60th Annual Report on Research 2015

During the 2014-2015 grant year I was able to support three students with my ACS PRF grant: Xiao Xiao (Amherst), John Stockman (Amherst), Daniela Garcia (Amherst) and Ariana Tribby (Pomona). This narrative will detail the progress made on the specific aims of the funded research. During this year, my research lab has been able to make significant progress one of the aims involving the studying the insertion of SO₂ into amido Pd(II) model complexes (aim 1).

Specific aim 1 focused on synthesizing a series of model Pd(II) aryl amido complexes and studying the mode of SO₂ insertion into either Pd–C or Pd–N bonds. Our initial target aryl amido Pd(II) complex was (dppf)Pd(4-FC₆H₅)(NPh₂) (3). A version of this complex was previously reported in the literature and provided the necessary cis orientation of the aryl and diphenylamido group needed for subsequent C–SO₂-NPh₂ bond-forming reductive elimination (Figure 1). We were able to successfully synthesize complex 3 in 36% yield (Figure 2).

The SO₂ insertion experiments first focused on observing changes of the p-FPh signal in the ¹⁹F NMR spectrum of 3 once subjected to SO₂ gas. Preliminary studies demonstrated a complex mixture of products (multiple fluorine signals in the ¹⁹F and ³¹P NMR) and significant Pd⁰ (Pd black precipitate). However, conducting the experiment in the presence of excess dppf or PPh₃ lead to one major product. This suppression of side products is most likely due to the extra phosphine ligand binding to the Pd(0) in solution and suppressing undesired reactions. In the future, we will revisit this experiment with DABSO and isolate the insertion product. The addition of excess phosphine is not ideal for our studies due to the unknown dynamics of the ligand exchange of the dppf ligand. Alternatively, we sought to synthesize Pd(II) aryl amido complexes with bidentate nitrogen ligands. The benefit of using nitrogen-based ligands versus phosphines is that they are more resistant to the oxidizing conditions of SO₂.

We synthesized two new Pd(II) aryl amido complexes (TMEDA)Pd(p-CF₃)(NPh₂) (4) and (tBubipy)Pd(P-CF₃)(NPh₂) (5) from their corresponding Pd(II) aryl complexes. Complex 4 was isolated in 81% yield and characterized by NMR spectroscopy and mass spectrometry. Complex  5 was only observed in solution by ¹H and ¹⁹F NMR spectroscopy and a protocol for isolation is pending. Notably both Pd(II) amido complexes are novel. SO₂ insertion experiments with (TMEDA)Pd(p–CF₃)(NPh₂) resulted in a mixture of products independent of time and temperature of the reaction. In retrospect, this was not surprising due to the higher lability (the nitrogen atoms comes on and off the Pd) versus phosphine ligands. This hypothesis is supported by protocols in the literature describing the synthesis of the aryl iodide 2 from the corresponding TMEDA aryl iodide complex and dppf indicating how readily TMEDA can be displaced off the Pd center. Finally, the dissociation of the sp³ nitrogen atom of TMEDA could lead to alternative pathways other than the desired SO₂ insertion by freeing up a coordination site providing another potential explanation for the complex mixture of products from exposing compound B to SO₂ gas.

In the literature, it is known that sp² nitrogen ligands serve as pi-acceptors bonding more strongly to palladium centers versus bidentate sp³ nitrogen ligands like TMEDA. Interestingly, the exposure of crude (tBubipy)Pd(P-CF₃)(NPh₂) (5) to SO₂ gaseous resulted in the complete consumption of 5 and the appearance of a new major peak by ¹⁹F NMR spectroscopy, giving us hope an insertion product is being formed.

Future Directions

Now that we have found a potential model system to study SO₂ insertion, we will next focus on studies observing SO₂ insertion using DABSO as the SO₂ source and purified compound 5. Pomona College students Neil Chan and Ashish Streatfield will conduct these studies this summer. In addition to ¹⁹F NMR spectroscopy to observe SO₂ insertion, we will also rely on solution-phase IR spectroscopy of which we have the instrumentation at Pomona College.