Funnel Complexes with CoII and NiII: New Probes into the Biomimetic Coordination Ability of the Calix[6]arene-Based Tris(imidazole) System

Electronic effects on polypyridyl Co complex-based water reduction catalysts

Three new isomeric cobalt complexes of TPA (tris(2-pyridylmethyl)amine) based on methoxy substitution at the ortho, meta and para positions, respectively, were constructed and their photocatalytic proton reduction efficiencies were compared. It was found that there are good linear correlations with the Hammett constants of the substituents for the computed Co–N bond lengths, redox potentials of Co^II/I and Co^I/0 events, and the photocatalytic activities of the complexes. The ortho-substituted Co complex distinguished itself from the others remarkably in all these comparisons, demonstrating the presence of a steric effect besides the electronic effect. For other examined complexes, a stronger electron-donating substituent may lead to a higher hydrogen evolution efficiency, suggesting that the formation of a Co(iii) hydride intermediate is the rate-limiting step.

Three isomeric Co complexes showed a significant substituent electronic effect in photocatalytic hydrogen production.

Electrocatalytic Proton Reduction by a Cobalt(III) Hydride Complex with Phosphinopyridine PN Ligands

Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [Co^II(PN)₂(MeCN)][BF₄]₂ 1, [Co^III(PN)₂(H)(MeCN)][PF₆]₂ 2, and [Co^III(PN)₂(H)(Cl)][PF₆] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. -1.4 V vs E(Fc⁺/Fc) in the presence of TFA yields H₂ with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.

Synthesis, Structure, and Characterization of Tris(1-ethyl-4-isopropyl-imidazolyl-κN)phosphine Nickel(II) Complexes

In this work we report the synthesis of five new nickel(II) complexes all coordinated to the tripodal ligand tris(1-ethyl-4- i Pr-imidazolyl)phosphine (TlEt4iPrIP). They are [Ni(T1Et4iPrIP)(CH3CN)2(OTf)](OTf) (1), [Ni(T1Et4iPrIP)(OTf)2] (2), [Ni(T1Et4iPrIP)(H2O)(OTf)](OTf) (3), [Ni(T1Et4iPrIP)Cl](OTf) (4), and [Ni(T1Et4iPrIP)Cl2] (5). The complexes serve as bioinorganic structural model complexes for histidine-coordinated nickel proteins. The X-ray structures have been determine for all complexes which feature coordination numbers 4-6. We investigated the spectroscopic interconversions for these compound in dichloromethane solution and demonstrate interconversion between 1-3 and conversion of 2 to 4. Complex 5 can be spectroscopically converted to the cation of 4 by dissolving it in dichloromethane. Fits of variable temperature magnetic susceptibility data yielded the following parameters: g = 1.944, D = -0.327 cm-1, E/D = 3.706 for 1; g = 2.280, D = -0.365 cm-1, E/D = 22.178 for 2; g = 2.000, D = -7.402 cm-1, E/D = -0.272 for 3; g = 2.176, D = -0.128 cm-1, E/D = -0.783 for 4; g = 2.258, D = 14.288 cm-1, E/D = 0.095 for 5. DFT structure optimizations afforded HOMO and LUMO energies indicating that complex 1 is the most stable.

Fluorescent phenanthridine-based calix[4]arene derivatives: synthesis and thermodynamic and computational studies of their complexation with alkali-metal cations

The novel calix[4]arenes exhibiting prominent fluorescence were shown to be potential sensitive fluorimetric cation sensors. Comprehensive experimental and computational studies provided detailed insight into the corresponding complexation reactions.

Coordination of lead(II) in the supramolecular environment provided by a "two-story" calix[6]arene-based N6 ligand

First insights into the coordination properties and host-guest behavior of a "two story" calix[6]aza-cryptand (1) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]arene small rim. The resulting N6 donor site coordinates Pb(II) ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb(II) with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth (1)H NMR studies confirm the host-guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and N-Me-formamide. Hence, in spite of the presence of a N6 donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb(II) to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described "one story" calix[6]aza-cryptands, ligand 1 displays several similar but also new features that are discussed.

Calorimetric study on coordination of tridentate imidazolyl calix[6]arene ligands to zinc ion in organic solvents

Complexation of three kinds of tris(imidazolyl)calix[6]arenes containing alternate p-substituents (Calix-tBu, R(1) = R(2) = tert-butyl; Calix-NH(2), R(1) = tert-butyl, R(2) = NH(2); Calix-NO(2), R(1) = tert-butyl, R(2) = NO(2)) with Zn(ClO(4))(2)(H(2)O)(6) in acetonitrile, methanol, and THF was investigated via isothermal titration calorimetry (ITC). For the coordination of these calixarene ligands to Zn(II) in acetonitrile, typical one-phase exothermic titration curves were obtained, indicating the formation of 1:1 ligand-Zn(II) complexes accompanied by large conformational changes of the ligands. In contrast, the complexation in methanol was endothermic and dominated by favorable entropy changes. The entropy gains were achieved by extensive desolvation from both Zn(II) and the ligands. ITC measurements suggest a 2:1 ligand-Zn(II) complex formation in THF in the presence of excess ligands (Calix-NH(2) and Calix-NO(2)). The 2:1 complexes were converted to 1:1 complexes upon further addition of Zn(ClO(4))(2)(H(2)O)(6). The results indicate the important role of a coordinating solvent (acetonitrile) for direct formation of the 1:1 complexes under the conditions of excess ligand. Complexation of a ditopic ligand (Calix-Tri) with three triazole moieties on the wider rim was also studied via ITC. The first coordination of the imidazole moieties to Zn(II) was an exothermic process. This was followed by the entropically favorable coordination of the triazole moieties to the divalent cation. We have also investigated exchange of the fourth ligand (H(2)O) of the Zn(II) complex of Calix-NH(2) with butylamine, heptylamine, acetonitrile, and acetamide in a noncompetitive solvent, THF. The ΔH(0) tended to decrease upon increasing the electron-pair-donating ability of the guest ligand, whereas it was also affected by an entropic term due to restricted rotation of the guest ligand inside the calixarene cavity.

Electrochemical behavior of the tris(pyridine)-Cu funnel complexes: an overall induced-fit process involving an entatic state through a supramolecular stress

The electrochemical behavior of the tris(pyridine) calix[6]arene Cu adducts is unique as compared to that of most classical Cu complexes in a strain-free environment. The presence of MeCN buried inside the cavity is a prerequisite for a quasi-reversible behavior in a dynamic mode. The CV behavior assisted by simulation outlines that the coordination adaptability of the Cu(II)/Cu(I) redox states is completely reversed, with a Td geometry enforced at either redox states. Hence, the supramolecular control of the Cu coordination by a protein-like pocket determines the dynamics of the electron transfer process, its thermodynamics, and the kinetics of the reorganizational barrier and generates a preorganized state for oxidation. This redox behavior corresponds to an overall induced-fit process generating a truly entatic highly oxidizing Cu(II) state through a protein-like strain by involvement of the secondary coordination sphere.