17334
J. Phys. Chem. 1996, 100, 17334-17336
Solvation of Palladium Diacetate in Trifluoroacetic Acid Ole Swang,* Richard Blom, and Olav B. Ryan Department of Hydrocarbon Process Chemistry, SINTEF Applied Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway
Knut Fægri, Jr. Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway ReceiVed: July 22, 1996; In Final Form: August 31, 1996X
The solvation of palladium diacetate in trifluoroacetic acid has been studied by ab initio quantum chemical calculations and infrared and NMR spectroscopy. We report evidence for the formation of a dicarboxylatedicarboxylic acid species, with square planar coordination around the metal center, through addition of two acid molecules.
Introduction Great effort has been spent in attempts to devise methods of methane activation that are more cost-efficient than present techniques, most of which involve synthesis gas as an intermediate.1 One possible approach is the heterolytic cleavage by homogeneous catalysts, as investigated, among others, by Sen et al.2 and Vargaftik et al.3 The catalysts described in these publications consist of metal carboxylate complexes dissolved in carboxylic acids. The mechanism(s) involved in such reactions are, however, less than clear. To establish reaction mechanisms, it is necessary to gain knowledge about the molecular and electronic structure of the catalyst in the actual solution. We have undertaken an investigation based on ab initio calculations, vibrational spectroscopy, and NMR spectroscopy of the solvation of palladium diacetate (Pd(Ac)2) in trifluoroacetic acid (HTFA). In the following, TFA will denote the trifluoroacetate anion, and Ac will denote the acetate anion. Computational and Experimental Details All calculations were performed at the restricted HartreeFock level using the GAMESS-US program package.4 The MIDI basis sets reported by Huzinaga5 were used, with the following extensions: For palladium, two p-type exponents of magnitude 0.083 and 0.03 and one d-type exponent of magnitude 0.106 were added. d-type polarization functions were added for carbon, oxygen, and fluorine, with exponents of 0.600, 1.154, and 1.496, respectively. For hydrogen, a p-type polarization function with the exponent 1.0 was added. The calculations were run on an Intel Paragon computer and demanded some 40000 h of CPU time. Infrared spectra were recorded on a Perkin Elmer S-2000 FTIR instrument. Spectra of solid Pd(Ac)2 were recorded in KBr pellets, while spectra of a 5 wt % solution of Pd(Ac)2 in HTFA were obtained by keeping the brown, homogeneous solution between two KBr windows. 1H-NMR spectra of a 2 wt % Pd(Ac)2 solution in HTFA were recorded on a Varian VXR 300 spectrometer at 25 °C. Attempts to dissolve Pd(Ac)2 in formic or acetic acid were unsuccessful. Results and Discussion In the following, results from ab initio calulations will be reported, followed by experimental NMR and IR results. Initial X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)02194-6 CCC: $12.00
calculations were carried out with palladium diformate as a model for Pd(TFA)2. The palladium diformate and Pd(TFA)2 molecules are depicted in Figure 1, while some calculated geometry data are listed in Table 1. According to the calculations, the monomeric formate complex has D2h symmetry, while the TFA complex has C2h symmetry. We note that the geometry around the metal atom, including the carboxyl groups, hardly changes when trifluoroacetate is substituted for the formate. When a geometry optimization is started from a palladium diformate molecule with two formic acid molecules at 8 Å above and below its plane, respectively, it converges to the diformatediformic acid complex (with Ci symmetry) depicted at the bottom of Figure 1. Some other configurations were investigated, but none of them were found to be more stable. We note that this species has a square planar coordination around palladium, as simple ligand field theory predicts for a d8 complex. The calculated Pd-O bond lengths are 2.12 Å for the formic acid ligands and 2.01 Å for the formate ligands. These values may be compared to the Pd-O distance of 2.08 Å in the unsolvated complex. The energy of formation is 66 kcal/ mol relative to a Pd(O2CH)2 molecule and two HCO2H molecules at infinite separation, corresponding to 33 kcal/mol/ acid molecule. Earlier experimental work6 indicated that HCO2H mainly exists as a cyclic dimer in the liquid phase, like many other carboxylic acids. To discern whether the formation of the adduct will take place in solution, it must be established whether the addition to the metal is more exothermic than the intermolecular association in the liquid acid itself. The calculated dimerization energy of formic acid is 32 kcal/mol, giving 16 kcal/mol/monomer. For an estimate of the energy gained by further association of the acid molecules, we conducted a geometry optimization on the hexamer of formic acid. This system, which contains 30 atoms, obviously has a large number of possible conformers. On the basis of some test calculations using semiempirical methods, we settled for a configuration with two cyclic dimers on top of each other and the remaining two molecules forming a hydrogen bond bridge between the dimers. The hexamer is 109 kcal/mol more stable than six monomers at infinite distance at the present computational level, which is 18 kcal/mol/monomer. Thus, only 2 additional kcal/mol/ monomer are gained from the further oligomerization. Comparing this 18 kcal/mol/monomer with the 33 kcal/mol/monomer © 1996 American Chemical Society
Solvatization of Palladium Diacetate in Trifluoroacetate Acid
J. Phys. Chem., Vol. 100, No. 43, 1996 17335
TABLE 1: Some ab Initio Geometrical Data for Pd(O2CH)2 and Pd(TFA)2a RPdC RPdO APdOC a
Pd(O2CH)2
Pd(TFA)2
2.40 2.08 88.9
2.40 2.09 88.2
Distances in angstroms, angles in degrees.
Figure 2. Spectra of liquid trifluoroacetic acid with and without dissolved Pd(TFA)2.
SCHEME 1
Figure 1. Palladium diformate, palladium bis(trifluoroacetate), and palladium diformate-diformic acid.
found for the addition to Pd(O2CH)2 gives an indication that the latter process will prevail. Turning to the HTFA system, we note that the calculated geometry of the bis(trifluoroacetate)-bis(trifluoroacetic acid) species, Pd(TFA)2(HTFA)2, is qualitatively the same as the formate analogue depicted in Figure 1. Its computed addition energy is 32 kcal/mol/monomer for the two acid molecules, which is very close to the formate value of 33 kcal/mol/ monomer. The computed dimerization energy of trifluoroacetic acid is 14 kcal/mol/monomer, and hence the same argument applies as for the formate: The addition of two acid molecules will be spontaneous. We have not carried out calculations for the species Pd(Ac)2(HTFA)2 or Pd(TFA)2(HAc)2, due to the very long computing times required. However, the similarity between the energetics of the formate and trifluoroacetate systems indicates that the assumption that two acid molecules add spontaneously is valid. Stephenson et al.7 have reported that Pd(TFA)2 is insoluble in HTFA. We find that upon dissolving Pd(Ac)2 in HTFA, a brown homogeneous solution is first obtained. After stirring overnight a brownish precipitate is formed, while the color of the solution becomes lighter. This process was monitored by 1H-NMR spectroscopy. Initially, the solution shows one single peak at δ2.56, corresponding to the methyl groups of the acetate ligands. This peak gradually disappears, and after 46 h, its intensity is reduced to 8%, indicating that acetate or acetic acid is contained in the precipitate. The observed exponential reduction suggests a first order rate of precipitation. At the same time there is an approximately linear change in the chemical shift, which decreases from δ2.56 to δ1.82 after 46 h. This is a consequence of the changes in heterogeneity of the sample as the relative number of dissolved complexes close to a precipitate particle increases with time. For a first order precipitation it can be shown that a linear change in chemical shift is expected.
We suggest that a three-step mechanism is involved, as illustrated in Scheme 1: First, Pd(Ac)2 is dissolved in HTFA by coordination of two HTFA molecules to the palladium center. Secondly, a hydrogen transfer from the coordinated HTFA molecules to the acetate ligands occurs. Preliminary ab initio results indicate that such a transfer proceeds with only a small energy barrier. Additionally, such a transfer is expected when considering the different basicities of acetate and trifluoroacetate. Hence, we suggest that both of the first two steps are fast. Finally, insoluble oligomeric or polymeric species are slowly formed. The vibrational spectrum of the initial homogeneous solution of Pd(Ac)2 dissolved in HTFA was recorded and compared to calculated spectra for Pd(TFA)2 and Pd(TFA)2(HTFA)2, respectively. Figure 2 shows the spectra of liquid trifluoroacetate with and without dissolved Pd(TFA)2. The main difference is a broad absorption, of medium strength, at about 1650 cm-1. Comparing this with the calculated spectra in Table 2, we note that this feature is hard to explain from the calculated spectrum of Pd(TFA)2, but is in line with the predicted spectrum of Pd(TFA)2(HTFA)2. The strong band at 1669 cm-1 in the calculated spectrum is a carbonyl stretch involving the oxygen atom bonded to palladium and the carbon on the acid ligands. Calculations were not carried out for Pd(TFA)2(HAc)2 or Pd(HTFA)2(Ac)2 (Vide supra), but the difference between the carbonyl stretch frequencies of those species and Pd(TFA)2(HTFA)2 should be negligible. We also note that the spectrum of the pure acid is slightly closer to the predicted spectrum for the dimer than for the monomer. This is in line with experimental results, as mentioned above.
17336 J. Phys. Chem., Vol. 100, No. 43, 1996
Swang et al.
TABLE 2: Vibrational Spectra As Calculated at the Hartree-Fock Level and Scaled with a Factor of 0.89a
a
HTFA
strength
(HTFA)2
strength
Pd(TFA)2
strength
Pd(TFA)4‚2H
strength
615 642 774 1128 1233 1262 1298 1405 1852 3652
w w w s m s m w s w
674 889 1164 1266 1287 1308 1454 1801 3378
w m s s s w w s s
485 1264 1302 1512 1523
w s s s s
679 779 945 1248 1297 1349 1370 1576 1669 2806 2915 2985
w w m m s w w m s m m s
Frequencies in inverse centimeters.
Concluding Remarks The computed energy of dimerization for HTFA is 32 kcal/ mol, while the energy for the reaction between two molecules of HTFA and one molecule of Pd(TFA)2 is 66 kcal/mol. Furthermore, the experimental vibrational spectrum of Pd(Ac)2 dissolved in HTFA is incompatible with the computed spectrum of palladium bis(trifluoroacetate), but compatible with the computed spectrum of the bis(trifluoroacetate)-bis(trifluoroacetic acid), Pd(TFA)2(HTFA)2. The major feature of interest in the latter computed spectrum is a carbonyl stretch frequency, which is likely to be very close to the corresponding frequency for Pd(TFA)2(HAc)2. From this, we might conclude that the palladium dicarboxylate adds two acid molecules per metal atom in the solution. Furthermore, the solution is unstable toward oligomerization, and the precipitate formed contains acetate groups. Acknowledgment. We would like to thank Eddy W. Hansen for performing the NMR experiments and assisting in their
interpretation and Duncan Akporiaye for comments on the manuscript. Financial support and a grant of computer time, both from the Norwegian Research Council (NFR), are gratefully acknowledged. O.S. wishes to acknowledge a travel grant from the VISTA project. References and Notes (1) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (2) (a) Kao, L. C.; Hutson, A. C. Sen, A. J. Am. Chem. Soc. 1991, 113, 700. (b) Sen, A. Platinum Met. ReV. 1991, 35, 126. (3) Vargaftik, M. N.; Stolarov, I. P.; Moiseev, I. I. J. Chem. Soc., Chem. Commun. 1990, 1049. (4) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, M.; Nguyen, K. A.; Su, S. J.; Windus, T. L. J. Comput. Chem. 1993, 14, 1347. (5) Huzinaga, S.; Andzelm, J. Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations; Elsevier: Amsterdam, 1984. (6) Milne, J. B. In The Chemistry of Nonaqueous SolVents; Lagowski, J. J., Ed.; Academic Press: London, 1978. (7) Stephenson, T. A.; Morehouse, S. M.; Powell, A. R.; Heffer, J. P.; Wilkinson, G. J. Chem. Soc. 1965, 3632.
JP9621944
