Atomically precise rhodium-indium carbonyl nanoclusters: synthesis, characterization, crystal structure and electron-sponge features

In this paper we present the investigation of the reactivity of [Rh7(CO)16]3- with InCl3, with the aim of expanding the more general study that allowed us to obtain, among other species, the icosahedral [Rh12E(CO)27]n- (n = 4 when E = Ge or Sn; n = 3 when E = Sb or Bi) family of clusters. Indeed, the study resulted in the isolation and characterization of the analogous In-centred icosahedral [Rh12In(CO)28]3- nanocluster (1), which is isoelectronic and isostructural with the [Rh12E(CO)27]n- congeners. During the course of the reaction two more new species, namely the octahedral [Rh6(CO)15InCl3]2- (2) and the dimeric [{Rh6(CO)15InCl2}2]2- (3) have also been identified. The reaction between [Rh7(CO)16]3- and InCl3 proved to be poorly selective; nevertheless, by fine tuning some reaction parameters it was possible to drive the reaction more towards one product or the other. Alternatively, [Rh6(CO)15InCl3]2- can be more selectively prepared by reacting either [Rh5(CO)15]- or, less efficiently, [Rh6(CO)15]2- with InCl3. As for the dimeric [{Rh6(CO)15InCl2}2]2- species, this was only isolated by carrying out the reaction with [Rh7(CO)16]3- under inert atmosphere, as opposed to under CO. All clusters were characterized by IR spectroscopy and ESI-MS, and their molecular structures were fully established by single-crystal X-ray diffraction studies. The [Rh12In(CO)28]3- species was also analysed by EDS via SEM, and further investigated through in situ infrared spectroelectrochemistry and CV experiments to check its multivalence nature. Indeed, [Rh12In(CO)28]3- can reversibly undergo two monoelectronic oxidation and one bi-electronic reduction processes, behaving like an electron sponge and, thus, giving rise to the further [Rh12In(CO)28]n- derivatives (n = 1, 2 and 5). These results parallel the findings for the [Rh12E(CO)27]n- series. The geometry variations of the metal framework associated with the changes in the cluster negative charge were investigated by means of DFT calculations.

Heterometallic rhodium clusters as electron reservoirs: Chemical, electrochemical, and theoretical studies of the centered-icosahedral [Rh12E(CO)27]n- atomically precise carbonyl compounds

In this paper, we present a comparative study of the redox properties of the icosahedral [Rh₁₂E(CO)₂₇]^n- (n = 4 when E = Ge or Sn and n = 3 when E = Sb or Bi) family of clusters through in situ infrared spectroelectrochemistry experiments and density functional theory computational studies. These clusters show shared characteristics in terms of molecular structure, being all E-centered icosahedral species, and electron counting, possessing 170 valence electrons as predicted by the electron-counting rules, based on the cluster-borane analogy, for compounds with such metal geometry. However, in some cases, clusters of similar nuclearity, and beyond, may show multivalence behavior and may be stable with a different electron counting, at least on the time scale of the electrochemical analyses. The experimental results, confirmed by theoretical calculations, showed a remarkable electron-sponge behavior for [Rh₁₂Ge(CO)₂₇]^4- (1), [Rh₁₂Sb(CO)₂₇]^3- (3), and [Rh₁₂Bi(CO)₂₇]^3- (4), with a cluster charge going from -2 to -6 for 1 and 3 and from -2 to -7 for cluster 4, making them examples of molecular electron reservoirs. The [Rh₁₂Sn(CO)₂₇]^4- (2) derivative, conversely, presents a limited ability to exist in separable reduced cluster species, at least within the experimental conditions, while in the gas phase it appears to be stable both as a penta- and hexa-anion, therefore showing a similar redox activity as its congeners. As a fallout of those studies, during the preparation of [Rh₁₂Sb(CO)₂₇]^3-, we were able to isolate a new species, namely, [Rh₁₁Sb(CO)₂₆]^2-, which presents a Sb-centered nido-icosahedral metal structure possessing 158 cluster valence electrons, in perfect agreement with the polyhedral skeletal electron pair theory.

Metal carbonyl clusters of groups 8-10: synthesis and catalysis

In this review article, we discuss advances in the chemistry of metal carbonyl clusters (MCCs) spanning the last three decades, with an emphasis on the more recent reports and those involving groups 8-10 elements. Synthetic methods have advanced and been refined, leading to higher-nuclearity clusters and a wider array of structures and nuclearities. Our understanding of the electronic structure in MCCs has advanced to a point where molecular chemistry tools and other advanced tools can probe their properties at a level of detail that surpasses that possible with other nanomaterials and solid-state materials. MCCs therefore advance our understanding of structure-property-reactivity correlations in other higher-nuclearity materials. With respect to catalysis, this article focuses only on homogeneous applications, but it includes both thermally and electrochemically driven catalysis. Applications in thermally driven catalysis have found success where the reaction conditions stabilise the compounds toward loss of CO. In more recent years, MCCs, which exhibit delocalised bonding and possess many electron-withdrawing CO ligands, have emerged as very stable and effective for reductive electrocatalysis reactions since reduction often strengthens M-C(O) bonds and since room-temperature reaction conditions are sufficient for driving the electrocatalysis.

Rh-Sb Nanoclusters: Synthesis, Structure, and Electrochemical Studies of the Atomically Precise [Rh20Sb3(CO)36]3- and [Rh21Sb2(CO)38]5- Carbonyl Compounds

The reactivity of [Rh₇(CO)₁₆]^3– with SbCl₃ has been deeply investigated with the aim of finding a new approach to prepare atomically precise metalloid clusters. In particular, by varying the stoichiometric ratios, the reaction atmosphere (carbon monoxide or nitrogen), the solvent, and by working at room temperature and low pressure, we were able to prepare two large carbonyl clusters of nanometer size, namely, [Rh₂₀Sb₃(CO)₃₆]^3– and [Rh₂₁Sb₂(CO)₃₈]^5–. A third large species composed of 28 metal atoms was isolated, but its exact formulation in terms of metal stoichiometry could not be incontrovertibly confirmed. We also adopted an alternative approach to synthesize nanoclusters, by decomposing the already known [Rh₁₂Sb(CO)₂₇]^3– species with PPh₃, willing to generate unsaturated fragments that could condense to larger species. This strategy resulted in the formation of the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]^3– heteroleptic cluster instead. All three new compounds were characterized by IR spectroscopy, and their molecular structures were fully established by single-crystal X-ray diffraction studies. These showed a distinct propensity for such clusters to adopt an icosahedral-based geometry. Their characterization was completed by ESI-MS and NMR studies. The electronic properties of the high-yield [Rh₂₁Sb₂(CO)₃₈]^5– cluster were studied through cyclic voltammetry and in situ infrared spectroelectrochemistry, and the obtained results indicate a multivalent nature.

The reactivity of [Rh₇(CO)₁₆]³⁻ with SbCl₃ has been deeply investigated as a new approach to prepare atomically precise metal nanoparticles. By varying the reaction conditions, we obtained three large carbonyl nanoclusters, [Rh₂₀Sb₃(CO)₃₆]³⁻, [Rh₂₁Sb₂(CO)₃₈]⁵⁻, and [Rh_28−xSb_x(CO)₄₄]⁶⁻, and the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]³⁻ species. They have all been characterized through X-ray diffraction, IR spectroscopy, and other techniques based on their specific nature. Spectroelectrochemical studies on [Rh₂₁Sb₂(CO)₃₈]⁵⁻ unravelled its multivalent nature.

From Mononuclear Complexes to Molecular Nanoparticles: The Buildup of Atomically Precise Heterometallic Rhodium Carbonyl Nanoclusters

Chemical research in synthesizing metal nanoparticles has been a major topic in the last two decades, as nanoparticles can be of great interest in many fields such as biology, catalysis, and nanotechnology. However, as their chemical and physical properties are size-dependent, the reliable preparation of nanoparticles at a molecular level is highly desirable. Despite the remarkable advances in recent years in the preparation of thiolate- or p-MBA or PA-protected gold and silver nanoclusters ( p-MBA = p-mercaptobenzoic acid; PA = phenylalkynyl), as well as the large palladium clusters protected by carbonyl and phosphine ligands that initially dominated the field, the synthesis of monodispersed and atomically precise nanoparticles still represents a great challenge for chemists. Carbonyl cluster compounds of high nuclearity have become more and more part of a niche chemistry, probably owing to their handling issues and expensive synthesis. However, even in large size, they are known at a molecular level and therefore can play a relevant role in understanding the structures of nanoparticles in general. For instance the icosahedral pattern, proper of large gold nanoparticles, is also found in some Au-Fe carbonyl cluster compounds. Rh clusters in general can also be employed as precursors in homo- and heterogeneous catalysis, and the possibility of doping them with other elements at the molecular level is an important additional feature. The fact that they can be obtained as large crystalline species, with dimensions of about 2 nm, allows one to place them not only in the nanometric regime, but also in the ultrafine-metal-nanoparticle category, which lately has been attracting growing attention. In fact, such small nanoparticles possess an even higher density of active catalytic sites than their larger (up to 100 nm) equivalents, hence enhancing atom efficiency and reducing the cost of precious-metal catalysts. Finally, the clusters' well-defined morphology could, in principle, contribute to expand the studies on the shape effects of nanocatalysts. In this Account, we want to provide the scientific community with some insights on the preparation of rhodium-containing carbonyl compounds of increasing nuclearity. Among them, we present the synthesis and molecular structures of two new heterometallic nanoclusters, namely, [Rh₂₃Ge₃(CO)₄₁]^5- and [Rh₁₆Au₆(CO)₃₆]^6-, which have been obtained by reacting a rhodium-cluster precursor with Ge(II) and Au(III) salts. The growth of such clusters is induced by redox mechanisms, which allow going from mononuclear complexes up to clusters with over 20 metal atoms, thus entering the nanosized regime.