The crystal and molecular structure of ammonium titanyl oxalate

Abstract

Ammonium titanyl oxalate monohydrate, (NH 4) 2 TiO(C 2O 4) 2·H 2O, is monoclinic with cell parameters a = 13.473(2), b = 11.329(1), c = 17.646(2) Å, β = 126.66(1)°. The space group is P2 1/c with Z = 8, d c = 1.808 g cm −3 and d m = 1.80 g cm −3. The crystal structure was determined from single-crystal diffractometer data and refined by least-squares methods using isotropic thermal parameters. The conventional R factor was 7.0% for 2466 used reflections. The anions consist of cyclic tetranuclear complexes [TiO 1 2 O 1 2 (C 2O 4) 2] 4 8− with 1 symmetry. The titanium atoms are six-coordinated with two bridging oxygen atoms cis to one another and four oxygen atoms of bidentate oxalate groups, together forming a distorted octahedron. The titanium atoms of the tetramer are linked through oxygen atoms at two neighbouring apices of each octahedron. The actual name of the compound should therefore be: ammonium cyclotetra-di-μ-oxo-cis-dioxalatotitanate(IV)tetrahydrate. The di-μ-oxo-tetratitanium unit is an eight-membered OTiO ring with Ti-O distances of successively 1.840(7), 1.785(7) and 1.855(6), 1.788(8) Å, repeated by the centre of symmetry, and the orientation of the atoms suggests the presence of dπpπ 3-center 2-electron bonds in each TiOTi set. The TiO distances trans to the bridging oxygen atoms are much longer: 2.060(7), 2.101(7), 2.081(7) and 2.116(7) Å respectively, as is to be expected from the charge displacement due to the dπpπ bonding. Most OTiO angles in the distorted octahedra differ considerably from 90°. As was found by difference Fourier synthesis and thermal analysis, half of the crystal water is held very loosely compared to the other half.