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

PdCl2 reacts with 2,6-diacetylpyridine (dap) (1:1) in refluxing MeOH to give the pincer complex [Pd(O1,N1,C1-L)Cl] (1) and (QH)2[{PdCl2(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 C5H3NH{C(OMe)2Me}2-2,6, the diketal of Hdap+. Reaction of 2 with NEt3 (1:2) in MeOH affords Q = C5H3N{C(OMe)2Me}2-2,6 (3). Complex 1 reacts with 2 equiv of RNC at 0 °C to give trans-[Pd(C1-L)Cl(CNR)2] (R = Xy = 2,6-dimethylphenyl (4a), tBu (4b)) but at room temperature affords [Pd(O2,C2-LR)Cl(CNR)] (R = Xy (5a), tBu (5b)). The ligand LR 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 (O2) and the inserted isocyanide carbon atom (C2). The reaction of 1 with 1 equiv of RNC at 0 °C leads to a mixture of [Pd(N1,C1-L)Cl(CNR)] (R = Xy (6a), tBu (6b); 85−90%), 1, and 4, but at room temperature gives the pincer complex [Pd(O1,N1,C2-LR)Cl] (R = Xy (7a), tBu (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(C2-LR)Cl(CNXy)2] (R = Xy (8a), tBu (8b)) or (2) with 1 equiv of tBuNC to afford 5b. The reaction of 1 (1) with [Tl(acac)] gives [Pd(N1,C1-L)(acac)] (9); (2) with chelating ligands N∧N affords [Pd(C1-L)Cl(N∧N)] (N∧N = 2,2′-bipyridine = bpy (10), 4,4′-di-tert-butyl-2,2′-bipyridine = dbbpy (11)); (3) with 1 equiv of PPh3 gives, in the same way as with isocyanides, an equilibrium mixture of [Pd(N1,C1-L)Cl(PPh3)] (12), 1, and trans-[Pd(C1-L)Cl(PPh3)2] (13), which is the only product when 2 equiv of PPh3 is added to the reaction mixture; and (4) with excess PPh3 affords the monoketal of dap, C5H3N{C(O)Me-2}{C(OMe)2Me-6} (14), and [Pd(PPh3)4]. The crystal structures of complexes 1, 2, 5b, 6a, and 7a have been determined.