Regioselective N- and C-Metalation of Neutral 2 ... - ACS Publications

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

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Regioselective N- and C‑Metalation of Neutral 2‑Halogenobenzimidazole Derivatives Rajorshi Das,†,‡ Jonas Blumberg,† Constantin G. Daniliuc,§ David Schnieders,§ Johannes Neugebauer,§ Ying-Feng Han,‡ and F. Ekkehardt Hahn*,†,‡

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Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany ‡ Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, College of Chemistry and Material Science, Northwest University, Xi’an 710127, People’s Republic of China § Organisch-Chemisches Institut and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: Neutral 2-chlorobenzimidazole (1) and its derivatives 8-chlorotheophylline (2) and 8-bromotheophylline (3) undergo at low temperature in THF an N−H polar oxidative addition reaction with Ni0 to give exclusively the nickel(II) hydrido complexes trans-[4]−trans-[6] featuring an N-metalated five-membered azolato heterocycle. In spite of the reactive C−halogen bond, no C-metalation was observed in any of these reactions. At 45 °C, however, 2chlorobenzimidazole (1) reacts in toluene/hexane in a polar oxidative addition of the C2−Cl bond to Ni0 to yield a mixture of the complexes trans-[8] and trans-[9] both bearing a Cmetalated benzimidazolato ligand. The related complex trans-[10] bearing a C-metalated azolato ligand is formed in the oxidative addition of 2-chlorocaffeine, featuring one alkylated ring nitrogen atom within the diaminoheterocycle to Ni0.

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INTRODUCTION The regioselective metalation of aromatic N-heterocycles is an important synthetic task owing to the various applications of such compounds in biological and medicinal chemistry.1 This is particularly true for the synthesis of functionalized arenes, which are essential building blocks for pharmaceuticals and agrochemicals.2 Significant efforts have been directed toward the regioselective lithiation of common N-heterocycles such as pyridines, quinolines, and pyrimidines.3 The regioselective C vs N metalation of the neutral benzimidazole scaffold, however, has been less explored. We described a versatile method for the regioselective C2metalation of neutral 2-halogeno-N-alkylazoles4 and of unsubstituted 2-halogenoazoles5 via the oxidative addition of the C−X (X = halogen) bond of the azole heterocycle to complexes of low-valent transition metals at elevated temperatures. The oxidative addition of otherwise unsubstituted 2halogenoazoles yields initially azolato complexes of type I which in the presence of an acid undergo protonation at the nucleophilic N3 ring-nitrogen atom to give complexes of type II, bearing protic NH,NH-NHC ligands (Scheme 1).5 Related C-metalation reactions via the oxidative addition of C2−X bonds (X = H, alkyl, halogen, SR) of neutral azoles or of N,N′disubstituted azolium cations to low-valent transition-metal complexes to give complexes bearing protic NH,NR-NHC6 or classical NR,NR-NHC7 ligands have also been described. © XXXX American Chemical Society

Scheme 1. Oxidative Addition of 2-Halogenobenzimidazoles to Zerovalent Transition Metals

While the oxidative addition of the C2−halogen bond of neutral azoles to zerovalent transition metals at elevated temperatures is nowadays a generally applicable procedure supported by multiple examples,6j the same reaction with Ni0 complexes under identical conditions constitutes a remarkable exception, yielding almost exclusively decomposition products (Scheme 1). We have therefore reinvestigated this reaction and describe here the unprecedented reaction of 2-halogenoazoles with Ni0 at low temperature in THF to yield NiII hydrido complexes via the regioselective oxidative addition of the N−H bond of the azoles. Received: May 27, 2019

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DOI: 10.1021/acs.organomet.9b00357 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION The reaction of 2-chlorobenzimidazole (1) with 1 equiv of [Ni(COD)2] in the presence of 2 equiv of triethylphosphine at 0 °C in THF over 2 h afforded the hydrido complex trans-[4] in 75% yield (Scheme 2, left). The complex trans-[4] was

Figure 1 (for full crystallographic details and a plot of the molecular structure of trans-[6] see the Supporting Informa-

Scheme 2. Regioselective N−H Polar Oxidative Addition of 2-Halogenoazoles to Ni0

Figure 1. Molecular structures of trans-[4] in trans-[4]·C7H5ClN2 (left) and of one of the three essentially identical molecules of trans[5] (right) in the asymmetric unit (50% probability; hydrogen atoms except for H1 have been omitted for clarity, and only the first atom of the P-ethyl substituents is shown). Selected bond lengths (Å) and angles (deg) for trans-[4]: Ni−P1 2.0388(9), Ni−P2 2.0639(9), Ni− N1 1.931(3), Ni−H1 1.36(4), N1−C2 1.337(4), N2−C2 1.311(4); P1−Ni−P2 157.56(5), P1−Ni−N1 100.84(8), P1−Ni−H1 83.2(14), P2−Ni−N1 101.56(7), P2−Ni−H1 74.5(14), N1−Ni−H1 175.6(14), N1−C2−N2 119.7(3). Selected bond lengths (Å) and angles (deg) for trans-[5] (molecule 1): Ni−P1 2.1612(4), Ni−P2 2.1701(4), Ni−N7 1.9183(11), Ni−H1 1.38(2), N7−C8 1.330(2), N9−C8 1.349(2); P1−Ni−P2 165.414(15), P1−Ni−N7 99.60(3), P1−Ni−H1 79.9(8), P2−Ni−N7 94.98(3), P2−Ni−H1 85.5(8), N7−Ni−H1 177.7(8), N7−C8−N9 118.07(12).

tion). The coordination geometry around the nickel atom in all three complexes is found to be significantly distorted square planar with bond angles N−Ni−H1 measuring 175.6(14)° (trans-[4]) and 177.7(8)° (trans-[5]). The different steric demands of the hydrido ligand and the diaminoheterocycle make the P1−Ni−P2 angles (trans-[4] 157.56(5)°, trans-[5] 165.414(15)°) deviate most significantly from linearity. The N−C−N angles within the five-membered diaminoheterocycle (trans-[4] 119.7(3)°, trans-[5] 118.07(12)°) are significantly larger than those observed in C-metalated azolato complexes.4,5 Treatment of trans-[4] featuring a basic hydrido ligand with another 1 equiv of 2-chlorobenzimidazole in THF at 60 °C yielded, with evolution of molecular hydrogen, complex trans[7] in 65% yield (Scheme 2). Complex trans-[7] is stable in air and was completely characterized by NMR spectroscopy and mass spectrometry (see the Supporting Information). An X-ray diffraction analysis with crystals of trans-[7] reveals the presence of a highly symmetrical complex which in the solid state resides on a crystallographic inversion center (Figure 2). The bond parameters observed for complex trans-[7] are consistent with those reported for related square-planar nickel(II) trans-bis-N-azolato complexes.10 The unique oxidative addition of the N−H bond of halogenoazoles to Ni0 at low temperature in the presence of a C−halogen bond in comparison to the routinely observed oxidative addition of the C−halogen bond to zerovalent transition metals at elevated temperature prompted us to investigate the reaction of 2-chlorbenzimidazole (1) with a Ni0 complex at elevated temperature. We reasoned that a slight increase in the reaction temperature might enable the oxidative addition of the C2−Cl bond to the Ni0 complex instead of formation of the hydrido complexes. Slow addition of 1 equiv of a mixture of [Ni(COD)2]/2PEt3 to a hexane/toluene (3:1) solution of 1 at 45 °C afforded a mixture of the C-azolato complexes trans-[8] (minor) and trans-[9] (major) (Scheme 3), clearly demonstrating the

isolated as a yellow solid which is soluble in THF and benzene. It is sensitive toward air and moisture and must be handled under argon. Similarly, 8-chlorotheophylline and 8-bromotheophylline react with 1 equiv of [Ni(COD)2]/2PEt3 at ambient temperature over 16 h to give the hydrido complexes trans-[5] (82%) and trans-[6] (69%), respectively (Scheme 2, center and right). Unexpectedly, the reactive C−halogen bond in 1−3 did not participate in any of these reactions. Oxidative additions of N−H bonds of aminoheterocycles to low-valent transition metals have been described,8 but never for 2halogenoazole derivatives featuring a reactive C−halogen bond. Formation of the hydrido complexes trans-[4]−trans-[6] was confirmed by NMR spectroscopy, ESI mass spectrometry, and X-ray diffraction studies (see the Supporting Information). The 1H NMR spectra for complexes trans-[4]−trans-[6], for example, show triplet resonances for the hydrido ligands at δ −22.15 ppm (t, 2JHP = 74.8 Hz), δ −22.61 ppm (t, 2JHP = 75.9 Hz), and δ −22.67 ppm (t, 2JHP = 75.6 Hz), respectively. The coupling to two chemically equivalent phosphorus atoms also implies the trans arrangement of the triethylphosphine ligands. In addition, doublets at δ 18.0 ppm (d, 2JPH = 74.8 Hz), δ 17.4 ppm (d, 2JPH = 75.9 Hz), and δ 17.5 ppm (d, 2JPH = 75.6 Hz) were detected in the 31P{1H} NMR spectra. The chemical shifts for the hydride resonances are in good agreement with previously recorded values for the NiII hydride complexes.9 The connectivity and coordination geometry of trans-[4]− trans-[6] were established by X-ray diffraction studies. The molecular structures of trans-[4] and trans-[5] are shown in B


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mass also corresponds to [[4] + H]+) and at m/z 563.1786 (calcd 563.1770 for [[9] + H]+) with the correct isotopic pattern. While the components of the complex mixture trans-[8]/ trans-[9] could not be separated by conventional methods, crystallization attempts (evaporation of the toluene/hexane solvents from a solution of the complex mixture) afforded a few crystals of trans-[9]. An X-ray diffraction analysis of these crystals confirmed the proposed composition of the complex (Figure 3).

Figure 2. Molecular structure of trans-[7] (50% probability; hydrogen atoms have been omitted for clarity, and only the first atom of the Pethyl substituents is shown). Selected bond lengths (Å) and angles (deg): Ni−P 2.2693(4), Ni−N1 1.8908(13), N1−C1 1.348(2), N2− C1 1.314(2); P−Ni−N1 88.47(4), P1−Ni−N1* 91.53(4), N1−C1− N2 118.61(15).

Scheme 3. Oxidative Addition of 2-Chlorobenzimidazole to Ni0 at Elevated Temperature

Figure 3. Molecular structure of trans-[9] (50% probability; hydrogen atoms except for H1 have been omitted for clarity, and only the first atom of the P-ethyl substituents is shown). Selected bond distances (Å) and angles (deg): Ni−P1 2.200(2), Ni−P2 2.216(2), Ni−N3 1.935(5), Ni−C1 1.872(6), N1−C1 1.397(8), N2−C1 1.341(9), N3−C11 1.329(10), N4−C11 1.334(9); P1−Ni−P2 171.86(8), P1− Ni−N3 94.2(2), P1−Ni−C1 86.5(2), P2−Ni−N3 93.0(2), P2−Ni− C1 86.2(2), N3−Ni−C1 179.0(3), N1−C1−N2 108.9(6), N3− C11−N4 119.5(7).

oxidative addition of the C−Cl bond under these conditions. Attempts to perform the reaction at ambient temperature led to a complex mixture containing large amounts of hydrido complex trans-[4] together with trans-[8] and trans-[9], while at higher temperature the yield of complex trans-[9] increased together with that of decomposition products. Due to their very similar solubility properties, complexes trans-[8] and trans-[9] could not be separated and therefore were isolated together as an orange-yellow solid. Separation of the complexes through column chromatography also proved impossible due to decomposition. Both complexes are only sparingly soluble in moderately polar organic solvents, while they show good solubility in highly polar solvents such as DMSO and DMF. The coexistence of complexes trans-[8] and trans-[9] in the product mixture was confirmed by NMR spectroscopy and mass spectrometry (see the Supporting Information), although the 1H and 13C{1H} NMR spectra do not provide fully conclusive information due to overlapping resonances for the rather similar complexes. The 1H NMR spectrum features a broad resonance for the N−H protons of the two complexes at δ ∼12.5 ppm. The 13C{1H} NMR spectrum shows two weak resonances in the typical range for C-metalated benzimidazolato ligands coordinated to nickel(II).4b Assignment of these resonances was achieved by 2D correlation spectroscopy. The resonance at δ 168.9 ppm (t, 2JCP = 45.6 Hz in DMSO-d6) was assigned to the Cbenzimidazolato atom of complex trans-[8] with the appearance of a triplet confirming the trans arrangement of the phosphine donors. The Cbenzimidazolato resonance for trans[9] was observed at δ 167.6 ppm (t, 2JCP= 42.6 Hz in DMSOd6). In addition, resonances at δ 17.6 (trans-[8]) and δ 14.1 ppm (trans-[9]) were detected in the 31P{1H} NMR spectrum. ESI mass spectroscopy (positive ion mode) showed peaks at m/z 447.1404 (calcd 447.1396 for [[8] + H]+; note that this

Complex trans-[9] adopts a slightly distorted square planar geometry. The nickel atom is coordinated by a C-metalated benzimidazolato ligand and a deprotonated N-metalated benzimidazolato ligand. Metric parameters found for the Cmetalated ligand closely resemble those found for the nickel NHC complex obtained by oxidative addition/N-protonation of 2-chloro-N-methylbenzimidazole. For example, the Ni−C1 (1.872(6) Å), Ni−P1 (2.200(2) Å), and Ni−P2 (2.216(2) Å) separations compare well with the equivalent parameters found in the diphosphine coordinated NiII complex bearing a protic NH,NMe-benzimidazolin-2-ylidene ligand.4b The bond angle N3−Ni−C1 is almost linear (179.0(3)°), while the P1−Ni−P2 angle measures 171.86(8)°, indicative of the difference in the steric demand of the two heterocyclic ligands. Also noticeable is the difference in the angles N1−C1−N2 (108.9(6)°) and N3−C11−N4 (119.5(7)°), reflecting the different metalation status of the diaminoheterocycles. We assume that trans-[8] and trans-[9] are formed by an initial oxidative addition of the C2−Cl bond of 1 to Ni0 to give trans-[8] followed by reaction with another molecule of 1 to yield complex trans-[9] after HCl elimination. In order to shed light on the reasons for the two different, temperature-dependent oxidative additions (N−H vs C−Cl) in the reaction of 1 with Ni0 complexes, a series of DFT calculations (PBE0-D3/def2-TZVP) was performed. These comprise an evaluation of the possible reaction mechanism for the formation of the N-azolato complexes. The reactions of 8chlorotheophylline (2) with [Ni(COD) 2 ]/PMe 3 , [Pd(COD)2]/PMe3, and [Pd(PPh3)4] have been studied next regarding the formation of N- or C-metalated reaction products. In addition, the reaction of 2-chlorobenzimidazole (1) with [Ni(COD)2]/PMe3 was also investigated (see the Supporting Information for full computational details). C


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Organometallics The reaction of 2-chlorotheophylline 2 and an excess of PMe3 with [Ni(COD)2] is discussed as a first example (Figure 4).

Figure 5. Reaction profiles toward the N-azolato and C-azolato reaction products, respectively, in the reaction of [Pd(COD)2] with 8chlorotheophylline (2) in the presence of an excess of PMe3. Depicted are the reaction complexes (RC), the transition states (TS,) and the reaction products (P) relative to the starting material (SM). Color code: nickel, gray; chloride, light green; carbon, dark green; nitrogen, blue.

Figure 4. Reaction profiles toward the N-azolato and C-azolato reaction products, respectively, in the reaction of [Ni(COD)2] with 8chlorotheophylline (2) in the presence of an excess of PMe3. Depicted are the reaction complexes (RC), the transition states (TS), and the reaction products (P) relative to the starting material (SM). Color code: nickel, purple; chloride, light green; carbon, dark green; nitrogen, blue.

product, while the C-azolato product will be formed in a thermodynamically controlled reaction. In comparison to the analogous reaction of [Ni(COD)2]/PMe3, the C-azolato pathway shows a very similar behavior, while the N-azolato pathway is much more favorable in the reaction of [Pd(COD)2]/PMe3. However, the calculations confirm that the C-azolato product is always thermodynamically favored at elevated temperature. In order to demonstrate that the C−Cl oxidative addition of chloroazoles to Ni0 proceeds cleanly in the absence of an N−H bond, the reaction was performed with 8-chlorocaffeine featuring an N9−CH3 group. This reaction proceeds cleanly under oxidative addition of the C8−Cl bond and formation of the C-azolato complex trans-[10] in 75% yield (Scheme 4). However, no N−H oxidative addition is possible in this reaction. A related azolato complex has been obtained from the rection of caffeine with [Co(CH3)(PMe3)4].11

In the transition state (TS) toward the formation of the Nazolato complex, the N−H proton of compound 2 has been added to the nickel(0) center. This is followed by coordination of the amido function of the heterocycles to yield complex trans-[5]. Alternatively, for the formation of the C-azolato complex, the C2 carbon atom of 2 adds oxidatively to the metal center followed by chloride coordination to the metal center. Comparison of the activation barriers for these two processes reveals a much lower barrier for the N−H oxidative addition (46.3 kJ/mol) in comparison to the C−Cl oxidative addition (62.1 kJ/mol). While the stability of the C-azolato complex is higher than that of the N-azolato complex, the low reaction temperature clearly favors the formation of the latter complex as the kinetic product. At higher temperatures, formation of the thermodynamically more stable product (the C-azolato complex) is expected, and this has also been observed experimentally. Figure 5 summarizes the reaction of [Pd(COD)(PMe3)2] in the presence of an excess of PMe3 with 8-chlorotheophylline (2). The starting materials, reaction complexes, transition states, and products are defined analogously to those in the reaction of [Ni(COD)2]/PMe3 with 8-chlorotheophylline (Figure 4). In order to obtain the N-azolato product, first a reaction complex is formed that is stabilized by 21.6 kJ/mol in comparison to the starting materials. A transition state is passed that is 7.1 kJ/mol less stable than the starting materials, resulting in a total activation barrier of 28.7 kJ/mol. The transition state again describes a proton transfer from 8chlorotheophylline to the Pd complex. A subsequent ligand exchange leads to the final product, stabilized by 37.0 kJ/mol. The C-azolato product is obtained by forming a reaction complex that is destabilized by 47.6 kJ/mol. A transition state of 70.4 kJ/mol is passed, resulting in the thermodynamically most stable C-azolato product, stabilized by 151.0 kJ/mol. Again, the N-azolato product is the kinetically controlled

Scheme 4. Oxidative Addition of 8-Chlorocaffeine to Ni0

The composition of and connectivity in trans-[10] was established by NMR spectroscopy (see the Supporting Information) and was confirmed by an X-ray diffraction analysis (Figure 6). The metric parameters of the caffeinederived azolato ligand match those previously reported for a related platinum(II) complex, obtained by oxidative addition of 8-chlorocaffeine to a Pt0 complex.6i Comparison of the metric parameters within the diaminoheterocycle upon Nmetalation (trans-[5]) or C-metalation (trans-[10]) reveals that the N7−C8−N9 angle shrinks significantly upon C8metalation from 118.07(12)° in trans-[5] to 110.85(11)° in trans-[10]. D


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temperature for 10 min, after which time the solution had turned orange-red. After the color change, the solution was added dropwise to a THF solution (5 mL) of 2-chlorobenzimidazole (17 mg, 0.11 mmol) at 0 °C. The solution was stirred for an additional 2 h at 0 °C, during which the color changed from orange-red to orange-yellow. Subsequently, the solution was concentrated (4 mL) and absolute hexane (20 mL) was added. The resulting suspension was kept at −20 °C for 48 h. Over this time yellow crystals of trans-[4] formed, which were isolated by filtration. Yield: 37 mg (0.083 mmol, 75%). 1H NMR (400 MHz, THF-d8): δ 7.52 (s br, 1H, H6), 7.32 (s br, 1H, H9), 6.85 (s br, 2H, H7, H8), 1.46 (br, 12H, PCH2CH3), 1.05 (br, 18H, PCH2CH3), − 22.15 (t, 2JH,P = 74.8 Hz, 1H, H10) ppm. 13C{1H} NMR (100 MHz, THF-d8): δ 148.6 (C4), 148.3 (C2), 145.6 (C5), 119.3 (C7), 119.1 (C8), 118.3 (C9), 113.3 (C6), 18.1 (virtual-t, 1,3 JC,P = 12.9 Hz, PCH2CH3), 8.7 (PCH2CH3) ppm. 31P{1H} NMR (162 MHz, THF-d8): δ 18.0 (d, 2JP,H = 74.8 Hz) ppm. HRMS (ESI, positive ions): m/z 447.1388 (calcd for [[4] + H]+ 447.1396). Synthesis of Complex trans-[5].

Figure 6. Molecular structure of trans-[10] (hydrogen atoms have been omitted for clarity, and only the first atom of each of the P-ethyl substituents is shown; only one of the two essentially identical molecules in the asymmetric unit is depicted). Selected bond lengths (Å) and angles (deg): Ni−P1 2.1969(4), Ni−P2 2.2038(4), Ni−Cl 2.2021(4), Ni−C8 1.8569(13), N7−C8 1.363(2), N9−C8 1.359(2); P1−Ni−P2 178.33(2), P1−Ni−Cl 90.666(14), P1−Ni−C8 88.97(4), P2−Ni−Cl 90.600(15), P2−Ni−C8 89.91(4), Cl−Ni−C8 172.97(4), N7−C8−N9 110.85(11).

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CONCLUSION We described the temperature-dependent, site-selective metalation of unsubstituted 2-halogenoazoles with nickel. At low temperature, the unique polar oxidative addition of the N−H bond of the azoles with formation of a NiII hydride complexes was observed with no participation of the C−halogen bond. At an elevated temperature (45 °C), the C−Cl oxidative addition of 2-chlorobenzimidazole to Ni0 proceeds to give a mixture of the C-azolato complexes trans-[8] and trans-[9]. DFT calculations confirm the experimentally observed preferences for the N−H and C−Cl oxidative addition at different reaction temperatures. The site-selective metalation of halogenoazoles by other transition metals is currently under investigation. This procedure might have ramifications for the normally cumbersome preparation of heterobimetallic complexes from azole scaffolds,12 since the hydrido complexes obtained by the N−H oxidative addition still feature a reactive C−halogen bond capable of undergoing another oxidative addition reaction.

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A solution of [Ni(COD)2] (30 mg, 0.11 mmol) in n-hexane (10 mL) was mixed with a solution of triethylphosphine in THF (1 M, solution, 0.22 mL, 0.22 mmol). The reaction mixture was stirred at ambient temperature for 10 min, after which time the solution turned orange-red. Subsequently, 8-chlorotheophylline (23 mg, 0.11 mmol) was added and the mixture was stirred for 16 h at ambient temperature. Over this time the color changed from orange-red to yellow. The reaction mixture was then kept at −30 °C for 7 days, which led to the precipitation of yellow crystals of trans-[5]. These were isolated and dried in vacuo. Yield: 46 mg (0.090 mmol, 82%). 1 H NMR (400 MHz, THF-d8): δ 3.35 (s, 3H, N3-CH3), 3.26 (s, 3H, N1-CH3), 1.49 (m, 12H, PCH2CH3), 1.07 (m, 18H, PCH2CH3), −22.61 (t, 2JH,P = 75.9 Hz, Ni−H) ppm. 13C{1H} NMR (100 MHz, THF-d8): δ 157.1 (C6), 152.4 (C2), 151.1 (C4), 143.4 (C8), 115.5 (C5), 29.8 (N3-CH3), 27.7 (N1-CH3), 18.0 (virtual-t, 1/3JC,P = 12.9 Hz, PCH2CH3), 8.5 (PCH2CH3) ppm. 31P{1H} NMR (162 MHz, THF-d8): δ 17.4 (d, 2JP,H = 75.9 Hz) ppm. HRMS (ESI, positive ions): m/z 509.1492 (calcd for [[5] + H]+ 509.1512). Synthesis of Complex trans-[6].

EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under an argon atmosphere using standard Schlenk techniques or in a glovebox. Solvents were dried by standard methods under argon prior to use. 1 H, 13C{1H}, and 31P{1H} NMR spectra were measured on Bruker AVANCE I 400 or Bruker AVANCE III 400 spectrometers. Chemical shifts (δ) are expressed in ppm using the residual protonated solvent signal as an internal standard. Coupling constants are expressed in Hertz. Mass spectra were obtained with an Orbitrap LTQ XL (Thermo Scientific) spectrometer. 2-Chlorobenzimidazole, [Ni(COD)2], and triethylphosphine were purchased from commercial sources and were used as received. Consistent microanalytical data for the complexes were difficult to obtain due to the high sensitivity of the nickel complexes.13 A complete set of HRMS and NMR spectra is provided instead. Synthesis of Complex trans-[4].

A solution of [Ni(COD)2] (30 mg, 0.11 mmol) in n-hexane (10 mL) was mixed with a solution of triethylphosphine in THF (1 M solution, 0.22 mL, 0.22 mmol). The reaction mixture was stirred at ambient temperature for 10 min. Over this period the solution turned orangered. Subsequently, 8-bromotheophylline (28 mg, 0.11 mmol) was added and the mixture was stirred for 16 h at ambient temperature. During this period the color changed from orange-red to yellow. The reaction mixture was then kept at −30 °C for 7 days, which led to the precipitation of yellow crystals of trans-[6]. These were isolated by filtration and dried in vacuo. Yield: 42 mg (0.076 mmol, 69%). 1H NMR (400 MHz, C6D6): δ 3.57 (s, 3H, N1-CH3), 3.56 (s, 3H, N3CH3), 1.25 (m, 12H, PCH2CH3), 0.90 (m, 18H, PCH2CH3), − 22.67 (t, 2JH,P = 75.6 Hz, Ni−H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 156.9 (C6), 152.1 (C2), 151.6 (C4), 131.8 (C8), 116.7 (C5), 30.0 (N3-CH3), 28.0 (N1-CH3), 17.5 (virtual-t, 1/3JC,P = 13.1 Hz, PCH2CH3), 8.4 (PCH2CH3) ppm. 31P{1H} NMR (162 MHz, C6D6): δ 17.5 (d, 2JP,H = 75.6 Hz) ppm. The HR mass spectrum of

Samples of [Ni(COD)2] (30 mg, 0.11 mmol) and triethylphosphine (26 mg, 0.22 mmol) were stirred in THF (7 mL) at ambient E


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Organometallics trans-[6] could not be recorded, most likely due to decomposition of the complex during data acquisition. Synthesis of Complex trans-[7].

A solution of [Ni(COD)2] (30 mg, 0.11 mmol) in n-hexane (10 mL) was mixed with a solution of triethylphosphine in THF (1 M solution, 0.22 mL, 0.22 mmol). The mixture was stirred at ambient temperature for 10 min. Over this period the solution turned orange-red. Subsequently, 8-chlorocaffeine (25 mg, 0.11 mmol) was added and the mixture was stirred at 50 °C for 3 days. During this period the color changed from orange-red to yellow. The resulting solution was concentrated to about 4 mL and stored at ambient temperature for 1 day. Over this period yellow crystals of trans-[10] formed, which were isolated by filtration and dried in vacuo. Yield: 42 mg (0.082 mmol, 75%). 1H NMR (400 MHz, C6D6): δ 4.12 (s, 3H, N7-CH3), 3.47 (s, 3H, N3-CH3), 3.46 (s, 3H, N1-CH3), 1.25 (m, 12H, PCH2CH3), 0.95 (m, 18H, PCH2CH3) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 168.4 (t, 2JCP = 40.8 Hz, C8), 153.8 (C6), 151.6 (C2), 151.6 (t, 4JC,P = 2.5 Hz, C4), 109.9 (C5), 35.1 (N7-CH3), 29.3 (N3-CH3), 27.5 (N1-CH3), 14.0 (virtual-t, 1/3JC,P = 12.7 Hz, PCH2CH3), 8.1 (PCH2CH3) ppm. 31P{1H} NMR (162 MHz, C6D6): δ 17.5 (s) ppm. HRMS (ESI, positive ions): m/z 487.1894 (calcd for [[6] − Cl]+ 487.1902), 528.2161 (calcd for [[10] − Cl + MeCN]+ 528.2167). X-ray Crystallography. X-ray diffraction data were collected with a Nonius KappaCCD diffractometer equipped with a rotating anode using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Diffraction data were collected over the full sphere and were corrected for absorption. Programs used: data collection, COLLECT;14 data reduction, Denzo-SMN;15 absorption correction, Denzo;16 structure solution, SHELXS-97;17 structure refinement, SHELXL-97.18 If not noted otherwise, hydrogen atoms were added to the structure model on calculated positions. All six ethyl groups at phosphorus atoms P1 and P2 are disordered over two positions in trans-[4]·C7H5N2Cl. Several restraints (SADI, SAME, ISOR, and SIMU) were used in order to improve refinement stability. Compound trans-[9] crystallized with a disordered half hexane molecule in the asymmetrical unit. The solvent molecule could not be satisfactorily refined. The program SQUEEZE19 was therefore used to remove mathematically the effect of the solvent. The quoted formula and derived parameters do not include the squeezed solvent molecule. Crystal Data for trans-[4]·C7H5N2Cl. Yellow crystals of trans-[4]· C7H5N2Cl were obtained by cooling the original reaction mixture of trans-[4] to −20 °C. Complex trans-[4] cocrystallized together with one molecule of 2-chlorobenzimidazole: formula C26H40N4Cl2NiP2, Mr = 600.17, yellow plate, 0.15 × 0.10 × 0.03 mm, a = 14.4531(2) Å, b = 14.5156(2) Å, c = 14.0229(2) Å, β = 107.0080(10)°, V = 2813.27(7) Å3, ρcalc = 1.417 g cm−3, μ = 1.016 mm−1, empirical absorption correction (0.86 ≤ T ≤ 0.97), Z = 4, monoclinic, space group P21/n, T = 223(2) K, ω and φ scans, 18504 intensities collected (±h, k, ±l) (8.5 ≤ 2θ ≤ 52.7°), 5668 independent (Rint = 0.039) and 4901 observed intensities (I ≥ 2σ(I)), 434 refined parameters, R = 0.0484, Rw = 0.1035, Rall = 0.0589, Rw,all = 0.1120, max/min residual electron density 0.67/−0.28 e Å−3. The positional parameters of the hydrogen atoms attached to Ni and N4 were refined freely; all other hydrogen atoms were added to the structure model on calculated positions and were refined as riding atoms. Crystal Data for trans-[5]. Yellow, cubic shaped crystals of complex trans-[5] were obtained by slow evaporation of the solvent from a tetrahydrofuran solution of the complex at ambient temperature: formula C19H37N4ClNiO2P2, Mr = 509.62, yellow cube, 0.20 × 0.18 × 0.16 mm, a = 11.0585(4) Å, b = 14.8014(6) Å, c = 23.8061(10) Å, α = 104.902(2)°, β = 101.118(2)°, γ = 94.322(2)°, V = 3662.7(3) Å3, ρcalc = 1.386 g cm−3, μ = 1.057 mm−1, empirical absorption correction (0.815 ≤ T ≤ 0.852), Z = 6, triclinic, space group P1̅, T = 173(2) K, ω and φ scans, 68132 intensities collected (±h, ±k, ±l) (1.8 ≤ 2θ ≤ 64.6°), 23500 independent (Rint = 0.033) and 17948 observed intensities (I ≥ 2σ(I)), 820 refined parameters, R = 0.0348, Rw = 0.0807, Rall = 0.0526, Rw,all = 0.0872, max./min residual electron density 0.73/−0.51 e Å−3. The asymmetric unit contains three essentially identical molecules of trans-[5]. Crystal Data for trans-[7]. Orange crystals of complex trans-[7] were obtained by slow evaporation of the solvents from a

A 150 mL Schlenk flask charged with trans-[4] (23 mg, 0.052 mmol), 2-chlorobenzimidazole (8 mg, 0.052 mmol), and THF (15 mL) was fitted with an argon bubbler on top. The mixture was then heated with stirring for 4 h at 60 °C. When the evolution of hydrogen ceased, the reaction mixture was cooled to ambient temperature. The solution was concentrated to 3 mL and placed in an ice/water bath. Subsequently, hexane (20 mL) and diethyl ether (10 mL) were added. An orange-yellow precipitate of trans-[7] formed, which was collected via filtration and dried in vacuo. Yield: 20 mg (0.033 mmol, 65%). 1H NMR (400 MHz, CD2Cl2): δ 8.42 (d, 3JH,H = 7.77 Hz, 2H, H6), 7.48 (d, 3JH,H = 7.86 Hz, 2H, H9), 7.25 (t, 3JH,H = 7.39 Hz, 2H, H7), 7.13 (t, 3JH,H = 7.39 Hz, 2H, H8), 1.19−0.58 (m, 60H, PCH2CH3) ppm. 13C{1H} NMR (100 MHz, CD2Cl2): δ 147.9 (C2), 147.0 (C4), 143.0 (C5), 120.82 (C8), 120.76 (C7), 118.8 (C9), 114.0 (C6), 12.9 (virtual-t, 1,3JC,P = 11.4 Hz, PCH2CH3), 7.9 (PCH2CH3) ppm. 31P{1H} NMR (162 MHz, CD2Cl2): δ 7.3 (s) ppm. HRMS (ESI, positive ions): m/z 599.1341 (calcd for [[7] + H]+ 599.1380). Synthesis of the Complex Mixture trans-[8]/trans-[9].

Samples of [Ni(COD)2] (30 mg, 0.11 mmol) and triethylphosphine (27 mg, 0.23 mmol) were stirred in hexane (5 mL) at ambient temperature for 15 min, after which time the solution had turned orange-red. This solution was added dropwise to 10 mL of a hexane/ toluene solution (3/1) of 2-chlorobenzimidazole (1) (17 mg, 0.11 mmol) at 45 °C over the course of 4 h. The solution was subsequently stirred overnight at 45 °C (total reaction time 18 h). Over this period the color changed from orange-red to yellow. Subsequently, absolute hexane (10 mL) was added to the reaction mixture. The resulting cloudy solution was kept at 4 °C for 7 days. After this period a yellow precipitate had formed. This precipitate was subsequently characterized by NMR spectroscopy and mass spectrometry. The precipitate contained some crystals of trans-[9]. Apart from crystallization allowing the isolation of trans-[9], it was not possible to isolate one of the complexes individually from the complex mixture. Yield: 29 mg of the complex mixture trans-[8]/trans-[9]. Due to the similarities of complexes trans-[8]/trans-[9], 1H and 13C{1H} NMR could not be assigned. 31P{1H} NMR (162 MHz, DMSO-d6): δ 17.6 (s, trans-[8]), δ 14.1 (s, trans-[9]) ppm. HRMS (ESI, positive ions): m/z 447.1404 (calcd for [[8] + H]+ 447.1396, note that this mass also corresponds to the mass of cation [[4] + H]+), m/z 563.1786 (calcd for [[9] + H]+ 563.1770). Synthesis of Complex trans-[10].

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Article

Organometallics tetrahydrofuran/hexane solution of the complex at ambient temperature: formula C26H38N4Cl2N4NiP2, Mr = 598.15, orange plate, 0.30 × 0.20 × 0.18 mm, a = 11.4964(2) Å, b = 9.1137(2) Å, c = 13.5728(3) Å, β = 93.375(1)°, V = 1419.62(5) Å3, ρcalc = 1.399 g cm−3, μ = 1.007 mm−1, empirical absorption correction (0.75 ≤ T ≤ 0.84), Z = 2, monoclinic, space group P21/n (No. 14), λ = 0.71073 Å, T = 223(2) K, ω and φ scans, 7633 intensities collected (±h, ±k, ±l) (8.1 ≤ 2θ ≤ 57.0°), 3461 independent (Rint = 0.028) and 3219 observed intensities (I ≥ 2σ(I)), 163 refined parameters, R = 0.0326, Rw = 0.0813, Rall = 0.0356, Rw,all = 0.0842, max/min residual electron density 0.32/−0.28 e Å−3. The hydrogen atoms were added to the structure model on calculated positions and were refined as riding atoms. The complex resides on a crystallographic inversion center. The asymmetric unit contains 1/2 molecule of trans-[7]. Crystal Data for trans-[9]. Slow evaporation of the solvents from a toluene/hexane solution of the complex mixture trans-[8]/trans-[9] at ambient temperature afforded yellow crystals of complex trans-[9]: formula C26H39N4ClNiP2, Mr = 563.71, yellow needle, 0.30 × 0.07 × 0.05 mm, a = 25.0145(6) Å, c = 10.1055(3) Å, V = 6323.3(4) Å3, ρcalc = 1.184 g cm−3, μ = 0.819 mm−1, empirical absorption correction (0.79 ≤ T ≤ 0.96), Z = 8, tetragonal, space group I4̅, T = 223(2) K, ω and φ scans, 20158 intensities collected (±h, ±k, ±l) (8.3 ≤ 2θ ≤ 52.7°), 5848 independent (Rint = 0.061) and 4993 observed intensities (I ≥ 2σ(I)), 318 refined parameters, R = 0.0530, Rw = 0.1337, Rall = 0.0660, Rw,all = 0.1462, max/min residual electron density 0.34/−0.24 e Å−3. The positional parameters of the hydrogen atom attached to N1 were refined freely; all other hydrogen atoms were added to the structure model on calculated positions and were refined as riding atoms. Crystal Data for trans-[10]. Yellow crystals of complex trans-[10] were obtained by slow evaporation of the solvents from a tetrahydrofuran solution of the complex at ambient temperature: formula C20H39N4ClNiO2P2, Mr = 523.65, yellow block, 0.24 × 0.20 × 0.20 mm, a = 17.8792(2) Å, b = 12.8276(1) Å, c = 23.2575(2) Å, β = 94.332(1)°, V = 5318.80(9) Å3, ρcalc = 1.308 g cm−3, μ = 0.973 mm−1, empirical absorption correction (0.685 ≤ T ≤ 0.746), Z = 8, monoclinic, space group P21/c, T = 173(2) K, ω and φ scans, 95467 intensities collected (±h, ±k, ±l) (6.7 ≤ 2θ ≤ 61.0°), 16246 independent (Rint = 0.041) and 13366 observed intensities (I ≥ 2σ(I)), 559 refined parameters, R = 0.0307, Rw = 0.0752, Rall = 0.0413, Rw,all = 0.0806, max/min residual electron density 0.70/−0.49 e Å−3. The asymmetric unit contains two crystallographically independent, essentially identical molecules of trans-[10].

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Johannes Neugebauer: 0000-0002-8923-4684 Ying-Feng Han: 0000-0002-9829-4670 F. Ekkehardt Hahn: 0000-0002-2807-7232 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027).

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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.organomet.9b00357. Crystallographic data for trans-[6] and NMR spectra of all new complexes (PDF) Cartesian coordinates of the calculated structures (XYZ) Accession Codes

CCDC 1027900−1027902 and 1892979−1892981 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

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

David Schnieders: 0000-0002-8042-7930 G


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