Organometallic Complexes of Palladium(II) Derived from 2,6-Diacetylpyridine Dimethylketal

Abstract

PdCl 2 reacts with 2,6-diacetylpyridine (dap) (1:1) in refluxing MeOH to give the pincer complex [Pd(O 1 ,N 1 ,C 1 -L)Cl] (1) and (QH) 2 [{PdCl 2 ( μ-Cl)}] 2 (2), where L is the monoanionic ligand resulting from deprotonation of the acetyl methyl group of the monoketal of dap and QH is 5b)). The ligand L R results from the insertion of one isocyanide into the Pd-C bond plus a tautomerization process from β-ketoimine to β-ketoenamine and coordinates in 5 through the carbonyl oxygen atom (O 2 ) and the inserted isocyanide carbon atom (C 2 ). The reaction of 1 with 1 equiv of RNC at 0 °C leads to a mixture of [Pd(N 1 ,C 1 -L)Cl(CNR)] (R = Xy (6a), t Bu (6b); 85-90%), 1, and 4, but at room temperature gives the pincer complex [Pd(O 1 ,N 1 ,C 2 -L R )Cl] (R = Xy (7a), t Bu (7b)), resulting from insertion/tautomerization processes similar to that leading to 5. Complex 7 reacts at 0 °C (1) with 2 equiv of RNC to give trans-[Pd(C 2 -L R )Cl(CNXy) 2 ] (R = Xy (8a), t Bu (8b)) or ( 2) with 1 equiv of t BuNC to afford 5b. The reaction of 1 (1) with [Tl(acac)] gives [Pd(N 1 ,C 1 -L)(acac)] ( 9); (2) with chelating ligands N ∧ N affords [Pd(C 1 -L)Cl(N ∧ N)] (N ∧ N = 2,2 0bipyridine = bpy (10), 4,4 0 -di-tert-butyl-2,2 0 -bipyridine = dbbpy (11)); (3) with 1 equiv of PPh 3 gives, in the same way as with isocyanides, an equilibrium mixture of [Pd(N 1 ,C 1 -L)Cl(PPh 3 )] ( 12), 1, and trans-[Pd(C 1 -L)Cl(PPh 3 ) 2 ] (13), which is the only product when 2 equiv of PPh 3 is added to the reaction mixture; and (4) with excess PPh 3 affords the monoketal of dap, C 5 H 3 N{C(O)Me-2}{C(OMe) 2 Me-6} (14), and [Pd(PPh 3 ) 4 ]. The crystal structures of complexes 1, 2, 5b, 6a, and 7a have been determined.

Synthesis of a Palladium(IV) Complex by Oxidative Addition of an Aryl Halide to Palladium(II) and Its Use as Precatalyst in a CC Coupling Reaction

“…Recently, we reported the reaction of PdCl 2 with 2,6‐diacetylpyridine in MeOH at reflux to give [Pd( O , N , C ‐L)Cl] ( 1 ⋅Cl, Scheme )20 which reacts with Cl 2 or NaBr to give, respectively, the stable Pd IV complex [Pd( O , N , C ‐L)Cl 3 ] or [Pd( O , N , C ‐L)Br], which, in turn, reacts with Br 2 to afford [Pd( O , N , C ‐L)Br 3 ] 21. The stability of these Pd IV complexes (which was attributed to the pincer ligand) and the weakly coordinating ability of the MeO group in the Pd II complexes (which was expected to help its oxidation to Pd IV (see above)) moved us to attempt the synthesis of a stable and unprecedented aryl Pd IV complex from an aryl iodide and a [Pd( O , N , C ‐L)X] complex.…”

“…Because Pd IV complexes are thermally unstable, and because we also set out to use such a Pd IV complex as a precatalyst in a CC coupling process, we decided to prepare and test its catalytic ability at room temperature. We chose as reagents 2‐iodobenzoic acid and the new complex [Pd( O , N , C ‐L)(OAc)] ( 1 ⋅OAc, Scheme ), prepared by treating 1 ⋅Cl20 with AgOAc. Coordination of the benzoate moiety, after deprotonation, would bring the iodine atom close to the palladium atom, and this situation, along with the presence of the electron‐withdrawing ortho substituent,4, 22 would give the oxidative addition reaction a chance to occur at room temperature.…”

Providing Support in Favor of the Existence of a PdII/PdIV Catalytic Cycle in a Heck‐Type Reaction

“…[31] As expected, complex 1 can be replaced by its precursor 2·OAc in the catalytic reaction. ( 1 H NMR: d = 6.03 (X = Cl), 6.04 (X = Br), 6.29 and 6.01 ppm (X = I)) and 1 (d = 4.95, 4.55 ppm), [29,32,33]…”

“…, which was isolated by reacting 2·Cl (Scheme 1) [32] with one equivalent of AgClO 4 and behaves in acetone as a 1:1 electrolyte (L M = 81 W·cm 2 mol À1 ), [34] and 2ii from the stoichiometric reactions [29,32,33] in which L' is the monoanionic ligand resulting from monodeprotonation of one acetyl group of 2,6diacetylpyridine. The presence of adventitious water (from the acetone or the atmosphere) and the acidic medium (IBzOH and HClO 4 ; Scheme 2) must be responsible for this acid-catalyzed hydrolytic process.…”

Complejos pincer y su comportamiento como iniciadores en polimerización de etileno

“…En este contexto, Lan-Chang et al, (2006) publicaron la síntesis, caracterización y reactividad de complejos de niquel(II) derivados de ligandos pincer PNP. Complejos NOC (Vicente et al, 2010), complejos SNS ) y complejos NNN (Decken et al, 2005).…”