Effect of ligand substitution on the exchange interactions in {Mn12}-type single-molecule magnets

We investigate how the ligand substitution affects the intra-molecular spin exchange interactions, studying a prototypal family of single-molecule magnets comprising dodecanuclear cluster molecules [Mn12O12(COOR)16]. We identify a simple scheme based on accumulated Pauling electronegativity numbers (a.e.n.) of the carboxylate ligand groups (R). The redistribution of the electron density, controlled by a.e.n. of a ligand, changes the degree of hybridization between 3d electrons of manganese and 2p electrons of oxygen atoms, thus changing the exchange interactions. This scheme, despite its conceptual simplicity, provides a strong correlation with the exchange energies associated with carboxylate bridges, and is confirmed by the electronic structure calculations taking into account the Coulomb correlations in magnetic molecules.Comment: 18 pages, 1 table, 4 figures. Accepted to "Inorganic Chemistry

The Role of Dynamic Ligand Exchange in the Oxidation Chemistry of Cerium(III)

The CeIII/IV couple is useful for many applications in organic, inorganic, and materials chemistry. However, attaining a general method to access both oxidations states through reversible solution redox chemistry remains challenging. Herein we report the synthesis, characterization, and oxidation chemistry of the novel Ce/Li REMB heterochiral diastereomer, 1-Ce(het). The solution exchange processes of 1-RE(het) (RE ¼ Ce and Yb) were investigated to estimate rates of ligand and cation exchange relevant in homochiral and heterochiral frameworks. A detailed mechanistic investigation following the solution dynamics of 1-Ce(het) revealed reactivity controlled both by ligand reorganization and redistribution processes. Ligand reorganization was responsible for the kinetics associated with the chemical oxidation reaction, whereas ligand redistribution and exchange dictated the isolated product

Radioanalytical Methods for Characterization of the Surface Modification of Nanoparticles

The use of iron oxide nanoparticles for a variety of applications has grown over the past few decades. Manipulation of surface chemistry of these materials is critical to customizing the properties of the particles for desired applications. Ligand exchange is a common and versatile tool for surface modification. There are many factors which affect ligand exchange including ligand chain length, number of binding groups, binding group chemistry, and particle aging and oxidation. Furthermore, ligand exchange may not always occur to completion. Therefore, it is important to characterize the surface of the particles to determine the extent of exchange. Current techniques to confirm and monitor ligand exchange can be limited in sensitivity and versatility, and often these techniques must be used in combination to thoroughly characterize the exchange. To address this issue, radioanalytical techniques were developed to quantify ligand exchange on iron oxide nanoparticles and investigate the factors which affect ligand exchange. Oleic acid coated iron oxide nanoparticles were synthesized via thermal decomposition with trace amounts of 14C-oleic acid on the surface. The particles were modified via ligand exchange with a variety of hydrophilic ligands. The modified particles were measured using liquid scintillation counting (LSC) to determine the activity and ultimately, the total number of 14C-oleic acid chains remaining after exchange. These techniques were used to determine effects of head group chemistry with polymeric ligands and effects of head group chemistry, number of binding groups, and ligand exchange reaction parameters with small molecule ligands. Results revealed catechols displace the most oleic acid during exchange. Furthermore, multidenticity, or multiple binding groups, increases the displacement of the oleic acid. Particle aging and oxidation was investigated using these techniques. Unlabeled, oleic acid coated particles which were aged in solution for 2, 7, and 30 days were mixed with 14C-oleic acid in exchange reactions. Results revealed that aging of the particles at 30 days effected an increase in the amount of 14C-oleic acid adsorbed on the particles after exchange. Kinetic analysis of these results indicated an increase in the desorption rate constant and a decrease in the adsorption rate constant with age but with no profound change in the overall reaction rates. A follow-up study with oxidized particles suggested that this behavior may be due to oxidation during aging. Overall, the results signify an increase in the number of available binding sites, possibly due to formation of a defective oxide shell during aging and/or oxidation

Carboxylic-acid-passivated metal oxide nanocrystals : ligand exchange characteristics of a new binding motif

Ligand exchange is central in the processing of inorganic nanocrystals (NCs) and requires understanding of surface chemistry. Studying sterically stabilized HfO2 and ZrO2 NCs using H-1 solution NMR and IR spectroscopy as well as elemental analysis, this paper demonstrates the reversible exchange of initial oleic acid ligands for octylamine and self-adsorption of oleic acid at NC surfaces. Both processes are incompatible with an X-type binding motif of carboxylic acids as reported for sulfide and selenide NCs. We argue that this behavior stems from the dissociative adsorption of carboxylic acids at the oxide surface. Both proton and carboxylate moieties must be regarded as X-type ligands yielding a combined X-2 binding motif that allows for self-adsorption and exchange for L-type ligands

Synthesis, Design, and Environmental Fate of Metallic Nanoparticles

Rational design of nanoparticle surface chemistry offers the ability to control nanoparticle characteristics such as size, polydispersity, shape, dispersibility in various solvents, functionality and end fate. Ligand exchange has proved to be is a versatile method for modifying the surface of plasmonic nanoparticles. Ligand exchange has provided a “green” alternative to traditional biphasic syntheses that require large amounts of phase transfer catalysts. Ligand exchange can also be used to reduce the amount of post synthesis processing and waste when it is conducted on nanoparticles that have been synthesized with a method that affords control over nanoparticle size and polydispersity. Ligand exchange is also an important reaction to consider when determining the end fate of nanomaterials due to the fact that when nanoparticles enter the natural environment, they will be exposed to a variety of natural ligands and electrolytes. We have conducted a comprehensive review of ligand exchange literature and used isothermal titration calorimetry to investigate ligand binding and exchange on gold nanoparticles experimentally. We have also investigated the impact that citrate and natural organic matter surface chemistries have on the transport properties of silver nanoparticles. This work has led to a greater understanding of the influencing factors on the mechanism of nanoparticle ligand binding and exchange

Charge Transport in Trap-Sensitized Infrared PbS Quantum-Dot-Based Photoconductors: Pros and Cons

Control of quantum-dot (QD) surface chemistry offers a direct approach for the tuning of charge-carrier dynamics in photoconductors based on strongly coupled QD solids. We investigate the effects of altering the surface chemistry of PbS QDs in such QD solids via ligand exchange using 3-mercaptopropionic acid (MPA) and tetrabutylammonium iodide (TBAI). The roll-to-roll compatible doctor-blade technique was used for the fabrication of the QD solid films as the photoactive component in photoconductors and field-effect phototransistors. The ligand exchange of the QD solid film with MPA yields superior device performance with higher photosensitivity and detectivity, which is due to less dark current and lower noise level as compared to ligand exchange with TBAI. In both cases, the mechanism responsible for photoconductivity is related to trap sensitization of the QD solid, in which traps are responsible of high photoconductive gain values, but slow response times under very low incident optical power (100 pW), where traps are filled, both MPA- and TBAI-treated photodevices exhibit similar behavior, characterized by lower responsivity and faster response time, as limited by the mobility in the QD solid

Improvement of hydrothermal stability of zeolitic imidazolate frameworks

The metal-organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation. © 2013 The Royal Society of Chemistry

Emergence of On-Surface Magnetochemistry

The control of exchange coupling across the molecule–substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or “surface ligand” are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established “trans effect” to the notion of the “surface-trans effect”. As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, spin-switching has been demonstrated to be controlled collectively via the interaction with a ligand (chemical selectivity) and individually via local addressing at the interface

From ligands to binding motifs and beyond; the enhanced versatility of nanocrystal surfaces

Surface chemistry bridges the gap between nanocrystal synthesis and their applications. In this respect, the discovery of complex ligand binding motifs on semiconductor quantum dots and metal oxide nanocrystals opens a gateway to new areas of research. The implications are far-reaching, from catalytic model systems to the performance of solar cells

Functionalisation of colloidal transition metal sulphides nanocrystals: A fascinating and challenging playground for the chemist

Metal sulphides, and in particular transition metal sulphide colloids, are a broad, versatile and exciting class of inorganic compounds which deserve growing interest and attention ascribable to the functional properties that many of them display. With respect to their oxide homologues, however, they are characterised by noticeably different chemical, structural and hence functional features. Their potential applications span several fields, and in many of the foreseen applications (e.g., in bioimaging and related fields), the achievement of stable colloidal suspensions of metal sulphides is highly desirable or either an unavoidable requirement to be met. To this aim, robust functionalisation strategies should be devised, which however are, with respect to metal or metal oxides colloids, much more challenging. This has to be ascribed, inter alia, also to the still limited knowledge of the sulphides surface chemistry, particularly when comparing it to the better established, though multifaceted, oxide surface chemistry. A ground-breaking endeavour in this field is hence the detailed understanding of the nature of the complex surface chemistry of transition metal sulphides, which ideally requires an integrated experimental and modelling approach. In this review, an overview of the state-of-the-art on the existing examples of functionalisation of transition metal sulphides is provided, also by focusing on selected case studies, exemplifying the manifold nature of this class of binary inorganic compounds