Correlating the Activity of Rhodium(I)-Phosphite-Lariat Ether Styrene

Article pubs.acs.org/Organometallics

Correlating the Activity of Rhodium(I)-Phosphite-Lariat Ether Styrene Hydroformylation Catalysts with Alkali Metal Cation Binding through NMR Spectroscopic Titration Methods Justin R. Martin, Ethan C. Cagle, Aaron L. Lucius, and Gary M. Gray* Department of Chemistry, University of Alabama at Birmingham, 901 14th Street South, Birmingham, Alabama 35294-1240, United States S Supporting Information *

ABSTRACT: Alkali metal salts can affect both the activities and regioselectivities of alkene hydroformylation catalysts containing polyether-functionalized phosphorus-donor ligands; however, it is unclear whether these effects arise from direct alkali metal cation binding to the active catalysts. To gain more insight into these effects, a series of phosphite-lariat ether ligands derived from the alkali metal cation binding agents 2-hydroxymethyl-12-crown-4 and 2hydroxymethyl-15-crown-5 have been prepared. Rhodium(I) complexes of these ligands have been evaluated as styrene hydroformylation catalysts in the absence and presence of a variety of alkali metal salts. The activities of catalysts containing phosphites derived from 2,2′-biphenol or 1,1′-binaphthol increased significantly (up to 92%) in the presence of alkali metal cations that are “moderately oversized” for archetypal binding to the crown cavity. When this criterion are not met, a decrease in the catalytic activity is observed upon addition of an alkali metal salt. NMR titrations (31P{1H} and 1H) of two model cis-Mo(CO)4(phosphite-lariat)2 complexes in which the phosphite was derived from 2,2′-biphenol were carried out to gain insight into the manner in which the alkali metal cations interact with the ligands. Both model complexes bind Li+ through a 2:1 two-site binding mechanism, and the model complex with the larger crown ether also binds Na+ in this fashion. In contrast, 1:1 complexes are formed upon Na+ and K+ binding to the model complex containing the smaller crown ether and upon K+ binding to the model complex containing the larger crown ether. Correlation between increases in catalyst activity and binding mode in complexes containing cations “moderately oversized” for archetypal binding to the crown cavity strongly suggests that the increases are due to a specific type of alkali metal cation binding by the lariat ether groups in these catalysts.

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INTRODUCTION Alkene hydroformylation is a highly useful industrial method for converting alkenes into a mixture of isomeric aldehydes through the catalytic addition of a formaldehyde group (HCHO) across the double bond of an alkene. As shown in Scheme 1, the formyl group has the potential to add either to

for hydroformylation catalysis because they are predisposed to yield the chiral iso aldehyde product. Subsequent oxidation of the branched aryl aldehyde to the corresponding 2arylpropionic acid then yields popular analgesics such as ibuprofen, ketoprofen, and naproxen.1,3 Attempts to improve aryl alkene hydroformylation have largely focused on the simplest aryl alkene, styrene (R = phenyl), and have predominantly involved bulky bis(phosphite), bis(phosphine), and bis(phospholane) ligands.3−9 A common approach to the design of more active and selective alkene hydroformylation catalysts has been to perturb the environment of the phosphorus-donor ligands, namely, through changes in their steric and electronic effects.1,10,11 One recently examined approach to this perturbation has been the use of phosphorus-donor ligands with functional groups that have the ability to bind alkali metal cations. This particular method is of interest because a bound alkali metal cation could perturb the ligand environment of the active Rh(I) transition metal center either through direct interactions with the electron

Scheme 1. General Reaction Scheme for Alkene Hydroformylation

the terminal position of the double bond, forming the linear (n) aldehyde, or to the internal position, forming the chiral branched (iso) aldehyde. This process is of great commercial interest due to the fact that it is responsible for the production of more than 15 billion tons of aldehydes annually.1,2 Furthermore, aryl alkenes (R = aryl) are attractive substrates © XXXX American Chemical Society

Received: April 22, 2016

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DOI: 10.1021/acs.organomet.6b00325 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

obtained with the Rh(I) procatalyst, as well as with a Rh(I) complex of 2,2′-(C12H8O2)POCH3, a ligand that contains no cation binding moiety. The alkali metal salts chosen for study are LiBPh4·3dme (dme = 1,2-dimethoxyethane), NaBArF (BArF− = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), and KB(C6F5)4. These salts are soluble in organic solvents, have noncoordinating anions, and provide alkali metal cations with sufficiently different ionic radii to coordinate to the crown ethers in both archetypal and nonarchetypal coordination environments.35 Third, NMR spectroscopic titration methods have been used to evaluate alkali metal cation binding to cisMo(CO)4(ligand)2 model complexes. This binding was also studied by FTIR spectroscopy to observe any interactions of alkali metal cations with the carbonyl ligands in the model complexes.

pairs on the carbonyl or acyl intermediates in the catalytic cycle12−16 or through a change in the conformation of the phosphorus-donor ligands upon binding of the alkali metal cation.17−20 McLain first investigated the effect of various amounts of alkali metal salt cocatalysts on alkene hydroformylation by using catalysts containing phosphines functionalized with azacrown lariat ethers.21,22 He reported that the activities of these metallalariat catalysts were increased in the presence of alkali metal salts; however, the mechanism by which this occurred was not described. Further, because all the alkali metal salts studied had approximately the same effect on the activities of the catalysts, it was unclear whether the increases in activities were due to direct alkali metal cation binding by the phosphine azacrown lariat ligands or to a more general effect, such as an increase in the ionic strength of the solution. More recently, the groups of Gray,23−26 Pereira,27 and Vidal-Ferran20,28−30 have used a slightly different series of bis(phosphite) ligands containing a polyether linker to generate metallacrown ether catalysts capable of binding alkali metal cations. The combined results of these inquiries show that when alkali metal salts are added to the styrene hydroformylation system containing these catalysts, three effects are often observed: (1) the activity is decreased; (2) the iso selectivity is increased (1−14%); and (3) in some cases the enantioselectivity is increased. Recent computational studies have suggested that the increases in regioselectivities and enantioselectivities may arise from changes in the P−Rh−P bite angle upon cation binding to the polyether backbone.20 An additional study performed by Sheff et al. demonstrated that cation binding by metallacrown ether model systems does not occur in tetrahydrofuran (THF), which was used in nearly all of these hydroformylation studies.31 In a related study, Li et al. reported a 17% increase in the enantioselectivities of metallacrown ether catalysts in alkene hydrogenation in the presence of alkali metal salts. This increase appeared to arise from coordination of the alkali metal cation to the metallacrown ether catalyst because it can be reversed by the addition of a more tightly binding crown ether to the reaction mixture.32 It is necessary to use ligands with donor groups that exhibit tight cation binding under the catalytic reaction conditions to more clearly investigate the effect of alkali metal binding to hard base donor sites in hydroformylation catalysts containing functional phosphorus-donor ligands. The bis(phosphite)polyether ligands that have been studied in most detail are not ideal for this because the metallacrown ethers that they form do not exhibit sufficiently strong alkali metal cation binding under the catalytic reaction conditions.28,31 This is most likely due to the fact that the polyether bridge between the bis(phosphorus-donor) groups in metallacrown ethers is too flexible and not as entropically preorganized as are organic crown ethers. In contrast, transition metal complexes formed by the coordination of phosphorus-functionalized lariat ethers may be able to overcome this problem due to the enhanced alkali metal cation binding affinities of the lariat ether moieties.22,33,34 The research described in this article has three phases. First, four phosphite-lariat ether ligands derived from 2-hydroxymethyl-12-crown-4 or 2-hydroxymethyl-15-crown-5 have been synthesized and characterized. Second, styrene hydroformylation reactions with Rh(I) catalysts containing these ligands have been carried out in both the absence and presence of alkali metal salts. Catalytic results obtained with Rh(I) complexes of the phosphite-lariat ether ligands are compared to those

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RESULTS AND DISCUSSION Synthesis and NMR Characterization of PhosphiteLariat Ether Ligands L1−L5. The phosphite-donor ligands L1−L5 were prepared according to Scheme 2. Singlet 31P{1H} Scheme 2. Synthesis of Phosphite-Lariat Ether Ligands L1− L5

NMR resonances are observed for L1−L4, and these are approximately 40 ppm upfield from their respective phosphochlorodite ester precursors. Two singlets are observed in the 31 1 P{ H} NMR spectrum of L5, due to the presence of diastereomers. The 1H NMR spectra of L2−L5 exhibit a complex multiplet in the 3.80−4.00 ppm region that simplifies when decoupled from 31P, indicating that the methylene protons in the lariat arm are deshielded relative to those in the crown ether portion of the molecule. The crown ether protons are observed as a series of multiplets that integrate to 15H and 19H for the ligands derived from 2-hydroxymethyl-12-crown-4 and 2-hydroxymethyl-15-crown-5, respectively. Methine resonances were assigned based on two-dimensional 1H−1H COSY and 1H−13C HSQC spectra, which are given in the Supporting Information. The 13C{1H} NMR spectra of L2−L5 display a resonance for each crown ether carbon in the 70.00−71.00 ppm range. Both the methine and methylene carbons are observed as doublets due to respective |3JPC| and |2JPC| coupling. These resonances are positioned upfield and downfield from the crown ether carbons, respectively, and were assigned based on B


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on the reaction conditions.36−38 To gain some insight into the nature of the active catalyst, the hydroformylation reactions with the catalysts formed by the in situ reaction of L2 and Rh(CO)2(acac) under syngas at higher ligand to Rh ratios (4.8 and 9.6 equiv of L2 relative to Rh(CO)2(acac)) were studied. The regioselectivity of the reaction (80% iso) was independent of the [L2]/[Rh] ratio, while the activities decreased drastically as the [L2]/[Rh] ratio increased ([L2]/[Rh] = 2.4; k = 8.3 × 10−4 s−1; [L2]/[Rh] = 4.8; k = 0.8 × 10−4 s−1; [L2]/[Rh] = 9.6; k = 0.2 × 10−4 s−1). The fact that the increase in phosphite ligand concentration does not affect selectivity but severely lowers the catalyst activity suggests that the active catalyst is formed at low [L2]/Rh ratios and that the addition of additional ligand either converts this into a less active species or acts as a inhibitor for the active catalyst. The latter seems more likely in view of the lack of any effect of added ligand on the regioselectivity. A recent report by Vidal-Ferran and co-workers has demonstrated that RhH(CO)2(phosphite)2 forms under similar conditions to those used in our catalytic reactions,28 so it seems possible that the active catalysts are complexes of this type. There are only slight changes in the regioselectivities of styrene hydroformylation reactions catalyzed by Rh(I) complexes of ligands L1−L5. The data show that the bulky lariat moieties decrease the regioselectivities (approximately 81% with L2 or L3) when compared to a simple methoxy group (86% with L1). Similarly, the use of bulkier diol-derived substituents on the phosphorus-donor groups also decreases the regioselectivities of these catalysts (75% with L4 and 79% with L5). These results suggest that, for this particular reaction, less congestion about the phosphorus-donor group produces more selective catalysts. The addition of alkali metal salts only slightly affects the regioselectivities, and in most cases this effect results in a decrease in regioselectivity. It is interesting that the effect of alkali metal salt addition to catalytic systems with ligands able to bind these cations (L2−L5) seems to depend on the relative sizes of the metal cations and ligand cavities. Namely, significant decreases in regioselectivity are observed only when the metal cation is too large to fit into the crown cavity of the ligand.35 For instance, Li+ seems to have little effect on the regioselectivities of catalysts containing L1 or L2, while Na+ decreases the regioselectivities of the catalysts containing ligands derived from 2-hydroxymethyl-12-crown-4 (L2, L4, and L5), but has no effect on that of L3, which is derived from 2-hydroxymethyl-15-crown-5. Furthermore, K+ causes decreases in the regioselectivities of both L2 and L3, having a slightly greater effect on L3. The regioselectivity results are quite different from those reported for related catalysts in the literature. For instance, addition of alkali metal salts to metallacrown ether catalysts under identical reaction conditions generally caused low to moderate (1−14%) increases in the regioselectivities.20,23−28,39 McLain did not report regioselectivity data in his study of catalysts with azacrown ether tethered phosphine ligands, which also used a different substrate and very different reaction conditions.22 The relative sizes of the ligand cavities and alkali metal cations used35 have significant effects on the activities of the Rh(I) hydroformylation catalysts. For example, the activity of the catalyst containing the 2-hydroxymethyl-12-crown-4derived L2 decreases in the presence of either 1:1 or 2:1 mol ratios of either Li+ or K+. In contrast, the activity of the catalyst containing L2 sharply increases by 69−92% in the presence of a

two-dimensional 1H−13C HSQC spectra, which are given in the Supporting Information. Because no unexpected resonances were observed in the NMR spectra, the ligands were used as-is in styrene hydroformylation reactions and the syntheses of the Mo(CO)4 complexes 1 and 2 without further purification. Styrene Hydroformylation Catalyzed by Rh(I) Complexes of L1−L5. The activities and regioselectivities obtained with Rh(I) catalysts containing the phosphorus-donor ligands L1−L5 in both the absence and presence of alkali metal salts are given in Table 1. Two catalytic runs were performed for Table 1. Results of Variable Alkali Metal Salt Content in Styrene Hydroformylation Catalyzed by Rh(I) Complexes of L1−L5a ligand none L1

L2

L3

L4 L5

c

[M+]/[Rh] none 1 Li+ none 1 Li+ 1 Na+ 1 K+ none 1 Li+ 2 Li+ 1 Na+ 2 Na+ 1 K+ 2 K+ none 1 Li+ 2 Li+ 1 Na+ 2 Na+ 1 K+ 2 K+ none 1 Na+ None 1 Na+

% iso 47 48 86 83 86 86 82 81 81 78 79 79 78 81 81 82 80 82 77 77 75 76 79 75

(47) (48) (86) (82) (85) (87) (81) (81) (81) (78) (80) (78) (78) (80) (81) (82) (81) (82) (77) (79) (75) (77) (79) (75)

k × 104 (s−1) 0.76 0.13 8.3 7.8 3.4 6.1 8.3 7.0 5.0 13 5.1 6.6 3.9 8.1 5.9 6.0 4.9 5.8 11 4.4 8.4 6.2 6.8 11

(0.88) (0.17) (8.2) (7.7) (3.1) (6.1) (7.3) (6.7) (5.4) (14) (4.5) (7.2) (4.5) (8.4) (6.6) (5.2) (5.6) (6.2) (11) (4.9) (8.4) (5.2) (7.2) (12)

TOFob × 10−3 0.27 0.047 3.0 2.8 1.2 2.2 3.0 2.5 1.8 4.7 1.8 2.4 1.4 2.9 2.1 2.2 1.8 2.1 4.0 1.6 3.0 2.2 2.4 4.0

(0.32) (0.061) (3.0) (2.8) (1.1) (2.2) (2.6) (2.4) (1.9) (5.0) (1.6) (2.6) (1.6) (3.0) (2.4) (1.9) (2.0) (2.2) (4.0) (1.8) (3.0) (1.9) (2.6) (4.3)

a Results for duplicate experiments are shown in parentheses. Conditions (all ratios are molar): solvent = dichloromethane, CO/ H2 = 1:1, T = 80 °C, P = 20 atm, substrate/Rh = 1000:1, ligand/Rh = 2.4:1, % conversion > 99.99%. The pseudo-first-order rate constant, k, was obtained from a first-order fit of pressure drop versus time using Graphical Analysis software. 1H NMR was used to determine % iso after a pressure change was no longer observed. No hydrogenation was observed. bPseudo-first-order rate constants, k, were converted to initial turnover frequencies (TOF) using the equation TOFo = k[S]o/ [C], where [S]o is the initial substrate concentration and [C] is the catalyst concentration. By convention, the pseudo-first-order rate constants were first converted to units of h−1 to provide TOF = mol substrate/mol Rh/h. cThe only pro-catalytic species present is Rh(CO)2(acac).

each set of reaction conditions in order to ensure reproducibility. In all cases, the differences in rate constants and regioselectivities were less than 14% and 2%, respectively. The activities and regioselectivities were also measured under the same conditions in the absence of any phosphite ligand to determine the effects of the ligands. The active catalyst that forms in situ is not clearly known but possibilities include RhH(CO)2+n(phosphite)2−n (n = 1, 0), HRh(CO)n (n = 3, 4), and rhodium carbonyl clusers depending C


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Organometallics 1:1 mol ratio of Na+, but then decreases again in the presence of a 2:1 mol ratio of Na+. A very similar trend is observed with the activity of the Rh(I) catalyst containing the 2hydroxymethyl-15-crown-5-derived L3. A 1:1 mol ratio of K+ sharply increases the activity of this catalyst by 31−36%, but all other alkali metal salts, as well as a 2:1 mol ratio of K+, cause a decrease in the activity. These results suggest that interactions between catalysts containing L2 and L3 and alkali metal cations are quite different when the alkali metal cations are “moderately oversized” to adequately fit into the crown cavity of the ligands.35 This effect also appears to be dependent, at least in part, on the phosphorus-donor group substituents, as the trend is not observed with the catalyst containing the 3,3′,5,5′-tetratert-butylbiphenyl-2,2′-diyl-derived L4, but is observed with the catalyst containing the 1,1′-binaphthalene-derived L5 (activity increases by 53−76%), even though both of these ligands contain the same 12-crown-4 crown moiety. Finally, the Rh(I) catalyst containing L1, which contains no cation binding moiety, exhibits little change in activity in the presence of Li+ and decreases in activity in the presence of both Na+ and K+. This further supports the suggestion that the increases in catalytic activity are due to favorable interactions between the alkali metal cations and the crown ether moieties in ligands L2 and L3. McLain observed that propene hydroformylation carried out with a family of azacrown ether tethered phosphine ligands, which had various phosphorus-donor groups and tether lengths, had increased initial reaction rates in the presence of the alkali metal salt NaPF6, and to a lesser extent LiPF6 (reported in initial turnover frequencies (TOF)), compared to runs carried out in the absence of the alkali metal salts.22 However, unlike the results reported in this paper, McLain observed that the addition of the alkali metal salts resulted in approximately the same activities for all of the catalysts, despite the ligand used. Further, no study was carried out with a ligand that was not capable of binding alkali metal cations, which would be useful in determining if the increased activity of the catalyst was due to cation binding or to some other effect, such as the increased ionic strength of the solution. Alkali metal salts also have quite different effects on the activities of Rh(I) metallacrown ether catalysts20,23−25,28 and Rh(I) catalysts with related monodentate phosphite ligands40 than they do on the activities of the Rh(I) catalysts in this study. In general, the activities of the catalysts decrease with increasing alkali metal salt. Also, Li+ and Na+ salts have quite similar effects that do not appear to be a function of the size of the metallacrown ether ring. Rationale for Mo(CO)4L2 Model Complexes of L1, L2, and L3. The activities of hydroformylation catalysts formed by the reactions of Rh(CO)2(acac) with L2 or L3 under syngas can be dramatically increased by the addition of alkali metal salts. These increases occur when the alkali metal cation is “moderately oversized” relative to the crown cavity of the ligand. Insight into this effect may be gained from studies of cation binding to transition metal complexes of these ligands. The catalysts are not ideal for this because the Rh(I) can exhibit more than one coordination geometry and is somewhat labile to ligand exchange. Previous studies of alkali metal cation binding with metallacrown ethers have used octahedral cisMo(CO)4(L)2 model complexes.23,31,41 These complexes are ideal because they contain two crown ethers necessary for some alkali metal cation binding modes and have an inert octahedral coordination geometry with cis-coordinated phosphorus-donor

ligands. Therefore, we have prepared cis-Mo(CO)4(L)2 (L = L1, L2, or L3) model complexes in order to investigate the potential binding interactions between the various alkali metal cations used in the styrene hydroformylation experiments and the coordinated ligands. Synthesis and NMR Characterization of Model Complexes 1, 2, and 3. The synthesis of cis-Mo(CO)4(2,2′-(C12H8O2)POCH3)2, 1, is reported.42 Complexes 2 and 3 were prepared according to Scheme 3. These reactions Scheme 3. Synthesis of Model Complexes 2 and 3

were carried out by vigorously stirring the cis-Mo(CO)4(nbd) precursor and the appropriate ligand in a 1:1 mol ratio in a solution of dry dichloromethane (DCM) for approximately 20 min. After this time, the solvent was immediately removed in vacuo in order to prevent the formation of polymeric norbornadiene impurities. Any impurities that were formed were removed via hexane trituration and sequential recrystallization procedures. Complexes 2 and 3 each display singlets at approximately 170.30 ppm in their respective 31P{1H} NMR spectra. The 1H NMR spectra of these complexes are very similar to their respective ligand counterparts and did not indicate any significant overall change in the crown conformation upon coordination to a transition metal. The 13C{1H} NMR spectra of each complex display the expected apparent quintet (A portion of an AXX′ spin system) and triplet (A portion of an AX2 spin system) resonances in the downfield carbonyl region that are consistent with cis coordination of two equivalent phosphorus-donor ligands to a Mo(CO)4 center. It is interesting to note that, because the phosphite-lariat ether ligands are chiral, distinct 31P{1H} NMR resonances of two diastereomeric complexes are observed in highly resolved 31 1 P{ H} NMR spectra (Figure 1) of complexes 2 and 3.

Figure 1. 31P{1H} NMR spectrum (500 MHz) of 2 showing the presence of diastereomers. D


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Figure 2. Effects of Li+ binding to 2 and 3. (Upper) 31P{1H} NMR stack plot depicting observed changes in the chemical shift of 2 (left) and 3 (right) upon addition of LiBPh4·3dme. (Lower) Nonlinear least-squares fit of normalized 31P{1H} signal change [(σo − σi)/(σf − σi)] of 2 (left) and 3 (right) vs molar equivalent of LiBPh4·3dme. Blue and red data points are experimental titration data, and the solid lines represent nonlinear leastsquares regressions of the titration data using eq 1.

Investigation of Alkali Metal Cation Binding to 1−3 Using 31P{1H} and 1H NMR Spectroscopies. To better understand the trends in the catalytic data (Table 1), interactions of the salts used in the hydroformylation experiments (LiBPh4·3dme, NaBArF, and KB(C6F5)4), as well as a salt with a noncoordinating cation (nBu4NBPh4), with the model complexes 1, 2, and 3 have been studied using NMR spectroscopic titration methods. The titrations were performed by adding aliquots of a solution containing 1, 2, or 3 and a large excess of the appropriate salt in dichloromethane-d2 to a solution containing only 1, 2, or 3 (10 mM) in dichloromethane-d2 in a 5 mm gastight J-Young NMR tube. Dichloromethane-d2 was used to provide the best possible comparison between the binding investigations and the hydroformylation experiments. No changes were observed in titration experiments performed with the noncoordinating salt n Bu4NBPh4 and 1−3, indicating that any change in the NMR spectra of 1−3 upon addition of the alkali metal salts is not due to ionic strength effects and thus must be due to binding interactions between the alkali metal cations and the complexes. Additionally, no changes in NMR spectra were observed in

titration experiments performed with 1 and the alkali metal salts studied. This suggests that any change in the NMR spectra in titration experiments carried out with 2 and 3 must be due to cation binding to the lariat crown ether groups in these complexes. Thus, the following sections will primarily focus on interactions between the alkali metal cations and 2 and 3. Li+ Binding to 2 and 3 Followed by 31P{1H} and 1H NMR Spectroscopies. Binding of Li+ by the model complexes 2 and 3 results in very different 31P{1H} NMR spectra for the two complexes, as depicted in Figure 2. For instance, the 31 1 P{ H} NMR spectra of 2 (upper left portion of Figure 2) display a complex chemical shift change (Δδ) of 2 upon titration with LiBPh4·3dme. Upon initial addition of LiBPh4· 3dme, the superimposed singlet resonances of the two diastereomers of free 2 (δfree,2) shift in opposite directions. This trend continues with increasing concentration of LiBPh4· 3dme until a 1:1 molar ratio of Li+:2 is reached (δMX,2), at which point the Δδ of the downfield resonance reverses path, eventually becoming superimposed on the upfield resonance at Li+:2 molar ratios greater than 2:1. This superimposed resonance, representing the fully bound Li+:2 species (δbound,2), E


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Organometallics Table 2. Li+ Binding Association Constants to 2 and 3 According to Eq 1 2 β1 β2

downfield

upfield

3

2.5(0.5) × 108 M−1 1.2(0.4) × 1019 M−2

2.5(0.4) × 108 M−1 3.3(1.1) × 1019 M−2

7.9(0.1) × 109 M−1 3.4(0.5) × 1022 M−2

is approximately 0.8 ppm upfield from that of δfree,2. In stark contrast to this, the titration of 3 with LiBPh4·3dme (upper right portion of Figure 2) exhibits much simpler behavior. The superimposed singlet 31P{1H} NMR resonances of the two diastereomers of free 3 (δfree,3) both shift downfield with increasing LiBPh4·3dme concentration until a 1:1 molar ratio of Li+:3 is reached (δMX,3), at which point they reverse path and begin to shift upfield. The final singlet resonance of the fully bound Li+:3 species (δbound,3) is approximately 0.2 ppm upfield from that of δfree,3. The directions in which the resonances of both diastereomers of 3 shift with increasing LiBPh4·3dme parallel those of the downfield resonance in the titration of 2 with LiBPh4·3dme. The lower portion of Figure 2 depicts the nonlinear leastsquares fitting of the 31P{1H} NMR titrations of 2 (lower left) and 3 (lower right) with LiBPh4·3dme according to eq 1. The binding analyses are indicative of two successive binding events with an overall 2:1 stoichiometry between Li+ and 2, as well as between Li+ and 3. This mechanism is consistent with the structures of both 2 and 3, each of which contains two lariat ether moieties capable of binding an alkali metal cation. The observed binding affinity constants (β1 and β2) for 2 and 3 are given in Table 2 (association constants were calculated for both the downfield and upfield resonances of 2). The high affinity constants, along with the sharp curvatures at 1:1 and 2:1 concentrations in both graphs in the lower portion of Figure 2, indicate that stoichiometric binding conditions dominate throughout the experiments and, thus, that the calculated binding affinities represent only upper limits of the Kd.43−46 This suggests that stoichiometric binding should also be observed under catalytic concentrations (3.5 × 10−5 M) because the same number and type of binding sites are present in the catalysts and the model complexes. The observation of separate 31P{1H} NMR resonances for the diastereomers of 2 (Figure 2) upon titration with LiBPh4· 3dme suggests that the two 31P{1H} NMR resonances observed during the titration may be due to different sensitivities of the phosphorus nuclei to Li+ binding in the 1:1 adducts. This suggestion is supported by the fact that two 31P{1H} NMR resonances are observed at all Li+:2 molar ratios less than 2:1 and that NMR integrations of the resonances are the same (1:1) throughout the titration. The existence of a single resonance for each diastereomer at all Li+:2 molar ratios is consistent with fast exchange between Li + and both diastereomers of 2 on the NMR time scale.47 Lithium cation binding can also be followed by observing the shift in the methylene proton resonance (Δδ) in the 1H NMR of 2 upon addition of LiBPh4·3dme. As depicted in Figure 3, the trends in Δδ are much more simplistic than are those in the 31 1 P{ H} experiment, shown in Figure 2, due to the fact that only a single resonance is observed for the two diastereomers throughout the titration. In spite of these significant differences, the trend in Δδ is similar to that for the downfield 31P{1H} NMR resonance of 2, and the nonlinear least-squares regression fit to eq 1 gives similar stoichiometric binding association constants (β1 = 2.1(0.1) × 108 M−1; β2 = 5.0(0.3) ×

Figure 3. Nonlinear least-squares fit of normalized 1H methylene signal change [(σo − σi)/(σf − σi)] of 2 vs molar equivalent of LiBPh4· 3dme. Blue data points are experimental titration values, and the solid line represents nonlinear least-squares regression of the titration data to eq 1.

1019 M−2) when compared to those obtained from the 31P{1H} experiment. It is interesting that different Δδ’s of the diastereomers are observed in the titration of 2 with LiBPh4·3dme, but not in the titration of 3 with LiBPh4·3dme (Figure 2). This suggests that the stereogenic centers in 2 interact more closely in the 1:1 adduct of Li+:2 than they do in the 1:1 adduct of Li+:3. This is consistent with literature precedent on the interactions of crown ethers with Li+, for instance, the small cavity size of 12crown-4 (1.20−1.50 Å) prevents archetypal crown ether cavity binding of Li+ (ionic radius = 0.78 Å), and rather results in socalled ‘sandwich complexes’ in which two crowns encapsulate the Li+ in both 2:1 and 2:2 coordination modes. In the 1:1 Li+:2 complex, such sandwich-type coordination between the 12crown-4 lariat ethers and the cation would place the stereogenic methine carbons on the 12-crown-4 moieties in close proximity to one another, giving rise to separate 31P{1H} NMR resonances for each diastereomer of 2 (δMX,2). In contrast, the larger cavity size of 15-crown-5 (1.70−2.20 Å)35 allows Li+ to fit well into the macrocyclic cavity.48 Thus, binding of Li+ to a single 15-crown-5 lariat ether in the 1:1 Li+:3 complex would not place the stereogenic methine carbons within feasible proximity to strongly differentiate the 31 P{ 1 H} NMR resonances of the two diastereomers. Facile exchange of Li+ between the two crown ether cavities of 3 relative to the NMR time scale is also responsible for the observation of a single 31 1 P{ H} NMR resonance throughout the experiment. Based on the latter discussions, Figure 4 shows the proposed binding models for Li+ with 2 and 3. Na+ Binding to 2 and 3 Followed by 31P{1H} NMR Spectroscopy. The addition of NaBArF to a dichloromethane-d2 solution of 2 results in two resonances in the 31 1 P{ H} NMR spectra at 170.36 and 174.49 ppm (Figure 5). In contrast to the titration of 2 with LiBPh4·3dme discussed F


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Figure 4. Proposed mechanism of Li+ binding to both diastereomers of 2 (left) and 3 (right).

Figure 6. 13C{1H} NMR stack plot outlining differences in the carbonyl resonances of the free (bottom) and Na+-bound (top) complexes of 2.

Figure 5. Effects of Na+ binding to 2. 31P{1H} NMR stack plot depicting observed changes in the 31P{1H} NMR spectrum of 2 upon titration with NaBArF.

carbonyls trans to the phosphites and an upfield triplet (A portion of an AX2 spin system) that is assigned to the two carbonyls trans to one another. When an equivalent of NaBArF is added, the spectrum of the carbonyl region exhibits five distinct resonances (top spectrum in Figure 6): two downfield resonances in an approximately 1:1 ratio that are assigned to carbonyls trans to phosphites and three upfield resonances in an approximately 0.5:1:0.5 ratio that are assigned to carbonyls trans to one another. A possible explanation for this behavior is that Na+ binding to the 12-crown-4 lariat ether moieties results in a sandwich-type coordination mode with each diastereomer of 2 (Figure 7) due to the large radius of Na+ relative to 12-

previously, increasing the NaBArF concentration does not change the chemical shifts of these resonances, but instead increases their integrations relative to that of 2. Moreover, the NMR integrations of the new resonances remain equal (1:1) throughout the experiment. At a final 1:1 concentration of Na+:2, the resonance assigned to the unbound complex 2 has completely disappeared and only the two new resonances remain. These trends indicate that Na+ is binding to 2 in a 1:1 fashion and that there is slow exchange between Na+ and 2 on the NMR time scale.47 It should be noted that the most downfield resonance (174.49 ppm) is not associated with the trans isomer of 2, trans-Mo(CO)4(2,2′-(C12H8O2)POCH2(C8H15O4))2, because this isomer was observed to have a 31 1 P{ H} NMR chemical shift of 178.49 ppm. The carbonyl ligand resonances in the 13C{1H} NMR spectra of the free 2 and the Na+:2 complex (Figure 6) provide insight into the species giving rise to the two new 31P{1H} NMR resonances observed in the titration of 2 with NaBArF. Before any NaBArF is added to the solution of 2, two carbonyl resonances are observed: a downfield apparent quintet (A portion of an AXX′ spin system) that is assigned to the two

Figure 7. Proposed mechanism of Na+ binding to both diastereomers of 2. G


Article

Organometallics crown-4. 35,49 This would result in the R,R and S,S diastereomers (left portion of Figure 7) each having C2 point group symmetry, with the C2 axis running along the Mo and the Na+. This makes the carbonyls trans to carbonyls chemically equivalent, and this complex gives rise to the central and larger upfield triplet resonance in the top spectrum in Figure 6. On the other hand, the R,S diastereomer (right portion of Figure 7) has Cs point group symmetry with the σ plane containing the Mo, Na+, and the two carbonyls trans to carbonyls. This symmetry does not require the carbonyls trans to one another to be chemically equivalent, and these give rise to the smaller peripheral resonances in the top spectrum in Figure 6. This binding mechanism is very different from that proposed for the Li+ complexes of 2 and 3 and is a plausible explanation for the higher catalytic activity of the Rh(I) complex of L2 in the presence of Na+ relative to Li+. The binding interactions of 3 with Na+ are very different from those of 2 with Na+. Figure 8 shows that the 31P{1H}

diastereomers to shift downfield and to broaden. The downfield resonance broadens more rapidly than does the upfield resonance and is not observable at intermediate K+:2 ratios. As the K+:2 ratio approaches 1:1, the two resonances continue to shift and sharpen so that separate resonances are clearly seen. However, when the K+:2 ratio exceeds 1:1, both resonances broaden sufficiently to be unobservable. The 1H NMR spectra of the titration of 2 with KB(C6F5)4 provides a somewhat clearer picture of the cation binding than does the 31P{1H} NMR spectra. The left portion of Figure 9

Figure 9. Effects of K+ binding to 2. (Left) 1H NMR stack plot depicting observed changes in the methylene chemical shift of 2 upon addition of KB(C6F5)4. (Right) Plot of normalized 1H methylene signal change [(σo − σi)/(σf − σi)] of 2 vs molar equivalent of KB(C6F5)4. Blue data points represent the upfield resonance, and red data points represent the downfield resonance. Note the sharp break in both curves at 1:1 molar equiv, suggesting strong stoichiometric binding in this system.

Figure 8. Effects of Na+ binding to 3. (Left) 31P{1H} NMR stack plot depicting observed changes in the chemical shift of 3 upon addition of NaBArF. (Right) Nonlinear least-squares fit of normalized 31P{1H} signal change [(σo − σi)/(σf − σi)] of 3 vs molar equivalent of NaBArF. Blue data points are experimental titration values, and the solid line represents nonlinear least-squares regression of the titration data to eq 1.

depicts a downfield shift of the 1H methylene resonance of 2 upon addition of KB(C6F5)4. As aliquots of the KB(C6F5)4 are added, separate resonances for the methylene protons of the two diastereomers begin to be observed. The resonances continue to shift downfield until the ratio of K+:2 reaches 1:1. The sharp break in the binding curve (right portion of Figure 9) at a 1:1 K+:2 ratio is indicative of stoichiometric binding under these conditions. It is interesting to note that although the binding of either Na+ or K+ by 2 results in a 1:1 complex, the exchange rates of the two cations are very different, and increased catalytic activity is observed only when Na+ is added to the Rh(I) complex of L2. Studies of the binding of K+ to 3 are of particular interest because the Rh(I) catalyst of L3 exhibits a significant increase in activity in the presence of KB(C6F5)4. Unfortunately, 31 1 P{ H} NMR spectroscopy is not particularly useful to study the titration of 3 with KB(C6F5)4 due to a gradual broadening and eventual disappearance of the 31P{1H} NMR resonance of 3 upon increasing KB(C6F5)4 concentration. 1H NMR spectroscopy is equally unhelpful due to a similar broadening of both the methylene and crown proton resonances. However, the carbonyl ligand resonances in the 13C{1H} NMR spectrum of the 1:1 K+:3 complex (Supporting Information) exhibit the same splitting patterns as do the carbonyl resonances in the 13 C{1H} NMR spectrum of the 1:1 Na+:2 complex (Figure 6), indicating that the K+ binding by 3 is similar to that of Na+ by 2. This observation further supports the proposal that a binding mechanism in which the alkali metal cation is “moderately oversized” for the cavity size of the lariat ether moiety gives rise to the high activities observed in the Rh(I) catalysts containing the phosphite-lariat crown ether ligands.

NMR resonance of 3 continuously shifts upfield upon addition of NaBArF (left) and that nonlinear least-squares regression of the titration data to eq 1 is indicative of two successive binding events with an overall 2:1 stoichiometry between Na+ and 3. The observed binding affinities are β1 = 8.9(3.2) × 109 M−1 and β2 = 4.5(3.2) × 1020 M−2. As stated previously, the high affinity constants indicate that stoichiometric binding conditions dominate throughout the experiment and, thus, that the calculated binding affinities represent only upper limits of the Kd.43−46 Nevertheless, the association constants do suggest that stoichiometric binding should be conserved even at catalytic concentrations (3.5 × 10−5 M) and that tight alkali metal cation binding is occurring during the hydroformylation process. The titrations of 3 with NaBArF are very similar to those of 3 with LiBPh4·3dme in that one 31P{1H} NMR resonance is observed throughout the titration; however this resonance only shifts upfield in the titration of 3 with NaBArF without reversing direction. K+ Binding to 2 and 3 Followed by 31P{1H} and 1H NMR Spectroscopies. When following the binding interactions of K+ with 2 via 31P{1H} NMR spectroscopy, the 31 1 P{ H} chemical shift changes are very complex, and it is not possible to obtain accurate chemical shift values throughout the titration. Addition of KB(C6F5)4 to a dichloromethane-d2 solution of 2 causes the 31P{1H} NMR resonances of both H


Article

Organometallics FTIR Titrations of 1−3 with Alkali Metal Salts. Because coordination of a Lewis acid metal cation to a carbonyl oxygen lone pair is known to facilitate migratory insertion reactions involving a carbonyl ligand and an alkyl ligand on a transition metal,50,51 it is possible that such interactions are facilitating the increased activity in hydroformylation catalysts containing L2− L5. To investigate any interactions that may be occurring between a bound alkali metal cation and a carbonyl ligand, FTIR titrations of the model complexes 1−3 with the alkali metal salts used in the NMR titrations and styrene hydroformylation experiments have been carried out. The FTIR titrations followed changes in the carbonyl stretching bands as aliquots of a solution containing the appropriate alkali metal salt dissolved in dry DCM were added to a dry DCM solution containing 1, 2, or 3. A stacked plot showing the FTIR spectra obtained from a titration of 2 with NaBArF is shown in Figure 10 (additional plots showing FTIR titrations of 1, 2, or 3 with

crown ether moiety, demonstrating that alkali metal cation binding by the phosphite-lariat crown ether ligands is having a positive effect on the activity. Furthermore, the catalytic data demonstrate that the positive effects on activity occur only when the alkali metal cation added is “moderately oversized” to provide archetypal binding to the crown ether cavity in the ligand. It should also be noted that the trends in increased activities are dependent, at least in part, on the phosphorusdonor group substituents, as the trend is not observed in catalysts containing the 3,3′,5,5′-tetra-tert-butylbiphenyl-2,2′diyl-derived L4, but is observed in catalysts containing the 1,1′binaphthalene-derived L5, even though both of these ligands contain a lariat ether moiety. The phosphorus nuclei in 1−3 provide a very useful and quantitative NMR spectroscopic probe for alkali metal cation binding to the lariat ether moieties, and 31P{1H} and 1H NMR spectroscopies have been used to study the binding of Li+, Na+, and K+ to these systems. The lack of any change in 31P{1H} or 1 H NMR resonances of 1 in the presence of any the alkali metal salts studied indicates that this complex does not bind alkali metal cations. In contrast, each of the alkali metal cations binds very tightly to 2 and 3. Binding of the smaller alkali metal cations (2/3:Li+ and 3:Na+) resulted in distinct two-site binding systems in which both a 1:1 intermediate and a fully bound 2:1 product are observed. In contrast, binding of the larger alkali metal cations (2:Na+/K+ and 3:K+) resulted in only 1:1 complexes. The differences in the 31P{1H} NMR spectroscopic titrations clearly suggest that the preferred coordination modes of the alkali metal cations depend on the lariat cavity size:ionic radius ratio. Of particular importance is that the highest increases in catalytic activities upon addition of alkali metal salts correlate with 1:1 binding of the alkali metal cation by the model complex. In contrast, the decreases in catalytic activities upon addition of alkali metal salts correlate with both 1:1 and 1:2 binding. This suggests that the “moderately oversized” coordination mode to the lariat ether moieties is necessary for high activities in styrene hydroformylation experiments catalyzed by Rh(I) complexes of these ligands. These results demonstrate that phosphite-lariat crown ethers are an interesting new class of ligands for use in hydroformylation catalysis. Their ability to tightly bind alkali metal cations can result in high activities when the alkali metal cation is “moderately oversized” for the crown ether group. It may also be possible to affect the enantioselectivities of the catalyst with alkali metal cation binding because this appears to provide strong interaction between the two chiral phosphite-lariat crown ethers in some of the complexes. Investigations separated by over 30 years leave the advancement of these compounds in its infancy, but the studies described in this paper suggest that they are a much more robust class of alkali metal cation binders than are their metallacrown ether counterparts. Thus, further investigation into complexes of phosphite-lariat crown ethers may improve upon our current understanding of Lewis-acid-assisted hydroformylation catalysis.

Figure 10. Observed changes in the FTIR of 2 upon titration with NaBArF in DCM.

the alkali metal salts are provided in the Supporting Information). No changes in the carbonyl ligand peak positions were observed in any of the FTIR titrations of 1 with the alkali metal salts, and this is as expected because 1 contains no cationbinding groups. Only minor changes in the carbonyl ligand peak positions were observed in the FTIR titrations of 2 or 3 with the alkali metal salts. In all cases, the IR stretches of the carbonyl ligands of 2 and 3 shift to higher frequencies upon coordination of the alkali metal salts. These results are similar to that observed for Li+ binding to a metallacrown ether;31 however the shifts observed in 2 and 3 are significantly smaller. This suggests that there are no strong alkali metal cation− carbonyl ligand interactions occurring in either 2 or 3 that would lead to a change in the bonding of the carbonyl ligands to the Mo(0) center.

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CONCLUSION The first family of phosphite-lariat crown ether ligands has been prepared, and their Rh(I) complexes have been evaluated as catalysts for the hydroformylation of styrene. The activities of these catalysts can be greatly improved via the addition of appropriate Lewis acid alkali metal cations without significant hindrance of the regioselectivities of the catalysts. Significant changes in activities are observed only for Rh(I) complexes containing phosphite-lariat ether ligands but not for Rh(I) complexes of phosphite ligands that do not contain a lariat

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EXPERIMENTAL SECTION

Materials and Methods. Tetrahydrofuran was initially dried over MgSO4 for at least 12 hours, then distilled from CaH2 and finally distilled from Na/benzophenone. It was stored over molecular sieves (3 Å; 8−12 mesh) and used within a few hours. Toluene was dried by distillation from Na and stored over molecular sieves (3 Å; 8−12 I


Article

Organometallics

(s, crown-CH2), 70.91 (s, crown-CH2), 70.89 (s, crown-CH2), 70.70 (s, crown-CH2), 70.66 (s, crown-CH2), 70.61 (s, crown-CH2), 70.58 (s, crown-CH2), 70.55 (s, crown-CH2), 70.47 (s, crown-CH2), 64.70 ppm (d, CH2OP, |2JPC| = 6 Hz). 2,2′-(C28H40O2)POCH2(C8H15O4), L4. Using the procedure for L2, 1.11 g (2.34 mmol) of 3,3′,5,5′-tetra-tert-butylbiphenyl-2,2′-diyl phosphorochloridite ester, 0.480 g (2.33 mmol) of 2-hydroxymethyl12-crown-4, and 0.320 mL (2.30 mmol) of dry triethylamine yielded 1.50 g (99.9%) of spectroscopically pure L4 as a colorless oil. 31P{1H} NMR (chloroform-d): δ 137.53 ppm (s). 1H NMR (chloroform-d): δ 7.48−7.44 (m, 2H, Ar-H), 7.22−7.18 (m, 2H, Ar-H), 3.87−3.59 (m, 16H, H2COP + crown-H), 3.46−3.44 (m, 1H, crown-H), 1.54−1.50 (m, 18H, CH3), 1.40−1.37 ppm (m, 18H, CH3). 13C{1H} NMR (chloroform-d): δ 146.51 (s, C(CH3)3), 146.46 (s, C(CH3)3), 146.20 (d, OAr-Cq, |2JPC| = 6 Hz), 146.10 (d, OAr-Cq, |2JPC| = 5 Hz), 139.88 (s, C(CH3)3), 139.80 (s, C(CH3)3), 132.65 (d, ArAr-Cq, |3JPC| = 2 Hz), 132.58 (d, ArAr-Cq, |3JPC| = 2 Hz), 126.58 (s, Ar-CH), 126.56 (s, ArCH), 124.28 (s, Ar-CH), 124.26 (s, Ar-CH), 78.73 (d, CHCH2OP, |3JPC| = 2 Hz), 71.24 (s, crown-CH2), 70.77 (s, crown-CH2), 70.66 (s, crown-CH2), 70.64 (s, crown-CH2), 70.61 (s, crown-CH2), 70.39 (s, crown-CH2), 70.20 (s, crown-CH2), 63.71 (bs, CH2OP), 35.41 (s, C(CH3)3), 34.70 (s, C(CH3)3), 31.57 (s, C(CH3)3), 31.07 (s, C(CH3)3), 31.03 ppm (s, C(CH3)3). 2,2′-(C20H12O2)POCH2(C8H15O4), L5. Using the procedure for L2, 1.00 g (2.85 mmol) of 1,1′-binaphthalene-2,2′diyl phosphochloridite ester, 0.588 g (2.85 mmol) of 2-hydroxymethyl-12-crown-4, and 0.400 mL (2.88 mmol) of dry triethylamine yielded 1.40 g (94.6%) of spectroscopically pure L5 as a white solid. 31P{1H} NMR (chloroformd): δ 143.72 (s), 142.51 ppm (s). 1H NMR (chloroform-d): δ 8.00− 7.93 (m, 4H, Ar-H), 7.54−7.49 (m, 2H, Ar-H), 7.47−7.43 (m, 2H, ArH), 7.40−7.37 (m, 2H, Ar-H), 7.64 (m, 2H, Ar-H), 4.03−3.98 (m, 1H, H2COP), 3.88−3.81 (m, 2H, H2COP), 3.80−3.62 (m, 14H, crownH), 3.55−3.52 ppm (m, 1H, crown-H). 13C{1H} NMR (chloroformd): δ 148.63 (d, 0.5 OAr-Cq, |2JPC| = 5 Hz), 148.62 (d, 0.5 OAr-Cq, |2JPC| = 5 Hz), 147.54 (d, 0.5 OAr-Cq, |2JPC| = 7 Hz), 147.53 (d, 0.5 OAr-Cq, |2JPC| = 7 Hz), δ 132.89 (bs, Ar-Cq), 132.67 (bs, Ar-Cq), δ 131.59 (bs, Ar-Cq), 131.11 (s, 0.5 Ar-Cq), 131.08 (s, 0.5 Ar-Cq), 130.40 (d, 2 Ar-CH, |3JPC| = 5 Hz), 130.00 (d, 2 Ar-CH, |3JPC| = 4 Hz), 128.39 (s, 2 Ar-CH), 128.33 (s, 2 Ar-CH), 127.04 (s, Ar-CH), 127.01 (s, 3 ArCH), 126.29 (s, 2 Ar-CH), 126.26 (s, Ar-CH), 126.22 (s, Ar-CH), 125.09 (s, 2 Ar-CH), 124.94 (s, Ar-CH), 124.92 (s, Ar-CH), 124.20 (dd, 2 Ar-Cq, |3JPC| = |3JPC| = 5 Hz), 122.80 (dd, 2 Ar-Cq, |4JPC| = 4 Hz, |4JPC| = 2 Hz), 121.85 (s, Ar-CH), 121.82 (s, 2Ar-CH), 121.65 (s, 1 ArCH), 78.86 (d, CHCH2OP, |3JPC| = 3 Hz), 78.79 (d, CHCH2OP, |3JPC| = 3 Hz), 71.18 (s, crown-CH2), 71.12 (s, crown-CH2), 71.11 (s, crown-CH2), 71.04 (s, crown-CH2), 70.93 (s, crown-CH2), 70.88 (s, crown-CH2), 70.75 (s, crown-CH2), 70.73 (s, crown-CH2), 70.50 (s, crown-CH2), 70.48 (s, crown-CH2), 70.39 (s, crown-CH2), 70.34 (s, crown-CH2), 64.73 (d, CH2OP, |2JPC| = 7 Hz), 64.65 ppm (d, CH2OP, |2JPC| = 5 Hz). cis-Mo(CO)4(2,2′-(C12H8O2)POCH2(C8H15O4))2, 2. A solution of 0.265 g (0.630 mmol) of L2 and 0.095 g (0.317 mmol) of cisMo(CO)4(nbd) in dry DCM was stirred at room temperature under a nitrogen atmosphere for 20 min. After this time, the DCM was removed by rotary evaporation to yield a brown solid. The solid product was triturated with three 15 mL portions of hexanes to remove residual norbornadiene and dried in vacuo overnight to yield crude 2 as a brown powder. Sequential recrystallizations from DCM/ hexanes yielded 0.210 g (63.6%) of analytically pure 2 in the form of a white powder. 31P{1H} NMR (dichloromethane-d2): δ 170.23 ppm (s). 1H NMR (dichloromethane-d2): δ 7.56 (d, 4H, Ar-H, |3JHH| = 7 Hz), 7.46−7.43 (m, 4H, Ar-H), 7.37−7.32 (m, 8H, Ar-H), 3.90 (bs, 2H, H2COP), 3.86 (bs, 2H, H2COP), 3.72−3.48 (m, 28H, crown-H), 3.42−3.39 ppm (m, 2H, crown-H). 13C{1H} NMR (dichloromethaned2): δ 211.20−1.50 Å (aq, 2 trans-CO, |2JPC + 2JP′C| = 32 Hz), 206.30 (t, 2 cis-CO, |2JPC| = 13 Hz), 149.92 (aq, OAr-Cq, |2JPC + 4JP′C| = 9 Hz), 149.90 (aq, OAr-Cq, |2JPC + 4JP′C | = 9 Hz), 130.11 (bs, 2Ar-CH + 2ArAr-Cq), 129.59 (d, Ar-CH, |3JPC| = 3 Hz), 129.56 (d, Ar-CH, |3JPC| = 3 Hz), 125.63 (bs, 2Ar-CH), 122.31 (d, Ar-CH, |3JPC| = 7 Hz), 122.26 (d, Ar-CH, |3JPC| = 7 Hz), 78.13 (bs, CHCH2OP), 70.85 (s, crown-

mesh). Dichloromethane was dried by distillation from CaH2 and stored over molecular sieves (3 Å; 8−12 mesh). Triethylamine was initially dried over KOH for a minimum of 12 h, distilled from Na/ benzophenone, and stored over molecular sieves (3 Å; 8−12 mesh). Other solvents were reagent grade and were degassed using highpurity (99.998%) nitrogen before use. 2-Hydroxymethyl-12-crown-4 and 2-hydroxymethyl-15-crown-5 were distilled prior to use. Literature procedures were used to prepare cis-Mo(CO)4(nbd) (nbd = norbornadiene),52 Rh(CO)2(acac) (acac = acetylacetonate),53 2,2′(C12H8O2)POCH3 (L1),54 cis-Mo(CO)4(2,2′-(C12H8O2)POCH3)2 (1),42 2,2′-diphenylylenephosphochlorodite ester,55 3,3′,5,5′-tetratert-butylbiphenyl-2,2′-diyl phosphorochloridite ester,56 and 1,1′binaphthalene-2,2′diyl phosphochloridite ester.57 Characterization. NMR spectra were recorded on Bruker DRX400, Avance-500, or Avance-700 MHz spectrometers. Solutions of the complexes in deuterated solvents were prepared under a stream of N2 gas, and all NMR experiments were performed at room temperature. The 31P{1H} NMR spectra were referenced to external 85% H3PO4 in a coaxial tube that also contained CDCl3, and the 13C{1H} and 1H NMR spectra were referenced to internal SiMe4 (TMS). Some assignments were based on 2D NMR spectra (1H−13C HSQC, 1H−1H COSY). FTIR data were recorded on a Bruker Alfa FT-IR spectrometer capable of transmittance and ATR. Elemental analyses (C and H) were performed by Atlantic Microlabs, Inc. 2,2′-(C12H8O2)POCH2(C8H15O4), L2. A solution of 1.23 g (4.91 mmol) of 2,2′-diphenylylenephosphochlorodite ester dissolved in 25 mL of dry THF was added dropwise to a stirring solution containing 1.01 g (4.90 mmol) of 2-hydroxymethyl-12-crown-4 and 0.681 mL (4.90 mmol) of dry triethylamine in 50 mL of dry THF; the dropping funnel was washed with 10 mL of dry THF after addition was complete, and the solution was then allowed to stir for 2 h at room temperature under a constant stream of N2(g). The solution was then cannula transferred in portions into a dry 200 mL fritted funnel containing a mixture of Celite and basic alumina to remove the triethylammonium chloride precipitate byproduct and any phosphite hydrolysis byproducts. The solution was then filtered via positive N2 pressure into a 100 mL Schlenk flask, and the residue in the filter was washed with two 10 mL portions of dry THF. Removal of the solvent through rotary evaporation and vacuum drying yielded 1.68 g (84.1%) of spectroscopically pure L2 as a colorless oil. 31P{1H} NMR (chloroform-d): δ 141.40 ppm (s). 1H NMR (chloroform-d): δ 7.36 (d, 2H, Ar-H, |3JHH| = 8 Hz), 7.25 (dd, 2H, Ar-H, |3JHH| = |3JHH| = 8 Hz), 7.16 (dd, 2H, Ar-H, |3JHH| = |3JHH| = 8 Hz), 7.10 (d, 1H, Ar-H, |3JHH| = 8 Hz), 7.09 (d, 1H, Ar-H, |3JHH| = 8 Hz), 3.89−3.81 (m, 2H, H2COP), 3.75−3.73 (m, 1H, crown-H), 3.70−3.67 (m, 1H, HCCH2OP), 3.65−3.53 (m, 12H, crown-H), 3.44−3.42 ppm (m, 1H, crown-H). 13C{1H} NMR (chloroform-d): δ 149.87 (d, OAr-Cq, |2JPC| = 5 Hz), 149.83 (d, OAr-Cq, |2JPC| = 5 Hz), 131.10 (d, ArAr-Cq, |3JPC| = 3 Hz), 131.00 (d, ArAr-Cq, |3JPC| = 3 Hz), 130.00 (d, Ar-CH, |3JPC| = 1 Hz), 130.00 (d, Ar-CH, |3JPC| = 1 Hz), 129.28 (bs, 2 Ar-CH), 125.14 (bs, Ar-CH), 125.13 (bs, Ar-CH), 122.04 (d, Ar-CH, |4JPC| = 1 Hz), 121.95 (d, Ar-CH, |4JPC| = 1 Hz), 78.84 (d, CHCH2OP, |3JPC| = 3 Hz), 71.18 (s, crown-CH2), 71.06 (s, crown-CH2), 70.86 (s, crownCH2), 70.73 (s, crown-CH2), 70.47 (s, crown-CH2), 70.36 (s, crownCH2), 64.29 ppm (d, CH2OP, |2JPC| = 5 Hz). 2,2′-(C12H8O2)POCH2(C10H19O5), L3. Using the procedure for L2, 1.12 g (4.47 mmol) of 2,2′-diphenylenephosphochlorodite ester, 1.12 g (4.47 mmol) of 2-hydroxymethyl-15-crown-5, and 0.625 mL (4.50 mmol) of dry triethylamine yielded 1.72 g (86.1%) of spectroscopically pure L3 as a colorless oil. 31P{1H} NMR (chloroform-d): δ 142.55 ppm (s). 1H NMR (chloroform-d): δ 7.36 (d, 2H, Ar-H, |3JHH| = 8 Hz), 7.25 (dd, 2H, Ar-H, |3JHH| = |3JHH| = 8 Hz), 7.16 (dd, 2H, Ar-H, |3JHH| = |3JHH| = 8 Hz), 7.10 (d, 1H, Ar-H, |3JHH| = 8 Hz), 7.09 (d, 1H, Ar-H, |3JHH| = 8 Hz), 3.96−3.89 (m, 2H, H2COP), 3.76−3.73 (m, 1H, crown-H), 3.68−3.48 ppm (m, 18H, crown-H). 13C{1H} NMR (chloroform-d): δ 149.90 (d, OAr-Cq, |2JPC| = 5 Hz), 149.80 (d, OArCq, |2JPC| = 5 Hz), 131.10 (d, ArAr-Cq, |3JPC| = 3 Hz), 131.00 (d, ArArCq, |3JPC| = 3 Hz), 129.97 (bs, Ar-CH), 129.94 (bs, Ar-CH), 129.26 (bs, 2Ar-CH), 125.12 (bs, Ar-CH), 125.09 (bs, Ar- CH), 122.10 (bs, ArCH), 122.03 (bs, Ar-CH), 78.86 (d, CHCH2OP, |3JPC| = 3 Hz), 71.18 J


Article

Organometallics CH2), 70.72 (s, crown-CH2), 70.69 (s, crown-CH2), 70.58 (s, crownCH2), 70.48 (s, crown-CH2), 70.30 (s, crown-CH2), 70.25 (s, crownCH2), 66.91 ppm (t, CH2OP, |2JPC| = 4 Hz). Anal. Calcd for C46H50MoO18P2: C, 52.68; H, 4.81. Found: C, 52.34; H, 4.86. cis-Mo(CO)4(2,2′-(C12H8O2)POCH2(C10H19O5))2, 3. Using the procedure for 2, 0.720 g (1.55 mmol) of L3 and 0.233 g (0.776 mmol) of cis-Mo(CO)4(nbd) yielded 0.559 g (63.4%) of analytically pure 3 in the form of a slightly brown, tacky solid. 31P{1H} NMR (dichloromethane-d2): δ 170.30 ppm (s). 1H NMR (dichloromethaned2): δ 7.45 (d, 4H, Ar-H, |3JHH| = 8 Hz), 7.36−7.33 (m, 4H, Ar-H), 7.28−7.21 (m, 8H, Ar-H), 3.90 (bs, 4H, H2COP), 3.70−3.43 ppm (m, 38H, crown-H). 13C{1H} NMR (dichloromethane-d2): δ 211.30 (aq, 2 trans-CO, |2JPC + 2JP′C| = 32 Hz), 206.30 (t, 2 cis-CO, |2JPC| = 13 Hz), 149.90 (s, OAr-Cq), 149.83 (s, OAr-Cq), 130.13 (d, 2 Ar-CH, |3JPC| = 3 Hz), 130.06 (d, 2 ArAr-Cq), |3JPC| = 3 Hz), 129.65 (bs, Ar-CH), 129.61 (bs, Ar-CH), 125.65 (s, Ar-CH), 125.63 (s, Ar-CH), 122.34 (d, Ar-CH, |3JPC| = 7 Hz), 122.30 (d, Ar-CH, |3JPC| = 7 Hz), 78.22 (bs, CHCH2OP), 70.80 (s, crown-CH2), 70.53 (s, crown-CH2), 70.42 (s, crown-CH2), 70.34 (s, crown-CH2), 70.27 (s, crown-CH2), 70.25 (s, crown-CH2), 70.19 (s, crown-CH2), 70.17 (s, crown-CH2), 70.12 (s, crown-CH2), 70.11 (s, crown-CH2), 67.27 ppm (bs, CH2OP). Anal. Calcd for C50H58MoO20P2·CH2Cl2: C, 50.13; H, 4.95. Found: C, 49.99; H, 5.19. Hydroformylation of Styrene. The hydroformylation reactions were carried out using a Parr Series 4560 minireactor connected to a high-pressure gas buret that introduced gas to the reactor at a constant pressure. The digitized reactor temperature, buret temperature, and buret pressure were monitored using Agilent Benchlink Data Logger software on a PC connected to an Agilent data acquisition switch unit. In a typical run, 9.0 mg (0.035 mmol) of Rh(CO)2(acac) and the appropriate amount of alkali metal salt, if used, were weighed and added to a 50 mL Schlenk round-bottom flask, which was then sealed, evacuated, and filled with N2. To this flask were added the appropriate amounts of dry DCM and a stock solution containing a known concentration of the appropriate ligand dissolved in dry DCM so that the mole ratio of ligand:rhodium was 2.4:1 and the total volume was 22 mL. This solution was then added to the reactor via cannula transfer under positive N2 pressure through the substrate inlet valve. The reactor was then purged three times with a 1:1 H2:CO (syngas) mixture and then pressurized to 20 atm with syngas and heated to 80 °C with mechanical stirring. The reactor was maintained under these conditions for 45 min to allow for precatalyst equilibration and was then cooled to ∼38 °C before pressure was slowly vented. Approximately 4.0 mL (35 mmol) of styrene was then injected into the reactor through the substrate inlet valve, after which the reactor was again pressurized to 20 atm with syngas and reheated to 80 °C with mechanical stirring. The progress of the reaction was monitored using the Agilent software. The activity of the catalyst was expressed in terms of a pseudo-first-order rate constant (k). The pressure drop versus time data were fit to the natural exponential equation P = (Pd)e−kt + Pf, where Pd is the pressure drop (Pf − Pi), Pf is the final pressure, and k (s−1) is the pseudo-first-order rate constant, using Graphical Analysis version 3.4.58 The regioselectivities (% iso) were determined from integrations of the 1H NMR spectra of the reaction mixtures at the end of the reactions. Alkali Metal Salt Titrations with 1−3 Followed by 31P{1H} and 1H NMR Spectroscopies. Titrations of 1−3 with LiBPh4·3dme, NaBArF, and KB(C6F5)4 were carried out in dichloromethane-d2 and were followed by 31P{1H} and 1H NMR spectroscopies. In a typical experiment, aliquots of a titrant solution containing 1, 2, or 3 (10 mM) and a large excess of alkali metal salt (LiBPh4·3dme = 100 mM, NaBArF = KB(C6F5)4 = 30 mM) in dichloromethane-d2 were added to an analyte solution containing only 1, 2, or 3 (10 mM) in dichloromethane-d2 in a 5 mm gastight J-Young NMR tube. The sample was then inverted five times to ensure adequate mixing of the solution before the spectrum was recorded. All titrant additions were performed under a stream of N2, and the NMR tube was backfilled with N2 after each addition to ensure that the experiment remained under an inert atmosphere. Titration data were plotted as normalized

signal versus total alkali metal salt concentration. The normalized signal was obtained using the form (σo − σi)/(σf − σi), where σo is the observed 31P{1H} or 1H chemical shift, σi is the initial 31P{1H} or 1H chemical shift, and σf is the final 31P{1H} or 1H chemical shift. When specified, the titration data were fit using Scientist 3.0 and a nonlinear least-squares regression analysis to a 2:1 binding model (eq 1) in which the extent of binding term, X̅ , is a function of the equilibrium constants, β1 and β2,59 and the free alkali metal concentration, x. In addition, it was necessary to treat the observed normalized signal (δobs) as a weighted average of the free (δfree), singly bound (δMX), and fully bound (δbound) species according to eq 2.

X̅ =

β1x + 2β2x 2 1 + β1x + β2x 2

δobs = δfree

(1)

β1x 1 + δMX 2 1 + β1x + β2x 1 + β1x + β2x 2

+ δ bound

β2x 2 1 + β1x + β2x 2

(2)

Alkali Metal Salt Titrations Followed by FTIR. Titrations of 1, 2, and 3 with LiBPh4·3dme, NaBArF, and KB(C6F5)4 were carried out in dry DCM and were followed by FTIR. In a typical experiment, a series of solutions were prepared by mixing the appropriate amount of a dry DCM solution containing only 1, 2, and 3 (1.0 mM) with the appropriate amount of a dry DCM solution containing 1, 2, and 3 (1.0 mM) and the appropriate amount of alkali metal salt (LiBPh4·3dme = 10 mM, NaBArF = KB(C6F5)4 = 3.0 mM). This ensured that each solution contained a specific complex:alkali metal salt ratio. Once all of the solutions were prepared, each solution was placed into an FTIR solvent cell for acquisition. The cell was flushed with fresh dry DCM and then dried between each acquisition.

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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.6b00325. Statistics for all data presented, all data sets plotted, NMR spectra of ligands L2−L5 and complexes 2 and 3, 31 1 P{ H} NMR stack plots depicting observed changes in the 31P{1H} NMR spectra of 2 and 3 upon titration with KB(C6F5)4, and a 13C{1H} NMR spectrum of the carbonyl resonances of the K+-bound complex of 3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation (NSF) for support under grant EPS-1158862. J.R.M. gratefully thanks NSF Alabama EPSCoR for funding under a Graduate Research Scholars Program (GRSP) graduate fellowship.

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REFERENCES

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Correlating the Activity of Rhodium(I)-Phosphite-Lariat Ether Styrene

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