Theoretically, the gold atom’s normal oxidation states in compounds are +I and +III. In contrast, the divalent form (+II) largely forms polynuclear compounds or just gets modified into the monovalent and trivalent forms. Yet, the neighboring elements of gold in the periodic table are entirely distinct with respect to this matter.
The ions of copper(+II) and silver(+II), which are the coinage metals, normally exist in divalent form. It is exactly the same in the case of platinum(+II) and mercury(+II), which are located to the left and right of gold in the periodic table, respectively. It has been proposed that when gold is subjected to photochemical catalysis reactions, although there is a possibility for the +II state to be formed, there is no reliable proof for this to date. The corresponding evidence has just been put advanced by Researchers from the Johannes Gutenberg University Mainz (JGU) in a recent publication.
A group of Chemists headed by Professor Katja Heinze from the Institute of Inorganic Chemistry and Analytical Chemistry of JGU has been successful in isolating and investigating gold in the highly uncommon +II oxidation state. This brings out the missing links in the homologous sequence of the coinage metal ions copper(+II), silver(+II), gold(+II) and in the “relativistic” triplet of platinum(+II), gold(+II), and mercury(+II). “Fundamental data unknown to date such as ion size, preferred structural arrangement, and the reactivity of gold(+II) have now been made available,” stated Sebastian Preiß, doctoral candidate in Heinze’s team, who was the first Researcher to successfully isolate the gold(+II) complex in its purest form. The outcomes of the research have been reported in the journal Nature Chemistry.
The Researchers successfully stabilized the highly unstable gold(+II) ion by using a so-called porphyrin to enclose the gold(+II) ion. The porphyrin macrocycle, along with magnesium or iron ions at the center, exists in chlorophyll, the green pigment in plants, and in hemoglobin, the red pigment in the blood. Porphyrin has gold(+II) at its center, thereby inhibiting the usual reaction pathways of gold(+II), that is, the synthesis of polynuclear compounds or the transformation to the highly stable gold(+I) and gold(+III) complexes.
This enabled for the first time to investigate this unique class of stable mononuclear gold(+II) complexes and to describe them comprehensively.
Professor Katja Heinze
Fascinatingly, the placement of the four atoms neighboring the gold(+II) ion is not square planar in which the atoms are positioned at equal distances from the gold atom, which is the way it is in the corresponding structures of copper(+II), silver(+II), platinum(+II), and mercury(+II). By contrast, the structure exhibits a rhombic distortion including two long and two short distances. Technically, this phenomenon that was not spotted earlier for the gold(+II) ions may be due to a second-order Jahn-Teller effect brought about by the relativistic characteristics of gold.
Due to the fact that the new gold(+II) compound can be produced from the gold(+III) complex existing in powerful anti-cancer agents, the Scientists attempted to discover if the gold(+II) porphyrin also has a vital role in biological systems. They found out that the gold(+II) complex can be produced from a cytostatic gold(+III) agent under virtually physiological conditions. Exposure to atmospheric oxygen causes the gold(+II) porphyrin to form reactive oxygen species (ROS), which obviously cause apoptosis, that is, programmed cell death.
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. 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.
Professor Katja Heinze