Homo- and Heterodinuclear Head-to-Head or Head-to-Tail

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

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Homo- and Heterodinuclear Head-to-Head or Head-to-Tail Complexes of Rhodium(I) and Iridium(I) with C2,N3 or C8,N9 Bridging Azolato Ligands Steffen Cepa, Maximilian Böhmer, Florian Roelfes, Tristan Tsai Yuan Tan, Fabian Dielmann, and F. Ekkehardt Hahn* Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany Organometallics Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/16/19. For personal use only.

S Supporting Information *

ABSTRACT: Reaction of in situ lithiated N-methylimidazole or 1,5,6trimethylbenzimidazole with [M(cod)(μ-Cl)]2 (M = Rh, Ir) led to the exclusive formation of the homodinuclear head-to-head complexes [2]− [5], where the metal centers are bridged by two C,N-coordinated azolato ligands. The two metal centers are each coordinated by one cod ligand and either two carbon or two nitrogen donors of the azolato ligands. Access to a heterodinuclear Ir/Rh complex [6] was achieved via the lithiated iridium complex [7] and addition of an equimolar amount of [Rh(cod)(μ-Cl)]2. Deprotonation of caffeine with lithium diisopropylamide and addition of [M(cod)(μ-Cl)]2 (M = Rh, Ir) led to mixtures of the head-to-head (M = Ir: [8H−H]; M = Rh: [9H−H]) and head-to-tail isomers (M = Ir: [8H−T]; M = Rh: [9H−T]) of the homodinuclear complexes. The molecular structures of the homo- and heterodinuclear complexes [3], [5]·2C6H5F, [6]·2C6H5F, and [9H−T]·C4H8O have been determined by X-ray diffraction studies.

1. INTRODUCTION N-Heterocyclic carbenes (NHCs) and their transition metal complexes have been studied extensively over the last decade.1 Due to their electronic and steric properties, NHCs have found many applications as ligands in coordination chemistry,1,2 as building blocks for metallosupramolecular complexes,3 as organocatalysts4 and their transition metal complexes are used in a broad array of catalytic applications.5 While transition metal NHC complexes containing one metal center have been thoroughly studied, there are only a few examples for NHC (or azolato) ligands acting as a bridging ligands in dinuclear complexes, where one metal is bound at the carbene (or azolato) carbon atom and another one is bound at the unsubstituted ring-nitrogen (A in Figure 1).6 Such structures are possibly promising candidates as catalysts for unique transformations through cooperative action of the two metal centers.7 It has been shown that N-deprotonation of platinum or palladium complexes bearing protic NH,NR-NHC ligands leads to homodinuclear complexes (B in Figure 1) with a headto-tail (H−T) orientation of the bridging ligands.8 Similar dinuclear complexes of type B have been obtained from the oxidative addition of 2-chloro-N-methylbenzimidazole to [Pt(PPh3)4]9 and by treatment of an iron(II)−di-NHC complex with PhLi (C in Figure 1).10 The formation of homodinuclear head-to-head (H−H) complexes has been proposed for the reaction of [Ir(cod)(μ-Cl)]2 with lithium Nbenzylimidazolate. However, the cod complexes could not be characterized conclusively by NMR spectroscopy and only © XXXX American Chemical Society

Figure 1. Selected dinuclear complexes with C,N-bridging azolato ligands.

after substitution of the two cod ligands for four CO ligands was H−H complex D characterized by X-ray diffraction.11a Received: February 4, 2019

A

DOI: 10.1021/acs.organomet.9b00074 Organometallics XXXX, XXX, XXX−XXX

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Organometallics The reaction of the more bulky substituted N-mesityimidazole with [Ir(cod)(μ-Cl)]2 yielded a mixture of H−H and H−T complexes.11b Finally, a mononuclear ruthenium(II) complex bearing a tridentate ligand with two protic pNHC donors can bind a second metal center after deprotonation of the ring-NH functions with n-BuLi (E in Figure 1).12 We became interested in the preparation of homo- and heterodinuclear complexes with μ-C,N bridging NHC ligands and in a detailed NMR spectroscopic characterization of the possible isomeric H−H and H−T isomers. Herein, we report the preparation of homo- and heterodinuclear complexes of IrI and RhI obtained from C2/8-deprotonated N-methylimidazole, 1,5,6-trimethylbenzimidazole and caffeine and their detailed 1H NMR spectroscopic characterization.

metalated 1,5,6-trimethylbenzimidazolate ligand in [5] is indicated in the 1H NMR spectrum by the absence of a resonance for H2 proton, which was recorded at δ = 7.70 ppm in ligand precursor 1. In addition, the 13C{1H} NMR spectrum of [5] exhibited the typical resonance for the metalated azolato carbon atom at δ = 192.5 featuring a 1JC,Rh coupling constant of 1 J = 49.8 Hz, which clearly indicates the presence of a complex with a Rh−Cazolato bond. Distinction between the head-to-head versus the head-to-tail complexes can be made by spin correlation spectroscopy (1H,1H-COSY) as has been suggested for related complexes.11a For the head-to-tail complexes, featuring a C2-symmetry axis, one spin system is expected with 12 resonances for the two chemically equivalent cod ligands. In contrast, the head-tohead complexes feature a mirror plane (CS) that cuts through both of the chemically non-equivalent cod ligands. Thus, two spin systems, each comprising six resonances for each 1/2 cod ligand, are to be expected. The 1H,1H-COSY NMR spectrum of [5] (Figure 2) does exhibit two separate spin systems,

2. RESULTS AND DISCUSSION Two geometrical isomers can be formed if N1-alkylated, azoles (imidazoles and benzimidazoles) are deprotonated and used as C,N-bridging ligands for the generation of dinuclear complexes. The bridging ligands could be orientated head-totail (H−T, Figure 1, B,C) or head-to-head (H−H, Figure 1, D). Reaction of in situ generated lithium N-methylimidazolate or 1,5,6-trimethylbenzimidazolate with a stoichiometric amount of [M(cod)(μ-Cl)]2 (M = Rh, Ir) in tetrahydrofuran (THF) leads exclusively to the air-sensitive homodinuclear H− H complexes [2]−[5] (Scheme 1). The formation of a mixture Scheme 1. Synthesis of Complexes [2]−[5] and Possible Geometric Isomers

Figure 2. 1H,1H-COSY NMR spectrum of complex [5]. The blue and red lines are assigned to the two spin systems.

indicating the head-to-head arrangement of the azolato ligands. Every CH group experiences 3JHH coupling to the protons of its neighboring CH- and CH2-groups. For example, the resonance at δ = 5.08 ppm, which was assigned to a CHproton, couples with the resonances at δ = 4.04 ppm (CH) and δ = 2.85 and 2.05 ppm (CH2). Furthermore, the head-to-head arrangement of the azolato ligands in rhodium complexes [3] and [5] (Scheme 1) has been established by X-ray diffraction analyses (Figure 3), confirming the conclusions drawn from NMR spectroscopy. Both complexes feature a boat conformation of the sixmembered Rh−C−N−Rh−N−C ring, and the two rhodium atoms are coordinated in an approximately square-planar fashion. Any intramolecular interaction between the rhodium atoms can be excluded owing to the long Rh1···Rh2 separations of 3.288(2) and 3.195(1) Å found for [3] and

of dinuclear H−H and H−T complexes as described by Braunstein for the reaction of lithium N-mesitylimidazolate with [Ir(cod)(μ-Cl)]2 was not observed, most likely due to the different steric bulk of the N1-substituents (methyl vs mesityl).11b Since complexes [2]−[5] are isostructural, the analytical data are discussed exemplarily for dirhodium complex [5] (for analytical data of [2]−[4], see the Experimental Section and the Supporting Information). The presence of the doubly B


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Scheme 2. Synthesis of Complex [6] from 1, [Rh(cod)(μCl)]2, and [Ir(cod)(μ-Cl)]2

Figure 3. Molecular structures of complex [3] (top) and of complex [5] in [5]·2C6H5F (bottom). (50% displacement ellipsoids, solvent molecules, and hydrogen atoms have been omitted). Selected bond lengths (Å) and angles (deg) for [3] and [[5]·2C6H5F]: Rh1···Rh2 3.288(2) [3.195(1)], Rh2−N3 2.080(2) [2.101(2)], Rh2−N23 2.078(2) [2.098(2)], Rh1−C2 2.033(2) [2.024(3)], Rh1−C22 2.042(2) [2.024(3)], N3−Rh2−N23 87.30(7) [86.16(8)], C2− Rh1−C22 84.83(8) [82.64(10)], N1−C2−N3 107.1(2) [109.1(2)], N21−C22−N23 106.4(2) [108.7(2)].

[5], respectively.11b,13 The Rh2−N distances (2.080(2) and 2.078(2) Å) in complex [3] are unexceptional.11b,13 The Rh1− Cazolato distances (2.033(2) and 2.042(2) Å) fall in the range observed in RhI dicarbene complexes14 and the related headto-tail complex with bridging N-mesitylimadzolato ligands.11b Dinuclear complex [5], featuring two bridging 1,5,6trimethylbenzimidazolato ligands, exhibits slightly longer Rh2−N bond lengths than observed in [3]. The Rh1−Cazolato bond lengths in [5] are similar to those found in complex [3]. The cod ligands in both complexes [3] and [5] adopt a slightly twisted conformation. We next attempted to prepare heterobimetallic complexes from N-methylbenzimidazole 1 and RhI and IrI complex precursors. Treatment of 1 with n-BuLi followed by addition and 0.25 equiv each of [Rh(cod)(μ-Cl)]2 and [Ir(cod)(μCl)]2 led to the formation of a complex mixture composed of the heterodinuclear complex [6] (major) and the homodinuclear complexes [4] and [5] (minor, Scheme 2). The heterobimetallic complex [6] could not be obtained by reacting a homonuclear complex (e.g., iridium complexes [2] or [4]) with a second metal complex such as [Rh(cod)(μCl)]2. This indicates some stability of the Ir(C-azolato)2 and Rh(N-azolato)2 motifs. NHC transfer from IrI to RhI and vice versa is also no common procedure. The 1H NMR spectrum of the complex mixture [4]−[6] exhibits three sets of signals, which can be been assigned to the previously described homobimetallic complexes [4] and [5] and the new complex [6] (Figure 4). Complexes [4] and [5] in the complex mixture were identified by comparison with the spectra of authentical samples previously prepared. Integration

Figure 4. Section of the 13C{1H} NMR spectra of complex [5] (top), complex [4] (middle), and the mixture of complexes [4]−[6] (bottom).

of the signals of the aromatic protons allows the conclusion that complexes [4]−[6] were formed in a ratio of 0.4:0.3:1. Head-to-head coordination of both azolato carbon atoms to one iridium atom has been concluded from the 13C{1H} NMR data of [6] showing no 1JRhC coupling for the resonance of the coordinated azolato carbon atoms at δ = 188.1 ppm. Meanwhile, 1JRhC coupling was observed for the resonances at δ = 78.9 (d, 1JCRh = 3.9 Hz) and 78.7 (d, 1JCRh = 2.6 Hz) assigned to the CH groups of the cod ligand coordinated to the rhodium atom (see the Supporting Information Figure S14). These observations confirm that the rhodium atom in [6] is not coordinated by the azolato carbon atoms but by two nitrogen atoms instead in an H−H arrangement of the ligands. Suitable crystals of [6]·2C6H5F for X-ray diffraction studies were obtained from the reaction mixture by cooling a concentrated solution of complexes [4]−[6] in fluorobenzene. The molecular structure determination (Figure 5) confirms the head-to-head arrangement of the two azolato ligands as concluded from the NMR data. The position of the metal center coordinated by the two nitrogen atoms of the azolato ligands is occupied by disordered Rh and Ir atoms in a ratio of Rh0.67/Ir0.33 indicating that complex [4] (homobimetallic iridium complex) co-crystallizes together with [6]. Owing to the metal disorder, metric parameters for [6] are not discussed in detail here. Formation of complex [6] as the C


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Cl)]2 to complex [7] gave the heterodinuclear complex [6] as the only product in 90% yield. The NMR spectra agree with the resonances assigned to complex [6] present in the mixture of complexes [4]−[6]. Finally, caffeine was tested as ligand for the synthesis of homodinuclear C,N-azolato bridged complexes. This study was initiated in order to investigate the influence of the electron withdrawing backbone of caffeine on the complex formation and particularly on the geometrical isomers (H−H or H−T) obtained. While the influence of the N-substituents of azoles on the geometry of the resulting complexes has been demonstrated,11 the effect of the electronic situation within the azole has not been studied previously. The conditions employed for the deprotonation of the azoles used for the preparation of [2]−[5] could not be used for the deprotonation of caffeine as this leads to decomposition. When the base was changed from n-BuLi to lithium diisopropylamide, deprotonation of caffeine proceeded with no problems and the subsequent addition of [Ir(cod)(μ-Cl)]2 or [Rh(cod)(μ-Cl)]2 gave the homodinuclear complexes [8] and [9] as mixtures of the head-to-head [H−H] and head-totail [H−T] isomers (Scheme 4). It was also tempting to

Figure 5. Molecular structure of complex [6] in [6]·2C6H5F. (50% displacement ellipsoids, solvent molecules, and hydrogen atoms have been omitted for clarity).

main product can be explained using the HSAB concept. A Cazolato atom is a softer donor than the N-azolato donor.10 In addition, the iridium(I) ion is a softer Lewis acid than the rhodium(I) ion. Following HSAB rules, the coordination of the azolato carbon atoms to iridium and of the azolato nitrogen atoms to rhodium would lead to the most stable complex and thus to the preferred formation of complex [6]. In order to prepare a pure sample complex [6] with no homobimetallic complexes present, a different synthetic approach, inspired by the work reported by Cossairt,12 was used. This approach involved the initial preparation of the lithiated complex [7] which was prepared by treatment of 1 with n-BuLi followed by addition of only 0.25 equiv of [Ir(cod)(μ-Cl)]2 (Scheme 3). The subsequent addition of [Rh(cod)(μ-Cl)]2 to complex [7] yielded the heterobimetallic complex [6] by metalation of the nitrogen donor sites.

Scheme 4. Synthesis of Isomer Mixtures of Homodinuclear Complexes [8] and [9]

Scheme 3. Selective Synthesis of Heterodinuclear Complex [6]

prepare heterobimetallic complexes with bridging caffeine ligands similar to [6]. However, no attempts were made to react C8-deprotonated caffeine with an equimolar mixture of [Rh(cod)(μ-Cl)]2 and [Ir(cod)(μ-Cl)]2. The homodinuclear caffeine-bridged complexes already form as mixtures of H−H and H−T isomers and a total of 4 complexes would be expected. In addition, various isomers of the heterodinuclear complex could form, ultimately leading to an inseparable mixture of homo- and heterodinuclear complexes. The overall yield for the mixture of the homodinuclear iridium complexes [8H−H]/[8H−T] was 29% with an isomer ratio of approximately 0.5:0.5 ([H−H]/[H−T]). The mixture of the homodinuclear rhodium complexes [9H−H]/[9H−T] was obtained in an overall yield of 39% with an isomer ratio of approximately 0.3:0.7 ([H−H]/[H−T]). Determination of the

Formation of [7] was confirmed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry revealing a peak for the cation [[7]−Li + 2H]+ at m/z = 621. The 13 C{1H} NMR spectrum of [7] features only four signals at δ = 68.9, 67.0 (CH), 29.7, and 29.4 ppm (CH2) for the cod ligand, indicating the existence of a mirror plane bisecting the complex in solution. The signal assigned to the azolato carbon atom in [7] was detected in the typical range at δ = 192.6 ppm. The subsequent addition of an equimolar amount of [Rh(cod)(μD


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2D NMR spectroscopy and by X-ray diffraction studies with dirhodium complexes [3] and [5]. The reaction of in situ lithiated N-methylbenzimidazolate with an equimolar mixture of [Rh(cod)(μ-Cl)]2 and [Ir(cod)(μ-Cl)]2 led to a mixture composed of the heterodinuclear complex [6] as a major reaction product and the two homodinuclear complexes [4] and [5]. However, a stepwise reaction sequence via the lithiated monoiridium complex [7] followed by metalation of the two ring-nitrogen atoms with [Rh(cod)(μ-Cl)]2 allowed the preparation of pure [6] in good yield. The lithiated complex [7] constitutes a perfect precursor for the preparation of additional heterodinuclear complexes that might possess unique properties owing to the presence two different metal centers in close proximity to each other.7,15 In addition, mixtures of homodinuclear head-to-head and head-to-tail complexes [8] and [9] have been synthesized by using caffeine as the η 2 ,μ-C,N-azolato ligand precursor and lithium diisopropylamide as the base.

isomer ratio is based on the integrals of the resonances for the cod protons in the 1H NMR spectra of the complex mixtures (see the Supporting Information, Figures S15 and S17). Separation of the H−H from the H−T complexes was impossible by chromatography and all corresponding attempts led to decomposition. NMR spectroscopy data revealed two sets of resonances for each complex, which can be assigned to the different isomers of complexes [8] and [9]. Resonances for the azolato carbon atoms were recorded in the 13C{1H} NMR spectra at δ = 182.6 ppm ([8H−H]), 178.6 ppm ([8H−T]) and at δ = 188.7 ppm (d, 1 JRhC = 50.0 Hz, [9H−H]), 183.2 (d, 1JRhC = 48.2 Hz, [9H−T] (see the Supporting Information, Figures S16 and S18). Crystals of [9H−T]·C4H8O suitable for an X-ray diffraction study were obtained by slow diffusion of n-hexane into a saturated solution of the complex mixture [9H−T]/[9H−H] in THF at ambient temperature. The molecular structure is depicted in Figure 6. Any intramolecular interaction between the rhodium atoms can be excluded due to long Rh(1)···Rh(2) separation of 3.1878(2) Å.

4. EXPERIMENTAL SECTION 4.1. General Procedures. All preparations were carried out under an argon atmosphere using conventional Schlenk techniques or in a glovebox. Solvents were dried and degassed by standard methods prior to use. Compound 1-methylimidazole was purchased from Alfa Aesar, and [Rh(cod)(μ-Cl)]216 and [Ir(cod)(μ-Cl)]217 were prepared as described in the literature. NMR spectra were recorded on a Bruker AVANCE I 400 or a Bruker AVANCE III 400 NMR spectrometer. Chemical shifts (δ) are expressed in ppm downfield from tetramethylsilane or using the residual protonated solvent signal as an internal standard. Coupling constants are expressed in Hertz. ESIHRMS spectra were obtained with an Orbitrap XL spectrometer. MALDI spectra were obtained with a Bruker Reflex IV spectrometer. Assignment of the proton NMR resonances was made via spin correlation spectroscopy (COSY). 4.2. Synthesis of Compound 1. A sample of 5,6-dimethylbenzimidazole (1.90 g, 13.0 mmol) was dissolved in acetonitrile (40 mL). An aqueous solution of NaOH (20 mL, 25 wt %) was added and the mixture was cooled to 0 °C. Then, a solution of methyliodide (0.80 mL, 13.0 mmol) in acetonitrile (20 mL) was added slowly and the reaction mixture was stirred at ambient temperature for 3 d. The solvents were removed in vacuo and the resulting residue was extracted with dichloromethane (3 × 20 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure to give 1 as a white solid. Yield: 1.34 g (8.4 mmol, 65%). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.70 (s, 1H, H2), 7.54 (s, 1H, H5), 7.12 (s, 1H, H8), 3.73 (s, 3H, H12), 2.38 (s, 3H, H11), 2.36 (s, 3H, H10). 13C{1H} NMR (101 MHz, CDCl3): δ (ppm) 142.7 (C2), 142.3 (C4), 133.0 (C9), 131.9 (C6), 130.7 (C7), 120.1 (C5), 109.4 (C8), 30.8 (C12), 20.4 (C11), 20.1 (C10). MS (MALDITOF, matrix DCTB) m/z (%): 161 (100) [1 + H]+. Anal. Calcd for 1: C, 74.96; H, 7.55; N, 17.49. Found: C, 73.91; H, 7.36; N, 17.16. 4.3. General Procedure for the Synthesis of Complexes [2] and [3]. A sample of 1,5,6-trimethylbenzimidazole (16.0 mg, 0.10 mmol) was dissolved in THF (5 mL) and cooled to −78 °C before nBuLi (0.08 mL, 0.13 mmol of an 1.6 M solution in n-hexane) was added. The reaction mixture was stirred for 2 h at −78 °C. A sample of complex [Ir(cod)(μ-Cl)]2 or [Rh(cod)(μ-Cl)]2 (0.05 mmol, 33.6 mg or 24.7 mg, respectively) was added and the solution was stirred for 12 h while it was allowed to warm up to ambient temperature. The solvent was removed in vacuo and the resulting residue was washed with methanol (2 × 5 mL) and dried under reduced pressure. 4.3.1. Characterization of Complex [2]. Complex [2] was obtained as a red solid. Yield: 18 mg, 0.235 mmol, 47%. 1H NMR (400 MHz, THF-d8): δ (ppm) 6.88 (d, 3JHH = 1.5 Hz, 1H, H4), 6.78 (d, 3JHH = 1.5 Hz, 1H, H5), 4.22−4.14 (m, 1H, cod-CH), 4.00−3.90 (m, 1H, cod-CH), 3.63 (s, 3H, H6), 3.58−3.57 (m, 1H, cod-CH), 3.33−3.26 (m, 1H, cod-CH), 2.39−2.25 (m, 4H, cod-CH2), 2.04− 1.97 (m, 2H, cod-CH2), 1.71−1.63 (m, 2H, cod-CH2). 13C{1H}

Figure 6. Molecular structure of complex of [9H−T] in [9H−T]·C4H8O (50% displacement ellipsoids, hydrogen atoms have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Rh1···Rh2 3.1878(2), Rh1−N21 2.133(2), Rh1−C8 2.021(2), Rh2−N9 2.134(2), Rh2−C20 2.023(2), N9−C8 1.372(2), N7−C8 1.357(2), N19−C20 1.361(2), N21−C20 1.368(3); C8−Rh1−N21 82.95(7), N9−Rh2−C20 84.44(7), N7−C8−N9 108.5(2), N19−C20−N21 108.5(2).

The formation of complex mixtures containing both geometric isomers ([H−H] and [H−T]) when C8-deprotonated caffeine is used as bridging C,N-ligand must be caused by the weaker σ-donor properties of the caffeine N9-atom owing to the electron withdrawing backbone compared to the N3-atom in the bridging azolato ligands in complexes [2]−[5]. As a result, a metal atom coordinated by two ring-nitrogen atoms of the caffeine-derived bridging ligand is less stabilized than one coordinated by one ring-nitrogen atom and a carbon atom from the second bridging ligand, leading to the formation of the H−T isomer next to the H−H isomer. This effect is more pronounced for the rhodium complex, thus leading to a larger ration of 0.7:0.3 (H−T/H−H) compared to the iridium complex (0.5:0.5).

3. CONCLUSIONS Four homodinuclear complexes [2]−[5] have been prepared, possessing two bridging η2,μ-C,N-azolato ligands. The head-tohead geometry and the different coordination environments of the two metal centers of these complexes were confirmed by E


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Organometallics NMR (101 MHz, THF-d8): δ (ppm) 175.3 (C2), 125.7 (C4), 120.1 (C5), 70.8 (cod-CH), 70.4 (cod-CH), 64.8 (cod-CH), 59.6 (codCH), 35.5 (s, C6), 33.3 (cod-CH2), 33.0 (cod-CH2), 32.7 (codCH2), 32.6 (cod-CH2). HRMS (ESI, positive ions) m/z: 763.2086 [[2] + H]+ (calcd for [[2] + H]+ 763.2097). 4.3.2. Characterization of Complex [3]. Complex [3] was obtained as an orange solid. Yield: 12 mg, 0.205 mmol (41%). 1H NMR (400 MHz, THF-d8): δ (ppm) 6.70 (d, 3JHH = 1.1 Hz, 1H, H4), 6.57 (d, 3JHH = 1.1 Hz, 1H, H5), 4.51−4.45 (m, 1H, cod-CH), 4.28− 4.17 (m, 2H, cod-CH), 3.83−3.75 (m, 1H, cod-CH), 3.63 (s, 3H, H6) 2.69−2.63 (m, 1H, cod-CH2), 2.59−2.51 (m, 2H, cod-CH2), 2.46−2.40 (m, 1H, cod-CH2), 2.21−2.13 (m, 2H, cod-CH2), 1.95− 1.85 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8): δ (ppm) 180.6 (d, 1JCRh = 50.5 Hz, C2), 127.1 (s, C4), 119.1 (s, C5), 85.5 (d, 1JCRh = 7.7 Hz, cod-CH), 83.6 (d, 1JCRh = 8.4 Hz, cod-CH), 80.4 (d, 1JCRh = 13.2 Hz, cod-CH), 77.0 (d, 1JCRh = 11.4 Hz, codCH), 35.6 (s, C6), 32.4 (s, cod-CH2), 32.2 (s, cod-CH2), 31.9 (s, cod-CH2), 31.8 (s, cod-CH2). HRMS (ESI, positive ions) m/z: 584.0518 [3]+ (calcd for [3]+ 584.0894). 4.4. General Procedure for the Synthesis of Complexes [4] and [5]. Compound 1 (32.0 mg, 0.20 mmol) was dissolved in THF (5 mL) and the solution was cooled to −78 °C. Then, n-BuLi (0.2 mmol, 0.13 mL of a 1.6 M solution in n-hexane) was added. The reaction mixture was stirred for 2 h at −78 °C. Subsequently, [Ir(cod)(μ-Cl)]2 (0.1 mmol, 67.2 mg) or [Rh(cod)(μ-Cl)]2 (0.10 mmol, 49.3 mg) was added and the solution was stirred for 12 h while it was allowed to warm up to ambient temperature. The solvent was removed in vacuo and the resulting residue was washed with methanol (2 × 5 mL) and dried under reduced pressure. 4.4.1. Characterization of Complex [4]. Complex [4] was obtained as a red solid. Yield: 41 mg, 45 mmol, 45%. 1H NMR (400 MHz, THF-d8): δ (ppm) 7.40 (s, 1H, H5), 6.85 (s, 1H, H8), 4.82 (s br, 1H, cod-CH), 4.43 (s br, 1H, cod-CH), 3.85 (s br, 1H, cod-CH), 3.80 (s, 3H, H12), 3.58 (s br, 1H, cod-CH), 2.68−2.60 (m, 1H, cod-CH2), 2.49−2.41 (m, 2H, cod-CH2), 2.36 (s, 1H, cod-CH2), 2.25 (s, 3H, H11), 2.22 (s, 3H, H10), 2.20−2.18 (m, 1H, cod-CH2), 2.15−2.13 (m, 1H, cod-CH2), 1.80−1.76 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8): δ (ppm) 185.5 (C2), 142.2 (C4), 136.9 (C9), 129.0 (C6), 128.7 (C7), 115.6 (C5), 110.1 (C8), 74.4 (codCH), 72.2 (cod-CH), 62.8 (cod-CH), 61.0 (cod-CH), 33.4 (C12), 33.0 (cod-CH2), 32.9 (cod-CH2), 32.8 (cod-CH2), 32.7 (cod-CH2), 20.3 (C11), 20.1 (C10). MS (MALDI-TOF, matrix DCTB) m/z: 918 [4]+. 4.4.2. Characterization of Complex [5]. Complex [5] was obtained as an orange solid. Yield: 26 mg, 35 mmol, 35%. 1H NMR (400 MHz, THF-d8): δ (ppm) 7.36 (s, 1H, H5), 6.77 (s, 1H, H8), 5.08 (s br, 1H, cod-CH), 4.79 (s br, 1H, cod-CH), 4.50 (s br, 1H, cod-CH), 4.04 (s br, 1H, cod-CH), 3.82 (s, 3H, H12), 2.90−2.80 (m, 1H, cod-CH2), 2.78−2.71 (m, 1H, cod-CH2), 2.69−2.60 (m, 1H, cod-CH2), 2.63−2.54 (m, 1H, cod-CH2), 2.37−2.29 (m, 2H, codCH2), 2.25 (s, 3H, H11), 2.19 (s, 3H, H10), 2.08−2.02 (m, 1H, codCH2), 2.02−1.95 (m, 1H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8): δ (ppm) 192.5 (d, 1JCRh = 49.8 Hz, C2), 143.6 (s, C4), 136.5 (s, C9), 128.1 (s, C6), 127.6 (s, C7), 115.0 (s, C5), 109.2 (s, C8), 87.2 (d, 1JCRh = 8.4 Hz, cod-CH), 86.6 (d, 1JCRh = 7.1 Hz, codCH), 78.2 (d, 1JCRh = 10.2 Hz, cod-CH), 78.1 (d, 1JCRh = 8.9 Hz, codCH), 33.4 (s, C12), 32.2 (s, 2 × cod-CH2), 31.9 (s, cod-CH2), 31.7 (s, cod-CH2), 20.3 (s, C11), 20.1 (s, C10). HRMS (ESI, positive ions) m/z: 531.1989 [[5]−Rh(cod) + 2H]+ (calcd for [[5]−Rh(cod) + 2H]+ 531.1995). 4.5. Synthesis of the Complex Mixture [4]−[6]. Compound 1 (32.0 mg, 0.20 mmol) was dissolved in THF (5 mL) and cooled to −78 °C. Then, n-BuLi (0.2 mmol, 0.13 mL of a 1.6 M solution in nhexane) was added. The reaction mixture was stirred for 2 h at −78 °C. Subsequently, [Ir(cod)(μ-Cl)]2 (33.6 mg, 0.05 mmol) and [Rh(cod)(μ-Cl)]2 (24.7 mg, 0.05 mmol) were added and the solution was stirred for 12 h while it was allowed to warm up to ambient temperature. The solvent was removed in vacuo and the resulting residue was washed with methanol (2 × 5 mL) and dried under reduced pressure. The mixture of complexes [4]−[6] was obtained as

a red solid. Yield: 64 mg (of the complex mixture). 1H NMR (400 MHz, THF-d8, only resonances for the main component [6] are listed): δ (ppm) 7.39 (s, 1H, H5), 6.81 (s, 1H, H8), 5.13−5.02 (m, 1H, cod-CH), 4.46−4.40 (m, 1H, cod-CH), 4.02−3.92 (m, 2H, codCH), 3.78 (s, 3H, H12), 2.89−2.80 (m, 1H, cod-CH2), 2.64−2.54 (m, 2H, cod-CH2), 2.50−2.42 (m, 1H, cod-CH2), 2.27 (s, 3H, H11), 2.21 (s, 3H, H10), 2.19−2.14 (m, 2H, cod-CH2), 2.07−2.01 (m, 1H, cod-CH2), 2.00−1.93 (m, 1H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8, only resonances for the main component [6] are listed): δ (ppm) 188.1 (s, C2), 143.4 (s, C4), 137.1 (s, C9), 128.7 (s, C6), 128.2 (s, C7), 115.6 (s, C5), 109.9 (s, C8), 78.9 (d, 1JCRh = 3.9 Hz, cod-CH), 78.7 (d, 1JCRh = 2.6 Hz), 73.6 (s, cod-CH), 72.0 (s, codCH), 33.24 (s, cod-CH2), 33.22 (s, C12), 32.8 (s, cod-CH2), 32.3 (s, cod-CH2), 32.0 (s, cod-CH2), 20.5 (s C11), 20.2 (s, C10). HRMS (ESI, positive ions) m/z: 830.2405 [6]+ (calcd for [6]+ 830.2409). 4.6. Synthesis of Complex [7]. Compound 1 (16 mg, 0.10 mmol) was dissolved in THF (5 mL) and the solution was cooled to −78 °C. Then, n-BuLi (0.13 mmol, 0.08 mL of a 1.6 M solution in nhexane) was added. The reaction mixture was stirred for 2 h at −78 °C. Then, [Ir(cod)(μ-Cl)]2 (16.8 mg, 0.025 mmol) was added and the solution was stirred over night while it was allowed to warm up to ambient temperature. The solvent was removed in vacuo and the solid residue was dissolved in THF (2 mL). Addition of n-pentane led to precipitation of a white solid. The suspension was filtered through Celite and the solvent was removed from the filtrate under reduced pressure to give complex [7] as a reddish solid. Yield: 23 mg, 0.037 mmol, 74%. 1H NMR (400 MHz, THF-d8): δ (ppm) 6.93 (s, 1H, H5), 6.80 (s, 1H, H8), 3.93 (s, 3H, H12), 3.91 (m, 1H, cod-CH), 3.58 (m, 1H, cod-CH), 2.29 (m, 1H, cod-CH2), 2.22 (s, 3H, H11), 2.20 (s, 3H, H10), 2.05 (m, 1H, cod-CH2), 1.84 (m, 1H, cod-CH2), 1.72 (m, 1H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8): δ (ppm) 192.6 (s, C2), 144.0 (s, C4), 134.6 (s, C9), 123.8 (C6), 123.1 (s, C7), 113.3 (s, C5), 105.6 (s, C8), 68.9 (s, cod-CH), 67.0 (s, codCH), 30.3 (s, C112), 29.7 (s, cod-CH2), 29.4 (s, cod-CH2), 17.42 (s, C11), 17.40 (s, C10). 7Li NMR (155 MHz, THF-d8): δ (ppm) 1.6. MS (ESI, positive ions) m/z: 621.2566 [7−Li + 2H]+ (calcd for [7− Li + 2H]+ 621.2570). MS (MALDI-TOF, matrix DCTB) m/z: 621 [7−Li + 2H]+. 4.7. Synthesis of Complex [6] from [7]. A sample of complex [7] (27.7 mg, 0.04 mmol) was dissolved in THF (5 mL) and [Rh(cod)(μ-Cl)]2 (10.8 mg, 0.02 mmol) was added as a solid. The reaction mixture was stirred at ambient temperature for 12 h. The solvent was removed in vacuo and the solid residue was washed with methanol (2 × 5 mL) and dried under reduced pressure to give complex [6] as a reddish solid. Yield: 29.9 mg, 0.036 mmol, 90%. 1H NMR (400 MHz, THF-d8): δ (ppm) 7.38 (s, 1H, H5), 6.81 (s, 1H, H8), 5.11−5.03 (m, 1H, cod-CH), 4.47−4.40 (m, 1H, cod-CH), 4.02−3.92 (m, 2H, cod-CH), 3.78 (s, 3H, H12), 2.87−2.81 (m, 1H, cod-CH2), 2.62−2.55 (m, 2H, cod-CH2), 2.50−2.42 (m, 1H, codCH2), 2.27 (s, 3H, H11), 2.21 (s, 3H, H10), 2.19−2.14 (m, 2H, codCH2), 2.08−2.01 (m, 1H, cod-CH2), 1.99−1.93 (m, 1H, cod-CH2). 13 C{1H} NMR (101 MHz, THF-d8): δ (ppm) 188.1 (s, C2), 143.4 (s, C4), 137.1 (s, C9), 128.8 (s, C6), 128.2 (s, C7), 115.6 (s, C5), 109.9 (s, C8), 78.9 (d, 1JCRh = 3.9 Hz, cod-CH), 78.7 (d, 1JCRh = 2.6 Hz, cod-CH), 73.6 (s, cod-CH), 72.1 (s, cod-CH), 33.24 (s, cod-CH2), 33.22 (s, C12), 32.8 (s, cod-CH2), 32.3 (s, cod-CH2), 32.0 (s, codCH2), 20.5 (s C11), 20.2 (s, C10). MS (MALDI-TOF, matrix DCTB) m/z: 831 [[6] + H]+. 4.8. General Procedure for the Synthesis of Complexes [8] and [9]. Lithium diisopropylamide (16 mg, 0.15 mmol) was dissolved in THF (4 mL) and the solution was cooled to −78 °C. The caffeine (19.8 mg, 0.10 mmol) was added. The reaction mixture was stirred for 2 h at −78 °C. Subsequently, [Ir(cod)(μ-Cl)]2 (33.6 mg, 0.05 mmol) or [Rh(cod)(μ-Cl)]2 (24.7 mg, 0.05 mmol) was added and the solution was stirred for 12 h while it was allowed to warm up to ambient temperature. The solvent was removed in vacuo and the solid residue was washed with methanol (2 × 2 mL) and dried under reduced pressure. 4.8.1. Characterization of Complex [8]. Yield: 14 mg, 0.014 mmol, 28%, ratio [8H−H]/[8H−T] 50:50. 1H NMR (400 MHz, THFF


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Organometallics d8 for [8H−H]): δ (ppm) 4.67 (m, 2H, cod-CH), 4.22 (s, 6H, C-11), 4.04 (m, 2H, cod-CH), 4.03 (s, 6H, H-12), 3.60 (m, 2H, cod-CH), 3.56 (m, 2H, cod-CH), 3.15 (s, 6H, H-10), 2.50 (m, 2H, cod-CH2), 2.46 (m, 2H, cod-CH2), 2.45 (m, 2H, cod-CH2), 2.43 (m, 2H, codCH2), 2.30 (m, 2H, cod-CH2), 2.28 (m, 2H, cod-CH2), 1.76 (m, 2H, cod-CH2), 1.69 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THFd8 for [8H−H]): δ (ppm) 182.6 (C-8), 154.6 (C-6), 152.6 (C-2), 147.4 (C-4), 110.6 (C-5), 78.7 (cod-CH), 74.7 (cod-CH), 63.2 (cod-CH), 56.2 (cod-CH), 36.9 (C-12), 33.0 (cod-CH2), 32.8 (C-11), 32.5 (cod-CH2), 32.4 (cod-CH2), 32.1 (cod-CH2), 27.6 (C-10). 1H NMR (400 MHz, THF-d8 for [8H−T]): δ (ppm) 4.53 (m, 2H, cod-CH), 4.17 (s, 6H, H-11), 4.17 (m, 2H, cod-CH), 4.09 (s, 6H, H-12), 3.92 (m, 2H, cod-CH), 3.56 (m, 2H, cod-CH), 3.15 (s, 6H, H-10), 2.60 (m, 2H, cod-CH2), 2.53 (m, 2H, cod-CH2), 2.42 (m, 2H, cod-CH2), 2.32 (m, 2H, cod-CH2), 2.21 (m, 2H, cod-CH2), 2.10 (m, 2H, codCH2), 1.86 (m, 2H, cod-CH2), 1.76 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8 for [8H−T]): δ (ppm) 178.6 (s, C-8), 154.6 (s, C-6), 152.6 (s, C-2), 147.4 (s, C-4), 110.7 (s, C-5), 81.2 (s, codCH), 81.1 (s, cod-CH), 78.7 (s, cod-CH), 74.7 (s, cod-CH), 36.1 (s, C-12), 35.6 (s, cod-CH2), 33.6 (s, cod-CH2), 32.6 (s, cod-CH2), 32.5 (s, C-11), 29.3 (s, cod-CH2), 27.6 (s, C-10). MS (MALDI-TOF, matrix DCTB) m/z (%): 986 (100) [8]+. 4.8.2. Characterization of Complex [9]. Yield: 16 mg, 0.020 mmol, 40%, ratio [9H−H]/[9H−T] 30:70. 1H NMR (400 MHz, THFd8 for [9H−H]): δ (ppm) 5.01 (m, 2H, cod-CH), 4.66 (m, 2H, codCH), 4.33 (s, 6H, H-11), 4.32 (m, 2H, cod-CH), 4.06 (m, 2H, codCH), 4.03 (s, 6H, H-12), 3.11 (s, 6H, H-10), 2.76 (m, 2H, cod-CH2), 2.73 (m, 2H, cod-CH2), 2.65 (m, 2H, cod-CH2), 2.62 (m, 2H, codCH2), 2.40 (m, 2H, cod-CH2), 2.40 (m, 2H, cod-CH2), 1.98 (m, 2H, cod-CH2), 1.97 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THFd8 for [9H−H]): δ (ppm) 188.7 (d, 1JRhC = 50.0 Hz, C-8), 153.8 (s, C6), 152.5 (s, C-2), 148.7 (s, C-4), 110.2 (s, C-5), 90.8 (d, 1JRhC = 8.3 Hz, cod-CH), 88.1 (d, 1JRhC = 7.0 Hz, cod-CH), 81.7 (d, 1JRhC = 13.5 Hz, cod-CH), 80.1 (d, 1JRhC = 11.8 Hz, cod-CH), 36.7 (s, C-12), 33.0 (s, cod-CH2), 32.1(s, C-11), 32.0 (s, cod-CH2), 31.7 (s, cod-CH2), 30.0 (s, cod-CH2), 27.3 (s, C-10). 1H NMR (400 MHz, THF-d8 for [9H−T]): δ (ppm) 4.73 (m, 2H, cod-CH), 4.66 (m, 2H, cod-CH), 4.33 (s, 6H, H-11), 4.54 (m, 2H, cod-CH), 4.06 (s, 6H, H-12), 3.96 (m, 2H, cod-CH), 3.12 (s, 6H, H-10), 2.76 (m, 2H, cod-CH2), 2.69 (m, 2H, cod-CH2), 2.63 (m, 2H, cod-CH2), 2.55 (m, 2H, cod-CH2), 2.26 (m, 2H, cod-CH2), 2.16 (m, 2H, cod-CH2), 2.14 (m, 2H, codCH2), 2.14 (m, 2H, cod-CH2). 13C{1H} NMR (101 MHz, THF-d8 for [9H−T]): δ (ppm) 183.2 (d, 1JRhC = 48.2 Hz, C-8), 154.0 (s, C-6), 152.6 (s, C-2), 148.5 (s, C-4), 110.2 (s, C-5), 95.8 (d, 1JRhC = 6.9 Hz, cod-CH), 95.0 (d, 1JRhC = 7.5 Hz, cod-CH), 75.9 (d, 1JRhC = 14.4 Hz, cod-CH), 72.2 (d, 1JRhC = 12.1 Hz, cod-CH), 35.8 (s, C-12), 33.0 (s, cod-CH2), 32.9 (s, cod-CH2), 32.5 (s, C-11), 32.5 (s, cod-CH2), 29.8 (s, cod-CH2), 27.3 (s, C-10). MS (MALDI-TOF, matrix DCTB) m/z (%): 808 (75) [9]+, 700 (100) [9−cod]+. 4.9. X-ray Diffraction Studies. X-ray diffraction data were collected at T = 153 K for [3], [5]·2C6H5F and [6]·2C6H5F and at T = 100 K for [9H−T] using a Bruker AXS APEX II charge-coupled device diffractometer equipped with a microsource using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). Semiemperical multiscan absorption corrections were applied to all data sets.18 Structure solutions were found with SHELXT (intrinsic phasing)19a and were refined with SHELXL19b against |F2| of all data using first isotropic and later anisotropic thermal parameters for all nonhydrogen atoms. Hydrogen atoms were added to the structure models on calculated positions. 4.9.1. Crystallographic Data for [3]. Crystals suitable for an X-ray diffraction study were obtained by cooling a concentrated solution of [3] in fluorobenzene. Formula C24H34N4Rh2, M = 584.37, orange plates, 0.173 × 0.097 × 0.004 mm3, triclinic, space group P1̅, a = 9.8190(2), b = 9.9553(2), c = 12.5831(2) Å, α = 76.9880(10), β = 89.2410(10), γ = 70.0240(10), V = 1123.60(4) Å3, Z = 2, ρcalcd = 1.727 g·cm−3, μ = 1.487 mm−1, 20 499 intensities measured in the range 5.7° ≤ 2Θ ≤ 62.0°, 7005 independent intensities (Rint = 0.0289), 5885 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.824 ≤ T ≤ 0.890), refinement of 294

parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0292, wR = 0.0686 [I ≥ 2σ(I)], R = 0.0381, wR = 0.0715 (all data). The asymmetric unit contains one formula unit. 4.9.2. Crystallographic Data for [5]·2C6H5F. Crystals suitable for an X-ray diffraction study were obtained by cooling a concentrated solution of [5] in fluorobenzene. Formula C48H56N4F2Rh2, M = 932.78, red blocks, 0.22 × 0.14 × 0.12 mm3, triclinic, space group P1̅, a = 10.5486(2), b = 11.5988(2), c = 17.9751(3) Å, α = 85.6760(10), β = 80.7610(10), γ = 70.8570(10), V = 2050.10(6) Å3, Z = 2, ρcalcd = 1.511 g·cm−3, μ = 0.853 mm−1, 38 697 intensities measured in the range 6.0° ≤ 2Θ ≤ 63.9°, 13 127 independent intensities (Rint = 0.0393), 10 400 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.857 ≤ T ≤ 0.905), refinement of 489 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0399, wR = 0.1031 [I ≥ 2σ(I)], R = 0.0554, wR = 0.1132 (all data). The asymmetric unit contains one formula unit of complex [5] and two molecules of fluorobenzene. The fluorine atom of one molecule of fluorobenzene is disordered over two positions (occupancies 80:20). The entire second fluorobenzene molecule is disordered over two positions (occupancies 60:40) and the positional parameters for this molecule were refined with isotropic displacement parameters. No hydrogen positions were calculated for carbon atoms refined with isotropic displacement parameters. 4.9.3. Crystallographic Data for [6]·2C6H5F. Crystals suitable for an X-ray diffraction study were obtained by cooling a concentrated solution of [6] in fluorobenzene. Formula C48H56F2Ir1.33N4Rh0.67, M = 1051.54, purple plates, 0.443 × 0.300 × 0.113 mm3, triclinic, space group P1̅, a = 10.5666(2), b = 11.5860(2), c = 17.9775(4) Å, α = 85.8891(10), β = 80.7144(10), γ = 70.7500(10), V = 2050.20(7) Å3, Z = 2, ρcalcd = 1.703 g·cm−3, μ = 4.630 mm−1, 37 179 intensities measured in the range 5.9° ≤ 2Θ ≤ 61.1°, 12 420 independent intensities (Rint = 0.0347), 11 675 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.002 ≤ T ≤ 0.020), refinement of 559 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0444, wR = 0.1162 [I ≥ 2σ(I)], R = 0.0460, wR = 0.1182 (all data). The asymmetric unit contains 1 formula unit of complex [6], and two molecules of fluorobenzene. Complex [6] had a Rh/Ir substitutional disorder (occupancies Rh/Ir, 67:33) and the positional and anisotropic displacement parameters of the disordered Rh and Ir atoms were constrained to be the same. The fluorine atoms of both fluorobenzene molecules are disordered over two positions. 4.9.4. Crystal Data for Complex [9H−T]·C4H8O. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of nhexane into a saturated solution of the complex mixture [9H−T]/ [9H−H] in THF. Formula C36H50N8O5Rh2, M = 880.66, orange needles, 0.360 × 0.260 × 0.100 mm3, triclinic, space group P1̅, a = 12.1104(5), b = 12.4776(5), c = 13.3350(5) Å, α = 76.316(2), β = 63.622(2), γ = 87.914(2), V = 1748.52(12) Å3, Z = 2, ρcalcd = 1.673 g· cm−3, μ = 1.000 mm−1, 47 891 intensities measured in the range 6.5° ≤ 2Θ ≤ 59.2°, 9746 independent intensities (Rint = 0.0325), 8651 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.673 ≤ T ≤ 0.745), refinement of 490 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0274, wR = 0.0604 [I ≥ 2σ(I)], R = 0.0330, wR = 0.0627 (all data). The asymmetric unit contains one formula unit of [9H−T] and one THF molecule.

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CCDC 1895429−1895432 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. CCDC 1895429−1895432 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

F. Ekkehardt Hahn: 0000-0002-2807-7232 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027). REFERENCES

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Homo- and Heterodinuclear Head-to-Head or Head-to-Tail

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