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The optical and electrochemical properties of quadruply bonded dimolybdenum paddlewheel complexes (Mo

The thermoelectric properties of parallel arrays of organic molecules on a surface offer the potential for large-area, flexible, solution processed, energy harvesting thin-films, whose room-temperature transport properties are controlled by quantum interference (QI). Recently, it has been demonstrated that constructive QI (CQI) can be translated from single molecules to self-assembled monolayers (SAMs), boosting both electrical conductivities and Seebeck coefficients. However, these CQI-enhanced systems are limited by rigid coupling of the component molecules to metallic electrodes, preventing the introduction of additional layers which would be advantageous for their further development. These rigid couplings also limit our ability to suppress the transport of phonons through these systems, which could act to boost their thermoelectric output, without comprising on their impressive electronic features. Here, through a combined experimental and theoretical study, we show that cross-plane thermoelectricity in SAMs can be enhanced by incorporating extra molecular layers. We utilize a bottom-up approach to assemble multi-component thin-films that combine a rigid, highly conductive ‘sticky’-linker, formed from alkynyl-functionalised anthracenes, and a ‘slippery’-linker consisting of a functionalized metalloporphyrin. Starting from an anthracene-based SAM, we demonstrate that subsequent addition of either a porphyrin layer or a graphene layer increases the Seebeck coefficient, and addition of both porphyrin and graphene leads to a further boost in their Seebeck coefficients. This demonstration of Seebeck-enhanced multi-component SAMs is the first of its kind and presents a new strategy towards the design of thin-film thermoelectric materials.

Dialkynylferrocenes exhibit attractive electronic and rotational features that make them ideal candidates for use in molecular electronic applications. However previous works have primarily focussed on single-molecule studies, with limited opportunities to translate these features into devices. In this report, we utilise a variety of techniques to examine both the geometric and electronic structure of a range of 1,1'-dialkynylferrocene molecules, as either single-molecules, or as self-assembled monolayers. Previous single molecule studies have shown that similar molecules can adopt an 'open' conformation. However, in this work, DFT calculations, STM-BJ experiments and AFM imaging reveal that these molecules prefer to occupy a 'hairpin' conformation, where both alkynes point towards the metal surface. Interestingly we find that only one of the terminal anchor groups binds to the surface, though both the presence and nature of the second alkyne affect the thermoelectric properties of these systems. First, the secondary alkyne acts to affect the position of the frontier molecular orbitals, leading to increases in the Seebeck coefficient. Secondly, theoretical calculations suggested that rotating the secondary alkyne away from the surface acts to modulate thermoelectric properties. This work represents the first of its kind to examine the assembly of dialkynylferrocenes, providing valuable information about both their structure and electronic properties, as well as unveiling new ways in which both of these properties can be controlled.

Sonogashira reactions are utilized herein to react iodo-ferrocenes and -biferrocenes with terminal alkyne ligands, functionalized with both pyridine and thioanisole groups. High-yielding reactions generate both monoalkynyl and dialkynyl derivatives, the ratio of which can be altered through changes in the reaction stoichiometry. This methodology allowed us to synthesize a large family of derivatives, comprising four symmetrical derivatives (3xx, where x represents a phenyl-substituted terminal alkyne) and six less-studied asymmetrical derivatives (3xy, where x and y represent two different phenyl-substituted terminal alkynes), as well as a number of their biferrocenyl analogues (6x, 7xx, and 7xy), including the first known examples of asymmetrically disubstituted biferrocenes. We examined the electrochemical behavior of all the systems in solution through the use of cyclic voltammetry and demonstrate that these highly conjugated alkynyl ligands exert delicate redox control over the central ferrocene motif. We also note that these substituents display some control over the mixed-valence character present in biferrocene monocations, with thioanisole substituents imparting almost an order of magnitude higher Kc than their pyridyl analogues, and asymmetric systems displaying rare characteristic properties of mixed-valence isomers. The electronic structure of these systems was further elucidated through a combination of UV/vis spectroscopy and density functional theory calculations. Our methodology provides a facile and adaptable route toward the isolation of a number of novel ferrocene and biferrocene derivatives. From our perspective, the asymmetric nature of these systems, along with the delicate and predictable redox control that these ligands exert on the central ferrocene unit(s), could lead to applications in molecular electronics, where these properties have previously shown promise in the fabrication of diodes and rectifiers, as well as in the synthesis of donor-π-acceptor systems.

π‐Coordination of aromatic molecules to metals dramatically alters their reactivity. For example, coordinated carbons become more electrophilic and C−H bonds of coordinated rings become more acidic. For many years, this change in reactivity has been used to trigger reactions that would not take place for uncoordinated arenes, however, there has been a recent resurgence in use of this technique, in part due to the development of catalytic reactions in which π‐coordination is transient. In this Minireview, we describe the key reaction chemistry of arenes coordinated to a range of transition metals, including stereoselective reactions and industrially relevant syntheses. We also summarise outstanding examples of catalytic processes. Finally, we give perspectives on the future direction of the field, with respect to both reactions that are stoichiometric in activating metals and those employing catalytic metal., Easy as π: π‐Coordination of aromatic compounds to transition metals dramatically alters the reactivity of the arene. In this Minireview, the authors highlight reactions that exploit this increased activity, selecting examples that are both stoichiometric and catalytic in the activating metal. The advantages and limitations of the activation strategy are discussed and it is considered which areas of future research will benefit from this technique.

This chapter focuses on the advances made in the chemistry of metal–metal quadruply bonded complexes over the last 5 years and is split into three main themes: electron transfer, reactivity and coordination chemistry. The question of electron (de)localisation is examined through the study of ground-state mixed valence complexes and their photo-excited state dynamics. Fundamental studies of metal–metal bonding allow the exploration of new modes of reactivity and low-coordinate compounds which are finding use in catalysis. Additionally, coordination chemistry allows the tuning of molecular properties and this can have profound effects on the electronic structures of functional molecular materials.

Ferrocene-based Lewis bases have found utility as metalloligands in a wide variety of applications. The coordination chemistry of cyanoferrocenes however, is underexplored. Herein, we describe a new synthetic protocol for the generation of cyanoferrocenes. The coordination chemistry of these metalloligands to [Cu(NCMe)4][PF6], [(PPh3)2Cu(NCMe)2][PF6] and [(dppf)Cu(NCMe)2][PF6] salts has been explored, providing crystallographic evidence of cluster and polymeric forms of 1,1′- and 1,2-dicyanoferrocene complexes. The stability of the complexes and ligand dissociation were found to be strongly solvent-dependent.

Correction for ‘Molecular-scale thermoelectricity: as simple as ‘ABC’’ by Ali Ismael et al., Nanoscale Adv., 2020, 2, 5329–5334, DOI: 10.1039/D0NA00772B.

Metal complexes formed from π-coordination of aromatic groups to metal centres may undergo reactions at the coordinated arene that do not occur for the unbound arene. Upon completion of such reactions, the coordinated arene product can be liberated from the metal complex by thermal or photolytic methods. A catalytic approach can also be envisaged in which the arene transiently π-coordinates to the metal centre, reacts and then undergoes arene exchange for further starting arene. The significant challenge to this catalytic approach is to balance reactivity with arene exchange. In this review, we summarise the synthesis and reactivity of π-arene metal complexes. We go on to discuss the features of arene exchange and conclude with a comprehensive review of catalytic reactions proceeding via π-arene intermediates.

We present a C–H activation protocol for aromatic compounds that overcomes the current limitations of the need for a directing group or covalently bound activating groups, by exploiting the increase in C–H acidity of aromatic compounds on π-coordination to a Ru(II) center. The increased acidity facilitates catalytic concerted metalation–deprotonation and subsequent arylation reactions. We present the development and optimization of the C–H activation protocol and show the applicability of the reaction to a range of aromatic substrates, including the simplest of substrates (benzene). Furthermore, we demonstrate the recyclability of the activating Ru(II) fragment using photolysis and give a mechanistic study, which provides strong evidence that this reaction occurs via a silver-mediated C–H bond activation. This is the first time Ru complexes have been shown to allow C–H activation of arenes by a π-coordination mechanism.

Mixtures of silver salts AgX (X = NO3, CF3CO2, CF3SO3) with M[Au(C6F5)2] (M = NBu4, PPh4) gave respectively the ionic mixed-metal clusters [M2{(C6F5)4Au2Ag2X2}]n (1, X = NO3; a, M = NBu4; b, M = PPh4) and [M{(C6F5)4Au2Ag2X}]n, (2a,b, X = CF3CO2; 3a,b, X = CF3SO3). The degree of aggregation n of these cluster compounds depends strongly on the method of isolation (solvent evaporation or precipitation); for example, recrystallization of 1a gave a crystalline salt of the tetraanion [(C6F5)4Au2Ag2X2]24– as well as the polymer [(NBu4)2{(C6F5)4Au2Ag2(NO3)2}]n. The aurophilic Au···Au interactions strongly influence the photoemission wavelength. The anion X has remarkably little effect on the luminescence color but strongly influences the conformation of the polyanionic chains, leading to a variety of solid-state structures, from well-defined dimers (1a1) to linear (1b) and curved (1a2, 2a) polymeric chain aggregates.

Stabilisation of the mixed-valence state in [Mo2(TiPB)3(HDOP)]2(+) (HTiPB = 2,4,6-triisopropylbenzoic acid, H2DOP = 3,6-dihydroxypyridazine) by electron transfer (ET) is related to the proton coordinate of the bridging ligands. Spectroelectrochemical studies suggest that ET is slower than 10(9) s(-1). The mechanism has been probed using DFT calculations, which show that proton transfer induces a larger dipole in the molecule resulting in ET.

The compounds M2(TiPB)3(HDON) (TiPB = 2,4,6-triisopropylbenzoic acid; H2DON = 2,7-dihdroxy-1,8-napthyridine; M = Mo (1a) or W (1b)) and Mo2(TiPB)2(O2CCH2Cl)(HDON) (1c) which contain a pendant 2-pyridone functional group have been prepared. These compounds are capable of forming self-complementary hydrogen bonds, resulting in the formation of "dimers of dimers" ([1a-c]2) in CH2Cl2 solutions. Electrochemical studies reveal two successive one-electron redox processes for [1a-c]2 in CH2Cl2 solutions that correspond to successive oxidations of the dimetal core, indicating stabilization of the mixed-valence state. Only small changes in the value of Kc are observed upon changing the ancillary ligand or metal, implying that proton coupled mixed valency is responsible for the stabilization. Dimethylsulfoxide (DMSO) disrupts the hydrogen bonding interactions in these compounds, and a single oxidation process is observed in DMSO which shifts to lower potential as the number of HDON ligands increases. Further substitution of carboxylate ligands with HDON leads to the formation of Mo2(TiPB)2(HDON)2 (2) and Mo2(HDON)4 (3), which adopt trans-1,1 and cis-2,2 regioisomers in the solid-state. (1)H NMR spectroscopy indicates that there are at least two regioisomers present in solution for both compounds. The lowest energy transition in the electronic absorption spectra of these compounds corresponds to a M2-δ → HDON-π* transition. The electrochemical, spectroscopic and structural results were rationalized with the aid of density functional theory (DFT) calculations.

Fine particles are a major health issue as they remain suspended in the atmosphere for extended periods, are able to penetrate deep into the human lung and contain significant concentrations of heavy metals, such as Arsenic. They are also a significant component of the smog that limits the visibility in many cities and even in some national parks plus scientists believe they have an effect on global weather patterns. The Indigo Agglomerator enhances fine particle collection by attaching the fine particles to the larger particles. These large agglomerated particles are easily collected in existing control devices, such as Electrostatic Precipitator (ESP), fabric filters, scrubbers and cyclones. This paper concentrated on PM2.5 particles, that is particles less than 2.5 μm in diameter, including data that was collected on particles down to 50 nm in diameter. It was found that the reduction in fine particle emission from an Electrostatic Precipitator provided by installing an Indigo Agglomerator increases with reducing particle size from a factor of 5 at 2 μm to a factor of 10 at 100 nm. Reductions of this magnitude will have a significant effect on the impact of fine particles on both visibility and health. It will also result in a reduction in heavy metal emissions. Recent regulations in the US require Mercury emission control on coal fired power stations. Mercury is considered a major health hazard because it concentrates in the food chain and, in particular, may result in very high concentrations in some fish. The Indigo Agglomerator enhances Mercury collection by increasing the interaction between the Mercury, in the form of elemental or ionic molecules contained in the gas stream, and the adsorbent, either injected Activated Carbon or using the LOI from the combustion process. Initial tests have shown a factor of four enhancement of the Mercury removal when an Indigo Agglomerator is installed in front of an Electrostatic Precipitator. This is in compliance with the long term EPA requirements in the US.

A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs., A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility.

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect ofdestructiveQI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms ofconstructiveQI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.