A Highly Substitutable Palladium(II) Complex Stabilized by the Smallest Steric N‐Heterocyclic Carbene, IMe (IMe = 1,3‐Dimethylimidazole‐2‐ylidene)

Abstract

Palladium complexes supported by N-heterocyclic carbenes (NHCs) have been of great interests in catalysis because of their capability as efficient catalysts in organic synthesis. 1,2 Since the strong palladium-NHC bonds retain the stability of palladium complexes, and steric modification of the NHC ligands can be readily available, NHCs become more popular and important alternatives to phosphines as supporting ligands. 3,4 Thus, the synthetic direction of NHCs has been aimed to optimize both electronic and steric properties of NHCs, which allows one to maximize the reactivity of inorganic catalysts for desired results. 4 As the results, numerous Pd(II)-NHC complexes have been synthesized and applied in cross-coupling reactions, telomerization reactions, oxidation reactions including a catalytic conversion of methane to methanol. 5-9 Although significant efforts have been made to increase steric bulkiness of NHC in particular, 10 another simple approach to access a reactive palladium complex is to introduce labile ligands to a metal center. In addition, since mono-ligated palladium complexes have been also known as active catalysts in most palladium catalyzed organic reactions, efficient methods to generate monoligated NHC palladium complexes have been successfully developed and utilized for the organic synthesis. 3,11 However, most of mono-ligated NHC palladium complexes contain bulky NHCs with relatively strongly bound anions such as acetate anions, or halide-or carboxylate-bridged palladium dimers are obtained. 12,13 With weakly bound anions, the metalation of bulky NHC ligands has been observed in many cases and resulted in the deterioration of the palladium-NHC complexes. 14 Therefore, the access to a mono-ligated NHC complex with weakly coordinating ligands is very limited. 15,16 To overcome the problematic metalation of NHC, we have utilized the smallest steric NHC and successfully prepared mono-ligated palladium(II) complex with a weakly coordinating anion and tetrahydrofuran (THF). 17,18 Here, we report the preparation and full characterization of a mono-ligated palladium(II) complex supported by IMe (IMe = 1,3-dimethylimidazole-2-ylidene), triflate, and THF.Synthesis of Pd(IMe)(OTf ) 2 (THF) (2) complex involves the formation of iodo-bridged palladium(II) dimer (1), 19 followed by halide abstraction of 1 with silver triflate in THF. Both complexes were obtained in 94% yield (Scheme 1). In the process of formation of 2, iodide abstraction should be carried out within 1 min, followed by a quick filtration of the reaction mixture through a pad of Celite. Otherwise cationic polymerization of THF starts immediately, indicating strong interaction between THF and a highly naked palladium(II) center due to labile ligands. Addition of pentane to the filtrate affords immediately crystalline yellow 2, which is further collected on a frit. Unlike most known stable palladium(II) NHC complexes in a solid phase, 2 decomposes to a black tar solid even at 50 C under dynamic vacuum, as the stability of 2 is not secured be...

Mechanistic Studies for Pd(II)(O2) Reduction Generating Pd(0) and H2O: Formation of Pd(OH)2 as a Key Intermediate

“…We have previously described the synthesis and characterization of Pd(0) I Me 2 ( 1 ), Pd­( I Me) 2 O 2 ( 1O 2 ), and related complexes. , Exposing a yellow tetrahydrofuran (THF) solution of 1 to atmospheric O 2 generated a bisque-colored precipitate, which was determined to be a palladium­(II) peroxide complex, 1O 2 . Adding 9,10-dihydroanthracene (DHA) to a pyridine solution of 1O 2 generated anthracene (Anth), H 2 O, 1 , and the dihydroxide 1­(OH) 2 .…”