Structural characterisation and dynamics of a paramagnetic {Cr₁₂Ni₃} seahorse in non-crystalline phases

The complex [{Ni(cyclen)}₂Cr₁₂NiF₁₈(O₂CBu)₂₄] (where cyclen = 1,4,7,10-tetrazacyclododecane) crystallises as a fifteen-metal chain that is shaped like a seahorse. Given this is one of the longest finite, paramagnetic chains found, we were intrigued whether this unusual structure is induced during crystal growth or also maintained in other phases. We report electron paramagnetic resonance spectroscopy, small angle X-ray scattering and atomistic molecular dynamics simulations, demonstrating that the S-structure from crystal is stable in powder and solution. Using ion mobility mass spectrometry (IM-MS), we revealed the coexistence of S-shaped structures and a closed isomeric assembly in the gas phase. Collision-induced dissociation mass spectrometry studies monitored by IM-MS show the rearrangement of the cyclic seahorse to the S-shaped conformation, as well as the dissociation to a cyclic, seven-metal complex.