Abstract
Direct observation and structural characterization of a kinetic product and a thermodynamic product for complexes with an NO2S2 macrocycle (L) are reported. L reacts with copper(I) iodide to give a mononuclear complex [Cu(L)]2(Cu2I4)·2CH2Cl2 (1), featuring three separate units. When cadmium(II) iodide was reacted with L, an anion-coordinated complex [Cd(L)I]2(Cd2I6)·4CH3CN (2) with a needle-type crystal shape was formed as the kinetic product. Interestingly, when the needle-type kinetic product was left undisturbed in the mother solution it gradually transformed to the pseudo-dimer complex [Cd2(L)2I2](Cd2I6) (3) with a brick-type crystal shape as the thermodynamic product. The dissolution-recrystallization process resulted in the elimination of the lattice solvent molecules (aceto-nitrile) in 2 and the contraction of two neighboring macrocyclic complex units [Cd(L)I]+, forming the pseudo-dimer 3via an intermolecular Cd⋯I interaction between two monomers. For the entire process from kinetic to thermodynamic products, it was possible to obtain sequential photographic snapshots, single-crystal X-ray structures and powder X-ray diffraction patterns. For the copper(I) and cadmium(II) complexes, competitive NMR results agree with the solid-state data that show copper(I) has a higher affinity for L than does cadmium(II).
Highlights
Self-assembly of synthetic coordination processes affords thermodynamic products and kinetic products when the energy for the latter is trapped in local minima (Percec et al, 2011; Gammon et al, 2010; Hwang et al, 2004; Hasenknopf et al, 1998)
The reason for the two products is the difference in their activation energy: kinetic products form rapidly and they usually occur at lower temperature, while thermodynamic products form slowly or at higher temperatures (Fig. 1) (Martı-Rujas & Kawano, 2013; Martı-Rujas et al, 2011)
It is hard to recognize or separate these two products completely and structurally characterize them in the single-crystal state as pure forms due to the difficulty of growing single crystals, because fast precipitation so often leads to the kinetic product
Summary
Self-assembly of synthetic coordination processes affords thermodynamic products and kinetic products when the energy for the latter is trapped in local minima (Percec et al, 2011; Gammon et al, 2010; Hwang et al, 2004; Hasenknopf et al, 1998). The kinetic states in the self-assembly of coordination products play a crucial role in understanding the mechanism and final products as well as the fundamental aspects of functionalization (Percec et al, 2011; Gammon et al, 2010; Hwang et al, 2004; Hasenknopf et al, 1998; Martı-Rujas & Kawano, 2013; Martı-Rujas et al, 2011).
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