International Journal of Inorganic ChemistryVolume 2011 (2011), Article ID 606271, 9 pagesdoi:10.1155/2011/606271

Research Article

Synthesis and Structural Characterization of a New Tetranuclear Nickel(II) Sulfato Complex Containing the Anionic Form of Di-2-Pyridyl Ketone Oxime

Eleni Moushi,¹ Constantinos G. Efthymiou,² Spyros P. Perlepes,³ and Constantina Papatriantafyllopoulou2,3

¹Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus²Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA³Department of Chemistry, University of Patras, 265 04 Patras, Greece

Received 7 December 2010; Accepted 26 January 2011

Academic Editor: Daniel L. Reger

Copyright © 2011 Eleni Moushi et al. Eleni Moushi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The preparation and crystal structure of a tetranuclear Ni(II) sulfato cluster containing the anion of di-2-pyridyl ketone oxime, (py)₂CNO⁻, are reported. Treatment of NiSO₄·6H₂O with one equivalent of (py)₂CNOH and one equivalent of NEt₃ in MeOH leads to the compound [Ni₄{(py)₂CNO}₄(SO₄)₂(MeOH)₄] (1) in moderate yield. The metal ions are linked together by two 3.2111 and two 2.1110 (Harris notation) (py)₂CNO⁻ ligands, as well as two 2.1100 ions to create a rare metallacrown-type (12-MC-4) ring. Strong H-bond intermolecular interactions in 1 lead to the formation of a 1D chain along the axis. Characteristic IR bands are discussed in terms of the known structure of 1.

1. Introduction

There is currently a renewed interest in the coordination chemistry of oximes [1–20]. 2-pyridyl oximes (Scheme 1) are popular ligands in coordination chemistry [21–36]. The anions of these molecules are versatile ligands for a variety of research objectives, including μ ₂ and μ ₃ behaviour [21, 22]; the activation of 2-pyridyl oximes by 3d-metal centers towards further reactions is also becoming a fruitful area of research [21, 22, 26]. The majority of the metal complexes of these ligands have been prepared in the last 15 years and much of their chemistry remains to be explored in more detail [22].

Figure 1: The molecular structure of 1. H atoms have been omitted for clarity.

Figure 2: Top: the [Ni₄(μ-SO₄)₂(μ ₂-ONR)₂(μ ₃-ONR)₂] core of complex 1, where R- = -C(py)₂. The metallacrown-type ring is highlighted. Bottom: the core of 1 using only the μ ₃ oximate groups.

The structure of 1 consists of tetranuclear molecules [Ni₄{(py)₂CNO}₄(SO₄)₂(MeOH)₄] which lie on a crystallographic inversion center. The metal ions are held together by two 3.2111 and two 2.1110 (using Harris notation, [53], Scheme 3) (py)₂CNO⁻ ligands, as well as two 2.1100 ions. Four MeOH molecules act as terminal ligands and complete the coordination sphere of the four metal centers. The molecule has a metallacrown-type topology [61]. A pseudo 12-MC-4 ring forms; the true 12-MC-4 topology is “destroyed” by the bridging character of the oximate oxygen atoms O2 and O2′.

Figure 3: Representation of a part of the 1D chain formed in 1 along the axis. The dotted lines represent H bonds.

The molecule of 1 contains the [Ni₄(μ-SO₄)₂(μ ₂-ONR)₂(μ ₃-ONR)₂] core, where R- = -C(py)₂ (Figure 2, top). An alternative description of the core (using only the μ ₃ oximate groups) is [Ni₄(μ ₃-ONR)₂]⁶⁺ (Figure 2, bottom). The topology of the four NiII ions can be also described as “saddle-like,” and it is observed for the first time in Ni₄ clusters. The most common topologies of the metal ions in NiII₄ complexes are the cubanes [66–73] and the face-shared distorted dicubanes in which one of the corners of each cubane is missing [74–79], while there are few Ni₄ clusters in which the metal ions adopt less common topologies such as linear [80–82], rectangular [83–87], and chair-like [88, 89] as follows.

Complex 1 joins a small but growing family of structurally characterized Ni(II) complexes containing the neutral or anionic forms of di-2-pyridyl ketone oxime as ligands [34, 36, 88–92]. The special features of 1 compared to the other members of this family are (1) It is the first example of these species containing the sulfato ligand, and (2) it has a unique Ni₄ clusters “saddle-like” metal topology.

3.3. IR Spectra

The medium intensity bands at 1568 and 1094 cm⁻¹ in the spectrum of the free ligand (py)₂CNOH are assigned to v(C=N)oxime and v(N-O)oxime modes, respectively [51, 52, 93]. The 1094 cm⁻¹ band is shifted to a higher wavenumber (1118 cm⁻¹) in 1. This shift is in accord with the concept that upon deprotonation and oximate-O coordination, there is a higher contribution of N=O to the electronic structure of the oximate group; consequently, the v(N-O) vibration shifts to a higher wavenumber in the complex relative to (py)₂CNOH [36]. Somewhat to our surprise, the 1568 cm⁻¹ band is shifted to a higher wavenumber in the complex (1598 cm⁻¹), overlapping with an aromatic stretch. This shift may be indicative of the oxime nitrogen coordination [94]. Extensive studies on Schiff base complexes (which also contain a C=N bond) have shown [95] that a change in the s character of the nitrogen lone pair occurs upon coordination such that the s character of nitrogen orbital involved in the C=N bond increases; this change in hybridization produces a greater C=N stretching force constant relative to the free neutral ligand.

The in-plane deformation band of the 2-pyridyl ring of free (py)₂CNOH at 622 cm⁻¹ shifts upwards (641 cm⁻¹), confirming the involvement of the ring-N atom in coordination [96]. The presence of the 618 cm⁻¹ bond in the spectrum of 1 indicates that some 2-pyridyl rings are “free,” that is, uncoordinated, in accordance with the 2.1110 (py)₂CNO⁻ ligands that are present in the complex.

The IR spectrum of the free, that is, ionic, sulfate (the ion belongs to the point group) consists of two bands at ~1105 and ~615 cm⁻¹, assigned to the () stretching [(SO)] and bending [(OSO)] modes, respectively [41, 97]. The () stretching [(SO)] and () bending [(OSO)] fundamentals are not IR active. The coordination of to metal ions decreases the symmetry of the group, and the and modes are split [41, 97]. In the case when the -site symmetry is lowered from to (bidentate chelating or bridging coordination), which is the case in 1, both and appear in the IR spectrum, while and each splits into three IR-active vibrations [97]. Thus, the bands at 1219, 1130, and 1020 cm⁻¹ are attributed to the modes [97], while the bands at 591, 618, and 670 cm⁻¹ are assigned to the modes [13, 74–79] with the intermediate wavenumber band being superimposed by a ligand’s vibration. The band at 982 cm⁻¹ and the weak feature at 452 cm⁻¹ can be assigned [41, 97] to the and modes, respectively. These spectral features agree with the low symmetry for the sulfato ligand in the complex, as also confirmed crystallographically.

4. Conclusions

The present work extends the body of results that emphasize the ability of the sulfate ion to create unique structural types in 3d-metal cluster chemistry. The study of the coordination chemistry of the binary /(py)₂CNOH ligand system in the presence of base in MeOH has provided access to the novel tetranuclear Ni(II) cluster [Ni₄{(py)₂CNO}₄(SO₄)₂(MeOH)₄] (1). Complex 1 contains the [Ni₄(μ-SO₄)₂(μ ₂-ONR)₂(μ ₃-ONR)₂] core, where R- = -C(py)₂, with a unique saddle-like topology of the NiII ions; it is thus a valuable addition to the family of tetranuclear NiII clusters.

Analogues of 1 with 2-pyridinealdoxime [(py)C(H)NOH], methyl(2-pyridyl)ketone oxime [(py)C(Me)NOH], or phenyl(2-pyridyl)ketone oxime [(py)C(ph)NOH] (Scheme 1) are not known, until to date, and further research efforts are in progress to determine the appropriate reaction conditions that could possibly favor such species. It is likely that the preparation and stability of such tetranuclear complexes are dependent on the particular nature of the R substituent on the oximate carbon. We are currently working on the chemistry of the NiSO₄·6H₂O/(py)C(R)NOH (R=H, Me, Ph) reaction systems.

Supporting Information

CCDC 802606 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk.

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