Comparing Interactions of a Three-Coordinate Pd Cation with

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Comparing Interactions of a Three-Coordinate Pd Cation with Common Weakly Coordinating Anions Derek I. Wozniak, William A. Sabbers, Kushan C. Weerasiri, Luckym V. Dinh, Jacqueline L. Quenzer, Andrew J. Hicks, and Graham E. Dobereiner* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States

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S Supporting Information *

ABSTRACT: A series of complexes Pd(IPr)(C(O)C9H6N)X has been prepared where X is a weakly coordinating anion (WCA). [Pd(IPr)(C(O)C9H6N)]+ can be isolated as a 3coordinate, air- and moisture-stable Pd(II) cation when paired with sterically encumbered WCAs. Using NMR, IR, X-ray diffraction, and computational (DFT, NBO) techniques, the coordination ability of WCAs in both solid and solution state is compared. The IPr methine isopropyl 1H NMR chemical shift and the anion’s percent buried volume (%Vbur) are highly sensitive to the steric encumbrance of the anion, while IR and NBO analysis offer a measure of relative donor strength. On the basis of computed νCO, the coordinating abilities of anions studied are as follows BArF4− ∼ [(BCF)2(imid)]− < SbF6− < PF6− < BF4− < NTf2− < ClO4− < OTf− < Cl−.

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INTRODUCTION

and DFT, to determine the donor ability of WCAs paired with [NCr(NiPr2)2]+.21 Since coordinating ability is cation-specific, we sought a Pdbased system as a foundation to better understand the role of anion effects in Pd catalysis.22−24 We have prepared a series of complexes featuring an air- and moisture-stable T-shaped Pd (II) cation [Pd(IPr)(C(O)C9H6N)]+ (Scheme 1). The complexes were chosen for the study because they are simple

1

In 1973, Rosenthal outlined the difficulty in defining the coordinating ability of weakly coordinating anions (WCAs), acknowledging that “detailed analyses are difficult to carry out and have rarely been done.” Measurements of coordinating ability since then have yielded valuable insights, aiding understanding of ion-pairing phenomena in catalysis2,3 and informing design of new WCAs,4−7 yet the extent of coordination is dependent on many factors and remains challenging to measure, let alone predict.8 Even more complicated is distinguishing the multiple, competing roles counterion effects can play in a catalytic reaction.9,10 In lieu of a universal scale for coordinating ability, comparisons of relative ability are valuable tools for synthetic and catalytic development. Theoretical11,12 and statistical13 approaches have yielded useful scales, while empirical measurements require pairing various WCAs with a single cation of interest. The earliest studies used magnetic moment, UV−vis, or IR spectra to compare coordinating ability.1,14 In the 1980s, Beck and co-workers15 compared IR and NMR data for CpM(CO)3X (M = Mo, W) to construct a comprehensive relative donor strength series: AsF6− ∼ SbF6− ∼ PF6− < BF4− < SO3F− ∼ OTf− < ClO4− < OTeF5− ∼ ReO4− ≪ Cl−. Besides UV−vis, IR, Raman, and NMR, electrochemical methods (conductometry and potentiometry) have been used to investigate degree of cation−anion association. Gradient NMR techniques including NOESY, ROESY, HOESY, DOSY, and PGSE can probe the degree of ion pairing in solution.3,16−20 Interplay between experiment and theory can offer valuable insights; for example, Odom and co-workers used 14N NMR, in combination with two-dimensional NMR © XXXX American Chemical Society

Scheme 1. Preparation of a Series of Compounds 3a

a

Where X is a weakly coordinating anion.

Received: May 25, 2018

A

DOI: 10.1021/acs.organomet.8b00356 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics to synthesize, stable at ambient conditions, and resemble catalytically relevant Pd(II) acyl intermediates invoked in catalytic cycles.25−32 The cation scaffold can accommodate a wide range of anion coordinating abilities; if an anion does not bind in the primary coordination sphere, then the IPr ligand forms agostic interactions with Pd via an isopropyl C−H. Spectroscopic comparison of the series is straightforward via NMR (IPr isopropyl groups) and IR (acyl CO) measurements. In the present study NMR, IR, X-ray crystallography, and DFT calculations are employed to determine the strength of interaction between cation [Pd(IPr)(C(O)C9H6N)]+ and various WCAs. Percent buried volume (%Vbur) calculations, derived from X-ray and DFT structures, gauge how steric congestion affects inner-sphere coordination. The effects of solvent upon ion-pairing have also been explored. Our results generally agree with previous WCA scales and indicate that electronic properties of WCAs are an important factor in determining the degree of coordination, but steric properties ultimately dictate coordination behavior of an anion with a specific cation.

Figure 1. Thermal ellipsoid plots of 3BArF4 (left) and CH2Cl2-bound 3BArF4 (right). Ellipsoids shown at 50% probability. Anions hidden for clarity.

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RESULTS Synthesis and Characterization. Addition of N,N′bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) to dimer 1 yields [Pd(IPr)(C(O)C9H6N)Cl] 2, and subsequent addition of MX (M = Li, Na, Ag; X = WCA) to 2 in dichloromethane yields a series of complexes 3 featuring Pd fragment [Pd(IPr)(C(O)C9H6N)]+. Paired WCAs include PF6−, BF4−, SbF6−, NTf2−, ClO4−, OTf−, and BArF4−, as well as [(BCF)2(imid)]−, a derivative of a WCA first reported by LaPointe, Klosin, and co-workers33 (Chart 1). Na+ and Li+ Chart 1

Figure 2. Thermal ellipsoid plots of inner-sphere 3BF4, 3OTf, and 3ClO4. Ellipsoids shown at 50% probability.

In the solid state, 3BF4, 3OTf, and 3ClO4 are neutral complexes, while in solution they may be better described as inner-sphere ion pairs.3 Outer-sphere complexes 3NTf2, 3BArF4, and 3[(BCF)2(imid)] are T-shaped with acyl trans to the vacant coordination site. X-ray bond lengths and angles do not correlate with degree of anion association. For example, Pd−Cacyl bonds are within the range of 1.94−1.98 Å (Table 1) regardless of donor trans to acyl. Although [Pd(IPr)(C(O)C9H6N)]+ is coordinatively unsaturated, the bulky IPr precludes cis-binding of sterically encumbered anions. Inner-sphere anions coordinate via an atom attached to a tetrahedral center; octahedral or more substantial anions, e.g., NTf2, BArF4, and [(BCF)2(imid)] are found in the outer sphere. Percent buried volume (%Vbur) is a useful metric for assessing the steric properties of ligands, similar to the Tolman cone angle for phosphines.36 While frequently used for neutral ligands, such as NHCs, it can also be applied to metal-bound X-type ligands to determine relative steric demand.37 Values are easily computed from X-ray structures using the SambVca program developed by the Cavallo group.38 Table 2 lists %Vbur for neutral and anionic donors, calculated using the X-ray

salts of [BArF4]− and [(BCF)2(imid)]− efficiently abstract Cl−, while Ag+ salts are needed for introducing other anions. Isolated yields of 3PF6, 3BF4, 3SbF6, 3NTf2, 3ClO4, 3OTf, 3BArF4, and 3[(BCF)2(imid)] range from 56 to 84%. X-ray Crystallography. Slow diffusion of pentane into dichloromethane yielded X-ray quality crystals of complexes 3. Neutral donor adducts of [Pd(IPr)(C(O)C9H6N)]+ can be prepared via recrystallization in the presence of water, acetonitrile, or piperidine. In the absence of a neutral or anionic donor, recrystallization of 3BArF4 from dichloromethane and pentane affords Pd-bound CH2Cl2 (Figure 1, left). Recrystallization of 3BArF4 from toluene/pentane gave the 3-coordinate acyl cation (Figure 1, right).30−32,34 The closest distances between NHC isopropyl methyl C−H and the open coordination site of the 3-coordinate cation are within the sum of the van der Waals radii of Pd and H (2.83 Å), consistent with weak agostic interactions.30,31,35 Structures 3BF4, 3OTf, and 3ClO4 (Figure 2), as well as solvent adducts [3(dcm)][PF6], [3(MeCN)] [BArF4], [3(H2O)][ClO4], [3(H2O)][SbF6], and piperidine adduct [3(pip)][BArF4] all feature distorted square planar geometries. B


Article

Organometallics Table 1. Bond Lengths from X-ray Structures complex F

[3(MeCN)]BAr 4 [3(H2O)]ClO4 [3(pip)]BArF4 3ClO4 3BF4 3OTf [3(H2O)]SbF6 2 [3(dcm)]PF6 3NTf2 3BArF4 3[(BCF)2(imid)]

Pd−Cacyla

donor

Pd−(donor)

1.984(2) 1.951(7) 1.981(3) 1.94(3) 1.961(12) 1.958(2) 1.966(9) 1.971(3) 1.968(6) 1.969(2) 1.959(4) 1.962(2)

NCMe OH2 Npiperidine OClO3 FBF3 OSO2CF3 OH2 Cl ClCH2Cl

2.099(2) 2.206(5) 2.228(2) 2.243(7) 2.265(7) 2.2743(18) 2.276(8) 2.4210(9) 2.578(3)

a

All lengths in angstroms (Å).

Table 2. Percent Buried Volume (%Vbur) donor

input structure

%Vbur

H2O MeCN BF4− OTf− ClO4− SbF6− piperidine PF6− NTf2−

[3(H2O)]ClO4 (X-ray) [3(MeCN)]BArF4 (X-ray) 3BF4 (X-ray) 3OTf (X-ray) 3ClO4 (X-ray) IIISbF6 (DFT) [3(pip)]BArF4 (X-ray) IIIPF6 (DFT) IIINTf2 (DFT)

12.8 13.6 14.2 15.4 17.3 19.0 19.2 19.4 26.5

Figure 3. Methine 1H NMR resonances (CD2Cl2, 25 °C) of 3.

fluorine is less basic than the coordinating oxygens of ClO4− and OTf−. The effect of relative permittivity upon solution-state behavior of 2, 3BF4, 3PF6, and 3BArF4 was probed by measuring change in the methine 1H NMR chemical shift with various ratios of CD2Cl2 to toluene-d8 (Table 3). Representative 1H NMR spectra for 2 and 3BF4 are shown in Figure 4. Decreasing the relative permittivity had a more pronounced effect on 2 and 3BF4 than that on 3PF6 and 3BArF4. Since 2 and 3BF4 can form inner-sphere complexes, the larger changes in chemical shift may arise from tighter anion−cation coordination as the permittivity is decreased and charge separation becomes disfavored. However, the peak-to-peak distance of methine resonances of 2 is essentially the same in each CD2Cl2/toluene-d8 solvent mixture, suggesting the kinetics of NHC rotation do not change appreciably with relative permittivity of the solvent. Strength of chloride binding to the Pd cation was investigated through addition of various equivalents of an exogenous organic-soluble chloride source, PPh4Cl. As the amount of chloride was increased, the methine peak began to move downfield and broaden significantly; by adding 1.0 equiv of chloride to 3BArF4, complex 2 is formed (Figure 5). In an attempt to determine ΔG of binding, 0.5 equiv of PPh4Cl were added to a solution of 3BArF4 in deuterated dichloromethane, and spectra were recorded at various temperatures. It was expected that at lower temperatures, two distinct species, 2 and 3BArF4, would be observed; however, no single decoalescence point was found. Instead, at 205 K, what had previously been a very broad peak split into multiple peaks that could not be resolved. Rather than a simple equilibrium corrseponding to interconversion between these two species, multiple distinct species (for example, [3(dcm)] [BArF4], 3BArF4, [(3)2(μCl)][BArF4], etc.) are present at low temperatures, and the pathway between the two may involve several steps. IR Studies. The influence of anion on the CO stretching frequency (νCO; Table 4) was investigated in both the solid (ATR-IR thin film, under air) and solution states (DCM, NaCl solution cells, prepared in an N2 atmosphere). Frequencies

structures of [3(H2O)]ClO4, [3(MeCN)]BArF4, 3BF4, 3OTf, 3ClO4, and [3(pip)][BArF4], and DFT geometry-optimized structures for inner-sphere 3SbF6, 3PF6, and 3NTf2 complexes (vide infra). The largest %Vbur for a donor bound in the solid state, [3(pip)][BArF4], is 19.2%. NTf2 is significantly higher (26.5%) and likely sterically precluded from binding. PF6 (19.4%) and SbF6 (19.0%) have similar %Vbur values to the piperidine adduct, but their weak cation affinity apparently cannot overcome steric encumberment. %Vbur for the largest anions used (3BArF4, 3[(BCF)2(imid)]) were not determined. NMR Studies. Characterization of series 3 via 1H NMR (CD2Cl2, 25 °C) revealed the chemical shift of the IPr isopropyl methine protons is sensitive to the associated donor (Figure 3). The quinoline protons furthest downfield follow a similar trend but partially overlap with other aromatic resonances. The isopropyl methine and isopropyl methyl shifts of 2 are consistent with two inequivalent isopropyl environments, perhaps reflecting hindered NHC rotation, likely around the Pd−C bond,39−43 when chloride is bound to Pd. The single resonance observed upon halide abstraction reflects a lower rotational barrier, suggesting the behavior of the methine resonance is associated with the coordinative strength of the anion. Fluxional coordination of solvent or anion at a rate much faster than the NMR time scale is likely occurring with all complexes in this series, with the methine peak appearing as a weighted average of the anion/solvent coordinated species and the three-coordinate species. This coordination would also hinder NHC rotation; however, it is likely so fast that no inequivalence of the isopropyl groups is observed. Only 2 and [3(pip)]BArF4 exhibit slow exchange behavior at room temperature. Of the inner-sphere compounds characterized via X-ray (3BF4, 3OTf, and 3ClO4), the methine shift of 3BF4 is the most upfield, perhaps because the BF4− C


Article

Organometallics Table 3. Effect of Solvent upon 1H NMR Methine Chemical Shift of 2, 3BF4, 3PF6, and 3BArF4 2 CD2Cl2/toluene-d8

δ (ppm)

100:0 90:10 75:25 50:50

3.44 3.55 3.70 4.01

3BF4 Δδ (ppm)

δ (ppm)

0.11 0.26 0.57

2.89 3.07 3.30 3.67

3BArF4

3PF6 Δδ (ppm)

δ (ppm)

0.18 0.41 0.78

2.75 2.84 2.96 3.18

Δδ (ppm)

δ (ppm)

Δδ (ppm)

0.09 0.21 0.43

2.75 2.83 2.95 3.15

0.08 0.20 0.41

Figure 4. 1H NMR spectra of 2 (left) and 3BF4 (right) in CD2Cl2:toluene-d8 mixtures.

Table 4. Acyl CO Frequencies (νCO) of Complexes 3 2 3PF6 3BF4 3SbF6 3NTf2 3ClO4 3OTf 3BArF4 3[(BCF)2(imid)] [3(H2O)][ClO4] [3(pip)][BArF4] [3(MeCN)][BArF4] a

Figure 5. Addition of various equivalents of PPh4Cl to 3BArF4, resulting in the formation of 2.

νCO, ATR-IRa

νCO, DCM

νCO, DFT

1669 1759 1689 1694 1691 1684 1681 1776 1757 1687 1677 1689

1669 1760 1760 1760 1760 1695 1688 1760 1760

1682 1721 1718 1726 1714 1713 1711 (1777)b (1777)b

1679 1690

1709

All frequencies in cm−1. bCalculated from cationic structure III.

span a relatively narrow range (1.196−1.217 Å) relative to lengths obtained from X-ray (1.14−1.21 Å), which are likely influenced by cocrystallized solvent molecules and other crystal-packing effects not reflected in DFT calculations. DFT-optimized structures of proposed inner-sphere compounds IIINTf2, IIIPF6, and IIISbF6 and free cation III were all obtained using the cation in X-ray structure 3BArF4 as a starting point, and the resulting geometries largely mirror those of the other inner-sphere structures. Optimizations of innersphere variants of 3BArF4 and 3[(BCF)2(imid)] were not attempted. To confirm structures are minima on the potential energy surface, all optimized geometries were subjected to a frequency calculation at the same level of theory. Vibrational frequencies corresponding to the acyl CO stretching mode (νCO), corrected with a method-dependent scaling factor,44 are listed in Table 4. The computed frequencies, spanning a range of values (1682−1777 cm−1), are typically 10−30 cm−1 above the measured ATR-IR frequencies. Inner-sphere DFT structure IIIPF6 is an exception

tabulated are the strongest bands observed in the range 1650− 1800 cm−1. Formation of H2O adducts complicate ATR-IR measurements and multiple bands are visible in some cases (see Supporting Information). For most compounds in dichloromethane solution, νCO = 1760 cm−1, consistent with anions fully dissociated from the metal center. Solution-state νCO for 2, 3ClO4, 3OTf, and adducts 3(pip)BArF4 and 3(MeCN) BArF4 suggest inner-sphere coordination of anion or neutral donor. Computational Studies. Using X-ray crystal structures as a starting point, DFT geometry optimizations at the M06/631g(d,p)/SDD(+f) level of theory yielded structures II, III, IIIOTf, IIIClO4, and IIIBF4. Overall, there is good agreement between DFT and X-ray bond lengths. Pd−C bond lengths in the DFT structures come within 0.03 Å of the bond distances in the respective X-ray structures. DFT COacyl bond lengths D


Article

Organometallics Table 5. Wiberg Bond Index for Computed Structures II IIIOTf IIIClO4 IIINTf2 IIIBF4 IIIPF6 IIISbF6 III

COacyl

Pd−Cacyl

Pd−CNHC

Pd−N

Pd−Oacyl

Pd−anion

1.731 1.748 1.758 1.751 1.763 1.761 1.762 1.822

0.626 0.703 0.702 0.692 0.730 0.731 0.734 0.718

0.530 0.507 0.509 0.485 0.508 0.499 0.499 0.510

0.232 0.229 0.228 0.219 0.231 0.227 0.225 0.232

0.120 0.139 0.138 0.145 0.147 0.153 0.156 0.173

0.240 0.106 0.116 0.113 0.078 0.065 0.059

to the trend, with a νCO substantially lower than that of outersphere 3PF6 (IIIPF6 1721 cm−1 vs 3PF6 1759 cm−1). The order of computed νCO, from lowest to highest, is Cl < OTf < ClO4 < NTf2 < BF4 < PF6 < SbF6. To better understand the influence of anion coordination on acyl CO bond character, a natural bond orbital (NBO 6.0) analysis was performed on all computed structures. According to NAO-based Wiberg bond indices (Table 5),45 the COacyl bond order ranges from 1.731 to 1.822; Pd−anion bond indices range from 0.059 to 0.240. In general, as the Pd−anion bond index increases, νCO decreases, COacyl and Pd−Oacyl bond orders decrease, and NPA natural charges (Table 6) of

Table 7. Donor−Acceptor Stabilization Energies ΔE(2) (in kcal/mol) from Second-Order Perturbation Theory Analysis nOacyl → σ* (Pd−Cacyl)

nPd → π* (COacyl)

σ(Pd−CNHC) → σ*(Pd−Cacyl)

II

18.19

12.38

49.48

IIIOTf

24.32

12.52

39.56

IIIClO4

23.19

13.07

41.24

IIINTf2

25.46

13.45

43.69

IIIBF4

23.21

13.85

36.96

IIIPF6

24.10

14.31

37.39

IIISbF6

24.41

14.38

36.97

III

36.42

12.22

24.71

Table 6. NPA Natural Charges for Computed Structures II IIIOTf IIIClO4 IIINTf2 IIIBF4 IIIPF6 IIISbF6 III

Pd

Cacyl

CNHC

N

Oacyl

0.476 0.497 0.512 0.517 0.522 0.519 0.513 0.406

0.435 0.503 0.489 0.495 0.500 0.510 0.517 0.570

0.328 0.287 0.294 0.268 0.282 0.277 0.273 0.244

−0.444 −0.464 −0.454 −0.449 −0.454 −0.456 −0.458 −0.472

−0.609 −0.592 −0.583 −0.580 −0.579 −0.575 −0.573 −0.532 a

Oacyl become more negative. Pd−CNHC and Pd−N bond orders appear to be partly related to the steric demand of the bound anion. The sterically demanding NTf2− in IIINTf2 significantly distorts the Pd square planar geometry, making the Pd−CNHC and Pd−N bonds weaker than those in all other structures. Although the nature of donor lone-pair NBOs varies with anion, major donor−acceptor interactions between cation and anion can be compared from a second-order perturbation theory energy analysis (Table 7). Mirroring the relationship of Pd−anion and Pd−O bond orders, as the sum of nanion → σ* (Pd−Cacyl) interactions decreases, the strength of nOacyl → σ* (Pd−Cacyl) interactions increases. The strongest acyl CO bond (structure III), where no anion is coordinated, has the largest nOacyl → σ* (Pd−Cacyl) interaction (36.42 kcal mol−1). II has the weakest such interaction (18.19 kcal mol−1) and the lowest CO bond order. The nPd → π* (COacyl) changes only slightly with anion, while σ (Pd−CNHC) → σ* (Pd−Cacyl) generally decreases as the acyl CO bond order increases.

nanion → σ*(Pd−Cacyl)a 70.37 5.40 1.47 21.26 11.30 2.59 29.42 11.30 15.27 13.10 26.65 3.61 1.21 20.31 2.24 1.36 17.58 1.60

Only interactions >1 kcal/mol are listed.

νCO and NMR indicate outer-sphere BF4−. The effect of solvent dielectric upon the 3BF4 methine 1H NMR chemical shifts suggest significant ion pairing. Complexes of bulky outersphere anions are difficult to distinguish from one another, as they all have relatively similar NMR and IR features. While Xray structures are useful for calculating buried volumes for several anions and ligands, bond distances from solid-state structures of ion pairs do not correlate to the WCA being inner- or outer-sphere, consistent with the findings of Odom.21 No linear correlation exists between the observed NMR methine proton shifts and the solution state νCO for complexes 3 (Figure 6); there are two regimes in place, apparently corresponding to inner- versus outer-sphere anion behavior. Comparing νCO to %Vbur (Figure 7), larger buried volumes tend to lead to higher frequency νCO. From an NBO perspective, sterically hindered or electron-poor anions cannot donate electron density as effectively into the σ* (Pd−Cacyl) acceptor orbital. IR (νCO) loosely correlates with %Vbur for inner-sphere neutral or anionic donors, with exceptions when the bound species is either sterically large but strongly Lewis basic (piperidine) or sterically small but weakly Lewis basic (BF4−). In the case of BF4−, binding in the solid state is likely favorable based on lack of steric congestion, which can be attributed to the relatively small size and small buried volume of the anion

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DISCUSSION From IR (νCO), 1H NMR (methine δ), X-ray (%Vbur), and DFT data (Wiberg bond orders and NBO interactions), it is possible to qualitatively compare the coordinating ability for anions in solution versus the solid state. For example, thin-film Pd−acyl νCO and X-ray data for 3BF4 suggest solid-state innersphere BF4− coordination, while in dichloromethane solution, E


Article

Organometallics

SbF6−, PF6−) with characteristic behavior in the solid and solution states. BF4− is an exception, demonstrating weak association in solution and strong coordination in the solid state. To best understand cation−anion interactions with ions of interest, we recommend a suite of complementary experimental and computational approaches be used with a variety of sterically and electronically diverse counterions.

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EXPERIMENTAL SECTION

General Methods. Unless otherwise specified, all manipulations were performed under a dry N2 atmosphere using standard Schlenk techniques or a Vacuum Atmospheres inert atmosphere glovebox. Analytical data were obtained from the CENTC Elemental Analysis Facility at the University of Rochester, funded by NSF CHE-0650456. NMR spectra were collected on Bruker Avance III 500 and 400 MHz instruments. 1H NMR chemical shifts (δ, ppm) are referenced to residual protiosolvent resonances and 13C NMR chemical shifts are referenced to the deuterated solvent peak.46 IR spectra were collected on a Thermo Scientific Nicolet iS5 FT-IR benchtop spectrometer with either a iD5 Diamond ATR or iD1 Transmission accessory. Dichloromethane (DCM), tetrahydrofuran (THF), pentane, and toluene were purified using a commercial solvent purification system. All deuterated NMR solvents (Cambridge Isotope Laboratories) were dried over activated 4 Å molecular sieves for 48 h before use. Sodium tetrakis(3,5-bistrifuloromethyl)phenylborate (NaBArF4) was prepared using the procedure of Yakelis and Bergman.47 [Pd(μ-Cl)(C(O)C9H6N)]2 (1) was prepared as reported by Pregosin and coworkers.48 Tris(pentafluorophenyl)borane (B(C6F5)3) was purified via sublimation (100 mTorr, 90 °C) prior to use. Other chemicals were used as received from commercial suppliers. [Pd(IPr)(C(O)C9H6N)Cl] (2). A mixture of [Pd(μ-Cl)(C(O)C9H6N)]2 (1) (0.1066 g, 0.1788 mmol) and N,N′-bis(2,6diisopropylphenyl)imidazole-2-ylidene (0.1415 g, 0.3641 mmol) in 5 mL of toluene was stirred at room temperature for 24 h. The brown suspension was filtered to yield an orange solution. The solution was concentrated and layered with hexane. The resulting orange precipitate was vacuum filtered, washed with hexanes, and dried in vacuo to afford 2 (196 mg, 86% yield). 1H NMR (500 MHz, CD2Cl2): δ 1.02 (d, 3J = 6.5 Hz, 6 H), 1.11 (d, 3J = 6.5 Hz, 6 H), 1.21 (d, 3J = 6.5 Hz, 6 H), 1.40 (d, 3J = 6.5 Hz, 6 H), 3.21 (sept, 3J = 6.5 Hz, 2 H), 3.44 (sept, 3J = 6.5 Hz, 2 H), 7.21 (s, 2 H), 7.24 (m, 4 H), 7.39 (t, 3J = 7.5 Hz, 2 H), 7.52 (m, 2 H),7.80 (dd, 3J = 7.0, 4J = 1.5 Hz, 1 H), 7.91 (dd, 3J = 8.0, 4J = 1.5 Hz, 1 H), 8.31 (dd, 3J = 8.0, 4J = 1.5 Hz, 1 H), 9.77 (dd, 3J = 5.0, 4J = 1.5 Hz, 1 H). 13C NMR (500 MHz, CD2Cl2): δ 23.1, 24.1, 26.58, 28.3, 29.3, 122.9, 124.2, 124.6, 124.6, 124.7, 128.4, 129.0, 130.2, 130.2, 130.8, 138.1, 143.7, 147.2, 148.1, 150.1, 152.2, 179.7, 210.9. Anal. Calcd for C37H42ClN3OPd· 0.7C6H14: C, 66.25; H, 6.99; N, 5.63. Found: C, 66.51; H, 6.68; N, 5.26. IR (thin film, cm−1): νCO 1669; IR (CH2Cl2, cm−1): νCO 1669. [Pd(IPr)(C(O)C9H6N)][PF6] (3PF6). In an inert atmosphere glovebox, a scintillation vial was charged with AgPF6 (12.6 mg, 0.0495 mmol, 1.05 equiv) and stirred in DCM (1 mL). To this vial was added 2 (30 mg, 0.0473 mmol, 1 equiv) in DCM (1 mL) dropwise. After 5 min, the yellow suspension was filtered through Celite and concentrated under vacuum to afford a yellow solid (29.2 mg, 83% yield). Slow vapor diffusion of pentane into a concentrated CH2Cl2 solution yielded X-ray quality crystals. NMR shows the presence of a small amount of [Ag(IPr)2][X] where X = PF6 or Cl; running the reaction in THF suppresses the formation of this impurity. However, it results in the presence of 1 equiv of THF in the product. Recrystallization of the product helps to remove the Ag impurity; however, more forms upon recrystallization, along with significant yield reduction with each recrystallization. Neither the presence of the Ag impurity nor that of THF appears to affect the position of the major Pd peaks in the NMR nor IR. Anal. Calcd for PdC37H42F6N3OP: C, 55.82; H, 5.32; N, 5.28. Found: C, 56.067; H, 5.457; N, 5.384. 1H NMR (500 MHz, CD2Cl2) δ 8.55 (dd, 3J = 8.4, 4J = 1.2 Hz, 1H), 8.23 (dd, 3J = 5.0, 4J = 1.3 Hz, 1H), 8.15 (dd, 3J = 8.1, 4 J = 1.1 Hz, 1H), 7.88 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.70 (dd, 3J =

Figure 6. Methine 1H NMR shift vs νCO (solution-state).

Figure 7. Percent buried volume (%Vbur) vs νCO (solution-state). Trendline is shown only to direct the eye. *: obtained from calculated inner-sphere structures.

compared to larger fluorinated anions PF6−, SbF6−, NTf2−, and the aryl borates. BF4− binding is considerably weaker than ClO4− and OTf−, as shown by IR, NMR, and NBO nanion → σ* (Pd−Cacyl) interactions. An absolute scale for coordinating ability is not possible because ordering of binding affinities depends on the cation being paired. Since [Pd(IPr)(C(O)C9H6N)]+ has a vacant coordination site, it is more susceptible to dative anion bonding than coordinatively saturated cations used in solutionphase (ion-pairing) studies.38 In this vein, our measurements offer a complementary approach to exploring cation−anion interactions experienced by coordinatively unsaturated catalyst intermediates.27,29 We believe that valuable information can be learned from each technique employed here. In general, NMR and buried volume calculations gauge the steric nature of the binding species, while IR and DFT report the electronics of anion binding. On the basis of DFT-derived νCO, anion donor strength, from weakest to strongest, is BAr F 4 − ∼ [(BCF)2(imid)]− < SbF6− < PF6− < BF4− < NTf2− < ClO4− < OTf− < Cl−. BF4− can adopt inner or outer-sphere coordination and sits in the middle of the IR (νCO), 1H NMR (methine δ), X-ray (%V bur ), and DFT (NBO interaction) scales.

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CONCLUSION Our group has synthesized a variety of [Pd][WCA] salts with WCAs of varying coordinative preference, using IR, NMR, Xray, and DFT to compare the complexes prepared. The anions can be mostly grouped into strong donors (NTf2−, ClO4−, OTf−, Cl−) and weak donors (BArF4−, [(BCF)2(imid)]−, F


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Organometallics 8.4, 4J = 5.0 Hz, 1H), 7.60 (td, 3J = 7.8, 3J = 5.8 Hz, 3H), 7.41 (d, 3J = 7.8 Hz, 4H), 7.36 (s, 2H), 2.74 (hept, 3J = 6.8 Hz, 4H), 1.32 (d, 3J = 6.9 Hz, 12H), 1.27 (d, 3J = 6.9 Hz, 12H).13C NMR (126 MHz, CD2Cl2) δ 183.97, 174.50, 151.33, 149.34, 146.73, 145.68, 139.99, 134.54, 134.08, 133.62, 129.84, 129.46, 125.09, 124.95, 124.35, 34.53, 29.41, 28.94, 25.19, 24.95, 24.78, 24.10, 22.74, 14.22. IR (thin film, cm−1): νCO 1759; IR (CH2Cl2, cm−1): νCO 1760. [Pd(IPr)(C(O)C9H6N)][BF4] (3BF4). In an inert atmosphere glovebox, a scintillation vial was charged with AgBF4 (9.6 mg, 0.0495 mmol, 1.05 equiv) and stirred in DCM (1 mL). To this vial was added 2 (30 mg, 0.0473 mmol, 1 equiv) in DCM (1 mL) dropwise. Immediately, a white precipitate formed. After 5 min, the yellow suspension was filtered through Celite and concentrated under vacuum to afford a yellow solid (18.2 mg, 56% yield). Slow vapor diffusion of pentane into a concentrated CH2Cl2 solution yielded Xray quality crystals. NMR shows the presence of a small amount of [Ag(IPr)2][X] where X = BF4 or Cl. Running the reaction in THF suppresses the formation of this impurity; however, it results in the presence of 1 equiv of THF in the product. Recrystallization of the product helps to remove the Ag impurity; however, more forms upon recrystallization, along with significant yield reduction with each recrystallization. Neither the presence of the Ag impurity nor that of THF appears to affect the position of the major Pd peaks in the NMR nor IR. Anal. Calcd for PdC37H42F3N3OB + 0.25 CH2Cl2: C, 58.93; H, 5.64; N, 5.53. Found: C, 58.498; H, 5.817; N, 5.482. 1H NMR (500 MHz, CD2Cl2) δ 8.65 (dd, 3J = 5.0, 4J = 1.3 Hz, 1H), 8.50 (dd, 3 J = 8.4, 4J = 1.3 Hz, 1H), 8.10 (dd, 3J = 8.1, 4J = 1.2 Hz, 1H), 7.86 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.67 (dd, 3J = 8.4, 3J = 5.0 Hz, 1H), 7.61−7.50 (m, 3H), 7.38 (d, 3J = 7.8 Hz, 3H), 7.32 (s, 1H), 2.89 (hept, 3J = 6.8 Hz, 3H), 1.31 (d, 3J = 6.8 Hz, 9H), 1.22 (d, 3J = 6.8 Hz, 9H). 13C NMR (126 MHz, CD2Cl2) δ 176.26, 152.12, 149.18, 146.75, 139.32, 134.67, 133.28, 130.95, 128.95, 128.34, 124.86, 124.04, 29.14, 25.55, 24.27. IR (thin film, cm−1): νCO 1689; IR (CH2Cl2, cm−1): νCO 1760. [Pd(IPr)(C(O)C9H6N)(OH2)][SbF6] ([3(H2O)]SbF6). In an inert atmosphere glovebox, a 4 mL vial was charged with 50 mg (0.0786 mmol) of 2 and 2 mL of DCM and stirred. To this was added 27 mg (0.0786 mmol) of AgSbF6 in 0.5 mL of toluene. The solution immediately turned bright yellow with concurrent appearance of a white precipitate. The suspension was stirred for 10 min and then filtered through Celite to produce a yellow solution, which was then dried in vacuo. The product was recrystallized at ambient temperature via layer diffusion of pentane into a concentrated DCM solution. On the basis of X-ray data, it was determined that adventitious water had coordinated to the Pd cation, likely a trace impurity in the extremely hygroscopic AgSbF6 salt used. NMR shows the presence of a small amount of [Ag(IPr)2][X] where X = SbF6 or Cl. Running the reaction in THF suppresses the formation of this impurity; however, it results in the presence 1 equiv of THF in the product. Recrystallization of the product helps to remove the Ag impurity; however, more forms upon recrystallization, along with significant yield reduction with each recrystallization. Neither the presence of the Ag impurity nor that of THF appears to affect the position of the major Pd peaks in the NMR nor IR. Yield 72%. Anal. Calcd for PdC37N3OH42SbF6·1.1 H2O: C, 49.01%, H, 4.91%, N, 4.63%. Found: C, 48.697%, H, 5.085%, N, 4.547%. 1H NMR (500 MHz, CD2Cl2) δ 8.55 (dd, 3J = 8.4, 4J = 1.1 Hz, 1H), 8.26 (dd, 3J = 5.0, 4J = 1.2 Hz, 1H), 8.15 (dd, 3J = 8.1, 4J = 1.1 Hz, 1H), 7.89 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.69 (dd, 3J = 8.4, 3J = 5.0 Hz, 1H), 7.65−7.55 (m, 3H), 7.43−7.34 (m, 6H), 2.75 (hept, 3J = 6.7 Hz, 4H), 1.30 (dd, 3J = 25.2, 3J = 6.9 Hz, 24H). 13C NMR (126 MHz, CD2Cl2) δ 151.26, 149.36, 146.63, 139.94, 131.41, 125.14, 124.93, 124.25, 29.39, 25.16. IR (thin film, cm−1): νCO 1694; IR (CH2Cl2, cm−1): νCO 1760. [Pd(IPr)(C(O)C9H6N)][NTf2] (3NTf2). A scintillation vial was charged with AgNTf (19.2 mg, 0.0473 mmol, 1.05 equiv) and stirred in CH2Cl2 (1 mL). To this vial was dropwise added 2 (30 mg, 0.0495 mmol, 1 equiv) in CH2Cl2 (1 mL). After 1.5 h, the yellow suspension was filtered through Celite and concentrated in vacuum to afford a yellow solid (25.5 mg, 61% yield). Slow vapor diffusion of pentane into a concentrated CH2Cl2 solution yielded X-ray quality crystals.

Anal. Calcd for PdC39H42F6N4O5S2: C, 50.30; H, 4.55; N, 6.02. Found: C, 49.395; H, 4.712; N, 5.66. 1H NMR (500 MHz, CD2Cl2) δ 8.55 (dd, 3J = 8.4, 4J = 1.1 Hz, 1H), 8.27 (dd, 3J = 5.0, 4J = 1.3 Hz, 1H), 8.15 (dd, 3J = 8.1, 4J = 1.1 Hz, 1H), 7.90 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.68 (dd, 3J = 8.4, 3J = 5.0 Hz, 1H), 7.65−7.60 (m, 1H), 7.58 (t, J = 7.8 Hz, 2H), 7.40 (d, 3J = 7.8 Hz, 4H), 7.36 (s, 2H), 2.75 (hept, 3J = 6.8 Hz, 4H), 1.32 (d, 3J = 6.8 Hz, 12H), 1.27 (d, 3J = 6.9 Hz, 12H). 13C NMR (126 MHz, CD2Cl2) δ 151.25, 146.62, 139.95, 133.98, 133.72, 131.39, 129.86, 129.51, 125.13, 124.95, 124.20, 29.39, 25.16, 25.03, 22.75, 14.23. IR (thin film, cm−1): νCO 1691; IR (CH2Cl2, cm−1): νCO 1760. [Pd(IPr)(C(O)C9H6N)][ClO4] (3ClO4). In an inert atmosphere glovebox, a 16 mL vial was charged with 100 mg (0.157 mmol) of 2 and 15 mL of CH2Cl2 and stirred. To this was added 33 mg (0.157 mmol) of AgClO4 in 1 mL of toluene. Immediate precipitation of a yellow solid occurred. The suspension was stirred for 5 min and then filtered through Celite to produce a yellow suspension, which was then dried in vacuo. The product was recrystallized at −35 °C via slow vapor diffusion of pentane into a concentrated CH2Cl2 solution, producing yellow/orange crystals suitable for X-ray diffraction. Yield 84%. Anal. Calcd for PdC37N3O5H42Cl·0.5 CH2Cl2: C, 56.79%, H, 5.46%, N, 5.30%. Found: C, 56.634%, H, 5.392%, N, 5.108%. 1H NMR (500 MHz, CD2Cl2) δ 9.23 (d, 3J = 7.6 Hz, 1H), 8.38 (d, 3J = 8.3 Hz, 1H), 7.98 (d, 3J = 8.1 Hz, 1H), 7.80−7.76 (m, 1H), 7.59 (dd, 3 J = 8.0, 4J = 5.1 Hz, 1H), 7.53 (t, 3J = 7.7 Hz, 1H), 7.48−7.42 (m, 2H), 7.34−7.27 (m, 6H), 3.12 (p, 3J = 6.7 Hz, 4H), 1.31 (d, 3J = 6.7 Hz, 12H), 1.15 (d, 3J = 6.8 Hz, 12H). 13C NMR (126 MHz, CD2Cl2) δ 153.00, 149.45, 146.80, 138.66, 135.60, 132.10, 130.50, 128.95, 128.54, 126.60, 124.79, 124.67, 123.61, 28.95, 26.20, 23.49. IR (thin film, cm−1): νCO 1684; IR (CH2Cl2, cm−1): νCO 1695. [Pd(IPr)(C(O)C9H6N)][OTf] (3OTf). In an inert atmosphere glovebox, a scintillation vial was charged with AgOTf (12.9 mg, 0.0495 mmol, 1.05 equiv) and stirred in CH2Cl2 (1 mL). To this vial was dropwise added 2 (30 mg, 0.0473 mmol, 1 equiv) in CH2Cl2 (1 mL). After 1.5 h, the solution yellow suspension was filtered through Celite and concentrated in vacuum to afford a yellow solid (24.4 mg, 65% yield). Slow vapor diffusion of pentane into a concentrated CH2Cl2 solution yielded X-ray quality crystals. Anal. Calcd for PdC38H42F3N3O4S·0.25 C5H12: C, 56.51; H, 5.44; N, 5.04. Found: C, 56.159; H, 5.768; N, 4.953. 1H NMR (500 MHz, CD2Cl2) δ 9.15 (s, 1H), 8.33 (d, 3J = 6.4 Hz, 1H), 7.95 (d, 3J = 7.1 Hz, 1H), 7.79 (d, 3J = 7.1 Hz, 1H), 7.53 (t, 3J = 7.6 Hz, 2H), 7.44 (t, 3J = 7.8 Hz, 2H), 7.35−7.26 (m, 6H), 3.15−3.00 (m, 4H), 1.29 (d, 3J = 6.6 Hz, 12H), 1.16 (d, 3J = 6.8 Hz, 12H). 13C NMR (126 MHz, CD2Cl2) δ 178.17, 152.83, 149.32, 146.64, 140.08, 138.54, 135.48, 131.99, 130.53, 128.88, 128.55, 126.43, 124.78, 123.47, 122.12, 119.58, 28.90, 26.21, 23.51. IR (thin film, cm−1): νCO 1681; IR (CH2Cl2, cm−1): νCO 1688. Preparation of [Pd(IPr)(C(O)C9H6N)][BArF4] (3BArF4). A vial was charged with 2 (0.2101 g, 0.3061 mmol) and NaBArF4 (0.2802 g, 0.3162 mmol), and 10 mL of toluene at room temperature was added. The reaction mixture was stirred for 24 h. The mixture was filtered through Celite and the filtrate was concentrated and layered with hexane. A pale yellow precipitate formed, which was vacuum filtered, washed with hexane, and dried in vacuo to yield [Pd(IPr)(C(O)C9H6N)][BArF4] in 80%. 1H NMR (500 MHz,CD2Cl2): δ 1.24 (d, 3J = 7.0 Hz, 12 H), 1.30 (d, 3J = 7.0 Hz, 12 H), 2.82 (sept, 3J = 7.0 Hz, 4 H), 7.36 (m, 6 H), 7.56 (m, 8 H), 7.72 (br s, 8 H), 7.89 (dd, 3J = 7.0, 4 J = 1.0 Hz, 1 H), 8.07 (dd, 3J = 8.0, 4J = 1.0 Hz, 1 H), 8. (dd, 3J = 5.0, 4 J = 1.0 Hz, 1 H), 8.46 (dd, 3J = 8.5, 4J = 1.5 Hz, 1 H). 13C NMR (500 MHz, CD2Cl2): δ 162.26, 161.86, 161.47, 161.07, 149.89, 148.92, 146.09, 139.25, 134.71, 133.76, 132.74, 130.72, 129.01, 128.94, 128.92, 128.90, 128.88, 128.82, 128.70, 128.67, 128.65, 128.63, 128.44, 127.75, 125.58, 124.57, 124.44, 123.41, 117.38, 28.76, 25.03, 23.90. Recrystallization from DCM yielded [Pd(IPr)(C(O)C9H6N)(dcm)][BArF4]. Anal. Calcd for C69H54BF24N3OPd·0.3 CH2Cl2: C, 54.05; H, 3.57; N, 2.73. Found: C, 54.02; H, 3.73; N, 2.73. IR (thin film, cm−1): νCO 1776; IR (CH2Cl2, cm−1): νCO 1760. Lithium 2-Phenylimidazolide. In an inert atmosphere glovebox, a 20 mL vial was charged with 200 mg (1.388 mmol) of 2phenylimidazole and 8 mL of toluene and cooled to −35 °C. The G


Article

Organometallics suspension was then stirred, and 870 μL (1.392 mmol) of nbutyllithium (1.6 M in hexanes) was added dropwise via syringe. The suspension was stirred for 72 h, dried in vacuo, and used directly without purification. 1H NMR (500 MHz, DMSO-d6) δ 7.89 (dd, 3J = 8.3, 4J = 1.3 Hz, 2H), 7.17−7.10 (m, 2H), 6.89 (t, 3J = 7.3 Hz, 1H), 6.71 (s, 2H). Li[(BCF)2(imid)]. In an inert atmosphere glovebox, a 20 mL vial was charged with 1150 mg (2.246 mmol) of tris(pentafluorophenyl)borane and 10 mL of toluene and cooled to −35 °C. This solution was stirred, and 169 mg (1.125 mmol) of lithium 2-phenylimidazolide was added and stirred for 95 h. The vial was removed from the glovebox, and 5 mL of pentane was added to precipitate the desired product as a white solid. The solid was filtered, washed with pentane, and dried in vacuo. Purification via slow diffusion of pentane into DCM/THF yielded X-ray quality crystals of the product as the Li(THF)4 salt. Yield 45%. Anal. Calcd for LiC45N2F30B2H7·3 C4H8O: C, 49.24%, H, 2.25%, N, 2.01%. Found: C, 49.410%, H, 2.469%, N, 1.945%. 1H NMR (500 MHz, CD2Cl2) δ 7.17 (d, 3J = 3.0 Hz, 2H), 7.04 (t, 3J = 8.0 Hz, 1H), 6.71 (t, 3J = 8.0 Hz, 2H), 6.35 (d, 3J = 7.5 Hz, 2H. 13C NMR (126 MHz, CD2Cl2) δ 149.56, 147.56, 140.61, 138.77, 137.80, 135.71, 129.17, 128.68, 127.41, 124.66. 19F NMR (471 MHz, CD2Cl2) δ −121.51, −129.13, −155.32, −157.21, −160.95, −163.44. 11B NMR (161 MHz, CD2Cl2) δ −8.56. [Pd(IPr)(C(O)C9H6N)][(BCF)2(imid)] (3[(BCF)2(imid)]). A 16 mL vial was charged with 32 mg (0.050 mmol) of 2, 60 mg (0.051 mmol) of Li[(BCF)2(imid)], and 5 mL of DCM and stirred under air for 5 h. The solution was filtered through Celite and dried in vacuo. Purification via slow vapor diffusion of pentane into a saturated DCM solution at −15 °C yielded X-ray quality crystals. Yield 78%. Anal. Calcd for PdC82N5OF30B2H49: C, 54.16%, H, 2.72%, N, 3.85%. Found: C, 54.240%, H, 2.860%, N, 3.570%. 1H NMR (500 MHz, CD2Cl2) δ 8.51 (d, 3J = 8.3 Hz, 1H), 8.24 (d, 3J = 4.4 Hz, 1H), 8.12 (d, 3J = 8.4 Hz, 1H), 7.91 (dd, 3J = 7.3, 4J = 1.1 Hz, 1H), 7.65−7.59 (m, 2H), 7.56 (t, 3J = 7.8 Hz, 2H), 7.40 (d, 3J = 7.8 Hz, 4H), 7.36 (s, 2H), 7.17 (d, 3J = 2.9 Hz, 2H), 7.03 (t, 3J = 7.5 Hz, 1H), 6.70 (t, 3J = 8.0 Hz, 2H), 6.36 (d, 3J = 7.4 Hz, 2H), 2.75 (hept, 3J = 7.0 Hz, 4H), 1.32 (d, 3J = 6.8 Hz, 12H), 1.27 (d, 3J = 6.9 Hz, 12H).13C NMR (126 MHz, CD2Cl2) δ 175.69, 150.88, 149.54, 146.64, 139.45, 134.21, 133.29, 131.23, 129.45, 129.16, 128.65, 127.39, 125.06, 124.99, 124.65, 123.62, 34.53, 29.24, 25.38, 24.68, 22.74, 14.22. IR (thin film, cm−1): νCO 1757; IR (CH2Cl2, cm−1): νCO 1760. [Pd(IPr)(C(O)C9H6N)(OH2)][ClO4] ([3(H2O)]ClO4). An 8 mL vial was charged with 100 mg (0.157 mmol) of 2 and 1 mL of toluene and stirred under air. To this was added 39 mg (0.188 mmol) of AgClO4 in 1 mL of toluene. Immediate precipitation of a yellow solid occurred. The suspension was stirred for 5 min and then filtered through Celite. The product was then flushed through the Celite with 10 mL of acetone to produce a homogeneous yellow solution, which was then dried in vacuo. The product was dissolved in minimal DCM, layered with pentane, and stored at −15 °C, producing yellow/orange crystals suitable for X-ray diffraction. Due to adventitious water, the product crystallized as the water adduct of the Pd cation. Yield 63%. Anal. Calcd for PdC37N3O6H44Cl·1.25 CH2Cl2: C, 52.52%, H, 5.36%, N, 4.80%. Found: C, 52.549%, H, 5.227%, N, 4.668%. 1H NMR (500 MHz, CD2Cl2) δ 9.11 (dd, 3J = 5.0, 4J = 1.3 Hz, 1H), 8.41 (dd, 3J = 8.3, 4J = 1.3 Hz, 1H), 8.00 (dd, 3J = 8.1, 4J = 1.2 Hz, 1H), 7.79 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.63 (dd, 3J = 8.3, 3J = 5.0 Hz, 1H), 7.54 (dd, 3 J = 8.0, 3J = 7.3 Hz, 1H), 7.48−7.43 (m, 2H), 7.35−7.26 (m, 6H), 3.06 (p, 3J = 6.8 Hz, 4H), 1.85 (s, 6H), 1.31 (d, 3J = 6.7 Hz, 12H), 1.17 (d, 3J = 6.8 Hz, 12H). 13C NMR (126 MHz, CD2Cl2) δ 178.23, 152.68, 149.66, 146.89, 139.02, 135.55, 132.28, 130.88, 128.85, 126.68, 124.98, 124.90, 123.99, 121.92, 29.20, 26.52, 23.61. IR (thin film, cm−1): νCO 1687; IR (CH2Cl2, cm−1): νCO 1670. [Pd(IPr)(C(O)C9H6N)(piperidine)][BArF4] ([3(pip)]BArF4). The compound was prepared via recrystallization of 3BArF4 in the presence of piperidine. Anal. Calcd for C74H65BF24N4OPd: C, 55.57; H, 4.10; N, 3.50. Found: C, 55.29; H, 3.98; N, 3.14. IR (thin film, cm−1): νCO 1677; IR (CH2Cl2, cm−1): νCO 1679. [Pd(IPr)(C(O)C9H6N)(MeCN)][BArF4] ([3(MeCN)]BArF4). Recrystallization of 3BArF4 in the presence of acetontirile yielded

[Pd(IPr)(C(O)C9H6N)(MeCN)][BArF4]. 1H NMR (500 MHz, CD2Cl2) δ 8.58 (dd, 3J = 5.0, 4J = 1.2 Hz, 1H), 8.48 (dd, 3J = 8.3, 4 J = 1.3 Hz, 1H), 8.04 (dd, 3J = 8.1, 4J = 1.2 Hz, 1H), 7.88 (dd, 3J = 7.3, 4J = 1.2 Hz, 1H), 7.75−7.69 (m, 8H), 7.62 (dd, 3J = 8.0, 3J = 7.3 Hz, 1H), 7.58 (d, 3J = 5.0 Hz, 1H), 7.56 (d, 3J = 7.1 Hz, 4H), 7.48− 7.43 (m, 2H), 7.35−7.28 (m, 6H), 3.01 (p, 3J = 14.3, 6.8 Hz, 4H), 2.19 (s, 3H), 1.26 (d, 3J = 6.7 Hz, 12H), 1.18 (d, 3J = 6.8 Hz, 12H). 13 C NMR (126 MHz, CD2Cl2) δ 158.58, 146.52, 139.93, 135.20, 132.40, 130.86, 125.10, 124.84, 123.16, 117.86, 29.00, 26.14, 23.61, 3.79. Anal. Calcd for C71H57BF24N4OPd: C, 54.82; H, 3.69; N, 3.60. Found: C, 54.79; H,3.56; N, 3.53. IR (thin film, cm−1): νCO 1689; IR (CH2Cl2, cm−1): νCO 1690. Solvent Effect Studies. First, 600 μL of 24 μM solutions of 2, 3BF4, 3PF6, and 3BArF4 were each prepared in 100% CD2Cl2, 90:10 CD2Cl2/C7D8, 75:25 CD2Cl2/C7D8, and 50:50 CD2Cl2/C7D8, and 1 H NMR spectra were obtained for each. Computational Details. Density functional theory (DFT) calculations were performed using Gaussian 16, revision A.03 (keywords in parentheses below).49 Gas-phase optimized geometries were obtained using the M06 functional (M06)50 and 6-31g(d,p) basis set51 (6-31g(d,p)) for all nonmetal atoms except Sb, for which a Stuttgart−Dresden effective core potential52 (SDD) and SDB-ccpVTZ basis set53 were used; for Pd, a Stuttgart−Dresden effective core potential and basis set52 (SDD) was complimented with an fpolarization function.54 Tight convergence criteria (opt = tight) and an ultrafine grid (int = ultrafine) were specified. A frequency calculation on optimized structures revealed no imaginary frequencies, indicating the structures were minima on the potential energy surface. Optimized structures were then reevaluated using the M06 functional (M06), the def2-TZVP basis set55 (def2tzvp) with density fitting basis sets (auto) on all nonmetal atoms, and SDD+f on Pd. The resulting single-point calculations were used as input for a natural bond orbital (NBO) analysis using NBO version 6.0.49,56

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00356. Cartesian coordinates for all computed structures (XYZ) NMR spectra, IR spectra and crystal structure reports for all X-ray structures (PDF) Accession Codes

CCDC 1443895−1443896, 1443898, 1840688−1840697, and 1841477 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [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

Graham E. Dobereiner: 0000-0001-6885-2021 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (CHE-1565721). This research includes calculations carried out on Temple University's HPC resources and thus was supported in part by the National Science Foundation H


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Organometallics

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through major research instrumentation grant number 1625061 and by the US Army Research Laboratory under contract number W911NF-16-2-0189. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Support of the NMR facility at Temple University by a CURE grant from the Pennsylvania Department of Health is gratefully acknowledged. We thank Boulder Scientific for a gift of B(C6F5)3, Dr. Charles W. DeBrosse for assistance with NMR experiments, and Prof. Michael J. Zdilla and Dr. Michael Gau for assistance with X-ray crystallography.

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REFERENCES

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Article

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