Stable Mononuclear Organometallic Pd(III) Complexes and Their C−C Bond Formation Reactivity

Abstract

Organometallic Pd(III) complexes have been implicated as intermediates in a number of catalytic and stoichiometric transformations. While a few dinuclear organometallic Pd(III) complexes have been characterized, no mononuclear organometallic Pd(III) complexes have been isolated to date. Reported herein is the synthesis and characterization of a series of Pd(III) complexes supported by the tetradentate ligand N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (N4). Chemical or electrochemical oxidation of the Pd(II) complexes (N4)Pd(II)(R)(X) (R = Me, X = Cl: 1; R = Ph, X = Cl: 2; R = X = Me: 3) generates [(N4)Pd(III)MeCl](+) (1(+)), [(N4)Pd(III)PhCl](+) (2(+)), and [(N4)Pd(III)Me(2)](+) (3(+)). These stable Pd(III) complexes were isolated and characterized by X-ray diffraction, cyclic voltammetry, UV-vis, EPR, magnetic moment measurements, and DFT to confirm the presence of paramagnetic d(7) Pd(III) centers. Moreover, these Pd(III) complexes undergo light-induced C-C bond formation to give the corresponding homocoupled products ethane or biphenyl. Particularly remarkable is the observation for the first time of ethane formation from monomethyl Pd complexes.

Carbon–carbon and carbon–chlorine bond formation on reaction of iodine(III) reagents with the bis(alkynyl)palladium(II) motif, and structural chemistry of trans-Pd(C C-o-Tol)2(PMe2Ph)2] and trans-[PdCl(C C-o-Tol)(PMe2Ph)2]

“…The detection of C(sp)-halogen coupling reported here is complementary to earlier reports of C(sp 2 )-halogen coupling from isolated or detected Pd III [11] and Pd IV [27] complexes, and C(sp 3 )-halogen coupling from isolated or detected Pd III [9] and Pd IV complexes [28].…”

“…The distinctive red colouration observed during reaction of IPhCl 2 may be indicative of Pd III ePd III species, as reported complexes of this type are all red [10,11]. Well documented mononuclear Pd III species are green and paramagnetic [9], and we observed no indication of paramagnetic shifts in NMR spectra.…”

“…These complexes were obtained using IPh(C^CSiMe 3 ) (OTf) as a strong oxidising agent via elimination of PhI and formal transfer of 'C^CSiMe 3 þ ' to Pd II . Organopalladium(III) complexes have also been well characterised by X-ray crystallography, including mononuclear [9] and Pd III ePd III bonded species [10,11]. In view of an earlier report proposing undetected mono(alkynyl)palladium(IV) and bis(alkynyl)palladium(IV) intermediates in complex reactions of trans-[PdCl 2 (PBu 3 ) 2 ] with SnMe 3 (C^CR) (R ¼ Me, Bu t ) [12], we have explored the interaction of a typical trans-[Pd(alkynyl) 2 (phosphine) 2 ] reagent with IPh(C^CR)(OTf)(R ¼ Bu t , SiMe 3 ) and other iodine(III) reagents (IPh 2 (OTf), IPhCl 2 ) to explore potential higher oxidation state organopalladium chemistry.…”

Dinuclear Palladium(III) Complexes with a Single Unsupported Bridging Halide Ligand: Reversible Formation from Mononuclear Palladium(II) or Palladium(IV) Precursors

“…[3,7] Furthermore, this result suggests that stabilization of the Pd III oxidation state can also be achieved by a tridendate N-ligand and does not require a rigid tetradentate ligand. [6] The formation of dinuclear Pd III complexes 2 a and 2 b from the mononuclear precursors 1 a and 1 b is intriguing. The circa 65 % yield of product suggests that the bridging halide ion comes from another molecule of the Pd II precursor.…”

“…Furthermore, the interconversion between dinuclear Pd III and mononuclear Pd IV species parallels the proposed involvement of analogous intermediates in Pd-catalyzed CÀH oxidative functionalization reactions and suggests that for a given ligand environment both types of intermediates can be present. [3][4][5][6] The relatively low oxidation potentials of complexes 1 a,b have prompted us to investigate their reactivity in oneelectron redox reactions, such as the Kharasch addition of polyhaloalkanes to alkenes. [28] Both 1 a and 1 b exhibit catalytic activity in the addition of CCl 3 Br to a number of alkenes (methyl methacrylate, methyl acrylate, styrene, norbornene, cyclopentene) to give selectively the 1:1 addition product in good to high yields at 65 8C under N 2 (Supporting Information, Chart S1, Table S1).…”