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Di-μ-hydroxy macrocyclic ytterbium(III) complex
Inorganic Chemistry Communications 6 (2003) 593–597 www.elsevier.com/locate/inoche
Di-l-hydroxy macrocyclic ytterbium(III) complex Jerzy Lisowski *, Przemysław Starynowicz Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, Wrocław 50-383, Poland Received 2 December 2002; accepted 27 January 2003
Abstract The mononuclear ½YbracLCl3 2H2 O and dinuclear ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complexes of the macrocyclic ligand L derived from trans-1,2-diaminocyclohexane and 2,6-diformylpyridine have been obtained. The formation of the dinuclear species upon addition of hydroxide to the ½YbracLCl3 2H2 O or the enantiopure ½YbrrrrLðNO3 Þ2 ðNO3 Þ has been followed using 1 H NMR spectroscopy. The X-ray crystal structure of ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complex has been determined. The complex is a dimer consisting of two macrocyclic units. The Yb(III) ion is coordinated by six nitrogen atoms of the macrocyclic ligand, two bridging OH groups and chloride anions. The chiral macrocycle L in this complex exhibits twist-bent conformation of approximate C2 symmetry. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Chiral macrocycles; Lanthanide complexes; Hydroxo complexes; Crystal structure
1. Introduction The macrocyclic lanthanide complexes have been shown to be very effective catalysts for the cleavage of the phosphate ester bond [1] and can potentially function as artificial nucleases. It has been shown that lanthanide hydroxo derivatives play a pivotal role in this hydrolytic reaction. In particular the hydroxo-bridged dinuclear lanthanide complexes proved to be very active catalysts [2], that are more effective than corresponding mononuclear species. The development of lanthanide(III)-based synthetic nucleases stimulates investigation of the dinuclear and polynuclear hydroxo-bridged lanthanide complexes [3]. While most of these complexes are characterised in the solid state, dinuclear hydroxobridged lanthanide(III) complexes, that are well defined in solution, are relatively rare. The macrocyclic ligands obtained in a template 2+2 condensation of diamines and 2,6-diformylpyridine or 2,6-diacetylpyridine form stable complexes with lanthanide(III) ions [4]. When the trans-1,2-diaminocyclohexane is used as a precursor in the above template reaction, *
Corresponding author. Tel.: +48-71-3757-252; fax: +48-71-3282348. E-mail address: [email protected] (J. Lisowski).
chiral lanthanide(III) complexes ½LnLX3 nH2 O (where Ln is the lanthanide(III) ion, X is nitrate or chloride anion, and L ¼ 3,10,18,25,31,35-hexaazapentacyclo[25: 3:1:112;24 :04;9 :019;24 ]-dotriaconta-1(31), 2,10,12,14,16(32), 17,25,27,29-decaene) possessing four chiral carbon atoms are formed [5–9] (Fig. 1). The enantiopure forms of these complexes, ½LnrrrrLX3 nH2 O, have been studied using circularly polarised luminescence [5] and NMR methods [6]. It has been shown previously [6,10] that the exchange of axial ligand has a dramatic effect on the NMR spectra of the macrocyclic lanthanide complexes. In this report we take advantage of this dependence to study the interactions of hydroxide anion with the macrocyclic ytterbium(III) complex ½YbrrrrLðNO3 Þ2 ðNO3 Þ and the new racemic complex ½YbracLCl3 2H2 O by means of 1 H NMR spectroscopy.
2. Experimental 2.1. Synthesis The new racemic complex ½YbracLCl3 2H2 O, has been prepared in the same way as the respective La(III), Ce(III) and Pr(III) complexes [7] in 32% yield. The
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J. Lisowski, P. Starynowicz / Inorganic Chemistry Communications 6 (2003) 593–597
Fig. 1. The general structure and labelling scheme of the ytterbium(III) complexes of the macrocycle L (coordinated anions and/or solvent molecules are omitted).
loosely trapped water molecule, were refined anisotropically. O60 was isotropic, and the positions of the hydrogen atoms were constrained to the relevant carbon atoms. The isotropic temperature factors of the H atoms were set as 1.2 of the trace of the thermal vibration tensor of the respective C-atoms. Crystal data: C56 H82 Cl4 N12 O8 Yb2 , M ¼ 1539:22, monoclinic, space group C2/c, a ¼ 24:814 , b ¼ 16:758ð12Þ A , c ¼ 15:758ð11Þ A , b ¼ 99:27 ð19Þ A 3 ð6Þ°, V ¼ 6467ð8Þ A , Z ¼ 4, Dc ¼ 1:581 Mg=m3 , lðMoKaÞ ¼ 3:099 mm1 , 20,064 reflections measured, 7585 unique (Rint ¼ 0:0488), final R indices ½I > 2rðIÞ RðF Þ ¼ 0:0525, Rw ðF 2 Þ ¼ 0:0943, R indices (all data) RðF Þ ¼ 0:0627, Rw ðF 2 Þ ¼ 0:0954.
½YbrrrrLðNO3 Þ2 ðNO3 Þ complex has been obtained as previously described [6]. All compounds gave satisfactory C, H and N analyses.
3. Results and discussion
2.2. 1 H NMR study
The solutions of ½YbracLCl3 2H2 O complex in deuterated methanol or chloroform/methanol mixture give rise to eight signals in their 1 H NMR spectra and seven signals in 13 C NMR spectra (Fig. 2). Some of the signals are relatively broad and are superimposed at room temperature. As expected for paramagnetic complex, with increased temperature the signals narrow and move towards diamagnetic region so that all the lines can be observed easily at elevated temperatures. The
The NMR spectra were taken on Bruker Avance 500 and AMX 300 spectrometers. The chemical shifts were referenced to the residual solvent signal or DSS. HMQC spectra were recorded using BIRD preparation period and 1K 256 data points. The data were processed by using a square sine bell window in both dimensions and zero filled to 1K1K matrix. The formation of hydroxo species has been followed with NMR method by titrating the ½YbracLCl3 2H2 O solution in CD3 OD or 1:2 v/ v CD3 OD=CDCl3 mixture with a stock solution of NaOH in CD3 OD, or by titrating solutions of ½YbracL Cl3 2H2 O or ½YbrrrrLðNO3 Þ2 ðNO3 Þ in D2 O with a stock solution of NaOH in D2 O.
3.1. NMR spectroscopy
2.3. X-ray data collection The crystals of ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O were grown by slow diffusion of diethyl ether into 2:1 v/v chloroform/methanol solution obtained in NMR titration that corresponded to addition of 1 equivalent of NaOH into ½YbracLCl3 2H2 O solution. The appropriate crystal was cut from a larger one and mounted on a Kuma KM4 diffractometer equipped with a CCD counter and an Oxford Cryosystem appliance. The data were corrected for polarisation, Lorentz factor and absorption, the latter calculated from the crystal habit captured from a photo scan. The structure was solved routinely with SHELXS-97 program [11], and SH ELXL-97 [11]. The positions of the C-bonded hydrogen were calculated geometrically; in the case of methyl groups of the methanol molecules the additional criterion of maximising the electron density at the hydrogen sites was used. No attempt to localise the O-bonded H atoms was taken. The full matrix refinement was performed with SHELXL-97; all non-H atoms except O60, which probably represents a
Fig. 2. 1 H NMR spectra of: top ½YbracLCl3 2H2 O (CD3 OD=CDCl3 1:2 v/v solution, 305 K), middle ½YbracLCl3 2H2 O (D2 O solution, 298 K) and bottom ½YbracLCl3 2H2 O with 1 equivalent of NaOH added (D2 O solution, 298 K).
J. Lisowski, P. Starynowicz / Inorganic Chemistry Communications 6 (2003) 593–597
number of observed resonances is consistent with the presence of diastereotopic cyclohexane protons and indicates the effective D2 symmetry of the ½YbracL Cl3 2H2 O complex in the investigated solutions. The effective D2 symmetry is higher than expected for complex with unsymmetrical coordination of three chloride anions on two sides of the macrocycle plane. This higher symmetry arises from dynamic exchange of coordinated chloride anions, similarly as is observed in the case of La(III), Ce(III), Pr(III) and Eu(III) complexes [7]. This process is fast on NMR time scale and makes the two sides of the macrocycle equivalent. The dynamic exchange of chloride anions is responsible for relatively broad NMR signals observed for this complex as well as for some concentration dependence of the chemical shifts. Although the general 1 H NMR spectrum pattern of ½YbracLCl3 2H2 O is similar to that of ½YbrrrrL ðNO3 Þ2 ðNO3 Þ, the spectra of these two derivatives are distinctly different, indicating the coordination of counteranions in the above solutions. The resonances of the chloride derivative can be tentatively assigned on the basis of the linewidth dependence on the reverse sixth power of the proton–Yb(III) distance [12], as well as comparison with the previously assigned [6] spectrum of the ½YbrrrrLðNO3 Þ2 ðNO3 Þ complex. Thus the resonances at )2.25, )3.82, 4.82, 44.17, 11.01, 10.65, 10.28 and 7.95 ppm (305 K) correspond to protons a, b, c, d, e, e0 , f and f 0 , respectively (see Fig. 1 for the labelling scheme). The 1 H NMR spectrum of solution of ½YbracLCl3 2H2 O complex in D2 O consists of eight well-separated signals (Fig. 2) and is identical to that of ½YbrrrrL ðNO3 Þ2 ðNO3 Þ, that points to the existence of identical solvent coordinated ½YbLðH2 OÞn 3þ species in water solution of both derivatives (with n most likely being three [9]). The number of NMR resonances observed for water solution of ½YbracLCl3 2H2 O indicates a dynamic, fast on the NMR time scale, exchange of coordinated water molecules, analogous to the chloride exchange discussed above. The resonances can be as-
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signed as described above; the signals at )1.40, )1.83, 13.19, 57.22, 18.28, 16.31, 14.14 and 11.00 ppm correspond to positions a, b, c, d, e, e0 , f and f 0 , respectively. The pairs of the geminal protons e, e0 and f, f 0 can be additionally identified on the basis of the HMQC spectrum, since the signals of geminal protons are crosscorrelated to the same carbon-13 signal. Apart from the eight signals of the aquo complex, additional set of 15 signals is observed for the water solution of ½YbracLCl3 2H2 O, corresponding to ca. 5% intensity of the main spectrum. The presence of that set of signals is independent on the sample preparation, that excludes the possible presence of paramagnetic impurity. The reasonable reason for the presence of that set of additional signals is the hydrolysis of the ½YbLðH2 OÞ3 þ3 ion leading to hydroxo species. This hypothesis has been checked in titration experiments based on controlled addition of NaOH to water solution of ½YbracLCl3 2H2 O. Upon addition of NaOH to the solution of ½YbracL Cl3 2H2 O in D2 O, the set of 15 signals of equal intensity at 127.40, 95.55, 85.00, 71.92, 48.71, 45.63, 32.54, 31.04, 19.93, 10.64, 10.05, )7.22, )11.10, )16.85 and )43.18 ppm increase in intensity, while the intensity of signals of the starting complex decreases. When 1 equivalent of OH is added the signals of the starting complex disappear and only the 15 signals of the initial minor form are present (Fig. 2). The number of 15 observed resonances is in accord with the C2 symmetry of the complex and different axial ligation of the two sides of the macrocyle. This can be explained by the formation of either mononuclear hydroxo complex or dinuclear l-dihydroxo complex. The former possibility, however, is less likely, since such complex would likely exhibit fluxional behaviour leading to spectrum corresponding to effective D2 symmetry, similarly as discussed above for the complexes with axial positions occupied by water molecules or chloride anions. The final conformation of the dinuclear nature of the hydroxide derivative is based on the crystal structure discussed below and the formation of heterodinuclear
Scheme 1. Simplified scheme of formation of dinuclear l-hydroxo complexes. X denotes one or two chloride anions, one or two water molecules or one nitrate anion.
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complexes in the analogous NMR titration experiments in which the mixture of starting complexes of two different lanthanide ions was used [13]. Further addition of hydroxide results in gradual shift of the 15 resonances to final positions at 149.06, 110.13, 96.26, 82.51, 59.32, 47.96, 39.21, 38.42, 22.93, 15.20, 12.95, )6.42, )7.76, )14.56, and )42.65 ppm. This shift reflects exchange (fast on the NMR time scale) of further axial water molecules for the hydroxide anion (Scheme 1). Identical spectra are obtained in the NaOH NMR titrations when ½YbrrrrLðNO3 Þ2 ðNO3 Þ is used as the starting complex instead of ½YbracLCl3 2H2 O. The water solutions of both complexes are relatively stable, ca. 5% decomposition of the macrocycle is observed over one day period at room temperature. This process is somewhat faster in the presence of the base and in the above titration experiments ca. 20% of the sample decomposed after 24 h. Somewhat different situation is observed when the NaOH titrations are performed for the CD3 OD or 1:2 v/v CD3 OD=CDCl3 solutions of the starting complexes. Similarly as in the water solution, appearance of 15-line spectra are observed after addition of 1 equivalent of NaOH indicating formation of dinuclear hydroxo species, e.g., signals at 136.19, 99.27, 83.84, 75.36, 52.23, 46.16, 32.94, 30.07, 19.79, 10.83, 9.00, )10.56, )15.66, )19.55 and )50.58 ppm are observed when 1 equivalent of hydroxide is added to 1:2 v/v CD3 OD=CDCl3 solution of ½YbracLCl3 2H2 O. This time, however, the spectra obtained after addition of base (less than 3 equivalents) to the ½YbrrrrLðNO3 Þ2 ðNO3 Þ and ½YbracLCl3 2H2 O starting complexes are not identical due to counteranion coordination. Another difference is the slow or intermediate binding rate of the additional hydroxide anions after the initial formation of di-l-hydroxo bridged complexes, hence at least three different
15-line spectra are observed in these titrations when up to four equivalents of NaOH are added. It is important, however, that the species that dominates the 1 H NMR spectrum after addition of three or more equivalents of NaOH is identical for both the ½YbracLCl3 2H2 O and ½YbrrrrLðNO3 Þ2 ðNO3 Þ starting complexes. Thus the 15line spectra with resonances at 169.35, 127.80, 114.50, 96.36, 63.08, 61.68, 43.55, 43.14, 26.48, 13.02, 11.19, )9.79, )16.48, )25.00 and )56.94 ppm are obtained for CD3 OD solutions of both complexes. This is in accord with full occupation of axial positions by hydroxide anions and formation of dinuclear hexahydroxo ytterbium(III) species (Scheme 1). 3.2. X-ray crystal structure The crystal of the ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complex is built up from complex cation, methanol, water molecules and chloride anions. The complex cation (Fig. 3) is composed of two moieties connected by two rather symmetric hydroxy bridges. Each moiety is formed by ytterbium(III) cation surrounded by the six-dentate macrocyclic ligand located roughly on the equatorial plane, and Cl anion in the apical position. Two hydroxy groups shared by both moieties complete the coordination environment of either ytterbium ion. The complex cation possesses the 2 (C2 ) symmetry in the following way: both Yb and both Yb-bonded Cl atoms are located on a 2 symmetry axis, and only a half of each macrocycle ligand is symmetry independent. There is also only one symmetry independent bridging OH group. The macrocycle in ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O is bent, in contrast to what is observed in the case of ½NdrrrrL ðNO3 Þ2 ðNO3 Þ CH3 OH and ½TmrrrrLðNO3 Þ2 ðNO3 Þ CH3 OH complexes [8]. Both macrocycles are bent along
Fig. 3. Left: a side view of the complex dimer, right: a view of a constituent macrocycle moiety, together with atom numbering scheme (the numbers after the commas refer to the other macrocycle); only symmetry independent atoms have been labelled.
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the N1–(Yb1)–N10 , or N11–(Yb2)–N110 lines, respectively (the prime stands for the 1 x; y; 1=2 z symmetry operation). Namely the dihedral angle between the planes N3–C7–C6–N2–C5–C4–C3–C2–C1–N1 and N30 –C70 – C60 –N20 –C50 –C40 –C30 –C20 –C10 –N10 is 45.8° and between N11–C21–C22–N12–C23–C24–C25–C26–C27–N13 and N110 –C210 –C220 –N120 –C230 –C240 –C250 –C270 –C270 –N130 is 42.3°. The torsion angle N1–Yb1–Yb2–N11, which may be the measure of mutual twisting of both macrocycles, is 150.7°, i.e., the twisting is 29.3°. The distances are typical with the exception of the small OH–OH sep. The short OH–OH distance aration equal to 1.809(11) A is probably brought about by the stretching of the Yb– ðOHÞ2 –Yb fragment caused by the steric interaction of the two macrocylic ligands. Additionally, in the absence of hydrogen bond acceptors available within the appropriate vicinity of the OH groups, the hydrogen atoms may turn to their oxygen neighbours, forming in this way a roughly square, strong tandem hydrogen bond. In conclusion, we have shown controlled formation of hydroxo-bridged ytterbium(III) dinuclear complexes which are well defined in solution. It should be noted that most lanthanide(III) complexes readily decompose to insoluble hydroxides or polynuclear hydroxo agregates upon addition of hydroxides. In the described case, these undesirable reactions are prevented due to macrocyclic effect of the ligand L. Thanks to the high thermodynamic and/or kinetic stability of its complexes with lanthanide(III) ions, these ions are not easily removed from the ligand even in the presence of excess of hydroxide. Additionally, the steric bulk of the macrocycle precludes formation of higher aggregates.
Supplementary data Supplementary data for ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O structure are available from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-33
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6033; e-mail: [email protected]) on request, quoting the deposition number CCDC 179383. Acknowledgements We thank Dr. Jarosław Mazurek for preliminary work on the X-ray crystal structure determination. This work was supported by KBN Grant 3 T09A 11519. References [1] J.R. Morrow, L.A. Butterey, V.M. Shelton, K.A. Berback, J. Am. Chem. Soc. 114 (1992) 1903; D.M. Epstein, L.L. Chappell, H. Khalili, R.M. Supkowski, W.D. Horrocks, J.R. Morrow, Inorg. Chem. 39 (2000) 2130; R.W. Hay, N. Govan, Polyhedron 24 (1997) 4233; K.G. Ragunathan, H.-J. Schneider, Angew. Chem. Int. Ed. Engl. 35 (1996) 1219; D. Magda, R.A. Miller, J.L. Sessler, B.L. Iverson, J. Am. Chem. Soc. 116 (1994) 7439; D.M. Magda, M. Wright, S. Crofts, A. Lin, J.L. Sessler, J. Am. Chem. Soc. 119 (1997) 6947. [2] P. Hurst, B.K. Takasaki, J. Chin, J. Am. Chem. Soc. 18 (1996) 9982; P.E. Jurek, A.M. Jurek, A.E. Martell, Inorg. Chem. 39 (2000) 1016. [3] Z. Zheng, Chem. Commun. (2001) 2521; R. Wang, D. Song, S. Wang, Chem. Commun. (2002) 368. [4] V. Alexander, Chem. Rev. 95 (1995) 273. [5] T. Tsubomura, K. Yasaku, T. Sato, M. Morita, Inorg. Chem. 31 (1992) 447. [6] J. Lisowski, Magn. Reson. Chem. 37 (1999) 287. [7] J. Lisowski, J. Mazurek, Polyhedron 21 (2002) 811. [8] J. Lisowski, P. Starynowicz, Polyhedron 19 (2000) 465. [9] S.W.A. Bligh, N. Choi, W.J. Cummins, E.G. Evagorou, J.D. Kelly, M. McPartlin, J. Chem. Soc. Dalton Trans. (1994) 3369. [10] J. Lisowski, J.L. Sessler, V. Lynch, T.D. Mody, J. Am. Chem. Soc. 117 (1995) 2273. [11] G.M. Sheldrick, SHELXS-97. Program for structure solution. University of G€ ottingen, 1997, G.M. Sheldrick, SHELXL-97. Program for structure refinement. University of G€ ottingen,1997. [12] G.N. La Mar, W.D. Horrocks Jr., R.H. Holm (Eds.), NMR of Paramagnetic Molecules, Academic Press, New York, 1973; I. Bertini, C. Luchinat, NMR of Paramagnetic Molecules in Biological Systems, Benjamin/Cummings, Menlo Park, CA, 1986; I. Bertini, P. Turano, A.J. Vila, Chem. Rev. 93 (1993) 2833; I. Bertini, C. Luchinat, Coord. Chem. Rev. 150 (1996) 1. [13] J. Lisowski, to be submitted.
Di-l-hydroxy macrocyclic ytterbium(III) complex Jerzy Lisowski *, Przemysław Starynowicz Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, Wrocław 50-383, Poland Received 2 December 2002; accepted 27 January 2003
Abstract The mononuclear ½YbracLCl3 2H2 O and dinuclear ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complexes of the macrocyclic ligand L derived from trans-1,2-diaminocyclohexane and 2,6-diformylpyridine have been obtained. The formation of the dinuclear species upon addition of hydroxide to the ½YbracLCl3 2H2 O or the enantiopure ½YbrrrrLðNO3 Þ2 ðNO3 Þ has been followed using 1 H NMR spectroscopy. The X-ray crystal structure of ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complex has been determined. The complex is a dimer consisting of two macrocyclic units. The Yb(III) ion is coordinated by six nitrogen atoms of the macrocyclic ligand, two bridging OH groups and chloride anions. The chiral macrocycle L in this complex exhibits twist-bent conformation of approximate C2 symmetry. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Chiral macrocycles; Lanthanide complexes; Hydroxo complexes; Crystal structure
1. Introduction The macrocyclic lanthanide complexes have been shown to be very effective catalysts for the cleavage of the phosphate ester bond [1] and can potentially function as artificial nucleases. It has been shown that lanthanide hydroxo derivatives play a pivotal role in this hydrolytic reaction. In particular the hydroxo-bridged dinuclear lanthanide complexes proved to be very active catalysts [2], that are more effective than corresponding mononuclear species. The development of lanthanide(III)-based synthetic nucleases stimulates investigation of the dinuclear and polynuclear hydroxo-bridged lanthanide complexes [3]. While most of these complexes are characterised in the solid state, dinuclear hydroxobridged lanthanide(III) complexes, that are well defined in solution, are relatively rare. The macrocyclic ligands obtained in a template 2+2 condensation of diamines and 2,6-diformylpyridine or 2,6-diacetylpyridine form stable complexes with lanthanide(III) ions [4]. When the trans-1,2-diaminocyclohexane is used as a precursor in the above template reaction, *
Corresponding author. Tel.: +48-71-3757-252; fax: +48-71-3282348. E-mail address: [email protected] (J. Lisowski).
chiral lanthanide(III) complexes ½LnLX3 nH2 O (where Ln is the lanthanide(III) ion, X is nitrate or chloride anion, and L ¼ 3,10,18,25,31,35-hexaazapentacyclo[25: 3:1:112;24 :04;9 :019;24 ]-dotriaconta-1(31), 2,10,12,14,16(32), 17,25,27,29-decaene) possessing four chiral carbon atoms are formed [5–9] (Fig. 1). The enantiopure forms of these complexes, ½LnrrrrLX3 nH2 O, have been studied using circularly polarised luminescence [5] and NMR methods [6]. It has been shown previously [6,10] that the exchange of axial ligand has a dramatic effect on the NMR spectra of the macrocyclic lanthanide complexes. In this report we take advantage of this dependence to study the interactions of hydroxide anion with the macrocyclic ytterbium(III) complex ½YbrrrrLðNO3 Þ2 ðNO3 Þ and the new racemic complex ½YbracLCl3 2H2 O by means of 1 H NMR spectroscopy.
2. Experimental 2.1. Synthesis The new racemic complex ½YbracLCl3 2H2 O, has been prepared in the same way as the respective La(III), Ce(III) and Pr(III) complexes [7] in 32% yield. The
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00051-0
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J. Lisowski, P. Starynowicz / Inorganic Chemistry Communications 6 (2003) 593–597
Fig. 1. The general structure and labelling scheme of the ytterbium(III) complexes of the macrocycle L (coordinated anions and/or solvent molecules are omitted).
loosely trapped water molecule, were refined anisotropically. O60 was isotropic, and the positions of the hydrogen atoms were constrained to the relevant carbon atoms. The isotropic temperature factors of the H atoms were set as 1.2 of the trace of the thermal vibration tensor of the respective C-atoms. Crystal data: C56 H82 Cl4 N12 O8 Yb2 , M ¼ 1539:22, monoclinic, space group C2/c, a ¼ 24:814 , b ¼ 16:758ð12Þ A , c ¼ 15:758ð11Þ A , b ¼ 99:27 ð19Þ A 3 ð6Þ°, V ¼ 6467ð8Þ A , Z ¼ 4, Dc ¼ 1:581 Mg=m3 , lðMoKaÞ ¼ 3:099 mm1 , 20,064 reflections measured, 7585 unique (Rint ¼ 0:0488), final R indices ½I > 2rðIÞ RðF Þ ¼ 0:0525, Rw ðF 2 Þ ¼ 0:0943, R indices (all data) RðF Þ ¼ 0:0627, Rw ðF 2 Þ ¼ 0:0954.
½YbrrrrLðNO3 Þ2 ðNO3 Þ complex has been obtained as previously described [6]. All compounds gave satisfactory C, H and N analyses.
3. Results and discussion
2.2. 1 H NMR study
The solutions of ½YbracLCl3 2H2 O complex in deuterated methanol or chloroform/methanol mixture give rise to eight signals in their 1 H NMR spectra and seven signals in 13 C NMR spectra (Fig. 2). Some of the signals are relatively broad and are superimposed at room temperature. As expected for paramagnetic complex, with increased temperature the signals narrow and move towards diamagnetic region so that all the lines can be observed easily at elevated temperatures. The
The NMR spectra were taken on Bruker Avance 500 and AMX 300 spectrometers. The chemical shifts were referenced to the residual solvent signal or DSS. HMQC spectra were recorded using BIRD preparation period and 1K 256 data points. The data were processed by using a square sine bell window in both dimensions and zero filled to 1K1K matrix. The formation of hydroxo species has been followed with NMR method by titrating the ½YbracLCl3 2H2 O solution in CD3 OD or 1:2 v/ v CD3 OD=CDCl3 mixture with a stock solution of NaOH in CD3 OD, or by titrating solutions of ½YbracL Cl3 2H2 O or ½YbrrrrLðNO3 Þ2 ðNO3 Þ in D2 O with a stock solution of NaOH in D2 O.
3.1. NMR spectroscopy
2.3. X-ray data collection The crystals of ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O were grown by slow diffusion of diethyl ether into 2:1 v/v chloroform/methanol solution obtained in NMR titration that corresponded to addition of 1 equivalent of NaOH into ½YbracLCl3 2H2 O solution. The appropriate crystal was cut from a larger one and mounted on a Kuma KM4 diffractometer equipped with a CCD counter and an Oxford Cryosystem appliance. The data were corrected for polarisation, Lorentz factor and absorption, the latter calculated from the crystal habit captured from a photo scan. The structure was solved routinely with SHELXS-97 program [11], and SH ELXL-97 [11]. The positions of the C-bonded hydrogen were calculated geometrically; in the case of methyl groups of the methanol molecules the additional criterion of maximising the electron density at the hydrogen sites was used. No attempt to localise the O-bonded H atoms was taken. The full matrix refinement was performed with SHELXL-97; all non-H atoms except O60, which probably represents a
Fig. 2. 1 H NMR spectra of: top ½YbracLCl3 2H2 O (CD3 OD=CDCl3 1:2 v/v solution, 305 K), middle ½YbracLCl3 2H2 O (D2 O solution, 298 K) and bottom ½YbracLCl3 2H2 O with 1 equivalent of NaOH added (D2 O solution, 298 K).
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number of observed resonances is consistent with the presence of diastereotopic cyclohexane protons and indicates the effective D2 symmetry of the ½YbracL Cl3 2H2 O complex in the investigated solutions. The effective D2 symmetry is higher than expected for complex with unsymmetrical coordination of three chloride anions on two sides of the macrocycle plane. This higher symmetry arises from dynamic exchange of coordinated chloride anions, similarly as is observed in the case of La(III), Ce(III), Pr(III) and Eu(III) complexes [7]. This process is fast on NMR time scale and makes the two sides of the macrocycle equivalent. The dynamic exchange of chloride anions is responsible for relatively broad NMR signals observed for this complex as well as for some concentration dependence of the chemical shifts. Although the general 1 H NMR spectrum pattern of ½YbracLCl3 2H2 O is similar to that of ½YbrrrrL ðNO3 Þ2 ðNO3 Þ, the spectra of these two derivatives are distinctly different, indicating the coordination of counteranions in the above solutions. The resonances of the chloride derivative can be tentatively assigned on the basis of the linewidth dependence on the reverse sixth power of the proton–Yb(III) distance [12], as well as comparison with the previously assigned [6] spectrum of the ½YbrrrrLðNO3 Þ2 ðNO3 Þ complex. Thus the resonances at )2.25, )3.82, 4.82, 44.17, 11.01, 10.65, 10.28 and 7.95 ppm (305 K) correspond to protons a, b, c, d, e, e0 , f and f 0 , respectively (see Fig. 1 for the labelling scheme). The 1 H NMR spectrum of solution of ½YbracLCl3 2H2 O complex in D2 O consists of eight well-separated signals (Fig. 2) and is identical to that of ½YbrrrrL ðNO3 Þ2 ðNO3 Þ, that points to the existence of identical solvent coordinated ½YbLðH2 OÞn 3þ species in water solution of both derivatives (with n most likely being three [9]). The number of NMR resonances observed for water solution of ½YbracLCl3 2H2 O indicates a dynamic, fast on the NMR time scale, exchange of coordinated water molecules, analogous to the chloride exchange discussed above. The resonances can be as-
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signed as described above; the signals at )1.40, )1.83, 13.19, 57.22, 18.28, 16.31, 14.14 and 11.00 ppm correspond to positions a, b, c, d, e, e0 , f and f 0 , respectively. The pairs of the geminal protons e, e0 and f, f 0 can be additionally identified on the basis of the HMQC spectrum, since the signals of geminal protons are crosscorrelated to the same carbon-13 signal. Apart from the eight signals of the aquo complex, additional set of 15 signals is observed for the water solution of ½YbracLCl3 2H2 O, corresponding to ca. 5% intensity of the main spectrum. The presence of that set of signals is independent on the sample preparation, that excludes the possible presence of paramagnetic impurity. The reasonable reason for the presence of that set of additional signals is the hydrolysis of the ½YbLðH2 OÞ3 þ3 ion leading to hydroxo species. This hypothesis has been checked in titration experiments based on controlled addition of NaOH to water solution of ½YbracLCl3 2H2 O. Upon addition of NaOH to the solution of ½YbracL Cl3 2H2 O in D2 O, the set of 15 signals of equal intensity at 127.40, 95.55, 85.00, 71.92, 48.71, 45.63, 32.54, 31.04, 19.93, 10.64, 10.05, )7.22, )11.10, )16.85 and )43.18 ppm increase in intensity, while the intensity of signals of the starting complex decreases. When 1 equivalent of OH is added the signals of the starting complex disappear and only the 15 signals of the initial minor form are present (Fig. 2). The number of 15 observed resonances is in accord with the C2 symmetry of the complex and different axial ligation of the two sides of the macrocyle. This can be explained by the formation of either mononuclear hydroxo complex or dinuclear l-dihydroxo complex. The former possibility, however, is less likely, since such complex would likely exhibit fluxional behaviour leading to spectrum corresponding to effective D2 symmetry, similarly as discussed above for the complexes with axial positions occupied by water molecules or chloride anions. The final conformation of the dinuclear nature of the hydroxide derivative is based on the crystal structure discussed below and the formation of heterodinuclear
Scheme 1. Simplified scheme of formation of dinuclear l-hydroxo complexes. X denotes one or two chloride anions, one or two water molecules or one nitrate anion.
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complexes in the analogous NMR titration experiments in which the mixture of starting complexes of two different lanthanide ions was used [13]. Further addition of hydroxide results in gradual shift of the 15 resonances to final positions at 149.06, 110.13, 96.26, 82.51, 59.32, 47.96, 39.21, 38.42, 22.93, 15.20, 12.95, )6.42, )7.76, )14.56, and )42.65 ppm. This shift reflects exchange (fast on the NMR time scale) of further axial water molecules for the hydroxide anion (Scheme 1). Identical spectra are obtained in the NaOH NMR titrations when ½YbrrrrLðNO3 Þ2 ðNO3 Þ is used as the starting complex instead of ½YbracLCl3 2H2 O. The water solutions of both complexes are relatively stable, ca. 5% decomposition of the macrocycle is observed over one day period at room temperature. This process is somewhat faster in the presence of the base and in the above titration experiments ca. 20% of the sample decomposed after 24 h. Somewhat different situation is observed when the NaOH titrations are performed for the CD3 OD or 1:2 v/v CD3 OD=CDCl3 solutions of the starting complexes. Similarly as in the water solution, appearance of 15-line spectra are observed after addition of 1 equivalent of NaOH indicating formation of dinuclear hydroxo species, e.g., signals at 136.19, 99.27, 83.84, 75.36, 52.23, 46.16, 32.94, 30.07, 19.79, 10.83, 9.00, )10.56, )15.66, )19.55 and )50.58 ppm are observed when 1 equivalent of hydroxide is added to 1:2 v/v CD3 OD=CDCl3 solution of ½YbracLCl3 2H2 O. This time, however, the spectra obtained after addition of base (less than 3 equivalents) to the ½YbrrrrLðNO3 Þ2 ðNO3 Þ and ½YbracLCl3 2H2 O starting complexes are not identical due to counteranion coordination. Another difference is the slow or intermediate binding rate of the additional hydroxide anions after the initial formation of di-l-hydroxo bridged complexes, hence at least three different
15-line spectra are observed in these titrations when up to four equivalents of NaOH are added. It is important, however, that the species that dominates the 1 H NMR spectrum after addition of three or more equivalents of NaOH is identical for both the ½YbracLCl3 2H2 O and ½YbrrrrLðNO3 Þ2 ðNO3 Þ starting complexes. Thus the 15line spectra with resonances at 169.35, 127.80, 114.50, 96.36, 63.08, 61.68, 43.55, 43.14, 26.48, 13.02, 11.19, )9.79, )16.48, )25.00 and )56.94 ppm are obtained for CD3 OD solutions of both complexes. This is in accord with full occupation of axial positions by hydroxide anions and formation of dinuclear hexahydroxo ytterbium(III) species (Scheme 1). 3.2. X-ray crystal structure The crystal of the ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O complex is built up from complex cation, methanol, water molecules and chloride anions. The complex cation (Fig. 3) is composed of two moieties connected by two rather symmetric hydroxy bridges. Each moiety is formed by ytterbium(III) cation surrounded by the six-dentate macrocyclic ligand located roughly on the equatorial plane, and Cl anion in the apical position. Two hydroxy groups shared by both moieties complete the coordination environment of either ytterbium ion. The complex cation possesses the 2 (C2 ) symmetry in the following way: both Yb and both Yb-bonded Cl atoms are located on a 2 symmetry axis, and only a half of each macrocycle ligand is symmetry independent. There is also only one symmetry independent bridging OH group. The macrocycle in ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O is bent, in contrast to what is observed in the case of ½NdrrrrL ðNO3 Þ2 ðNO3 Þ CH3 OH and ½TmrrrrLðNO3 Þ2 ðNO3 Þ CH3 OH complexes [8]. Both macrocycles are bent along
Fig. 3. Left: a side view of the complex dimer, right: a view of a constituent macrocycle moiety, together with atom numbering scheme (the numbers after the commas refer to the other macrocycle); only symmetry independent atoms have been labelled.
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the N1–(Yb1)–N10 , or N11–(Yb2)–N110 lines, respectively (the prime stands for the 1 x; y; 1=2 z symmetry operation). Namely the dihedral angle between the planes N3–C7–C6–N2–C5–C4–C3–C2–C1–N1 and N30 –C70 – C60 –N20 –C50 –C40 –C30 –C20 –C10 –N10 is 45.8° and between N11–C21–C22–N12–C23–C24–C25–C26–C27–N13 and N110 –C210 –C220 –N120 –C230 –C240 –C250 –C270 –C270 –N130 is 42.3°. The torsion angle N1–Yb1–Yb2–N11, which may be the measure of mutual twisting of both macrocycles, is 150.7°, i.e., the twisting is 29.3°. The distances are typical with the exception of the small OH–OH sep. The short OH–OH distance aration equal to 1.809(11) A is probably brought about by the stretching of the Yb– ðOHÞ2 –Yb fragment caused by the steric interaction of the two macrocylic ligands. Additionally, in the absence of hydrogen bond acceptors available within the appropriate vicinity of the OH groups, the hydrogen atoms may turn to their oxygen neighbours, forming in this way a roughly square, strong tandem hydrogen bond. In conclusion, we have shown controlled formation of hydroxo-bridged ytterbium(III) dinuclear complexes which are well defined in solution. It should be noted that most lanthanide(III) complexes readily decompose to insoluble hydroxides or polynuclear hydroxo agregates upon addition of hydroxides. In the described case, these undesirable reactions are prevented due to macrocyclic effect of the ligand L. Thanks to the high thermodynamic and/or kinetic stability of its complexes with lanthanide(III) ions, these ions are not easily removed from the ligand even in the presence of excess of hydroxide. Additionally, the steric bulk of the macrocycle precludes formation of higher aggregates.
Supplementary data Supplementary data for ½Yb2 ðOHÞ2 Cl2 ðracLÞ2 Cl2 4CH3 OH 2H2 O structure are available from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-33
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6033; e-mail: [email protected]) on request, quoting the deposition number CCDC 179383. Acknowledgements We thank Dr. Jarosław Mazurek for preliminary work on the X-ray crystal structure determination. This work was supported by KBN Grant 3 T09A 11519. References [1] J.R. Morrow, L.A. Butterey, V.M. Shelton, K.A. Berback, J. Am. Chem. Soc. 114 (1992) 1903; D.M. Epstein, L.L. Chappell, H. Khalili, R.M. Supkowski, W.D. Horrocks, J.R. Morrow, Inorg. Chem. 39 (2000) 2130; R.W. Hay, N. Govan, Polyhedron 24 (1997) 4233; K.G. Ragunathan, H.-J. Schneider, Angew. Chem. Int. Ed. Engl. 35 (1996) 1219; D. Magda, R.A. Miller, J.L. Sessler, B.L. Iverson, J. Am. Chem. Soc. 116 (1994) 7439; D.M. Magda, M. Wright, S. Crofts, A. Lin, J.L. Sessler, J. Am. Chem. Soc. 119 (1997) 6947. [2] P. Hurst, B.K. Takasaki, J. Chin, J. Am. Chem. Soc. 18 (1996) 9982; P.E. Jurek, A.M. Jurek, A.E. Martell, Inorg. Chem. 39 (2000) 1016. [3] Z. Zheng, Chem. Commun. (2001) 2521; R. Wang, D. Song, S. Wang, Chem. Commun. (2002) 368. [4] V. Alexander, Chem. Rev. 95 (1995) 273. [5] T. Tsubomura, K. Yasaku, T. Sato, M. Morita, Inorg. Chem. 31 (1992) 447. [6] J. Lisowski, Magn. Reson. Chem. 37 (1999) 287. [7] J. Lisowski, J. Mazurek, Polyhedron 21 (2002) 811. [8] J. Lisowski, P. Starynowicz, Polyhedron 19 (2000) 465. [9] S.W.A. Bligh, N. Choi, W.J. Cummins, E.G. Evagorou, J.D. Kelly, M. McPartlin, J. Chem. Soc. Dalton Trans. (1994) 3369. [10] J. Lisowski, J.L. Sessler, V. Lynch, T.D. Mody, J. Am. Chem. Soc. 117 (1995) 2273. [11] G.M. Sheldrick, SHELXS-97. Program for structure solution. University of G€ ottingen, 1997, G.M. Sheldrick, SHELXL-97. Program for structure refinement. University of G€ ottingen,1997. [12] G.N. La Mar, W.D. Horrocks Jr., R.H. Holm (Eds.), NMR of Paramagnetic Molecules, Academic Press, New York, 1973; I. Bertini, C. Luchinat, NMR of Paramagnetic Molecules in Biological Systems, Benjamin/Cummings, Menlo Park, CA, 1986; I. Bertini, P. Turano, A.J. Vila, Chem. Rev. 93 (1993) 2833; I. Bertini, C. Luchinat, Coord. Chem. Rev. 150 (1996) 1. [13] J. Lisowski, to be submitted.