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Aust. J. Chem. 2009, 62, 1614–1621
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A ‘Butterfly’-Shaped Water Tetramer in a Cu4 Complex Supported by a Hydrazone Ligand: Synthesis, Crystal Structure, Magnetic Properties, and Quantum Chemical Study Sambuddha Banerjee,A Soma Sen,A Joy Chakraborty,A Ray J. Butcher,B Carlos J. Gómez García,C Ralph Puchta,D,E and Samiran MitraA,F A Department
of Chemistry, Jadavpur University, Kolkata-700 032, India. of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC, 20059, USA. C Instituto de Ciencia Molecular (ICMol). Parque Científico. Universidad de Valencia, 46980 Paterna (Valencia) Spain. D Inorganic Chemistry, Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany. E Computer Chemistry Center, Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Nägelsbachstr 25, 91052 Erlangen, Germany. F Corresponding author. Email:
[email protected] B Department
A potentially tetradentate NOOO donor hydrazone ligand, LH2 (condensation product of benzhydrazide with O-vanillin) generates a tetranuclear CuII complex [Cu4 (L)4 ]·4H2 O (1), whose void spaces are occupied by water tetramers presenting a ‘butterfly’ conformation with the highest dihedral angle reported to date, as revealed by its X-ray crystal structure. 1 has also been characterized using various spectroscopic techniques, including IR, UV-vis, and elemental analysis. Variable temperature magnetic susceptibility measurements reveal the presence of moderate antiferromagnetic intratetramer coupling between the four CuII centres connected through simple oxo groups of the hydrazone ligand with two different coupling constants (J1 = −61.7(3) cm−1 and J2 = −92(1) cm−1 ) corresponding to the two different CuII tetramers identified in the X-ray structure. We also report a quantum chemical study (MP2(full)/6–311+G(3df,2p)//B3LYP/6– 311+G(3df,2p)) to calculate the stability of the water tetramers. Manuscript received: 6 April 2009. Manuscript accepted: 10 July 2009.
Introduction The synthesis and characterization of polynuclear CuII coordination complexes with different nuclearties has been, and remains, one of the most important areas in coordination chemistry, due to the huge number of different structures, functionalities, and magnetic properties observed in CuII coordination complexes.[1,2] From the structural point of view, there are thousands of characterized CuII complexes with one, two, and three dimensions; presenting weak, moderate, or strong magnetic couplings (either ferro or antiferromagnetic). If we restrict our search to tetranuclear CuII complexes, we can find several hundreds, of which only 24 present exclusively simple oxo-bridges. In this area, we have recently reported a CuII tetranuclear complex with the same hydrazone ligand having a distorted cubane structure with a µ3 -phenoxo bridge.[3] In our ongoing research on phenoxo-bridged transition metal complexes,[4–6] here we describe the synthesis of a completely different metalligand arrangement, showing µ2 -phenoxo bridges, obtained with the same metal and ligand but with a slight modification of the experimental set up which yielded a very different tetranuclear
complex. As reported earlier,[7a] in phenoxo-bridged CuII complexes, the J value depends on (i) the Cu–O(phenolate)–Cu angle, (ii) the Cu–Cu distance, (iii) the geometry around the phenolate oxygen atom, and (iv) the copper coordination geometry. A very comprehensive overview on tetranuclear CuII complexes is also available.[7b] Variable temperature magnetic (VTM) susceptibility measurements of the title complex reinforced these points. Another research area enjoying increased interest, is the study of small water clusters that act as bridges between single water molecules and liquid water or ice. The aim of such research is many fold: from understanding numerous ‘anomalous’ behaviours of bulk water, to probe its role in the stabilization and action of bio-molecules; from unravelling the roles in chemical processes, to the design of new materials and the exploration of their possible structures and stability in different environments. H-bonding interactions and their fluctuations determine the properties of water in bulk, with associated high directionality leading to a reduced number of neighbours (∼4) around each water molecule, in contrast with other solvents
© CSIRO 2009
10.1071/CH09192
0004-9425/09/121614
‘Butterfly’-Shaped Water Tetramer in a Cu4 Complex Supported by a Hydrazone Ligand
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C(13A) C(12A)
C(14A)
C(15A)
C(11A) C(10A)
C(9A) O(3A) N(2A)
Cu(1)
C(8A)
C(3B) O(2B)
N(1A)
C(2B) C(1B)
C(4B)
O(1B)
O(1A) C(7A)
Cu(2)
C(1A)
O(3B) C(6A)
C(7B)
C(5B) C(6B)
C(8B) N(1B)
N(2B)
C(9B)
C(2A)
O(2A)
C(14B) C(10B) C(13B)
C(11B) C(5A)
C(3A)
C(15B)
C(12B)
C(4A)
Fig. 1. ORTEP diagram of [Cu4 (L)4 ] cluster of 1 showing the Cu(1) and Cu(2) centres with atom numbering scheme. H-atoms are omitted for clarity.
where this number is 10–11.[8] Theoretical and experimental studies of water clusters in restricted environments have been carried out and their structures have been predicted by theoretical ab initio calculations. In order to account for the heat capacity of liquid water (which largely exceeds that of the gas phase), examination of small and long-lived water clusters has become a major concern in recent studies. Water tetramers play a crucial role in such studies as they are taken as a standard model for theoretical calculations of the heat capacity of bulk water.[8] Moreover, it has been possible to calculate different configurations of water tetramers embedded in different matrices, with some even being characterized by far infrared vibration rotation tunnelling spectroscopy.[9,10] Theoretical calculations of the lowest energy conformation for small water clusters show that water trimers, tetramers, and pentamers prefer cyclic and quasi-planar structures. In the present work we report a cyclic water tetramer, having a ‘butterfly’ conformation, stabilized by a tetranuclear CuII hydrazone complex [Cu4 (L)4 ]·4H2 O (1). The calculations show that the observed conformation of the ‘butterfly’ shaped tetrameric water clusters aquire some of their stability from the confinement of the hydrazone ligands of 1. Here we report the synthesis, X-ray crystal structure, spectral properties, and quantum chemical study showing the stability of a water tetramer in a [Cu4 (L)4 ]·4H2 O (1) complex along with
its magnetic properties that show a moderate antiferromagnetic coupling with two distinct coupling constants, corresponding to the two different CuII tetramers present in the structure of 1. Results and Discussion Crystal Structure of Complex 1 The structure of 1 consists of two crystallographically independent CuII units where each tetranuclear unit lies on a crystallographic symmetry operation thus giving only two unique CuII centres per unit and are designated as Cu(1)–Cu(2) and Cu(3)–Cu(4) (Fig. 1). All the CuII ions in the Cu4 core are fivecoordinated with a CuO4 N coordination environment. The bond lengths and angles in both CuII tetramers are listed in Table 1. The CuII ions are linked in the tetranuclear cluster by µ2 -phenoxo bridges of the hydrazone ligand H2 L. The four crystallographically independent CuII ions are present in distorted square pyramidal geometries, as confirmed by the trigonality Addison indices (τ) of 0.0695, 0.1111, 0.1032, and 0.0231 for Cu(1) to Cu(4) atoms, respectively (τ = |β − α|/60, where α and β are the two greatest basal angles when the polyhedron is viewed as a square-pyramid. For a perfectly square-pyramidal geometry, τ is equal to zero and it becomes unity for a perfect trigonal bipyramidal geometry).[11]
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Table 1. Selected bond lengths [Å] and bond angles [◦ ] around Cu(1), Cu(2), Cu(3), and Cu(4) ions in 1 Bond lengths [Å] Cu(1)
Cu(2)
Cu(1)–N(1A) Cu(1)–O(3A) Cu(1)–O(1A) Cu(1)–O(2B)#1 Cu(1)–O(1B)#
1.913(3) 1.9316(19) 1.9623(19) 2.289(2) 1.983(2)
Cu(3)
Cu(2)–N(1B) Cu(2)–O(3B) Cu(2)–O(1A) Cu(2)–O(2A) Cu(2)–O(1B)
1.917(3) 1.939(3) 1.991(2) 2.287(2) 1.959(2)
Cu(4)
Cu(3)–N(1C) Cu(3)–O(3C) Cu(3)–O(1C) Cu(3)–O(2D)#2 Cu(3)–O(1D)#2
1.922(3) 1.917(2) 1.969(2) 2.322(2) 1.989(2)
Cu(4)–N(1D) Cu(4)–O(3D) Cu(4)–O(1C) Cu(4)–O(2C) Cu(4)–O(1D)
1.922(3) 1.909(2) 2.005(2) 2.336(2) 1.922(2)
Bond angles [◦ ] N(1A)–Cu(1)–O(3A) N(1A)–Cu(1)–O(1A) O(3A)–Cu(1)–O(1A) N(1A)–Cu(1)–O(1B)#1 O(3B)–Cu(2)–O(1B) N(1B)–Cu(2)–O(1A) O(1B)–Cu(2)–O(2A) O(1A)–Cu(2)–O(2A) O(3C)–Cu(3)–N(1C) O(3C)–Cu(3)–O(1C) N(1C)–Cu(3)–O(1C) N(1C)–Cu(3)–O(1D)#2 N(1C)–Cu(3)–O(2D)#2 O(3D)–Cu(4)–O(1D) N(1D)–Cu(4)–O(1C) O(3D)–Cu(4)–O(1D) N(1D)–Cu(4)–O(1C) O(1D)–Cu(4)–O(1C) O(3D)–Cu(4)–O(2C)
81.78(10) 92.36(10) 171.33(10) 175.50(12) 171.38(9) 178.18(10) 99.58(8) 77.13(9) 81.13(11) 170.03(10) 92.25(11) 176.22(10) 108.00(11) 173.87(10) 172.48(11) 173.87(10) 172.48(11) 88.80(9) 84.58(9)
c b
a
Fig. 2.
H-bonding diagram of 1.
The water molecules are present in the ‘exo-pockets’ of the tetrameric CuII units holding the individual tetranuclear metalclusters to form 1D polynuclear chains (Fig. 2) along the c-axis. The formation of a small water cluster can be anticipated following the vibrational spectra of the complex. The
intermolecular O–H bending peak was observed ∼80 cm−1 , blue-shifted with respect to a mononuclear lattice water, whereas the appearance of a sharp band at nearly 500 cm−1 , red-shifted from the standard band for lattice water, indicated the probable presence of bridging H-atoms.[8b]
‘Butterfly’-Shaped Water Tetramer in a Cu4 Complex Supported by a Hydrazone Ligand
Table 2. The inter-water separations [Å] in O(1W)–O(3W) and O(2W)–O(4W) cluster O(1W)–O(3W) O(3W)–O(1W) O(2W)–O(4W) O(4W)–O(2W)
2.830 2.779 2.849 2.858
There are in total four crystallographically independent water molecules in the void space of the tetranuclear CuII cores whose oxygen atoms are designated as O(1W), O(2W), O(3W), and O(4W) (refer to Table 2 for inter-water distances). These four water molecules are arranged in two different water tetramers (first as O(1W)–O(3W)–O(1W)#4–O(3W)#3, referred to as the O(1W)–O(3W) cluster; and second as O(2W)– O(4W)–O(2W)#5–O(4W)#3, referred to as the O(2W)–O(4W) cluster). Each water molecule in the cluster forms one Hbond with its immediate neighbouring water molecule and thus, for each water, one remaining hydrogen does not participate in an H-bonding interaction within the water cluster. These ‘non-interacting’ hydrogens are involved in H-bonding interactions with the N(2A) and N(2B) of the Cu(1)–Cu(2) core, and with the N(2C) and N(2D) of the Cu(3)–Cu(4) core. The geometrical arrangements of these latter hydrogen atoms, with respect to the ones involved in H-bonding interactions within the water clusters, are also important and will be treated in detail in the quantum chemical study result section. In both water tetramers (O(1W)–O(3W) and O(2W)–O(4W)), the hydrogen atoms forming H-bonds with the ligand N atoms, are arranged in an up-down-up-down fashion, leading to the formation of 1D H-bonded polynuclear clusters formed by Cu4 cores, linked by cyclic water oligomers (the O(1W)–O(3W) cluster links the Cu(1)–Cu(2) tetramers, whereas the O(2W)–O(4W) cluster links the Cu(3)–Cu(4) core). The two tetrameric water clusters differ in their geometry and symmetry and thus, are discussed separately. The O(1W)–O(3W) water cluster is more planar with a dihedral angle of 17.10◦[10] between the mean planes, whereas for the O(2W)–O(4W) water cluster this angle is 32.63◦ . According to our literature survey, an inter-planer angle of 32.55◦ between the mean planes of water tetramers is yet to be reported, and this is therefore, the first water tetramer with a ‘butterfly’ conformation with such a large dihedral angle. Each water molecule in the tetrameric water cluster is tri-coordinated, linking the CuII metal clusters by H-bonds with uncoordinated hydrazide nitrogen atoms of the ligand fragment (N(2A) and N(2B) for the O(1W)– O(3W) cluster and N(2C) and N(2D) for the O(2W)–O(4W) cluster). The H-bond distances are, as expected, quite different for the two water tetramers (Table 3 and Fig. 3). The O· · ·O distances in both clusters are quite different (2.858 and 2.849 Å in the O(2W)–O(4W) cluster compared with 2.779 and 2.830 Å in the O(1W)–O(3W) cluster), but they are all in the range of those observed in solid phase water tetramers.[8b,12] Other researchers have already shown that the major factor contributing to the stabilization of H-bonds is the charge transfer (CT) interaction, and not the electrostatic interaction for many small cluster systems, where the latter is cancelled by exchange interactions. The CT interaction was described as the stabilization of a O–H antibonding level by acceptance of a lone pair from oxygen. This leads to both enthalpic and entropic stabilization too. The other consequences of such CT interaction is
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Table 3. Hydrogen bonds for 1 [Å and ◦ ] Symmetry transformations used to generate equivalent atoms: #1 −x + 1/2, y, −z + 1/2; #2 −x + 1/2, y, −z + 3/2; #3 x, y − 1, z; #4 −x + 1/2, y + 1, −z + 1/2; #5 −x + 1/2, y + 1, −z + 3/2 D–H· · ·A
d(H· · ·A) d(D· · ·A) 2σ(I)] R indices (all data) Largest diff. peak and hole [e Å−3 ]
C60 H56 Cu4 N8 O16 1399.29 Monoclinic P 1 2/n 1 21.1297(7) 13.4375(4) 21.6104(9) 90 104.767(4) 90 5933.2(4) 4 1.566 1.491 2864 0.2925 × 0.0947 × 0.0511 4.50 to 25.18◦ 57648 19627 (Rint = 0.1498) 0.740 R1 = 0.0502, wR2 = 0.0476 R1 = 0.2314, wR2 = 0.0678 0.646 and −0.685
(λ = 0.71073 Å) source in the ϕ and ω scan modes at 200 K. Data were processed using the CrysAlis-CCD programs.[31] The structure was determined by direct methods procedures in SHELXS [32a] and refined by full-matrix least-squares methods, on F2 ’s, in SHELXL.[32b] The crystallographic data and the refinement results are listed in Table 4. Supplementary Information CCDC number for 1 is CCDC 673139. These data can be obtained free of charge at www.ccdc.cam.ac.uk (or from Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; email:
[email protected]). Accessory Publication The Gaussian archive entries of the frequency calculations are provided in the Accessory Publication on the Journal’s website. Acknowledgement The authors sincerely acknowledge DRDO and AICTE New Delhi, the European Union (MAGMANet network of excellence) and the Spanish Ministerio de Educación y Ciencia (Projects MAT2007–61584 and Consolider-Ingenio 2010 CSD 2007–00010 in Molecular Nanoscience) for funding this project. We also thank Professor Tim Clark, of University of Erlangen-Nürnberg, Germany, for hosting this work in the CCC and the Regionales Rechenzentrum Erlangen (RRZE) for a generous allotment of computer time. We also thank the Generalitat Valenciana (Project PROMETEO/2009/095).
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‘Butterfly’-Shaped Water Tetramer in a Cu4 Complex Supported by a Hydrazone Ligand
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