| Summary: | Unexpectedly, a Zeise’s salt analogue, the dinuclear, butadiene-bridged complex, trans-eta^2:eta^2-1,3-butadiene-bis(trichloroplatinate(II)) was isolated when [AuCl4] and [PtCl4]2- were reacted together in the presence of the tetrabutylammonium cation. Early observations ruled out the involvement of the gold(III) species in generating the butadiene directly, but it was found to be acting as an oxidising agent facilitating conversion of [PtIICl4]2- to [PtIVCl6]2-, which turned out to be a key observation. Further studies identified the source of the C4 fragment of butadiene as the tetrabutylammonium cation, suggesting a C-H activation, perhaps via some Shilov chemistry, linked perhaps to a Hofmann-like elimination. The reaction is photochemical and proceeds directly from (NBu4)2[PtCl6] in the total absence of gold. A computational study and related literature precedent regarding the excited state of [PtCl6]2- showed the formation of PtIII as well as chlorine radicals, revealing the possibility for radical induction of the reaction. Indeed, PtIII and N-based radicals were found in the electron paramagnetic spectroscopy (EPR) spectrum of an irradiated frozen solution. Given the extrusion of a butadiene fragment from the tetrabutylammonium cation, the possible involvement of Hofmann elimination mechanism was considered. One possibility would be the elimination of butene in the first step with concomitant reduction to PtII leading to a butene analogues of Zeise’s salt, which could then react further to give the observed product. However, this possibility was not supported by the observed unreactivity of [PtCl3(butene)]- (butene = 1-butene or 2-butene) under the prevailing reaction conditions. This led to the proposal of an intermediate complex in which (Bu3N(butene))+ is pi-bound to [PtCl3]-, which was supported by mass spectrometric evidence from a photochemical study. The same reaction was carried out with the [PtCl6]2- salt of other tetraalkylammonium cations (NPr4+ and NPe4+) and alkene-PtII complexes were again found with NPe4+ salts but not with NPr4+ salts. All of these results and observation were collected together to propose an outline mechanism for the reaction. In a totally separate piece of work, some examples of polycatenar 2,5-diphenylpyridine ligands (LH) were prepared and bound to palladium(II) to give complexes with an orthometallated 2-phenylpyridine and an acac co-ligand - [Pd(L)(acac)]. These are analogous to related PtII complexes prepared in the group which were found to be liquid-crystalline and emissive. While some liquid-crystalline examples were prepared, none of the complexes showed evidence for room-temperature triplet emission, which can be a feature of palladium congeners. However, in the related platinum chemistry it was observed that on formation of the complexes with three alkyloxy groups on the metallating phenyl ring, there was a Pt-promoted C-O bond cleavage. The milder conditions required for formation of the palladium meant that the same C-O bond cleavage was not observed here.
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