Amphiphilic five-coordinate iron(III) complexes with {N₂O₂Cl} and {N₂O₃} coordination spheres are studied to elucidate the roles of electronic structure on the mechanisms for current rectification. The presence of an apical chlorido or phenolato ligand plays a crucial role, and the [Fe^III{N₂O₂Cl}] species supports an asymmetric mechanism while its [Fe^III{N₂O₃}] counterpart seems to allow for unimolecular mechanism. The effects of electron-donating and electron-withdrawing substituents in the ligand frameworks are also considered.

A new pentadentate oxime has been designed to drive the preferential coordination favored by Co^I in catalysts used for proton/water reduction. The ligand incorporates water upon metal coordination and is water soluble. This Co^III species is doubly reduced to Co^I and exhibits H⁺ reduction activity in the presence of weak acids in MeCN and evolves H₂ upon protonation suggesting that the ligand design increases catalyst effectiveness. Superior catalysis is observed in water with a turnover number (TON) of 5700 over 18 h. However, the catalyst yields Co-based nanoparticles, indicating that the solvent media may dictate the nature of the catalyst.

A new pentadentate oxime has been designed to drive the preferential coordination favored by Co^I in catalysts used for proton/water reduction. The ligand incorporates water upon metal coordination and is water soluble. This Co^III species is doubly reduced to Co^I and exhibits H⁺ reduction activity in the presence of weak acids in MeCN and evolves H₂ upon protonation suggesting that the ligand design increases catalyst effectiveness. Superior catalysis is observed in water with a turnover number (TON) of 5700 over 18 h. However, the catalyst yields Co-based nanoparticles, indicating that the solvent media may dictate the nature of the catalyst.

A series of cobalt complexes with pentadentate pyridine-rich ligands is studied. An initial Co^II amine complex 1 is prone to aerial oxidation yielding a Co^III imine complex 2 that is further converted into an amide complex 4 in presence of adventitious water. Introduction of an N-methyl protecting group to the ligand inhibits this oxidation and gives rise to the Co^II species 5. Both the Co^III 4 and Co^II 5 show electrocatalytic H₂ generation in weakly acidic media as well as in water. Mechanisms of catalysis seem to involve the protonation of a Co^IIH species generated in situ.

A series of cobalt complexes with pentadentate pyridine-rich ligands is studied. An initial Co^II amine complex 1 is prone to aerial oxidation yielding a Co^III imine complex 2 that is further converted into an amide complex 4 in presence of adventitious water. Introduction of an N-methyl protecting group to the ligand inhibits this oxidation and gives rise to the Co^II species 5. Both the Co^III 4 and Co^II 5 show electrocatalytic H₂ generation in weakly acidic media as well as in water. Mechanisms of catalysis seem to involve the protonation of a Co^IIH species generated in situ.

Langmuir–Blodgett films of metallosurfactants were used in Au|molecule|Au devices to investigate the mechanisms of current rectification.

Langmuir–Blodgett films of metallosurfactants were used in Au|molecule|Au devices to investigate the mechanisms of current rectification.

The redox-active ligand N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phenylenediamine (H₄L), known for supporting catalytic processes with both late and early transition metals, unexpectedly undergoes an unusual and thus far unobserved cyclization. Upon equimolar treatment of a cobalt(II) salt in the presence of Et₃N under aerobic conditions, a ligand rearrangement occurs to afford an unprecedented phenoxazinyl radical moiety (L′). The resulting bisligated Co complex, thoroughly characterized structurally and electronically and analyzed by multiple experimental and computational approaches, is presented. It is shown to traverse oxidation states from 2+ to 3–, spanning a thermodynamic window of approximately 2 V.

Three six-coordinate cobalt(III) complexes containing electron-rich phenolato pentadentate [N₂O₅] ligands were synthesized and characterized, namely, [Co^III(L¹)(MeOH)] (1), [Co^III(L²)(MeOH)] (2) and [Co^III(L³)(MeOH)] (3), where L¹, L² and L³ are the triply deprotonated, triply negative form of(E)-6,6′-[({2-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol), (E)-6,6′-[({3-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]naphthalen-2-yl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol) and (E)-6,6′-[({2-[(2-hydroxy-3-methoxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol), respectively. Crystal structures were obtained for 1–3 and reveal a hexacoordinate cobalt(III) ion bound to the [N₂O₃] donors of each ligand and a methanol molecule occupying the sixth position. The complexes exhibited comparable electronic behavior dominated by phenolatecobalt charge transfer processes and four redox-accessible states involving three distinct phenolato/phenoxyl radical couples and a fourth process associated with the Co^II/Co^III couple. The redox processes were cycled 30 times without major decomposition at the surface of the electrode for 1 and 2, indicating that the oxidized species should be substitutionally inert and do not degrade significantly upon cycling. Electronic-structure DFT calculations on models 1′ and 2′ favor the generation of localized phenoxyl radicals and suggest distinctive oxidation sequences associated to the nature of the ligands.

The metallosurfactants [Ni^II(L^tBuODA)(OAc)] (1), [Ni^II(L^tBuODA)₂](2), [Cu^II(HL^tBuODA)(L^tBuODA)]ClO₄·CH₃OH (3), and [Zn^II(HL^tBuODA)₂](ClO₄)₂ (4) {where HL^tBuODA = 2,4-di-tert-butyl-6-[(octadecyl(pyridin-2-ylmethyl)amino)methyl]phenol}were synthesized and characterized in an attempt to establish how coordination modes and protonation preferences relate to amphiphilic behavior. The archetypical compounds [Ni^II(L^tBuI)(OAc)]·CH₃OH (5), [Ni^II(L^A)₂]·CH₃OH·H₂O (6), [Ni^II(L^tBuA)₂]·2CH₃OH (7), [Cu^II(HL^tBuA)(L^tBuA)]ClO₄ (8), and [Zn^II(HL^tBuA)(L^tBuA)]ClO₄ (9) were synthesized to model the stoichiometric, coordination, and protonation chemistry in the waxy metallosurfactants 1–4. Detailed data analysis and comparison between 1–4 and 5–9 involved mass spectrometric and spectroscopic methods along with crystallographic determination of 5 (P2₁/c), 6 (P), 7 (P2₁/c), 8′ (the analogue of 8 with tetraphenylborate counterions, P2₁/c), and 9 (P). DFT calculations were used to identify the frontier orbitals, polarizability, and dipole moments. Species 1–4 had their compression isotherms measured and monitored by means of Brewster angle miscroscopy. The nickel-containing 1 is square planar, while 2 has a neutral octahedral core with two deprotonated ligands. The five-coordinate copper-containing 3 has a monocationic core associated with one protonated ligand, whereas the dicationic zinc-containing 4 has a four-coordinate core with protonated ligands. It was observed that Langmuir films of 1 display approximately half of the average molecular area observed for 2–4 and that the flexibility and coordination number of the cores foster distinctive collapse mechanisms. Therefore, careful choice of the metal ion leads to control of surfactant-to-metal ratio, selection of coordination modes and structural properties, and the understanding of the protonation preferences of the ligands. This information will play an important role in the development of metal-containing responsive films. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture shows an artistic rendition of the main chemical motifs in the paper, namely, a metalloamphiphile, compression isotherms, and Brewster angle micrographs, along with model complexes with distinctive protonation states. Details are discussed in the article by C. N. Verani et al. on p. 345 ff.

The metalloamphiphiles [(L^PM18)₂Cu₂Cl₂]·H₂O (1), [(L^PMtax)₂Cu₂Cl₂]·2MeOH (2), [(L^OF18)₂Cu₂Cl₂] (3), [(L^OF18)₂Cu₄(μ₄-O)(μ₂-OAc)₄] (4), [(L^OF14)₂Cu₄(μ₄-O)(μ₂-OAc)₄] (5) were synthesized and characterized by means of several spectroscopic and spectrometric methods. The acronyms L^PMC18, L^PMtax, L^OF18, and L^OF14 indicate the deprotonated forms of pseudomacrocyclic (PM) and open-frame (OF) ligands based on 2,6-bis(iminomethyl)-4-methylphenol attached to octadecyl (18), trialkoxyamine (tax) and tetradecyl (14) moieties, whereas Cl^– and OAc^– indicate chlorido and acetato coligands. Crystal structures were obtained for 4 and 5 that crystallize in triclinic crystal systems with P space groups. The average Cu–O_phenolate and Cu–N_imine bond lengths are approximately 2.0 Å. Species 1–5 were analyzed by means of isothermal compression and Brewster angle microscopy (BAM). Moderate collapse pressures of approximately 15 mN m^–1 were observed for 3, 4, and 5, while 1 collapses at approximately 30 mN m^–1. Species 1, 3, 4, and 5 are responsive to several subphase changes such as temperature and addition of terephthalate ions, which leads to considerable Langmuir film enhancement. In presence of terephthalate the collapse pressure for 3, 4, and 5 nearly doubles, whereas a minimal effect was observed for 1. It appears that as temperature decreases, collapse pressure increases. The variation of pH produced inconclusive data. It is suggested that terephthalate ion interacts with the amphiphiles at a molecular level to generate discrete molecules rather than extended systems.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)