CRYSTAL AND MOLECULAR-STRUCTURE OF PENTAAQUOHEXA(DIPHENYL PHOSPHATO)TRIMAGNESIUM(II), [(C6H5O)2P(O)O]6MG3(H2O)5, A HYDRATED MAGNESIUM PHOSPHODIESTER SALT WITH PENTACOORDINATE AND HEXACOORDINATE METAL-IONS

Anhydrous magnesium diphenyl phosphate is prepared by the reaction of diphenyl methyl phosphate with tris-(tetrahydrofuran)magnesium(II) bromide. A solution of the salt in benzene and moist ether deposits triclinic crystals of pentaaquohexa(diphenyl phosphato)trimagnesium(II), space group PI, with two formula units, [(C6H5-O)2P(O)O]6Mg3(H2O)5, in the unit cell (Z = 2β) of dimensions: α = 15.173 (23); b = 23.625 (17); c = 12.896 (9) Å; a = 104.85 (6); β = 112.56 (10);γ = 86.23 (10)°. Reflections were collected on a Pi diffractometer (Mo Kα) by ω step scan mode. The structure was solved by Patterson superposition methods and by iterative Fourier maps, and refined by full-matrix least-squares methods to a final R factor of 8.3% for 8126 reflections. Average interatomic distances (Å) are: 2.154, 2.111 for Mg-W (water); 2.042,1.987 for Mg-O (phosphoryl) for coordination numbers 6 and 5, respectively; and 4.702 for Mg⋯Mg. The compound can be formulated as [(ArO)2P(O)O]2Mg(W)2-[(ArO)2P(O)-O]2Mg(W)-[(Aro)2P(O)O]2Mg(W)2, where Ar = C6H6 and W = H2O. The structure consists of infinite chains of phosphodiester molecules linked through Mg2+ ions and oriented along the z axis. The chains are joined together by the Mg2+ ions to give a sequence of eight-membered rings fused in a spiro configuration at the metal, each ring with two Mg, two P, and four O atoms. There are two types of magnesium ions, coordination number 6 and coordination number 5. There are two of the former and one of the latter in the asymmetric unit. Each magnesium ion is coordinated to four oxygen atoms from four different phosphate groups and by one or two water molecules. The geometry about the CN6 Mg atoms is nearly octahedral, but that of the CN5 Mg atom is neither regular trigonal bipyramidal (TBP) nor regular tetragonal pyramidal (TP). The structure suggests models for some possible interactions which may contribute to the conformation of hydrated metal salts of some polynucleotides. © 1978, American Chemical Society. All rights reserved.