Synthesis and Characterization of [(NHC)Ni(styrene)2] Complexes

Jun 8, 2018 - therefore can have a significant influence in catalysis.64,78−86. Evaluating the steric properties of the isolated methylated and chlori...
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Synthesis and Characterization of [(NHC)Ni(styrene)2] Complexes: Isolation of Monocarbene Nickel Complexes and Benchmarking of %VBur in (NHC)Ni‑π Systems Stephanie Felten, Sarah F. Marshall, Alisa J. Groom, Ryan T. Vanderlinden, Ryan M. Stolley, and Janis Louie* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States Downloaded via UNIV OF TEXAS AT EL PASO on October 22, 2018 at 15:27:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The investigation of the stereoelectronic influence of N-aryl substituted NHC ligands on Ni(0) is reported. The structural analysis of a family of [(NHC)Ni(styrene)2] complexes are correlated to known literature parameters (TEP and %VBur). The analysis involved NMR spectroscopic techniques and X-ray crystallography analysis of the isolated [(NHC)Ni(styrene)2] complexes. The synthesis and characterization of this user-friendly and easy accessible Ni(0) precatalysts can be realized in a two-step synthesis starting from deprotonation of the imidazolium salt followed by direct coordination onto nickel in the presence of excess styrene.

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INTRODUCTION The discovery of stable N-heterocyclic carbene ligands (NHCs) in 1991 benchmarked a new area in homogeneous catalysis resulting in a broad variety of remarkable catalytic transformations.1−8 A primary reason for the predominance of NHCs lies in the relatively simple synthetic modifications available that allows for a wide range of steric and electronic properties.2,9−12 The substitution of the N-moiety of the imidazole/ imidazoline ring by alkyl or aryl substituents can be easily accessed synthetically and is typically chosen to tune the steric environment of the ligand. Furthermore, backbone substitution at the C4−C5 position of the imidazole framework generates a straightforward method to modify the electronic properties of the NHC ligand, which greatly impacts the electronic properties of the metal center.10−14 These stereoelectronic properties make NHCs to a powerful tool to influence the efficiency in transition metal mediated catalysis.3,15−19 The combination of NHCs with group 10 transition metals, in particular, has matured into a family of compounds of great impact in homogeneous catalysis.20−26 Whereas NHC−Pd systems are well-studied, less attention has been given to NHC− Ni systems despite their prevalent use. Due to the rapid growth of nickel catalysis in recent years as both a complementary and competitive catalyst to palladium, the fundamental understanding of the stereoelectronic effect of NHC ligands on nickel is crucial for future development (Figure 1).27−31 The evaluation of the stereoelectronic properties of NHC ligands on nickel can be made by looking at the well-established parameters, such as percent buried volume (%VBur) and Tolman Electronic Parameter (TEP). Both parameters are commonly used to describe NHC ligands.32−37 While literature reports evaluating stereoelectronic influences of NHCs on Ni(0) are known, they are typically of [(NHC)Ni(CO)3] complexes, which may not be an accurate analogues for pertinent reaction intermediates.38−40 © XXXX American Chemical Society

Figure 1. NHC−Ni mediated Catalysis.

Over the years, our group was able to develop and illustrate the high efficiency of a number of Ni/NHC-catalyzed systems in various regio- and stereoselective cyclization reactions to synthetically valuable 5-, 6-, 7-, and 8-membered carbo- and heterocycles.41 We, like others, have observed subtle nuances in selectivity and yield relative to the applied NHC ligand.41−45 For instance, in the numerous (NHC)2Ni catalyzed cycloadditions developed in our group, a distinct trend in the efficacy of a particular ligand remains elusive. Indeed, no difference in the selectivity or conversion was observed in the (NHC)2Ni-catalyzed cycloaddition of diynes and tropones (eq 1) when employing either IPr versus SIPr (99% conversion, 92% yield).41,46 However, in the cycloaddition of unsaturated hydrocarbons with carbonyl compounds, we observed a distinct difference in reactivity between the two ligands (eq 3). In the reaction of diynes and benzaldehyde, the use of SIPr affords slightly higher conversion than for the unsaturated analogue IPr (98% yield versus 87%, respectively). In comparison, the reaction between enynes and ketones in comparison shows IPr to be superior to SIPr (eq 3).41,47 Even more striking is the reported [2+2+2] cycloaddition of Received: June 8, 2018

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the [(IPr)Ni(styrene)2] (3a) and [(SIPr)Ni(styrene)2] (3b) analogues, wherein in situ formation of the free carbene is followed by the addition of a previous equilibrated mixture of Ni(COD)2 and excess styrene (Scheme 1).53,57,58 Upon

tethered diynes and cyanamides in which SIPr and IMes outperform IPr in terms of both conversion and yield of the reaction (eq 2). However, the three-component cycloaddition of terminal diynes and cyanamides requires the use of SIPr over IMes (eq 4).41,48−50 These examples are just a few illustrations of the need for a deeper fundamental understanding of the NHC−Ni relationship in π-systems. The prevalent role of N-aryl NHC ligands (e.g., IMes, IPr, SIPr, etc.) lead us to focus our study on the investigation of commonly used N-aryl NHC ligands with increased sterics in the ortho position as well as the effect of backbone saturation and substitution at the C4−C5 position of the imidazole ring.1,34,51−55 Furthermore, the growing impact of NHC−Ni systems in homogeneous catalysis involving π-systems demands an increased understanding of ligand effects on the metal center, which would enable further developments in the field of NHC−Ni catalysis in general. To close the gap we present the synthesis and structural characterization of readily accessible [(NHC)Ni-π] complexes to investigate the NHC− Ni relationship. In addition, the structural analysis are correlated to the stereoelectronic parameters the percent buried volume (%VBur) and Tolman Electronic Parameter (TEP).

Scheme 1. General Synthesis of [(NHC)Ni(styrene)2] Complexes 1−4

completion of the reaction, volatiles were removed to isolate these compounds as yellow-orange solids in good to excellent yields. The compounds were characterized and evaluated primarily by NMR-spectroscopy and single crystal X-ray diffractometry. The majority of the complexes are thermally stable in the solid state in an inert gas atmosphere, similar to the reported complexes. However, the chlorinated variants decomposed over several weeks at room temperature in an inert atmosphere but were stable in a −38 °C freezer under an inert atmosphere. Although the [(IPr)Ni(styrene)2] (3a) and [(SIPr)Ni(styrene)2] (3b) analogues were reported to be formed from a 1:1 ratio of NHC:Ni(COD)2, we observed unreacted Ni(COD)2 in the final product in some cases.58 For cleaner isolations, reaction conditions for each compound were optimized for each ligand (see the Supporting Information). NMR Spectroscopic Analysis of [(NHC)Ni(styrene)2] Complexes. Overall, the 1H NMR spectra of the isolated complexes displayed distinct splitting patterns for the ortho groups on the N-aryl moiety upon coordination to nickel into separate signals. In the IPr series (3a−d), the complexation to Ni(0) yields in the splitting of the ortho 2,5-isopropyl groups into 4 separate doublets integrating to 6 protons each (Figure 2a). Similar observations can be made for IMes-, IEt-, and IPentanalogues. The 1H NMR analysis of the isolated complexes in C6D6 also showed an indication of strong π-back bonding between styrene and nickel which results in an upfield shift of the olefinic proton resonances of the styrene moiety from δ 5.00−6.55 ppm to δ 2.00−3.50 ppm. A similar phenomenon was observed in the 13C{1H} NMR congruent to analogous reported complexes.58,59 Further 13C{1H} NMR analysis of the isolated [(NHC)Ni(styrene)2] complexes (1−4) in C6D6 indicate a characteristic NHC−C1 carbon shift upfield upon coordination to Ni(0) in comparison to the C1 carbon shift of the free ligand (Table 1).38,60,62 The difference in NHC−C1 carbon shift can be rationalized by illustration of the orbital interactions. In case of the free NHC, the σ-orbital lone pair at the NHC−C1 carbon donates electron density into the pπ-orbital, affecting the singlet−triplet energy gap of the given NHC ligand. Unsaturated NHCs (e.g., IMes, IEt, IPr) display large singlet−triplet energy gaps resulting in NHC−C1 carbon

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RESULTS Preparation and Characterization of [(NHC)Ni(styrene)2] Complexes. In general, the presumed active catalyst, (NHC)nNi, for our cycloaddition and other reactions is formed in situ from free carbene and Ni(COD)2 due to practicality and simplicity. However, in situ generation of the catalyst does have significant challenges compared to an isolated and well-defined NHC−Ni(0) catalyst. First, the in situ formation of the desired catalytic species is limited by the steric demands of the ligand. For instance, NHCs larger than (S)IPr have tremendous difficulty in binding to the Ni center.56 Second, the dynamic equilibria between Ni(0)/free NHC and the NHC−Ni(0) complex often results in difficulty in true identification of the active species as well as high catalyst loadings requirements. To avoid these issues, we prepared a family of readily accessible, thermally stable [(NHC)Ni(styrene)2] complexes. Complexes 1−4 were synthesized via modified literature procedure from Nicasio and co-workers for B

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Figure 2. 1H and 13C{1H} NMR spectra of IPrMe carbene (top,blue) and [(IPrMe)Ni(styrene)2] (3d) (bottom, red) in C6D6 at 126 MHz.

between δ 206.3−209.5 ppm (IMesCl (1c) vs IPrCl (3c), respectively). The stronger upfield C1 carbon resonance for methyl-substituted [(NHC)Ni(styrene)2] complexes (1d and 3d) implies an increased shielding of the probed nucleus by introduction of electron-donating substituents in the backbone (Figure 2b). Surprisingly, [(IPent)Ni(styrene)2] complex (4a) showed a C1 carbon shift at δ 204.0 ppm, which is in between the C1 chemical shift values for the IMes (1a) and IPr (3a) analogues. Similar observations can be made for the C4−C5 chlorinated [(IPentCl)Ni(styrene)2] complex (4b), which shows also a C1 carbon resonance in between the C1 chemical shifts of the IMesCl (1c) and IPrCl (3c) complexes (Table 1). Due to the increased steric bulk of the IPent and IPentCl ligands, the C1 chemical shift in the 13C{1H} NMR spectra was expected to be at lower field (e.g., IPent: around δ 206.0 ppm; IPentCl: around δ 210.0 ppm), to correlate with the hypothesis described above, that increased steric bulk at the imidazole ortho N-aryl position is influencing the C1 chemical shift of the given NHC ligand. Crystallographic Analysis of [(NHC)Ni(styrene)2] Complexes. We also evaluated the X-ray crystal structures of each [(NHC)Ni(styrene)2] complex and compared significant bond lengths and angles (Table 2). Single crystals were grown by slow diffusion of a concentrated THF mixture into pentane at −38 °C for all species. The [(NHC)Ni(styrene)2] complexes crystallize as yellow-orange single crystals and dissolve quickly at higher temperature (Table S3). The analysis of the X-ray structures show an isostructural relationship to the previous reported [(IPr)M(styrene)2] analogues (M = Ni, Pd).57,58 The Ni−C1 bond length for all complexes ranges between 1.89 and 1.92 Å and with a single coordinated NHC ligand to a Ni(0) center.38,58,65−67 Furthermore, the metal center exhibits two η2-bound styrene ligands with olefinic bond lengths between 1.398 and 1.409 Å, which is slightly longer than for free styrene (1.346 Å).58,68 In agreement with previous reports, all isolated complexes exhibit the same coordination of styrene pseudo planar to the nickel center as illustrated in Figure 3 (X-ray structures 1−3 are illustrated in Figure S21).57,58 Furthermore, the [(NHC)Ni(styrene)2] complexes (1−3) have similar nickel to styrene bond lengths and angles which indicates that the Ni(η2-styrene)2 coordination is not significantly affected by the stereoelectronic alteration of the imidazole/imidazoline framework. The X-ray structure of saturated versus unsaturated [(NHC)Ni(styrene)2] complexes

Table 1. 13C{1H} NMR Spectroscopy Data for the Carbenic Carbon of the Free NHC Ligand, [(NHC)Ni(styrene)2] Complex (1−4), and Their TEP Values10,11,61−64 13

[(NHC) Ni(styrene)2] IMes SIMes IMesCl IMesMe IEt SIEt IPr SIPr IPrCl IPrMe IPent IPentCl

1a 1b 1c 1d 2a 2b 3a 3b 3c 3d 4a 4b

C NMR δ CfreeC1 C6D6 [ppm] 219.7 243.8 221.4 215.4 218.4 243.7 220.6 244.0 220.6 217.4 221.5 222.8

13

C NMR δ CcoordC1 C6D6 [ppm] 202.9 230.7 206.3 200.1 204.2 231.8 205.1 232.0 209.5 202.8 204.0 207.3

TEP [IrCl(NHC)(CO)2] DCM [cm−1]

ref

2050.7 2051.5 2052.6a 2048.0 2051.1

10 10 11 61, 62 63

2051.5 2052.3 2054.0 2049.3 2049.6 2053.0

64 64 64 64 64 64

a

TEP measured as [RhCl(CO)2(NHC)].

shift upfield. In contrast, the saturated analogues (e.g., SIMes, SIEt, SIPr) exhibit smaller singlet−triplet energy gaps (NHC− C1 carbon downfield shifted). In the case of NHC-to-metal coordination, the NHC−C1 lone pair (σ-orbital) donates its electron into the empty metal-centered orbital, which yields in a significant NHC−C1 carbon shift upfield the NMR time scale.38,60,62 The observed shift of the C1 carbon was shown to be influenced by modification of the imidazole backbone as well as the steric at the ortho N-aryl position. Saturated [(NHC)Ni(styrene)2] complexes resonate at around δ 230.00 ppm and differ by roughly δ 1.00 ppm with increasing steric of the ortho N-aryl substituent (SIMes (1b): δ 230.7 ppm, SIEt (2b): δ 231.8 ppm, SIPr (3b): δ 232.0 ppm) which is also observed for their unsaturated analogues (IMes (1a): δ 202.9 ppm, IEt (2a): δ 204.2 ppm, IPr (3a): δ 205.1 ppm). The 13C{1H} NMR analysis of the C4−C5 backbone modified [(NHC)Ni(styrene)2] complexes showed different C1 carbon resonances for electron-donating substituents (−Me) versus electron-withdrawing substituents (−Cl). [(NHC)Ni(styrene)2] complexes bearing methyl groups in the C4−C5 position of the imidazole ring exhibit C1 carbon shifts between δ 200.1− 202.8 ppm (IMesMe (1d) vs IPrMe (3d), respectively), whereas the chloro-substituted analogues resonate further downfield C

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Table 2. Selected Bond Lengths [Å], Angles [°], and Torsion Angles [°] for Isolated [(NHC)Ni(styrene)2] Complexes (1−3): Ni−C1, CAr−N−C1, N−C1−N, N−C4−C5−N [(NHC)Ni(styrene)2] IMes SIMes IMesCl IMesMe IEt SIEt IPra SIPr IPrCl IPrMe

1a 1b 1c 1d 2a 2b 3a 3b 3c 3d

Ni−C1 [Å]

CAr−N−C1 angle [°]

N−C1−N angle [°]

N−C4−C5−N angle [°]

1.912(2) 1.904(4) 1.900(2) 1.914(2) 1.900(1) 1.908(2) 1.899(0) 1.897(7) 1.910(0) 1.924(0)

126.30/126.17 127.96/127.32 124.23/123.94 123.77/122.99 124.72/125.99 126.28/124.88 126.40/125.11 126.62/126.32 125.48 125.07

102.57 106.10 103.31 102.67 101.64 106.00 101.36 105.46 102.44 102.05

0.38 −22.50 −0.29 −0.80 −0.10 −15.11 0.56 26.72 2.54 2.97

a

The structural data were collected from the reported X-ray structure of the given complex, from the CCDC.58

Figure 3. ORTEP diagram of the isolated [(IPrMe)Ni(styrene)2] complex (3d). Ellipsoids are set at 30% probability level. Selected bond lengths [Å] and angles [°]: Ni−C1, CAr−N−C1, N−C1−N, N−C4−C5−N. Most hydrogens were omitted for clarity. From the left to the right side: front view; side view, 90° rotation of C2-axis; bottom view along C2-axis.

imply a significant influence of the N−C1−N angle upon backbone saturation by roughly 4° (IMes 1a 102.57° vs SIMes 1b 106.10°; IEt 2a 101.64° vs SIEt 2b 106.00°; IPr 3a 101.36° vs SIPr 3b 105.46°). The data also indicate a slight decrease in the N−C1−N angle (α) with increased sterics at the ortho N-aryl position (IMes 1a > IEt 2a > IPr 3a; SIMes 1b > SIEt 2b > SIPr 3b). In addition, we observe a greater N−C4−C5−N torsion angle (β) for saturated [(NHC)Ni(styrene)2] complexes than for their unsaturated analogues (Table 2). Both observations are in good agreement with literature reports.42,69,70 The X-ray structures for the methylated and chlorinated [(NHC)Ni(styrene)2] complexes exhibit differences in the Ni−C1 bond length as well as the CAr−N−C1 angles (γ). The IPr series implies an overall increase of the Ni− C1 bond length from −H < −Cl < −Me in conjunction with a decrease in the CAr−N−C1 angle (γ) (Table 2). In contrast, the IMes-series showed that the Ni−C1 bond length does not correlate with the decrease in the CAr−N−C1 angle (γ). However, the crystallographic data indicate that the CAr− N−C1 angle (γ) has a more significant decrease for the IMes family than for the IPr family (IMes 1a: 126.30° to IMesMe 1d: 123.77° vs IPr 3a: 126.40° to IPrMe 3d: 125.07°), which can be reasoned by the increased steric demand of the ortho N-aryl position. To our disappointment, the crystallization of the [(NHC)Ni(styrene)2] complexes 4a and 4b were unsuccessful in yielding defined single crystals, despite several attempts. In Silico Structural Analysis. Backbone saturation of NHC ligands has been shown to have a significant impact on

catalyst activity in terms of selectivity, conversion and yield. As previously mentioned, a relevant difference in reactivity can be observed between the saturated and unsaturated NHC analogues in similar reactions.6−8,41−44 From that point of view we decided to look closer into the stereoelectronic differences of the saturated versus unsaturated [(NHC)Ni(styrene)2] complexes (NHC = IMes/SIMes 1a/b, IEt/SIEt 2a/b, IPr/ SIPr 3a/b) in regard to the environment at ortho N-aryl moiety, which appears to be the most impactful and easy modified structural feature. Structural differences of the isolated [(NHC)Ni(styrene)2] complexes were evaluated by comparing significant bond lengths and angles from the X-ray structures and were further correlated to the calculated %VBur (Table 3). Both qualitatively and the contained data suggest that [(NHC)Ni(styrene)2] complexes bearing saturated NHC ligands are sterically more demanding than their unsaturated analogues, which is in agreement with literature reports for different complexes coordinating the same ligands.42,45,69−72 The increased N−C1−N angle (α) of the saturated [(NHC)Ni(styrene)2] complexes causes the N-substituents to bend closer to the metal center and results in a more crowded environment. In addition, the greater torsion angle (β) of saturated [(NHC)Ni(styrene)2] complexes implies a higher degree of flexibility in the imidazoline backbone, which will greatly affect the overall activity of the nickel catalyst in reaction (Scheme 2). The electronic analysis of the isolated [(NHC)Ni(styrene)2] complexes were evaluated by looking into literature reported TEP values for a [IrCl(NHC)(CO)2] in CH2Cl2, which shows D

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substituents can also affect the sterics of the given complex and therefore can have a significant influence in catalysis.64,78−86 Evaluating the steric properties of the isolated methylated and chlorinated [(NHC)Ni(styrene)2] complexes indicates that the steric demand of the given complex increases in order of backbone substitution from −H < −Cl < −Me. Furthermore, the backbone steric effect becomes more pronounced with an increase in the ortho N-aryl position of the NHC (Table 3). Consistent with the observed trend are the van der Waals bond radii for the given backbone substituent −H (1.20 Å) < −Cl (1.75 Å) < −Me (2.00 Å) as well as the CAr−N−C1 bond angles (γ) (Scheme 3).87−89 Furthermore, substitution of the imidazole backbone seems to not only influence the CAr−N−C1 bond angle (γ) but also significantly impact the dynamic of the N-aryl substituents. Thus, dynamic rotational behavior can be compromised by substitution in the imidazole backbone and could yield in significant effects on the steric crowding around the metal center. Unsubstituted NHC ligands (e.g., IMes, IPr, IPent) should exhibit relatively free rotation of the N-aryl groups at the imidazole ring whereas substitution of the backbone would result in a disruption of rotational behavior. We hypothesize that large backbone substituents like methyl groups (vdW = 2.00 Å) lock down the rotation in comparison to medium sized substituents, e.g. chloro groups (vdW = 1.75 Å), which allow compromised rotation (Scheme 4). The appearance of dynamic properties of NHC ligands at a given complex is known to play an important role in catalysis. However, the analysis of NHC dynamics when ligated to nickel warrants further investigation to gain specific insight. Overall, recent literature reports from Cavallo and co-workers for NHC-Ru complexes in metathesis support our hypothesis.90 We correlated the proposed findings to the calculated %VBur for the given backbone modified [(NHC)Ni(styrene)2] complexes 1−3 (Table 3). The calculations were conducted with the free online software SambVca. 2.0, developed by Cavallo and co-workers.33 Overall, we observe that the resulting %VBur values agree with the proposed trends. Our analysis showed that [(NHC)Ni(styrene)2] complexes bearing saturated NHCs are slightly more sterically demanding than their unsaturated

Table 3. X-ray Calculated %VBur Parameters for Different Ni−C1 Bond Lengths of [(NHC)Ni(styrene)2] Complex (1−3) [(NHC) Ni(styrene)2] IMes SIMes IMesCl IMesMe IEt SIEt IPr SIPr IPrCl IPrMe

1a 1b 1c 1d 2a 2b 3a 3b 3c 3d

Ni−C1 [Å]

%VBur [%]a

Ni−C1 [Å]

%VBur [%]a

1.912(2) 1.904(4) 1.900(2) 1.914(2) 1.900(1) 1.908(2) 1.899(0) 1.897(7) 1.910(0) 1.924(0)

35.3 36.0 36.0 35.9 37.0 36.8 38.5 39.9 40.7 40.9

2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

33.7 34.3 34.2 34.4 35.2 36.7 36.9 38.1 39.1 39.6

a

X-ray structure. SambVca. parameters: sphere radius, 3.50 Å; mesh spacing, 0.05; bond radii, 1.17. H atoms are excluded.33

no significant difference between the saturated vs unsaturated versions of the same ligand (Table 1).10,11,34,38,45,65,73 However, literature reports investigating the donor strength of NHC ligands by for example 13C{1H} NMR spectroscopy support that saturated NHC ligands are stronger donor ligands than their unsaturated analogues.10,11,62,74−76 Overall, our analysis supports that modification of the N-aryl moiety of the NHC ligand has a great impact on the steric environment.34,38,71,77 We also evaluated the effects of saturation of the imidazole backbone as well as substitution at the C4−C5 position of the imidazole ring with electron-withdrawing groups (EWG) (e.g., Cl) and electron-donating groups (EDG) (e.g., Me) since this substitution pattern is a simple yet significant variation of the NHC framework and can greatly impact catalytic activity. The substitution/alteration of the imidazole backbone is well-known as a common method to modify the electronic properties of the NHC ligand.10,12,59,69,71 The TEP values shown in Table 1, suggest that imidazole rings bearing chloro-substitution at the C4−C5 position have higher wave numbers in the Ir−CO band than their methylated analogues and indicates that the latter are better donor ligands.10,11,34,71 However, recent reports in the organometallic community suggest that introduction of backbone

Scheme 2. Evaluating the N−C1−N Angle (Left) and N−C4−C5−N Torsion Angle (Right) of ORTEP Diagram from Isolated [(NHC)Ni(styrene)2] Complexesa

NHC = IMes 1a vs SIMes 1b. Ellipsoids are set at 30% probability level. Selected bond lengths [Å] and angles [°]: N−C1−N, N−C4−C5−N. Most hydrogens were omitted for clarity. a

E

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reported differences between %VBur values for IPr and SIPr are on the same order of magnitude and result in dramatic swings in reactivity. The difference in %VBur values between Ni and Pd could be explained by the quality of the X-ray structures used for the calculation, but it also reflects that there is a distinct difference for ligands coordinated on Pd or on Ni, which is an important factor when trying to use ligands effective in Pd catalyzed reactions on Ni. The case becomes more clear if looking at the %VBur value for the same ligand on different complexes. The steric evaluation of new NHCs will be commonly performed on a linear [(NHC)MCl] complex (M = Cu, Ag, Au), which allows the evaluation of the ligand without steric clashes from other ligands. Furthermore, the simple preparation of the [(NHC)MCl] complex (M = Cu, Ag, Au) results in a broad variety of reported %VBur values for different NHCs on this specific complex.69,94,95 However, the extrapolation of these values to other metal complexes bearing the same NHC ligands, like nickel in our case, is misleading and not recommended. A great illustration is the comparison of the IPr series on different metal complexes reported in the literature (Scheme 5).58,64,69,71,96 The stated examples in Scheme 5

Scheme 3. Effect of Backbone Substitution on Nickel Center in [(NHC)Ni(styrene)2]a

Scheme 4. Hypothesized Dynamic Behavior of BackboneSubstituted NHCs and Its Effect on the Nickel Center in [(NHC)Ni(styrene)2] (1−4)

Scheme 5. Comparison the %VBur Values for the IPr Series across Different Metal Complexes (M = Ni, Pd, and Au)a

analogues and demonstrates that the increase in torsion angle affects the steric environment of the ligand and the metal (%VBur [2.0 Å]: IMes 1a = 33.7% vs SIMes 1b = 34.3%; IEt 2a = 35.2% vs SIEt 2b = 36.7%; IPr 3a = 36.9% vs SIPr 3b = 38.1%). Furthermore, [(NHC)Ni(styrene)2] complexes containing backbone modification by methylation or chlorination showed an increase in %VBur from −H < −Cl < −Me and with that correlating well with the aforementioned crystallographic observations (e.g., %VBur [2.0 Å]: IPr 3a = 36.9% < IPrCl 3c = 39.1% < IPrMe 3d = 39.6%). Not surprisingly, the trend for the different backbone modifications becomes more dominant with increase in steric demand in the ortho N-aryl position. It has been suggested that the accuracy of the %VBur value highly depends on the quality of the data used for the calculations.33,69 To probe the accuracy of the %VBur values from our X-ray structures, we turned to DFT calculations. The optimized [(NHC)Ni(styrene)2] (1−4) structures correlate with the %VBur trend discussed above (Table S1).33,69,91−93 Furthermore, the quality of the input data is an important factor for evaluating %VBur values of a given complex, but the correlation to reported %VBur values in literature can be misleading if not specific to the exact complex with the same geometry (coordination sphere) as well as the same metal center.33,67 For example, the comparison of the %VBur values from the [(IPr)Ni(styrene)2] complex isolated by Nicasio and co-workers to the Pd analogue reported by Belderrain and co-workers show different values for the same ligand in the same coordination sphere but with a different metal center (Table S2).57,58 While the difference is apparently low, the

a

SambVca parameters: sphere radius, 3.50 Å, Ni−C1. 2.00 Å; mesh spacing, 0.05; bond radii, 1.17. H atoms are excluded.33

demonstrate only small variations of the %VBur value in the same complex; however, the resulting steric trend for the F

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quartet, quintet, and multiplet in that order. All 13C NMR spectra were proton decoupled. The percent buried volume (%VBur) for given [(NHC)Ni(styrene)2] complexes 1−4 were calculated by using the free available SambVca. 2.0 online software (https://www.molnac. unisa.it/OMtools/sambvca2.0/)33 with the following parameters: sphere radius, 3.50 Å, Ni−C1. 2.00 Å; mesh spacing, 0.05; bond radii, 1.17. H atoms are excluded. The discussed ORTEP diagrams of the X-ray structures 1−3 were created using Mercury.99 X-ray Structure. X-ray data was collected and analyzed by Dr. Ryan T. Vanderlinden at the University of Utah. Single-crystal X-ray diffraction data was collected on a Nonius KappaCCD diffractometer equipped with Mo KR radiation (λ = 0.71073 Å) and a BRUKER APEXII CCD detector. The APEX3100 software suite was used to manage data collection, integration, scaling, and absorption correction by the Multiscan method (SADABS),101 structure determination via direct methods (SHLEXT)102 and model refinement (SHELXL).103 All data was collected at 103(2) K. PLATON SQUEEZE104 was used to account for seriously disordered solvent molecules not represented in the structure. The crystallographic parameters for X-ray structures of [(NHC)Ni(styrene)2] (1−3) are provided in Table S3. Computational Details. The DFT geometry optimizations and frequency calculations were conducted with Gaussian 09,91 using a B3LYP92,93 density functional method together with a 6-31G* basis set.105 The X-ray structures were used as start structure for the optimization steps. The Cartesian coordinates for all optimized structures are provided in the Supporting Information. The computations were performed at the Center for High Performance Computing at the University of Utah. General Synthesis of [(NHC)Ni(styrene)2] Complexes.58 In the first step, the NHC salt was dissolved in THF, followed by the addition of base. The reaction mixture was stirred at room temperature between 2 and 24 h (depending on the NHC salt) affording a pale yellow solution which was then filtered through a Celite plug. In the second step, Ni(COD)2 and styrene were equilibrated in THF for 15 min at room temperature affording a red solution. To this solution was added the previously prepared carbene solution in a single portion, which led to an immediate color change from red to orange/yellow. The reaction was stirred for 2 h at room temperature. All volatiles were then removed in vacuo. The crude yellowish product was dissolved in THF and filtered through a Celite filter, dried in vacuo, and triturated with pentane yielding the desired [(NHC)Ni(styrene)2]. [(IMes)Ni(styrene)2], 1a. IMesHCl (1.0 equiv, 0.03 g, 0.088 mmol) and KOtBu (1.0 equiv, 0.010 g, 0.088 mmol) in THF (1.0 mL) were stirred for 4 h at room temperature before being filtered through Celite. Ni(COD)2 (0.9 equiv, 0.024 g, 0.087 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.09 mL, 0.704 mmol). 1a (0.048 g, 0.084 mmol, 96%).1H NMR (500 MHz, C6D6, ppm) δ 7.06−6.96 (m, 6H), 6.82 (s, 2H), 6.75 (s, 2H), 6.50− 6.44 (m, 4H), 6.27 (s, 2H), 3.30 (dd, J = 12.4, 9.6 Hz, 2H), 2.78− 2.69 (m, 4H), 2.21 (s, 6H), 2.08 (s, 6H), 2.00 (s, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 202.89, 147.64, 138.85, 138.12, 136.55, 135.90, 129.92, 129.85, 128.68, 128.58, 128.39, 128.30, 128.20, 128.17, 124.70, 123.64, 123.17, 72.87, 49.57, 21.46, 18.82. [(SIMes)Ni(styrene) 2 ], 1b. SIMesHCl (1.0 equiv, 0.03 g, 0.087 mmol), KOtBu (1.0 equiv, 0.0097 g, 0.087 mmol) and KH (1.0 equiv, 0.0035 g, 0.087 mmol) in THF (1.0 mL) were stirred for 4 h at room temperature before being filtered through Celite. Ni(COD)2 (0.55 equiv, 0.013 g, 0.048 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.08 mL, 0.69 mmol). 1b (0.045 g, 0.079 mmol, 91%).1H NMR (500 MHz, C6D6, ppm) δ 6.99 (t, J = 9.7 Hz, 6H), 6.79 (t, J = 8.9 Hz, 4H), 6.38 (d, J = 6.9 Hz, 4H), 3.25−3.13 (m, 6H), 2.75 (dd, J = 18.4, 11.1 Hz, 4H), 2.29 (s, 6H), 2.20 (d, J = 7.9 Hz, 6H), 2.11 (d, J = 10.6 Hz, 6H). 13 C{1H} NMR (126 MHz, C6D6, ppm) δ 230.66, 147.65, 138.47, 137.91, 137.33, 136.67, 130.27, 130.12, 128.68, 128.58, 128.39, 128.20, 128.11, 124.71, 123.65, 73.26, 51.66, 49.79, 37.96, 21.46, 18.92, 18.78, 18.68. [(IMesCl)Ni(styrene)2], 1c. IMesClHCl (1.0 equiv, 0.03 g, 0.073 mmol) and KOtBu (1.0 equiv, 0.0082 g, 0.073 mmol) in

IPr-series is changing for each complex. Herein, a conclusive interpretation of the IPr-series steric demand is not possible. Thus, examples should demonstrate that the extrapolation from reported %VBur values should be done carefully.

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CONCLUSION

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EXPERIMENTAL SECTION

In conclusion, we report the synthesis and characterization of a family of [(NHC)Ni(styrene)2] complexes. We evaluated the influence of commonly used N-aryl NHC ligands on nickel in π-systems as well as the effects of increased sterics in ortho position of the N-aryl moiety at the imidazole/imidazoline ring and the effect of backbone saturation and substitution at the C4−C5 position of the imidazole ring. Saturation of the backbone has a large effect on the torsion angle and N−C1−N angle of the ligand which yields in a greater steric bulk of saturated NHCs as well as influences the dynamics of the ligand. The trend was shown to become more dramatic with increased steric hindrance at the ortho N-aryl position of the imidazole/ imidazoline ring. Furthermore, we investigated the effect of backbone substitution at the C4−C5 position of the imidazole framework by methylation or chlorination. The results of our study showed that the stereoelectronic of the NHC ligands are greatly effect by the backbone substitution. Upon substitution of the backbone, we observed a decrease in the CAr−N−C1 angle from −H < −Cl < −Me yielding a more crowded metal center. The steric trends of the given complexes are in good correlation with the calculated %VBur, which shows an increase in steric demands through manipulation of the imidazole backbone. Though the calculated %VBur values correlate well with our experimental findings, the parameters are highly sensitive to data input. Thus, our findings are a reminder for the scientific community that the %VBur parameter is a useful tool in homogeneous catalysis but should not be extrapolated across coordination sphere, metal, and oxidation state. The presented study illustrates a benchmark of [NHC−Ni-π] systems in homogeneous catalysis. The results lead to our hypothesis that the dynamic of the N-aryl moiety of the NHC ligand is greatly affected by modification of the ligand framework which is proposed to influence the catalytic activity, in terms of conversion and selectivity. The dynamic property of the presented [(NHC)Ni(styrene)2] complexes is current research in our laboratory.

General Experimental. All reactions were conducted under an atmosphere of N2 using standard Schlenk techniques or in a N2 filled glovebox unless otherwise noted. Styrene was purchased from SigmaAldrich, freshly distilled and degassed before usage in the reaction, but not dried. THF, benzene, ether, and pentane were dried over neutral alumina under N2 using a Grubbs type solvent purification system. Deuterated benzene was purchased from Cambridge, dried over CaH2, and degassed before use. Ni(COD)2 was purchased from Strem and used without further purification. NHC ligands 1−4 were synthesized according to literature procedure.2,63,64,78,97,98 NHCs 1b and 4a were purchased from Strem and used without further purifications. All other reagents were purchased and used without further purification unless otherwise noted. [(NHC)Ni(styrene)2] complexes 1−4 were prepared by an optimized literature procedure.57,58 1H and 13 C{1H} NMR spectra of pure compounds were acquired at 500 MHz, respectively, unless otherwise noted. Chemical shifts were referenced to the deuterated solvents (C6D6: 1H: δ 7.16 ppm, 13 C: δ 128.38 ppm). The abbreviations s, d, dd, ddd, dt, dq, t, q, quint, and m stand for singlet, doublet, doublet of doublets, doublet of doublet of doublets, doublet of triplets, doublet of quartets, triplet, G

DOI: 10.1021/acs.organomet.8b00394 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

[(IPrCl)Ni(styrene)2] complex (3c) showed in the 1H and 13C{1H} NMR spectra traces of other signals. The low concentration and the instability of the complex in solution made the determination of the origin of the observed signals challenging. However, structural analysis of the complex by X-ray diffraction identified only [(IPrCl)Ni(styrene)2] complex (3c). [(IPrMe)Ni(styrene) 2 ], 3d. IPrMeHCl (1.0 equiv, 0.03 g, 0.066 mmol) and KOtBu (1.0 equiv, 0.0074 g, 0.066 mmol) in THF (1.0 mL) were stirred for 2 h at room temperature before being filtered through Celite. Ni(COD)2 (1.0 equiv, 0.018 g, 0.066 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.66 mL, 0.53 mmol). 3d (0.046 g, 0.068 mmol, 94%). 1 H NMR (500 MHz, C6D6, ppm) δ 7.15 (t, J = 7.2 Hz, 2H), 6.99 (d, J = 6.7 Hz, 4H), 6.83 (d, J = 7.2 Hz, 6H), 6.26 (d, J = 6.1 Hz, 4H), 3.26−3.11 (quin, 2H), 3.03 (t, J = 10.7 Hz, 2H), 2.74−2.61 (m, 2H), 2.54 (dd, J = 19.6, 11.0 Hz, 4H), 1.47 (s, 6H), 1.24 (d, J = 6.0 Hz, 6H), 0.92 (d, J = 6.1 Hz, 6H), 0.80 (dd, J = 23.9, 6.2 Hz, 12H). 13 C{1H} NMR (126 MHz, C6D6, ppm) δ 202.76, 147.66, 147.44, 146.73, 136.61, 130.24, 129.91, 129.51, 128.57, 128.47, 128.28, 128.08, 127.43, 127.27, 127.13, 125.33, 125.22, 124.99, 124.68, 124.40, 123.49, 123.19, 73.10, 50.98, 28.99, 28.52, 26.05, 25.02, 24.86, 24.81, 23.39, 11.05. [(IPent)Ni(styrene) 2 ], 4a. IPentHCl (1.0 equiv, 0.03 g, 0.056 mmol), KOtBu (1.0 equiv, 0.0062 g, 0.056 mmol), and NaH (1.1 equiv, 0.0015 g, 0.61 mmol) in THF (1.0 mL) were stirred for 24 h at room temperature before being filtered through Celite. Ni(COD)2 (0.95 equiv, 0.0145 g, 0.053 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.51 mL, 0.45 mmol). 4a (0.048 g, 0.084 mmol, 95%). 1H NMR (500 MHz, C6D6, ppm) δ 7.30 (t, J = 7.8 Hz, 2H), 7.10 (dd, J = 11.7, 7.8 Hz, 4H), 7.04−6.96 (m, 6H), 6.69 (s, 2H), 6.45 (d, J = 6.9 Hz, 4H), 3.24 (dd, J = 12.8, 9.4 Hz, 2H), 2.90−2.81 (m, 2H), 2.80−2.69 (m, 4H), 2.52 (dd, J = 5.9, 3.7 Hz, 2H), 2.03−1.94 (m, 2H), 1.81−1.70 (m, 2H), 1.66−1.47 (m, 6H), 1.43 (dd, J = 13.4, 7.0 Hz, 2H), 1.37−1.29 (m, 4H), 1.05 (t, J = 7.3 Hz, 6H), 0.84 (t, J = 7.5 Hz, 6H), 0.77 (t, J = 7.4 Hz, 6H), 0.70 (t, J = 7.4 Hz, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 203.95, 147.28, 145.48, 145.06, 139.21, 129.20, 128.68, 128.58, 128.39, 128.30, 128.24, 128.20, 126.49, 125.98, 124.89, 124.84, 123.69, 72.39, 50.24, 42.06, 41.71, 37.24, 29.23, 28.56, 27.49, 27.00, 14.23, 12.39, 12.09, 11.07. [(IPentCl)Ni(styrene)2], 4b. IPentClHCl (1.0 equiv, 0.03 g, 0.049 mmol) and C2CO3 (5.0 equiv, 0.081 g, 0.25 mmol) in THF (1.0 mL) were stirred for 24 h at room temperature before being filtered through Celite. Ni(COD)2 (0.90 equiv, 0.012 g, 0.044 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.045 mL, 0.39 mmol). 4b (0.039 g, 0.047 mmol, 94%). 1 H NMR (500 MHz, C6D6, ppm) δ 7.30 (t, J = 7.7 Hz, 2H), 7.12 (t, J = 7.2 Hz, 4H), 6.98 (s, 6H), 6.42 (s, 4H), 3.32 (dd, J = 12.2, 9.9 Hz, 2H), 3.09 (s, 2H), 2.84 (dd, J = 27.8, 11.0 Hz, 4H), 2.61 (s, 2H), 1.99−1.87 (m, 2H), 1.87−1.70 (m, 6H), 1.65 (dd, J = 12.4, 6.5 Hz, 2H), 1.57 (dd, J = 13.8, 6.9 Hz, 4H), 1.51−1.39 (m, 2H), 1.06 (t, J = 7.0 Hz, 6H), 0.86 (dt, J = 21.1, 7.2 Hz, 12H), 0.72 (t, J = 7.1 Hz, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 207.28, 146.67, 146.17, 145.27, 135.96, 129.75, 128.68, 128.58, 128.39, 128.20, 127.76, 126.93, 126.83, 125.01, 124.14, 119.82, 74.67, 50.91, 42.50, 41.09, 28.45, 27.05, 26.55, 26.50, 13.86, 12.21, 11.31, 11.26.

THF (1.0 mL) were stirred for 2 h at room temperature before being filtered through Celite. Ni(COD)2 (0.75 equiv, 0.017 g, 0.055 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.07 mL, 0.068 mmol). 1c (0.040 g, 0.063 mmol, 86%). 1 H NMR (500 MHz, C6D6, ppm) δ 7.06−6.92 (m, 6H), 6.75 (d, J = 17.8 Hz, 4H), 6.46−6.33 (m, 4H), 3.28 (d, J = 10.1 Hz, 2H), 2.81− 2.69 (m, 4H), 2.16 (s, 6H), 2.13 (s, 6H), 2.01 (s, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 206.29, 147.15, 139.94, 137.05, 136.89, 134.91, 130.22, 130.04, 128.58, 128.39, 128.23, 128.20, 124.80, 123.98, 117.83, 74.32, 50.87, 21.52, 18.72, 18.44. [(IMesMe)Ni(styrene)2], 1d. IMesMeHCl (1.0 equiv, 0.03 g, 0.081 mmol) and KOtBu (1.0 equiv, 0.0093 g, 0.081 mmol) in THF (1.0 mL) were stirred for 2 h at room temperature before being filtered through Celite. Ni(COD)2 (0.66 equiv, 0.015 g, 0.054 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.074 mL, 0.65 mmol). 1d (0.042 g, 0.070 mmol, 88%). 1 H NMR (500 MHz, C6D6, ppm) δ 7.03 (d, J = 6.1 Hz, 6H), 6.82 (d, J = 21.8 Hz, 4H), 6.48 (d, J = 4.3 Hz, 4H), 3.28 (t, J = 10.2 Hz, 2H), 2.76 (t, J = 11.0 Hz, 4H), 2.23 (s, 6H), 2.09 (s, 6H), 2.02 (s, 6H), 1.49 (s, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 200.07, 148.07, 138.66, 136.92, 136.62, 136.40, 130.05, 129.92, 128.68, 128.58, 128.39, 128.30, 128.20, 128.13, 126.12, 124.77, 123.50, 72.73, 50.05, 21.54, 18.75, 18.60, 9.40. [(IEt)Ni(styrene)2], 2a. IEtHCl (1.0 equiv, 30 mg, 0.081 mmol) and KHMDS (1.1 equiv, 0.018 g, 0.089 mmol) in THF (1.0 mL) were stirred for 4 h at room temperature before being filtered through Celite. Ni(COD)2 (0.7 equiv, 0.0156g, 0.058 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.074 mL, 0.65 mmol). 2a (0.042 g, 0.07 mmol, 87%). 1H NMR (500 MHz, C6D6, ppm) δ 7.24 (d, J = 7.7 Hz, 2H), 7.06 (dd, J = 7.5, 3.4 Hz, 4H), 7.00 (dt, J = 6.8, 4.4 Hz, 6H), 6.47 (s, 2H), 6.41 (d, J = 7.0 Hz, 4H), 3.28 (dd, J = 12.1, 9.9 Hz, 2H), 2.69 (dd, J = 11.0, 3.2 Hz, 4H), 2.62 (dt, J = 15.4, 7.7 Hz, 4H), 2.42 (ddd, J = 22.1, 14.8, 7.5 Hz, 4H), 1.11 (t, J = 7.6 Hz, 6H), 0.94 (t, J = 7.6 Hz, 6H).13C{1H} NMR (126 MHz, C6D6, ppm) δ 204.16, 147.44, 142.62, 141.87, 139.24, 129.73, 128.68, 128.58, 128.39, 128.29, 128.20, 127.48, 127.06, 124.65, 123.74, 123.68, 73.17, 49.91, 25.10, 25.10, 15.33, 15.21. The 1 H and 13C{1H} NMR spectra of [(IEt)Ni(styrene)2] complex (2a) show residues of KHMDS from the first step of the reaction. [(SIEt)Ni(styrene)2], 2b. SIEtHCl (1.0 equiv, 0.03 g, 0.081 mmol) and KHMDS (1.1 equiv, 0.018 g, 0.089 mmol) in THF (1.0 mL) were stirred for 4 h at room temperature before being filtered through Celite. Ni(COD)2 (0.9 equiv, 0.019 g, 0.072 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.074 mL, 0.65 mmol). 2b (0.047 g, 0.078 mmol, 97%). 1H NMR (500 MHz, C6D6, ppm) δ 7.24 (t, J = 7.7 Hz, 2H), 7.09 (dd, J = 18.9, 7.6 Hz, 4H), 7.01−6.92 (m, 6H), 6.29 (d, J = 7.1 Hz, 4H), 3.41−3.31 (m, 4H), 3.15−3.07 (m, 2H), 2.98−2.87 (m, 2H), 2.73 (dd, J = 15.1, 7.5 Hz, 2H), 2.67 (dd, J = 17.0, 9.1 Hz, 6H), 2.60−2.50 (m, 2H), 1.23 (t, J = 7.6 Hz, 6H), 1.01 (t, J = 7.6 Hz, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 231.79, 147.39, 143.46, 142.45, 139.78, 128.89, 128.68, 128.58, 128.39, 128.26, 128.20, 127.59, 127.04, 124.62, 123.69, 73.37, 53.00, 49.97, 25.07, 24.65, 24.56, 15.92, 15.73, 14.91, 7.76, 7.56, 7.35. The 1H and 13C{1H} NMR spectra of [(SIEt)Ni(styrene)2] complex (2b) residues of KHMDS from the first step of the reaction. [(IPrCl)Ni(styrene)2], 3c. IPrClHCl (1.0 equiv, 0.03 g, 0.061 mmol) and KOtBu (1.0 equiv, 0.0068 g, 0.061 mmol) in THF (1.0 mL) were stirred for 2 h at room temperature before being filtered through Celite. Ni(COD)2 (1.0 equiv, 0.017 g, 0.061 mmol) was dissolved in THF (1.2 mL), followed by addition of styrene (8.0 equiv, 0.056 mL, 0.48 mmol). 3c (0.040 g, 0.056 mmol, 94%). 1H NMR (500 MHz, C6D6, ppm) δ 7.29 (t, J = 7.8 Hz, 2H), 7.17−7.12 (m, 4H), 6.98 (d, J = 3.1 Hz, 6H), 6.41−6.35 (m, 4H), 3.34 (quin, 2H), 3.27−3.19 (m, 2H), 2.86 (s, 2H), 2.76 (dd, J = 27.6, 11.2 Hz, 4H), 1.39 (d, J = 5.3 Hz, 6H), 1.23 (d, J = 7.2 Hz, 6H), 1.06 (d, J = 6.9 Hz, 6H), 0.98 (d, J = 6.0 Hz, 6H). 13C{1H} NMR (126 MHz, C6D6, ppm) δ 209.50, 148.09, 147.48, 146.90, 135.19, 131.23, 128.88, 128.78, 128.58, 128.49, 128.39, 125.86, 125.41, 124.96, 124.29, 119.26, 75.15, 51.91, 29.88, 29.19, 26.55, 25.43, 24.86, 23.31. The NMR analysis of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00394. Characterization of [(NHC)Ni(styrene)2] complexes 1−4 via 1H and 13C{1H} NMR spectra (PDF) Cartesian coordinates of all geometry optimized complexes (XYZ) Accession Codes

CCDC 1847653−1847661 contain the supplementary crystallographic data for this paper. These data can be obtained H

DOI: 10.1021/acs.organomet.8b00394 Organometallics XXXX, XXX, XXX−XXX

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free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Janis Louie: 0000-0003-3569-1967 Present Address

A.J.G.: Department of Chemistry, University of Rochester, Rochester, New York 14627, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation (CHE1213774) and the National Institute of Health (GM076125) for financial support. Calculations were performed on the Extreme Science and Engineering Discovery Environment (XSEDE) which is supported by the National Science Foundation (ACI-1053575).

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