CâH Activation Reactions of a Nucleophilic Palladium Carbene

Article pubs.acs.org/Organometallics

C−H Activation Reactions of a Nucleophilic Palladium Carbene Cezar C. Comanescu and Vlad M. Iluc* Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The reactivity of a nucleophilic palladium carbene, [PC(sp2)P]Pd(PMe3) (1; [PC(sp2)P] = bis[2(diisopropylphosphino)phenyl]methylene), toward the C−H bonds of CH3COCH3, CH3CN, Ph−CCH, fluorene, and 9,10dihydroanthracene was investigated. All surveyed substrates reacted with 1. However, there was no detectable reaction of 1 with Ph2CH2. It is proposed that the pKa values of the studied C−H bonds govern their reactivity toward 1: our results show that substrates with a pKa higher than 29, such as Ph2CH2 (pKa = 32.2), do not react even with prolonged heating.

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recently reported nucleophilic palladium carbene, [PC(sp2)P]Pd(PMe3) (1; [PC(sp2)P] = bis[2-(diisopropylphosphino)phenyl]methylene).13 The reactivity of the analogous [PC(sp2)P]Ni(PMe3) toward E−H (E−H = O−H, N−H, C−H) bonds was discussed by Piers et al.9b

INTRODUCTION C−H bond activation at transition-metal centers has important practical applications, as it could replace the current petrochemical feedstocks by inexpensive but relatively inert alkanes.1 While highly reactive species, such as free radicals and enzymatic systems, are capable of C−H bond activation, the mechanistic study of analogous reactions carried out by metal complexes allows the design of synthetic catalysts that mimic and improve the scope and selectivity of biological systems.2 The reactivity of metal carbene complexes has proved important in areas ranging from organic chemistry to biological synthesis.3 Although carbenes have been distinguished for a long time as nucleophilic or electrophilic,4 recently, examples that feature characteristics that diverge from these classifications have emerged.5 Ultimately, the reactivity of a carbene metal complex is dictated by an interplay between the ability of the carbene substituents and of the metal to release electrons into the empty p orbital of the carbene carbon.6 In the area of nucleophilic carbenes of late transition metals, these definitions become blurry, since nucleophilic carbenes are usually associated with early transition metals in the highest oxidation states.7 For example, for a long time, isolated and characterized examples of group 10 metal alkylidenes were absent from the scientific literature. The situation changed dramatically when the group of Hillhouse reported, in 2002, the synthesis and characterization of (dtbpe)Ni(CPh2) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane).8 Since then, examples of analogous nickel carbenes have been discussed and their reactivity has been studied.9 In this area, group transfer reactions were particularly prominent, with several examples of carbon−carbon and carbon−heteroatom bond formation being reported.10 However, early- to late-transition-metal alkylidenes and alkylidynes also show C−H activation reactivity, as demonstrated amply by the groups of Mindiola11 and Legzdins.12 Since C−H activation reactions are at the forefront of sustainable catalysis efforts, we became interested in the reactivity toward C−H bonds of a © XXXX American Chemical Society

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RESULTS AND DISCUSSION

The development of transition-metal-catalyzed C−H bond activation is an active research area, which could open up new avenues in hydrocarbon processing to economically valuable products (alcohols, acids, ketones, etc.). Transition-metal catalysis enables selectivity control; for example, (C5Me5)Rh(PMe3)H2 produces the C−H activation product of CH3−CN, (C5Me5)Rh(PMe3)(CH2CN)H, while [Ni(dippe)H]2 (dippe = bis(diisopropylphosphino)ethane) activates, in contrast, the C−C bond.14 From such studies, it has been found that the relative ease with which C−H bond activation and the subsequent heterolytic C−H bond splitting occur is dependent on various conditions that, in certain cases, can be quantified by the pKa value of the substrate of interest. In order to determine the strength of the C−H bonds that can be activated by the nucleophilic carbene complex 1, the following substrates were surveyed (bond dissociation energies (BDEs) and pKa values for the C−H bonds shown in the formula are given in parentheses): CH3COCH3 (BDE = 93.9 kcal/mol15 or 95.9 ± 0.716 kcal/mol, pKa = 26.515), CH3CN (BDE = 93 ± 2 kcal/mol, pKa = 2517), Ph−CCH (BDE = 125 kcal/mol,18 pKa = 28.719), fluorene (for the methylene hydrogen: BDE = 80 ± 5 kcal/mol,15,20 pKa = 22.6 or 2315,20a), and 9,10-dihydroanthracene (for the methylene hydrogen: BDE = 76.3 kcal/mol,21 pKa = 2719b). Special Issue: Gregory Hillhouse Issue Received: May 14, 2015

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DOI: 10.1021/acs.organomet.5b00414 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Treatment of [PC(sp2)P]Pd(PMe3) (1) with acetone, in THF at ambient temperature (eq 1), affords the acetonyl

resonate at 2.29 ppm as a singlet. 31P NMR data confirm that the two phosphorus nuclei are equivalent, as only one resonance at 48.64 ppm was found in the spectrum. The 13C NMR spectrum exhibits a downfield peak at 207.3 ppm (t, 3JCP = 1.4 Hz) corresponding to the carbonyl carbon −CH2COCH3; the backbone carbon can be found at 57.7 ppm (t, 2JCP = 1.3 Hz), while the methylene and methyl carbons from acetonyl resonate at 25.7 ppm (t, 2JCP = 7.1 Hz) and 31.5 ppm (s), respectively. The reaction of [PC(sp2)P]Pd(PMe3) (1) with acetonitrile in THF at ambient temperature (eq 2) afforded, within less

complex [PC(H)P]Pd(CH2COCH3) (2) after 20 min. The first report of C−H activation of acetone by a Pt(II) complex, leading to [Pt(CH2COCH3)Cl(bipy)] (bipy = 2,2′-bipyridyl), dates from 1997, when Falvello et al. treated an acetone solution of cis-Cl2Pt(NCCH3)2 with bipy for 15 days at ambient temperature.22 Since then, several examples of C−H bond activation of acetone to form the corresponding acetonyl metal complex have been reported.23 In particular, several palladium acetonyl complexes are known, some of which originate from acetone through C−H bond activation.24 The solid-state molecular structure of 2 (Figure 1) features a Pd−CH2COCH3 distance of 2.163(7) Å, the longest Pd−C

than 1 h, the acetonitrile complex [PC(H)P]Pd(CH2CN) (3), which features a backbone proton resonance in the 1H NMR spectrum at 5.84 ppm, shifted upfield with respect to the closely related [PC(H)P]PdCl (6.23 ppm).13b,c The methylene group resonates at 1.48 ppm as a triplet (3JHP = 5.7 Hz), due to virtual coupling to two equivalent phosphorus nuclei. The nitrile carbon can be found at 129.1 ppm as a singlet, and the methylene carbon is a triplet at −21.1 ppm (2JCP = 10.2 Hz) in the corresponding 13C NMR spectrum. The 31P NMR spectrum confirms the chemical equivalence of the two phosphorus nuclei and exhibits a single resonance at 49.7 ppm. A literature survey indicates that there are two other examples of complexes containing the Pd(CH2CN) moiety.26 Roberts et al. reported trans-chloro(cyanomethyl)bis(triphenylphosphine)palladium(II), trans-[PdCl(CH2CN)(PPh3)2], as an acetone, acetonitrile, or benzene solvate, with Pd−CH2CN distances of 2.064−2.089 Å, CH2−CN distances of 1.317−1.424 Å, and C−N distances of 1.150−1.174 Å.26a The synthesis of trans-[PdCl(CH2CN)(PPh3)2] was reported initially by Yamamoto et al. from the reaction of Pd(PPh3)4 with ClCH2CN for 3 h at ambient temperature in benzene.26b The other example of a Pd(CH2CN) complex, L2Pd(CH2CN)2 (L2 = 1,2-bis(diphenylphosphino)ethane), was reported by Bauer et al. and was synthesized from the reaction of the CH 2 CN anion (obtained by n BuLi deprotonation of [CH3CN]−) with L2PdCl2; L2Pd(CH2CN)2 underwent a subsequent reductive elimination to form succinonitrile, NCCH2CH2CN, when treated with oxygen.26c Characterization of L2Pd(CH2CN)2 by single-crystal X-ray diffraction indicated a Pd−CH2CN distance of 2.110 Å, a CH2−CN distance of 1.434 Å, and a C−N distance of 1.139 Å.26c Similarly, the pincer complex 3 (Figure 2) features a square-planar palladium(II) center (sum of angles 361.51(10)°) with Pd−C distances for the two molecules present in the unit cell of 2.136(3) and 2.155(3) Å, CH2−CN distances of 1.423(6) and 1.430(6) Å, and C−N distances of 1.158(6) and 1.148(5) Å; these parameters are similar to those of the other two analogous palladium complexes.26 The C−H activation reaction of PhCC−H by 1 (eq 3) occurs quickly and produces cleanly a red product, [PC(H)P]Pd(CCPh) (4). Spectroscopic investigations confirmed the expected solution structure, indicating that the backbone proton resonates in the corresponding 1H NMR spectrum at

Figure 1. Thermal ellipsoid (50% probability level) representation of one of the crystallographically independent molecules of [PC(H)P]Pd(CH2COCH3) (2). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd(1)−C(1) = 2.101(8), Pd(1)−C(51) = 2.163(7), C(52)−O(1) = 1.253(9), Pd(1)−P(11) = 2.2690(18), Pd(1)−P(12) = 2.3094(19), C(1)−C(11) = 1.512(10), C(1)−C(21) = 1.528(10); P(11)−Pd(1)−P(12) = 161.48(8), C(1)− Pd(1)−C(51) = 179.5(3), C(11)−C(1)−Pd(1) = 117.5(5), C(21)− C(1)−Pd(1) = 108.8(5), C(11)−C(1)−C(21) = 114.9(6).

distance reported for palladium acetonyl complexes (2.046(10) Å24e to 2.131(10) Å24g). In addition, the Pd(1)−C(1) distance of 2.101(8) Å is shorter than a typical Pd−C(sp3) bond, while the C(52)−O(1) distance of 1.253(9) Å is in line with C(sp2)O distances in other Pd−CH2COCH3 moieties (1.218(3)−1.285(12) Å).25 The backbone proton of 2 resonates in the corresponding 1H NMR spectrum as a singlet at 5.89 ppm, shifted upfield in comparison to the value of 6.23 ppm in the related [PC(H)P]PdCl. 13b,c The methylene protons, Pd(CH2)COCH3, were found at 2.93 ppm as a triplet (3JHP = 5.3 Hz), and the methyl protons from the acetonyl substituent B


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distance of 2.033(4) Å is within the range of 1.9410−2.0920 Å found for related Pd(II) complexes.27 Similarly, the C(sp)− C(sp) distance of 1.200(6) Å falls within 1.0030−1.2280 Å, while the C(sp)−C(sp2) distance of 1.438(6) Å is within the range 1.4090−1.5450 Å.27,28 As was the case with cyanomethyl compounds, although numerous CCPh palladium complexes exist, they were not prepared by C−H activation reactions; only a small number of literature reports involve C−H activation of HCCPh. For example, a similar nickel complex, (PCcarbeneP)NiPPh3 (PCH2P = bis[2-(diisopropylphosphino)phenyl]methane), reacts in a similar manner to generate the phenylacetylide product.9b The majority of synthesized complexes employ the corresponding lithium phenylacetylenide to react in a metathesis reaction with a palladium(II) halide. The reaction of carbene 1 with fluorene only occurs with thermal activation, after which it proceeds relatively quickly and produces cleanly the Pd−(η1-fluorenyl) product [PC(H)P]Pd(η1-fluorenyl) (5; eq 4) by heterolytic C−H bond cleavage of Figure 2. Thermal ellipsoid (50% probability level) representation of one of the crystallographically independent molecules of [PC(H)P]Pd(CH2CN) (3). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd(1)−C(1) = 2.115(3), Pd(1)−C(11) = 2.136(3), C(11)−C(12) = 1.423(6), Pd(1)−P(11) = 2.2964(10), Pd(1)−P(12) = 2.2904(14), C(12)−N(1) = 1.158(6); P(11)−Pd(1)−P(12) = 157.78(10), C(1)−Pd(1)−C(11) = 174.3(2), Pd(1)−C(11)−C(12) = 110.0(3), C(11)−C(12)−N(1) = 175.7(4).

the substrate. While there are no other crystallographically characterized examples of Pd−(η1-fluorenyl) or Pd−(η5fluorenyl) derivatives, some examples of η1-fluorenyl (Fl) complexes exist when supported by other metals: Ge (η1-Fl);29 Ga (η1-Fl);30 Mn (η1-Fl and η5-Fl);31 Li, Na, K, Cs, Ca, Ba;32 Zn;33 Zr;34 Ti;35 Mo;36 Ni;37 Cu;38 Au.39 Examples of indenyl palladium complexes (with variable hapticity: 1, 3, or 5) have been reported.40 The backbone proton of 5 resonates at 5.89 ppm as a singlet in the corresponding 1H NMR spectrum, while the 9-fluorenyl proton appears as a multiplet in the 6.16−6.01 ppm range. The two phosphorus nuclei are nonequivalent due to the dihedral angle between the fluorenyl ligand and the P−C−P plane and thus display two doublets in the 31P{1H} NMR spectrum (2JPP = 361.3 Hz), while the backbone carbon resonates at 56.8 ppm in the 13C{1H} NMR spectrum as a singlet. The solution structure agrees with the solid-state molecular structure (Figure 4), which reveals a square-planar Pd(II) center (the sum of angles around palladium is 362.72(10)°), a slightly elongated Pd−C backbone distance of 2.125(4) Å (in comparison to Pd− C(sp2) = 2.086(4) Å in 1) and a Pd−C(51) distance of 2.177(3) Å, similar to reported Pd−C(η1-indenyl) distances.40b,f The reaction between 1 and 9,10-dihydroanthracene in C6D6 (Scheme 1) in a sealed NMR tube led to the formation of anthracene, and both H atoms transferred to the former carbene carbon to form a trigonal-planar Pd(0) species, [PC(H)2P]Pd(PMe3) (6). This reaction could be stopped at a hydrido species intermediate, [PC(H)P]PdH (7), or could be driven thermally to 6. We reason that the reaction of 1 to give 7 may proceed through two mechanisms: either hydrogen atom abstraction or deprotonation. In order to probe the formation of a radical species that would abstract a hydrogen atom, 1 was heated in a J. Young NMR tube in C6D6 up to 80 °C; no sign for the formation of a paramagnetic species (zero magnetic susceptibility, Evans method41) was observed. However, these

5.79 ppm as a singlet, while the backbone carbon can be found at 54.5 ppm in the 13C{1H} NMR spectrum. The two phosphorus nuclei have equivalent environments, exhibiting a single resonance at δ 55.11 ppm (s) in the 31P{1H} NMR spectrum. A single-crystal X-ray diffraction study (Figure 3) indicates a square-planar geometry at palladium (the sum of angles around the metal center is 360.06(12)°). The Pd−C(sp)

Figure 3. Thermal ellipsoid (50% probability level) representation of [PC(H)P]Pd(CCPh) (4). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.112(4), Pd−C(51) = 2.033(4), C(51)−C(52) = 1.200(6), Pd−P(1) = 2.2680(13), Pd−P(2) = 2.2877(11), C(52)−C(61) = 1.438(6); P(1)−Pd−P(2) = 162.56(10), C−Pd−C(51) = 179.7(2), Pd− C(51)−C(52) = 177.6(4), C(51)−C(52)−C(61) = 175.5(6). C


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Organometallics

Figure 5. Thermal ellipsoid (50% probability level) representation of [PC(H)2P]Pd(PMe3) (6). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−P(1) = 2.3212(6), Pd−P(2) = 2.3309(5), Pd−P(3) = 2.2994(6); P(1)− Pd−P(2) = 118.15(2), P(1)−Pd−P(3) = 114.59(2), P(2)−Pd−P(3) = 126.99(2).

Figure 4. Thermal ellipsoid (50% probability level) representation of [PC(H)P]Pd(η1-fluorene) (5). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.125(4), Pd−C(51) = 2.177(3), C(51)−C(52) = 1.490(5), C(51)−C(55) = 1.481(5), Pd−P(1) = 2.2812(9), Pd−P(2) = 2.3328(9); P(1)−Pd− P(2) = 157.66(3), C−Pd−C(51) = 168.00(14), Pd−C(51)−C(52) = 123.1(2), Pd−C(51)−C(55) = 112.0(2), C(55)−C(51)−C(52) = 103.9(3).

spectrum for the three equivalent −CH3 protons at 1.34 ppm, due to H−P coupling (2JHP = 3.5 Hz), while the 31P{1H} NMR spectrum shows an AX2 spin system, with δ(X2) 27.17 ppm (d, 2 JPP = 103.2 Hz) and δ(A) −34.67 ppm (t, 2JPP = 103.7 Hz). The hydride resonance for 7 was identified in the corresponding 1H NMR spectrum at −4.88 ppm as a triplet of doublets, due to 2JHP = 17.2 Hz and 3JHH = 5.6 Hz, while the backbone proton resonates at 5.70 ppm as a doublet due to coupling to the hydride ligand (3JHH = 5.5 Hz). The two phosphorus nuclei are equivalent and display a sharp singlet in the 31P{1H} NMR spectrum at δ 65.72 ppm, shifted downfield from 50.20 ppm in [PC(H)P]PdCl. The backbone carbon shifted downfield to 58.0 ppm in the 13C{1H} NMR spectrum from 51.8 ppm in the parent [PC(H)P]PdCl.13b,c The solidstate molecular structure of 7 (Figure 6) was investigated by Xray crystallography, and it revealed a square-planar Pd(II) center. The hydride atom was found in the electron density map at 1.57(4) Å from palladium, similar to the Pd−H distance (1.57 Å) found by Wendt et al. in a related Pd(II) hydride,42 while the Pd−C distance of 2.152(5) Å is slightly longer than

Scheme 1. Reaction of 1 with 9,10-Dihydroanthracene and Subsequent Studies

results are inconclusive; given the nucleophilicity of carbene 1, it is possible that transformation of 1 to 7 proceeds through deprotonation of 9,10-dihydroanthracene, but a radical mechanism cannot be ruled out at this point.. The compound [PC(H)P]PdH (7) was independently synthesized by a salt metathesis reaction from [PC(H)P]PdCl and Li[HBEt3] in THF at −35 °C (Scheme 1). The molecular structure of 6 was investigated by singlecrystal X-ray diffraction (Figure 5) and revealed a trigonalplanar Pd(0) center, bound to the diphosphine ligand and PMe3 (the sum of angles around palladium is 359.73(2)°). The PMe3 ligand displays a doublet in the corresponding 1H NMR

Figure 6. Thermal ellipsoid (50% probability level) representation of [PC(H)P]PdH (7). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.152(5), Pd−H(2) =1.57(4), Pd−P(1) = 2.250(2), Pd−P(2) = 2.300(2); P(1)−Pd−P(2) = 160.20(10), C−Pd−H(2) = 177.5(14), P(1)−Pd−H(2) = 96.0(14), P(2)−Pd−H(2) = 97.8(14), C−Pd−P(1) = 84.80(16), C−Pd−P(2) = 81.93(15). D


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Pd−C(sp) 2.033(4), Pd−CHAr2 2.112(4) 55.11 (s)

34.0 (t, 2JHP = 5.9 Hz, −Pd−C(H)(Flu)), 56.8 (s, backbone C)

6.16−6.01 (m, 1H, Pd−CH(Flu)), 5.89 (s, backbone H) −4.88 (td, 2JHP = 17.2, 3JHH = 5.6 Hz, Pd− H), 5.70 (d, 3JHH = 5.5 Hz, backbone H)

2

CONCLUSIONS All surveyed substrates (Table 1 and Table 2) reacted with carbene 1; while phenylacetylene and acetone reacted within 30

min at ambient temperature, CH3CN took 1 h under similar conditions and fluorene and 9,10-dihydroanthracene required hours (6 and 36 h, respectively) of heating at 90−100 °C. However, there was no detectable reaction of 1 with Ph2CH2 (BDE = 82−88 kcal/mol,47 pKa = 32.219b) likely because the C−H bond of diphenylmethane is not acidic enough. Therefore, the value of pKa likely governs the reactivity of carbene 1 toward C−H bonds: our results show that substrates with pKa higher than 29, such as Ph2CH2 (pKa = 32.2), do not react even with prolonged heating. While all investigated substrates led to the expected C−H activated products, the reaction between 1 and 9,10dihydroanthracene led to [PC(H)2P]Pd(PMe3) (6) and anthracene. We proposed that the transformation of 1 to 6 takes place by deprotonation of 9,10-dihydroanthracene to an intermediate hydride, [PC(H)P]PdH (7), although a radical mechanism cannot be ruled out at this point. It is important to mention that although examples of complexes similar to the products obtained here have been reported, with the exception of the acetone reaction, none or few were synthesized by the C−H activation of the substrates discussed here. E

C NMR

46.91 (d, 2JPP = 361.1 Hz), 43.95 (d, 2JPP = 361.6 Hz) 65.72 (s)

[PC(H)P] Pd(CH2COCH3) (2) [PC(H)P] Pd(CH2CN) (3) [PC(H)P] Pd(CCPh) (4) [PC(H)P]Pd(η1fluorene) (5) [PC(H)P]PdH (7)

19 20 19b

13

16

3

ref 15, 17 18, 15, 21,

H NMR

26.5 25 28.7 22.6−23 27

3

95.9 ± 0.7 93 ± 2 125 80 ± 5 76.3

complex

pKa

CH3COCH3 CH3CN PhCCH fluorene 9,10-dihydroanthracene

1

BDE (kcal mol−1)

Table 2. Relevant NMR Shifts and Metrical Parameters for the Complexes

compound

significant peak (ppm)

Table 1. BDE and pKa Values for Selected Organic Substrates

58.0 (s, backbone C)

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Pd−C(η1-fluorene) 2.177(3), Pd−CHAr2 2.125(4) Pd−H(2) 1.57(4), Pd−CHAr2 2.152(5)

Pd−CH2CN 2.136(3) Pd−CHAr2 2.115(3) 49.71 (s)

129.1 (s, −Pd−CH2−CN), −21.1 (t, 2JCP = 10.2 Hz, −Pd−CH2), 56.8 (s, backbone C) 113.9 (t, 2JCP = 20.4 Hz, C6H5−C(sp)−C(sp)−Pd), 54.5 (s, backbone C) 1.48 (t, 3JHP = 5.7 Hz, Pd−CH2CN), 5.84 (backbone H) 5.79 (backbone H)

distance (Å) 31

P NMR

Pd−CH2COCH3 2.163(7), Pd−CHAr2 2.101(8) 48.64 (s) 207.3 (t, JCP = 1.4 Hz, −CH2−COCH3), 57.7 (t, JCP = 1.3 Hz, backbone C), 25.7 (t, 2JCP = 7.1 Hz, −CH2−COCH3), 31.5 (s, −CH2−COCH3).

the corresponding distance in the chloride [PC(H)P]PdCl (2.0738(19) Å).11b In the absence of PMe3, heating or UV treatment of 7 (Scheme 1) leads to a mixture containing 7 and a dimeric Pd(0) species, {[PC(H)2P]Pd}2 (8), reported by us recently.13a A similar rearrangement was observed by Campora et al., who reported that ( i P r PCP)Pd−H ( i P r PCP = 2,6-bis[(diisopropylphosphino)methyl]phenyl) undergoes reductive C−H coupling to dimeric or polymeric [Pd(μ-iPrPCHP)]2, which was crystallographically characterized, exhibiting infinite chains of two-coordinate Pd(0) centers bridged by iPrPCHP units in a near-planar zigzag arrangement.43 As mentioned earlier, hydride 7 converts under heating in the presence of excess PMe3, through reductive elimination, to [P(CH)2P]Pd(PMe3) (6). We also reported recently the conversion of (tPCCHP)NiH (tPCHCHP = 2,2-bis(diisopropylphosphino)-trans-stilbene) to a Ni(0) complex, (tPCHCHP)Ni, similarly to the reductive elimination exhibited by 7.44 Furthermore, while the lack of hydrogen acceptor ability of a Ni(II) bis(amido)amide pincer, (MeN2N)Ni−H,45 led to decomposition, arenes capable of accepting a hydrogen aid Ni-to-arene hydrogen transfer reactions, as observed with terphenyl diphosphine Ni(II) complexes reported by the Agapie group.46

2.93 (t, JHP = 5.3 Hz, Pd(CH2)COCH3), 5.89 (backbone H)

Organometallics


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Organometallics

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C6D6, 298 K): δ 49.71 (s). 13C{1H} NMR (101 MHz, C6D6, 298 K): δ 159.5 (t, 1JCP = 14.7 Hz, ArC), 135.2 (t, 2JCP = 17.7 Hz, ArC), 132.1 (s, ArC), 130.2 (s, ArC), 129.1 (s, −CH2−CN), 127.9−127.7 (m, ArC), 124.6 (t, 3JCP = 3.2 Hz, ArC), 56.8 (s, backbone −Pd−C(H)−), 25.9 (t, 1JCP = 10.1 Hz, −CH(CH3)2), 25.0 (t, 1JCP = 11.9 Hz, −CH(CH3)2), 19.3 (t, 2JCP = 3.0 Hz, −CH(CH3)2), 19.0 (t, 2JCP = 2.6 Hz, −CH(CH3)2), 18.3 (d, 2JCP = 0.9 Hz, −CH(CH3)2), −21.1 (t, 2 JCP = 10.2 Hz, −Pd(CH2)CN). Anal. Calcd for C27H39NP2Pd: C, 59.40; H, 7.20; N 2.57. Found: C, 59.35; H, 7.28; N 2.52. Synthesis of [PC(H)P]Pd(CCPh) (4). In a 20 mL scintillation vial, 58.1 mg of 1 [PC(sp2)P]Pd(PMe3) (C28H45P3Pd, 0.1 mmol) was stirred in 5 mL of THF prior to the addition of 1 mL of a 0.1 M phenylacetylene solution in THF (0.1 mmol). The solution turned dark orange within 5 min. After 30 min, the volatiles were removed under reduced pressure. The residue was triturated with n-pentane (3 × 5 mL). A 55.2 mg amount of complex 4 (C33H42P2Pd, 607.07 g mol−1) was obtained as an off-gray powder, in 91% yield. The product was considered to be pure by 1H and 31P{1H} NMR spectroscopy. An analytically pure sample and X-ray-quality crystals were obtained by recrystallization from a saturated Et2O solution at −35 °C. Data for 4 are as follows. 1H NMR (400 MHz, C6D6, 298 K): δ 7.69−7.64 (m, 2H, ArH), 7.39 (ddd, 3JHH = 2.8, 3JHH = 2.0, 4JHH =1.0 Hz, 2H, ArH), 7.18−7.05 (m, 7H, ArH), 7.00−6.94 (m, 1H, ArH), 6.90 (ddd, 3JHH = 7.3, 3JHH = 4.1, 4JHH = 0.8 Hz, 2H, ArH), 5.79 (s, 1H, backbone −Pd− C(H)−), 2.41 (qdd, 2JHP = 9.6, 3JHH = 6.9, 4JHH = 2.7 Hz, 2H, −CH(CH3)2), 2.36−2.25 (m, 2H, −CH(CH3)2), 1.39 (dd, 3JHP = 15.5, 3JHH = 7.9 Hz, 12H, −CH(CH3)2), 1.05 (dd, 3JHP = 14.9, 3JHH = 7.7 Hz, 6H, −CH(CH3)2), 1.00 (dd, 3JHP = 14.7, 3JHH = 7.3 Hz, 6H, −CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6, 298 K): δ 55.11 (s). 13 C{1H} NMR (101 MHz, C6D6, 298 K): δ 159.9 (t, 1JCP = 14.9 Hz, ArC), 135.8 (t, 2JCP = 16.6 Hz, ArC), 132.1 (s, ArC), 131.1 (s, ArC), 130.5 (s, ArC), 130.3 (s, ArC), 128.3 (s, ArC), 128.1 (t, 2JCP = 13.3 Hz, ArC), 124.7 (s, ArC), 124.7−124.6 (m, ArC), 117.2 (s, C6H5−C(sp)C(sp)−Pd), 113.9 (t, 2JCP = 20.4 Hz, C6H5−C(sp)−C(sp)−Pd), 54.5 (s, backbone −Pd−C(H)−), 26.4 (t, 1JCP = 10.6 Hz, −CH(CH3)2), 25.3 (t, 1JCP = 12.8 Hz, −CH(CH3)2), 19.7 (t, 2JCP = 2.7 Hz, −CH(CH3)2), 18.6 (t, 2JCP = 2.3 Hz, −CH(CH3)2), 18.4 (t, 2JCP = 1.3 Hz, −CH(CH3)2). Anal. Calcd for C33H42P2Pd: C, 65.29; H, 6.97. Found: C, 64.93; H, 6.73. Synthesis of [PC(H)P]Pd(η1-fluorenyl) (5). In a 20 mL scintillation vial, 58.1 mg of 1 (C28H45P3Pd, 0.1 mmol) was stirred with 16.6 mg of fluorene (C13H10, 166.22 g mol−1, 0.1 mmol) in 5 mL of toluene. After 1 h at ambient temperature, an aliquot analyzed by 1 H and 31P NMR spectroscopy indicated that the reaction does not occur at a reasonable rate at room temperature. The solution was then transferred to a Schlenk flask, brought outside the glovebox, and heated in an oil bath at 100 °C. After 6 h, the solution turned yellow. The flask was brought back in the glovebox, and the volatiles were removed under reduced pressure. The residue was triturated with npentane (3 × 5 mL). A 54.4 mg amount of complex 5 (C38H46P2Pd, 671.15 g mol−1) was obtained as a bright yellow powder, in 81% yield. The product was found to be pure by 1H and 31P{1H} NMR spectroscopy. An analytically pure sample was isolated by recrystallization from a concentrated Et2O solution at −35 °C. Data for 5 are as follows. 1H NMR (500 MHz, C6D6, 298 K): δ 8.07 (d, 3JHH = 7.6 Hz, 2H, ArH), 7.92 (d, 3JHH = 7.7 Hz, 1H, ArH), 7.73 (d, 3JHH = 7.6 Hz, 1H, ArH), 7.38 (td, 3JHH = 7.4, 4JHH = 1.2 Hz, 1H, ArH), 7.32 (td, 3JHH = 7.4 Hz, 4JHH = 1.3 Hz, 1H, ArH), 7.30−7.23 (m, 4H, ArH), 7.16− 7.11 (m, 1H, ArH), 7.05 (t, 3JHH = 7.3 Hz, 1H, ArH), 6.99 (q, 3JHH = 6.5 Hz, 2H, ArH), 6.91 (t, 3JHH = 7.4 Hz, 1H, ArH), 6.75 (t, 3JHH = 7.4 Hz, 1H, ArH), 6.16−6.01 (m, 1H, −Pd−CH(Flu)), 5.89 (s, 1H, backbone −Pd(CH)−), 2.63−2.50 (m, 1H, −CH(CH3)2), 2.41 (qd, 2 JHP = 11.2 Hz, 3JHH = 7.0 Hz, 1H, −CH(CH3)2), 1.31 (dd, 3JHP = 15.2 Hz, 3JHH = 7.2 Hz, 3H, −CH(CH3)2), 1.27 (d, 2JHP = 2.5 Hz, 1H, −CH(CH3)2), 1.24 (dd, 3JHP = 15.8 Hz, 3JHH = 7.1 Hz, 3H, −CH(CH3)2), 1.16−1.06 (m, 7H, −CH(CH3)2 and −CH(CH3)2), 0.72 (ddd, 3JHP = 18.9 Hz, 3JHP = 13.3 Hz, 3JHH = 7.0 Hz, 6H, −CH(CH3)2), 0.52 (dd, 3JHP = 17.0 Hz, 3JHH = 7.0 Hz, 3H, −CH(CH3)2), 0.28 (dd, 3JHP = 17.0 Hz, 3JHH = 7.1 Hz, 3H,

EXPERIMENTAL SECTION

General Considerations. Unless noted otherwise, all experiments were performed under a nitrogen atmosphere in a glovebox. Molecular sieves (4 Å, 1−2 mm beads) were ordered from Alfa Aesar and were activated by heating at 250 °C for 2 days under vacuum on a Schlenk line and were then brought into a N2-filled glovebox. Hexanes, npentane, toluene, diethyl ether, tetrahydrofuran, and toluene were stored over molecular sieves in an N2-filled glovebox. Deuterated solvents were purchased from Cambridge Isotope Laboratories. C6D6 was brought in the glovebox, stirred and dried over CaH2 for 24 h, filtered, and stored over molecular sieves. [PC(sp2)P]Pd(PMe3) (1) was synthesized according to a published procedure.13b The other reagents were used as received. NMR spectra were recorded on either a Bruker 400 MHz (1H NMR, 400 MHz; 13C NMR, 100 MHz; 31P NMR, 162 MHz) or Bruker 500 MHz (1H NMR, 500 MHz; 13C NMR, 126 MHz; 31P NMR, 200 MHz) spectrometer; peaks are reported in ppm. For 1H and 13C NMR spectra, the residual solvent peak was used as an internal reference. For 31P NMR spectra, an external reference of PPh3 was prepared and locked to −5.0 ppm in C6D6. Single-crystal X-ray diffraction was conducted on Bruker Kappa or Duo instruments with Mo Kα X-ray sources. CHN analyses were performed on a CE-440 Elemental Analyzer or by Midwest Microlab, LLC. Synthesis of [PC(H)P]Pd(CH 2 COCH 3) (2). In a 20 mL scintillation vial, 58.1 mg of 1 (C28H45P3Pd, 0.1 mmol) was stirred in 5 mL of THF prior to the addition of 1.5 mL of a 0.1 M (CH3)2C O solution in Et2O (0.15 mmol). The solution turned red within 5 min, and after 15 min more, the volatiles were removed under reduced pressure and the resulting residue was triturated three times with 5 mL of n-pentane. A 47.3 mg amount of complex 2 (C28H42OP2Pd, 563.01 g mol−1) was obtained as an off-gray powder, in 84% yield. The product was considered to be pure by 1H and 31P{1H} NMR spectroscopy. Data for 2 are as follows. 1H NMR (400 MHz, C6D6, 298 K): δ 7.36−7.31 (m, 2H, ArH), 7.22−7.16 (m, 2H, ArH), 7.11− 7.06 (m, 2H, ArH), 6.91 (t, 2JHH = 7.4 Hz, 2H, ArH), 5.89 (s, 1H, backbone −Pd−C(H)−), 2.93 (t, 3JHP = 5.3 Hz, 2H, −Pd(CH2)COCH3), 2.72−2.63 (m, 2H, −CH(CH3)2), 2.50−2.39 (m, 2H, −CH(CH3)2), 2.29 (s, 3H, −Pd−CH2(CO)CH3), 1.29 (dd, 3JHP = 15.0, 3JHH =7.7 Hz, 6H, −CH(CH3)2), 1.21 (td, 3JHP = 8.6, 3JHH = 7.2 Hz, 6H, −CH(CH3)2), 1.07 (dd, 3JHP =13.4, 3JHH = 6.5 Hz, 6H, −CH(CH3)2), 0.93 (dd, 3JHP = 14.9, 3JHH =7.8 Hz, 6H, −CH(CH3)2). 31 1 P{ H} NMR (162 MHz, C6D6, 298 K): δ 48.64 (s). 13C{1H} NMR (101 MHz, C6D6, 298 K): δ 207.3 (t, 3JCP = 1.4 Hz, −Pd− CH2(CO)CH3), 159.6 (t, 3JCP = 14.6 Hz, ArC), 135.9 (t, 2JCP = 17.1 Hz, ArC), 132.22 (s, ArC), 130.0 (s, ArC), 127.9 (s, ArC), 124.5 (t, 3 JCP = 3.2 Hz, ArC), 57.7 (t, 2JCP = 1.3 Hz, backbone −Pd(CH)), 31.5 (s, −Pd−CH2(CO)CH3), 25.70 (t, 2JCP = 7.1 Hz, −Pd−CH2(CO)CH3), 25.5 (d, 1JCP = 11.8 Hz, −CH(CH3)2), 24.9 (t, 1JCP = 9.5 Hz, −CH(CH3)2), 20.2 (t, 2JCP = 3.6 Hz, −CH(CH3)2), 19.00 (t, 2JCP = 2.1 Hz, −CH(CH3)2), 17.6 (s, −CH(CH3)2). Anal. Calcd for C28H42OP2Pd: C, 59.73; H, 7.52. Found: C, 59.81; H, 7.25. Synthesis of [PC(H)P]Pd(CH2CN) (3). In a 20 mL scintillation vial, 58.1 mg of 1 (C28H45P3Pd, 0.1 mmol) was stirred in 5 mL of THF prior to the addition of 1.5 mL of a 0.1 M acetonitrile solution in THF (0.15 mmol). The solution turned red within 15 min. After 1 h, the volatiles were removed under reduced pressure and the residue was triturated with n-pentane (3 × 5 mL). A 46.9 mg amount of complex 3 (C27H39NP2Pd, 545.98 g mol−1) was obtained as an off-brown powder, in 86% yield. The product was considered to be pure by 1H and 31 1 P{ H} NMR spectroscopy. An analytically pure sample was isolated by recrystallization from a concentrated Et2O solution at −35 °C. Data for 3 are as follows. 1H NMR (400 MHz, C6D6, 298 K): δ 7.39 (d, 3 JHH = 8.2 Hz, 2H, ArH), 7.12−7.05 (m, 4H, ArH), 6.91 (t, 3JHH = 7.4 Hz, 2H, ArH), 5.84 (s, 1H, backbone −Pd−C(H)−), 2.34 (dt, 2JHP = 20.4, 3JHH = 6.9 Hz, 4H, −CH(CH3)2), 1.48 (t, 3JHP = 5.7 Hz, 2H, −Pd−CH2−CN), 1.26 (dd, 3JHP = 15.6, 3JHH = 8.0 Hz, 6H, −CH(CH3)2), 1.11 (dd, 3JHP = 15.5, 3JHH =7.9 Hz, 6H, −CH(CH3)2), 1.00 (dd, 3JHP = 14.5, 3JHH = 7.2 Hz, 6H, −CH(CH3)2), 0.91 (dd, 3JHP = 14.5, 3JHH = 7.2 Hz, 6H, −CH(CH3)2). 31P{1H} NMR (162 MHz, F


Article

Organometallics

1.4 Hz, −CH(CH3)2). Anal. Calcd for C25H38P2Pd: C, 59.23; H, 7.56. Found: C, 59.31; H, 7.64. Reaction of 1 with 9,10-Dihydroanthracene. A 58.1 mg portion of 1 (C28H45P3Pd, 0.1 mmol) was dissolved in toluene, and 18.1 mg of 9,10-dihydroanthracene (C14H12, 0.1 mmol) was added slowly. After 2 h at ambient temperature, no reaction took place, as indicated by an aliquot analyzed by 1H and 31P NMR spectroscopy. The reaction mixture was then placed in a Schlenk flask, brought outside the glovebox, and heated in an oil bath at 90 °C for 36 h, during which time the color gradually changed to muddy orange. 1H and 31P NMR spectroscopic data showed that the reaction was complete, and anthracene was the major organic product present in the mixture. The Pd-containing product was [PC(H)2P]Pd(PMe3) (6; 88% based on 1H NMR spectroscopy), with a small amount of the intermediate hydride [PC(H)P]PdH (7; 12%) also being obtained. Conversion of 7 to 8. Placing 30.5 mg of 7 (0.6 mmol) in C6D6 in a quartz J. Young NMR tube and exposing it to UV radiation from a 450 W mercury lamp led to a mixture of 7 and {[PC(H2)P]Pd}2 (8), containing about 40% 8. Similar results were obtained by prolonged heating of the NMR tube at 80 °C.

−CH(CH3)2). 31P{1H} NMR (202 MHz, C6D6, 298 K): δ 46.91 (d, 2 JPP = 361.1 Hz), 43.95 (d, 2JPP = 361.6 Hz). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ 159.6 (d, 2JCP = 29.3 Hz, ArC), 158.4 (d, 2JHP = 28.3 Hz, ArC), 156.7 (s, ArC), 155.0 (s, ArC), 138.1 (s, ArC), 137.4 (s, ArC), 136.2 (d, 2JCP = 33.8 Hz, ArC), 134.7 (d, 2JCP = 35.7 Hz, ArC), 132.5 (s, ArC), 131.7 (s, ArC), 130.3 (s, ArC), 129.6 (s, ArC), 127.8− 127.7 (m, ArC), 127.5−127.4 (m, ArC), 124.7 (s, ArC), 124.6 (s, ArC), 124.5−124.5 (m, ArC), 124.2−124.2 (m, ArC), 124.1 (s, ArC), 123.9 (s, ArC), 121.4 (s, ArC), 121.3 (s, ArC), 120.3 (s, ArC), 120.0 (s, ArC), 56.8 (s, backbone −Pd−C(H)Ar2), 34.0 (t, 3JHP = 5.9 Hz, −Pd− C(H)(Flu)), 26.1−25.7 (m, −CH(CH3)2), 25.7−25.4 (m, −CH(CH3)2), 25.4−25.0 (m, −CH(CH3)2), 24.8−24.2 (m, −CH(CH3)2), 19.8−19.6 (m, −CH(CH3)2), 19.6−19.4 (m, −CH(CH3)2), 19.4−19.2 (m, −CH(CH3)2), 18.5−18.2 (m, −CH(CH3)2). Anal. Calcd for C38H46P2Pd: C, 68.01; H, 6.91. Found: C, 67.90; H, 7.02. Synthesis of [PC(H)2P]PdPMe3 (6). A 40.5 mg portion of [PC(H)P]PdH (7; 0.08 mmol) was dissolved in 5 mL of THF and transferred to a Schlenk flask. A 0.16 mL portion of a solution of PMe3 (1 M in THF) was placed in the flask and the solution heated for 3.5 h at 60 °C. The mixture turned light orange. After removal of volatiles under reduced pressure, the orange powder was dissolved in n-pentane and filtered. Analytically pure 6 was isolated by crystallization from a concentrated n-pentane solution at −35 °C, in the glovebox. Yield for 6 (C28H47P3Pd, 583.03 g mol−1): 21.4 mg (46%). The room temperature 1H, 31P{1H}, and 13C{1H} NMR spectra were broad. Spectra taken at lower or higher temperatures were not resolved. Data for 6 are as follows. 1H NMR (500 MHz, C6D6, 298 K): δ 7.40 (dd, 3 JHH = 7.6 Hz, 4JHH = 1.6 Hz, 2H, ArH), 7.34 (dd, 3JHH = 7.6 Hz, 4JHH = 1.7 Hz, 2H, ArH), 7.11 (br s, endo −Ar2CH(H)), 7.08 (td, 3JHH = 7.4 Hz, 4JHH = 1.4 Hz, 2H, ArH), 7.03 (td, 3JHH = 7.4 Hz, 4JHH = 1.5 Hz, 2H, ArH), 3.40 (br s, exo −Ar2CH(H)), 2.11 (ddt, 2JHP = 10.6 Hz, 2 JHP = 8.7 Hz, 3JHH = 3.5 Hz, 4H, −CH(CH3)2), 1.34 (d, 2JHP = 3.5 Hz, 9H, −Pd−P(CH3)3), 1.18 (br s, 12H, −CH(CH3)2), 0.92 (dd, 3 JHP = 12.5 Hz, 3JHH = 6.5 Hz, 12H, −CH(CH3)2). 31P{1H} NMR (202 MHz, C6D6, 298 K): δ 27.17 (d, 2JPP = 103.2 Hz), −34.67 (t, 2JPP = 103.7 Hz). 13C NMR (126 MHz, C6D6, 298 K): δ 156.0 (t, 2JCP = 2.9 Hz, ArC), 148.7 (t, 1JCP = 10.5 Hz, ArC), 132.0 (s, ArC), 132.0 (t, 3 JCP = 3.0 Hz, ArC), 127.9 (s, ArC), 125.1 (s, ArC), 38.2 (t, 3JCP = 19.4 Hz, backbone Ar2CH2), 29.8−26.2 (br s, −CH(CH3)2), 22.5 (dd, 1JCP = 7.4 Hz, 3JCP = 3.2 Hz, −CH(CH3)2), 20.8 (t, 3JCP = 8.5 Hz, −P(CH3)3), 20.6−17.9 (br s, −CH(CH3)2). Anal. Calcd for C28H47P3Pd: C, 57.68; H, 8.13. Found: C, 57.55; H, 7.99. Synthesis of [PC(H)P]PdH (7). A 54.1 mg portion of [PC(H)P]PdCl (C25H37ClP2Pd, 541.39 g mol−1, 0.1 mmol) was dissolved in 5 mL of Et2O and cooled to −35 °C. To this chilled solution was added 1 mL of a solution of Li[HBEt3] (0.1 M in THF). The mixture turned cloudy and was stirred for 2 h at −35 °C. The volatiles were removed under reduced pressure, and the residue was extracted with 10 mL of n-pentane and filtered over Celite. A bright yellow solution was obtained, from which the volatiles were removed under reduced pressure again. A 44.1 mg amount of 7 (C25H38P2Pd, 506.95 g mol−1) was obtained in 87% yield. An analytically pure sample was isolated by recrystallization from a concentrated n-pentane solution at −35 °C. Data for 7 are as follows. 1H NMR (400 MHz, C6D6, 298 K): δ 7.55 (d, 3JHH = 8.4 Hz, 2H, ArH), 7.20−7.08 (m, 4H, ArH), 6.91 (t, 3JHH = 7.3 Hz, 2H, ArH), 5.70 (d, 3JHH = 5.5 Hz, 1H, backbone −Pd−C(H)), 2.20−2.05 (m, 4H, −CH(CH3)2), 1.26 (dd, 3JHP = 16.1 Hz, 3JHH = 7.7 Hz, 6H, −CH(CH3)2), 1.13 (dd, 3JHP = 16.2 Hz, 3JHH = 7.7 Hz, 6H, −CH(CH3)2), 0.95 (dd, 3JHP = 14.4 Hz, 3JHH = 7.1 Hz, 6H, −CH(CH3)2), 0.83 (dd, 3JHP = 15.1 Hz, 3JHH = 7.2 Hz, 6H, −CH(CH3)2), −4.88 (td, 2JHP = 17.2, 3JHH = 5.6 Hz, 1H, −Pd−H). 31 1 P{ H} NMR (162 MHz, C6D6, 298 K): δ 65.72 (s). 13C{1H} NMR (101 MHz, C6D6, 298 K): δ 160.6 (t, 2JCP = 14.9 Hz, ArC), 137.5 (t, 2 JCP = 16.5 Hz, ArC), 132.1 (s, ArC), 129.9 (s, ArC), 128.5 (t, 3JCP = 7.9 Hz, ArC), 123.5 (t, 3JCP = 3.1 Hz, ArC), 58.0 (s, backbone −Pd− C(H)), 25.9 (t, 1JCP = 10.8 Hz, −CH(CH3)2), 24.0 (t, 1JCP = 12.9 Hz, −CH(CH3)2), 20.3 (t, 3JCP = 4.0 Hz, −CH(CH3)2), 18.9 (t, 3JCP = 1.8 Hz, −CH(CH3)2), 18.7 (t, 3JCP = 4.2 Hz, −CH(CH3)2), 18.4 (t, 3JCP =

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures, tables, and CIF files giving NMR spectra and crystallographic data and details for compounds 2−7. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00414.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.M.I.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Dr. Allen Oliver for crystallographic assistance. This work was supported by the University of Notre Dame and by the donors of the American Chemical Society Petroleum Research Fund (ACS PRF # 53536-DNI3).

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DEDICATION Dedicated to the memory of Gregory L. Hillhouse, an extraordinary scientist and wonderful mentor. REFERENCES

(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (2) Duetz, W. A.; Van Beilen, J. B.; Witholt, B. Curr. Opin. Biotechnol. 2001, 12, 419. (3) (a) Dorwald, F. Z. Metal carbenes in organic synthesis; Wiley-VCH: Weinheim, New York, 1999. (b) Scott, J.; Mindiola, D. J. Dalton Trans. 2009, 8463. (c) N-heterocyclic carbenes in transition metal catalysis; Springer: Berlin, 2007. (4) (a) Stander-Grobler, E.; Strasser, C. E.; Schuster, O.; Cronje, S.; Raubenheimer, H. Inorg. Chim. Acta 2011, 376, 87. (b) Broring, M.; Brandt, C. D.; Stellwag, S. Chem. Commun. 2003, 2344. (c) Schuster, O.; Raubenheimer, H. G. Inorg. Chem. 2006, 45, 7997. (d) Oulié, P.; Nebra, N.; Saffon, N.; Maron, L.; Martin-Vaca, B.; Bourissou, D. J. Am. Chem. Soc. 2009, 131, 3493. (e) López-Alberca, M. P.; Mancheño, M. J.; Fernández, I.; Gómez-Gallego, M.; Sierra, M. A.; Torres, R. Org. Lett. 2007, 9, 1757. (f) Stander-Grobler, E.; Schuster, O.; Heydenrych, G.; Cronje, S.; Tosh, E.; Albrecht, M.; Frenking, G.; Raubenheimer, H. G. Organometallics 2010, 29, 5821. (g) Weng, W.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. Organometallics 2007, 26, 3315. (h) Kessler, F.; Szesni, N.; Põhako, K.; Weibert, B.; Fischer, H. Organometallics 2009, 28, 348. (i) Taubmann, C.; Ö fele, K.; Herdtweck, E.; Herrmann, W. A. Organometallics 2009, 28, 4254.

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