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

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 SO42− ions to create a rare metallacrown-type (12MC-4) ring. Strong H-bond intermolecular interactions in 1 lead to the formation of a 1D chain along the c axis. Characteristic IR bands are discussed in terms of the known structure of 1.

Recently, we have begun a program which can be considered as a modification of the above-mentioned binary "ligand blend" approach.We have been exploring the use of other inorganic ions, such as chemistry with 2-pyridyloximate ligands.The sulfate ion [41] is a ligand with great coordinative flexibility (μ 2 , μ 3 , μ 4 , μ 5 , μ 6 , μ 8 , or μ 10 potential), see Scheme 2. Metal-sulfato complexes have been studied for their roles in the field of porous framework materials [42,43], in catalysis [44], in the construction of luminescent molecular materials [45,46], and in medicinal [47], environmental [48], and bioinorganic [49] chemistry.The possible advantages of using SO 4 2− instead of R CO 2 − include (i) the possibility of triggering aggregation of preformed smaller cationic species into new, higher-nuclearity products and (ii) the possible diversion of known reaction systems developed using monoanionic carboxylates to new species as a result of the higher charge and higher denticity/bridging capability of sulfates.Thus, the initial employment of the sulfate ion in Ni II /(py)C(R)NOH (R = Me, Ph, NH 2 ) chemistry has led to the isolation and characterization of high-nuclearity Ni II compounds, such as Ni 12 [50] and Ni 6 [51,52] clusters which possess interesting structural properties.
In this work, we expand our efforts to a different member of 2-pyridyl oximes which is di-2-pyridyl ketone oxime, (py) 2 CNOH, and report the synthesis and characterization of the new tetranuclear compound [Ni 4 {(py) 2 CNO} 4 (SO 4 ) 2 (MeOH) 4 ].The structure of the compound has been determined by single-crystal X-ray diffraction.The IR data are discussed in terms of the nature of bonding and the structure of the complex.

Experimental
2.1.General and Physical Measurements.All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received.
Microanalyses (C, H, N) were performed by the University of Ioannina (Greece) Microanalytical Laboratory using an EA 1108 Carlo Erba analyzer.IR spectra (4000-400 cm −1 ) were recorded on a Perkin-Elmer 16 PC FT-spectrometer with samples prepared as KBr pellets.1).NEt 3 (0.139 ml, 1.00 mmol) was added to a colourless solution of (py) 2 CNOH (0.199 g, 1.00 mmol) in MeOH (25 ml).Subsequently, solid NiSO 4 •6H 2 O (0.263 g, 1.00 mmol) was added, and the resulting red solution was stirred for 1 h at room temperature.A small quantity of undissolved material was   The structure was solved by direct methods using SIR92 [54] and refined by full-matrix least-squares techniques on F 2 with SHELXL-97 [55].Some residual electron density in the accessible voids of the structure was too disordered to refine as solvent molecules; therefore, the SQUEEZE procedure [56] of PLATON was employed to remove the contribution of the electron density in the solvent region from the intensity data.The solvent-free model and intensity data were used for the final results reported here.The non-H atoms were treated anisotropically.The H atoms of the (py) 2 CNOH ligands and the methyl groups of the methanol molecules were placed in calculated, ideal positions and International Journal of Inorganic Chemistry refined as riding on their respective C atoms.The H atom of the OH group of one independent methanol molecule (O(8)H) was located in difference Fourier maps and was refined isotropically, but the H atom of the OH group of the second independent methanol molecule (O(7)H) could not be located.The programs used were CRYSALIS CCD [57] for data collection, CRYSALIS RED [57] for cell and data refinement, WINGX [58] for crystallographic calculations, and MERCURY [59] and DIAMOND [60] for molecular graphics.

Results and Discussion
3.1.Synthetic Comments.Our general synthetic approach for the isolation of Ni II /2-pyridyloximate/sulfato clusters has been to treat the metal sulfate "salt" with the appropriate ligand and a base in a variety of solvents.The addition of base is necessary for the deprotonation of the oxime ligand.
Treatment of NiSO  ( As expected, the nature of the base is not crucial for the identity of the product, and it affects only its crystallinity, and in some cases its purity; we were able to isolate 1 by using a plethora of different bases such as NaOMe, NMe 4 OH, NEt 4 OH, and LiOH•H 2 O. Small changes in the molar ratio of the reactants, the crystallization method, and the presence of counterions do not seem to affect the identity of the isolated product.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 .
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][89][90][91][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 Ni 4 clusters "saddle-like" metal topology.

IR Spectra.
The medium intensity bands at 1568 and 1094 cm −1 in the spectrum of the free ligand (py) 2 CNOH 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) 2 CNOH [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) 2 CNOH 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) 2 CNO − ligands that are present in the complex.
The IR spectrum of the free, that is, ionic, sulfate (the ion belongs to the T d point group) consists of two bands at ∼1105 and ∼615 cm −1 , assigned to the v 3 (F 2 ) stretching [ν d (SO)] and v 4 (F 2 ) bending [δ d (OSO)] modes, respectively [41,97].The v 1 (A 1 ) stretching [ν s (SO)] and v 2 (E) bending [δ d (OSO)] fundamentals are not IR active.The coordination of SO 4 2− to metal ions decreases the symmetry of the group, and the v 3 and v 4 modes are split [41,97].In the case when the SO 4 2− -site symmetry is lowered from T d to C 2v (bidentate chelating or bridging coordination), which is the case in 1, both v 1 and v 2 appear in the IR spectrum, while v 3 and v 4 each splits into three IR-active vibrations [97].Thus, the bands at 1219, 1130, and 1020 cm −1 are attributed to the v 3 modes [97], while the bands at 591, 618, and 670 cm −1 are assigned to the v 4 modes [13,[74][75][76][77][78][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 v 1 and v 2 modes, respectively.These spectral features agree with the low C 2v symmetry for the sulfato ligand in the complex, as also confirmed crystallographically.

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 SO

Scheme 2 :
Scheme 2: The up to now crystallographically established coordination modes of the sulfato ligand and the Harris notation[53] which describes these modes.

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

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

Table 1 :
Summary of crystal data, data collection, and structure refinement for the X-ray diffraction study of complex 1.
Anal.Calc.for C 49 H 52 Ni 4 N 24 O 17 S . removed by filtration and the dark red filtrate layered with Et 2 O (50 ml).Slow mixing gave X-ray quality, orange crystals which were collected by filtration, washed with Et 2 O (2 × 3 ml), and dried in air; yield 57%.The dried solid was analyzed satisfactorily as 1•MeOH.2:C,38.02; H, 3.99; N, 21.72.Found: C, 38.45; H, 3.87; N, 21.37%.IR (KBr pellet): v = 33882.3.Single-Crystal X-Ray Crystallography.A crystal of 1 with appropriate dimensions 0.08 × 0.03 × 0.01 mm was attached to a glass fiber using silicone grease.Data were collected on an Oxford Diffraction Xcalibur-3 diffractometer, equipped with a Sapphire CCD area detector, at 100 K, using a graphite monochromated Mo Kα radiation.Complete crystal data and parameters for data collection and processing are listed in Table1. 4 •6H 2 O with one equivalent of (py) 2 CNOH and one equivalent of NEt 3 in MeOH gave a red solution which, upon crystallization, gave orange crystals of the new tetranuclear cluster which can be written as [Ni 4 {(py) 2 CNO} 4 (SO 4 ) 2 (MeOH) 4 ] (1).