Complexes of Palladium IV

Compared with the plethora of platinum(IV) compounds, the palladium(IV) complexes are as yet relatively few in number [10, 11]. When isolable, they tend to resemble the corresponding platinum compounds. 

The PdXg (X = halogen) complexes are described in section 3.3.1. A limited range of complexes have been made by oxidation of palladium(II) species [188]  

The limited stability of these compounds is shown by the fact that other tertiary phosphines and arsines do not yield isolable products. 

With chelating phosphine and arsine ligands, two types of complex have been isolated  

Just as many palladium(IV) complexes are produced by halogen oxidation of the corresponding palladium(II) complex, so the palladium(IV) compounds tend to decompose by the reverse process, usually on heating  

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Complexes of Palladium(IV) Containing Group VI Donor Atoms 1124... 

Complexes of Palladium(IV) with N, P and As Donor Atoms 51,6,3,1 PalladiumQV) complexes containing N donor atoms... 

Coordination Compounds. Palladium forms numerous complexes with ammonia and with simple amines. Examples are [Pd(NH3)4]2+ [15974-14-8], [PdCl(dien)]+ [17549-31-4], cis-[PdCL (NH3)2] [15684-18-1], and trans-[PdC (NH3)2] [13782-33-7]. Monoammine complexes such as [PdCl3 (NH3)] [15691-32-4] are stable but less common. Examples of aromatic amine complexes include trans- [PdCf (pyr)2] [14052-12-1], [PdCl2(bipy)] [14871-92-2], and nucleosides such as [PdCl(dien)(guanosine)]+ [73601-42-0] (193). Complexes of Pd(IV) such as [PdCl2(NH3)4]2+ [70491-81-5] and [PdCl4(bipy)] [57209-01-5] may be prepared by chlorine oxidation of the corresponding Pd(2+). The aromatic amine Pd(IV) complexes are more stable than ammine and aliphatic amine species, which are reduced to Pd(II) in water or thermally (194). 

The decomposition of the dimethyl palladium(II) carbene complex with excess methyl iodide is a stepwise process. Although the authors [282] propose oxidative addition of methyl iodide on the palladium centre forming an octahedral palladium(IV) complex, it seems much more likely, with respect to the rarity of palladium(IV) compounds [284,285], that the first step is reductive elimination of an imidazolium salt, a decomposition pathway found to be fairly common after the initial publication of McGuinness et al. [283], Oxidative addition of methyl iodide followed by reductive elimination of ethane would account for the accumulation of iodide ligands on the palladium centre and a Pd(0)/Pd(II) redox couple. However, in the last step, a six coordinate Pd(IV) centre still seems to be necessary (see Figure 3.91). 

Considerably less is known about the chemistry of palladium and platinum 1,1-dithio complexes. Of late, there has been only one report that dealt with the synthesis of a large number of palladium dithiocar-bamates (392). Twenty-five yellow palladium dithiocarbamate complexes were obtained by reaction of PdCL with NaR2dtc in methanol solution. Several other reports have appeared in which a few dithiocarbamate complexes of palladium were synthesized. Thus, the novel [Pd (OH)2dtc 2], which is soluble in water, was isolated (393). The synthesis of optically active palladium(II) complexes of iV-alkyl-a-phen-ethyldithiocarbamates, similar to (XXIV), via the reaction between the optically active amine, CS2, and PdCL, has been described. From ORD and CD spectra, it has been established that the vicinal contribution of a remote, asymmetric carbon center could give rise to optical activity of the d—d transitions of palladium (394). Carbon disulfide has been shown to insert into the Pt-F bond of [PtF(PPh3)3]HF2, and X-ray studies indicated the structure (XXIX). 

Here, there is no evidence that masking interferes in any way with these reactions. When the complexes of palladium (II) were examined for a similar reactivity, no complexes containing coordinated chloramines could be isolated (43, 44)- The reaction of ammines of platinum (IV) with bromine can be used to introduce bromoamine in the coordination sphere, but the products are somewhat less stable than those obtained with chlorine. The general nature of the processes is similar. One example of a bromination reaction established by Kukushkin is ... 

Since the first X-ray crystal structure of a palladium(IV) complex was published by Canty [29, 30], a number of groups have undertaken the study of these relatively uncommon intermediates. In an effort to elucidate the role of palladium(IV) and palladium(II) intermediates in the reaction mechanism, Catellani prepared a number of isolable palladacycle complexes in both the (II) and (IV) oxidation states using 1,10-phenanthroline as the ligand (Scheme 8) [31]. Catellani successfully isolated palladium(IV) palladacycle 22, which was subsequently characterized by... 

Ikegami and co-workers have prepared assembled complexes of palladium and a non-cross-linked amphiphilic polymer (Scheme 64).The material, denoted as PdAS, was prepared by reaction of (NH4)2PdCl4 with poly-[iV-isopropylacrylamide)-fo-(4-diphenylstyrylphosphine)], itself made by random co-polymerization of 4-diphenyl-... 

As mentioned before (Eqs. 16 and 17), this secondary reaction is connected to the presence of a protonic source in the reaction mixture or simply to direct hydrogen transfer from the arene to the norbornane site of the complex through palladium(IV). 

Unlike the platinum analogs, direct observation of palladium(IV) alkyls has been limited to those with nitrogen-based ligands such as 2,2 -bipyridine and a-diimines [49]. However, reductive elimination from palladium(IV) complexes has... 

Transition-Metal-Promoted Reactions of Unsaturated Hydrocarbons. IV. Reactions of Norbornenyl Complexes of Palladium(II) with Group V Donor Ligands, Olefins, 1,3-Dienes and 1,2-Dienes, E. Ban, R. P. Hughes, and J. Powell, J. Organometal. Chem., 69, 455 (1974). 

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