Last Month's Most Accessed Feature: Gold too? Ions in the +II oxidation state

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  • Published: Oct 3, 2017
  • Categories: Atomic
thumbnail image: Last Month's Most Accessed Feature: Gold too? Ions in the +II oxidation state

Aural oxidation

Gold in its divalent form is stable in the center of porphyrins. Credit: Ill. Katja Heinze, JGU

When it comes to atoms, there are several seemingly immutable facts about many of them - their oxidation states or valencies are seemingly fixed. However, whereas gold is generally considered to exist as only gold(I) and gold(III), theoretically, a 2+ oxidation state should exist to fill the gap in the homologous series of the coinage metal ions copper(+II), silver(+II), gold(+II), and in the "relativistic" triad of platinum(+II), gold(+II), and mercury(+II). That gap has now been filled as chemists isolate and analyse gold in this rare oxidation state.

The stabilisation and isolation of a divalent gold complex could offer new insights into the chemistry of precious metals as well as leading to an improved understanding of the mode of action of cytostatic gold(+III) porphyrin drugs used as anticancer agents.

A standard inorganic chemistry textbook will discuss the oxidation states of gold and its chemical cousins, citing the +I and +III states as being the only ones known. There are known examples of compounds containing the gold(II) ion, but these are usually polynuclear compounds. Any attempt to make a simple gold(II) complex usually leads to it transforming into the far more common mono- and tri-valent forms. This is in stark contrast to gold's neighbours in the periodic table. Above gold, copper and silver are both well known in the +II state as too are the neighbours either side, platinum and mercury. Chemists had theorized that photochemical reactions of gold might generate the +II state, but there was no definitive evidence of this until work by researchers at Johannes Gutenberg University Mainz (JGU).

Katja Heinze of the Institute of Inorganic Chemistry and Analytical Chemistry of JGU and her colleagues have reported a gold(II) complex in the journal Nature Chemistry. The work now allows fundamental data about the Au2+ ion such as its size, preferred structural arrangement, and reactivity, to be examined.

Stabilisation, that's the name of the game

The team was able to stabilize the very "labile" gold(+II) ion using a porphyrin molecule to surround it. Porphyrins are well known as the molecules that keep a grip on magnesium ions in chlorophyll and on iron in the haem unit of blood. Related molecules are present in other systems, such as the cobalt at the heart of vitamin B12 with its corrin ring and the haem-type group in cytochrome enzymes.

In this complex the porphyrin ring inhibits the usual reaction pathways that gold(II) would otherwise take in progressing to the higher or lower oxidation state or the formation of polynuclear systems. "This enabled for the first time to investigate this unique class of stable mononuclear gold(+II) complexes and to describe them comprehensively," explains Heinze. The team points out that the four atoms closest to the gold(+II) ion are not arranged in a planar square as one would expect with each atom placed equidistant from the metal centre. Complexes of copper(+II), silver(+II), platinum(+II), and mercury(+II) would all adopt a symmetrical square planar system in such circumstances. Instead, the team's structure reveals a rhombic distortion with two short and two long distances. This previously unobserved effect is due to the second-order Jahn-Teller effect, which is caused by the effects of general relativity on the properties of gold. Gold is "yellow" in colour because of relativity and similarly mercury is liquid at room temperature.

Given that this new gold(+II) compound might form from a gold(+III) complex present in potent anti-cancer agents, the team also investigated the nature of the gold(+II) porphyrin complex and whether it might have a biological role. They found that the gold(+II) complex can be generated under conditions not dissimilar to physiological conditions from one such cytostatic gold(+III) agent. When this compound is exposed to the air, the gold(+II) porphyrin forms reactive oxygen species, which are known to induce cell death, apoptosis.

Plausible atomic

"We thus have a plausible functional chain starting with a cytostatic agent and leading to targeted cell death with the gold(+II) porphyrin acting as an important link in the chain," explains Heinze. "A major impetus for us to continue with research in this field is that curiosity-driven fundamental research about unusual species enabled us to reach insights that could well be relevant to medical applications," she adds.

The gold(II) complex was characterized by electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray, ultraviolet–visible (UV-Vis) and infrared absorption spectroscopy, as well as by magnetic susceptibility and X-ray diffraction (XRD) analyses, the team reports.

Related Links

Nature Chem 2017, online: "Structure and reactivity of a mononuclear gold(II) complex"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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