Triethylammonium bis(2-oxido-2,2-diphenylacetato-κ2 O 1,O 2)antimonate(III)

The coordination around the Sb atom in the title compound, (C6H16N)[Sb(C14H10O3)2], is fourfold in a pseudo-trigonal-bipyramidal pattern in which one of the equatorial sites is occupied by the stereoactive lone pair of electrons. The four ligating atoms comprise the hydoxylate and carboxylate O atoms from two independent benzilate ligands, each of which forms a five-membered chelating ring, spanning an axial and an equatorial site about the Sb atom. The hydroxylate atoms occupy the two equatorial sites, and the carboxylate atoms are in the pseudo-axial sites; the O—Sb—O angle is 147.72 (5)°. One carboxylate group shows quite different bond lengths from those of the other group; one O atom is clearly the carbonyl atom and the other O atom the hydroxylate atom. In the other ligand, there is less distinction in the C—O bonds. This is presumably related to the carbonyl O atom being the acceptor atom of a strong N—H⋯O hydrogen bond, which links the ammonium cation to the Sb complex anion.

The coordination around the Sb atom in the title compound, (C 6 H 16 N) [Sb(C 14 H 10 O 3 ) 2 ], is fourfold in a pseudo-trigonalbipyramidal pattern in which one of the equatorial sites is occupied by the stereoactive lone pair of electrons. The four ligating atoms comprise the hydoxylate and carboxylate O atoms from two independent benzilate ligands, each of which forms a five-membered chelating ring, spanning an axial and an equatorial site about the Sb atom. The hydroxylate atoms occupy the two equatorial sites, and the carboxylate atoms are in the pseudo-axial sites; the O-Sb-O angle is 147.72 (5) . One carboxylate group shows quite different bond lengths from those of the other group; one O atom is clearly the carbonyl atom and the other O atom the hydroxylate atom. In the other ligand, there is less distinction in the C-O bonds. This is presumably related to the carbonyl O atom being the acceptor atom of a strong N-HÁ Á ÁO hydrogen bond, which links the ammonium cation to the Sb complex anion.

Comment
In an ongoing study of metal-carboxylate and -alkoxide complexes (Reza et al., 1998(Reza et al., , 1999(Reza et al., , 2003Tarafder et al., 2008), with particular interest in their antimicrobial and catalytic properties, we are exploring the coordination chemistry of some α-hydroxycarboxylate ions which contain both the ligating groups of interest and whose parent acids have important roles in enzyme reactions. Often, the α-hydroxycarboxylate ligand forms a simple chelating ring with a metal (e.g. Vergopoulos et al., 1995), but terminal and bridging modes have also been found (e.g. Carballo et al., 2004aCarballo et al., , 2004b. The bridging modes, using the three (or more) available oxygen donor atoms, allow the formation of oligomeric complexes, for example the macrocyclic rings formed between benzilate ions and AlMe 3 (Redshaw et al., 2005) or ZnEt 2 (Redshaw et al., 2007), and the cages formed by 9-hydroxy-9-fluorenecarboxylic acid with n-BuSn(O) OH (Chandrasekhar et al., 2005). We recall also the series of citrate complexes in which the citrate ion chelates an antimony ion and bridges to a range of simple cations (alkali metals, copper, silver, etc) to form a variety of polymeric structures (Hartley et al., 1991, Smith et al., 1992, 1993, Bott et al., 2000. We have now prepared an Sb(III) complex of the benzilate (diphenylglycolate) ion, with Et 3 NH + as the counterion.
This cation is non-coordinating and has a single N-H group with potential for formation of hydrogen bonds. The product, the title compound (I), shows ( Figure 1) the antimony atom coordinated by two independent benzilate ligands with very similar conformations, each with a five-membered chelating ring which bridges an axial and equatorial site of the pseudo trigonal bipyramidal coordination polyhedron; the fifth, equatorial coordination site is occupied by a stereoactive lone-pair of electrons in the style typical of antimony(III) complexes of citrate and other α-hydrocarboxylate ions. In all these Sb(III) complexes, the alkoxide O atom is in an equatorial position and one of the carboxylate O atoms is in an axial site, and the axial Sb-O bonds are slightly longer than the equatorial bonds. Slight differences in the dimensions of the carboxylate groups in our complex are thought to result from the participation of one of these groups as acceptor of a good, almost linear hydrogen bond, N(7)-H(7)···O(11), from the Et 3 NH + cation. The Sb(OOCCOPh 2 ) 2 anion and Et 3 NH + cation thus appear as a discrete ion-pair unit in the crystal.
Molecular dimensions are available in the archived CIF.

Experimental
The title complex, (Et 3 NH) [Sb(Ph 2 COCOO) 2 ] (1) was synthesized by adding, with stirring, a mixture of benzilic acid (2 mmol) and triethylamine (2 mmol) in acetonitrile (50 ml) to 30 ml of an aqueous solution of SbCl 3 (1 mmol). The stirring was continued for another 30 min at 60 C. Precipitates initially formed were filtered and the filtrate was concentrated to one-third of its original volume (ca 30 ml). The colourless single crystals of 1 which appeared after a week were collected, washed with water and dried in air at room temperature, Mp. 558 K.

Refinement
The non-hydrogen atoms were refined with anisotropic thermal parameters. The ammonium hydrogen atom, H (7), was located in a difference map and was refined freely; the remaining hydrogen atoms were included in idealized positions and their U iso values were set to ride on the U eq values of the parent carbon atoms. The largest difference peak (near Sb) and hole were 0.41 and -0.42 e.Å -3 . Fig. 1. View of a molecule of (Et 3 NH)[Sb(Ph 2 COCOO) 2 ] (1), indicating the atom numbering scheme. Hydrogen atoms (except that involved in the hydrogen bond) have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Triethylammonium bis(2-oxido-2,2-diphenylacetato-κ 2 O 1 ,O 2 )antimonate(III)

Figures
Crystal data (C 6  as those based on F, and R-factors based on ALL data will be even larger.