SOLID-STATE WATER-CATALYZED TRANSFORMATION AT ROOM-TEMPERATURE OF A NONLUMINESCENT LINEAR-CHAIN URANYL PHENYLPHOSPHONATE INTO A LUMINESCENTONE

Single crystals of a recently prepared, at room temperature non-lumine scent, linear-chain cl-uranyl phenylphosphonate (alpha-UPP), [UO2(HO3P C6H5)(2)(H2O)](2) . 8H(2)O, transform at room temperature and moderate humidity into a new strongly luminescent linear-chain beta-uranyl phe nylphosphonate (beta-UPP), UO2(HO3PC6H5)(2)(H2O). 2H(2)O. Both compoun ds have also been synthesized separately and characterized by single-c rystal methods. The alpha-phase crystallizes in the triclinic space gr oup P (1) over bar, with a = 11.724(4) Angstrom, b = 16.676 Angstrom, c = 11.375 Angstrom, alpha = 101.61(5)degrees, beta = 106.76(3)degrees , gamma = 102.57(4)degrees, and Z = 2, whereas the beta-phase crystall izes in the monoclinic space group P2(1)/c with a = 7.766(2) Angstrom, b = 11.255(2) Angstrom, c = 20.959(4) Angstrom, beta = 100.38(3)degre es, and Z = 4. In both compounds, the coordination of the uranium atom s is pentagonal bipyramidal, and the metal-to-phosphonate ratio is 1:2 . The most obvious difference between the two linear-chain compounds i s the manner in which their phenyl rings are arranged. When viewed alo ng the appropriate axes, phenyl rings of alpha-UPP arrange in two rows in a cis conformation, whereas those of beta-UPP arrange in two rows in a trans conformation. The suggested mechanism of transformation con sists of breaking a U-O-equatorial bond, followed by a rotation of a p henylphosphonate group. During this rotation, the protonated phosphona te oxygen is deprotonated by one of the lattice water molecules and th e detaching phosphonate oxygen is protonated. The deprotonation appear s to be the rate-determining step of the transformation, not the U-O-e quatorial bond breaking. The linear uranyl phenylphosphonate chains re main uninterrupted during the phase transformation process, but they s hift and rearrange to form a different lattice as their shape changes from cis to trans.