6,6′-{[Ethane-1,2-diylbis(azaneylylidene)]bis(methaneylylidene)}bis[2-(hexyloxy)phenolato] Nickel(II)

: Polymeric nickel complexes of salen ligands meet a wide range of applications in catalysis and electrochemistry. Because these polymers usually form very rigid ﬁlms, the introduction of the conformationally ﬂexible fragments in the corresponding monomers favors the amorphization and, thus, the mass transport. Herein we report a preparation of the hexyloxy-substituted monomeric NiSalen complex by the direct alkylation of the hydroxy-substituted complex. The resulting product was characterized by the 1 H and 13 C nuclear magnetic resonance (NMR), ESI-high resolution mass spectrometry (ESI-HRMS, and Fourier-transform infrared spectroscopy (FTIR). The crystal structure of the product was examined by an XRD, indicating the formation of the solvate with dichloromethane. The obtained product was found to be highly soluble in non-polar solvents, in contrast to typical NiSalens.

These polymeric complexes are known for unique properties, such as reversible electrochemical oxidation in a wide range of potentials, thermal stability, high redox and electronic conductivity, and high specific capacities [10,14,15]. Polymeric NiSalens met their application in Li-ion batteries as cathode active materials [16] and binders [17].

Results
The [Ni(HOSalEn)] complex was subjected to an alkylation with 1-bromohexane while using different reaction conditions (Scheme 1). Attempts of alkylation using the widely used system, NaH in THF [19], failed due to the low solubility of the starting material. K 2 CO 3 or Cs 2 CO 3 in DMF or DMSO, which are frequently exploited for the alkylation of phenols [20], afforded the desired product in low yields (<10%). Addition of NaI had no effect on the reaction yield. Finally, NaH in DMF at 60 • C afforded the [Ni(HexOSalEn)] complex in 79% yield after 2 days.

Results
The [Ni(HOSalEn)] complex was subjected to an alkylation with 1-bromohexane while using different reaction conditions (Scheme 1). Attempts of alkylation using the widely used system, NaH in THF [19], failed due to the low solubility of the starting material. K2CO3 or Cs2CO3 in DMF or DMSO, which are frequently exploited for the alkylation of phenols [20], afforded the desired product in low yields (<10%). Addition of NaI had no effect on the reaction yield. Finally, NaH in DMF at 60 °C afforded the [Ni(HexOSalEn)] complex in 79% yield after 2 days. The product was purified by flash chromatography, followed by the crystallization from hexane-dichloromethane mixture. The melting point of the product comprises 172-174 °C, which is significantly lower as compared with typical NiSalen complexes, indicating an influence of the hexyloxy groups. The 1 H-NMR spectrum of the resulted complex ( Figure S1) shows the typical set of hexyloxy signals: 6H triplet at 0.74 ppm with 3 JH-H = 6.8 Hz, 8H multiplet at 0.93-1.13 ppm, two 4H multiplets in the ranges of 1.28-1.42 and 1.42-1.59 ppm, and smoothed triplet at 4.74 ppm, which is noticeably shifted upfield when compared with the normal position of alkoxy methylene group. Additionally, the ethylene bridge 4H singlet appears at 3.66 ppm, along with the 2H singlet of imine protons at 7.87 ppm and a set of aromatic signals, 2H triplet at 6.49 ppm, and two 2H doublets at 6.66 and 6.81 ppm, all with 3 JH-H = 7.9 Hz. 13  We demonstrate the possibility for the direct alkylation of [Ni(HOSalEn)] with hexyl bromide, which provides the facile one step route to the diversity of alkoxy substituted NiSalens and the polymers obtained thereof. The obtained complex may be used further in order to synthesize the polymeric materials for energy storage and electrocatalysis. The product was purified by flash chromatography, followed by the crystallization from hexane-dichloromethane mixture. The melting point of the product comprises 172-174 • C, which is significantly lower as compared with typical NiSalen complexes, indicating an influence of the hexyloxy groups. The 1 H-NMR spectrum of the resulted complex ( Figure S1) shows the typical set of hexyloxy signals: 6H triplet at 0.74 ppm with 3 J H-H = 6.8 Hz, 8H multiplet at 0.93-1.13 ppm, two 4H multiplets in the ranges of 1.28-1.42 and 1.42-1.59 ppm, and smoothed triplet at 4.74 ppm, which is noticeably shifted upfield when compared with the normal position of alkoxy methylene group. Additionally, the ethylene bridge 4H singlet appears at 3.66 ppm, along with the 2H singlet of imine protons at 7.87 ppm and a set of aromatic signals, 2H triplet at 6.49 ppm, and two 2H doublets at 6.66 and 6.81 ppm, all with 3 J H-H = 7.9 Hz. 13  We demonstrate the possibility for the direct alkylation of [Ni(HOSalEn)] with hexyl bromide, which provides the facile one step route to the diversity of alkoxy substituted NiSalens and the polymers obtained thereof. The obtained complex may be used further in order to synthesize the polymeric materials for energy storage and electrocatalysis.

General Considerations
Reagents of "reagent grade" purity were purchased from Sigma-Aldrich (Europe). [Ni(HOSalEn)] was obtained, as described in literature [21]. The Fourier transform infrared spectra were recorded on Shimadzu IRaffinity-1 FTIR spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in KBr pellets. UV-Vis spectra were recorded in CHCl3 solutions on Shimadzu UV-1800 double-beam spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in 1 cm quartz cuvettes. 1 H and 13 C-NMR spectra were acquired on a Bruker Avance 400 spectrometer (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) at 400 and 101 MHz, respectively, in CDCl3 while using a residual solvent peak as internal standard. HRMS of the positive ions was recorded from the methanolic solution of the sample using electrospray ionization of on a Bruker micrOTOF apparatus (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) in positive mode.
X-ray single crystal analyses were performed on Agilent Technologies «Xcalibur» diffractometer (Agilent Technology, Santa Clara, CA, USA) with monochromated Mo Kα radiation. The temperature was kept at 100 K during data collection. The structure was solved while using Olex2 [22] and refined with the SHELXL [23] refinement package (G. Sheldrick, Gottingen, Germany) using Least Squares minimization. CCDC 2045743 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk.

General Considerations
Reagents of "reagent grade" purity were purchased from Sigma-Aldrich (Europe). [Ni(HOSalEn)] was obtained, as described in literature [21]. The Fourier transform infrared spectra were recorded on Shimadzu IRaffinity-1 FTIR spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in KBr pellets. UV-Vis spectra were recorded in CHCl 3 solutions on Shimadzu UV-1800 double-beam spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in 1 cm quartz cuvettes. 1 H and 13 C-NMR spectra were acquired on a Bruker Avance 400 spectrometer (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) at 400 and 101 MHz, respectively, in CDCl 3 while using a residual solvent peak as internal standard. HRMS of the positive ions was recorded from the methanolic solution of the sample using electrospray ionization of on a Bruker micrOTOF apparatus (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) in positive mode.
X-ray single crystal analyses were performed on Agilent Technologies «Xcalibur» diffractometer (Agilent Technology, Santa Clara, CA, USA) with monochromated Mo Kα radiation. The temperature was kept at 100 K during data collection. The structure was solved while using Olex2 [22] and refined with the SHELXL [23] refinement package (G. Sheldrick, Gottingen, Germany) using Least Squares minimization. CCDC 2045743 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk.

Synthesis of 6,6 -{[Ethane-1,2-diylbis(azaneylylidene)]bis(methaneylylidene)}bis[2-(hexyloxy)phenolato] Nickel(II)
To a stirred suspension of [Ni(HOSalEn)] (179 mg, 0.5 mmol) in 5 mL of dry DMF, NaH (60% susp. in oil, 44 mg, 1.1 mmol) was added in one portion under Ar, and then the mixture was stirred at 60 • C for 1 h. At this point, 1-bromohexane (155 µL, 182 mg, 1.1 mmol) was added in one portion, and the reaction mixture was stirred at 60 • C for 48 h under Ar. The reaction mixture was cooled to RT and then poured in 50 mL of H 2 O with 200 µL of AcOH. Sticky precipitate was collected by decantation, washed with water, and dried at 50 • C on air. The dried residue was refluxed in hexane, cooled to RT, filtered, and washed with hexane. The obtained precipitate was purified on a short chromatographic column (SiO 2 , DCM → DCM + 2% MeOH), evaporated, and crystallized while using layer-by-layer crystallization technique from 5 mL of DCM layered with 25 mL of hexane to obtain a desired product as brown crystalline solid (207 mg, 0.395 mmol, 79%).