Coordination Chemistry of Azacryptands

Argentophilic Interactions, Flexibility, and Dynamics of Pyrrole Cages Encapsulating Silver(I) Clusters

Recently, pyrrole cages have been synthesized that encapsulate ion pairs and silver(I) clusters to form intricate supramolecular capsules. We report here a computational analysis of these structures using density functional theory combined with a semiempirical tight-binding approach. We find that for neutral pyrrole cages, the Gibbs free energies of formation provide reliable predictions for the ratio of bound ions. For charged pyrrole cages, we find strong argentophilic interactions between Ag ions on the basis of the calculated bond indices and molecular orbitals. For the cage with the Ag4 cluster, we find two minimum-geometry conformations that differ by only 6.5 kcal/mol, with an energy barrier <1 kcal/mol, suggesting a very flexible structure as indicated by molecular dynamics. The predicted energies of formation of [Agn⊂1]n-3+ (n = 1-5) cryptands provide low energy barriers of formation of 5-20 kcal/mol for all cases, which is consistent with the experimental data. Furthermore, we also examined the structural variability of mixed-valence silver clusters to test whether additional geometrical conformations inside the organic cage are thermodynamically accessible. In this context, we show that the time-dependent density functional theory UV-vis spectra may potentially serve as a diagnostic probe to characterize mixed-valence and geometrical configurations of silver clusters encapsulated into cryptands.

Fully Delocalized Mixed-Valent Cu1.5 Cu1.5 Complex: Strong Cu-Cu interaction and Fast Electron Self-Exchange Rate Despite Large Structural Changes

A flexible macrocyclic ligand with two tridentate {CNC} compartments can host two Cu ions in reversibly interconvertible states, Cu^I Cu^I (1) and mixed-valent Cu^1.5 Cu^1.5 (2). They were characterized by XRD and multiple spectroscopic methods, including EPR, UV/Vis absorption and MCD, in combination with TD-DFT and CASSCF calculations. 2 features a short Cu⋅⋅⋅Cu distance (≈2.5 Å; compared to ≈4.0 Å in 1) and a very high delocalization energy of 13 000 cm^-1 , comparable to the mixed-valent state of the biological Cu_A site. Electron self-exchange between 1 and 2 is rapid despite large structural reorganization, and is proposed to proceed via a sequential mechanism involving an active conformer of 1, viz. 1'; the latter has been characterized by XRD. Such electron transfer (ET) process is reminiscent of the conformationally gated ET proposed for biological systems. This redox couple is a unique pair of flexible dicopper complexes, achieving fast electron self-exchange closely related to the function of the Cu_A site.

Study on the Photoluminescent and Thermal Properties of Zinc Complexes with a N₆O₄ Macrocyclic Ligand

Reactions between a N₆O₄ macrocyclic ligand (L¹) and several Zn(II) salts (trifluoromethane sulfonate, p-toluenesulfonate, acetate, benzoate, o-, m- or p-hydroxybenzoate) led to the formation of seven complexes, [Zn₂L¹ (DMSO)₄](OSO₂CF₃)₄ (1), [Zn₂(p-OSO₂PhCH₃)₄L¹] (2), [Zn₂(OCOCH₃)₄L¹] (3), [Zn₂(OCOPh)₄L¹] (4), [Zn₂(o-OCOPhOH)₄L¹] (5), [Zn₂(m-OCOPhOH)₄ L¹] (6) and [Zn₂(p-OCOPhOH)₄ L¹] (7), which were characterized by elemental analysis, ¹H-NMR, ¹³C-NMR, IR, fluorescence spectroscopies and single crystal X-ray diffraction. In 1, the Zn atom is pentacoordinated with a N₃O₂ irregular trigonal bipyramidal coordination environment, like the geometries in compounds 3–7, whereas in structure 2 the metal atom is envisaged as possessing a distorted N₃O₃ octahedronal environment. All the compounds show interesting photoluminescent properties in solid states and solutions in DMF and DMSO, which are reported along with their TG-DTA thermal decomposition processes, UV-vis absorption spectroscopy and fluorescence quantum yields in DMF and DMSO.

Improved Access to 1,8-Diformyl-carbazoles Leads to Metal-Free Carbazole-Based [2 + 2] Schiff Base Macrocycles with Strong Turn-On Fluorescence Sensing of Zinc(II) Ions

Development of a new and high yielding synthetic route to 1,8-diformyl-carbazoles 3 (3a 3,6-di- tert-butyl substituted; 3b 3,6-unsubstituted) is reported. Use of a Heck coupling reaction, followed by ozonolysis, has greatly facilitated the preparation of these interesting head units in useful quantities. An initial foray into the new generations of Schiff base macrocycles that ready access to these head units (3) opens up, has led to the direct (i.e., metal-free) synthesis of two [2 + 2] macrocycles from 3a or 3b with 1,2-diaminoethane, H₂L^tBu (4a) and H₂L^H (4b), respectively, obtained as yellow powders in high yields (87-88%). The dizinc complex [Zn₂L^H(OAc)₂] (5b) was isolated as a bright yellow solid in 83% yield, by 1:2:2 reaction of H₂L^H with zinc(II) acetate and triethylamine. Aldehydes 3a and 3b, macrocycle H₂L^H, and complex [Zn₂L^H(OAc)₂] (5b) have been structurally characterized. The carbazole NH makes bifurcated hydrogen bonds with the pair of flanking 1,8-diformyl-moieties in 3, or 1,8-diimine-moieties in H₂L^H, leading to a flat, all- cis conformation. The stepped conformation of the metal-free macrocycle H₂L^H is retained in [Zn₂L^H(OAc)₂], despite deprotonation and binding of two zinc(II) centers within the two tridentate pockets. The N₃O₂ coordination of the zinc ions is completed by one μ_1,1- and one μ_1,3- bridging acetate anion. Excitation of nanomolar [Zn₂L^H(OAc)₂] in DMF at 335 nm results in clearly visible blue fluorescence (λ_max = 460 nm). Further studies on the H₂L^H macrocycle revealed turn-on fluorescence, with selectivity (over Ca²⁺, Mg²⁺ and a range of 3d dications) and nanomolar sensitivity for zinc(II) ions, highlighting one of the many potential applications for these new carbazole-based Schiff base macrocycles.

Steric complementarity in carboxylate anion recognition

A 2,6 pyridino-linked aminocryptand acts as a good host for carboxylate anions, oxalate, malonate, acetate and lactate at low protonation levels as shown by aqueous complexation constants. With oxalate, at the hexaprotonated level of the host, the crystal structure shows formation of a dimeric unit linked by an oxalate/semioxalate short (2.474 A) H-bond. The protonated cryptand uses a cleft-binding mode in preference to the cavity-binding mode used by other aminocryptand hosts.

Coordination features of ditopic oxa-azamacrocycles toward Ni(ii) and Co(ii). Dioxygen uptake by their dinuclear Co(ii) complexes

The coordination properties of the ditopic oxa-aza macrocycles L1-L3 toward Ni(II) and Co(II) have been investigated by means of potentiometric and UV-vis spectrophotometric measurements. L1-L3 contain two triamine and/or tetraamine chains separated by two dioxa chains and form both mono- and dinuclear complexes in aqueous solution. In the [ML]2+ complexes, the metal ion is coordinated by one of the two polyamine moieties, while the other does not participate in the coordination. In the dinuclear complexes each metal ion is coordinated, almost independently, to a single polyamine moiety. Under aerobic conditions the binuclear Co(II) complexes of the ligands L1-L3 are able to bind molecular oxygen, with a bridging coordination of O2 between the two metals.