Methylpalladium complexes with pyrimidine-functionalized N-heterocyclic carbene ligands

A series of methylpalladium(II) complexes with pyrimidine-NHC ligands carrying different aryl- and alkyl substituents R ([((pym)^(NHC-R))PdII(CH3)X] with X = Cl, CF3COO, CH3) has been prepared by transmetalation reactions from the corresponding silver complexes and chloro(methyl)(cyclooctadiene)palladium(II). The dimethyl(1-(2-pyrimidyl)-3-(2,6-diisopropylphenyl)imidazolin-2-ylidene)palladium(II) complex was synthesized via the free carbene route. All complexes were fully characterized by standard methods and in three cases also by a solid state structure.


Introduction
Palladium complexes have been shown to be versatile homogeneous catalysts in a variety of reactions [1]. One of the most prominent examples are the palladium catalyzed cross-coupling reactions [2], but in addition palladium complexes recently received much attention also in the field of selective CH oxidations [3][4][5][6]. The catalytic conversion of alkanes and especially of methane into a value-added products while avoiding overoxidation to carbon dioxide is still one of the most difficult tasks and considered to be one of the "Holy Grails" of chemistry [7]. Early work by Bergman [8,9] and Graham [10,11] showed that the activation of methane is possible, but their systems provided only stoichiometric reactions. The Shilov system [12][13][14][15][16][17], had the disadvantage of using stoichiometric amounts of platinum for the reoxidation of the active catalyst and the platinum bispyrimidine system [18][19][20] was not successful because of the large amounts of diluted sulfuric acids as the byproduct of the methanol synthesis. Sen reported the activity of palladium(II) catalysts for the oxidation of methane [21] and several groups contributed to the progress in the field which is summarized in recent reviews [22,23]. The system based on N-heterocyclic carbene (NHC) ligands developed in our group uses a mixture of trifluoroacetic acid (TFA) and its anhydride (TFAA) as solvents. It has the advantage that the formed ester can be separated and hydrolyzed, followed by recycling of the free acid which potentially can be reused in the process [24][25][26][27][28][29][30]. Contrary to the "Catalytica" bispyrimidine system, for the Scheme 1: Synthesis of the monomethylpalladium(II) complexes 9-11 (in DCM) and 12 (in CH 3 CN).
biscarbene system the palladium complexes turned out to be more stable than the corresponding platinum complexes under the reaction conditions [31,32], although Peter Hofmann had recently shown the stability and reactivity of the platinum-alkyl complexes with bis-NHC ligand [33]. To study the differences between both systems we synthesized palladium [34] and platinum [35] "hybrid complexes" with ligands combining both structural elements the pyrimidine as well as the NHC fragment. We also used density functional theory (DFT) calculations to investigate the mechanism and potential intermediates.
Quantum chemical (QC) investigations have been very helpful during the last years in elucidating the mechanisms of palladium-catalyzed reactions [26,36]. According to our QC calculations the catalytic cycle for the alkane activation by bis(NHC) palladium complexes like the bis(1,1'-dimethyl-3,3'-methylenediimidazoline-2,2'-diylidene)palladium(II) dibromide [L 2 PdBr 2 ] consists of three steps: electrophilic substitution, oxidation and reductive elimination involving a palladium(IV) intermediate [26]. But we also experimentally set out to investigate potential intermediates of the catalytic cycle [37]. One of the proposed intermediates in the catalytic cycle is a monomethyl complex which is formed from the starting material after methane activation and which could either carry a bromo [L 2 PdBr(CH 3 )] or a trifluoroacetato [L 2 Pd(CF 3 COO)(CH 3 )] ligand. These intermediates should be active according to the proposed catalytic cycle, which starts with an electrophilic substitution reaction. We therefore set out to synthesize the corresponding methyl complexes for the "hybrid ligand" with chloroand trifluoroacetato ligands.

Results and Discussion
Synthesis Some time ago we reported the synthesis of the imidazolium salts 1-4 and their corresponding silver complexes 5-8. The reaction with dichloro(cyclooctadiene)-palladium(II) [(COD)Pd II Cl 2 ] allowed for the isolation and characterization of the [((pym)^(NHC-R))Pd II Cl 2 ]-complexes [34]. For the synthesis of the corresponding monomethyl complexes we could rely on earlier work by Byers and Canty which reported the synthesis of methyl-and dimethylpalladium(II) complexes with various nitrogen donor systems [38]. The palladium complexes [(pym)^(NHC-R)Pd II (CH 3 )Cl] 9-12 have been synthesized in good yields from the corresponding silver complexes 5-8 by transmetalation with chloro(methyl)(cyclooctadiene)palladium(II) [(COD)Pd II (CH 3 )Cl] either in dichloromethane (A) or acetonitrile (B, Scheme 1).
Although two isomers could be formed we only observed the isomer B, where the methyl group is located cis to the carbene carbon atom (Figure 1). This can be explained by the strong donor character of the carbene which makes the trans isomer less favorable. NMR analysis of the reaction products confirms that only one complex is formed. Additional proof comes from density functional theory (DFT) calculations which predict isomer B to be more stable for all complexes 9-12. At the B3LYP/6-311+G** level of theory the isomer with the methyl group coordinated cis to the carbene carbon atom the B isomers are thermodynamically favored by 5-11 kcal/mol ( Table 1). For complexes 9 and 12 ( Figure 2 and Figure 3) we were able to obtain solid state structures, which show the predicted geometry and confirm that the B isomers are formed.   It is interesting to note that the two structures show very similar bond lengths and angles with the exception of the palladium-carbene bond lengths (9: 2.03 Å; 12: 1.97 Å), which might be an indication of the different donor character. The experimentally determined geometrical parameters are in good agreement with the computed results.
Interestingly, complexes 9, 10 and 12 show only one discrete doublet for the m-pyrimidine proton signals in the 1 H NMR spectrum in DMSO-d 6 indicating that the pyrimidine ring might rotate at room temperature and therefore a weak coordination of the pyrimidine nitrogen atom to the palladium center. The broader signal in case of complex 11 indicates a stronger coordination of the nitrogen and a hindered rotation of the pyrimidine ring. Within some hours, complexes 9 and 12 decompose in DMSO-d 6 by reductive cis-elimination [40] leading to imidazolium salts with the methyl group on the former carbene carbon atom and palladium black. The weaker coordination is most probably caused by the stronger trans effect of the methyl group. The decomposition is significantly slower in CDCl 3 , where two different signals for the m-pyrimidine protons are observed.
We recently reported that the dihalogenato complexes are active catalysts of the methane CH activation in trifluoroacetic acid [34]. Under the reaction conditions it seems likely, that an exchange of the chloro against a trifluoroacetato ligands occurs [29], which is present in the solution in large excess. This potential intermediate of the catalytic cycle, the [(pym)^(NHC-R)Pd II (CH 3 )(CF 3 COO)]-complex 13 (with R = 2,6-diisopropylphenyl), could be synthesized by the reaction of complex 11 with silver trifluoroacetate (Scheme 2). We could also confirm the formation of the desired product by a solid state structure ( Figure 4). Analyzing the solution we could confirm the formation of methyl trifluoroacetate and a 'methyl free' Pd(II) complex. This complex showed similar NMR spectra like the bistrifluoroacetate complex 14 (Scheme 3), which was prepared by the reaction of the corresponding dichloro complex [34] with silver trifluoroacetate for comparison (see experimental details). The  result indicates that an oxidation/reductive elimination cycle took place (Scheme 3, upper pathway). The direct reductive elimination of methyl trifluoroacetate from complex 13 by heating complex 13 in DMSO-d 6 up to 90 °C in the presence of sodium trifluoroacetate [30,34], yielding complex 15, could not be observed (Scheme 3). This indicates that a P(II)/Pd(0)mechanism is unfavorable. A more representative simulation of the reaction conditions of the catalytic methane activation by using trifluoroacetic acid as solvent was not possible as the protonation and dissociation of the methyl group occurs very quickly and quantitatively. The results obtained are in agreement with the computational results [41] that for this ligand system a Pd(II)/Pd(IV)-mechanism might be more favorable than a Pd(II)/Pd(0)-pathway for the formation of methyl trifluoroacetate from methylpalladium(II) complexes like 13 (Scheme 3).
It has previously been shown that the oxidation of palladium(II) complexes is feasible [42]. Several papers reported the successful oxidation of Pd(II) complexes by hypervalent iodo reagents [43][44][45][46][47][48]. Sanford, Arnold and co-workers could also show, that the Pd(IV)(NHC) complexes prepared by this reaction tend to reductively eliminate at temperatures around −35 °C [49]. As we also consider a Pd( 3 ] was synthesized by reaction of 13 with iodobenzene bistrifluoroacetate at room temperature. We observed the formation of complex 14 as the only product (Scheme 4), while at a temperature of −78 °C no reaction was observed and complex 13 was reisolated from the reaction mixture. We followed the reaction by NMR by mixing the reagents at −78 °C while measuring the 1 H NMR spectra at −10 °C. After 5 minutes we observed a 1:1 mixture of complexes 13 and 14.
When the mixture warmed up to room temperature, the formation of complex 14 and methyl trifluoroacetate was detected. We believe that the reaction mechanism is similar to what was observed before by Sanford, Arnold and co-workers [49].
After we could successfully synthesize all the monomethyl complexes we were curious whether also the corresponding dimethyl complex is accessible. For this synthesis we decided to use the reaction of the free carbene with dimethyl (N,N,N  spectrometer at 298 K. Elemental analyses were performed by the microanalytical laboratory of our institute using an EuroVektor Euro EA-300 Elemental Analyzer. Chemicals were supplied by Acros, Fluka and Aldrich and used as received; solvents were dried by standard procedures before use. Imidazolium salts 1-4 [34], silver complexes 5-8 [27], chloro(methyl)(cyclooctadiene)palladium(II) [54] and dimethyl-(N,N,N',N'-tetramethyl-1,2-ethylendiamine)palladium(II) [38] were prepared according to literature procedures.
Solid-state structure determination of 9, 12 and 13 Preliminary examination and data collection were carried out on an area detecting system (Kappa-CCD; Nonius) at the window of a sealed X-ray tube (Nonius, FR590) and graphite monochromated Mo Kα radiation (λ = 0.72073 Å). The reflections were integrated. Raw data were corrected for Lorentz and polarization and, arising from the scaling procedure, for latent decay. An absorption correction was applied using SADABS [76]. After merging, the independent reflections were all used to refine the structures, which were solved by a combination of direct methods and difference Fourier synthesis. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in calculated positions and refined using the riding model. Full-matrix least-squares refinements were carried out by minimizing Σw(F o 2 − F c 2 ) 2 .

Supporting Information
Supplementary crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.