“Jumping Crystals”: Oxygen-Evolving Metal-Nitroxide Complexes

The crystals of heterospin complexes [M(hfac)2L2] (where M = Cu, Ni, Co, or Mn; hfac = hexafluoroacetylacetonate; and L = nitronyl nitroxide, 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl) were found to make unusual jumping motions. Under ambient conditions, the jumping and various displacements of crystals lasted for several weeks. The mechanical motion was accompanied by the cracking and disintegration of crystals, and a solid [M(hfac)2(L1)2] complex with the corresponding imino nitroxide 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-1-oxyl (L1) was detected. The jumping was accompanied by the spontaneous elimination of oxygen, the source of which was the nitronyl nitroxyl fragment of coordinated L. An X-ray study of [M(hfac)2L2] (where M = Cu, Ni, Co, or Mn) showed that the molecular structure of all [M(hfac)2L2] and their packing in the solid state were identical. The packing of [M(hfac)2L2] was concluded to be critical to the mechanical effect. In complexes with different stoichiometries or different sets of diamagnetic ligands ([Cu(hfac)2L]2, [Cu(hfac)(acac)L]·EtOH, [CuPiv2L2]·2CH2Cl2, and [Cu(hfac)2L2Cu2Piv4]·3C7H8 (where acac is acetylacetonate and Piv is trimethylacetate), or free L), the effect vanished when the packing changed.