From M6 to M12, M19 and M38 molecular alloy Pt-Ni carbonyl nanoclusters: selective growth of atomically precise heterometallic nanoclusters

Heterometallic Chini-type clusters [Pt_6-xNi_x(CO)₁₂]^2- (x = 0-6) were obtained by reactions of [Pt₆(CO)₁₂]^2- with Ni-clusters such as [Ni₆(CO)₁₂]^2-, [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, or from [Pt₉(CO)₁₈]^2- and [Ni₆(CO)₁₂]^2-. The Pt/Ni composition of [Pt_6-xNi_x(CO)₁₂]^2- (x = 0-6) depended on the nature of the reagents employed and their stoichiometry. Reactions of [Pt₉(CO)₁₈]^2- with [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, as well as reactions of [Pt₁₂(CO)₂₄]^2- with [Ni₆(CO)₁₂]^2-, [Ni₉(CO)₁₈]^2- and [H₂Ni₁₂(CO)₂₁]^2-, afforded [Pt_9-xNi_x(CO)₁₈]^2- (x = 0-9) species. [Pt_6-xNi_x(CO)₁₂]^2- (x = 1-5) were converted into [Pt_12-xNi_x(CO)₂₁]^4- (x = 2-10) upon heating in CH₃CN at 80 °C, with almost complete retention of the Pt/Ni composition. Reaction of [Pt_12-xNi_x(CO)₂₁]^4- (x ≈ 8) with HBF₄·Et₂O afforded the [HPt_14+xNi_24-x(CO)₄₄]^5- (x ≈ 0.7) nanocluster. Finally, [Pt_19-xNi_x(CO)₂₂]^4- (x = 2-6) could be obtained by heating [Pt_9-xNi_x(CO)₁₈]^2- (x = 1-3) in CH₃CN at 80 °C, or [Pt_6-xNi_x(CO)₁₂]^2- (2-4) in DMSO at 130 °C. The molecular structures of these new alloy nanoclusters have been determined by single crystal X-ray diffraction. The site preference of Pt and Ni within their metal cages has been computationally investigated. The electrochemical and IR spectroelectrochemical behavior of [Pt_19-xNi_x(CO)₂₂]^4- (x = 3.11) has been studied and compared to the isostructural homometallic nanocluster [Pt₁₉(CO)₂₂]^4-.

From Small Metal Clusters to Molecular Nanoarchitectures with a Core-Shell Structure: The Synthesis, Redox Fingerprint, Theoretical Analysis, and Solid-State Structure of [Co38As12(CO)50]4

The cluster [Co₃₈As₁₂(CO)₅₀]^4- was obtained by pyrolysis of [Co₆As(CO)₁₆]^-. The metal cage features a closed-packed core inside a Co/As shell that progressively deforms from a cubic face-centered symmetry. The redox and acid-base reactivities were determined by cyclic voltammetry and spectrophotometric titrations. The calculated electron density revealed the shell-constrained distribution of the atomic charges, induced by the presence of arsenic.

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.

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.

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

The reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.5 equiv of PCl₃ affords the monophosphide [Ni₁₁P(CO)₁₈]^3- that in turn further reacts with PCl₃ resulting in the tetra-phosphide carbonyl cluster [HNi₃₁P₄(CO)₃₉]^5-. Alternatively, the latter can be obtained from the reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.8-0.9 equiv of PCl₃. The [HNi₃₁P₄(CO)₃₉]^5- penta-anion is reversibly protonated by strong acids leading to the [H₂Ni₃₁P₄(CO)₃₉]^4- tetra-anion, whereas deprotonation affords the [Ni₃₁P₄(CO)₃₉]^6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni₃₁P₄(CO)₃₉]^7- hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [Ni₁₁P(CO)₁₈]^3-, [HNi₃₁P₄(CO)₃₉]^5-, and [H₂Ni₃₁P₄(CO)₃₉]^4-. The reversible formation of the stable [Ni₁₁P(CO)₁₈]^4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni₁₁P(CO)₁₈]^3-. The redox changes of [HNi₃₁P₄(CO)₃₉]^5- show features of chemical reversibility and the vibrational spectra in the ν_CO region of the nine redox states of the cluster [HNi₃₁P₄(CO)₃₉]^n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H₂Ni₃₁P₄(CO)₃₉]^4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H₂Ni₃₁P₄(CO)₃₉]^n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi₃₁P₄(CO)₃₉]^5- and [H₂Ni₃₁P₄(CO)₃₉]^4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by ¹H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.

Structural rearrangements induced by acid-base reactions in metal carbonyl clusters: the case of [H(3-n)Co15Pd9C3(CO)38]n- (n = 0-3)

The new bimetallic [HCo15Pd9C3(CO)38](2-) tri-carbide carbonyl cluster has been obtained from the reaction of [H2Co20Pd16C4(CO)48](4-) with an excess of acid in CH2Cl2 solution. The mono-hydride di-anion can be reversibly protonated and deprotonated by means of acid-base reactions leading to closely related [H(3-n)Co15Pd9C3(CO)38](n-) (n = 0-3) clusters. The crystal structures of the three anionic and the neutral clusters have been determined as their H3Co15Pd9C3(CO)38·2thf, [NEt4][H2Co15Pd9C3(CO)38]·0.5C6H14, [NMe3(CH2Ph)]2[HCo15Pd9C3(CO)38]·C6H14 and [NEt4]3[Co15Pd9C3(CO)38]·thf salts. They are composed of a Pd9(μ3-CO)2 core stabilised by three Co5C(CO)12 organometallic fragments. The poly-hydride nature of these clusters has been indirectly inferred via chemical, electrochemical and magnetic measurements. Besides, cyclic voltammetry shows that the [H(3-n)Co15Pd9C3(CO)38](n-) (n = 1-3) anions are multivalent, since they undergo two or three reversible oxidations. SQUID measurements of [HCo15Pd9C3(CO)38](2-) indicate that this even electron cluster is paramagnetic with two unpaired electrons, giving further support to its hydride nature. Finally, structural studies show that the Pd9 core of [H(3-n)Co15Pd9C3(CO)38](n-) (n = 0,1) is a tri-capped octahedron, which becomes a tri-capped trigonal prism in the more charged [H(3-n)Co15Pd9C3(CO)38](n-) (n = 2,3) anions. Such a significant structural rearrangement of the metal core of a large carbonyl cluster upon protonation-deprotonation reactions is unprecedented in cluster chemistry, and suggests that interstitial hydrides may have relevant stereochemical effects even in large carbonyl clusters.

Selective synthesis of the [Ni36Co8C8(CO)48]6- octa-carbide carbonyl cluster by thermal decomposition of the [H2Ni22Co6C6(CO)36]4- hexa-carbide

The thermal decomposition in thf solution of [H2Ni22Co6C6(CO)36](4-) results in the new [HNi36Co8C8(CO)48](5-) bimetallic Ni-Co octa-carbide, which can be converted into the closely related [H6-nNi36Co8C8(CO)48](n-) (n = 3-6) polyhydrides by means of acid-base reactions. The structure of the [Ni36Co8C8(CO)48](6-) hexa-anion has been established via X-ray crystallography, showing that the eight interstitial carbide atoms are lodged within different metal cages. Thus, two C-atoms are enclosed within regular square anti-prismatic Ni8C cages, four within irregular Ni8C square anti-prismatic cages, and the last two within mono-capped trigonal prismatic Ni5Co2C cages. The structure of [Ni36Co8C8(CO)48](6-) is non-compact and closely related to [Ni32C6(CO)38](6-) and [HNi38C6(CO)44](5-). [Ni36Co8C8(CO)48](6-) approaches the nanosize regime and the whole molecular ion has a diameter (measured from the outer oxygen atoms) of ca. 1.61 nm.

Ni-Cu tetracarbide carbonyls with vacant Ni(CO) fragments as borderline compounds between molecular and quasi-molecular clusters

The reaction of [Ni(9)C(CO)(17)](2-) with [Cu(CH(3)CN)(4)][BF(4)] (1.1-1.5 equiv.) afforded the first Ni-Cu carbide carbonyl cluster, i.e., [H(2)Ni(30)C(4)(CO)(34){Cu(CH(3)CN)}(2)](4-) ([H(2)1](4-)). This has been crystallised in a pure form with miscellaneous [NR(4)](+) (R = Me, Et) cations, as well as co-crystallised with [H(2)Ni(29)C(4)(CO)(33){Cu(CH(3)CN)}(2)](4-) ([H(2)2](4-)) which differs from [H(2)1](4-) by a missing Ni(CO) fragment. By increasing the [Cu(CH(3)CN)(4)](+)/[Ni(9)C(CO)(17)](2-) ratio to 1.7-1.8, the closely related [H(2)Ni(30)C(4)(CO)(35){Cu(CH(3)CN)}(2)](2-) ([H(2)3](2-)), [H(2)Ni(29)C(4)(CO)(34){Cu(CH(3)CN)}(2)](2-) ([H(2)4](2-)), and [H(2)Ni(29)C(4)(CO)(32)(CH(3)CN)(2){Cu(CH(3)CN)}(2)](2-) ([H(2)5](2-)) dianions have been obtained. Replacement of Cu-bonded CH(3)CN with p-NCC(6)H(4)CN afforded, after protonation of the tetra-anion, mixtures of [H(3)Ni(30)C(4)(CO)(34){Cu(NCC(6)H(4)CN)}(2)](3-) ([H(3)6](3-)) and [H(3)Ni(29)C(4)(CO)(33){Cu(NCC(6)H(4)CN)}(2)](3-) ([H(3)7](3-)). The species 1-7 display a common Ni(28)C(4)Cu(2) core and differ for the charge, the presence of additional Ni atoms, the number and nature of the ligands, even though they are obtained under similar experimental conditions and often in mixtures. Their nature in solution has been investigated via FT-IR, chemical and electrochemical methods. Electrochemical studies, besides confirming the poly-hydride nature of these clusters, show that they undergo different redox processes with features of chemical reversibility, and this might be taken as proof of the incipient metallisation of their metal cores.

Surface decorated platinum carbonyl clusters

Four molecular Pt-carbonyl clusters decorated by Cd-Br fragments, i.e., [Pt(13)(CO)(12){Cd(5)(μ-Br)(5)Br(2)(dmf)(3)}(2)](2-) (1), [Pt(19)(CO)(17){Cd(5)(μ-Br)(5)Br(3)(Me(2)CO)(2)}{Cd(5)(μ-Br)(5)Br(Me(2)CO)(4)}](2-) (2), [H(2)Pt(26)(CO)(20)(CdBr)(12)](8-) (3) and [H(4)Pt(26)(CO)(20)(CdBr)(12)(PtBr)(x)](6-) (4) (x = 0-2), have been obtained from the reactions between [Pt(3n)(CO)(6n)](2-) (n = 2-6) and CdBr(2)·H(2)O in dmf at 120 °C. The structures of these molecular clusters with diameters of 1.5-2 nm have been determined by X-ray crystallography. Both 1 and 2 are composed of icosahedral or bis-icosahedral Pt-CO cores decorated on the surface by Cd-Br motifs, whereas 3 and 4 display a cubic close packed Pt(26)Cd(12) metal frame decorated by CO and Br ligands. An oversimplified and unifying approach to interpret the electron count of these surface decorated platinum carbonyl clusters is suggested, and extended to other low-valent organometallic clusters and Au-thiolate nanoclusters.

Nickel poly-acetylide carbonyl clusters: structural features, bonding and electrochemical behaviour

The reactions of [NEt(4)](2)[Ni(6)(CO)(12)] with miscellaneous carbon halides (e.g. CCl(4), C(4)Cl(6)) in CH(2)Cl(2) have been extensively investigated particularly focusing on the stoichiometric ratio of the reagents and reaction temperature. This allowed the preparation of the previously known acetylide clusters [Ni(16)(C(2))(2)(CO)(23)](4-), [HNi(25)(C(2))(4)(CO)(32)](3-) and [Ni(22)(C(2))(4)(CO)(28)Cl](3-), as well as isolation and full characterisation of the closely related [Ni(17)(C(2))(2)(CO)(24)](4-) and [Ni(25)(C(2))(4)(CO)(32)](4-) tetraanions. From a structural point of view, all these clusters are based on a Ni(16) square orthobicupola which contain interstitial C(2), Ni(η(2)-C(2))(4) or Ni(2)(μ-η(2)-C(2))(4) moieties, displaying rather short C-C bonds. Electrochemical studies reveal that all these species undergo different redox processes, even if their stability is rather limited. This is corroborated by an extensive analysis of the interaction between interstitial C(2) acetylide units and the metal cluster cage by Extended Huckel Molecular Orbital (EHMO) calculations, which indicates that tightly bonded C-C units are less effective than isolated C-atoms in stabilising the cluster cage.