Giuliano Longoni

High Nuclearity Metal Carbonyl Clusters

Publisher Summary The chapter focuses on the compounds containing five or more metal atoms. The slow rate of publication in this area is mainly due to the number of steps required by this research, i.e., synthesis, crystallization, structural identification, and chemical characterization. More than fifty different examples of high nuclearity carbonyl clusters (HNCC) arc presently known, all of which contain Group VIII transition metals. In post-transition metals the increased separation between the (n - 1) d and ns-np orbital is probably responsible for the low stability of their bonds with the highly π-acidic carbon monoxide ligand; a number of high nuclearity clusters with less π -acidic ligands, such as tertiary phosphines or organic donor groups, is known. Approximate calculations on some of the more crowded clusters, such as [Fe 4 (CO) 13 ] 2- , Fe 5 (CO) 15 C, and Ru 6 (CO) 18 H 2 , show that, at the level of the carbon atoms, about 96% of the available surface is occupied. This figure seems very high particularly if the distribution of the carbonyl groups is not considered as homogeneous. The high Nuclearity clusters of the transition metals that precede Group VIII are, therefore, expected to be destabilized by steric crowding, although some carbides and mixed nitrosyl-carbonyl derivatives should be sterically possible.

Chelate organometallic compounds of nickel(II), palladium(II) and platinum(II) derived from N,N-dialkyl- benzylamines

Abstract The preparation of organometallic compounds of nickel(II), palladium(II) and platinum(II) containing two chelate five-membered rings of type (A) (M = Ni, R=Me; M=Pd, R=Me, Et; M=Pt, R=Me, Et) are described. The platium(II) derivatives are monomeric, and the corresponding cis and trans isomers have been characterized. The palladium(II) derivatives are also monomeric, but only the cis derivatives have been isolated. There is a sharp change of reactivity in going from the nickel to the platinum derivatives, as well as of solubility in going from the N,N -dimethyl to N,N -diethyl derivatives. The reactions with carbon monoxide have been studied in detail, and several different types of acyl complexes of platinum(II) have been isolated.

Physical properties of metal cluster compounds I: Magnetic measurements on high-nuclearity nickel and platinum carbonyl clusters

Abstract Metal-carbonyl cluster compounds are composed of macromolecules that consist of a core of metal atoms coordinated by a “shell” of CO ligands. We present susceptibility and high-field magnetization measurements on various high-nuclearity platinum and nickel clusters. The compounds display very unusual low-temperature magnetic behaviour, which we ascribe to quantum-size-effects arising from the discreteness of the electronic energy levels due to the small dimensions of the metal clusters (10–40 atoms). We compare our results with predictions from molecular orbital calculations.

Stereochemical non-rigidity of a metal polyhedron; carbon-13 and platinum-195 fourier transform nuclear magnetic resonance spectra of [Ptn(CO)2n]2- (n = 3, 6, 9, 12 or 15)

Abstract Platinum-195 spectra are reported for [Ptn(CO)2n]2- (n = 3, 6, 9, 12 and 15) and carbon-13 spectra are reported for n = 6, 9 and 12 over a range of temperatures. The spectra provide evidence for (a) intramolecular rotation of the Pt3-triangles about the principal three-fold axis, (b) inter-exchange of Pt3-triangles, (c) lack of terminal/edge carbonyl exchange within the Pt3(CO)3(μ-CO)3 group. Evidence is also presented for the formation of [Ni3Pt3(CO)12]2- on mixing [Pt6(CO)12]2- [Ni6(CO)12]2-.

Hydroformylation of olefins under mild conditions: Part I: the Co4−nRhn(CO)12 + x L (n = 0, 2, 4 ; x = 0 - 9) system and preformed Rh4(CO)12−xLx clusters (x = 1 – 4)

Abstract The hydroformylation of cyclohexene, 1-pentene and styrene under mild conditions (25–50 °C, 1 atm equimolar mixture of CO and H 2 ) has been investigated using as catalyst precursor either the Co 4- n Rh n (CO) 12 + x L ( n = 0, 2, 4; x = 0 – 9) system or preformed Rh 4 CO) 12− x L x ( x = 1 – 4) substituted clusters, where L is a trisubstituted phosphine or phosphite. The activity of these systems increases as a function of x , and reaches a maximum for a L/Co 4− n Rh n (CO) 12 ( n = 2, 4) molar ratio of ca . 5 – 6. A further increase in this ratio corresponds to a smooth decrease in the activity. This ratio has apparently a negligible effect on the regioselectivity in the hydroformylation of both 1-pentene and styrene. In contrast, both the activity and the regioselectivity are significantly affected by the nature of the ligand employed as cocatalyst. When working with Rh 4 (CO) 12 as well as Rh 6 (CO) 16 , and trisubstituted phosphites as ligands, infrared spectroscopy and 31 P NMR invariably show the presence of Rh 4 (CO) 9 L 3 as the most substituted rhodium carbonyl species present in solution, and there is no evidence of fragmentation of the tetranuclear cluster during the catalytic process. In contrast, when using phosphine ligands such as PPh 3 , evidence of fragmentation to Rh 2 (CO) 6 (PPh 3 ) 2 or to Rh 2 (CO) 4 (PPh 3 ) 4 species has been obtained at the higher PPh 3 /Rh 4 (CO) 12 molar ratios. Degradation of the ligand employed as cocatalyst, particularly the arylsubstituted phosphines, is observed, and this is probably at the origin of the loss of catalytic activity of some of these systems with time.

An Organometallic Approach to Gold Nanoparticles: Synthesis and X‐Ray Structure of CO‐Protected Au21Fe10, Au22Fe12, Au28Fe14, and Au34Fe14 Clusters

Ligand-stabilized quasi-molecular gold nanoparticles, consisting of chunks of cubic close-packed lattice featuring “magic numbers” of gold atoms, are being intensively investigated both experimentally and theoretically, owing to their potential use in miniaturized basic devices in electronics, models and precursors of metallic catalysts, stains of biological samples, and imaging nanoprobes for drug screening and diagnosis. The monodispersity of several of the above thiol/thiolate or phosphine monolayer protected gold nanoparticles has been disputed. Furthermore, STM experiments and DFT calculations of adsorption of methylthiolate on Au(111) showed formation of linear RS-Au-SR staple motives. This feature was later confirmed by calculations on an Au38 cluster of Oh symmetry, [8] and has been experimentally demonstrated by the first structural characterizations of gold–thiolate clusters, namely [Au25(SCH2CH2Ph)18] [9] and the giant [Au102(p-MBA)44] (pMBA = p-mercaptobenzoic acid) cluster. The only other structurally characterized gold particles exceeding 1 nm in at least one dimension are the ligand-protected [Au16(AsPh3)8Cl6], [11] [Au25(PPh3)10(SEt)5Cl2] , and [Au39(PPh3)14Cl6]Cl2 clusters, [13] which do not display such a feature. Herein, we report an organometallic approach to a new kind of molecular ligand-stabilized gold nanoparticle, consisting of the synthesis of Au–Fe colloidal nanoparticles, in which {Fe(CO)x} (x= 3, 4) moieties take the place of thiol or thiolate ligands in protecting and stabilizing the gold kernel. These iron carbonyl groups share and may also exceed the bonding versatility of thiols/thiolates. The synthesis of CO-protected Au–Fe clusters involves the oxidation of [Fe3(CO)11] 2 with [AuCl4] salts in acetone and under an inert atmosphere. After formation of the previously unknown yellow-orange [Au5{Fe(CO)4}4] 3 cluster, the reaction affords brown solutions of colloidal Au–Fe nanoparticles, which display broad and unresolved IR carbonyl absorptions shifting from 1960 to 2010 cm 1 as a function of the starting ratio of the reagents. The colloidal nature of these solutions was confirmed by dynamic light scattering (DLS) measurements in acetonitrile for two samples. The first sample (nCO at 1980 cm ) revealed the presence of two sets of nanoparticles displaying hydrodynamic diameters in the ranges 10–30 and 100–200 nm. The second sample (nCO at 1990 cm ) showed particles with nominal diameters of 35–60 and 110–300 nm. No evidence of smaller particles could be gathered. These results parallel recent measurements of solutions from which monodispersed [Au25(SCH2CH2Ph)x] was obtained in good yields. [15] The addition of Au salts in excess gives rise to separation of gold powder and the formation of the dark-green [Au{Fe2(CO)8}2] cluster, two isomers of which have previously been isolated and characterized by other routes. In our investigations of the above two samples we have so far isolated five molecular species, namely, [NEt4]3[Au5{Fe(CO)4}4] (nCO in CH3CN at 1945s, 1861s cm ), [NEt4]6[Au21{Fe(CO)4}10]·Cl (nCO in CH3CN at 1982s, 1937sh, 1889sh cm ), [NEt4]6[Au22{Fe(CO)4}12]·(CH3)2CO·0.5C6H14 (nCO in CH3CN at 1980s, 1925sh, 1880sh cm ), [NEt4]8[Au28{Fe(CO)3}4{Fe(CO)4}10]·6CH3CN (nCO in CH3CN at 1985s, 1927sh, 1887sh cm ) and [NEt4]10[Au34{Fe(CO)3}6{Fe(CO)4}8]·2Cl·7.6CH3CN (nCO in CH3CN at 1990s, 1932sh, 1900sh cm ). Their structures have been determined by single-crystal X-ray diffraction studies. The [Au5{Fe(CO)4}4] 3 cluster (1) is isostructural with the corresponding [Cu5{Fe(CO)4}4] 3 [18] and [Ag5{Fe(CO)4}4] 3 [14] species (see Figure S1 in the Supporting Information). As shown in Figure 1, [Au22{Fe(CO)4}12] 6 (2) may be formally envisioned to derive by sandwiching two [Au5] 3+ fragments between three [Au4{Fe(CO)4}4] 4 [19] moieties in a tripledecker fashion. The outer {Fe(CO)4} groups adopt C3v local symmetry of the carbonyl groups and behave as triply bridging (m3) ligands, whereas the central {Fe(CO)4} groups adopt C2v local symmetry of the carbonyl groups and behave as m4 ligands. The [Au21{Fe(CO)4}10] 5 structure (3) may be envisioned as a molecular model of LicurgoCs cup (Figure 2). The metal frame consists of an inner Au-centered pentagonal antiprism, at the top and bottom of which two pentagonal Au5Fe5 rings are condensed. As a result the whole metal frame may be described as deriving from two fused concave cups generated by a Au5Fe5-Au5-Au-Au5-Au5Fe5 sequence of layers, sharing the unique Au atom and with opposite orientations. In spite of [*] Dr. C. Femoni, Prof. M. C. Iapalucci, Prof. G. Longoni, C. Tiozzo, Dr. S. Zacchini Dipartimento di Chimica Fisica ed Inorganica Universit. di Bologna Viale Risorgimento 4, 40136 Bologna (Italy) Fax: (+39)051-209-3690 E-mail: femoni@ms.fci.unibo.it

Crystal structure of [Me3NCH2Ph][Fe4(CO)13H]. A ‘butterfly’ metal cluster with an unusually bonded carbonyl group

A single-crystal X-ray analysis has shown the [Fe4(CO)13H]– anion to contain a butterfly arrangement of metal atoms; twelve carbonyl groups are terminally bonded, three to each iron atom, whereas the thirteenth carbonyl group interacts with all the four iron atoms and behaves as a four-electron ligand.

[Pt19(CO)21(NO)]3− and [Pt38(CO)44]2−: Nitrosyl Bending through Intramolecular Electron Transfer as an Intermediate Step in the Nucleation Process from Polydecker to ccp Platinum Carbonyl Clusters

The intermediacy of CO/NO substitution in the condensation of [Pt19(CO)22]4− into [Pt38(CO)44]2− (structure shown) has been demonstrated. Two high-nuclearity carbonyl metal clusters, including one with an unprecedented nitrosyl ligand, have been synthesized and structurally characterized.

Iron, cobalt and nickel carbide-carbonyl clusters by CO scission

A new approach to the synthesis in good yields of known cobalt and iron carbidecarbonyl clusters by CO cleavage in mild conditions is reported. Cleavage of CO results from attaching an acetyl or benzoyl carbocation to the oxygen atom, and by transfer of electrons from an external source. This synthetic approach to carbide molecular clusters may be of some significance with respect to the formation of carbide atoms on to metal crystallites. Attempts to synthesize nickel carbide clusters with the same approach have only been partly successful. The new [Ni9C(CO)17]2~ and [Ni8C(CO)13]2~ have been obtained more conveniently from the reaction of [Ni6(CO)12]2- with CC14. The related reaction of [Ni6(CO)12]2- with Co3(CO)9CCl results in the formation of the mixed-metal carbide cluster [Co3Ni9C(CO)20]3_. This compound is degraded under a carbon monoxide and hydrogen mixture (25 °C, 1 atm) to Ni(CO)4, [Co(CO)4]~ and ethane. Intermediate formation of [Co3Ni7(C-C) (CO)15]3-, in which the two carbide atoms show an interatomic separation of 1.43 A, or of a related species, would provide a possible pathway for C-C bond formation.

Self-Assembly of [Pt3n(CO)6n]2− (n = 4−8) Carbonyl Clusters: from Molecules to Conducting Molecular Metal Wires

A comprehensive study discussing the different parameters that influence the self-assembly of [Pt(3n)(CO)(6n)](2-) (n = 4-8) clusters with miscellaneous mono- and dications into 0-D, 1-D, 2-D, and 3-D materials is herein reported. As an unexpected bonus, the use of Ru(II) dications allowed the first structural characterization of the previously unknown [Pt(21)(CO)(42)](2-) dianion. 0-D structures, which contain isolated ions, are electrical insulators in solid form. Conversely, as soon as infinite chains of clusters are formed, the electrical resistivity, measured in pressed pellets, decreases to 10(5)-10(6), 10(4), and 10(2) ohms cm for discontinuous, semicontinuous, and continuous chains, respectively. Therefore, the resemblance of these materials to molecular metal wires is not only morphological but also functional. Preliminary results of possible self-assembly phenomena in a solution of [Pt(15)(CO)(30)](2-) and [Pt(18)(CO)(36)](2-) according to dynamic light scattering experiments are also reported.