methanimine Ligands in Organonickel(II) Complexes - ACS Publications

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Unsymmetrical N‑Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2′-Bipyridine and N,N‑Diaryl-α-diimines? Christian Biewer,† Claudia Hamacher,† Andre Kaiser,† Nicolas Vogt,† Aaron Sandleben,† Mason T. Chin,‡ Siqi Yu,‡ David A. Vicic,‡ and Axel Klein*,† †

Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States

‡

S Supporting Information *

ABSTRACT: The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me2-, 3,5-Me2-, 2,4,6-Me3-, 2,6-iPr2-, 3,5-(OMe)2-, 2NO2-4-Me-, 4-NO2-, 2-CF3-, and 2-CF3-6-F-phenyl; Mes = 2,4,6trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh3)2Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Nibased and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the NPy^Nmethanimine ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2′-bipyridine (bpy; NPy^NPy) and N,N-diaryl-substituted aliphatic α-diimines (Nmethanimine^Nmethanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

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INTRODUCTION Organometallic nickel(II) complexes with bidentate heteroaromatic ligands, such as the prototypical 2,2′-bipyridine (bpy), or aliphatic α-diimines, such as the diaza-1,3-butadienes (RDAB), have recently gained enormous interest because of their role in important catalytic processes such as olefin oligo- or polymerization, olefin/CO copolymerization,1−6 C−C crosscoupling using Grignard or zinc reagents (Kumada and Negishi),7−11 photoredox-driven C−C coupling reactions,12 and electrocatalytic C−C coupling reactions.13−21 In contrast to their vast use in catalysis, only a few fundamental studies on the structures and electronic properties of such organonickel complexes have been conducted.22−29 In the past few years, we have investigated the structures and reactivity on some mononuclear organonickel complexes [(N^N)Ni(Mes)Br] containing symmetrical heteroaromatic or aliphatic α-diimine ligands N^N (Mes = mesityl = 2,4,6trimethylphenyl) and binuclear complexes [(μ-N^N){Ni(Mes)Br}2] with unsymmetrical bis(arylimino)-1,4-pyrazine © XXXX American Chemical Society

(bpip) ligands (Chart 1) or bridging 2,2′-bipyrimidine (bpym) using various methods.30−36 Especially, their redox chemistry was investigated, in view of the application of such systems in (electro)catalytical C−C coupling reactions.16,21,34−36 The most important results of these studies are as follows: (i) Strongly donating solvents like nitriles, N,N-dimethylformamide, or dimethyl sulfoxide slowly replace the bromido ligand (eq 1), while in acetone, ethers, or chlorinated solvents, the complexes are completely stable toward such substitution.31,32 [(N^N)Ni(Mes)Br] + L ⇌ [(N^N)Ni(Mes)(L)]+ + Br − L = strongly donating solvents

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(ii) Examination of the lowest excited states of [(bpy)Ni(Mes)Br] by UV−vis−near-IR (NIR) absorption spectroscopy Received: August 9, 2016

A

DOI: 10.1021/acs.inorgchem.6b01874 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

been previously used in organonickel chemistry,40,41 with applications in polymerization catalysis.42−52 Bifunctional and oligofunctional PyMA derivatives have found application in supramolecular chemistry53−57 and biomolecular recognition.58−61 Interestingly, PyMA complexes of transition metals have also been prepared from pyridine-2-carbaldehyde and the corresponding aniline in the presence of suitable metal precursors and a Lewis catalyst, thus circumventing synthesis of the ligands.62−64 We prepared a number of mononuclear complexes [(RPyMA)Ni(Mes)X] [Chart 2; R-PyMA = N-aryl-1-(pyridin-2-

Chart 1. Comparison of Chelate Ligands with Symmetric N^N (bpy and DAB) and Unsymmetric N^N′ (PyMA and bpip) Coordination Sitesa

Chart 2. Arbitrarily Chosen Cis and Trans Assignment of the Two Stereoisomers of the Nickel Complexes [(RPyMA)Ni(Mes)X]

a

bpy = 2,2′-bipyridine, bpz = 2,2′-bipyrazine, iPr-DAB = N,N′diisopropyldiaza-1,3-butadiene, Ph-DAB = N,N′-diphenyldiaza-1,3butadiene, R-PyMA = N-aryl-1-(pyridin-2-yl)methanimines, and bpip = bis(arylimino)-1,4-pyrazines.

and quantum-chemical calculations revealed mixed Ni/Mes/Br contributions to the highest occupied molecular orbital (HOMO) and bpy-based π* lowest unoccupied molecular orbitals (LUMOs).32,33 (iii) The one-electron-reduced complexes [(N^N)Ni(Mes)Br]•− rapidly cleave the bromide coligand (eq 2).16,33−36 [(N^N)Ni(Mes)Br]

yl)methanimines (Chart 3); Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I]. For a square-planar arrangement of the ligands around the Ni atom (d8, strong ligands) two possible stereoisomers (cis and trans) should be observed, and we have chosen the nomenclature shown in Chart 2 to discriminate between them properly. As the cis stereoisomer, we chose the one having the pyridinyl N atom and the Mes group standing cis to each other, and in the trans isomer, Py and Mes stand trans to each other (Chart 2). With no substituents on the Py function and the methanimine aryl group, the cis isomers might be sterically less favored. However, steric bulk in the 2 and 6 positions of the aryl group might render the trans isomer sterically less favored. From the viewpoint of the individual character of the two N atoms, the C atom, and the X ligand functions (see above), a preference for one of the two isomers remains difficult to predict. From the general considerations described above, for the PyMA complexes, the cis derivative might be electronically favored from having the strong σ-donor Mes in the trans position to the better π-acceptor N-arylmethanimine. However, recent work, including density functional theory (DFT) calculations, has shown that the π-accepting properties of such N-aryl-substituted alkyldiimines strongly depends on the tilt angle between the binding plane N−metal−N and the aryl group.37−39 For example, quantum-chemical calculations modeling the nickel-catalyzed ethylene polymerization using either the aliphatic, Brookhart-type ligand N,N′-(2,6dimethylphenyl)ethylenediimine or 2,6-Me2-PyMA (Chart 3) have revealed that in the cationic intermediate [(2,6-Me2PyMA)Ni(Me)]+ a coplanar arrangement of the aryl groups with the Ni−N−N−C coordination plane is preferred for electronic reasons, in line with the catalytic results.39 In turn, this arrangement allows strong agnostic interactions of the Naryl-2-methyl sustituents and the nickel and stabilizes an orientation of the methyl (Me) coligands trans to the imine N atom. Thus, it can also be said that the Me trans to N imine situation is only stabilized for this coplanar arrangement. We have therefore introduced various substituents to the Naryl part of the PyMA ligand with widely varied steric and

+e −

HoooI [(N^N)Ni(Mes)Br]•− ⇌ [(N^N)Ni(Mes)]• + Br −

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[(N^N)Ni(Mes)]• + solvent ⇌ [(N^N)Ni(Mes)(solv)]• (3)

[(N^N)Ni(Mes)]• + [(N^N)Ni(Mes)]• ⇌ [(N^N)(Mes)Ni−Ni(Mes)(N^N)]

(4)

The resulting undercoordinated radical species [(N^N)Ni(Mes)]• might either add a coordinating solvent molecule (eq 3) or dimerize (eq 4). The results obtained so far for complexes of symmetric ligands (Chart 1) suggest that the better σdonating heteroaromatic ligands bpy and 1,10-phenanthroline (phen) have a higher preference to form mononuclear radical complexes [(N^N)Ni(Mes)(Solv)]•, while the excellent πaccepting 2,2′-bipyrazine (bpz) and iPr-DAB derivatives rapidly form dimers.16,35 (iv) The complexes [(N^N)Ni(Mes)Br] and the binuclear derivatives [(μ-N^N){Ni(Mes)Br}2] were active in Negishi-like C−C coupling reactions.36 In this contribution, we report of the use of unsymmetric N^N′ ligands of the N-aryl-1-(pyridin-2-yl)methanimine (PyMA) type (Chart 1). Combining the established two prevailing types of unsaturated N,N chelate ligands, the aromatic derivatives of the bpy type and the aliphatic diimines of the N,N′diaryldiazabutadiene (DAB) type, the PyMA ligands should allow mononuclear complexes with two different N ligand functions in terms of σ-donating and π-accepting ability and steric bulk (Chart 1).37−39 They are also related to the bridging bpip ligands, which have recently been used to generate binuclear complexes.36 From general considerations, the pyridine moiety should be a stronger σ-donating ligand than the N-arylmethanimine function, and, in turn, the latter should be a better π acceptor. Such unsymmetric PyMA [alternatively called N-aryl-1-(pyridin-2-ylmethylene)aniline] ligands have B


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Inorganic Chemistry Chart 3. R-PyMA Ligands and Abbreviations

under reduced pressure gave reddish violet, microcrystalline solids in 80−98% yield. Details on the yields, elemental analysis, NMR, and MS are provided in the SI. Syntheses of cis/trans-[(PyMA)Ni(Mes)Cl]. An amount of 0.737 g (1 mmol) of trans-[(Ph3P)2Ni(Mes)Cl] was suspended in 40 mL of toluene, and 365 mg (2 mmol) of PyMA was added. After stirring for 30 min, the reaction mixture turned from yellow to dark violet and vast portions of the products precipitated. The addition of 40 mL of nheptane led to complete precipitation, and a supernatant solvent was decanted. The dark-violet residue was washed three times with 20 mL of n-pentane and dried under reduced pressure. Yield: 325 mg, 0.82 mmol, 82%. Elem anal. Found (calcd for C21H21N2NiCl, M = 395.56 g mol−1): C, 63.59 (63.77); H, 5.28 (5.35); N, 7.02 (7.08). EI-MS: m/z 394 ([M]+). Cis/trans (NMR) = 1.25/1. 1H NMR (300 MHz, CD2Cl2, cis): δ 8.33 (s, 1H, CH−), 7.91 (t, 1H, H5py), 7.59 (s, 1H, H6py), 7.43−7.32 (m, 4H, H3py, H2,4,6anil), 7.30 (t, 1H, H4py), 7.04 (t, 2H, 3,5anil), 6.45 (s, 2H, H3,5Mes), 2.96 (s, 6H, CH32,6Mes), 2.20 (s, 3H, CH34Mes). 1H NMR (300 MHz, CD2Cl2, trans): δ 9.33 (d, 1H, H6py), 8.32 (s, 1H, CH−), 7.99 (t, 1H, H4py), 7.70 (t, 1H, H5py), 7.73−7.30 (m, 3H, H3py, H2,6anil), 7.11 (t, 1H, H4anil), 6.73 (t, 2H, H3,5anil), 6.08 (s, 2H, H3,5Mes), 2.81 (s, 6H, CH32,6Mes), 1.94 (s, 3H, CH34Mes). Synthesis of cis/trans-[(PyMA)Ni(Mes)F]. Method 1: To a solution of 0.12 g (0.27 mmol) of [(PyMA)Ni(Mes)Br] in 20 mL of THF was added 0.1 g (0.44 mmol, 1.6 equiv) of thallium fluoride, and the reaction mixture was stirred for 24 h at 60 °C. After the volume was reduced to 10 mL, a grayish precipitate (TlBr) was filtered off and 30 mL of n-pentane was added to the filtrate. The resulting red precipitate was filtered off, washed with 3 × 20 mL of n-pentane, and dried under reduced pressure. Yield: 96 mg (0.25 mmol, 94%) of red-violet microcrystalline material. Method 2: To a solution of 0.12 g (0.27 mmol) of [(PyMA)Ni(Mes)Br] in 20 mL of THF was added 0.09 g (0.27 mmol) of Ag[SbF6] under the strict exclusion of light, and the reaction mixture was stirred for 30 min. After filtration, the red-violet filtrate was added to a solution of 0.025 g (0.27 mmol) of (Me4N)F in 10 mL of THF. After stirring for 30 min, a grayish solid was filtered off and the filtrate was evaporated to dryness. The crude product was washed with 3 × 20 mL of n-pentane and dried under reduced pressure, yielding 42 mg (0.11 mmol, 41%) of the product. Elem anal. Found (calcd for C21H21N2NiF, M = 379.10 g mol−1): C, 66.59 (66.53); H, 5.61 (5.58); N, 7.42 (7.39). EI-MS: m/z 378 ([M]+). Cis/trans (NMR) = 1/1. 1H NMR (300 MHz, CD2Cl2, cis): δ 8.52 (s, 1H, CH−), 7.99 (s, 1H, H6py), 7.82 (t, 1H, H5py), 7.47− 7.32 (m, 4H, H3py, H2,4,6anil), 7.32 (t, 1H, H4py), 7.11 (t, 2H, H3,5anil), 6.49 (s, 2H, H3,5Mes), 2.96 (s, 6H, CH32,6Mes), 2.21 (s, 3H, CH34Mes). 19F NMR (282 MHz, CD2Cl2, cis): δ −251.6 (s, 1F). 1 H NMR (300 MHz, CD2Cl2, trans): δ 9.33 (d, 1H, H6py), 8.46 (s, 1H, CH−), 7.99 (t, 1H, H4py), 7.82−7.32 (m, 4H, H5py, H3py,

electronic properties (Chart 3) and studied the structures of the unsymmetric [(R-PyMA)Ni(Mes)X] complexes in the solution state by multiple NMR spectroscopy, in the solid state from single-crystal X-ray diffraction (XRD), and in the gas phase through quantum-chemical calculations. Detailed photophysical and electrochemical investigations, including UV−vis and spectroelectrochemistry (SEC), were carried out to probe for the influence of the ligand substitution pattern and the role of the coligand X on both the reactivity and electronic states. Finally, we have made preliminary C−C cross-coupling reactions under Negishi conditions and will briefly report on this.

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EXPERIMENTAL SECTION

Methods and Instrumentation. See the Supporting Information (SI). Syntheses. Synthesis of the N-aryl-1-(pyridin-2-yl)methanimine ligands (R-PyMA) is described in detail in the SI. For all of these ligands, related preparation methods and analytical data have been reported elsewhere.40−47,65−68 Synthesis of trans-[(PPh3)2Ni(Mes)Cl]. Portions of 10.49 g (40 mmol) of PPh3 and 2.6 g (20 mmol) of NiCl2 were suspended in 100 mL of tetrahydrofuran (THF), stirred for about 1 h, and then cooled to 0 °C. To this dark-green mixture was added a THF solution containing about 40 mmol of MesMgCl [freshly prepared from 1.5 g (62 mmol) of Mg and 6.9 g (44.5 mmol) of MesCl], and the mixture was stirred for 2.5 h, during which the color turned from green to reddish-brown. This mixture was poured into 600 mL of ethanol, which gave a dark suspension. Storing this suspension at −18 °C overnight lead to precipitation of a yellow solid. The supernatant solution was removed and the yellow solid washed with 3 × 20 mL of diethyl ether and 3 × 20 mL of n-pentane. Drying the yellow material gave 10 g (13.6 mmol). Yield: 68%. Elem anal. Found (calcd for C45H41ClNiP2, M = 737.90 g mol−1): C, 73.26 (73.25); H, 5.71 (5.69). 1 H NMR (300 MHz, C6D6): δ 7.96 (m, 6H, PPh3), 7.31 (m, 12H, PPh3), 7.11 (m, 12H, PPh3), 6.05 (s, 2H, H3,5Mes), 2.83 (s, 6H, CH32,6Mes), 2.15 (s, 3H, CH34Mes). 31P NMR (121 MHz, C6D6): δ 24.8. EI-MS: m/z 736 ([M]+). Syntheses of Complexes [(R-PyMA)Ni(Mes)Br]: General Description. An amount of 780 mg (1 mmol) of trans-[(Ph3P)2Ni(Mes)Br] was suspended in 40 mL of toluene (for 2,4,6-Me3-PyMA and 3,5Me2-PyMA, THF was used). Then 2 mmol (2 equiv) of the corresponding R-PyMA ligand was added, and the yellow reaction mixtures were stirred at ambient temperatures until they turned completely dark violet, which took between 30 min (PyMA) and 24 h (for 2-CF3-6-F-PyMA). The precipitation of the products was completed by adding 40 mL of n-heptane. Filtration, recrystallization from CH2Cl2/n-heptane (1/1), washing with n-pentane, and drying C


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Inorganic Chemistry

Figure 1. (Left) Crystal structure of trans-[(PyMA)Ni(Mes)Br] (viewed along the crystallographic b axis). (Right) Molecular structure (thermal ellipsoids at the 50% probability level and H atoms omitted for clarity). H2,6anil), 7.00 (t, 1H, H4anil), 6.65 (t, 2H, H3,5anil), 6.19 (s, 2H, H3,5Mes), 2.87 (s, 6H, CH32,6Mes), 1.90 (s, 3H, CH34Mes). 19F NMR (282 MHz, CD2Cl2, trans): δ −275.2 (s, 1F). Safety Note! Thallium(I) f luoride is highly toxic and should be handled with great care. The same is true for the thallium-containing precipitates. Synthesis of cis/trans-[(PyMA)Ni(Mes)I]. To a solution of 0.15 g (0.38 mmol) of [(PyMA)Ni(Mes)Cl] in 20 mL of acetone was added 0.24 g (1.6 mmol, 4 equiv) of NaI, and the reaction mixture was heated to reflux for 20 h. The solvent was evaporated and the resulting solid extracted using 5 × 10 mL of CH2Cl2. Yield: 112 mg (0.23 mmol, 60%) of a dark-violet microcrystalline material. Elem anal. Found (calcd for C21H21N2NiI, M = 487.01 g mol−1): C, 51.59 (51.79); H, 4.31 (4.35); N, 5.72 (5.75). EI-MS: m/z 486 ([M]+). Cis/trans (NMR) = 1.5/1. 1H NMR (300 MHz, acetone-d6, cis): δ 8.22 (s, 1H, CH−), 7.83 (t, 1H, H5py), 7.49 (s, 1H, H6py), 7.40−7.22 (m, 4H, H3py, H2,4,6anil), 7.19 (t, 1H, H4py), 6.92 (t, 2H, H3,5anil), 6.34 (s, 2H, H3,5Mes), 2.95 (s, 6H, CH32,6Mes), 2.20 (s, 3H, CH34Mes). 1H NMR (300 MHz, acetone-d6, trans): δ 9.27 (d, 1H, H6py), 8.21 (s, 1H, CH−), 7.81 (t, 1H, H4py), 7.60 (t, 1H, H5py), 7.63−7.24 (m, 3H, H3py, H2,6anil), 7.03 (t, 1H, H4anil), 6.61 (t, 2H, H3,5anil), 6.04 (s, 2H, H3,5Mes), 2.80 (s, 6H, CH32,6Mes), 1.95 (s, 3H, CH34Mes).

deshielded by the close-by halide coligand. It is interesting to note that this effect shows almost the same magnitude for all halide coligands. We found mixtures of cis and trans isomers in solution in an almost 1/1 ratio for all complexes having no substituent at the sterically crucial 2 position of the N-aryl group (Chart 1). The ratio was determined from the integrals of the o-CH3 groups of the Mes coligand. It is a bit surprising that even only one substituent in this position, as for the 2-CF3-PyMA and 2-NO24-Me-PyMA ligands, leads exclusively to the sterically less hindered cis isomer, although a simple rotation along the N− Cα axis would remove the steric bulk. Upon cooling from 293 to 213 K, the cis-to-trans ratio for [(PyMA)Ni(Mes)Br] changed from 1.1/1 to 1.5/1, preferring the sterically less hindered cis isomer (Figure S1 in the SI) Analogous to our observation, in the corresponding palladium complexes [(R-PyMA)Pd(Me)Cl] with no 2substituents on the N-aryl moiety, mixtures of cis and trans isomers were observed in solution.67 Introduction of a 2-Me substituent on the N-aryl shifted the stereochemistry completely to the isomer in which the Me coligands lies trans to the pyridine group. For the cationic derivatives [(RPyMA)Pd(Me)(MeCN)]+, the introduction of a 2-Me substituent on the N-aryl group only slightly changed the cis/ trans ratio, but a 2′-Me substituent on the pyridine shifts the structure completely to the conformer with Me trans to Py. In the complexes [(R-PyMA)Pd(Me)L]0/+ (L = Cl or MeCN) with the bulky R = pyrene substituent, only the isomers with the Me coligands trans to pyridine were observed.68 The same is true for [(2,6-iPr2-PyMa)Pd(Me)Cl].69 From electronic considerations, one might expect a superior match of the strongly σ-donating Me coligand trans to the presumably better π-acceptor, the N-arylmethanimine group over the other isomer; however, these examples show that even with small coligands the steric bulk on the N-aryl and pyridine groups drive the formation of stereoisomers rather than the electronic trans influence, in line with our observations. Very recently, for related Pd(Me)Cl complexes with the unsymmetrical N^N′ ligand 3-methoxy-N-1-(pyridin-2-yl)ethylidenepropan-1-amine and related ligands in solution, also cis and trans mixtures were observed by NMR.70 DFT calculations revealed a very small energy difference of 0.731 kJ mol−1 in favor of the isomer with

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RESULTS AND DISCUSSION Preparation and Analyses. The complexes [(R-PyMA)Ni(Mes)X] (X = Cl or Br) were prepared from trans[(PPh3)2Ni(Mes)X] and the R-PyMA ligands in a way similar to that previously described for the bpy derivatives.28−31 The complexes [(R-PyMA)Ni(Mes)X] (X = F or I) were obtained from [(R-PyMA)Ni(Mes)Br] by a salt metathesis reaction using TlF or NaI, respectively. All complexes were obtained as an air-stable violet microcrystalline material in good yield, and the stabilities of the [(R-PyMA)Ni(Mes)X] complexes in solution very much resemble those of the bpy and related derivatives studied previously (for details, see the SI).31−34 NMR Spectroscopy: Molecular Structures in Solution. The occurrence of mixtures of cis and trans isomers for the complexes [(R-PyMA)Ni(Mes)X] was unequivocally concluded from 1H NMR spectroscopy (including NOESY and H−H COSY). The cis isomer is defined by the neighborhood (cis) of the Mes coligands to the pyridyl group (see Chart 2), leading to a marked low-field shift of its H6 proton through shielding by the π system of the aromatic Mes coligand. For the trans isomer, the H6 proton of the pyridyl group is markedly D


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Inorganic Chemistry

Figure 2. (Left) Crystal structure of cis-[(2-NO2-4-Me-PyMA)Ni(Mes)Br]·CH2Cl2 (viewed along the crystallographic a axis). (Right) Molecular structure (thermal ellipsoids at the 50% probability level and H atoms omitted for clarity).

Ni−ligand distances were in the range of related complexes.5,6,26−36 Remarkably, for the cis isomers, the Ni−NPy distance is markedly shorter (∼1.9 Å) than the Ni−Nmethanimine distance (∼2 Å), and for the trans isomers, this is more or less exactly reversed, reflecting the trans influence of the Br (weak) versus Mes (strong) coligands but revealing no preference for the cis and trans configurations. The Mes groups are tilted to the binding plane. While for the trans-configured complexes the N−Ni−C−C dihedral angle varies from 70 to 84°, the Mes group of the cis derivatives always lies almost perpendicular to the binding plane (angles from 80 to 90°). Similar values have been found for other NiMes complexes.30−32,36 The tilt angle between the N-aryl cores and the binding plane ranges from approximately 50 to 90° with large angles up to 90° for the cis derivatives with the Br coligands but only 48° for cis-[(PyMA)Ni(Mes)Cl], close to the value found for the uncoordinated ligand 4-NO2-PyMA. A dihedral angle of about 50° has been also observed in the bpip (Chart 1) ligands, and this angle seems to represent an electronically favored orientation.36,74,75 Steric bulk at the Naryl core and the C1 position shift this angle to higher values, presumably reducing thus the electronic influence of substituents on the N-aryl group to the metal-coordinating plane. Quantum-Chemical Calculation on the Molecular Structures and Isomerization. DFT calculations were initiated to rationalize the occurrence of cis and trans isomers for the [(R-PyMA)Ni(Mes)Br] complexes observed in solution by NMR, which showed approximately a 1/1 cis/trans mixture for R-PyMA complexes having no ortho substituent on the Naryl group and exclusive cis geometry for all complexes with at least one substituent in the sensitive ortho position. The molecular structures of the complexes were calculated in the gas phase on the (RI-)BP86/def-SV(P) level and refined on the (RI-)BP86/def2-TZVP level (for details, see the SI). Frequency calculations were carried out in order to check for the absence of imaginary frequencies, which confirms an energetic minimum for the geometry. The calculated energies are listed in Table S1 (in the SI) and show upon first view a generally higher stability of the cis configuration; only trans-[(3,5(OMe)2-PyMA)Ni(Mes)Br] is more stable than its cis isomer, in line with the electron-donating substituents in this complex. Within the group of complexes having no ortho substituent on the N-aryl unit, the differences between the cis and trans isomers are quite small, in line with their observation in a fluid solution in an almost 1:1 ratio (see NMR). The introduction of one ortho substituent and even more of two such groups leads

the Me coligand trans to the pyridine unit, in line with the molecular structure in the solid state. Crystal and Molecular Structures in the Solid. From the ligand 4-NO2-PyMA and the complexes cis-[(PyMA)Ni(Mes)Cl], trans-[(PyMA)Ni(Mes)Br], trans-[(4-NO2-PyMA)Ni(Mes)Br], trans-[(3,5-Me2-PyMa)Ni(Mes)Br], cis-[(2-CF3PyMA)Ni(Mes)Br], cis-[(2-NO2-4-Me-PyMA)Ni(Mes)Br]· CH2Cl2, cis-[(Mes-PyMA)Ni(Mes)Br], and cis-[(2,6-iPr2PyMA)Ni(Mes)Br] single crystals were obtained. The crystal and molecular structures were solved and refined from XRD data with the results visualized in Figures 1 and 2 and summarized in Tables S1 and S2 in the SI (full data and further figures also in the SI). Although all complexes were crystallized from a concentrated solution in CH2Cl2, only cis-[(2-NO2-4-Me-PyMA)Ni(Mes)Br]·CH2Cl2 contains a cocrystallized solvent. In the complexes, also no intermolecular interactions were found, with the exception of the complex [(PyMA)Ni(Mes)Br], in which intermolecular π stacking of the pyridine rings occurs. With a centroid distance of 3.46 Å and an interplanar angle of 23°, they are classified as parallel offset stacking.71,72 From the four complexes showing a mixture of cis and trans isomers in solution as inferred from NMR spectroscopy (vide supra), three crystallized in the trans form. A further five complexes crystallized in the cis form, although for one of them, cis-[(PyMA)Ni(Mes)Cl], both isomers were observed in solution. Assuming that in each case the superior crystallization properties of the observed structure guided the crystallization process, there seems to be no control concerning the stereoisomers. The powder XRD of a microcrystalline sample of [(PyMA)Ni(Mes)Br] (Figure S9) is in good agreement with the calculated 2θ plot of the crystallized trans-[(PyMA)Ni(Mes)Br] showing that the cis derivative represents at least no crystalline phase impurity. We therefore conclude that the complete phase of this material represents the trans isomer (and not only some crystals), although in solution (NMR) we observe cis and trans isomers. The same has been concluded for comparable palladium complexes [(PyMHA)Pd(Me)Cl] [PyMHA = N(pyridin-2-ylmethyl)amines].73 All complexes show an almost perfect planar coordination of the Ni atom with a sum of angles very close to 360°. Because of the N−Ni−N bite angles with a quite invariable value of about 82° and quite regular X−Ni−C angles of 88−90°, the N−Ni−X and N−Ni−C angles were widened to about 95°, which represents not exactly square-planar coordination. Overall, the E


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Inorganic Chemistry Table 1. Selected Electrochemical Data for Complexes [(R-PyMA)Ni(Mes)Br]a compound [(iPr-DAB)Ni(Mes)Br]b [(bpy)Ni(Mes)Br]b [(bpy)Ni(Mes)2]b [(R-PyMA)Ni(Mes)Br] PyMA 4-NO2 2-NO2-4-Me 2-CF3-6-F 2-CF3 2,6-Me2 2,6-iPr2 3,5-Me2 3,5-(OMe)2 Mes

EpaOx1 irr.

E1/2Red1 rev. (Ipa/Ipc)

EpcRed2 irr.

EpcRed3 irr.

0.30 0.21 −0.14

−1.55 (0.28) −1.92 (0.10) −2.19 (1.00)

−2.25 −2.08c −2.97c

−2.91

−1.70 −1.51 −1.70 −1.62 −1.66 −1.63 −1.69 −1.73 −1.76 −1.79

−2.46 −2.00 −2.70 −2.24 −2.20 −2.82 −2.14 −2.50 −2.51 −2.60

0.10 0.11 0.11 0.25 0.24 0.29 0.32 0.10 0.14 0.31

(0.09) (0.14) (0.10) (0.07) (0.40) (1.00) (1.00) (0.99) (0.25) (0.30)

−2.25 −2.91 −2.54 −2.47

From CV in 0.1 M nBu4NPF6/THF solutions at a 100 mV s−1 scan rate. Potentials/V versus ferrocene/ferrocenium. Half-wave potentials E1/2 for reversible or semireversible processes (rev.) with the peak current ratio (Ipa/Ipc) in parentheses. Anodic (Epa) or cathodic (Epc) peak potentials for irr. = irreversible processes. bFrom ref 16 and 35. cReversible waves. a

Figure 3. Cyclic voltammogramms of [(PyMA)Ni(Mes)Br] (left) and [(2,6-iPr2-PyMA)Ni(Mes)Br] (right) in a 0.1 M nBu4NPF6/THF solution at 298 K and 100 mV s−1 scan rate. Potentials/V versus ferrocene/ferrocenium.

coordination site at Ni with the o-CH3 group.39 We conclude that the trigonal-planar transition state of [(PyMA)Ni(Ph)Br] is largely governed by the ionic interaction, while in the absence of an “active”, coordinating anion such as Br−, agostic interactions can dominate the structure, in line with similar findings.76−78 This nicely underlines the necessity of noncoordinating anions such as [B(Ar F ) 4 ] − = [B(3,5(CF3)2C6H3)4]− for effective olefin polymerizations using such cationic catalysts.1,76 Electrochemistry. In the cyclic voltammograms, the complexes [(R-PyMA)Ni(Mes)Br] show one or two irreversible oxidation waves: the first lies at around 0.2 V and the other at around 0.8 V. For related arylnickel complexes, the first oxidation has been ascribed to a NiII/NiIII couple. However, this assignment was based on a comparison (see, e.g., Table 1) and not on direct experimental proof.20,40 This oxidation process and also the second wave might as well represent oxidation of the bromido ligand; thus, a Br−/1/2Br2 redox couple. Several reduction waves were observed at potentials lower than about −1.5 V. Through the range of PyMA complexes, the first wave exhibits various degrees of reversibility, as can be seen when the plots in Figure 3 and the data in Table 1 are compared. When the experiments are run exactly under the same conditions (298 K, 100 mV s−1 scan rate, concentration ∼10−4 mmol L−1), the reversibility, as derived from the peak current ratio Ipa/Ipc, ranges from

to a marked stabilization of the cis isomer, in line with the observation of only this isomer in solution. Clearly, the steric hindrance has the biggest influence on the stereochemistry, while electronic effects seem to be much weaker. The dominating influence of the steric bulk of ortho substituents on the N-aryl has also been recognized in DFT (ADF) calculations compared to the so-called Brookhart catalyst [(2,6R-DAB)Ni(Me)]+ (DAB = 1,4-diazabutadiene; see Chart 1; R = Me or H) with the R-PyMA derivative [(2,6R-PyMA)Ni(Me)]+,39 completely in line with our results. From the NMR experiments, it was clear that dynamic isomerization occurs between the cis and trans forms, and we thus calculated the energies and species during isomerization for the model complex [(PyMA)Ni(Ph)Br] and found that a trigonal (PyMA)Ni-Ph arrangement with a partially dissociated Br− coligand (quasi ion pair) is the species at lowest energy (Figure S10), almost equal to a sawhorse-like structure with a Ph−Ni−Br angle of 180° perpendicular to the PyMA Ni binding plane. Tetrahedral transition states were far higher in energy. The ion-pair-like structure is in line with our observation that polar solvents easily can dissociate from the Br− ligand. Interestingly, the calculated structure of the [(2,6Me2-PyMa)Ni(Me)]+ intermediate (active in olefin polymerization) shows a T-shaped structure, with the Me coligands trans to the N-aryl group (thus a cis isomer in our nomenclature) and an agostic interaction of the open F


Article

Inorganic Chemistry completely reversible (1.0) to almost irreversible (0.07) behavior. As has been shown previously16,35 for the related derivatives with symmetric unsaturated N,N ligands [(bpy)Ni(Mes)Br] and [(iPr-DAB)Ni(Mes)Br] (iPr-DAB = N,N′diisopropyldiaza-1,3-butadiene), cleavage of the bromido ligand after one-electron reduction leads to this reduced reversibility and the rate of cleavage, and therefore the degree of reversibility depends on the type of N,N ligand and can be enhanced at low temperatures and upon the addition of excess bromide.16,35 Replacing the Br− coligand by the nonleaving Mes group leads to completely reversible reduction in [(bpy)Ni(Mes)2] (Table 1). Table 1 shows that the bpy complex [(bpy)Ni(Mes)Br] shows a rather irreversible reduction wave under the chosen conditions (Ipa/Ipc = 0.1), while the wave for the iPr-DAB complex is partially reversible (Ipa/Ipc = 0.28). The R-PyMA complexes range from almost completely irreversible (0.07) to completely reversible (1.00); the latter were the derivatives with 2-Me- or 2-iPr substituents on the N-aryl group and surprisingly also the 3,5-Me2 derivative. The potentials of the first reduction reflect only a very weak electronic influence of the substituents, in line with the results from XRD showing the N-aryl group strongly tilted from the coordination plane and thus hampering the electronic influence. The rather high (less negative) first reduction of the 4-NO2-substituted complex is due to the general electron-withdrawing effect on the N-aryl group, and the marked lower potential of the 2-NO2-4-Me derivative is in line with this. At the same time, the processes might represent the direct reduction of the nitro group, as has been observed in similar cases with ligands such as NO2bpy79,80 or CCPh-NO2.81−83 When the first reduction potentials of the [(N^N)Ni(Mes)Br] complexes are compared (Table 1), the R-PyMA complexes lie between the bpy and iPr-DAB derivatives. Assuming a LUMO of π* character as the target for this reduction, the potentials are in line with a mixed character of this LUMO with contributions from both the pyridine and the N-arylimine functions (for a blend of bpy and DAB, see Chart 1). At potentials below −2 V, further big irreversible reduction waves were observed in addition to smaller reversible waves (compare the plots in Figure 3). UV−Vis Absorption Spectroscopy. The new complexes all exhibit two long-wavelength absorptions of moderate intensity (2500−4700 M−1 cm−1) covering big parts of the visible range (data in Table S4 in the SI). These bands are strongly negative solvatochromic, as Figure 4 illustrates, and can thus be assigned to metal-to-ligand charge-transfer (MLCT) transitions, in line with the assignments of similar complexes.30−36 The intense absorptions in the UV range are due to intraligand π−π* transitions and were also observed in the uncoordinated ligands. A comparison with [(bpy)Ni(Mes)Br] (λmax in CH2Cl2 = 483 nm)32,33 and [(iPr-DAB)Ni(Mes)Br] (λmax in CH2Cl2 = 569 nm)38 puts them aside the diazabutadiene complex, and we can thus conclude that the LUMO is more centered on the N-arylimine moiety rather than on the pyridine. Support for this comes from the marked influence of the R substituent because alkyl substituents shift the bands to higher energy (Table S4). The assignment is also in line with the very long-wavelength MLCT absorptions for the binuclear bpip complexes having the excellent π-accepting 1,4-pyrazine unit as the charge acceptor in the MLCT.36

Figure 4. UV−vis absorption spectrum of [(2,6-iPr2-PyMA)Ni(Mes)Br] in CH2Cl2. Inset: Solvatochromic effects on the long-wavelength bands.

The absorption maxima of the complexes [(PyMA)Ni(Mes)X] do not vary upon alteration of the X coligands. The longwavelength bands undergo a slight red shift in the series from F (536 nm) to Cl (553 nm) to Br (570 nm) to I (578 nm) (Figure S11 and Table S5). This shift is in line to what has been observed for [(bpy)Ni(Mes)X] derivatives32 and supports the MLCT assignment for this band. Electron Paramagnetic Resonance (EPR) and UV−Vis SEC. Solutions of the complex [(PyMA)Ni(Mes)Br] were electrolyzed at potentials above −2 V versus ferrocene/ ferrocenium in THF/nBu4NPF6 solutions. In the course of the reductions at 298 K, the red-violet color of the solution bleached slowly and a broad unresolved signal appeared at g = 2.0012 (Figure 5). In the course of the next few minutes, this

Figure 5. X-band EPR spectra of [(PyMA)Ni(Mes)Br] after electrochemical reduction in THF/nBu4NPF6 at 258 K. An initial spectrum appeared at −2 V and a second spectrum after 30 min. A third spectrum appeared after prolonged electrolysis at about −3 V.

spectrum was progressively replaced by a sharper, more intense signal at g = 2.0043 (Figure 5, center). When the potential was lowered further to about −3 V, the signal at g = 2.0043 was gradually replaced by a slightly broader and partially structured signal at g = 2.0031 (Figure 5, top). Reduction using cobaltocene at 258 K, immediate freezing of the solutions in liquid N2, and measurement at 110 K allowed one to observe a broad EPR signal (Figure 6). Simulation gave g1 = 2.007, g2 = 2.003, g3 = 1.993, an averaged g value (gav) of 2.001, and a g anisotropy (Δg = g1 − g3) of 0.014. The careful reduction procedure was meant to obtain the undissociated halido complex radical [(PyMA)Ni(Mes)Br]•− G


Article

Inorganic Chemistry

signal recorded for the PyMA complex has a g value of 2.003 and a total width of about 15 G. We assign this to the superior ability of PyMA to localize the unpaired electron in the ligand, which is in line with the superior π-accepting ability of PyMA compared with that of bpy. The appearance of the dimeric species, however, points to a behavior different from that of the excellent π-accepting ligands bpz or DAB; thus, PyMA lies between these two groups, in line with the electrochemical data and in excellent agreement with its structure representing a blend of DAB and bpy. Reductive UV−vis absorption SEC of selected samples (Figure S12 and Table S6 in the SI) reveals that for complexes with PyMA and 3,5-(OMe)2-PyMA the first reduction occurs irreversibly on the slower time scale (minutes) of the SEC experiment even if under the cyclic voltammetry (CV) conditions (time scale of seconds) the processes are partially reversible. For the 2NO2-4Me-PyMA complex, the first reduction occurs reversibly (Figure S13), and the observed long-wavelength band at about 715 nm (14000 cm−1) is quite similar to that observed for the reduced platinum complex [Pt(4-NO2-bpy)Cl2]•−. Thus, we assume that the first reduction of [(2-NO2-4-Me-PyMA)Ni(Mes)Br] is largely centered on the nitro group of the N-aryl moiety, in line with the electrochemical data. Catalytic Negishi Cross-Coupling Reactions. With the new complexes in hand, we wanted to demonstrate the proofin-principle that such unsymmetric N,N′ ligand frameworks can support catalytic reactions of current interest at nickel. Nickel has been especially useful in the cross-coupling of aryl halides with alkylzinc nucleophiles to generate new C(sp2)−C(sp3) bonds.8,9,11 Basically, all complexes were able to catalytically mediate the Negishi reactions of iodotoluene with either pentylzinc bromide or 2-(1,3-dioxolan-2-yl)ethylzinc bromide (Table S7) under mild room temperature conditions. Moderate-to-high (20−65%) yields were observed for the cross-coupling products, with [(PyMA)Ni(Mes)Br] being the highest yields in the series. A comparison with [(bpy)Ni(Mes)Br] reveals that the latter provides better yields for both reactions. The nickel complexes bearing unsymmetric PyMA ligands produced varying amounts of homocoupled products, sometimes even with yields that were competitive with the desired cross-couplings.

Figure 6. X-band EPR spectrum of [(PyMA)Ni(Mes)Br] after reduction using cobaltocene in THF at 258 K and measured at 110 K in a glassy frozen matrix with a simulation below. The following parameters were used for the simulation: g1 = 2.007, g2 = 2.003, and g3 = 1.993 and Gaussian line broadenings of 20, 15, and 9 G, respectively. gav = 2.001, and Δg = 0.014.

and the gav value of 2.001 is almost identical with that seen for the first species observed during electrolysis (Figure 5, bottom). We are thus quite confident that these two spectra represent [(PyMA)Ni(Mes)Br]•−. In the next few minutes of the electrolysis at ambient temperature, the signal sharpens and shifts to lower field, which is probably due to the loss of the bromido coligand and formation of [(PyMA)Ni(Mes)(Solv)]• (eq 2). This is in line with the EPR spectra of cathodically electrolyzed solutions of the complexes [(bpy)Ni(Mes)2], [(bpy)Ni(Mes)Br], and [(bpym)Ni(Mes)Br] (bpym = 2,2′bipyrimidine). They showed narrow lines with hyperfine splitting (HFS; coupling to N and H atoms of the heteroaromatic ligand) at ambient and low temperature.16,34,35 For the radical [(PyMA)Ni(Mes)Br]•−, the bromido ligand (79/81Br, I = 3/2, and Q = 30.5 and 25.4 fm2 respectively) should couple with the unpaired electron and lead to HFS or, if not resolved, broaden the lines in the EPR spectra. It was thus concluded that the EPR spectra of electrolyzed solutions of the parent bromido complexes of bpy and bpym represent the solvated radical complexes [(N^N)Ni(Mes)(Solv)]• after cleavage of the bromide, in line with the observation for the anion radical complex [(bpy)Ni(Mes)2]•−.16,34,35 Therefore, the second species observed during electrolysis of [(PyMA)Ni(Mes)Br] represents the radical [(PyMA)Ni(Mes)(Solv)]•. The missing HFS is probably due to the unsymmetric nature of the PyMA ligand in contrast to the symmetric bpy or bpym. The second signal does not increase even after prolonged electrolysis, which is probably due to dimerization of the lower coordinate species [(PyMA)Ni(Mes)]•, which is in equilibrium with [(PyMA)Ni(Mes)(Solv)]• (eq 3) to the diamagnetic dimer [(PyMa)(Mes)Ni−Ni(Mes)(PyMa)] (eq 4). In previous work, we concluded from EPR and UV−vis−NIR SEC that for bpy or phen this process is favored and prolonged electrolysis at very negative potentials allowed one to observe the reduced dimer [(bpy)(Mes)Ni−Ni(Mes)(bpy)]•−. In contrast to this, for superior π-accepting ligands such as bpz or DAB, the monomeric species are more stable and no dimer was observed.16,35 PyMA reduction at very negative potentials allowed one to observe a third signal, which might thus be due to the reduced dimer [(PyMa)(Mes)Ni−Ni(Mes)(PyMa)]•−. On the other hand, the signals observed for the bpy and PyMA derivatives are quite different. The species observed for bpy has a g value of 2.140 and a line width of over 150 G, while the

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CONCLUSIONS A series of organonickel complexes [(R-PyMA)Ni(Mes)X] (Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I) with the unsymmetric unsaturated N,N′ ligands R-PyMA [N-(Rphenyl)-1-(pyridin-2-yl)methanimine); R = H, 2,6-Me2, 3,5Me2, 2,4,6-Me3, 2,6-iPr2, 3,5-(OMe)2, 2-NO2-4-Me, 4-NO2, 2CF3, and 2-CF3-6-F] were synthesized and studied. The complexes were obtained as cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from precursors trans-[(PPh3)2Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although DFT calculations show a preference for the sterically favored cis isomers, both isomers could be observed in solution for the bromido complexes [(R-PyMA)Ni(Mes)Br] with R-PyMA ligands without an ortho substituent on the Naryl moiety in almost equal amounts. In three cases, singlecrystal XRD reveals superior crystallization of the trans isomers H


Inorganic Chemistry

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in the solid. For all derivatives carrying at least one ortho substituent exclusively, the cis isomers were observed in solution (by NMR) and in the solid (XRD). For [(PyMA)Ni(Mes)Cl], both cis and trans were observed in solution, but this time the cis isomer crystallized, showing again that beyond molecular preferences in solution the superior ability to crystallize is a determining factor. Possible mechanisms for the isomerization were investigated by DFT calculations using a [(PyMA)Ni(Ph)Br] model complex showing a strong ionic interaction between the cationic trigonal-planar [(PyMA)NiPh]+ fragment and the partially dissociated Br− coligand. The small overall energy difference between the cis and trans isomers for the sterically less hindered derivatives (without ortho substituents) also means that the physical or chemical properties were not too different. While NMR spectroscopy is able to detect the differences between the two isomers, other methods such as UV−vis absorption or CV fail to do so. For potential applications in catalysis, which were initiated by cleavage of the bromido ligand, it is very probably not important that an isomeric mixture is found for the precatalyst. The first electrochemical reduction of the [(R-PyMA)Ni(Mes)Br] complexes occurs at about −1.7 V, and when the first reduction potentials of [(N^N)Ni(Mes)Br] complexes are compared, the R-PyMA complexes lie straight between the bpy (−1.9 V) and iPr-DAB (−1.55) derivatives, in line with their mixed character. These potentials do not vary largely with the R substituent; however, the reversibility ranges from completely irreversible to almost completely reversible on the time scale of the CV experiment (seconds) depending on the R group. The irreversibility is due to rapid cleavage of the Br− coligands after reduction, and on the time scale of the spectroelectrochemical (UV−vis absorption) experiment, the cleavage is complete within minutes; thus, all reduction processes occur irreversible. In contrast to the reduction potentials, the long-wavelength MLCT absorptions vary markedly with the R substituent of the ligand but are almost invariable upon variation of X. Both the electrochemical data and UV−vis spectroscopy is in line with largely nickel-based HOMOs with small contributions from the X coligands and with pyridine- and N-arylimine-based π* LUMOs. A detailed look concerning the LUMOs reveals that the conclusion from electrochemistry was that the unsymmetric R-PyMA ligands with their different NPy and Nmethanimine donor functions lie straight between the symmetric bpy (NPy function) and DAB (1,4-diaza-1,3-butadiene; Nmethanimine function) ligands (from which they were formally blended), while the conclusion from the optical spectroscopy is that they behave rather like DAB ligands. Very probably, several close-lying π* LUMOs are available for electron uptake through electron or charge transfer, and details such as orbital overlap with electron-donating moieties or geometrical relaxation decide on the contributions of these LUMOs, thus making the R-PyMA ligands a very interesting alternative to symmetric ligands such as bpy or DAB. Additionally, the N-aryl core can be easily substituted, allowing electronic and steric fine-tuning. Future detailed quantum-chemical calculations shall give some more insight. The first attempts to use these complexes in Negishi-type cross-coupling reactions were successful, but more work is needed to assess how the unsymmetrical R-PyMA complexes perform in catalysis compared with those of symmetric heteroaromatic and aliphatic α-diimine derivatives.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01874. Experimental information, further figures showing 1H NMR spectra, crystal and molecular structures, powder XRD, results from quantum-chemical calculations, UV− vis absorption SEC, catalytic cross-coupling reactions, and structural data (PDF) Full crystallographic data (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Axel Klein: 0000-0003-0093-9619 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the computing center of the University of Cologne (RRZK) for providing CPU time on the DFG-funded supercomputer CHEOPS, as well as for support. We are also grateful for financial support by Deutsche Forschungsgemeinschaft Projects KL 1194/5-1 and -6-1. N.V. acknowledges support from the University of Cologne, PROMI programme.

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REFERENCES

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