Nature vol.455 (7216), (23 Oct 2008) Letters to Nature p.1093
doi:10.1038/nature07356

Catalytic oxidation: a platinum standardMany important biological and chemical processes, including photocatalytic water oxidation to molecular oxygen (of interest as a route to artificial photosynthesis) and the activation of dioxygen on metal surfaces, are thought to involve transition metal terminal oxo complexes. Poverenov et al. now report the synthesis of a platinum based oxidizing reagent with potentially useful characteristics. It's a d6 Pt(IV) terminal oxo complex that is not stabilized by an electron accepting ligand framework, and so exhibits reactivity as both an inter- and intra-molecular oxygen donor and as an electrophile. It also undergoes water activation to produce a terminal dihydroxo complex, which may be of relevance to the mechanism of water oxidation and other catalytic reactions.

Certain transition-metal complexes are thought to exist only fleetingly, perhaps as intermediates in reactions. So the discovery of one such complex that is stable at room temperature is provocative.
During the past 40 years, few inorganic compounds have been more discussed or sought after than a curious class of transition-metal complexes. Known as late transition metal oxo (LTMO) complexes, these compounds are thought to be intermediates in all sorts of oxygen-dependent processes. For example, they could be involved in reactions promoted by copper-containing enzymes; in catalytic-converter processes; in reactions at the cathode of fuel cells; and in industrial oxidation reactions that use 'noble metal' catalysts (such as gold, platinum and silver) on solid supports. But for a long time, there was no evidence for their existence because LTMO complexes are intrinsically unstable. However, a few have recently been isolated and characterized1, 2, 3, 4. The surprising stability of these compounds might be due to the ligand molecules that bind to the metal, which lower the electron density in the metal–oxygen unit.
On the paper, Poverenov et al.5 report the first example of an LTMO complex in which the ligands are not electron-withdrawing. The complex undergoes reactions that provide insight into how the above-mentioned catalytic processes might work.
LTMO complexes are characterized by a multiple bond between the metal and oxygen atoms (Fig. 1a); in this context, the oxygen atom is known as a terminal oxo ligand. The metal–oxygen bond is destabilized by repulsion between the bonding electrons from the oxo ligand and the 'd electrons' on the metal. The amount of repulsion depends on the number of d electrons. Those transition metals on the left-hand side of the periodic table (the early transition metals, such as vanadium and tungsten) have few or no d electrons, so terminal oxo complexes are stable and common for these elements. The transition metals in the middle of the periodic table (such as manganese and iron) have more d electrons, and their terminal oxo complexes are highly reactive. The late transition metals (such as platinum, silver and gold) are found towards the right-hand side of the periodic table, and have the most d electrons of all; their complexes are therefore generally so reactive that, for many years, none could be isolated.

But in 1993, the first LTMO complex1 — an iridium complex — was isolated. Iridium, although technically a late transition metal, has fewer d electrons than platinum. The ligands in this complex adopt a tetrahedral arrangement around the metal, which permits iridium's d electrons to be accommodated without destabilizing the terminal oxo group. Subsequently isolated LTMO complexes incorporated electron-poor ligands, which probably withdraw electron density from the metal–oxo unit, thus reducing electron–electron repulsion. In this way, terminal oxo complexes of platinum2, palladium3 and gold4 were made.
But Poverenov et al.5 have made a platinum terminal oxo complex that has non-electron-withdrawing ligands. The authors treated a conventional platinum complex that lacks oxygen ligands with a compound (dimethyl dioxirane) that acts as a potent oxygen-atom donor. The resulting product (Fig. 1b) is sufficiently stable, both as a powder and in solution, to be extensively characterized at room temperature. Remarkably, the ligand (an aromatic ring) opposite the oxo group on the metal is not an electron-withdrawing group — in fact, it actually donates electrons to the platinum. The authors used quantum-mechanical calculations to show that empty orbitals on this aryl ligand are too high in energy to receive electron density from the metal. Furthermore, the other ligands in the complex are also more electron-donating than electron-accepting in character.
The stability of Poverenov and colleagues' complex is intriguing. The most widely used strategy for stabilizing reactive moieties — both in chemistry and biology — is to encapsulate them with other groups so that they can't interact and react with neighbouring molecules. But the authors show that such 'steric' protection doesn't happen in their compound: analytical data and structural predictions, along with the reactivity of the complex, all indicate that the platinum–oxygen bond is at least partly exposed. Thus, no convincing electronic or steric argument can be made to explain why the compound is stable.
Of course, extraordinary claims require extraordinary evidence. In this case, given that the reactivity of the authors' compound is lower than that expected for LTMO complexes, the proposed structure seems highly improbable — but it has been characterized by a commensurately extensive and convincing ensemble of techniques. Although the authors were not able to obtain an X-ray structure of the compound, several lines of evidence establish that their proposed structure, complete with a partly exposed terminal oxo ligand that lies out of the plane formed by the other ligands, is correct. Specifically, their X-ray spectroscopy data agree with predictions from computational modelling of the proposed structure; the infrared spectrum of the complex contains a signal characteristic of a multiple platinum–oxygen bond; and mass spectrometry data show that the molecular mass of the complex is the same as that of the proposed structure.
A similar battery of evidence strongly suggests that the oxidation state of the platinum atom in the complex is consistent with Poverenov and colleagues' suggested structure. And as if all this weren't enough, the observed chemical reactions of the compound also support the authors' interpretation of the data: it transfers its oxo oxygen from the platinum atom to the adjacent phosphorus ligand, reacts with water, and takes part in other reactions that collectively are best explained by assuming a terminal oxo structure.
Poverenov and colleagues' findings5 go a long way to substantiating the previous reports of platinum2, palladium3 and gold4 oxo complexes. This, in turn, makes it hard to argue that LTMO complexes don't exist, as might reasonably have been argued before on the basis of conventional wisdom about modes of chemical bonding. On the contrary, it seems that they might be fairly widespread. They can certainly now be regarded as realistic candidates for intermediates in many important catalytic reactions that involve oxygen.
The report of this remarkable platinum complex5 also raises many questions. What is it that stabilizes certain LTMO compounds, and how common are these stabilization mechanisms in other complexes that contain different ligands and/or different arrangements of ligands? Do terminal ligands other than oxo groups exist, in which atoms such as nitrogen or sulphur form multiple bonds to noble metals? And how might LTMO complexes take part in the widely used technologies that involve noble metals as catalysts? The answers to these questions will be of great fundamental and practical interest.

References

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• Poverenov, E. et al. Nature 455, 1093–1096 (2008). | Article |

学习了，谢谢

thank you!!!!!!!!

唉，还是没有晶体结构证明啊。如果有的话，就Perfect了。

nice

good!

Thanks!!

看了原文没有?   作者给出三个晶体结构.

The Pt=O one is not the crystal structure. It is only DFT calculated structure.