Platinum Metals Rev., 1993, 37, (1), 2
The +IV Oxidation State in Organopalladium Chemistry
Recent Advances and Potential Intermediates in Organic Synthesis and Catalysis
The organometallic chemistry of palladium is dominated by the +II oxidation state, and the chemistry of complexes containing simple organic groups bonded to palladium in the +IV oxidation state has developed only recently. Organic synthesis and catalytic reactions that may involve undetected palladium(IV) intermediates have been suggested frequently, and the new oxidation state +IV chemistry provides some support for these proposals, and gives encouragement for the development of new systems involving palladium(IV). The chemistry of organopalladium(IV) is reviewed here, and possible catalytic roles forpalladium(IV) are discussed. The synthesis and decomposition reactions of palladium(IV) complexes provide “models” for catalytic proposals. The palladium(IV) complexes are formed by oxidative addition of organohalides to palladium(II) complexes, and most complexes decompose under mild conditions by carbon-carbon bond formation in reductive elimination reactions, for example, for methyl(pheny1) (2,2’-bipyridyl)palladium(II) as a substrate, oxidative addition of benzyl bromide gives PdIV BrMePh(CH2Ph) (bpy), which reductively eliminates toluene to form the complex PdIIBr(CH2Ph)(bpy).
The organometallic chemistry of palladium is dominated by the oxidation state +II, and in the catalytic applications of palladium complexes the most common oxidation states are +II and 0 (1). On many occasions, possible roles for palladium (IV) in organic synthesis and catalysis have been proposed (2, 3) but, apart from several pentafluorophenylpalladium(IV) complexes isolated in the mid-1970s (4), organopalladium (IV) complexes were not characterised until the report of the preparation and crystal structure of a 2,2’-bipyridyl complex in 1986 (5):
The organometallic chemistry of palladi-um(IV) has developed rapidly since 1986 (6–8) and has been reviewed recently (8). The new chemistry includes reaction systems that are ideal for studies of mechanisms in organometallic chemistry, and that model some of the proposed roles for palladium(IV) in catalysis.
Synthesis is based on the oxidative addition reaction of organohalides with palladium(II) complexes, as illustrated in Equation (i). The bromine atom in PdBrMe2(CH2Ph)(bpy) may also be replaced on reaction with silver salts and anions to give a range of complexes PdXMe2(CH2Ph)(bpy), where X may be F, N3 or O2CPh (9). Typical complexes are shown in Figure 1, and X-ray structural studies of several complexes show the expected octahedral co-ordination characteristic of organoplatinum(IV) chemistry (8).
Kinetic studies of the oxidative addition of MeI and PhCH2Br to PdMe2(bpy) and PdMe2(phen) are consistent with the occurrence of the classical SN2 mechanism, involving Pd(II) as the nucleophile (13). 1H NMR spectra at low temperature allow detection of cationic intermediates, see Equation (ii) (14), and, in support of this mechanism, PdMe2(NMe2CH2CH2NMe2) reacts with methyl triflate in CD3CN to form [PdMe3(NMe2-CH2CH2NMe2)(NCCD3)]+OSO2CF3– (6a)
In contrast to platinum chemistry, the cations are fluxional, and another illustration of the greater lability of palladium (IV) is the occurrence of an equilibrium between PdIMe3(bpy) and [PdMe3(bpy)(NCMe)]+I- in acetonitrile (8).
Most palladium(IV) complexes undergo facile reductive elimination in the solid state and at moderate temperatures in solution, for example, Reaction (iii) occurs at 0°C in acetone. Differential scanning calorimetry of some of these systems in the solid state has allowed the first estimate of palladium-carbon bond energies, for example ~130 kJ/mol for Pd-Me (13a). There is a high selectivity for methyl elimination, see Equations (iii), (iv) and (v).
Kinetic studies of reductive elimination from PdIMe3(bpy) indicate that a pre-equilibrium occurs to form a cation, and that reductive elimination occurs from a five-co-ordinate cation (or octahedral complex involving solvent coordination) (13a), for example, for PdBrMe2(CH2Ph)(bpy) in acetone:
Related work suggests that cation formation is also a key step in new redox reaction systems for alkyl halide transfer from Pd(IV) to Pt(II) or Pd(II), as illustrated in Figure 2. In a mechanism directly related to oxidative addition, such as in Equation (vi) for transfer from Pd(IV) to Pd(II), cation formation is expected to enhance nucleophilic attack at an axial benzyl group to give PdMePh(bpy) as the leaving group and a Pd(IV) cation which reacts rapidly with bromide.
Proposals for the involvement of Pd(IV) intermediates in organic synthesis and catalysis fall into two categories: those that require oxidative addition-reductive elimination sequences resulting in carbon-carbon bond formation (2), reactions now established as characteristic of Pd(IV) chemistry (8), and those that require other types of reaction (3), in particular C-H activation via oxidative addition.
Coupling reactions to form C-C bonds that are catalysed by palladium complexes generally proceed via a Pd(0)-Pd(II) oxidative addition-reductive elimination cycle (15), but the development of an extensive chemistry of Pd(IV) indicates that Pd(II)-Pd(IV) cycles are feasible. For catalyses that may involve Pd(II)-Pd(IV) sequences, intermediates may be either octahedral, with PdIMe3(bpy) and related complexes as models, or five-co-ordinate with cationic species such as those shown in Equations (ii) and (v) as models.
Examples of proposed catalyses are illustrated in Figures 3a and 4a. For the coupling of acetanilide with iodomethane (Figure 3a) the Pd(II) cyclometallated complex is known to be an intermediate, and formation of the product could occur via either a benzonium Pd(II) intermediate or an oxidative addition Pd(IV) intermediate (2e). Choosing between these possibilities is not straightforward, although it is interesting to note that theoretical calculations (16a) suggest that related Pt(II) benzonium species (16b) are formed via oxidative addition (Figure 3b). The benzenonium species may also be regarded as possible intermediates in reductive elimination from aryl(alkyl)palladium(TV) complexes, such as in Reactions (iii) and (iv).
The formation of hexahydromethanotriph-enylenes and related molecules using palladium(0) catalysts is believed to involve both Pd(II) and Pd(rV) intermediates (2g, 2h, 2j), as illustrated in Figure 4a. Model reactions for the Pd(n
However, the model reactions to date employ alkyl or allyl halides for oxidative addition, whereas the catalysis proposal requires aryl halide oxidative addition. The interaction of aryl halides with palladium(II) has yet to be explored, although precedents do exist for oxidative addition of aryl halides to platinum(IT) (17).
Organopalladium(rvr) chemistry is providing new comparisons of structure, solution dynamics, and reactivity among the nickel triad elements, and its development is commencing some 80 years after the synthesis of plat-inumCPV) complexes by Pope and Peachey (18). The new chemistry is providing a firmer basis for proposals involving palladium(rV) in catalysis, in particular the occurrence of oxidative addition-reductive elimination sequences for carbon-carbon bond formation.
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This work was funded by grants from the Australian Research Council, and travel grants for collaborative studies from the Ian Potter Foundation (Australia), the Department of Industry, Technology, and Commerce (Australia), the Australian National University, the Natural Sciences and Engineering Research Council (Canada), and the Netherlands Organisation for Scientific Research. Johnson Matthey provided generous loans of palladium salts.