Chapter 1
Fe, Ru, and Os Nanoparticles

Madhu Kaushik, Yuting Feng, Nathaniel Boyce and Audrey Moores

1.1 Introduction

Although Fe, Ru, and Os are transition metals all belonging to Group 8 of the periodic table, their prevalence, chemistry, and applications differ greatly. Iron is an earth abundant element, playing a key role in life with implications in some of the most difficult biological processes [1]. Iron is a current topic of interest in the context of catalysis, both in its homogeneous [2, 3] and heterogeneous forms [4, 5], as a means to replace more toxic and less abundant transition metals. Iron has interesting properties in this context, including magnetic properties, a rich redox chemistry, with implications both in molecular and material sciences, an affordable price and nontoxicity. Iron complexes, oxides, and metal-based materials have been applied to a wide array of chemical processes, including oxidation processes, hydrogenation, CC coupling, and aromatic substitutions [3, 6-9]. Ruthenium belongs to the platinum series and is obtained as a by-product to platinum or nickel mining. Within the platinum series, ruthenium is comparably cheaper than its counterparts and thus desirable as a replacement to more expensive, catalytically active transition metals. Compared to iron, it is less prone to oxidation when in its reduced form. Ruthenium has established itself as an important and industrially relevant catalyst, in both its homogeneous and heterogeneous forms for a number of important processes including the Haber-Bosch process (dinitrogen to ammonia) [10, 11], the Fischer-Tropsch process (syngas to hydrocarbons) [12, 13], hydrogenation reactions [14], including the partial and selective hydrogenation of benzene and phenol, olefin metathesis [15], CH activation [16], and organic oxidations just to name a few. Osmium has been comparatively less explored. Osmium tetroxide and related compounds have demonstrated early on their ability to catalyze the oxidative cleavage of olefins with molecular oxygen [17]. Other osmium complexes have now been developed for a number of reactions including complete and partial hydrogenation, dehydrogenation, and hydroformylation [18, 19]. Nanoparticulate osmium counterparts have been studied for their catalytic properties toward Fischer-Tropsch reaction, the homologation of alkenes under H2 and the hydrogenolysis of alkanes [20], and electrocatalytic activity useful in fuel cell research [21].

Ionic liquids (ILs) are defined as salts with melting points below 100 °C. Over the last two decades, research on ILs has developed with a focus on liquid ones at or near room temperature [22, 23]. Typically such systems are constituted of flexible organic ion pairs with delocalized charges and tunable lipophilic domains [24]. Because of their liquid nature and specific properties, ILs are used as solvents to substitute volatile molecular liquids and specialty materials for a number of important applications, including synthesis, catalysis and biocatalysis, separation technology, biomass processing and transformation, electrochemical devices including capacitors, fuel cells and solar cells, nanotechnology, sensing, lubricants, hypergolic materials, and pharmaceutics [22, 25-30]. ILs rapidly emerged as a privileged environment for catalysis because of their unique intrinsic properties. ILs are attractive as potential solvents for a number of reasons: (i) they are nonvolatile under ambient conditions; (ii) they are colorless and little viscous; (iii) they possess good solvation properties for a large number of species; (iv) they are immiscible with many conventional solvents; (v) The properties of ILs can be easily tuned by a careful choice of the cation and anion entities, making them "designer solvents" [31]; and (vi) they are commercially available [30]. The combination of properties (i), (iii), (iv), and (v) explain that they are perfectly suited as catalyst stabilizers in biphasic systems [32, 33]. ILs have also been developed in supported versions to afford heterogenized systems of interest in catalysis [27].

Metal nanoparticles (NPs) have been intensely researched in the context of catalysis [34, 35]. Their high surface-over-volume ratio and unique effects linked to their nano size (role of defects, photon, and electron-linked properties) explain unique activities at the crossroads of homogeneous and heterogeneous catalysis [36-38]. They are appealing as highly active catalysts and materials easily amenable to recovery and recycling strategies [37]. Metal NPs and ILs, as privileged catalysts and catalysis media respectively, have thus been naturally explored in partnership and have opened a unique and rich research field, which has already been largely reviewed. Among the papers published over the last 10 years, the reader is directed to reviews on ILs in catalysis [25, 39], functionalized ionic liquids (FILs) [40, 41], FILs in catalysis [42], NPs in ILs [36, 43-46], NPs in ILs for catalysis [26, 47-55], and Ru NPs in IL [56]. In this chapter, we focused on the synthesis, stabilization properties, and catalytic applications of Fe, Ru, and Os NPs in ILs. For Fe, we extended the review to iron oxide NPs as well, as under aerobic condition, reduced iron NPs lead to such species.

1.2 Synthesis of Fe, Ru, and Os NPs in ILs

In general, the synthetic methods to access metal and metal oxide NPs in ILs have focused on providing materials with key features relevant for catalysis, that is, small sizes, good monodispersity, and high purity [26]. These properties are achieved using bottom-up synthetic approaches, either by reduction of metal salts, by the direct use of zero-valent molecular species, which can be freed from their ligands via hydrogenation, or simply decomposed (Scheme 1.1). The use of additional stabilizers [46] or functional ILs [42] has been reported to improve the properties of the end product. Top-down approaches are known for the synthesis of Au, Rh, Pd, and Pt NPs in ILs but are not a classic approach to access NPs of metals of Group 8 [26]. Also, although synthetic routes via phase transfer [57] from another organic or aqueous medium have been reported for other metals such as gold [58] and rhodium [59], Group 8 NPs are typically prepared directly in neat ILs. Because of the preorganized structure of ILs via electrostatic, hydrogen bonding, and van der Waals interactions, especially those with imidazolium and phosphonium functionalities, the IL medium is described as made of polar and nonpolar nanodomains. Those tunable domains act as nanoreactors and stabilizing chambers via noncovalent interactions [44, 46]. Thanks to the polarity, thermal stability, and preorganized supermolecular structure of ILs, the synthesis of Ru, Fe, and Os NPs could be attained in ambient conditions [44, 60]. In ILs, such strategies have unlocked access to metal NPs with precise size control and narrow size distributions [56]. Further comments on the stabilization properties of ILs for NPs are provided in Section 1.3.

Scheme 1.1 Summary of the synthetic schemes for accessing Fe, Ru, and Os NPs in ILs.

A complete list of Ru NPs synthesis in ILs was established by Campbell et al. in 2013 [56]. In the following sections, we focused on the most distinctive and recent examples for synthesis of Fe, Ru, and Os NPs. One of the specifics of the synthesis of metal NPs directly within an IL is the difficulty in washing the resulting material from any salt by-product generated during the reaction. Hence it is important to select "salt-free" precursors or organometallic precursors decomposing into easily washable organic species or volatile [61, 62] ones, such as [Ru(COD)(2-methylallyl)2], [Ru(COD)(COT)], or carbonyl compounds such as (COD = 1,5-cyclooctadiene and COT = 1,3,5-cyclooctatriene) [63], as discussed below.

1.2.1 Synthesis via Reduction of Metal Precursors or Ligands

Despite the long list of reducers used with this metal, Ru precursors in ILs are reduced by H2 in reported procedures [36, 48, 56, 64]. In a common scheme, a Ru precursor, such as [Ru(COD)(2-methylallyl)2] or [Ru(COD)(COT)] is dissolved or suspended into a specific IL under argon and exposed to mild H2 pressure and heating (<90 °C) from a few hours to days to obtain a black suspension. The size of the resulting Ru NPs - usually between 1.0 and 3.0 nm - and their size distribution may be tuned depending on reaction conditions, namely, stirring, temperature, and IL cations/anions (Figure 1.1) [56, 65-70]. From a formal standpoint, [Ru(COD)(2-methylallyl)2] and [Ru(COD)(COT)] precursors differ in that the latter is a Ru(0) complex which should not necessitate the use of a reducer. Under H2 pressure, however, the COD and COT ligands are hydrogenated to release atomic Ru(0) and allow the growth of Ru NPs [61, 62]. RuCl3 and RuO2 have also been reported as precursors being easily reduced by H2 to access the desired NPs. In some ILs, precursor solubility may be a limitation that has been circumvented via the use of an auxiliary solvent. Our group showed that tetrahydrofuran (THF) could be successfully used to mix [Ru(COD)(2-methylallyl)2] with phosphonium and imidazolium ILs, before being easily removed in vacuo. Subsequent reduction under H2 pressure afforded small Ru NPs (between 1.5 and 2.5 nm) [71]. Although not necessarily required for subsequent catalytic applications, the separation of the obtained NPs from imidazolium ILs may be performed and depends on the anion in the IL [65]. Prechtl et al. also showed that imidazolium ILs containing...