Anionic Nickel(II) Complexes with Doubly Deprotonated PNP Pincer

Dec 28, 2012 - deprotonated PNP-pincer ligands coordinated to a nickel(II) center. DFT calculations, as well as multinuclear NMR spectroscopy and the ...
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Anionic Nickel(II) Complexes with Doubly Deprotonated PNP PincerType Ligands and Their Reactivity toward CO2 Matthias Vogt,†,§ Orestes Rivada-Wheelaghan,†,§ Mark A. Iron,‡ Gregory Leitus,‡ Yael Diskin-Posner,‡ Linda J. W. Shimon,‡ Yehoshoa Ben-David,† and David Milstein*,† †

Department of Organic Chemistry and ‡Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: The aromatization−dearomatization reaction of pincer-type complexes prompted by protonation−deprotonation of the pincer “arm” is a key step in bond activation chemistry and atom-economic catalytic transformations. However, the possibility of double deprotonation of ancillary pincer ligands is rarely discussed in the literature. Here we report on square-planar cationic nickel(II) complexes of PNPR type ligands (PNP = 2,6-bis[(dialkylphosphino)methyl]pyridine with R = iPr, tBu), which can be readily transformed into the doubly deprotonated anionic species. The complexes [Ni(PNPR)Cl]Cl (3, R = iPr; 4, R = tBu) are readily prepared from the reaction of NiCl2·6H2O and the PNPR ligand in THF. Treatment of the cationic chloro complexes 3 and 4 with 2 equiv of MeLi gives the nickel(II) methyl complexes [Ni(PNPR*)Me] (7, R = iPr; 8, R = tBu), the asterisk indicates the deprotonated pincer arm). Reaction of 7 and 8 with an additional 1 equiv of MeLi gives the anionic complexes [Li(DME)3][Ni(PNPiPr**)Me] (9-DME, DME = 1,2-dimethoxyethane) and [Li(Et2O)2][Ni(PNPtBu**)(Me)] (10-Et2O), respectively. Single-crystal X-ray diffraction studies exhibit doubly deprotonated PNP-pincer ligands coordinated to a nickel(II) center. DFT calculations, as well as multinuclear NMR spectroscopy and the X-ray structures, suggest a conjugated π-system with delocalization of the negative charge throughout the carbon backbone of the pincer ligand. The electrophilic attack of complex 9 by CO2 and tautomerization gives [Li][Ni(PNPiPr*COO)(Me)] (11). The dearomatized complex that is formed contains an exocyclic methylene carbon atom and a carboxylate moiety adjacent to the second pincer arm.

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INTRODUCTION Electron-rich transition-metal (TM) complexes decorated with bulky, tridentate, pyridine- and acridine-based LNL′ pincertype ligands have been shown to have extraordinary properties in bond activation chemistry (e.g., S−H,1,2 N−H,3,4 O−H,5 H− H,6−8 C−H,7,9−12 C−C,13 and CO214,15)16−20 and have gained major importance in atom-economic catalytic transformations, such as the dehydrogenative coupling (DHC) of alcohols to esters,21−24 the DHC of alcohol and amines to amides25−27 or imines,28 the synthesis of primary amines directly from alcohols and ammonia,29,30 the hydrogenations of esters to alcohols,31,32 the direct hydrogenations of amides to alcohols and amines,33 the hydrogenation of organic crabonates/carbamates34 or urea derivatives35 to methanol, and the hydrogenation of CO2.36−39 Metal−ligand cooperation (MLC) via an aromatization− dearomatization reaction of the pincer ligand plays an essential role throughout the reactions (Scheme 1A).19,20,40−42 Deprotonation of both arms of the ancillary pincer ligand is possible, yet sparsely discussed in the literature.43 An early publication by Venanzi and co-workers describes dianionic [PtIIPCP] alkyl complexes (PCP = 2,6-bis[(diphenylphosphino)methyl]phenyl) as in situ generated species in methylation reactions of the pincer arms.44 Examples of doubly deprotonated PNP−RhI complexes are given by © XXXX American Chemical Society

Scheme 1. (A) Reversible Deprotonation of a PNP Pincer (a) To Give the Dearomatized Form (b) and the DoubleDeprotonation Product (c); (B) Resonance Structures of Compound c with Phosphorus Ylide Structures Not Shown

Taube and co-workers and were characterized as in situ intermediates by NMR spectroscopy.45 Our group recently reported anionic PNP-type Pd(II) and Pt(II) complexes (PNP = 2,6-bis[(di-tert-butylphosphino)methyl]pyridine) as rare examples of anionic TM complexes lacking strong π-acceptor Received: November 12, 2012

A

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Figure 1. The PNPiPr pincer ligand chelates the nickel(II) center via the phosphine and pyridine moieties; a chloride

ligands such as CO or olefins, including a DFT study, which suggested the formation of a conjugated π-system spread throughout both the deprotonated arms and the pyridine backbone (Scheme 1B).46 However, to the best of our knowledge, no crystallographically characterized doubly deprotonated complex was reported. Along these lines, we herein report the fully structurally characterized neutral and anionic organo-nickel(II) complexes [Ni(PNPiPr*)Me] (7), [Li(solvent)n][Ni(PNPR**)Me] (9, R = iPr; n = 3, solvent = 1,2-dimethoxyethane (DME); 10, R = tBu, n = 2, solvent = Et2O; PNPtBu = 2,6-bis[(di-tert-butylphosphino)methyl]pyridine, PNPiPr = 2,6-bis[(di-iso-propylphosphino)methyl]pyridine; one asterisk indicates single deprotonation and two asterisks indicate double deprotonation). Complex 9 reacts with CO2 via C−C bond formation.

Figure 1. ORTEP drawing of [Ni(PNPiPr) (Cl)]Cl (3) with ellipsoids at the 50% probability level (isopropyl groups are drawn as wire frames). Hydrogen atoms (except for pincer arms) and the Cl− counterion are omitted for clarity.

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RESULTS AND DISCUSSION The synthesis of the cationic and neutral PNP nickel pincer compounds is a modification of a previously reported procedure by van der Vlugt and co-workers.1 The cationic complex [Ni(PNPiPr)Cl]Cl (3) was readily prepared by the reaction of nickel(II) chloride hexahydrate and PNPiPr in THF at ambient temperature (Scheme 2). The 31P{1H} NMR

ligand completes the slightly distorted square planar coordination sphere. A second chloride is the counteranion and does not show any close contacts to the metal center within the crystal packing. When the cationic complex 3 was suspended in ether and reacted with 1 equiv of LiHMDS (HMDS = − N(SiMe3)2), the yellow suspension turned into a dark brown-green solution. The neutral dearomatized species [Ni(PNPiPr*)Cl] (5) was isolated via extraction with n-pentane, and a crystalline solid was obtained from a concentrated solution. The 31P{1H} NMR shows an AB spin system indicating two chemically inequivalent phosphorus nuclei with two doublets (2Jpp = 304 Hz) at 44.7 and 41.3 ppm. The 1H NMR spectrum of 5 has three distinct signals for the protons of the pyridine backbone shifted to lower frequencies (6.32 (vt), 6.16 (d), 5.21 ppm (d)), signifying a reduction of symmetry (i.e., loss of the C2 rotational axis from the quasi-C2v symmetric complex) due to dearomatization of the complex. The deprotonation gives rise to two doublet signals for the pincer arms in the 1H NMR spectrum coupled to two different phosphorus nuclei. The doublets correspond to the methine proton (CH) at 3.09 ppm (d, 2JHP = 5.0 Hz, 1H) and the two equivalent methylene protons at 2.15 ppm (d, 2JHP = 8.2 Hz). The organo-nickel(II) complex [Ni(PNPiPr*)(Me)] (7) was prepared either via substitution of the chloro ligand of 5 with a methyl group using 1 equiv of MeLi as a nucleophile or directly from 3 by using 2 equiv of MeLi. The significant resonance in the 1H NMR spectrum for the introduced methyl group is a doublet of doublets (dd) at −0.30 ppm coupled to the two phosphorus nuclei with 3JHP = 9.6 Hz and 3JHP = 8.1 Hz. In the 13 C{1H} NMR spectrum the resonance for the Ni−CH3 moiety was identified as a virtual triplet at −23.3 ppm (2JCP = 24.3 Hz). Complex 7 remains dearomatized; the three individual resonances for the pyridine protons are shifted to lower frequencies (6.50 (vt) ppm for p-CH and two doublets at 6.35 and 5.38 ppm for m-CH). The 13C QDEPT NMR spectrum clearly indicates a methine carbon resonance at 59.1 ppm and a methylene resonance at 33.0 ppm for the pincer arms. Single crystals of 7 were obtained from a concentrated n-pentane solution. The X-ray structure shows a square-planar coordination sphere around the Ni(II) center formed by the PNPiPr and methyl ligands (Figure 2). The dearomatized structure is clearly indicated by the considerably shorter C1−C2 bond (by 0.115 Å) compared to C6−C7. Notably, P1−C1 is shorter (by 0.070 Å) than P2−C7, indicating a contribution from the aromatic

Scheme 2. Reaction Scheme for the Synthesis of Cationic, Neutral, and Anionic Nickel(II) PNP Pincer Complexes 3− 10

spectrum in CDCl3 has a singlet centered at 48.5 ppm indicating two phosphorus nuclei in indistinguishable chemical environments. The 1H NMR spectrum has two resonances for the pyridine backbone in the aromatic region, namely a virtual triplet at 7.92 ppm for the p-CH and a doublet, with an integral value equal to two protons, at 7.53 ppm. The complex was recrystallized from a CH2Cl2/n-pentane mixture, and orange crystals suitable for X-ray diffraction analysis were obtained. The ORTEP plot of the determined structure of 3 is shown in B

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corresponding 13C{1H} NMR chemical shift (CH−P) is observed at 49.0 ppm as a broad singlet consistent with two methine carbon atoms (confirmed by 13C QDEPT spectroscopy). The double deprotonation, observed in the NMR spectroscopy studies, was also established by X-ray crystallography. An ORTEP plot of the molecular structure of 9-DME is shown in Figure 3. There are two separated ions in the crystal

Figure 2. ORTEP drawing of [Ni(PNPiPr*)(Me)] (7) with ellipsoids at the 50% probability level (isopropyl groups are drawn as wire frames). Hydrogen atoms (except for pincer arms and Ni−Me) are omitted for clarity.

ylide structure, although the dearomatized structure is dominant (Table 1). Treatment of a cold suspension (−40 °C) of 3 with a 3-fold excess of MeLi afforded the doubly deprotonated organo-nickel species [Li(solv)x][Ni(PNPiPr**)Me] (9-solvent). Alternatively, subjection of the brownish green solution of 7 to a strong base (MeLi in Et2O at −40 °C) resulted in a color change to orange and led to the same organo-nickel complex (9-solvent). The doubly deprotonated anionic species that is formed is thermally stable and can be crystallized after workup from a n-pentane-layered DME solution to give red crystals of [Li(DME)3][Ni(PNPiPr**)Me] (9-DME). Trace amounts of water protonate 9-solvent to give complex 7. For 9-solvent, two chemically equivalent phosphorus nuclei give rise to only one sharp singlet in the 31P {1H} NMR spectrum with a chemical shift of 46.1 ppm. The 13C QDEPT and 1H NMR spectra suggest the formation of a symmetric but dearomatized complex; only two signals are observed for the pyridine backbone (6.54 ppm (m, 3JHH = 7.5 Hz, JHP = 1.6 Hz, 1H) and 5.34 ppm (d, 3JHH = 7.5 Hz, 2H)). The deprotonation of both PNP arms is indicated in the 1H NMR spectrum by a broad single resonance for both methine CH moieties at 2.57 ppm with an integration value consistent with two protons. The

Figure 3. ORTEP drawing of [Li(DME)3][Ni(PNPiPr**)(Me)] (9DME) with ellipsoids at the 50% probability level (isopropyl groups are drawn as wire frames). Hydrogen atoms (except for pincer arms and Ni−Me) are omitted for clarity.

lattice. Three chelating DME molecules form the octahedral coordination sphere around the Li+ countercation ([Li(DME)3]+) of the anionic [Ni(PNPiPr**)Me]− fragment. The Ni(II) center resides in a square-planar coordination sphere formed by the doubly deprotonated PNP pincer and the methyl groups. On the basis of NMR data and DFT calculations on the related group 10 anionic complexes [M(PNP)Me]− (M = Pd, Pt), we previously concluded that upon the second deprotonation a π-system with a delocalized negative charge is formed throughout the arms and the pyridine carbon backbone (Scheme 1).46 With the X-ray structure of 9-DME in hand, we can now analyze the experimental bond lengths for the C1−C7 scaffold (average C−C bond length 1.40 ± 0.02 Å)

Table 1. Selected Bond Lengths for the Crystal Structures of Complexes 3, 7, 9, and 10 and the DFT Calculated Bond Lengths for the Optimized Structures of [Ni(PNPiPr**)Me]− (9) and [Ni(PNPtBu**)Me]− (10) bond length (Å)

Ni1−N1 Ni1−P1 Ni1−P2 Ni1−Xa C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C7 N1−C2 N1−C6 P1−C1 P2−C7 a

[Ni(PNPiPr)Cl]Cl (3)

[Ni(PNP *)Me] (7)

[Li(DME)3][Ni(PNPiPr**)Me] (9-DME)

[Li(Et2O)2] [Ni(PNPtBu**)Me] (10-Et2O)b

DFT (9)

DFT (10)

1.915(3) 2.192(1) 2.194(1) 2.163(1) 1.507(5) 1.382(5) 1.391(6) 1.390(5) 1.385(5) 1.512(5) 1.372(5) 1.371(5) 1.822(4) 1.822(4)

1.954(2) 2.191(1) 2.162(1) 1.955(2) 1.385(3) 1.440(3) 1.363(3) 1.404(3) 1.375(3) 1.500(3) 1.397(2) 1.368(2) 1.758(2) 1.828(2)

1.947(2) 2.184(1) 2.182(1) 1.954(2) 1.406(2) 1.418(2) 1.384(3) 1.378(3) 1.420(2) 1.403(3) 1.395(2) 1.395(2) 1.740(2) 1.735(2)

1.958(1) 2.215(1) 2.205(1) 1.959(2) 1.391(2) 1.434(2) 1.368(2) 1.398(2) 1.414(2) 1.433(2) 1.389(2) 1.379(2) 1.755(2) 1.763(2)

1.964 2.223 2.223 1.971 1.430 1.440 1.410 1.410 1.440 1.429 1.414 1.413 1.764 1.765

1.971 2.202 2.209 1.976 1.426 1.440 1.410 1.410 1.440 1.426 1.413 1.413 1.764 1.764

iPr

X = C20 (complexes 7, 9, and 10) or Cl1 (complex 3). bLi1−C5 = 2.587(4) Å; Li1−C6 = 2.392(3) Å; Li1−C7 = 2.200(3) Å. C

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Wiberg bond indices, and partial atomic charges (Truhlar’s Charge Model 5 (CM5); see Table 2 and Tables S5−S12 in the

suggesting a similar delocalization of charge for the organonickel species 9, in line with DFT calculations (vide infra). A further example of a doubly deprotonated Ni-PNP species was obtained when the brownish green solution of [Ni(PNPtBu*)(Me)] (8) was subjected to an excess of MeLi in Et2O at −40 °C. Warming the mixture to ambient temperature resulted in a drastic color change to orange. The anionic compound that formed was crystallized from a solution of npentane layered with Et2O. The highly moisture sensitive, red single crystals of [Li(Et2O)2][Ni(PNPtBu**)Me] (10-Et2O) were subjected to a X-ray diffraction analysis. An ORTEP plot of the molecular structure of 10 is shown in Figure 4.

Table 2. Selected Wiberg Bond Indices and Partial Atomic Charges (Truhlar’s CM5 Model) for Optimized Structures of [Ni(PNPtBu**)Me]− (10′) and [Ni(PNPiPr**)Me]− (9′)

CM5 charge

Wiberg index

atom

9

10

bond

9

10

Ni1 P1 P2 N1 C1 C2 C3 C4 C5 C6 C7 C20 ∑C20H3

0.314 −0.046 −0.046 −0.408 −0.281 0.096 −0.194 −0.151 −0.194 0.097 −0.278 −0.431 −0.243

0.307 −0.043 −0.043 −0.409 −0.272 0.098 −0.193 −0.151 −0.193 0.098 −0.272 −0.430 −0.239

Ni1−P1 Ni1−P2 Ni1−N1 P1−C1 P2−C7 C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C7 N1−C1 N1−C2 Ni1−C20

0.64 0.64 0.45 1.08 1.08 1.36 1.26 1.44 1.44 1.26 1.36 1.17 1.17 0.66

0.55 0.55 0.36 1.07 1.07 1.36 1.26 1.44 1.44 1.23 1.36 1.17 1.17 0.59

Supporting Information), the structures of the anions [Ni(PNPR**)(Me)]− (R = iPr (9), tBu (10)) are best described as an amide (R2N−) ligand bonded to the nickel(II) center with an additional negative charge delocalized over the meta, para, and exocyclic arm carbons of the PNP ligand. Similar results were observed for the analogous platinum(II) and palladium(II) systems.46 Reactivity toward Carbon Dioxide. A solution of complex 9 in THF, placed in an NMR tube and fitted with a rubber septum, rapidly reacts upon injection of 1 equiv of CO2 (Scheme 3). A yellow solid precipitates, which immediately redissolves to give a clear orange solution.

Figure 4. ORTEP drawing of [Li(Et2O)2][Ni(PNPtBu**)(Me)] (10Et2O) with ellipsoids at the 50% probability level (tert-butyl groups are drawn as wire frames). Hydrogen atoms (except for pincer arms and Ni−Me) are omitted for clarity.

Unlike the structure of 9-DME, the structure of 10-Et2O shows the formation of an ion pair that exhibits close contacts between the Li+ cation (Li1) and carbon atoms C5 (2.587(4) Å), C6 (2.392(3) Å), and C7 (2.200(3) Å) of the [Ni(PNPtBu**)Me]− anion, as well as partial solvation of Li1 by two Et2O molecules. However, multinuclear NMR spectroscopy in solution suggests the formation of a C2v-symmetric species, most likely due to the dissociation of the ion pair. In particular, the 31P{1H} NMR spectrum of 10-Et2O reveals two chemically equivalent phosphorus nuclei showing only one sharp singlet with a chemical shift of 46.1 ppm. Likewise, the 13 C{1H} and 1H NMR spectra indicate a symmetric complex: two resonances are observed for the three protons of the pyridine backbone in the 1H NMR spectrum, as well as a halfset of 13C resonances for the pyridine ring (1H NMR, 6.54 ppm (1H, p-CH) and 5.34 ppm (2H, m-CH); 13C{1H} NMR, 172.9 ppmn (2C, quat-C) 135.1 ppm (1C, p-CH), 92.4 ppm (2C, mCH)). Correspondingly, only a single resonance for the exocyclic methine moieties is detected in the 1H (2.82 ppm (br s, 2H)) and 13C{1H} NMR (49.0 ppm (br s, 2C)) spectra. DFT Calculations. The geometries of [Ni(PNPR)(X)]+ (X = Cl, R = iPr (3), tBu (4); X = Me, R = iPr (3a′), tBu (4a′)) and their singly and doubly deprotonated products were optimized using density functional theory (DFT) at the DFPBE/SDD(d) level of theory. The optimized bond lengths (Table 1 and Tables S1−S4 in the Supporting Information) are in reasonable agreement with the corresponding crystallographically characterized structures. From the bond distances,

Scheme 3. Reaction of Complex 9 and CO2 To Form 11a

a

Proposed reaction pathway: electrophilic attack by CO2 on an exocyclic methine carbon and subsequent tautomerization. Hydrogen bonding between the meta proton of the pyridine ring and the carbonyl of the carboxylate moiety is indicated by a dashed line.

A systematic NMR spectroscopy study (heteronuclear and multidimensional) identified the anionic complex [Li][Ni(PNPiPr*-COO−)(Me)] (11). The 31P{1H} NMR spectrum has an AB spin system with two doublets at 48.9 and 45.9 ppm (2JPP = 252.0 Hz), suggesting two inequivalent phosphorus nuclei. The methyl ligand is observed as a virtual triplet in the D

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H NMR spectrum at −0.44 ppm (3JHP = 8.0 Hz) and in the C{1H} NMR spectrum at −21.6 ppm (2JCP = 23.4 Hz). Significantly, the 1H NMR spectrum exhibits only one broadened doublet resonance at 3.05 ppm (2JPP = 8.10 Hz) for the protons of the exocyclic pincer arm integrating to a value of two hydrogen atoms. The QDEPT 13C NMR confirms a CH2 moiety and 1H−13C HSQC correlation spectroscopy confirms that both protons are adjacent to a methylene carbon at 32.5 ppm (d, 1JCP = 16.3 Hz). Proton resonances for the second exocyclic pincer arm are absent. Instead, a new quaternary carbon moiety was identified by 13C QDEPT NMR for the second arm at 77.2 ppm. In contrast to all other compounds described, one of the three distinct resonances for the pyridine backbone in 11 is drastically shifted to higher frequencies (doublet at 8.51 ppm). We assign this shift to the existence of hydrogen bonding between the meta proton of the pyridine ring and the in-plane oxygen atom of the carboxylate group adjacent to the pincer arm. Significantly, the 13C QDEPT spectrum of 11 shows, alongside the two resonances for both quaternary carbon nuclei of the pyridine ring (172.2 ppm and 157.2 ppm), an additional quaternary signal at high frequencies (178.1 ppm). This resonance shows no cross-peaks in the 1 H−13C HMBC and 1H−13C HSQC two-dimensional spectra and is assigned to the COO carboxylate carbon atom. Recently, we reported a novel activation mode of CO2 in a ruthenium pyridine based PNP pincer complex, where MLC plays an essential role in reversible CO2 binding.14 However, metal− ligand cooperation promoting reversible CO2 binding via both the nickel center and the methine exocyclic carbon may not be involved in the case of 9. A conceivable pathway for the observed addition of CO2 to 9 might proceed via the direct electrophilic attack of CO2 on the methine carbanion (Fukui plot in Figure 5, vide infra) without participation of the Ni(II)

CO2, unlike the dearomatized PNP12 and PNN13 ruthenium complexes.47 The Fukui function, specifically the negative version f−(r), indicates sites on a molecule that are susceptible to electrophilic attack.48,49 The f− function for this complex is shown in Figure 5 and clearly shows two potentially reactive sites: the pincer arms and the ring meta carbons (note that the complex has quasi-C2v symmetry). An attack on the latter by CO2 would result in an initial shortening of the conjugated system in the pincer backbone; hence, the former is more likely to undergo electrophilic attack by CO2.

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SUMMARY AND CONCLUSIONS A series of cationic, neutral, and anionic nickel(II) PNP pincer complexes were synthesized and characterized (2−11). The phenomenon of double deprotonation was investigated in detail. The molecular structures of the complexes [Li(DME)3][Ni(PNP iPr **)(Me)] (9-DME) and [Li(Et 2 O) 2 ][Ni(PNPtBu**)(Me)] (10-Et2O) were obtained from single-crystal X-ray diffraction studies. Multinuclear NMR spectroscopy results are in agreement with the determined crystal structures of the anionic nickel complexes 9 and 10, highlighting a doubly deprotonated PNP pincer ligand ancillary to a nickel(II) center. Computational DFT studies (i.e., bond lengths, Wiberg bond indices, and partial atomic charges) corroborate the existence of a π-system with delocalized negative charge throughout the carbon backbone in an amido (rather then pyridino) PNP pincer ligand. Topological analysis of the negative Fukui function shows that the complex is susceptible to electrophilic attack at the pincer exocyclic carbons, as well as at the pyridine meta carbon atoms. The reactivity of complex 9 toward CO2 was examined, and the formation of [Li][Ni(PNPiPr*-COO−)(Me)] (11) was established by NMR spectroscopy. Unlike the previously observed bonding of CO2 to a RuII−PNP pincer complex via metal−ligand cooperation, such a bond activation phenomenon was not observed for the reaction of 9 and CO2.

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EXPERIMENTAL SECTION

All experiments were carried out under an atmosphere of purified nitrogen in a MBraun Unilab 1200/780 glovebox or using standard Schlenk techniques. All solvents were purchased “reagent grade” or better. All solvents, except methylene chloride, were refluxed over sodium/benzophenone and distilled under an argon atmosphere. Methylene chloride was used as received from the supplier and dried over 4 Å molecular sieves. Deuterated solvents were used as received and degassed with argon and kept in the glovebox over 4 Å molecular sieves. The PNPtBu 50 and PNPiPr 51 ligands were prepared according to the literature procedures. NiCl2·6H2O, LiHMDS, and MeLi were used as received from Sigma-Aldrich without any further purification. 1H, 13 C{1H}, and 31P{1H} NMR spectra were recorded on Bruker AMX300, AMX-400, and AMX-500 NMR spectrometers. 1H, 13C{1H}, and 13 C{1H}-DEPTQ NMR chemical shifts are reported in ppm relative to tetramethylsilane referenced using the residual solvent signals. 31P{1H} NMR chemical shifts are reported in ppm referenced to an external 85% solution of phosphoric acid in D2O. Elemental analyses were performed on a Thermo Finnigan Italia S.p.A-FlashEA 1112 CHN elemental analyzer by the Unit of Chemical Research Support, Weizmann Institute of Science. [Ni(PNPiPr)Cl]Cl (3). NiCl2·6H2O (100 mg, 0.421 mmol) was stirred with 1 equiv of PNPiPr (143 mg, 0.421 mmol) in THF for 8 h, affording a yellow suspension. The solvent was removed in vacuo, and the yellow solid was dissolved in dichloromethane and subsequently filtered through a plug of Celite. The desired product was precipitated from the filtrate with n-pentane. The yellow product was filtered off, washed with n-pentane, and dried under high vacuum. Yield: 170 mg,

Figure 5. Fukui minus function ( f−) showing sites of susceptibility to electrophilic attack in [(PNP**)NiMe]− (9). The color red indicates values of zero, and the value of f− increases from orange to yellow to green to cyan to blue.

center, resulting in C−C bond formation to generate a carboxylate group, bearing an α-proton with increased acidity. Subsequent tautomerization gives the dearomatized complex 11, furnishing the planarization and the extension of the conjugation within the complex’s backbone. Note that the singly dearomatized complexes 5, 6 and 7, 8 do not react with E

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−0.30 (dd, 3JHH = 9.6 Hz, 3JHP = 8.1 Hz, 3H, Ni−CH3). 13C{1H} NMR (126 MHz, C6D6, 25 °C): δ 173.3 (d, 2JPC = 17.1 Hz, 1C, C− Pyquat), 157.8 (s, 1C, C−Pyquat), 132.3 (s, 1C, CH-Pypara), 113.2 (d, 3 JCP = 17.0 Hz, 1C, CH-Pymeta), 98.2 (d, 3JCP = 9.3 Hz, 1C, CH-Pymeta), 59.1 (d, 2JCP = 51.6 Hz, CH−P), 33.0 (d, 2JCP = 18.8 Hz, CH2−P), 25.0 (d, 2JCP = 27.6 Hz, CH(CH3)2), 22.4 (d, 2JCP = 19.1 Hz, CH(CH3)2), 19.4 (s, 1C, CH(CH3)2), 18.4 (s, 1C, CH(CH3)2), 18.2 (s, 1C, CH(CH3)2), 17.6 (s, 1C, CH(CH3)2), −23.3 (t, 2JCP = 24.3 Hz, Ni−CH3). 31P{1H} NMR (122 MHz, C6D6, 25 °C): δ 45.9 (d, 2JPP = 263 Hz, 1P, PiPr2), 39.35 (d, 2JPP = 263 Hz, 1P, PiPr2). Anal. Calcd for C20H37ClNNiP2: C, 58.28; H, 9.05; N, 3.40. Found: C, 58.47; H, 9.23; N, 3.37. Crystal data: C20H37NP2Ni, orange prism, 0.09 × 0.05 × 0.02 mm3, monoclinic, P21/c (No. 14), a = 11.0278(5) Å, b = 13.6180(6) Å, c = 30.0929(13) Å, β = 106.252(2)°, from 25 degrees of data, T = 100(2) K, V = 4338.7(3) Å3, Z = 8, fw = 412.16, Dc = 1.262 Mg m−3, μ = 1.044 mm−1. Data collection and processing: Bruker APEX Kappa CCD diffractometer, Mo Kα (λ = 0.71073 Å), graphite monochromator, 67963 reflections collected, −14 ≤ h ≤ 14, −17 ≤ k ≤ 17, −39 ≤ l ≤ 38, frame scan width 0.5°, scan speed 1.0° per 60 s, typical peak mosaicity 0.63°, 9963 independent reflections (Rint = 0.0530). The data were processed with Denzo-Scalepack. Solution and ref inement: structure solved by direct methods with SHELXS-97, full-matrix least-squares refinement based on F2 with SHELXL-97, 507 parameters with 0 restraints, final R1 = 0.0348 (based on F2) for data with I > 2σ(I) and R1 = 0.0502 on 9963 reflections, goodness of fit on F2 = 1.046, largest electron density peak 0.605 e Å−3, deepest hole −0.396 e Å−3. [Li(THF)4[Ni(PNPiPr**)Me] (9·4THF). To a cold solution (−40 °C) of 7 (100 mg, 0.242 mmol) in diethyl ether (5 mL) was added a solution of MeLi (1.6 M in diethyl ether, 150 μL, 0.242 mmol) via microsyringe. The reaction mixture was stirred for 1 h at room temperature. The color changed from dark orange to deep red. The solvent was removed in vacuo, and the crude product was washed with pentane (3 × 10 mL). The obtained solid was dried and subsequently recrystallized from THF/n-pentane by slow diffusion to afford red crystals (60% yield, 58 mg). 1H NMR (500 MHz, THF-d8, 25 °C): δ 6.54 (m, 3JHH = 7.5 Hz, JHP = 1.6 Hz, 1H, CH-Pypara), 5.34 (d, 3JHH = 7.5 Hz, 2H, CH-Pymeta), 2.57 (br s, 2H, CH−P), 2.07 (m, 4H, CH(CH3)2), 1.43 (m, partially obscured by THF, 24 H, CH(CH3)2), −0.08 (t, 3JHP = 7.2 Hz, 3H, Ni−CH3). 13C{1H} NMR (75 MHz, C6D6, 25 °C): δ 172.9 (t, 2JCP = 11.6 Hz, 2C, C-Pyquat), 135.1 (s, 1C, CH-Pypara), 92.4 (s, 2C, CH-Pymeta), 49.0 (br s, 2C, CH−P), 24.9 (br s, 4C CH(CH3)2), 19.6 (s, 4C, CH(CH3)2), 18.1 (s, 4C, CH(CH3)2), −22.2 (vt, 2JCP = 22.5 Hz, 1C, Ni−CH3). 31P{1H} NMR (122 MHz, C6D6, 25 °C): δ 46.1 (s, 2P, PiPr2). The extraordinary sensitivity of the complex toward moisture precluded elemental analysis and mass spectrometry. Crystal data: [Li(DME)3[Ni(PNPiPr**)Me], C20H36NNiP2 + C12H30LiO6, red plate, 0.26 × 0.20 × 0.04 mm3, monoclinic, P21/n, a = 11.1865(7) Å, b = 16.160(1) Å, c = 21.979(2) Å, α = 90°, β = 103.460(2)°, γ = 90° from 18 degrees of data, T = 100(2) K, V = 3864.0(5) Å3, Z = 4, fw = 688.45, Dc = 1.183 Mg m−3, μ = 0.623 mm−1. Data collection and processing: Bruker KappaApexII CCD diffractometer, Mo Kα (λ = 0.71073 Å), MiraCol optics, graphite monochromator, −9 ≤ h ≤ 14, −20 ≤ k ≤ 21, −28 ≤ l ≤ 20, 2θmax = 55.08°, frame scan width 0.5°, scan speed 1.0° per 180 s, typical peak mosaicity 0.67, 48057 reflections collected, 8861 independent reflections (Rint = 0.054). The data were processed with Bruker Apex2. Solution and ref inement: structure solved with SHELXS-97, full-matrix least-squares refinement based on F2 with SHELXL-97, 419 parameters with 1 restraint gave final R1 = 0.0333 (based on F2) for data with I > 2σ(I) and R1 = 0.0607 on 8861 reflections, goodness of fit on F2 = 1.012, largest electron density peak 0.369 e Å−3, largest hole −0.314 e Å−3. [Li(Et2O)2][Ni(PNPtBu**)Me] (10·2Et2O). To a cold solution (−40 °C) of [Ni(PNPtBu*)Cl)] (6; 100 mg, 0.205 mmol) in diethyl ether (15 mL) was added a solution of MeLi (1.6 M in diethyl ether, 280 μL, 0.450 mmol) via microsyringe. The reaction mixture was stirred for 2 h at room temperature. The color changed from green to deep red. The solvent was removed in vacuo and the product extracted with pentane (2 × 25 mL). After filtration, the solvent was evaporated to leave an

86%. Single crystals were obtained from a solution of 3 in dichloromethane layered with n-pentane. 1H NMR (500 MHz, CD3CN, 25 °C): δ 7.92 (vt, 3JHH = 7.7 Hz, 1H, CH-Pypara), 7.53 (d, 3 JHH = 7.7 Hz, 1H, CH-Pymeta), 3.67 (br s, 4H, CH2-P), 2.39 (br m, 4H, CH(CH3)2), 1.49 (br d, 3JHH = 7.4 Hz, 12H, CH(CH3)2), 1.32 (br d, 3JHH = 7.0 Hz, 12H, CH(CH3)2)). 13C{1H} NMR (126 MHz, CDCl3, 25 °C): δ 165.6 (s, 2C, C-Pyquart), 141.5 (s, 2C, CH-Pymeta), 124.4 (s, 1C, CH-Pypara), 33.2 (vt, JCP = 6.3 Hz, 2C, CH2-P), 23.6 (d, JCP = 9.4 Hz, 4C, CH(CH3)2), 18.5 (s, 4C, CH(CH3)2)., 18.0 (s, 4C, CH(CH3)2). 31P{1H} NMR (202 MHz, CDCl3, 25 °C): δ 48.5 (s, 2P, P(iPr)2). Anal. Calcd for C19H35Cl2NNiP2: C, 48.65; H, 7.52; N, 2.99. Found: C, 48.61; H, 7.61; N, 2.83. Crystal data: 2C19H35ClNNiP2 + 2CH2Cl2 + 2Cl−, yellow needle, 0.20 × 0.05 × 0.05 mm3, monoclinic, P21 (checked symmetry with Platon/Addsym), a = 15.8560(10) Å, b = 11.12400(10) Å, c = 16.0637(2) Å, β = 108.2250(5)°, from 6494 reflections, T = 120(2) K, V = 2690.81(4) Å3, Z = 4, fw = 553.96, Dc = 1.367 Mg m−3, μ = 1.245 mm−1. Data collection and processing: Nonius KappaCCD diffractometer, Mo Kα (λ = 0.71073 Å), graphite monochromator, −20 ≤ h ≤ 20, −13 ≤ k ≤ 14, −20 ≤ l ≤ 20, frame scan width 1.0°, scan speed 1.0° per 270 s, typical peak mosaicity 0.486°, 51791 reflections collected, 6504 independent reflections (Rint = 0.057). The data were processed with HKL2000. Solution and ref inement: structure solved with SIR-97, full-matrix leastsquares refinement based on F2 with SHELXL-97 on 533 parameters with 10 restraints to give final R1 = 0.0478 (based on F2) for data with I > 2σ(I) and R1 = 0.0577 on 12180 reflections, goodness of fit on F2 1.040, largest electron density peak 0.785 e Å−3, largest hole −0.891 e Å−3. [Ni(PNPiPr*)Cl] (5). LiHMDS (35 mg, 0.213 mmol) was added to a suspension of 1 (100 mg, 0.213 mmol) in diethyl ether (15 mL) and stirred for 4 h at ambient temperature. The color changed from yellow to green-brown. All volatiles were removed in vacuo, and the product was extracted with pentane (2 × 25 mL). The extract was filtered through a plug of Celite. Subsequently, the solvent was evaporated to leave the pure green crystalline product in 80% yield (74 mg). 1H NMR (300 MHz, C6D6, 25 °C): δ 6.32 (vt, 3JHH = 9.0 Hz, 1H, CHPypara), 6.16 (d, 2JHH = 8.8 Hz, 1H, CH-Pymeta), 5.21 (d, 2JHH = 6.5 Hz, 1H, CH-Pymeta), 3.09(d, 2JHP = 5.0, 1H, CH−P), 2.15 (d, 2JH,P = 8.2 Hz, 2H; CH2-P), 2.11 (m, 2H, CH(CH3)2), 1.74 (m, 2H, CH(CH3)2), 1.66, 1.67 (ddd, 3JHP = 14.0 Hz, 3JHH = 7.1 Hz, 3JHP = 2.9 Hz, 6H, CH(CH3)2), 1.32 (d(vt), 3JHP = 11.8 Hz, J = 7.4 Hz, J = 2.8 Hz, 12H, CH(CH3)2), 0.91 (ddd, 3JHP = 11.5 Hz, 3JHH = 7.0 Hz, 3JHH = 2.7 Hz, 6H, CH(CH3)2). 13C{1H} NMR (126 MHz, C6D6, 25 °C): δ 174.9 (dd, 2JPC = 20.1 Hz, 4JCP = 6.7 Hz, 1C, C−Pyquat), 159.9(br s, 1C, C− Pyquat), 132.3 (s, 1C, CH-Pypara), 113.9 (d, 3JCP = 17.2 Hz, 1C, CHPymeta), 99.7 (d, 3JCP = 10.4 Hz, 1C, CH-Pymeta), 60.4 (d, 1JCP = 49.6 Hz, CH-P), 30.7 (d, 1JCP = 18.0 Hz, CH2-P), 25.6 (d, 1JCP = 27.0 Hz, CH(CH3)2), 22.4 (d, 1JCP = 18.9 Hz, CH(CH3)2), 19.0 (s, 1C, CH(CH3)2), 18.3 (s, 1C, CH(CH3)2), 17.9 (s, 1C, CH(CH3)2), 17.3 (s, 1C, CH(CH3)2). 31P{1H} NMR (122 MHz, C6D6, 25 °C): δ AB system 44.7 (d, 2JPP = 304 Hz, 1P, PiPr2), 41.3 (d, 2JPP = 304 Hz, 1P, PiPr2). Anal. Calcd for C19H34ClNNiP2: C, 52.75; H, 7.92; N, 3.24. Found: C, 53.10; H, 7.69; N, 3.16. [Ni(PNPiPr*)Me] (7). To a cold solution (−40 °C) of 3 (100 mg, 0.213 mmol) in THF (10 mL) was added MeLi (1.6 M solution in diethyl ether, 130 μL, 0.213 mmol) via a microsyringe. The reaction was stirred for 1 h and warmed to ambient temperature. The color changed from yellow to dark orange. All volatiles were removed in vacuo, and the product was extracted in pentane (2 × 25 mL). After filtration through a plug of Celite, the solvent was evaporated to leave an orange solid, which was eventually recrystallized from pentane at −40 °C to afford orange crystals of 7 (60% yield, 58 mg). 1H NMR (500 MHz, C6D6, 25 °C): δ 6.50 (vt, 1H, 3JHH = 7.5 Hz, CH-Pypara), 6.35 (d, 2JH,H = 8.7 Hz, 1H, CH-Pymeta), 5.38 (d, 2JHH = 6.4 Hz, 1H, CH-Pymeta), 3.28 (d, 2JHP = 4.4 Hz, 1H P−CH), 2.36 (d, 2JHP = 9.3 Hz, 2H, P−CH2), 1.93 (m, J = 14.6, 6.9, 3.4 Hz, 2H, CH(CH3)2), 1.58 (m, J = 14.0, 7.1, 3.6 Hz, 2H, CH(CH3)2), 1.38 (dd, 3JHH = 15.4 Hz, 3 JHP = 7.1 Hz, 6H, CH(CH3)2), 1.27 (dd, 3JHH = 12.9 Hz, 3JHP = 6.9 Hz, 6H, CH(CH3)2), 1.00 (dd, 3JHH = 15.4 Hz, 3JHP = 7.2 Hz, 6H, CH(CH3)2), 0.83 (dd, 3JHH = 12.6 Hz, 3JHP = 7.1 Hz, 6H, CH(CH3)2), F

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Organometallics

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orange solid, which was recrystallized from diethyl ether/n-pentane via slow diffusion to afford red crystals (60% yield, 58 mg). 1H NMR (500 MHz, C6D6, 25 °C): δ 6.47 (t, 3JHH = 7.6 Hz, 1H, CH-Pypara), 5.33 (d, 3 JHH = 7.6 Hz 2H, CH-Pymeta), 2.9 (br s, 2H, CH−P), 1.46 (vt, 3JHP = 7.0 Hz, 36H, C(CH3)3), 0.17 (t, 3JHP = 8.3 Hz, 3H, Ni−CH3). 13 C{1H} NMR (126 MHz, C6D6, 25 °C): δ 171.6 (vt, 2JCP = 11.3 Hz, 2C, C-Pyquat), 134.7 (s, 1C, CH-Pypara), 91.1 (s, 2C, CH-Pymeta), 53.6 (vt, 1JCP = 19.0 Hz, 2C, CH−P), 36.1 (br s, 4C, C(CH3)3), 15.1 (s, 12C, C(CH3)3),−24.8 (vt, 2JCP = 24.9 Hz, 1C, Ni−CH3). 31P{1H} NMR (122 MHz, C6D6, 25 °C): δ 45.4 (s, 2P, PtBu2). The extraordinary sensitivity of the complex toward moisture precluded elemental analysis and mass spectrometry. Crystal data: C32H64O2NLiP2Ni (C24H44NP2Ni + Li + 2 OC4H10), red prism, 0.09 × 0.05 × 0.05 mm3, triclinic, P1̅ (No. 2), a = 11.633(2) Å, b = 11.750(2) Å, c = 14.916(3) Å, α = 84.54(2)°, β = 70.92(3)°, γ = 66.44(3)° from 27 degrees of data, T = 120(2) K, V = 1764.7(8)Å3, Z = 2, fw = 622.43, Dc = 1.171 Mg m−3, μ = 0.667 mm−1. Data collection and processing: Nonius KappaCCD diffractometer, Mo Kα (λ = 0.71073 Å), graphite monochromator, 67984 reflections collected, −16 ≤ h ≤ 16, −14 ≤ k ≤ 16, −19 ≤ l ≤ 20, frame scan width 1°, scan speed 1.0° per 20 s, typical peak mosaicity 0.435°, 9581 independent reflections (Rint = 0.0285). The data were processed with DenzoScalepack. Solution and ref inement: structure solved by direct methods with SHELXS-97, full-matrix least-squares refinement based on F2 with SHELXL-97, 404 parameters with 0 restraints, final R1 = 0.0392 (based on F2) for data with I > 2σ(I) and R1 = 0.0503 on 9581 reflections, goodness of fit on F2 = 1.069, largest electron density peak 0.509 e Å−3, deepest hole −0.505 e Å−3. [Li][Ni(PNPiPr-COO)Me] (11). To a solution of 9 (50 mg, 0.102 mmol) in dry THF (1 mL) was added approximately 1 equiv (0.102 mmol, 2.75 mL) of CO2 gas via syringe through a rubber septum, resulting in an immediate reaction. A yellowish solid precipitated from the deep red solution, which subsequently redissolved to give a clear orange solution. The solvent was removed in vacuo, and the product was washed with pentane (3 × 3 mL). Further attempts to isolate the product resulted in complex mixtures due to the extraordinary air and moisture sensitivity of 11. 1H NMR (400 MHz, THF-d8): δ 8.51 (br d, 2 JHH = 8.0 Hz, 1H, CH-Pypara), 6.74 (t, 2JHH = 7.4 Hz, 1H, CHPymeta‑COO), 5.88 (br d, 2JHH = 7.0 Hz, 1H, CH-Pymeta‑CH2), 3.15 (br s, 2H, CH(CH3)2), 3.05 (br d, 2JHP = 8.1 Hz, 2H, P−CH2), 2.23 (dq, 3 JHH = 13.5, 2JHP = 6.7 Hz, 2H, CH(CH3)2), 1.49−1.19 (m, 24H, CH(CH3)2 not resolved, 1H{31P} decoupled NMR spectrum shown in the Supporting Information), −0.44 (vt, 3JHP = 8.6 Hz, 3H, Ni−CH3). 13 C{1H} NMR (126 MHz, THF-d8, 25 °C): δ 178.1 (br s, 1C, COO), 172.2 (d, 2JCP = 26.7 Hz, 1C, C−Pyquat‑COO), 157.2 (s, 1C, C− Pyquat‑CH2), 132.4 (s, 1C, CH-Pypara), 117.44 (s, 1C, CH-Pymeta‑COO), 103.6 (s, 1C, CH-Pymeta‑CH2), 77.2 (d, 1JCP = 44.4 Hz, 1C, P− C(COO)), 32.5 (d, 1JCP = 16.3 Hz, 1C, CH2−P), 22.53 (br s, 2C, CH(CH3)2),, 22.38 (br s, 2C, CH(CH3)2), 20.20 (br s, 2C, CH(CH3)2), 19.61 (br s, 2C, CH(CH3)2), 18.13 (br s, 2C, CH(CH3)2), 17.25 (br s, 2C, CH(CH3)2), −21.6 (vt, 2JCP = 23.4 Hz; Ni−CH3). 31P{1H} NMR (202 MHz, THF-d8): δ 48.86 (d, 2JPP = 252.0 Hz, 1P, PiPr2), 45.87 (d, 2JPP = 252.0 Hz, 1P, PiPr2). Computational Details. All calculations were carried out using Gaussian 09 Revision C.01.52 Two DFT exchange-correlation functionals were used. The first is the Perdew−Burke−Ernzerhof (PBE) GGA functional.53,54 The second is Adamo and Barone’s hybrid version thereof (i.e., PBE0).55 The former was used for geometry optimizations and frequency calculations, while the latter was used for more accurate energy calculations. With these functionals, two basis sets were used. SDD(d) is the combination of the Huzinaga−Dunning double-ζ basis set56 on lighter elements, with extra polarization functions (i.e., D95(d)) on second-row elements, with the Stuttgart− Dresden basis set-RECP combination57 on transition metals. cc-pV(D +d)Z is Dunning’s cc-pVDZ58 on the main-group elements and nickel with Wilson’s cc-pV(D+d)Z59 modification on second-row elements. Density fitting basis sets (DFBS),60,61 as implemented in Gaussian09, were employed in order to improve the computational efficiency of the calculation. The automatic DFBS generation algorithm built in to

Gaussian09 was employed. Bulk solvent effects were approximated by single-point energy calculations using a polarizable continuum model (PCM),62−65 specifically the integral equation formalism model (IEFPCM)62,63,66 with diethyl ether as the solvent as in the experiments. Specifically, Truhlar and co-workers’ empirically parametrized Solvation Model-Dispersion (SMD)67 version was used. Geometries were optimized using the default pruned (75,302) grid, while the “ultrafine” (i.e., a pruned (99,590)) grid was used for energy and solvation calculations. Four types of charges were considered: (i) natural population analysis (NPA) charges68 derived from natural bond order (NBO) analyses,68 (ii) Löwdin charges,69−71 (iii) atomic polar tensor (APT) charges,72 and (iv) Truhlar’s Charge Model 5 (CM5) charges73 based on the Hirshfeld population analysis.74 Wiberg bond orders68,75 are also presented. These properties were calculated using Gaussian09. CM5 charges were calculated using CM5PAC.76 The Fukui function f−(r)48,49 was calculated at the SMD(Et2O)PBE0/cc-pV(D+d)Z//DF-PBE/SDD(d) level of theory and visualized using GaussView 577 by mapping the f− function onto the electron density isosurface (contour value of 0.04) of [Ni(PNPiPr**)(Me)]−.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving NMR spectra, tables containing partial atomic charges and Cartesian coordinates of DFT structures, and CIF files giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Author Contributions §

These authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the European Research Council under the FP7 framework (ERC No. 246837) and by the MINERVA Foundation. M.V. would like to thank the Swiss Friends of the Weizmann Institute of Science for a postdoctoral fellowship. D.M. is the Israel Matz Professorial Chair of Organic Chemistry.

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ABBREVIATIONS DME,dimethoxyethane; s,singlet; d,doublet; dd,doublet of doublets; ddd,doublet of doublets of doublets; t,triplet; vt,virtual triplet; q,quartet; m,multiplet

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REFERENCES

(1) van der Vlugt, J. I.; Lutz, M.; Pidko, E. A.; Vogt, D.; Spek, A. L. Dalton Trans. 2009, 1016−1023 The syntheses of complexes 4-BF4, 6, and 7 were previously reported by van der Vlugt and co-workers, starting most likely from [Ni(NCCH3)6](BF4)2; however, the original article refers to [Ni(NCCH3)4](BF4)2 as the starting material. (2) van der Vlugt, J. I.; Pidko, E. A.; Bauer, R. C.; Gloaguen, Y.; Rong, M. K.; Lutz, M. Chem. Eur. J. 2011, 17, 3850−3854. (3) Khaskin, E.; Iron, M. A.; Shimon, L. J. W.; Zhang, J.; Milstein, D. J. Am. Chem. Soc. 2010, 132, 8542−8543. (4) Feller, M.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-Ari, E.; Milstein, D. Organometallics 2012, 31, 4083−4101. (5) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74−77. G

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dx.doi.org/10.1021/om3010838 | Organometallics XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/om3010838 | Organometallics XXXX, XXX, XXX−XXX