Preparation of Rhodium (III) Di-NHC Chelate Complexes Featuring

Article pubs.acs.org/Organometallics

Preparation of Rhodium(III) Di-NHC Chelate Complexes Featuring Two Different NHC Donors via a Mild NaOAc-Assisted C−H Activation Francisco Aznarez,† Pablo J. Sanz Miguel,‡ Tristan T. Y. Tan,† and F. Ekkehardt Hahn*,† †

Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany ‡ Departamento de Quı ́mica Inorgánica-ISQCH, Universidad de Zaragoza-CSIC, Plaza San Francisco S/N, 50009 Zaragoza, Spain S Supporting Information *

ABSTRACT: Two molecules of 4,5-dichloroimidazole react with dibromomethane to give the diimidazole 1. Reaction of 1 with iodomethane or 2-fluorobenzyl bromide yields the monoalkylated imidazolium/imidazole salts [2]I and [3]Br, respectively. Salt [2]I reacts with Ag2O followed by transmetalation to [Rh(Cp*)(Cl)2]2 to give the RhIII-NHC complex [4], bearing an NHC ligand with a pendant imidazole group. The pendant imidazole can be deprotonated using NaOAc to yield complex [5], bearing a doubly C-metalated C∧C chelate ligand. Reaction of [2]I or [3]Br with NaOAc and [Rh(Cp*)(Cl)2]2 yields the C∧C chelate complexes [7] and [9], respectively, in a one-pot reaction. The imine ring nitrogen atom in complexes [7] and [9] can be protonated using HBF4·Et2O to give complexes [8]BF4 and [11]BF4, each bearing a C(NHC)∧C(pNHC) chelate ligand (pNHC = protic NH,NR-NHC ligand). Alkylation of the imine ring nitrogen atom in [9] yields complex [10]BF4, bearing a unique unsymmetrical (NHC)∧C(NHC′) chelate ligand.

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INTRODUCTION Over the last two decades, N-heterocyclic carbenes (NHCs) have been shown to be versatile ligands in organometallic chemistry.1 Due to their distinct electronic and steric properties2 they have found various applications as spectator ligands in catalytically active complexes,3 as organocatalysts,4 and for additional uses.5 The vast majority of NHCs are prepared from azolium cations featuring two alkylated or arylated ring nitrogen atoms. Some derivatives of NHCs such as mesoionic or cyclic alkylaminocarbenes (CAACs) have also been described.6 We developed an interest in complexes bearing protic NHC (pNHC) ligands which possess one (NH,NR-NHC) or two (NH,NH-NHC) protonated instead of alkylated ring nitrogen atoms.7 Initially, such complexes were obtained by the metaltemplate-assisted cyclization of suitably β-functionalized isocyanides.8 More recently, we found that neutral 2halidoazoles oxidatively add to suitable transition metals with formation of negatively charged azolylato ligands, which can be N-protonated to give the complexes A, bearing protic NHC ligands (Figure 1, top).9 The more conventional oxidative addition of azolium cations to transition metals has previously been described for C2−X, C2−H, and C2−R bonds.10 The related C2−H oxidative addition of neutral azoles has so far been observed only once to give the RhI complex B.11 However, neutral C2−H azoles, if substituted at one of the ring nitrogen atoms with a donor group, can be converted into coordinated pNHC∧donor chelate ligands in complexes of types C,12 D,13 E,14 and F (Figure 1).15 It has been proposed that this “formal tautomerization”, particularly in the absence of © XXXX American Chemical Society

a base for the C2−H deprotonation of the azole, proceeds by an initial coordination of the N-tethered donor to the metal center, followed by the oxidative addition of the C2−H bond and finally reductive elimination of a proton with protonation of the ring nitrogen atom.9c,16 This proposal has so far not been confirmed experimentally. In fact, we know of only one example where the initial reaction product of the C2−H oxidative addition, namely the hydrido complex, has been isolated.17 Alternative synthetic protocols and mechanistic proposals for the formation of complexes bearing protic NH,NR-NHC ligands have been put forward.18 Complexes bearing protic pNHC ligands are promising catalysts for selected transformations, as the N−H group of the pNHC ligand can function as a molecular recognition unit.19 In addition, the N−H unit in pNHC complexes can potentially be employed in cooperative catalytic transformations, as has been described for complexes featuring primary or secondary amines.20 In this contribution we describe the preparation of methylene-linked imidazolium/imidazole cations and the metalation of the imidazolium moiety by the classical Ag2O method. Subsequently, the activation of the C2−H bond of the imidazole leading to complexes of type G (Figure 1, bottom) with a chelating coordinated (NHC)∧C(pNHC) ligand was investigated. Received: December 5, 2015

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DOI: 10.1021/acs.organomet.5b00993 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

reaction of 1 with 2 equiv of iodomethane or 10 equiv of 2fluorbenzyl bromide, respectively, in refluxing CH3CN for 24 h. The monomethylation of 1 to give an imidazolium/imidazole derivative has been described previously,21 and this property was the reason for using the 4,5-dichloroimidazole for the preparation of the ligand precursors. The pale yellow solid [2]I was obtained in a moderate yield of 65%, while [3]Br was isolated in 40% yield. The 2-fluorobenzyl substituent in [3]Br was introduced, since the resulting imidazolium/imidazole compound precipitates from the reaction mixture and thus can be isolated easily. Both compounds were characterized by NMR spectroscopy and by high-resolution electrospray ionization (HR-ESI) mass spectrometry. The NMR data for [2]I are essentially identical with those reported in the literature (considering the difference in solvents used for spectral recording).21 The resonance for the H5 proton in [3]Br (see numbering in Scheme 1) appeared as a singlet at δ 10.08 ppm, which is consistent with the chemical shifts reported for related imidazolium salts,21,22 and the resonance for the C5 carbon atom in the 13C{1H} NMR spectrum was detected at δ 138.2 ppm, also falling in the expected range.21,22 In addition, the HR-ESI mass spectrum (positive ions) of [3]Br featured a strong peak at m/z 394.96127 (calcd for [3]+ m/z 394.96150). Reaction of 2 equiv of [2]I with 1 equiv of Ag2O with exclusion of light in CH3CN for 4 h yielded an AgI-NHC complex (presumably by selective metalation of the imidazolium group),23 and this complex was then added without isolation to a solution of 1 equiv of [Rh(Cp*)(Cl)2]2 in CH3CN. Complex [4] was thus obtained by transmetalation from silver to rhodium in 85% yield as a dark red solid (Scheme 2). It was characterized by NMR spectroscopy (1H, 13C{1H}) and by HR-ESI mass spectrometry. In the 1H NMR spectrum of [4], the singlet corresponding to the H5 proton in [2]I was missing, while the resonance for the H1 proton was still observed at δ 7.65 ppm (for atom numbering see Scheme 1). The resonance for the H4 protons was not detected at ambient temperature. However, lowtemperature NMR spectroscopy (T = 255 K) revealed two doublets at δ 7.22 and 6.00 ppm (2JH,H = 12.6 Hz) indicative of a restrained rotation about the N−C4 bond, which resulted in diastereotopic H4 protons (Figure 2). The difference in chemical shifts between the two H4 doublets (Δδ = 1.22 ppm) is remarkably large. In the 13C{1H} NMR spectrum, the signal corresponding to the C(NHC) carbon atom C5 appeared as a doublet at δ 177.5 ppm (1JC,Rh = 58.5 Hz) due to the coupling to 103Rh. As expected, the resonance of C1 was observed as a singlet at δ 135.6 ppm, only slightly shifted upfield from the C1 resonance in [2]I (δ 137.5 ppm). These observations indicate that indeed only the mono-NHC-RhIII complex [4] had formed. The conclusions drawn from NMR spectroscopy were confirmed by an X-ray diffraction analysis with crystals of [4], obtained by slow vapor diffusion of Et2O into a saturated CH2Cl2 solution of [4] at ambient temperature. The diffraction analysis showed that [4], in the solid state, adopts the classical piano-stool geometry (Figure 3). Comparable bond distances and angles fall in the range previously observed for related NHC-RhIII compounds (Figure 3).24 The two diazaheterocycles, however, feature different metric parameters. The NHC ligand shows two almost identical N−C5 separations (N3−C5 1.379(3) Å and N4−C5 1.361(3) Å) and an N3−C5−N4 angle of 103.8(2)°, which is typical for imidazolin-2-ylidenes. The C1−N bond distances found for the pendant imidazole group

Figure 1. Synthesis of different complexes bearing pNHC ligands.

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RESULTS AND DISCUSSION The azolium/azole ligand precursors [2]I and [3]Br were obtained following a two-step procedure starting from 4,5dichloroimidazole (Scheme 1 and the Supporting Information). Treatment of 4,5-dichloroimidazole with KOH and CH2Br2 at 120 °C for 12 h gave compound 1 as a light brown solid in 82% yield.21 The salts [2]I and [3]Br were subsequently obtained by Scheme 1. Synthesis of Ligand Precursors [2]I and [3]Br

B


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Organometallics Scheme 2. Synthesis of Complexes [4] and [5]

followed by reductive elimination of a proton and protonation of the ring nitrogen atom. Such an oxidative addition would naturally be hampered by the +3 oxidation state of rhodium, although a related reaction was recently observed with IrIII.13 However, all attempts to generate a RhIII complex with a C(NHC)∧C(pNHC) chelate ligand by heating of [4] in different solvents (CH3CN, CH2Cl2, CH3OH) failed. Alternatively, treatment of [4] with the weak base NaOAc at ambient temperature gave [5] in 89% yield (Scheme 2), apparently by deprotonation of the N−CH−N group. Complex [5] was identified by NMR spectroscopy. The 1H NMR spectrum in CD2Cl2 showed the signals for the diastereotopic H4 protons as two doublets at δ 6.03 and 5.15 ppm both with 2JH,H = 12.9 Hz coupling (for atom numbering see Scheme 2). A resonance for the H1 proton was no longer detected. The 13C{1H} NMR spectrum exhibited two doublets in the downfield region at δ 176.7 ppm (1JC,Rh = 54.9 Hz) and δ 161.5 ppm (1JC,Rh = 47.9 Hz) for the carbene carbon atom C5 and C1, respectively. These observations confirm the formation of an unprecedented chelate ligand composed of a classical NHC and an N-deprotonated NHC ligand forming a sixmembered metallacycle. The difference in the chemical shifts for the C(NHC) resonances nicely reflects the differing electronic situations within the heterocycles, with the more electron rich diazaheterocycle containing atom C1. The formation of complex [5] was confirmed by an X-ray diffraction study. Single crystals of [5]·1.5CH2Cl2 were obtained by slow evaporation of the solvents from a CH2Cl2/ hexane solution of [5]. The asymmetric unit contains two molecules of [5], one disordered CH2Cl2 molecule, and 1/2 CH2Cl2 molecule on a special position. The structure analysis shows the RhIII atom surrounded in a pseudooctahedral fashion by the two carbene carbon atoms (C1 and C5), a chlorido ligand, and the midpoint of the Cp* ligand (Figure 4). The Rh1−C(NHC) distances (2.026(2) Å) are identical in molecule 1 (and very close in magnitude in molecule 2). The bite angle C1−Rh1−C5 of the dicarbene ligand measures 86.06(10)°. The N3−C5−N4 bond angle of 104.4(2)° found in the classical NHC ligand is significantly smaller than the corresponding N1−C1−N2 angle in the N-deprotonated NHC ligand (109.4(2)°), demonstrating the electronic differences between the NHC donors in spite of the similar Rh1−C1/C5 bond distances. Noticeably, complex [5] is chiral at the metal center. Since we did not observe the oxidative addition of the C2−H bond of the imidazole in [4], a reaction which was observed multiple times for N-phosphine tethered benzimidazoles and RhI,16 we decided to study the reactivity of [2]I with RhI instead of RhIII complexes. Compound [2]I was therefore reacted with Ag2O.23 Transmetalation of the NHC ligand to rhodium(I) using [Rh(COD)Cl]2 gave complex [6] in a good yield of 79% (Scheme 3).

Figure 2. Section of the VT 1H NMR spectra of complex [4] in CD2Cl2.

Figure 3. Molecular structure of complex [4]. Displacement ellipsoids are shown at the 50% probability level, and hydrogen atoms (except for H1) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh−Cl5 2.4165(7), Rh−Cl6 2.4300(7), Rh−C5 2.057(2), Rh−CCp* range 2.142(3)−2.206(3), N1−C1 1.311(3), N2−C1 1.372(3), N3−C5 1.379(3), N4−C5 1.361(3); Cl5−Rh− Cl6 88.28(2), Cl5−Rh−C5 92.85(7), Cl6−Rh−C5 95.64(7), C1− N1−C2 104.6(2), C1−N2−C3 105.2(2), C5−N3−C7 110.6(2), C5− N4−C6 111.0(2), N1−C1−N2 112.5(2), N3−C5−N4 103.8(2).

differ significantly (N1−C1 1.311(3) Å and N2−C1 1.372(3) Å), and the N1−C1−N2 angle is enlarged to 112.5(2)°, in comparison to the N3−C5−N4 angle of the NHC ligand (103.8(2)°). Furthermore, the C−N−C angle at the unsubstituted ring nitrogen atom N1 is much smaller (C1−N1−C2 104.6(2)°) than the C−N−C angles within the ring of the NHC ligand (C5−N3−C7 110.6(2)° and C5−N4−C6 111.0(2)°). Next, we attempted to C1-metalate the free imidazole group of [4]. In principle, complex [4] consists of an NHCsubstituted imidazole where the NHC is coordinated to RhIII. Complexes featuring a donor-tethered imidazole are believed to be intermediates in the formation of complexes bearing protic NHC ligands such as B−F (Figure 1). We therefore tried to initiate tautomerization of the imidazole to a protic NHC and coordination of this pNHC to the RhIII center. Alternatively, the formation of a RhIII complex with a chelating bidentate C(NHC)∧C(pNHC) ligand from [4] could proceed by the oxidative addition of the C1−H bond in [4] to rhodium, C


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Organometallics

Figure 4. Molecular structure of [5] in [5]·1.5CH2Cl2. Only one of the two molecules in the asymmetric unit is shown. Displacement ellipsoids are shown at the 50% probability level, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh1−Cl5 2.4067(7), Rh1−C1 2.026(2), Rh1−C5 2.026(2), Rh1−CCp* range 2.153(3)−2.246(2), N1−C1 1.336(3), N2−C1 1.379(3), N3−C5 1.364(3), N4−C5 1.353(3); Cl5−Rh1−C1 89.99(7), Cl5−Rh1−C5 89.54(7), C1−Rh1−C5 86.06(10), C1− N1−C2 106.0(2), C1−N2−C3 108.1(2), C5−N3−C7 111.4(2), C5− N4−C6 110.4(2), N1−C1−N2 109.4(2), N3−C5−N4 104.4(2).

Figure 5. Molecular structure of complex [6] in [6]·CH2Cl2. Displacement ellipsoids are shown at the 50% probability level, and hydrogen atoms (except for H1) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh−Cl1 2.3986(5), Rh− C5 2.004(2), Rh−CCOD range 2.111(2)−2.233(2), N1−C1 1.316(3), N2−C1 1.365(2), N3−C5 1.368(2), N4−C5 1.348(2); Cl1−Rh−C5 88.50(5), C1−N1−C2 104.2(2), C1−N2−C3 105.8(2), C5−N3−C7 110.5(1), C5−N4−C6 110.8(1), N1−C1−N2 112.8(2), N3−C5−N4 104.6(2).

Scheme 3. Synthesis of RhI Complex [6]

Å; for [6], H1···Cl1 2.785 Å). The values of the chemical shift for the H1 proton in [4] (δ(H1) 7.65 ppm) and [6] (δ(H1) = 8.43 ppm), however, differ significantly. We assume that the reason for this behavior is the more linear intramolecular C1− H1···Cl1 angle of 166.5° in [6], allowing for a strong H···Cl hydrogen bond interaction, in comparison to a C1−H1···Cl5 angle of 114.4° for [4], where only a weak hydrogen bond interaction is possible. Subsequently, samples of complex [6] were stirred in different solvents at varying temperatures. In none of these procedures was the oxidative addition/tautomerization of the neutral imidazole group to give the coordinated pNHC ligand observed. Such a transformation proceeded readily for rhodium(I) complexes bearing phosphine-tethered benzimidazoles and yielded rhodium(I) complexes of types D and E (Figure 1).16 Alternatively, reaction of the di-NHC precursor [2]I with [Rh(Cp*)(Cl)2]2 in the presence of 2 equiv of NaOAc cleanly gave complex [7], bearing a doubly metalated C∧C chelate ligand in 92% yield in a one-pot reaction (Scheme 4). We assume that the imidazolium group is deprotonated first and coordinated to the rhodium(III) center. In a second reaction step, the imidazole is deprotonated and coordinated to the metal center, leading to the complex bearing a chelate ligand. Due to the different synthetic protocols employed, complexes [5] and [7] differ in the halido coligands. This difference influences the NMR spectra only marginally, as can be seen from the resonances for the C1 and C5 carbon atoms (for atom numbering see Scheme 4: [5], δ 176.7 ppm (d, 1JC,Rh = 54.9 Hz, C5), δ 161.5 ppm (d, 1JC,Rh = 47.9 Hz, C1); [7], δ 177.4 ppm (d, 1JC,Rh = 55.1 Hz, C5), δ 158.8 ppm (d, 1JC,Rh = 47.2 Hz, C1)). Protonation of the C-metalated imidazole in [7] would lead to a chelate ligand composed of a classical NHC ligand and a protic NHC ligand (pNHC). Protonation reactions of Cmetalated azolates have been described to proceed with mild acids such as NH4Cl and NH4BF4.13−16 Due to the presence of the chloro substituents at the heterocycles of complex [7], we selected a stronger acid for the protonation of the imine nitrogen atom. Complex [7] reacts cleanly with HBF4·Et2O (1.2 equiv) in CH2Cl2 to give complex [8]BF4, bearing an unprecedented NHC∧pNHC chelate ligand, in 96% yield.

The 1H NMR spectrum of [6] features the resonance for the H1 proton (for atom numbering see Scheme 3) at δ 8.43 ppm. The 13C{1H} NMR resonance for the carbene carbon atom C5 was observed as a doublet at δ 188.1 ppm (1JC,Rh = 54.9 Hz), while the resonance for the imidazole C1 carbon atom appeared as a singlet at δ 136.6 ppm, only slightly shifted from the equivalent resonance in the RhIII complex [4] (δ(C1) 135.6 ppm). Crystals of [6]·CH2Cl2 were obtained by slow evaporation of the solvent from a saturated dichloromethane solution of [6] at ambient temperature. The coordination geometry around the rhodium atom in [6] is best described as slightly distorted square planar (Figure 5). Comparable metric parameters of the NHC ligand in complexes [4] and [6] are very similar, as the only difference between the two complexes is the C5 coordination of the NHC to a {RhIII(Cp*)(Cl)2} unit for [4] and to a {RhI(COD)Cl} unit for [6]. Thus, a large N1−C1− N2 angle (112.8(2)°) and a small N3−C5−N4 angle (104.6(2)°) were also found in [6]. The major differences in the bond parameters between [4] and [6] are found for the bonds involving the rhodium atom. The Rh−C5 bond length is shorter in [6] (2.004(2) Å) than in [4] (2.057(2) Å). The same holds for the Rh−Cl bond lengths, where a shorter value (Rh− Cl1 2.3986(5) Å) was observed for [6] in comparison to the two Rh−Cl bonds in [4] (Rh−Cl5 2.4165(7) Å, Rh−Cl6 2.4300(7) Å). These differences are easily explained by the different coordination numbers and oxidation states of the rhodium atoms in [4] and [6]. Interestingly, the separation between the imidazole proton H1 and one of the chlorido ligands in [4] and [6] are very similar (for [4], H1···Cl5 2.779 D


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Organometallics Scheme 4. Synthesis of Complexes [7] and [8]BF4

a chelating fashion (Figure 6). Equivalent metric parameters for [9] and the related complex [5] (obtained in a two-step

The formation of [8]BF4 was confirmed by NMR spectroscopy. The 1H NMR spectrum showed the resonance for the N−H proton at δ 10.62 ppm. The resonances for the two C(NHC) atoms in [8]BF4 were detected at δ 172.7 ppm (d, 1 JC,Rh = 53.8 Hz, C5) and δ 168.5 ppm (d, 1JC,Rh = 52.6 Hz, C1) in the 13C{1H} NMR spectrum. Note that the protonation of the imine nitrogen atom in [7] to give [8]BF4 causes an upfield shift of the C5 resonance by about 5 ppm and a downfield shift of the resonance for C1 by about 10 ppm. A reactivity similar to that of [2]I was observed for the imidazolium/imidazole salt [3]Br. Reaction of [3]Br with NaOAc and [Rh(Cp*)(Cl)2]2 gave, in analogy to the formation of [7] (and [5]), the chelate complex [9] in 81% yield (Scheme 5). Complex [9] was identified by the two doublet resonances Scheme 5. Synthesis of Complexes [9], [10]BF4, and [11]BF4

Figure 6. Molecular structure of complex [9]. Displacement ellipsoids are shown at the 50% probability level, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh− Br 2.5292(12), Rh−C1 2.023(5), Rh−C5 2.019(5), Rh1−CCp* range 2.160(5)−2.253(5), N1−C1 1.339(6), N2−C1 1.375(6), N3−C5 1.365(6), N4−C5 1.359(6); Br−Rh−C1 88.46(14), Br−Rh−C5 89.37(13), C1−Rh−C5 85.4(2), C1−N1−C2 105.8(4), C1−N2−C3 107.7(4), C5−N3−C7 111.2(4), C5−N4−C6 110.6(4), N1−C1−N2 109.3(4), N3−C5−N4 104.3(4).

procedure, Figure 4) are identical within experimental error. As was observed for [5], the two diazaheterocycles in [9] feature slightly different metric parameters. As was previously observed with complex [7] and related complexes,13−16 the imine nitrogen atom found in one of the diazaheterocycles of [9] can also act as a base and nucleophile. The nucleophilicity and basicity of this nitrogen atom in [9] is, however, reduced owing to the chlorine substitution of the heterocycle. Thus, only rather active alkylation agents are capable to alkylate the imine nitrogen of [9]. Reaction of [9] with Et3OBF4 in CH2Cl2 gave complex [10]BF4 in 73% yield, featuring an C(NHC)∧C(NHC′) chelate ligand composed of two different classical NR,NR-NHC donors (Scheme 5). The formation of [10]BF4 was established by 1H and 13 C{1H} NMR spectroscopy and by HR-ESI mass spectrometry. In the 13C{1H} NMR spectrum two doublet resonances for the carbene carbon atoms are found at δ 173.6 ppm (1JC,Rh = 53.3 Hz, C5) and δ 171.9 ppm (1JC,Rh = 53.0 Hz, C1) (for atom numbering see Scheme 5). In contrast to the situation in [9], the C(NHC)∧C(NHC′) chelate ligand in [10]+ contains two classical NHC donors and thus the chemical shifts for the C(NHC) carbon atoms are very similar. The resonances for the ethyl group are detected at δ 47.5 ppm (C15) and 16.0 ppm (C16). Single crystals of [10]BF4·CH2Cl2 were obtained by slow vapor diffusion of Et2O into a saturated CH2Cl2 solution of the

for the metalated carbon atoms detected at δ 178.4 ppm (d, JC,Rh = 55.3 Hz, C5) and δ 160.4 ppm (d, 1JC,Rh = 46.9 Hz, C1) in the 13C{1H} NMR spectrum. These values are, as expected, very similar to the chemical shifts detected for the metalated carbon atoms in [7] and [5]. Single crystals of [9] were obtained by slow evaporation of the solvent from an acetonitrile solution of the complex at ambient temperature. An X-ray diffraction study with these crystals revealed the expected coordination of the C∧C ligand in 1

E


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Organometallics complex. The structure analysis confirmed the formation of the C(NHC)∧C(NHC′) chelate ligand (Figure 7). Since both

Figure 8. Carbene carbon resonances in the 13C{1H} NMR spectrum of [11]BF4 (in CD2Cl2).

Figure 7. Molecular structure of complex cation [10]+ in [10]BF4· CH2Cl2. Displacement ellipsoids are shown at the 50% probability level, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh−Br 2.5227(6), Rh−C1 2.039(3), Rh−C5 2.047(3), Rh1−CCp* range 2.164(3)−2.226(3), N1−C1 1.352(4), N2−C1 1.357(4), N3−C5 1.358(4), N4−C5 1.356(4); Br−Rh−C1 90.91(8), Br−Rh−C5 89.17(8), C1−Rh−C5 85.34(12), C1−N1−C2 109.7(3), C1−N2−C3 111.0(3), C5−N3−C7 111.6(2), C5−N4−C6 110.6(2), N1−C1−N2 105.1(3), N3−C5−N4 104.3(2).

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CONCLUSIONS

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EXPERIMENTAL SECTION

We have prepared the imidazolium/imdazole salts [2]I and [3]Br. The salt [2]I reacts with Ag2O followed by transmetalation to [Rh(Cp*)(Cl)2]2 to give the RhIII complex of an NHC possessing a pendant imidazole group. The pendant imidazole can be deprotonated using NaOAc, and complex [5] bearing a doubly C metalated chelate ligand has been obtained this way. This reaction resembles previously reported NaOAcmediated cyclometalation reactions.28 Reaction of [2]I or [3]Br with NaOAc and [Rh(Cp*)(Cl)2]2 yields the C∧C chelate complexes [7] and [9], respectively, in a one-pot reaction. The imine nitrogen atom in complexes [7] and [9] can be protonated to give complexes with an C(NHC)∧C(pNHC) chelate ligand. Alkylation of the imine nitrogen atom in [9] yields complex [10]BF4, bearing a unique unsymmetrical C(NHC)∧C(NHC′) chelate ligand. Since complexes [9] and [10]BF4 are both chiral-at-metal complexes, the N-alkylation reaction using enantiopure alkylation agents possibly gives access to enantiopure NHC complexes which are chiral at the metal center and bear a chiral N substituent. The preparation of such complexes is currently being studied.

NHC donors are classical NR,NR-NHCs, the bond distances within the diazaheterocycles are now very similar. Both N− C(NHC)−N angles fall in the typical range for imidazolin-2ylidenes (∼105°),1 and the N−C(NHC) bond distances in both diazaheterocycles are also identical within experimental error. While a number of different dicarbene ligands have been described, only select ones are capable of forming chelate complexes25 and only recently were the first dicarbene chelate ligands described which feature two different NHC donors.26 The generation of an unsymmetrical di-NHC ligand at a metal template such as found in [10]BF4 by alkylation of a coordinated diazaheterocycle is unprecedented. This method appears to be a promising pathway for the generation of unique chiral-at-metal NHC complexes.27 Complex cation [10]+ is chiral at the metal center. Reaction of a complex of type [9] (or [7]) with an enantiopure alkylation agent thus leads to diastereomeric complexes which upon resolution would yield enantiopure chiral-at-metal NHC complexes. Apart form the N-alkylation, complex [9] (as was already demonstrated for [7]) can also be protonated at the imine nitrogen atom of one of the diazaheterocycles. Strong acids such as HBF4·Et2O and HCl·Et2O have to be used as the proton source. Most likely due to the electron-withdrawing chloro substituents at the diazaheterocycles, no N-protonation was observed with weak acids such as NH4BF4. Reaction of [9] with HBF4·Et2O in CH2Cl2 gave complex [11]BF4 in 91% yield (Scheme 5). This complex was also investigated by NMR spectroscopy. The resonance for the N−H proton was detected at δ 12.23 ppm. The resonances for the carbene carbon atoms in [11]BF4 (δ 174.6 ppm (d, 1JC,Rh = 53.3 Hz, C5) and δ 172.8 ppm (d, 1JC,Rh = 51.1 Hz, C1)) are shifted slightly downfield in comparison to the equivalent resonances for the N-alkylated complex [10]BF4. Both carbene carbon resonances in [11]BF4 are detected as doublets with almost identical 1JC,Rh coupling constants (Figure 8).

General Procedures. All manipulations were carried out under an argon atmosphere using standard Schlenk techniques. Glassware was oven-dried at 120 °C. Solvents were distilled by standard procedures prior to use. 1H and 13C{1H} NMR spectra were recorded at 298 K on Bruker AVANCE I 400, Bruker AVANCE III 400, and Bruker AVANCE II 200 spectrometers. Chemical shifts (δ) are expressed in ppm downfield from tetramethylsilane or using the residual protonated solvent as an internal standard. All coupling constants are expressed in hertz. Mass spectra were obtained with an Orbitrap LTQ XL (Thermo Scientific) spectrometer. 5,6-Dichloroimidazole, iodomethane, Ag2O, and 2-fluorobenzyl bromide were used as received from commercial sources. [Rh(Cp*)(Cl)2]2 was prepared as described previously in the literature.29 Satisfactory microanalytical data for [3]Br, [8]BF4, [9], [10]BF4, and [11]BF4 could not be obtained, due to the sensitivity of the compounds or the fluorine content. A complete set of NMR spectra is provided in the Supporting Information instead. For assignment of the NMR resonances see the numbering scheme at the molecular plots. F


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Organometallics

C13), 119.6 (C6), 119.4 (d, 2JC,F = 13.8 Hz, C9), 119.1 (C7), 115.7 (d, 2JC,F = 20.7 Hz, C11), 113.0 (C3), 55.4 (C4), 46.1 (d, 3 JC,F = 4.1 Hz, C8). HRMS (ESI, positive ions): m/z 394.96127 (calcd for [3]+ 394.96150). Synthesis of [4].

Synthesis of 1.

Compound 1 was prepared according to a procedure reported previously by Peris et al.21 A mixture of 4,5-dichloroimidazole (1.50 g, 11.0 mmol), KOH (0.926 g, 16.5 mmol), and CH2Br2 (0.39 mL, 0.97 g, 5.6 mmol) was heated in a high-pressure Schlenk tube at 120 °C for 12 h. Subsequently, the reaction mixture was extracted with CH2Cl2 (3 × 10 mL). Evaporation of the solvent from the combined extracts gave 1 as a light brown solid. Yield: 1.29 g (4.51 mmol, 82%). 1H NMR (400 MHz, CD2Cl2): δ 7.71 (s, 2H, H1), 5.98 (s, 2H, H4). 13C{1H} NMR (100 MHz, CD2Cl2): δ 135.5 (C1), 128.0 (C2), 113.4 (C3), 53.6 (C4). Anal. Calcd for C7H4N4Cl4: C, 29.40; H, 1.41; N, 19.60. Found: C, 30.35; H, 2.00; N, 19.03. HRMS (ESI, positive ions): m/z 286.92741 (calcd for [1 + H]+ 286.92390). Synthesis of [2]I.

To a solution of [2]I (50 mg, 0.12 mmol) in CH3CN (10 mL) was added Ag2O (14 mg, 0.060 mmol) with exclusion of light. The resulting suspension was stirred for 4 h at ambient temperature. Subsequently, [Rh(Cp*)(Cl)2]2 (36 mg, 0.058 mmol) was added, and the reaction mixture was stirred for an additional 12 h at ambient temperature. A red suspension was obtained, which was filtered through Celite to yield a dark red solution. The solvent from the filtrate was removed in vacuo to give [4] as a dark red solid. Analytically pure [4] was obtained by column chromatography using silica gel and a CH2Cl2/ MeOH (95/5, v/v) or a CH2Cl2/(CH3)2CO (80/20, v/v) solvent mixture. Yield: 60 mg (0.099 mmol, 85%). 1H NMR (400 MHz, CD2Cl2): δ 7.65 (s, 1H, H1), 7.22 (d, 2JH,H = 12.6 Hz, 1H, H4a), 6.00 (d, 2JH,H = 12.6 Hz, 1H, H4b), 4.09 (s, 3H, H8), 1.58 (s, 15H, H10). 13C{1H} NMR (100 MHz, CD2Cl2): δ 177.5 (d, 1JC,Rh = 58.5 Hz, C5), 135.6 (C1), 126.5 (C2), 121.7 (C6), 118.2 (C7), 113.5 (C3), 97.6 (d, 1JC,Rh = 7.1 Hz, (C9), 58.6 (C4), 38.7 (C8), 9.6 (C10). Anal. Calcd for C18H21N4Cl6Rh: C, 35.50; H, 3.48; N, 9.20. Found: C, 34.87; H, 3.56; N, 8.99. HRMS (ESI, positive ions): m/z 572.92330 (calcd for [[4] − Cl]+ 572.92356). Synthesis of [5].

A sample of compound 1 (400 mg, 1.40 mmol) and iodomethane (0.174 mL, 397 mg, 2.80 mmol) were dissolved in CH3CN (10 mL) ,and the resulting solution was heated to reflux for 24 h.21 After cooling of the reaction mixture to ambient temperature, the solvent was removed in vacuo and the yellow solid obtained was washed with CH2Cl2 (2 × 5 mL). The solid residue was dried in vacuo to give the salt [2]I as a pale yellow solid. Yield: 389 mg (0.909 mmol, 65%). 1H NMR (400 MHz, DMSO-d6): δ 9.74 (s, 1H, H5), 8.20 (s, 1H, H1), 6.61 (s, 2H, H4), 3.86 (s, 3H, H8). 13C{1H} NMR (100 MHz, DMSO-d6): δ 137.6 (C5), 137.5 (C1), 125.6 (C2), 120.4 (C6), 117.9 (C7), 112.9 (C3), 55.1 (C4), 35.3 (C8). Anal. Calcd for C8H7N4Cl4I: C, 22.46; H, 1.65; N, 13.10. Found: C, 22.59; H, 1.92; N, 13.16. HRMS (ESI, positive ions): m/z 300.93856 (calcd for [2]+ 300.93956). Synthesis of [3]Br.

A sample of [4] (50 mg, 0.082 mmol) was dissolved in 10 mL of CH3CN, and NaOAc was added (13 mg, 0.16 mmol). The reaction mixture was stirred for 2 h at ambient temperature and slowly filtered through a pad of Celite until a clear filtrate was obtained. The solvent was then removed in vacuo, and the solid obtained was washed with diethyl ether (3 × 5 mL). Complex [5] was obtained as a red solid. Yield: 42 mg (0.073 mmol, 89%). 1H NMR (400 MHz, CD2Cl2): δ 6.03 (d, 2JH,H = 12.9 Hz, 1H, H4a), 5.15 (d, 2JH,H = 12.9 Hz, 1H, H4b), 3.93 (s, 3H, H8), 1.69 (s, 15H, H10). 13C{1H} NMR (100 MHz, CD2Cl2): δ 176.7 (d, 1JC,Rh = 54.9 Hz, C5), 161.5 (d, 1JC,Rh = 47.9 Hz, C1), 126.7 (C2), 118.8 (C6), 115.9 (C7), 109.6 (C3), 99.4 (d, 1 JC,Rh = 5.1 Hz, C9), 55.8 (C4), 37.0 (C8), 9.9 (C10). Anal. Calcd for C18H20N4Cl5Rh: C, 37.76; H, 3.52; N, 9.79. Found: C, 36.77; H, 3.58; N, 9.18. HRMS (ESI, positive ions): m/z 572.92369 (calcd for [[5] + H]+ 572.92301). Synthesis of [6].

A solution of 1 (0.500 g, 1.75 mmol) in 5 mL of CH3CN was added to a solution of 2-fluorobenzyl bromide (2.10 mL, 3.29 g, 17.4 mmol) in 5 mL of CH3CN. The resulting mixture was heated at 75 °C for 24 h. During this period the salt [3]Br precipitated as a white solid. The solid was collected by filtration, washed with cold CH3CN (3 × 10 mL), and dried in vacuo. Yield: 0.331 g (0.70 mmol, 40%). 1H NMR (400 MHz, DMSO-d6): δ 10.08 (s, 1H, H5), 8.27 (s, 1H, H1), 7.53 (m, 1H, H14), 7.51 (m, 1H, H12), 7.33 (m, 1H, H11), 7.29 (m, 1H, H13), 6.66 (s, 2H, H4), 5.65 (s, 2H, H8). 13C{1H} NMR (100 MHz, DMSO-d6): δ 160.1 (d, 1JC,F = 247.7 Hz, C10), 138.2 (C5), 137.7 (C1), 131.6 (d, 3JC,F = 8.4 Hz, C12), 130.9 (d, 3JC,F = 2.9 Hz, C14), 125.6 (C2), 124.9 (d, 4JC,F = 3.7 Hz,

To a solution of [2]I (50 mg, 0.12 mmol) in CH3CN (10 mL) was added Ag2O (14 mg, 0.060 mmol) with exclusion of light. G


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Organometallics

MHz, CD2Cl2): δ 10.62 (s, 1H, NH), 6.34 (d, 2JH,H = 14.5 Hz, 1H, H4a), 5.50 (d, 2JH,H = 14.5 Hz, 1H, H4b), 3.78 (s, 3H, H8), 1.97 (s, 15H, H10). 13C{1H} NMR (100 MHz, CD2Cl2): δ 172.7 (d, 1JC,Rh = 53.8 Hz, C5), 168.5 (d, 1JC,Rh = 52.6 Hz, C1), 119.5 (C2), 119.2 (C6), 117.2 (C7), 116.1 (C3), 101.4 (C9), 57.4 (C4), 40.3 (C8), 11.1 (C10). HRMS (ESI, positive ions): m/z 664.85827 (calcd for [8]+ 664.85865). Synthesis of [9].

The resulting suspension was stirred for 4 h at ambient temperature. Subsequently, [Rh(COD)Cl]2 (29 mg, 0.059 mmol) was added and the reaction mixture was stirred for an additional 12 h. The obtained suspension was filtered through Celite to yield an orange solution. The solvent from the filtrate was removed in vacuo to give [6] as a pale orange solid. Analytically pure [6] was obtained by column chromatography using silica gel and a CH2Cl2/MeOH (95/5, v/v) or a CH2Cl2/ (CH3)2CO (80/20, v/v) solvent mixture. Yield: 52 mg (0.095 mmol, 79%). 1H NMR (400 MHz, CD2Cl2): δ 8.43 (s, 1H, H1), 6.99 (d, 2JH,H = 13.6 Hz, 1H, H4a), 6.33 (d, 2JH,H = 13.7 Hz, 1H, H4b), 5.12 (s br, 2H, H16 and H9), 3.36 (m, 1H, H12), 3.01 (m, 1H, H13), 2.53−2.32 (m, 4H, H10, H11, H14, H15), 2.05−1.91 (m, 4H, H10, H11, H14, H15). 13C{1H} NMR (100 MHz, CD2Cl2): δ 188.1 (d, 1JC,Rh = 52.9 Hz, C5), 136.6 (C1), 126.9 (C2), 119.3 (C6), 115.7 (C7), 113.4 (C3), 101.6 (d d, 1JC,Rh = 5.0 Hz, C16 and C9), 70.8 (d, 1JC,Rh = 14.2 Hz, C12), 69.8 (d, 1JC,Rh = 14.2 Hz, C13), 57.8 (C4), 37.8 (C8), 33.5 (C11), 32.7 (C14), 29.5 (C15), 28.7 (C10). Anal. Calcd for C16H18N4Cl5Rh: C, 35.16; H, 3.32; N, 10.25. Found: C, 34.36; H, 3.75; N, 9.19. HRMS (ESI, positive ions): m/z 510.93033 (calcd for [[6] − Cl]+ 510.93123), 568.88907 (calcd for [[6] + Na]+ 568.88983). Synthesis of [7].

A sample of [3]Br (75 mg, 0.16 mmol) was dissolved in 10 mL of CH3CN, and to this solution was added NaOAc (26 mg, 0.32 mmol). The reaction mixture was stirred for 10 min, and then [Rh(Cp*)(Cl)2]2 (49 mg, 0.079 mmol) was added. The resulting suspension was stirred for 2 h at ambient temperature. Subsequently, the mixture was filtered through Celite to give a clear solution. The filtrate was concentrated to 1 mL, and cold diethyl ether (5 mL) was added, resulting in the precipitation of a red solid. The solid was collected and washed with diethyl ether (3 × 5 mL). After drying of the solid in vacuo compound [9] was obtained as a red solid. Yield: 95 mg (0.13 mmol, 81%). 1H NMR (400 MHz, CD2Cl2): δ 7.31 (m, 1H, H12), 7.11 (m, 1H, H11), 7.07 (m, 1H, H13), 6.96 (m, 1H, H14), 6.11 (d, 2JH,H = 12.8 Hz, 1H, H4a), 5.74 (s, 2H, H8), 5.22 (d, 2 JH,H = 12.8 Hz, 1H, H4b), 1.68 (m, 15H, H16). 13C{1H} NMR (100 MHz, CD2Cl2): δ 178.4 (d, 1JC,Rh = 55.3 Hz, C5), 160.4 (d, 1JC,Rh = 46.9 Hz, C1), 160.2 (d, 1JC,F = 246.4 Hz, C10), 129.9 (d, 3JC,F = 8.1 Hz, C12), 129.0 (d, 3JC,F = 3.5 Hz, C14), 127.0 (d, 3JC,Rh = 2.0 Hz, C2), 124.5 (d, 4JC,F = 3.6 Hz, C13), 123.1 (d, 2JC,F = 13.3 Hz, C9), 118.7 (C6), 117.1 (C7), 115.4 (d, 2JC,F = 20.9 Hz, C11), 110.0 (C3), 99.7 (d, 1JC,Rh = 5.1 Hz, C15), 55.1 (C4), 48.8 (d, 3JC,F = 6.6 Hz, C8), 10.1 (C16). HRMS (ESI, positive ions): m/z 710.89596 (calcd for [[9] + H]+ 710.89534). Synthesis of [10]BF4.

A sample of [2]I (50 mg, 0.12 mmol) was dissolved in 10 mL of CH3CN, and to this solution was added NaOAc (19 mg, 0.23 mmol). The reaction mixture was stirred for 10 min, and then [Rh(Cp*)(Cl)2]2 (36 mg, 0.058 mmol) was added. The resulting suspension was stirred for 2 h at ambient temperature. Thereafter the mixture was filtered through Celite to give a clear solution. The filtrate was concentrated to about 1 mL, and cold diethyl ether (5 mL) was added, resulting in the precipitation of a red solid. The solid was isolated, washed with diethyl ether (3 × 5 mL), and dried in vacuo. Yield: 70 mg (0.11 mmol, 92%). 1H NMR (400 MHz, CD2Cl2): δ 6.03 (d, 2 JH,H = 12.7 Hz, 1H, H4a), 5.19 (d, 2JH,H = 12.7 Hz, 1H, H4b), 3.85 (s, 3H, H8), 1.84 (s, 15H, H10). 13C{1H} NMR (100 MHz, CD2Cl2): δ 177.4 (d, 1JC,Rh = 55.1 Hz, C5), 158.8 (d, 1 JC,Rh = 47.2 Hz, C1), 127.0 (C2), 118.5 (C6), 115.9 (C7), 109.7 (C3), 99.6 (d, 1JC,Rh = 5.2 Hz, C9), 55.8 (C4), 40.1 (C8), 10.6 (C10). Anal. Calcd for C18H20N4ClIRh: C, 32.56; H, 3.04; N, 8.44. Found: C, 32.09; H, 3.31; N, 7.44. HRMS (ESI, positive ions): m/z 664.85857 (calcd for [[7] + H] + 664.85865). Synthesis of [8]BF4.

A sample of [9] (50 mg, 0.070 mmol) was dissolved in 10 mL of CH2Cl2, and to this solution was added Et3OBF4 (16 mg, 0.084 mmol). The reaction mixture was stirred for 1 h at ambient temperature. Subsequently, the resulting suspension was filtered through Celite to give a clear solution. The filtrate was concentrated to 1 mL, and cold diethyl ether (5 mL) was added. This resulted in the precipitation of a red solid, which was isolated, washed with diethyl ether (3 × 5 mL), and dried in vacuo to give [10]BF4 as a red solid. Yield: 42 mg (0.051 mmol, 73%). 1H NMR (400 MHz, CD2Cl2): δ 7.34 (m, 1H, H12), 7.14 (m, 1H, H11), 7.11 (m, 1H, H13), 6.89 (m, H14), 6.47 (d, 2JH,H = 14.3 Hz, 1H, H4a), 5.75 (d, 2JH,H = 16.3 Hz, 1H, H8a), 5.60 (d, 2JH,H = 14.3 Hz, 1H, H4b), 5.48 (d, 2JH,H = 16.3 Hz, 1H, H8b), 4.30 (m, 1H, H15a), 4.19 (m, 1H, H15b), 1.76 (s, 15H, H18), 1.53 (t, 3H, H16). 13C{1H} NMR (100 MHz, CD2Cl2): δ 173.6 (d, 1JC,Rh = 53.3 Hz, C5), 171.9 (d,

To a sample of [7] (50 mg, 0.075 mmol) in CH2Cl2 (10 mL) was added HBF4·Et2O (22 μL, 26.0 mg, 0.090 mmol). The reaction mixture was stirred for 1 h at ambient temperature. Then, the mixture was concentrated to 1 mL and cold diethyl ether (5 mL) was added, resulting in the precipitation of a red solid which was isolated and dried in vacuo to give [8]BF4 as a red solid. Yield: 54 mg (0.072 mmol, 96%). 1H NMR (400 H


Organometallics

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1

JC,Rh = 53.0 Hz, C1), 160.0 (d, 1JC,F = 246.0 Hz, C10), 130.2 (d, 3JC,F = 8.2 Hz, C12), 128.4 (d, 3JC,F = 3.4 Hz, C14), 124.8 (d, 4JC,F = 3.2 Hz, C13), 122.4 (d, 2JC,F = 13.4 Hz, C9), 119.9 (d, 3JC,Rh = 1.5 Hz, C6), 119.2 (d, 3JC,Rh = 1.5 Hz, C2 and C7), 118.7 (d, 3JC,Rh = 1.4 Hz, C3), 115.5 (d, 2JC,F = 20.7 Hz, C11), 101.6 (d, 1JC,Rh = 5.5 Hz, C17), 57.9 (C4), 49.1 (C8), 47.5 (C15), 16.0 (C16), 10.0 (C18) ppm. HRMS (ESI, positive ions): m/z 738.92742 (calcd for [10]+ 738.92611). Synthesis of [11]BF4.

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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

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A sample of [9] (50 mg, 0.070 mmol) was dissolved in 10 mL of CH2Cl2, and to this solution was added HBF4·Et2O (21 μL, 25 mg, 0.084 mmol). The reaction mixture was stirred for 1 h at ambient temperature, and the resulting suspension was filtered through Celite. The solvent was removed from the filtrate in vacuo to give [11]BF4 as an orange solid. Yield: 51 mg (0.064 mmol, 91%). 1H NMR (400 MHz, CD2Cl2): δ 12.23 (s, 1H, NH), 7.35 (m, 1H, H12), 7.14 (m, 1H, H11), 7.12 (m, 1H, H13), 6.89 (m, 1H, H14), 6.43 (d, 2JH,H = 14.2 Hz, 1H, H4a), 5.76 (d, 2JH,H = 17.0 Hz, 1H, H8a), 5.62 (d, 2JH,H = 14.2 Hz, 1H, H4b), 5.44 (d, 2JH,H = 17.0 Hz, 1H, H8b) 1.78 (s, 15H, H16). 13C{1H} NMR (100 MHz, CD2Cl2): δ 174.6 (d, 1JC,Rh = 53.3 Hz, C5), 172.8 (d, 1JC,Rh = 51.1 Hz, C1), 159.9 (d, 1JC,F = 246.0 Hz, C10), 130.3 (d, 3JC,F = 8.0 Hz, C12), 128.3 (d, 3JC,F = 2.9 Hz, C14), 124.8 (d, 4JC,F = 3.2 Hz, C13), 119.6 (C6), 119.0 (C3), 118.3 (C2), 117.0 (C7), 115.6 (d, 2JC,F = 20.5 Hz, C11), 101.8 (C15), 57.8 (C4), 49.2 (C8), 10.1 (C16). HRMS (ESI, positive ions): m/z 710.89420 (calcd for [11]+ 710.89478). X-ray Diffraction Studies. X-ray diffraction data for compounds [4], [5]·1.5CH 2 Cl 2 , [6]·CH 2 Cl 2 , [9], and [10]BF4·CH2Cl2 were recorded on an APEX-II diffractometer equipped with an area detector and graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The data reduction was performed using APEX2 software.30 Structure solutions were found by direct methods and refined by full-matrix least-squares methods based on F2 using SHELXL-97 and WinGX software.31 Hydrogen atoms were positioned geometrically in idealized positions and refined with isotropic displacement parameters as riding atoms. If not noted otherwise (see the Supporting Information), all on-hydrogen atoms were refined anisotropically. Geometrical calculations were performed using the SHELXL-97 and WinGX programs.31

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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.5b00993. Nuclear magnetic resonance (NMR) spectra for all new compounds and high-resolution electrospray ionization (HR-ESI) mass spectra for selected complexes (PDF) X-ray crystallographic data for [4], [5]·1.5CH2Cl2, [6]· CH2Cl2, [9], and [10]BF4·CH2Cl2 (CIF) I


Article

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