Complexation, Computational, Magnetic, and Structural Studies of the Maillard Reaction Product Isomaltol Including Investigation of an Uncommon π Interaction with Copper(II)

Abstract

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL(2) are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L(2))(2)]·8H(2)O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato)copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions.

Shedding Light on Intermolecular Metal–Organic Ring Interactions by Theoretical Studies

“…All compounds appear to be of particular intrinsic interest by forming almost planar five-and sixmembered chelated rings which are stacked similarly to p-p aromatic systems, in the absence of any aromatic ring. Very recently, [16] a similar pattern of interactions was found to be pres-ent in bis(isomaltolato) Cu II complexes combining hydrogen bonding and dispersion forces that dominate the interaction in the p system of the furan moiety to chelated Cu II ion. Interestingly, in the (malonamide-N,N'-dicarboxylato) Cu II compounds reported here, according to the EDA analysis, the main contributions to the intermolecular MORi supramolecular arrangement arise from dispersion, electrostatic interactions, and hydrogen bonds.…”

“…Several interactions between chelated and aromatic rings have been reported; [11][12][13][14][15][16] Lindoy et al have recently demonstrated that such kind of stacking interactions can be concluded to be primarily dispersive in nature. [16] Besides, evidence of chelate-chelate stacking interactions was obtained by analyzing crystal structures of square-planar transition-metal complexes from the Cambridge Structural Database by Zarić et al [17] . Meanwhile, theoretical calculations from the same group show that none of the complexes that contain acetylacetonate (acac) chelate rings satisfy the magnetic criterion for aromaticity.…”

“…In addition, we have tried to authenticate the nature of this interaction through theoretical investigations that have been performed for 1 and 2 as well for a previously reported compound formulated as [Ca 4 (CuL) 4 (H 2 O) 16 ] 6H 2 O (3·6H 2 O). [19]…”

A mixed valent heterometallic CuII/NaI coordination polymer with sodium–phenyl bonds

“…The copper-π distances of between 3.1 and 3.5 Å are similar to those observed in a series of related complexes. 26 Adjacent polymeric chains interact with their neighbours through a series of hydrogen bonds (Fig. 3, Table S1 †).…”

Unusual Absence of Head-to-Tail Chains in the Crystal Structure of Glycyl-l-glutamyl-l-phosphoseryl-l-leucine

“…All quoted equilibrium constants are concentration constants. A pK w value of 13.78 was used for the calculations, and all determinations were obtained under identical conditions [25]. During the titrations, NaOH (0.2 M) was added incrementally to 4.5 ml of a solution of HNO 3 (15 mM) and the tetrapeptide (3 mM).…”