivative [14]. The outer sphere composition in this case is the same as the dependent amino groups and the coordinate water positions are coincident. 5. Supplementary data A summary of the crystal data, data collection, and structure refinement is presented in Table 1; CCDC Nos. 296826, 296827 and 296828 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic
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