Hydrogen-Bonding Network in Metal-Pterin Complexes: Synthesis and Characterization of Water-Soluble Octahedral Nickel and Cadmium Pterine Derivatives
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1597-1601
Alessandra Crispini,* Daniela Pucci,* Anna Bellusci, Giovanna Barberio, Massimo La Deda, Antonella Cataldi, and Mauro Ghedini Department of Chemistry, University of Calabria, Via P. Bucci, Arcavacata di Rende (CS), Italy Received March 1, 2005;
Revised Manuscript Received May 13, 2005
ABSTRACT: New octahedral pterine complexes of the general formula [M(en)(pterin)2] (M ) Ni (1), Cd (2)) have been synthesized by reaction between the pterin ligand and ethylenediamine complexes of Ni(II) and Cd(II) in a basic aqueous solution. The newly synthesized complexes are surprisingly water soluble, despite the scarce solubility shown by most pteridine complexes reported up to now. In particular, the Ni(II) derivative (1) has been crystallized from water solution, giving rise to very regular brown-yellow needles suitable for single-crystal X-ray analysis. Noncovalent interactions can be identified in the molecular organization of 1 in the 3D space, such as hydrogen bonds of O-H- - -O, N-H- - -N, and N-H- - -O types and π-π interactions, which led to a new metallosupramolecular system. The photophysical characterization of these new pterin derivatives highlights their excellent luminescent properties in solution, confirming the metal-binding ability of the pterin ring and the stability of these systems in solution. Introduction Control of the supramolecular solid-state organization of individual molecules is one of the most important ways to develop novel functional materials. In this field the use of molecular self-organization processes for the development of self-assembled and self-organized structures has attracted attention both in biology and in materials chemistry.1 Molecular organization into one-, two-, or three-dimensional networks is mainly driven by noncovalent interactions, of which hydrogen bonding (of both the strong and the weak variations) plays an important role.2,3 Weak hydrogen bonding can play a role in enzymatic reactions because it is capable of modulating the bonding situation in catalytic sites to a significant degree, thus influencing catalytic properties.2b The construction of metallosupramolecular systems based on these types of intermolecular interactions is becoming of wide interest nowadays. Pteridine ligands have been used to construct metallosupramolecular H-bonded systems, which gave insights in order to explain the cooperative interaction between electron transfer and proton transfer in hydrogen-bonded chargetransfer systems.4 Pterins can form redox-active metal chelates through the O(4) and N(5) atoms (Chart 1) and information on pterin-metal interactions and spectral and electrochemical properties of the various coordination complexes has been recently reviewed.5 Pteridines and their structurally related flavins and pterins are biochemically important as enzyme cofactors, capable of undergoing one- and two-electron-transfer reactions,6 and they can also act as physiological receptors of UV radiation.7 The notoriously poor solubility of metal* Towhomcorrespondenceshouldbeaddressed.(A.C.)Tel: 390984492888. Fax: 390984482066. E-mail:
[email protected]; (D.P.) E-mail: d.pucci@ unical.it.
Chart 1.
Pterin and Pterin Complexes 1 and 2
containing pterin complexes, caused by the presence of multiple intermolecular hydrogen bonds, represents a serious limitation in the studies of correlation between properties and solid-state organization. For this reason there are few X-ray-determined structures of complexes containing pterin ligands.4,8 Most of the structural characterizations of metal complexes have been carried out using pterins in which the amino function in the 2-position has been modified with alkyl groups9 or, in the case of studies by Moreno-Carretero and co-workers, using lumazine methylated ligands, whose solubility in organic solvents or water enabled them to isolate crystals of a series of metal-pteridine complexes.10-13 We have recently been interested in the synthesis of new Ni(II) octahedral complexes containing N,O chelating ligands whose ability to be intercalators (extended aromatic systems) can promote in these new metal derivatives some of the chemical and physical properties necessary for metallointercalation.14 As a variation of these studies, we decided to build up similar octahedral systems of Ni(II) and Cd(II) but containing pterins as N,O chelating ligands. Ni(II) and
10.1021/cg050075k CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005
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Cd(II) were selected for our studies, since they are wellknown to be highly toxic to living organisms and their mechanism of action can be found in the studies of the interaction between heavy metals and nucleic acids and proteins and in the modifications of their functions.15 Therefore, we report here the synthesis and the characterization of the new pterin complexes [M(en)(pterin)2], where M ) Ni, Cd and en ) ethylenediamine (Chart 1). The Ni(II) complex 1 has been structurally characterized, and its solid-state features are discussed with reference to the presence within the molecule of several synthons for the formation of a metallosupramolecular system. Moreover, the photophysical characterization of the new complexes reported herein has pointed out the excellent luminescent properties in solution of these new pterin derivatives. Experimental Section General Methods. All reactions and manipulations were performed in air with reagent-grade solvents unless otherwise noted. The infrared spectra were recorded on a Spectrum One FT-IR Perkin-Elmer diffuse reflectance spectrometer. Elemental analyses were performed with a Perkin-Elmer 2400 microanalyzer by the Microanalytical Laboratory at the Universita` della Calabria. Spectrofluorimetric grade water (Acros Organics) was used for the photophysical investigations in solution, at room temperature. A Perkin-Elmer Lambda 900 spectrophotometer was employed to obtain the absorption spectra, while the corrected emission spectra, all confirmed by excitation spectra, were recorded with a Perkin-Elmer LS 50B spectrofluorimeter, equipped with a Hamamatsu R928 photomultiplier tube. Emission quantum yields were determined using the optically dilute method16 on aerated solutions where absorbance at excitation wavelengths was 2σ(I) no. of refined params goodness of fit R indicesa,b R indices (all data) a
C14H26N12O7Ni 533.18 monoclinic P2/c 7.8179(16) 9.4709(19) 15.568(3) 91.16(3) 1152.5(4) 2 1.536 0.904 298 556 2.15-26.05 2465 2293 (R(int) ) 0.0272) 1650 157 1.029 R1 ) 0.0650, wR2) 0.1828 R1 ) 0.0862, wR2) 0.1949
R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.
systematic absences, attempts to solve the structure in both acentric and centric systems were made. The best solution was found in the P2/c space group. The structure solution and fullmatrix least-squares refinements based on F2 were performed with the XS and XL routines in the SHELXTL program package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included as idealized atoms riding on the respective nitrogen, oxygen, and carbon atoms, with N-H, O-H, and C-H bond lengths appropriate to the atom hybridization.
Results and Discussion Synthesis. The reaction between [M(en)2Cl2] (M ) Ni, Cd) and 2 equiv of pterin was conducted in a basic aqueous solution in order to deprotonate the ligand and at the same time solubilize it in water. New octahedral complexes of general formula [M(en)(pterin)2] (1, 2) were synthesized and obtained in good yield. The coordination of the pterin ligand has been confirmed by IR spectroscopy. In fact, the IR spectra of both complexes 1 and 2 showed substantial shifts to lower frequencies in the region 1730-1550 cm-1 compared to the free ligand (CdO stretching at 1727 and 1693 cm-1) and the appearance of new intense absorptions in the region 1400-1600 cm-1. These general features are typical of deprotonation of the neighboring protonated endocyclic nitrogen atom in pteridine ligands and, therefore, complexation.9 In both cases the results of the elemental analysis were correct for the [M(en)(pterin)2] stoichiometry and indicated that two chlorine ligands and one ethylenediamine ligand from the precursors were substituted by two pterinates. In the case of 1 the presence of cocrystallized water molecules was also shown by the elemental analysis data. Rather surprisingly for a pterin derivative, complex 1 was found to be very soluble and stable in water, from which it crystallized easily, always giving rise to very regular brown-yellow needles suitable for single-crystal X-ray analysis. Crystal Structure of 1‚5H2O. Single-crystal X-ray analysis performed on 1 confirmed the stoichiometry initially postulated from the analytical data and the presence of five cocrystallized water molecules. A view
Metal-Pterine Complexes
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Figure 2. Packing diagram of 1 viewed down the b axis showing π-π interactions and N-H- - -N hydrogen bond types. Figure 1. Perspective view of [Ni(en)(pterin)2] (1) with thermal ellipsoids (59% probability) and numbering scheme. Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 1 Ni-N(1) Ni-N(11) N(1)-Ni-O(1) O(1)-Ni-N(11) N(1)-Ni-O(1a)
2.109(4) 2.056(4)
Ni-O(1) O(1)-C(7)
80.10(14) 94.24(16) 91.22(14)
N(1)-Ni-N(11) O(1)-Ni-O(1a) N(1)-Ni-N(11a)
2.098(3) 1.267(5) 95.44(16) 168.16(18) 174.49(17)
of the structure of complex 1 is shown in Figure 1. Selected bond distances and angles are reported in Table 2. The coordination about Ni, located on the C2 axis, is a distorted octahedron. Two pterinate molecules are bound to nickel through their nitrogen and oxygen atoms in a chelate fashion and in a cis conformation. In fact, the two five-membered chelate rings Ni-N-C-C-O, obtained through chelation of the pterinate ligands, are nearly orthogonal to each other, with a dihedral angle between the two best planes of 79.6(8)°. The cis arrangement of the pteridine ligands in octahedral complexes is not common and, up to now, has been found in two Cu(II) derivatives of general formula [M(bidentate)2(monodentate)2], containing 1,3dimethyllumazine and 1-methyllumazine as chelating ligands and two water molecules as monodentate ligands.11,19 Within the five-membered ring Ni-N-C-C-O, the Ni-N(1) bond distance is 0.012 Å longer than that of Ni-O(1) and both distance values as well as the bite angle of 80.1(1)° are in agreement with those observed in 8-quinolinate Ni(II) octahedral complexes and in the Ni(II) 4-hydroxyacridine derivative recently reported by us.14 The octahedral coordination sphere of the Ni(II) ion in 1 is completed by one molecule of ethylendiamine in a λ conformation. The beauty of the crystal structure of 1 became evident on analyzing the way in which the molecules of 1 organize themselves in the 3D space together with the five cocrystallized water molecules. The crystal packing of 1 contains various different types of noncovalent interactions (from strong to weak hydrogen bonds, π-π interactions) which all contribute to generate a splendid example of a metallosupramolecular system.
Table 3. Geometrical Data of the Hydrogen-Bonding Network for Complex 1 N-H‚‚‚Na
d(H‚‚‚N)/Åb d(N‚‚‚N)/Å ∠N-H‚‚‚N/deg
N(12)-H(12b)‚‚‚N(8)i
2.16 (2.00) N(11)-H(11a)‚‚‚N(10)ii 2.51 (2.41) N-H‚‚‚Oa N(12)-H(12a)‚‚‚O(3)i N(11)-H(11b)‚‚‚O(2)iii
2.993(6) 3.224(6)
162.0 137.2
d(H‚‚‚N)/Å d(N‚‚‚O)/Å ∠N-H‚‚‚O/deg 2.30 (2.14) 2.53 (2.42)
3.123(9) 3.314(5)
159.7 146.6
O-H‚‚‚Oa
d(H‚‚‚O)/Å
d(O‚‚‚O)/Å
∠O-H‚‚‚O/deg
O(2)-H(2w)‚‚‚O(3)iv
1.90 (1.99)
2.860(10)
150(4)
Symmetry codes: (i) -x, -y, 2 - z; (ii) 1 - x, -y, 2 - z; (iii) 1 - x, y, 1 + (1/2 - z); (iv) x, y, z. b Normalized values in brackets. a
In the crystal packing the coordinated pterin ligands of 1 contribute to determine a two-dimensional network of intermolecular interactions, as presented in Figure 2. The presence of the amino group in an ortho position with respect to the N(8) atom in the pterin rings is wellknown to be an important synthon for the formation of hydrogen-bonded self-assembling patterns.20,21 In fact, neighboring molecules of 1 are linked through pairs of hydrogen bonds of N-H‚‚‚N type, affording an eightmembered ring (an R22(8) ring in the graph-set notation22) (Table 3). The dimers thus formed communicate to each other through π-π interactions between the aromatic rings of the pterin ligands, which are stacked at an average interplanar distance of 3.5 Å (Figure 2). The 3D nature of the supramolecular architecture is better understood by observing the crystal packing in the bc plane. As shown in Figure 3a, along the c direction there is a continuous row of water molecules, connected to each other between strong hydrogen bonds (Table 3). The repetition motif along the b axis is such that two rows of molecules of 1 are separated and held together by a row of water molecules. The interactions between water molecules and 1 are based on N-H‚‚‚O hydrogen bonding, which originates from the molecular functionality NH2 on both the pterin and the ethylendiamine ligands of 1 (Table 3). The view along the b direction highlights the way in which the π-π interactions propagate through the crystal. The offset face-to-face (OFF) stacking, which is a primary motif between
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Figure 3. Packing diagram of 1 viewed down the b axis showing (a) the hydrogen bond network and (b) the OFF zigzag chain motif between planar aromatic units.
Figure 4. Absorption and emission spectra in water solution. Table 4. Electronic Absorption and Emission Spectral Data for Synthesized Complexes water soln
1 2
abs λ/nm (/M-1cm-1)
em λ/nm (Φ)
solid state abs λ/nm
255 (23 500), 365 (10 050), 585 (10) 250 (23 580), 358 (10 150)
455 (0.20) 450 (0.12)
263, 384 260, 378
planar aromatic units,23 generates an extended OFF zigzag chain motif where the cleft between pterine planes is perpendicular to the propagation axis (Figure 3b). Photophysical Characterization. The spectral properties in the UV and visible range have been examined in water solution and in the solid state, to study the electronic structures of the synthesized complexes in a common biological solvent in relation to their crystalline properties. The absorption spectrum of 1 in water showed two principal bands, at 255 and 365 nm, and a very weak band at about 585 nm; these intense bands in the UV range are slightly blue-shifted in the case of the cadmium derivative 2 (Table 4 and Figure 4). These spectral features are almost identical with those found in the electronic spectrum of the pterinate (Figure 4) and are therefore assigned to π-π* excitations in the aromatic ligand fragment.24 The weak band at 585 nm ( ) 10) in the spectral pattern of 1 is attributable to a d-d transition, due to the presence of the Ni(II) ion,
Figure 5. Absorption spectra of complexes 1 and 2 in the solid state (KBr disks).
and it is typical of Ni(II) complexes, as confirmed by the absence of the same band in the spectrum of complex 2. An interesting aspect of these pterin complexes is their luminescence in solution. Both compounds showed an emission peak at about 450 nm, with a 5420 and 5710 cm-1 Stokes shift for 1 and 2, respectively (Figure 4). In both cases the luminescence is attributed to a deactivation from the lower π-π* ligand-localized state, as suggested from the emission spectrum of the pterinate (Figure 4); the presence of the d-d level in complex 1 does not perturb the radiative deactivation, an aspect that would be worthy of a deeper investigation. The luminescence quantum yield (Φ) of these complexes in solution (Table 4) is slightly less than that measured for the pterinate (Φ ) 0.30) and, as expected, this attenuation is caused by the presence of the heavy atom. Due to the relevance of the crystal structure, the photophysical investigation was also performed in the solid crystalline state. Even if the absorption bands are now found to be slightly red-shifted (Figure 5), the absorption spectra of complexes 1 and 2 showed the same features recorded in solution, confirming that the electronic structure in both complexes remains unvaried. Therefore, we believe that the intermolecular interactions may play the same role in both aggregation states, with the only difference being the strength and the number of the contacts in the solid state in com-
Metal-Pterine Complexes
parison with the solution state. Unfortunately, no luminescence was recorded from the solid crystalline samples of complexes 1 and 2, since in the way in which the samples have been prepared the main observation was the scattered light. Conclusions In this work, we have proved that through reaction between the pterin ligand and ethylenediamine complexes of Ni(II) and Cd(II) in a basic aqueous solution, it is possible to obtain and isolate octahedral pterin complexes in good yields. The newly synthesized complexes 1 and 2 retain one ethylenediamine molecule in the structure. This is confirmed by the analysis of the molecular structure of the nickel derivative 1. In the molecular organization of 1 in the 3D space various noncovalent interactions can be identified: hydrogen bonds of O-H- - -O, N-H- - -N, and N-H- - -O types and π-π interactions with an average interplanar distance of 3.5 Å, which led to an offset face-to-face (OFF) zigzag chain motif along the c direction (Figure 3b). The choice of the ethylenediamine ligand in the octahedral coordination sphere of these complexes has had two important consequences: (i) to improve or induce solubility (particularly in water) in pterin complexes that are otherwise extremely insoluble and (ii) to introduce additional synthons in the generation of supramolecular arrangements. Complexes 1 and 2 have been spectroscopically characterized in water solution, and their electronic molecular properties are very similar and comparable with those of the deprotonated pterin ligand. In particular, the luminescence of the pterinate is retained in the derived complexes, and this is a further confirmation of the metal-binding ability of the pterin ring and the stability of these systems in solution. The study of the solid samples of 1 and 2 showed no significant differences in comparison with their spectra recorded in solution, confirming that the molecular electronic features, on the basis of the intermolecular interactions observed in the solid crystalline state of 1, are the same in both aggregation states. Moreover, since the absorption spectra of complexes 1 and 2 are almost identical, it can be inferred that the analogous intermolecular interactions found in 1 are present in the molecular structure of the cadmium complex 2, giving rise to similar photophysical properties. Acknowledgment. This work was partly supported by the Ministero dell’Istruzione, dell’Universita` e della Ricerca through the Centro di Eccellenza CEMIF.CAL grant. Supporting Information Available: X-ray crystallographic information file (CIF) for complex 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG050075K
