Zwitterion coordination to hard cations: Synthesis and characterization of betaine coordination compounds with divalent Mg, Ca, Sr and Ba and of trivalent Y, La and Pr

Abstract

We report the synthesis and structure of coordination compounds in which betaine acts as a ligand towards Pearson-hard cations. The overall neutral zwitterion was reacted with the corresponding nitrate salts in polar protic solvents such as water or methanol or mechanochemically by grinding the reaction partners. Reaction of MgII, CaII, SrII and BaII resulted in the first alkaline earth derivatives of betaine. The products comprise mono- and oligonuclear cations for the light, and cationic chain polymers for the heavy group2 metals. In the case of CaII, the betaine stoichiometry was decisive for the outcome of the reaction; solids with 1:4 and the less intuitive 3:16 cation to ligand ratio have been structurally characterized. Coordination numbers increase from six for MgII and CaII over seven and eight for the two independent SrII to eight for BaII cations. Coordination of betaine to trivalent Y, La and Pr results in the formation of structurally related dinuclear cations, with coordination numbers eight for Y and nine for the larger lanthanides. Water and betaine compete for coordination; aqua ligands prevail for the smaller, betaine for the larger cations. Betaine coordination to all these hard cations occurs in various modes, similar to those of other carboxylates. Bridging betaine subtends cation⋯cation distances between 4.5 and 6.1 Å. Nitrate coordination was only observed in the BaII chain polymer.