A high-spin nickel(II) borohydride complex in dehalogenation

Abstract

A nickel(II)–borohydride complex bearing a macrocyclic tridentate N-donor ligand, [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (Me₃-TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane), was prepared, isolated, and characterized by various physicochemical methods, including UV-vis, ESI-MS, IR and X-ray analyses. The structural and spectroscopic characterization clearly shows that the borohydride ligand is bound to the high-spin nickel(II) center in an η²-manner. Density functional theory calculations provided geometric information of 2, showing that the η²-binding of borohydride to the nickel center is more favorable than the η³-binding mode in CH₃CN. The complex is paramagnetic with an effective magnetic moment of 2.9μ_B consistent with a d⁸ high-spin system. The reactivity of the high-spin nickel(II)–borohydride complex was examined in dehalogenation with numerous halocarbons. A kinetic isotope effect value of 1.7 was observed in the dehalogenation of CHCl₃ by the nickel(II)–borohydride complex. Kinetic studies and isotopic labeling experiments implicate that hydride ion or hydrogen atom transfer from the borohydride group is the rate determining step. The positive Hammett ρ value of 1.2, obtained in the reactions of [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ and para-substituted benzoyl chloride, indicates that the dehalogenation by the nickel(II)–borohydride species occurs via a nucleophilic reaction.

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Jaeheung Cho

Dr Jaeheung Cho received his B.S. (in 2000) and M.S. (in 2002) degrees in Chemistry from Pukyong National University in Korea and his PhD degree in Inorganic Chemistry from Kanazawa University in Japan under the guidance of Professor Masatatsu Suzuki in 2005, in which he examined the dinuclear metal-active oxygen species in aliphatic C–H bond functionalization. After graduation, he moved to the group of Professor Charles G. Riordan at the University of Delaware in USA as a Postdoctoral Fellow. Then, he joined Ewha Womans University in Korea as a Full-Time Lecturer and a Professor by special appointment where he investigated models of mononuclear metal-active oxygen intermediates, in collaboration with Professor Wonwoo Nam. In 2010, he got the Distinguished Lectureship Award from the Chemical Society of Japan. In 2012, he began his independent career at DGIST in Korea as an Assistant Professor. His research interests focus on the synthesis, characterization and reactivity of biomimetic complexes, especially those mimicking active sites of metalloenzymes.

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Introduction

The nickel–hydrogen interactions are of significant importance in industrial and biological catalytic systems. Nickel–hydrogen complexes are involved in catalytic hydrogenation for organic syntheses,^1–4 nickel hydrogen batteries for rechargeable cells,⁵ and hydrogen storage for transportation applications.⁶ In nickel-containing enzymes such as hydrogenases, which catalyse the reversible two electron reduction of protons to hydrogen gas, nickel–hydrogen adducts are key intermediates in the proposed mechanisms of the enzymatic reactions.^7–9 One of the eight known nickel enzymes, methyl coenzyme M reductase (MCR), catalyzes the reductive cleavage of the thioether cofactor methyl coenzyme M to produce methane.^10–13

Among the nickel–hydrogen adducts, much attention has recently been focused on nickel–borohydride species, since such adducts are potential materials for nickel-catalyzed hydrogenation.¹⁴ In synthetic chemistry, numerous nickel–borohydride complexes have been prepared and characterized by various spectroscopic and X-ray crystallographic methods, which provided considerable information. The well characterized mononuclear nickel–borohydride species show a variety of hapticities for the borohydride ligand (Scheme 1).^1,15 Cramer and co-workers reported the first example of a single crystal structure of the nickel–borohydride complex bearing a tridentate monoanionic N-donor macrocyclic ligand, [Ni(Tp*)(BH₄)], in an η³-binding mode, where the importance of ancillary ligands in stabilizing nickel–borohydride species was emphasized.¹⁶ Its reactivity was also investigated for dehalogenation reactions.

Scheme 1 Mononuclear nickel–BH₄ adducts with three different coordination modes.

With PCP pincer ligands, Guan et al. described (η²-BH₄) nickel(II) complexes by the reaction of the corresponding nickel(II)–hydride complexes and BH₃.¹⁷ Recently, the reactivity of a similar complex reported by Kirchner and co-workers has been examined in CO₂ reduction.¹⁸ McGrady's group on the other hand reported the (η²-BH₄) nickel(I) complex with a tridentate P-donor ligand.¹⁹ Churchard and co-workers were able to isolate two isomers of nickel(II)–borohydride complexes bearing a tetradentate N-donor macrocycle where such complexes were suggested as hydrogen storage materials.²⁰ Herein we report the preparation and characterization of a mononuclear high-spin (η²-BH₄) nickel(II) complex bearing a tridentate N-donor ligand, [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (Me₃-TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane). To the best of our knowledge, the present study reports not only the first example of a high-spin nickel borohydride complex that has an η² binding mode but also the first well investigated kinetics in dehalogenation reactions of nickel–borohydride species.

Results and discussion

Nickel(II) precursor complex

The starting complex, [Ni(Me₃-TACN)(CH₃CN)₂](ClO₄)₂ (1-(ClO₄)₂), was synthesized by reacting Ni(ClO₄)₂·6H₂O and Me₃-TACN in CH₃CN. The UV-vis spectrum of 1 in CH₃CN exhibits several d–d bands at 340 (ε = 25 M⁻¹ cm⁻¹), 540 (ε = 16 M⁻¹ cm⁻¹), 800 (ε = 23 M⁻¹ cm⁻¹), and 880 nm (ε = 28 M⁻¹ cm⁻¹) (Fig. 1a). The electrospray ionization mass spectrum (ESI-MS) of 1 shows intense isotope envelopes at a mass-to-charge (m/z) of 155.6 for {Ni(Me₃-TACN)(CH₃CN)₂}²⁺, 176.2 for {Ni(Me₃-TACN)(CH₃CN)₃}²⁺ and 369.2 for {Ni(Me₃-TACN)(ClO₄)}⁺ (see Fig. S1, ESI†). The spin state of 1 in CH₃CN solution was determined using the ¹H NMR spectroscopy method of Evans, and the room temperature magnetic moment of 3.2μ_B indicates a high-spin state (S = 1) of the nickel(II) species.²¹

Fig. 1 (a) UV-vis spectrum of 1 in acetonitrile (red) and the reflectance spectrum of a powdered sample of 1 (black, intensity scale is arbitrary). (b) X-ray structure of the [Ni(Me₃-TACN)(CH₃CN)₃]²⁺ cation in 1-(BPh₄)₂ showing 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Violet crystals were obtained upon addition of NaBPh₄ to a solution of 1-(ClO₄)₂ in CH₃CN. The crystal structure of the cationic part of 1-(BPh₄)₂ is shown in Fig. 1b and Table 1, and selected bond distances and angles are listed in Table S1 (ESI†). Complex 1 has a distorted octahedral geometry based on facially coordinated Me₃-TACN and three CH₃CN solvent molecules.

Table 1 Crystal data and structure refinements for 1-(BPh₄)₂ and 2-(BPh₄)

	1-(BPh4)2	2-(BPh4)
Empirical formula	C63H70B2N6Ni	C35H48B2N4Ni
Formula weight	991.58	605.10
Temperature (K)	100(2)	100(2)
Crystal system	Monoclinic	Orthorhombic
Space group	P21/c	Pbca
Unit cell dimensions
a (Å)	18.0900(2)	20.1159(6)
b (Å)	11.65560(10)	15.8862(5)
c (Å)	25.8637(3)	20.4047(6)
β (°)	90.1750(10)	90
Volume (Å^3)	5453.33(10)	6520.6(3)
Z	4	8
d calc (g cm^−3)	1.208	1.233
μ (mm^−1)	0.401	0.625
Reflections collected	95 457	110 259
Independent reflections	13 591	8116
Refinement method	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2
F(000)	2112	2592
Goodness-of-fit on F^2	1.030	1.070
Final R indices [I > 2σ(I)]	R 1 = 0.0303	R 1 = 0.0524
	wR2 = 0.0775	wR2 = 0.1251
R indices (all data)	R 1 = 0.0361	R 1 = 0.0573
	wR2 = 0.0775	wR2 = 0.1291

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Nickel(II)–borohydride complex and characterization

The nickel(II)–borohydride complex, [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (2), was prepared by adding 1.5 equiv. of NaBH₄ to a solution containing 1 in CH₃CN at −20 °C under an N₂ atmosphere, where the color changed from violet to purple (Scheme 2). Complex 2 is thermally stable in CH₃CN under N₂, which allowed us to isolate crystals by adding NaBH₄ that were used in structural and spectroscopic analyses and reactivity studies. The crystalline solid is stable to air and moisture for several days. The UV-vis spectrum of 2 in CH₃CN shows three absorption bands at 355 (ε = 23 M⁻¹ cm⁻¹), 545 (ε = 18 M⁻¹ cm⁻¹) and 881 nm (ε = 17 M⁻¹ cm⁻¹) (Fig. 2a), which are characteristic bands expected for a high-spin d⁸ nickel(II) in a distorted octahedral geometry and can be attributed to three possible d–d transitions, ³A_2g → ³T_1g (P), ³A_2g → ³T_1g (F) and ³A_2g → ³T_2g, respectively.^22,23 The reflectance spectrum of a powder sample prepared from crystals of 2 shows that the spectral feature of 2 in the solid state is the same as in the solution state (Fig. 2a). The room temperature magnetic moment of 2.9μ_B, determined using the ¹H NMR Evans method, is consistent with a high-spin state (S = 1) of the nickel(II) species.²¹

Scheme 2 A synthetic route for a mononuclear nickel(II)–borohydride complex.

Fig. 2 (a) UV-vis spectrum of 2 in acetonitrile (red) and the reflectance spectrum of a powdered sample of 2 (black, intensity scale is arbitrary). (b) ESI-MS of 2 in CH₃CN under N₂. Inset shows observed isotope distribution patterns for {Ni(Me₃-TACN)(BH₄)}⁺ at m/z 244.2 (lower) and {Ni(Me₃-TACN)(BD₄)}⁺ at m/z 248.2 (upper). (c) FT-IR spectra of 2 prepared with BH₄⁻ (red line) and BD₄⁻ (blue line).

The ESI-MS of 2 exhibits a prominent ion peak at a m/z of 244.2, whose mass and isotope distribution patterns correspond to {Ni(Me₃-TACN)(BH₄)}⁺ (calculated m/z of 244.2) (Fig. 2b). When the reaction was carried out with isotopically labeled NaBD₄, its mass corresponding to {Ni(Me₃-TACN)(BD₄)}⁺ appeared at a m/z of 248.2 (calculated m/z of 248.2) (Fig. 2b, inset). The four mass unit shift on substitution of BH₄⁻ with BD₄⁻ proves that 2 contains a borohydride unit. The Fourier transform infrared (FT-IR) spectrum of 2 shows several isotopically sensitive features (Fig. 2c). [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (2^H) exhibits a strong band at 2428 cm⁻¹ (doublet, 41 cm⁻¹ splitting), which shifts to 1809 cm⁻¹ (doublet, 74 cm⁻¹ splitting) in [Ni(Me₃-TACN)(BD₄)(CH₃CN)]⁺ (2^D). This band has been tentatively assigned to be v(B–H)_terminal and the observed doublet is characteristic of an η² manner of BH₄⁻ binding to the metal center as previously reported.^1,18,20 The observed v(B–H)/v(B–D) ratio of 0.75 for v(B–H)_terminal is in good agreement with the calculated ratio of 0.74 based on Hooke's law. According to the literature,^1,20 the bidentate binding of BH₄⁻ is of a broad band at 1650–2150 for v(B–H)_bridging. In 2^H, three isotopically sensitive bands at 2043, 1964 and 1943 cm⁻¹ are observed in the region of v(B–H)_bridging, which shift to a spectral region obscured by the ligand fingerprint bands in 2^D. It was also difficult to observe the lower frequencies for 2 due to the vibrational modes of the supporting ligand. The solution IR spectrum of 2 recorded in CH₃CN exhibits that the spectral behavior of 2 in solution is the same as that in the solid state (Fig. S2, ESI†), consistent with the result from UV-vis spectroscopy (vide supra).

The X-ray crystal structure of [Ni(Me₃-TACN)(BH₄)(CH₃CN)](BPh₄) (2-(BPh₄)) reveals that the borohydride ligand coordinates in a symmetrical η²-fashion (Fig. 3). The presence of a counter anion, BPh₄⁻, supports the nickel(II) oxidation state assignment. The distorted octahedral nickel center of 2 is bound by three nitrogen atoms of Me₃-TACN, two bridging hydrogen atoms of BH₄⁻, and one nitrogen atom of CH₃CN. To the best of our knowledge, this is the first crystal structure of a high-spin nickel(II)–borohydride complex in an η²-manner. The Ni⋯B distance of 2 (2.225(3) Å) is longer than that of the tridentate borohydride nickel(II) complex, [Ni(Tp*)(η²-BH₄)] (2.048 Å);¹⁶ and slightly shorter than that of the bidentate borohydride nickel(I) complex, [Ni(triphos)(η²-BH₄)] (2.24 Å).¹⁹ However, the Ni⋯B distance of 2 is comparable to that of the bidentate nickel(II) complex, [Ni(PCP^Me–iPr)(η²-BH₄)] (2.218 Å).¹⁸ The average Ni–H bond length of 2 (1.745 Å) is comparable to that of [Ni(PCP^Me–iPr)(η²-BH₄)] (1.78 Å) and lies well within the nickel–borohydride category (1.59–1.94 Å). Based on the structural and spectroscopic characterization, complex 2 is assigned as a high-spin nickel(II)–borohydride complex in an η²-fashion.

Fig. 3 X-ray structure of the [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ cation in 2-(BPh₄) showing 30% probability thermal ellipsoids. Hydrogen atoms except those attached to boron are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni–N1 2.083(2), Ni–N2 2.1430(18), Ni–N3 2.0701(18), Ni–N4 2.0945(19), Ni–H1h 1.72(3), Ni–H2h 1.77(3), Ni⋯B1 2.225(3), N1–Ni–N2 83.84(7), N2–Ni–N3 84.40(7), N3–Ni–N1 84.92(8).

Reactivity

It is known that mononuclear metal–borohydride complexes are capable of performing reductive dehalogenation of halocarbons.^16,19,24,25 In order to examine the dehalogenation reactivity of 2, we carried out the reaction of CHCl₃ with 2. Upon the addition of CHCl₃ to 2 in CH₃CN at 25 °C, the UV-vis absorption bands of 2 disappeared with a pseudo-first-order decay (Fig. 4a, inset). Product analysis of the final reaction mixture revealed the formation of CH₂Cl₂ and very little H₂, where the volatile products were confirmed by ¹H NMR.^16,26 In addition, [Ni(Me₃-TACN)(Cl)]⁺ was found in the reaction solutions as a product of 2 (Fig. S3 for ESI-MS analysis, ESI†). Although several attempts to isolate the product as crystals have not been successful, an intense band at 342 nm of the product solution is similar to that of previously reported tetrahedral Ni(II)–Cl complexes (Fig. 4a).²⁷ These bands have been assigned to the Cl pπ → Ni dπ* LMCT band on the basis of theoretical calculations (Fig. S4, ESI†).²⁸

Fig. 4 Reactions of 2 with chloroform (CHCl₃) in CH₃CN. (a) UV-vis spectral changes of 2 (4 mM) upon addition of 125 equiv. of CHCl₃ at 25 °C. Inset shows the time course of the absorbance at 545 nm. (b) Plots of k_obs against the concentration of CHCl₃ to determine second-order rate constants for the reactions of 2^H (red line with ●) and 2^D (blue line with ) at 25 °C. (c) Plot of second-order rate constants against 1/T to determine activation parameters for the reaction of 2 and CHCl₃.

The pseudo-first-order rate constants, monitored at 545 nm, increased proportionally with the CHCl₃ concentration, giving a second-order rate constant (k₂) of 2.7(1) × 10⁻³ M⁻¹ s⁻¹ (Fig. 4b). The rates were dependent on the reaction temperature, where a linear Eyring plot was observed in the range of 278–308 K with activation parameters of ΔH^‡ = 33 kJ mol⁻¹ and ΔS^‡ = −182 J mol⁻¹ K⁻¹ (Fig. 4c). The observed negative entropy value and second-order kinetics suggest that a bimolecular mechanism is performing in the reduction of CHCl₃ by 2 (Scheme 3). The reactivity of 2 in dehalogenation is much higher than that of the low-spin (η³-BH₄) nickel(II) complex, [Ni(Tp*)(BH₄)], where the latter reacted with CHCl₃ only at elevated temperatures (>50 °C).^16,18

Scheme 3 Proposed mechanism of the dehalogenation of CHCl₃ by 2.

We also observed a kinetic isotope effect (KIE) value of 1.7 in the reduction of CHCl₃ by 2 when 2^H was replaced with 2^D (Fig. 4b). This KIE value is comparable to that of [Ni(Tp*)(BH₄)] (k_H/k_D = 3),¹⁶ in which hydride ion or hydrogen atom transfer was proposed to be rate limiting.

The reactivity of 2 was further investigated using para-substituted benzoyl chloride, which has a series of electron-donating and -withdrawing substituents at the para-position of the phenyl group (para-X-Ph-COCl; X = MeO, ^tBu, H, Cl, NO₂). 2 disappeared immediately upon addition of benzoyl chloride (10 equiv.) in CH₃CN at −40 °C. Thus, the reactivity of 2 with para-substituted benzoyl chloride was examined using a stopped-flow spectrometer. The Hammett plot of the second-order rate constants vs. σ_p⁺ gave a ρ value of 1.2 (Fig. 5). The positive ρ value is consistent with the process involving a nucleophilic character. Product analysis of the final reaction mixture revealed the formation of para-substituted benzaldehydes.

Fig. 5 Hammett plot of logk₂ against σ_p⁺ of benzoyl chloride derivatives in the reactions of 2 with para-substituted benzoyl chloride, para-X-Ph-COCl (X = OMe, ^tBu, H, Cl, NO₂) in CH₃CN at −40 °C.

Density functional theory results

To compare the stability of the Ni(II)–borohydride complexes in different coordination modes at a quantitative level, we performed spin-polarized density functional theory (DFT) calculations of the complexes in the gas phase using a non-local hybrid exchange–correlation functional (B3LYP).²⁹ Two different coordination modes, η² and η³, were considered (Fig. 6). Both complexes are in a high-spin state (S = 1). For the [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ cation in an η²-fashion, the Ni⋯B distance is calculated to be 2.19 Å and the average Ni–H bond length is 1.75 Å, in good agreement with the experimental values. For the [Ni(Me₃-TACN)(BH₄)]⁺ cation in an η³-fashion, the Ni⋯B distance is shortened to 1.97 Å, while the Ni–H bonds are elongated with the average value of 1.85 Å. We found that the η²-complex is 35 kJ mol⁻¹ more stable than the η³-complex, indicating that the Ni(II)–borohydride will exist in an η²-fashion in a solution at room temperature.

Fig. 6 (a) DFT structure of 2 in an η²-fashion (gray, C; blue, N, cyan, H; orange, B; green, Ni). Hydrogen atoms except those attached to boron are omitted for clarity. Selected bond lengths (Å) and angles (deg) are listed below for a direct comparison with the result in Fig. 3: Ni–N1 2.14, Ni–N2 2.20, Ni–N3 2.15, Ni–N4 2.17, Ni–H1h 1.76, Ni–H2h 1.75, Ni⋯B1 2.19, N1–Ni–N2 83.26, N2–Ni–N3 82.95, N3–Ni–N1 83.85. (b) DFT structure of the metastable [Ni(Me₃-TACN)(BH₄)]⁺ cation in an η³-fashion.

Conclusions

A mononuclear high-spin (η²-BH₄) nickel(II) complex, [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (2), has been successfully prepared and characterized by various physicochemical methods. In particular, FT-IR spectroscopy together with X-ray crystallography clearly shows that a borohydride is bound to the nickel center in an η²-fashion. 2 is capable of performing dehalogenation reactions. Kinetic studies and isotope labelling experiments suggest that bimolecular collision between 2 and halocarbons is the rate-limiting step in dehalogenation reactions.

Experimental section

Materials

All chemicals obtained from Aldrich Chemical Co. were of the best available purity and used without further purification unless otherwise indicated. Solvents were dried according to published procedures and distilled under Ar prior to use.³⁰

Physical methods

UV-vis spectra were recorded on a Hewlett Packard 8453 diode array spectrophotometer equipped with UNISOKU Scientific Instruments for low-temperature experiments or with a circulating water bath. Fast reactions were monitored using a Hi-Tech Scientific SF-61 DX2 cryogenic stopped-flow spectrometer equipped with a Xe arc lamp and a KinetaScan diode array rapid scanning unit. Electrospray ionization mass spectra (ESI-MS) were collected on a Waters (Milford, MA, USA) Acquity SQD quadrupole mass instrument, by infusing samples directly into the source using a manual method. The spray voltage was set at 2.5 kV and the capillary temperature at 80 °C. Infrared spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer equipped with a diamond attenuated total reflectance (ATR) accessory. The effective magnetic moments were determined using the modified ¹H NMR method of Evans at room temperature.^21,31,32 A WILMAD® coaxial insert (sealed capillary) tube containing the blank acetonitrile-d₃ solvent (with 1.0% TMS) only was inserted into the normal NMR tubes containing the complexes dissolved in acetonitrile-d₃ (with 0.03% TMS). The chemical shift of the TMS peak (and/or the solvent peak) in the presence of the paramagnetic metal complexes was compared with that of the TMS peak (and/or the solvent peak) in the inner coaxial insert tube. The effective magnetic moment was calculated using the equation, μ = 0.0618(ΔνT/2fM)^1/2, where f is the oscillator frequency (MHz) of the superconducting spectrometer, T is the absolute temperature, M is the molar concentration of the metal ion, and Δv is the difference in frequency (Hz) between the two reference signals. Product analysis was performed on a Thermo Fisher Trace 1310 gas chromatograph system equipped with a flame ionization detector (GC). ¹H NMR spectra were recorded with a Bruker AVANCE III-400 spectrometer at CCRF in DGIST.

Syntheses

[Ni(Me₃-TACN)(CH₃CN)₂](ClO₄)₂ (1-(ClO₄)₂). Me₃-TACN (0.1 g, 0.58 mmol) was added to an acetonitrile solution (30 mL) of Ni(ClO₄)₂·6H₂O (0.212 g, 0.58 mmol) and tetramethylethylenediamine (0.067 g, 0.58 mmol). The resulting solution was refluxed for 12 h, affording a pink solution. After cooling to room temperature, the solvent was removed under reduced pressure to give a pink powder. The solid was filtered and the filtrate was then washed with diethyl ether and dried in a vacuum. Yield: 0.29 g (90%). ESI-MS (in CH₃CN): m/z 155.6 for {Ni(Me₃-TACN)(CH₃CN)₂}²⁺ and 176.2 for {Ni(Me₃-TACN)(CH₃CN)₃}²⁺. Anal. Calcd for C₁₃H₂₇Cl₂N₅NiO₈: C, 30.56; H, 5.33; N, 13.71. Found: C, 30.58; H, 5.49; N, 13.99. X-ray crystallographically suitable crystals were obtained by addition of NaBPh₄.

[Ni(Me₃-TACN)(BH₄)(CH₃CN)](BPh₄) (2-(BPh₄)). Treatment of 1 (0.051 g, 0.1 mmol) in CH₃CN (1 mL) with 1.5 equiv. of NaBH₄ (0.006 g, 0.15 mmol) at −20 °C under N₂ afforded the formation of a purple solution, to which was added NaBPh₄ (0.034 g, 0.1 mmol). The resulting solution was allowed to stand for several days at −20 °C, giving dark purple crystals suitable for X-ray crystallography. Crystalline yield: 0.043 g (71%). ESI-MS (in CH₃CN): m/z 244.2 for {Ni(Me₃-TACN)(BH₄)}⁺. Anal. Calcd for C₃₅H₄₈B₂N₄Ni: C, 69.47; H, 8.00; N, 9.26. Found: C, 69.12; H, 7.71; N, 9.55.

X-ray crystallography

Single crystals of [Ni(Me₃-TACN)(CH₃CN)₃](BPh₄)₂ (1-(BPh₄)₂) and [Ni(Me₃-TACN)(BH₄)(CH₃CN)](BPh₄) (2-(BPh₄)) were picked from solutions by using a nylon loop (Hampton Research Co.) on a handmade copper plate mounted inside a liquid N₂ Dewar vessel at ca. −40 °C and mounted on a goniometer head in a N₂ cryostream. Data collections were carried out on a Bruker SMART APEX II CCD diffractometer equipped with a monochromator in the Mo Kα (λ = 0.71073 Å) incident beam. The CCD data were integrated and scaled using the Bruker-SAINT software package, and the structure was solved and refined using SHELXTL V 6.12.³³ Hydrogen atoms were located in the calculated positions except those of borohydride, which were found from the Fourier difference map. All non-hydrogen atoms were refined with anisotropic thermal parameters. Crystal data for 1-(BPh₄)₂: C₆₃H₇₀B₂N₆Ni, monoclinic, P2₁/c, Z = 4, a = 18.0900(2), b = 11.65560(10), c = 25.8637(3) Å, β = 90.1750(10)°, V = 5453.33(10) ų, μ = 0.401 mm⁻¹, ρ_calcd = 1.208 g cm⁻³, R₁ = 0.0303, wR₂ = 0.0775 for 13 591 unique reflections, 655 variables. Crystal data for 2-(BPh₄): C₃₅H₄₈B₂N₄Ni, orthorhombic, Pbca, Z = 8, a = 20.1159(6), b = 15.8862(5), c = 20.4047(6) Å, V = 6520.6(3) ų, μ = 0.625 mm⁻¹, ρ_calcd = 1.233 g cm⁻³, R₁ = 0.0524, wR₂ = 0.1251 for 8116 unique reflections, 399 variables. The crystallographic data for 1-(BPh₄)₂ and 2-(BPh₄) are listed in Table 1, and Table S1† lists the selected bond distances and angles. CCDC 1423623 for 1-(BPh₄)₂ and 1423622 for 2-(BPh₄) contain the supplementary crystallographic data for this paper.

Reactivity

All reactions were run in a 1 cm UV cuvette by monitoring UV-vis spectral changes of reaction solutions, and rate constants were determined by fitting the changes in absorbance at 545 nm for [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (2). For the stopped-flow experiments, all reaction traces were collected at −40 °C. The raw kinetic data were treated with KinetAsyst 3 (Hi-Tech Scientific) and Specfit/32 Global Analysis System software from Spectrum Software Associates. Reactions were run at least in triplicate, and the data reported represent the average of these reactions. Crystalline samples of 2 were used in kinetic studies, such as the reduction of chloroform in CH₃CN at 25 °C. After the completion of reactions, pseudo-first-order fitting of the kinetic data allowed us to determine k_obsvalues. Products formed in the reduction of chloroform by 2 in CD₃CN at 25 °C were analyzed by ¹H NMR. Products were identified by comparing with authentic samples.

Computational details

Spin-polarized DFT calculations were performed using a non-local hybrid exchange–correlation functional (B3LYP).²⁹ We used the projector-augmented wave (PAW) method³⁴ and a plane wave cutoff of 400 eV, as implemented in the VASP code.^35,36 Atomic structures of the Ni(II)–borohydride complexes were relaxed within 0.03 eV Å⁻¹. Geometry optimization and time-dependent DFT (TD-DFT) calculations for [Ni(Me₃-TACN)(Cl)]⁺ were performed with the Gaussian 09 package.

Acknowledgements

The research was supported by NRF (2014R1A1A2056051), the Ministry of Science, ICT and Future Planning (DGIST R&D Program 15-BD-0403 & 15-HRLA-02, KCRC 2014M1A8A1049320, and 2015M3D3A1064890) and the Ministry of Oceans and Fisheries (Marine Biotechnology Program 20150220) of Korea (J.C.). We are grateful to Prof. Kiyoung Park (KAIST) and Mr Donghyun Jeong (DGIST) for valuable discussions on TD-DFT. This research used capabilities of the DGIST Supercomputing and Bigdata Center.

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Footnote

† Electronic supplementary information (ESI) available: [Ni(Me₃-TACN)(CH₃CN)₃](BPh₄)₂ and [Ni(Me₃-TACN)(BH₄)(CH₃CN)](BPh₄). CCDC 1423622 and 1423623. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00206k

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