A pyrazine bis-adduct of a binuclear rhodium(II) carboxylate containing 3,4,5-triethoxybenzoate as the equatorial ligand

INQUIMAE, DQIAyQF, FCEyN, Universidad de Buenos Aires, Pab. II, Ciudad Universitaria, (C1428EHA) Buenos Aires, Argentina, Departamento de Fõ Âsica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata and IFLP (CONICET), CC 67, (1900) La Plata, Argentina, and Instituto de Fisica de Sa Äo Carlos, Universidade de Sa Äo Paulo, CP 369, 13560 Sa Äo Carlos (SP), Brazil Correspondence e-mail: marciar@qi.fcen.uba.ar


Comment
Lantern-type dirhodium carboxylates of the general formula [Rh 2 II,II (O 2 CR) 4 L 2 ] (L are axial ligands) have been the subject of numerous investigations focusing on their catalytic properties, antitumour activity, molecular and electronic structure, spectroscopic features and chemical reactivity (Norman et al., 1979;Christoph & Koh, 1979;Boyar & Robinson, 1983;Cotton & Walton, 1992;Kitamura et al., 2000). Moreover, their use as building blocks in the synthesis of advanced materials has received increasing attention (Marchon et al., 1992;Barbera Â et al., 1992;Bonar-Law et al., 2000;Rusjan et al., 2002). We have recently found that coordination polymers formed from dirhodium subunits linked by axial pyrazine ligands have interesting liquid-crystalline properties if the equatorial ligands are bulky carboxylates. We have suggested supramo-lecular models to explain the structural features of the mesophases formed (Rusjan et al., 2002).
Complex (I) is located on a crystallographic inversion centre. Each Rh atom is in a slightly distorted octahedral environment ( Fig. 1 , 1989]. The distances and angles in (I) are also in the normal range for analogous amine bis-adducts (Cotton & Walton, 1992;Bonar-Law et al., 2000;Kitamura et al., 2000).
The phenyl rings of the equatorial ligands are tilted by ca 10 with respect to the planes of their carboxylate groups. Moreover, the ethoxy substituents have different conformations in the two independent benzoate groups. In one of them, the 3-and 5-alkoxy chains are in the plane of the phenyl ring, in a zigzag trans conformation [C23ÐO23ÐC231ÐC232 and C25ÐO25ÐC251ÐC252 torsion angles of 169.1 (3) and 169.0 (2) , respectively], while the 4-alkoxy chain adopts a gauche conformation [C24ÐO24ÐC241ÐC242 torsion angle of 68.3 (3) ]. In the other benzoate ligand, the 3-and 5-alkoxy chains adopt gauche conformations [C13ÐO13ÐC131ÐC132 and C15ÐO15ÐC151ÐC152 torsion angles of 66.3 (3) and 73.6 (4) , respectively], pointing towards opposite sides of the phenyl-ring plane, while the 4-substituent has a trans conformation [C14ÐO14ÐC141ÐC142 torsion angle of 168.5 (2) ].
The pyrazine (Pz) rings have a torsion angle of ca 18.5 around the RhÐN31 bond with respect to the plane of the nearer carboxylate group, and they are non-coplanar with the RhÐRh axis [RhÐRhÐN31 and RhÐN31ÐN32 angles of 174.57 (6)  1991; Cotton et al., 1992;Miyasaka et al., 2001), but not seen for Viossat's pyrazine-derivative bis-adduct (Viossat et al., 1993). No comparisons can be made with the [Rh 2 -(TTB) 4 (C 5 H 5 N) 2 ] compound, where the axial pyridines are highly disordered (Callot et al., 1989). The deviation from linearity of the RhÐRhÐN31 angle is comparable with that found in other systems exhibiting hydrogen bonds (Kitamura et al., 2000) or H±% interactions (Bonar-Law et al., 2000). However, no evidence of such interactions has been found in (I). Nevertheless, it supports the hypothesis suggested for the mesophase of the long-chain polymeric analogues, where nonlinear Rh±Pz±Rh arrangements were proposed. The explanation of these features still remains open.

Experimental
The precursor compound, [Rh 2 {O 2 CC 6 H 2 (OC 2 H 5 ) 3 } 4 ], was synthesized from commercial [Rh 2 (O 2 CCH 3 ) 4 ] by a metathesis reaction in a twofold excess of the molten ligand for 4 h under an N 2 atmosphere. The compound was puri®ed by washing with hot methanol, and then by dissolution in CHCl 3 and addition of methanol until precipitation of the light-blue methanolic bis-adduct occurred. Finally, the precipitate was dried under vacuum at 353 K until a green solid was obtained, due to the release of the axial ligands. When an excess of pyrazine was added to a solution of the complex in acetone, the solution immediately turned red. Red crystals of (I) were obtained by slow evaporation of the solvent and were kept in contact with the mother liquor in order to prevent loss of pyrazine and subsequent degradation. This instability made other physicochemical assays dif®cult. All H atoms were located among the ®rst 45 peaks of a difference Fourier map, but were positioned stereochemically and re®ned with a riding model, with CÐH distances in the range 0.95±0.99 A Ê . Methyl H atoms were treated as rigid bodies and were allowed to rotate around the corresponding CÐC bond. The largest peak and trough of the ®nal difference map were located 1.23 and 0.94 A Ê , respectively, from the Rh atom.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: BK1651). Services for accessing these data are described at the back of the journal.  (6) Symmetry code: (i) 1 À xY 1 À yY 1 À z.