Structure investigation of dimethyl-aluminium, -gallium and -indium O,OЈ-chelate complexes in solution and the solid state. Molecular structure of five-co-ordinated [Me 2 M( -OC 6 H 4 CO 2 Me-2)] 2 adducts

The reaction of trimethyl-aluminium, -gallium and -indium with an equimolar amount of methyl salicylate, 2-(HO)C 6 H 4 CO 2 Me (Hmesal), gives [Me 2 M(mesal)] n chelate complexes in high yield (where n = 1 or 2, M = Al 1 or Ga 2; n = 2, M = In 3). The resulting compounds have been characterised in solution by NMR and IR spectroscopy as well as by cryoscopic molecular weight determinations in benzene; their molecular structures in the solid state were determined by single-crystal X-ray diffraction techniques. All three compounds are dimeric in the solid state, with five-co-ordinated metal centres. The deformation of the metal centre co-ordination sphere is discussed. In solution compounds 1 and 2 are monomeric, four-co-ordinated chelate complexes while 3 retains its dimeric structure. On the basis of the structural observation resulting from this study and with respect to related compounds the following sequence of the Group 13 metal centre Lewis acidity, in four-co-ordinated diorganometallic O,OЈ-chelate complexes is proposed: In у Al > Ga. The past decade has brought a considerable increase in the number of examples of simple neutral monomeric five-and six-co-ordinated organometallic complexes of the Group 13 metals. 1, 2 The results indicate that the formation of compounds with higher co-ordination numbers is not limited to those containing macrocyclic ligands, and Group 13 organometallic chemistry has considerably broadened beyond the sphere of four-co-ordinated compounds that dominated previously. A large group of [R 2 M(O,X)] 2 -type chelate complexes (M = Al, Ga or In, and O,X = O, OЈ-or O,N-bidentate ligand), where diorganometallic alkoxides form oxygen-bridged dimers consisting of five-co-ordinated metal centres, are known. 3,4 These Group 13 organometallic chelate complexes potentially create interesting possibilities for comparative studies of the reactivity of four-and five-co-ordinated compounds. In a previous paper we discussed the influence of O,OЈ-bidentate ligands on the structure of dialkylaluminium O,OЈ-chelate complexes in solution and in the solid state. 4 Although this group of compounds has a tendency to form [R 2 Al(O,O)] 2 -type adducts with five-coordinated metal centres in the solid state, in solution they exhibit a considerably greater structural variety depending on the nature of the chelating ligand. For instance, we have elucidated the electronic factors determining the rearrangement from the five-co-ordinated adduct to the four-co-ordinated monomeric chelate complex upon dissolution (Scheme 1). 4 Such a rearrangement occurs for compounds with unsaturated O,OЈ-bidentate ligands, where the π interaction of the alkoxide oxygen lone electron pair with the chelate ligand unsaturated bond system considerably weakens the Lewis basicity of the bridging oxygen and simultaneously strengthens the basicity of the chelating oxygen owing to π delocalization. The role played by the nature of the bidentate ligand in related gallium and indium derivatives is less clear, because there are very few examples of structurally well characterised dialkylgallium O,Ochelate complexes 3l-3r and only two examples of related indium compounds. 3s,t In view of this, it is of interest to know whether the factors affecting the structure of dialkylaluminium chelate complexes can be extended to gallium and indium derivatives. In this paper we present the structural investigation of the † E-Mail: [email protected] Group 13 metal (M = Al, Ga or In) diorganometallic chelate complexes with the methyl salicylate anion (an unsaturated O,OЈ-bidentate ligand with a π conjugated system) in solution and in the solid state. Results and Discussion The interaction of Me 3 M (M = Al, Ga or In) with an equimolar amount of methyl salicylate (Hmesal) results in the quantitative evolution of methane and formation of diorganometallic O,OЈ-chelate complexes according to equation (1). We reported previously that the addition of 2 equivalents of Hmesal to Me 3 Al yielded a monomeric five-co-ordinated chelate complex MeAl(mesal) 2 . 2b It should be noted that when similar reactions were attempted under identical conditions with trimethylgallium and trimethylindium, only the products corresponding to a 1 : 1 stoichiometry were obtained. Compounds 1, 2 and 3 are stable as solids or in solution under an inert atmosphere. The resulting compounds have been characterised in solution by NMR and IR spectroscopy and by cryoscopic molecular weight measurements. The molecular structures of 1, 2 and 3 have been determined by X-ray crystallography; selected bond lengths and angles are given in Structure of [Me 2 Al(mesal)] n 1 Th