Cyclometalated Nickel Complexes as Key Intermediates in C(sp2)âH

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Cyclometalated Nickel Complexes as Key Intermediates in C(sp2)−H Bond Functionalization: Synthesis, Catalysis, Electrochemical Properties, and DFT Calculations Yulia B. Dudkina,† Robert R. Fayzullin,† Konstantin A. Lyssenko,‡ Aidar T. Gubaidullin,† Kirill V. Kholin,† Alina I. Levitskaya,† Marina Yu. Balakina,† and Yulia H. Budnikova*,†

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Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS, 8 Arbuzov Street, Kazan 420088, Russian Federation ‡ A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, 28 Vavilov Street, Moscow 119991, Russian Federation S Supporting Information *

ABSTRACT: This paper elucidates the redox properties and reactivity of a number of cyclometalated nickel(II) complexes in oxidation processes. Original and robust procedures for the synthesis of nickelacycles with two cyclometalated moieties and halogenated nickelacycles were developed. The complexes were studied by means of electrochemical, ESR, DFT, and singlecrystal XRD methods. Notable features of the nickelacycles are unusually low oxidation potentials. Upon electrochemical oxidation the complexes produce carbon−carbon and carbon− heteroatom coupled products, the ratio of which can be tuned by the conditions applied.

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INTRODUCTION 2

Hence, our questions are as follows. (i) Are the redox trends and regularities revealed for palladacycles true for nickel complexes? (ii) How do potentials of NiII/NiIII/NiIV redox couples vary depending on their structure? (iii) How can nickelacycles be applied in a catalytic cycle to produce products with new carbon−carbon and carbon−heteroatom bonds?

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Nickel-mediated C(sp )−H or C(sp )−X bond activation and functionalization reactions have become important methods for the construction of new carbon−carbon and carbon− heteroatom bonds.1−6 In many cases, cyclometalated nickel complexes are proposed to be intermediates of these reactions to explain the regioselectivity of C−H functionalizations.7 While there have been some reports on cyclometalated nickel(II) complexes,8−13 there is still a lack of detailed studies on the properties and reactivities of nickelacycles in different oxidation states that are necessary to discuss the difference between Ni and Pd in mediating bond-formation processes. Only a few well-defined NiIII and NiIV complexes with solitary cyclometalated examples have been reported14−20 and shown to undergo reductive elimination to form functionalized products. While a wide range of cyclometalated palladium(II) complexes of different structures has been investigated by electrochemical and ESR techniques,21−26 including electrochemical studies on their reactivity in catalytic reactions,23−25 electrochemical and ESR data for nickelacycles are rare and incomplete.7,11,18 The aim of this work was to study a series of cyclometalated nickel(II) complexes with the benzo[h]quinoline ligand, in order to develop new procedures for their synthesis, elucidation, and comparison of their redox properties, preparative oxidations, and the design of a catalytic cycle of benzo[h]quinoline functionalization reactions involving these complexes. © XXXX American Chemical Society

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RESULTS AND DISCUSSION While cyclometalated palladium(II) complexes are easily formed in C−H activation reactions by a mixture of palladium(II) salts and ligands containing directing groups, for instance, bhq (benzo[h]quinoline),23,24 this procedure does not work with nickel. The most common way to obtain nickelacycles is a direct reaction of Ni0(COD)2 (where COD is cycloocta-1,5-diene) with brominated ligands, affording bromide nickel(II) complexes,9−11,18 and bromide can be further replaced by other groups: e.g., acetate18 and trifluoromethyl.11,12 The former reaction being a typical oxidation addition of aryl bromides to Ni0, aryl chlorides can also be used to obtain cyclometalated NiII complexes. We used these approaches to synthesize nickelacycles with different structures: mononuclear Special Issue: Organometallic Electrochemistry: Redox Catalysis Going the Smart Way Received: July 28, 2018

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DOI: 10.1021/acs.organomet.8b00536 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 1 and 2 and binuclear clamshell complex 3 (Scheme 1). Figures 1 and 2 illustrate the structures of complexes 1 and 2, respectively. Selected interatomic distances are given in the caption. Scheme 1. Synthesis of Nickelacycles 1−3

Figure 2. ORTEP projection of complex 2 showing anisotropic displacement ellipsoids at the 50% probability level. Selected interatomic distances (Å): Ni1−Cl1 2.2499(3), Ni1−N1 1.9196(9), Ni1−N2 1.8789(10), Ni1−C111 1.9014(11), N1−C113 1.3639(14), C111−C112 1.4146(15), C112−C113 1.4145(15).

Obtained nickelacycles 1, 3, and 4 are highly reactive and air-sensitive; complexes 3 and 4 decompose even in CH3CN solutions under an inert atmosphere. Such high reactivity might be related to low oxidation potentials, and that has been confirmed by electrochemical studies (vide infra). Then, nickelacycles containing more electron withdrawing anionic ligands are supposed to be more stable: for example, complexes with chloride or trifluoroacetate ligands. The corresponding trifluoroacetate complex 6 was synthesized according to the reaction in Scheme 3. Unexpectedly, the mononuclear trifluoroacetate nickelacycle 6 with a pyridine ligand forms instead of a binuclear clamshell analogue of acetate 3, although conversion of binuclear metallacycles into mononuclear species in the presence of nitrogen-donor ligands is known for palladium(II) complexes.23,26 The structure of 6 was determined by X-ray analysis and is shown in Figure 5. It is worth mentioning that, among the first obtained nickelacycles, the pyridine-containing species 1, 2, and 6 are characterized by the trans positions of two nitrogen atoms toward the NiII centers with an almost ideal square-planar geometry (τ4 = τ4′ = 0.10 for 1 and 2; τ4 = 0.08 and τ4′ = 0.06 for 632−35). In contrast, the double-cyclometalated complex 4 adopts a cis configuration while the cyclometalated moieties labeled in Figure 3 form a remarkable angle of 28.20(8)° due to steric hindrance on the periphery of the ligands. However, according to the τ parameter (τ4 = τ4′ = 0.26) this nickelacycle shows a distorted-square-planar geometry. Electrochemical and DFT Studies. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques were used to elucidate the redox properties of the cyclometalated nickel(II) complexes. Cyclic voltammograms of 1−4 and 6 in CH3CN and THF solutions are depicted in Figures 6 and 7, respectively, and the results are summarized in Table 1. Analysis of the electrochemical data in both solvents shows several findings. Thus, the electrochemical gap value (the difference between oxidation and reduction potentials) is much lower in CH3CN than in THF for all of the studied complexes (Figures 6 and 7). All reductions of the nickelacycles are irreversible and include an adsorption peak of

Figure 1. ORTEP projection of complex 1 showing anisotropic displacement ellipsoids at the 50% probability level. Selected interatomic distances (Å): Br1−Ni1 2.3865(4), Ni1−N1 1.925(2), Ni1−N2 1.877(2), Ni1−C111 1.899(3), N1−C113 1.362(3), C111− C112 1.415(4), C112−C113 1.422(3).

Interestingly, attempts to obtain a mononuclear nickelacycle containing a bpy (2,2′-bipyridine) unit afforded the doublecyclometalated complex 4 and the byproduct 5 (Scheme 2). The latter is insoluble in THF and can be easily removed from the reaction mixture. Structures of both 4 and 5 were confirmed by single-crystal X-ray diffraction data (Figures 3 and 4, respectively). Notably, octahedral complex 5 was isolated as a cis isomer similar to an analogous bpy2NiCl2 complex.27−29 Although nickel complexes with two cyclometalated moieties have been reported previously,8−10 this reaction is an interesting one-step and high-yield alternative to the known procedures of their synthesis. The reaction occurs in the same way with tridentate ligands (2,2′:6′,2″-terpyridine, tris(3,5dimethyl-1-pyrazolyl)borate). Analogous double-cyclometalated complexes with bhq ligands are also known for other group 10 metals.30,31 B


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Organometallics Scheme 2. Synthesis of Double-Cyclometalated Nickelacycle 4

Scheme 3. Synthesis of Trifluoroacetate Nickelacycle 6

Figure 3. ORTEP projection of complex 4 showing anisotropic displacement ellipsoids at the 50% probability level. Selected interatomic distances (Å): Ni1−N1 1.981(2), Ni1−C111 1.889(2), N1−C113 1.364(3), C111−C112 1.414(3), C112−C113 1.422(3), Ni1−N2 1.977(2), Ni1−C211 1.895(2), N2−C213 1.363(3), C211− C212 1.414(3), C212−C213 1.433(3).

Figure 5. ORTEP projection of complex 6 showing anisotropic displacement ellipsoids at the 50% probability level. The minor component of the rotationally disordered trifluoromethyl group is omitted for clarity. Selected interatomic distances (Å): Ni1−N2 1.885(2), Ni1−N1 1.910(2), Ni1−O31 1.9674(19), Ni1−C111 1.891(3), N1−C113 1.363(3), C111−C112 1.416(3), C112−C113 1.416(4).

metallic Ni0 oxidation on the reverse scan at low negative potentials that is also typical for some unsaturated NiII complexes with bpy ligands.36−38 All of the nickelacycles are characterized by extremely low oxidation potentials (−0.5 to +0.3 V vs Fc+/Fc), which corresponds directly with their stability: the most reactive complexes 3 and 4 are the first ones to be oxidized. Reversibility is also featured, to a greater or lesser degree, only for the most stable nickelacycles 1, 2, and 6. The obtained results are in good agreement with those for the previously studied similar nickelacycles [(Phbpy)NiBr] and [(Phbpy)NiCF3]11 and diverge from data reported for [bhqNi(OAc)]2 (3),18 where a Bu4NPF6/Bu4NCl mixture was used as the supporting electrolyte. The environment, in which transformation of 3 into chloride 2 is possible, results in the oxidation potential of the latter being much closer to the reported value.18

Figure 4. ORTEP projection of complex 5 showing anisotropic displacement ellipsoids at the 50% probability level. The solvent molecules are omitted for clarity. Selected interatomic distances (Å): Br1−Ni1 2.5640(4), Br2−Ni1 2.6028(4), Ni1−N1 2.0803(19), Ni1− N2 2.0880(19), Ni1−N4 2.0811(19), Ni1−N3 2.0760(19).

C


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and LUMO, respectively (Table S1). The energy level diagrams of calculated DFT (Table S6) and electrochemically estimated (Table S1) frontier orbitals of the nickelacycles are given in Figures 8 and 9.

Figure 6. Cyclic voltammograms of nickelacycles in CH3CN.

Figure 8. Frontier orbital energy values of nickelacycles in CH3CN from DPV experiments (black) and from calculations at the M06-L/ TZP-DKH (blue) or B3LYP/M06-L/TZVFs (purple) computational level.

Figure 7. Cyclic voltammograms of nickelacycles in THF.

Table 1. Electrochemical Properties of Nickelacycles CV potential (V)a complex 1 2 3 4 6 1 2 3 4 6

oxidation Acetonitrileb f r Ep = 0.089; Ep = −0.120; q-rev Epf = 0.175; Epr = −0.114; q-rev Epf = −0.099; irrev not availablec Epf = 0.328; Epr = −0.088; q-rev THFd Epf = 0.108; Epr = −0.048; q-rev Epf = 0.255; Epr = −0.089; q-rev Epf = −0.106; Epr = −0.284; q-rev Epf = −0.556; irrev Epf = 0.254; irrev

reduction Epf = −1.690; irrev Epf = −1.974; irrev Epf = −2.091; irrev Epf = −1.711; irrev Epf Epf Epf Epf Epf

= = = = =

−2.091; −2.261; −2.396; −2.348; −2.091;

irrev irrev irrev irrev irrev

Figure 9. Frontier orbital energy values of nickelacycles in THF from DPV experiments (black) and from calculations at the M06-L/TZPDKH (blue) or B3LYP/M06-L/TZVFs (purple) computational level.

a

General conditions: 0.005 M solutions; Pt working electrode (diameter 3 mm). Potentials are referred vs the Fc+/Fc couple. Epf is the forward peak potential, Epr is the reverse peak potential, rev is a reversible process, q-rev is a quasi-reversible process;and irrev is an irreversible process. b0.1 M Bu4NPF6 supporting electrolyte. c Complex undergoes fast destruction in CH3CN solution. d0.2 M Bu4NPF6 supporting electrolyte.

According to the DFT calculations performed for complexes 1 and 4, there is no pronounced effect of the structure on the frontier orbital energies and the values of the band gaps for the studied compounds. The agreement between the computational estimations and the values calculated from DPV potentials is rather good, when the M06-L density functional is used. This functional has no HF exchange fraction,40 which conforms to the recommendation for transition-metal complexes;41 the B3LYP density functional with a moderate

Obtained DPV potentials of the first oxidation and reduction were further recalculated39 to give the energies of the HOMO D


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Organometallics percentage of HF exchange (20%) essentially overestimates the values of band gaps, stabilizing the HOMO significantly and destabilizing the LUMO in comparison with the corresponding values given by the M06-L functional (see Tables S3−S5). It is worth noting that, in contrast to the estimations made from DPV potentials, the theoretical data demonstrate almost no dependence of the HOMO and LUMO energies and the values of the band gaps on the solvent: the estimations obtained in CH3CN and THF are quite close. Cyclometalated nickel(II) complexes are about 0.6−0.7 V more easily oxidized in comparison with their palladium analogues under the same conditions26 (about +0.5 V vs Fc+/ Fc). To date, the scope of nickelacycles is insufficient to prove or disprove whether they demonstrate the same dependence of the electrochemical properties on the structure as is the case for cyclometalated palladium(II) complexes. However, in contrast to palladacycles, anionic ligands have a much lower effect on the oxidation potentials of nickel complexes. For example, for benzo[h]quinoline palladacycles in CH3CN the difference in oxidation peak potentials between the binuclear acetate [bhqPd(OAc)]2 and mononuclear bhqPd(CH3CN)(OCOCF3) is 730 mV, while nickelacycles 3 ([bhqNi(OAc)]2) and 6 (bhqNi(py)(OCOCF3)) differ by 430 mV. This is due to two reasons. First, we found26 that a metal− metal interaction contributes to the difference in oxidation potential values between binuclear and mononuclear palladacycles, while Diao18 showed such an interaction in a clamshell is much weaker for nickelacycles than for palladium complexes. Second, DFT calculations (Figures S9−S16) reveal that the electron density of the HOMO is localized mainly on the metal atoms in cyclometalated nickel complexes, while in cyclometalated palladium complexes it is shifted from metal atoms to the aromatic ligand, depending on the structure. As the HOMO energy is directly linked to the standard oxidation potential value,39,42−46 the similar distributions of electron density indicate the similar oxidation potentials of the nickelacycles. ESR and Preparative Oxidations. Although the first oxidations of the nickelacycles are partially reversible, our attempts to obtain stable nickel(III) complexes upon electrochemical oxidation of nickel(II) were unsuccessful. Preparative electrolysis carried out at room temperature led only to their decomposition productssubstituted benzo[h]quinoline. During the oxidative electrolysis of 1−3 and 6 in CH3CN or THF we were able to detect ESR signals (Table S2) with g factors close to 2.15 at 255 K (2.18 for 1 in THF at 235 K). At room temperature the signals that appeared had low intensity in most cases because of the instability of the intermediates formed; consequently, the paramagnetic species were generated at low temperatures of 235−275 K, where a balance between generation and decomposition was achieved. The signals are typical of both NiIII and NiI species with no clear attribution. It can be assumed that NiIII is formed and registered under oxidative conditions. The recently reported18 ESR of the mixed-valent complex [bhqNi2.5+(OAc)(THF)]2, which should be formed upon oxidation of 3 in one of our ESR experiments, reveals g⊥ > g∥ at 10 K. In contrast, in all of our ESR experiments at low temperatures the signals are typical for axially symmetric metal centers with g∥ > g⊥ (Figure 10 and Figures S1−S8 and Table S2). A possible explanation is that we observe ESR signals of NiI intermediates as parallel and perpendicular g factors of NiI and NiIII are reciprocal. The NiI species are probably produced in reductive elimination

Figure 10. ESR spectra obtained upon oxidation of 1 in CH3CN at 135 K: g∥ = 2.251, ΔH = 50 G, g⊥ = 2.086, ΔH = 60 G.

reactions of NiIII intermediates initially formed upon electrochemical oxidation whose lifetimes are significantly shorter. Previously, strong oxidants9,10,18 such as PhICl2 and TDTT (often taken in excess) were applied to study the formation of substituted products upon oxidation of nickelacycles, which might promote the reaction to pass through high-valent NiIV intermediates. Our studies focused on electrochemical oxidation of cyclometalated nickel(II) complexes with an anode being the oxidant, whose strength can be easily tuned by the applied potential. First, oxidations of bhqNi(py)Br (1) were tested (Scheme 4, top). One-electron oxidation in CH3CN was found Scheme 4. Electrochemical Oxidation of Nickelacycles 1 and 6

to lead to quantitative formation of a dimeric bhq-bhq product regardless of the applied potential (0.2−0.6 V vs Fc+/Fc). Under the same conditions in THF two products were formed, 10-Br-bhq and dimeric bhq-bhq, in a 1:1 ratio according to 1H NMR. Introducing external bromide (Et4NBr, 5 equiv) to CH3 CN reaction mixtures increases the ratio of the brominated product 10-Br-bhq to 40−50% but decreases the total yield to 63%, probably because of a competing side reaction of bromide oxidation. Increasing the amount of the electricity passed to 2 e mol−1 improves the yield of 10-Br-bhq to 78%, while other products are dimers and unidentified E


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To obtain a stable cyclometalated product, 2 equiv of pyridine was added to a (DME)NiBr2 and 10-Br-bhq solution in THF. After joint electrochemical reduction of this mixture the electrolyte color turned from blue to orangish yellow and a CV scan (Figure 12) showed lack of the initial peak C1 of

products. Oxidations of chloride 2 (bhqNi(py)Cl) reveal the same trends. Oxidation of trifluoroacetate 6 affords no dimeric product but rather trifluoroacetoxylated bhq (Scheme 4, bottom). These results agree with our previous report of C−H functionalization reactions of 2-phenylpyridine,47 where similar fluorocarboxylated complexes were proposed to be key intermediates in the catalytic cycle but were not isolated or characterized. Electrochemical Approach to Synthesis of Nickelacycles. Ni0 complexes are known to react with aryl halides upon reduction of NiII precursors to form (σ-aryl)nickel(II) intermediates or to catalyze coupling reactions to afford biarylic products.48,49 Both processes can be detected and elucidated by CV, revealing new peaks of arylnickel(II) complexes37,38 or catalytic currents due to regeneration of NiII on the electrode surface in coupling reactions.48,49 With this knowledge we explored new synthetic procedures to cyclometalated nickel(II) complexes, which could allow us to replace the expensive and unstable Ni0(COD)2 starting material with more convenient and available NiII sources. First, we tested electrochemically generated Ni0 from a (DME)NiBr2 (DME is 1,2-dimethoxyethane) precursor in a reaction with 10-Br-bhq. In the presence of increasing amounts of aryl bromide the CVs (Figure 11) show catalytic currents at

Figure 12. CVs illustrating formation of nickelacycle 1 after 2 e was passed through a mixture of (DME)NiBr2,10-bhq-Br, and pyridine in THF.

(DME)NiBr2 reduction and appearance of peaks C2 and A1 inherent to nickelacycle 1 reduction/oxidation, which was formed according to Scheme 5, pathway 2.

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CONCLUSION In summary, a family of cyclometalated nickel(II) complexes with a benzo[h]quinoline ligand and Br, Cl, OCOCH3, and OCOCF3 anionic ligands has been obtained. According to the single-crystal X-ray diffraction data, all of the complexes, except the acetate-bridged nickelacycle, are mononuclear and are characterized by definite (1, 2, 6) or distorted (4) squareplanar geometries. An original and robust synthetic procedure for double-cyclometalated complexes was developed. Moreover, we showed that the halogenated nickelacycles can be synthesized via electrochemical joint reduction of a NiII precursor, a halogenated ligand, and pyridine. The stabilities and redox properties of the complexes have been estimated by voltammetry, ESR, and preparative (controlled-potential) electrolysis. Oxidation of the nickelacycles occurs through high-valent nickel(III) intermediates and affords two main products, substituted benzo[h]quinoline and dimeric bhq-bhq, whose ratio can be tuned by the conditions applied. In contrast to the case for Pd, anionic ligands have a much lower effect on the oxidation potentials of the nickel complexes, while a notable feature of the nickelacycles is an unusually low oxidation potential. Thus, even such a weak oxidant such as air50 easily oxidizes these complexes and might be an appropriate oxidant for the target carbon−carbon and carbon−heteroatom bond forming reactions. The design of more resistant nickelacycles is our future goal.

Figure 11. CVs illustrating catalytic currents upon addition of increasing amounts of 10-bhq-Br to (DME)NiBr2 in THF.

reduction potentials of NiII according to the reaction in Scheme 5, pathway 1. As expected, preparative Ni-mediated reduction of an equimolar mixture of (DME)NiBr2 and 10-Brbhq leads only to dimeric bhq-bhq. Scheme 5. Electrochemical Synthesis of Nickelacyclesa

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out using standard Schlenk line and glovebox techniques under an atmosphere of dry nitrogen. All solvents employed were purified and dried according to standard methods prior to use. All chemicals were purchased from commercial suppliers and used as received. 10Bromobenzo[h]quinoline (10-Br-bhq)51 10-chlorobenzo[h]quinoline

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Solvent coligands are omitted for clarity. F


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Organometallics

MHz, THF-d8): δ 9.32 (d, J = 7.0 Hz, 1H); 8.32 (d, J = 7.2 Hz, 1H); 7.90 (br, 2H); 7.71, 7.56 (ABq, 2H, JAB = 8.9 Hz, 2H); 7.44 (d, J = 7.8 Hz, 2H); 7.48−7.43 (m, 3H); 7.07 (t, J = 7.5 Hz; 1H); 5.70 (d, J = 6.1 Hz, 1H). 13C NMR (100.6 MHz, THF-d8): δ 149.6, 147.7, 143.0, 136.2, 134.6, 132.8, 128.7, 128.4, 128.2, 127.4, 126.1, 125.3, 123.7, 123.2, 122.5, 122.1; CF3 and CO peaks could not be resolved. 19F NMR (470.6 MHz; THF-d 8 ): δ −76.08. Anal. Calcd for C20H13F3N2NiO2: C, 55.99; H, 3.05; N, 6.53. Found: C, 56.07; H, 3.12; N, 7.01. Procedure for Nickelacycle Electrochemical Oxidation. Controlled-potential electrolyses were carried out with a BASi Epsilon potentiostat/galvanostat (USA) at room temperature in a divided cell with a Pt working electrode, a platinum auxiliary electrode, and an Ag/AgNO3 reference electrode with vigorous stirring under a dry nitrogen atmosphere. The cell was charged with 0.1 mmol of nickel complex, 0.5 mmol of a supporting electrolyte, and 5 mL of solvent. The counter electrode chamber was charged with 27 mg of FcBF4, 0.1 mmol of supporting electrolyte, and 1 mL of solvent. The supporting electrolyte was Et4NPF6 or Et4NX (Et4NBr with complex 1 or Et4NCl with complex 2) for CH3CN and Bu4NPF6 for THF. After the electrolysis was complete, the solution was concentrated, washed with aqueous NH4Cl (15 mL), and extracted with benzene (three times with 5 mL). The organic layer was dried over magnesium sulfate and filtered. The residual solution was concentrated under reduced pressure, and the products were analyzed by 1H NMR. Single-Crystal X-ray Investigation. The X-ray diffraction (XRD) data for single crystals of 1, 2, 5, and 6 were collected on a Bruker Kappa Apex II CCD diffractometer (ω-/φ-scan mode). The XRD data for 4 were collected on a Bruker Smart Apex II CCD diffractometer (ω-scan mode). The performance mode of the sealed X-ray tubes was 50 kV and 30 mA; graphite-monochromated Mo Kα (0.71073 Å) radiation was used. The diffractometers were equipped with Oxford Cryostream LT devices. Except for sample 1, which was measured at 100(2) K, the X-ray data were obtained at 150(2) K. Suitable crystals of appropriate dimensions were mounted on glass fibers or cactus needles in random orientations. Preliminary unit cell parameters were determined with 3 or 4 sets of a total of 12 narrow frame scans. The data were collected according to recommended strategies. For data collection, images were indexed and integrated using the APEX3 data reduction package (v2015.9-0, Bruker AXS). Final cell constants were determined by global refinement of reflections from the complete data set. Analysis of the integrated data did not show any decay. Data were corrected for systematic errors and absorption by means of SADABS-2014/5 on the basis of the point group symmetry using equivalent reflections. XPREP-2014/ 2 and the ASSIGN SPACEGROUP routine of WinGX-2018.1 were used for analysis of systematic absences and space group determination. The structures were solved by direct methods using SHELXT2018/252 and refined by full-matrix least squares on F2 using SHELXL-2018/3.53 Calculations were mainly performed using the WinGX-2018.1 suite of programs.54 All non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms of methyl groups were found using a rotating group refinement with idealized tetrahedral angles. The other hydrogen atoms were inserted at calculated positions and refined as riding atoms. It should be mentioned that compound 5 crystallizes as a solvate with DMF molecules in the asymmetric cell, one of which is substitutionally disordered by a diethyl ether molecule (the 5:DMF:Et2O ratio in the crystal studied was 1:1.665:0.335). The trifluoromethyl group of 6 appeared to be disordered as well. The disorder was resolved using free variables and reasonable restraints on geometry and anisotropic displacement parameters. The compounds studied have no unusual bond lengths and angles. Crystallographic data for 1: C18H13BrN2Ni, orange prism, formula weight 395.92, monoclinic, P21/c (No. 14), a = 12.9059(7) Å, b = 10.2371(5) Å, c = 12.4695(6) Å, β = 117.150(2)°, V = 1465.94(13) Å3, Z = 4, Z′ = 1, T = 100(2) K, dcalc = 1.794 g cm−3, μ(Mo Kα) = 4.045 mm−1, F(000) = 792; Tmax/min = 0.7462/0.5284; 23431 reflections collected (3.269° ≤ θ ≤ 29.150°), 3894 of which were

(10-Cl-bhq),51 and nickelacycle [bhqNi(OAc)]2 (3)18 were synthesized according to literature procedures. NMR experiments were carried out with a Bruker AVANCE-400 spectrometer. Chemical shifts are reported on the δ (ppm) scale relative to the residual solvent signals for 1H and 13C and to external C6F6 (−164.9 ppm) for 19F NMR spectra. Electrochemistry and ESR. Voltammograms were recorded with a BASi Epsilon potentiostat/galvanostat (USA) at room temperature in THF/0.2 M Bu4NPF6 or CH3CN/0.1 M Bu4NPF6 solution with 0.005 or 0.002 M substrate concentration under a dry nitrogen atmosphere. All measurements were carried out with a platinum working electrode (0.07 cm2), a platinum auxiliary electrode, and an Ag/AgNO3 reference electrode. All potentials were referenced against the ferrocenium/ferrocene redox couple. The ESR spectra were recorded in degassed CH3CN/0.1 M Bu4NPF6 or THF/0.2 M Bu4NPF6 solutions on a setup including an ELEXSYS E500 ESR spectrometer of the X-range and a three-electrode (Pt) cell. The spectra were simulated using the WinSim 0.96 program (developed by NIEHS). Synthesis of bhqNi(py)Br (1) and bhqNi(py)Cl (2). 10-Br-bhq/ 10-Cl-bhq (1 mmol) and pyridine (100 μL, 1.2 mmol) were dissolved in 7 mL of THF. Ni(COD)2 (275 mg, 1 mmol) was dissolved in 10 mL of THF. Both solutions were cooled to −35 °C, and then the ligand solution was added to the Ni(COD)2 slurry. The mixture was stirred for 1 h, upon which the color changed to orange. The solvent was removed, and the product was dried under vacuum. Data for 1 are as follows. Orange solid, 304 mg, 77% yield. 1H NMR (400.1 MHz, THF-d8): δ 9.63 (d, J = 5.1 Hz, 1H); 8.28 (d, J = 7.9 Hz, 1H); 8.03 (br, 5H); 7.71, 7.55 (ABq, 2H, JAB = 8.8 Hz, 2H); 7.45 (d, J = 7.8 Hz, 1H); 7.40 (dd, J = 7.9, 5.5 Hz, 1H); 7.09 (t, J = 7.6 Hz; 1H). 13C NMR (100.6 MHz, THF-d8): δ 155.3; 155.0; 150.2; 143.0; 137.7; 132.9; 132.4; 129.0; 128.8; 126.6; 123.9; 123.2; 122.0; pyridine fragment peaks could not be resolved. Anal. Calcd for C18H13BrN2Ni: C, 54.61; H, 3.31; N, 7.08. Found: C, 54.73; H, 3.40; N, 7.10. Data for 2 are as follows. Yellow solid, 240 mg, 68% yield. 1H NMR (400.1 MHz, THF-d8): δ 9.37 (d, J = 4.1 Hz, 1H); 8.03 (br, 2H); 8.27 (d, J = 7.8 Hz, 1H); 7.83 (br, 1H); 7.69, 7.53 (ABq, 2H, JAB = 8.7 Hz, 2H); 7.43−7.37 (m, 4H); 7.07 (t, J = 7.5 Hz; 1H); 5.70 (d, J = 7.4 Hz, 1H). 13C NMR (100.6 MHz, THF-d8): δ 155.5, 153.8, 152.7, 149.5, 143.11, 138.0, 137.6, 132.9, 132.7, 128.8, 128.9, 126.4, 125.36, 123.8, 123.0, 121.7. Anal. Calcd for C18H13ClN2Ni: C, 61.51; H, 3.73; N, 7.97. Found: C, 61.71; H, 3.82; N, 8.10. Synthesis of bhq2Ni (4) and bpy2NiBr2 (5). 10-Br-bhq (258 mg, 1 mmol) and 2,2′-bipyridine (156 mg, 1 mmol) were dissolved in 7 mL of THF. Ni(COD)2 (275 mg, 1 mmol) was dissolved in 10 mL of THF. Both solutions were cooled to −35 °C, and then the ligand solution was added to the Ni(COD)2 slurry. The mixture was stirred for 1 h, upon which the color changed to dark red and a precipitate formed. The reaction mixture was filtered, the solvent was removed from the filtrate, and the residue (4) was dried under vacuum. The precipitate (5) was washed with THF (three times with 10 mL) and dried under vacuum. The reaction occurs in the same way with tridentate ligands (2,2′:6′,2″-terpyridine, tris(3,5-dimethyl-1pyrazolyl)borate). Data for 4 are as follows. Dark red solid, 176 mg, 85% yield. 1H NMR (400.1 MHz, THF-d8): δ 7.95−7.91 (m, 3H); 7.77 (dd, J = 4.2, 1.8 Hz, 1H); 7.65−7.62 (m, 2H); 7.36 (dd, J = 7.1, 1.1 Hz, 1H); 7.6.97 (dd, J = 7.9, 4.3 Hz, 1H). 13C NMR (100.6 MHz, THF-d8): δ 148.3, 147.2, 146.9, 135.30, 135.27, 131.2, 129.5, 129.2, 129.2, 127.6, 127.1, 125.8, 120.8. Anal. Calcd for C26H16ClN2Ni: C, 75.23; H, 3.88; N, 6.75. Found: C, 75.28; H, 3.72; N, 6.79. Data for 5 are as follows. Green solid, 258 mg, 93% yield. Anal. Calcd for C20H16Br2N4Ni: C, 45.25; H, 3.04; N, 10.55. Found: C, 45.22; H, 2.99; N, 10.52. Synthesis of bhqNi(py)(OCOCF3) (6). AgOCOCF3 (167 mg, 0.76 mmol) in THF was added dropwise to a solution of 1 (300 mg, 0.76 mmol) in THF, resulting in a color change to dark red and precipitation of a gray solid. After it was stirred for 1 h, the reaction mixture was filtered, the solvent was removed from the filtrate, and the residue was dried under vacuum to afford a red solid. Data for 6 are as follows. Yellow solid, 325 mg, 84% yield. 1H NMR (400.1 G


Organometallics

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unique; Rint = 0.0526, Rσ = 0.0393; completeness to a θ value of 25.242° 99.3%. The refinement of 199 parameters with no restraints converged to R1 = 0.0391 and wR2 = 0.1044 for 3307 reflections with I > 2σ(I) and R1 = 0.0489 and wR2 = 0.1093 for all data with S = 1.069 and residual electron density ρmax/min = 1.582 and −1.092 e Å−3. Crystallographic data for 2: C18H13ClN2Ni, yellow prism, formula weight 351.46, monoclinic, P21/c (No. 14), a = 12.8988(7) Å, b = 10.2995(5) Å, c = 12.2616(6) Å, β = 117.720(2)°, V = 1442.02(13) Å3, Z = 4, Z′ = 1, T = 150(2) K, dcalc = 1.619 g cm−3, μ(Mo Kα) = 1.526 mm−1, F(000) = 720; Tmax/min = 0.6499/0.5140; 46971 reflections collected (2.663° ≤ θ ≤ 33.780°), 5775 of which were unique; Rint = 0.0363, Rσ = 0.0208; completeness to a θ value of 25.242° 99.9%. The refinement of 199 parameters with no restraints converged to R1 = 0.0266 and wR2 = 0.0626 for 5049 reflections with I > 2σ(I) and R1 = 0.0334 and wR2 = 0.0659 for all data with S = 1.046 and residual electron density ρmax/min = 0.535 and −0.529 e Å−3. Crystallographic data for 4: C26H16N2Ni, purple prism, formula weight 415.12, monoclinic, P21/n (No. 14), a = 13.488(3) Å, b = 7.3724(14) Å, c = 18.160(3) Å, β = 103.970(4)°, V = 1752.4(6) Å3, Z = 4, Z′ = 1, T = 150(2) K, dcalc = 1.573 g cm−3, μ(Mo Kα) = 1.123 mm−1, F(000) = 856; Tmax/min = 0.7462/0.6373; 15185 reflections collected (1.699° ≤ θ ≤ 27.896°), 4187 of which were unique; Rint = 0.0573, Rσ = 0.0543; completeness to a θ value of 25.242° 100%. The refinement of 262 parameters with no restraints converged to R1 = 0.0404 and wR2 = 0.0916 for 3153 reflections with I > 2σ(I) and R1 = 0.0602 and wR2 = 0.1012 for all data with S = 1.029 and residual electron density ρmax/min = 0.438 and −0.392 e Å−3. Crystallographic data for 5: C20H16Br2N4Ni·1.665C3H7NO· 0.335C4H10O, green prism, formula weight 677.43, monoclinic, C2/ c (No. 15), a = 28.8233(16) Å, b = 14.9218(9) Å, c = 14.4579(9) Å, β = 113.441(2)°, V = 5705.1(6) Å3, Z = 8, Z′ = 1, T = 150(2) K, dcalc = 1.578 g cm−3, μ(Mo Kα) = 3.515 mm−1, F(000) = 2742; Tmax/min = 0.6473/0.4101; 129187 reflections collected (2.730° ≤ θ ≤ 27.800°), 6742 of which were unique; Rint = 0.0595, Rσ = 0.0254; completeness to a θ value of 25.242° 99.8%. The refinement of 386 parameters with 95 restraints converged to R1 = 0.0275 and wR2 = 0.0645 for 5662 reflections with I > 2σ(I) and R1 = 0.0389 and wR2 = 0.0683 for all data with S = 1.075 and residual electron density ρmax/min = 0.930 and −0.370 e Å−3. Crystallographic data for 6: C20H13F3N2NiO2, yellow prism, formula weight 429.03, monoclinic, P21/n (No. 14), a = 14.679(3) Å, b = 7.8331(16) Å, c = 15.753(3) Å, β = 108.458(10)°, V = 1718.2(6) Å3, Z = 4, Z′ = 1, T = 150(2) K, dcalc = 1.659 g cm−3, μ(Mo Kα) = 1.180 mm−1, F(000) = 872; Tmax/min = 0.5735/0.4463; 30477 reflections collected (2.984° ≤ θ ≤ 25.698°), 3263 of which were unique; Rint = 0.0771, Rσ = 0.0458; completeness to a θ value of 25.242° 99.8%. The refinement of 281 parameters with 78 restraints converged to R1 = 0.0379 and wR2 = 0.0900 for 2619 reflections with I > 2σ(I) and R1 = 0.0521 and wR2 = 0.0976 for all data with S = 1.024 and residual electron density ρmax/min = 0.678 and −0.532 e Å−3. Computational Details. Quantum-chemical calculations were performed by density functional theory (DFT) with the Gaussian 1655 and Jaguar56,57 program packages. The calculation scheme successfully used in our previous work to elucidate HOMO/LUMO energies for palladium complexes26 failed for nickelacycle calculations because of significant underestimation of HOMO/LUMO energies. According to the literature58−61 and preliminary calculations (for complexes bhqNi(py)Br (1) and bhq2Ni (4) in CH3CN or THF; compared with the electrochemically obtained values (Tables S3− S5)), the M06-L and B3LYP functionals and TZP-DKH62 (Gaussian 16) and TZV(F)s63,64 (Jaguar) basis sets were employed. Although the well-known B3LYP functional is good for determining geometry characteristics, the frontier orbital energy values were significantly underestimated in our calculations. Hence, this functional was used only for geometry optimization. However, on the bsis of reports58,59,65 the M06-L functional40,41 performs well in determining the geometric and energetic parameters of organometallic molecules. The two solvents (CH3CN, THF) were accounted for by PCM66 (Gaussian 16) and PBF67,68 (Jaguar) models.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00536. DPV, ESR, DFT, and NMR data (PDF) Cartesian coordinates for calculated structures (XYZ) Accession Codes

CCDC 1858230, 1858232, 1858236, and 1858238−1858239 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.H.B.: [email protected]. ORCID

Robert R. Fayzullin: 0000-0002-3740-9833 Yulia H. Budnikova: 0000-0001-9497-4006 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially (the electrochemical experiments) supported by the Russian Science Foundation (Grant No. 1423-00016).

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REFERENCES

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