Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Aug 7;43(17):1904–1911. doi: 10.1021/acs.organomet.4c00263

Synthesis of a Palladium Dimer Supported by a C-Bound Trifluoroacetonate Bridge Formed by Cleavage of a Hexafluoroacetylacetonate Ligand

Paul Byrne †, Hugh Burgoon †, Jessica Koester †, Wei-Yuan Chen ‡, Christopher J Ziegler ‡, Emilian Tuca §, Gino A DiLabio §, Larry F Rhodes †,*
PMCID: PMC11389688  PMID: 39268182

Abstract

graphic file with name om4c00263_0007.jpg

Palladium(II) hexafluoroacetylacetonate (Pd(Hfacac)2) is known to form adducts of bases, such as lutidine (2,6-dimethylpyridine). When treated with approximately 3 equiv of lutidine, Pd(Hfacac)2 yields a 1:1 complex as reported in the literature, Pd(O,O-Hfacac)(C-Hfacac)(lutidine), 1. However, when the amount of excess lutidine is increased, a new complex, 2, is formed. A single-crystal X-ray structure of 2 proves it is a rare example of a dimeric palladium complex containing two Pd(Hfacac)(lutidine) fragments bridged by a dianionic trifluoroacetonate ligand, μ-CHC(O)CF3. The formation of 2 is accompanied by a white precipitate determined to be a mixture of trans-Pd(O2CCF3)2(lutidine)2 (3), confirming the fate of the missing trifluoroacetate fragment from the cleavage of the Hfacac ligand, and [lutidinium][Hfacac] (4). Subsequent experiments revealed the determinative role that water played in this reaction. The mechanism of cleavage of the Hfacac ligand was explored by DFT methods.

Introduction

Ligands derived from acetylacetone have become a classic in transition metal chemistry ever since the recognition by Werner of its actual coordination geometry.1,2 Acetylacetone (also known as β-diketone) exists in an equilibrium between the keto and the enol form and can be readily deprotonated by base to form the anionic acetylacetonate (typically termed acac), which coordinates to metals. The most common coordination mode for acac ligands is as a bidentate ligand through two oxygen atoms. Other bonding modes of this versatile ligand have been described in the literature.3

The characteristics of metal acac complexes can be tuned by a suitable choice of substituents on the ligand. The electron-donating, -withdrawing, and -steric properties of the acac ligand can be readily tailored. Substitution of CH3 by CF3, for example, makes transition metal complexes a more volatile and stable precursor for MOCVD applications.4 Chemists have manipulated the acac ligands to favor generation of supramolecular metalloclusters.5 Such assemblies can be photoactive as well.6

Acac complexes have been used as precursor compounds that catalyze a number of chemical transformations such as hydroformylation, oxidation, hydrogenation, carbon–carbon bond formation, and isomerization reactions.7

Our interest in late transition metal acac complexes stems from the observation that they can catalyze the oligomerization of olefins8−10 and polymerization of norbornene.11,12 In 2003, we reported that both Ni and Pd acac derivatives initiate norbornene polymerization when reacted with a suitable activator such as B(C6F5)3.13 Since then, Suslov et al. noted that cationic Pd acac derivatives polymerize norbornene with and without activation by BF3·OEt2.14−18

Recently Promerus researchers discovered that Pd acac compounds promote the UV-initiated polymerization of norbornene monomers in the presence of photoacid generators and photosensitizers.19 The fact that the stability of such compositions can be enhanced by addition of amine compounds prompted us to evaluate the suitability of Pd acac adducts with pyridine ligands.20 In the process of this study, we discovered a rare example of a palladium Hfacac dimer complex supported by a dianionic acetonate bridging ligand derived from the cleavage of a Hfacac ligand.

Results and Discussion

Siedle and Pignolet reported the synthesis of 1, a 1:1 adduct of Pd(Hfacac)2 and lutidine as the result of the reaction of the palladium starting material with an unspecified excess of lutidine in 80% yield as yellow needles.21 Repeating the procedure with ∼3 equiv of lutidine gave the expected yellow needles after recrystallization, albeit in a lower yield than previously reported. However, when the reaction was run with ∼7.5-fold molar excess of lutidine, a new compound was ultimately isolated as well-formed red–orange crystals, 2, in modest yield as well as a white solid (Scheme 1).

Scheme 1. Reaction of Pd(Hfacac)2 with Different Amounts of Lutidine (L).

Scheme 1

Open in a new tab

The 19F NMR spectrum of 2 is similar to but distinct from that observed by Siedle and Pignolet for 1. For example, both complexes exhibit three resonances in the −73 to −78 ppm range. Compound 1 yields peaks at −75.07, −75.12, and −77.50 ppm, while compound 2 exhibits signals at −74.09 (broad), −75.55 (sharp), and −77.14 ppm (broad). However, the integration for 1 is 1:1:2 while the integration for 2 is 2:2:1, respectively. The sharp singlet observed at −75.55 ppm for 2 is akin to the CF3 peaks assigned to the O,O-bonded Hfacac ligands in 1. The −77.14 ppm resonance for 2 is similar to that observed for the C-bonded Hfacac ligand in 1 (Figure S2). However, the FT-IR spectrum of 2 does not exhibit CO stretches above 1700 cm–1 inconsistent with the presence of a C-bonded Hfacac ligand (Figure S11). These data are in agreement with the supposition that 2 likely contains O,O-bonded Hfacac ligands but not a C-bonded Hfacac ligand.

The 1H NMR spectrum of 2 revealed a sharp singlet at 6.24 ppm assigned to the methine proton of an O,O-bonded Hfacac ligand. As expected, based on the 19F NMR results, there is no evidence of a C-bonded Hfacac in the 1H NMR spectrum. For compound 1, this type of ligand has a methine proton resonance at 4.99 ppm; this region of the spectrum is blank for 2. However, there is a peak at 3.83 ppm, which is split into a 1:3:3:1 quartet with a very small coupling of ∼1.5 Hz (Figures 1 and S1).

Figure 1.

Figure 1

Open in a new tab

1H NMR spectrum of 2.

Further upfield, two singlets are observed at 3.20 and 3.05 ppm and are assigned to the lutidine methyl groups; the downfield resonance is significantly broader than the upfield one. The presence of two resonances is consistent with two inequivalent methyl groups associated with the lutidine. The analogous but equivalent methyl groups in 1 were found at 3.26 ppm.

Salient features of the 13C{1H} NMR spectrum of 2 include three quartets in the far downfield region at 198.92, 175.29, and 173.95 ppm with intensities of 1, 2, and 2, respectively (Figure S4). The chemical shifts of the two latter peaks are suggestive of carbonyl carbons in Hfacac ligands; Siedle reported that the 13C NMR resonances of the carbonyl carbons in the starting material, Pd(Hfacac)2, resonate at 176.6.22 Interestingly, there is only one Hfacac methine carbon resonance observed at 89.84 ppm (Figure S6). One interpretation of these observations is that there are two Hfacac ligands, each of which contains the two inequivalent carbonyl carbons. The peak at 198.92 ppm is quite distinct from an O,O-Hfacac ligand.

In the upfield portion of the 13C{1H} NMR spectrum, two singlets are found at 27.67 and 27.24 ppm with equal intensity and by analogy with 1 are assigned to the methyl carbons of a coordinated lutidine (Figure S6).

An unusually high-field resonance is observed at 20.15 ppm in the 13C NMR spectrum. While the exact nature of this peak was unclear at this juncture, the 2D HSQC NMR of 2 clearly showed that this peak is associated with the 1H NMR resonance at 3.83 ppm (Figure S7) which exhibited an additional coupling fine structure.

A single-crystal X-ray structure determination was undertaken for 2, the asymmetric unit of which is shown in Figure 2. Compound 2 is a rare example of a palladium dimer in which two Pd(II) atoms are bridged by a dianionic acetonate ligand, μ-CHC(O)CF3. The distance between the palladium atoms, at ∼3.25 Å, is too long to suggest any bonding interaction. Each palladium atom is coordinated to one bidentate Hfacac anion and one lutidine ligand. The bridging acetonate ligand completes the square planar coordination geometry around each palladium. The angles of the inner coordination sphere of each palladium approach 90° (between 87.22(17)° and 93.64(18)° for Pd(1) and between 87.70(18)° and 92.43(17)° for Pd(2)).

Figure 2.

Figure 2

Open in a new tab

Single-crystal X-ray structure of 2 with 35% thermal ellipsoids (hydrogens have been omitted for the sake of clarity).

The bonding angles around carbon C(18) of the μ-CHC(O)CF3 bridging ligand suggest a distorted tetrahedron in which the O5–C(18)-Pd(1) and Pd(2) angles are splayed out from the idealized 109.5° to 119.8(5)° and 112.6(4)°, respectively. In response, the Pd(1)–C(18)–Pd(2) angle is diminished somewhat and only reaches 107.0(2)°.

The Pd–O bond distances for the oxygen atoms of the Hfacac ligands that are trans to the N atoms of the lutidines are significantly shorter (Pd(1)–O(2) = 2.049(3) Å and Pd(2)–O(3) = 2.047(3) Å) than those that are trans to the bridging acetonate ligand (Pd(1)–O(1) = 2.122(3) Å and Pd(2)–O(4) = 2.134(3) Å). The bridging Pd–C bond distances are essentially identical at 2.024(5) Å for Pd(1) and 2.023(5) Å for Pd(2).

A related, albeit mononuclear, square planar palladium structure was reported by Okeya in which one Hfacac ligand derived from the Pd(Hfacac)2 starting material is coordinated to Pd in a traditional bidentate fashion, while the other Hfacac forms part of a chelating N ∼ O ligand from the reaction with acetaldoxime. The bond distance between the Pd and the coordinated methine of the chelate was ∼2.05 Å.23

The dihedral angle between the two square planes defined by the inner coordination sphere surrounding Pd(1) and Pd(2) is approximately 54°. The more open dihedral angle accommodates the Hfacac ligands, allowing them to slip past one another forming a stacked pair of Hfacac ligands. From this configuration, one CF3 from both Hfacac ligands could be considered proximal to the palladium dimer core, while the other one would be distal (Figure S29).

Once the structure of 2 was solved, the unidentified peaks in the 19F, 1H, and 13C NMR spectra could be assigned to the μ-CHC(O)CF3 ligand in addition to other resonances. For example, the 3.83 ppm signal in the 1H NMR spectrum is ascribable to the methine H of the μ-CHC(O)CF3 ligand forming a quartet due to four-bond coupling (∼1.5 Hz) to the fluorines of the CF3 substituent. The quartet collapses to a singlet in the 1H{19F} spectrum (Figure S12). Similar 4JHF coupling (∼1 Hz) was observed in an organic proxy for the acetonate ligand, namely, CF3COCH3.24 Simulation of the 4JHF coupling based on the solid-state X-ray crystal structure of 2 at the B3LYP/6-311+G* level of theory indicates that the observed ∼1.5 Hz coupling is expected. The three simulated 4JHF couplings were 0.74, 1.13, and 6.31 Hz suggesting a rotational average of ∼2.8 Hz, similar to the observed coupling. The complementary 4JHF coupling is not observed in the 19F at room temperature due to the broad nature of the CF3 peak associated with μ-CHC(O)CF3 (width at half-height ∼5.8 Hz). The coupling was observed in the high-temperature spectrum (see below).

In the 19F NMR spectrum, the broader of the two CF3 substituents of the Hfacac ligands is assigned to the CF3 proximal to the Pd dimer core. From a space-filling representation, it appears that the crowding around this substituent may limit the free rotation of the CF3 group, leading to some broadening of this resonance. The CH3 substituents of each lutidine are rendered inequivalent due to the asymmetry induced by the μ-CHC(O)CF3 ligand. The space-filling model suggests that the two CH3 substituents that cover the axial positions of the Pd atoms are quite crowded. This may limit the free rotation of this group, thus leading to the broadened peak at 3.20 ppm in the 1H NMR spectrum.

The low-temperature 19F NMR spectrum of 2 at −60 °C shows that the two peaks due to the CF3 substituents of the Hfacac ligands resolve into four distinct peaks (Figure S13). Examination of the solid-state structure of 2 would predict that these substituents are inequivalent, yielding four signals for each of the two Hfacac ligands. The μ-CHC(O)CF3 ligand renders the substituents above the plane defined by the pseudotetrahedral bridging carbon and the two palladium atoms distinct from those below the plane. In the −60 °C 1H NMR spectrum of 2, the methyl groups of the lutidines resolve into four distinct resonances: the doublet due to aromatic hydrogens ortho to the methyl substituents of the lutidines similarly evolve into four doublets (Figure S14).

In the 80 °C 19F NMR spectrum, a complementary 4JHF coupling of 1.22 Hz in the μ-CHC(O)CF3 ligand is observed as the broad peak at −77.14 ppm sharpens considerably and resolves into a doublet as expected due to coupling with the acetonate methine proton (Figure S16a,b).

Next, our attention turned to the identity of the white solid in Scheme 1. In the 19F NMR spectrum of the white solid, peaks due to some residual 2 are observed along with two sharp resonances at −76.45 ppm and −74.10 ppm (Figure S18). Fortunately, clear crystals could be isolated from this white solid, which were identified by a single-crystal X-ray structure determination (Figure S30) and an independent synthesis (method b in the Experimental Section) to be trans-Pd(O2CCF3)2(lutidine)2 (3). Complex 3 is responsible for the resonance at −74.10 ppm.

The identification of 3 helps explain the origin of the μ-CHC(O)CF3 ligand in 2. Okeya reported that a monomeric Hfacac palladium compound complexed by 1,8-bis(dimethylamino)naphthalene slowly reacts with water to form an anionic palladium dimer bridged by a μ-CHC(O)CF3 ligand and a μ-O(O)CCF3 ligand.25 Similarly, Fackler found that Cu(II) Hfacac in the presence of pyridine and water produces the pyridine adduct of Cu(O2CCF3)2.26

Since water was implicated in the cleavage of the Hfacac ligand in prior references, we explored the reaction of one equiv of Pd(Hfacac)2 in the presence of water and 2 equiv of dry lutidine (∼60 ppm water based on Karl Fischer measurements) using 19F NMR spectrometry. Within the time of mixing and running the 19F NMR spectrum (<10 min), peaks due to 2, a small quantity of 3, and unidentified resonances at −76.45, −75.18, −74.23, and −74.13 ppm appeared. Within 1 h, the peaks at −75.18, −74.23, and −74.13 ppm disappeared, while the peak at −76.45 and those for 2 and 3 remained (Figure S17). Throughout this time, a very small amount of 1 was present. The peak at −76.45 ppm was also observed in the white powder (Figure S18). The compound associated with this resonance is implicated in the balanced reaction in Scheme 2 as [lutidinium][Hfacac] (4) which was confirmed by an independent synthesis (Figures S24–S27). Integration of the 19F NMR spectrum after 5 h shows that for every 0.4 equiv of 2, the amounts of 3 and 4 formed (Figure S17) are approximately as expected according to the balanced equation in Scheme 2. Thus, 2 is responsible for the red–orange crystals, and 3 and 4 are responsible for the white solid formed in Scheme 1. The identities of the fleeting intermediates observed in the 19F NMR spectrum upon mixing are unclear at this time.

Scheme 2. Reaction of Pd(Hfacac)2 with H2O in the Presence of Lutidine (L).

Scheme 2

Open in a new tab

The balanced reaction in Scheme 2 along with the supporting 19F NMR data made us suspicious of the need for 7.5 equiv of lutidine for the reaction in Scheme 1. In fact, the more likely reason for 7.5 equiv of lutidine was to ensure that sufficient water was present to effect the observed transformation. The lutidine in Scheme 1 was used as received and subsequently found to contain a much higher concentration of water than the dry lutidine used in Scheme 2 (∼6000 ppm vs ∼60 ppm water) amounting to ∼24 equiv of water per palladium.

In order to understand more fully the role of water and lutidine in the cleavage of the Hfacac ligand bound to palladium, we turned to density functional theory (DFT). The simulations (see Supporting Information) were conducted in order to determine the likely path of cleavage for Hfacac, the key step in the formation of 2. Since Hfacac is present as a ligand in both Pd(Hfacac)2 and 1, we considered the possibility that either one is the precursor to cleavage. Scheme 3 shows the three pathways for which we were able to identify the key cleavage transition state (TS): path A - water attack on one of the ligands in Pd(Hfacac)2, followed by complexation of lutidine to Pd; path B - water attack on the C-bound Hfacac of 1; and paths C1/C2 - water attack on the O,O-bound Hfacac of 1, followed by O,O-rearrangement of the C-bound ligand. A more detailed picture of our DFT exploration of the Hfacac cleavage is shown in Figure S31, together with the optimized structures of each species.

Scheme 3. Possible Pathways for the Cleavage of Hfacac Identified from DFT Modeling (ΔGBH = Free Energy Barrier of Reaction in kcal/mol).

Scheme 3

Open in a new tab

We were able to identify the TS associated with each pathway, provided that water initiates the attack in the form of a hydrogen-bonded H2O-lutidine complex. Path C can take two distinct routes (C1 and C2), depending on the orientation of the C-bound Hfacac ligand at the time of attack. This ligand can rotate around the C–Pd bond axis, leading to two energetically distinct rotamers for compounds 1: 1–1 and 1–2. (see files 23_C1.xyz and 24_C2.xyz in the Supporting Information). The carbonyl oxygens of the C-bound Hfacac are proximal to the lutidine ligand in 1–1 and distal in 1–2. While rotomer 1–2 is roughly 5 kcal/mol less stable than 1–1, the free energy barrier height (BH) for C2 is 5.5 kcal/mol lower than that for C1. Therefore, the most likely route for C is C2, via 1–1 → 1–2.

We also considered the possibility that lutidine alone could cleave Hfacac. However, our attempts to identify the trifluoroacetic acid (Tfac) elimination TS without water failed, reinforcing the conclusion that water plays a crucial role in the cleavage.

Scheme 4 continues the reaction path and, together with Scheme 3, paints a global picture of the formation of compound 2. The free energy for this global reaction was calculated to be −1.2 kcal/mol via path A (starting from Pd(Hfacac)2) and −10.1 kcal/mol for paths B, C1, and C2 (starting from 1–1).

Scheme 4. Stabilization of the Anion Intermediate via a Pd–C–C–O Cycle and Formation of 2 Obtained from DFT Modeling (Free Energies of Reaction Shown in kcal/mol).

Scheme 4

Open in a new tab

Assuming that Hfacac cleavage is the rate-limiting step in the formation of 1, the calculated BHs are a good indicator of the activation energies for each path, with the lowest value corresponding to the fastest and therefore most probable, route. While path A has the lowest BH, it is close to that of C2 (16.9 vs 17.4 kcal/mol) and is thermodynamically disfavored relative to C2, based on the aforementioned free energies for the global reactions. On the other hand, C2 requires the rotation of the C-bound ligand in 1, which is an additional endergonic step prior to cleavage. These kinetic considerations suggest that A is likely the fastest and therefore the main route by which 2 is formed.

A noteworthy discovery from the DFT simulations is the presence of an intermediate Pd anion species with a four-atom Pd–C–C–O cycle, compound 5, as shown in Scheme 4. Within the four-atom cycle, the negative charge resulted from Hfacac cleavage is stabilized by 5.4 kcal/mol relative to the acyclic species. This stabilization likely plays a crucial role in the formation of 2 by allowing the Pd anion to exist long enough to interact with a second Pd complex.

While the DFT results do not clearly identify the route by which the cleavage occurs, they strongly suggest that the attack is initiated by water on a O,O-bound Hfacac, in either Pd(Hfacac)2 or compound 1, solidifying further the importance of water in the formation of 2, and the role Pd plays in stabilizing the intermediate species.

Finally, we examined the possibility that compound 1 might be involved in the formation of 2 as suggested by the DFT simulation and by the presence of a minute quantity of 1 observed in Figure S17 when Pd(Hfacac)2 is reacted with 2 equiv of lutidine and 1 equiv of water. Thus, the reaction of one equiv of 1, dry lutidine, and water was monitored by 19F NMR spectrometry. After 5 h, only about 14% of 1 was converted to 2, 3, and 4, which is considerably slower than when Pd(Hfacac)2 is used as the starting material (Figure S19). The slower kinetics for the reaction is consistent with the higher barrier heights calculated for routes B and C1 and with the lower stability of the cleavage precursor in route C2 from the DFT simulations.

Conclusions

In conclusion, we have deconstructed the roles of water and lutidine in the cleavage of the Hfacac ligand associated with Pd(Hfacac)2 to form a unique dimeric Pd complex held together by a bridging dianionic acetonate ligand, namely, 2. Formation of 2 is accompanied by the production of trans-Pd(O2CCF3)2(lutidine)2 (3) which accounts for the remaining portion of the cleaved Hfacac ligand. The balanced reaction is accommodated by creation of [lutidinium][Hfacac] (4) which is identified in the reaction mixture and confirmed by independent synthesis. The DFT simulations corroborate the role of water as the initiator of the cleavage responsible for the formation of the dimers’ μ-bridge and point to a Pd–C–C–O cycle as the stabilizing structure for the intermediate species.

Experimental Section

Materials and Methods

All manipulations were carried out using standard Schlenk or drybox techniques. Anhydrous pentane and hexanes were purchased from Sigma-Aldrich, sparged with N2, and used without further purification. Pd(Hfacac)2 and lutidine were purchased from Strem and Sigma-Aldrich, respectively. NMR spectra were recorded at 298 K on a Bruker Avance III HD spectrometer operating at 500.15 (1H), 470.61 (19F), and 125.77 (13C). Chemical shifts are reported relative to SiMe4 (1H, 13C, 0 ppm) or CFCl3 (19F, 0 ppm). FT-IR spectra were recorded by using a Nicolet iS50 instrument.

The DFT simulations were performed at the BP86/6-31+G** level of theory,27−29 using the Gaussian31 software to optimize the structure of each species. The electronic energy of each optimized structure was used in stoichiometric reactions to determine the thermochemistry of the reaction. An entropic correction associated with the internal degrees of freedom of each species was included in order to estimate the free energy of reaction ΔG. For simulation of the 4JHF coupling of 2, we used the experimental crystal structure of the compound and the B3LYP/6-311+G* level of theory within the GIAO approximation.30

X-ray intensity data for 2 and 3 were measured on a Bruker PHOTON II CPAD-based diffractometer with dual Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 Å; Mo Kα radiation, λ = 0.71073 Å). Crystals were mounted on a cryoloop using Paratone oil. The detector was placed at a distance of 5.00 cm from the crystal. The data were merged, corrected for absorption, and the structures refined using the Bruker SHELXTL Software Package (Version 6.1) and were solved using direct methods until the final anisotropic full-matrix, least-squares refinement of F2 converged.

Synthesis of Compound 1

Pd(Hfacac)2 (0.200 g, 0.384 mmol) was added to a 50 mL Schlenk tube equipped with a magnetic stir bar. The tube was purged with nitrogen and then sealed with a rubber septum. Anhydrous pentane (5 mL) was added to the tube, and the mixture was stirred until the Pd complex dissolved (∼5–10 min) to give a light yellow–orange solution. After the Pd complex dissolved, lutidine (0.133 g, 1.29 mmol) was added to the solution. A slightly darker yellow–orange solution resulted. The solution was stirred overnight (∼17 h). The resulting yellow precipitate was collected and washed with cold hexanes. The yellow precipitate was recrystallized from refluxing hexanes to give 0.105 g (44%) yellow needles of 1. The NMR spectra of the resulting material matched previously published data for this compound.21

Synthesis of Compound 2

Pd(Hfacac)2 (0.325 g, 0.625 mmol) was added to a 50 mL Schlenk tube equipped with a magnetic stir bar. The tube was purged with nitrogen and then sealed with a rubber septum. Anhydrous pentane (8 mL) was added to the tube, and the mixture was stirred until the Pd complex dissolved (∼5–10 min) to give a light yellow–orange solution. After the Pd complex dissolved, wet (∼6000 ppm) lutidine (0.55 mL, 4.7 mmol) was added to the solution. The solution turned dark brown. After the mixture was stirred for 3 days, orange and white materials precipitated which were collected and washed with hexanes. The solids were heated to reflux in 10 mL of hexanes. The hot solution was filtered to remove the white material, which gave a yellow filtrate. The filtrate was cooled to room temperature and was allowed to stand overnight. Red–orange crystals formed, which were collected, and washed with hexanes to give 69.9 mg (23%) of 2. Anal. Calc’d for C27H21F15N2O5Pd2: C, 34.09; H, 2.23; N, 2.94. Found: C, 34.30; H, 2.07; N, 2.91. 1H NMR (500.15 MHz, C6D6): 6.52 (dd, apparent triplet, JHH ∼ 7.7 Hz, 2 H), 6.25 (v. br. d, overlapped with 6.24 ppm peak, 2 H), 6.24 (s, 2 H), 6.17 (br. d, 2 H), 3.83 (q, 3JHF = 1.5 Hz, 1 H), 3.20 (br s, 6 H), 3.05 (s, 6 H). 13C {1H} NMR (125.77 MHz, C6D6): 198.92 (q, 2JCF = 29.3 Hz), 175.29 (q, 2JCF = 34.3 Hz), 173.95 (q, 2JCF = 34.2 Hz, 2 C), 161.82 (s), 160.47 (s), 137.79 (s),122.87 (s), 121.49 (s,), 118.10 (overlapping quartets, 1JCF = 286.2 Hz), 113.33 (q, 1JCF = 303.6 Hz), 89.84 (s), 27.67 (s), 27.24 (s), 20.15 (s). 19F NMR (470.56 MHz, C6D6): −74.09 (br s, 2 F), −75.53 (s, 2 F), −77.14 (s, 1 F). IR (neat): 1678 s, 1635 s, 1551 s, 1518 s.

Synthesis of Compound 3

Method a

Pd(Hfacac)2 (0.199 g, 0.384 mmol) was added to a 50 mL Schlenk tube equipped with a magnetic stir bar. The tube was purged with nitrogen and then sealed with a rubber septum. Anhydrous pentane (8 mL) was added to the tube, and the mixture was stirred until the Pd complex dissolved (∼5–10 min) to give a light yellow–orange solution. After the Pd complex was dissolved, lutidine (0.28 mL, 2.40 mmol) was added to the solution. The solution turned dark brown. After stirring for 3 days, orange and white materials precipitated which were collected and washed with hexanes. The solids were heated to reflux in 10 mL of hexanes. The hot solution was filtered to remove the white material. The white material was dissolved in acetone. Heptanes layered onto the acetone solution vial was placed inside a larger vial containing ∼2 mL of heptanes and sealed. After 3 months, crystals of compound 3 suitable for X-ray analysis were formed.

Method b

Trans-Pd(OAc)2(lutidine)2 was prepared according to a literature procedure.31 To a 20 mL vial with a magnetic stirrer was added 0.0792 g (0.180 mmol) of trans-Pd(OAc)2(lutidine)2 followed by 5 mL of dichloromethane. Trifluoroacetic acid (1.0 g, 8.8 mmol) was added to the dichloromethane slurry which caused the Pd complex to dissolve. The solution was stirred for 2 h; then, 10 mL of methanol was added to the vial forming a precipitate. The mixture was stirred for an additional hour and was allowed to stand overnight (∼16 h). The light yellow precipitate was collected and washed with methanol, and 47.5 mg (48.1%) of compound 3 was obtained. Anal. Calc’d for C18H18F6N2O4Pd: C, 39.54; H, 3.32; N, 5.12. Found: C, 39.81; H, 3.31; N, 4.98. 1H NMR (500.15 MHz, C6D6): 6.41 (t, JHH = 7.7 Hz, 2 H), 6.07 (d, 4 H), 3.67 (s, 12 H). 19F NMR (470.56 MHz, C6D6): −74.10. 13C NMR (125 MHz, tetrachloroethane-d2): δ 162.15 (q, JCF = 36.7 Hz), 161.2, 139.3, 123.1, 113.3 (q, JCF = 288.7 Hz), 25.2 ppm. IR (neat): 1690 s, 1650 w, 1610 w, 1579 w.

Synthesis of Compound 4

In a nitrogen glovebox, hexafluoroacetylacetone (0.768 g, 3.69 mmol) was added to a 50 mL Schlenk tube equipped with a magnetic stir bar. Hexanes (5 mL) was added to the Schlenk tube via a syringe. To the hexanes solution was added 0.44 g (4.11 mmol) of lutidine. A white precipitate was formed. This mixture was stirred overnight. The white precipitate was collected and washed with hexanes, 0.92 g (79%). Anal. Calc’d for C12H11F6NO2: C, 45.72; H, 3.52; N, 4.44. Found: C, 45.96; H, 3.51; N, 4.37. 1H NMR (500.15 MHz, C6D6): 14.51 (v br s, 1 H), 6.52 (t, JHH = 7.9 Hz, 1 H), 6.46 (s, 1 H), 5.91 (d, JHH = 7.9 Hz, 2 H), 2.00 (s, 6 H). 13C {1H} NMR (125.77 MHz, C6D6): 175.42 (q, 2JCF = 31.2 Hz), 153.32, 142.85, 120.80, 119.65 (q, 1JCF = 290.3 Hz), 86.84, 19.58. 19F NMR (470.56 MHz, C6D6): −76.45 IR (neat): 2617 bm, 1665 s, 1537 s.

Reaction of Pd(Hfacac)2 with Dry Lutidine and Water Followed by 19F NMR

Water (6.4 mg, 0.36 mmol) and dry (∼60 ppm) lutidine (0.105 g, 0.980 mmol) were added to a vial. After mixing, 5.349 g of C6D6 was added to the vial. To this C6D6 solution, Pd(Hfacac)2 (0.196 g, 0.376 mmol) was added causing the solution to become dark yellow–orange immediately. The solution was shaken for ∼1 min, and then an aliquot was taken for NMR analysis.

Reaction of Compound 1 with Dry Lutidine and Water Followed by 19F NMR

A stock solution was prepared from water (3.4 mg, 0.19 mmol), lutidine (0.0537 g, 0.501 mmol), and 14.96 g of C6D6. The solution was sonicated for 1 m, vortexed for 1 m, and was allowed to stand for 30 m. Compound 1 (49.2 mg, 0.0784 mmol) was added to a portion of the stock solution (2.61 g). The vial was shaken until compound 1 was dissolved. An aliquot of the solution was taken for NMR analysis.

Acknowledgments

Dr. G. Meyer recorded the IR spectra. G.A.D. wishes to thank the National Sciences and Engineering Research Council of Canada, the Digital Research Alliance of Canada, and the UBC Advanced Research Computing Centre for generous financial support and access to computing facilities.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.4c00263.

  • NMR spectra, IR spectra, and X-ray crystallography (PDF)

  • Structures obtained from DFT calculations (ZIP)

The authors declare no competing financial interest.

Supplementary Material

om4c00263_si_001.pdf (7.3MB, pdf)
om4c00263_si_002.zip (30.7KB, zip)

References

  1. Werner A. About Acetyl Acetone Compounds of Platinum. Ber. Deut. Chem. Ges. 1901, 34, 2584–2593. 10.1002/cber.190103402206. [DOI] [Google Scholar]
  2. Kauffman G. B. Alfred Werner’s Research on Optically Active Coordination Compounds. Coord. Chem. Rev. 1974, 12, 105–149. 10.1016/S0010-8545(00)82025-1. [DOI] [Google Scholar]
  3. Sodhi R.; Paul S. An Overview of Metal Acetylacetonates: Developing Areas/Routes to New Materials and Applications in Organic Synthesis. Catal. Surv. Asia 2018, 22, 31–62. 10.1007/s10563-017-9239-9. [DOI] [Google Scholar]
  4. Condorelli G. G.; Malandrino G.; Fragal̀a I. L. Engineering of Molecular Architectures of β-diketonate Precursors Toward New Advanced Materials. Coord. Chem. Rev. 2007, 251, 1931–1950. 10.1016/j.ccr.2007.04.016. [DOI] [Google Scholar]
  5. Aromi G.; Gamez P.; Reedijk J. Poly Beta-Diketones: Prime Ligands to Generate Supramolecular Metalloclusters. Coord. Chem. Rev. 2008, 252, 964–989. 10.1016/j.ccr.2007.07.008. [DOI] [Google Scholar]
  6. Olivier J.-H.; Harrowfield J.; Ziessel R. 3-Substituted-2,4-Pentanedionates: Ligands for Photoreactive Supramolecular Assemblies. Chem. Commun. 2011, 47, 11176–11188. 10.1039/c1cc12771c. [DOI] [PubMed] [Google Scholar]
  7. Westcott S. A.Catalysis Using β-diketonato Metal Complexes in Chemistry of Metal Enolates, Part 1; Zabicky J., Ed.; John Wiley & Sons, 2009; pp 551–577. [Google Scholar]
  8. Keim W.; Hoffmann B.; Lodewick R.; Peuckert M.; Schmitt G.; Fleischhauer J.; Meier U. Linear Oligomerization of Olefins via Nickel Chelate Complexes and Mechanistic Consideration Based on Semi-Empirical Calculations. J. Mol. Catal. 1979, 6, 79–97. 10.1016/0304-5102(79)80016-4. [DOI] [Google Scholar]
  9. Behr A.; Falbe V.; Freudenberg U.; Keim W. 1,3-Diketones as Active Ligands in the Nickel-Catalyzed Linear Oligomerization of Olefins. Isr. J. Chem. 1986, 27 (3), 277–279. 10.1002/ijch.198600041. [DOI] [Google Scholar]
  10. Keim W.; Behr A.; Kraus G. Influences of olefinic and Dikeontate Ligands in the Nickel-Catalyzed Linear Oligomerization of 1-Butene. J. Organomet. Chem. 1983, 251, 377–391. 10.1016/S0022-328X(00)98784-9. [DOI] [Google Scholar]
  11. Janiak C.; Lassahn P. G. Metal Catalysts for the Vinyl Polymerization of Norbornene. J. Mol. Catal. A: Chem. 2001, 166, 193–209. 10.1016/S1381-1169(00)00475-1. [DOI] [Google Scholar]
  12. Blank F.; Janiak C. Metal Catalysts for the Vinyl/Addition Polymerization of Norbornene. Coord. Chem. Rev. 2009, 253, 827–861. 10.1016/j.ccr.2008.05.010. [DOI] [Google Scholar]
  13. Barnes D. A.; Benedikt G. M.; Goodall B. L.; Huang S. S.; Kalamarides H. A.; Lenhard S.; McIntosh L. H.; Selvy K. T.; Shick R. A.; Rhodes L. F. Addition Polymerization of Norbornene-Type Monomers Using Neutral Nickel Complexes Containing Fluorinated Aryl Ligands. Macromolecules 2003, 36, 2623–2632. 10.1021/ma030001m. [DOI] [Google Scholar]
  14. Suslov D. S.; Bykov M. V.; Abramov P. A.; Pakhomova M. V.; Ushakov I. A.; Voronov V. K.; Tkach V. S. [Pd(acac)(MeCN)2]BF4: Air-Tolerant, Activator-Free Catalyst for Alkene Dimerization and Polymerization. RSC Adv. 2015, 5, 104467–104471. 10.1039/C5RA24591E. [DOI] [Google Scholar]
  15. Suslov D. S.; Bykov M. V.; Abramov P. A.; Pakhomova M. V.; Ushakov I. A.; Voronov V. K.; Tkach V. S. Synthesis, Characterization, and Application for Addition Polymerization of Norbornene of Novel Acetylacetonate Bis(anilines) Palladium (II) Complexes. Inorg. Chem. Commun. 2016, 66, 1–4. 10.1016/j.inoche.2016.01.022. [DOI] [Google Scholar]
  16. Suslov D. S.; Bykov M. V.; Abramov P. A.; Pakhomova M. V.; Ushakov I. A.; Voronov V. K.; Tkach V. S. [Pd(acac)(L)2][BF4] (L = morpholine, diethylamine, dibutylamine, dioctylamine): Synthesis, Structure and Their Catalytic Activity. J. Mol. Struct. 2017, 1133, 411–421. 10.1016/j.molstruc.2016.12.025. [DOI] [Google Scholar]
  17. Suslov D. S.; Bykov M. V.; Kuzman A. V.; Abramov P. A.; Kravchenko O. V.; Pakhomova M. V.; Rokhin A. V.; Ushakov I. A.; Tkach V. S. Cationic Acetylacetonate Palladium Complexes/Boron Trifluoride Etherate Catalyst Systems for Polymerization of 5-Methoxycarbonylnorbornene. Catal. Commun. 2018, 106, 30–35. 10.1016/j.catcom.2017.12.010. [DOI] [Google Scholar]
  18. Bykov M. V.; Abramov Z. D.; Pakhomova M. V.; Borodina T. N.; Smirnov V. I.; Suslov D. S. Structure and Catalytic Properties of Palladium(II) (Acetylacetonato-κ2O,O′) Bis(Tris(Diethylamino)Phosphine) Tetrafluoroborate. J. Struct. Chem. 2022, 63 (1), 125–139. 10.1134/S0022476622010139. [DOI] [Google Scholar]
  19. Deng G.; Burtovyy O.; Hayakawa S.. UV-Active Derivatives of Pd(acac)2L Catalyzed Polycycloolefin Compositions as Optical Materials. US Patent 11,746,166, 2023.
  20. Hayakawa S.; Deng G.; Burtovyy R.; Langsdorf L.; Rhodes L. F.. Hindered Amine Stabilized UV-Active Organopalladium-Catalyzed Polycycloolefin Compositions as Optical Materials. US Patent 11,299,566, 2022.
  21. Siedle A. R.; Pignolet L. H. 1:1 Lewis Base Adducts of Palladium Bis(Hexafluoroacetylacetonate) and the Structure of Pd(F6acac-C)(F6acac-O,O)·(CH3)2NH. Inorg. Chem. 1981, 20, 1849–1853. 10.1021/ic50220a047. [DOI] [Google Scholar]
  22. Siedle A. R.; Newmark R. A.; Pignolet L. H. Stucture of Palladium Bis(hexafluoroacetylacetonate) and the Systematics of Its Acid-Base Chemistry. Inorg. Chem. 1983, 22, 2281–2286. 10.1021/ic00158a012. [DOI] [Google Scholar]
  23. Okeya S.; Taniguchi K.; Nishimura J.; Kusuyama Y.; Nagasawa I.; Kushi Y. Synthesis of a New Pd(II) Complex with One hfac-O, O’ and One hfac-Monoxime-C, N as Ligands; Unexpected C-O Bond Formation Between C3-Bonded hfac and N-Bonded Methine. Chem. Lett. 1997, 26, 1095–1096. 10.1246/cl.1997.1095. [DOI] [Google Scholar]
  24. Abraham R. J.; Jones A. D.; Warne M. A.; Rittner R.; Tormena C. F. Conformational Analysis. Part 27. NMR, Solvation and Theoretial Investigation of Conformational Isomerization in Fluoro- and 1,1-Difluoro-acetone. J. Chem. Soc., Perkin Trans. 1996, 2, 533–539. 10.1039/p29960000533. [DOI] [Google Scholar]
  25. Yamasaki T.; Ozaki N.; Saika Y.; Ohta K.; Goboh K.; Nakamura F.; Hashimoto M.; Okeya S. First Transition Metal Complex of 1,8-Bis(dimethylamino)naphthalene (Proton Sponge). Chem. Lett. 2004, 33 (7), 928–929. 10.1246/cl.2004.928. [DOI] [Google Scholar]
  26. Pradilla J.; Chen H. W.; Koknat F. W.; Fackler J. P. Structure and Electron Paramagnetic Resonance Spectrum of the Product of the Reaction of Aqueous Pyridine with Copper(II) Hexafluoroacetylacetonate. Tetrakis(Pyridine) bis (Trifluoroacetato) Copper(II). Inorg. Chem. 1979, 18, 3519–3522. 10.1021/ic50202a045. [DOI] [Google Scholar]
  27. Becke A. D. Density-functional thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652. 10.1063/1.464913. [DOI] [Google Scholar]
  28. Lee C.; Yang W.; Parr R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785–789. 10.1103/PhysRevB.37.785. [DOI] [PubMed] [Google Scholar]
  29. Vosko S. H.; Wilk L.; Nusair M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: a Critical Analysis. Can. J. Phys. 1980, 58, 1200–1211. 10.1139/p80-159. [DOI] [Google Scholar]
  30. Frisch M. J.; Trucks G. W.; Schlegel H. B.; Scuseria G. E.; Robb M. A.; Cheeseman J. R.; Scalmani G.; Barone V.; Petersson G. A.; Nakatsuji H.; Li X.; Caricato M.; Marenich A. V.; Bloino J.; Janesko B. G.; Gomperts R.; Mennucci B.; Hratchian H. P.; Ortiz J. V.; Izmaylov A. F.; Sonnenberg J. L.; Williams-Young D.; Ding F.; Lipparini F.; Egidi F.; Goings J.; Peng B.; Petrone A.; Henderson T.; Ranasinghe D.; Zakrzewski V. G.; Gao J.; Rega N.; Zheng G.; Liang W.; Hada M.; Ehara M.; Toyota K.; Fukuda R.; Hasegawa J.; Ishida M.; Nakajima T.; Honda Y.; Kitao O.; Nakai H.; Vreven T.; Throssell K.; Montgomery J. A. Jr.; Peralta J. E.; Ogliaro F.; Bearpark M. J.; Heyd J. J.; Brothers E. N.; Kudin K. N.; Staroverov V. N.; Keith T. A.; Kobayashi R.; Normand J.; Raghavachari K.; Rendell A. P.; Burant J. C.; Iyengar S. S.; Tomasi J.; Cossi M.; Millam J. M.; Klene M.; Adamo C.; Cammi R.; Ochterski J. W.; Martin R. L.; Morokuma K.; Farkas O.; Foresman J. B.; Fox D. J.. Gaussian 16, Revision C.01; Gaussian, Inc.: Wallingford CT, 2016. [Google Scholar]
  31. Zhang Y.-H.; Shi B.-F.; Yu J.-Q. Pd(II)-Catalyzed Olefination of Electron-Deficient Arenes Using 2,6-Dialkylpyridine Ligands. J. Am. Chem. Soc. 2009, 131, 5072–5074. 10.1021/ja900327e. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

om4c00263_si_001.pdf (7.3MB, pdf)
om4c00263_si_002.zip (30.7KB, zip)

Articles from Organometallics are provided here courtesy of American Chemical Society

RESOURCES