Novel Cage-Like Hexanuclear Nickel(II) Silsesquioxane. Synthesis, Structure, and Catalytic Activity in Oxidations with Peroxides

New hexanuclear nickel(II) silsesquioxane [(PhSiO1.5)12(NiO)6(NaCl)] (1) was synthesized as its dioxane-benzonitrile-water complex (PhSiO1,5)12(NiO)6(NaCl)(C4H8O2)13(PhCN)2(H2O)2 and studied by X-ray and topological analysis. The compound exhibits cylinder-like type of molecular architecture and represents very rare case of polyhedral complexation of metallasilsesquioxane with benzonitrile. Complex 1 exhibited catalytic activity in activation of such small molecules as light alkanes and alcohols. Namely, oxidation of alcohols with tert-butylhydroperoxide and alkanes with meta-chloroperoxybenzoic acid. The oxidation of methylcyclohexane gave rise to the isomeric ketones and unusual distribution of alcohol isomers.

To use the advantage of good solubility of cage metallasilsesquioxanes in organic solvents and the ability of bringing unusual effects in the catalytic act due to specific structures of catalytic centers [19] Molecules 2016, 21, 665 2 of 12 we decided to study some other (not copper-containing) types of CLMSs. Here we present preliminary results on first examination of new Ni(II)-CLMS under oxidation conditions.

Synthesis
Recently, some of us reported on ability of 1,4-dioxane molecules to serve as bridging linkers, combining individual CLMSs into an entire supramolecular system [20], and we were interested in the synthesis of new dioxane-CLMS complexes. Performing the synthesis of Ni-phenylsilsesquioxane [starting from PhSi(OEt) 3 ] in dioxane-containing media allowed us to isolate (in 20% yield) a new cage-like hexanuclear product [(PhSiO 1.5 ) 12 (NiO) 6 (NaCl)] (1) in the form of its adduct with dioxane/benzonitrile/water solvating ligands (PhSiO 1,5 ) 12 (NiO) 6  To use the advantage of good solubility of cage metallasilsesquioxanes in organic solvents and the ability of bringing unusual effects in the catalytic act due to specific structures of catalytic centers [19] we decided to study some other (not copper-containing) types of CLMSs. Here we present preliminary results on first examination of new Ni(II)-CLMS under oxidation conditions.

Structure
The age skeleton of the product belongs to a CLMS of the cylinder type [9], characterized by the presence of three layers: two 12-membered siloxane cycles mutually connected through a metal-oxide (Ni6O6) cycle ( Figure 1; see Supplementary Materials for more details). The inner void of the cylinder moiety contains a chloride anion which occupies the crystallographic center of inversion. Each Ni(II) ion of 1 adopts distorted octahedral coordination. The axial positions of the octahedron are occupied by a Clanion and oxygen or nitrogen atoms of coordinated dioxane and benzonitrile molecules, respectively. Four nickel ions of 1 are coordinated by dioxane molecules, while the remaining two ions are bonded to benzonitrile molecules ( Figure 2

Structure
The age skeleton of the product belongs to a CLMS of the cylinder type [9], characterized by the presence of three layers: two 12-membered siloxane cycles mutually connected through a metal-oxide (Ni 6       This is just the second evidence that benzonitrile ligands could participate in aggregation of a CLMS structure. The first observation of such unusual coordination was presented by some of us [21] for the copper-containing CLMS.

Topological Analysis and Supramolecular Assembly
Following the procedure of a metal cluster notation [22] implemented into the ToposPro package (the Samara Center for Theoretical Materials Science, Samara, Russia) [23] we obtained that nickel atoms in compound 1 form in terms of the NDk-m notation the discrete 5M6-1clusters, where 5 is the coordination number of topologically non-equivalent nodes, M denotes a discrete cluster, 6 is the number of metal atoms in the cluster, and 1 is a classification number to distinguish topologically-distinct clusters with equal NDk parameters. A database of topological representations of polynuclear nickel compounds [24] contains representatives of the nickel clusters with the 5M6-1 topology, and µ 6 -coordinated Hal´and S 2´a nions; recently, some of us have synthesized a nickel-silsesquioxane encapsulating the O 2´a nion [13]. Nevertheless, to our knowledge, complex 1 is only the sixth known representative of Ni 6 clusters with the 5M6-1 topology.
An additional attractive feature of synthesized complex is a formation of supramolecular structure where cage components are assembled into infinite chains ( Figure 3) via H-bonds between water molecules bonded to sodium anions and oxygen atoms of siloxane cycles. The r(O¨¨¨O) and =OHO are equal to 3.364(6)-3.488(6) Å and 135.7˝-149.0˝. The connection between ions is additionally supported by C-H¨¨¨O interactions between 1,4-dioxane and silsesquioxane as short as 3.62(2) and 3.91(2) Å for r(O¨¨¨C). As a consequence, cylinder cage fragments and complex cations Na(H 2 O) 2 (O 2 C 4 H 8 ) 2 share the same pseudo two-fold axis parallel to the [100]-crystallographic direction. The chains are packed as the hexagonal rod packing, with the distances between two-fold axes of 16.1 and 16.3 Å and non-parallel disposition of Ni 6 metal rings. Only weak C-H¨¨¨O and C-H¨¨¨π bonding between neighboring chains, or chains and solvent molecules, were found. Worth noting is that the shortest distance between two oxygen atoms of 1,4-dioxanes connected with Ni is equal to 8.6 Å, which is only slightly longer than the distance between nitrogen atoms of 4,4 1 -bipyridine, at 7.1 Å. In principle, this means that bipyridine and its analogues can be used to obtain coordination polymers connected by linkers through d-metals even for bulky phenylsilsesquioxanes. This opportunity will be a subject of our further investigations. supported by C-H···O interactions between 1,4-dioxane and silsesquioxane as short as 3.62(2) and 3.91(2) Å for r(O···C). As a consequence, cylinder cage fragments and complex cations Na(H2O)2(O2C4H8)2 share the same pseudo two-fold axis parallel to the [100]-crystallographic direction. The chains are packed as the hexagonal rod packing, with the distances between two-fold axes of 16.1 and 16.3 Å and non-parallel disposition of Ni6 metal rings. Only weak C-H···O and C-H···π bonding between neighboring chains, or chains and solvent molecules, were found. Worth noting is that the shortest distance between two oxygen atoms of 1,4-dioxanes connected with Ni is equal to 8.6 Å, which is only slightly longer than the distance between nitrogen atoms of 4,4′-bipyridine, at 7.1 Å. In principle, this means that bipyridine and its analogues can be used to obtain coordination polymers connected by linkers through d-metals even for bulky phenylsilsesquioxanes. This opportunity will be a subject of our further investigations.
The oxidation of n-octane obtained earlier for other catalysts (c-f). Indeed, concentrations of isomers P8 and P9 is noticeably low in comparison with amounts of both P6, P7 and P10, P11. This effect has not been found for other catalysts (c-f) and is apparently due to sterical hindrance around catalytic centers in 1. Like in the cyclohexane oxidation, chromatograms of oxygenates obtained from methylcyclohexane (Figure 6a,b) before and after reduction with PPh 3 are very similar and this indicates that alkyl hydroperoxides are also not formed in this experiment.   . It can be clearly seen that the ratio of isomeric alcohols in the case of this catalytic system (a, b) is different from that obtained earlier for other catalysts (c-f). Indeed, concentrations of isomers P8 and P9 is noticeably low in comparison with amounts of both P6,P7 and P10,P11. This effect has not been found for other catalysts (c-f) and is apparently due to sterical hindrance around catalytic centers in 1. Like in the cyclohexane oxidation, chromatograms of oxygenates obtained from methylcyclohexane (Figure 6a,b) before and after reduction with PPh3 are very similar and this indicates that alkyl hydroperoxides are also not formed in this experiment.     [19]); (η 6 -p-cym)Os(py)Cl 2 /py/H 2 O 2 (f, [53]).
The oxidation of cis-1,2-dimethylcyclohexane with m-CPBA in MeCN catalyzed complex 1 proceeds non-stereoselectively: the ratio of formed tertiary trans and cis alcohols t/c was 0.88 before reduction with PPh 3 and 0.93 after the reduction (yield was 12% based on cis-1,2-DMCH; TON = 34). In the blank experiment (without complex 1) the t/c ratio was 0.77 after reduction with PPh 3 (yield was 5%).

Synthesis of Compound 1
Compound PhSi(OEt) 3 and solvents were purchased from Acros Organics (Moscow, Russia) and were used as received.
Compound PhSi(OEt) 3 (3 g, 12.48 mmol), water (0.45 g, 24.96 mmol) and NaOH (0.5 g, 12.50 mmol) in 20 mL of methanol were placed into a flask, equipped with a magnetic stirrer and condenser. After total dissolution of NaOH, the solution was heated to reflux for 1.5 h. Afterwards solution was cooled down to room temperature and mixed with 85 mL of dioxane. Then Ni(NH 3 ) 6 Cl 2 (1.4 g, 6.04 mmol) was added at once. Mixture was brought to reflux along with simultaneous distillation of the solution to remove methanol from reaction mixture. When 18 mL of distillate was collected, mixture was heated to reflux for additional 4 h and then left stirring at room temperature overnight. Then reaction mixture was filtered into an evaporation flask containing benzonitrile (8 mL). The flask was equipped with a septum and needle to allow solvents to evaporate under a slow current of nitrogen. Immediately after yellow-colored crystals began to form, the flask was transferred to the fridge and stored there until the crystal fraction growth (two weeks) ceased, as visually determined.

X-ray Diffraction Study
X-ray diffraction studies were carried out on Bruker APEX DUO diffractometer (Madison, WI, USA). The structure was solved by direct method and refined in anisotropic approximation against F2. The positions of hydrogen atoms were calculated from geometrical point of view and refined in isotropic approximation (the C-H and O-H distances and displacement parameters of hydrogen atoms are constrained). All calculations were carried out with SHELX (Gottingen, Germany) [54,55] and OLEX2 software (Durham, UK) [56]. The experimental parameters and crystal data are summarized in Table 2.

Catalytic Oxidation of Alkanes and 1-Phenylethanol
Typically, the catalyst and the co-catalyst (nitric or trifluoroacetic acid) were introduced into the reaction mixture in the form of stock solutions in acetonitrile. The reactions of alcohols and hydrocarbons were carried out in air in thermostated Pyrex cylindrical vessels with vigorous stirring and using MeCN as solvent. The substrate (alcohol or hydrocarbon) was then added and the reaction started when the oxidant was introduced in one portion (CAUTION: the combination of air or molecular oxygen and peroxides with organic compounds at elevated temperatures may be explosive!). The reactions with 1-phenylethanol were analyzed by 1 H-NMR method (solutions in acetone-d 6 ; "Bruker AMX-400" instrument, 400 MHz, Billerica, MA, USA). Areas of methyl group signals were measured to quantify oxygenates formed in oxidations of 1-phenylethanol. As stated previously, the samples obtained in the alkane oxidation were typically analyzed twice (before and after their treatment with PPh 3 ) by GC. This method (an excess of solid triphenylphosphine is added to the samples 10-15 min before the GC analysis) was proposed by one of us earlier [48][49][50][51]. Samples of the reaction mixture were analyzed by GC (Agilent 6890, Santa Clara, California, United States, N 2 was carrier gas, FID) and GC-MS (Shimadzu QP-2010 Plus, Nishinokyo-Kuwabara-cho, Nakagyo-ku, Kyoto 604-8511, Japan; He was carrier gas); in both instruments the column was BP-20 (SGE; polyethyleneglycol 30 mˆ250 µmˆ0.25 µm). Assignment of peaks was made by comparison with chromatograms of authentic samples and by GC-MS.