English

• 1. INTRODUCTION

  Due to the coexistence of mildly acidic hydroxyl group and slightly basic imine unit, the name of oxime is derived from the combination of hydroxyl and imine, C=NOH. Since Chugaev discovered the reaction between Ni(Ⅱ) salts and dimethylglyoxime to form the [4N]-coordinated complex in 1905, a great deal of work on oxime and their coordination complexes has been published^[1-3]. Subsequently, complexes of Co(Ⅲ) vic-dioximes have been extensively investigated as a result of their similarity to vitamin B₁₂^[4]. In recent years, the oxime-containing transition metal complexes have drawn great attention as catalysts for electro- and photocatalytic H₂-evolving reaction (HER), not only due to the relatively high turnover frequencies and moderate overpotentials, but also to their designable and tunable structures^[5-7]. Especially, these complexes containing alkyl bridged diimine-dioxime ligands exhibit much better stabilities and activities than those of bis-glyoximato complexes^[8-11]. The redox properties of model complexes could be adjusted via modifying the alkyl bridge and the R groups on C=N unites.

  As shown in Fig. 1, the bridged diimine-dioxime ligands could coordinate with metal atoms through N and/or O atoms in versatile coordination modes. In most complexes, the tetradentate ligand just coordinates via four N atoms, and the two oxygen atoms do not participate. The H-bridged coordination mode I and BF₂-analogue mode Ⅱ are quite common^[12-14]. O-binding is generally found in binuclear and multinuclear complexes as bridging oximato mode Ⅲ, wherein the N atoms of oxime units are also involved in bonding^{[15, 16]}. Only a limited number of complexes are known to belong to mode IV at present, in which the oximate anions are linked to the metal atom through O atoms of oxime and N atoms of imine^[17-19]. However, most of these complexes were merely characterized by UV-vis and IR spectra, without more structure details such as ¹H NMR or single crystal structure by X-ray diffraction.

  Figure 1

  

  Figure 1.  Coordination modes of diimine-dioxime

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  Herein, a diimine-dioxime ligand ^PhdoenH₂ was synthesized from α-benzilmonoxime and ethylenediamine. The Ni(Ⅱ) complex 1 via the complexation of ^PhdoenH₂ with Ni(Ⅱ) salts (Ni(ClO₄)₂, Ni(BF₄)₂, NiCl₂, Ni(acac)₂ and NiSO₄) exhibits the rare coordination mode IV which is different from those reported Ni(Ⅱ) complexes, such as [Ni(^MedoenH)]⁺ (2; ^MedoenH₂ = 1, 8-dihydroxyimino-1, 2, 7, 8-tetramethyl-3, 6-diazocta-2, 6-diene)^[20]. The structure of complex 1 has been carefully characterized by ¹H NMR spectrum, elemental analysis, and X-ray single-crystal diffraction. Theoretical density functional theory (DFT) investigations revealed that the coordination mode should be attributed to the different relative thermodynamic stabilities between the oxime cis- and trans-isomers of ^PhdoenH₂ and ^MedoenH₂.

  2. EXPERIMENTAL

  2.1 Materials and methods

  α-Benzilmonoxime and [Ni(^MedoenH)]ClO₄ were prepared according to the literature methods, respectively^{[20, 21]}. Ethylenediamine and Ni(Ⅱ) salts were purchased from Aladdin Reagent Inc. (China).

  ¹H NMR spectra were collected in deuterated dimethyl sulfoxide (DMSO-d₆) using Bruker AC-400 at room temperature, and chemical shifts were reported in ppm units with respect to the reference frequency of TMS. Mass spectrometry (MS) analyses were carried out on an electrospray ionization time-of-flight mass spectrometer (ESI-TOF MS, UltiMate 3000, Dionex Company). UV-vis spectra were measured on a Shimadzu UV1800 spectro-photometer. Elemental analyses (C, H and N) were performed by Vario EL-Ⅲ CHNOS instrument. Infrared spectra were obtained on a BEQ UZNDX 550-FTIR spectrophotometer (Bruker) with KBr pellets. Thermal gravimetric analysis was performed on a NETZSCH STA449C instrument.

  2.2 Synthesis of ^PhdoenH₂

  α-Benzilmonoxime (2.25 g, 10 mmol) and ethylenediamine (0.30 g, 5 mmol) were refluxed in 15 mL methanol for 48 h. After cooling to room temperature, the white solid precipitated was filtered, washed with methanol, and dried under vacuum (1.49 g, yield: 63%). ¹H NMR (400MHz, DMSO-d₆): δ, ppm, 11.73 (s, 2H, 2 OH), 7.75~7.25 (m, 20H, 4 Ph), 3.90~3.50 (m, 4H, CH₂CH₂).

  2.3 Synthesis of [Ni(^Phdoen)] (1)

  [(^PhdoenH₂)] (0.47 g, 1 mmol) was added into 10 mL EtOH, and the mixture was brought to reflux. Then the solution of Ni(ClO₄)₂·6H₂O (0.37 g, 1 mmol) in 10 mL EtOH was added dropwise into the suspension. The mixture was refluxed overnight. After evaporation of solvents, the obtained solid was purified by chromatography on silica gel with CH₂Cl₂/EtOH (50:1, v/v) as eluent. An orange solid was obtained from the collected red band after removing the solvent (0.16 g, yield: 30%). Single crystals suitable for X-ray diffraction were obtained by layering n-hexane over a CH₂Cl₂ solution containing the crude complex at room temperature. ¹H NMR (400MHz, DMSO-d₆): δ, ppm, 7.40~7.00 (m, 20H, 4 Ph), 3.09 (s, 4H, CH₂CH₂). Elemental analysis (%): calcd. for C₃₀H₂₄N₄NiO₂: C, 67.83; H, 4.55; N, 10.55. Found: C, 67.72, H, 4.35, N, 10.21. HR-MS (ESI): m/z⁺ = 531.1326 (M+H⁺), calcd.: 530.1253.

  2.4 X-ray structure determination

  The diffraction data were collected using a graphite-monochromated CuKα radiation (λ = 1.54178 Å) at 100.00(10) K with a SuperNova, Dual, Cu at zero, AtlasS2 diffractometer. The structure was solved by direct methods using the SHELXS and refined with SHELXL of the SHELXTL package^[22]. Anisotropic thermal factors were assigned to all non-hydrogen atoms. H atoms attached to C were added theoretically. Complex 1 is of monoclinic system, space group P2₁/c with a = 12.8234(4), b = 13.0403(4), c = 15.7134(6) Å, β = 106.622(4)º, V = 2517.81(15) ų, Z = 4, S = 1.034, F(000) = 1104, R = 0.0577 and wR = 0.1484 (I > 2σ(I)). R = Σ||F_o| − |F_c||/Σ|F_o|; wR = [Σ[w(F_o² − F_c²)²]/Σw(F_o²)²]^1/2. The selected bond lengths and bond angles are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ni(1)–O(1)	1.853(2)		Ni(1)–N(3)	1.844(2)		N(1)–O(1)	1.309(3)
  Ni(1)–O(2)	1.826(2)		Ni(1)–N(4)	1.853(2)		N(2)–O(2)	1.313(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Ni(1)–O(2)	81.98(9)		O(1)–Ni(1)–N(3)	94.72(10)		Ni(1)–O(1)–N(1)	127.38(17)
  N(3)–Ni(1)–N(4)	88.31(10)		O(2)–Ni(1)–N(4)	94.90(9)		Ni(1)–O(2)–N(2)	129.01(17)

  2.5 Electrochemical experiment

  Electrochemistry measurements were carried out with a CHI650E electrochemical workstation. A glassy carbon disk (diameter 3 mm) was used as the working electrode, a platinum column as the counter electrode, and Ag/AgNO₃ electrode as the reference electrode. All potentials were referenced against ferrocene/ferrocenium as an external standard. The solution was purged with argon for 15 min before measurement. A solution of 0.1 mol·L⁻¹ n-Bu₄NPF₆ in MeCN was used as the supporting electrolyte.

  2.5 Computational details

  The geometries were optimized at density functional TPSS^[23] level of theory. The SDD type ECP together with the valence basis functions^[24] was chosen for the metal element and def2-SVP for the rest (TPSS/SDD/def2-SVP)^[25]. Solvation effects in ethanol were treated by the implicit solvation model IEFPCM^[26]. Study showed that the hybrid functional with small percentage of exchange performed well for late transition metal compounds^[27]. The geometry was optimized at the TPSSh/SDD/def2-SVP level of theory in acetonitrile solution. The def2-SVP part of basis set was replaced by def2-TZVP in the TD-DFT calculations. The natural transition orbital (NTO) analysis was performed on the excited states^[28]. The free energies of the compounds and their reduction species were obtained at the TPSS/SDD/def2-TZVP level of theory using the ideal-gas approximation at 298 K and 1 atm. SMD model was employed to compute the solvation energy^[29]. The global minimum search was performed with Spartran'10 program^[30]. The low-lying conformers were first picked up using both the systematic and Monte Carlo search methods and the energies were evaluated with the MMFF94 force field method^[31]. The configurations with molecular mechanics energies within 5 kcal·mol⁻¹ were selected for further DFT B3LYP-D3/6-31G(d) optimizations and finally DLPNOCCSD(T)/cc-pVTZ single point energy calculations (E) at the B3LYP-D3/6-31G(d) level^{[32, 33]}. The solvation energies (E_sol) with SMD model at the DFT optimized geometries were computed at the B3LYP-D3/6-311+G(d) level and free energy corrections (∆G) at the B3LYP-D3/6-31G(d) level with the ideal gas approximation. The free energy is finally expressed as G = E + E_sol + ∆G. The EDA calculation was performed with ADF 2016 program and bond orders with Multiwfn 3.6^[34]. ORCA 4.2 was used for wavefunction calculations^{[35, 36]}. All other calculations were performed with the Gaussian09 program^[37].

  3. RESULTS AND DISCUSSION

  3.1 Synthesis and characterization

  In 1964, Uhlig and Friedrich reported the diimine-dioxime ligand ^MedoenH₂ reacted with Ni(ClO₄)₂·6H₂O to form complex 2 in coordination mode I ([NiN₄]⁺)^[20]. When the analogue ligand ^PhdoenH₂ was used to react with Ni(Ⅱ), it is interesting to find that the only isolable product is [Ni(^Phdoen)] belonging to the rare coordination mode IV ([NiN₂O₂]) (Scheme 1). In the ¹H NMR spectrum of 1, the OH proton and hydrogen bridge (O–H···O) band were not observed, which indicates that coordination took place via the oxygen atoms of oxime group rather than nitrogen atoms. And the result of elemental analyses is also in good agreement with the theoretical value.

  Scheme1

  

  Scheme1.  Syntheses of complexes 1 and 2

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  3.2 X-ray crystal structure

  The structure of complex 1 was further confirmed by X-ray diffraction analysis of single crystal obtained by layering n-hexane over a CH₂Cl₂ solution. Fig. 2 shows the structure of monomeric unit of 1. The nickel left of 1 is coordinated with two N and two O atoms of the ligand, and any atoms of solvent molecules or adjacent units do not bind to the metal left from the axial sites. The NiN₂O₂ unit displays a slightly distorted square planar geometry with a 5-6-5 ring pattern, and Ni(1) lies 0.012 Å from the N₂O₂ plane. The X-ray crystal structure of complex 2 which features a non-symmetrical hydrogen-bond has been reported by Peters^[16]. In complex 2, the nickel left is also in the distorted square planar environment, but is ligated by four N atoms of the diimine-dioxime ligand. The distances of Ni–N_imine bonds in complex 1 are slightly longer than those in 2. The distances of N–O^– bonds in 1 are almost identical with that of N(2)–O(2) bond in the deprotonated oximate unit of 2, and significantly shorter than the N(1)–O(1) bond distance of oxime unit in 2. The O(1)−Ni(1)−O(2) angle in 1 is constrained in the present structure to the square 81.98(9)°, while the N(1)−Ni(1)−N(2) angle in 2 is 102.45(14)°.

  Figure 2

  

  Figure 2.  ORTEP drawing of complex 1 showing thermal ellipsoids at the 50% probability level

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  3.3 Electrochemistry

  Redox properties of complexes 1 and 2 were studied by cyclic voltammetry in MeCN under an Ar atmosphere. The ferrocene (F_c) was used as the reference, and the potentials are given versus F_c^0/+. The cyclic voltammogram (CV) of 2 exhibits a reduction peak at E_pc = −1.12 V and an associated re-oxidation peak at E_pa = −1.04 V, as shown in Fig. 3. The peak-to-peak separation for this couple is ΔE_p = 72 mV at a scan rate of 100 mV·s⁻¹, while ΔE_p is also 75 mV for the couple of Fc^0/+ under the same condition. This process is attributed to the one-electron redox of Ni^Ⅱ/Ni^I couple. For comparison, the cyclic voltammogram (CV) of 1 exhibits an irreversible reduction peak at E_pc = −2.05 V, which is more negative than that of 2. It indicates the strong electrondonating ability of O⁻ atoms renders the reduction of nickel core in 1 more difficultly. The coordination bonds were analyzed for complexes 1 and 2. The energy decomposition shows that Ni–O/N bonds have balanced contributions from both ionic and covalent interactions. The bond orders are less than 1. The Mulliken charges on Ni are +0.20 and +0.39 e for 1 and 2, respectively. The smaller positive charges of Ni atom in 1 make it difficult to be reduced than those in 2, and this result is consistent with the phenomena of cyclic voltammetry. The calculated values of the relative reduction potentials are −1.819 and −0.935 V for 1 and 2, respectively, which are in good agreement with the experimental values. Note that the spin density of the Ni(I) complex is delocalized over the ligand (Fig. S10).

  Figure 3

  

  Figure 3.  CVs of 1 and 2 in MeCN under Ar atmosphere. Conditions: 1 mM analyte in 0.1 mol⋅L⁻¹ n-Bu₄NPF₆/MeCN; glassy carbon working electrode, Pt wire counter electrode, Ag/AgNO₃ reference electrode; referenced to internal Fc standard; scan rate 100 mV⋅s⁻¹

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  3.4 DFT calculation

  The different coordination modes of 1 and 2 can be attributed to the different geometry properties of ligands. The C=N double bond of the oxime group leads to either cis or trans isomers. The coordination mode of complexes 1 or 2 requires the ligand to be a cis or trans isomer, respectively. Consider the hypothetical reaction of mode I/IV interconversion:

  $ \left[\mathrm{NiN}_4\right]^{+}+cis-\text{ligand } \rightarrow\left[\mathrm{NiN}_2 \mathrm{O}_2\right]+[\mathrm{H}(trans-\text{ligand})]^{+} $

  (I)

  For ligand ^MedoenH₂, the free energy (ΔG) of reaction I is 9.47 kJ⋅mol⁻¹, indicating that the complex [NiN₄]⁺ is more stable. Meanwhile, the global minimization shows that the energy of cis isomer of ^MedoenH₂ is 22 kJ⋅mol⁻¹ higher than that of the trans isomer (Fig. 4). Therefore, the trans isomer would be the dominant existence of ^MedoenH₂, which results in the formation of 2 with coordination mode I in the reaction of ^MedoenH₂ and Ni(Ⅱ). In contrast, ΔG is negative (−0.11 kJ⋅mol⁻¹) for the ligand ^PhdoenH₂, implying that complex [NiN₂O₂] is preferred. However, the energy gap between the cis- and trans-isomers of ^PhdoenH₂ is only 0.8 kJ⋅mol⁻¹, which means the coexistence of these two isomers. So, with very small reaction free energies, the reaction between ^PhdoenH₂ and Ni(Ⅱ) would be quite complicated, accompanied with the formation of various products, where [NiN₂O₂] is the most abundant one. This result is consistent with the experimental phenomena.

  Figure 4

  

  Figure 4.  The lowest trans and cis isomers of ligand ^MedoenH₂ (left) and ^PhdoenH₂ (right). Relative energies are in kJ⋅mol⁻¹

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  4. CONCLUSION

  Herein, the synthesis and structural characterization of a mononickel complex 1 were reported, in which the diimine-dioxime ligand links to the Ni atom through two O atoms of oxime and two N atoms of imine. As far as we know, it is the first time that the crystal structure and ¹H NMR spectra of this unusual [2N2O]-coordination mode were reported. Recently, the alkyl bridged diimine-dioxime ligands have been widely applied into the synthesis of molecular catalysts for HER. However, when designing the [4N]-coordinated transition-metal complexes, the effect of diimine-dioxime ligand should be considered. There are two isomers, cis and trans, for the C=N double bond of the oxime group. And the [2N2O]- and [4N]-coordination modes require the ligands to be a cis or trans isomer of the C=N double bond in oxime group, respectively. The equilibrium abundance of each isomer is determined by the relative free energies.

  
  
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  Figure 1  Coordination modes of diimine-dioxime

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  Scheme1  Syntheses of complexes 1 and 2

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  Figure 2  ORTEP drawing of complex 1 showing thermal ellipsoids at the 50% probability level

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  Figure 3  CVs of 1 and 2 in MeCN under Ar atmosphere. Conditions: 1 mM analyte in 0.1 mol⋅L⁻¹ n-Bu₄NPF₆/MeCN; glassy carbon working electrode, Pt wire counter electrode, Ag/AgNO₃ reference electrode; referenced to internal Fc standard; scan rate 100 mV⋅s⁻¹

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  Figure 4  The lowest trans and cis isomers of ligand ^MedoenH₂ (left) and ^PhdoenH₂ (right). Relative energies are in kJ⋅mol⁻¹

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ni(1)–O(1)	1.853(2)		Ni(1)–N(3)	1.844(2)		N(1)–O(1)	1.309(3)
  Ni(1)–O(2)	1.826(2)		Ni(1)–N(4)	1.853(2)		N(2)–O(2)	1.313(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Ni(1)–O(2)	81.98(9)		O(1)–Ni(1)–N(3)	94.72(10)		Ni(1)–O(1)–N(1)	127.38(17)
  N(3)–Ni(1)–N(4)	88.31(10)		O(2)–Ni(1)–N(4)	94.90(9)		Ni(1)–O(2)–N(2)	129.01(17)

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