Configurational Stability and Stereochemistry of P-Stereogenic Nickel

Article pubs.acs.org/Organometallics

Configurational Stability and Stereochemistry of P‑Stereogenic Nickel POCOP-Pincer Complexes Anubendu Adhikary, Jeanette A. Krause, and Hairong Guan* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States S Supporting Information *

ABSTRACT: The P-stereogenic nickel complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2) has been synthesized via cyclometalation of the POCOP-pincer ligand 1,3[(t-Bu)(Ph)PO]2C6H4 (1) with NiCl2. The initially isolated 2 consists of a 1:1 mixture of racemic and meso isomers that are separable through repeated crystallization and is configurationally stable even at 110 °C. Upon mixing with t-BuOK, the meso isomer (2-meso) displays a higher ligand substitution rate than the racemic isomer (2-rac), likely because its nickel center is sterically more accessible. Complex 2, as either pure 2-rac or a 2-rac/2-meso mixture, can be converted to the nickel triflate complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiOTf (3) or the nickel formate complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiOCHO (7) without epimerization at the phosphorus centers. Under a dynamic vacuum at 90 °C, decarboxylation of 7-meso is faster than that of 7-rac, suggesting that in the transition state the formato hydrogen approaches the nickel center from the axial site rather than the equatorial site. The structure of 2-rac has been studied by X-ray crystallography.

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INTRODUCTION Transition metal complexes bearing a bis(phosphinite)-based POCOP-pincer ligand (Figure 1, R, R′ = alkyl or aryl group)

supported by a POCOP-pincer ligand are sterically more accessible, as suggested by smaller Cipso−M−P angles.1b,9 These differences in electronic and steric properties have profound influences on the reactivity of the pincer complexes. In processes such as iridium-catalyzed transfer dehydrogenation of alkanes,2 palladium-catalyzed allylation of aldehydes and imines,10 and rhodium-catalyzed dimerization of terminal alkynes,7 POCOP-pincer complexes have proven to be more active catalysts than the PCP-pincer complexes. One perceived challenge of using POCOP-pincer complexes as homogeneous catalysts is the lack of strategies to develop enantioselective catalysts. Unlike the PCP-pincer complexes, where chirality can be built into the pincer backbone adjacent to the phosphorus donors (Figure 1),11 POCOP-pincer complexes can be made chiral only through manipulation of the phosphorus substituents (R and R′) or conceivably at the sites between the oxygen atoms and the ipso carbon. To date, the known chiral POCOP-pincer complexes are bis(phosphite)- and bis(phosphoramidite)-based, which are derived from optically active diols12 and amino alcohols,13 respectively. While effective at times, these complexes often display reactivities different from those with a bis(phosphinite)-based pincer ligand. An alternative strategy is to keep one phosphinite arm of a POCOP-pincer ligand intact and replace the other one with a chiral auxiliary such as a chiral oxazoline14 or imidazoline,15 assuming that changing the donor group would not negatively impact the reactivity. The main objective of this work was to synthesize bis(phosphinite)-based nickel POCOP-pincer complexes with stereogenic phosphorus centers (R ≠ R′). Although our longer-term goal is to utilize these P-stereogenic complexes for

Figure 1. POCOP- and PCP-pincer complexes.

have gained tremendous popularity in recent years as catalysts for a wide range of applications.1 One of the attractive features of POCOP-pincer complexes is that the ligand synthesis is high yielding and rather straightforward, and compared with the synthesis of the related bis(phosphine)based PCP-pincer ligands, it is usually less costly. For example, 1,3-[(t-Bu)2PO]2C6H4, one of the most widely used POCOPpincer ligands, is readily available from the reaction between doubly deprotonated resorcinol and (t-Bu)2PCl.2 Preparation of the analogous PCP-pincer ligand 1,3-[(t-Bu)2PCH2]2C6H4 would require the use of 1,3-(BrCH2)2C6H4 and the more expensive3 and air-sensitive (t-Bu)2PH.4 Electrochemical and infrared data for nickel,5 iridium,6 and rhodium7 systems seem to support the notion that POCOP-pincer ligands in general are less donating than PCP-pincer ligands; however, density functional theory calculations on iridium pincer complexes have suggested that {2,6-[(t-Bu)2PO]2C6H3}Ir has a more electron-rich iridium center than {2,6-[(t-Bu)2PCH2]2C6H3}Ir.8 Crystallographic studies have consistently shown that metals © 2015 American Chemical Society

Received: May 11, 2015 Published: July 10, 2015 3603

DOI: 10.1021/acs.organomet.5b00402 Organometallics 2015, 34, 3603−3610

Article

Organometallics

NMR spectrum of the same sample revealed two well-separated resonances at 127.5 and 127.6 ppm with equal intensities, suggesting a 1:1 ratio of the racemic and meso isomers. Cyclometalation of 1 with NiCl2 was accomplished by refluxing the mixture in toluene, and the crude product was identified as a 1:1 mixture of the racemic and meso pincer chloride complexes 2-rac and 2-meso. The phosphorus chemical shifts (158.0 and 158.9 ppm) for the two isomers were further separated than those for the ligand, and the proton resonances of 2-rac and 2-meso were also readily distinguishable from each other. Repeated recrystallization of 2 from 1:1 CH2Cl2/pentane provided 2-rac with high isomeric purity (98%; see Figure S1 in the Supporting Information). Removal of solvent from the mother liquor usually yielded a 2-meso-enriched sample (80−93%). The morphologies of the two isomers are different; 2-rac is crystalline, while 2-meso appears as a fine powder. Single crystals of 2-rac were obtained from slow evaporation of a saturated solution in CH2Cl2 and studied by X-ray crystallography (Figure 2A). The Ni−Cipso bond of 2-rac [1.895(2) Å] is almost identical to those of other nickel POCOP-pincer chloride complexes.18 For the Ni−P bond distance, 2-rac [2.1741(6) and 2.1774(6) Å] is ranked in between [2,6(Ph2PO)2C6H3]NiCl [2.1556(6) and 2.1582(7) Å]18a and [2,6(t-Bu2PO)2C6H3]NiCl [2.1868(7) and 2.1912(7) Å],18c reflecting the relative sizes of the phosphorus substituents. Compared with most C2v-symmetric nickel POCOP-pincer complexes, the core structure of 2-rac deviates more from planarity; the dihedral angle between the plane containing C1 through C6 and the P1−Ni−P2−Cl plane is 6.23(6)°. This is likely due to an intermolecular CH···π interaction (Figure 2B) in the crystal packing19 that leads to the asymmetric bending of the pincer arms. The 31P{1H} NMR spectrum of the single crystals dissolved in CDCl3 showed a single resonance at 158.9 ppm, suggesting that 2-rac appeared 0.9 ppm downfield from 2-meso

asymmetric catalysis, our initial interest was to examine the configurational stability of the phosphorus centers. The meso and racemic forms of the nickel POCOP-pincer complexes were also studied for both ligand substitution and decarboxylation reactions, and the differences in rates provide useful information on the transition states for these processes.

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RESULTS AND DISCUSSION Synthesis of P-Stereogenic Nickel POCOP-Pincer Complexes. Optically active P-stereogenic mono(phosphinite)-16 and bis(phosphine)-based PCP-pincer ligands17 are known in the literature; however, the methods to introduce the chirality in these ligands could not be easily adopted and applied to the synthesis of optically active P-stereogenic bis(phosphinite)-based POCOP-pincer ligands. Instead, a diastereomeric mixture of a POCOP-pincer ligand was synthesized first, followed by cyclometalation with NiCl2 and separation of the resulting nickel pincer complex. Following the synthetic procedure for 1,3-[(tBu)2PO]2C6H4,2 the ligand 1,3-[(t-Bu)(Ph)PO]2C6H4 (1) was prepared in 85% yield from doubly deprotonated resorcinol and commercially available racemic PhP(t-Bu)Cl (Scheme 1). The 1H NMR spectrum of 1 in CDCl3 showed only one set of resonances expected for the ligand (e.g., only one doublet at 1.03 ppm for the tert-butyl resonance). In contrast, the 31P{1H} Scheme 1. Synthesis of the Nickel POCOP-Pincer Chloride Complexes

Figure 2. (A) ORTEP drawing of rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2-rac) at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni−Cl 2.2087(6), Ni−C1 1.895(2), Ni−P1 2.1774(6), Ni−P2 2.1741(6), P1−Ni−P2 162.46(3), C1−Ni−Cl 179.27(7), C1−Ni−P1 81.88(6), C1−Ni−P2 81.43(6). (B) Illustration of an intermolecular CH···π interaction between two pincer molecules. H14A···Ct = 3.08 Å and C14−H14A···Ct = 125°, where Ct denotes the centroid of the plane containing C1 to C6. 3604


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Organometallics Scheme 2. Attempts To Resolve the Enantiomers via Chiral Carboxylate Complexes

(δP = 158.0 ppm). The 1H and 13C chemical shifts of the two isomers are fairly close except for the protons (2-rac, δH = 1.30 ppm; 2-meso, δH = 1.37 ppm) and the quaternary carbon (2-rac, δC = 37.0 ppm; 2-meso, δC = 37.5 ppm) of the tert-butyl groups. Attempts were made to resolve the enantiomers of 2-rac by removal of the chloride ligand with AgOTf followed by substitution with a chiral carboxylate (Scheme 2). (S)-O-acetylmandelate and gibberellate were chosen as the chiral auxiliaries for two reasons. First, they are readily available through deprotonation of the corresponding carboxylic acids, which are commercially available. Second, the resulting nickel carboxylate complexes 4 and 5 were expected to be stable enough to sustain various recrystallization conditions, yet they could be reverted back to the chloride complex 2 (the optically active form) once separation of the diastereomers was complete. Both 4 and 5 were isolated as roughly 1:1 mixtures of diastereomers, as judged by 31P{1H} NMR spectroscopy. Unfortunately, despite different solvents and solvent combinations for recrystallization trials, there was no appreciable separation of the diastereomers of 4 or 5. Future efforts to resolve 2-rac will focus on the use of a chiral HPLC column. Configurational Stability. The success of the utilization of P-stereogenic POCOP-pincer complexes for asymmetric catalysis would rely on high configurational stability at the phosphorus centers. Epimerization of these pincer complexes is likely to erode the enantioselectivity. To test whether 2-rac and 2-meso would interconvert, a toluene solution of a 2-racenriched sample (2-rac:2-meso = 50:1) and another one of a 2-meso-enriched sample (2-rac:2-meso = 1:5) were heated at 110 °C. After 1 week, neither sample showed any change in the isomeric ratio. This observation does not necessarily rule out the possibility that a pincer arm dissociates from the nickel center because the reverse step in which the pincer arm recoordinates to nickel could be much faster than the pyramidal inversion and P−O bond rotation steps (occurring in either order) required for the epimerization process (Scheme 3). Nevertheless, the result does establish that phosphorus centers are configurationally stable even at an elevated temperature. The conservation of the isomeric ratio for the reactions described in the next section further suggests that under ambient conditions the racemic and meso isomers of the nickel POCOP-pincer complexes do not interconvert readily. Reactivity Differences between the Racemic and Meso Isomers. Although P-stereogenic bis(phosphine)-based pincer complexes have been described in the literature,17 the current work represents the first pincer system for which both racemic and meso isomers are reported. Considering that the pincer

Scheme 3. Interconversion between 2-rac and 2-meso

backbone and the substituents on the two phosphorus donors are identical, it is reasonable to assume that the racemic and meso nickel POCOP-pincer complexes have approximately the same electronic properties at the nickel center. However, the spatial arrangements of the phosphorus substituents in the two isomers are quite different. As illustrated in Figure 3, the two

Figure 3. Spatial arrangements of the P-substituents in the racemic and meso nickel POCOP-pincer complexes.

axial sides of the racemic isomer are sterically equivalent, both guarded by a tert-butyl group and a phenyl group. The two axial sides of the meso isomer, on the other hand, are in different steric environments. Inspection of the space-filling models of the related nickel POCOP-pincer chloride complexes (Figure 4) led to the conclusion that the phenyl side of the meso isomer is the less hindered side for an incoming ligand. We were thus curious to see whether these steric differences would influence the reactivity of the nickel complexes. Substitution of the chloride ligand of 2 with tert-butoxide was investigated first. To ensure that epimerization would not take place during ligand substitution, the reaction of pure 2-rac with t-BuOK (2 equiv with respect to 2-rac) in C6D6 was studied by NMR spectroscopy. After 12 h, the racemic nickel tert-butoxide complex rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiO(t-Bu) (6-rac) was obtained quantitatively without noticeable isomerization 3605


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Figure 4. Space-filling models of (left) [2,6-(Ph2PO)2C6H3]NiCl, (center) rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl, and (right) [2,6-(tBu2PO)2C6H3]NiCl.

2-meso would imply that kPhPh + ktButBu > 2kPhtBu or kPhPh − kPhtBu > kPhtBu − ktButBu. In other words, the phenyl side of 2-meso is disproportionately more reactive. Decarboxylation of metal formate complexes is a crucial step in many catalytic processes, such as transfer hydrogenation of alkynes20 and dehydrogenation of formic acid.21 For the nickel POCOP-pincer system, it has been proposed as a kinetically relevant step in catalytic reduction of CO2 with boranes,9b,22 although the reverse step, namely, insertion of CO2 into nickel hydride complexes, is more favorable. Investigating the transitionstate structures for the decarboxylation of nickel formate complexes should lead to a better understanding of the CO2 insertion process, which according to the principle of microscopic reversibility proceeds via the same transition state. The nickel formate complex rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiOCHO (7-rac) was readily available from rac-{2,6[(t-Bu)(Ph)PO]2C6H3}NiOTf (3-rac) and sodium formate (eq 2). Like the reaction between 2-rac and t-BuOK, the configuration of the phosphorus atoms during the synthesis of 7-rac was preserved. A 1:1 7-rac/7-meso mixture was obtained from the 1:1 2-rac/2-meso mixture by treatment with AgOTf to generate a mixture of the triflate complexes 3-rac and 3-meso in situ followed by the addition of sodium formate. In both cases,

to 6-meso. The 1:1 mixture of 2-rac and 2-meso was then treated with t-BuOK similarly (eq 1). Monitoring the reaction over a period of several hours clearly established that 2-meso reacted faster than 2-rac (Figure 5). After 2 h, 73% of 2-meso was converted to 6-meso, whereas only 30% of 2-rac was converted to 6-rac. When 2-meso was fully converted (after ∼5 h of reaction), at least 17% of the 2-rac remained unreacted. For substitution reactions of square-planar complexes, an associative or interchange mechanism is expected, especially in a nonpolar solvent such as benzene. Thus, how easily the tertbutoxide approaches the nickel center should determine how fast the ligand substitution is. If kPhPh, ktButBu, and kPhtBu are the rate constants for the ligand substitution reactions occurring from the phenyl side of 2-meso, the tert-butyl side of 2-meso, and either side of 2-rac, respectively, the higher reactivity of

Figure 5. 31P{1H} NMR spectra of the reaction between 1:1 2-rac/2-meso and t-BuOK. 3606


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Organometallics

The faster decarboxylation of 7-meso can be rationalized by the possibility of pushing the Ni−O bond to the less hindered side while directing the hydrogen to the tert-butyl side of 7-meso (boxed structure in Figure 7). Such a transition-state structure the isolated nickel formate complex was contaminated with a small amount (∼4%) of nickel hydride species, suggesting that the decarboxylation process occurred when the product was dried under vacuum. Heating a C6D6 solution of 1:1 7-rac/7-meso in a J. Young NMR tube at 60 °C for 10 days did not result in any appreciable reaction. This is not surprising because CO2 would be trapped in this closed system and decarboxylation of 7-rac/7-meso could be a thermodynamically uphill process. To drive the reaction in the direction of hydride formation, a solid sample of 1:1 7-rac/7-meso was heated at 90 °C under a dynamic vacuum (eq 3), which removed CO2 from the system. Figure 7. Transition-state structures for the decarboxylation of 7-rac and 7-meso.

is similar to those calculated for CO2 insertion into squareplanar nickel hydride complexes.22b,24

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CONCLUSIONS We have synthesized several P-stereogenic nickel POCOPpincer complexes and have successfully separated the racemic and meso isomers of the chloride complex through repeated recrystallization. The phosphorus centers of these diastereomers are configurationally stable at 110 °C over a prolonged period of time, suggesting either no pincer arm dissociation or very slow pyramidal inversion and P−O bond rotation. In addition, the phosphorus configuration in this POCOP-pincer system is retained during the manipulation of the fourth ligand on nickel, whether it is for substitution of chloride by tert-butoxide or decarboxylation of the nickel formate complex. The high configurational stability provides the prerequisite for the use of these pincer complexes in asymmetric catalysis. We have also shown that the nickel POCOP-pincer complexes in the meso form react faster than the racemic isomers in ligand substitution and decarboxylation reactions. These results provide experimental evidence supporting an axial approach of

After 30 min, the residue was dissolved in C6D6 for NMR analysis. As shown at the bottom of Figure 6, decarboxylation of 7-meso was noticeably faster than that of 7-rac. The formateto-hydride conversion was calculated to be 79% for the meso isomer and 68% for the racemic one. Experimentally probing the transition-state structure for the decarboxylation (or the CO2 insertion) process is challenging. Our kinetic study of the decarboxylation of C2v-symmetric nickel formate complexes [2,6-(R2PO)2C6H3]NiOCHO gave negative ΔS⧧ values, consistent with a crowded transition state directing the formato hydrogen toward the nickel center.23 However, ambiguity arose as to the orientation of the formato group. The reactivity difference observed for 7-rac and 7-meso would argue against equatorial H− abstraction (Figure 7) because the void space at the equatorial site is comparable for these two isomers. The result is more consistent with a transition state involving abstraction of H− through the axial sites.

Figure 6. 31P{1H} NMR spectra of (top) a 1:1 7-rac/7-meso mixture dissolved in C6D6 and (bottom) a solid sample of 1:1 7-rac/7-meso that was heated while being evacuated for 30 min and then dissolved in C6D6. 3607


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167.8 (t, JP−C = 10.6 Hz, ArC). 31P{1H} NMR (162 MHz, CDCl3, δ): 158.0 (s). Synthesis of rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiOTf (3-rac). Under an argon atmosphere, AgOTf (308 mg, 1.2 mmol) was added to a solution of 2-rac (532 mg, 1.0 mmol) in 25 mL of CH2Cl2. The reaction mixture was stirred at room temperature in the dark for 2 h and then filtered through Celite. Removal of the solvent from the filtrate yielded the product as a yellow solid (520 mg, 81% yield). 1 H NMR (400 MHz, CDCl3, δ): 1.32 (vt, JP−H = 8.0 Hz, CH3, 18H), 6.48 (d, JH−H = 8.0 Hz, ArH, 2H), 7.03 (t, JH−H = 8.0 Hz, ArH, 1H), 7.53−7.57 (m, ArH, 6H), 8.00−8.03 (m, ArH, 4H). 13C{1H} NMR (101 MHz, CDCl3, δ): 25.5 (t, JP−C = 3.2 Hz, CH3), 37.2 (t, JP−C = 12.4 Hz, C(CH3)3), 106.7 (t, JP−C = 5.9 Hz, ArC), 112.3 (t, JP−C = 22.0 Hz, ArC), 119.2 (q, JF−C = 319.8 Hz, CF3), 128.8 (t, JP−C = 4.9 Hz, ArC), 129.7 (t, JP−C = 17.2 Hz, ArC), 130.3 (s, ArC), 131.4 (t, JP−C = 6.8 Hz, ArC), 131.8 (s, ArC), 167.9 (t, JP−C = 9.2 Hz, ArC). 31P{1H} NMR (162 MHz, CDCl 3 , δ): 160.5 (s). Anal. Calcd for C27H31O5F3P2SNi: C, 50.26; H, 4.84. Found: C, 50.50; H, 4.68. Synthesis of rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiO(t-Bu) (6-rac). To a J. Young NMR tube containing a solution of 2-rac (13.3 mg, 0.025 mmol) in C6D6 (0.5 mL) was added potassium tert-butoxide (5.6 mg, 0.050 mmol). The color of the reaction mixture changed gradually from yellow to red. The conversion of 2-rac to 6-rac was complete within 12 h, as confirmed by 1H and 31P{1H} NMR spectroscopy. Attempts to isolate 6-rac in solid form led to decomposition of the product, mainly as a result of rapid hydrolysis by adventitious water. 1 H NMR (400 MHz, C6D6, δ): 1.11 (s, OC(CH3)3, 9H), 1.34 (vt, JP−H = 8.0 Hz, PC(CH3)3, 18H), 6.60 (d, JH−H = 8.0 Hz, ArH, 2H), 6.93 (t, JH−H = 8.0 Hz, ArH, 1H), 7.11−7.15 (m, ArH, 6H), 8.15−8.19 (m, ArH, 4H). 13C{1H} NMR (101 MHz, C6D6, δ): 26.6 (s, PC(CH3)3), 35.9 (s, OC(CH3)3), 37.1 (t, JP−C = 10.1 Hz, PC(CH3)3), 69.1 (s, OC(CH3)3), 105.7 (t, JP−C = 6.0 Hz, ArC), 122.7 (t, JP−C = 25.3 Hz, ArC), 128.6 (s, ArC), 130.6 (s, ArC), 132.3 (t, JP−C = 5.6 Hz, ArC), 133.7 (t, JP−C = 14.1 Hz, ArC), 168.3 (t, JP−C = 10.1 Hz, ArC); one resonance was obscured by solvent resonances. 31P{1H} NMR (162 MHz, C6D6, δ): 148.5 (s). Synthesis of rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiOCHO (7-rac). Under an argon atmosphere, sodium formate (51 mg, 0.75 mmol) was added to a solution of 3-rac (400 mg, 0.62 mmol) in 40 mL of THF. After the mixture was stirred at room temperature for 1 h, the volatiles were removed under vacuum, and the residue was extracted with pentane (40 mL first and then 20 mL). The combined pentane extracts were evaporated to dryness, providing the product as a yellow solid (244 mg, 73% yield). 1H NMR (400 MHz, C6D6, δ): 1.20 (vt, JP−H = 8.0 Hz, CH3, 18H), 6.59 (d, JH−H = 8.0 Hz, ArH, 2H), 6.87 (t, JH−H = 8.0 Hz, ArH, 1H), 7.04−7.16 (m, ArH, 6H), 8.25−8.28 (m, ArH, 4H), 8.67 (s, OCHO, 1H). 13C{1H} NMR (101 MHz, C6D6, δ): 24.9 (t, JP−C = 3.0 Hz, CH3), 36.1 (t, JP−C = 12.6 Hz, C(CH3)3), 106.7 (t, JP−C = 5.8 Hz, ArC), 120.9 (t, JP−C = 23.2 Hz, ArC), 128.6 (t, JP−C = 5.0 Hz, ArC), 129.6 (s, ArC), 131.2 (t, JP−C = 16.2 Hz, ArC), 131.4 (s, ArC), 132.2 (t, JP−C = 6.8 Hz, ArC), 168.2 (s, OCHO), 168.3 (t, JP−C = 10.0 Hz, ArC). 31P{1H} NMR (162 MHz, C6D6, δ): 160.0 (s). ATR-IR (solid): νCO = 1629 cm−1. Anal. Calcd for C27H32O4P2Ni: C, 59.92; H, 5.96. Found: C, 60.03; H, 6.01. Synthesis of {2,6-[(t-Bu)(Ph)PO]2C6H3}NiOCHO (7-rac:7-meso = 1:1). The 1:1 mixture of 7-rac/7-meso was prepared in 72% yield from 2-rac/2-meso by generation of a mixture of the triflate complexes 3-rac and 3-meso in situ followed by the addition of sodium formate. The workup procedures were the same as those used for the purification of 7-rac. 1H NMR of 7-meso (400 MHz, C6D6, δ): 1.31 (vt, JP−H = 7.6 Hz, CH3, 18H), 6.59 (d, JH−H = 8.0 Hz, ArH, 2H), 6.87 (t, JH−H = 8.0 Hz, ArH, 1H), 6.97−7.04 (m, ArH, 6H), 8.03−8.07 (m, ArH, 4H), 8.65 (s, OCHO, 1H). 13C{1H} NMR of 7-meso (101 MHz, C6D6, δ): 24.8 (s, CH3), 36.7 (t, JP−C = 12.4 Hz, C(CH3)3), 106.5 (t, JP−C = 5.8 Hz, ArC), 120.9 (t, JP−C = 16.9 Hz, ArC), 129.6 (s, ArC), 131.2 (s, ArC), 131.4 (t, JP−C = 16.2 Hz, ArC), 131.7 (t, JP−C = 6.8 Hz, ArC), 168.4 (t, JP−C = 10.0 Hz, ArC), 168.6 (s, OCHO); one resonance was obscured by solvent resonances. 31P{1H} NMR of 7-meso (162 MHz, C6D6, δ): 158.7 (s).

an incoming ligand (including the formato hydrogen) to the nickel center. Our future study will be focused on the resolution of racemic isomers of P-stereogenic nickel POCOP-pincer complexes for the development of enantioselective catalysts, particularly for the hydrosilylation of ketones18c and Michael addition of secondary phosphines to enones.11c,d

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EXPERIMENTAL SECTION

General Comments. All of the organometallic compounds were prepared and handled under an argon atmosphere using standard Schlenk and inert-atmosphere box techniques. Dry and oxygen-free solvents [tetrahydrofuran (THF), pentane, toluene, and CH2Cl2] were collected from an Innovative Technology solvent purification system and used throughout all experiments. C6D6 was distilled from Na and benzophenone under an argon atmosphere. CDCl3 was purchased from Cambridge Isotope Laboratories, kept under an argon atmosphere, and used without further purification. 1H, 13C{1H}, and 31 1 P{ H} NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Chemical shift values in the 1H and 13C{1H} NMR spectra were referenced internally to the residual solvent resonances. Chemical shifts in the 31P{1H} spectra were referenced externally to 85% H3PO4 (0 ppm). Infrared spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR spectrometer equipped with a Smart Orbit diamond attenuated total reflectance (ATR) accessory. Synthesis of 1,3-[(t-Bu)(Ph)PO]2C6H4 (1). Under an argon atmosphere, a suspension of sodium hydride (252 mg, 10.5 mmol) in 20 mL of THF was added slowly to a solution of resorcinol (550 mg, 5.0 mmol) in 40 mL of THF. The resulting mixture was refluxed for 6 h. After the mixture was cooled to room temperature, a solution of PhP(t-Bu)Cl (1.98 mL, 10.5 mmol) in 20 mL of THF was added, and the mixture was refluxed for another 6 h. The volatiles were removed under reduced pressure, and the residue was extracted with pentane (60 mL first and then 20 mL). Evaporation of pentane yielded the product as a light-yellow oil (1.86 g, 85% yield). 1H NMR (400 MHz, CDCl3, δ): 1.03 (d, JP−H = 13.2 Hz, CH3, 18H), 6.70 (d, JH−H = 8.4 Hz, ArH, 2H), 6.86 (s, ArH, 1H), 7.06 (t, JH−H = 8.4 Hz, ArH, 1H), 7.35−7.38 (m, ArH, 6H), 7.49−7.52 (m, ArH, 4H). 13 C{1H} NMR (101 MHz, CDCl3, δ): 25.0 (d, JP−C = 16.2 Hz, CH3), 33.4 (d, JP−C = 13.1 Hz, C(CH3)3), 109.56 (t, JP−C = 9.1 Hz, ArC2 of one isomer), 109.62 (t, JP−C = 9.1 Hz, ArC2 of the other isomer), 112.3 (d, JP−C = 11.1 Hz, ArC), 127.9 (d, JP−C = 7.1 Hz, ArC), 129.4 (s, ArC), 129.8 (s, ArC), 130.4 (d, JP−C = 22.2 Hz, ArC), 138.5 (d, JP−C = 29.3 Hz, ArC), 159.0 (d, JP−C = 9.1 Hz, ArC). 31P{1H} NMR (162 MHz, CDCl3, δ): 127.5 (s), 127.6 (s). Synthesis of {2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2). Under an argon atmosphere, 1 (1.84 g, 4.2 mmol) and anhydrous NiCl2 (545 mg, 4.2 mmol) were mixed with 40 mL of toluene. The resulting mixture was heated to reflux for 36 h. Removal of the solvent under vacuum gave a yellow solid, which was washed with cold pentane (10 mL). After drying, the desired product (2-rac:2-meso = 1:1) was isolated as a yellow powder (1.58 g, 71% yield). rac-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2-rac). 1H NMR (400 MHz, CDCl3, δ): 1.30 (vt, JP−H = 8.0 Hz, CH3, 18H), 6.52 (d, JH−H = 8.0 Hz, ArH, 2H), 6.99 (t, JH−H = 8.0 Hz, ArH, 1H), 7.48−7.50 (m, ArH, 6H), 8.13−8.15 (m, ArH, 4H). 13C{1H} NMR (101 MHz, CDCl3, δ): 25.4 (s, CH3), 37.0 (t, JP−C = 13.0 Hz, C(CH3)3), 106.1 (t, JP−C = 6.1 Hz, ArC), 124.3 (t, JP−C = 22.3 Hz, ArC), 128.3 (t, JP−C = 4.5 Hz, ArC), 129.1 (s, ArC), 130.9 (t, JP−C = 16.8 Hz, ArC), 131.1 (s, ArC), 131.8 (t, JP−C = 6.2 Hz, ArC), 167.7 (t, JP−C = 10.4 Hz, ArC). 31P{1H} NMR (162 MHz, CDCl3, δ): 158.9 (s). Anal. Calcd for C26H31O2P2NiCl: C, 58.74; H, 5.88. Found: C, 58.90; H, 6.06. meso-{2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2-meso). 1H NMR (400 MHz, CDCl3, δ): 1.37 (vt, JP−H = 8.0 Hz, CH3, 18H), 6.53 (d, JH−H = 8.0 Hz, ArH, 2H), 7.00 (t, JH−H = 8.0 Hz, ArH, 1H), 7.42−7.46 (m, ArH, 6H), 8.10−8.12 (m, ArH, 4H). 13C{1H} NMR (101 MHz, CDCl3, δ): 25.5 (t, JP−C = 2.7 Hz, CH3), 37.5 (t, JP−C = 12.5 Hz, C(CH3)3), 105.9 (t, JP−C = 6.1 Hz, ArC), 124.2 (t, JP−C = 22.2 Hz, ArC), 128.2 (t, JP−C = 4.8 Hz, ArC), 129.1 (s, ArC), 130.7 (t, JP−C = 17.5 Hz, ArC), 131.1 (s, ArC), 131.7 (t, JP−C = 6.3 Hz, ArC), 3608


Organometallics

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Ligand Substitution Reaction of 2 with tert-Butoxide. In a J. Young NMR tube, the 1:1 mixture of 2-rac and 2-meso (13.3 mg, 0.025 mmol) was dissolved in 0.6 mL of C6D6, resulting in a yellow solution. Potassium tert-butoxide (5.6 mg, 0.050 mmol) was then added and mixed thoroughly with the solution. The progress of the reaction was monitored by 1H and 31P{1H} NMR spectroscopy. Decarboxylation of the Nickel Formate Complex To Form the Nickel Hydride Complex. A J. Young NMR tube was loaded with a 1:1 mixture of nickel formate complexes 7-rac and 7-meso (13.5 mg, 0.025 mmol) and then attached to a vacuum line. The sample was subjected to continuous evacuation (to remove any gases or volatiles that were generated) while being heated at 90 °C with an oil bath. After 30 min, the residue was dissolved in C6D6 (∼0.5 mL), and the NMR spectra were recorded. Extended heating resulted in decomposition of the hydride species. Attempts to isolate the nickel hydride complex 8-rac or a mixture of 8-rac and 8-meso in analytically pure form failed because of instability of the hydride complex. 1 H NMR of 8-rac (400 MHz, C6D6, δ): −7.14 (t, JP−H = 52.0 Hz, NiH, 1H), 1.14 (vt, JP−H = 8.0 Hz, CH3, 18H), 6.94 (d, JH−H = 8.0 Hz, ArH, 2H), 7.04−7.11 (m, ArH, 7H), 8.08−8.13 (m, ArH, 4H). 13 C{1H} NMR of 8-rac (101 MHz, C6D6, δ): 25.7 (t, JP−C = 3.8 Hz, CH3), 35.3 (t, JP−C = 14.8 Hz, C(CH3)3), 105.7 (t, JP−C = 6.2 Hz, ArC), 129.4 (s, ArC), 130.8 (s, ArC), 132.6 (t, JP−C = 18.2 Hz, ArC), 133.2 (t, JP−C = 7.4 Hz, ArC), 141.7 (t, JP−C = 17.6 Hz, ArC), 167.5 (t, JP−C = 10.6 Hz, ArC); one resonance was obscured by solvent resonances. 31P{1H} NMR of 8-rac (162 MHz, C6D6, δ): 193.8 (s). Hydride 1H NMR resonance of 8-meso (400 MHz, C6D6, δ): −7.21 (t, JP−H = 52.0 Hz). 31P{1H} NMR of 8-meso (162 MHz, C6D6, δ): 193.3 (s). X-ray Structure Determination. Single crystals of 2-rac were grown by slow evaporation of a saturated solution in CH2Cl2 at room temperature. Crystal data collection and refinement parameters are summarized in Table S1 in the Supporting Information. Intensity data were collected at 150 K on a Bruker SMART6000 CCD diffractometer using graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å). The data frames were processed using the program SAINT. The data were corrected for decay, Lorentz, and polarization effects as well as absorption and beam corrections based on the multiscan technique. The structures were solved by a combination of direct methods in SHELXTL and the difference Fourier technique and refined by fullmatrix least-squares procedures. Non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms were either located directly or calculated and subsequently treated with a riding model. No solvent of crystallization was present in the lattice.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Complete details of the crystallographic study (PDF, CIF, and xyz formats), NMR spectra of nickel pincer complexes, and experimental details of the separation of stereoisomers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00402.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0952083), the Alfred P. Sloan Foundation (Research Fellowship to H.G.), and the University of Cincinnati (Doctoral Enhancement Research Fellowship to A.A.) for support of this research. Crystallographic data were collected on a Bruker SMART6000 diffractometer, which was funded by an NSF-MRI Grant (CHE-0215950). 3609


Article

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Configurational Stability and Stereochemistry of P-Stereogenic Nickel

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