Rational design of carbonitrile-carboxaldehyde cation receptor models: probing the nature of the heteroatom-metal interaction

Rational Design of a Fluorescent Chromophore as a Calcium Receptor via DFT and Multivariate Approaches

Computational and experimental approaches were adopted to utilize a chromophore diglycolic functionalized fluorescein derivative as a Ca²⁺ receptor. Fluorescein diglycolic acid (Fl-DGA, 1) was synthesized and used in multivariate determination of Ca²⁺ and K⁺. Full-structure computation shows that the complexes of 1 and Ca²⁺ have comparable energies regardless of additional interaction with lactone moiety. The initial formation of diglycolic-Ca²⁺ complex followed by macrocyclization is thermodynamically disfavored. A U-shaped pre-organized 1 allows Ca²⁺ to interact simultaneously with diglycolic and lactone motifs. Both motifs actively participate in Ca²⁺ recognition and the eleven methylene units in the undecyl arm provides excellent flexibility for reorganization and optimum interaction. Principal component analysis (PCA) of computational molecular properties reveals a simple method in evaluating motifs for cation recognition. Fragment models support full-structure results that negative charge causes significant structural changes, but do not reproduce the full extent of C-O bond breaking observed in the latter. Experimental optical responses show that 1 is selective towards Ca²⁺ and discriminates against K⁺ and Mg²⁺. PCA of emission intensities affords distinct clusters of 0.01, 0.1 and 1 mM Ca²⁺ and K⁺, and suggests applicability of this technique for simultaneous determination of cationic plant macronutrients in precision agriculture and a wide variety of other applications.

Computational evaluation of unsaturated carbonitriles as neutral receptor model for beryllium(II) recognition

Design of neutral receptor molecules (ionophores) for beryllium(II) using unsaturated carbonitrile models has been carried out via density functional theory, G3, and G4 calculations. The first part of this work focuses on gas phase binding energies between beryllium(II) and 2-cyano butadiene (2-CN BD), 3-cyano propene (3-CN P), and simpler models with two separate fragments; acrylonitrile and ethylene. Interactions between beryllium(II) and cyano nitrogen and terminal olefin in the models have been examined in terms of geometrical changes, distribution of charge over the entire π-system, and rehybridization of vinyl carbon orbitals. NMR shieldings and vibrational frequencies probed charge centers and strength of interactions. The six-membered cyclic complexes have planar structures with the rehybridized carbon slightly out of plane (16° in 2-CN BD). G3 results show that in 2-CN BD complex participation of vinyl carbon further stabilizes the cyclic adduct by 16.3 kcal mol(-1), whereas, in simpler models, interaction between beryllium(II) and acetonitrile is favorable by 46.4 kcal mol(-1) compared with that of ethylene. The terminal vinyl carbon in 2-CN BD rehybridizes to sp (3) with an increase of 7 % of s character to allow interaction with beryllium(II). G4 calculations show that the Be(II) and 2-CN BD complex is more strongly bound than those with Mg(II) and Ca(II) by 98.5 and 139.2 kcal mol(-1) (-1), respectively. QST2 method shows that the cyclic and acyclic forms of Be(II)-2-CN BD complexes are separated by 12.3 kcal mol(-1) barrier height. Overlap population analysis reveals that Ca(II) can be discriminated based on its tendency to form ionic interaction with the receptor models.