Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni11P(CO)18]3- and [H6-nNi31P4(CO)39]n- (n = 4 and 5)

Atomically Precise Ni-Pd Alloy Carbonyl Nanoclusters: Synthesis, Total Structure, Electrochemistry, Spectroelectrochemistry, and Electrochemical Impedance Spectroscopy

The molecular nanocluster [Ni36-xPd5+x(CO)46]6- (x = 0.41) (16-) was obtained from the reaction of [NMe3(CH2Ph)]2[Ni6(CO)12] with 0.8 molar equivalent of [Pd(CH3CN)4][BF4]2 in tetrahydrofuran (thf). In contrast, [Ni37-xPd7+x(CO)48]6- (x = 0.69) (26-) and [HNi37-xPd7+x(CO)48]5- (x = 0.53) (35-) were obtained from the reactions of [NBu4]2[Ni6(CO)12] with 0.9-1.0 molar equivalent of [Pd(CH3CN)4][BF4]2 in thf. After workup, 35- was extracted in acetone, whereas 26- was soluble in CH3CN. The total structures of 16-, 26-, and 35- were determined with atomic precision by single-crystal X-ray diffraction. Their metal cores adopted cubic close packed structures and displayed both substitutional and compositional disorder, in light of the fact that some positions could be occupied by either Ni or Pd. The redox behavior of these new Ni-Pd molecular alloy nanoclusters was investigated by cyclic voltammetry and in situ infrared spectroelectrochemistry. All three compounds 16-, 26-, and 35- displayed several reversible redox processes and behaved as electron sinks and molecular nanocapacitors. Moreover, to gain insight into the factors that affect the current-potential profiles, cyclic voltammograms were recorded at both Pt and glassy carbon working electrodes and electrochemical impedance spectroscopy experiments performed for the first time on molecular carbonyl nanoclusters.

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.

Structural Diversity in Molecular Nickel Phosphide Carbonyl Nanoclusters

The reaction of [Ni₆(CO)₁₂]^2– as a [NBu₄]⁺ salt in CH₂Cl₂ with 0.8 equiv of PCl₃ afforded [Ni₁₄P₂(CO)₂₂]^2–. In contrast, the reactions of [Ni₆(CO)₁₂]^2– as a [NEt₄]⁺ salt with 0.4–0.5 equiv of POCl₃ afforded [Ni_22–xP₂(CO)_29–x]^4– (x = 0.84) or [Ni₃₉P₃(CO)₄₄]^6– by using CH₃CN and thf as a solvent, respectively. Moreover, by using 0.7–0.9 mol of POCl₃ per mole of [NEt₄]₂[Ni₆(CO)₁₂] both in CH₃CN and thf, [Ni_23–xP₂(CO)_30–x]^4– (x = 0.82) was obtained together with [Ni₂₂P₆(CO)₃₀]^2– as a side product. [Ni_23–xP₂(CO)_30–x]^4– (x = 0.82) and [Ni₂₂P₆(CO)₃₀]^2– were separated owing to their different solubility in organic solvents. All the new molecular nickel phosphide carbonyl nanoclusters were structurally characterized through single crystal X-ray diffraction (SC-XRD) as [NBu₄]₂[Ni₁₄P₂(CO)₂₂] (two different polymorphs, P2₁/n and C2/c), [NEt₄]₄[Ni_23–xP₂(CO)_30–x]·CH₃COCH₃·solv (x = 0.82), [NEt₄]₂[Ni₂₂P₆(CO)₃₀]·2thf, [NEt₄]₄[Ni_22–xP₂(CO)_29–x]·2CH₃COCH₃( x = 0.84) and [NEt₄]₆[Ni₃₉P₃(CO)₄₄]·C₆H₁₄·solv. The metal cores’ sizes of these clusters range from 0.59 to 1.10 nm, and their overall dimensions including the CO ligands are 1.16–1.63 nm. In this respect, they are comparable to ultrasmall metal nanoparticles, molecular nanoclusters, or atomically precise metal nanoparticles. The environment of the P atoms within these molecular Ni–P–CO nanoclusters displays a rich diversity, that is, Ni₅P pentagonal pyramid, Ni₇P monocapped trigonal prism, Ni₈P bicapped trigonal prism, Ni₉P monocapped square antiprism, Ni₁₀P sphenocorona, Ni₁₀P bicapped square antiprism, and Ni₁₂P icosahedron.

Five new molecular nickel phosphide carbonyl clusters have been obtained displaying overall sizes in the range 1.16−1.63 nm and very diverse environments for the phosphide atoms.

Redox active Ni-Pd carbonyl alloy nanoclusters: syntheses, molecular structures and electrochemistry of [Ni22-xPd20+x(CO)48]6- (x = 0.62), [Ni29-xPd6+x(CO)42]6- (x = 0.09) and [Ni29+xPd6-x(CO)42]6- (x = 0.27)

A redox active Ni-Pd alloy nanocluster [Ni_22-xPd_20+x(CO)₄₈]^6- (x = 0.62) ([1]^6-) was obtained from the redox condensation of [NBu₄]₂[Ni₆(CO)₁₂] with 0.7-0.8 equivalents of Pd(Et₂S)₂Cl₂ in CH₂Cl₂. Conversely, [Ni_29-xPd_6+x(CO)₄₂]^6- (x = 0.09) ([2]^6-) and [Ni_29+xPd_6-x(CO)₄₂]^6- (x = 0.27) ([3]^6-) were obtained by employing [NEt₄]₂[Ni₆(CO)₁₂] and 0.6-0.7 equivalents of Pd(Et₂S)₂Cl₂ in CH₃CN. The molecular structures of these high nuclearity Ni-Pd carbonyl clusters were determined by single-crystal X-ray diffraction (SC-XRD). [1]^6- adopted an M₄₀ccp structure comprising five close-packed ABCAB layers capped by two additional Ni atoms. Conversely, [2]^6- and [3]^6- displayed an hcp M₃₅ metal core composed of three compact ABA layers. [1]^6-, [2]^6- and [3]^6- showed nanometric sizes, with the maximum lengths of their metal cores being 1.3 nm ([1]^6-) and 1.0 nm ([2]^6- and [3]^6-), which increased up to 1.9 and 1.5 nm, after including also the CO ligands. Ni-Pd distribution within their metal cores was achieved by avoiding terminal Pd-CO bonding and minimizing Pd-CO coordination. As a consequence, site preference and partial metal segregation were observed, as well as some substitutional and compositional disorders. Electrochemical and spectroelectrochemical studies revealed that [1]^6- and [2]^6- were redox active and displayed four and three stable oxidation states, respectively. Even though several redox active high nuclearity metal carbonyl clusters have been previously reported, the nanoclusters described herein represent the first examples of redox active Ni-Pd carbonyl alloy nanoclusters.

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.