Thiocyanate ‘Prussian Blues’

Left: the structure of Fe[Bi(SCN)₆] determined using single crystal X-ray diffraction. Right: the optical absorption spectrum of M[Bi(SCN)₆], showing which wavelengths of light each compound absorbs.

Prussian Blue is an iconic material. Discovered around 1706, ostensibly by adding iron sulfate to a concotion of boiled animal blood and potassium carbonate, it was the first modern purely synthetic pigment. It made blue paint that wouldn’t fade available to artists at a low cost and perhaps most famously dyed the uniforms of the Royal Prussian Army from the early 18th century through to the 20th. Since then Prussian Blue and related compounds have been found to be useful for all kinds of applications, including electrochromics (materials that change colour when a voltage is applied to them), batteries, and even as magnets. Prussian Blue is also one of the most simple examples of a coordination polymer: a material where metal atoms are connected by multi-atom linkers into infinite chains or frameworks. In this case, iron atoms are connected by cyanide (CN^–) into a three dimensional cubic network, with the chemical formula (Fe³⁺₄[Fe²⁺(CN)₆]₃)· x H₂O.

In this paper we report the first examples of “Prussian Blue”-like frameworks made with thio-cyanate (S-CN^–) and find that these new thiocyanate compounds are very strongly coloured. Fortunately, unlike our Prussian forebears, we did not find it necessary to boil any animal blood to make these compounds, as we were able to purchase our starting materials from ordinary chemical suppliers (rather than a butchers). Perhaps our most visually striking finding was that these new thiocyanate compounds are very strongly coloured. The thiocyanate frameworks created are closely related to the cyanide Prussian Blues, but there are a number of key differences due to the extra sulfur atom.

Firstly, the sulfur and nitrogen ends of thiocyanate are much more different from one another than the carbon and nitrogen ends of cyanide. This means to connect a thiocyanate into a 3D networks usually requires two different metals, one for the S-end (in this case bismuth) and one for the N-end (in this case iron, scandium or chromium). The sulfur also likes to bond with bent angle and the resultant distortion means that despite the extra atom thiocyanate frameworks are in fact only slightly larger than corresponding cyanide frameworks.

Secondly, a further key difference is how the colour comes about in these materials. The colour of Prussian Blue comes about because it is ‘mixed valent’: it has both Fe²⁺ and Fe³⁺. This means when visible light is shone on it, it is absorbed and pushes an electron from the Fe²⁺ onto Fe³⁺, making them Fe³⁺ and Fe²⁺, respectively. This ‘charge-transfer’ absorption is very strong and is responsible for the deep blue colour of the pigment.

The three thiocyanate frameworks we report are not mixed valent, as all the metals are 3+, though. Their color comes about because light can temporarily move an electron from the SCN^– molecule onto the metals. This ’ligand to metal charge transfer’ can happen because the sulfur atom raises the energy of electrons in the ligand to better match the energy levels of the metal. If the electron hops to Fe³⁺, it produces a vivid dark red colour (sometime used as a way of making trick blood); however if it hops to Bi³⁺ it produces a vivid yellow orange colour. These effects are what produce the striking colours of M[Bi(SCN)₆] frameworks: Fe[Bi(SCN)₆] is very dark green, Sc[Bi(SCN)₆] is orange, and the most stable example Cr[Bi(SCN)₆] is brick red. The strong colours suggest that these compounds might be interesting as materials which harness light to useful ends - though we will need to investigate them further to discover how!