Probing the effect of nitro-substituents in the modulation of LUMO energies for directional electron transport through 4d6 ruthenium(II)-based metallosurfactants.

Electron-withdrawing nitro-substituents were installed onto terpyridine- and phenanthroline-based metallosurfactants with 4d6 ruthenium(II), which were deposited as Langmuir-Blodgett monolayers aiming to study the feasibility of charge transport in Au|LB|Au junctions. The nitro groups are intended to modulate the energy of the frontier molecular orbitals to near to, or match that of Fermi levels in the gold electrodes. A series of heteroleptic metallosurfactants [RuII(C18OPh-terpy)(X-terpy)](PF6)2 and [RuII(C18OPh-terpy)(X-phen)Cl]PF6 were synthesized, where C18OPh-terpy is the 4'-[4-(octadecyloxy)phenyl]-2,2':6',2''-terpyridine amphiphile common to all species, X-terpy is a terpyridine with-H (1) or-phenyl-NO2 (2) and X-phen is a phenanthroline with-H (3) or-NO2 (4) groups. These metallosurfactants were characterized by experimental and computational methods, and the presence of nitro groups affect more affordable reductions at less negative potentials, as well as slightly more positive oxidations, these changes are less pronounced in species 2 than in 4. Species 1 and 2 showed limited Pockels-Langmuir and Langmuir-Blodgett film formation with lower collapse pressure of 27 mN m-1. In contrast, metallosurfactants 3 and 4 showed enhanced hydrophilicity indicated by higher collapse pressures of ca. 36 mN m-1. The LB monolayers of 3 and 4 were deposited on gold electrodes to form Au|LB|Au junctions and electron transport was measured as I/V curves. The NO2-bearing species 4 showed asymmetric curves associated with directional electron transport with amplitudes up to -2.0 nA and rectification ratios from 5 to 26 between -1 to +1 V and from 3 to 14 between -3 to +3 V.

A bioinspired cobalt catalyst based on a tripodal imidazole/pyridine platform capable of water reduction and oxidation.

The prototypical drug carrier [CoII(L1)Cl]PF6 (1), where L1 is a tripodal amine bound to pyridine and methyl-imidazoles, had its electrocatalytic water splitting activity studied under different pH conditions. This species contains a high-spin 3d7 CoII metal center, and is capable of generating both H2 from water reduction and O2 from water oxidation. Turnover numbers reach 390 after 3 h for water reduction. Initial water oxidation activity is molecular, with TONs of 71 at pH 7 and 103 at pH 11.5. The results reveal that species 1 can undergo several redox transformations, including reduction to the 3d8 CoI species that precedes a LS3d6 hydride for water reduction, as well as nominal CoIVO and CoIII-OOH species required for water oxidation. Post-catalytic analyses confirm the molecular nature of reduction and support initial molecular activity for oxidation.

Bioinspired oxidation of benzyl alcohol: The role of environment and nuclearity of the catalyst evaluated by multivariate analysis.

Inspired by copper-containing enzymes such as galactose oxidase and catechol oxidase, in which distinct coordination environments and nuclearities lead to specific catalytic activities, we summarize here the catalytic properties of dinuclear and mononuclear copper species towards benzyl alcohol oxidation using a multivariate statistical approach. The new dinuclear [Cu2(µ-L1)(µ-pz)]2+ (1) is compared against the mononuclear [CuL2Cl] (2), where (L1)- and (L2)- are the respective deprotonated forms of 2,6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol, and 3-((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzaldehyde and (pz)- is a pyrazolato bridge. Copper(II) perchlorate (CP) is used as control. The catalytic oxidation of benzyl alcohol is pursued, aiming to assess the role of the ligand environment and nuclearity. The multivariate statistical approach allows for the search of optimal catalytic conditions, considering variables such as catalyst load, hydrogen peroxide load, and time. Species 1, 2 and CP promoted selective production of benzaldehyde at different yields, with only negligible amounts of benzoic acid. Under normalized conditions, 2 showed superior catalytic activity. This species is 3.5-fold more active than the monometallic control CP, and points out to the need for an efficient ligand framework. Species 2 is 6-fold more active than the dinuclear 1, and indicates the favored nuclearity for the conversion of alcohols into aldehydes.

Electron transport through a (terpyridine)ruthenium metallo-surfactant containing a redox-active aminocatechol derivative.

Aiming to develop a new class of metallosurfactants with unidirectional electron transfer properties, a (terpyridine) ruthenium complex containing a semiquinone derivative L2, namely [RuIII(Lterpy)(L2)Cl]PF6 (1), was synthesized and structurally characterized as a solid and in solution. The electronic and redox behaviour of 1 was studied experimentally as well as by means of DFT methods, and is indicative of significant orbital mixing and overlap between metal and ligands. The complex forms stable Pockels-Langmuir films at the air-water interface and allows for the formation of thin films onto gold electrodes to prepare nanoscale Au|LB 1|Au junctions for current-voltage (I/V) analysis. Complex 1 shows asymmetric electron transfer with a maximum rectification ratio of 32 based on tunnelling through MOs of the aminocatechol derivative.

Distinct Bimetallic Cooperativity Among Water Reduction Catalysts Containing [CoIII CoIII ], [NiII NiII ], and [ZnII ZnII ] Cores.

Three binuclear species [LCoIII 2 (µ-Pz)2 ](ClO4 )3 (1), [LNiII 2 (CH3 OH)2 Cl2 ]ClO4 (2), and [LZnII 2 Cl2 ]PF6 (3) supported by the deprotonated form of the ligand 2,6-bis[bis(2-pyridylmethyl) amino-methyl]-4-methylphenol were synthesized, structurally characterized as solids and in solution, and had their electrochemical and spectroscopic behavior established. Species 1-3 had their water reduction ability studied aiming to interrogate the possible cooperative catalytic activity between two neighboring metal centers. Species 1 and 2 reduced H2 O to H2 effectively at an applied potential of -1.6 VAg/AgCl , yielding turnover numbers of 2,820 and 2,290, respectively, after 30 minutes. Species 3 lacked activity and was used as a negative control to eliminate the possibility of ligand-based catalysis. Pre- and post-catalytic data gave evidence of the molecular nature of the process within the timeframe of the experiments. Species 1 showed structural, rather than electronic cooperativity, while species 2 displayed no obvious cooperativity. DFT methods complemented the experimental results determining plausible mechanisms.

Dual anticancer and antibacterial activities of bismuth compounds based on asymmetric [NN'O] ligands.

Two new bismuth(III) complexes, [BiL1Cl2] (1) and [BiL2Cl2] (2), in which L1 is (2-hydroxy-4-6-di-tert-butylbenzyl-2-pyridylmethyl)amine and L2 is 2,4-diiodo-6-((pyridine-2-ylmethylamino)methyl)phenol, were synthesized and characterized by elemental and conductivity analyses, atomic absorption spectrometry, infrared and 1H NMR spectroscopies. The molecular structure of 1 reveals that the NN'O ligand forms a 1:1 complex with bismuth through coordination via the nitrogen of the aliphatic amine, the nitrogen of the pyridine ring and the oxygen of the phenolate. The coordination sphere is completed with two chloride anions in a distorted square pyramidal geometry. Bismuth exhibits the same coordination mode in compound 2. The cytotoxic activity of 1 and 2 was investigated in a chronic myelogenous leukemia cell line. The complexes are approximately three times more potent than the corresponding free ligands, with the IC50 values 0.30 and 0.38 µM for complex 1 and 2, respectively. To address the cellular mechanisms underlying cell demise, apoptosis was quantified by flow cytometry analysis. From 0.1 µM, both complexes induce apoptosis and there is a remarkable concentration-dependent increase in the population of cells in apoptosis. The complexes were also evaluated against Gram-positive and Gram-negative bacteria. Both inhibited the bacterial growth in a concentration-dependent way, with remarkable activity in some of the tested strains, for example, complex 2 was more active than its free ligand against all bacterial strains and approximately fourteen times more potent against S. dysenteriae and S. typhimurium.

Reactivity and Mechanisms of Photoactivated Heterometallic [RuII NiII ] and [RuII NiII RuII ] Catalysts for Dihydrogen Generation from Water.

Two heterometallic photocatalysts were designed and probed for water reduction. Both [(bpy)2 RuII NiII (L1 )](ClO4 )2 (1) and [(bpy)2 RuII NiII (L2 )2 RuII (bpy)2 ](ClO4 )2 (2) can generate the low-valent precursor involved in hydride formation prior to dihydrogen generation. However, while the bimetallic [RuII NiII ] (1) requires the presence of an external photosensitizer to trigger catalytic activity, the trimetallic [RuII NiII RuII ] (2) displays significant coupling between the catalytic and light-harvesting units to promote intramolecular multielectron transfer and perform photocatalysis at the Ni center. A concerted experimental and theoretical effort proposes mechanisms to explain why 1 is unable to achieve self-supported catalysis, while 2 is fully photocatalytic.

Electrochemical Quantification of Corrosion Mitigation on Iron Surfaces with Gallium(III) and Zinc(II) Metallosurfactants.

We have recently described a new potential use for Langmuir-Blodgett films of surfactants containing redox-inert metal ions in the inhibition of corrosion and have shown good qualitative results for both iron and aluminum surfaces. In this study we proceed to quantify electrochemically the viability of gallium(III)- and zinc(II)-containing metallosurfactants [GaIII(LN2O3)] (1) and [ZnII(LN2O2)H2O] (2) as mitigators for iron corrosion in saline and acidic media. We evaluate their charge transfer suppression and then focus on potentiodynamic polarization and impedance spectroscopy studies, including detailed SEM data to interrogate their metal dissolution/oxygen reduction rate mitigation abilities. Both complexes show some degree of mitigation, with a more pronounced activity in saline than in acidic medium.

A Molecular Approach for Mitigation of Aluminum Pitting based on Films of Zinc(II) and Gallium(III) Metallosurfactants.

The use of metallosurfactants to prevent pitting corrosion of aluminum surfaces is discussed based on the behavior of the metallosurfactants [ZnII (LN2O2 )H2 O] (1) and [GaIII (LN2O3 )] (2). These species were deposited as multilayer Langmuir-Blodgett films and characterized by IR reflection absorption spectroscopy and UV/Vis spectroscopy. Scanning electron microscopy images, potentiodynamic polarization experiments, and electrochemical impedance spectroscopy were used to assess corrosion mitigation. Both metallosurfactants demonstrate superior anticorrosion activity due to the presence of redox-inactive 3d10 metal ions that enhance the structural resistance of the ordered molecular films and limit chloride mobility and electron transfer.

Influence of nitro substituents on the redox, electronic, and proton reduction catalytic behavior of phenolate-based [N2O3]-type cobalt(iii) complexes.

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt(iii) complexes, namely [CoIII(L1)MeOH] (1) and [CoIII(L2)MeOH] (2). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences.

Molecular rectifiers based on five-coordinate iron(iii)-containing surfactants.

In this article we review the state-of-the-art of metallorganic-based molecular rectification with an emphasis on our research in five-coordinate FeIII-containing surfactants. We place rectification in the broader context of molecular electronics, and include the description of methodology used in electrode|LB film|electrode assemblies, concluding with an outlook on future directions for metallosurfactants.

Observation of current rectification by the new bimetallic iron(iii) hydrophobe [Fe(LN4O6)] on Au|LB-molecule|Au devices.

Targeting the development of stimulus-responsive molecular materials with electronic functionality, we have synthesized and studied the redox and electronic properties of a new bimetallic iron hydrophobe [FeIII2(LN4O6)] (1). The new H6LN4O6 ligand displays bicompartmental topology capable of accomodating two five-coordinate HSFeIII ions bridged by tetraaminobenzene at a close distance of ca. 8 Å. We show that the metal-based reduction processes in (1) proceed sequentially, as observed for electronically coupled metal centers. This species forms a well-defined Pockels-Langmuir film at the air-water interface, with collapse pressure of 32 mN m-1. Langmuir-Blodgett monolayers were deposited on gold substrates and used to investigate current-voltage (I-V) measurements. This unprecedented bimetallic hydrophobe [FeIII2(LN4O6)] (1) shows unquestionable molecular rectification and displays a rectification ratio RR between 2 and 15.

Immobilization of an Amphiphilic Molecular Cobalt Catalyst on Carbon Black for Ligand-Assisted Water Oxidation.

We have prepared the amphiphilic molecular catalyst [CoIII(LOC18)(pyrr)2]ClO4 (1), where LOC18 is the deprotonated form of N, N'-[4,5-bis(octadecyloxy)-1,2-phenylene]dipicolinamide. Species 1 can be anchored onto a carbon black support to yield the assembly 1@CB, which can catalyze water oxidation at an affordable onset overpotential of 0.32 V, with a current density of 10 mA/cm2 at 0.37 V. Moreover, 1@CB displays TOF = 3850 h-1. A mechanism is proposed based on the experimental and density-functional-theory-calculated data.

Observation of current rectification by a new asymmetric iron(iii) surfactant in a eutectic GaIn|LB monolayer|Au sandwich.

In this paper we expand on the search for molecular rectifiers of electrical current and report on a hexacoordinate metallosurfactant [FeIII(LN3O)(OMe)2], where (LN3O)- is the deprotonated form of the new asymmetric ligand 2-((E)-((4,5-bis(2-methoxyethoxy)-2-(((E)-pyridin-2-ylmethylene)amino)phenyl)imino)methyl)-4,6-di-tert-butyl-phenol. This species rectifies current when deposited as a Langmuir-Blodget film in a "EGaIn/Ga2O3|LB|Au" sandwich with rectification ratios ranging from 25 to 300 at 1 Volt.

An in situ spectroelectrochemical study on the orientation changes of an [FeiiiLN2O3] metallosurfactant deposited as LB Films on gold electrode surfaces.

In this paper we analyze the changes in molecular orientation triggered by electrochemical reduction of an iron-containing surfactant in Langmuir-Blodgett films deposited onto gold electrodes. The metallosurfactant [Feiii(LN2O3)] (1) is an established molecular rectifier capable of unidirectional electron transfer between two electrodes. A gradual decrease in the activity is observed in sequential current vs. potential curves upon repeated cycles. Here we evaluate the redox response associated with the reduction of the Feiii/Feii couple in a single monolayer, as well as in a 5-layer LB film of 1. We use polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) to follow structural and orientation changes associated with such applied potential scans. We observe that the reduction of the Fe center becomes increasingly irreversible because an Fe-Ophenolate bond is cleaved. This transformation is accompanied by an almost vertical change in the orientation of metallosurfactant molecules in LB films.

A pentadentate nitrogen-rich copper electrocatalyst for water reduction with pH-dependent molecular mechanisms.

The new pentadentate 3d9 complex [CuII(LN2Py3)](PF6)2 (1) based on a nitrogen-rich framework acts as an electrocatalyst toward dihydrogen production from water. This species is active at pHs 7 and 2.5 yielding respective TON3h values of 1670 and 3900. Comparison of the molecular structure of 1 with that of the reduced [CuI(LN2Py3)]PF6 (2) evidences elongated Cu-N bond lengths resulting from an increased electron density around the 3d10 CuI center. The absence of nanoparticulate formation indicates that molecular mechanisms prevail at both pHs. Furthermore, experimental and DFT data support that distinct mechanisms are operative: while the metal center plays a key role at pH 7, one dangling pyridine moiety gets protonated at pH 2.5 and becomes actively involved in a relay mechanism. In both cases the CuIII-H- intermediate seems to be bypassed by PCET processes.

Bimetallic Cooperativity in Proton Reduction with an Amido-Bridged Cobalt Catalyst.

The bimetallic catalyst [CoII2 (L1 )(bpy)2 ]ClO4 (1), in which L1 is an [NN'2 O2 ] fused ligand, efficiently reduced H+ to H2 in CH3 CN in the presence of 100 equiv of HOAc with a turnover number of 18 and a Faradaic efficiency of 94 % after 3 h of bulk electrolysis at -1.6 V (vs. Ag/AgCl). This observation allowed the proposal that this bimetallic cooperativity is associated with distance, angle, and orbital alignment of the two Co centers, as promoted by the unique Co-Namido -Co environment offered by L1 . Experimental results revealed that the parent [CoII CoII ] complex undergoes two successive metal-based 1 e- reductions to generate the catalytically active species [CoI CoI ], and DFT calculations suggested that addition of a proton to one CoI triggers a cooperative 1 e- transfer by each of these CoI centers. This 2 e- transfer is an alternative route to generate a more reactive [CoII (CoII -H- )] hydride, thus avoiding the CoIII -H- required in monometallic species. This [CoII (CoII -H- )] species then accepts another H+ to release H2 .