Lanthanide(III) Complexation with an Amide Derived Pyridinophane

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Lanthanide(III) Complexation with an Amide Derived Pyridinophane Goretti Castro,† Rufina Bastida,‡ Alejandro Macías,‡ Paulo Pérez-Lourido,*,† Carlos Platas-Iglesias,*,§ and Laura Valencia† †

Departamento de Química Inorgánica, Facultad de Ciencias, Universidade de Vigo, As Lagoas, Marcosende, 36310, Pontevedra, Spain ‡ Departamento de Química Inorgánica, Facultad de Química, Universidad de Santiago de Compostela, Avenida de las Ciencias s/n, E-15782, Santiago de Compostela, Spain § Departamento de Química Fundamental, Facultade de Ciencias, Universidade da Coruña, A Coruña, Spain S Supporting Information *

ABSTRACT: Herein we report a detailed investigation of the solid state and solution structures of lanthanide(III) complexes with the 18-membered pyridinophane ligand containing acetamide pendant arms TPPTAM (TPPTAM = 2,2′,2″-(3,7,11-triaza-1,5,9(2,6)-tripyridinacyclododecaphane3,7,11-triyl)triacetamide). The ligand crystallizes in the form of a clathrated hydrate, where the clathrated water molecule establishes hydrogen-bonding interactions with the amide NH groups and two N atoms of the macrocycle. The X-ray structures of 13 different Ln3+ complexes obtained as the nitrate salts (Ln3+ = La3+−Yb3+, except Pm3+) have been determined. Additionally, the X-ray structure of the La3+ complex obtained as the triflate salt was also obtained. In all cases the ligand provides 9-fold coordination to the Ln3+ ion, ten coordination being completed by an oxygen atom of a coordinated water molecule or a nitrate or triflate anion. The bond distances of the metal coordination environment show a quadratic change along the lanthanide series, as expected for isostructural series of Ln3+ complexes. Luminescence lifetime measurements obtained from solutions of the Eu3+ and Tb3+ complexes in H2O and D2O point to the presence of a water molecule coordinated to the metal ion in aqueous solutions. The analysis of the Ln3+-induced paramagnetic shifts indicates that the complexes are ten-coordinated throughout the lanthanide series from Ce3+ to Yb3+, and that the solution structure is very similar to the structures observed in the solid state. The complexes of the light Ln3+ ions are fluxional due to a fast Δ(λλλλλλ) ↔ Λ(δδδδδδ) interconversion that involves the inversion of the macrocyclic ligand and the rotation of the acetamide pendant arms. The complexes of the small Ln3+ ions are considerably more rigid, the activation free energy determined from VT 1H NMR for the Lu3+ complex being ΔG⧧298 = 72.4 ± 5.1 kJ mol−1.

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INTRODUCTION Macrocycles represent an important class of molecules that allow designing ligands for specific applications through creative and focused efforts.1 For instance, macrocyclic ligands play a key role in lanthanide(III) coordination chemistry,2 as they often provide complexes with high thermodynamic stability and kinetic inertness. Among the important biomedical applications of lanthanide(III) complexes are their use as contrast agents in magnetic resonance imaging (MRI)3,4 and as luminescent probes for biomedical analysis and imaging.5 Of particular interest is the case of Gd3+ complexes with poly(aminocarboxylate) ligands, since they are commonly used as contrast agents in MRI. With a size approximating Ca2+ but with a higher charge, the free Gd3+ ion is very toxic, as it is able to disrupt critical Ca2+ signaling.6 Thus, it is clear that the integrity of Gd3+ complexes must be maintained in vivo in order to create safe and efficient contrast agents. The issue of thermodynamic stability and kinetic inertness is therefore also critical for the application of luminescent Ln3+ complexes for imaging purposes in vivo. © XXXX American Chemical Society

Polyazamacrocycles with coordinating pendant arms form very stable complexes with a wide range of metal ions, including the lanthanide ions.7 These ligands encapsulate the metal ion in the macrocyclic cavity, which often results in high kinetic inertness with respect to complex dissociation. Besides, macrobicyclic structures such as the famous cryptates reported by Lehn have found application in homogeneous fluoroimmunoassays thanks to their very low dissociation rates in biological media.8 Most of the stable macrocyclic Ln3+ complexes studied so far are based on the tetraaza 12-membered macrocycle cyclen (1,4,7,10-tetraazacyclododecane), which can be functionalized with different pendant groups. Among the different arms that are often appended to cyclen are acetate,9 methylenephosphonate,10 acetamide,11 and picolinate groups,12 which give a strong binding to the Ln3+ ions while providing a good solubility in aqueous media. The most important ligand of this family in biomedical imaging is H4DOTA [1,4,7,10Received: November 3, 2014

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DOI: 10.1021/ic502653r Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tetraazacyclododecane-1,4,7,10-tetraacetic acid, see Chart 1],13 which forms lanthanide complexes of exceptionally high thermodynamic stability and kinetic inertness.14

However, these ligands exhibit a very important selectivity for the large Ln3+ and An3+ ions, which might find applications in the separation of these metal ions.22 Complexes with 18membered hexaazamacrocycles such as Py2N6Ac423 (Chart 1) and derivatives containing different pendant arms24 have been also reported. However, detailed thermodynamic and kinetic studies on these systems have not been conducted yet. Aiming to expand the array of macrocyclic platforms providing stable Ln3+ complexation for biomedical applications, in a recent work we have reported the synthesis and characterization of the lanthanide complexes with the macrocyclic ligand 2,11,20-triaza-[3.3.3]-(2,6) pyridinophane (TPP, Chart 1).25 These complexes were found to be stable in aqueous solutions around neutral pH in spite of the absence of any coordinating pendant arm in the structure of the ligand. Following these encouraging results, in this paper we report the related macrocyclic ligand TPPTAM (see Chart 1), which incorporates three acetamide pendant arms. The structures of the ligand and 14 different Ln3+ complexes have been determined using X-ray diffraction. Furthermore, a combination of NMR spectroscopy, luminescence lifetime measurements, and DFT calculations was used to establish the structure of the complexes in solution.

Chart 1

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EXPERIMENTAL SECTION

Measurements. Infrared (IR) spectra were recorded as KBr disks on a Bruker VECTOR 22 spectrometer. Melting points were determined using a Stuart SMP3 melting point apparatus. ESI experiments were performed on an microTOF (focus) mass spectrometer (Bruker Daltonics, Bremen, Germany). Ions were generated using an ApolloII (ESI) source, and ionization was achieved by electrospray. Absorption UV−vis spectra were recorded on a PerkinElmer Lambda 900 spectrophotometer using 1.0 cm path quartz cells. Excitation and emission spectra were recorded on a PerkinElmer LS-50B spectrometer. Luminescence lifetimes were calculated from the monoexponential fitting of the average decay data, and they are averages of at least 3−5 independent determinations. 1H, 13C, COSY, and HSQC NMR spectra were recorded in D2O solutions (pD = 7.0) on a Bruker ARX400 NMR spectrometer (9.4 T). Materials. All chemicals purchased from commercial sources were of the highest available purity and were not purified further. 2Bromoacetamide and hydrated lanthanide(III) nitrates and triflates were obtained from Aldrich. Solvents used were of reagent grade and purified by usual methods. Preparation of the Ligand TPPTAM (2,2′,2″-(3,7,11-Triaza1,5,9(2,6)-tripyridinacyclododecaphane-3,7,11-triyl)triacetamide). The synthesis of the ligand TPPTAM was achieved starting from TPP.26 A solution of 2-bromoacetamide (0.420 g, 3.044 mmol) in dry acetonitrile (20 mL) was added dropwise to a mixture of TPP (0.361 g, 1.001 mmol) and K2CO3 (4.45 g, 32.20 mmol) heated under reflux in the same solvent (40 mL). The mixture was heated under reflux for 5 h and allowed to cool down to room temperature. The precipitate formed was isolated by filtration and washed with small portions of water (3 × 5 mL) to remove inorganic salts. The white solid was air-dried and recrystallized from water. Yield: 0.416 g (65%). TPPTAM·6H2O: C27H45N9O9 (639.61), calcd C 50.8, H 6.4, N 19.4; found C 50.7, H 7.0, N 19.7. Mp: 209 °C. IR (KBr, cm−1): 1631 (s), 1402 (s) [ν(CC) and ν(CN)py], 1651 (s) [ν(CO)], 3265 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 532 (532) [TPPTAM + H]+. 1H NMR (δ, ppm): H1 7.61 (t, 3H), H2 7.23 (d, 6H, 3J = 7.7 Hz), H4 4.85 (s, 12H), H5 3.20 (s, 6H). Preparation of the Complexes. General Procedure. A solution of Ln(NO3)3·xH2O (0.24−0.40 mmol) (or La(CF3SO3)3·H2O, 0.24 mmol) in methanol (5 mL) was added to a stirred solution of 0.589 equiv of TPPTAM·6H2O in the same solvent (10 mL). The addition of the metal salt does not lead to the precipitation of the complexes. Slow concentration of the methanolic solutions resulted in the

Some recent efforts have been conducted to expand the array of macrocyclic frameworks that are used in the design of Ln3+ complexes for biomedical applications. The strategies followed for this purpose include the substitution of two nitrogen atoms of the cyclen unit in trans positions by oxygen atoms,15 and the increase of the rigidity of the 12-membered macrocycle of cyclen through the introduction of a piperidine ring,16 or one or two pyridine rings,17 in the macrocyclic framework. A detailed thermodynamic and kinetic study of the [Ln(PCTA)] complexes (Chart 1) revealed a high thermodynamic stability and kinetic inertness. Furthermore, complex formation was found to be at least ten times faster than that of the well-studied Ln3+−DOTA complexes, suggesting that they may have great potential for biomedical applications.18 Regarding the size of the macrocycle, the kinetic inertness of Ln3+ complexes with tetraazamacrocycles decreases by several orders of magnitude on increasing the ring size from the 12membered ligand DOTA to the 13- and 14-membered analogues TRITA and TETA.19 Some efforts have been also made in order to synthesize ligands based on larger macrocyclic rings to obtain lanthanide complexes with potential applications as MRI contrast agents.20 For example, a 15-membered macrocycle incorporating a bipyridine ring in the macrocyclic structure and three acetate pendant arms was shown to form very stable Ln3+ complexes in solution.21 Several 15- and 18membered diaza crown ethers incorporating different pendant arms have been also reported. The thermodynamic stability of the corresponding Ln3+ complexes was found to be much lower than those with similar ligands based on 12-membered rings. B


Article

Inorganic Chemistry formation of precipitates, which were isolated by filtration and airdried. Slow evaporation of the aqueous solutions afforded single crystals that were adequate for X-ray diffraction measurements in all cases except Lu−TPPTAM. [La(TPPTAM)(NO3)][La(NO3)3](NO3)2·3CH3OH (La−TPPTAM). TPPTAM·6H2O (0.088 g, 0.139 mmol) and La(NO3)3·5H2O (0.099 g, 0.236 mmol).Yield: 0.133 g (88%). IR (KBr, cm−1): 1604 (s), 1462 (s) [ν(CC) and ν(CN)py], 1663 (s) [ν(CO)], 1448 (m), 1384 (s), 1329 (s), 1089 (m), 1040 (m), 817 (m), 797 (m), 739 (m) [ν(NO 3 − )], 3288 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 366 (366) [La(TPPTAM)(NO3)]2+. C30H45La2N15O24 (1282.42): calcd C 28.1, H 3.5, N 16.4; found C 28.7, H 3.4, N 16.6. Crystals suitable for X-ray diffraction were obtained with formula [(La(TPPTAM))2-μ-NO3][La(NO3)6](NO3)2·2.5H2O. [Ce(TPPTAM)(NO3)][Ce(NO3)3](NO3)2·3CH3OH (Ce−TPPTAM). TPPTAM·6H2O (0.100 g, 0.156 mmol) and Ce(NO3)3·6H2O (0.123 g, 0.283 mmol). Yield: 0.100 g (55%). IR (KBr, cm−1): 1604 (m), 1467 (m) [ν(CC) and ν(CN)py], 1661 (s) [ν(CO)], 1447 (s), 1384 (s), 1316 (s), 1088 (m), 1034 (m), 818 (m), 796 (m), 735 (m) [ν(NO 3 − )], 3217 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 367 (367) [Ce(TPPTAM)(NO3)]2+. C30H45Ce2N15O24 (1279.1): calcd C 28.2, H 3.5, N 16.4; found C 28.4, H 3.4, N 16.2. Crystals suitable for X-ray diffraction were obtained with formula [(Ce(TPPTAM))2-μ-NO3][Ce(NO3)6](NO3)2·2.5H2O. [Pr(TPPTAM)(NO3)][Pr(NO3)3](NO3)2·4CH3OH (Pr−TPPTAM). TPPTAM·6H2O (0.101 g, 0.158 mmol) and Pr(NO3)3·5H2O (0.117 g, 0.281 mmol). Yield: 0.150 g (81%). IR (KBr, cm−1): 1605 (m), 1467 (m) [ν(CC) and ν(CN)py], 1662 (s) [ν(CO)], 1447 (s), 1384 (s), 1318 (s), 1089 (m), 1036 (m), 818 (m), 795 (m), 737 (m) [ν(NO 3 − )], 3227 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 367 (367) [Pr(TPPTAM)(NO3)]2+. C31H49Pr2N15O25 (1313.1): calcd C 28.3, H 3.6, N 16.0; found C 28.7, H 3.4, N 16.4. Crystals suitable for X-ray diffraction were obtained with formula [(Pr(TPPTAM))2-μ-NO3](NO3)5·2H2O. [Nd(TPPTAM)(NO 3 )][Nd(NO 3 ) 3 ](NO 3 ) 2 ·2CH 3 OH (Nd− TPPTAM). TPPTAM·6H2O (0.101 g, 0.158 mmol) and Nd(NO3)3· 6H2O (0.122 g, 0.278 mmol). Yield: 0.124 g (71%). IR (KBr, cm−1): 1605 (m), 1467 (m) [ν(CC) and ν(CN)py], 1662 (s) [ν(C O)], 1446 (s), 1384 (s), 1319 (s), 1089 (m), 1033 (m), 818 (m), 796 (m), 738 (m) [ν(NO3−)], 3223 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 369 (369) [Nd(TPPTAM)(NO 3 )] 2+ . C29H41Nd2N15O23 (1255.5): calcd C 27.7, H 3.3, N 16.7; found C 28.0, H 3.5, N 16.7. Crystals suitable for X-ray diffraction were obtained with formula [(Nd(TPPTAM))2-μ-NO3](NO3)5·2H2O. [Sm(TPPTAM)(NO 3 )][Sm(NO 3 ) 3 ](NO 3 ) 2 ·2CH 3 OH (Sm− TPPTAM). TPPTAM·6H2O (0.101 g, 0.158 mmol) and Sm(NO3)3· 6H2O (0.124 g, 0.279 mmol). Yield: 0.143 g (81%). IR (KBr, cm−1): 1606 (m), 1467 (m) [ν(CC) and ν(CN)py], 1660 (s) [ν(C O)], 1447 (s), 1384 (s), 1324 (s), 1089 (m), 1033 (m), 817(m), 796 (m), 740 (m) [ν(NO3−)], 3236 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 373 (372) [Sm(TPPTAM)(NO 3 )] 2+ . C29H41Sm2N15O23 (1267.7): calcd C 27.5, H 3.2, N 16.6; found C 27.9, H 3.5, N 16.7. Crystals suitable for X-ray diffraction were obtained with formula [(Sm(TPPTAM))2-μ-NO3](NO3)5·2H2O. [Eu(TPPTAM)(NO3)][Eu(NO3)3](NO3)2·4CH3OH (Eu−TPPTAM). TPPTAM·6H2O (0.122 g, 0.191 mmol) and Eu(NO3)3·5H2O (0.148 g, 0.346 mmol). Yield: 0.119 g (52%). IR (KBr, cm−1): 1613 (m), 1473 (m) [ν(CC) and ν(CN)py], 1655 (s) [ν(CO)], 1384 (s), 1331 (s), 1090 (m), 1038 (m), 796 (m) [ν(NO3−)], 3210 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 682 (683) [Eu(TPPTAM)]+, 745 (746) [Eu(TPPTAM)(NO3)]+, 808 (808) [Eu(TPPTAM)(NO3)2]+. C31H49Eu2N15O25 (1337.1): calcd C 27.9, H 3.7, N 15.7; found C 27.7, H 3.4, N 15.3. Crystals suitable for X-ray diffraction were obtained with formula [(Eu(TPPTAM))2-μ-NO3](NO3)4.75·Br0.25·2H2O. [Gd(TPPTAM)(NO 3 )][Gd(NO 3 ) 3 ](NO 3 ) 2 ·7CH 3 OH (Gd− TPPTAM). TPPTAM·6H2O (0.100 g, 0.156 mmol) and Gd(NO3)3· 6H2O (0.126 g, 0.279 mmol). Yield: 0.102 g (51%). IR (KBr, cm−1): 1632 (m), 1474 (m) [ν(CC) and ν(CN)py], 1655 (s) [ν(C

O)], 1462 (m), 1384 (s), 1331 (m), 1040 (m), 825 (m), 796 (m), 746 (m) [ν(NO3−)], 3223 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 687 (688) [Gd(TPPTAM)]+, 750 (751) [Gd(TPPTAM)(NO 3 )] + , 813 (813) [Gd(TPPTAM)(NO 3 ) 2 ] + . C34H61Gd2N15O28 (1443.2): calcd C 28.3, H 4.3, N 14.6; found C 28.5, H 3.6, N 14.6. Crystals suitable for X-ray diffraction were obtained with formula [(Gd(TPPTAM))2-μ-NO3](NO3)4.5·Br0.5· 2H2O. [Tb(TPPTAM)(NO3)][Tb(NO3)3](NO3)2·5CH3OH (Tb−TPPTAM). TPPTAM·6H2O (0.097 g, 0.152 mmol) and Tb(NO3)3·6H2O (0.124 g, 0.274 mmol). Yield: 0.073 g (39%). IR (KBr, cm−1): 1613 (m), 1470 (m) [ν(CC) and ν(CN)py], 1658 (s) [ν(CO)], 1444 (s), 1384 (s), 1334 (s), 1091 (m), 1037 (m), 825 (m), 797 (m), 747 (w) [ν(NO3−)]. MS (ESI-MS, m/z, found (calculated)): 688 (690) [Tb(TPPTAM)]+, 753 (753) [Tb(TPPTAM)(NO3)]+, 814 (815) [Tb(TPPTAM)(NO3)2]+. C32H53Tb2N15O26 (1380.8): calcd C 27.8, H 3.8, N 15.2; found C 28.2, H 3.6, N 15.1. Crystals suitable for X-ray diffraction were obtained with formula [(Tb(TPPTAM))2-μ-NO3](NO3)4.6·Br0.4·2H2O. [Dy(TPPTAM)(NO3)][Dy(NO3)3](NO3)2·5CH3OH (Dy−TPPTAM). TPPTAM·6H2O (0.106 g, 0.166 mmol) and Dy(NO3)3·5H2O (0.131 g, 0.299 mmol). Yield: 0.088 g (42%). IR (KBr, cm−1): 1607 (s), 1474 (s) [ν(CC) and ν(CN)py], 1659 (s) [ν(CO)], 1444 (m), 1384 (m), 1331 (m), 1089 (m), 1033 (m), 825 (m), 797 (m), 747 (m) [ν(NO 3 − )], 3197 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 694 (694) [Dy(TPPTAM)]+, 755 (756) [Dy(TPPTAM)(NO3)]+. C32H53Dy2N15O26 (1388.0): calcd C 27.7, H 3.8, N 15.1; found C 27.5, H 3.6, N 15.1. Crystals suitable for X-ray diffraction were obtained with formula [Dy(TPPTAM)(H2O)](NO3)3·2H2O. [Ho(TPPTAM)(NO 3 )][Ho(NO 3 ) 3 ](NO 3 ) 2 ·4CH 3 OH (Ho− TPPTAM). TPPTAM·6H2O (0.097 g, 0.152 mmol) and Ho(NO3)3· 5H2O (0.120 g, 0.272 mmol). Yield: 0.058 g (31%). IR (KBr, cm−1): 1607 (m), 1475 (m) [ν(CC) and ν(CN)py], 1659 (s) [ν(C O)], 1383 (s), 1329 (s), 1089 (m), 1034 (m), 826 (m), 798 (m), 750 (m) [ν(NO3−)], 3175 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 695 (697) [Ho(TPPTAM)]+, 759 (759) [Ho(TPPTAM)(NO3)]+. C31H49Ho2N15O25 (1360.8): calcd C 27.3, H 3.6, N 15.4; found C 27.4, H 3.9, N 15.2. Crystals suitable for X-ray diffraction were obtained with formula [(Ho(TPPTAM))2-μ-NO3](NO3)4.74·Br0.26·2H2O. [Er(TPPTAM)(NO3)][Er(NO3)3](NO3)2·4CH3OH (Er−TPPTAM). TPPTAM·6H2O (0.142 g, 0.222 mmol) and Er(NO3)3·5H2O (0.177 g, 0.399 mmol). Yield: 0.152 g (56%). IR (KBr, cm−1): 1613 (m), 1479 (m) [ν(CC) and ν(CN)py], 1659 (s) [ν(CO)], 1383 (s), 1328 (s), 1090 (m), 1036 (m), 826 (m), 797 (m), 751 (m) [ν(NO 3 − )], 3213 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 760 (761) [Er(TPPTAM)(NO3)]+, 823 (823) [Er(TPPTAM)(NO3)2]+. C31H49Er2N15O25 (1365.5): calcd C 27.2, H 3.6, N 15.4; found C 27.5, H 3.9, N 15.1. Crystals suitable for X-ray diffraction were obtained with formula [(Er(TPPTAM))2-μ-NO3](NO3)4.5·Br0.5·2H2O. [Tm(TPPTAM)(NO 3 )][Tm(NO 3 ) 3 ](NO 3 ) 2 ·5CH 3 OH (Tm− TPPTAM). TPPTAM·6H2O (0.106 g, 0.166 mmol) and Tm(NO3)3· 5H2O (0.133 g, 0.299 mmol). Yield: 0.070 g (33%). IR (KBr, cm−1): 1635 (m), 1485 (m) [ν(CC) and ν(CN)py], 1661 (s) [ν(C O)], 1454 (s), 1384 (s), 1344 (s), 1039 (m), 825 (m), 750 (m) [ν(NO 3 − )], 3201 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 700 (701) [Tm(TPPTAM)]+, 763 (763) [Tm(TPPTAM)(NO3)]+. C32H53Tm2N15O26 (1400.9): calcd C 27.4, H 3.8, N 15.0; found C 27.5, H 4.6, N 15.4. Crystals suitable for X-ray diffraction were obtained with formula [Tm(TPPTAM)(H2O)](NO3)3·2H2O. [Yb(TPPTAM)(NO3)][Yb(NO3)3](NO3)2·7CH3OH (Yb−TPPTAM). TPPTAM·6H2O (0.093 g, 0.145 mmol) and Yb(NO3)3·5H2O (0.117 g, 0.260 mmol). Yield: 0.060 g (31%). IR (KBr, cm−1): 1613 (m), 1480 (m) [ν(CC) and ν(CN)py], 1662 (s) [ν(CO)], 1384 (s), 1341 (s), 1088 (m), 1037 (m), 825 (m), 798 (m) [ν(NO3−)], 3242 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 705 (705) [Yb(TPPTAM)]+, 766 (767) [Yb(TPPTAM)(NO3)]+. C


Article

Inorganic Chemistry

split bands associated with ν(CN) and ν(CC) vibrations of the pyridine rings indicate the coordination of the Ln3+ ions to the pyridyl nitrogen atoms.35 Additionally, the spectra of the nitrate complexes show several bands between 739 and 1448 cm−1 due to the presence of free and coordinated nitrate groups,36 while the bands attributable to free and coordinated triflate groups appear at 1651, 1252, 1174, 1091, and 1031 cm−1.37 In most of the cases, bands in the range 3200−3500 cm−1 corresponding to the NH groups present in the molecule can be also observed. However, these bands are often masked by a broad signal centered at 3500 cm−1 due to the presence of water molecules. The mass spectra (ESI+) of the compounds display peaks corresponding to the [Ln(TPPTAM)(NO3)]2+ or [Ln(TPPTAM)(NO 3 ) 2 ] + entities ([La(TPPTAM)(CF 3SO3 )2]+ for LaT−TPPTAM), which confirms the formation of all the lanthanide complexes. X-ray Crystal Structures. The crystal structure of TPPTAM·3H2O has been determined using X-ray diffraction. The asymmetric unity shows a TPPTAM molecule and three disordered water molecules. The ligand shows a truncated cone conformation with all the acetamide pendant groups in a syn conformation directed to the small base of the cone. The central position of the ligand is occupied by a clathrated water molecule establishing hydrogen bond interactions with the amide NH groups of the pendant arms. Simultaneously, the water molecule acts as a hydrogen bond donor by forming hydrogen bonds with two N atoms of the macrocyclic unit (N3 and N5, Figure 1). Thus, TPPTAM·3H2O forms a clathrate

C34H61Yb2N15O28 (1473.1): calcd C 27.7, H 4.1, N 14.3; found C 27.5, H 4.2, N 14.0. Crystals suitable for X-ray diffraction were obtained with formula [Yb(TPPTAM)(H2O)](NO3)3·2H2O. [Lu(TPPTAM)(NO3)][Lu(NO3)3](NO3)2·5H2O (Lu−TPPTAM). TPPTAM·6H2O (0.110 g, 0.172 mmol) and Lu(NO3)3·H2O (0.118 g, 0.311 mmol). Yield: 0.104 g (50%). IR (KBr, cm−1): 1608 (m), 1464 (m) [ν(CC) and ν(CN)py], 1662 (s) [ν(CO)], 1384 (s), 1334 (m), 1088 (m), 1035 (m), 826 (m), 797 (m), 749 (m) [ν(NO 3 − )], 3213 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 384 (384) [Lu(TPPTAM)(NO3)]2+. C27H43Lu2N15O26 (1343.55): calcd C 24.2, H 3.2, N 15.6; found C 25.6, H 3.6, N 14.8. [La(TPPTAM)(CF3SO3)][La(CF3SO3)3](CF3SO3)2·CH3OH (LaT− TPPTAM). TPPTAM·6H2O (0.085 g, 0.133 mmol) and La(CF3SO3)3·H2O (0.144 g, 0.238 mmol). Yield: 0.092 g (45%). IR (KBr, cm−1): 1604 (m), 1470 (m) [ν(CC) and ν(CN)py], 1666 (s) [ν(CO)], 1651 (m), 1252 (s), 1174 (s), 1091 (m), 1031 (s) [ν(CF3SO3−)], 3288 (m) [ν(NH)]. MS (ESI-MS, m/z, found (calculated)): 410 (410) [La(TPPTAM)(CF3SO3)]2+, 968 (969) [La(TPPTAM)(CF3SO3)2]+. C34H37La2N9O22F18S6 (1734.6): calcd C 23.5, H 2.1, N 7.3; found C 23.5, H 2.4, N 7.1. Crystals suitable for Xray diffraction were obtained with formula [La(TPPTAM)(CF3SO3)](CF3SO3)2·2H2O. Crystal Structure Determinations. Measurements were made on a BRUKER Smart-CCD-1000 diffractometer. Graphite monochromated Mo Kα radiation was used. All data were corrected by Lorentz and polarization effects. Empirical absorption corrections were also applied.27 Complex scattering factors were taken from the program package SHELX-97.28 The structures were solved by direct methods using SIR-92,29 which revealed the position of all non-hydrogen atoms. All the structures were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all nonhydrogen atoms. Molecular graphics were generated using WebLAB ViewerPro 4.0 and ORTEP-3.30 Computational Methods. All calculations reported in this work were performed employing the Gaussian 09 package (Revision B.01).31 Full geometry optimizations of the [Ln(TPPTAM)(H2O)]3+ complexes (Ln3+ = La3+, Pr3+, Eu3+−Dy3+, Er3+−Lu3+) were performed in aqueous solution employing DFT within the hybrid meta generalized gradient approximation with the TPSSh exchangecorrelation functional.32 The large-core quasirelativistic effective core potential (LCECP) of Dolg et al. and its associated [5s4p3d]-GTO valence basis set was used for the lanthanides,33 while the ligand atoms were described by using the standard 6-31G(d,p) basis set. Input geometries were constructed from crystallographic structures. The stationary points found on the potential energy surfaces as a result of the geometry optimizations were confirmed to represent energy minima rather than saddle points via frequency analysis. Solvent effects were included by using the integral equation formalism variant of the polarizable continuum model (IEFPCM), as implemented in Gaussian 09.34

■

RESULTS AND DISCUSSION Synthesis and Characterization of the Ligand and Metal Complexes. The TPPTAM ligand was obtained in 65% yield by N-alkylation of the TPP precursor with 2bromoacetamide. The Ln3+ complexes of TPPTAM were obtained with 31−88% yields by direct reaction between the ligand and the appropriate hydrated lanthanide nitrate or triflate salt in methanol. All complexes are soluble in this solvent. The complexes were characterized by IR, ESI-MS, and NMR spectroscopy. Additionally, DFT calculations were also carried out to gain insight into the structure of the complexes in solution. The IR spectra (KBr disks) of all complexes show a band between 1655 and 1666 cm−1 associated with the ν(C O) vibration of the carbonyl groups of the acetamide pendant arms. This band is shifted with respect to the one observed for TPPTAM (1651 cm−1), suggesting interaction between the carbonyl groups and the metal ions. The expected shifted and

Figure 1. X-ray crystal structure of TPPTAM·3H2O. Hydrogen atoms, except those of amide groups and water molecules, are omitted for simplicity. The ORTEP plot is at the 30% probability level.

hydrate in which the guest water molecule is held by up to five hydrogen bond interactions.38 Additional hydrogen bond interactions are also established between the hydration water molecules and the amide groups of the ligand (See Table S1, Supporting Information). Intermolecular π−π stacking interactions involving pyridine rings of adjacent ligands are also established, the distance between centroids being 3.72 Å and the distance between planes 3.5 Å (Figure S1, Supporting Information). D


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for TPPTAM·3H2O and Its Ln3+ Complexes formula mol wt cryst syst space group a (Å), α (deg) b (Å), β (deg) c (Å), γ (deg) V (Å3) Z D(calc) (Mg/m3) μ (mm−1) Rint R1a wR2 (all data)b

La−TPPTAM

Ce−TPPTAM

Pr−TPPTAM

Nd−TPPTAM

Sm−TPPTAM

C27H38N13.5O19La1.5 1064.07 triclinic P1̅ 12.8230(15), 106.508(2) 12.9297(15), 110.376(2) 13.6859(16), 91.085(2) 2021.8(4) 2 1.748 1.663 0.0544 0.0569 0.1751 Eu−TPPTAM

C27H38N13.5O19Ce1.5 1065.89 triclinic P1̅ 12.809(5), 106.526(5) 12.900(5), 110.461(6) 13.678(5), 91.104(7) 2012.3(12) 2 1.759 1.776 0.0174 0.0345 0.1017 Gd−TPPTAM

C27H35N12O13Pr 876.58 monoclinic C2/c 19.508(2) 16.766(2), 93.247(2) 21.153(2) 6907.4(12) 8 1.686 1.493 0.0341 0.0400 0.1196 Tb−TPPTAM

C27H35N12O13Pr 879.91 monoclinic C2/c 19.478(2) 16.838(2), 92.812(2) 21.111(3) 6915.5(15) 8 1.690 1.584 0.0243 0.0304 0.0853 Dy−TPPTAM

C27H35N12O13Sm 886.02 monoclinic C2/c 19.448(1) 16.866(1), 92.606(1) 21.089(1) 6910.6(8) 8 1.703 1.782 0.0281 0.0300 0.0846 Ho−TPPTAM

formula mol wt cryst syst space group a (Å), α (deg) b (Å), β (deg)

C27H35N11.88O12.63Br0.13Eu 889.87 monoclinic C2/c 19.478(2) 16.640(2) 93.150(2)

C27H35N11.75O12.25Br0.25Gd 897.40 monoclinic C2/c 19.459(3) 16.473(2) 93.463(2)

C27H35N11.75O12.25Br0.25Tb 899.07 monoclinic C2/c 19.445(5) 16.539(5) 93.255(4)

c (Å), γ (deg) V(Å3) Z D(calc) (Mg/ m3) μ (mm−1) abs struct param Rint R1a wR2 (all data)b

21.093(2) 6826.2(12) 8 1.732

21.086(3) 6746.9(16) 8 1.767

21.094(6) 6773(3) 8 1.763

2.066

2.343

2.464

0.0233 0.0449 0.1256

0.0604 0.0553 0.1605

0.0883 0.0598 0.1961 Yb−TPPTAM

formula mol wt cryst syst space group a (Å), α (deg) b (Å), β (deg) c (Å), γ (deg) V (Å3) Z D(calc) (Mg/m3) μ (mm−1) abs struct param Rint R1a wR2 (all data)b a

Er−TPPTAM

Tm−TPPTAM

C27H35N11.75O12.25Br0.25Er 907.41 monoclinic C2/c 19.423(2) 16.466(2) 93.385(2) 21.141(3) 6749.7(14) 8 1.786 2.864

C27H39N12O15Tm 940.63 monoclinic Pn 10.657(1) 11.195(1) 105.717(1) 15.686(2) 1801.3(3) 2 1.734 2.549 0.000(7) 0.0169 0.0234 0.0605

0.0692 0.0498 0.1455

C27H39N12O15Yb 944.74 monoclinic Pn 10.6586(11) 11.1919(12)105.733(2) 15.6731(17) 1799.6(3) 2 1.743 2.685 −0.016(8) 0.0182 0.0234 0.0606

C27H39N12O15Dy 934.20 monoclinic Pn 10.720(2) 11.246(2) 105.894(3) 15.646(3) 1814.0(6) 2 1.710

C27H35N11.88O12.63Br0.13Ho 902.84 monoclinic C2/c 19.391(3) 16.432(2) 93.467(3) 21.159(3) 6729.7(17) 8 1.782

2.145 0.005(10) 0.0186 0.0275 0.0736 LaT−TPPTAM

2.583 0.0481 0.0464 0.1353 TPPTAM·3H2O

C30H37N9O14F9S3La 1153.78 monoclinic P2(1)/c 13.710(1) 22.543(2) 109.505(1) 15.437(1) 4497.3(6) 4 1.704 1.197

C27H39N9O6 585.67 triclinic P1̅ 8.973(4) 117.856(8) 14.210(7) 95.342(10) 14.926(7) 92.016(9) 1668.7(14) 2 1.166 0.085

0.0290 0.0364 0.1074

0.0643 0.0945 0.3466

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(||Fo|2 − |Fc|2|)2]/∑[w(Fo4|)]}1/2.

The crystal structures of all complexes except Lu−TPPTAM were determined by X-ray diffraction. Single crystals were obtained thorough slow evaporation of aqueous solutions of the complexes. Crystal data are collected in Table 1, while selected bond lengths of the lanthanide(III) coordination environments are given in Tables 2 and 3. Inspection of the crystal data indicates that three groups of isostructural compounds are formed along the lanthanide series: La−TPPTAM and Ce− TPPTAM crystallize in the centrosymmetric triclinic space group P1̅, compounds Pr−TPPTAM to Tb−TPPTAM, Ho−

TPPTAM, and Er−TPPTAM crystallize in the centrosymmetric monoclinic space group C2/c, and Dy−TPPTAM, Tm− TPPTAM, and Yb−TPPTAM crystallize in the chiral monoclinic space group Pn. The series of compounds that crystallize in the P1̅ and C2/c space groups contain half of the dinuclear entity [(Ln(TPPTAM))2-μ-(NO3)]5+ in the asymmetric unit (Ln3+ = La3+−Er3+, except Dy3+), while those compounds that crystallize in the Pn space group contain the [Ln(TPPTAM)(H2O)]3+ complex in the asymmetric unit (Ln3+ = Dy3+, Tm3+, or Yb3+). Finally, the asymmetric unit of E


Article

Inorganic Chemistry

Table 2. Selected Bond Lengths (Å) of the Metal Coordination Environments Obtained from the X-ray Crystal Structures La− TPPTAM to Gd−TPPTAM Ln−N(1) Ln−N(2) Ln−N(3) Ln−N(4) Ln−N(5) Ln−N(6) Ln−O(1) Ln−O(2) Ln−O(3) Ln−O(1N)

La−TPPTAM

Ce−TPPTAM

Pr−TPPTAM

Nd−TPPTAM

Sm−TPPTAM

Eu−TPPTAM

Gd−TPPTAM

2.714(7) 2.817(7) 2.736(7) 2.803(7) 2.703(6) 2.797(7) 2.467(6) 2.481(6) 2.519(6) 2.575(16)

2.715(4) 2.795(4) 2.731(4) 2.799(4) 2.697(3) 2.775(4) 2.453(3) 2.478(3) 2.515(3) 2.547(8)

2.689(4) 2.759(4) 2.686(4) 2.755(5) 2.711(4) 2.739(4) 2.441(3) 2.427(4) 2.423(4) 2.594(11)

2.683(3) 2.745(3) 2.681(3) 2.748(3) 2.702(3) 2.726(3) 2.423(3) 2.420(3) 2.410(3) 2.569(9)

2.670(3) 2.729(3) 2.670(3) 2.734(4) 2.685(3) 2.707(3) 2.392(3) 2.396(3) 2.378(3) 2.558(9)

2.671(5) 2.725(5) 2.662(5) 2.730(6) 2.680(5) 2.695(5) 2.385(4) 2.384(4) 2.367(4) 2.496(5)

2.670(8) 2.708(8) 2.657(8) 2.723(9) 2.673(8) 2.689(8) 2.370(7) 2.371(7) 2.354(6) 2.481(9)

Table 3. Selected Bond Lengths (Å) of the Metal Coordination Environments Obtained from the X-ray Crystal Structures Tb− TPPTAM to Yb−TPPTAM and LaT−TPPTAM Ln−N(1) Ln−N(2) Ln−N(3) Ln−N(4) Ln−N(5) Ln−N(6) Ln−O(1) Ln−O(2) Ln−O(3) Ln−O(1N) Ln−O(3N) Ln−O(1W) Ln−O(1T)

Tb−TPPTAM

Dy−TPPTAM

Ho−TPPTAM

Er−TPPTAM

Tm−TPPTAM

Yb−TPPTAM

LaT−TPPTAM

2.664(8) 2.706(8) 2.644(8) 2.703(9) 2.672(8) 2.675(8) 2.365(7) 2.352(7) 2.339(7) 2.474(9)

2.672(6) 2.692(6) 2.666(5) 2.669(5) 2.664(4) 2.751(5) 2.357(5) 2.334(4) 2.371(4)

2.661(7) 2.663(7) 2.652(7) 2.702(8) 2.641(7) 2.688(7) 2.321(6) 2.327(6) 2.341(6) 2.460(7)

2.670(8) 2.661(8) 2.657(8) 2.700(9) 2.630(8) 2.692(8) 2.311(6) 2.332(7) 2.331(6) 2.431(8)

2.657(4) 2.683(4) 2.658(4) 2.735(4) 2.658(3) 2.655(4) 2.317(4) 2.336(3) 2.304(3)

2.657(6) 2.674(6) 2.660(5) 2.649(6) 2.660(4) 2.736(5) 2.314(5) 2.285(4) 2.327(4)

2.724(3) 2.807(4) 2.713(3) 2.777(3) 2.701(3) 2.789(3) 2.485(3) 2.466(3) 2.480(3)

2.332(4)

2.321(5)

2.383(5)

2.574(3)

LaT−TPPTAM contains the [La(TPPTAM)(CF3SO3)]2+ cation. Half of the anionic complex [Ln(NO3)6]3− is also present in the asymmetric units of La−TPPTAM and Ce− TPPTAM. Independent nitrate ions (triflate ions for LaT− TPPTAM) and different water molecules of crystallization are present in all crystal lattices. In some cases (Eu−TPPTAM, Tb−TPPTAM, Ho−TPPTAM, and Er−TPPTAM), the nitrogen atom of an independent nitrate ion is sharing its position with a bromide ion, with occupancy factors for the nitrate ion of 75−87%. This small amount of bromide ions is likely to come from impurities of the TPPTAM ligand, which was obtained by reaction of TPP and 2-bromoacetamide. All the dimeric entities [(Ln(TPPTAM))2-μ-(NO3)]5+ present in most of the crystal lattices (Ln3+ = La3+−Tb3+, Ho3+, and Er3+) are very similar. Each cationic dimer is composed of two [Ln(TPPTAM)]3+ units joined by a disordered nitrate group. The metal ion is placed into the macrocyclic cavity coordinated by the six nitrogen atoms of the ligand backbone and the oxygen atoms of the acetamide fragments. The conformation of the ligand resembles a truncated cone with the three pendant groups in a syn conformation directed toward the smaller base of the cone regardless of the size of the ion. An oxygen atom of the bridging nitrate group placed on the opposite side completes coordination number ten in all cases. The two [Ln(TPPTAM)]3+ units are encapsulating the bridging nitrate group between the two metal centers, conferring a ball shape to the cationic dimer [(Ln(TPPTAM))2-μ-(NO3)]5+ (Figure 2). As previously observed for [(Ln(TPP))2-μ-(NO3)(H2O)6]5+ (Ln3+ = La3+ or Ce3+),25 the nitrogen atom of the bridging

Figure 2. Crystal structure of the cation [(La(TPPTAM))2-μ-NO3]5+. Hydrogen atoms are omitted for simplicity. The ORTEP plot is at the 30% probability level.

nitrate group is located in an inversion center that relates the two [Ln(TPPTAM)]3+ subunits, and it is disordered into two positions with 50% occupancy factors. The [Ln(TPPTAM)(H2O)]3+ (Ln3+ = Dy3+, Tm3+, or Yb3+) and [La(TPPTAM)(CF3SO3)]2+ cations show similar tencoordinated coordination environments for the metal ions (Figure 3), where the ligand provides the six nitrogen donor atoms of the macrocycle and three oxygen atoms of the pendant arms. Ten-coordination is completed by an oxygen atom of a water molecule or a triflate anion. F


Article

Inorganic Chemistry

nitrogen atoms of the ligand (the Ln3+−Nam bond distances being slightly longer than the Ln3+−Npy ones), which are similar to those previously reported for Ln3+ complexes containing pyridyl units.24,25,42 A plot of the average Ln3+− Nam, Ln3+−Npy, and Ln3+−Oam bond distances versus the number of f electrons of the Ln3+ ion (Figure 4) shows that the

Figure 4. Variation of the bond distances of the metal coordination environments observed in the X-ray structures of the series of TPPTAM complexes crystallizing in the P21/c, P1̅, C2/c, and Pn space groups. Average bond distances are provided for the three amine nitrogen atoms (Nam), pyridyl nitrogen atoms (NPy), and oxygen atoms of the acetamide groups (Oam), and average bond distances of the coordination environment (Avr.). The solid lines represent quadratic fits of the data as described in the text.

Figure 3. Crystal structure of the [Yb(TPPTAM)(H2O)]3+ cation. Hydrogen atoms are omitted for simplicity. The ORTEP plot is at the 30% probability level.

The Ln3+ complexes with the previously reported TPP platform showed different conformations of the ligand along the lanthanide series depending on the size of the Ln3+ ion.25 In the complexes of the large Ln3+ ions the ligand showed a truncated cone conformation close to C3 symmetry, with the NH groups in a syn conformation. As the size of the Ln3+ ion is reduced, different conformations for the TPP moiety were observed with ideal symmetries varying from C2 to Cs. However, the presence of the three acetamide pendants in TPPTAM imposes a truncated cone conformation all along the lanthanide series due to the syn conformation of the arms. The syn conformation of the ligand in the TPPTAM complexes results in the presence of two different sources of chirality: One is associated with the layout of the three acetamide pendant groups (often represented as Δ or Λ) and the second one related to the conformations of the six fivemembered chelate rings formed upon coordination of the macrocyclic moiety.39,40 Depending on the sign of the NPy−C− C−NAm torsion angle (NPy and Nam represent pyridyl and amine nitrogen atoms, respectively), the conformation of each five-membered chelate ring in the macrocyclic ligand can be left-handed, designated as λ (negative N−C−C−N torsion angle), or right-handed, designated as δ (positive N−C−C−N torsion angle). Inspection of the crystal data shows that in the case of La−TPPTAM and Ce−TPPTAM the enantiomeric forms Δ(λλλλλλ) and Λ(δδδδδδ) are both present in the crystal lattice due to the centrosymmetric nature of the P1̅ triclinic space group. The same situation occurs for the family of isostructural compounds that crystallize in the C2/c space group, while for the complexes that crystallize in the noncentrosymmetric Pn monoclinic space group only one enantiomer is present in the crystal: Δ(λλλλλλ) for Dy− TPPTAM and Λ(δδδδδδ) for Tm−TPPTAM and Yb− TPPTAM. The Ln3+−N and Ln3+−O bond distances decrease rather regularly from La3+ to Yb3+ in agreement with the lanthanide contraction (see Table 2).41 The distances between the Ln3+ ion and the oxygen atoms of the nitrate groups or water molecules are shorter than those between the metal ion and the

isostructural series of compounds crystallizing in the C2/c space group provide a smooth variation of the Ln3+−donor bond distances along the series, while the data obtained for the P1̅ and Pn series fall out of the expected quadratic trend. However, the average values of the ten bond distances of the metal coordination environment obtained for the C2/c and Pn series give an excellent quadratic fit of the form y = a + bx + cx2 with R2 > 0.997. This indicates that the somewhat longer average Ln3+−Nam, Ln3+−NPy, and Ln3+−Oam distances observed for the Pn series are related to a short bond distance to the oxygen atom of the coordinated water molecule [Ln3+−O(1W), Table 3]. The average bond distances of La−TPPTAM and Ce− TPPTAM fall clearly out of the trend observed for the C2/c and Pn, reflecting a somewhat different structure of the complex. However, the crystallographic data of [La(TPPTAM)(CF3SO3)]2+ fit well with the quadrated trend expected for the Ln3+−donor distances. A careful inspection of the structures of La−TPPTAM and Ce−TPPTAM shows that two of the three Nam−C−C−Oam torsion angles have a different sign with respect to the third Nam−C−C−Oam dihedral (Figure S2, Supporting Information), which results in λδδ or δλλ conformations. However, the complexes of the C2/c and Pn series present identical conformations for the three chelate rings formed due to the coordination of the pendant arms [δδδ or λλλ]. As a consequence, La−TPPTAM and Ce−TPPTAM show similar values for the three possible dihedral angles between pyridine rings (between 58.4 and 61.7°), while in the remaining compounds these angles differ up to 12−14°. The quadratic fit of the average bond distances of the metal coordination environment observed for the C2/c and Pn series gives a = 2.650(1), b = −1.39(5) × 10−2, and c = 3.52(32) × 10−4, which results in normalized parameters of b* = b/a = −5.26 × 10−3, c* = c/a = 1.33 × 10−4 and c/b = −2.53. These values are very similar to those obtained for the isostructural G


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Inorganic Chemistry [Ln(H2O)9](EtOSO3)3 series (b* = b/a = −6.83 × 10−3, c* = c/a = 1.52 × 10−4, and c/b = −2.22),41 which suggests that the decrease of the ionic radius of the Ln3+ ion across the 4f series does not result in an increased constraint for the coordination of the TPPTAM ligand to the metal ion.43 The assignment of a coordination polyhedron to describe the metal coordination environment in Ln3+ complexes of TPPTAM is not straightforward. Among the different coordination polyhedra often observed for ten-coordinate metal complexes are the staggered dodecahedron, the capped triangular cupola, the bicapped square antiprism, the pentagonal antiprism, and the sphenocorona.44 Thus, we performed continuous shape measures [S(A)] with the assistance of the SHAPE program.45 The shape measure S(A) = 0 for a structure fully coincident in shape with the reference polyhedron, while the maximum allowed value of S(A) is 100. The analysis of the coordination polyhedra in the different TPPTAM complexes provides shape measures for a sphenocorona in the range 1.32−2.66, while a bicapped square antiprism provides shape measures typically >4.6 (Table S3, Supporting Information). The remaining reference polyhedra considered in the analysis of the metal coordination environments give even higher S(A) values.46 Thus, the metal coordination environment in Ln3+ complexes of TPPTAM can be best described as a distorted sphenocorona (Figure 5). One of the quadrangular faces of the polyhedron is described by N1, N2, N3, and O1W, while the second quadrangular face is defined by O1W, N1, N6, and N5.

Figure 6. Absorption (dotted line), excitation (λem = 544 nm) and emission spectra of the Tb3+ complex of TPPTAM as recorded in H2O solution (10−5 M, pH = 7) at room temperature.

by the 5D0 → 7F2 transition, which points to a low symmetry of the ligand field around the Eu3+ ion.47 The excitation spectra recorded upon metal centered emission are very similar to the corresponding absorption spectra, indicating that the coordinated pyridyl moieties provide an efficient energy transfer to the Eu3+ and Tb3+ ions (antenna effect).48 The emission lifetimes of the Eu(5D0) and Tb(5D4) excited levels have been measured in D2O and H2O solutions of the complexes and were used to calculate the number of coordinated water molecules q.49 This methodology is based on the fact that O−H oscillators of coordinated water molecules quench the Ln3+ emission more efficiently than O−D oscillators, using empirical relationships to relate the emission lifetimes measured in D2O and H2O solutions and the hydration number. The emission lifetimes determined for the Tb3+ complex amount to τH2O = 1.37 ms and τD2O = 2.40 ms, which provide hydration numbers of 1.30 and 1.25 using the equations reported by Horrocks49 and Parker,50 respectively. These values are somewhat higher than 1, which suggests that the N−H oscillators of amide groups contribute significantly to quench the Tb(5D4) excited state. The lifetimes of the Eu(5D0) excited state measured in H2O and D2O solutions of the complex are τH2O = 0.57 ms and τD2O = 1.90 ms. The use of the equation proposed by Horrocks49 gives q = 1.3, in good agreement with the results obtained for the Tb3+ complex. In the case of Eu3+ complexes more refined equations have been proposed to take into account the influence of N−H oscillators of amide groups (qN) on the luminescence lifetimes.50,51 However, the primary amide N−H oscillators of TPPTAM are diastereotopic, and they are likely to quench with different efficiencies the Eu(5D0) excited state.50 Assuming that each of the six amide N−H oscillators contributes 0.075 ms−1 to the quenching of the Eu(5D0) excited state we obtain q values of 0.5−0.6, which indeed suggests that three N−H oscillators contribute only marginally to quench the Eu3+-centered emission. Inspection of the X-ray structure of the Eu3+ complex shows that three H atoms of the amide groups are ca. 0.37 Å further away from the metal ion than the second set of 3. In spite of the difficulties to determine accurately the number of coordinated water molecules due to the presence of N−H oscillators in the ligand structure, taken together the results obtained for the Tb3+ and Eu3+ complexes point to the presence of one water molecule in the first coordination sphere of the Ln3+ ion.

Figure 5. View of the coordination polyhedron in Yb−TPPTAM with the shape of a sphenocorona.

Electronic Absorption and Emission Spectra. The absorption spectra of the Eu3+ and Tb3+ complexes recorded in water at pH 7 and 298 K show a band with a maximum at ca. 267 nm (ε ∼ 5000 M−1 cm−1), which can be assigned to a combination of π → π* and n → π* transitions centered on the pyridyl units of the macrocyclic fragment. The corresponding emission spectra of ca. 10−5 M solutions of the complexes, obtained under excitation through the ligand bands at 267 nm, display the 5D0 → 7FJ (Eu3+, J = 0−4) or 5D4 → 7FJ (Tb3+, J = 6−3) transitions characteristic of the particular Ln3+ ion (Figure 6). The emission spectrum of the Eu3+ complex is dominated H


Article

Inorganic Chemistry DFT Calculations. The [Ln(TPPTAM)(H2O)]3+ complexes have been investigated using DFT calculations at the TPSSh/LCRECP/6-31G(d,p) level. The optimized geometries corresponding to the δδδ isomer present a nearly undistorted C3 symmetry in which the symmetry axis contains the Ln3+ ion and the oxygen atom of the inner-sphere water molecule. The calculated bond distances of the metal coordination environments show an excellent agreement with those observed in the solid state structures. The differences between experimental and calculated distances are typically lower than ∼0.05 Å (Table S4, Supporting Information), with average unsigned deviations 0.9997). The normalized parameters of b* = b/a = −5.65 × 10−3, c* = c/a = 1.30 × 10−4, and c/b = −2.30 are very close to those obtained from the analysis of the solid state structures of the C2/c and Pn series (vide supra). Full geometry optimizations of the λδδ forms of the complexes were also performed to analyze a possible structural change along the lanthanide series in aqueous solution, as suggested by the X-ray structures of the P1̅ series. The relative free energies of the λδδ and δδδ isomers calculated for the different [Ln(TPPTAM)(H2O)]3+ complexes (Figure S3, Supporting Information) indicate that the λδδ form is more stable than the δδδ one at the TPSSh/LCRECP/6-31G(d,p) level by 1.2−0.3 kcal mol−1. These energy differences are likely to be within the error margin of the computational method. However, the calculations point to a stabilization of the δδδ form along the lanthanide series, which is in line with the observed X-ray structures. Thus, our DFT calculations suggest that the [Ln(TPPTAM)(H2O)]2+ complexes of the large Ln3+ ions may adopt a λδδ (or δλλ) conformation of the acetamide pendant arms in solution, as observed in the X-ray crystal structures of the La3+ and Ce3+ complexes crystallizing in the P1̅ space group. NMR Spectra of the Diamagnetic Complexes. The 1H NMR spectra of D2O solutions of compounds La−TPPTAM (Figure S4, Supporting Information) and LaT−TPPTAM in D2O (pD 7.0) are virtually identical, indicating that the nitrate bridge and the triflate ion coordinated to the Ln3+ ion in the solid state structures of La−TPPTAM and LaT−TPPTAM are replaced by water molecules upon dissolution of the complexes in water.52 They consist of five resonances (see Table 4 and Chart 1), which points to an effective C3v symmetry of the complexes in solution. This is confirmed by the 13C NMR spectrum, which shows six signals for the 27 carbon nuclei of the ligand skeleton. The X-ray structure of LaT−TPPTAM presents a nearly undistorted C3 symmetry in solid state, which is expected to provide nine signals in the 1H NMR spectra, while the solid state structure of La−TPPTAM is more distorted (see above). Thus, most likely dynamic intramolecular exchange processes result in an effective C3v symmetry in solution. This causes the averaging of the two proton signals expected for H2 within the C3 point group. However, protons H4 are observed as an AB spin system with 2J = 15 Hz. The proton nuclei of the acetamide pendants H5 are observed as a singlet, which indicates a fast interconversion between the optical isomers arising from the different orientation of the three acetamide groups. On the contrary, the 1H NMR spectrum of the Lu3+ complex recorded in D2O at 298 K (pD 7.0) shows 8 signals, which indicates a C3 symmetry of the complexes in solution. A second

Table 4. NMR Shifts (ppm) Observed for Ln3+ Complexes of TPPTAM at 298 K (pD 7.0)a La3+

Ce3+

Pr3+

Nd3+

Sm3+

8.49 8.18 6.41 6.88 4.60

9.09 9.22 10.58 9.55

8.99 9.24 9.97 8.51

8.00 7.55 4.09 4.28

1

H H1 H2 H4a H4b H5

7.99 7.54 4.22 4.36 3.80

13

C C1 C2 C3 C4 C5 C6

a

141.0 123.7 156.0 62.4 60.6 178.3

See Chart 1 for labeling.

set of 1H NMR signals are also observed, which is particularly evident in the aromatic region. These signals are probably related to the presence of a second complex species in solution. Assignment of the 1H NMR spectrum of the major species was carried out on the basis of the 13C, COSY, and HSQC NMR spectra. The H4 protons give 4 signals observed as two AB spin systems, two of them corresponding to the CH2 group pointing to the side of the truncated cone where the coordinated water molecule is placed (H4ic), and the other two for the CH2 group pointing outside cone (H4oc). The methylenic protons of the acetamide pendants are observed as a singlet, presumably due to the small chemical shift difference between the two environments expected for these protons (axial and equatorial). The four doublets observed for H4ic/H4oc gradually broaden on increasing the temperature above 273 °C reflecting the presence of intramolecular exchange processes (Figure 7). Coalescence of these signals occurs in the temperature range 341−351 K. These temperature-dependent spectral changes have been analyzed by measuring the line widths Δν1/2 of the H4ax oc signal at 3.93 ppm in the temperature range 283−338 K, which are related to the exchange rate of the dynamic process k through k = π(Δν1/2 − Δν1/2(0)), where Δν1/2(0) is the line width in the absence of exchange. A plot of −ln(k/T) versus 1/T [k = (kbT/h) exp(ΔS⧧/R − ΔH⧧/RT)] in which kb and h are the Boltzmann and Planck constants respectively, T is the absolute temperature, and k is the rate constant yields the following activation parameters for the interconversion process: (ΔG⧧298 = 72.4 ± 5.1 kJ mol−1, ΔH⧧ = 83.6 ± 3.2 kJ mol−1, ΔS⧧ = 41.1 ± 1.6 J mol−1K−1, k298 = 1.93 ± 0.14 s−1). We attribute the dynamic process observed for the Lu3+ complex to a Δ(λλλλλλ) ↔ Λ(δδδδδδ) enantiomerization process, which involves both the inversion of the macrocyclic ring and the rotation of the three pendant arms of the ligand. This process is fast on the NMR time scale for the La3+ complex in the temperature range 273−335 K, which results in an effective C3v symmetry in solution. The inversion of the macrocyclic unit exchanges the pairs of protons H4eq(ic) ↔ H4ax(oc) and H4ax(ic) ↔ H4eq(oc), and as a result only two signals are observed for protons H4. The Δ(λλλλλλ) ↔ Λ(δδδδδδ) interconversion is slow on the NMR time scale at low temperature for the Lu3+ complex, which points to an increasing rigidity of the complexes in solution upon decreasing the ionic radius of the Ln3+ ion. This effect has been observed previously for different series of Ln3+ complexes,53 and may be I


Article

Inorganic Chemistry

For a given nucleus i, the isotropic paramagnetic shift induced by a lanthanide ion j (δpara ij ) is the result of two effects, 57 dip the Fermi contact (δcon ij ) and dipolar (δij ) contributions: δijpara = δijexp − δijdia = δijcon + δijdip

(1)

where δexp represents the experimentally observed chemical ij shift and δdia ij denotes the diamagnetic contribution, which can be estimated by measuring the chemical shifts for analogous diamagnetic complexes (La3+, Lu3+, or Y3+). In the present case, the 1H NMR shifts observed for the La3+ complex were used to estimate the diamagnetic contribution. Contact shifts arise from through-bond transmission of unpaired electron-spin density from the Ln3+ ion to the nucleus under study, and they can be expressed as in eq 2: μ A 6 10 = ⟨Sz⟩j Fi δijcon = ⟨Sz⟩j B 3kTγI ℏ (2) where ⟨Sz⟩ is the reduced value of the average spin polarization, μB the Bohr magneton, k the Boltzmann constant, γI the gyromagnetic ratio of the observed nucleus, and A/ℏ the hyperfine coupling constant (HFCC, in rad·s−1); δijcon is expressed in ppm. The pseudocontact (or dipolar) contribution results from the local magnetic field induced in the nucleus under study by the magnetic moment of the Ln3+ ion and for a system with axial symmetry can be written as in eq 3: δijdip = Figure 7. Top: 1H NMR spectra of the Lu−TPPTAM complex recorded in D2O solution at different temperatures (pD 7.0, 400 MHz). Bottom: Eyring plot for the dynamic process of Lu−TPPTAM based upon line-broadening data for H4ax oc.

CjμB 2 ⎡ A 20⟨r 2⟩(3 cos2 θ − 1) ⎤ ⎥ ⎢ 60k 2T 2 ⎣ r3 ⎦

(3)

58

Here Cj is the Bleaney’s constant, characteristic of the Ln3+ ion, and A02⟨r2⟩ is the ligand field coefficient of the second degree. If the principal magnetic axis system is used as the coordinate system, combination of eqs 2 and 3 gives

δijpara = ⟨SZ⟩j Fi + CjGi

attributed to the combination of two effects: (i) The increased charge density of the metal ion results in a stronger electrostatic interaction between the ligand and the Ln3+ ion. (ii) The more compact structure of the complexes with the small Ln3+ ions hinders conformational changes of the ligand. The free energy barrier determined for the Δ(λλλλλλ) ↔ Λ(δδδδδδ) interconversion in [Lu(TPPTAM)(H2O)]3+ (ΔG⧧298 = 72.4 ± 5.1 kJ mol−1) is somewhat higher than those reported for the ring-inversion and arm-rotation processes in Lu3+ complexes with cyclen-based ligands containing four pendant arms,54 which typically fall within the range 56−66 kJ mol−1.55 NMR Spectra of the Paramagnetic Complexes. The 1H NMR spectrum of the Ce3+ complex shows five paramagnetically shifted resonances, which is in line with an effective C3v symmetry of the complex in solution (Figure S4, Supporting Information; Table 4). In the case of the Pr3+, Nd3+, and Sm3+ complexes only four signals are observed at 298 K in the 1H NMR spectra. Signals were assigned on the basis of line width analysis, as no cross peaks were observed on the 2D COSY spectra. The broader resonances were associated with proton nuclei closer to the paramagnetic metal ion.56 No signals for the methylenic protons of the pendant arms H5 are observed for these complexes at 298 K. Variable temperature experiments performed in a solution of the Pr3+ complex allowed this signal to be located at ca. 6 ppm (Figure S5, Supporting Information). Thus, most likely the signals due to H5 protons are not observed due to extreme line broadening because they are close to coalescence at room temperature.

(4)

where Gi is proportional to the (3 cos2 θ − 1)/r3 term in eq 3. Equation 4 can be rewritten in the linear form given by eq 5: δijpara Cj

=

⟨SZ⟩j Cj

Fi + Gi

(5)

Since ⟨Sz⟩ and Cj are characteristic of the Ln3+ ion but independent of the ligand, whereas Fi and Gi are characteristic of the nucleus under study, but independent of the Ln3+ ion, plots according to eq 5 for a series of isostructural complexes should exhibit linear trends provided that the crystal field coefficients are invariant. The 1H paramagnetic shifts observed for the Ce3+, Pr3+, Nd3+, and Sm3+ complexes of TPPTAM plotted according to eq 5 indeed give straight lines (Figure 8, R2 > 0.976), which allowed a separation of the contact and pseudocontact contributions to the paramagnetic shifts. The slopes of the straight lines shown in Figure 8 provided the Fi values listed in Table 5. The contact contribution to the different paramagnetic shifts observed for the Pr3+ complex were obtained with eq 2 with ⟨Sz⟩ = −2.956, which allowed us to estimate the pseudocontact contribution as well with the use of eq 1. The results show that contact and pseudocontact mechanisms provide comparable contributions to the observed paramagnetic shifts. A plot of the pseudocontact shifts obtained by this procedure for the Pr3+ complex versus the geometrical factors (3 cos2 θ − 1)/r3 obtained from the DFT structure of the δδδ isomer of [Pr(TPPTAM)(H2O)]3+ gives a straight line J


Article

Inorganic Chemistry

Figure 9. 1H NMR spectrum of Yb−TPPTAM recorded in D2O solution at 298 K and pD 7.0 (400 MHz). Figure 8. Plot of the paramagnetic shifts observed for Ln3+ complexes of TPPTAM (Ln3+ = Ce3+, Pr3+, Nd3+, and Sm3+) according to eq 5 (R2 > 0.976).

61 and 75 Hz and two signals with line widths of 25 and 26 Hz (at 400 MHz and 298 K). These two sets of signals correspond to two sets of Yb3+−proton distances, the broader resonances being associated with the axial protons, which are closer to the metal ion.56 The paramagnetic 1H shifts induced by Yb3+ are largely dominated by the pseudocontact contribution, and therefore they can be analyzed directly without separating the contact contribution. A plot of the experimental paramagnetic shifts 2 3 (δexp i , Table 6) versus the geometric factors (3 cos θ − 1)/r

(R2 > 0.974), which indicates that our DFT calculations provide good models for the structure in solution of these complexes. The geometrical factors obtained for the λδδ isomer of [Pr(TPPTAM)(H2O)]3+ do not correlate linearly with the pseudocontact shifts (R2 ∼ 0.63), which suggests that these complexes adopt a δδδ structure in solution. However, we notice that the chemical shifts observed for protons H5 are significantly different in the case of the diamagnetic La3+ (3.80 ppm) and Lu3+ complexes (4.29 ppm), which suggests that the λδδ isomer might be dominant in solution for the complex of the large La3+ ion. The 1H NMR spectra of the paramagnetic Eu3+−Yb3+ complexes recorded in D2O at pD 7.0 show similar trends. All of them show 8−9 signals indicating a C3 symmetry of the complexes in solution, in line with the corresponding X-ray structures. Unfortunately the 1H NMR spectra of the Eu3+− Tm3+ complexes are poorly resolved, which prevents the assignment of the spectra. However, the 1H NMR spectrum of the Yb−TPPTAM complex shows 9 well-resolved signals corresponding to the 9 possible different magnetic environments of proton nuclei expected for a C3 symmetry (Figure 9). The assignments of the proton signals were based on standard 2D homonuclear COSY experiments, which gave strong cross peaks between ortho coupled pyridyl protons and between the geminal CH2 protons. The H4 protons give rise to four signals, as observed for the Lu3+ analogue. These four signals can be grouped into two different sets according to their relative line broadening: two resonances with line widths at half-height of

Table 6. 1H NMR Shifts (ppm) for Lu−TPPTAM and Yb− TPPTAM and Comparison of the Experimental and Calculated Paramagnetic 1H NMR Shifts for the Yb− TPPTAM Complex at 298 K (pD 7.0)a Yb3+ H

Lu3+

Yb3+

δexp i

δcalc i

(3 cos2 θ − 1)/r3b

H1 H2 ic H2 oc H4ax ic H4eq ic H4ax oc H4eq oc H5ax H5eq

7.94 7.46 7.46 3.45 4.03 3.93 4.41 4.29 4.29

9.46 9.34 11.28 1.96 10.60 18.43 12.34 8.48 11.81

−1.52 −1.88 −3.82 1.49 −6.57 −14.50 −7.93 −4.19 −7.52

−1.20 −1.43 −3.22 1.42 −6.47 −12.54 −8.89 −5.72 −9.58

−1.553 −1.841 −4.137 1.826 −8.319 −16.11 −11.42 7.347 −12.31

1

a

See Chart 1 for labeling. bGeometric factors obtained from the X-ray structure of Yb−TPPTAM (×103 Å−3). Values for symmetry equivalent nuclei have been averaged.

Table 5. Ln3+-Induced 1H NMR Paramagnetic Shifts, Contact and Pseudocontact (Dipolar) Contributions (ppm), and Calculated Geometrical Factors for Pr−TPPTAM Complex at 298 K a δpara i

Pr

H

Ce3+

Pr3+

Nd3+

Sm3+

Fib (Ce → Sm)

δcon i

δdip i

(3 cos2 θ − 1)/r3c

H1 H2 H4a H4b H5

0.50 0.64 2.19 2.52 0.80

1.10 1.68 6.36 5.19

1.00 1.70 5.75 4.15

0.01 0.01 −0.13 −0.08

−0.16 −0.29 −1.12 −0.77

0.485 0.858 3.324 2.298

0.615 0.822 3.036 2.892

−1.301 −2.671 −10.873 −8.017

1

Diamagnetic contribution estimated by using the shifts observed for the La3+ complex. bObtained from the linear fits of the data according to eq 5. Geometric factors obtained from the structure of the δδδ isomer of Pr−TPPTAM optimized in aqueous solution at the TPSSh/LCRECP/631G(d,p) level (×103 Å−3). Values for symmetry equivalent nuclei have been averaged.

a c

K


Article

Inorganic Chemistry

Rodríguez-Ubis, J.-C. Curr. Chem. Biol. 2007, 1, 11−39. (d) Xu, J.; Corneillie, T. M.; Moore, E. G.; Law, G.-L.; Butlin, N. G.; Raymond, K. N. J. Am. Chem. Soc. 2011, 133, 19900−19910. (e) Parker, D. Coord. Chem. Rev. 2000, 205, 109−130. (6) Langhorst, M. F.; Schwarzmann, N.; Guse, A. H. Cell. Signalling 2004, 16, 1283−1289. (7) Mewis, R. E.; Archibald, S. J. Coord. Chem. Rev. 2010, 254, 1686− 1712. (8) (a) Alpha, B.; Lehn, J.-M.; Mathis, G. Angew. Chem., Int. Ed. 1987, 26, 266−267. (b) Mathis, G. Clin. Chem. 1993, 39, 1953−1959. (9) (a) Spirlet, M. R.; Rebizant, J.; Desreux, J. F.; Loncin, M. F. Inorg. Chem. 1984, 23, 359−363. (b) Brittain, H. G.; Desreux, J. F. Inorg. Chem. 1984, 23, 4459−4469. (10) (a) Geraldes, C. F. G. C.; Sherry, A. D.; Cacheris, W. P. Inorg. Chem. 1989, 28, 3336−3341. (b) Lazar, I.; Hrncir, D. C.; Kim, W. D.; Kiefer, G. E.; Sherry, A. D. Inorg. Chem. 1992, 31, 4422−4424. (c) Rodovsky, J.; Cigler, P.; Kotek, J.; Hermann, P.; Vojtisek, P.; Lukes, I.; Peters, J. A.; Vander Elst, L.; Muller, R. N. Chem.Eur. J. 2005, 11, 2373−2384. (11) (a) Aime, S.; Barge, A.; Bruce, J. I.; Botta, M.; Howard, J. A. K.; Moloney, J. M.; Parker, D.; de Sousa, A. S.; Woods, M. J. Am. Chem. Soc. 1999, 121, 5762−5771. (b) Di Bari, L.; Pintacuda, G.; Salvadori, P.; Dickins, R. S.; Parker, D. J. Am. Chem. Soc. 2000, 122, 9257−9264. (c) Zhang, S.; Wu, K.; Sherry, A. D. J. Am. Chem. Soc. 2002, 124, 4226−4227. (d) Amin, S.; Morrow, J. R.; Lake, C. H.; Churchill, M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 773−775. (12) (a) Regueiro-Figueroa, M.; Bensenane, B.; Ruscsák, E.; EstebanGómez, D.; Charbonnière, L. J.; Tircsó, G.; Tóth, I.; de Blas, A.; Rodríguez-Blas, T.; Platas-Iglesias, C. Inorg. Chem. 2011, 50, 4125− 4141. (b) Rodríguez-Rodríguez, A.; Esteban-Gómez, D.; de Blas, A.; Rodríguez-Blas, T.; Fekete, M.; Botta, M.; Tripier, R.; Platas-Iglesias, C. Inorg. Chem. 2012, 51, 2509−2521. (c) Rodríguez-Rodríguez, A.; Esteban-Gómez, D.; de Blas, A.; Rodríguez-Blas, T.; Botta, M.; Tripier, R.; Platas-Iglesias, C. Inorg. Chem. 2012, 51, 13419−13429. (13) Stasiuk, G. J.; Long, N. J. Chem. Commun. 2013, 49, 2732−2746. (14) Toth, E.; Brucher, E.; Lazar, I.; Toth, I. Inorg. Chem. 1994, 33, 4070−4076. (15) (a) Mato-Iglesias, M.; Roca-Sabio, A.; Pálinkás, Z.; EstebanGómez, D.; Platas-Iglesias, C.; Tóth, E.; de Blas, A.; Rodríguez-Blas, T. Inorg. Chem. 2008, 47, 7840−7851. (b) Palinkas, Z.; Roca-Sabio, A.; Mato-Iglesias, M.; Esteban-Gómez, D.; Platas-Iglesias, C.; de Blas, A.; Rodríguez-Blas, T.; Tóth, É. Inorg. Chem. 2009, 48, 8878−8889. (16) Chong, H. S.; Garmestani, K.; Bryant, L. H.; Milenic, D. E.; Overstreet, T.; Birch, N.; Le, T.; Brady, E. D.; Brechbiel, M. W. J. Med. Chem. 2006, 49, 2055−2062. (17) (a) Kim, W. D.; Kiefer, G. E.; Maton, F.; McMillan, K.; Muller, R. N.; Sherry, A. D. Inorg. Chem. 1995, 34, 2233−2243. (b) Aime, S.; Botta, M.; Crich, S. G.; Giovenzana, G. B.; Jommi, G.; Pagliarin, R.; Sisti, M. Inorg. Chem. 1997, 36, 2992−3000. (18) Tircso, G.; Kovacs, Z.; Sherry, A. D. Inorg. Chem. 2006, 45, 9269−9280. (19) Balogh, E.; Tripier, R.; Ruloff, R.; Toth, E. Dalton Trans. 2005, 1058−1065. (20) Zheng, Q.; Dai, H. Q.; Merritt, M. E.; Malloy, C.; Pan, C. Y.; Li, W. H. J. Am. Chem. Soc. 2005, 127, 16178−16188. (21) Nasso, I.; Galaup, C.; Havas, F.; Tisnes, P.; Picard, C.; Laurent, S.; Vander Elst, L.; Muller, R. N. Inorg. Chem. 2005, 44, 8293−8305. (22) (a) Roca-Sabio, A.; Mato-Iglesias, M.; Esteban-Gómez, D.; de Blas, A.; Rodríguez-Blas, T.; Platas-Iglesias, C. Dalton Trans. 2011, 40, 384−392. (b) Roca-Sabio, A.; Mato-Iglesias, M.; Esteban-Gómez, D.; Toth, E.; de Blas, A.; Platas-Iglesias, C.; Rodríguez-Blas, T. J. Am. Chem. Soc. 2009, 131, 3331−3341. (c) Jensen, M. P.; Chiarizia, R.; Shkrob, I. A.; Ulicki, J. S.; Spindler, B. D.; Murphy, D. J.; Hossain, M.; Roca-Sabio, A.; Platas-Iglesias, C.; de Blas, A.; Rodríguez -Blas, T. Inorg. Chem. 2014, 53, 6003−6012. (d) Chang, C. A.; Rowland, M. E. Inorg. Chem. 1983, 22, 3866−3869. (e) Chang, C. A.; Ochaya, V. O. Inorg. Chem. 1986, 25, 355−358. (f) Brücher, E.; Györi, B.; Emri, J.; Solymosi, P.; Sztanyik, L. B.; Varga, L. J. Chem. Soc., Chem. Commun. 1993, 574−575.

obtained from the X-ray structure of the complex provides a straight line passing through the origin (R2 > 0.971) with slope 778 ± 47 ppm × Å3. The paramagnetic shifts calculated using the geometric factors and the slope obtained from the fitting (δicalc, Table 6) show an excellent agreement with the experimental values, which confirms (i) that the paramagnetic shifts are dominated by the pseudocontact contribution and (ii) that the structure of the complex in the solid state is retained in aqueous solution.

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CONCLUSIONS The TPPTAM ligand forms ten-coordinate Ln3+ complexes throughout the lanthanide series. The macrocyclic unit adopts a cone-like conformation with the three acetamide pendants being oriented to the same side of the macrocycle. A water molecule or oxygen atoms of nitrate or triflate anions coordinate from the opposite side of the macrocycle. The average bond distances of the metal coordination environment follow a quadratic trend across the series, which is typical of isostructural complexes. The structure of the complexes in the solid state is maintained in solution, as demonstrated by the analysis of the Ln3+ induced paramagnetic shifts. An increasing rigidity of the complexes in solution is observed across the series.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in cif format, hydrogen bond interactions in TPPTAM, dihedral angles Nam−C−C−Oam in Ln−TPPTAM complexes, shape measures, and comparison of experimental (X-ray) and calculated (DFT) bond distances. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ministerio de Ciencia e Innovación, Plan Nacional de I+D+i (CTQ2011-24487), for financial support. The authors are indebted to Centro de Supercomputación of Galicia (CESGA) for providing the computer facilities and to CACTI (Universidade de Vigo) for the X-ray measurements.

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REFERENCES

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