A team of scientists from the US, the UK and Germany has been the first to deliberately use a bifluoride building block to make a three-dimensional coordination polymer. 

The structure, made by Jamie Manson at the Eastern Washington University, Cheney, US, and colleagues, contains copper ions bound to pyrazine molecules in a planar square.  Bifluoride ions (HF₂^-) sit above and below the copper ions. Each pyrazine molecule can bond to one copper ion at each end, to give a potentially infinite copper-pyrazine plane. Bifluoride ions act as bridges between the planes.

The structure is very thermally stable (up to 200 ºC) due to the exceptional strength of the hydrogen bonds within it - bifluoride contains the strongest known hydrogen bond.

The biflouride ions (shown in green) bridge the copper-pyrazine planes (copper ions are shown in red)

The researchers studied the magnetic properties of the material by a technique that uses muons as mini-magnetometers. Muons are sub-atomic particles that are heavier than electrons but have the same charge and magnetic spin. 

The magnetic properties of a material arise from the way the magnetic spins of unpaired electrons line up. In Manson's coordination polymer, the spins of the unpaired electrons on the copper ions are disordered at ambient temperature, so that there is no overall magnetism - this is known as paramagnetism. As the temperature drops, magnetic spins of the electrons begin to align, but in opposite directions so that, again, there is no overall magnetic field - known as antiferromagnetism. The researchers found that below a temperature of 1.54 K complete antiferromagnetic ordering of the spins occurs.

The team hopes that the magnetic studies will help them understand to what extent bifluoride units and their hydrogen bonds influence the spin arrangement on neighbouring magnetic centres.

Lars Öhrström, an expert in molecular networks at the Chalmers University of Technology, Göteborg, Sweden, appreciates the team's work. 'The ingenious generation, under mild conditions, of the bifluroide anion, and the subsequent preparation of the three-dimensional coordination polymer, open up a route to a range of new solid state coordination compounds that may prove to be as diverse as that of other small linear bridging anions such as cyanide,' he said.

Robin Forder