Dipodal, Tripodal, and Discoidal Coordination Modes of Kemp’s Triacid Anions
Résumé
In order to probe the conformational preferences of the anions of Kemp's triacid (cis,cis-1,3,5-trimethyl-1,3,5-cyclohexanetricarboxylic acid, H3kta) in their metal ion complexes, and their influence on the structures thereof, six complexes with widely different metal cations (alkali, alkaline-earth, 3d-block and actinide) were synthesized and crystallographically characterized. All these complexes crystallize as diperiodic assemblies, but with three different conformations of the kta$^{3-}$/Hkta$^{2-}$/H$_3$kta ligands. Only the tripodal form, with the cyclohexyl ring in chair conformation and the three carboxylic/ate groups axial is found in [Na$_5$(kta)(Hkta)(H$_2$O)], [Sr$_3$(H$_3$kta)(kta)$_2$], [Mn$_3$(kta)$_2$(H$_2$O)$_3$], and [Mn(H$_2$O)$_6$][Mn$_5$(kta)$_4$(H$_2$O)$_4$]. Particularly notable are hexanuclear ring subunits formed around two ligands in 2. The tripodal form coexists with the discoidal one, with the ring in the chair conformation and the three carboxylate groups equatorial, in [Mn$_6$(kta)$_4$(nmp)$_3$], with nmp = $N$-methyl-2-pyrrolidone, which displays hexanuclear ring subunits. Finally, the so-called dipodal form, with the ring in boat conformation, two carboxylate groups axial and one equatorial, is only found in [UO$_2$Cs(kta)], in which uranyl cations and ligands alone form a honeycomb assembly, the caesium cations being decorating species.
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