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

Authors: Eleni Moushi; Constantinos G. Efthymiou; Spyros P. Perlepes; Constantina Papatriantafyllopoulou
Journal: International Journal of Inorganic Chemistry
 (2011)

---

## 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 SO42- 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 c axis.  Characteristic IR bands are discussed in terms of the known structure of 1.


---

## Body

## 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 μ2 and μ3 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].Scheme 1
General structural formula and abbreviations of simple 2-pyridyl oximes.We have been exploring “ligand blend” reactions involving carboxylates (R′CO2-) and various 2-pyridyloximates with (ternary “ligand blends”) or without (binary “ligand blends”) additional inorganic monoanions (Cl−, Br−, NO3-, N3-, SCN−) as a means to high-nuclearity species. The presence of a deprotonated oxime group leads to a great coordinative flexibility due to the well-known ability of the oximate group to bridge two or three metal ions. On the other hand, carboxylates are able to deprotonate the oxime group of 2-pyridyloximes under mild conditions (the use of external hydroxides often perplexes the reactions). Besides their deprotonating ability, the R′CO2- ions are flexible ligands, a consequence of their ability to adopt a number of different ligation modes, both terminal and bridging as well as both bidentate and tridentate. The additional inorganic monoanions in the ternary “ligand blends” often behave as terminal ligands and help the formation of clusters (and not coordination polymers). However, sometimes they act as bridging ligands, and this may eventually lead to clusters with complicated structures; the formation of coordination polymers cannot be ruled out in such a case. Thus, a variety of Cr, Mn, Fe, Co, Ni, and Cu clusters [22–39] with nuclearities ranging from 3 to 12 have been characterized from our [23–36] and other [21, 37–39] groups, some of them possessing interesting magnetic properties, including single-molecule magnetism behaviour [33, 40].Recently, we have begun a program which can be considered as a modification of the above-mentionedbinary “ligand blend” approach. We have been exploring the use of other inorganic ions, such as SO42-, instead of the carboxylato ligand, R′CO2-, in the 3d-metal cluster chemistry with 2-pyridyloximate ligands. The sulfate ion [41] is a ligand with great coordinative flexibility (μ2, μ3, μ4, μ5, μ6, μ8, or μ10 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 SO42- instead of R′CO2- 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 NiII/(py)C(R)NOH (R = Me, Ph, NH2) chemistry has led to the isolation and characterization of high-nuclearity NiII compounds, such as Ni12 [50] and Ni6 [51, 52] clusters which possess interesting structural properties.Scheme 2
The up to now crystallographically established coordination modes of the sulfato ligand and the Harris notation [53] which describes these modes.In this work, we expand our efforts to a different member of 2-pyridyl oximes which is di-2-pyridyl ketone oxime, (py)2CNOH, and report the synthesis and characterization of the new tetranuclear compound [Ni4{(py)2CNO}4(SO4)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.


## 2. 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.


### 2.2. Compound Preparation

#### 2.2.1. [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1)

NEt3 (0.139 ml, 1.00 mmol) was added to a colourless solution of (py)2CNOH (0.199 g, 1.00 mmol) in MeOH (25 ml). Subsequently, solid NiSO4·6H2O (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 removed by filtration and the dark red filtrate layered with Et2O (50 ml). Slow mixing gave X-ray quality, orange crystals which were collected by filtration, washed with Et2O (2×3 ml), and dried in air; yield 57%. The dried solid was analyzed satisfactorily as 1·MeOH. Anal. Calc. for C49H52Ni4N24O17S2: C, 38.02; H, 3.99; N, 21.72. Found: C, 38.45; H, 3.87; N, 21.37%. IR (KBr pellet): ṽ=3388 sb, 2902 w, 1654 w, 1598 m, 1460 m, 1430 m, 1376 w, 1340 w, 1282 w, 1219 m, 1130 m, 1118 s, 1086 s, 1045 s, 1020 m, 982 m, 896 w, 788 w, 748 m, 702 m, 670 m, 641 m, 618 s, 591 w, 452 w cm−1.


### 2.3. Single-Crystal X-Ray Crystallography

A crystal of1 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 Table 1.Table 1
Summary of crystal data, data collection, and structure refinement for the X-ray diffraction study of complex1.
Complex1Empirical formulaNi4C48H48N24O16S2Formula weight1515.97Colour, habitorange, rodCrystal systemorthorhombicSpace groupPccna (Å)14.2969 (10)b (Å)20.9678 (14)c (Å)18.5395 (13)V (Å3)5557.7 (7)Z4ρcalc (g cm-3)1.609Radiation,λ (Å)0.71073μ (mm-1)1.488F (000)2760Temperature (K)100 (2)2θmax [°]61Rangesh−18 → 18k−21 → 27l−27 → 23Measured reflections42848Unique reflections6377Reflections used3703(I>2σ(I))Parameters refined374GoF (onF2)0.903R1 a (I>2σ(I))0.0464wR2 b (I>2σ(I))0.1042(Δρ)max/(Δρ)min (e Å-3)0.861/−0.621aR1=∑(|Fo|-|Fc|)/∑(|Fo|).bwR2={∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]}1/2.The structure was solved by direct methods using SIR92 [54] and refined by full-matrix least-squares techniques on F2 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)2CNOH  ligands and the methyl groups of the methanol molecules were placed in calculated, ideal positions and 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.


## 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.


## 2.2. Compound Preparation

### 2.2.1. [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1)

NEt3 (0.139 ml, 1.00 mmol) was added to a colourless solution of (py)2CNOH (0.199 g, 1.00 mmol) in MeOH (25 ml). Subsequently, solid NiSO4·6H2O (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 removed by filtration and the dark red filtrate layered with Et2O (50 ml). Slow mixing gave X-ray quality, orange crystals which were collected by filtration, washed with Et2O (2×3 ml), and dried in air; yield 57%. The dried solid was analyzed satisfactorily as 1·MeOH. Anal. Calc. for C49H52Ni4N24O17S2: C, 38.02; H, 3.99; N, 21.72. Found: C, 38.45; H, 3.87; N, 21.37%. IR (KBr pellet): ṽ=3388 sb, 2902 w, 1654 w, 1598 m, 1460 m, 1430 m, 1376 w, 1340 w, 1282 w, 1219 m, 1130 m, 1118 s, 1086 s, 1045 s, 1020 m, 982 m, 896 w, 788 w, 748 m, 702 m, 670 m, 641 m, 618 s, 591 w, 452 w cm−1.


## 2.2.1. [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1)

NEt3 (0.139 ml, 1.00 mmol) was added to a colourless solution of (py)2CNOH (0.199 g, 1.00 mmol) in MeOH (25 ml). Subsequently, solid NiSO4·6H2O (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 removed by filtration and the dark red filtrate layered with Et2O (50 ml). Slow mixing gave X-ray quality, orange crystals which were collected by filtration, washed with Et2O (2×3 ml), and dried in air; yield 57%. The dried solid was analyzed satisfactorily as 1·MeOH. Anal. Calc. for C49H52Ni4N24O17S2: C, 38.02; H, 3.99; N, 21.72. Found: C, 38.45; H, 3.87; N, 21.37%. IR (KBr pellet): ṽ=3388 sb, 2902 w, 1654 w, 1598 m, 1460 m, 1430 m, 1376 w, 1340 w, 1282 w, 1219 m, 1130 m, 1118 s, 1086 s, 1045 s, 1020 m, 982 m, 896 w, 788 w, 748 m, 702 m, 670 m, 641 m, 618 s, 591 w, 452 w cm−1.


## 2.3. Single-Crystal X-Ray Crystallography

A crystal of1 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 Table 1.Table 1
Summary of crystal data, data collection, and structure refinement for the X-ray diffraction study of complex1.
Complex1Empirical formulaNi4C48H48N24O16S2Formula weight1515.97Colour, habitorange, rodCrystal systemorthorhombicSpace groupPccna (Å)14.2969 (10)b (Å)20.9678 (14)c (Å)18.5395 (13)V (Å3)5557.7 (7)Z4ρcalc (g cm-3)1.609Radiation,λ (Å)0.71073μ (mm-1)1.488F (000)2760Temperature (K)100 (2)2θmax [°]61Rangesh−18 → 18k−21 → 27l−27 → 23Measured reflections42848Unique reflections6377Reflections used3703(I>2σ(I))Parameters refined374GoF (onF2)0.903R1 a (I>2σ(I))0.0464wR2 b (I>2σ(I))0.1042(Δρ)max/(Δρ)min (e Å-3)0.861/−0.621aR1=∑(|Fo|-|Fc|)/∑(|Fo|).bwR2={∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]}1/2.The structure was solved by direct methods using SIR92 [54] and refined by full-matrix least-squares techniques on F2 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)2CNOH  ligands and the methyl groups of the methanol molecules were placed in calculated, ideal positions and 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.


## 3. Results and Discussion

### 3.1. Synthetic Comments

Our general synthetic approach for the isolation of NiII/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 NiSO4·6H2O with one equivalent of (py)2CNOH and one equivalent of NEt3 in MeOH gave a red solution which, upon crystallization, gave orange crystals of the new tetranuclear cluster which can be written as [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1). Its formation can be summarized in (1)
(1)4NiSO4⋅6H2O+4(py)2CNOH+4NEt3+4MeOH→MeOH[Ni4{(py)2CNO}4(SO4)2(MeOH)4]+2(HNEt3)2SO4+24H2O.
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, NMe4OH, NEt4OH, and LiOH·H2O. 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.


### 3.2. Description of Structure

Partially labeled plots of the complete structure and the core of the molecule [Ni4{(py)2CNO}4(SO4)2(MeOH)4] that is present in complex 1 are shown in Figures 1 and 2, respectively. Selected interatomic distances and angles are listed in Table 2.Table 2
Selected interatomic distances (Å) and angles (°) for1.
Ni1-O12.046 (3)Ni2-O22.071 (3)Ni1-O22.046 (3)Ni2-O32.042 (3)Ni1-O42.033 (3)Ni2-O72.163 (3)Ni1-O82.048 (3)Ni2-N12.090 (3)Ni1-N52.090 (3)Ni2-N21.997 (4)Ni1-N62.074 (3)Ni2-N42.053 (3)O1-Ni1-O289.59 (11)O2-Ni2-O394.82 (11)O1-Ni1-O4101.65 (11)O2-Ni2-O788.69 (10)O1-Ni1-O8173.53 (11)O2-Ni2-N1169.13 (12)O1-Ni1-N582.89 (11)O2-Ni2-N290.44 (12)O1-Ni1-N683.38 (12)O2-Ni2-N484.07 (12)O2-Ni1-O484.24 (11)O3-Ni2-O7176.29 (10)O2-Ni1-O888.62 (11)O3-Ni2-N188.91 (12)O2-Ni1-N5107.12 (11)O3-Ni2-N295.11 (12)O2-Ni1-N6170.67 (12)O3-Ni2-N487.08 (12)O4-Ni1-O884.36 (12)O7-Ni2-N187.86 (11)O4-Ni1-N5167.92 (13)O7-Ni2-N286.05 (12)O4-Ni1-N691.16 (12)O7-Ni2-N492.09 (12)O8-Ni1-N591.70 (12)N1-Ni2-N279.04 (13)O8-Ni1-N699.02 (12)N1-Ni2-N4106.36 (13)N5-Ni1-N678.16 (13)N2-Ni2-N4174.25 (13)Figure 1
The molecular structure of1. H atoms have been omitted for clarity.Figure 2
Top: the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-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 μ3 oximate groups.The structure of1 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 SO42- 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′.Scheme 3
The two different coordination modes of the (py)2CNO− ligands present in complex 1 and the Harris notation [53] which describes them.A distorted octahedral environment is created about each metal center; the chromophores are represented by the following formulas Ni(1,1′)(Npy)(Nox)(Oox)2(Osulf)(Omet) and Ni(2,2′)(Npy)2(Nox)(Oox)(Osulf)(Omet), where the abbreviations “py”, “ox”, “sulf,” and “met” are for the 2-pyridyl, oximate, sulfato, and methanolic donor atoms, respectively. The average Ni–Oox, Ni–Nox, and Ni–Npy bond lengths of 2.054(3), 2.043(3), and 2.071(3) Å, respectively, agree well with the values expected for high-spin NiII ions in octahedral environment [34, 35, 62–64]. The Ni–Osulf bond lengths are typical [41, 51, 62, 65].  The fact that S-(O3, O4) (average 1.474 Å) > S-(O5, O6) (average 1.450 Å) reflects the coordinating nature of O3 and O4 and the noncoordinating character of O5 and O6; as expected, the sulfur to “free” oxygen bond lengths are the shortest.The crystal structure of1is stabilized by strong inter- and intramolecular hydrogen bonds. The intramolecular hydrogen bonds involve the O atom (O7 and its symmetry equivalent) belonging to a methanol ligand as donor and the O atom (O1 and its symmetry equivalent) of the doubly bridging organic ligand as acceptor [O1 ⋯ O7 = 2.721(3) Å]. The O atom of the remaining methanol ligand (O8 and its symmetry equivalent) is participating as donor in an intermolecular hydrogen bond with the acceptor being the pyridyl N atom (N3 and its symmetry equivalent)  of the doubly bridging organic ligand [O8 ⋯ N3 = 2.851(3) Å, H(O8) ⋯ N3 = 1.987(3) Å, and O8-H(O8)-N3 = 172.1(1)]. This hydrogen bonding leads to the formation of a 1D chain along the c axis (Figure 3).Figure 3
Representation of a part of the 1D chain formed in1 along the c axis. The dotted   lines represent H bonds.The molecule of1 contains the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-ONR)2] core, where R- = -C(py)2 (Figure 2, top). An alternative description of the core (using only the μ3 oximate groups) is [Ni4(μ3-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.Complex1joins 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 theTd point group) consists of two bands at ~1105 and ~615 cm−1, assigned to the v3 (F2) stretching [νd(SO)] and v4(F2) bending [δd(OSO)] modes, respectively [41, 97]. The v1(A1) stretching [νs(SO)] and v2(E) bending [δd(OSO)] fundamentals are not IR active. The coordination of SO42- to metal ions decreases the symmetry of the group, and the v3 and v4 modes are split [41, 97]. In the case when the SO42--site symmetry is lowered from Td to C2v (bidentate chelating or bridging coordination), which is the case in 1, both v1 and v2 appear in the IR spectrum, while v3 and v4 each splits into three IR-active vibrations [97]. Thus, the bands at 1219, 1130, and 1020 cm−1 are attributed to the v3 modes [97], while the bands at 591, 618, and 670 cm−1 are assigned to the v4 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 v1 and v2 modes, respectively. These spectral features agree with the low C2v symmetry for the sulfato ligand in the complex, as also confirmed crystallographically.


## 3.1. Synthetic Comments

Our general synthetic approach for the isolation of NiII/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 NiSO4·6H2O with one equivalent of (py)2CNOH and one equivalent of NEt3 in MeOH gave a red solution which, upon crystallization, gave orange crystals of the new tetranuclear cluster which can be written as [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1). Its formation can be summarized in (1)
(1)4NiSO4⋅6H2O+4(py)2CNOH+4NEt3+4MeOH→MeOH[Ni4{(py)2CNO}4(SO4)2(MeOH)4]+2(HNEt3)2SO4+24H2O.
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, NMe4OH, NEt4OH, and LiOH·H2O. 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.


## 3.2. Description of Structure

Partially labeled plots of the complete structure and the core of the molecule [Ni4{(py)2CNO}4(SO4)2(MeOH)4] that is present in complex 1 are shown in Figures 1 and 2, respectively. Selected interatomic distances and angles are listed in Table 2.Table 2
Selected interatomic distances (Å) and angles (°) for1.
Ni1-O12.046 (3)Ni2-O22.071 (3)Ni1-O22.046 (3)Ni2-O32.042 (3)Ni1-O42.033 (3)Ni2-O72.163 (3)Ni1-O82.048 (3)Ni2-N12.090 (3)Ni1-N52.090 (3)Ni2-N21.997 (4)Ni1-N62.074 (3)Ni2-N42.053 (3)O1-Ni1-O289.59 (11)O2-Ni2-O394.82 (11)O1-Ni1-O4101.65 (11)O2-Ni2-O788.69 (10)O1-Ni1-O8173.53 (11)O2-Ni2-N1169.13 (12)O1-Ni1-N582.89 (11)O2-Ni2-N290.44 (12)O1-Ni1-N683.38 (12)O2-Ni2-N484.07 (12)O2-Ni1-O484.24 (11)O3-Ni2-O7176.29 (10)O2-Ni1-O888.62 (11)O3-Ni2-N188.91 (12)O2-Ni1-N5107.12 (11)O3-Ni2-N295.11 (12)O2-Ni1-N6170.67 (12)O3-Ni2-N487.08 (12)O4-Ni1-O884.36 (12)O7-Ni2-N187.86 (11)O4-Ni1-N5167.92 (13)O7-Ni2-N286.05 (12)O4-Ni1-N691.16 (12)O7-Ni2-N492.09 (12)O8-Ni1-N591.70 (12)N1-Ni2-N279.04 (13)O8-Ni1-N699.02 (12)N1-Ni2-N4106.36 (13)N5-Ni1-N678.16 (13)N2-Ni2-N4174.25 (13)Figure 1
The molecular structure of1. H atoms have been omitted for clarity.Figure 2
Top: the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-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 μ3 oximate groups.The structure of1 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 SO42- 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′.Scheme 3
The two different coordination modes of the (py)2CNO− ligands present in complex 1 and the Harris notation [53] which describes them.A distorted octahedral environment is created about each metal center; the chromophores are represented by the following formulas Ni(1,1′)(Npy)(Nox)(Oox)2(Osulf)(Omet) and Ni(2,2′)(Npy)2(Nox)(Oox)(Osulf)(Omet), where the abbreviations “py”, “ox”, “sulf,” and “met” are for the 2-pyridyl, oximate, sulfato, and methanolic donor atoms, respectively. The average Ni–Oox, Ni–Nox, and Ni–Npy bond lengths of 2.054(3), 2.043(3), and 2.071(3) Å, respectively, agree well with the values expected for high-spin NiII ions in octahedral environment [34, 35, 62–64]. The Ni–Osulf bond lengths are typical [41, 51, 62, 65].  The fact that S-(O3, O4) (average 1.474 Å) > S-(O5, O6) (average 1.450 Å) reflects the coordinating nature of O3 and O4 and the noncoordinating character of O5 and O6; as expected, the sulfur to “free” oxygen bond lengths are the shortest.The crystal structure of1is stabilized by strong inter- and intramolecular hydrogen bonds. The intramolecular hydrogen bonds involve the O atom (O7 and its symmetry equivalent) belonging to a methanol ligand as donor and the O atom (O1 and its symmetry equivalent) of the doubly bridging organic ligand as acceptor [O1 ⋯ O7 = 2.721(3) Å]. The O atom of the remaining methanol ligand (O8 and its symmetry equivalent) is participating as donor in an intermolecular hydrogen bond with the acceptor being the pyridyl N atom (N3 and its symmetry equivalent)  of the doubly bridging organic ligand [O8 ⋯ N3 = 2.851(3) Å, H(O8) ⋯ N3 = 1.987(3) Å, and O8-H(O8)-N3 = 172.1(1)]. This hydrogen bonding leads to the formation of a 1D chain along the c axis (Figure 3).Figure 3
Representation of a part of the 1D chain formed in1 along the c axis. The dotted   lines represent H bonds.The molecule of1 contains the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-ONR)2] core, where R- = -C(py)2 (Figure 2, top). An alternative description of the core (using only the μ3 oximate groups) is [Ni4(μ3-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.Complex1joins 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 theTd point group) consists of two bands at ~1105 and ~615 cm−1, assigned to the v3 (F2) stretching [νd(SO)] and v4(F2) bending [δd(OSO)] modes, respectively [41, 97]. The v1(A1) stretching [νs(SO)] and v2(E) bending [δd(OSO)] fundamentals are not IR active. The coordination of SO42- to metal ions decreases the symmetry of the group, and the v3 and v4 modes are split [41, 97]. In the case when the SO42--site symmetry is lowered from Td to C2v (bidentate chelating or bridging coordination), which is the case in 1, both v1 and v2 appear in the IR spectrum, while v3 and v4 each splits into three IR-active vibrations [97]. Thus, the bands at 1219, 1130, and 1020 cm−1 are attributed to the v3 modes [97], while the bands at 591, 618, and 670 cm−1 are assigned to the v4 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 v1 and v2 modes, respectively. These spectral features agree with the low C2v 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 thebinarySO42-/(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(μ2-ONR)2(μ3-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 of1 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 for1. 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.



---

*Source: 606271-2011-04-20.xml*