DOI: 10.1021/cg9014465
A Copper(II)-Cytidine Complex as a Building Unit for the Construction of an Unusual Three-Dimensional Coordination Polymer
2010, Vol. 10 1757–1761
Nadia Marino,†,# Donatella Armentano,† Teresa F. Mastropietro,† Miguel Julve,‡ Francesc Lloret,‡ and Giovanni De Munno*,† †
Centro di Eccellenza CEMIF.CAL, Dipartimento di Chimica - Universit a della Calabria, via P. Bucci anica/Instituto de 14/c, 87030, Arcavacata di Rende (CS), Italy, and ‡Departament de Quı´mica Inorg Ciencia Molecular (ICMol), Facultat de Quı´mica de la Universitat de Val encia, Polı´gono La Coma s/n, # 46980 Paterna (Val encia), Spain. Current address: Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100. Received November 19, 2009; Revised Manuscript Received February 22, 2010
ABSTRACT: The chiral [Cu(cyd)4]6- anion acts as a multiarmed complex-ligand toward auxiliary copper(II) centers leading to the first example of cytidinate-bridged three-dimensional (3D) coordination polymer of formula [Cu6(H2O)7(ClO4)3Cu(cyd)4](ClO4)3 (1). Single crystal X-ray analysis of 1 shows a unique 3D covalent network supported by the exclusive hypercoordination of the cytidinate ligand that bridges four crystallographically independent copper(II) ions via the N3, O2, O20 , O30 , and O50 set of atoms. Magnetic susceptibility measurements in the temperature range 1.9-295 K reveal the coexistence of ferro- and antiferromagnetic interactions within the hexacopper(II) core of 1, the exchange pathways being the di-μ-aqua and the singleand double-alkoxo bridges.
Introduction The design and synthesis of metal organic frameworks from biological ligands have been recently recognized as the interconnection between materials and living sciences, promoting the development of new materials for technological applications.1,2 Supramolecular structures incorporating biomolecules and metal ions as building blocks are relevant for several reasons. For instance, biological ligands can in principle be used to easily assemble controlled nanostructures or to generate acentric/chiral units in the coordination complexes that they form. Moreover, the presence of metal centers can account for the additional functional properties conferred upon such kinds of assemblies. Among the biomolecules, the constituents of the nucleic acids definitely represent resourceful ligands for the construction of polymers with tailored architectures and tunable properties, owing to the presence of several accessible donor sites and their ability to self-organize in high-ordered structures.2-5 Transition metal complexes containing natural nucleobases and their biologically more relevant derivatives have been thoroughly investigated, in particular as biomimetic systems. Most of these compounds are mononuclear compounds or discrete low-nuclearity polymeric species in which the supramolecular three-dimensional (3D) structure is achieved by means of noncovalent interactions.6 Some examples of polymeric structures containing nucleobases as terminal ligands have also been reported.3 On the contrary, the number of examples of covalent nD (n = 1-3) complexes based on bridging interactions of nucleobases is quite limited.2 Our strategy in this field is based on the use of mononuclear nucleoside-containing 3d metal ions as building blocks for the rational design of nucleoside-bridged high-nuclearity coordination compounds and high-dimensionality coordination
polymers. In the framework of our current research work concerning the reactivity of divalent first-row transition metal ions toward the cytidine nucleoside (H2cyd), we observed unprecedented metal-nucleoside coordination modes, which accounted for the formation of octanuclear calixarene-like4a and dodecanuclear globular-shaped copper(II) complexes.4b In both compounds, the monodeprotonated Hcyd- group acts as a bridging ligand between three different metal centers, the oxygen atoms of the ribose moiety being also unconventionally involved in the coordination. Aiming at enriching this family of cytidine-containing complexes with higher dimensionality species, we have recently explored the possibility to assemble the chiral mononuclear complex [Cu(H2cyd)4](ClO4)2 3 5H2O4b into extended structures, the idea being that it could be used as a multiarm metal-centered building block if the oxygen atoms of the ribose moiety were engaged in the metal coordination. Our attemtps along this line afforded the first cytidinate-bridged copper(II) 3D compound of formula [Cu6(H2O)7(ClO4)3Cu(cyd)4](ClO4)3 (1) whose preparation, crystal structure determination, and variable-temperature magnetic study are the subject of the present work. Experimental Section
*Corresponding author. Phone (þ39) 0984 492068. Fax: (þ39) 0984 493309. E-mail:
[email protected].
Materials and Methods. All chemicals were purchased from commercial sources and used as received without further purification. Elemental analyses (C, H, N) were performed by the Microanalytical Service of the Universit a della Calabria. The Cu:Cl molar ratio (7:6) in 1 was determined by electron probe X-ray microanalysis at the Servicio Interdepartamental of the University of Valencia. The water contents of 1 was determined by thermogravimetric analysis with a Perkin-Elmer Thermogravimetric Analyzer Pyris 6 TGA. The IR spectrum of 1 was recorded on a Perkin-Elmer 1750 FTIR spectrophotometer as KBr pellets in the 4000-400 cm-1 range. Magnetic measurements on a polycrystalline sample of 1 were carried out with a Quantum Design SQUID magnetometer in the temperature range 1.9-295 K, operating at 500 G (T e 100 K) and 1 T (T > 100 K). Diamagnetic corrections of all the constituent
r 2010 American Chemical Society
Published on Web 03/19/2010
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Crystal Growth & Design, Vol. 10, No. 4, 2010
Results and Discussion Description of the Structure of [Cu6(H2O)7(ClO4)3Cu(cyd)4](ClO4)3 (1). Compound 1 exhibits a unique 3D network with the chiral [Cu(cyd)4]6- unit acting as a multiarmed complex-ligand toward auxiliary copper(II) ions, whose coordination sphere is completed by either water molecules or monodentate perchlorate groups. Free perchlorate anions are also present in 1. The mononuclear [Cu(cyd)4]6- unit contains four cytidinate ligands chelating the metal ion through the usual N(3)/ O(2) binding mode4a,13 [2.839(5) and 2.036(5) A˚ for Cu(4)O(2) and Cu(4)-N(3), respectively] with the same unusual 4 þ 40 coordination around the Cu(4) atom that was observed in the cationic precursor [Cu(H2cyd)4]2þ.4b No sensible modifications of the bond distances and angles at the Cu(4) metal center are produced in the mononuclear unit. Each cytidinate coordinates simultaneously to the other threecrystallographically independent copper atoms [Cu(2), Cu-
Table 1. Crystal Data and Structure Refinement for Compound 1 empirical formula formula weight temperature crystal system space group unit cell dimensions volume Z density (calculated) absorption coefficient F(000) crystal size theta range for data collection index ranges
C36H58Cl6Cu7N12O51 533.11 293(2) K orthorhombic F222 a = 11.039(1) A˚ b = 22.784(3) A˚ c = 31.280(4) A˚ 7867.2(18) A˚3 4 1.800 Mg/m3 2.170 mm-1 4284 0.16 0.12 0.08 mm3 1.79 to 26.83° -13 < h < 13, -28 < k < 28, -38 < l < 39 36 496 4110 [R(int) = 0.0333] 97.8% semiempirical from equivalents 0.856 and 0.786 full-matrix least-squares on F2 4110/2/247 1.056 R1a = 0.0637, wR2b,c = 0.2075 R1a = 0.0740, wR2b,c = 0.2190 0.02(3) 0.00084(16) 1.135 and -1.087 e A˚-3
reflections collected independent reflections completeness to theta = 26.83° absorption correction max and min transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute structure parameter extinction coefficient largest diff peak and hole P P P a R1 = Fo| - |Fc / |Fo|. b wR2 ={ [w(Fo2 - Fc2)2]/[(w(Fo2)2]}1/2. c 2 2 2 w = 1/[σ (Fo ) þ (aP) þ bP] with P = [Fo2 þ 2Fc2]/3, a = 0.1732 and b = 13.2624. )
atoms of 1 were estimated from Pascal’s constants7 as -540 10-6 cm3 mol-1 [per seven copper(II) ions]. Corrections for the tip of the copper(II) ion (60 10-6 cm3 mol-1) and for the sample holder were also applied. Preparation of [Cu6(H2O)7(ClO4)3Cu(cyd)4](ClO4)3 (1). The reaction of Cu(ClO4)2 3 6H2O (0.7410 g, 2 mmol) and H2cyd (0.2432 g, 1 mmol) in water (15 cm3) afforded a blue solution whose the starting pH value was 6.0. X-ray quality crystals of the mononuclear complex [Cu(H2cyd)4](ClO4)2 3 5H2O as violet parallelepipeds were formed by slow evaporation at room temperature after 5 days. Lowquality blue crystals, that were identified as compound 1 according to a preliminary structural and elemental analysis, appeared after several weeks, in the solution as well as all over the violet crystals, which resulted almost totally replaced by microcrystals of 1 in ca three months. The polymerization reaction of the mononuclear complex [Cu(H2cyd)4](ClO4)2 3 5H2O in the presence of an excess of Cu(ClO4)2 3 6H2O under the same conditions led to 1 as a microcrystalline blue solid in a moderate yield (ca. 40%). X-ray quality blue rhombuses of 1 were then obtained by slow vapor diffusion of diethyl ether into a 1:1 (v/v) water/ethanol mixture containing Cu(ClO4)2 3 6H2O and H2cyd in a 2:1 Cu(II)/cytidine molar ratio. Yield about 50%. Anal. Calcd for C36H58Cl6Cu7N12O51 (1): C, 20.28; H, 2.74; N, 7.88. Found: C, 20.85; H, 3.02; N, 7.45%. Crystal Data Collection and Refinement. X-ray diffraction data on a single crystal of 1 were collected at room temperature on a Bruker-Nonius X8-APEXII CCD area detector diffractometer by using monochromatized Mo-KR radiation (λ = 0.71073 A˚), and they were processed through the SAINT8 reduction and SADABS9 absorption software. The structure was solved by the Patterson method and subsequently completed by Fourier recycling using the SHELXTL10 software packages and refined by the full-matrix leastsquares techniques on F2 with all observed reflections. All nonhydrogen atoms were refined anisotropically except some perchlorate-oxygen atoms (O4P and all the oxygen atoms from the Cl2 and Cl3/Cl3A perchlorate anions, respectively). The hydrogen atoms of the cytidinate ligands were set in calculated positions and refined as riding atoms. One of the perchlorate anions (Cl3-O) was found to be highly disordered in two positions, and two sets of chlorine and oxygen atoms were found with an occupancy factor of 0.5 and 0.25, respectively. It was located on a ΔF map and fixed after a first refinement of positions and thermal factor. Furthermore, a good model was found refining a water molecule (O3W) with an occupancy factor of 0.25 that was found to be disordered near the Cl3AO positions. The overall water and chlorine contents are in agreement with TGA and SEM analysis, respectively. The residual maximum and minimum in the final Fourier-difference maps were 1.135 and -1.087 e A˚-3. The final geometrical calculations and the graphical manipulations were carried out with the PARST9711 and DIAMOND12 programs, respectively. A summary of the crystallographic data and structure refinement is given in Table 1, and selected bond lengths and angles are listed in Table 2.
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)
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(3), and Cu(4)] by using all the hydroxyl-oxygen atoms of the ribose moiety [(O(20 ), O(30 ), and O(50 )] in an unprecedented fashion (Figure 1 and Scheme 1). Upon deprotonation, the O(20 ) and O(30 ) atoms adopt a bis-monodentate bridging mode within the Cu(2)/Cu(3) and Cu(1)/Cu(2) pairs, respectively [mean value of the Cu-O(0 ) distance of 1.915(4) A˚]. In addition, the remaining hydroxyl group on the ligand hooks up the Cu(1) atom [Cu(1)-(O50 ) = 2.172(7) A˚]. Consequently, two fused five-membered chelate rings occur at Cu(1) and Cu(2). The resulting hyper-coordination of the cytidinate (-2) ligand is significant, and it could be invoked among the driving-forces that allow the self-assembling process of the 3D structure. Although the chelation through the O(20 ) and O(30 ) hydroxyl groups of ribonucleosides is known,4a,14 this is the less frequent coordination mode. Moreover, the bis-monodentate bridging mode of O(30 ) for nucleosides is very uncommon, and as far as we know, only one previous example has been reported for an adenosine-containing mononuclear species.14e Concerning the cytidine nucleoside, this bismonodentate bridging mode was observed by some of us in an octanuclear and a dodecanuclear copper(II) species, but it concerned the O(20 ) ribose-oxygen atom.4 Another example in the solid state was reported for an octanuclear uridinatecontaining compound.14c The coordination of O(50 ) in 1 is noteworthy, because it was never observed, as far as we know, in transition metal complexes with unmodified nucleosides. To the best of our knowledge, only one example of this coordination mode exists in literature, and it concerns a one-dimensional polymer containing the guanosine-20 monophosphate nucleotide as bridging ligand.15 However, the chelation through the O(20 ) and O(30 ) atoms in such a case is precluded because the O(20 ) site is blocked by a phosphate group.
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Table 2. Selected Bond Lengths [A˚] and Angles [°] for Compound 1a Cu(1) environment Cu(1)-O(30 ) Cu(1)-O(50 ) O(30 )-Cu(1)-O(1w) O(30 )-Cu(1)-O(50 ) O(30 )-Cu(1)-O(1wb) O(30 )-Cu(1)-O(30 a) O(30 )-Cu(1)-O(50 a)
1.902(4) 2.172(7) 91.4(2) 91.1(2) 83.5(2) 173.1(2) 93.6(2)
Cu(1)-O(1w)-Cu(1b)
94.5(3)
Cu(2) environment Cu(2)-O(20 ) Cu(2)-O(3w) O(20 )-Cu(2)-O(30 ) O(20 )-Cu(2)-O(20 b) O(20 )-Cu(2)-O(30 b) O(20 )-Cu(2)-O(3w) O(30 )-Cu(2)-O(3w) O(30 )-Cu(2)-O(8PA)
1.890(4) 2.79(3) 86.4(2) 82.9(3) 162.3(2) 92.3(6) 96.4(6) 110.5(2)
Cu(3) environment Cu(3)-O(20 ) Cu(3)-O(2w) O(20 )-Cu(3)-O(2w) O(20 )-Cu(3)-O(6P) O(20 b)-Cu(3)-O(20 ) O(2wb)-Cu(3)-O(6P)
1.946(5) 1.954(6) 96.3(2) 90.6(2) 80.0(3) 80.9(5)
Cu(4) environment Cu(4)-N(3) N(3)-Cu(4)-O(2) N(3e)-Cu(4)-N(3) N(3c)-Cu(4)-O(2) N(3d)-Cu(4)-O(2)
2.036(5) 52.1(2) 93.7(3) 91.5(2) 144.1(2)
Cu(1)-O(1w) 0
2.245(5)
O(5 )-Cu(1)-O(1w) O(50 )-Cu(1)-O(50 a) O(50 )-Cu(1)-O(1wb) O(1wb)-Cu(1)-O(1w)
89.8(3) 95.4(4) 172.7(2) 85.5(3)
Cu(2)-O(30 ) Cu(2)-O(8PA) O(30 b)-Cu(2)-O(8PA) O(30 )-Cu(2)-O(30 b) O(20 b)-Cu(2)-O(3w) O(30 b)-Cu(2)-O(3w) O(20 )-Cu(2)-O(8PA) O(20 b)-Cu(2)-O(8PA)
1.923(4) 2.8057(4) 87.0(2) 107.1(2) 98.0(6) 75.3(6) 77.4(2) 80.9(2)
Cu(3)-O(6P) 0
2.4758(3)
O(2 )-Cu(3)-O(2wb) O(20 b)-Cu(3)-O(6P) O(2w)-Cu(3)-O(6P) O(2wb)-Cu(3)-O(2w)
170.3(4) 96.2(2) 92.8(4) 88.6(4)
Cu(4)-O(2) N(3c)-Cu(4)-N(3) N(3d)-Cu(4)-N(3) N(3e)-Cu(4)-O(2)
2.839(5) 88.7(3) 163.4(3) 77.2(2)
a Symmetry transformations used to generate equivalent atoms: (a) -x, y, -z; (b) 2 -x, -y, z; (c) 3 -x - 1/2, -y þ 1/2, z; (d) -x - 1/2, y, -z þ 1/2; (e) x, -y þ 1/2, -z þ 1/2.
Scheme 1. Coordination Modes of Hcyd- (a-c) and cyd2- (d) in the Dodecanuclear (a and b), Octanuclear (c) and Three-Dimensional (d) Copper(II) Compounds of Formula [Cu(H2O)6][Cu8(Hcyd)12(CO3)2](ClO4)8 3 11H2O,4b [Cu12(Hcyd)8(CF3SO3)4](CF3SO3)6 3 12H2O,4a and [Cu6(H2O)7(ClO4)3Cu(cyd)4](ClO4)3 (1), respectively
Figure 1. A view of the chiral [Cu(cyd)4]6- unit showing the multiple coordination of the cytidinate dianion with the atom numbering scheme.
The 3D framework of 1 which is shown in Figure 2 can be alternatively described as a periodic arrangement of cationic [Cu6(cyd)4(ClO4)3(H2O)7]þ hexanuclear units connected by copper(II) ions (Figure 3). Within the hexanuclear core, a diμ-aquadicopper(II) motif is formed by two symmetry-related copper atoms [Cu(1)-O(1w) = 2.245(5) A˚] exhibiting an overall rare compressed octahedral environment. Concerning the environment of the other two crystallographically independent copper(II) ions of this core [Cu(2) and Cu(3)], the equatorial positions are defined by O(20 ), O(30 ), O(20 a), and O(20 ) [at Cu(2)] and O(30 a), O(2w), O(20 a), and O(2wa)
[at Cu(3)] set of atoms. Because of disorder, it was not possible to clearly define the axial positions, which involves perchlorate anion and water molecules. Our best model is
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Figure 4. Temperature dependence of χMT for 1: (O) experimental data; (b) and (—) best-fit curves through eqs 1-3 with θ = 0 (b) and -0.6 K (__) (see text). The inset shows the magnetic coupling scheme within the hexanuclear core.
Figure 2. (a) A view along the crystallographic a axis showing a fragment of the 3D framework of 1. The perchlorate groups have been omitted for clarity. (b) Schematic representation of the connectivity of the copper(II) ions in 1 with the copper atom from the [Cu(cyd)4]6- building block being highlighted in dark blue: (left) a view of the connectivity among the copper atoms along the crystallographic [110] direction; (right) a simplified view of the network along the same direction.
Magnetic Properties. The magnetic properties of 1 in the form of χMT versus T plot [χM is the magnetic susceptibility per seven copper(II) ions] are shown in Figure 4. At room temperature, χMT is 2.50 cm3 mol-1 K, a value which is below that expected for seven magnetically isolated spin doublets (χMT = 2.89 cm3 mol-1 K with SCu = 1/2 and g = 2.10). This value continuously decreases upon cooling, and it exhibits an incipient plateau at ca. 30 K and further decreases to a value of 0.96 cm3 mol-1 K at 1.9 K. This curve is typical of an overall antiferromagnetic coupling. In the light of the structure and the magnetic data of 1, its spin coupling pattern could be described by the Hamiltonian of eqs 1-3 H ¼ Hex þ HZee
ð1Þ
Hex ¼ -J11 ðSCu1a : SCu1 Þ -J12 ðSCu1a : SCu2 þ SCu1 : SCu2 þ SCu1 : SCu2a þ SCu1a : SCu2a Þ -J23 ðSCu2a : SCu3a þ SCu2 : SCu3 Þ ð2Þ HZee ¼ gβHðSCu1 þ SCu2 þ SCu3 þ SCu1a þ SCu2a þ SCu3a þ SCu4 Þ
Figure 3. View of the hexanuclear [Cu6(cyd)4(ClO4)3(H2O)7]þ core in 1.
represented by three terminally bound perchlorate groups and a coordinated water molecule around the four axial positions at Cu(2)/Cu(2a) and Cu(3)/Cu(3a) with a 4 þ 1 coordination.
ð3Þ
the magnetic interactions being restricted to the hexanuclear core shown in the inset of Figure 4. An additional term (θ) is introduced to account for the magnetic coupling between this hexanuclear unit and the outer Cu(4) atom. Least-squares best-fit parameters are J11 = þ6.3 cm-1, J12 = -84.5 cm-1, J23 = -22.5 cm-1, θ = -0.6 K, and g = 2.08. In order to avoid overparametrization, a common g value was assumed for the four crystallographically independent copper(II) ions. The theoretical curve (solid line in Figure 4) matches very well the magnetic data in the whole temperature range investigated. For this set of coupling values, the hexameric core would exhibit a triplet as ground spin state with a singlet as first excited level. This is illustrated by the dotted curve in Figure 4 (calculated plot using the above parameters with θ = 0 K), where χMT at the lowest temperature tends to ca. 1.50 cm3 mol-1 K [a value which corresponds to the sum of the spin triplet (ca. 1.1 cm3 mol-1 K) and the magnetically isolated spin doublet (ca. 0.40 cm3 mol-1 K)]. The presence of a minimum of χMT in this calculated curve is due to the existence of the excited spin singlet. The computed value of θ (-0.6 K) has to be
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considered as the upper limit for the interaction between the hexameric units through the Cu(4) atoms because the zerofield splitting of the spin triplet was discarded. The strongest antiferromagnetic coupling in 1 (J12 = -84.5 cm-1) corresponds to the magnetic coupling between the Cu(1) and Cu(2) atoms through the single O(30 ) riboseoxygen bridge, the angle at the bridge being 128.9(3)°. This relatively strong antiferromagnetic coupling is in line with those reported for the parent single hydroxo-bridged dicopper(II) complexes (-J values covering the range 303-1000 cm-1 with values of the angle at the hydroxo bridge varying between 128.1 and 145.7°).16 As far as the magnetic interactions through the double ribose-oxygen [J23 = -22.5 cm-1 with a value of the angle at the bridgehead O(20 ) atom of 98.5(3)°] and di-μ-aqua- [J11 = þ6.3 cm-1 with a value of the angle at the bridgehead O(1W) atom of 94.5(3)°] dicopper(II) fragments, their nature is as expected in the context of the well-known magneto-structural correlation concerning the bis(μ-hydroxo)dicopper(II) complexes.17,18 For such a correlation, singlet and triplet ground spin states are predicted for values of the angle at the hydroxo bridge above and below 97.5°, respectively. The pattern of the magnetic couplings within the hexanuclear unit of 1 leads to a low-lying spin triplet whose zero-field splitting and/or magnetic interaction with the spin doublet of the outer Cu(4) ion would account for the decrease of χMT in the domain of the very low temperatures. Conclusions In summary, we provided the first example of a metal organic framework showing a unique 3D architecture based on a multiple coordinated nucleoside. It exhibits an unprecedent deprotonation state of cytidine (-2), which affords a unique hyper-coordination of the nucleoside via N3, O2 and O20 , O30 , O50 atoms . Undoubtedly, the multiple coordination of the cytidine ligand is significant and could be invoked among the driving-forces that allow the self-assembling process of the extended 3D covalent framework. Apart from the aesthetic structure of 1, this work supports the idea that nucleosides can be easily used in the construction of polynuclear compounds. The mononuclear [Cu(H2cyd)4]2þ cation as metal assembling unit would constitute a good strategy, having in mind that the four “arms” can be extended by using two or more nucleoside-containing ligands, to regulate the network topology. Acknowledgment. Financial support was received from the Ministero dell’Istruzione, dell’Universit a e della Ricerca Scientifica (MiUR) through the Centro di Eccellenza CEMIF. CAL (CLAB01TYEF) and the Spanish Ministerio de Ciencia e Innovaci on through the projects CTQ2007-61690 and Consolider Ingenio CSD2007-00010 and the Generalitat Valenciana through the project Prometeo/2009/108. Supporting Information Available: X-ray crystallographic file in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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