Development of Platinum(II) and -(IV) CNC Pincer Complexes and

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Development of Platinum(II) and -(IV) CNC Pincer Complexes and Their Application in a Hydrovinylation Reaction Daniel Serra,† Peng Cao,† Jose Cabrera,† Robin Padilla,† Frank Rominger,‡ and Michael Limbach*,†,§ †

CaRLa, Catalysis Research Laboratory, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany Organisch-Chemisches Institut, Ruprecht-Karls-Universit€at Heidelberg, 69120 Heidelberg, Germany § BASF SE, Basic Chemicals Research, GCB/C-M313, 67056 Ludwigshafen, Germany ‡

bS Supporting Information ABSTRACT: A series of new, dicationic platinum(II) CNC pincer complexes were prepared and characterized by NMR and X-ray diffraction analysis. Oxidative addition of methyl iodide and iodine yielded the mostly unstable platinum(IV) complexes, which readily underwent reductive elimination to yield the platinum(II) precursors. Nonetheless, a platinum(IV) iodide adduct was isolated and was characterized by X-ray diffraction analysis. Additionally, a platinum(II) ethylene complex was isolated and found to be a moderately active yet highly selective catalyst in the co-dimerization of 2-methyl-2-butene with ethylene.

’ INTRODUCTION In 1976 Shaw et al. were the first ones to use tridentate ligands of the PCP type1 to prepare what van Koten et al. later incidentally named pincer complexes.2 The field developed fast and culminated in a flourish of pincer ligand motifs, e.g., of the NCN (van Koten et al.3), PCP, PNN (Milstein et al.4,5), and PNP type (Ozerov and Hahn et al.6). In particular, the introduction of stable NHC ligands by Arduengo et al.7 in 1991 paved the way for new tridentate pincer complexes of various metals and ligand motifs, e.g., CCC and CNC type (Crabtree et al.8,9). Pincer complexes have enabled synthetic transformations that, at the time of their discovery in 1976, had been considered difficult or even impossible.10 Pincer metallacycles with the CNC and CCC ligand framework in particular show a remarkable stability toward oxygen, moisture, and heat.11 This stability arises from the strong σdonor properties of the NHC moieties. Crabtree, Faller, and Eisenstein et al. have shown that the temporary decoordination of the central 2,6-lutidine or pyridine moiety in a palladium CNC pincer complex, even under mild reaction conditions, allows for potential catalytic pathways.8d The low fluxionality barrier for CNC pincer complexes in comparison to their monoanionic CCC analogues makes the neutral CNC ligand an interesting target for catalysis studies. Furthermore, the NHC backbone in CNC pincer complexes can be easily modified so as to enable the fine-tuning of stability vs reactivity.9d Only a small number of CNC pincer complexes have been used to tailor the metal’s coordination sphere,9f however. Palladium is the most represented metal in CNC pincer metallacycles,8d,12 followed by ruthenium,7b,13 iron,14 nickel,15 r 2011 American Chemical Society

cobalt,14c,16 and chromium.14c,17 To date, only a handful of platinum CNC pincer complexes are known. They have been used mainly as photoluminescence emitters18 and, to the best of our knowledge, not as catalysts in synthetic transformations. The prominence of palladium pincer complexes over their platinum analogues in catalytic studies is quite understandable for several reasons. Platinum tolerates a narrower set of substrates, commonly giving lower yields,19 and platinum(II) is one of the most kinetically inert metal species known, making carboplatination more challenging compared to carbopalladation. Transformations involving platinum are therefore often limited to activated substrates such as allenenes,20 1,3-dienes,21 or alkylidenecyclopropanes.22 On the other hand, platinum often shows complementary reactivity to palladium,23 and nonpincer complexes of platinum have been successfully applied as catalysts in cycloisomerization,24 hydroamination,25 and hydrovinylation reactions.24b,26 In light of our own work on C-H activation reactions27 and considering that there are few reports on the synthesis and reactivity of dicationic platinum(II) CNC complexes, we became very interested in synthesizing various platinum CNC pincer complexes to exploit their potential in catalysis, i.e., in hydrovinylation reactions.

’ RESULTS AND DISCUSSION Synthesis of Ligand Precursors 7-11 and of Platinum(II) CNC Pincer Complexes 12-16. Alkylation of the N-substituted Received: December 1, 2010 Published: March 09, 2011 1885

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Scheme 1. Synthesis of Bis-imidazolium Salts 7-11 from Pyridines 1-3 and Imidazoles 4-6 and Bis-N-heterocyclic Pincer Complexes 12-16

Figure 1. X-ray structure of 12 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms have been omitted for clarity.

imidazoles 4-6 (R = Me, nBu, Mes) with bis-2,6-(chloromethyl)pyridine (1),8d bis-2,6-(bromomethyl)pyridine (2),28 or 2,6-dibromopyridine (3)29 yielded, in one step, the bisimidazolium salts 7,30 8,28 9,29 10,31 and 1131 in 85-95% yield, respectively (Scheme 1). Contrary to the published procedures, 8 and 9 were prepared under milder reaction conditions (i.e., at room temperature with extended reaction times). Under the original conditions (heating to reflux in dioxane), pink-colored byproduct were formed. 2,6-Bis(3-methylimidazol-1-yl)pyridine dibromide (11) was prepared in a sealed ampule from neat 3 and 6 under harsher reaction conditions (5 d at 140 °C).31 The crude bis-imidazolium salts were precipitated from MeOH/Et2O and recrystallized to give 7-11 as white or gray powders. All arylsubstituted derivatives with the exception of 10 and 11 turned out to be highly hygroscopic. Attaching ligand precursors 7-11 to platinum turned out to be difficult, as the reaction of 7, first with Ag2O and then with various platinum sources (such as Pt(COD)Cl2, Pt(PhCN)2Cl2, or PtCl2), led only to the reisolation of 7 (DMSO-d6, 100 °C, 12 h) instead of forming the desired 12. Similar results were observed when first the bis Ag-carbene of 7 was isolated and reacted with PtCl2.32 The reaction of 7 with [Pt4(OAc)8] 3 2HOAc33 in an analogous procedure reported by Crabtree et al.8d,28 also failed to yield complex 12. Surprisingly, the reaction of 7 with Ag2O and PtBr2 instead of PtCl2 (DMSO, 100 °C, 12 h) gave the chlorido complex 12 in 55% yield instead of the expected bromo derivative 13. This result shows that the resulting halide ligand on the metal as well as the outer-sphere anion of the platinum complex are derived from the counterion of the imidazolium ligand precursor. Suitable crystals for X-ray analysis were obtained by slow diffusion of Et2O into a concentrated MeOH solution of 12. The complex shows the expected twisted structure (Figure 1). Relevant bond lengths and angles are given in Table 1. The geometry around the platinum center is square planar, with the molecule exhibiting crystallographic C2 symmetry, similar to the Pd analogue described by Crabtree et al.28

The reaction of 8 with PtBr2 (Ag2O, DMSO, 100-150 °C, 12 h) gave the hitherto unknown complex 13 in 84% yield (Scheme 1). Complexes 12 and 13 are both highly hygroscopic, and their solubility in solvents other than MeOH or DMSO is poor. The reaction of the bis-n-butyl or bis-mesityl imidazolium bromides 9 and 10 with PtBr2 produced complexes 14 and 15 in 52% and 73% yield, respectively, as pale yellow solids. Both complexes are highly soluble in acetone and CH2Cl2 and are less hygroscopic than 12 and 13. In fact, upon exposure to air for a couple of hours, 15 shows no sign of decomposition, whereas 14 becomes noticeably moist after an hour. The methylene protons of the lutidine backbone of 12-15 appear as a doublet of doublets in the 1H NMR (5.43 and 5.53 ppm). This property has been observed for analogous palladium pincer complexes,31 due to the diastereotopicity that results from the complexes’ twisted conformation. The new platinum CNC pincer complex 16, which has no methylene bridge between the central pyridine ligand and the NHC ligands, was produced in good yield (75%) in a manner similar to 12-15. Compound 16 is a yellow powder that does not show any sign of decomposition after exposure to moisture and air for weeks. The 1H NMR spectrum of 16 is comparable to the Pd analogue reported by Danopoulos et al.31 The 12 o-methyl and the aromatic protons of the mesityl ring appear as sharp singlets (at 1.98 and 6.95 ppm, respectively), revealing a symmetry plane in the molecule. Anion Metathesis and Exchange of the Halide Ligand with N-Donor Ligands to Yield Pyridine Adduct 18 and Acetonitrile Adducts 19-21. Tetrafluoroborate derivative 17 was obtained from 16 via anion exchange with an excess of NH4BF4 (CH2Cl2, 24 h, room temperature) in 91% yield and gave crystals suitable for X-ray analysis (Scheme 2, Figure 2, Table 1). As already observed for 12, 17 adopts a distorted square-planar geometry at the platinum center (Npy-Pt-Br = 178.8(2)° and CNHC-Pt-CNHC = 158.8(2)°) defined by the two carbenes in trans configuration and the bromido ligand trans to the central pyridine moiety. The X-ray 1886

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analysis proves the (noncrystallographic) C2v symmetry, which had previously been deduced from 1H NMR data.

The reaction of the bis-chloride 12 with two equivalents of AgBF4 in pyridine yielded the corresponding dicationic pyridine adduct 18 in 70% yield (Scheme 2). Slow diffusion of Et2O into a CH2Cl2 solution of 18 gave crystals suitable for X-ray analysis (Figure 3, Table 1). Similar to 12 and 17, complex 18 adopts a slightly distorted square-planar geometry at the Pt(II) center, defined by the two pyridine ligands in trans arrangement and the two trans-configured carbene ligands. The Pt-Npy bond length in 18 is shorter than in 12 (2.045(3) vs 2.054(3) Å), probably due to the different trans influence of pyridine vs chloride. On the other hand, the bond length of the freely coordinated pyridine to Pt in 18 is significantly shorter than the bond from Pt to Npy of the pincer ligand (2.013(3) vs 2.045(3) Å), which points to a strong coordination of the pyridine ligand to Pt. The ease of the ligand exchange from halide to an N-donortype ligand was observed during the attempted anion metathesis

Scheme 2. Anion Exchange of Complex 16 to Give 17 and of 12 to Give 18



Table 1. Selected Pt-CNHC, Pt-Npy, and Pt-X (X = Cl, Br, Npy) Distances (Å) and Selected Bond Angles (deg) for Npy-PtXtrans, Npy-Pt-CNHC, and CNHC-Npy-Pt-CNHC for 12, 17, 18, 20, 22, 24, 25, and 26 CNHC-Pt-CNHC

Pt-X

Npy-Pt-Xtrans

Npy-Pt-CNHC

Npy-Pt-CNHC

178.8(2)

87.78(15)

87.73(14)

178.82(17)

79.1(2)

79.7(3)

175.95(14)

177.04(12)

88.10(4)

87.88(3)

175.45(9)

180

87.72(4)

complex

Pt-CNHC

Pt-CNHC

Pt-Npy

12

2.025(5)

2.016(4)

2.054(3)

2.3099(9)

175.49(15)

17

2.000(7)

2.021(7)

1.971(5)

2.4181(7)

158.8(3)

18

2.018(4)

2.015(4)

2.045(3)

2.013(3)

20

2.027(3)

2.053(2)

1.951(2)

22

2.03(2)

2.05(2)

1.938(9)

2.546(2), 2.613(2), 2.621(2)

157.0(8)

179.2(5)

78.4(7)

78.6(7)

24 25

2.004(3) 2.027(3)

2.005(4) 2.031(3)

1.983(3) 1.988(3)

2.5509(4) 2.171(3), 2.176(4)

158.23(14) 157.34(13)

179.13(8) 161.43(16)

79.11(13) 78.59(11)

79.24(13) 78.75(12)

26

2.002(9)

2.003(9)

2.03(2)

2.285(4)

158.4(3)

177.7(3)

78.8(4)

79.7(4)

2.019(9)

2.019(8)

1.97(1)

2.272(4)

157.8(3)

176.1(3)

78.1(4)

79.8(4)

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Organometallics from Br- to TfO- in acetonitrile-d3. The reaction of the bromides 14-16 with two equivalents of AgOTf yielded the triflate complexes 19-21 as acetonitrile-d3 adducts in excellent yields (>90%) instead of the anticipated bromido-platinum triflate complexes (Scheme 3). Complexes 19-21 were isolated by simple filtration to remove AgBr and characterized via NMR spectroscopy. The pyridine and imidazole moieties show the characteristic 1H NMR shifts in the range δ = 8.16-7.27 ppm. The diastereotopic protons at δ = 5.40 ppm appear as sharp doublets. For complex 20, the three different singlets for the various methyl groups (at δ = 2.31, 2.05, and 1.98 ppm) and the four aromatic protons of the mesityl ring (two singlets at δ = 7.08 and 7.02 ppm) point to a twisted C2 symmetry. This supposition was confirmed by X-ray crystallographic analysis (Figure 4, Table 1), which shows the mesityl rings wrap around the distorted square-planar platinum center. Oxidation of 17 to Form Platinum(IV) Complexes 22 and 23 and Their Decomposition via Reductive Elimination to 24. While examples for platinum(IV) NHC complexes are rare,34 they do show an interesting potential in cycloplatination reactions.35 The oxidation of platinum(II) complex 17 with three equivalents of iodine36 in dichloromethane-d2 at room temperature cleanly produced the platinum(IV) oxidation product 22 in 80% yield. Upon recrystallization from the reaction mixture, triiodide was found to be the counterion instead of the expected tetrafluoroborate (Scheme 4, Figure 5, Table 1). As with the previously mentioned related complexes, X-ray analysis of 22 shows a C2 distorted octahedral geometry. Two

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trans-iodido ligands point out of the plane defined by the CNC ligand. Due to the bulkiness of iodido and bromido ligands, the Pt-Br bond in 22 is conspicuously elongated compared to 17, and the Pt-I bonds are even longer. The length of the Pt-Npy bond in 22 is comparable to that observed in 17 (Table 1). The reaction of 17 with 1.8 equivalents of methyl iodide is an equilibrium reaction and required elevated temperature (CDCl2, 100 °C), as no oxidative addition was observed at room temperature. When the reaction mixture was heated, the volatile methyl bromide, formed by reductive elimination from the platinum(IV) complex 23, was detected by 1H NMR in addition to the platinum(II) iodido complex 24 (shift of the CH3 group from δ = 2.14 to 2.63 ppm). Complex 24 adopts a distorted square-planar geometry at the platinum(II) center (Figure 6, Table 1) with bond angles (Npy-Pt-I = 179.13° and CNHCPt-CNHC = 158.2(1)°) in the range of those observed for chlorido complex 26 and bromido complex 17. The Pt-Npy bond is slightly elongated in 24 compared to 17 (1.983(3) vs 1.971(5) Å). Synthesis of Ethylene CNC-Platinum(II) Complex 25 and Its Activity in the Hydrovinylation of 2-Methyl-2-butene. Positively charged platinum(II) or palladium(II) alkene com-

Scheme 3. Synthesis of Acetonitrile Adducts 19-21 from Complexes 14-16


Scheme 4. Oxidation of 17 with Iodine and Methyl Iodide to Yield 22, 23, and the Reductive Elimination Product 24

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plexes have been shown to be active in oligomerization37 and polymerization reactions.38 The attempted synthesis of the corresponding alkene complexes from the acetonitrile complexes 20, 21, and the pyridine adduct 18 was not successful with either ethylene or excess (i.e., 10 equiv) styrene as the alkenyl substrate over a range of temperatures and solvents (80-150 °C, THF, MeCN, CH2Cl2). The reaction of 16 with two equivalents of silver tetrafluoroborate in nitromethane in a pressurized atmosphere of ethylene (5 bar) led to the quantitative formation of ethylene complex 25 within 3 h. Complex 25 crystallized out of the reaction mixture as

yellow crystals, suitable for X-ray analysis (Scheme 5, Figure 7, Table 1). The geometry around platinum(II) is square planar. Both CNHC-Pt bonds are equivalent (2.027(3) vs 2.031(3) Å), and the η2-bound ethylene molecule is symmetrically coordinated at the platinum CNC fragment. The distances between Pt and the alkenyl carbons are identical (2.171(3) vs 2.176(4) Å), respectively. These bond lengths are significantly longer than those observed for Zeise’s salt [Pt(C2H4)Cl3]- (2.128(3) and 2.135(3) Å).39 The 1H NMR spectrum of 25 shows a sharp singlet with platinum satellites at δ = 4.18 ppm (JPt-C = 70 Hz, JPt-H = 31 Hz in CD2Cl2), which were assigned to the ethylene bound to the metal center. For complex 25, the experimental ethylene C-C bond length is 1.371(7) Å, which falls between the value of free ethylene

Figure 5. Molecular structure of 22 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms have been omitted for clarity.


Scheme 5. Synthesis of the Alkene Complex 25 and Byproduct 26 Observed in CH2Cl2 from the Bromide 16

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Scheme 6. Catalytic Hydrovinylation of 2-Methyl-2-butene (27) with Ethylene in the Presence of Platinum Pincer Complex 25 to Selectively Form 3,4-Dimethyl-1-pentene (28)

Scheme 7. Potential Mechanistic Pathways Leading to Either the Hydrovinalytion Product 28 (pathway A) or the Cyclopropanation Product 29 (pathway B)



(1.3391(13) Å) and the ethane value of 1.534(2) Å. We take this as an indication of π-donation, as the length of the C-C double bond in coordinated alkenes increases with the increasing degree of π-back-donation from the metal (and thus with the π-basic character of the platinum(II) center in the CNC complex).40,41 The length of the C-C double bond in other cationic platinum alkene complexes is even shorter, e.g., 1.359(10) Å for [Pt(PNP)C2H4](BF4)2.41b Nevertheless, their reactivity toward nucleophilic attack is sufficient for the catalytic co-dimerization of ethylene and internal alkenes.26a Formation of 25 in solvents other than nitromethane proved challenging. The reaction of 16 with two equivalents of AgOTf in CH2Cl2 under ethylene, for example, led to significant amounts of the chlorido complex 26 (Scheme 5), which is formed by chloride abstraction from the solvent. X-ray quality crystals of 26 were grown by slow diffusion of Et2O into a solution of 26 in CH2Cl2 (Figure 8, Table 1). Complex 26 crystallized with two separate molecules in the unit cell and displays a distorted squareplanar geometry at platinum(II) defined by the two transcarbenes and the chlorido ligand trans to the pyridine moiety. When complex 25 was heated in nitromethane under an atmosphere of ethylene with 2-methyl-2-butene (27) to 100 °C for 8 d,

the hydrovinylation product 3,4-dimethyl-1-pentene (28) was formed with 68% conversion in >99% selectivity based on 27 (Scheme 6). The high selectivity of this reaction was confirmed by GC-MS and 1H NMR measurements. The preference for compound 28 is remarkable considering the numerous other products that could potentially be formed, such as linear or cyclic homo-oligomers (e.g., 1,1-dimethyl-2-methylcyclohexane). Among those byproducts, only 1,1-methyisopropylcyclopropane (29) and 3,4,4trimethyl-1-hexene (30) were detected in traces via GC-MS (28:29:30 = 1074:7:1, GC-area %). At a lower temperature and shorter reaction time (60 °C for 3 d), there was no reaction with isolated ethylene complex 25. The reason for the catalyst’s clear preference for hydrovinylation over cyclopropanation and oligomerization is, as of yet, not well understood. From a mechanistic point of view both reaction pathways are initiated by the attack of an electron-rich olefin on a Pt-coordinated ethylene molecule 1 leading to a δ-carbocationic platinum alkyl species 2 (Scheme 7). This intermediate is prone to rearrangement by 1,2-hydride shift to give a γ-carbocation species 3. The pathway to the hydrovinylation product 28 (pathway A) has been well explored for PNP pincer ligands.26a With PPP ligands, the γcationic Pt-alkyl intermediate 3 is trapped by the Pt-C bond and gives cyclopropane product 29 (pathway B).26d As proposed by Vitagliano et al.,26c this switch in selectivity might be a consequence of the different trans influence of the central donor atom on each ligand. The hydride shift to give olefin complex might be preferred when the resulting Pt-olefin complex 4 is more stable, as would be the case for a nitrogen donor atom with lower trans influence (as in a CNC pincer ligand). Conversely, phosphorus or other donor atoms with higher trans influence than N-donors might increase the basicity of the Pt-C bond, which would favor the trapping of the γ-cation by cyclopropanation. 1890


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’ CONCLUSIONS We have synthesized and characterized new platinum(II) and platinum(IV) complexes with a common set of CNC pincer ligands. Selected platinum(II) CNC pincer complexes were easily oxidized with iodine or methyl iodide, whereas in the latter case the reductive elimination from the platinum(IV) complex was fast. Some of these complexes catalyze the hydrovinylation of 2-methyl-2-butene with ethylene. A key intermediate in the catalytic hydrovinylation cycle, i.e., the platinum(II) η2-ethylene complex, was isolated and characterized. The activity of the platinum(II) η2-ethylene complex in the hydrovinylation of 2-methyl-2-butene is low, but the selectivity is high and the reaction is clearly catalytic. Future work will be aimed at elucidating the origin of this phenomenon as well as the application of this selective hydrovinylation reaction to relevant target molecules in a variety of settings. ’ EXPERIMENTAL SECTION General Procedures. All reactions and manipulations were performed under an argon atmosphere using standard Schlenk techniques. Ethyl ether (Et2O) was dried by distillation from Na/Ph2CO. Hexane, tetrahydrofuran (THF), and dichloromethane (CH2Cl2) were dried with an MBraun solvent purification system. Methanol (MeOH), acetonitrile (CH3CN), dimethylsulfoxide (DMSO), nitromethane (CH3NO2), 1,4-dioxane, and chloroform (CHCl3) were purchased in anhydrous solvent purity. All solvents were degassed with argon prior to use. All deuterated solvents for NMR measurements were degassed via freeze-pump-thaw cycles and stored over molecular sieves (4 Å). 1H and 13C{1H} NMR spectra were recorded at room temperature on a Bruker 250 spectrometer operating at 200 and 50 MHz, respectively, with chemical shifts (δ, ppm) reported relative to the solvent peaks (1H NMR, 13C NMR). For X-ray crystal structure analyses, suitable crystals were mounted with a perfluorinated polyether oil on nylon loops and cooled immediately on the goniometer head. Data collections were performed with Mo KR radiation (graphite monochromator) on a Bruker Smart CCD (17, 20, 22, 26) or a Bruker APEX (12, 18, 24, 25) at 200 K. Structures were solved by direct methods and refined by full-matrix least-squares against F2. All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were included in calculated positions. Calculations were performed using the SHELXTL software package.42 GC analyses were carried out on a Agilent 6890N modular GC base equipped with a split-mode capillary injection system and flame ionization detector using a standard HP-5 capillary column (30 m 0.32 mm 0.25 mm; He flow: 2.0 mL/min; program: 50 °C (1 min), 20 °C/min to 250 °C (10 min)). Elemental analyses and mass spectra were obtained from the Organic Chemistry Institute of the University of Heidelberg. All other starting materials were purchased in reagent grade purity from Acros, Aldrich, Fluka, or Strem and used without further purification. 2,6-Bis[(3-methylimidazolium-1-yl)methyl]pyridine Dichloride (7). A solution of 2,6-bis(chloromethyl)pyridine (1, 2.00 g, 11.36 mmol) and 1-methylimidazole (4, 2.14 g, 37.5 mmol) were stirred in dioxane (80 mL) at 100 °C for 12 h. After cooling to room temperature, the solid was filtered, repeatedly precipitated from MeOH/Et2O, and recrystallized from CH2Cl2/Et2O. Gray solid, 3.3 g, 85% yield. 1H NMR (DMSO-d6): δ 9.54 (s, 2H), 7.96 (t, 1H, J = 7.8 Hz), 7.79 (d, 4H), 7.51 (d, 2H, J = 7.8 Hz) 5.60 (s, 4H), 3.94 (s, 6H). further analytical data match those previously published.30

2,6-Bis[(3-methylimidazolium-1-yl)methyl]pyridine Dibromide (8). A solution of 2,6-bis(bromomethyl)pyridine (2, 2.00 g, 7.54 mmol) and 1-methylimidazole (4, 1.37 g, 16.8 mmol) was stirred in dioxane (80 mL) at 100 °C for 48 h. The white precipitate was filtered,

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washed with dioxane (20 mL) and Et2O (100 mL), and recrystallized from CH2Cl2/Et2O. Colorless solid, 2.98 g, 92% yield; mp 185 °C. 1H NMR (DMSO-d6): δ 9.24 (s, 2H), 7.97 (t, 1H, J = 7.8 Hz), 7.73 (d, 4H), 7.49 (d, 2H, J = 7.8 Hz) 5.57 (s, 4H), 3.92 (s, 6H). Further analytical data match those previously published.28

2,6-Bis[(3-butylimidazolium-1-yl)methyl]pyridine Dibromide (9). A solution of 2,6-bis(bromomethyl)pyridine (2, 2.00 g, 7.54 mmol) and 1-butylimidazole (5, 2.81 g, 22.64 mmol) was stirred in dioxane (80 mL) at room temperature for 48 h. The precipitate was filtered, washed with dioxane (20 mL) and Et2O (100 mL), and recrystallized from CH2Cl2/Et2O. Colorless solid, 3.69 g, 95% yield. 1 H NMR (DMSO-d6): δ 9.48 (s, 2H), 7.97 (t, 1H, J = 7.7 Hz), 7.90 (m, 2H), 7.80 (m, 2H), 7.51 (d, 2H, J = 7.7 Hz) 5.61 (s, 4H), 4.27 (t, 4H, J = 7.0 Hz), 1.79 (m, 4H), 1.25 (m, 4H), 0.90 (t, 6H). Further analytical data match those previously published.29

2,6-Bis[(3-mesitylimidazolium-1-yl)methyl]pyridine Dibromide (10). A solution of 2,6-bis(bromomethyl)pyridine (2, 2.00 g, 7.54 mmol) and 1-mesitylimidazole (6, 4.21 g, 22.6 mmol) was stirred in dioxane (80 mL) at 100 °C for 2 days. The white precipitate was filtered, washed with dioxane (20 mL) and Et2O (100 mL), and recrystallized from CH2Cl2/Et2O. Gray solid, 4.23 g, 88% yield; mp 160 °C. 1H NMR (DMSO-d6): δ 9.73 (s, 2H), 8.14-8.01 (m, 5H), 7.56 (d, 2H, J = 7.8 Hz), 7.15 (s, 4H), 5.72 (s, 4H), 2.33 (s, 6H), 2.05 (s, 12H). Further analytical data match those previously published.31

2,6-Bis[(3-mesityl)imidazolium]pyridine Dibromide (11). A sealed glass ampule immersed in an oil bath containing a mixture of 2,6-dibromopyridine (3, 1.50 g, 6.33 mmol) and 1-(mesityl)imidazole (6, 2.94 g, 15.8 mmol) was heated to 140 °C for 5 days. After cooling to room temperature, the brown residue was triturated with Et2O (15 mL) and filtered. The resulting light brown solid was stirred overnight in Et2O (300 mL) to remove soluble impurities. The remaining solid was isolated via vacuum filtration to afford the product. Beige solid, 3.47 g, 90% yield; mp 140 °C. 1H NMR (DMSO-d6): δ 10.68 (s, 2H), 9.22 (s, 2H), 8.71 (t, 1H, J = 8 Hz), 8.49 (d, 2H, J = 8 Hz), 8.32 (s, 2H), 7.20 (s, 4H), 2.35 (s, 6H), 2.14 (s, 12H). Further analytical data match those previously published.31

(η3-C,C0 ,N0 )(2,6-Bis{[N-methyl-N0 -methylene]imidazol-2ylidene}pyridine)chloridoplatinum(II) Chloride (12). A solu-

tion of 7 (190 mg, 560 μmol), Ag2O (130 mg, 560 μmol), and PtBr2 (200 mg, 560 μmol) in DMSO (5 mL) was heated to 100 °C for 12 h. The mixture was filtered through a pad of Celite, and the filtrate was diluted with CH2Cl2 (20 mL) and Et2O (200 mL). The resulting precipitate was filtered, washed with Et2O until the rinses became colorless, and dried under vacuum. The solid was suspended in CHCl3 (40 mL) and filtered through a pad of Celite. The filtrate was concentrated (∼2 mL), and Et2O (10 mL) was added. The precipitate was filtered and dried under vacuum. White powder, 1.93 g, 55% yield. 1 H NMR (DMSO-d6): δ 8.21 (t, 1H, J = 7.8 Hz), 7.87 (d, 2H, J = 7.8 Hz), 7.61 (d, 2H, J = 1.8 Hz) 7.36 (d, 2H, J = 1.8 Hz), 5.72 (br s, 4H), 3.94 (s, 6H). 13C{1H} NMR (DMSO-d6): δ 161.6, 155.4, 141.1, 126.0, 123.3, 121.1, 54.3, 36.5. HRMS (ESI): calcd for C15H17ClN5Pt m/z 497.0820 (M - X)þ, found 497.0813. X-ray quality crystals were grown by slow diffusion of Et2O into a solution of 12 in MeOH: Colorless crystal (polyhedron), dimensions 0.34 0.05 0.05 mm3, crystal system monoclinic, space group Pc, Z = 2, a = 10.5717(14) Å, b = 9.0406(12) Å, c = 11.9346(16) Å, R = 90°, β = 110.695(2)°, γ = 90°, V = 1067.0(2) Å3, F = 1.859 g/cm3, T = 200(2) K, θmax = 28.32°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 11 110 reflections measured, 5165 unique (R(int) = 0.0286), 4744 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 6.85 mm-1, Tmin = 0.20, Tmax = 0.73. 251 parameters refined, Flack absolute 1891


Organometallics structure parameter -0.018(9), goodness of fit 1.09 for observed reflections, final residual values R1(F) = 0.028, wR(F2) = 0.060 for observed reflections, residual electron density -0.97 to 1.37 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis{[N-methyl-N0 -methylene]imidazol-2ylidene}pyridine)bromidoplatinum(II) Bromide (13). A solu-

tion of 8 (730 mg, 1.70 mmol), Ag2O (390 mg, 1.70 mmol), and PtBr2 (600 mg, 1.70 mmol) in DMSO (5 mL) was stirred at room temperature for 2 h and then heated to 100 °C for 12 h. The mixture was filtered through a pad of Celite, and the filtrate was diluted with CH2Cl2 (20 mL) and Et2O (200 mL). The precipitate was filtered, washed with Et2O (100 mL), dried under vacuum, and recrystallized from MeOH/ Et2O. White powder, 0.88 g, 84% yield. 1H NMR (DMSO-d6): δ 8.20 (t, 1H, J = 7.8 Hz), 7.83 (d, 2H, J = 7.7 Hz), 7.57 (br s, 2H), 7.39 (br s, 2H), 5.58-5.35 (m, 4H), 3.90 (s, 6H). 13C{1H} NMR (DMSO-d6): δ 161.0, 155.0, 141.3, 125.9, 123.4, 121.3, 54.4, 37.4. HRMS (ESI): calcd for C15H17BrN5Pt m/z 491.0315 (M-Br)þ, found 491.0316.

(η3-C,C0 ,N0 )(2,6-Bis{[N-n-butyl-N0 -methylene]imidazol-2ylidene}pyridine)bromidoplatinum(II) Bromide (14). A solu-

tion of 9 (1.38 g, 2.67 mmol), Ag2O (710 mg, 3.09 mmol), and PtBr2 (1.00 g, 2.81 mmol) in DMSO (5 mL) was heated to 100 °C for 12 h. The mixture was filtered through a pad of Celite, and the filtrate was diluted with CH2Cl2 (20 mL) and Et2O (200 mL). The precipitate was filtered, washed with Et2O (100 mL), and dried under vacuum. Pale yellow powder, 0.98 g, 52% yield; mp 180-182 °C. 1H NMR (DMSOd6): δ 8.22 (t, 1H, J = 7.8 Hz), 7.85 (d, 2H, J = 7.8 Hz), 7.59 (br s, 2H), 7.47 (br s, 2H), 5.57 (d, 2H, J = 15.0 Hz), 5.32 (d, 2H, J = 15.0 Hz), 4.50 (m, 2H), 4.21 (m, 2H), 1.83 (m, 4H), 1.21 (m, 4H), 0.93 (t, 6H, J = 7.2 Hz). 13C{1H} NMR (DMSO-d6): δ 159.9, 155.3, 143.9, 125.6, 123.7, 123.0, 54.4, 50.2, 32.1, 18.7, 13.2. HRMS (ESI): calcd for C21H29BrN5Pt m/z 625.1254 (M - Br)þ, found 625.1256.

(η3-C,C0 ,N0 )(2,6-Bis{[N-mesityl-N0 -methylene]imidazol-2ylidene}pyridine)bromidoplatinum(II) Bromide (15). A solu-

tion of 10 (1.00 g, 1.56 mmol), Ag2O (400 mg, 1.72 mmol), and PtBr2 (610 mg, 1.72 mmol) in DMSO (5 mL) was heated to 100 °C for 12 h. The mixture was filtered through a pad of Celite, and the filtrate was diluted with CH2Cl2 (20 mL) and Et2O (200 mL). The precipitate was filtered, washed with Et2O (100 mL), and dried under vacuum. Pale gray powder, 0.95 g, 73% yield; mp 325 °C. 1H NMR (DMSO-d6): δ 8.33 (t, 1H, J = 7.4 Hz), 7.96 (d, 2H, J = 7.4 Hz), 7.80 (s, 2H), 7.36 (s, 2H), 6.90 (s, 2H), 6.86 (s, 2H), 5.73 (d, 2H, J = 15.4 Hz), 5.33 (d, 2H, J = 15.4 Hz), 2.23 (s, 6H), 2.00 (s, 6H), 1.82 (s, 6H). 13C{1H} NMR (DMSO-d6): δ 162.2, 154.6, 141.5, 137.6, 135.2, 133.7, 128.5, 128.2, 126.2, 123.9, 121.7, 54.8, 20.5, 18.2, 17.8. HRMS (ESI): calcd for C31H33BrN5Pt m/z 749.1567 (M - Br)þ, found 749.1570.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) bromidoplatinum(II) Bromide (16). In the glovebox, a solution of

11 (2.00 g, 3.28 mmol), Ag2O (0.84 g, 3.61 mmol), and PtBr2 (1.28 g, 3.61 mmol) in DMSO (40 mL) was shielded from light with aluminum foil and heated to 100 °C for 12 h and then at 150 °C for 1 h. The mixture was filtered through a pad of Celite, and the filtrate was diluted with CH2Cl2 (20 mL) and Et2O (200 mL). The precipitate was filtered, washed with Et2O (30 mL), and dried under vacuum. Yellow powder, 1.97 g, 75% yield; mp 380 °C (dec). 1H NMR (DMSO-d6): δ 8.70-8.61 (m, 3H), 8.14 (d, 2H, J = 8.2 Hz), 7.74 (d, 2H, J = 2.2 Hz), 6.94 (s, 4H), 2.24 (s, 6H), 1.95 (s, 12H). 13C{1H} NMR (DMSO-d6): δ 166.9, 151.3, 145.5, 138.7, 134.0, 133.8, 128.6, 125.5, 119.4, 108.4, 20.6, 17.3. HRMS (ESI): calcd for C29H29BrN5Pt m/z 721.1254 (M - Br)þ, found 721.1250.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) bromidoplatinum(II) Tetrafluoroborate (17). Under argon in a Schlenk flask, 16 (0.50 g, 0.62 mmol) and NH4BF4 (0.14 g, 1.37 mmol) were dissolved in CH2Cl2 (10 mL) and stirred for 24 h at room temperature. The resulting suspension was then filtered through Celite and washed with CH2Cl2 (5 mL). The solvent was concentrated to

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3 mL, and Et2O was added to precipitate the product. The resulting solid was filtered, washed with Et2O, and dried under vacuum. Yellow solid, 0.40 g, 91% yield; mp 360 °C. 1H NMR (200 MHz, CD2Cl2): δ 8.44 (t, J = 8.2 Hz, 1H), 8.15 (d, J = 2.2 Hz, 2H), 7.95-7.84 (m, 2H), 7.03 (m, 2H), 6.86 (s, 4H), 2.22 (s, 6 H), 1.95 (s, 12H). 13C{1H} NMR (50 MHz, DMSO-d6): δ 166.9, 151.3, 145.5, 138.7, 134.0, 133.8, 128.6, 125.4, 119.3, 108.3, 20.6, 17.3. HRMS (ESI): calcd for C29H29BrN5Pt m/z 721.1254. (M - BF4)þ, found 721.1250. Yellow crystal (plate), dimensions 0.28 0.22 0.02 mm3, crystal system monoclinic, space group P21/c, Z = 4, a = 18.6098(3) Å, b = 10.8408(2) Å, c = 14.5687(1) Å, R = 90°, β = 92.658(1)°, γ = 90°, V = 2936.00(7) Å3, F = 1.831 g/cm3, T = 180(2) K, θmax = 47.47°, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 28 706 reflections measured, 6699 unique (R(int) = 0.0933), 4722 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 6.19 mm-1, Tmin = 0.28, Tmax = 0.89; 376 parameters refined, goodness of fit 1.059 for observed reflections, final residual values R1(F) = 0.043, wR(F2) = 0.090 for observed reflections, residual electron density -1.13 to 0.37 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis[N-methylimidazol-2-ylidene]pyridine) pyridineplatinum(II) Tetrafluoroborate (18). A solution of 12

(100 mg, 160 μmol) and AgBF4 (68 mg, 350 μmol) in pyridine (2 mL) was stirred at room temperature for 3 h in the dark. After removal of the solvent, the residue was extracted with CHCl3. The solvent was removed, the residue was taken up in CH2Cl2, and Et2O was added to precipitate the product, which was isolated via filtration and dried under vacuum. Gray powder, 0.70 g, 70% yield. 1H NMR (DMSO-d6): δ 9.23-8.96 (m, 2H), 8.25 (t, 2H, J = 7.8 Hz), 7.88 (d, 2H, J = 7.8 Hz), 7.77-7.68 (m, 4H), 7.39 (d, 2H, J = 1.7 Hz), 5.80-5.60 (m, 4H), 2.98 (s, 6H). 13C{1H} NMR (DMSO-d6): δ 162.5, 155.9, 154.5, 142.3, 140.8, 127.9, 126.2, 123.6, 121.4, 54.2, 34.8. HRMS (ESI): calcd for C20H22BrF4N6Pt m/z 628.1582 (M - BF4)þ, found 628.1576. Anal. Calcd for C20H22BrF4N6Pt: C 33.59, H 3.10, N 11.75. Found: C 35.35, H 3.58, N 11.13. Colorless crystal (polyhedron), dimensions 0.21 0.10 0.07 mm3, crystal system monoclinic, space group P21/n, Z = 4, a = 11.0836(9) Å, b = 15.1933(12) Å, c = 14.0305(11) Å, R = 90°, β = 98.222(2)°, γ = 90°, V = 2338.4(3) Å3, F = 2.031 g/cm3, T = 200(2) K, θmax = 27.87°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 23 541 reflections measured, 5572 unique (R(int) = 0.0300), 4957 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 6.09 mm-1, Tmin = 0.36, Tmax = 0.68; 361 parameters refined, goodness of fit 1.07 for observed reflections, final residual values R1(F) = 0.028, wR(F2) = 0.065 for observed reflections, residual electron density -0.64 to 1.67 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis{[N-n-butyl-N0 -methylene]imidazol-2ylidene}pyridine)acetonitrileplatinum(II) Tosylate (19). To a

solution of 14 (200 mg, 283 μmol) in CH2Cl2 (10 mL) was added AgOTf (145 mg, 564 μmol). The mixture was stirred for 12 h at room temperature in the dark. Filtration through a pad of Celite and removal of the solvent yielded the tosylate (215 mg, 92%) as a pale yellow solid. Spectral data of 19 were obtained as a solvent adduct in CD3CN by heating the tosylate (20 mg, 28 μmol) to 70 °C in CD3CN in an NMR tube. 1H NMR (CD3CN): δ 8.03 (t, 1H, J = 7.8 Hz), 7.69 (d, 2H, J = 7.8 Hz), 7.37 (d, 2H, J = 1.8 Hz) 7.19 (d, 2H, J = 1.8 Hz), 5.34-5.26 (m, 4H), 4.12-4.00 (m, 4H), 1.82-1.70 (m, 4H), 1.26-1.18 (m, 4H), 0.84 (t, 6H, J = 7.2 Hz). 13C{1H} NMR (CD3CN): δ 158.9, 155.4, 142.5, 126.2, 121.8, 120.8, 54.6, 49.1, 32.3, 19.0, 12.5. HRMS (ESI): calcd for C24H29D3F3N6O3PtS m/z 739.2045 (M - OTf)þ, found 739.2037. 1892


Organometallics (η3-C,C0 ,N0 )(2,6-Bis{[N-mesityl-N0 -methylene]imidazol-2ylidene}pyridine)acetonitrileplatinum(II) Tosylate (20). To a

solution of 15 (200 mg, 240 μmol) in CH2Cl2 (10 mL) was added AgOTf (123 mg, 478 μmol). The mixture was stirred for 12 h at room temperature in the dark. Filtration through a pad of Celite and removal of the solvent yielded the tosylate (218 mg, 91%) as a pale gray solid. Spectral data of 20 were obtained as a solvent adduct in CD3CN by heating the tosylate (20 mg, 24 μmol) to 70 °C in CD3CN in an NMR tube. 1H NMR (DMSO-d6): δ 8.38 (t, 1H, J = 7.8 Hz), 8.01 (d, 2H, J = 7.8 Hz), 7.92 (d, 2H, J = 1.6 Hz), 7.63 (d, 2H, J = 1.6 Hz), 7.09 (s, 4H), 5.84 (d, 2H, J = 15.6 Hz), 5.47 (d, 2H, J = 15.6 Hz), 2.28 (s, 6H), 1.98 (s, 6H), 1.94 (s, 6H). 13C{1H} NMR (DMSO-d6): δ 158.1, 155.4, 143.4, 139.1, 143.5, 134.1, 133.4, 129.6, 128.8, 127.1, 124.1, 122.6, 117.4, 54.5, 20.4, 17.9, 17.2. HRMS (ESI): calcd for C34H33D3F3N6O3PtS m/z 863.2357 (M - OTf)þ, found 863.2350. Colorless crystal (polyhedron), dimensions 0.22 0.14 0.13 mm3, crystal system orthorhombic, space group Fdd2, Z = 8, a = 42.1158(4) Å, b = 8.2226(1) Å, c = 23.8465(2) Å, R = 90°, β = 90°, γ = 90°, V = 8258.08(15) Å3, F = 1.691 g/cm3, T = 200(2) K, θmax = 27.49°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 19 798 reflections measured, 4694 unique (R(int) = 0.0417), 4219 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 3.58 mm-1, Tmin = 0.51, Tmax = 0.65; 272 parameters refined, Flack absolute structure parameter 0.031(6), goodness of fit 1.08 for observed reflections, final residual values R1(F) = 0.022, wR(F2) = 0.049 for observed reflections, residual electron density -0.44 to 0.34 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) acetonitrileplatinum(II) Tosylate (21). To a solution of 16 (200

mg, 249 μmol) in CH2Cl2 (10 mL) was added AgOTf (127 mg, 498 μmol), and the mixture was stirred for 12 h at room temperature in the dark. Filtration through a pad of Celite and removal of the solvent yielded the tosylate (212 mg, 91%) as a bright yellow solid. Spectral data of 21 were obtained as a solvent adduct in CD3CN by heating the tosylate (20 mg, 21 μmol) to 70 °C in CD3CN in an NMR tube. 1H NMR (CD3CN): δ 8.42 (t, 1H, J = 8.4 Hz), 8.06 (d, 2H, J = 2.2 Hz), 7.81-7.69 (m, 2H), 7.36-7.34 (m, 2H), 7.00 (s, 4H), 2.21 (s, 6H), 1.99 (s, 12H). 13C{1H} NMR (50 MHz, CD2Cl2þDMSO-d6): δ 169.2, 159.9, 152.4, 144.0, 138.2, 136.0, 132.7, 128.5, 124.1, 124.0, 113.2, 23.9, 20.6. HRMS (ESI): calcd for C32H29D3F3N6O3PtS m/z 834.2024 (M OTf)þ, found 834.2018.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) bromidodiiodidoplatinum(II) Triiodide (22). Complex 17 (70

mg, 86 μmol) was dissolved in CH2Cl2 (20 mL), and I2 (66 mg, 260 μmol) was added in one portion. The reaction was stirred at room temperature for 24 h. The resulting dark solution was concentrated, and the residue was washed with Et2O until the ethereal phase was colorless. The residue was dissolved in CH2Cl2 and precipitated with Et2O. Dark brown solid, 0.80 g, 80% yield. 1H NMR (CD2Cl2): δ 8.58 (td, 1H, J = 8.6 Hz, J = 2.0 Hz), 8.35-8.33 (m, 2H), 8.26-8.15 (m, 2H), 7.25-7.18 (m, 2H), 6.87 (s, 4H), 18.00 (br s, 18H). Suitable crystals of 22 for X-ray analysis were obtained by slow diffusion of Et2O into the CH2Cl2 solution. Black crystal (polyhedron), dimensions 0.27 0.16 0.04 mm3, crystal system triclinic, space group P1, Z = 2, a = 8.5193(1) Å, b = 11.9768(1) Å, c = 21.0599(1) Å, R = 78.0370(10)°, β = 85.6620(10)°, γ = 70.7660(10)°, V = 1984.75(3) Å3, F = 2.695 g/cm3, T = 200(2) K, θmax = 25.35°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 17 566 reflections measured, 7255 unique (R(int) = 0.2705), 4847 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the

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reciprocal space, μ = 10.01 mm-1, Tmin = 0.17, Tmax = 0.69; 352 parameters refined, goodness of fit 1.05 for observed reflections, final residual values R1(F) = 0.096, wR(F2) = 0.234 for observed reflections, residual electron density -2.76 to 5.29 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) iodidoplatinum(II) Tetrafluoroborate (24). Complex 17 (15 mg,

18 μmol) was dissolved in CH2Cl2 (1 mL), and MeI (2 μL, 32 μmol) was added in one portion. The reaction was stirred at room temperature for 2 h and then at 100 °C for 24 h. The solution was concentrated, and the product was precipitated with Et2O. Bright yellow solid, 78% yield; mp 370 °C. 1H NMR (CD2Cl2): δ 8.50 (t, 1H, J = 8.4 Hz), 8.27 (d, 2H, J = 2.0 Hz), 7.97-7.87 (m, 2H), 7.03-6.98 (m, 2H), 6.86 (s, 4H), 2.21 (s, 6H), 1.91 (s, 12H). 13C{1H} NMR (CD2Cl2): δ 168.7, 151.5, 145.7, 140.7, 135.2, 135.0, 129.6, 125.5, 119.2, 108.8, 21.2, 18.1. HRMS (ESI): calcd for C29H29IN5Pt m/z 769.1150 (M - BF4)þ, found 769.1113. Crystals suitable for X-ray analysis were obtained by slow diffusion of Et2O into the CH2Cl2 solution of 24. Red crystal (polyhedron), dimensions 0.34 0.16 0.05 mm3, crystal system monoclinic, space group P21/c, Z = 4, a = 19.131(2) Å, b = 7.8884(9) Å, c = 20.700(2) Å, R = 90°, β = 107.167(2)°, γ = 90°, V = 2984.7(6) Å3, F = 1.906 g/cm3, T = 200(2) K, θmax = 28.32°, radiation Mo KR, λ = 0.71073 Å, 0.3° ωscans with CCD area detector, covering a whole sphere in reciprocal space, 30 240 reflections measured, 7417 unique (R(int) = 0.0330), 6561 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 5.79 mm-1, Tmin = 0.24, Tmax = 0.76; 381 parameters refined, goodness of fit 1.08 for observed reflections, final residual values R1(F) = 0.029, wR(F2) = 0.065 for observed reflections, residual electron density -1.20 to 1.48 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) ethyleneplatinum(II) Tetrafluoroborate (25). In a high-pressure

NMR tube, complex 16 (24 mg, 30 μmol) and AgBF4 (12 mg, 60 μmol) were dissolved in CD3NO2 (1 mL). After degassing, the mixture was charged with ethylene (5 bar) and allowed to stand for 3 h at room temperature. The mixture was filtered through a pad of Celite, the solvent was removed, and the residue was dissolved in dichloromethane (1 mL). The mixture was filtered again to remove the insoluble impurities. Evaporation of the solvent gave 25 as a bright yellow solid, 23 mg, 90% yield; mp 265 °C (dec). 1H NMR (200 MHz, CD2Cl2): δ 8.56 (t, J = 8.4 Hz, 1H), 8.24 (d, J = 1.8 Hz, 2H), 8.04 (d, J = 8.4 Hz, 2H), 7.16 (d, J = 1.8 Hz, 2H), 7.07 (s, 4H). 4.20 (t, JPt-H = 34 Hz, 4H), 2.35 (s, 6H), 2.14 (s, 12H). 13C{1H} NMR (50 MHz, CD3NO2): δ 161.4, 151.0, 150.9, 142.2, 134.6, 131.6, 129.8, 126.2, 120.2, 109.7, 83.0, 19.8, 16.0. Anal. Calcd for C31H33B2F8N5Pt: C 44.10, H 3.94, N 8.29. Found: C 43.67, H 4.28, N 8.43. Yellow crystal (polyhedron), dimensions 0.28 0.25 0.19 mm3, crystal system triclinic, space group P1, Z = 2, a = 11.4207(16) Å, b = 11.4940(16) Å, c = 18.532(3) Å, R = 94.301(3)°, β = 104.912(2)°, γ = 116.299(2)°, V = 2056.6(5) Å3, F = 1.638 g/cm3, T = 200(2) K, θmax = 28.34°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 21 770 reflections measured, 10 114 unique (R(int) = 0.0185), 9172 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 3.74 mm-1, Tmin = 0.42, Tmax = 0.54. 541 parameters refined, except of the ethylen hydrogen atoms, which were refined isotropically, goodness of fit 1.05 for observed reflections, final residual values R1(F) = 0.030, wR(F2) = 0.078 for observed reflections, residual electron density -0.65 to 1.59 e Å-3.

(η3-C,C0 ,N0 )(2,6-Bis[N-mesitylimidazol-2-ylidene]pyridine) chloridoplatinum(II) Triflate (26). Under argon in a Schlenk flask,

16 (20 mg, 25 μmol) and AgOTf (13 mg, 49 μmol) were dissolved in CD2Cl2 (1 mL) and stirred for 4 h at room temperature. The resulting 1893


Organometallics solution was carefully filtered into a high-pressure NMR tube, which was then charged with ethylene (5 bar). After mixing at room temperature, the NMR tube was kept at ambient temperature for 24 h and then heated to 80 °C for 2 h. Crystallization by diffusion of Et2O into the reaction mixture yielded 26. Yellow crystal (polyhedron), dimensions 0.24 0.14 0.08 mm3, crystal system monoclinic, space group Pc, Z = 4, a = 15.271(2) Å, b = 13.924(3) Å, c = 16.834(3) Å, R = 90°, β = 102.961(11)°, γ = 90°, V = 3488.3(11) Å3, F = 1.737 g/cm3, T = 200(2) K, θmax = 27.78°, radiation Mo KR, λ = 0.71073 Å, 0.3° ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 35 646 reflections measured, 14 504 unique (R(int) = 0.0824), 10 259 observed (I > 2σ(I)), intensities were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using SADABS43 based on the Laue symmetry of the reciprocal space, μ = 4.37 mm-1, Tmin = 0.42, Tmax = 0.72; 848 parameters refined, Flack absolute structure parameter 0.704(7), goodness of fit 0.98 for observed reflections, final residual values R1(F) = 0.045, wR(F2) = 0.084 for observed reflections, residual electron density -0.78 to 0.75 e Å-3.

Oligomerization with Complex 25 to 3,4-Dimethyl-1-pentene, 28. A solution of 25 (10 mg, 12.2 μmol) and 2-methyl-2-butene

(13.4 mg, 192 μmol) in CD2Cl2 (1.0 mL) was degassed and pressurized with ethylene (5 bar). After heating at 100 °C for 8 d, the solution was cooled and the ethylene pressure released. After filtration through a pad of Celite, the filtrate was subjected to GC-MS and 1H NMR analysis, which showed 68% conversion to 28.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic information files (CIF) of compounds 12, 17, 18, 20, 22, 24, 25, and 26 and NMR spectra of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Files CCDC 797313 (12), 797314 (17), 797315 (18), 797316 (20), 797320 (22), 797321 (24), 797317 (25), and 797318 (26) contain the 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.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.S., P.C., J.C., R.P., and M.L. work at CaRLa of the University of Heidelberg, being cofinanced by the University of Heidelberg, the state of Baden-W€urttemberg, and BASF SE. Support of these institutions is greatly acknowledged. ’ REFERENCES (1) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020–1024. (2) van Koten, G. Pure Appl. Chem. 1989, 61, 1681–1694. (3) (a) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem. Res. 1998, 31, 423–431. (b) van de Kuil, L. A.; Grove, M. D.; Gossage, R. A.; Zwikker, J. W.; Jenneskens, L. W.; Drenth, W.; van Koten, G. Organometallics 1997, 16, 4985–4994. (c) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–3781. (4) (a) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840–841. (b) Xu, W.-W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; Krogh-Jespersen, K.;

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Development of Platinum(II) and -(IV) CNC Pincer Complexes and

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