Palladium(II) Agostic Complex: Exchange of Aryl–Pd and Alkyl–Pd

Article pubs.acs.org/Organometallics

Palladium(II) Agostic Complex: Exchange of Aryl−Pd and Alkyl−Pd Bonds Helen R. Thomas, Robert J. Deeth, Guy J. Clarkson, and Jonathan P. Rourke* Department of Chemistry, Warwick University, Coventry, U.K. CV4 7AL S Supporting Information *

ABSTRACT: Cyclopalladation of 2-tbutyl-6-(4-fluorophenyl)pyridine with palladium acetate cleanly gives cyclometalated complex 3 with an aryl−Pd bond. Replacement of the acetates by chloride gives the monomeric species 4 with a crystal structure confirming the presence of an agostic interaction from the alkyl group. Reaction of 3 with Na(acac) initially gives the monomeric acac complex 5 which maintains the aryl−Pd bond. Complex 5 shows fluxional behavior of the acac group, and this provides a pathway for its isomerization to complex 6 which has an alkyl−Pd bond; complex 6 was characterized crystallographically. Reaction of agostic 4 with triphenylphosphine gives an initial complex that maintains the aryl−Pd bond, but this complex isomerizes to crystallographically characterized 8 which has an alkyl−Pd bond.

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acetate (relative integral 3), six signals (each of relative integral 1) in the aromatic region, and one singlet for the tbutyl group (relative integral 9). We can postulate three likely structures for this product: one with a monodentate acetate and an agostic interaction, 1, one with a bidentate chelating acetate, 2, and one with a bridged dimeric structure, 3, Scheme 1. A survey of the Cambridge Crystallographic Database 9 suggests considerable precedent for all three structural types. It reveals 161 structures containing palladium coordinated with a monodentate acetate (though admittedly none of these have additional agostic interactions), 11 monomeric structures with a bidentate acetate, and some 204 structures with bridging acetates (105 of these are dimeric with a further N donor, and an additional 30 are dimers with a P donor ligand). The dimeric structure 3 has the two halves of the molecule related by a C2 rotation (but not an inversion): the bridging acetates fold the two halves of the molecule back on each other, rather like an open book, and this can lead to Pd−Pd interactions.10 This arrangement would allow the tbutyl groups to be in a sterically undemanding position pushed away from the central core, and such dimeric open book structures are known to persist in solution.11 NMR and mass-spectral data support the dimeric structure 3 but cannot be considered definitive. In particular, the peak of highest intensity in the high-resolution ESI mass spectrum corresponds to (dimer 3 − OAc)+, with other high intensity peaks corresponding to (dimer-OAc-Me)+ and (dimer-2OAc + O−H)+; a strong peak corresponding to (monomer-OAc)+ is

INTRODUCTION The problem of realizing a selective and general method for the activation of C−H bonds remains one of considerable topical interest.1 Cyclometalation reactions provide one widely studied route into the study of such reactions as the prior coordination of a ligating group can bring C−H bonds into close proximity to a metal.2 Agostic complexes, where a C−H bond interacts directly with a metal center are seen as intermediates along the pathway to full cleavage of the bond,3 and it is generally recognized that a preference exists for the activation of aryl over alkyl C−H bonds,4 though the balance between the two can be quite fine.5 Palladium is probably the metal most studied in this area,6 with recognition of its role in synthetic organic chemistry coming in 2010 with the award of the Nobel Prize for palladium catalyzed C−C bond formation. Recently Sanford has used the cyclopalladation reaction to oxidatively functionalize arenes,7 and Ritter has made extensive studies into the role of dimeric palladium complexes in organic transformations.8 In this paper we present details of an aryl C−H bond activation with palladium to give a new agostic complex with the agostic interaction coming from an alkyl group. Subsequent reactions convert this alkyl agostic interaction, at the expense of the aryl−Pd bond, to give complexes with alkyl−Pd bonds.

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RESULTS AND DISCUSSION Direct reaction of palladium acetate with 2- tbutyl-6-(4fluorophenyl)pyridine, in acetic acid, activates one of the sp 2 C−H bonds of the fluorinated phenyl ring to give a single cyclometalated species, cleanly, and in good yield. The room temperature 1H NMR shows the product to contain one © 2011 American Chemical Society

Received: May 26, 2011 Published: October 12, 2011 5641

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Scheme 1

also present. Variable temperature NMR shows no fluxional processes over the temperature range investigated (+30 to −64 °C): in particular there is no additional broadening of the tbutyl resonances compared with the rest of the resonances on reducing the temperature (contrast this with the case for the agostic 4, see below). One-dimensional nuclear Overhauser effect (NOE) experiments support a folded dimer: irradiation of the tbutyl group induces a small effect on the resonance of the proton ortho to both the Pd and the F in the metalated phenyl ring; such effects are not seen in complexes we know to be monomeric. Whatever the actual structure, the acetate complex reacts with chloride (in the form of KCl) to give a chloride 4.

resonances. However, even at −64 °C (the lowest temperature we were able to go to before the solvent froze), the tbutyl resonance had by no means separated out into the individual resonances implied by an agostic structure. We were able to grow crystals of 4, Figure 1, and gain considerable further insight into its structure. Unfortunately, all the crystals we were able to grow had very weak diffractions due to being multicomponent. At least four components could be located with the twin option in CrysAlis12 (ratio of twins 55/22/18/15 with overlaps of up to 30%). There were high residual electron density peaks around the palladium as no successful twin model could be found; consequently the hydrogens on the methyl C(16) could not be directly located, and these were placed at calculated positions. It is clear from the positions of the Pd, N(1), C(8), Cl(1), and C(16) that, without any agostic interactions from the hydrogens on C(16), the palladium would formally be a Tshaped, three coordinate, 14e species. However, the closeness of C(16) to the Pd (2.51 Å), a distance substantially shorter than the sum of the van der Waals radii of the two (1.70 and 1.63 Å, respectively), 13 indicates there definitely is an interaction between the two. A comparison of the positions of the heavy atoms of 4 with those of its platinum analogue (where the hydrogens were directly located) shows their positions to be very similar.5a In particular the Pt−C(agostic)

Compared with the acetate precursor, solution NMR data does suggest a monomeric structure for 4, complete with an agostic interaction. Variable temperature NMR does suggest the beginnings of the freezing out of a fluxional process: as the temperature of the sample is reduced, the resonance of the t butyl group broadens significantly more than the rest of the

Figure 1. X -ray crystal structure of 4; thermal ellipsoids at 50% probability. Note that due to poor crystal quality, the hydrogens on C(16) could not be directly located. Selected bond lengths (Å) and angles (deg): Pd(1)−C(8) 1.953(8); Pd(1)−N(1) 2.018(7); Pd(1)−Cl(1) 2.308(2); Pd(1)− C(16) 2.507(9); C(8)−Pd(1)−N(1) 83.0(3); C(8)−Pd(1)−Cl(1) 96.8(2); N(1)−Pd(1)−Cl(1) 179.1(2); C(8)−Pd(1)−C(16) 159.2(3); N(1)− Pd(1)−C(16) 76.3(3); Cl(1)−Pd(1)−C(16) 103.9(2). 5642

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2.78 and 2.71 Å,15 2.84 and 2.88 Å,16 2.71 and 2.83 Å,17 and 2.47 Å;18 our Pd···C distance is thus well within this range. Reaction of acetate 3 with sodium acetylacetonate (acac) in acetone cleanly yielded a single compound. Within this compound, it is apparent from the 1H NMR that the phenyl to palladium bond is maintained and that the tbutyl group is not bonded to the palladium (two sets of three mutually coupled signals of relative intensity 1, one set with additional coupling from the fluorine, in the aromatic region, and a singlet of relative intensity 9 in the aliphatic region). At low temperatures, the resonances for the acac group are as expected: two signals of relative integral 3 and one of relative integral 1, and this allows us to assign the structure 5 to this product, Scheme 2. All other evidence for the structure of 5 points to a monomeric square-planar structure, i.e., high-resolution massspectral data shows no major peaks with m/z greater than the sodiated mass ion. However, the structure of 5, as drawn, does require the close proximity of one side of the acac to the tbutyl group, and this might be problematic. In fact, we observe some fluxional behavior for 5 with the resonance for the two methyl groups of the acac interchanging in the 1H NMR. At 25 °C on a 400 MHz spectrometer, the resonances for the acac methyls are barely visible (it would appear that this is actually the coalescence temperature) but on cooling, two broad signals are clear at 10 °C, and the signals are very sharp at −25 °C. Conversely, on heating, a single broad peak is clear at +40 °C, and there is some additional narrowing of the resonance by +60 °C (just below the boiling point of the solvent). Applying a standard analysis, the barrier to the interconversion of the two acac methyl resonances can be determined to be 60.7 ± 0.5 kJ mol−1. The interconversion of the acac methyls in the NMR suggests either a very distorted coordination plane at the palladium whereupon a twist of the acac (via an approximately tetrahedral transition state) results in swapping the sides of the acac or a pathway where one of the coordinating atoms of one of the two chelating ligands dissociates from the palladium before re-entering in a different position. Thermally induced isomerisations of square planar palladium and platinum complexes via tetrahedral transition states are currently unknown19 (though photochemically induced reactions have been reported20). In addition, tetrahedral complexes would have unpaired electrons and thus would be paramagnetic, but we observe no unexpected features in any of our NMR spectra that would suggest such species. We therefore rule out an isomerization that proceeds via a tetrahedral transition state. A dissociative pathway could involve either chelate ligand, but it is apparent that the group most likely to dissociate would be the acac oxygen trans to the aryl group. This group will experience the strongest trans influence, and in addition it is rather sterically constrained by the tbutyl group. Acac ligands

distance of 2.472(4) in the platinum analogue (which actually had a bifurcated dual agostic interaction) is close to the Pd− C(16) distance of 2.507(9) Å seen in 4. It is impossible to say for certain whether the agostic interaction in 4 is actually a bifurcated one between two of the hydrogens on C(16) and the Pd center, rather than the more common single C−H···Pd one, but it must be a real possibility. Molecules of 4 in the crystal are essentially flat, with only two of the carbons, and the hydrogens, of the tbutyl group being out of plane. These flat molecules pack in infinite stacks along the c axis of the crystal related by the c glide. The cores are aligned with all the Pd−Cl bonds facing in a similar direction, but each molecule in the stack is alternating along the long axis of the complex with a separation of 3.47 Å, a Pd−Pd distance of 3.572 0(3) Å, and a Pd−Pd−Pd angle of 152°, Figure 2.

Figure 2. Stacking arrangement of the heavy atoms of 4 in the crystal. The Pd−Pd distance is 3.572 0(3) Å, and the Pd−Pd−Pd angle is 152°.

If we are careful to distinguish agostic and anagostic interactions,3 actual agostic complexes of palladium(II) are relatively rare. A survey of the Cambridge Structural Database 9 shows only 10 published examples of T-shaped palladium complexes with an additional agostic interaction completing the coordination sphere of the palladium. Within these other examples, the Pd···C distances are reported as 2.74−2.94 Å, 14 Scheme 2

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Scheme 3

are known to be able to bond with only one oxygen bound to a particular metal,21 so dissociation of the oxygen is not unlikely. Furthermore, dissociation of this oxygen would give rise to a transition state that could be stabilized by agostic interactions from the tbutyl group; certainly we22 and others3 have seen such a stabilization. For all these reasons, we believe the most likely scenario is that depicted in Scheme 3. Subsequently 5 isomerizes in solution on a time scale of weeks to give 6, Scheme 2. Clear evidence for this transformation comes from 1H NMR data whereupon there are two signals in the aliphatic region (relative intensities 2 and 6) and two signals for the fluorinated phenyl ring (each of relative intensity 2). At room temperature, the 1H resonances of the acac methyls in 6 show no unusual features and variable temperature NMR does not indicate any fluxional processes: whatever process was interconverting the two sides of the acac ligand in 5 is not operating in complex 6. The transformation of 5 to 6 results in the replacement of a strong aryl C−Pd bond by a weaker alkyl C−Pd bond, but this unfavorable factor is presumably more than compensated for by the relief of steric strain caused by the interaction of the acac with the tbutyl group. A DFT optimization of the structures of 5 and 6 suggests that the sp3 cyclometalated 6 is favored by some 11 kJ mol−1. Finally, we were able to grow crystals of 6 and solve the structure, see Figure 3. The crystal structure of 6 is relatively undistorted at the palladium: the mean plane defined by Pd(1), N(1), C(16), and the two oxygens is close to flat with none of these five atoms being more than 0.06 Å away from it. With the fluorinated phenyl ring twisted out of the plane with respect to the pyridine, there does not appear to be any unfavorable steric interactions with the acac group. We can therefore attribute for the lack of any fluxional behavior in the NMR of 6 to this uncomplicated arrangement. In a manner similar to that we have previously seen for the platinum analogue of 4, reaction with 2 equiv of triphenylphosphine rapidly leads to cleavage of the pyridine to palladium bond with the formation of a complex containing two triphenylphosphine ligands,23 Scheme 4. Evidence for a trans arrangement of two PPh3 ligands comes from the 31P NMR (a single resonance) and the 1H NMR (integrals show the ratio of PPh3 to tBu-pyridine to be 2:1). In fact, 7 forms even when only 1 equiv of triphenylphosphine is added to 4, whereupon an equimolar mixture of 4 and 7 forms. We might have expected the reaction of 4 with a single equivalent of PPh3 to give a complex that remains cyclometalated via the phenyl ring, and this complex must be the intermediate 9 in the reaction, but it would appear that steric constraints render it unstable with respect to further reaction. Thus displacement of the pyridine by a second PPh3 results in the tbutyl pyridine moiety being able to rotate away, and this

Figure 3. X-ray crystal structure of 6; thermal ellipsoids at 50% probability. Note that the two hydrogens on C(16) were located in a difference map and allowed to refine freely (but given thermal parameters equal to 1.2 times that of the carbon to which they are attached). Selected bond lengths (Å) and angles (deg): Pd(1)−C(16) 1.9950(18); Pd(1)−O(18) 2.0263(12); Pd(1)−N(1) 2.0567(14); Pd(1)−O(20) 2.1390(12); C(16)−Pd(1)−O(18) 88.95(6); C(16)− Pd(1)−N(1) 80.53(7); O(18)−Pd(1)−N(1) 169.36(5); C(16)− Pd(1)−O(20) 175.70(6); O(18)−Pd(1)−O(20) 89.69(5); N(1)− Pd(1)−O(20) 100.92(5).

relief of steric strain is sufficient to stabilize 7 with respect to the unseen complex 9 by a greater amount than the unseen 9 is stabilized with respect to agostic 4; reactions of this type have been seen before.24 The equimolar mixture of 4 and 7 that results from the addition of a single equivalent of triphenylphosphine to 4 transforms slowly (a time scale of days) to another new palladium compound, complex 8, Scheme 5. The new complex is a singly cyclometalated species with a single phosphine ligand per palladium center, but it is apparent from the NMR spectra that the new complex is cyclometalated via the tbutyl group, rather than via the phenyl ring. Evidence for this formulation comes from two sets of resonances (each of relative integral 2) for the fluorophenyl ring, and two resonances (of relative integral 2 [with coupling to phosphorus] and 6) in the aliphatic region (rather than one resonance of integral 9). We were able to crystallize 8 and solve the X-ray structure, see Figure 4. The coordination geometry of the palladium in 8 is recognizably square-planar though rather distorted. Thus, while the P−Pd−C angle of 91.5° is close to 90°, all other 5644

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Scheme 4

Scheme 5

relatively small stabilization is presumably a function of the fact that the favored isomer is still very much distorted away from an ideal geometry. Variable temperature NMR was used to investigate the possibility of fluxional processes occurring within complex 8. The signals that we anticipated showing effects are the CH 2 protons (at room temperature they are a sharp doublet caused by coupling to the 31P of the phosphine ligand) and the two methyl groups. We reasoned that the distortions to the coordination plane around the palladium would result in sufficient asymmetry such that these two CH2 protons (and the two methyl groups) would become inequivalent, if we were able to freeze out the buckling motion that would equilibrate them via a ring inversion. Reducing the temperature of the sample (on a 400 MHz NMR spectrometer) did indeed result in the broadening of the CH2 resonance giving a coalescence at around −56 °C and separation into two broad peaks at −64 °C (the lowest temperature we were able to reach in CDCl3 solution). Since we were unable to reach a true lowtemperature limiting spectrum, we can only estimate the final separation of the two resonances and calculate a relatively imprecise barrier to the inversion of the metallacycle of 40 ± 4 kJ mol−1. It is pertinent to consider the mechanism of the conversion of 5 to 6 and of the formation of 8. In both cases, an unfavorable steric interaction is driving the formation of an alkyl−Pd bond at the expense of an aryl−Pd bond. A crucial step of the transformations will be the transfer of a hydrogen from the alkyl group to the aryl group. We have previously5a,23 extensively studied the reaction that gives the platinum analogue of 8 from the analogues of 4 and 7. We were able to demonstrate that a hydrogen moves from the alkyl group to the aryl group in an intramolecular transfer across the platinum, in a process that is strongly reminiscent of the sigma-CAM mechanism proposed by Perutz and Sabo-Etienne,26 and that the most likely reaction route was one in which the rate determining step was the isomerization of the analogue of agostic 4 to the analogue of another agostic complex (i.e., 10, with an interaction from the aryl ring). Assuming the reactions with palladium go via similar intermediates, we propose Scheme 6 as our favored route to the formation of 8. An initial estimate of the transition state between 4 and 10 was obtained from a linear transit starting from 4 where the agostic C−H bond was lengthened from 1.1 to 3 Å. A full

Figure 4. X-ray crystal structure of 8; thermal ellipsoids at 50% probability. Note that the two hydrogens on C(116) were directly located in a difference map and allowed to refine freely (but given isotropic displacement parameters equal to 1.2 times the equivalent isotropic displacement parameter of C116). Selected bond lengths (Å) and angles (deg): Pd(1)−C(116) 2.0305(17); Pd(1)−N(101) 2.1462(15); Pd(1)−P(201) 2.2369(5); Pd(1)−Cl(1) 2.4492(4) C(116)−Pd(1)−N(101) 78.15(7); C(116)−Pd(1)−P(201) 91.53(6); N(101)−Pd(1)−P(201) 165.61(4); C(116)−Pd(1)−Cl(1) 160.33(6); N(101)−Pd(1)−Cl(1) 95.69(4); P(201)−Pd(1)−Cl(1) 97.250(17).

angles at the Pd center are between 6 and 20° away from ideal values of 90 or 180° with the angles of the mutually trans ligands being 165.6 and 160.3°. The Pd, N(101), Cl(1), P(201), and C(116) atoms are by no means coplanar: the N, Cl, P and C are, respectively, 0.250, 0.214, 0.316, 0.202 Å away from the calculated mean plane defined by these atoms and the Pd. The platinum analogue of 8 has a similarly distorted squareplanar geometry,23 with bond lengths that are slightly longer than for related undistorted complexes.25 A DFT optimization suggests that the sp3 cyclometalated 8 is more stable than its unseen sp 2 cyclometalated isomer (intermediate 9 in Scheme 4) by some 16 kJ mol−1. This 5645

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Scheme 6

force for these reactions is a relief of steric strain caused by the bulky tbutyl group.

transition state optimization (DZP in the gas phase) was performed starting from the maximum in the linear transit path to give the structure shown in Figure 5. This structure was

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Figure 5. Calculated transition state between 4 and 10; the arrow indicates the transition state vector.

confirmed as a first order saddle point with a single imaginary frequency of −400 cm−1. The free energy for this process was then calculated with TZ2P and COSMO corrections and showed the process to be endoergic by 6.3 kJ mol−1 with a barrier of 167 kJ mol−1. This barrier, as calculated, is an upper limit but is sufficiently large to throw some doubt on the probability of this route being viable. It is also important to note that DFT calculations are not perfect and ours did not include factors such as tunnelling, which will reduce the barrier, maybe substantially.27 We must therefore be alert to other possibilities such as a metal assisted acid−base reaction. By analogy, the transformation of the acac complex 5 to 6, would then proceed via the intermediate we proposed in Scheme 3, instead of 4. This agostic intermediate would be set up to transfer a hydrogen across from the alkyl to aryl group, followed by a recoordination of the second acac oxygen to give the final product.

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EXPERIMENTAL SECTION

All chemicals were used as supplied, unless noted otherwise. All NMR spectra were obtained on a Bruker Avance 300, 400, 500, or 600 MHz spectrometer in CDCl3 or D6-acetone; 1H and 13C shifts are referenced to external trimethylsilane (TMS), assignments being made with the use of decoupling, NOE and the heteronuclear multiple bond correlation (HMBC), heteronuclear single quantum correlation (HSQC), distortionless enhancement by polarization transfer (DEPT), and correlation spectroscopy (COSY) pulse sequences. 19F chemical shifts are quoted from the directly observed signals and are referenced to external CFCl3. All accurate mass spectra were run on a Bruker MaXis mass spectrometer, and X-ray crystal structures were collected on an Oxford Diffraction Gemini four-circle system with Ruby CCD area detector. The synthesis of the starting 2- tbutyl-6-(4fluorophenyl)pyridine has been reported previously.5a Synthesis of Palladium Acetate Complex 3. Palladium acetate (0.209 g, 0.93 mmol, 2 equiv) and 2-tbutyl-6-(4-fluorophenyl)pyridine (0.110 g, 0.48 mmol, 1 equiv) were refluxed in glacial acetic acid (25 mL) in air (2 h). During this time, there was an observable color change from a red solution to a dark green mixture containing some metallic palladium. The mixture was filtered to give a dark brown filtrate. The solvent was removed, and the resultant brown solid was dissolved in diethyl ether and chromatographed. The second fraction yielded the product as a pale orange solid. Yield, 0.140 g, (0.36 mmol, 75%). δ H: 7.79 (1H, t, 3J 8 Hz, central py), 7.44 (1H, d, 3J 8 Hz, py adjacent to ph), 7.31 (1H, dd, 3J 9 Hz, 4J(H−F) 5 Hz, ph, meta to F), 7.26 (1H, d, 3J 8 Hz, py, adjacent to tBu), 6.87 (1H, td, 3J(H−H), (H−F) 9 Hz, 4J 2 Hz, ph, ortho to F), 6.85 (1H, dd, 3J(H−F) 8 Hz, 4J 2 Hz, ph, ortho to Pd), 2.14 (3H, s, OAc), 1.64 (9H, s, tBu). δ C: 183.8 (CO), 169.9 (py, adjacent to N, with tBu attached), 162.1 (py, adjacent to N, with ph attached), 160.5 (d, 1J(C−F) 257 Hz, ph with F directly attached), 145.0 (d, 3J(C−F) 6 Hz, ph, attached to Pd), 141.3 (d, 4J(C−F) 3 Hz, ph, attached to py), 139.6 (central py), 125.0 (d, 3J(C−F) 8 Hz, ph, meta to F and Pd), 120.1 (py adjacent to py adjacent to tBu), 117.0 (py adjacent to ph), 116.9 (d, 2J(C−F) 21 Hz, ph, adjacent to F and Pd), 113.4 (d, 2J(C−F) 23 Hz, ph, adjacent to F), 39.6 (tBu nonprotonated), 30.5 (OAc Me), 23.5 (tBu Me). δ F: −107.5. HR-MS (ESI): m/z 334.0224, calculated for C 15H15F14N106Pd = (monomer-OAc) + 334.0226; 351.0489, calculated for C15H18F14N2106Pd = (monomerOAc + NH3)+ 351.0472; 685.0330, calculated for C30H29F215N2O106Pd2 = (dimer-2OAc-H + O)+ 685.0334; 715.0427, calculated for C31H31F215N2O2106Pd2 = (dimer-OAc-CH3)+ 715.0436; 729.0584, calculated for C 32H3315N2F2O2106Pd2 = (dimer-OAc)+ 729.0585. Synthesis of Palladium Chloride Agostic Complex 4. Palladium acetate complex 3 (50 mg, 0.13 mmol) was dissolved in

CONCLUSIONS

Cyclopalladation of 2-tbutyl-6-(4-fluorophenyl)pyridine with palladium acetate has been shown to proceed cleanly to give a cyclometalated complex with an aryl−Pd bond. This compound is likely to have a dimeric structure with two bridging acetates completing the square planar coordination. However, replacement of the acetates by chlorides breaks up this dimer and gives a monomeric species with an agostic interaction from the alkyl group completing the coordination sphere. Subsequent reactions give complexes that only temporarily maintain the aryl−Pd bond: these complexes isomerize to more thermodynamically stable complexes with alkyl−Pd bonds. The driving 5646

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Table 1. X-ray Data for Complexes 4, 6, and 8 complex crystal form dimensions/mm3 empirical formula Mw crystal system space group a/Å b/Å c/Å α/° β/° γ/° U /Å3 T/K Z Dcalc/Mg m−3 F(000) μ(MoKα)/mm−1 θ max/deg refln measured unique data R1 [I > 2σ(I)] wR2 data/rest/param

agostic 4

acac 6

yellow block 0.20 × 0.6 × 0.06 C15H15ClFNPd 370.13 monoclinic P2(1)/c 8.915 7(4) 22.923 9(9) 6.941 5(3) 90 105.875(5) 90 1364.62(10) 100(2) 4 1.802 736 1.551 30.22 12607 3689 0.0911 0.2545 3689/0/ 175

colorless block 0.40 × 0.35 × 0.10 C20H22FNO2Pd 433.79 monoclinic P2(1)/n 11.200 67(18) 14.281 2(2) 11.411 99(18) 90 92.630 5(15) 90 1823.53(5) 100(2) 4 1.580 880 1.040 30.55 17506 5071 0.0243 0.0570 5071/0/ 236

PPh3 8 colorless block 0.35 × 0.25 × 0.10 C34H31Cl4FNPPd 751.77 monoclinic P2(1)/c 19.257 4(4) 9.889 60(10) 18.584 7(4) 90 116.374(10) 90 3171.01(10) 100(2) 4 1.575 1520 1.005 30.14 18306 8229 0.0280 0.0632 8229/0/ 385

124.3 (d, 3J(C−F) 8 Hz, ph, meta to F and Pd), 121.4 (py, ortho to tBu), 116.9 (d, 2J(C−F) 18 Hz, ph, ortho to Pd and F), 114.9 (py, ortho to ph), 112.1 (d, 2J(C−F) 19 Hz, ph, ortho to F), 99.9 (central acac), 38.5 (tBu nonprotonated), 31.4 (tBu Me), 27.5 (acac Me) ppm. δ F: −110.5 ppm. HR-MS (ESI): m/z 456.0570, calculated for C20H22F14NNaO2106Pd = (M + Na)+ 456.0583; 769.0897, calculated for C35H37F215N2O2106Pd2 = (2M-acac)+ 769.0932. Synthesis of sp3 Metalated Palladium acac Complex 6. A solution of complex 5 in chloroform was left for 1 month at room temperature, by which time much of the material had isomerized and crystals of 6 had separated out. δ H: 8.11 (2H, m, ph, meta to F), 7.76 (1H, t, 3J 8 Hz, central py), 7.26 (1H, dd, 3J 8 Hz, 4J 1 Hz, py), 7.15 (1H, dd, 3J 8 Hz, 4J 1 Hz, py), 7.10 (2H, t, 3J(H−F), (H−H) 8 Hz, ph, ortho to F), 5.51 (1H, s, central acac), 2.39 (2H, s, CH2), 2.29 (3H, s, acac Me), 2.15 (3H, s, acac Me), 1.61 (6H, s, Me2). δ F: −113.2 ppm. Synthesis of bis-PPh3 Palladium Complex 7. Triphenylphosphine (7.1 mg, 0.027 mmol, 2 equiv) was added to a solution of the palladium agostic complex 4 (5 mg, 0.014 mmol) in chloroform. An immediate color change from pale yellow to colorless was observed. Yield: 10.3 mg (0.012 mmol, 85%). δ H: 8.43 (1H, d, 3J 8 Hz, py), 7.44 (1H, t, 3J 8 Hz, central py), 7.35 (12H, dd, 3J(H−P) 12 Hz, 3J 6 Hz, PPh3), 7.24 (6H, t, 3J 6 Hz, PPh3), 7.14 (12H, t, 3J 7 Hz, PPh3), 7.08 (2H, m, py and ph meta to F), 6.32 (1H, dd, 3J(H−F) 9 Hz, 4J 2 Hz, ph ortho to Pd and F), 6.09 (1H, td, 3 J(H−F), (H−H) 8 Hz, 4J 2 Hz, ph ortho to F), 1.11 (9H, s, Me3) ppm. δ F: −117.1 ppm. δ P: 21.9 ppm. HR-MS (ESI): m/z 857.2062, calculated for C51H45F14NP2105Pd = (M-Cl)+ 857.2502 Synthesis of sp3 Metalated PPh3 Palladium Complex 8. Triphenylphosphine 3.5 mg, 0.014 mmol, 1 equiv) was added to a solution of the palladium agostic complex 4 in chloroform 5.0 mg, 0.014 mmol) and left to react for 3 days at room temperature. The solvent was removed to give the product as a pale yellow solid. Yield 7.4 mg (0.012 mmol, 87%) δ H: 8.01 (2H, m, ph meta to F), 7.70 (1H, t, 3J 8 Hz, central py), 7.56 (6H, m, 3J(H−P) 11 Hz, 3J 8 Hz, 4J 2 Hz, PPh3, ortho to P), 7.31 (10H, m, py and PPh3), 7.10 (3H, m, py and ph ortho to F), 1.82 (2H, d, 3J(H−P) 4 Hz, CH2), 1.38 (6H, s, Me) ppm. δ C: 173.2 (py with alkyl attached), 163.8 (d, 1J(C−F) 254 Hz, ph with F attached), 159.2 (py

glacial acetic acid (25 mL) together with excess KCl (0.5 g). The mixture was stirred under reflux (30 min), with an almost immediate color change from pale orange to pale yellow. The reaction solution was cooled and filtered, and the solvent was removed to leave a pale yellow solid. The solid was purified by column chromatography, loading and eluting with diethyl ether: the product was the first colored fraction. Yield: 40 mg (0.11 mmol, 85%). δ H: 7.78 (1H, t, 3J 8 Hz, central py), 7.42 (1H, dd, 3J(H−F) 9 Hz, 4J 3 Hz, ph, adjacent to Pd), 7.40 (1H, d, 3J 8 Hz, py, adjacent to tBu), 7.27 (1H, dd, 3J 9 Hz, 4J(H−F) 5 Hz, ph, meta to F and Pd), 7.17 (1H, d, 3J 8 Hz, py, adjacent to ph), 6.80 (1H, td, 3J(H−H), (H−F) 8 Hz, 4J 3 Hz, ph, adjacent to F), 1.44 (9H, s, tBu). δ C: 169.2 (py, adjacent to N, with t Bu attached), 161.1 (py, adjacent to N, with ph attached), 160.7 (d, 1 J(C−F) 258 Hz, ph with F directly attached), 148.0 (ph, directly bonded to py), 142.5 (ph with Pd directly bonded), 139.1 (central py), 124.9 (d, 3J(C−F) 9 Hz, ph, meta to F and Pd), 122.6 (d, 2J(C−F) 21 Hz, ortho to F and Pd), 120.5 (py, adjacent to ph), 116.8 (py, adjacent to t Bu), 113.1 (d, 2J(C−F) 23 Hz, ph, ortho to F), 42.4 ( tBu nonprotonated), 28.5 (tBu Me) ppm. δ F: −106.0 ppm. HR-MS (ESI): m/z 334.0224, calculated for C15H15F14N106Pd = (M-Cl)+ 334.0225; 352.0330, calculated for C15H17F14NO106Pd = (M-Cl + H2O)+ 352.0325; 705.0140, calculated for C30H3035ClF215N2106Pd2 = (2M-Cl)+ 705.0145. Synthesis of sp2 Metalated Palladium acac Complex 5. Sodium acetylacetonate (48 mg, 0.40 mmol) was added to a solution of complex 3 (50 mg, 0.13 mmol) in chloroform. The mixture was refluxed overnight. After cooling, the solution was filtered and the solvent removed to give a pale yellow solid. Yield: 46 mg (0.11 mmol, 85%). δ H(263K): 7.72 (1H, t, 3J 8 Hz, central py), 7.43 (1H, dd, 3J 8 Hz, 4 J 1 Hz, py, ortho to ph), 7.36 (1H, dd, 3J 8 Hz, 4J 1 Hz, py, ortho to t Bu), 7.29 (1H, dd, 3J 9 Hz, 4J(H−F) 5 Hz, ph, meta to F and Pd), 7.15 (1H, dd, 3J(C−F) 9 Hz, 4J 3 Hz, ph, ortho to Pd and F), 6.82 (1H, td, 3 J(H−H), (H−F) 9 Hz, 4J 3 Hz, ph, ortho to F), 5.37 (1H, s, central acac), 2.09 (3H, s, acac Me), 1.96 (3H, s, acac Me), 1.70 (9H, s, tBu). δ C: 187.3 (CO), 174.8 (py, with tBu attached), 164.1 (py, with Ph attached), 160.3 (d, 1J(C−F) 254 Hz, ph, with F attached), 153.1 (d, 3 J(C−F) 6 Hz, ph with Pd attached), 142.3 (Cd), 138.0 (central py), 5647

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Organometallics

Article

with ph attached), 138.4 (central py), 134.5 (d, 3J(C−P) 11 Hz, PPh3), 131.9 (d, 1J(C−P) 52 Hz, ipso PPh3) 130.8 (ph meta to F), 130.1 (PPh3), 128.1 (d, 4J(C−P) 10 Hz, PPh3), 123.0 (py), 119.0 (py), 115.0 (d, 3J(C−F) 22 Hz, ph), 50.4 (nonprotonated alkyl), 47.9 (CH2), 32.4 (Me) ppm. δ F: −112.5 ppm. δ P: 35.5 ppm. HR-MS (ESI): m/z 596.1142, calculated for C33H30F14NP106Pd = (M-Cl)+ 596.1160. X-ray Crystallographic Studies. X-ray data for complexes 4, 6, and 8 are shown in Table 1. DFT Calculations. All DFT calculations used the Amsterdam density functional (ADF) code version 2008.01.28 The general features available in ADF have been described.29 Here, we have used scalar zero-order regular approximation (ZORA) relativistic corrections with the OPBE functional30 and the supplied frozen-core, triple-ζ plus polarization ZORA basis sets. Solvation effects were included via the conductor-like screening model (COSMO) as implemented in ADF. Default SCF and geometry optimization convergence criteria were used.

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(13) Bondi, A. J. Phys. Chem. 1964, 68, 441. (14) Stambuli, J. P.; Incarvito, C. D.; Buhl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 1184. (15) Yamashita, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 5344. (16) Yamashita, M.; Takamiya, I.; Jin, K.; Nozaki, K. Organometallics 2006, 25, 4588. (17) Watanabe, C.; Iwamoto, T.; Kabuto, C.; Kira, M. Angew. Chem., Int. Ed. 2008, 47, 5386. (18) Walter, M. D.; White, P. S.; Brookhart, M. Chem. Commun. 2009, 6361. (19) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Addison Wesley Longman Limited: Harlow, U.K., 1999. (20) Annibale, G.; Bonivento, M.; Canovese, L.; Cattalini, L.; Michelon, G.; Tobe, M. L. Inorg. Chem. 1985, 24, 797. (21) Alvarez, B.; Bois, C.; Jeannin, Y.; Miguel, D.; Riera, V. Inorg. Chem. 1993, 32, 3783. (22) (a) Crosby, S. H.; Clarkson, G. J.; Deeth, R. J.; Rourke, J. P. Organometallics 2010, 29, 1966. (b) Crosby, S. H.; Clarkson, G. J.; Deeth, R. J.; Rourke, J. P. Dalton Trans. 2011, 40, 1227. (23) Crosby, S. H.; Clarkson, G. J.; Rourke, J. P. Organometallics 2011, 30, 3603. (24) (a) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; Koten, G. v. J. Am. Chem. Soc. 2000, 122, 11822. (b) Rodriguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127. (c) Crespo, M.; Granell, J.; Solans, X.; Font-Bardia, M. J. Organomet. Chem. 2003, 681, 143. (d) Crespo, M.; Solans, X.; FontBardia, M. J. Organomet. Chem. 1996, 509, 29. (25) (a) Newman, C. P.; Casey-Green, K.; Clarkson, G. J.; Cave, G. W. V.; Errington, W.; Rourke, J. P. Dalton Trans. 2007, 3170. (b) Mamtora, J.; Crosby, S. H.; Newman, C. P.; Clarkson, G. J.; Rourke, J. P. Organometallics 2008, 27, 5559. (26) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578. (27) Schreiner, P. R.; Reisenauer, H. P.; Ley, D.; Gerbig, D.; Wu, C.H.; Allen, W. D. Science 2011, 332, 1300. (28) Baerends, E. J. B., A.; Bo, C.; Boerrigter, P. M.; Cavallo, L.; Deng, L.; Dickson, R. M.; Ellis, D. E.; Fan, L.; Fischer, T. H.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Groeneveld, J. A.; Gritsenko, O. V.; Harris, F. E.; van den Hoek, P.; Jacobsen, H.; van Kessel, G.; Kootstra, F.; van Lenthe, E.; Osinga, V. P.; Philipsen, P. H. T.; Post, D.; Pye, C. C.; Ravenek, W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.; Snijders, J. G.; Sola, M.; Swerhone, D.; te Velde, G.; Vernooijs, P.; Versluis, L.; Visser, O.; van Wezenbeek, E.; Wiesenekker, G.; Wolff, S. K.; Woo, T. K.; Ziegler, T. ADF 2008.01; Scientific Computing and Modelling NV: Free University, Amsterdam, The Netherlands, 2008. (29) Velde, G. t.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84. (30) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (b) Handy, N. C.; Cohen, A. J. Mol. Phys. 2001, 99, 403.

ASSOCIATED CONTENT

S Supporting Information *

DFT optimized coordinates and energies for 4, 5, 6, 8, 9, 10 and the TS in Figure 5; CIF files; and complete sets of structural data for 4, 6, and 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS

We are grateful for support from Advantage West Midlands (AWM) (partially funded by the European Regional Development Fund) for the purchase of a high-resolution mass spectrometer and the XRD system that was used to solve the crystal structures of 4, 6, and 8.

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REFERENCES

(1) (a) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (b) Bergman, R. G. Nature 2007, 446, 391. (2) Albrecht, M. Chem. Rev. 2009, 110, 576. (3) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908. (4) Goldman, A. S.; Goldberg, K. I. In Activation and Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; ACS Symposium Series, Vol. 885; American Chemical Society: Washington, DC, 2004. (5) (a) Crosby, S. H.; Clarkson, G. J.; Rourke, J. P. J. Am. Chem. Soc. 2009, 131, 14142. (b) Garner, A. W.; Harris, C. F.; Vezzu, D. A. K.; Pike, R. D.; Huo, S. Chem. Commun. 2011, 47, 1902. (6) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (7) (a) Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234. (b) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 15618. (c) Ye, Y. D.; Ball, N. D.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2010, 132, 14682. (8) (a) Powers, D. C.; Geibel, M. A. L.; Klein, J.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 17050. (b) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302. (c) Powers, D. C.; Xiao, D. Y.; Geibel, M. A. L.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 14530. (d) Chuang, G. J.; Wang, W. K.; Lee, E.; Ritter, T. J. Am. Chem. Soc. 2011, 133, 1760. (9) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf. Comput. Sci. 1996, 36, 746. (10) Bercaw, J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.; Winkler, J. R. Inorg. Chem. 2010, 49, 1801. (11) Evans, P. G.; Brown, N. A.; Clarkson, G. J.; Newman, C. P.; Rourke, J. P. J. Organomet. Chem. 2006, 691, 1251. (12) Crysalis, Agilent Technologies Ltd.: Yarnton, U.K., 2010.

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NOTE ADDED AFTER ASAP PUBLICATION

In the version of this paper published on October 12, 2011, the DFT data mentioned in the paragraph describing the Supporting Information were missing. The version that appears as of November 7, 2011, has the data correctly deposited as Supporting Information.

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dx.doi.org/10.1021/om200451v | Organometallics 2011, 30, 5641−5648