CRYSTAL GROWTH & DESIGN
Self-Assembly of an Infinite Copper(II) Chiral Metallohelicate Edgar Mijangos,† Jose´ Sa´nchez Costa,‡ Olivier Roubeau,§ Simon J. Teat,| Patrick Gamez,‡ Jan Reedijk,‡ and Laura Gasque*,† Departamento de Quı´mica Inorga´nica y Nuclear, Facultad de Quı´mica, UniVersidad Nacional Auto´noma de Me´xico, Ciudad UniVersitaria, Me´xico D. F. 04510, Me´xico, Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, UniVersite´ Bordeaux 1, Centre de Recherche Paul Pascal-CNRS, 115 aVenue du dr. A. Schweitzer, 33600 Pessac, France, and ALS, Berkeley Laboratory, 1 Cyclotron Road, MS2-400, Berkeley, California 94720
2008 VOL. 8, NO. 9 3187–3192
ReceiVed September 12, 2007; ReVised Manuscript ReceiVed May 29, 2008
ABSTRACT: The self-assembly between a chiral polydentate N8O4 ligand, containing a bis-imidazole unit substituted by 4 L-valine groups, and copper(II) ions leads to an optically pure, infinite M-helical chain. The X-ray diffraction studies revealed that the compound crystallizes in the hexagonal chiral space group P6122 with a single [HValbiim]5- unit coordinating three different copper(II) centers. Cu1 is in a distorted square-pyramidal environment, Cu2 is involved as well in a five-coordinated CuN2O3 chromophore but with a coordination geometry intermediate between a square pyramid and a trigonal bipyramid, and the third copper(II) ion, namely, Cu3, is in a slightly distorted square planar environment defined by three nitrogen atoms and one oxygen atom from two [HValbiim]5ligands. The resulting double bridge (namely, Cu2-O42′′ and Cu2′′-O42) gives rise to the formation of a hexanuclear entity. The found antiferromagnetic behavior, successfully modeled by an isosceles triangle, is consistent with the strong coupling of Cu1 and Cu2 through a hydroxido group and the coupling of Cu3 with each one of Cu1 and Cu2 through one of the imidazolato groups from the Biim2- moiety of the ligand.
1. Introduction Self-assembly is a fundamental process elegantly and efficiently used by nature to generate beautiful and intricate supramolecules. Helices represent outstanding examples of such supramolecular architectures, which can be found in various biochemical systems.1 During the past two decades, the study of helicates, that is, metal-containing helices,2 has turned into an important area of supramolecular chemistry.3,4 Helical complexes are not only the subject of much scientific interest because of aesthetic reasons but increasingly also for their potential applications in catalysis,5 magnetochemistry,6 or DNA groove binding.7 From the different synthetic approaches to generate artificial, self-organized helices, the use of coordination chemistry has proven to be quite effective.8 The formation of helicates relies on the nature of the organic ligands and the metal ion, the latter often being involved in the induction of chiral information, through its specific coordination geometry.9 The challenge of introducing chirality into the material can primarily be met by using chiral ligands.10 Multicarboxylate ligands have proven to be prime ligands to produce remarkable helices,11 including chiral ones.12,13 Optically pure aminoacids14 such as L-histidine have been successfully used to create chiral helical silver(I) coordination polymers.15 We previously reported16 a new potentially dodecadentate N8O4-donor ligand, that is, H6Valbiim · 9H2O (Chart 1, the potential donor atoms are labeled), and its tetranuclear copper(II) compounds as well as their solution behavior. Herein, the reaction of 2 equiv of CuCO3 · Cu(OH)2 with 1 equiv of this polynucleating ligand in methanol to yield the coordination compound [Cu3(HValbiim)(µ-OH)(H2O)](H2O)4 (1) is described. * To whom correspondence should be addressed. Tel/Fax: +52 55 5622 3811. E-mail:
[email protected]. † Universidad Nacional Auto´noma de Me´xico. ‡ Leiden University. § Centre de Recherche Paul Pascal-CNRS. | Berkeley Laboratory.
Chart 1. Schematic Representation of the Ligand H6Valbiim
2. Experimental Section 2.1. Materials. All reagents and solvents used in the syntheses were reagent grade and used without further purification. For the characterization of the ligand H6Valbiim and complexes 1, spectroscopic grade solvents were used. 2.2. Methods. C, H, and N analyses were carried out using an automatic Perkin-Elmer 2400 Series II CHNS/O analyzer. UV-visible spectra were recorded on an 8453 Agilent diode array spectrophotometer operating in the range of 190-1100 nm with quartz cells. Diffuse reflectance spectra were collected on a Varian Cary 5E UV-vis-NIR spectrophotometer. NMR spectra were recorded on a VARIAN Unity Inova Spectrometer (300.2 MHz) at 25 °C. 2.3. Magnetic Measurements. Variable temperature and field magnetic measurements were performed in the range of 2-300 K using a Quantum Design MPMS-7XL SQUID magnetometer in fields of 0.05-7 T. Corrections for diamagnetic portions of the complex, deduced from Pascal’s tables,17 and for the sample holder were applied. 2.4. X-ray Crystallography. For single-crystal X-ray diffraction, a tiny single crystal of 1 of size approximately 0.03 × 0.08 × 0.08 mm3 was selected for the X-ray measurements and mounted onto the diffractometer at ca. 150(2) K. The measurements were made using Si(111) monochromated synchrotron radiation (λ ) 0.69110 Å) and a Bruker APEX II CCD diffractometer using standard procedures and programs for Station 9.8 of Daresbury SRS.18 The intensity data were processed using SAINT version 7.06a.19 The structure was solved using direct methods with the SHELXTL program package.20,21 All nonhydrogen atoms were refined anisotropically. Hydrogens were found in difference Fourier maps and placed geometrically on their riding atom,
10.1021/cg700876d CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
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Table 1. Crystal Data and Structure Refinement for Coordination Compound 1 complex
1
empirical formula fw space group unit cell dimensions (Å, deg)
C30H56Cu3N8O14 933.40 P6122 a ) 15.270(1) b ) 15.270(1) c ) 61.093(5) R ) 90.00 β ) 90.00 γ ) 120.00 12336.6(14), 12 150(2) 0.69110 1.508 1.589 R1 ) 0.0399 wR2 ) 0.1039 R1 ) 0.0422 wR2 ) 0.1053
V (Å3), Z temp (K) λ (Å) Dcalcd (mg m-3) µ (mm-1) final R indices [I > 2σ(I)]a final R indices (all data)a a
R1 ) Σ|F0 - Fc|/|F0|. R2 ) {Σ[w(F0 - Fc ) ]/Σ[wF0 ]} . 2
2 2
4
1/2
except H12A, H21A, H30A, H39A, H39B, and H47A, which were refined with distance constraints with their carrying atom (hydroxide O47 and N12, N21, N30, and N39) and for H39A and H39B also with neighboring atoms (C38 and C40). Hydrogen atoms of the four water molecules could not be found and were therefore not included in the structural model. Several disordered solvent (water) molecules could not be modeled satisfactorily and were taken into account with the SQUEEZE option of PLATON. The absolute conformation of the chiral carbon atoms of the ligand (S) was in agreement with the used reagent. The function minimized was [w(Fo2 - Fc2)] with reflection weights w - 1 ) [2Fo2 + (g1P)2 + (g2P)] where P ) [max Fo2 + 2Fc2]/3. Details of the X-ray single-crystal analysis refinement are listed in Table 1. Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 649293. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, United Kingdom [fax: (+44)1223- 336-033; e-mail:
[email protected]]. 2.5. Preparation of 4,5-(L-Valine-N-methyl)-2-[4,5-(L-valineN-methyl)-1H-imidazol-2-yl]-1H-imidazole (H6Valbiim). 2-(1H-imidazol-2-yl)-1H-imidazole (H2Biim) was synthesized according to the preparation previously described by Schugar et al.22 H6Valbiim · 9H2O was prepared by condensation of H2Biim and 23 L-valine through a Mannich reaction (Scheme 1). A 7.03 g (60 mmol) amount of L-valine was introduced in a round-bottomed flask containing 100 mL of water. Aqueous KOH was added until the amino acid dissolved. To the resulting amino acid solution, 50 mL of an aqueous solution of formaldehyde (7.3 mL 37%, 90 mmol) was added dropwise. The pH of the ensuing mixture was adjusted to 12.5 with concentrated aqueous KOH. A 2.012 g (15 mmol) amount of 2,2′-bisimidazole (H2Biim) was subsequently added, and the mixture was stirred for 12 h at 55 °C and at room temperature for 5 days. The pH of the clear yellow solution obtained was adjusted to 5 with glacial acetic acid. The white precipitate formed was filtered and purified by Soxhlet extraction with water. Yield ) 17% (2.1 g). Optical activity in methanol at 25 °C: [R] ) +42. 1H NMR (300 MHz, D2O/DCl pH ) 1, TMS): δ 4.21 (s, 2H, Biim-CH2-N), 3.86 (d, J ) 3.9 Hz, 1H, R-CH), 2.15 (m, 1H, β-CH),
Figure 1. X-ray structure of the trinuclear repeating unit [Cu3(HValbiim)(µOH)(H2O)](H2O)4 (1). 0.83 (d, J ) 7.2 Hz, 3H, -CH3), 0.76 (d, J ) 6.9 Hz, 3H, -CH3) ppm. 13C NMR (75.5 MHz, D2O/DCl pH ) 1): δ 19.0 (-CH3), 20.6 (-CH3), 31.8 (β-CH), 43.3 (-CH2), 68.1 (R-CH), 130.4 (C 4 and 5 Biim), 140.4 (C 2 Biim), 172.6 (COOH) ppm. Anal. calcd for C30H50N8O8 · 9H2O: C, 44.3%; H, 8.4%; N, 13.8%. Found: C, 44.9%; H, 8.2%; N, 13.8%. TG analysis confirmed the water content by a loss of 20% of total mass at 150 °C. 2.6. Preparation of [Cu3(HValbiim)(µ-OH)(H2O)](H2O)4.1 A suspension of 0.25 mmol of CuCO3 · Cu(OH)2 (55 mg) and 0.125 mmol of H6Valbiim · 9H2O in 100 mL of methanol was refluxed for 4 days, after which the remaining undissolved material was filtered and the solution was left to rest. Lath-shaped green single crystals of 1 were obtained by slow evaporation of the methanolic solution. Yield ) 35% (45 mg). [Cu3(HValbiim)(µ-OH)(H2O)](H2O)4 (1): Anal. calcd for Cu3C30H48N8O10 · 4H2O: C, 38.19; H, 5.98; N, 11.88. Found: C, 38.48; H, 5.13; N, 11.48. Diffuse reflectance: λmax ) 346 and 386 nm (CT); 545 and 655 nm (d-d). TG analysis confirmed the water content (see Figure S1 of the Supporting Information).
3. Results 3.1. X-ray Crystallography. The enantiomerically pure polynucleating ligand H6Valbiim is prepared from L-valine and 2-(1H-imidazol-2-yl)-1H-imidazole (H2Biim), via a straightforward Mannich reaction. This ligand reacts with 2 equiv of CuCO3 · Cu(OH)2 in methanol and yields the coordination compound [Cu3(HValbiim)(µ-OH)(H2O)](H2O)4 (1), whose trinuclear unit is depicted in Figure 1. Small lath-shaped green single crystals of 1, suitable for X-ray diffraction analysis, were obtained by slow evaporation of a methanolic solution of the reactants after the undissolved material was filtered out. Compound 1 crystallizes in the hexagonal chiral space group P6122 and exhibits three different copper(II) centers, namely, Cu1, Cu2, and Cu3, coordinated by a single [HValbiim]5- ligand (exhibiting a protonated amine N39 function; see Figure 1). Details for the structure solution and refinement are summarized in Table 1, and selected bond lengths and angles are given in Table 2.
Scheme 1. Synthetic Route of H6Valbiim
Infinite Copper(II) Chiral Metallohelicate
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Table 2. Selected Bond Lengths (Å) and Angles (°) for 1 Cu1 Cu1-N1 Cu1-O15 Cu1-O48 N1-Cu1-N12 O15-Cu1-O47 N1-Cu1-O15 O48-Cu1-N1
1.979(5) 1.954(4) 2.457(5) 83.7(2) 92.9(2) 154.7(2) 93.8(2)
Cu1-N12 Cu1-O47 Cu1-Cu2 N12-Cu1-O15 O47-Cu1-N1 O15-Cu1-O48
2.032(5) 1.909(4) 3.540(1) 83.4(2) 101.3(2) 106.6(2)
Cu2 Cu2-N7 Cu2-O33 Cu2-O47 N7-Cu2-N30 O42′′-Cu2-O47 N7-Cu2-O42′′ O33-Cu2-N7
2.030(5) 2.153(5) 1.899(5) 81.4(2) 90.0(2) 146.0(2) 102.5(2)
Cu3-N4 Cu3-N21′ N4-Cu3-N10 N21′-Cu3-O24′
2.028(5) 2.042(5) 81.4(2) 83.4(2)
Cu2-N30 Cu2-O42′′
2.039(5) 2.077(4)
N30-Cu2-O42′′ O47-Cu2-N7 O42′′-Cu2-O33
92.5(2) 97.0(2) 109.4(2)
Cu3-N10 Cu3-O24′ N10-Cu3-N21′ O24′-Cu3-N4
1.970(5) 1.953(4) 91.4(2) 105.5(2)
Cu3
Within the trinuclear basic unit (Figure 1), Cu1 is in a distorted square-pyramidal environment (τ ) 0.33),24 whose basal plane is formed by two nitrogen atoms, one imidazolic nitrogen (N1) and one amino nitrogen (N12), one carboxylato oxygen atom (O15) belonging to one [HValbiim]5- ligand, and a bridging hydroxido oxygen atom (O47). The axial position is occupied by a water molecule (O48). The Cu-N and Cu-O bond distances are in normal ranges for such coordination geometry.25 The coordination angles reflect the distortion that most likely arises from the small bite angles of the tridentate N1,N12,O15unit[N1-Cu1-N12)83.7(2)°andN12-Cu1-O15 ) 83.4(2)°]. Cu2 is involved as well in a five-coordinated CuN2O3 chromophore but with a coordination geometry intermediate between a square pyramid and a trigonal bipyramid (τ ) 0.52).24 The coordination sphere of Cu2 consists of the donor atoms N7, N30, and O33 from the [HValbiim]5- ligand (which also coordinates to Cu1), and the bridging hydroxido oxygen atom O47 [the Cu1-Cu2 distance amounts to 3.540(1) Å]. The pentacoordination around Cu2 is completed by a carboxylato oxygen atom, that is, O42′′, from an adjacent trinuclear unit (Figures 1 and 2). The Cu-N and Cu-O bond lengths (see Table 2) can be considered as normal for this type of coordination environment.26 As mentioned above, Cu2 is connected to an adjacent
Figure 2. Dimerization of 1 generated by a double bridge via the coordination contacts Cu2-O42′′ and Cu2′′-O42.
trinuclear complex via the atom O42′′ of a different ligand; similarly, the neighboring Cu2′′ atom is coordinated by the oxygen atom O42 of the initial ligand. The resulting double bridge (namely, Cu2-O42′′ and Cu2′′-O42) gives rise to the formation of a hexanuclear entity, which is illustrated in Figure 2. The coordination of a donor atom (O42′′) from a second [HValbiim]5- ligand to Cu2 generates a stronger distortion (as compared to Cu1), which is reflected by the coordination mode of the N7, N30, and O33 unit (Figure 1). Indeed, while the atoms N1, N12, and O15 of the equivalent tridentate unit are almost in the same plane for Cu1, it is not the case for N7, N30, and O33 for Cu2. Actually, the atom O33 is now out of the plane enclosing the nitrogen atoms N7 and N30 (Figure 1). The angle between the N30 Cu2 N7 plane and the N30 Cu2 O33 plane amounts to 76°, whereas the corresponding value for Cu1 is 22°. The third copper(II) ion, namely, Cu3, is in a slightly distorted square planar environment defined by three nitrogen atoms and one oxygen atom from two [HValbiim]5- ligands (Figure 1). The Cu-N and Cu-O distances are typical of what is found for comparable CuIIN3O chromophores.27 As pointed out above, Cu3 is coordinated by a second ligand; as a result, Cu3 is bridging the hexanuclear unit to an adjacent hexanuclear unit, yielding a fascinating one-dimensional (1D) chain (Figure 3, left, and Figure S2 of the Supporting Information). This infinite polymeric chain is thus generated from hexacopper building units connected via two copper(II) ions (Cu3/Cu3′, Cu3′′/Cu3′′′, etc; Figure 3, left, and Figure S2 of the Supporting Information). The crystal packing of 1 along the c-axis (Figure 3, right, and Figure S3 of the Supporting Information) shows that the 1D chains are packed in a parallel manner. A close examination of the copper(II) ions along the chains reveals a remarkable arrangement of these metal centers around the polymer axis (Figure 3, left). Indeed, the CuII ions are bridged by the ligands into a lefthanded (M) helix, as evidenced in Figures 3 and 4. It is interesting to notice that all helices are M-configurated (see Figure 3), which obviously arises from the fact that an enantiopure chiral ligand has been used to produce these outstanding supramolecules. This feature clearly demonstrates that the stereochemistry of helical structures can indeed be controlled through the use of chiral ligands. Each loop of the helix is formed by 12 [HValbiim]5- ligands (Figure S4 of the Supporting Information). The parallel helices are wellseparated from each other, most likely as a result of steric constraints created by the isopropyl peripheral shields (from the valine groups). The consequent voids between the chains are fully occupied by water molecules (Figure 4), which are involved in an intricate hydrogen bonding network resembling those observed in protein crystal structures (see Figure S5 and Table S1 of the Supporting Information). 3.2. Magnetic Measurements. The magnetic susceptibility of the complex has been measured in the temperature range of 2-300 K. The magnetic properties of 1 are shown in Figure 5 under the form of χMT product vs T and M vs applied field at 2 K plots (χM being the molar magnetic susceptibility, e.g., per Cu3 unit, and M the magnetization). At 350 K, χMT is ca. 0.84 cm3 mol-1 K, a value significantly below that expected for three isolated S ) 1/2 spins (1.125 cm3 mol-1 K for g ) 2), indicative of dominant strong antiferromagnetic interactions. Indeed, χMT decreases upon lowering the temperature to reach a plateau at ca. 0.41 cm3 mol-1 K below 60 K, a value in agreement with a doublet spin state (0.375 cm3 mol-1 K for g ) 2). Below 20 K, a further decrease down to 0.22 cm3 mol-1 K at 2 K is
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Figure 3. (Left) Spatial arrangement of the copper(II) ions along the polymer, generating a left-handed infinite M-helix and (right) view along the c-axis.
Figure 4. Crystal packing of 1 along the c-axis showing the water molecules filling the voids between the helical chains.
observed, likely due to weaker antiferromagnetic interactions among these remaining doublet spins. A Curie-Weiss fit of the high temperature area (250-350 K, see Figure S6 of the Supporting Information) results in a large negative θ of -290 K and a low Curie-Weiss constant of 1.54 cm3 mol-1, both in agreement with strong antiferromagnetic interactions active at these temperatures. The presence in the trinuclear core of 1 of a single hydroxido bridge with a rather large Cu-O-Cu angle [Cu1-47-u2 ) 137.0(2)°] would account for the dominant strong antiferromagnetic coupling observed,28 while exchange coupling through the Biim2- bridges is also expected to be antiferromagnetic in nature, albeit weaker.29 These interactions would then result in an isosceles Cu(II) triangle (considering the Cu1-Cu3 and Cu2-Cu3 pairs to be similar) presenting at low temperatures a doublet ground state, as observed down to 20 K. The further decrease at lower temperatures can then only be ascribed to weak carboxylato bridges along the helix between Cu3 units. In agreement with these assumptions
based on magneto-structural considerations, the experimental data were correctly simulated with a model contemplating isosceles triangles of exchange-coupled S ) 1/2 spins weakly interacting in one dimension, this latter interaction being treated in the mean-field approximation. The Hamiltonian for an isosceles triangle of S ) 1/2 Cu(II) ions H ) -J1(S1 · S2 + S1′ · S2) - J2(S1 · S1′) was used to model the experiment susceptibility data. J2 represents here the coupling through the wide open hydroxido bridge, while we assume that both Cu1-Cu3 and Cu2-Cu3 interactions through the Biim2- moiety are similar and described by the interaction constant J1. The energies of the different state are then:
E(S,S ′ ) ) -J1S(S + 1) - (J2 - J1)S ′ (S ′ + 1) with S′ ) S1 + S1′, which yields the following expression for the susceptibility using the van Vleck equation:
Infinite Copper(II) Chiral Metallohelicate
χtri )
Ng2β2 4kBT
[ ( ) [ ( )
Crystal Growth & Design, Vol. 8, No. 9, 2008 3191
( )] ( )]
J1 - J2 3J1 + 10exp kBT 2kBT J1 - J2 3J1 1 + exp + 2exp kBT 2kBT
1 + exp
Note that for limiting the number of variable parameters, we consider identical Lande´ factors for the three Cu(II) ions. After including interaction among the [Cu3] units through mean-field approximation and a term for TIP, the expression
χ)
χtri + TIP 2zJ 1χ tri NAg2µB2
was used to reproduce the experimental data. The resulting best set of variable parameters was g ) 2.11(1), J1/kB ) -260(10) K describing both Cu1-Cu3 and Cu2-Cu3 interactions, J2/kB ) -335(5) K describing the Cu1-OH-Cu2 interaction, and zJ/kB ) 1.3(1) K corresponding to inter-[Cu3] interaction. The value of J2 is consistent with the value of the hydroxido bridge and the coordination environments of Cu1 and Cu2.28 Thus, while the high temperature behavior of 1 is dominated by the strong antiferromagnetic interaction through the hydroxide bridge, below ca. 55 K, 1 can be depicted as a unidimensional assembly of antiferromagnetically coupled S ) 1/2 spins. Confirmation of this magnetic chain behavior is found
Figure 5. χMT vs T plot for 1 (circles) and best fit to a model contemplating isosceles triangles of Cu(II) ions coupled in 1D (full line). The inset shows the magnetization vs field measurements at 2K saturating toward ca. 1 Bohr magneton, in agreement with a doublet spin ground state for the [Cu3] units.
Figure 6. Ln(χMT) vs 1/T plot for 1 in the 60-2000 range, showing a linear variation indicative of chainlike behavior.
in the linear lnχMT vs T-1 plot in this temperature range (Figure 6),30 in agreement with the rather isolated helices in the structure of 1.
4. Conclusions In summary, an unprecedented infinite M-helicate has been obtained by self-assembly between CuII ions and a chiral, amino acid-derived ligand. This study is now being extended to other ligands prepared from various natural amino acids (such as alanine or methionine). The investigations of the enantioselective catalytic and DNA-binding properties of compound 1 are currently carried out. Acknowledgment. We acknowledge the provision of time on the Small Molecule Crystallography Service at the CCLRC Daresbury Laboratory via support by the European Community. L.G. and E.M. thank DGAPA UNAM for economic support through Grant IN106003. Supporting Information Available: CIF file, TGA plot, crystal packing plots, H bond distances, χM-1 vs T plot for 1, and EPR spectrum of 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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CG700876D
