Bridging Allyl Dimers - ACS Publications - American Chemical Society

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Mechanistic Studies of the Insertion of CO2 into Palladium(I) Bridging Allyl Dimers Damian P. Hruszkewycz,† Jianguo Wu,† Jennifer C. Green,‡ Nilay Hazari,*,† and Timothy J. Schmeier† †

The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States The Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom

‡

S Supporting Information *

ABSTRACT: In contrast to the chemistry of momomeric η1-Pd allyls, which act as nucleophiles, and monomeric η3-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that PdI dimers containing two bridging allyl ligands react with one equivalent of CO2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of PdI bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO2. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO2. An alternative pathway for CO2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylateand chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η1-Pd allyls.

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INTRODUCTION There has been increasing interest in utilizing CO2 as a C1 source for the synthesis of both fine and commodity chemicals because CO2 is cheap, abundant, nontoxic, and relatively easy to transport.1 Unfortunately, activating and subsequently converting CO2 into more valuable products under mild conditions is difficult due to the kinetic and thermodynamic stability of CO2 . Transition metal complexes have been explored extensively to this end, as several transition metal complexes are known to bind CO2 and weaken its strong C−O double bond.2 Alternatively, transition metal complexes can promote the formation of C−E bonds (E = H, C, N, or O) through the insertion of CO2 into M−E bonds,2a and this elementary step has been utilized in several catalytic cycles for CO2 conversion.1f,k−n Our group has studied the reaction of Pd and Ni allyl complexes with CO2 in detail and developed a catalytic reaction for the coupling of CO2 with allylstannanes and allylboranes.3 Recently, we discovered that CO2 cleanly inserts into one of the bridging allyl ligands in PdI dimers containing two bridging allyl ligands (eq 1),4 the first examples of a reaction between CO2 and a bridging allyl ligand on any metal. Despite the numerous examples of PdI dimers supported by bridging allyl ligands, little has been done to study their reactivity and the chemical properties of these species remain unclear.5 © 2011 American Chemical Society

This is in contrast to the well-characterized reactivity of monomeric η1-Pd allyls, which can act as nucleophiles,6 and monomeric η3-Pd allyls, which can act as electrophiles.7 Figure 1 depicts a general classification scheme for PdI bridging allyl dimers and related molecules that have been reported.4a,8−17 Complexes supported by bridging cyclopentadienyl (types IV− VII) and indenyl ligands (types VIII−IX) are included in Figure 1 because spectroscopic, crystallographic, and theoretical evidence indicates that the cyclopentadienyl and indenyl ligands Received: November 21, 2011 Published: December 22, 2011 470

dx.doi.org/10.1021/om201163k | Organometallics 2012, 31, 470−485

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Figure 2. Illustration of the back-bonding interactions described by Kurosawa and co-workers for type III dimers involving (a) donation from the Pd−Pd σ orbital to the allyl π* orbital, (b) donation from the Pd−Pd π orbital to the allyl π* orbital, and (c) donation from the allyl nπ orbital to the Pd−Pd σ* orbital.10j

electron-withdrawing substituents were found to coordinate more strongly to the PdI dimer than the mononuclear complex, although the mechanism of exchange is still unclear. Recently we described the first well-defined reactions of type I complexes (containing two bridging allyl ligands). These included reactions with electrophiles such as HCl, thiols, and carboxylic acids, as well as the unusual CO2 insertion (eq 1).4a Subsequently, Lin and co-workers published a preliminary computational study on the reaction of one of our type I dimers with CO2.20 They proposed a pathway that involved direct insertion of CO2 into the bridging allyl ligand. Here, we present a combined experimental and computational study that builds on our previous results. We discuss the structure, bonding, and reactivity of type I, II, and III complexes, propose a different mechanism from that described by Lin for the reaction of type I complexes with CO2, and discuss the effect of weakly coordinating ligands on the rate of CO2 insertion. Our mechanistic studies provide a clear basis for understanding the reactivity of type I species with CO2 and suggest that doubly bridged allyl dimers have similar reactivity to η1-Pd allyls.

Figure 1. Classification of PdI dimers containing bridging allyl or related ligands.

bridge the Pd centers in an analogous fashion to conventional bridging allyl ligands.5a,15,16 Seminal work investigating the reactivity of compounds with bridging cyclopentadienyl and indenyl ligands, in particular type IV, V, and VII complexes, was performed by Werner and coworkers. They found that two types of reaction occurred: (i) reactions in which two mononuclear products are formed and (ii) reactions in which the binuclear core is retained. The reactions of type IV (containing one bridging allyl and one bridging cyclopentadienyl ligand) and type V complexes (containing two bridging cyclopentadienyl ligands) with electrophiles such as MeI, HCl, or I2 resulted in cleavage of the Pd−Pd bond and the formation of monomeric products.9c,11c,14b In contrast carboxylic acids and thiols were shown to selectively protonate the cyclopentadienyl ligand in type IV and V species to form products with a bridging carboxylate or thiolate ligand.9c Subsequently, type VII complexes (containing one bridging cyclopentadienyl and one bridging halide ligand) were shown to undergo an unusual reaction in which the bridging ligands could be replaced by small neutral ligands, such as CO or MeCN, to form species with bridging CO or MeCN ligands and terminal Cp and halide ligands.14c,d In a rare example of catalytic reactivity, several groups have demonstrated that type III dimers are in equilibrium with monomeric compounds, which are active catalysts for crosscoupling reactions.18,19 Alongside these reactivity studies, calculations have been performed investigating the electronic structure of closely related type III (containing one bridging allyl and one bridging halide ligand) and VII (containing one bridging cyclopentadienyl and one bridging halide ligand) complexes. Kurosawa and coworkers reported that there is a significant back-bonding interaction from the dσ−dσ and dπ−dπ orbitals of the Pd−Pd bond to the π* orbital of the bridging allyl ligand in type III complexes (Figure 2) and proposed that the allyl ligand is negatively charged.5c,10j,k However, at this stage no reactivity has been found that reflects this theoretical finding. Interestingly, these type III complexes undergo facile ligand exchange with complexes of the type Pd(η3-allyl)(L)(Cl) containing substituted allyl ligands (eq 2). Allyl ligands bearing

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RESULTS AND DISCUSSION Synthesis of Allyl Dimers and Reactivity with CO2. One of the main reasons for the lack of previous studies on type I PdI dimers with two bridging allyl ligands has been the difficulty associated with synthesizing these species. In the past they have been synthesized through the direct reaction of thermally unstable Pd(allyl)2 with free ligand8d or via the decomposition of isolated species of the type bis(allyl)Pd(L) (these species are intermediates in the synthesis starting from Pd(allyl)2).8a,h Recently we discovered that type I dimers could be prepared through the treatment of complexes of the type (η3-allyl)Pd(Cl)(L) with (allyl)MgCl.4a In order to perform detailed mechanistic studies of the insertion of CO2 into type I allyl dimers, a series of new and previously prepared phosphine and N-heterocyclic carbene (NHC) supported dimers4a,8b−d,g,h were synthesized, using our improved synthetic method.4a The compounds prepared are summarized in Figure 321 and represent an electronically and sterically diverse family. All new complexes were fully characterized. Unfortunately, both the old and new synthetic routes were unsuccessful for the preparation of dimers with a phenyl substituent on the 2-position of the allyl group and for the preparation of related Ni species. However, serendipitously we were able to grow crystals of a type I Ni dimer supported by the IPr ligand, I-Ni-IPr-A (vide inf ra),23 which demonstrates that in principle it is possible to form Ni complexes of this type. 471


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Figure 3. Type I PdI dimers prepared as part of this work. The new compounds are I-IiPr-A, I-IMes-A, and I-SIPr-A. Other compounds were previously prepared in refs 4a and 8d,h.

Figure 4. (a) General reactivity pattern observed for type I and II dimers with CO2 and (b) general reactivity pattern observed for type III dimers with CO2.

this species no CO2-inserted product is observed prior to decomposition. The PCy3-supported complex I-PCy3-A appears to react with CO2 at room temperature, but we were unable to isolate or characterize a CO2-inserted product. Table 1 summarizes the reactivity of all type I complexes studied in our group, including those previously reported.4a Qualitatively all complexes supported by NHC ligands react with CO2 at a similar rate, which is markedly faster than the complexes supported by phosphine ligands, and appear to give cleaner reactivity. The replacement of the bridging allyl ligands with 2-methylallyl ligands results in faster reactions in all cases. Notably, once the type II products are formed, no further reaction with CO2 is observed even at elevated temperature and pressure. This suggests that the presence of a single bridging carboxylate ligand deactivates the second bridging allyl ligand for CO2 insertion. Similarly no CO2 insertion was observed into the bridging allyl ligand of type III complexes, suggesting that the bridging Cl− ligand has a similar effect to the bridging carboxylate ligand.

Several new type III complexes were also synthesized and characterized.21 These compounds were prepared through the treatment of type I complexes with one equivalent of anhydrous HCl (eq 3). When these reactions were performed in a J. Young NMR tube, we were able to detect propene by 1H NMR spectroscopy. Presumably one of the allyl ligands is protonated to form propene gas, and a choride anion replaces the protonated allyl ligand. Compound III-IPr-A is the first example of a type III complex supported by an NHC ligand instead of a phosphine.

We exposed all the newly synthesized complexes to 1 atm of CO2 in benzene. Our observed general reactivity pattern is summarized in Figure 4. Consistent with previous observations,4a all of the new type I complexes supported by NHC ligands react with CO2 according to eq 1. The products were fully characterized. In contrast, the decomposition of I-PiPr3-A through the reductive elimination of 1,5-hexadiene, to form Pd(PiPr3)2 and Pd black (eq 4), is competitive with the rate of CO2 insertion. As a result, we could not cleanly isolate the CO2-inserted product II-PiPr3-A. This decomposition pathway is also evident in the reactions of I-PPh3-A with CO2, and for

The kinetics of CO2 insertion into I-IPr-A, I-IiPr-A, and I-PMe3-2Me were investigated using 1H NMR spectroscopy 472


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Table 1. Qualitative Comparisons of the Reactivity of Type I Bridging Allyl Dimers with CO2 (Based on This Work and Previous Work)4a compound

reactivitya

I-NHC-Ab I-IPr-2Me I-PMe3-A I-PMe3-2Me I-PEt3-A I-PEt3-2Me I-PPh3-A I-PPh3-2Me I-PiPr3-A I-PCy3-A

1−2 h, rt 1 h 15 min, rt; significant side product formation 96 h, 45 °C (no observable reaction at rt) 90 h, rt 70 h, 45 °C 43 h, rt no reaction before decomposition at 45 °C 12 h, rt product observed with significant decomposition at rt no product characterized

Figure 6. Plot of observed rate constant as a function of CO2 pressure. The solid line denotes the linear best fit.

a

Time indicates when complete conversion of starting material was observed by 1H NMR spectroscopy. bNHC = IPr, IiPr, IMes, or SIPr (all rates approximately the same).

Figure 7. Eyring plot for the reaction of I-IPr-A with CO2. The solid line denotes the linear best fit.

Table 2. Eyring Parameters for CO2 Insertion into I-IPr-A, I-IiPr-A, and I-PMe3-2Me complex

ΔH⧧ (kJ/mol)

ΔS⧧ (J/(K mol))

ΔG298⧧ (kJ/mol)

I-IPr-A I-IiPr-A I-PMe3-2Me

38 ± 3 30 ± 2 61 ± 8

−180 ± 9 −202 ± 6 −127 ± 24

90 ± 4 90 ± 2 98 ± 11

in benzene at different temperatures are consistent with this assumption.24 Fitting the data to the Eyring equation gave a linear plot (see Figure 7 for an example), from which the activation parameters were calculated (Table 2). The large negative ΔS⧧ found for all systems is consistent with a transition state that is bimolecular. On the basis of the strikingly similar activation parameters of I-IPr-A and I-IiPr-A, we conclude that steric bulk is not a major factor in the insertion of CO2. We were interested in testing if the addition of an L-type ligand to the reaction mixture affected the rate of CO2 insertion. We expected that if CO2 precoordination was required prior to insertion, then there would be a decrease in the rate. Alternatively excess ligand could enhance the rate of CO2 insertion, by stabilizing a reactive η1-allyl intermediate, which could then react with CO2 (eq 6). Qualitative rate studies were performed with I-PEt3-A, where the rate of CO2 insertion into I-PEt3-A in the presence of various additives was compared with the rate of CO2 insertion in the absence of additive. The reactions were heated at 70 °C and monitored using 31P NMR spectroscopy, except in the case where PEt3 was used as the additive, when the reaction was monitored by 1 H NMR spectroscopy. The starting material and product peak integrations were compared to the integration of an internal standard. The results of this qualitative study are summarized in Table 3. The addition of pyridine had no affect on the rate of reaction, while other hard ligands such as triethylamine and tetrahydrofuran

Figure 5. Plot showing the natural logarithm of [I-IPr-A] versus time at 26.8 °C. The solid line denotes the linear best fit.

under the conditions shown in eq 5. The reactions were followed for three half-lives under pseudo-first-order conditions with a large excess of CO2. In all three systems linear pseudofirst-order plots were obtained when the natural logarithm of the starting material concentration versus time was plotted. A typical plot is shown in Figure 5. This suggests that the reaction is first-order in Pd dimer, which was supported by the observation that the rate constant for the reaction did not change when the initial concentration of I-IPr-A was varied.22 Furthermore, the observed rate for CO2 insertion into I-IPrA is directly proportional to the pressure of CO2, which indicates that the reaction is also first-order in CO2 (Figure 6). Thus the overall rate law for insertion into the dimers is rate = k[CO2][dimer]. Eyring plots were constructed for all three systems based on the pseudo-first-order rate constants obtained at several temperatures. The concentration of CO2 was assumed to be constant over the temperature range studied. Recent measurements by Wendt and co-workers of the concentration of CO2 473


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Table 3. Qualitative Comparison of the Rate of CO2 Insertion into I-PEt3-A in the Presence of Various L-Type Additives additive acetonitrile (25 equiv) propionitrile (25 equiv) benzonitrile (25 equiv) pyridine (25 equiv) triethylamine (25 equiv) tetrahydrofuran (25 equiv) 1,5-hexadiene (25 equiv) 1-octene (25 equiv) PEt3 (1 equiv)

Figure 8. Graph of kobs versus number of equivalents of d3-MeCN for the insertion of CO2 into I-IPr-A. The solid line denotes the best linear fit.

effect on reactivitya 9% more product at 32% conversion 4% more product at 32% conversion 4% more product at 49% conversion, significant decomposition 1% more product at 49% conversion 20% less product at 49% conversion, significant decomposition 29% less product at 49% conversion, significant decomposition 29% less product at 37% conversion, significant decomposition 6% more product at 32% conversion 32% more product at 40% conversion

“9% more product at 32% conversion” means that at 32% conversion (conversion is measured by integrating I-PEt3-A against the internal standard) there is 9% more product (II-PEt3-A) in the reaction with the additive than in the reaction without the additive. This shows that the additive is increasing the rate of the reaction. In many cases measuring the effect of the additive at higher conversion is complicated by decomposition. a

Figure 9. X-ray structure of I-PiPr3-A (hydrogen atoms have been omitted for clarity). Selected bond lengths (Å) and angles (deg) (two independent molecules in unit cell): Pd(1)−Pd(2) 2.7701(8) and 2.7639(8), Pd(1)−P(1) 2.3042(14) and 2.3065(14), Pd(2)−P(2) 2.3060(12) and 2.3045(12), Pd(1)−C(1) 2.159(5) and 2.178(6), Pd(1)−C(2) 2.528(5) and 2.520(5), Pd(1)−C(4) 2.157(6) and 2.154(6), Pd(1)−C(5) 2.570(6) and 2.621(6), Pd(2)−C(3) 2.163(7) and 2.147(7), Pd(2)−C(2) 2.583(5) and 2.599(5), Pd(2)−C(6) 2.173(6) and 2.183(6), Pd(2)−C(5) 2.553(6) and 2.509(6), P(1)− Pd−(1)−Pd(2) 144.34(4) and 148.69(4), P(2)−Pd(2)−Pd(1) 149.76(4) and 149.09(4).

inhibited the formation of the CO2 insertion product and accelerated decomposition. Most of the softer nitrile, phosphine, and olefin additives accelerated the reaction, although in many cases accelerated decomposition was also observed, which limited the number of equivalents of additives that could be utilized and made comparison at high conversion difficult. Interestingly, the chelating olefin, 1,5-hexadiene, inhibited the reaction. When we changed the starting complex from the phosphine-supported species I-PEt3-A to the NHCsupported species I-IPr-A, no decomposition was observed upon CO2 addition in the presence of acetonitrile. Thus, the effect of d3-MeCN on the reaction of CO2 with I-IPr-A was quantified by recording the rate constant at several d3-MeCN concentrations. At the highest measured concentration of acetonitrile (136 equiv), a rate enhancement of 60% was achieved. Overall the observed rate constant was proportional to the concentration of d3-MeCN, although the graph of kobs versus number of equivalents of d3-MeCN had a nonzero intercept (Figure 8). The value obtained for the nonzero intercept was consistent with the rate of the reaction in the absence of MeCN. This is indicative of a two-term rate law in the presence of MeCN, with one term being first-order in [MeCN] and the other term zero-order. Thus, two pathways for CO2 insertion must be present when MeCN is added to the reaction mixture. Comparison of X-ray Structures of Type I, II, and III Bridging Allyl Dimers. The compounds I-PiPr3-A (Figure 9), I-IiPr-A (Figure 10), I-Ni-IPr-A (Figure 11), II-IPr-A (Figure 12), II-SIPr-A (Figure 13), and III-IPr-A (Figure 14) were

Figure 10. X-ray structure of I-IiPr-A (hydrogen atoms and solvent of crystallization have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−Pd(2) 2.6987(5), Pd(1)−C(7) 2.048(5), Pd(2)−C(8) 2.043(5), Pd(1)−C(1) 2.122(6), Pd(1)− C(2) 2.632(5), Pd(1)−C(4) 2.133(6), Pd(1)−C(5) 2.412(5), Pd(2)−C(3) 2.159(6), Pd(2)−C(2) 2.398(5), Pd(2)−C(6) 2.118(5), Pd(2)−C(5) 2.645(5), C(7)−Pd−(1)−Pd(2) 147.25(14), C(8)−Pd(2)−Pd(1) 154.83(15).

characterized by X-ray crystallography. Examples of crystallographically characterized type I and II dimers are rare,4a,8c and there are also a limited number of reports of structurally characterized type III allyl dimers.8f,10a,e,g,h,j,l Thus our new structures allow us to perform detailed structural comparisons between the different types of dimers for the first time. In all 474


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Figure 11. X-ray structure of I-Ni−IPr-A (hydrogen atoms and isopropyl groups have been omitted for clarity). Selected bond lengths (Å) and angles (deg) (inversion center is present in molecule): Ni(1)− Ni(1A) 2.6274(4), Ni(1)−C(1) 1.959(7), Ni(1)−C(2) 2.216(5), Ni(1)−C(4) 1.852(4), Ni(1A)−C(3) 1.941(6), C(4)−Ni(1)−Ni(1A) 162.73(13).

Figure 14. X-ray structure of III-IPr-A (hydrogen atoms and isopropyl groups have been omitted for clarity). In the structure of III-IPr-A the chloride atom and the bridging allyl ligand are disordered and occupy two sites. Only one site is depicted. Selected bond lengths (Å) and angles (deg) (′ represents disordered position): Pd(1)−Pd(2) 2.5849(5), Pd(1)−C(1) 2.057(5), Pd(1)−C(4) 2.024(4), Pd(1)− Cl(1) 2.433(15), Pd(1)-Cl(1′) 2.419(14), Pd(2)−Cl(1) 2.423(11), Pd(2)−Cl(2′) 2.438(10), Pd(1)−C(2) 2.350(6), Pd(1)−C(2′) 2.482(16), Pd(2)−C(2) 2.402(9), Pd(2)−C(2′) 2.397(20), Pd(2)− C(3) 2.053(5), C(4)−Pd(1)−Pd(2) 164.84(12), C(5)−Pd(2)−Pd(1) 164.88(12).

the Pd atoms in closest proximity. The two allyl ligands in the type I complexes are oriented in a syn manner, where the central carbons are oriented on the same side of the molecule. In all complexes, the phosphine or NHC ligands are oriented away from the central carbon of the bridging allyl ligand, so that the Pd(1)−Pd(2)−L (L = phosphine or NHC) bond angle is significantly less than 180°. The structure of the Ni complex I-Ni-IPr-A is similar to the structure of the analogous Pd complex I-IPr-A, which we reported previously,4a and also the structure of I-IiPr-A. In an analogous fashion to the Pd species, the allyl ligands are oriented in a syn manner and the NHC ligands are pointing away from the central carbon of the bridging allyl ligand. As expected, the major difference between the Ni species and the Pd species is a contraction in the metal− metal and metal−ligand bond lengths. For example the Ni−Ni bond distance in I-Ni-IPr-A (2.6274(4) Å) is significantly shorter than the Pd−Pd bond distance in I-IPr-A (2.72002(15) Å).4a Similarly, the Ni−NHC and Ni−bridging allyl bond lengths in I-Ni-IPr-A are almost 0.1 Ǻ shorter than the corresponding distances in I-IPr-A. There is a large difference in the bond lengths between the allyl ligand and the Pd centers in type I, II, and III complexes. Table 4 summarizes the Pd−allyl bond lengths taken from the type I, II, and III structures in the literature and this work.4a,8c,f,10d,l These data show that type I complexes generally exhibit longer Pd bridging allyl bond lengths than type II and III species. Average Pd−allyl bond lengths are ∼0.1 Å shorter for type II and III complexes compared with type I species. The results are most dramatic when analyzing only the terminal allyl position. The maximum bond lengths of type II and III complexes are not as long as the overall minimum bond length observed for type I species. The crystallization of II-IPr-A and III-IPr-A allowed us to compare bond lengths in type I, II, and III systems supported by the same ligands for the first time. The average terminal and central Pd bridging allyl bond lengths in II-IPr-A and III-IPr-A are 0.12, 0.13 Å and 0.07, 0.10 Å, shorter than those of I-IPr-A. We believe that these differences in bond length can be explained by the trans influence of the bridging ligand. The bridging allyl ligand exerts a stronger trans influence than either the bridging chloride or carboxlyate, and

Figure 12. X-ray structure of II-IPr-A (hydrogen atoms and isopropyl groups have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−Pd(2) 2.6650(7), Pd(1)−C(1) 1.996(6), Pd(1)− C(2) 2.369(7), Pd(1)−C(4) 2.014(7), Pd(2)−C(2) 2.376(6), Pd(2)− C(3) 2.018(6), Pd(2)−C(5) 2.059(6), Pd(1)−O(1) 2.166(4), Pd(2)− O(2) 2.187(4), C(6)−C(7) 1.538(9), C(7)−C(8) 1.500(10), C(8)− C(9) 1.326(9), O(1)−C(6) 1.248(9), O(2)−C(6) 1.242(9), C(4)− Pd(1)−Pd(2) 170.00(16), C(5)−Pd(2)−Pd(1) 160.15(14).

Figure 13. X-ray structure of II-SIPr-A (hydrogen atoms and isopropyl groups have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−Pd(2) 2.6766(8), Pd(1)−C(1) 1.992(6), Pd(1)−C(2) 2.401(6), Pd(1)−C(4) 2.033(8), Pd(2)−C(2) 2.345(6), Pd(2)−C(3) 1.985(6), Pd(2)−C(5) 2.048(8), Pd(1)−O(1) 2.157(4), Pd(2)−O(2) 2.163(4), C(6)−C(7) 1.531(10), C(7)−C(8) 1.501(11), C(8)−C(9) 1.320(10), O(1)−C(6) 1.274(9), O(2)−C(6) 1.250(9), C(4)−Pd(1)−Pd(2) 166.85(17), C(5)−Pd(2)−Pd(1) 160.96(16).

cases the central carbon of the bridging allyl ligand is bound to both Pd centers, while the terminal carbons are bound only to 475


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Table 4. Average Pd Bridging Allyl Bond Lengths for Type I, II, and III Structures average Pd−allyl (Å)

maximum Pd−allyl (Å)

minimum Pd−allyl (Å)

type

terminal

central

terminal

central

terminal

central

ref

I II III

2.141 2.020 2.071

2.537 2.412 2.43

2.183 2.047 2.102

2.677 2.494 2.48

2.105 1.984 2.052

2.390 2.344 2.350

4a, 8c, this work 4a, this work 8f, 10d, 10l, this work

hence the Pd bridging allyl bond lengths are longest in type I complexes. Our results are consistent with theoretical calculations by Lin and co-workers20 and also standard trends in trans influence for transition metal complexes.25 Another notable difference between the three types of species is that the Pd−Pd distance is longer in I-IPr-A compared to either II-IPr-A or III-IPr-A. The type III complex IIIIPr-A (Pd−Pd 2.5849(5) Ǻ ) has the shortest distance, presumably because the single atom chloride bridge forces the metal centers to be closer together. The carboxylate ligand also appears to constrain the Pd−Pd distance, as the Pd−Pd bond length in II-IPr-A of 2.6650(7) Ǻ is shorter than that in IIPr-A (Pd−Pd 2.72002(15) Å).4a Although there is still not enough data available to generalize this trend, examination of other related structures in the literature supports this observation.4a,8c Some of the geometrical features of the dimers provide insight into the bonding in these complexes. Previously, Kurosawa and co-workers noted that the dihedral angle between the allyl plane and the M−M−X plane in their phosphine-supported type III complexes is significantly less than 90° (θ in Figure 15a),10j

which provides evidence for the back-bonding interaction from the Pd−Pd bond to the LUMO of the allyl ligand (see Figure 2). We also observe this distortion in the dihedral angle in II-IPr-A and III-IPr-A (θ is 69.71° in II-IPr-A, 69.05° in II-SIPr-A, and 74.15° in III-IPr-A), suggesting that there is also significant back bonding in these NHC-supported complexes. A related phenomenon is evident in the type I complexes. The two bridging allyl moieties are oriented in a syn manner, but planes containing the allyl ligands are not parallel to each other. Instead the central carbons of the allyl ligands are canted inward toward the Pd−Pd bond such that the planes of the allyl ligands intersect in this direction (ϕ in Figure 15b). This dihedral angle is 29.5° and 23.3° for I-PiPr3-A and I-IiPr-A, respectively. Our calculations on the type I system indicate that the system adopts a syn orientation in order to maximize the overlap between the p orbital of the central carbon and the Pd−Pd bond (vide inf ra). The allyl groups may be canted inward in order to further enhance this overlap with the Pd−Pd bond. Computational Studies of the Structure and Reactivity with CO2 of Type I, II, and III Dimers. Structure, Bonding, And Thermodynamics of CO2 Insertion. To gain further insight into the structure and bonding of allyl-bridged dimers and to understand their reactivity with CO2, DFT calculations were performed on a range of different type I, II, and III compounds (Figure 16). Given our experimental results indicated that the steric bulk on the NHC ligand did not play a large role in reactions between type I dimers and CO2, when modeling compounds with NHC ligands, a simplified ligand that contained methyl groups on the two nitrogen atoms of the imidazole was used; this ligand is denoted as NHC′. For calculations involving PPh3, QMMM was used to model the

Figure 15. (a) Dihedral angle formed between the allyl plane and the M−M−X plane in type II and III complexes. (b) Dihedral angle formed between the two allyl planes in type I complexes.

Figure 16. Compounds whose structures were optimized using DFT. 476


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phenyl rings of the phosphines. In general good agreement was obtained between calculated and experimental structures. The crystal structures of all type I dimers show that the orientation of the two allyl groups relative to each other is always syn. In order to understand why the syn isomer is preferred, the structures of a series of type I dimers were optimized with the allyl groups oriented both syn and anti to each other. The energies are compared in Table 5. In all cases the syn isomer is favored and the smallest difference between the two isomers is 8.6 kJ mol−1 for I-NHC′-2Me. The large thermodynamic difference in energy between the conformations is consistent with only one isomer being observed by 1H NMR spectroscopy. Comparison of the molecular orbitals of the syn and anti isomers reveals that the HOMO−1 is significantly more stable in the syn isomer (Figure 17). The HOMO−1 is a Pd−Pd bonding orbital, which also has a bonding interaction with the p orbitals of the central carbons of the allyl groups (as well as an anti-bonding interaction between the metal and the ancillary ligand). In the syn isomer the central carbons of the allyl group are directly aligned with each other, whereas in the anti isomer they are offset. As a result, the overlap is greater in the syn isomer, which leads to an orbital that is lower in energy.

Table 6. Calculated Gibbs Free Energies for the Insertion of CO2 into the Allyl Group of Different Allyl-Bridged Dimersa

compound

syn

anti

0 0 0 0 0 0

13.6 14.7 12.3 8.9 8.7 8.6

allyl substituent

PMe3

PEt3

PPh3

NHC

I I II II III III

A 2-Me A 2-Me A 2-Me

−45.8 −39.1 54.9 59.7 129.3 108.9

−46.5 −55.6 55.3 63.2 115.8 108.1

−58.1 −52.8

−49.2 −48.5 44.4 22.1 95.7 85.2

a

Energies given in kJ mol−1.

CO2 insertion). In contrast CO2 insertion into both type II and III dimers is thermodynamically uphill. Lin and co-workers recently proposed that the thermodynamic favorability of CO2 insertion into bridging allyl dimers is related to the trans influence of the ligand trans to the bridging allyl that is undergoing CO2 insertion, and our results are consistent with this hypothesis.20 There are also differences in the HOMOs of the type II and III dimers compared with type I dimers. For type I species the HOMO is localized on the terminal carbons of the bridging allyl groups, whereas for the chloride and carboxylate complexes the HOMO is based on the central carbon of the bridging allyl and the Pd centers (Figure 18). This difference is caused by the presence of two allyl groups in type I dimers. The principal orbital of a π-allyl group employed for bonding is the π2, which contains one node between the π orbitals.25 In the type I dimers these form in-phase and out-of-phase linear combinations. The in-phase combination forms a strong bond with the Pd 4d orbitals, and the resulting MO is stabilized and lies low in the orbital manifold. The same occurs with the single allyl π2 orbital of the type II and type II dimers. However the out-of-phase combination of the π2 orbitals in the type I dimers may interact effectively only with Pd 5p orbitals, which are high in energy and inefficient at bonding. The consequence is that the out-of-phase combination is not stabilized and forms the HOMO. The localization of the HOMO on the bridging allyl ligand in type I dimers is similar to the localization of the HOMO on the terminal olefin in Pd species containing η1-allyl ligands and is a potential source of nucleophilicity located on the terminal carbons.6 Mechanism of CO2 Insertion. Various mechanisms can be envisaged for the reaction of type I dimers with CO2. As part of our computational study three mechanisms have been considered: (a) decoordination of a bridging allyl ligand from one Pd center to form an η1-allyl, (b) direct attack of CO2 on the bridging allyl ligand, and (c) a mechanism that involves initial dissociation of the dimer to form two monomers. In order to assess the plausibility of our computational models, we have compared the calculated barriers with the experimental barriers for CO2 insertion into I-NHC-A and I-PMe3-2Me (ΔG ⧧ = 90 ± 4 kJ mol−1 for I-NHC-A and 98 ± 11 kJ mol−1 for I-PMe3-2Me at 298 K).

Table 5. Relative SCF Energies of syn and anti Isomers of [(allyl)2Pd2(L)2] and [(2-methylallyl)2Pd2(L)2]a

I-PMe3-A I-PEt3-A I-NHC′-A I-PMe3-2Me I-PEt3-2Me I-NHC′-2Me

compound

Relative energies are given in kJ mol−1 with the lowest energy structures assigned as being at 0 kJ mol−1.

a

Figure 17. Comparison of the HOMO−1 for syn and anti I-PMe32Me.

Experimentally CO2 insertion occurred for all type I dimers that were tested, except for I-PPh3-A and I-PCy3-A, but no insertion was observed into any type II or III dimers. Our calculations indicate that CO2 insertion is thermodynamically favorable only for type I dimers (Table 6). In this case the reaction is favorable by between 40 and 60 kJ mol−1 for all ligand systems that we explored. This suggests that the lack of reactivity between I-PPh3-A and CO2 is purely related to kinetic factors (the decomposition of the dimer is faster than 477


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Figure 18. Comparison of the HOMOs in I-PMe3-2Me, II-PMe3-2Me, and III-PMe3-2Me.

Scheme 1. Pathway for CO2 Insertion Involving Initial Formation of an η1-Allyl-Containing Species

ligand and the Pd center. In the proposed rate-determining step (TS2Lin) these agostic interactions are broken to allow one of the oxygen atoms of the carboxylate to coordinate to Pd, resulting in a complex (INT2Lin) containing an anionic η1-carboxylate bound to one Pd and a coordinated olefin bound to the other. In INT2Lin the Pd−Pd bond is formally a dative interaction between one Pd0 center and a PdII center. A facile rotation of the carboxylate (TS3Lin) results in the displacement of the olefin and coordination of the carboxylate to both Pd centers. Our experimental barrier for CO2 insertion into I-PMe3-2Me is lower than Lin’s calculated barrier of 118.8 kJ mol−1.20 In order to investigate why Lin’s mechanism is not consistent with our experimental work, we recalculated Lin’s pathway for both I-PMe3-2Me and I-NHC′-A using our computational method. The energies are summarized in Table 8. Our computational results for I-PMe3-2Me are analogous to Lin’s and give an identical barrier, which is higher than experiment. Furthermore, an internal reaction coordinate (IRC) calculation starting at TS2Lin for I-PMe3-2Me failed to locate a pathway connecting TS2Lin to either INT1Lin or INT2Lin, and a transition state corresponding to TS2Lin could not be found for I-NHC′-A. As TS2Lin is proposed to be the highest point on the potential energy surface, we do not have an accurate calculated barrier for CO2 insertion into I-NHC′-A. The calculated energy for TS1Lin of I-NHC′-A is 96.4 kJ mol−1, slightly higher than the experimental value of 90 kJ mol−1, suggesting that there is not a good agreement between experiment and theory for the NHC-supported compound either. Overall, although we are unable unambiguously to rule out the Lin mechanism, our results suggest that it is unlikely. In the Lin pathway CO2 approaches the bridging allyl dimer so that the new C−C bond is essentially formed in the plane containing the Pd atoms and the carbons of the bridging allyl (Figure 20);20 the Pd(1)−C(4)−C(7) bond angle is 151.1°. We were able to locate an alternative transition state for C−C bond formation in which CO2 attacks the dimer so that the C−C bond is formed in a plane perpendicular to that containing the Pd atoms and the carbons of the bridging allyl (Figure 20);

Previously, it has been demonstrated that the reaction of Pd η1-allyls with CO2 is facile.3a−c,24 One possible mechanism for CO2 insertion into type I dimers involves the initial formation of an η1-allyl, followed by nucleophilic attack of the η1-allyl on CO2 (Scheme 1). To test the validity of this mechanism, the structures of Pd dimers with one bridging allyl and one η1-allyl were optimized. The optimized structures of these species contain an agostic interaction between the C−H bond of the η1-allyl and the Pd center not directly bound to the η1-allyl, as shown in Figure 19. This presumably stabilizes the coordinatively unsaturated Pd center. Energetically the formation of an η1-allyl ligand is unfavorable but still lower in energy than the experimental barrier for CO2 insertion, in cases where direct comparison is possible (Table 7). As expected the η1-allyl moiety is nucleophilic, with the HOMO being partially localized on the terminal olefin of the η1-allyl, suggesting that it can interact with electrophilic CO2 in an analogous fashion to monomeric systems (Figure 19).3a−c,24 However, the energy of the TS for the reaction of the η1-allyl with CO2 is significantly larger than the experimental barrier for CO2 insertion, eliminating this mechanism (Table 7 and Figure 19). We postulated that the formation of an η1-allyl-containing dimer could be stabilized by the addition of another neutral ligand, which could coordinate to the Pd center not directly bound to the η1-allyl, replacing the agostic interaction. Surprisingly, our calculations predict that the binding of MeCN to an η1-allylcontaining dimer is highly unfavorable relative to both the starting dimeric compound and the species containing an η1allyl ligand (Table 7 and Figure 19). This is consistent with no visible change being observed in the 1H NMR spectra when an excess of THF or MeCN is added to solutions of type I dimers in toluene or benzene. Recently Lin et al. suggested that CO2 insertion into I-PMe32Me proceeds through a multistep mechanism involving nucleophilic attack by one of the terminal carbons of the bridging allyl group on CO2 to form a C−C bond (Scheme 2).20 The resulting intermediate (INT1Lin) is stabilized by two agostic interactions between the two hydrogens of the carboxylate 478


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Figure 19. (a) Optimized structure and HOMO of η1-allyl of I-PMe3-2Me (selected hydrogen atoms omitted for clarity). Selected bond distances (Ǻ ) and angles (deg): Pd(1)−Pd(2) 2.67, Pd(1)−C(1) 2.16, Pd(1)−C(2) 2.46, Pd(2)−C(3) 2.14, Pd(2)−C(2) 2.75, Pd(1)−C(4) 2.17, Pd(2)− H(1) 2.06, C(1)−C(2) 1.42, C(2)−C(3) 1.42, C(4)−C(5) 1.49, C(5)−C(6) 1.35, C(1)−Pd(1)−C(4) 161.3, Pd(1)−C(4)−C(5) 111.4, C(4)− C(5)−C(6) 124.1. (b) Transition state for insertion of CO2 into η1-allyl of I-PMe3-2Me (selected hydrogen atoms omitted for clarity). Selected bond distances (Ǻ ) and angles (deg): Pd(1)−Pd(2) 2.68, Pd(1)−C(1) 2.11, Pd(1)−C(2) 2.49, Pd(2)−C(3) 2.09, Pd(2)−C(2) 2.57, Pd(1)−C(4) 2.30, Pd(2)−H(1) 2.04, C(1)−C(2) 1.43, C(2)−C(3) 1.43, C(4)−C(5) 1.43, C(5)−C(6) 1.39, C(6)−C(7) 2.15, C(1)−Pd(1)−C(4) 153.4, Pd(1)−C(4)−C(5) 101.5, C(4)−C(5)−C(6) 124.2, O(2)−C(7)−O(1) 151.6. (c) Optimized structure of MeCN adduct of I-PMe3-A (hydrogen atoms omitted for clarity). Selected bond distances (Ǻ ) and angles (deg): Pd(1)−Pd(2) 2.79, Pd(1)−C(1) 2.10, Pd(1)−C(2) 2.59, Pd(2)−C(3) 2.11, Pd(2)−C(2) 2.42, Pd(1)−C(4) 2.21, Pd(2)−N(1) 2.32, C(1)−C(2) 1.45, C(2)−C(3) 1.41, C(4)−C(5) 1.47, C(5)−C(6) 1.35, C(1)− Pd(1)−C(4) 167.7, Pd(1)−C(4)−C(5) 110.7, C(4)−C(5)−C(6) 127.8.

Table 7. Calculated Gibbs Free Energies for the Formation of an η1-Allyl from Type I Dimers, in the Presence and Absence of MeCNa compound

η1-allyl

TS-CO2-η1-allylb

η1-allyl-MeCNc

I-PMe3-A I-NHC′-A I-PMe3-2Me I-NHC′-2Me

69.6 63.7 79.3 60.3

135.8 120.4 138.1 111.5

109.3 98.6

I-NHC′-A than the Lin mechanism (Table 9), and there appears to be slightly better agreement between the computational and experimental barriers. The final mechanism we considered involved the initial splitting of the type I dimer into two monomers. Our kinetics experiments indicate that the overall rate law for the reaction is rate = k[CO2][dimer]. For this rate law to be consistent with initial monomer formation, a situation where the dimer splits into two monomers, one of which reacts with CO2, followed by rate-determining association between a monomer that has reacted with CO2 and a monomer that has not reacted with CO2, is required. There are several different monomer/dimer equilibria that could be occurring, and these are summarized in Figure 21. In case (i) homolytic dissociation of the Pd−Pd single bond is coupled with decoordination of each of the bridging allyls from the opposite Pd to give two identical PdI monomers. Alternatively in (ii) heterolytic dissociation of the Pd−Pd single bond is coupled with decoordination of each of the bridging allyls from the opposite Pd to give one positively charged PdII complex and one negatively charged Pd0 species. Finally disproportionation of the dimer could occur to give a singly ligated Pd0 species and a bis(allyl)PdII(L) species, as depicted in (iii). The reverse of this process has been proposed as a mechanism for the formation of type I dimers.5a,8a,g Calculations on the monomer/dimer equilibria indicate that the formation of a monomer is highly unfavorable for all three

d

100.5

Energies are given in kJ mol−1 relative to the dimeric starting material. bEnergy relative to the dimeric starting material and free CO2. cEnergy relative to the dimeric starting material and free MeCN. d Convergence on an optimized structure with MeCN coordinated was not achieved. a

the Pd(1)−C(4)−C(7) bond angle is 70.1°. Apart from the variation in this crucial Pd(1)−C(4)−C(7) bond angle, most of the other geometric parameters are similar between the two transition states, and in both cases the nucleophilic bridging allyl is attacking electrophilic CO2. Importantly, examination of the potential energy surface starting from our new TS demonstrates that there is a low-energy pathway connecting the TS to the products, involving a single intermediate (Scheme 3). This intermediate is analogous to INT2Lin in the Lin mechanism, and therefore our new mechanism avoids the formation of the unusual complex with two agostic interactions (INT1Lin above). Furthermore the calculated activation energy for our new pathway is lower in energy for both I-PMe3-2Me and 479


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Scheme 2. Pathway for CO2 Insertion Proposed by Lin Co-workers18

combined with the unfavorable formation of the monomer (70 kJ mol−1 for I-NHC′-A), it strongly suggests that this mechanism is not feasible for CO2 insertion into the dimer. An interesting aspect of our kinetic studies is the increase in the rate of CO2 insertion when MeCN or an olefin, such as 1-octene, is present as an additive. Although we have not performed exhaustive calculations to investigate this effect, we believe the increase in rate occurs because the addition of the extra L-type ligand assists in the stabilization of monomer/ dimer equilibrium (iii). The extra L-type ligand coordinates to the Pd0 species that is formed after the dimer splits (eq 7), and as a result, it is not as unfavorable for the dimer to form monomeric products (Table 11). The presence of both MeCN and 1-octene results in the formation of more stable Pd0

Table 8. Calculated Gibbs Free Energies for CO2 Insertion into Type I Dimers Using the Pathway Proposed by Lin and Co-workers20a compound I-NHC′-A I-PMe3-2Me

TS1Lin 96.4 105.6

INT1Lin

TS2Lin

INT2Lin

TS3Lin

79.7 92.5

b

−9.3 6.0

−3.2 10.0

118.3

Energies are given in kJ mol−1 relative to the dimeric starting material. bConvergence on an optimized transition state was not achieved. a

cases (Table 10). In the cases of (i) and (ii) the energy required to form the monomers is higher than the experimental barrier for CO2 insertion, indicating that these mechanisms are not viable. In the case of (iii) the calculated energy for the formation of a Pd0 species and bis(allyl)PdII(L) is lower than the experimental value for CO2 insertion. However, we have previously calculated the barrier for CO2 insertion into complexes of the type bis(allyl)PdII(L) as being at least 40 kJ mol−1 for a NHC-supported species.3a When this activation energy is

Figure 20. (a) TS1Lin for CO2 insertion into I-PMe3-2Me using mechanism proposed by Lin (hydrogen atoms omitted for clarity). Selected bond distances (Ǻ ) and angles (deg): Pd(1)−Pd(2) 2.79, Pd(1)−C(1) 2.09, Pd(1)−C(2) 2.51, Pd(2)−C(3) 2.10, Pd(2)−C(2) 2.58, Pd(1)−C(4) 2.43, Pd(1)−C(5) 2.96, Pd(2)−C(6) 2.34, Pd(2)−C(5) 2.63, C(1)−C(2) 1.43, C(2)−C(3) 1.43, C(4)−C(5) 1.43, C(5)−C(6) 1.38, C(4)−C(7) 2.01, C(1)−Pd(1)−C(4) 167.2, Pd(1)−C(4)−C(5) 96.6, Pd(1)−C(4)−C(7) 151.1, C(4)−C(5)−C(6) 123.5, O(1)−C(7)−O(2) 147.6. (b) TS1 for insertion of CO2 into I-PMe3-2Me with our new mechanism (selected hydrogen atoms omitted for clarity). Selected bond distances (Ǻ ) and angles (deg): Pd(1)−Pd(2) 2.78, Pd(1)−C(1) 2.08, Pd(1)−C(2) 2.63, Pd(2)−C(3) 2.13, Pd(2)−C(2) 2.49, Pd(1)−C(4) 2.46, Pd(1)−C(5) 2.78, Pd(2)−C(6) 2.34, Pd(2)−C(5) 2.63, C(1)−C(2) 1.44, C(2)−C(3) 1.42, C(4)−C(5) 1.46, C(5)−C(6) 1.38, C(4)−C(7) 2.13, C(1)−Pd(1)−C(4) 162.7, Pd(1)−C(4)−C(5) 86.8, Pd(1)−C(4)−C(7) 70.1, C(4)−C(5)−C(6) 122.5, O(1)−C(7)−O(2) 147.6. 480


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Scheme 3. Our Proposed Lowest Energy Pathway for CO2 Insertion

Table 10. Calculated Gibbs Free Energies for the Formation of Different Monomers from Type I Dimersa compound

equilibrium (i)

equilibrium (ii)b

equilibrium (iii)


114.5 115.8 100.5 110.9 129.4 87.6

353.6 359.6 334.3 346.6 343.8 316.7

95.3 98.8 76.9 96.9 91.8 62.5

Energies are given in kJ mol−1 relative to the dimeric starting material. bEnergy includes solvent correction (for benzene) due to the formation of charged products. a

Table 11. Calculated Gibbs Free Energies for the Formation of bis(allyl)Pd(L) and a Pd0 Monomer Supported by an Additional Ligand from Type I Dimersa added ligand

Table 9. Calculated Gibbs Free Energies for CO2 Insertion into Type I Dimers Using Our New Mechanism20a compound

TS1

INT1

TS2

I-NHC′-A I-PMe3-2Me

83.3 113.1

−9.3 6.0

−3.2 10.0

allyl dimer

MeCN

1-octene

benzene


63.8 71.4 43.6 65.4 66.1 29.3

56.5 60.6 30.3 58.0 53.5 16.0

70.5 77.6 52.5 72.0 70.6 38.1

Energies are given in kJ mol−1 relative to the dimeric starting material and the additional ligand. a

Energies are given in kJ mol−1 relative to the dimeric starting material. a

the stabilized Pd0 species to generate the bridging carboxylate product. The trapping of the CO2-inserted product by the Pd0 species is presumably the rate-determining step, as our experiments indicate the reaction is first-order in dimer. When the activation energy for CO2 insertion into bis(allyl)Pd(L) is combined with the energy required for the formation of the monomers, the energies are slightly lower than the direct insertion into the dimeric product, which is consistent with our hypothesis. The feasibility of monomer/dimer equilibrium (iii) in the presence of coordinating ligands could also explain why we see increased decomposition in many reactions between CO2 and type I dimers in the presence of additives. Complexes of the type bis(allyl)Pd(L) are quite unstable in the presence of other coordinating ligands, especially ligands such as phosphines and tetrahydrofuran, and decomposition is therefore more likely to be observed when these additives are present. Overall, on the basis of our calculations we conclude that in the absence of a coordinating additive, CO2 insertion into type I dimers is a two-step process involving initial rate-determining nucleophilic attack on CO2, followed by a facile ligand substitution to give the observed product (Scheme 3). The calculated barriers are in reasonable agreement with the experimentally determined barriers, especially given the inherent problems using gas phase entropy calculations for solution reactions.20 Our proposed mechanism is similar to the reaction of monomeric η1-Pd and Ni allyls with CO23a,b,d,24 and strongly suggests that bridging allyls are able to act as nucleophiles like η1-allyls. In the presence of a weakly coordinating ligand we believe two pathways are possible for CO2 insertion: (i) direct insertion in which the dimer stays intact as described above or (ii) insertion via a monomer/dimer equilibrium in which the coordinating ligand stabilizes one of the monomers. Only the

Figure 21. Potential monomer/dimer equilibria for type I dimers.

complexes compared with coordinated benzene (Table 11), which probably stabilizes the Pd0 species in the absence of an additive. This disproportionation was observed experimentally, as Pd(IPr)2 was seen slowly growing in the 1H NMR spectrum when one equivalent of free IPr was added to I-IPr-A. After formation of the stabilized Pd0 monomers, we then propose that the bis(allyl)Pd(L) species reacts with CO2, via the mechanism we have previously described.3a Subsequently, the CO2-inserted monomer or an intermediate along this pathway is trapped by 481


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triphenyl phosphate, ferrocene, phenolphthalein, and PiPr3 (4.7% wt in hexane) were purchased from Aldrich. 1,5-Hexadiene was purchased from TCI America, while 1-octene and palladium chloride were purchased from Acros Organics. Anhydrous CO2 was obtained from Airgas Inc. and was not dried prior to use. Deuterated solvents were obtained from Cambridge Isotope Laboratories. C6D6 was dried over sodium metal. CD3CN, triethylamine, and pyridine were dried over calcium hydride prior to use. NMR spectra were recorded on Bruker AMX-400 and -500 spectrometers at ambient probe temperatures unless noted. Chemical shifts are reported with respect to residual internal protio solvent for 1H and 13C{1H} NMR spectra and to an external standard for 31P{1H} spectra (85% H3PO4 at δ 0.0 ppm). Atom numbering for the peak assignments is given below. All assignments are based on two-dimensional 1H,13C-HMQC and 1H,13C-HMBC experiments. IR spectra were measured using a diamond Smart Orbit ATR on a Nicolet 6700 FT-IR instrument. Robertson Microlit Laboratories, Inc. performed the elemental analyses (inert atmosphere). Literature procedures were followed to prepare the following compounds: {(η3allyl)PdCl} 2 , 26 (η 3 -allyl) 2 Pd, 26 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride (IMesHCl),27 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr),27 allyl magnesium chloride,28 salicaldehyde phenylhydrazone,29 anhydrous HCl in ether,30 I-PMe3-A,4a I-PEt3-A,4a I-IPr-A,4a I-PEt3-2Me,4a and II-IPr-A.4a The concentration of allyl magnesium chloride was determined through a titration using salicylaldehyde phenylhydrazone as an indicator.29 The concentration of HCl was determined through an acid−base titration using phenolphthalein as an indicator. Experimental Procedures and Characterizing Data for New Compounds. I-SIPr-A. KOtBu (45 mg, 0.40 mmol) was added to a suspension of SIPrHBF4 (150 mg, 0.31 mmol) in 4 mL of THF. The mixture was stirred at rt for 0.5 h, and then the volatiles were removed by vacuum evaporation. The resulting residue was extracted using toluene (2 × 4 mL) and filtered through Celite. The toluene solution containing SIPr was added to a solution of {(η3-allyl)PdCl}2 (57 mg, 0.31 mmol) in 2 mL of THF (100% conversion to SIPr was assumed). The mixture was stirred at rt for 0.5 h, and then (allyl)MgCl (1.4 mL, 0.32 M in ether, 0.45 mmol) was introduced. After 4 h the volatiles were removed under reduced pressure. The resulting residue was extracted with benzene (3 × 5 mL) and filtered through Celite. The solution was evaporated, washed with cold pentane, and dried under reduced pressure to give I-SIPr-A as pale yellow solid. Yield: 141 mg (0.13 mmol, 84%). Anal. Calcd (found) for C60H86N4Pd2: C, 66.96 (67.04); H, 8.05 (7.88); N, 5.21 (4.88).

monomer/dimer equilibrium pathway has a dependence on [MeCN] or [1-octene], which is consistent with our experimentally observed two-term rate law in the presence of these additives.

■

CONCLUSIONS We have prepared a range of PdI dimers that contain two bridging allyl ligands, one bridging allyl and one bridging carboxlyate ligand, and one bridging allyl and one bridging chloride ligand, respectively. Structurally the three different types of dimers are similar, but the Pd bridging allyl bond lengths are strongly influenced by the nature of the other bridging ligand. A bridging allyl ligand has the strongest trans influence, and as a result, the Pd carbon bond lengths in compounds with two bridging allyl ligands are longer than in carboxylate- or chloride-bridged species. The three different types of dimers also have different electronic structures. The HOMO in complexes with two bridging allyl ligands is localized on the terminal carbon atoms of the bridging allyl ligands, while in complexes with one bridging allyl and one bridging carboxylate ligand, and one bridging allyl and one bridging chloride ligand, the HOMO is a Pd−Pd bonding orbital, which also has a bonding interaction with the p orbitals of the central carbons of the allyl groups. The relative weakness of the Pd bridging allyl bond in doubly bridged allyl dimers means that CO2 insertion into the bridging allyl ligand is thermodynamically favorable for these species, whereas it is unfavorable for species that contain a bridging carboxylate or chloride ligand. Our detailed experimental and computational studies indicate that CO2 insertion into doubly bridged allyl dimers can occur via two different pathways. In one pathway the bridging allyl ligand acts like a nucleophile and attacks electrophilic CO2. This is similar to the pathway through which CO2 inserts into monomeric Pd and Ni η1-allyls.3a,b,d,24 In an alternative pathway, which occurs only in the presence of a weakly coordinating ligand, the doubly bridged dimer splits into bis(allyl)Pd(L) and a Pd0 species stabilized by the weakly coordinating ligand. The bis(allyl)Pd(L) species then reacts with CO2, before recombining with the Pd0 species to form the dimeric product. Thus, in the specific case of CO2 insertion reactions, doubly bridged allyl dimers can act either as a source of η1-Pd allyls in the presence of weakly coordinating ligands or as analogues of η1-Pd allyls in the absence of coordinating ligands. In future work we plan to test both if PdI dimers with two bridging allyl ligands can act as more stable surrogates for monomeric η1-Pd allyls in other reactions and also if PdI dimers with bridging cyclopentadienyl and indenyl ligands will react with CO2.

■

1

H NMR (500 MHz, C6D6): 7.11 (d, J = 10 Hz, 4H, H9), 6.98 (d, J = 10 Hz, 8H, H8), 3.57 (s, 8H, H5), 3.54 (m, 8H, H10), 2.89 (m, 2H, H2), 1.95 (d, J = 10 Hz, 4H, H1 and H3), 1.14 (d, J = 5 Hz, 24H, H11 or H12), 1.09 (d, J = 10 Hz, 24H, H11 or H12), 0.79 (d, J = 10 Hz, 4H, H1 and H3). 13C{1H} NMR (125.8 MHz): 227.1, 147.4, 138.9, 128.9, 124.6, 85.1, 54.4, 28.9, 26.8, 24.4, 24.1. I-IMes-A. To a suspension of IMesHCl (150 mg, 0.44 mmol) in 3 mL of THF was added KOtBu (48 mg, 0.63 mmol). The mixture was stirred for 0.5 h at rt. Then the volatiles were removed under reduced pressure, and the residue was extracted using toluene (2 × 2 mL). The toluene solution containing IMes was added to a solution of {(η3-allyl)PdCl}2 (80 mg, 0.22 mmol) in 2 mL of THF (100% conversion to IMes was assumed). The mixture was stirred for 0.5 h at rt, and then (allyl)MgCl (1.8 mL, 0.32 M in ether, 0.58 mmol) was added. After 1 h the volatiles were removed under reduced pressure. The residue was extracted with benzene (2 × 3 mL) and filtered through Celite. The benzene solution was heated at 50 °C for 12 h and filtered through Celite, and then the volatiles were removed under reduced pressure. The resulting residue was washed with cold pentane (2 × 1 mL) and dried under reduced pressure to give I-IMes-A as a pale yellow solid. Yield: 83 mg (0.092 mmol, 42%). Anal.

EXPERIMENTAL SECTION

General Procedures. Experiments were performed under a dinitrogen atmosphere in an M-Braun drybox or using standard Schlenk techniques. (Under standard glovebox conditions purging was not performed between uses of hexane, diethyl ether, benzene, toluene, and THF; thus when any of these solvents were used, traces of all these solvents were in the atmosphere and could be found intermixed in the solvent bottles.) Moisture- and air-sensitive liquids were transferred by stainless steel cannula on a Schlenk line or in a drybox. The solvents for air- and moisture-sensitive reactions were dried by passage through a column of activated alumina followed by storage under dinitrogen. All commercial chemicals were used as received except where noted. KOtBu, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium tetrafluoroborate (SIPrHBF4), 1,3-diisopropylimidazolium tetrafluoroborate (IiPrHBF4), triethylphosphine, pyridine, triethylamine, 482


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Anal. Calcd (found) for C49H58N4O2Pd2: C, 62.09 (61.87); H, 6.17 (5.68); N, 5.91 (5.86).

Calcd (found) for C48H58N4Pd2: C, 63.79 (64.21); H, 6.47 (6.39); N, 6.20 (6.60).

1

H NMR (500 MHz, C6D6): 6.71 (s, 8H, H8), 6.38 (s, 4H, H5), 3.18 (m, 2H, H2), 2.15 (d, J = 10 Hz, 4H, H1 and H3), 2.12 (s, 12H, H10), 2.09 (s, 24H, H11), 0.91 (d, J = 10 Hz, 4H, H1 and H3). 13 C{1H} NMR (125.8 MHz): 196.6, 138.4, 137.4, 136.0, 129.3, 122.5, 84.5, 24.6, 21.4, 19.0. I-IiPr-A. IiPrHBF4 (104.9 mg, 0.44 mmol) and KOtBu (54 mg, 0.48 mmol) were suspended in 5 mL of THF. This mixture was stirred for 0.5 h at rt. The volatiles were then removed under reduced pressure, and the residue was extracted with toluene (2 × 2 mL) and filtered using a cannula. The toluene solution containing IiPr was added to a solution of (η3-allyl)2Pd (83.4 mg, 0.44 mmol) in 2 mL of toluene (100% conversion to IiPr was assumed). The mixture was stirred for 0.5 h at rt and then stirred in an oil bath at 70 °C for 12 h. The resultant reaction mixture was filtered, and the volatiles were removed under reduced pressure. The remaining residue was extracted with pentane (2 × 15 mL), and the pentane was removed under reduced pressure to give I-IiPr-A as a yellow solid. Yield: 52 mg (0.087 mmol, 39%). The product could be further purified through recrystallization from a saturated solution of cold pentane at −35 °C. Crystals for X-ray analysis were grown from a saturated benzene solution at rt. Anal. Calcd (found) for C24H42N4Pd2: C, 48.09 (49.16); H, 7.06 (7.41); N, 9.35 (9.03). It is unclear why the combustion analysis on this compound is slightly outside the acceptable range; a 1H NMR spectrum is provided as part of the Supporting Information to support that this compound was in fact made in high purity. 1 H NMR (300 MHz, C6D6): 6.57 (d, J = 1.8 Hz, 2H, imidazole H), 6.52 (d, J = 1.8 Hz, 2H, imidazole H), 5.11 (septet, J = 6.8 Hz, 2H, iPr CH), 4.44 (septet, J = 6.8 Hz, 2H, iPr CH), 4.12 (m, 2H, allyl-central H), 2.87 (d, J = 7.8 Hz, 4H, allyl-end H), 1.80 (d, J = 12.4 Hz, 4H, allyl-end H), 1.09 (d, J = 6.8 Hz, 12H, iPr CH3), 0.90 (d, J = 6.8 Hz, 12H, iPr CH3). 13C{1H} NMR (75 MHz, C6D6): 195.0, 116.3, 116.3, 81.2, 52.0, 51.7, 23.4, 23.1. II-SiPr-A. Excess 1 atm CO2 was added via a dual-manifold Schlenk line to a suspension of I-SiPr-A (11.0 mg, 0.010 mmol) in 0.4 mL of benzene at rt. The mixture was stirred for 2 h and then dried under reduced pressure to give II-SiPr-A as an off-white solid. Yield: 11.1 mg (0.010 mmol, 97%). Anal. Calcd (found) for C61H86N4O2Pd2: C, 65.40 (65.18); H, 7.74 (7.78); N, 5.00 (4.84).

IR (cm−1): 1575 (νCO2), 1395 (νCO2). 1H NMR (500 MHz, C6D6): 6.80 and 6.78 (8H, H10), 6.26 (s, 4H, H7), 6.18 (m, 1H, H2), 5.08 (m, 2H, H1), 2.97 (d, J = 10 Hz, 2H, H3), 2.19 (s, 36H, H12 and H13), 2.01 (d, J = 10 Hz, 2H, H4), 1.66 (m, 1H, H5), 0.27 (d, J = 10 Hz, 2H, H4′). 13C{1H} NMR (125.8 MHz, C6D6): 192.8, 181.2, 138.1, 137.7, 137.6, 136.3, 129.2, 128.7, 125.1, 121.3, 114.0, 43.8, 43.0, 24.2, 21.54, 18.8, 18.7. II-IiPr-A. Excess 1 atm CO2 was added via a dual-manifold Schlenk line to a solution of I-IiPr-A (9.8 mg, 0.016 mmol) in 0.45 mL of benzene at rt. The mixture was stirred for 3 h, and the volatiles were removed under reduced pressure. The resulting residue was extracted with pentane (2 × 2 mL), and the pentane was removed under reduced pressure to give II-IiPr-A as a pale yellow solid. Yield: 4.1 mg (0.006 mmol, 39%). Anal. Calcd (found) for C25H42N4O2Pd2: C, 46.66 (44.47); H, 6.58 (6.22); N, 8.71 (8.09). It is unclear why the combustion analysis on this compound is slightly outside the acceptable range; a 1H NMR spectrum is provided as part of the Supporting Information to support that this compound was in fact made in high purity.

IR (cm−1): 1577 (νCO2), 1370 (νCO2). 1H NMR (300 MHz, C6D6): 6.40 (s, 4H, imidazole H), 6.34 (m, 1H, H4), 5.48 (septet, J = 6.8 Hz, 4H, iPr CH), 5.11−4.91 (m, 2H, H5), 3.30 (dt, J = 7.0, 1.5 Hz, 2H, H3), 2.87−2.67 (m, 3H, H1 and H2), 1.45 (d, J = 9.5 Hz, 2H, H1), 1.10 (broad, 24H, iPr CH3). 13C{1H} NMR (75 MHz, C6D6): 188.43, 183.08, 136.76, 115.19, 114.27, 52.12, 44.15, 43.18, 23.76, 23.67. III-PMe3-A. HCl in diethyl ether (0.0736 M, 0.76 mL, 0.056 mmol) was added dropwise to a solution of I-PMe3-A (25.1 mg, 0.044 mmol) in 5 mL of THF at rt. This solution was stirred for 20 min. The volatiles were removed under reduced pressure to yield III-PMe3-A as a dark yellow solid. Yield: 19.4 mg (0.044 mmol, 78%). The product was further purified through recrystallization from a saturated solution of cold pentane at −35 °C. Anal. Calcd (found) for C9H23ClP2Pd2: C, 24.48 (23.99); H, 5.25 (5.02). 1 H NMR (300 MHz, C6D6): 3.11 (m, 2H, allyl-end H), 2.69 (m, 1H, allyl-central H), 1.65 (d, J = 11.7 Hz, 2H, allyl-end H), 1.02 (s, 18H, PCH3). 13C{1H} NMR (75 MHz, C6D6): 63.88, 33.85, 17.87. 31 1 P{ H} NMR (121 MHz, C6D6): −23.2. III-PEt3-A. HCl in diethyl ether (0.95 mL, 0.0736 M in ether, 0.070 mmol) was added dropwise to a solution of I-PEt3-A (37.0 mg, 0.070 mmol) in 5 mL of THF at rt. This solution was stirred for 20 min. The volatiles were removed under reduced pressure. The resulting residue was washed with pentane (1 mL) and dried in vacuo to yield III-PEt3-A as a brown solid. Yield: 30.1 mg (0.057 mmol, 82%). The product was further purified through recrystallization from a saturated solution of cold pentane at −35 °C. Anal. Calcd (found) for C15H35ClP2Pd2: C, 34.27 (34.22); H, 6.71 (6.46). 1 H NMR (300 MHz, C6D6): 3.21 (m, 2H, allyl-end H), 2.60 (m, 1H, allyl-central H), 1.71 (d, J = 12.4 Hz, 2H, allyl-end H), 1.38 (m, 12H, PCH2CH3), 0.99 (m, 18H, PCH2CH3). 13C{1H} NMR (125.8 MHz, C6D6): 63.0, 32.7, 19.7, 9.1. 31P{1H} NMR (121 MHz, C6D6): 17.4.

IR (cm−1): 1575 (νCO2), 1392 (νCO2). 1H NMR (500 MHz, C6D6): 7.22 (t, J = 7.6 Hz, 4H, H11), 7.10 (d, J = 7.6 Hz, 8H, H10), 6.28 (m, 1H, H2), 5.17−5.11 (m, 2H, H1), 3.54 (septet, J = 6.8 Hz, 4H, H12), 3.45−3.36 (m, 12H, H12 and H7), 1.95 (t, J = 1.5 Hz, 1H, H3), 1.94 (t, J = 1.5 Hz, 1H, H3′), 1.84 (d, J = 10 Hz, 2H, H4), 1.44 (m, 1H, H5), 1.34 (d, J = 5 Hz, 12H, H13 or H14), 1.29 (d, J = 5 Hz, H13 or H14), 1.25 (m, 24H, H13 or H14), 0.40 (d, J = 15 Hz, 2H, H4′). 13 C{1H} NMR (125.8 MHz, C6D6): 218.8, 181.9, 147.9, 147.2, 138.3, 128.9, 124.4, 113.8, 54.4, 44.7, 43.8, 28.9, 26.9, 26.4, 24.7, 24.6. II-IMes-A. Excess 1 atm CO2 was added via a dual-manifold Schlenk line to a suspension of I-IMes-A (7 mg, 0.008 mmol) in 0.4 mL of benzene at rt. The mixture was stirred for 1 h and then dried to give II-IMes-A as a white solid. Yield: 6.9 mg (0.007 mmol, 95%). 483


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III-IPr-A. HCl in diethyl ether (0.60 mL, 0.0736 M in ether, 0.044 mmol) was added dropwise to a solution of I-IPr-A (47.1 mg, 0.044 mmol) in 5 mL of THF at rt. This solution was stirred for 20 min. The volatiles were removed under reduced pressure to yield III-IPr-A as a yellow solid. Yield: 46.6 mg (0.044 mmol, 99%). The product was further purified through recrystallization from a saturated solution of a pentane/toluene mix at −35 °C. Crystals for X-ray analysis were grown through slow evaporation of a saturated solution in pentane/ toluene. Anal. Calcd (found) for C57H77ClN4Pd2: C, 64.19 (64.20); H, 7.28 (7.01); N, 5.25 (5.11).

compounds to demonstrate that a motion connecting the reactant and product was present. Solvent was modeled on reactions involving charged species using the IEPCM model (benzene) as implemented in Gaussian 09. Iso-surfaces were generated using Gaussian 09. The coordinates and energies for optimized structures are given in the Supporting Information.

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ASSOCIATED CONTENT * Supporting Information X-ray crystallographic information on the structures of I-PiPr3A, I-IiPr-A, I-Ni-IPr-A, II-IPr-A, II-SIPr-A, and III-IPr-A, details on kinetics experiments, and optimized coordinates and energies of the structures used for calculations are available free of charge via the Internet at http://pubs.acs.org. S

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1 H NMR (400 MHz, C6D6): 7.21 (t, J = 7.7 Hz, 4H, H2), 7.11− 7.03 (m, 8H, H1), 6.60 (s, 4H, H3), 3.15 (septet, J = 6.9 Hz, 4H, H4), 3.03 (septet, J = 6.8 Hz, 4H, H4), 2.27 (d, J = 7.8 Hz, 2H, allyl-end), 1.55 (m, 1H, allyl-central), 1.31 (d, J = 6.8 Hz, 12H, H5), 1.26 (d, J = 6.8 Hz, 12H, H5), 1.12 (d, J = 6.9 Hz, 12H, H5), 1.08 (d, J = 6.9 Hz, 12H, H5), 0.49 (d, J = 12.6 Hz, 2H, allyl-end). 13C{1H} NMR (125.8 MHz, C6D6): 192.7, 146.4, 146.3, 137.5, 129.3, 123.8, 123.8, 122.7, 54.0, 28.9, 28.8, 27.3, 25.7, 25.6, 23.7, 23.5. III-PEt 3 -2Me. HCl in diethyl ether (0.068 M, 1.04 mL, 0.070 mmol) was added dropwise to a solution of I-PEt3-2Me (39.4 mg, 0.070 mmol) in 2 mL of diethyl ether at −78 °C in a dry ice/ 2-propanol bath. The reaction mixture was stirred at −78 °C for 20 min and was then removed from the cold bath. The reaction mixture was left to stir for 5 min while it warmed, and then the volatiles were removed under reduced pressure. The residue was dissolved in pentane and filtered through Celite. The pentane was removed under reduced pressure, and the residue was then washed with 1 mL of pentane, leaving III-PEt3-2Me as a yellow powder. Yield: 16.2 mg (0.030 mmol, 43%). The product was further purified through recrystallization from a saturated solution of cold pentane at −35 °C. Anal. Calcd (found) for C16H37ClP2Pd2: C, 35.61 (35.37); H, 6.91 (6.83). 1 H NMR (400 MHz, C6D6): 3.29 (m, 2H, allyl-end), 1.88 (m, 2H, allyl-end), 1.52 (s, 3H, allyl-methyl), 1.39 (m, 12H, PCH2CH3), 1.01 (m, 18H, PCH2CH3). 13C{1H} NMR (126 MHz, C6D6): δ 82.50, 38.63, 26.88, 19.72, 9.23. 31P{1H} NMR (121 MHz, C6D6): 19.0 (s). X-ray Crystallography. Crystal samples were mounted in MiTeGen polyimide loops with immersion oil. The diffraction experiments were carried out on a Rigaku SCXMini diffractometer using filtered Mo Kα radiation (λ = 0.71073 Å) or a Rigaku RAPID II image plate area detector using filtered Cu Kα radiation (λ = 1.54187 Å). The data frames were processed using Rigaku CrystalClear31 and corrected for Lorentz and polarization effects. The structures were solved by direct methods32 and expanded using Fourier techniques.33 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were treated as idealized contributions. In the structure of IIIIPr-A both the chloride atom and the central atom of the bridging allyl ligand were disordered. A DELU restraint was applied to C1, C2′, and C3 during refinement to assist with structure solution. Details of the crystal and refinement data for I-PiPr3-A, I-IiPr-A, I-Ni-IPr-A, II-IPrA, II-SIPr-A, and III-IPr-A are given in the Supporting Information. Computational Details. All geometry optimizations were performed using Gaussian 09 Revision A.02.34 The hybrid functional m062X was employed along with the LANL2DZ basis set for Pd and the 6-31G++(d,p) basis set for all other atoms.12a,35 The LANL2DZ pseudopotential was used for Pd. For calculations on compounds with PPh3 ligands, QMMM calculations were performed using ONIOM(m062X:UFF).36 In these calculations the phenyl groups on the phosphine ligands were calculated at the UFF level and the rest of the molecule was calculated using DFT. Initial geometries were obtained using the coordinates from X-ray structures, and all optimized structures were verified using frequency calculations to check that they were true minima. Entropy effects were calculated in the gas phase at 298 K and 1 atm. Calculated transitions states all showed one imaginary frequency, and IRC calculations were performed on representative

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS We gratefully acknowledge support through a Doctoral New Investigator Grant from the ACS Petroleum Research Fund (51009-DNI3). D.P.H. thanks the NSF for support as a NSF Graduate Research Fellow. This work was supported in part by the Yale University Faculty of Arts and Sciences High Performance Computing Facility (and staff).

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G. A.; Johansson Seechurn, C. C. C.; Li, H.; Colacot, T. J.; Chou, J.; Woltermann, C. J. J. Org. Chem. 2010, 75, 6477. (b) Johansson Seechurn, C. C. C.; Parisel, S. L.; Colacot, T. J. J. Org. Chem. 2011, 76, 7918. (20) Wang, M.; Fan, T.; Lin, Z. Polyhedron 2011, in press, doi = 10.1016/j.poly.2011.05.016. (21) Throughout this paper the following labeling scheme is used to identify compounds X-L-R, where X denotes the type of compound, doubly bridged allyl dimer, chloride-bridged allyl dimer, carboxylatebridged allyl dimer, etc. (see Figure 1); L denotes the ancillary phosphine or NHC ligand, PH3, PMe3, NHC, etc.; and R denotes the substitution on the allyl, either A for an unsubstituted allyl or 2Me for a compound containing a methyl group in the 2 position of the allyl. Hence a compound labeled I-PMe3-2Me refers to a doubly bridged allyl dimer, with PMe3 ancillary ligands and a methyl group in the 2 position of the allyl. From this point forward for type III compounds the bridging X substituent is always a chloride. (22) See Supporting Information for more details. (23) Despite repeated attempts, we were unable to reproduce the synthesis that generated crystals of the type I Ni dimer on a scale that allowed for isolation and complete characterization of this complex. Almost all our synthetic efforts led to the formation of monomeric Ni0 complexes of the type (hexadiene)Ni(L). For more information see: Wu, J.; Hazari, N.; Incarvito, C. D. Organometallics 2011, 30, 3142. (24) Johnson, M. T.; Johansson, R.; Kondrashov, M. V.; Steyl, G.; Ahlquist, M. S. G.; Roodt, A.; Wendt, O. F. Organometallics 2010, 29, 3521. (25) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: New York, 2009. (26) Goliaszewski, A.; Schwartz, J. Tetrahedron 1985, 41, 5779. (27) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523. (28) O’Brien, S.; Fishwick, M.; McDermott, B.; Wallbridge, M. G. H.; Wright, G. A. Inorg. Synth. 1971, 13, 73. (29) Love, B. E.; Jones, E. G. J. Org. Chem. 1999, 64, 3755. (30) Maxson, R. N. Inorg. Synth. 1939, 1, 147. (31) Fleckenstein, C. A.; Plenio, H. Chem. Soc. Rev. 2010, 39, 694. (32) Wuertz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (33) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (35) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (36) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber, S.; Morokuma, K. J. J. Phys. Chem. 1996, 100, 19357.

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Bridging Allyl Dimers - ACS Publications - American Chemical Society

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