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

Abstract

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(-Cl)}]2 (2), where L is the monoanionic ligand resulting from the 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 the diketal of dap, Q = C5H3NC(OMe)2Me}2-2,6 (3). Complex 1 reacts with two equiv of RNC at 0 ºC to give trans-[Pd(C1-L)Cl(CNR)2] (R = Xy (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 ketoimine to -ketoenamine, and coordinates in 5 through the carbonyl oxygen atom (O2) and the inserted isocyanide carbon atom (C2). The reaction of 1 with one equiv of RNC at 0 ºC leads a mixture of [Pd(N1,C1-L)Cl(CNR)] (R = Xy (6a), tBu (6b); 8590%), 1 and 4, but at room temperature gives the pincer complex [Pd(O1,N1,C2-LR)Cl] (R = Xy (7a), tBu (7b)) resulting from the same insertion/tautomerization processes that lead to 5. Complex 7 reacts at 0 ºC (1) with 2 equiv of RNC to give trans-[Pd(C2LR)Cl(CNXy)2] (R = Xy (8a), tBu (8b)) or (2) with one 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 L^L affords [Pd(C1-L)Cl(N^N)] (N^N = 2,2’-bipyridine = bpy (10), 4,4’-di-tertbutyl-2,2’-bipyridine = dbbpy (11)), (3) with one 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 two equiv of PPh3 is added to the reaction mixture, and (4) with excess of 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.