Activation of Aryl Halides by Nickel(I) Pincer Complexes: Reaction

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Activation of Aryl Halides by Nickel(I) Pincer Complexes: Reaction Pathways of Stoichiometric and Catalytic Dehalogenations Christoph A. Rettenmeier, Jan Wenz, Hubert Wadepohl, and Lutz H. Gade* Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: Homolytic C−X bond cleavage of organohalides by the T-shaped nickel(I) complexes [LigNiI] 1 bearing the iso-PyrrMeBox ligand had been found previously to be the crucial activation step in the asymmetric hydrodehalogenation of geminal dihalides. Here, this mechanistic investigation is extended to aryl halides, which allowed a systematic study of the activation process by a combination of experimental data and density functional theory modeling. While the activation of both aryl chlorides and geminal dichlorides appears to proceed via an analogous transition state, the generation of a highly stabile nickel(II)aryl species in the reaction of the aryl chlorides for the former represents a major difference in the reactive behavior. This difference was found to have a crucial impact on the activity of these nickel pincer systems as catalysts in the dehalogenation of aryl chlorides compared to geminal dichlorides and highlights the importance of the regulatory pathways controlling the nickel(I) concentration throughout the catalysis. These results along with the identification and characterization of novel nickel(II)aryl species are presented.

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INTRODUCTION

corresponding nickel(I) complexes and thus allows the systematic investigation of their reactivity. The aim of this study was to further our understanding of the activation of organohalides by the T-shaped nickel(I) complex 1. This had previously been found to be the key activation step in the asymmetric hydrodehalogenation of geminal dihalides (Scheme 1).8 However, the high rates of reaction between these dihalide substrates and the nickel(I) complex led us to extend our investigation to aryl halides, allowing us to gain a more complete picture of these transformations. In this work, we report a combined experimental and computational study of

The activation of organic halogen compounds by low-valent metal complexes presents one of the most fundamental reaction steps in organometallic chemistry and has been investigated extensively.1 Reactions with molecular nickel(0) and nickel(I) species have been studied in great detail in pioneering work by Cassar,2 Kochi,3 and Espenson,4 which has shaped our understanding of such reaction steps. These feature in more sophisticated reaction sequences of nickel-catalyzed transformations such as dehalogenations5 or C−C cross-coupling reactions.6 In comparison to its heavier homologue palladium, nickel has shown a greater variety of accessible oxidation states, frequently favoring one-electron redox processes in the activation of halogen compounds over concerted two-electron steps.7 This greater diversity in the reactive behavior of nickel may lead to a change in the reaction mechanism of a given type of transformation upon going from one catalyst to another. Detailed insight into such reaction pathways is thus essential for the development of more effective catalysts. Recently, we reported the asymmetric hydrodehalogenation of geminal dihalides8 catalyzed by nickel(I)/nickel(II) hydride complexes bearing the iso-PyrrMeBox ligand.9 This ligand system combines the advantageous sterical features of a chiral bisoxazoline pincer ligand with a delocalized electronic πsystem almost identical to that of hydrocorphins found in cofactor F430 of methyl coenzyme M reductase.10 In contrast to most ligand systems with isolated hard nitrogen donor atoms, the delocalized π-electron system stabilizes the © XXXX American Chemical Society

Scheme 1. Proposed Mechanism for the Catalytic Cycle of the Hydrodehalogenation of Geminal Dihalogenides

Received: June 15, 2016

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DOI: 10.1021/acs.inorgchem.6b01448 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

coordination plane spanned by the three nitrogens and the aryl carbon donors, thus reducing the extent of steric repulsion of the two Ph substituents and the aryl ligand. The latter is oriented in a near-perpendicular manner with respect to the coordination plane to allow efficient π-back-bonding and minimal interligand repulsion [N−Ni−C−C torsion angles of −84.0° (4-F), −77.3° (4-H), and −84.6° (4-OMe)]. In all three cases, the aryl ligand is slightly tilted out of the coordination plane [176.5° (4-F), 174.3° (4-H), and 166.6° (4-OMe)]. The presence of neither the electron-withdrawing fluoro nor the electron-donating methoxy para substituent at the phenyl ligand causes significant differences in the Ni−C bond lengths. In all three complexes, the protons in α-position to the oxazolines’ substituents are located in close proximity of the face of the aryl ligand. The corresponding chemical shifts of the 1 H nuclear magnetic resonance (NMR) spectra were found to be strongly affected by the electronic nature of the different aryl ligands, which proved useful for their identification and quantification in mixtures. Stoichiometric and Catalytic C−X Bond Activation Using iso-PyrrMeBox−Nickel(I) Complex 1. The nature of the halide had a strong influence on the reaction rate of the stoichiometric transformation depicted in Scheme 2, the general trend being the following: I > Br > Cl > F. In fact, C−F activation could be observed only for the reactive hexafluorobenzene, while monofluorobenzene was stable even at elevated temperatures.13b Among the para-substituted phenyl chlorides, those bearing electron-withdrawing groups reacted faster than the derivatives with electron-donating groups (OMe < Me < H < F < CF3). A Hammett correlation of this series of reactions gave a ρ value of 3.0 (Figure 2a). The value is lower than that obtained by Kochi et al. in the case of nickel(0) complexes (ρ = 5.4)3 but is similarly interpreted to reflect the feasibility of accepting electron density from the electron rich nickel(I) complex during the transition state (see below). Initial rate kinetics for the reaction of p-chlorofluorobenzene and nickel(I) complex 1 showed a linear dependency on both the complex and chloroaryl concentrations [k = (5.0 ± 0.5) × 10−3 M−1 s−1 (Figure 2b,c)]. The Eyring plot based on the reaction in the temperature range from 10 to 50 °C gave values for the activation barrier of 20.4 kcal mol−1 for ΔG⧧295 K and 14.2 kcal mol−1 for ΔH (Figure 2d). Upon reaction of 2,4,6-tri-tert-butyl1-bromobenzene with nickel(I) complex 1, isomerization was observed,8 consistent with an intermediate formation of the organo radical, and the alkyl species 5 was the organometallic product formed as reported previously (Scheme 4).8 These results are consistent with the observations made by Kochi et al.3 and Espenson et al.4a on low-valent nickel complexes and strongly support the idea of a C−X bond activation process involving a partial electron transfer to the aryl halides resulting in the formation of organic radical intermediates (vide infra). However, a key aspect of whether nickel(III) species are involved in the activation process remained unclear. To shed light on this issue, a computational study [density functional theory (DFT), ub3lyp/6-311G(d,p)] of this initial reaction step was performed. A transition state was found (S = 1/2) for the activation of p-chlorofluorobenzene (Figure 4) analogous to that described by Budzelaar et al.15 for cobalt(I) complexes in which the nickel(I) complex attacks the halogen atom with an almost linear Ni···Cl···C atom arrangement (Ni···Cl, 2.309 Å; C···Cl, 2.289 Å; Ni···Cl···C, 154.4°). The elongation of the C···Cl bond indicates that a significant

these reactions along with the synthesis and characterization of novel nickel(II) aryl species.

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RESULTS AND DISCUSSION This study focuses first on the preparation and characterization of arylnickel(II) complexes via substitution of the corresponding halide precursors, followed by a mechanistic study of their generation via the stoichiometric reaction of NiI complexes with aryl halides. This is followed by an investigation into the corresponding catalytic conversion and the way in which the accumulation of aryl−nickel species may be developed into a preparative method. Finally computational modeling of reaction pathways governing the hydrodehalogenation of geminal alkyl dihalides provides additional mechanistic insight into this previously reported reaction. Synthesis and Structural Characterization of isoPyrrMeBox−Nickel−Aryl Complexes. Nickel(I) complex 1 reacted with aryl halides in a formal bimolecular oxidative addition giving the corresponding nickel(II)halogenido and aryl complexes in equimolar quantities (Scheme 2).11 Scheme 2. Bimolecular Oxidative Addition of Aryl Halides to Two Independent Nickel(I) Centers

As will be discussed below, this type of transformation not only models a key step in the catalytic hydrodehalogenation of halocarbons but also may be developed into an efficient synthetic protocol for the preparation of aryl−nickel complexes that are difficult to access via conventional substitution of the halide compounds using aryllithium, Grignard, or related aryl transfer reagents. The nickel halogenido complexes 2 and 3 described previously8 represent a convenient starting point for the synthesis of aryl and alkyl12 nickel(II) complexes via reaction with the corresponding Grignard reagents as shown in Scheme 3. Accordingly, a series of diamagnetic para-substituted Scheme 3. Synthetic Route to Nickel(II)aryl Complexes

nickel(II)phenyl complexes 4-OMe, 4-Ph, and 4-F as well as pentafluorophenyl species 4-F5 were synthesized and characterized. The molecular structures of complexes 4-OMe, 4-Ph, and 4-F were determined by X-ray structure analysis and are shown in Figure 1. The molecular structures show the typical characteristics of nickel(II) complexes of the iso-PyrrMeBox ligand13 and of other nickel(II)aryl pincer complexes.14 The backbone of the pincer is slightly twisted relative to the B


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Inorganic Chemistry

Figure 1. Molecular structures of aryl complexes 4-F, 4-H, and 4-OMe. Only one of the independent molecules is shown for 4-F and 4-H. Hydrogen atoms have been omitted for the sake of clarity. Footnote a denotes that the averages over values of all the independent molecules in the unit cell are given and that standard uncertainties are those of the individual values.

organoradical generated in the initial step. Therefore, the catalytic hydrodehalogenation of geminal dihalides occurs efficiently only at a relatively high concentration of hydrido complex 6 and a very low concentration of nickel(I) complex 1, which is continuously regenerated through the elimination of H2 from two nickel hydrido complexes 6.8 In the case of geminal dihalides, the nickel(I) concentration is decreased by the reaction of the organoradical with the nickel(I) complex itself. The reaction of 2,2-dichlorotetrahydronaphthalene with two molecules of nickel(I) complex 1 gives almost quantitatively chlorido complex 2, which is part of the actual catalytic cycle of the hydrodehalogenation.8 On the other hand, the aryl chlorides react with the nickel(I) complex in the bimolecular oxidative addition described above leading to nickel(II)aryl complexes that were found to be stable under the reaction conditions [no reaction was observed when complex 4-F was exposed to H2 (5 bar), nickel hydrido complex 6 (in the presence of 5 bar of H2), or LiEt3BH even at 100 °C]. In the course of the catalysis, these aryl complexes accumulate at the expense of the catalytically active nickel species and thereby lead to the experimentally observed low level of conversion in the actual hydrodehalogenation (Figure 5). In fact, a near-quantitative formation of the aryl species from the nickel halogenido complex can be achieved under these conditions. For simple aryl halides that lead to complexes that are often directly accessible by the reaction of the corresponding Grignard reagent with a nickel halogenido complex (Scheme 2), this methodology is of little synthetic use. However, we were able to apply the bimolecular oxidative addition to obtain more sophisticated target molecules, whose synthesis proved to be difficult using organomagnesium species. The di- and tetrametalated aryl compounds 7 and 8 were synthesized using the corresponding aryl bromides and nickel(I) complex 1 (Figure 6). In the case of the latter, suitable single crystals for a singlecrystal X-ray structure analysis were obtained (Figure 7). Besides minor deviations, all four LigNi fragments are equivalent and are tilted with respect to the central tetraazaperopyrene plane. The homochiral centers at the oxazoline ring of the pincers generate the overall D2 symmetry of the molecule with a broken local C2 symmetry in the LigNi fragments that is reflected in the NMR spectra of 8.

electron transfer from the nickel(I) center to the antibonding orbital has occurred (Figure 3). After the homolytic cleavage of the C−Cl bond, the initial products formed are the chlorido complex and the pfluorophenyl radical. Interestingly, the addition of the latter to the nickel(II)halogenido complex giving a nickel(III) species (Scheme 5) is energetically favored {Δ(ΔG) = 16 kcal mol−1 [S = 1/2 (Figure 3)]} and appears to proceed in an almost barrierfree manner (see the Supporting Information). However, attempts to locate a transition state that directly links the NiI species to such a nickel(III) complex were not successful. Given these results, a stabilization of the organoradical formed in the initial activation by coordination to the simultaneously generated nickel(II) chlorido complex appears to be likely to occur (Scheme 5). Possibly, the capture of the phenyl radical by the nickel(II) complex competes with its diffusion out of the solvent cage immediately after the C−Cl bond activation step. This would be analogous to the scenario described by Kochi et al., who observed the partial formation of nickel(I) species in the reaction of aryl halides with the tetrakis(triethylphosphine)nickel(0) complex.3 Nevertheless, in both cases, diffusion or capture of the phenyl radical, the subsequent reaction with the second molecule of the nickel(I) complex is expected to give the bimolecular oxidative addition products observed experimentally (see above). DFT modeling generally overestimates the energy barriers for the activation of aryl chlorides [ΔG⧧298 K = 27.0 kcal/mol (DFT) compared to ΔG⧧295 K = 20.4 kcal/mol (Eyring analysis)] for chlorofluorobenzene; however, the relative reaction rates (krel, Hammett plot) correlate well with the calculated values (Figure 4). Catalytic Dehalogenation of Aryl Halides: Accumulation of Aryl−Nickel Species. The hydrodehalogenation of the aryl chlorides can be performed catalyically using LiEt3BH as a reductant and the nickel(I) complex as catalyst; however, the activity of the catalyst for this class of substrates is poor compared to the previously reported hydrodehalogenation of geminal dichlorides (Scheme 1).8 The main reason for the decrease in activity becomes apparent only when considering the regulating pathways that control the concentration of the active nickel(I) species in the hydrodehalogenation of geminal dihalides (Scheme 6). The latter not only is crucial for the activation of the substrate itself but also competes very effectively with the nickel hydrido species to react with the C


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Inorganic Chemistry

Scheme 4. Isomerization during the Bimolecular Oxidative Addition of the Sterically Very Demanding 2,4,6-Tri-tertbutyl-1-bromobenzene to Nickel(I) Complexes 18

Figure 3. Results of the DFT modeling [UB3LYP/6-311G(d,p)] on the activation of p-chlorofluorobenzene by nickel(I) complex 1: (top) geometry and SOMO (natural orbital) of the transition state and (bottom) energy profile of the total activation process (brackets indicate a close association of both molecules). The organoradical formed may either diffuse into solution or be captured to form a corresponding nickel(III) species.

Scheme 5. Proposed Reversible Coordination of the Aryl Radical to the Nickel Chlorido Complex To Give a Nickel(III) Intermediate

Figure 2. (a) Hammett plot using the relative rates (krel) of the reaction of nickel(I) complex 1 with the para-substituted chlorophenyls (-CF3, -F, -H, -Me, and -OMe) in C6D6 at 295 K. (b and c) Initial rate kinetics on the activation of chlorofluorobenzene by varying the p-chlorofluorobenzene and complex concentrations, respectively. (d) Eyring plot of the reaction over the temperature range of 10−50 °C. D


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Inorganic Chemistry

Figure 4. Correlation between relative reaction rates krel and the activation energies of the rate-determining step obtained from the modeling.

Figure 5. Course of the catalytic hydrodehalogenation of pchlorofluorobenzene with LiEt3BH and chlorido complex 2 as a catalyst (10 mol %) monitored by 19F NMR spectroscopy. After an induction period, in which the nickel(I) complex is generated via the elimination of H2, the catalytic hydrodehalogenation takes place, during which nickel(II)aryl complex 4-F accumulates.

Scheme 6. Regulatory Pathways for the Concentration of the Nickel(I) Species in the Case of (A) Geminal Dichlorides and (B) Aryl Chlorides

Figure 6. Dimetalated biphenyl 7 and tetrametalated tetraazaperopyrene 8 synthesized via the direct reaction of nickel(I) complex 1 with the corresponding aryl bromides.

obenzene], reflecting the experimentally observed rapid reaction of geminal dichlorides with the nickel(I) complex. Notably, the corresponding transition states for the activation of 2-chlorotetrahydronaphthalene (both enantiomers) were disfavored by roughly 5−7 kcal/mol [21.8 kcal/mol (S) and 21.4 kcal/mol (R) (see the Supporting Information)], which is in agreement with the relatively slow reaction of 2chlorotetrahydronaphthalene with 1 and high selectivity for the monohydrodehalogenation during the catalytic reduction of geminal dihalides observed experimentally.8 Besides the differences in the activation energies for the aryl and alkyl chlorides, additional insights, which concern the subsequent H-transfer as it occurs during the catalytic hydrodehalogenation of geminal dichlorides, were gained by the DFT study. While the coordination of the aryl radical to nickel(II)chlorido complex 2 forming the nickel(III) species leads to a large stabilization (Figure 3), in the case of the alkyl radical, on the other hand, only a relatively small energy gain results from its coordination to the metal center (Figures 9 and 10). Assuming a low barrier for the homolytic Ni−C bond dissociation [as found in the case of Ni3−Cl-dia_r (see the

Computational Modeling of the Hydrodehalogenation of Geminal Alkyl Dihalides. The computational studies were extended to the activation of 2,2-dichlorotetrahydronaphthalene by nickel(I) complex 1, which was found to react rapidly to a mixture of 1,2-dihydronaphthalene and 1,4dihydronaphthalene as well as the nickel(II)chlorido complex8 to establish whether similar initial activation paths exist in both cases. Indeed, two diastereomeric transition states analogous to those described above were found (Figure 8). For both, the activation barrier was calculated to be significantly lower than for the aryl chlorides [17.0 kcal/mol (pro-R) and 15.3 kcal/mol (pro-S) compared to 27.0 kcal/mol for the p-fluorochlorE


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Inorganic Chemistry

Figure 9. DFT-modeled geometries of nickel(III) species Ni3−Cldia_r and Ni3−H-dia_r (one of two diastereomeric structures is depicted).

The consequences of the significant difference in the stability between aryl and alkyl nickel(III) complexes toward homolytic cleavage of the Ni−C bond for the hydrodehalogenation are not yet fully understood. A reversible addition of the corresponding organic radicals to the nickel(II) halogenido complexes may be accessible in both cases; however, the degree to which such a stabilizing interaction actually occurs may determine the mechanism subsequent to the activation step of the halogen compound. In the case of the aryl chlorides, the possibility that the relatively high stability of the nickel(III) species allows a chloride/hydride exchange at the nickel(III) center by LiEt3BH followed by a subsequent elimination of the corresponding benzene cannot be ruled out [attempts to optimize the modeled structure of a nickel(III) species with a hydride and pfluorophenyl ligand led directly to the elimination of fluorobenzene].

Figure 7. Molecular structure of tetrametalated tetraazaperopyrene 8. Only one of two independent molecules is shown. Hydrogen atoms have been omitted for the sake of clarity. Selected bond lengths (angstroms) and angles (degrees): Ni−C, 1.881(5)−1.940(5); Ni− N(pyr), 1.916(5)−1.982(5); Ni−N(oxaz), 1.857(6)−1.912(5); C− Ni−N(pyr), 158.8(2)−174.4(2); C−Ni−N(oxaz), 86.3(2)−91.6(2).

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CONCLUSION The experimental and computational investigation of the activation of aryl halides by nickel(I) complex 1 provided insight into the basic steps involved in this transformation. Furthermore, by extending the DFT investigation to geminal dichlorides that appear to react via an analogous activation step, we were able to model a complete catalytic cycle for the enantioselective hydrodehalogenation of geminal dihalides that is consistent with the previous experimental study. Besides the main reaction cycle, other regulatory pathways that control the crucial ratio between the nickel(I)- and nickel(II)hydrido species 1 and 6 during the reaction exist. In the case of the aryl halides, this regulation is dysfunctional, and instead of the nickel(I) species being transformed to the hydrido complex, a stable nickel(II)aryl complex is formed that accumulates and does not re-enter the catalytic cycle, leading to the eventual halt of the catalytic conversion.

Figure 8. DFT-modeled transtion states for the C−Cl bond activation and the subsequent H-transfer (one of two diastereomeric transition states is depicted).

Supporting Information)], a rapid diffusion of the alkyl radical into solution would be expected to occur. These findings are in accordance with the experimental investigation into the reversible addition of alkyl radicals to (1,4,8,11tetraazacyclotetradecan)nickel(II) complexes by Espenson et al.16 and by Meyerstein et al.17 as well as the theoretical investigation by Kozlowski et al. on bipyridine nickel complexes as catalysts in photoredox cross-coupling reactions.18 Attempts to synthesize aryl or alkyl nickel(III) species of this type were not successful;19 however, the potential involvement of nickel(III) intermediates was indicated by the in situ observation of EPR resonances in frozen tetrahydrofuran (THF) of the ongoing hydrodehalogenation of 2,2-dichlorotetrahydronaphthalene with LiEt3BH and nickel(I) complex 1 as a catalyst (Supporting Information), although the actual nature of the EPR active species as well as its role in the catalysis remains elusive.

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EXPERIMENTAL SECTION

General Procedures. All manipulations of air- and moisturesensitive materials were performed under an inert atmosphere of dry argon (Argon 5.0 purchased from Messer Group GmbH and dried over Granusic phosphorpentoxide granulate) using standard Schlenk techniques or by working in a glovebox. The solvents were dried over sodium (toluene), potassium (hexane), or a sodium/potassium alloy (pentane and diethyl ether), distilled, and degassed prior to being used.20 Deuterated solvents were purchased from Deutero GmbH and F


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Inorganic Chemistry

Figure 10. Energy and free energy profiles of the activation of 2,2-dichlorotetrahydronaphthalene by nickel(I) complex LigNiI (1) and the subsequent H-transfer by hydrido complex LigNiH (6) to give 2-chlorotetrahydronaphthalene. The geometries of the crucial steps of the reaction path marked in blue are depicted (representing only one of the two diastereomeric geometries in each case; brackets indicate a close association of both molecules). or from Euriso-Top GmbH, dried over potassium (C6D6, toluene-d8, and THF-d8), vacuum distilled, degassed, and stored in Teflon valve ampules under argon. Hydrogen 5.0 (Messer Group GmbH) was used as purchased without further purification. The phenyl-substituted 2,5bis(oxazolinylmethyl)pyrrole protioligands9a as well as complexes LigNiI (1),8 LigNiCl (2),8 and LigNiBr (3)8 were synthesized according to literature procedures. All other reagents were obtained from commercial sources and used as received unless explicitly stated otherwise. Air-sensitive samples for NMR spectroscopy were prepared under argon in 5 mm Wilmad tubes equipped with J. Young Teflon valves. 1H and 13C NMR spectra were recorded on Bruker Avance (200 MHz), Bruker Avance II (400 MHz), and Bruker Avance III (600 MHz, equipped with a CryoProbe) NMR spectrometers and were referenced internally using the residual protio solvent (1H) or solvent (13C).21 The appearance of the signals was described using the following abbreviations: s, singlet; d, doublet; dd, doublet of doublet; ddd, doublet of doublet of doublet; dt, doublet of triplets; t, triplet; q, quartet; m, multiplet; b, broad signal. Continuous-wave X-band (∼9 GHz) EPR spectra were recorded using a Bruker Biospin Elexsys E500 EPR spectrometer fitted with a super high Q cavity. The magnetic field and the microwave frequency were calibrated with a Bruker ER 041XK Teslameter and a Bruker microwave frequency counter. The temperature of the sample was adjusted using a flow-through cryostat in conjunction with a Eurotherm (B-VT-2000) variable-temperature controller. EPR spectral simulations were conducted using the XSophe software (Bruker, version 1.1.4). Elemental analyses were performed

by the analytical service of the Heidelberg Chemistry Department using the vario EL and vario MIKRO cube analytical devices. Mass spectra were recorded on Bruker ApexQe hybrid 9.4 T FT-IVR (HRESI, HR-DART) and JEOL JMS-700 magnetic sector (HR-FAB, HREI, LIFDI) spectrometers at the mass spectrometry facility of the Organic Department at the University of Heidelberg. Either 3nitrobenzyl alcohol (NBA) or o-nitrophenyloctyl ether (NPOE) was used as the matrix in the FAB-MS measurements. X-ray diffraction analysis was performed at the laboratory for structural analysis of the Inorganic Chemistry Department at the University of Heidelberg under the supervision of Prof. Dr. Wadepohl. An Agilent Technologies Supernova-E CCD (Cu Kα or Mo Kα X-radiation, microfocus tube, and multilayer mirror optics) and a Bruker AXS Smart 1000 CCD diffractometer (Mo Kα radiation, graphite monochromator, and λ = 0.71073 Å) were used for data acquisition. UV/vis spectra were recorded on a Varian Cary 5000 UV/vis/NIR spectrometer. Preparation of Complex LigNiC6F5 (4-F5). A solution of bromido complex 3 (69 mg, 0.13 mmol), chloropentafluorobenzene (0.70 mmol, 5.3 equiv), and magnesium (0.82 mmol, 6.2 equiv) in 5 mL of THF was stirred at room temperature for 30 min. After removal of the solvents, a toluene/pentane mixture (1/15) was added, and the reaction mixture was stirred for 4 h and subsequently filtered. After removal of the solvents, the crude was recrystallized from a THF/ pentane mixture at −40 °C to give a yellow crystalline solid in 84% yield (68 mg). The synthesis of 4-F5 was also performed using previously prepared Grignard reagent BrMgC6F5 in THF instead of G


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Inorganic Chemistry

4 H, H1,1′); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 171.4 (C2), 165.0 (C4), 157.2 (C14), 144.7 (C7), 144.6 (C11), 137.3 (C12), 128.3 (C9), 127.1 (C10), 126.2 (C8), 111.7 (C13), 81.3 (C3), 73.4 (C5), 69.4 (C 6 ), 54.8 (C 15 ), 31.0 (C 1 ); HR-MS (FAB+) calcd for C31H29N358NiO2 m/z 549.1562, found m/z 549.1541. Elemental Anal. Calcd for C31H29N3NiO2: C, 67.66%; H, 5.31%; N, 7.64%. Found: C, 67.68%; H, 5.20%; N, 7.65. Preparation of Dinuclear Complex 4,4′-[LigNi]2biphenyl (7). To a solution of 4,4′-dibromobiphenyl (15 mg, 0.048 mmol) and 3 mL of THF was added a solution of nickel(I) complex 1 (0.202 mmol, 4.2 equiv in 2 mL of THF) at room temperature. After 1 h, LiEt3BH (0.1 mmol, 2.1 equiv) was added and the solution was concentrated and stirred at 60 °C for 30 min to convert generated nickel hydrido species 6 into nickel(I) complex 1. Additional 4,4′-dibromobiphenyl (6.4 mg, 0.02 mmol, 0.42 equiv) was added, and the reaction mixture was stirred at room temperature for 30 min. Again, LiEt3BH was added (0.07 mmol), and the solvents were removed. The residue was treated with a toluene/pentane mixture (2/1), and the solution was filtered. After removal of the solvents, the residue was dissolved in toluene and the product was precipitated with pentane at −40 °C to give a yellow solid in 50% yield (52 mg): 1H NMR (C6D6, 600.130 MHz, 295 K) δ 7.44 (d, 3J = 7.7 Hz, 4 H, H12), 7.21 (d, 3J = 7.6 Hz, 4 H, H13), 7.11− 7.02 (m, 12 H, H9,10), 6.90 (d, 3J = 7.8 Hz, 8 H, H8), 5.21 (s, 4 H, H3), 4.16 (dd, 3J = 8.2 Hz, 3J = 8.5 Hz, 4 H, H6), 3.57 (dd, 2J = 8.4 Hz, 3J = 8.4 Hz, 4 H, H5), 3.43 (dd, 2J = 8.2 Hz, 3J = 2.3 Hz, 4 H, H5′), 2.19 (m, 8 H, H1′1); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 171.6 (C2), 166.0 (C4), 156.5 (C14), 144.7 (d, C7), 138.0 (C12), 136.5 (C11), 128.4 (C9), 127.2 (C10), 126.3 (C8), 123.2 (C13), 81.3 (C3), 73.4 (C5), 69.4 (C6), 31.0 (C1); HR-MS (FAB+) calcd for C60H52N658Ni2O4 m/z 1036.2757, found m/z 1036.2710. Elemental Anal. Calcd for C60H52N6Ni2O4: C, 69.39%; H, 5.05%; N, 8.09%. Found: C, 68.92%; H, 5.03%; N, 7.98. Preparation of Tetranuclear Complex 4,7,11,14-[LigNi]4TAPP (8). To a solution of 9-bisperfluoropropyl 4,7,11,14tetrabromo-1,3,8,10-tetraazaperopyrene (29 mg, 0.030 mmol) in 30 mL of THF was added a solution of nickel(I) complex 1 (132 mg, 0.298 mmol, 8.9 equiv) at 0 °C, and the mixture was stirred for 10 h. After removal of the solvents, the residue was treated with toluene and stirred and pentane was added to precipitate the product. After filtration, the crude was recrystallized from a THF/Et2O/pentane mixture at room temperature to give a red crystalline solid in 38% yield (28 mg): 1H NMR (C6D6, 600.130 MHz, 295 K) δ 9.44 (s, 4 H, HTAPP), 7.83 (d, 3J = 7.5 Hz, 8 H, H8), 7.58 (dd, 3J = 7.4 Hz, 3J = 7.3 Hz, 8 H, H9), 7.42 (t, 3J = 7.3 Hz, 4 H, H10), 6.49 (t, 3J = 7.2 Hz, 4 H, H20), 6.37 (dd, 3J = 7.4 Hz, 3J = 7.3 Hz, 8 H, H19), 5.72 (d, 3J = 7.4 Hz, 8 H, H18), 5.39 (s, 4 H, H3/13), 5.31 (s, 4 H, H3/13), 4.57 (dd, 3J = 8.5 Hz, 3J = 2.5 Hz, 4 H, H6), 4.18 (dd, 2J = 8.2 Hz, 3J = 8.6 Hz, 4 H, H5), 3.87 (dd, 2J = 8.2 Hz, 3J = 8.6 Hz, 4 H, H5′), 3.60 (dd, 3J = 8.0 Hz, 3J = 1.6 Hz, 4 H, H16), 3.45 (dd, 2J = 8.2 Hz, 3J = 8.3 Hz, 4 H, H15), 2.98 (dd, 2J = 8.0 Hz, 3J = 1.9 Hz, 4 H, H15′), 2.53−2.29 (m, 16 H, H1,1′,11,11′); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 172.5 (C2/1212), 172.4 (C2/12), 166.1 (C14), 165.7 (C4), 160.7 (CTAPP), 160.4 (CTAPP), 144.9 (C7), 142.2 (C17), 138.2 (CHTAPP), 129.4 (C9), 127.8 (C10), 127.6 (CTAPP), 127.3 (C19), 126.7 (C8), 126.4 (C20), 124.7 (C18), 122.1 (CTAPP), 119.0 (CTAPP), 112.7 (CTAPP), 81.8 (C3/13), 80.8 (C3/13), 73.8 (C5), 73.6 (C15), 69.4 (C6,16), 31.1 (C1,11); 19F NMR (C6D6, 376.273 MHz, 295 K) δ −79.7 (t, 3J = 10.6 Hz, 6 F), −106.2 to −110.3 (m, 4 F), −123.5 (m, 4 H); HR-MS (ESI+) calcd for C124H92F14N1658Ni4O8 m/z 2430.4474, found m/z 2430.4484. Elemental Anal. Calcd for C124H92F14N16Ni4O8: C, 61.17%; H, 3.81%; N, 9.20. Found: C, 61.38%; H, 4.17%; N, 9.30. Catalytic Hydrodehalogenation of p-Chlorofluorobenzene (NMR experiment). A NMR sample was prepared using pchlorofluorobenzene (13.1 mg, 0.1 mmol), LiEt3BH (0.21 mmol), complex 2 (4.8 mg, 0.01 mmol, 10 mol %), and 1,4-bis(trifluoromethyl)benzene as an internal standard in 0.5 mL of THFd8, and the reaction was monitored by 19F NMR spectroscopy. Initial Rate Determination of the Reaction of Nickel(I) Complex 1 with p-Chlorofluorobenzene. The order of the reaction of 1 with p-chlorofluorobenzene was determined by initial

the one-pot methodology described above: 1H NMR (C6D6, 600.130 MHz, 295 K) δ 7.10 (m, 4 H, H9), 7.05 (m, 2 H, H10), 6.65 (m, 4 H, H8), 5.03 (s, 2 H, H3), 3.70 (dd, 3J = 9.0 Hz, 3J = 3.0 Hz, 2 H, H6), 3.45 (dd, 2J = 8.3 Hz, 3J = 8.5 Hz, 2 H, H5), 3.45 (dd, 2J = 8.3 Hz, 3J = 3.0 Hz, 2 H, H5′), 2.14−2.00 (m, 4 H, H1,1′); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 172.2 (C2), 166.0 (C4), 149.1 (C12/13/14, 1JCF = 231.3 Hz), 142.9 (C7), 138.4 (C12/13/14, 1JCF = 231.3 Hz), 135.3 (C12/13/14, 1JCF = 231.3 Hz), 128.8 (C9), 127.9 (C10), 125.2 (C8), 116.1 (C11), 81.6 (C3), 73.4 (C5), 69.9 (C6), 30.7 (C1); 19F NMR (C6D6, 376.273 MHz, 295 K) δ −116.6 (m, 2 F, F12/13), −161.2 (m, 1 F, F14), −163.9 (m, 2 F, F12/13); HR-MS (FAB+) calcd for C30H22F5N358NiO2 m/z 609.0986, found m/z 609.1052. Elemental Anal. Calculated for C30H22F5N3NiO2: C, 59.05%; H, 3.63%; N, 6.89%. Found: C, 59.05%; H, 3.78%; N, 6.96%. Preparation of p-Fluorphenyl Complex LigNiC6H4F (4-F). To a solution of chlorido complex 2 (192 mg, 0.40 mmol) in 20 mL of THF was added p-fluorophenylmagnesium bromide (0.48 mmol, 1.2 equiv) at 0 °C. After 10 min, the cooling bath was removed and the reaction mixture was stirred for an additional 5 min. After removal of the solvents, the residue was treated with a toluene/pentane mixture (1/15) and the solution was filtered. After removal of the solvents, the crude was recrystallized from a toluene/pentane mixture at −40 °C to give a yellow crystalline solid in 77% yield (165 mg): 1H NMR (C6D6, 600.130 MHz, 295 K) δ 7.10 (dd, 3J = 8.2 Hz, 3JFH = 6.8 Hz, 2 H, H12), 7.08−7.00 (m, 6 H, H9,10), 6.79 (d, 3J = 6.7 Hz, 4 H, H8), 6.54 (dd, 3J = 8.2 Hz, 4JFH = 9.9 Hz, 2 H, H13), 5.17 (s, 2 H, H3), 3.84 (dd, 3 J = 8.5 Hz, 3J = 2.0 Hz, 2 H, H6), 3.55 (dd, 3J = 8.5 Hz, 2J = 8.1 Hz, 2 H, H5), 3.43 (dd, 2J = 8.1 Hz, 3J = 2.3 Hz, 2 H, H5′), 2.34−2.10 (m, 4 H, H1,1′); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 171.5 (C2), 166.1 (C4), 161.6 (d, 1JCF = 238.6 Hz, C14), 149.7 (d, 4JCF = 2.9 Hz, C11), 144.4 (C7), 137.5 (d, 2JCF = 5.0 Hz, C12), 128.4 (C9), 127.3 (C10), 126.1 (C8), 111.8 (d, 3JCF = 17.8 Hz, C13), 81.3 (C3), 73.4 (C5), 69.3 (C6), 31.0 (C1); 19F NMR (C6D6, 376.273 MHz, 295 K) δ 123.9 (m); HR-MS (FAB+) calcd for C30H26FN358NiO2 m/z 537.1363, found m/ z 537.1350. Elemental Anal. Calcd for C30H26FN3NiO2: C, 66.94%; H, 4.87%; N, 7.81%. Found: C, 66.43%; H, 4.53%; N, 7.68. Preparation of Phenyl Complex LigNiPh (4-H). To a solution of chlorido complex 2 (113 mg, 0.24 mmol) in 5 mL of THF was added PhMgCl (0.31 mmol, 1.3 equiv) at 0 °C. After 5 min, the cooling bath was removed and the reaction mixture was stirred for an additional 5 min. After removal of the solvents, the residue was treated with a toluene/pentane mixture (1/20) and the solution was filtered. After removal of the solvents, the crude was recrystallized from a toluene/pentane mixture (1/30) at −40 °C to give a yellow crystalline solid in 41% yield (50 mg): 1H NMR (C6D6, 600.130 MHz, 295 K) δ 7.35 (m, 2 H, H12), 7.11−7.01 (m, 6 H, H9,10), 6.90−6.84 (m, 5 H, H8,14), 6.74 (m, 2 H, H13), 5.19 (s, 2 H, H3), 4.00 (dd, 3J = 8.6 Hz, 3J = 2.1 Hz, 2 H, H6), 3.56 (dd, 3J = 8.6 Hz, 2J = 8.0 Hz, 2 H, H5), 3.45 (dd, 2 J = 8.0 Hz, 3J = 2.2 Hz, 2 H, H5′), 2.18 (m, 4 H, H1,1′); 13C NMR (C6D6, 150.903 MHz, 295 K) δ 171.4 (C2), 166.1 (C4), 158.9 (C14), 144.7 (C7), 137.8 (C12), 128.4 (C9), 127.2 (C10), 126.2 (C8), 125.1 (C13), 122.4 (C11), 81.3 (C3), 73.4 (C5), 69.4 (C6), 54.8 (C15), 31.0 (C1); HR-MS (FAB+) calcd for C30H27N358NiO2 m/z 519.1457, found m/z 519.1465. Elemental Anal. Calcd for C30H27N3NiO2: C, 69.26%; H, 5.23%; N, 8.08%. Found: C, 69.09%; H, 5.24%; N, 7.94. Preparation of p-Methoxyphenyl Complex LigNiC6H4OMe (4-OMe). To a solution of chlorido complex 2 (100 mg, 0.21 mmol) in 10 mL of Et 2 O and 5 mL of toluene was added pmethoxyphenylmagnesium bromide (0.25 mmol, 1.2 equiv) at −78 °C. After 5 min, the cooling bath was removed and the reaction mixture was stirred for an additional 30 min. After removal of the solvents, the residue was treated with a toluene/pentane mixture (1/6) and the solution was filtered. After removal of the solvents, the crude was recrystallized from a toluene/Et2O/pentane mixture at −40 °C to give a yellow crystalline solid in 88% yield (101 mg): 1H NMR (C6D6, 600.130 MHz, 295 K) δ 7.15 (d, 3J = 8.6 Hz, 2 H, H12), 7.10−7.05 (m, 4 H, H9), 7.05−7.02 (m, 2 H, H10), 6.88 (d, 3J = 7.2 Hz, 4 H, H8), 6.49 (d, 3J = 8.6 Hz, 2 H, H13), 5.20 (s, 2 H, H3), 4.04 (dd, 3J = 8.5 Hz, 3J = 2.1 Hz, 2 H, H6), 3.60 (dd, 2J = 8.1 Hz, 3J = 8.5 Hz, 2 H, H5), 3.49 (s, 3 H, H15), 3.47 (dd, 2J = 8.1 Hz, 3J = 2.1 Hz, 2 H, H5′), 2.26−2.12 (m, H


Article

Inorganic Chemistry rate measurements in a series of kinetic experiments using 19F NMR spectroscopy. Therefore, the concentration of 1 and later of pchlorofluorobenzene was varied in two series of NMR measurements at 295 K in C6D6. In the first series, a solution of p-chlorofluorobenzene (5 mg) and 1,4-dimethoxybenzene as an internal standard in 0.2 mL of C6D6 was added to a solution of nickel(I) complex 1 (1.25, 2.5, 5.0, 7.5, 10.0, 12.5, and 15.0 mg) in 0.4 mL of C6D6, respectively, and the reaction was monitored by 19F NMR spectroscopy. In a second series, a solution of nickel(I) complex 1 (5.0 mg) in 0.2 mL of C6D6 was added to a solution of p-chlorofluorobenzene (2.5, 5, 7.5, 10, 12.5, and 15 mg) and 1,4-dimethoxybenzene in 0.4 mL of C6D6, respectively. Additionally, the temperature dependence of the initial rate was probed using 5 mg of 1 and 5 mg of p-chlorofluorobenzene (0.6 mL of C6D6) at 283, 303, 313, and 333 K. The initial rate of each run was determined by fitting (least-squares, no constraints) the general function of a second-order reaction (reaction 1) to the data obtained from the NMR measurements (intensity of the 19F NMR signal corresponding to aryl complex 4-F vs time). The value of the first derivative of the fitted function at y = 0 was used as the initial rate that was plotted versus the concentration of 1 or p-chlorofluorobenzene.

a − {a(a − b) exp[k(a − b)(x − c)]} /{a × exp[k(a − b)(x − c)] − b}

Figure 11. Observed diffraction patterns (simulated precession pictures) of 8·xTHF: (a) (h0l), a* horizontal, c* vertical; (b) (hk0); (c) (hk1); and (d) (hk2). Intensities in panels c and d are multiplied by 23 with respect to those in panel b. The overall intensity distribution of (hk3) in panel c is similar to that of (hk4) in panel b. No background or LP corrections were applied.

(1)

Determining the Relative Rates of Reaction of Differently Substituted Chlorobenzenes with the Nickel(I) Complex and the Hammett Correlation. The relative rates of the reaction of parasubstituted chlorobenzenes (R = MeO, CH3, H, F, or CF3) with nickel(I) complex 1 under pseudo-first-order conditions were determined in three separately performed competition experiments (A, F and MeO; B, F, H, and CH3; C, F and CF3). Therefore, an excess of the corresponding p-chlorobenzenes (0.339 mmol each, 10 equiv each) was dissolved in 12 mL of THF and the mixture held at 295 K. A solution of the nickel(I) complex (15.0 mg, 0.034 mmol) in 2 mL of THF was added, and the reaction mixture was stirred for 16 h. The solvent was removed, and the residue was analyzed by NMR spectroscopy in C6D6. The ratio of the integrals of the H6 proton signals [for R = MeO, H, and F, see above; the complexes with R = Me (H6 = 4.07 ppm) and CF3 (H6 = 3.75 ppm) were not separately prepared and isolated] was used for a Hammett plot (Figure 2a). Crystallographic Data. Crystal data and details of the structure determinations are listed in Table S1 (Supporting Information). Full shells of intensity data were collected at low temperatures with an Agilent Technologies Supernova-E CCD diffractometer (Cu Kα radiation, microfocus X-ray tube, multilayer mirror optics). Detector frames (typically ω-scans, occasionally φ-scans, scan width of 1°) were integrated by profile fitting.22,23 Data were corrected for air and detector absorption and Lorentz and polarization effects23 and scaled essentially by application of appropriate spherical harmonic functions.24,25 Absorption by the crystal was treated with a semiempirical multiscan method (as part of the scaling process) and augmented by a spherical correction,25 analytically23,26 or numerically (Gaussian grid).23,27 The structures were determined by intrinsic phasing28 (complex 8) or by the charge flip procedure29 (all others) and refined by full-matrix least-squares methods based on F2 against all unique reflections.30 All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were input at calculated positions and refined with a riding model.30 The diffraction pattern of complex 8·xTHF clearly showed weak superstructure reflections in the c* direction (Figure 11). The structure could be determined nevertheless in both the sub- and supercells (with c quadrupled) in space groups P3121 (or P3221) and P31 (or P32), to reveal the molecular core of 8 with the C3F7 groups completely or partially missing and many of the phenyl substituents badly resolved. In any case, difference Fourier syntheses showed much rather diffuse additional electron density not associated with the complex molecules, apparently from heavily disordered THF solvent. Final integration of the diffraction intensities and all further calculations were then performed in the large supercell (Table S1).

Although lower-symmetry space group P31 could not be ruled out, we chose to refine the structure with the higher symmetry (space group P3121 with two independent complex molecules). This choice of space group was made in view of the large amount of disorder, very obvious with the C3F7 groups and numerous THF solvent molecules, and considerably reduced complexity without imposing too much bias on the molecular structure of 8. In particular, the resolution of the C3F7 and phenyl groups did not improve with the lower symmetry. The tetranickelated tetraazaperopyrene core was well-defined [some residual electron density (maximum of 1.79 e Å−3), very close to some of the Ni atoms, may indicate that the lower symmetry of the solvent environment has a minor effect on the effective symmetry of the nonsolvent part of the structure]. Some rotational disorder (or libration) of the phenyl substituents on the iso-PyrrMeBox ligand could be battled by suitable geometry constraints. However, the apparent extensive multisite disorder of the perfluorinated propyl substituents could not be modeled satisfactorily with a split-atom approach. We therefore resorted to a rigid group refinement of a single C3F7 moiety each, the site occupation factor of which was refined (to values below 1.0) to achieve reasonable Ueq values. Hence, the refined orientations of these substituents resemble the most populated conformations only. Rigid body restraints were applied throughout. The electronic contribution of the THF solvent molecules was finally removed from the Fobs with the smtbx solvent masking procedure.31−33 Final refinement was then performed against appropriately modified Fobs. A satisfactory final absolute structure parameter34 of 0.012(5), in line with what was expected from the configuration of 1 used for the synthesis, further confirms the validity of our approach. Computational Data. DFT calculations were performed using Gaussian 09, revision D.01,35 on the bwforcluster JUSTUS. Geometry optimization and the harmonic frequency analysis were conducted at the unrestricted B3LYP/6-311G(d,p)36 level of theory, including Grimme D3 dispersion correction37 and SMD polarizable continuum model38 using the “tight” convergence criteria for SCF calculations. The connection between the transition states and minimum geometries was verified by two separate optimizations starting from the transition state geometry with a slight distortion along either direction of the negative vibrational mode. I


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Inorganic Chemistry

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T.; Desrochers, P. J.; Pulay, P.; Vicic, D. A. J. Am. Chem. Soc. 2006, 128, 13175. (e) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.; Percec, V. Chem. Rev. 2011, 111, 1346. (f) Han, F.-S. Chem. Soc. Rev. 2013, 42, 5270. (7) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Nature 2014, 509, 299. (8) Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Chem. - Eur. J. 2014, 20, 9657. (9) (a) Konrad, F.; Lloret Fillol, J.; Wadepohl, H.; Gade, L. H. Inorg. Chem. 2009, 48, 8523. (b) Mazet, C.; Gade, L. H. Chem. - Eur. J. 2003, 9, 1759. (c) Konrad, F.; Lloret Fillol, J.; Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Eur. J. Inorg. Chem. 2009, 2009, 4950. (d) Deng, Q.H.; Wadepohl, H.; Gade, L. H. Chem. - Eur. J. 2011, 17, 14922. (e) Mazet, C.; Gade, L. H. Organometallics 2001, 20, 4144. (f) Langlotz, B. K.; Lloret Fillol, J.; Gross, J. H.; Wadepohl, H.; Gade, L. H. Chem. - Eur. J. 2008, 14, 10267. Review: (g) Deng, Q.-H.; Melen, R. L.; Gade, L. H. Acc. Chem. Res. 2014, 47, 3162. (10) (a) Knittel, K.; Boetius, A. Annu. Rev. Microbiol. 2009, 63, 311. (b) Mayr, S.; Latkoczy, C.; Krüger, M.; Günther, D.; Shima, S.; Thauer, R. K.; Widdel, F.; Jaun, B. J. Am. Chem. Soc. 2008, 130, 10758. (c) Jaun, B.; Thauer, R. K. Methyl-Coenzyme M Reductase and its Nickel Corphin Coenzyme F430 in Methanogenic Archaea. In Nickel and Its Surprising Impact in Nature; John Wiley & Sons, Ltd.: New York, 2007; pp 323. (11) Anton, M.; Clos, N.; Müller, G. J. Organomet. Chem. 1984, 267, 213. (12) Rettenmeier, C. A.; Wadepohl, H.; Gade, L. H. Chem. Sci. 2016, 7, 3533. (13) (a) Rettenmeier, C. A.; Wadepohl, H.; Gade, L. H. Angew. Chem., Int. Ed. 2015, 54, 4880. (b) Wenz, J.; Rettenmeier, C. A.; Wadepohl, H.; Gade, L. H. Chem. Commun. 2016, 52, 202. (14) (a) Kieltsch, I.; Dubinina, G. G.; Hamacher, C.; Kaiser, A.; Torres-Nieto, J.; Hutchison, J. M.; Klein, A.; Budnikova, Y.; Vicic, D. A. Organometallics 2010, 29, 1451. (b) Cariou, R.; Graham, T. W.; Dahcheh, F.; Stephan, D. W. Dalton Trans. 2011, 40, 5419. (15) Zhu, D.; Korobkov, I.; Budzelaar, P. H. M. Organometallics 2012, 31, 3958. (16) Kelley, D. G.; Marchaj, A.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1991, 113, 7583. (17) Sauer, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1988, 27, 4578. (18) Gutierrez, O.; Tellis, J. C.; Primer, D. N.; Molander, G. A.; Kozlowski, M. C. J. Am. Chem. Soc. 2015, 137, 4896. (19) (a) Spasyuk, D. M.; Zargarian, D.; van der Est, A. Organometallics 2009, 28, 6531. (b) Zheng, B.; Tang, F.; Luo, J.; Schultz, J. W.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2014, 136, 6499. (c) Lee, C.-M.; Chen, C.-H.; Liao, F.-X.; Hu, C.-H.; Lee, G.-H. J. Am. Chem. Soc. 2010, 132, 9256. (d) Grove, D. M.; Van Koten, G.; Zoet, R.; Murrall, N. W.; Welch, A. J. J. Am. Chem. Soc. 1983, 105, 1379. (e) Zhang, C.-P.; Wang, H.; Klein, A.; Biewer, C.; Stirnat, K.; Yamaguchi, Y.; Xu, L.; Gomez-Benitez, V.; Vicic, D. A. J. Am. Chem. Soc. 2013, 135, 8141. (f) Yu, S.; Dudkina, Y.; Wang, H.; Kholin, K. V.; Kadirov, M. K.; Budnikova, Y. H.; Vicic, D. A. Dalton Trans. 2015, 44, 19443. (g) Higgs, A. T.; Zinn, P. J.; Simmons, S. J.; Sanford, M. S. Organometallics 2009, 28, 6142. (20) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals; Butterworth-Heinemann: Oxford, U.K., 2009. (21) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176. (22) Kabsch, K. In International Tables for Crystallography; Rossmann, M. G., Arnold, E., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001; Vol. F, Chapter 11.3. (23) CrysAlisPro; Agilent Technologies UK Ltd.: Oxford, U.K.; Rigaku Oxford Diffraction, Rigaku Polska Sp.z o.o.: Wrocław, Poland, 2015−2016. (24) Blessing, R. H. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 33.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01448. Complete experimental procedures and characterization data of complexes 1−8, including their atom labeling schemes; listings of the crystal data and structural parameters of all compounds characterized by X-ray diffraction; and additional computational details (PDF) Cartesian coordinates of all relevant minimum structures of this study (XYZ) Crystallographic data for compounds 4-F, 4-H, 4-OMe, and 8 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Anorganisch-Chemisches Institut, Universität of Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Julio Lloret-Fillol (ICIQ, Tarragona, Spain) for his support of the computational study and Lena Hahn for the Tapp-Br synthesis and experimental support. We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, Ga 488/9-1). The computational studies were supported in part by bwGRiD, a member of the German D-Grid initiative, funded by the Ministry for Education and Research and the Ministry for Science, Research and Arts Baden-Württemberg and in part by the bwHPC initiative and the bwHPC-C5 project provided through associated computing services of the JUSTUS HPC facility at the University of Ulm. bwHPC and bwHPC-C5 (http://www.bwhpc-c5.de) are funded by the Ministry of Science, Research and the Arts Baden-Württemberg (MWK) and the Germany Research Foundation (DFG).

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REFERENCES

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