C5 BORCHELATE WITH ORGANIC LIGANDS CONTAINING AROMATIC RING; CRYSTAL STRUCTURE OF AMMONIUM BIS(o-OXYMETHYLPHENOLATO)BORATE NH₄[(C₆H₄OCH₂O)₂B]

Irena Zviedre¹, Jevgenija Schwartz¹, Vladimir Fundamensky²

¹Institute of Inorganic Chemistry of the Riga Technical University, Miera str., 34, Salaspils, LV-2169 Latvia, ²Departament of Crystallography, St. Petersburg State University, 199034 St. Petersburg, Russia
E-mail: zviedre@dati.lv

Keywords: Crystal structure, tetragonal boron, complex.

Interaction of boric acid with o-hydroxymethylphenol (saligenin) and ammonia in the ethanol solutions leads to formation of the ammonium bis(o­oxymethylphenolato)borate of NH₄[(C₆H₄OCH₂O)₂B] (I). Good quality crystals of the title compound for X-ray diffraction were grown by slow evaporation of aqueous solution. Crystal structure has been determined using the X-ray diffraction method (Syntex P2_1, l-MoKa radiation).

Crystals are orthorhombic. The unit cell parameters are: a=13.344(4), b=13.640(4), c=7.265(2) A, V=1322.3A³, Z=4, r_c=1,73 g/nm³, space group Pbcn, final R=0.058, R_w=0.062.

Crystal structure is formed by [NH₄]⁺ cations and spiran-type complex anions [(C₆H₄OCH₂O)₂B]^- . The ammonium ion is surrounded by four oxygen atoms of four different complex anions. The complex anion is formed by two o­oxymethylphenol radicals, each doubly attached to the symmetrically distorted central BO₄-terahedron (B-O(hydroxyl) 1.461(5)A , B-O(phenyl) 1.479(5)A and C-O(hydroxyl) 1.445(5)A , C-O(phenyl) 1.373(4)A ). The heterocycles are nonplanar. The two carbon, one oxygen and boron atoms are coplanar, but hydroxyl O atom and tetrahedral carbon atom is apart -0.526(2) A and +0.179(1)A from this plane. X-ray investigations of boron complex with polyhydroxycarboxylic acids (citrato-; (D,L)malato-; xylotrihydroxyglutarato-; tartratoborates) have shown that angles between the boron - coordination planes in the complex anions are to 90^o. However this angle in the borchelates of polyoxyorganic ligands contain aromatic ring such as salicylic acid and their derivatives and also o-oxymethylphenol differ from 90^o. This can be explained with the structure complex anions where all oxygen atoms are located in the small central BO₄-tetrahedron and the contact with cation is restricted.

The authors synthesized and determined the structures [Zn(H₂O)₆][(C₆H₄OCOO)₂B]^.H₂O (II), K[(NH₂C₆H₃OCOO)₂B]^.H₂O (III), K[(C₇H₄O₃)₂B]^.C₇H₆O₃ (IV), Cs[(CH₃C₆H₃OCOO)₂B]^.H₂O (V), Cs[C₇H₄O₃)₂B (VI) by X-ray analysis. The angle between the boroxygen ring planes in the I-VI structures depends on the nature of the cation, coordination manner with anion, existence of supplemental coordination partner, for example molecules H₂O, H₂Sal a.o. The angle among the two identical (from the viewpoint of crystal chemistry) sides of anion decreases from 90.5^o (II); 83^o (III); 77.8^o (IV); 76.9(5)^o (V); 73.95(5)^o (VI); to 62.78(1)^o (I). Substitution of the COOH group by CH₂OH group in the aromatic ring diminishes the oxygen atoms number in the complex anion.

In the crystal structure (I) complex anions and ammonium are linked together by hydrogen bonds. The hydrogen bond distances are 2.778(5) A and 2.919(4) A.