Weakly Coordinating yet Ion Paired: Anion Effects on an Internal

Article pubs.acs.org/Organometallics

Weakly Coordinating yet Ion Paired: Anion Effects on an Internal Rearrangement Kelly E. Aldrich, Brennan S. Billow, Daniel Holmes, Ross D. Bemowski, and Aaron L. Odom* Department of Chemistry, Michigan State University, 578 South Shaw Lane, East Lansing, Michigan 48824, United States S Supporting Information *

ABSTRACT: “Weakly coordinating anions” such as tetraarylborates are ubiquitous in applications of inorganic and organometallic chemistry, with great industrial importance. In this work, we probe the ion-pairing ability of these weakly coordinating anions using the highly sensitive chromium(VI) nitrido bis(diisopropylamido) system NCr(N-i-Pr2)2X, with one variable coordination site (X). This system is being used in the quantification of ligand donor ability to high-valent metal centers and has simply been called the ligand donor parameter (LDP). The donor ability of the variable ligand can be measured by solution-state rotational barrier studies via NMR spectroscopy. If the variable ligand is neutral, the chromium complex is cationic, {NCr(N-i-Pr2)2L}+, with its pendant anion. Despite the weakly coordinating nature of the counteranions employed, a significant amount of ion pairing has been noted in solution, the result of which is substantial enough to perturb the sensitive LDP measurement. These effects have been noted for many commonly used counteranions, including hexafluoroantimonate(V), hexafluorophosphate(V), tetraphenylborate, and tetrakis(bis(3,5-trifluoromethyl)phenyl)borate (BArF24). Using diffusion ordered (DOSY) and rotating-frame Overhauser effect (ROESY) NMR spectroscopy and LDP values, we have shown, predictably, that the extent of ion pairing is solvent dependent and appears to be minimized by increasing the dielectric constant of the NMR solvent utilized. Additionally, we have gained insight into differences in the nature of ion pairing dependent upon the identity of the weakly coordinating anion employed. It was found that the tetraarylborate anions appear to be fully ion paired in CDCl3 but affect amido rotation less in comparison to other anions. We postulate that the smaller effect on the internal rearrangement by these fluorinated tetraarylborate anions is due to a lack of specificity in the interaction with the cation rather than a lack of ion pairing, which may be a general feature of these anions.

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INTRODUCTION The importance of ion-pairing effects on solution-state properties of ionic compounds has been noted many times across varied disciplines of chemistry.1 The relative association of ion pairs in solution plays a significant role in such diverse processes as supramolecular assembly, charge transfer in solution, and substitution reactions vital to both catalytic and noncatalytic organometallic substitution reactions.2 As a result of the great significance of ion pairing on so many different areas of chemistry, efforts to more precisely identify and characterize the role of ion pairing have increased in number dramatically over the last few decades. Traditional techniques to measure ion pairing include conductance, potentiometry, and dielectric relaxation methods and other spectroscopic techniques.1,3−5 Even X-ray crystallographic data have been employed to illustrate the likelihood of coordination between a transition-metal cation and an associated anion.6 More recently, chemists have begun to explore NMR spectroscopy as a means of studying ion pairing in solution.7−9 Such studies can take many forms. In the case of contact ion pairs, spectral shifts in peak position or emergence of new features can indicate pairing. However, when ion pairing demonstrates less dramatic effects on nuclear magnetic environments, 2D correlation studies such as nuclear Over© 2017 American Chemical Society

hauser effect spectroscopy (NOESY or ROESY) and diffusion ordered spectroscopy (DOSY) have provided insight into ionpairing behaviors.10−12 In this report, the effects of ion pairing on chromium(VI) cationic complexes and the ligand donor parameter (LDP) platform are studied. The LDP system is founded upon chromium(VI) complexes of the general formula NCr(NiPr2)2X, where X is a variable ligand of interest (Figure 1).13 The orientation of the diisopropylamido ligands in the compound’s ground state allows donation of the lone pair electron density into the acceptor orbitals in the xy-plane of the chromium atom. These orbitals are concurrently receiving

Figure 1. Chromium(VI) nitrido bis(diisopropylamido) scaffold used to measure donor ability to a high-valent metal in the ligand donor parameter (LDP) system. Received: November 4, 2016 Published: January 25, 2017 1227

DOI: 10.1021/acs.organomet.6b00839 Organometallics 2017, 36, 1227−1237

Article

Organometallics electron density from X, which creates a competition between the diisopropylamides and X for these acceptor orbitals. Consequently, if X donates strongly, the diisopropylamido lone pairs are less donating, and the Cr−N(amido) bond is closer to a single bond. If X donates poorly, the diisopropylamido lone pairs are more fully donated to chromium, and the Cr−N(amido) bond is closer to a double bond. The extent of lone pair donation from the amide to chromium is directly observable as variance in the barrier to rotation around the chromium−amide bond. The rate constant of this rotation can be measured experimentally with 1H NMR spin saturation transfer spectroscopy. Using the Eyring equation, the rate constant is used to calculate the free energy barrier to rotation. With the assumption that the entropy is a constant (ΔS⧧ = −9 eu)14 for all X, the estimated enthalpic barrier to rotation is dubbed the ligand donor parameter (LDP) and is roughly ΔH⧧. The new parameter is specifically designed for use with high-valent systems where there are vacant σ- and π-acceptor orbitals on the metal. LDP was first introduced in 2012, with X = monodentate anionic ligands. Since that time, additional ligands have been examined.14 In this paper, neutral donor ligands (L) are used in place of X, which gives cationic complexes. Because the solution-state NMR experiments used to measure the rotation barrier are highly stereoelectronically sensitive, any interference from the anion via association in solution alters the LDP value obtained experimentally. In an effort to further understand how ion pairing can affect LDP within neutral ligand complexes, a series of studies were conducted by which effects of changing the counterion and solvent polarity were examined. Two neutral ligand complexes were utilized in these studies, where L is dimethylphenylphosphine (PPhMe2) or trimethylphosphine (PMe3). The extent of ion pairing is drastically changed by solvent choice and counterion identity; in turn, these effects appear to render large changes to experimentally measured LDP values.

Table 1. Synthetic Approach for Generation of [Cr(N)(N-iPr2)2L]X Salts

PR3

M−X

solvent

isolated yield (%)

PMe2Ph (1)

Ag{SbF6} Tl{PF6} Tl{BArF24} K{BArF20} Ag{Al(OtBuF9)4} Ag{BPh4} Ag{SbF6} Tl{BArF24} Ag{BPh4}

MeCN MeCN CH2Cl2 CH2Cl2 MeCN MeCN/CH2Cl2 MeCN CH2Cl2 MeCN/CH2Cl2

56.4 64.8 38.4 27.8 52.5 12.7 52.4 60.4 20.5

PMe3 (2)

reaction can be performed as a one-pot procedure, adding the phosphine to the initial solution of NCr(N-i-Pr2)2I. The solvent can also be changed to a less coordinating solvent on the basis of the solubility of the weakly coordinating anion starting material. For example, CH2Cl2 was found to be a suitable solvent for reactions utilizing the BArFx− anions (Chart 1). A strongly coordinating solvent, such as MeCN, results in competition for the vacant coordination site on chromium, which can greatly reduce the yield of the phosphine complex. Chart 1. Structures of Anions Used in This Study Along with Their Abbreviations

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RESULTS AND DISCUSSION Synthesis of Chromium(VI) Nitrido Complexes. Cationic chromium(VI) species are scarce in the literature. To our knowledge, only a handful of chromium(VI) cations have been isolated and structurally characterized to date.15,16 As examples, Wilkinson and co-workers reported the dicationic bis(imide) [Cr(N-t-Bu)2(bpy)2]2+ and Che and co-workers reported the monocation [Cr(N-t-Bu)2(tacn)Cl]+. In our case, we stabilize the potentially very oxidizing Cr(VI) cation with a strongly donating nitrido group, whereas the two systems above use strongly donating imides. [NCr(NiPr2)2(PMe2Ph)]+ (1) and [NCr(NiPr2)2(PMe3)]+ (2) derivatives were synthesized with a variety of different counterions fitting different experimental needs. The syntheses of both compounds, with a variety of different anions, are similar. The general synthetic procedure is shown in Table 1. Starting from the neutral species NCr(N-i-Pr2)2I, a precipitation reaction can be performed using thallium, silver, or potassium salts of weakly coordinating anions, resulting in the formation of MI (M = Tl, Ag, K) and cationic 1 or 2. When a AgX salt is used, the reaction must be performed in two steps in acetonitrile, to prevent coordination of the phosphine to silver. This generates {NCr(NiPr2)2(NCCH3)}+ in situ, which upon addition of phosphine quickly converts to the phosphine complex. When a Tl or K salt is utilized to achieve the precipitation of the respective iodide salt, the

For this study, we included several common weakly coordinating anions used in inorganic and organometallic chemistry, most of which are commercially available. The relatively inexpensive and noncoordinating PF6− and SbF6− were used most often. Three different borate salts were also employed: BPh4−, B[3,5-(CF3)2C6H3]4− (BArF24−), and B(C6F5)4− (BArF20−). Finally, the readily prepared perfluoroaluminate Al(OtBuF9)4− (Al(ORF)4−, where RF = C(CF3)3) was also investigated (Chart 1).17 X-ray Crystallography and Structural Characterizations. Structurally, 1 and 2 are very similar. The Cr−N, Cr−P, and Cr−N bond lengths in both complexes are not significantly different. One slight difference between the structures is in the N1−Cr1−P1−C14 dihedral angle. For the PMe2Ph derivative, 1228


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chemical shifts of NCr(NiPr2)(NPh) (CrN123), which bears nominally single, double, and triple bonds between nitrogen and Cr(VI).20 In this complex, the N−Cr amido, imido, and nitrido chemical shifts are found at 214.33, 559.6, and 963.25 ppm, respectively. The 14N NMR chemical shifts for the diisopropylamide nitrogens in complexes 1 and 2 are observed at 445.9 and 452.6 ppm. This is quite different from the amido resonances in CrN123 (214.3 ppm). The drastic downfield shift correlates well with expected Cr(VI)−imide 14N chemical shifts, which typically fall near 500 ppm.18,19 Indeed, this shift in 1 and 2 is much closer to the formal imido shift in CrN123 (559.3 ppm). Subsequently, this shift in the amide 14N resonances of 1 and 2 suggests a very large degree of double-bond character between the diisopropylamido ligands and chromium, indicative of poor donation by the phosphine ligand. The chemical shifts of the nitride 14N signals for the complexes of 1 and 2 appear in the range of 980.6−1008.5 and 1005.9−1012.4 ppm, respectively, shifting slightly depending on which anion is used. These shifts are within the same range as the nitride resonance in CrN123 (963 ppm). In order to further understand the nature of the Cr−N multiple bonds in these neutral ligand complexes, the calculated bond orders were investigated. To simplify the system, [NCr(NH2)2(PPhMe2)]+ and [NCr(NH2)2(PMe3)]+ were chosen as models for 1 and 2, respectively. Natural resonance theory analyses of the bond orders was carried out by first optimizing the geometry of the models with DFT using the 6-311G(d,p) basis set at the B3PW91 level of theory.21 The optimized structure was then analyzed by NBO6 to give a local NRT analysis, which included the Cr, P, N, and first atom of the substituents on phosphorus.22 Mayer bond orders were also calculated by Mayer’s program BORDER. The results for both sets of calculations are given in Table 2.23,24

the dihedral angle is 171.6° (Figure 2), while for the PMe3 complex this angle is 178.6° (Figure 3). This may be due to the

Figure 2. Crystal structure of {1}PF6 with thermal ellipsoids drawn at the 50% probability level. H atoms in calculated positions are omitted. The inset shows the N1−Cr1−P1−C14 dihedral angle.

Figure 3. Crystal structure of {2}BArF24 with thermal ellipsoids drawn at the 50% probability level. H atoms in calculated positions are omitted. The inset shows the N1−Cr1−P1−C14 dihedral angle.

Table 2. Calculated Bond Orders for the Model Complexes 1

additional steric bulk of the phenyl substituent, requiring a slightly twisted geometry, whereas the smaller PMe3 ligand is almost perfectly staggered with respect to the CrN3 fragment. This would allow the phenyl group more room as it shifts slightly away from the amide isopropyl group toward the open space available near the nitrido group, the least sterically encumbering ligand on chromium. Multiple iterations of crystal structures of 1 with different weakly coordinating anions resulted in changes to the space group and crystal packing. However, the bond lengths and angles within the chromium cation remained largely uniform on comparison among structures of the same cation. Among the solid-state structures with various anions there are no overwhelming trends suggesting the mechanism of ion pairing, i.e., there are no repetitive instances of close contacts between the ions in the solid state. While this finding is inconclusive, it also leaves open the possibility of different pairing behaviors existing for different counterion combinations. By 14N NMR spectroscopy, the NCr(N-i-Pr2)2X compounds generally exhibit two distinct peaks in the 14N spectrum.18,19 A broad signal downfield, in the range of 900−1100 ppm, correlates to the nitrido nitrogen, and a sharper signal typically in the 100−300 ppm range correlates to the diisopropylamido nitrogens. Our group previously reported the 14N NMR

2 CrN123

a

bond type

NRT bond ordera

Mayer bond order

CrN Cr−NiPr2 CrN Cr−NiPr2 CrN CrNPh Cr−NiPr2

2.69 1.15 2.72 1.13 2.63 1.61 0.67

2.84 1.31 2.87 1.28 2.84 1.73 0.78

Natural resonance theory (NRT).

When compounds 1 and 2 are compared to CrN123, all three show very similar bond orders for the nitrido nitrogen by both NRT and Mayer calculations. However, the amides in CrN123 are quite different from the amides in 1 and 2. The calculations suggest that, for 1 and 2, the amido ligands have Cr−N bond orders significantly higher than those in CrN123 and approach the imide bond order in CrN123. This analysis agrees with the large downfield shifted value for the 14N NMR resonance of the amides in 1 and 2. Additionally, a very notable spectral difference emerges when 1 is examined with the SbF6− counterion. There is an observable peak for the amide at ∼449 ppm (Figure 4); however, there is no nitrido peak visible in the spectrum, even with extended transients. The 14N NMR spectra of both 1 and 2 with alternative counterions show clear nitrido and amido peaks. With the chromium cation and solvent remaining 1229


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quadrupolar, proximity of antimony to the nitrido nitrogen in solution could induce fast relaxation, which would broaden the signal into the baseline of the spectrum.25,26 This rapid relaxation is observed in the 19F NMR resonance of the SbF6− anion as well, which decays so rapidly that it could not be studied by DOSY or NOESY NMR. The fluorine and the antimony are bound, which justifies the manner in which the quadrupolar relaxation affects the 19F signal. For the 14N signal of the nitrido to be so strongly affected by the antimony quadrupole, the nitrido must be spatially close to the counterion in solution. However, the nitrido resonance is clearly seen for the PF6− anion in the 14N NMR resonances, which presumably resides near the same place as valence isoelectronic SbF6− around the cation but does not contain a quadrapolar nucleus (Figure 4). Consequently, we propose that SbF6−, and likely PF6− as well, reside near the nitrido nitrogen in solution. To explore this issue further, we turned to computational analysis of the ions to probe for a preferred orientation of the PF6 − anion relative to the chromium cations.27 DFT calculations were performed on the full molecule [NCr(N i Pr 2 ) 2 (PMe 3 )] + PF 6 − ({2}PF 6 ) using B3PW91 and wB97XD with the 6-31G(d,p) basis set. The latter functional includes dispersion corrections, but both calculations gave virtually identical results. Optimization of the input geometry led to the orientation sketched in Figure 5. In an attempt to

Figure 5. Spatial arrangement of the cation and anion in [NCr(NiPr2)2(PMe3)]+PF6− ({2}PF6) by DFT with intermolecular interactions shown by dotted lines.

eliminate bias from the optimization, multiple starting positions of the anion were examined. Calculations from which the anion was started anti to the nitrido failed to optimize after extended computational time. Starting the anion syn to the nitrido was the only conformation from which the optimization converged. Further analysis by NBO and Mayer bond order revealed significant interactions between the PF6− anion and both the nitrido nitrogen and methyl groups of the diisopropylamido ligands on the cation. Specifically, the fluorine (F1) oriented toward the nitrido group shows lone pair donation into the Cr−N π* with an interaction energy of ∼1 kcal/mol from the perturbation theory analysis. This fluorine also shows ∼5 kcal/ mol interaction into three of the closest neighboring C−H σ* orbitals. A second fluorine on the PF6− ion (F2), cis to the fluorine interacting with the nitrido group, also shows about a 3 kcal/mol interaction with its closest neighboring C−H σ* orbital. Examination of bond orders of these two fluorines shows small nonzero terms by Mayer calculations between the F and H’s.

Figure 4. 14N NMR spectra of {1}SbF6 (top) and {1}PF6 (bottom). The sharp peak for dissolved N2 is marked with an asterisk (*). For {1}SbF6 there is a single peak, a, which corresponds to the diisopropylamido nitrogen. For {1}PF6 there are two peaks, a, corresponding to the diisopropylamido nitrogen and b, corresponding to the nitrido nitrogen.

constant among the spectra for 1, the difference in the appearance of the nitrido peak is likely attributable to an effect of the SbF6− counterion. The nitrido resonance is readily observed for other anions, including closely related PF6−. It is well-known that the presence of quadrupolar nuclei near the nuclei under observation by NMR can induce accelerated relaxation effects. Given that antimony’s two stable isotopes are 1230


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Organometallics The most notable indication that these fluorines heavily donate their lone pair density to the cation, however, comes by observing the differences in the P−F bond orders. The six Mayer bond orders calculated for each P−F bond are shown in Table 3. The table shows F5 and F6 with bond orders of 0.88.

Table 4. Summary of LDP Values Determined for the {[Cr]PR3}X− Complexes with a Variety of Counterions LDP (kcal/mol) phosphine complex

anion

CDCl3

CD3CN

1 (L = PMe2Ph)

SbF6 PF6 BArF24 BArF20 BPh4 Al(ORF)4 SbF6 BPh4 BArF24

16.99 16.96 16.60 16.64 16.57 16.66 17.24 16.71 16.86

16.53 16.53 16.58 16.62 16.47 16.62 16.64 16.66 16.65

Table 3. Summary of Mayer Bond Order Calculations for {2}PF6− P−F Bonds bond

Mayer bond order

P−F1 P−F2 P−F3 P−F4 P−F5 P−F6

0.74 0.80 0.94 0.94 0.88 0.88

2 (L = PMe3)

The polarity gradient between the two pure solvents was achieved by mixing the solvents in different ratios, and the LDP was measured in each mixture. The results of these measurements are shown in Table 5. Generally, as the solvent mixture

These fluorine atoms have no interaction with the chromium cation. F1 and F2 show notable interaction with empty antibonding orbitals on the chromium cation, mentioned above. The lone pair donation from these two fluorines is reflected by the ∼0.1−0.15 bond order reduction with phosphorus. The two fluorines trans to F1 and F2 are F3 and F4, respectively. The bond orders for these trans fluorines, as expected, increase to compensate for the reduced donation from F1 and F2 in the three-center−four-electron bonds. These results, noted by DFT, NBO, and Mayer bond order calculations, support the proposed preferential orientation of the cation−anion pair, with the small PF6− and SbF6− sitting near the nitrido group. This is in strong agreement with 14N NMR. LDP Measurements. Measuring the LDP for PMe3 and PPhMe2 complexes reveals that they are much poorer donors toward a high-valent metal than other ligands reported thus far. Due to the relatively poor donor ability of the phosphines, the diisopropylamide barriers to rotation are high. Therefore, higher temperatures are needed in order to achieve experimentally measurable bond rotation. As an alternative to CDCl3, which has been the default NMR solvent for LDP measurements,13,14 CD3CN was the most acceptable substitute. The phosphine complexes are highly soluble in CD3CN, and the boiling point of the solvent exceeds the range of thermal stability for the chromium scaffold. Thermal decomposition of the cationic chromium complexes in solution does not begin until ∼75 °C. As some of the LDP values measured in CDCl3 were repeated in CD3CN, it became evident that the values are very sensitive to solvent choice for these salts. This is a departure from the behavior of previously reported neutral complexes, NCr(NiPr2)2X, where X is a monoanionic ligand, as their LDP values are largely insensitive to solvent change. The results shown in Table 4 demonstrate that changing from CDCl3 to CD3CN alters the LDP value. The effects of solvent on the rotational barriers are most pronounced for PF6− and SbF6−. For these anions, the LDP in CDCl3 is much higher than that in acetonitrile. While the other counterions also follow this trend, it seemed relevant to verify that the solvent effects on LDP were related to some solvent property which could be tracked continuously between the two pure solvents. Due to the ionic nature of the complexes and our hypothesis that the main difference between the two solvents depends on how charge separation is supported in each solvent, the polarity was examined on a gradient scale.

Table 5. [NCr(NiPr2)2(PMe2Ph)]+PF6− ({1}PF6) LDP Values as a Function of Estimated Solvent Dielectric Constant CDCl3:CD3CN (μL:μL)

LDP (kcal/mol)

esta solvent dielectric constant

pure CD3CN 1:1 19:11 11:4 14:1 pure CDCl3

16.53 16.58 16.65 16.71 16.81 16.96

37.5 24.6 20.2 16.6 8.08 4.81

a

Estimated by following the ideal behavior of mixing, assuming additive behavior in a weighted fashion proportionate to mole fraction (see the Supporting Information for additional details).

became more polar (approaching pure CD3CN), the LDP value of the model compound [NCr(NiPr2)2(PMe2Ph)]+ PF6− ({1}PF6) decreased. This can be correlated to the dielectric constant, as shown in Figure 6. In terms of the polarity of the

Figure 6. Correlation between the [NCr(NiPr2)2(PMe2Ph)]+PF6− ({1}PF6) LDP value of the complex and the log of the dielectric constant.

solvent, there is a lower limit to LDP, which is approached in CD3CN. On extrapolation to the polarity of water (ε = 78), the LDP value for 1 without ion pairing should be 16.36 kcal/ mol.28 The difference of 0.17 kcal/mol is close to the ±0.1 kcal/mol experimental error intrinsic to the rotation barrier measurement. Additionally, CD3CN remains the most polar common NMR solvent in which compounds 1 and 2 are stable. Effects of Solvent on Ion Pairing. The manner in which solvent affects LDP was further investigated by directly probing the presence and extent of ion pairing, as well as how ion 1231


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Table 6. Diffusion Coefficients for the PMe3 (1) and PPhMe2 (2) Complexes with Various Weakly Coordinating Anionsa complexb 1 (L = PMe2Ph)

anion PF6

solvent

ion

Dstdc

Diond

Dion:Dstd

D+/s:D−/se

CDCl3

+ − + − + − + − + − + − + − + − + − + − + − + − + − + −

17.53 14.92 25.81 28.20 14.79 15.69 21.69 20.27 16.38 12.25 20.26 19.75 15.17 13.65 24.02 20.46 14.68 14.68 22.53 22.53 15.72 15.72 21.29 21.29 14.75 14.75 21.76 21.76

10.05 8.73 17.90 29.63 6.35 6.47 14.21 11.68 7.34 5.10 13.36 11.30 6.08 5.35 15.70 14.48 6.07 6.03 14.89 15.12 6.47 6.34 11.68 15.03 6.27 6.32 15.36 14.75

0.57 0.58 0.69 1.05 0.43 0.41 0.66 0.58 0.45 0.42 0.66 0.57 0.40 0.39 0.65 0.71 0.41 0.41 0.66 0.67 0.41 0.40 0.56 0.73 0.43 0.43 0.71 0.68

0.98 ± 0.05

CD3CN BArF24

CDCl3 CD3CN

BArF20

CDCl3 CD3CN

Al(OtBuF9)4

CDCl3 CD3CN

BPh4

CDCl3 CD3CN

2 (L = PMe3)

BArF24

CDCl3 CD3CN

BPh4

CDCl3 CD3CN

0.66 ± 0.04 1.04 ± 0.07 1.14 ± 0.03 1.08 ± 0.04 1.15 ± 0.06 1.01 ± 0.04 0.92 ± 0.04 1.01 ± 0.02 0.98 ± 0.02 1.02 ± 0.04 0.76 ± 0.02 0.99 ± 0.02 1.04 ± 0.03

See the Supporting Information for molecular volumes. Diffusion coefficients are reported in units of 1 × 10−10 m2/s. bAll DOSY measurements were made at a concentration of 0.025 M, the same concentration used for LDP measurements. cMeasured diffusion constant for 1,3,5-(CF3)3C6H3 standard by DOSY under conditions identical with those of ion measurement. dMeasured diffusion constant for ion from DOSY. eRatio of the cation to anion standardized values. a

diffusion delay and gradient lengths were adjusted to achieve a reliable exponential attenuation of the 1H signal (∼90% at highest gradient strength) but generally fell between 25 and 50 ms and between 1.5 and 2.5 ms, respectively. The gradient strength was varied from 2.15 to 53.9 G/cm. All DOSY measurements were made on solutions of 0.025 M concentration of the salts of 1 and 2, to match the concentrations used for LDP measurements. The cations 1 and 2 could be analyzed for their diffusion coefficients in this manner. For those anions which possessed clean, identifiable 1H signals, the diffusion coefficient of the counterion was determined from the same 1H DOSY spectrum. However, for those counterions which lacked the necessary features to identify the diffusion coefficient from the 1H DOSY, it was necessary to identify the diffusion coefficient from 19F DOSY. Similar to the case for the 1H DOSY experiments, the 19 F DOSY data were collected with a relaxation delay of 4 s and 64, 128, or 256 transients. Again, the diffusion delay and gradient length were varied to provide the best exponential signal attenuation across 15 gradient intervals, with parameter ranges similar to those applied for the 1H experiments. The 19F DOSY NMR for {1}BArF20 was acquired using a different pulse sequence. The data for these two experiments were collected using the CHORUS OneShot DOSY experiment, which has been established to provide better diffusion-based attenuation in complexes affected by coupling and other relaxation issues.31 (For additional details, see the Supporting Information.)

pairing varies between solvents. Generally, for the complexes of {1}X and {2}X, there is a significant difference in size between the cation and the anion. This size difference should manifest itself as differences in the diffusion rate of each ionic species in solution, provided the ions are unpaired. If the ions remain paired, the pair will diffuse through solution as a single molecular entity, and the cation and anion should exhibit identical diffusion coefficients.7−9,29 The relationship between diffusion coefficient and molecular size is shown in eq 1, the simplest form of the Stokes−Einstein equation, where k = Boltzmann constant, T = temperature, η = viscosity of the solvent, and rH = hydrodynamic radius. The first term in eq 1 is then a group of constants. The second term says that the diffusion coefficient, Dt, is proportional to the temperature and inversely proportional to the size (hydrodynamic radius) of the diffusing species. While this relationship can be modified to provide a more sophisticated treatment of diffusion,9,30 this treatment is sufficient for the discussion here.

Dt =

k T 6πη rH

(1)

The diffusion coefficients for a variety of salts of 1 and 2 were determined in CDCl3 and CD3CN by 1H DOSY experiments using the Dbppste_cc (DOSY bipolar pulse pair stimulated echo convection corrected) pulse sequence. The relaxation delay was set to 4 s, and 64, 128, or 256 transients were collected as needed to obtain adequate signal to noise. The 1232


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the syn-isopropyl groups on the diisopropylamides (Figures 2 and 3).13 As stated above, we propose that small anions such as PF6− can approach less congested areas surrounding the chromium center on the cation, specifically the area near the nitrido nitrogen. We also propose that the large anions are too bulky to approach a specific site on the cation, despite their observed tight pairing behavior. In either scenario, the close approach of the paired anion presents hindrance to amido group rotation, which raises the LDP. Due to differences in the specifics of anion orientation, this effect is more noticeable for the smaller anions in comparison to the bulky, diffuse anions. Due to the fast relaxation of fluorine signals and the molecular weights of the compounds of interest, adequate 1 H−19F HOESY (heteronuclear Overhauser effect spectroscopy) signals could not be obtained.32 However, the larger anions that also have 1H NMR resonances were studied by ROESY NMR (rotating-frame Overhauser effect spectroscopy). To eliminate aromatic overlap, {2}BArF24 and {2}BPh4 were chosen. The resulting ROESY spectra in CDCl3 and CD3CN were analyzed for correlation between the anion and cation. In CDCl3, correlation peaks are observed between the aromatic 1 H NMR signals for BArF24− and the aliphatic peaks for both NiPr2 and PMe3 on 2. This suggests that the ion pairing is not specific to one site on the cation and that BArF24− can pair on the PMe3 side of the cation, on the NiPr2 side of the cation, and anywhere intermediate. With more ambiguous pairing behavior between the anion and cation, the anion does not always affect amido rotation. Specifically, the paired ion resides, at least in some portion of the ion pairs, away from the NiPr2 groups and allows free rotation. As a result, the increase in LDP value is smaller, even when complete pairing is observed by DOSY NMR, due to the averaging effect of LDP. In CD3CN, there are no correlation peaks observed between cation and anion. This by itself is not conclusive of a lack of ion pairing but is consistent with the reduction of ion pairing in this solvent observed using other methods (vide supra). DOSY Molecular Weight Determination. Another useful application of DOSY is to utilize the relationship of the diffusion coefficient with size to estimate molecular weights of unknown compounds in solution.33,34 This is potentially a useful tool for examination of ion pairs, as paired versus unpaired ions should correlate to largely different molecular weights. In order to make this estimation, the diffusion coefficients of standard compounds were determined and plotted as a function of molecular weight, related by eq 2:

To correct for intrinsic deviations in the diffusion coefficient measurements between different nuclei where the different time scales of the experiment can lead to differences induced by convection, for example, an internal standard was used to calibrate the diffusion coefficients. For 1H DOSY and 19F DOSY, 1,3,5-tris(trifluoromethyl)benzene was selected, as it gives sharp, singlet signals for both nuclei that do not overlap with the signals of the chromium complexes. In a given sample, the standard diffuses with the same rate; therefore, deviations in diffusion coefficients between the 1H and 19F measurements are caused by parameter differences between the experiments. Thus, we corrected for these differences by taking each diffusion coefficient and comparing it as a ratio to the internal standard’s diffusion coefficient. The results are shown in Table 6. From the ratios of the standardized cation and anion diffusion constants, the behaviors of the complexes with various weakly coordinating anions are the same in CDCl3. The ratios for all of the different salts are ∼1 in CDCl3, suggesting that, in chloroform, the cation and anion are 100% ion paired. We postulate that, because the rotational barrier enthalpy is very sensitive to steric hindrance, the congestion caused by a closely associated anion in solution raises the barrier to rotation. The results agree with this concept, as the LDP values measured in chloroform for various anions are always higher in comparison to the LDP values in CD3CN. The ratios in Table 6 are quite different in CD3CN for the pairs of ions. When the DOSY experiment is carried out in a more polar solvent, the diffusion coefficients diverge, suggesting that the ions diffuse at rates proportional to their individual sizes, resulting in different diffusion constants. This result is indicative of unpaired ions in CD3CN. There are two notable exceptions to this trend: {1}BPh4 and {2}BPh4. In both of these salts, the anion and cation are nearly the same size, within ∼10% molecular volume. Therefore, it is expected that the diffusion coefficients of the two ions will be very close, even when the ions are unpaired. On comparison of the relative diffusion coefficients in the two different solvents, normalized against the standard, the ions move much more quickly in CD3CN than in CDCl3. The larger diffusion constants in the more polar solvent are indicative of smaller particle sizes (eq 1), suggesting that the ions are, in fact, unpaired in the more polar solvent for the BPh4− salts as well. Of note is the dramatic difference in the magnitude of this effect depending on the identity of the anion. Specifically, the LDP value in CDCl3 for {1}PF6 is nearly 0.5 kcal/mol higher than that measured in CD3CN, whereas for the larger anions, the difference in LDP values in the two solvents is much smaller. For {1}BArF24, the difference between solvents is within the reported margin of error for the LDP measurements. Again, both PF6− and BArF24− are ion paired in CDCl3; consequently, this suggests something fundamentally different about the way ion pairing occurs between cation 1 and the two anions PF6− and BArF24−. Along with this difference in the magnitude of the impact of solvent effects through ion pairing, there are a few other observations that suggest a major difference in the amount of steric hindrance induced by each anion. The chromium cation’s ligands vary substantially in size. The nitrido ligand is small, while the diisopropylamido group is large, and the phosphine is intermediate in size. This leaves more space available near the nitrido group, as evidenced by the ground state orientation of

log D = a log FW + B

(2)

The internal references must have a range of molecular weights and distinct 1H NMR resonances in the DOSY spectrum. Once the diffusion coefficients for the references are known, the unknown’s diffusion coefficient is used to solve for the predicted molecular weight. This has been shown to work well with other compounds. Typically, lower errors in the formula weight are obtained in less dense solvents.33−35 To perform these experiments with {1}PF6 (molecular weight 549.49 g/mol), diethyl ether, ferrocene, and tetrakis(trimethylsilyl)silane were chosen as standards. The 1H DOSY molecular weight determination was performed in CDCl3, CD3CN, and chlorobenzene-d5 to give a range of polarities. The results (Table 7) were not as clear as molecular weight determination experiments reported in the literature, but 1233


Article

Organometallics Table 7. Molecular Weight of {1}PF6 in Solution by 1H DOSY NMR solvent

concentration (M)

dielectric constant

CDCl3 C6D5Cl CD3CN CD3CN CD3CN

0.025 0.025 0.025 0.01 0.10

4.81 5.62 37.5 37.5 37.5

concentration used for typical LDP measurements). This suggests that salt aggregation is not occurring in solution. In conjunction with the other studies in CD3CN, which suggest very little to no ion pairing in this solvent, this suggests that the high molecular weight predictions are likely the result of solvation differences between the charged species of interest and the neutral molecular weight standards in solution and that aggregation is not significant in the system. Upon repetition of the molecular weight calibration at high temperature, where faster exchange of solvent molecule associated with the chromium cation is expected, the calibrated weight remained unchanged within error.

solution MW (g/mol) 693 628 542 578 525

± ± ± ± ±

114 21 76 93 68

certain trends emerged from the data that help illuminate the ion pairing and solvation environment in each solvent. As the solvent becomes more polar, the molecular weight of the chromium cation in solution decreases. This is expected, on the basis of the standard 1H DOSY experiments, showing ion pairing in CDCl3 and unpaired ions in CD3CN. However, it is important to note that the calibrated molecular weight in CDCl3 is higher than that of the salt {1}PF6− by a substantial amount. The molecular weight in CD3CN, similarly, is greater than the molecular weight of the free cation 1 (404 g/mol). Reported uses of DOSY molecular weight calibrations primarily have focused on neutral substances.35 In addition, all of the molecular weight standards are neutral species, as ionic internal standards would be noninnocent in solution. This leads us to suspect that, again, this difference in behavior with the complex was a manifestation of its ionic character. The two likely possibilities for the higher than expected molecular weight in CD3CN are (1) aggregation of several ions11,36 and (2) solvent molecules pairing to and moving with the cation in solution slowing diffusion. Both processes are equilibria involving relatively weak, dynamic interactions between the cation and other species in solution. As such, both aggregation and solvation may be difficult to observe directly. However, if the salts are aggregating, the observed molecular weight should decrease with dilution. If the weight is high due to solvent pairing with the cation, then the observed molecular weight should not change with concentration. To verify whether or not the ions aggregate in solution, the concentration of {1}PF6 solutions was varied in CD3CN, and the molecular weight calibrated at each concentration. An example of the plots obtained from these types of measurements is shown in Figure 7. There was no significant difference ranging from 0.01 to 0.10 M (including the 0.025 M

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CONCLUSIONS A novel series of chromium(VI) cations has been generated of formula {NCr(N-i-Pr2)2(PR3}+X−, where PR3 = PMe3, PMe2Ph while X− is a common weakly coordinating anion (Chart 1) such as SbF6−, PF6−, BArF20−, BArF24−, BPh4−, or Al(OtBuF9)4−. The effect of different anions on the LDP measurement was pronounced for many of these salts. We ascribe the higher LDP values in CDCl3 versus more polar solvents to ion pairing and resulting steric effects from interaction of the cation and anion in solution. As may be anticipated, the ubiquitous “noncoordinating” anions BArF20− and BArF24− had the smallest effect on LDP in CDCl3, which may lead one to believe that these large anions are not ion pairing. However, DOSY and ROESY clearly show that these anions are ion paired in CDCl3, much like the case for the other anions listed. The smaller effect of these anions on LDP was apparently due to a lack of specif icity in where they ion pair to the cation. We postulate that the BArFx− anions are often ion paired in such a way as to not impede amide rotation and thus minimally affect LDP measurement. Other anions, e.g., SbF6− and PF6−, seem to pair with the cation in a specific location, which happens to impede amide rotation. In the case of SbF6− and PF6−, the anions are localized near the nitrido nitrogen. In these cases, one of the F lone pairs donates into the Cr−N π*-orbital (∼1 kcal/mol), which is consistent with the observed quadrupolar broadening of the nitrido nitrogen in the 14 N NMR. By DFT, the fluorines of these anions also show strong interactions, with fluorine lone pair donation to the C− H σ* orbitals of the methyl groups on the diisopropylamido ligands syn to the nitrido group. These intermolecular interactions between the cation and anion certainly suggest a preferred location for ion pairing for SbF6− and PF6−, which is placed in such a way that impedes amide rotation and increases the LDP value when paired. These interactions seem to be mitigated in solution state measurements by conducting NMR experiments in a sufficiently polar solvent. Phenomena less easily explained than the differences between LDP in different solvents with different anions are the absolute values of LDP (Table 4). One expects that the lower LDP values better represent the free cation rotation barrier with no ion pairing. However, the LDP values in polar CD3CN are elevated for the supposedly less coordinating anions relative to SbF6− and PF6−. Indeed, there seems to be little difference between LDP values in lower polarity and higher polarity solvents with anions such as BArF20− not because both values are near the lowest LDP value (16.53 kcal/ mol) but because both values are elevated. This may indicate that LDP is reporting on some weaker, but still present, ion pairing in CD3CN with these anions that DOSY and ROESY are not. It is possible that these larger, more lipophilic anions

Figure 7. Representative plot of diffusion constant versus molecular weight for {1}PF6 in CD3CN. Red circles show the molecular weight standards. The black square shows the predicted molecular weight of the in situ chromium species. 1234


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Organometallics

Synthesis of {1}BArF24. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (50 mg, 0.127 mmol, 1 equiv), PPhMe2 (35 mg, 0.253 mmol, 2 equiv), 3 mL of DCM, and a magnetic stir bar. To this stirred solution was added a solution of TlBArF24 (135 mg, 0.127 mmol, 1 equiv) in 1 mL of DCM. Upon addition, copious amounts of yellow precipitate formed, and the solution went from dark red-orange to transparent bright orange. The solution was stirred for 3 h at room temperature. The TlI precipitate was filtered through Celite, and the bright orange filtrate was concentrated in vacuo. The concentrated solution in DCM (∼1 mL) was layered with pentane and chilled to −30 °C overnight to obtain X-ray-quality orange crystals (62 mg, 38.5%). Mp: 115 °C dec. 1H NMR (500 MHz, CDCl3): δ 7.70 (s, 8H), 7.52 (s, 5H), 7.51−7.40 (m, 4H), 4.95 (sept, J = 12.8, 6.4 Hz, 2H), 3.88 (sept, J = 12.5, 6.3 Hz, 2H), 1.87 (d, J = 10.3 Hz, 6H), 1.65 (d, J = 6.3 Hz, 6H), 1.56 (d, J = 6.3 Hz, 6H), 1.15 (d, J = 6.4 Hz, 6H), 1.10 (d, J = 6.4 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 164.34− 159.48 (m), 134.76 (s), 132.65 (s), 130.15 (d), 129.65 (d), 128.87 (d), 127.77 (s), 125.60 (s), 123.43 (s), 121.26 (s), 117.46 (s), 59.70 (s), 59.00 (s), 31.85 (s), 29.71 (s), 23.07 (s), 22.27 (s), 14.86 (d).19F NMR (470 MHz, CDCl3): δ −62.38 (s). 31P NMR (202 MHz, CDCl3): δ 12.74 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1008.5 (s), 445.8 (s). Synthesis of {1}BArF20. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (60 mg, 0.153 mmol, 1 equiv), PPhMe2 (42 mg, 0.303 mmol, 2 equiv), 3 mL of DCM, and a magnetic stir bar. To this stirred solution was added a solution of KBArF20 (110 mg, 0.153 mmol) in 1 mL of DCM. This reaction mixture was stirred for 8 h at room temperature. Over the course of the reaction, the dark redorange, cloudy solution slowly cleared and became bright orange as a yellow precipitate formed. The KI precipitate was removed by filtration through Celite, and the bright orange solution was concentrated in vacuo. The concentrated filtrate (∼1 mL) was layered with pentane (∼2 mL) and chilled to −30 °C overnight to yield X-rayquality orange crystals (46 mg, 27.8%). Mp: 135 °C dec. 1H NMR (600 MHz, CDCl3): δ 7.61−7.40 (m, 4H), 5.01 (sept, J = 12.8, 6.4 Hz, 2H), 3.91 (sept, J = 12.5, 6.2 Hz, 2H), 1.90 (d, J = 10.3 Hz, 6H), 1.66 (d, J = 6.3 Hz, 6H), 1.58 (d, J = 6.3 Hz, 6H), 1.21 (d, J = 6.4 Hz, 6H), 1.16 (d, J = 6.4 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 148.30 (d), 137.28 (t), 132.78 (s), 130.31 (d), 129.92 (d), 128.52 (d), 59.97 (d), 59.15 (s), 32.08 (d), 29.88 (s), 23.35 (s), 22.47 (d), 14.88 (d). 19F NMR (470 MHz, CDCl3): δ −132.56 (s), −163.03 (d, J = 17.7 Hz), −166.84 (s). 31P NMR (202 MHz, CDCl3): δ 12.92 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1010.3 (s), 448.5 (s). Synthesis of {1}Al(OtBuF9)4. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (100 mg, 0.254 mmol, 1 equiv), 3 mL of acetonitrile, and a magnetic stir bar. To this was added a solution of AgAl(OtBuF9)4 (273 mg, 0.254 mmol, 1 equiv) in 1 mL of acetonitrile. The resultant solution was stirred for 1 h at room temperature, during which time a yellowish precipitate formed, and the solution darkened from red-orange to brown. The AgI was removed by filtration through Celite, and the filtrate was once again stirred. To the stirring filtrate was added a solution of PPhMe2 (35 mg, 0.254 mmol, 1 equiv) in 1 mL of acetonitrile. This solution was stirred at room temperature for 2 h, changing slightly in color from dark brown to dark orange-brown. The solution was dried in vacuo, and the residue was washed with several aliquots of cold pentane. The solids were again dried in vacuo and dissolved in a minimal amount of chloroform. This concentrated solution was layered with pentane and chilled to −30 °C, which afforded yellow crystals (183 mg, 52.5%). Removal of all traces of solvent from the aluminate complex (without decomposing the complex) was difficult, and solvent peaks were identified in the NMR spectra. Mp: 136−138 °C. 1H NMR (500 MHz, CDCl3): δ 7.68−7.43 (m, 5H), 5.02 (sept, J = 6.4 Hz, 2H), 3.93 (sept, J = 6.1 Hz, 2H), 1.94 (d, J = 10.3 Hz, 7H), 1.68 (d, J = 6.3 Hz, 6H), 1.60 (d, J = 6.3 Hz, 6H), 1.23 (d, J = 6.4 Hz, 8H), 1.20 (d, J = 6.4 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 132.85 (d), 130.35 (d), 129.88 (d), 128.51 (d), 121.35 (q), 60.00 (d), 59.19 (s), 32.03 (d), 29.85 (s), 23.22 (s), 22.40 (d), 14.80 (s), 14.21 (s). 19F NMR (470 MHz, CDCl3): δ −75.49 (s). 31 P NMR (202 MHz, CDCl3): δ 12.71 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 980.5 (s), 452.6 (s).

are less stabilized in a more polar solvent in comparison to SbF6− and PF6− and therefore are still interacting with the lipophilic cation. In ionic metal-based systems, where an internal rearrangement or metal access is vital to some process (i.e., catalyst turnover), the close association of a large counterion via ion pairing can wield a large effect. Congestion around the active site of a catalyst where associative or dissociative processes of substrates are occurring can affect the rate of the reaction dramatically and even catalyst enantioselectivity.2,37 Consequently, a better understanding of these ion-pairing effects in their many guises is an important, and often neglected, aspect of catalyst design.

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EXPERIMENTAL SECTION

General Considerations. All procedures were carried out under an N2 atmosphere in an MBraun glovebox or using Schlenk techniques. NCr(N-i-Pr2)2I was prepared as previously described.38 The phosphines PMe3 and PPhMe2 were purchased from Strem Chemical and used as received. AgSbF6,TlPF6, and AgBPh4 were purchased from Sigma-Aldrich and used as received. TlBArF24 was prepared from NaBArF24 and TlNO3 following the literature procedure.39 The KBArF20 was supplied as a gift from Boulder Chemical Co. and was used as received. AgAl(OtBuF9)4 was prepared following the literature procedure.40 All deuterated solvents were purchased from Cambridge Isotopes. CD3CN was dried over CaH2, distilled under N2, and stored over 3 Å molecular sieves. CDCl3 was dried over P2O5, distilled under N2, and stored over 3 Å molecular sieves. C6D5Cl was sparged with N2 and stored over activated 3 Å molecular sieves. Diethyl ether, dichloromethane, acetonitrile, and pentane were dried using activated alumina columns, sparged with N2, and stored over activated 3 Å molecular sieves. Due to the extreme air and moisture sensitivity of these chromium(VI) cation complexes, accurate elemental analysis could not be obtained through standard techniques. Samples were often noted to change mass while measurements were being made prior to sample analysis. As a result, the complexes were characterized by NMR spectroscopy and melting point. Crystal structures of 1 and 2 were also obtained with multiple counterions to verify any ion effects on bond distances and angles. Additionally, the complex {2}SbF6 is particularly unstable in solution. As a result, 14N NMR could not be attained, as the long acquisition times required exceeded the maximum time frame of stability of the complex in solution. The LDP measurements in CD3CN and the 13C NMR were both obtained from a sample of {2}SbF6 generated in situ, with a slight excess of PMe3 stabilizing the complex in solution. Synthesis of {1}PF6. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (50 mg, 0.127 mmol, 1 equiv), a magnetic stir bar, PPhMe2 (35 mg, 0.253 mmol, 2 equiv), and 3 mL of acetonitrile. To this stirred solution was added a solution of TlPF6 (44.3 mg, 0.127 mmol, 1 equiv) in 1 mL of acetonitrile. Upon addition, copious amounts of yellow precipitate formed, and the solution went from dark red-orange to a lighter orange. This solution was stirred for 3 h at room temperature. The TlI precipitate was filtered off of the bright orange solution through Celite, and the filtrate was pumped to dryness in vacuo. The residue was crystallized with CH2Cl2/pentane at −30 °C, which afforded X-ray-quality orange crystals (64.8 mg, 48.3%). Mp: 172−174 °C dec. 1H NMR (500 MHz, CDCl3): δ 7.69−7.58 (m, 2H), 7.52 (dd, J = 5.6, 2.9 Hz, 3H), 5.39 (sept, J = 12.5, 6.3 Hz, 2H), 3.91 (sept, J = 12.5, 6.2 Hz, 2H), 1.99 (d, J = 10.3 Hz, 6H), 1.56 (dd, J = 8.7, 6.4 Hz, 12H), 1.24 (d, J = 6.2 Hz, 12H). 13C NMR (126 MHz, CDCl3): δ 132.26 (d), 130.68 (d), 130.13 (d), 59.98 (d), 58.94 (s), 32.28 (d), 29.92 (s), 23.65 (s), 22.89 (s), 14.69 (d). 19F NMR (470 MHz, CDCl3): δ −72.21 (d, J = 719.7 Hz). 31P NMR (202 MHz, CDCl3): δ 12.35 (s), −130.77 to −156.42 (sept). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1016.1 (s), 451.1 (s). 1235


Article

Organometallics Synthesis of {1}BPh4. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (100 mg, 0.254 mmol, 1 equiv), 3 mL of acetonitrile, and a magnetic stir bar. To this was added a suspension of AgBPh4 (108 mg, 0.254 mmol, 1 equiv) in 1 mL of DCM. The resultant solution was stirred for 3 h at room temperature, during which time an off-white precipitate formed, and the solution darkened from redorange to brown. The AgI was removed by filtration through Celite, and the filtrate was once again stirred. To the stirred filtrate was added a solution of PPhMe2 (70 mg, 0.506 mmol, 2 equiv) in 1 mL of acetonitrile. This solution was stirred at room temperature for 1 h, changing slightly in color from dark brown to dark orange-brown. The solution was dried in vacuo, and the residue was rinsed with several small aliquots of diethyl ether. The residue was dried again and dissolved in a minimal amount of DCM. This concentrated solution was layered with diethyl ether and chilled to −30 °C overnight, affording bright orange crystals (23.4 mg, 12.7%). Mp: 84 °C dec. 1H NMR (600 MHz, CDCl3): δ 7.49 (dt, J = 6.5, 3.2 Hz, 1H), 7.43 (dd, J = 11.4, 3.6 Hz, 11H), 7.31−7.25 (m, 2H), 7.00 (t, J = 6.1 Hz, 9H), 6.86 (s, 4H), 4.81 (dt, J = 12.7, 6.4 Hz, 2H), 3.79 (dt, J = 12.5, 6.3 Hz, 2H), 1.52 (dd, J = 11.9, 6.3 Hz, 13H), 1.39 (d, J = 10.3 Hz, 6H), 1.14 (d, J = 6.4 Hz, 6H), 1.04 (d, J = 6.3 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 164.38 (q), 136.49 (d), 132.16 (d), 130.23 (m), 125.65 (d), 121.80 (d), 59.88 (s), 58.75 (s), 32.06 (s), 29.74 (s), 23.48 (s), 23.71 (s), 14.08 (s). 31P NMR (202 MHz, CDCl3): δ 13.60 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1006.8 (s), 445.9 (s). Synthesis of {1}SbF6. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (50 mg, 0.127 mmol, 1 equiv), 3 mL of acetonitrile, and a magnetic stir bar. To this stirred solution was added a solution of AgSbF6 (44 mg, 0.127 mmol, 1 equiv) in 1 mL of acetonitrile. This solution was stirred for 1 h at room temperature, forming an off-white precipitate and changing color from dark red-orange to dark brown. The AgI was removed by filtration through Celite, and the filtrate was once again stirred. To the stirred solution of filtrate was added a solution of PPhMe2 (35 mg, 0.253 mmol, 2 equiv) in 1 mL of acetonitrile. The dark brown solution changed color, taking on an orange hue. The solution was stirred for 1 h and was dried in vacuo and rinsed with several small aliquots of diethyl ether. The solids were once again dried, and the residue was dissolved in chloroform, layered with diethyl ether, and chilled to −30 °C overnight to afford X-rayquality orange-brown crystals (56.4 mg, 68.4%) Mp: 152−154 °C dec. 1 H NMR (500 MHz, CDCl3): δ 7.62 (dd, J = 8.7, 3.1 Hz, 2H), 7.53 (s, 3H), 5.33 (sept, J = 23.2 Hz, 2H), 3.91 (dt, J = 12.0, 5.9 Hz, 2H), 1.99 (d, J = 10.3 Hz, 6H), 1.57 (dd, J = 11.1, 6.2 Hz, 12H), 1.31−1.17 (m, 13H). 13C NMR (126 MHz, CDCl3): δ 132.07 (s), 130.38 (d), 129.93 (d), 59.81 (d), 58.73 (s), 32.05 (s), 29.68 (s), 23.40 (s), 22.63 (s), 14.56 (s), 14.32 (s). 19F NMR (470 MHz, CDCl3): δ −122.13 (d, J = 5077.9 Hz). 31P NMR (202 MHz, CDCl3): δ 11.89 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 448.8 (s). Synthesis of {2}BArF24. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (50 mg, 0.127 mmol, 1 equiv), PMe3 (0.301 mmol, 2.4 equiv), 3 mL of DCM, and a magnetic stir bar. To the stirred solution was added a solution of TlBArF24 (135 mg, 0.127 mmol, 1 equiv) in 1 mL of DCM. Upon addition, a copious amount of yellow precipitate formed, and the solution went from dark red to bright orange. This solution was stirred for 3 h at room temperature. The TlI precipitate was removed by filtration through Celite, and the bright orange filtrate was concentrated in vacuo. The concentrated solution was layered with pentane and chilled to −30 °C, which afforded X-ray-quality crystals (92.6 mg, 60.4%). Mp: 93 °C dec. 1H NMR (500 MHz, CDCl3): δ 7.69 (s, 8H), 7.53 (s, 4H), 5.01 (sept, J = 12.8, 6.4 Hz, 2H), 3.95 (sept, J = 12.5, 6.3 Hz, 2H), 1.82 (d, J = 6.3 Hz, 6H), 1.59 (d, J = 6.3 Hz, 6H), 1.51 (d, J = 10.6 Hz, 9H), 1.22 (dd, J = 14.2, 6.4 Hz, 14H). 13C NMR (126 MHz, CDCl3): δ 161.62 (q), 134.73 (s), 129.60−128.24 (m), 124.50 (q), 117.46 (s), 59.61 (s), 59.04 (s), 31.92 (s), 30.23 (s), 22.97 (s), 15.89 (s), 15.64 (s). 19F NMR (470 MHz, CDCl3): δ −62.35 (s). 31P NMR (202 MHz, CDCl3): δ 7.16 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1010.7 (s), 449.4 (s). Synthesis of {2}BPh4. A scintillation vial was charged with NCr(N-i-Pr2)2I (75 mg, 0.191 mmol, 1 equiv), 3 mL of acetonitrile,

and a magnetic stir bar. To this stirred solution was added a suspension of AgBPh4 (82 mg, 0.191 mmol, 1 equiv) in 1 mL of DCM. This solution was stirred for 3 h at room temperature. During this time the solution changed color from dark red-orange to dark brown, and an off-white precipitate was formed. The precipitate was removed by filtration through Celite. The dark brown filtrate was stirred, and a solution of 30 mg of PMe3 (0.394 mmol) in 1 mL of acetonitrile was added. This solution was stirred for 1 h at room temperature and then dried in vacuo. The residue was rinsed with several small aliquots of diethyl ether and dried in vacuo once again. The complex was dissolved in a minimal amount of DCM, layered with diethyl ether, and chilled to −30 °C, which afforded orange crystals (25.8 mg, 20.5%). Mp: 93−95 °C dec. 1H NMR (500 MHz, CDCl3): δ 7.44 (s, 8H), 7.05 (s, 8H), 6.90 (s, 4H), 4.84 (sept, J = 12.6, 6.3 Hz, 2H), 3.88 (dt, J = 12.6, 6.1 Hz, 2H), 1.76 (d, J = 6.2 Hz, 6H), 1.55 (d, J = 6.2 Hz, 7H), 1.18 (t, J = 6.1 Hz, 13H), 1.08 (d, J = 10.8 Hz, 9H). 13C NMR (126 MHz, CDCl3): δ 164.37 (q), 136.47 (s), 125.78 (s), 121.89 (s), 59.56 (s), 58.79 (s), 32.11 (s), 30.41 (s), 23.39 (s), 15.70 (s), 15.45 (s). 31P NMR (202 MHz, CDCl3): δ 8.02 (s). 14N NMR (36 MHz, CDCl3, 25 °C): δ 1015.9 (s), 446.4 (s). Synthesis of {2}SbF6. A 20 mL scintillation vial was charged with NCr(N-i-Pr2)2I (89 mg, 0.226 mmol, 1 equiv), 3 mL of acetonitrile, and a magnetic stir bar. To this stirred solution was added a solution of AgSbF6 (78 mg, 0.226 mmol, 1 equiv) in 1 mL of acetonitrile. The solution was stirred for 1 h at room temperature, over which time it turned from dark red-orange to dark brown and formed an off-white precipitate. The AgI precipitate was removed by filtration through Celite. The dark brown filtrate was stirred, and a solution of PMe3 (35 mg, 0.460 mmol, 2 equiv) in 1 mL of acetonitrile was added. The solution was stirred for 1 h at room temperature, and the solution took on an orange hue. The solution was dried in vacuo, and the residue was rinsed with several small aliquots of diethyl ether. The solids were once again dried, and the residue was dissolved in chloroform, layered with diethyl ether, and chilled to −30 °C overnight, affording X-rayquality crystals (68.2 mg, 52.4%). Mp: 111−113 °C. 1H NMR (500 MHz, CDCl3): δ 5.42−5.29 (sept, 2H), 4.08−3.93 (sept, 2H), 1.86 (t, J = 4.6 Hz, 6H), 1.69 (d, J = 10.9 Hz, 9H), 1.61 (d, J = 6.3 Hz, 6H), 1.39 (d, J = 6.3 Hz, 6H), 1.27 (d, J = 9.0 Hz, 6H). 13C NMR (126 MHz, CD3CN): δ 59.77 (s), 59.16 (s), 32.06 (s), 30.44 (s), 23.20 (d), 16.79 (s), 16.14 (s), 15.89 (s). 19F NMR (470 MHz, CDCl3): δ −123.1 (d). 31P NMR (202 MHz, CDCl3): δ 6.86 (s).

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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.6b00839. Details of the DOSY experiments and data for solution molecular weight experiments (PDF) Structural information (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.L.O.: [email protected]. ORCID

Aaron L. Odom: 0000-0001-8530-4561 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly appreciate the financial support of the NSF under CHE-1562140. Boulder Scientific generously donated the K[BArF20] used in the project. We also thank Dr. Richard Staples for his expert consultation on disorder and pseudo symmetry encountered in the crystallographic data. 1236


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Organometallics

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