Heteroleptic Chini-Type Platinum Clusters: Synthesis and Characterization of Bis-Phospine Derivatives of [Pt3n(CO)6n]2- (n = 2-4)

2-D Molecular Alloy Ru-M (M = Cu, Ag, and Au) Carbonyl Clusters: Synthesis, Molecular Structure, Catalysis, and Computational Studies

The reactions of [HRu3(CO)11]- (1) with M(I) (M = Cu, Ag, and Au) compounds such as [Cu(CH3CN)4][BF4], AgNO3, and Au(Et2S)Cl afford the 2-D molecular alloy clusters [CuRu6(CO)22]- (2), [AgRu6(CO)22]- (3), and [AuRu5(CO)19]- (4), respectively. The reactions of 2-4 with PPh3 result in mixtures of products, among which [Cu2Ru8(CO)26]2- (5), Ru4(CO)12(CuPPh3)4 (6), Ru4(CO)12(AgPPh3)4 (7), Ru(CO)3(PPh3)2 (8), and HRu3(OH)(CO)7(PPh3)3 (9) have been isolated and characterized. The molecular structures of 2-6 and 9 have been determined by single-crystal X-ray diffraction. The metal-metal bonding within 2-5 has been computationally investigated by density functional theory methods. In addition, the [NEt4]+ salts of 2-4 have been tested as catalyst precursors for transfer hydrogenation on the model substrate 4-fluoroacetophenone using iPrOH as a solvent and a hydrogen source.

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.

Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field

Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)-protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR-protected metal NCs has been achieved in 2005. Since then, research on SR-protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR-protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the "synthesis" section, recent knowledge on the reactivity of NCs in solution is highlighted in the "understanding" section, and the applications of NCs in the energy and environmental field are highlighted in the "application" section. This review provides insight on the current state of research on SR-protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.

Heterometallic Ni-Pt Chini-Type Carbonyl Clusters: An Example of Molecular Random Alloy Clusters

The direct reactions of homometallic [Ni₆(CO)₁₂]^2– and [Pt₆(CO)₁₂]^2– Chini carbonyl clusters result in heterometallic Ni–Pt Chini-type clusters of the general formula [Pt_6–xNi_x(CO)₁₂]^2– (x = 0–6). Their molecular structures have been determined by single-crystal X-ray diffraction (SC-XRD), showing a common octahedral (staggered, D_3d) structure analogous to that of [Ni₆(CO)₁₂]^2–, whereas [Pt₆(CO)₁₂]^2– displays a trigonal-prismatic (eclipsed, D_3h) structure. This structural change after replacing one single Pt with Ni may be classified as an alloying effect, and it has been theoretically investigated by DFT methods. Spectroscopic (IR and ¹⁹⁵Pt and ¹³C NMR) and ESI-MS studies indicate that mixtures of [Pt_6–xNi_x(CO)₁₂]^2– (x = 0–6) clusters are actually present in solution, whose compositions may be varied in an almost continuous way. Thus, they may be viewed as random alloy clusters whose overall compositions depend on the stoichiometry of the reagents.

Molecular Pt−Ni random alloy clusters adopting a Chini-type structure have been obtained upon mixing related homometallic clusters and investigated by structural, spectroscopic, and theoretical methods.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Polymerization Isomerism in Co-M (M = Cu, Ag, Au) Carbonyl Clusters: Synthesis, Structures and Computational Investigation

The reaction of [Co(CO)4]- (1) with M(I) compounds (M = Cu, Ag, Au) was reinvestigated unraveling an unprecedented case of polymerization isomerism. Thus, as previously reported, the trinuclear clusters [M{Co(CO)4}2]- (M = Cu, 2; Ag, 3; Au, 4) were obtained by reacting 1 with M(I) in a 2:1 molar ratio. Their molecular structures were corroborated by single-crystal X-ray diffraction (SC-XRD) on isomorphous [NEt4][M{Co(CO)4}2] salts. [NEt4](3)represented the first structural characterization of 3. More interestingly, changing the crystallization conditions of solutions of 3, the hexanuclear cluster [Ag2{Co(CO)4}4]2- (5) was obtained in the solid state instead of 3. Its molecular structure was determined by SC-XRD as Na2(5)·C4H6O2, [PPN]2(5)·C5H12 (PPN = N(PPh3)2]+), [NBu4]2(5) and [NMe4]2(5) salts. 5 may be viewed as a dimer of 3 and, thus, it represents a rare case of polymerization isomerism (that is, two compounds having the same elemental composition but different molecular weights) in cluster chemistry. The phenomenon was further studied in solution by IR and ESI-MS measurements and theoretically investigated by computational methods. Both experimental evidence and density functional theory (DFT) calculations clearly pointed out that the dimerization process occurs in the solid state only in the case of Ag, whereas Cu and Au related species exist only as monomers.

Creation of active water-splitting photocatalysts by controlling cocatalysts using atomically precise metal nanoclusters

With global warming and the depletion of fossil resources, our fossil-fuel-dependent society is expected to shift to one that instead uses hydrogen (H₂) as clean and renewable energy. Water-splitting photocatalysts can produce H₂ from water using sunlight, which are almost infinite on the earth. However, further improvements are indispensable to enable their practical application. To improve the efficiency of the photocatalytic water-splitting reaction, in addition to improving the semiconductor photocatalyst, it is extremely effective to improve the cocatalysts (loaded metal nanoclusters, NCs) that enable the reaction to proceed on the photocatalysts. We have thus attempted to strictly control metal NCs on photocatalysts by introducing the precise-control techniques of metal NCs established in the metal NC field into research on water-splitting photocatalysts. Specifically, the cocatalysts on the photocatalysts were controlled by adsorbing atomically precise metal NCs on the photocatalysts and then removing the protective ligands by calcination. This work has led to several findings on the electronic/geometrical structures of the loaded metal NCs, the correlation between the types of loaded metal NCs and the water-splitting activity, and the methods for producing high water-splitting activity. We expect that the obtained knowledge will lead to clear design guidelines for the creation of practical water-splitting photocatalysts and thereby contribute to the construction of a hydrogen-energy society.

One-, Two-, and Three-Dimensional Self-Assembly of Atomically Precise Metal Nanoclusters

Metal nanoclusters (NCs), which consist of several, to about one hundred, metal atoms, have attracted much attention as functional nanomaterials for use in nanotechnology. Because of their fine particle size, metal NCs exhibit physical/chemical properties and functions different from those of the corresponding bulk metal. In recent years, many techniques to precisely synthesize metal NCs have been developed. However, to apply these metal NCs in devices and as next-generation materials, it is necessary to assemble metal NCs to a size that is easy to handle. Recently, multiple techniques have been developed to form one-, two-, and three-dimensional connected structures (CSs) of metal NCs through self-assembly. Further progress of these techniques will promote the development of nanomaterials that take advantage of the characteristics of metal NCs. This review summarizes previous research on the CSs of metal NCs. We hope that this review will allow readers to obtain a general understanding of the formation and functions of CSs and that the obtained knowledge will help to establish clear design guidelines for fabricating new CSs with desired functions in the future.

Organophosphines in PtP4 derivatives; structural aspects

In this review, we classify and analyze the structural data of more than 80 monomeric platinum coordination complexes with an inner coordination sphere of PtP4 in which only organophosphines are involved. On the basis of the coordination mode of respective organophosphines, these complexes can be divided into six subgroups: Pt(PL)4, Pt(PL)2(PL′)2, Pt(η2-P2L)(PL)2, Pt(η2-P2L)2, Pt(η2-P2L)(η2-P2L′)2, and Pt(η4-P4L). The chelating ligands forming wide varieties of metallocycles: (P=P), (PNP), (PCP), (PC2P), (PP2P), (PC2NP), (PNCNP), (PNPNP), (POHOP), (POBOP), (PCNCP), (PC3P), (PC4P), and (PC2OC2P). The effects of both steric and electronic factors reflect on the values of P-Pt-P chelate angles. The total mean values of Pt-P elongate in the order: 2.289 Å (tetradentate)<2.306 Å (monodentate)<2.320 Å (bidentate). The same order shows the respective covalent bond weaknesses.

Synthesis of [Pt12(CO)20(dppm)2]2- and [Pt18(CO)30(dppm)3]2- Heteroleptic Chini-type Platinum Clusters by the Oxidative Oligomerization of [Pt6(CO)12(dppm)]2

The reactions of [Pt₆(CO)₁₂]^2- with CH(PPh₂)₂ (dppm), CH₂═C(PPh₂)₂ (P^P), and Fe(C₅H₄PPh₂)₂ (dppf) proceed via nonredox substitution and result in the heteroleptic Chini-type clusters [Pt₆(CO)₁₀(dppm)]^2-, [Pt₆(CO)₁₀(P^P)]^2-, and [Pt₆(CO)₁₀(dppf)]^2-, respectively. Conversely, the reactions of [Pt₆(CO)₁₂]^2- with Ph₂P(CH₂)₄PPh₂ (dppb) and Ph₂PC≡CPPh₂ (dppa) can be described as redox fragmentation that afford the neutral complexes Pt(dppb)₂, Pt₂(CO)₂(dppa)₃, and Pt₈(CO)₆(PPh₂)₂(C≡CPPh₂)₂(dppa)₂. The oxidation of [Pt₆(CO)₁₀(dppm)]^2- results in its oligomerization to yield the larger heteroleptic Chini-type clusters [Pt₁₂(CO)₂₀(dppm)₂]^2-, [Pt₁₈(CO)₃₀(dppm)₃]^2-, and [Pt₂₄(CO)₄₀(dppm)₄]^2- (for the latter there is only IR spectroscopic evidence). All the clusters were characterized by means of IR and ³¹P NMR spectroscopies and electrospray ionization mass spectrometry. Moreover, the crystal structures of [NEt₄]₂[Pt₆(CO)₁₀(dppm)]·CH₃CN, [NEt₄]₂[Pt₁₂(CO)₂₀(dppm)₂]·2CH₃CN·2dmf, [NEt₄]₂[Pt₁₂(CO)₂₀(dppm)₂]·4dmf, [NEt₄]₂[Pt₆(CO)₁₀(dppf)]·2CH₃CN, Pt₂(CO)₂(dppa)₃·0.5CH₃CN, Pt₈(CO)₆(PPh₂)₂(C≡CPPh₂)₂(dppa)₂·2thf, and Pt(dppb)₂ were determined by single-crystal diffraction (dmf = dimethylformamide; thf = tetrahydrofuran).

Water soluble derivatives of platinum carbonyl Chini clusters: synthesis, molecular structures and cytotoxicity of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2-

The reactions of [Pt3n(CO)6n]2- (n = 2-5) homoleptic Chini-type clusters with increasing amounts of 1,3,5-triaza-7-phosphaadamantane (PTA) result in the stepwise substitution of one terminal CO ligand per Pt3 triangular unit up to the formation of [Pt3n(CO)5n(PTA)n]2- (n = 2-5). Competition between the nonredox substitution with retention of the nuclearity and the redox fragmentation to afford lower nuclearity heteroleptic Chini-type clusters is observed as a function of the amount of PTA and the nuclearity of the starting cluster. Because of this, [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- are more conveniently obtained via the oxidation of [Pt9(CO)15(PTA)3]2-. All the new species were spectroscopically characterized, and the structures of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- were determined by single-crystal X-ray diffraction. These clusters may be viewed as heteroleptic Chini-type clusters composed of stacks of four and five Pt3(μ-CO)3(CO)2(PTA) units, respectively. The solubility in water of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- has been determined and their cytotoxicity towards human ovarian (A2780) cancer cells and their cisplatin-resistant strain (A2780cisR) has been evaluated.

Reactions of Platinum Carbonyl Chini Clusters with Ag(NHC)Cl Complexes: Formation of Acid-Base Lewis Adducts and Heteroleptic Clusters

The reactions of anionic platinum carbonyl Chini clusters [Pt_3n(CO)_6n]^2- [n = 2 (1), 3 (2), 4 (3)] with Ag(IPr)Cl [IPr = C₃N₂H₂(C₆H₃ⁱPr₂)₂] afford the neutral acid-base Lewis adducts [Pt₉(CO)₁₈(AgIPr)₂] (4) and [Pt₆(CO)₁₂(AgIPr)₂] (5). These are thermally transformed into the homometallic heteroleptic neutral cluster [Pt₃(CO)₄(IPr)₂] (6). Alternatively, 6 can be obtained from the reactions of 1-3 with an excess of the free IPr carbene ligand. The formation of 6 is sometimes accompanied by trace amounts of [Pt₄(CO)₄(IPr)₃] (7). The reaction of 6 with free IPr affords the closely related [Pt₃(CO)₃(IPr)₃] (8) heteroleptic cluster by substitution of the unique terminal CO ligand with a third IPr ligand. The reactions of 1-3 with Ag(IMes)Cl [IMes = C₃N₂H₂(C₆H₂Me₃)₂] proceed differently from those involving Ag(IPr)Cl. Indeed, the only product isolated after workup is the bimetallic tetranuclear cluster [Pt₃(CO)₃(IMes)₃(AgCl)] (9). 9 slowly reacts under a CO atmosphere, resulting in the pentanuclear [Pt₅(CO)₇(IMes)₃] (10) complex. All of the new clusters 4-10 have been spectroscopically characterized and their molecular structures determined by X-ray crystallography. 4 and 5 retain the original trigonal-prismatic structures of the parent anionic Chini clusters, which are capped by two [Ag(IPr)]⁺ moieties. Conversely, 6-9 are based on a Pt₃ triangular core decorated by CO and N-heterocyclic carbene ligands as well as Pt(CO) (in the case of 7) and AgCl (9) moieties. 10 displays an edge-bridged tetrahedral geometry.