Lithium Complexes of Neutral Bis-NHC Ligands - Organometallics

Vitaly NesterovDominik ReiterPrasenjit BagPhilipp FrischRichard HolznerAmelie PorzeltShigeyoshi Inoue. Chemical Reviews 2018 Article ASAP. Abstract | ...
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Lithium Complexes of Neutral Bis-NHC Ligands Matthias Brendel, Jan Wenz, Igor V. Shishkov,† Frank Rominger, and Peter Hofmann* Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: The employment of lithium hexamethyldisilazide for the deprotonation of methylene-bridged bis(imidazolium) salts led to the formation of lithium carbene adducts. Depending on the crystallization method and the substituents of the ligands, monomeric, dimeric, or polymeric solid-state structures were obtained. These lithium carbene complexes represent the first examples of lithium complexes bearing neutral bis(N-heterocyclic carbene) ligands.

N

-heterocyclic carbenes (NHCs) are one of the most important ligand types in organometallic chemistry.1 Although their complexes have been known for a long time,2 their broad usage did not start until the isolation of a stable, crystalline NHC by Arduengo and co-workers.3 Already in the first example of an application of NHC complexes in homogeneous catalysis, chelating bis-NHC ligands have been used.4 Complexes bearing this ligand type are stabilized due to the chelate effect and offer various possibilities for tuning their geometric and electronic properties.5 Especially aryl-substituted, methylene-bridged bis-NHCs are sometimes employed as isolated compounds in the syntheses of metal complexes, and one of these bis(carbenes) could even be characterized by X-ray crystallography.6 Nevertheless, in most cases these ligands are prepared in situ due to their temperature sensitivity. In the synthesis of nickel(0) and platinum(0) bis-NHC chelate complexes bearing Lt‑Bu and LDipp (Figure 1), we employed

Figure 2. Examples of structurally characterized lithium NHC complexes by Arduengo (A),9a Arnold (B),9b and Hofmann (C).10c

Lt‑BuH2Br2 at room temperature yielded colorless single crystals. The solid-state structure reveals the formation of the lithium carbene complex [Lt‑BuLiBr]2 (1; Scheme 1, top). The Ci-symmetric dimeric compound resembles complex B, with two lithium centers each coordinated in a distortedtetrahedral manner by a bis-NHC ligand in a chelating fashion and two bridging bromo ligands (Figure 3, left).9b In comparison to the 97.6(4)° angle in C bearing an anionic borate-bridged Scheme 1. Formation of Bis-NHC Lithium Complexes 1−5

Figure 1. Bis-NHC ligands employed in this study (Dipp = 2,6diisopropylphenyl).

lithium hexamethyldisilazide (LiHMDS) as base to generate the bis(carbenes) in situ at room temperature, starting from bis(imidazolium) dibromides.7 In comparison to the free carbenes formed by deprotonation with potassium hexamethyldisilazide (KHMDS), the carbene signals in the 13C NMR spectra were shifted to higher field after deprotonation with LiHMDS, suggesting the formation of lithium carbene adducts.8 Structurally characterized lithium complexes bearing neutral NHC ligands are rare and were first described by Arduengo et al. (Figure 2).9 Lithium adducts of polydentate NHCs are exclusively known with ligands containing anionic backbones.10 After the reaction of the bis(imidazolium) dibromides Lt‑BuH2Br2 and LDippH2Br2 with an excess of LiHMDS, mass spectra of the product solutions did not provide hints to the formation of lithium NHC complexes. However, condensation of pentane into a saturated THF solution of deprotonated © 2015 American Chemical Society

Received: December 2, 2014 Published: January 22, 2015 669

DOI: 10.1021/om501229b Organometallics 2015, 34, 669−672

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Organometallics

Figure 4. Solid-state structure of 3 with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Li−C1 2.223(7), Li−O 1.961(10), Li−F7 1.934(10), P−F7 1.638(3), P−F8 1.580(4), P−F9 1.555(4), P−F10 1.582(4), C1−Li−C1′ 94.3(4), F7−Li−O 111.0(5), Li−F7−P 146.0(4), N5−C6−N5′ 113.6(4).

Figure 3. Solid-state structures of 1 and 2 with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): 1, Li−C1 2.212(8), Li−C6 2.255(8), Li−Br 2.499(6), Li−Br′ 2.523(6), Li−Li′ 3.234(12), C1−Li− C6 91.9(3), Br−Li−Br′ 99.8(2), N5−C11−N7 112.4(3); 2, Li−C1 2.291(6), Li−C6 2.253(6), Li−Br 2.585(5), Li−O 1.984(6), C1−Li− C6 92.0(2), Br−Li−O 101.9(2), N5−C11−N7 113.1(2).

the bromo complexes, where two different species were crystallized, but the NMR spectra show the clean formation of a single compound after deprotonation. The reaction of the aryl-substituted bis(imidazolium) salt LDippH2Br2 with LiHMDS in THF led to a yellow solution. By condensation of pentane into the solution, colorless single crystals were obtained after 1 day. In contrast to the case for compounds 2 and 3, the THF ligand in the monomer [LDippLiBr(thf)] (4) lies on the opposite side of the bis-NHC ligand’s methylene bridge (Figure 5). The Li−C distances are in a range similar to those in the other compounds; the bite angle of 91.0(3)° is slightly smaller.

ligand,10c the bis-NHC bite angles of 91.9(3)° in 1 are smaller. The Li−C distances of 2.212(8) and 2.255(8) Å are longer than those in most lithium NHC adducts reported so far and are only surpassed in complexes with an NHC bridging two lithium centers.10a,11 When the crystallization was performed at 8 °C with n-hexane instead of pentane, a different complex was obtained (Figure 3, right). [Lt‑BuLiBr(thf)] (2) represents the monomer of 1, where the vacant coordination site is saturated by a THF molecule. Both compounds exhibit nearly the same bite angles. The Li−C distances of 2.291(6) and 2.253(6) Å are longer than in compound 1. The coordinated bromo ligand is pointing toward the bulky tert-butyl groups, whereas the THF ligand is located on the side of the bis-NHC ligand’s methylene bridge. Ong et al. reported a similar complex bearing an aminofunctionalized NHC chelate ligand instead of the bis-NHC.9c To examine the influence of the counterion on the formation of lithium NHC complexes, Lt‑BuH2(PF6)2 was prepared by treatment of the corresponding dibromide with ammonium hexafluorophosphate. Deprotonation with LiHMDS in THF led to a colorless solution. In addition to minor differences in the 1H NMR spectrum, the signal of the carbene carbon atoms in the 13 C NMR spectrum at 205.9 ppm is shifted toward higher field in comparison to the signal for the free carbene (214.7 ppm7). The LIFDI mass spectrum also corroborates the formation of a lithium carbene adduct with [Lt‑BuLi(thf) + H]+ as the base peak. Condensation of n-hexane into a saturated THF solution at 8 °C eventually led to long, colorless needles. The Cs-symmetric complex [Lt‑BuLi(PF6)(thf)] (3) is similar to the bromo monomer 2, with the hexafluorophosphate anion coordinated to the lithium center via a fluorine atom (Scheme 1, middle). The bite angle of 94.3(4)° is larger than that in the bromo complexes, and the Li−C distances of 2.223(7) Å are slightly shorter in comparison to the average bond lengths in 1 and 2 (Figure 4). The coordination of a hexafluorophosphate anion to a lithium atom is rare and, to the best of our knowledge, has only been observed in two other cases.12 Due to the coordination to the lithium center, the P−F7 distance of 1.638(3) Å is longer than the average lengths of the other P−F bonds (1.57 Å). While the crystals were stable in the mother liquor at 8 °C for several days, they decomposed quickly when they were isolated. The 1H NMR spectrum of the dissolved crystals in THF-d8 differs slightly from that of the in situ generated species. In contrast to the latter, which is stable for days, 3 decomposed in solution within a few hours at room temperature (see the Supporting Information). Obviously, the crystalline compound is not identical with the lithium carbene adduct formed in situ. This is also the case with

Figure 5. Solid-state structure of 4 with ellipsoids drawn at the 50% probability level. Hydrogen atoms and noncoordinated THF molecule are omitted for clarity. Selected bond lengths (Å) and angles (deg): Li− C1 2.227(7), Li−C6 2.219(7), Li−Br 2.528(7), Li−O 1.946(7), C1− Li−C6 91.0(3), Br−Li−O 110.2(3), N5−C11−N7 112.4(3).

After 2 days more, a further species crystallized as colorless needles (Scheme 1, bottom). In contrast to the case for complexes 1−4, the ratio of lithium atoms to bis-NHC ligands is 2:1 in this C2-symmetric coordination polymer [LDipp(LiBr)2(thf)2]n (5) (Figure 6). The bis-NHC ligands do not coordinate in a chelating fashion but bridge lithium bromide dimers. In addition to the 2-fold rotation axes through the C6 atoms, the molecule exhibits inversion centers within the lithium bromide entities. As in the other complexes, the vacant coordination sites are saturated by THF molecules. In comparison to those in the monomer 4, the Li−C distances in 5 are slightly longer. The formation of the dimer 1 and the coordination polymer 5 is a result of the different steric properties of the bis-NHC ligands. In comparison to the tert-butyl groups in Lt‑Bu, the aryl substituents in LDipp are sterically more demanding and therefore impede the formation of a dimeric compound. To summarize, we have shown that the employment of a lithium base in the deprotonation of methylene-bridged bis(imidazolium) salts leads to the formation of various lithium NHC complexes. The potential of these species as in situ formed, convenient transmetalation agents has already been demon670

DOI: 10.1021/om501229b Organometallics 2015, 34, 669−672

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methanol (150 mL). The colorless suspension was stirred for 5 h at room temperature. A colorless solid was isolated by filtration, washed with methanol (2 × 50 mL), and refluxed for 4 h with NH4PF6 (2.5 g, 15.34 mmol) in methanol (150 mL). After slow cooling to room temperature, the solid was isolated by filtration and washed with methanol (2 × 40 mL). The product was refluxed in methanol (120 mL) and filtered hot. The colorless, crystalline solid was washed with methanol (2 × 20 mL) and diethyl ether (2 × 50 mL) and dried in vacuo. Yield: 2.4 g (61%). Mp: 256−257 °C dec. Anal. Calcd for C15H26F12N4P2 (552.33): C, 32.62; H, 4.75; N, 10.14. Found: C, 32.64; H, 4.84; N, 9.99. 1H NMR (500.13 MHz, DMSO-d6): δ 1.60 (s, 18H, CH3), 6.51 (s, 2H, CH2), 8.01 (s, 2H, CH-Im), 8.13 (s, 2H, CH-Im), 9.46 (s, 2H, N2CH-Im). 13C{1H} NMR (125.77 MHz, DMSO-d6): δ 28.66 (CH3), 58.20 (C(CH3)3), 60.25 (CH2), 121.05 (CH-Im), 122.44 (CH-Im), 136.02 (N2CH-Im). MS (ESI, MeOH): m/z (%) 205.5 (100) [M − t-Bu − 2 PF6]+, 261.6 (84) [M − H − 2 PF6]+, 407.4 (38) [M − PF6]+. Deprotonation of Lt‑BuH2(PF6)2. LiHMDS (3.3 mg, 20 μmol) was dissolved in THF-d8 (0.5 mL) and added to Lt‑BuH2(PF6)2 (4.4 mg, 8 μmol). 1H NMR (600.24 MHz, THF-d8): δ 1.55 (s, 18H, CH3), 6.06 (s, 2H, CH2), 7.15 (s, 2H, CH-Im), 7.25 (s, 2H, CH-Im). 13C{1H} NMR (150.93 MHz, THF-d8): δ 31.20 (CH3), 56.73 (C(CH3)3), 64.80 (CH2), 117.23 (CH-Im), 119.87 (CH-Im), 205.92 (N2C-Im). MS (LIFDI, THF-d8): m/z (%) 340.8 (100) [Lt‑BuLi(thf) + H]+, 260.4 (45) [Lt‑Bu]+. [Lt‑BuLi(PF6)(thf)] (3). LiHMDS (58.4 mg, 349 μmol) was dissolved in THF (6 mL), Lt‑BuH2(PF6)2 (87.9 mg, 159 μmol) was added, and the reaction mixture was stirred at room temperature for 5 min. Long, colorless needles were obtained by condensation of n-hexane into the colorless solution at 8 °C. The isolated and dried crystals decompose rapidly at ambient temperature. 1H NMR (600.24 MHz, THF-d8): δ 1.54 (s, 18H, CH3), 6.08 (s, 2H, CH2), 7.13 (s, 2H, CH-Im), 7.14 (s, 2H, CH-Im). Anal. Calcd for C19H32F6LiN4OP: C, 47.11; H, 6.66; N, 11.57. Found: C, 47.18; H, 6.58; N, 12.05 (due to the fast decomposition, the values are outside the range associated with analytical purity but are supplied as the best data which have been obtained so far). Deprotonation of LDippH2Br2. LiHMDS (3.3 mg, 20 μmol) was dissolved in THF-d8 (0.5 mL) and added to LDippH2Br2 (5.0 mg, 8 μmol). 1H NMR (600.24 MHz, THF-d8): δ 1.07 (d, 3J = 6.9 Hz, 12H, CH3), 1.16 (d, 3J = 6.8 Hz, 12H, CH3), 2.60 (sept, 3J = 6.9 Hz, 4H, CH(CH3)2), 6.83 (s, 2H, CH2), 6.96 (d, 3J = 1.6 Hz, 2H, CH-Im), 7.24 (d, 3J = 7.7 Hz, 4H, m-CH-Ar), 7.35 (t, 3J = 7.7 Hz, 2H, p-CH-Ar), 7.88 (s, 2H, CH-Im). 13C{1H} NMR (150.93 MHz, THF-d8): δ 24.22 (CH3), 24.23 (CH3), 28.78 (CH(CH3)2), 64.53 (CH2), 120.61 (CHIm), 123.35 (CH-Im), 123.90 (m-CH-Ar), 129.28 (p-CH-Ar), 138.97 (i−C-Ar), 146.78 (o−C-Ar), 211.08 (N2C-Im). MS (LIFDI, THF-d8): m/z (%) 469.0 (100) [LDipp + H]+. [LDippLiBr(thf)] (4). LiHMDS (58.4 mg, 349 μmol) was dissolved in THF (5 mL), LDippH2Br2 (100.0 mg, 159 μmol) was added, and the reaction mixture was stirred at room temperature for 5 min. Colorless single crystals were obtained after 1 day by condensation of pentane into the yellow solution at room temperature. The isolated and dried crystals decompose rapidly at ambient temperature. Anal. Calcd for C39H56BrLiN4O2: C, 66.94; H, 8.07; N, 8.01. Found: C, 67.07; H, 7.73; N, 8.28 (the one noncoordinated THF molecule visible in the crystal structure is included). [LDipp(LiBr)2(thf)2]n (5). After 2 days more, small needles crystallized from the mother liquor of 4. The isolated and dried crystals decomposed too fast to allow a correct elemental analysis. X-ray Diffraction Studies. Intensity data were collected at 200(2) K with Mo Kα irradiation (λ = 0.71073 Å), intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS13a on the basis of the Laue symmetry of the reciprocal space, structures were refined against F2 with a full-matrix least-squares algorithm using the SHELXL software package,13b and hydrogen atoms were treated using appropriate riding models. CCDC 983371 (1), 1035545 (2), 1035546 (3), 983370 (4), and 1035547 (5) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Figure 6. Segment of the solid-state structure of 5 with ellipsoids drawn at the 50% probability level. Hydrogen atoms, isopropyl groups, and noncoordinated THF molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Li−C1 2.24(2), Li−O 1.97(2), Li−Br 2.58(2), Li−Br′ 2.51(2), Li−Li′ 3.26(4), C1−Li−O 117.3(11), Br−Li− Br′ 110.1(7), N2−C6−N2′ 112.5(14).

strated in a previous study on the synthesis of bis-NHC nickel and platinum chelate complexes.7 Depending upon the substitution pattern of the imidazol-2-ylidene moieties and the crystallization conditions, monomeric, dimeric, or polymeric structures were observed. These complexes represent the first examples of structurally characterized lithium complexes bearing neutral bis-NHC ligands.

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EXPERIMENTAL SECTION

General Methods. All experiments were carried out under an argon atmosphere using standard Schlenk and glovebox techniques. Diethyl ether, n-hexane, and THF were taken from an MBraun SPS-800 solvent purification system, and pentane was distilled from sodium. All solvents were degassed by freeze−pump−thaw cycles, saturated with argon, and stored over molecular sieves (3 Å). Lt‑BuH2Br2 and LDippH2Br2 were synthesized according to published procedures.7 Lithium hexamethyldisilazide was purified by Kugelrohr distillation. NMR data are reported in units of δ relative to tetramethylsilane referenced to the residual solvent resonance as internal standard. Deprotonation of Lt‑BuH2Br2. LiHMDS (3.3 mg, 20 μmol) was dissolved in THF-d8 (0.5 mL) and added to Lt‑BuH2Br2 (3.4 mg, 8 μmol). 1 H NMR (600.24 MHz, THF-d8): δ 1.55 (s, 18H, CH3), 6.13 (s, 2H, CH2), 7.08 (s, 2H, CH-Im), 7.14 (s, 2H, CH-Im). 13C{1H} NMR (150.93 MHz, THF-d8): δ 31.37 (CH3), 56.44 (C(CH3)3), 65.80 (CH2), 117.10 (CH-Im), 118.42 (CH-Im), 211.91 (N2C-Im). MS (LIFDI, THF-d8): m/z (%) 340.6 (36) [Lt‑Bu + 2H + Br]+, 260.4 (45) [Lt‑Bu]+. [Lt‑BuLiBr]2 (1). LiHMDS (133.3 mg, 797 μmol) was dissolved in THF (12 mL), Lt‑BuH2Br2 (84.1 mg, 199 μmol) was added, and the reaction mixture was stirred at room temperature for 2 h. Colorless single crystals were obtained by condensation of pentane into the yellow solution at room temperature. The isolated and dried crystals decompose rapidly at ambient temperature. Anal. Calcd for C30H48Br2Li2N8: C, 51.89; H, 6.97; N, 16.14. Found: C, 51.66; H, 7.20; N, 14.44. [Lt‑BuLiBr(thf)] (2). LiHMDS (58.6 mg, 350 μmol) was dissolved in THF (6 mL), Lt‑BuH2Br2 (67.2 mg, 159 μmol) was added, and the reaction mixture was stirred at room temperature for 2 h. Colorless single crystals were obtained by condensation of n-hexane into the yellow solution at 8 °C. The isolated and dried crystals decompose rapidly at ambient temperature. Anal. Calcd for C19H32BrLiN4O: C, 54.42; H, 7.69; N, 13.36. Found: C, 54.48; H, 7.56; N, 13.79 (due to the fast decomposition, the values are outside the range associated with analytical purity but are supplied as the best data which have been obtained so far). Lt‑BuH2(PF6)2. Ammonium hexafluorophosphate (2.4 g, 14.72 mmol) was added to a stirred solution of Lt‑BuH2Br2 (3.0 g, 7.11 mmol) in 671

DOI: 10.1021/om501229b Organometallics 2015, 34, 669−672

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Crystallographic data for 1: colorless crystal (polyhedron), 0.30 × 0.14 × 0.13 mm3, monoclinic, P21/c, Z = 2, a = 9.903(3) Å, b = 11.156(3) Å, c = 16.309(4) Å, α = 90°, β = 101.682(5)°, γ = 90°, V = 1764.6(8) Å3, ρ = 1.307 g cm−3, θmax = 26.42°, 0.3° ω scans with CCD area detector, covering a whole sphere in reciprocal space up to a resolution of 0.80 Å, 15712 reflections measured, 3624 unique reflections (Rint = 0.0337), 2710 observed reflections (I > 2σ(I)), μ = 2.33 mm−1, Tmin = 0.54, Tmax = 0.75, 246 parameters refined, goodness of fit 1.06 for observed reflections, final residue values R1(F) = 0.053, wR2(F2) = 0.152 for observed reflections, residual electron density −0.34 to 0.71 e Å−3. Crystallographic data for 2: colorless crystal (polyhedron), 0.22 × 0.16 × 0.14 mm3, monoclinic, P21/c, Z = 4, a = 13.1261(4) Å, b = 14.2984(4) Å, c = 12.0267(3) Å, α = 90°, β = 103.1189(7)°, γ = 90°, V = 2198.29(11) Å3, ρ = 1.267 g cm−3, θmax = 25.063°, 0.5° ω scans with CCD area detector, covering the asymmetric unit in reciprocal space with a mean redundancy of 5.18 and a completeness of 99.8% to a resolution of 0.84 Å, 20573 reflections measured, 3891 unique reflections (Rint = 0.0262), 3183 observed reflections (I > 2σ(I)), μ = 1.88 mm−1, Tmin = 0.63, Tmax = 0.68, 252 parameters refined, goodness of fit 1.07 for observed reflections, final residue values R1(F) = 0.039, wR2(F2) = 0.090 for observed reflections, residual electron density −0.41 to 0.65 e Å−3. Crystallographic data for 3: colorless crystal (needle), 0.21 × 0.19 × 0.14 mm3, orthorhombic, Pnma, Z = 4, a = 11.4626(6) Å, b = 14.2987(8) Å, c = 14.8866(9) Å, α = 90°, β = 90°, γ = 90°, V = 2439.9(2) Å3, ρ = 1.319 g cm−3, θmax = 25.702°, 0.5° ω scans with CCD area detector, covering the asymmetric unit in reciprocal space with a mean redundancy of 11.85 and a completeness of 100.0% to a resolution of 0.82 Å, 31233 reflections measured, 2418 unique reflections (Rint = 0.0661), 1850 observed reflections (I > 2σ(I)), μ = 0.18 mm−1, Tmin = 0.79, Tmax = 0.99, 179 parameters refined, goodness of fit 1.21 for observed reflections, final residue values R1(F) = 0.083, wR2(F2) = 0.147 for observed reflections, residual electron density −0.42 to 0.43 e Å−3. Crystallographic data for 4: colorless crystal (polyhedron), 0.35 × 0.25 × 0.22 mm3, monoclinic, Cc, Z = 4, a = 21.4410(17) Å, b = 14.6684(12) Å, c = 15.7400(13) Å, α = 90°, β = 129.0936(10)°, γ = 90°, V = 3842.0(5) Å3, ρ = 1.210 g cm−3, θmax = 29.641°, 0.5° ω scans with CCD area detector, covering the asymmetric unit in reciprocal space with a mean redundancy of 4.48 and a completeness of 99.9% to a resolution of 0.72 Å, 24370 reflections measured, 10390 unique reflections (Rint = 0.0362), 7730 observed reflections (I > 2σ(I)), μ = 1.11 mm−1, Tmin = 0.70, Tmax = 0.82, 434 parameters refined, Flack absolute structure parameter 0.023(4), goodness of fit 1.01 for observed reflections, final residue values R1(F) = 0.047, wR2(F2) = 0.107 for observed reflections, residual electron density −0.41 to 0.59 e Å−3. Crystallographic data for 5: colorless crystal (needle), 0.34 × 0.04 × 0.03 mm3, monoclinic, C2/c, Z = 4, a = 22.395(4) Å, b = 18.564(3) Å, c = 14.160(3) Å, α = 90°, β = 128.579(5)°, γ = 90°, V = 4602.3(15) Å3, ρ = 1.135 g cm−3, θmax = 19.027°, 0.5° ω scans with CCD area detector, covering the asymmetric unit in reciprocal space with a mean redundancy of 4.20 and a completeness of 44.8% to a resolution of 1.09 Å, 7986 reflections measured, 1870 unique reflections (Rint = 0.1290), 1145 observed reflections (I > 2σ(I)), μ = 1.79 mm−1, Tmin = 0.73, Tmax = 0.96, 240 parameters refined, goodness of fit 1.02 for observed reflections, final residue values R1(F) = 0.068, wR(F2) = 0.179 for observed reflections, residual electron density −0.36 to 0.57 e Å−3.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.H.: [email protected]. Present Address

† BASF Coatings GmbH, EC/ITI, Glasuritstraße 1, D-48165 Münster, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this work by the Deutsche Forschungsgemeinschaft (through SFB 623 “Molecular Catalysts: Structure and Functional Design”) is gratefully acknowledged.

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REFERENCES

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S Supporting Information *

Text, figures, and CIF files giving NMR spectra and crystallographic data of Lt‑BuH2(PF6)2 and of compounds 1−5. This material is available free of charge via the Internet at http://pubs. acs.org. 672

DOI: 10.1021/om501229b Organometallics 2015, 34, 669−672