Synthesis of Arylpalladium(II) Complexes Derived from Benzyl Alcohol, Reactivity toward Alkyl Halides, and Synthesis of Dinuclear Arylpalladium(II) Complexes

Abstract

The aryl palladium complexes [PdI­(C6H4CH2OH-2)­(N∧N)] (N∧N = bpy = 2,2′-bipyridyl (1a), tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine (1b), tmeda = N,N,N′,N′-tetramethylethylenediamine (1c)) were synthesized by oxidative addition of 2-iodobenzyl alcohol to one equivalent of “[Pd­(dba)2]” (dba = dibenzylideneacetone) in the presence of the N∧N ligands. By reaction of 1a with three equivalents of XyNC (Xy = 2,6-dimethylphenyl) the insertion complex trans-[PdI­{C­(NXy)­(C6H4CH2OH-2)}­(CNXy)2] (2) was formed. The reaction of 1a with KOtBu resulted in the formation of the chelate complex [Pd­(κ2-C,O-C6H4CH2O-2)­(bpy)] (3), which crystallizes as pairs of molecules bridged by hydrogen bonds to water of crystallization. Complex 3 reacts with XyNC, forming the cyclic imidate N-(2,6-dimethylphenyl)-2-benzofuran-1­(3H)-imine (4). By reaction of 3 with various primary alkyl halides RCH2X, the complexes [PdX­(C6H4CH2OCH2R-2)­(bpy)] (X = I, R = H (5a), X = Br, R = Ph (5b), p-C6H4CH2Br (5c), p-C6H4Br (5d), and p-C6H4I (5e)) were obtained. When the reaction of 3 with p-C6H4(CH2Br)2 was carried out in a 2:1 ratio, the dinuclear arylpalladium complex [{(bpy)­BrPd­(C6H4CH2OCH2-2)}2­(C6H4-1,4)] (6) formed. An halide exchange reaction on 5e, using AgOTf and an excess of NaI, afforded [PdI­{C6H4(CH2OCH2­(C6H4I-4))-2}­(bpy)] (5f), which by oxidative addition to [Pd­(dba)2] in the presence of bpy formed another dinuclear arylpalladium complex, [(bpy)­IPd­(C6H4CH2-2)­O­(CH2C6H4-4)­PdI­(bpy)] (7). All the complexes have been extensively characterized by NMR spectroscopy. The crystal structures of 1a, 3·H2O, and 5e were determined by X-ray diffraction studies.

Reactivity towards nitriles, cyanamides, and carbodiimides of palladium complexes derived from benzyl alcohol. Synthesis of a mixed Pd2Ag complex

“…We have described similar reactions of II with alkyl halides (bromides and iodides) in a previous paper. 14…”

“…1.97-2.00 Å). 4,11,14,22 The two Pd-C bond distances for 5 are slightly longer, 2.014(7) Å for Pd(1)-C(1) and 2.009 Å for Pd(2)-C(31). The Pd-N (trans to aryl) bond distances are very similar for the three complexes (between 2.115(5) and 2.118(2) Å), and they are longer than the Pd-N (trans to N) bond distances (in the range 2.026(2)-2.070(5) Å), as expected for the stronger trans influence of the aryl ligand with respect to N-donor ligands.…”

“…[10][11][12] With carbodiimides, 11 the addition of the O-H group to one of the CvN bonds of the substrate, together with the coordination of the other N to the Pd atom, was an alternative reaction to the insertion. 11 We have recently extended this research to ortho-palladated hydroxymethylphenyl complexes, 14 where the methylene link in the alcoholic substituent might significantly influence the reactivity towards unsaturated molecules. There are very few reports of 2-hydroxymethylphenyl palladium complexes 15 or oxapalladacycles derived from them.…”

“…16 These compounds have been used as precatalysts in Heck and cross-coupling reactions, 17 but their reactivity towards unsaturated molecules had not been systematically investigated, with a single report on a reaction of a palladacycle with tert-butyl isocyanide, yielding an imidate, while the reaction of the same compound with maleic anhydride resulted in the coordination of the olefin to the Pd(II) centre. 16 In our recent work 14 we synthesized the complex [PdI(C 6 H 4 CH 2 OH-2)(bpy)] (I) and investigated its reactivity towards alkynes, alkenes, nitriles, cyanamides, allenes, and carbon monoxide, which did not result in clean insertion (C-Pd bond) or addition (O-H bond) reactions. 14 Only the reaction of I with XyNC gave a clean insertion product, trans-[PdI{C(vNXy)C 6 H 4 CH 2 OH-2}(CNXy) 2 ].…”

“…14 Only the reaction of I with XyNC gave a clean insertion product, trans-[PdI{C(vNXy)C 6 H 4 CH 2 OH-2}(CNXy) 2 ]. 14 By deprotonation of complex I we prepared the chelate complex [Pd{κ 2 -C,O-C 6 H 4 (CH 2 O)-2}(bpy)] (II), which displayed an interesting reactivity towards primary alkyl halides, via a nucleophilic attack of the coordinated oxygen at the alkyl group of the halide. 14 We have now extended our research to the chelate complex II, which, in the presence of AgOTf, reacts with acetonitrile, cyanamides, and carbodiimides to give novel complexes containing a [Pd{κ 2 -C,N-C 6 H 4 {CH 2 OC-(vNX)Y}-2}] chelate ring, resulting, unexpectedly, from insertion reactions of the CuN or CvN bonds into the O-Pd bond of II.…”