High-yield syntheses of [Rh7(CO)16] and [Rh14(CO)25] working in ethylene glycol solution under 1 atm of CO

The anionic rhodium carbonyl clusters [Rh₇(CO)₁₆]³⁻ and [Rh₁₄(CO)₂₅]⁴⁻ can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl₃·nH₂O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl₃·nH₂O under 1 atm of CO at 50 °C to give [Rh(CO)₂Cl₂]⁻; (ii) addition of a base (CH₃CO₂Na or Na₂CO₃) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh₇(CO)₁₆]³⁻; 150 °C for [Rh₁₄(CO)₂₅]⁴⁻). These new syntheses are more convenient than those previously reported, especially since such clusters are not accessible via silica surface-mediated reactions. This different behavior is due to the particular stabilization on the silica surface and under 1 atm of CO of an anionic carbonyl cluster, called A, which does not allow the formation of a higher nuclearity carbonyl cluster, called B, which was shown to be the key-intermediate in the synthesis of [Rh₁₄(CO)₂₅]⁴⁻ working in ethylene glycol solution. Although it was not possible to isolate crystals of A and B suitable for X-ray structural determination, a combination of cyclovoltammetry, one of the few examples so far available of the use of this technique for anionic rhodium carbonyl clusters, infrared spectroscopy and elemental analyses suggest that A and B are probably the never reported [Rh₇(CO)₁₄]⁻ and [Rh₁₅(CO)₂₈]³⁻ clusters, respectively. In particular the tentative formulation of the two clusters was carried out by a non-conventional method based on the existence of a linear correlation between carbonyl frequencies of the main band and the [(charge/Rh atoms)/CO number] ratio.     

Surface supported metal cluster carbonyls. Chemisorption decomposition and reactivity of Rh4(CO)12 supported on silica and alumina

Chemisorption of Rh₄(CO)₁₂ on to a highly divided silica (Aerosil “0” from Degussa), Leads to the transformation: 3 Rh₄(CO)₁₂ → 2 Rh₆(CO)₁₆ + 4 CO. Such an easy rearrangement of the cluster cage implies mobility of zerovalent rhodium carbonyl fragments on the surface. Carbon monoxide is a very efficient inhibitor of this reaction, and Rh₄(CO)₁₂ is stable as such on silica under a CO atmosphere. Both Rh₄(CO)₁₂ and Rh₆(CO)₁₆ are easily decomposed to small metal particles of higher nuclearity under a water atmosphere and to rhodium(I) dicarbonyl species under oxygen. From the Rh^I(CO)₂ species it is possible to regenate first Rh₄(CO)₁₂ and then Rh₆(CO)₁₆ by treatment with CO (P_co ? 200 mm Hg) and H₂O (P_H2O ? 18 mm Hg). The reduction of Rh^I(CO)₂ surface species by water requires a nucleophilic attack to produce an hypothetical [Rh(CO)_n]_m species which can polymerize to small Rh₄ or Rh₆ clusters in the presence of CO but which in the absence of CO lead to metal particles of higher nuclearity. Similar results are obtained on alumina.     

ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity

An ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3-, [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- and [Co₆(CO)₈(μ₂-CO)₆C]^-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3- and of [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- are found to be mainly comprised of metal d_z² atomic orbitals, while for [Co₆(CO)₈(μ₂-CO)₆C]^- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis.     

Rhodium carbonyl cluster chemistry. Synthesis and chemical characterization of the anion [Rh6(CO)14]4−

The anion [Rh₆(CO)₁₄]^4? has been isolated from the reaction of [Rh₆(CO)₁₆] with alkali hydroxides in aqueous solution. It shows high reactivity towards electrophiles and in redox condensations with other rhodium clusters; and is rapidly decomposed by carbon monoxide.     

Reaktionen von phosphor-yliden mit übergangsmetallverbindungen : IX. Komplexe mit unsymmetrisch substituierten iminen

The novel anion [Rh₁₄H(CO)₂₅]^3? has been obtained by reaction of the tetra-anion [Rh₁₄(CO)₂₅]^4? with acids. Its structure can be related to that of the parent species, but there are differences in the cluster geometry and in the carbonyl stereochemistry. The metal atom polyhedron can be described as an array intermediate between a cubic body-centered packing and a cubic close-packed arrangement.     

Structural characterization of the anions [Rh6(CO)15X]− [X = COEt and CO(OMe)]

The anions [Rh₆(CO)₁₅X]^?, with X = COEt and CO(OMe), have been studied by single-crystal X-ray diffraction. They contain octahedral rhodium clusters, with mean metalmetal distances of 2.779 and 2.765 », respectively. The carbonyl stereochemistry in the two anions is similar to that of Rh₆(CO)₁₆, with one terminal CO group replaced by the X ligand. The RhC(carbomethoxy) bond distance (1.96(2) ») is significantly shorter than the RhC(acyl) distance (2.06(2) »).     

New Bimetallic Ni–Rh Carbonyl Clusters: Synthesis and X-ray Structure of the [Ni7Rh3(CO)18]3?, [Ni3Rh3(CO)13]3? and [NiRh8(CO)19]2? Cluster Anions

The reaction of the [Ni₆(CO)₁₂]²⁻ dianion with [Rh(COD)Cl]₂ (COD = cyclooctadiene) in acetone affords a mixture of bimetallic Ni–Rh clusters, mainly consisting of the new [Ni₇Rh₃(CO)₁₈]³⁻ and [Ni₈Rh(CO)₁₈]³⁻ trianions. A study of the reactivity of [Ni₇Rh₃(CO)₁₈]³⁻ led to isolation of the new [Ni₃Rh₃(CO)₁₃]³⁻ and [NiRh₈(CO)₁₉]²⁻ anions. All these new bimetallic Ni–Rh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and have been characterised by elemental analysis, X-ray diffraction studies, ESI-MS and electrochemistry. The unit cell of the [NEt₄]₃[Ni₇Rh₃(CO)₁₈] salt contains two orientationally-disordered ν₂-tetrahedral [Ni₇Rh₃(CO)₁₈]³⁻ trianions with occupancy factors of 0.75 and 0.25. Besides, their inner Ni₃Rh₃ octahedral moieties show two cis sites purely occupied by Rh atoms, two trans sites purely occupied by Ni atoms and the remaining two cis sites are disordered Ni and Rh sites with respective occupancy fraction of 0.5. At difference from the parent [Ni₇Rh₃(CO)₁₈]³⁻, the octahedral [Ni₃Rh₃(CO)₁₃]³⁻ displays an ordered distribution of Ni and Rh atoms in two staggered triangles. The [NiRh₈(CO)₁₉]²⁻ dianion adopts an isomeric metal frame with respect to that of the [PtRh₈(CO)₁₉]²⁻ congener. As a fallout of this work, new high-yield synthesis of the known [Ni₆Rh₃(CO)₁₇]³⁻ and [Ni₆Rh₅(CO)₂₁]³⁻, as well as other currently-investigated bimetallic Ni–Rh clusters have been obtained.     

Mössbauer study of some Fe-containing M6 and M5 clusters

Isomer shift (δ) and quadrupole splitting (Δ) parameters have been assigned to the iron sites in [FeRh₅(CO)₁₆]⁻, trans- and cis-[Fe₂Rh₄(CO)₁₆]²⁻, [Fe₃-Rh₃(CO)₁₇]³⁻, [FeRh₄(CO)₁₅]²⁻, [Fe₃Pt₃(CO)₁₅]²⁻ and [Fe₄M(CO)₁₆]²⁻ (M = Pd or Pt) from ⁵⁷Fe Mössbauer spectra recorded at 78 K. The data for the closo compounds [FeRh₅(CO)₁₆]⁻ and [Fe₂Rh₄(CO)₁₆]²⁻ are compared with those for [Fe₆(CO)₁₆C]²⁻. In [Fe₃Rh₃(CO)₁₇]³⁻, the three major Fe sites were identified. For both [Fe₄M(CO)₁₆]²⁻ compounds two isomers were shown to be present in the solid state.     

High nuclearity PtRh carbonyl clusters. Synthesis and X-ray characterization of the [Pt2Rh9(CO)22]3− trianion

The new mixed PtRh cluster trianion [Pt₂Rh₉(CO)₂₂]^3? has been isolated as a minor product of the pyrolysis of [PtRh₅(CO)₁₅]^?, and has been characterized by X-ray diffraction. The metal skeleton, which has ideal D_3h symmetry, consists of three face-to-face condensed octahedra, as previously found in the isoelectronic species [Rh₁₁CO)₂₃]^3?, with the Pt atoms on the three-fold axis, in the positions of maximum MM connectivity.     

Controlled Expansion of a Strong‐Field Iron Nitride Cluster: Multi‐Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity

Multimetallic clusters have long been investigated as molecular surrogates for reactive sites on metal surfaces. In the case of the μ₄‐nitrido cluster [Fe₄(μ₄‐N)(CO)₁₂]^?, this analogy is limited owing to the electron‐withdrawing effect of carbonyl ligands on the iron nitride core. Described here is the synthesis and reactivity of [Fe₄(μ₄‐N)(CO)₈(CNAr^Mes2)₄]^?, an electron‐rich analogue of [Fe₄(μ₄‐N)(CO)₁₂]^?, where the interstitial nitride displays significant nucleophilicity. This characteristic enables rational expansion with main‐group and transition‐metal centers to yield unsaturated sites. The resulting clusters display surface‐like reactivity through coordination‐sphere‐dependent atom rearrangement and metal–metal cooperativity.     

Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant d_π orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled d_π orbitals. Both ligands readily form bridges that span M? M bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)₃], [CpW(CO)₂], [Co(CO)₃], and CH are related by the isolobal analogy. Thus the compounds [(RO)₃W ? W(OR)₃], [Cp(CO)₂W?W(CO)₂Cp], hypothetical [(CO)₃Co?Co(CO)₃], and HC?CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W₃(μ₃-CR)(OR′)₉] versus [Co₃(μ₃-CR)(CO)₉] and [W₄(C)(NMe)(OiPr)₁₂] versus [Fe₄(C)(CO)₁₃]—but differ with respect to M? M bonding. The carbonyl clusters use e_g-type orbitals for M? M bonding whereas the alkoxide clusters employ t_2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M? M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M₂(OR)₆] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.     

Salts of Anionic Metal Carbonyl Clusters with Cryptand[2.2.2](Na+), DB‐18‐crown‐6(Na+), and Paramagnetic Cp*2Cr+ Cations Obtained by Reduction

Reduction of neutral metal clusters (Co₄(CO)₁₂, Ru₃(CO)₁₂, Fe₃(CO)₁₂, Ir₄(CO)₁₂, Rh₆(CO)₁₆, {CpMo(CO)₃}₂, {Mn(CO)₅}₂) by decamethylchromocene (Cp*₂Cr) or sodium fluorenone ketyl in the presence of cryptand[2.2.2] and DB‐18‐crown‐6 was studied. Nine new salts with paramagnetic Cp*₂Cr⁺, cryptand[2.2.2](Na⁺), and DB‐18‐crown‐6(Na⁺) cations and [Co₆(CO)₁₅]^2– ( 1 , 2 ), [Ru₆(CO)₁₈]^2– ( 3 – 4 ) dianions, [Rh₁₁(CO)₂₃]^3– ( 6 ) trianions, and new [Ir₈(CO)₁₈]^2– ( 5 ) dianions were obtained and structurally characterized. The increase of nuclearity of clusters under reduction was shown. Fe₃(CO)₁₂ preserves the Fe₃ core under reduction forming the [Fe₃(CO)₁₁]^2– dianions in 7 . The [CpMo(CO)₃]₂ and [Mn(CO)₅]₂ dimers dissociate under reduction forming mononuclear [CpMo(CO)₃]^– ( 8 ) and [Mn(CO)₅]^– ( 9 ) anions. In all anions the increase of negative charge on metal atoms shifts the bands attributed to carbonyl C–O stretching vibrations to smaller wavenumbers in agreement with the elongation of the C–O bonds in 1 – 9 . In contrast, the M–C(CO) bonds are noticeably shortened at the reduction. Magnetic susceptibility of the salts with Cp*₂Cr⁺ is defined by high spin Cp*₂Cr⁺ (S = 3/2) species, whereas all obtained anionic metal clusters and mononuclear anions are diamagnetic. Rather weak magnetic coupling between S = 3/2 spins is observed with Weiss temperature from –1 to –11 K. That is explained by rather long distances between Cp*₂Cr⁺ and the absence of effective π–π interaction between them except compound 7 showing the largest Weiss temperature of –11 K. The {DB‐18‐crown‐6(Na⁺)}₂[Co₆(CO)₁₅]^2– units in 2 are organized in infinite 1D chains through the coordination of carbonyl groups of the Co₆ clusters to the Na⁺ ions and π–π stacking between benzo groups of the DB‐18‐crown‐6(Na⁺) cations.     

Iridium Cluster Chemistry: The Synthesis and the Solid-State Structure of [Ir11(CO)23]3−

The cluster [Ir₁₁(CO)₂₃]^3– was obtained by reaction of [Ir₁₀(CO)₂₁]^2– and [Ir(CO)₄]^– in refluxing MeCN, and its solid-state structure was determined on the salt [NEt₄]₃[Ir₁₁(CO)₂₃]. The metallic framework of D_3h symmetry is composed by three face-fused octahedra, all sharing a common edge. The cluster contains 9 edge bridging and 14 terminal carbonyl ligands, a disposition different from that of the two isomeric forms of the isoelectronic [Rh₁₁(CO)₂₃]^3–, both having, in the solid-state, more edge-bridging COs. Naked clusters of non-transition metals, found in binary and ternary materials, such as Cs₁₁O₃, display very similar trioctahedral polyedra.     

Double deoxygenation of PPN(NO2) using metal carbonyl clusters of cobalt,rhodium, and iridium

The reaction of PP(NO₂) with M₄(CO)₁₂ (M = Co, Rh) gives the nitrido clusters [M₆N(CO)₁₅]^? in 13 and 21% yields, respectively. A high yield synthesis (77%) of [Rh₆N(CO)₁₅)]^? directly from Rh₆(CO)₁₆ and PPN(NO₂) is also presented. PPN(NO₂) reacts with Ir₄(CO)₁₂ to give the new isocyanato cluster, [Ir₄(NCO)(CO)₁₁]^? in 34% yield, while the direct synthesis of this isocyanate product occurs in 77% yield from PPN(N₃) and Ir₄(CO)₁₂. Modifications of published procedures for the preparation of [N(C₂H₅)₄]₂ [Ir₆(CO)₁₅] and Ir₆(CO)₁₆ are reported that allow shorter reaction times and give higher yields. The reaction of Ir₆(CO)₁₆ with one equivalent of PPN(NO₂) generates a new cluster, PPN[Ir₆(CO)₁₅(NO)], in 57% yield which is proposed to contain a bent nitrosyl ligand. An additional equivalent of PPN(NO₂) gives (PPN)₂[Ir₆(CO)₁₅] in 84% yield with the evolution of N₂O as well as CO₂.     

The effect of solvents on the stability of the products of electrochemical reactions of iron, ruthenium, and osmium carbonyl cluster compounds

The electrochemical reactions of carbonyl cluster compounds M₃(CO)₁₂ (M = Fe, Ru, Os) were compared with those of [Fe₅C(CO)₁₄]^2? and [Fe₆C(CO)₁₆]^2? in different aprotic solvents. The stability of electrochemically generated products of these clusters in the studied solvents was shown to grow in the following order: tetrahydrofuran < acetone < dichloromethane < acetonitrile.     

Trimethylphosphite derivatives of the mixed cobalt—iridium tetranuclear cluster Co2Ir2(CO)12

Up to four carbonyl groups of Co₂Ir₂(CO)₁₂ have been replaced by trimethylphosphite to form tetranuclear clusters of formula Co₂Ir₂(CO)_12?n[P(OMe)₃]_n. The clusters do not exhibit the redistribution of the metal core which is observed in the case of mixed cobalt—rhodium clusters. Attachment of three or four trimethylphosphites to the metal skeleton of the cluster inhibits the scrambling of the carbonyl groups.     

Reactions of nitrosonium ion with anionic carbonyl monomers and clusters. Crystal and molecular structure of FeCo2(μ3-NH)(CO)9

The reactions of several mono- and poly-nuclear carbonyl metallates with nitrosonium ion have been studied. Besides simple substitution of a carbon monoxide with NO⁺ some reactions yielded products containing other nitrogeneous ligands. When [CoRu₃(CO)₁₃]^? reacts with NO⁺, low yields of the new nitrido cluster CoRu₃N(CO)₁₂ are formed. Prior conversion of [CoRu₃(CO)₁₃]^? to the new hydrido cluster [H₂CoRu₃(CO)₁₂]^? under hydrogen, followed by nitrosylation, forms the new imido cluster H₂Ru₃(NH)(CO)₉ in very low yield. The reaction of [FeCO₃(CO)₁₂]^? with NO⁺ also generates an imido cluster, FeCo₂(NH)(CO)₉, in 15% yield. This cluster has been characterized by X-ray crystallography and was found to be similar to the tricobalt alkylidyne clusters. (Triclinic crystal system, P$\text{1}$ space group, Z=2, a 6.787(1), b 8.016(1), c 13.881(2) Å, α 95.50(1), β 100.77(1), γ 107.93(1)°. Modifications of the nitrosylations using NO⁺ were studied. In particular, the addition of triethylamine or N-t-butylbenzaldimine allowed the use of NO⁺ in THF without solvent decomposition. With [CpMo(CO)₃]^? and [CpFe(CO)₂]^? the N-nitrosoiminium species appears to form transient alkylmetals which further react to give the dimers [CpMo(CO)₃]₂ and [CpFe(CO)₂]₂.     

Synthesis and structures of new heteronuclear cluster complexes [PPh4][Fe4Rh3Se2(CO)16] and [PPh4]2[Fe3Rh4Te2(CO)15]

The reaction of the K₂[Fe₃Q(CO)₉] clusters (Q = Se or Te) with Rh₂(CO)₄Cl₂ under mild conditions is accompanied by complicated fragmentation of cores of the starting clusters to form large heteronuclear cluster anions. The [PPh₄][Fe₄Rh₃Se₂(CO)₁₆] and [PPh₄]₂[Fe₃Rh₄Te₂(CO)₁₅] compounds were isolated by treatment of the reaction products with tetraphenylphosphonium bromide. The structures of the products were established by X-ray diffraction. In both compounds, the core of the heteronuclear cluster consists of two octahedra fused via a common Rh₃ face. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 775–778, May, 2006.     

The reduction of triosmium dodecacarbonyl by sodium borohydride. The preparation and X-ray structure of the cluster anion [H2Os4(CO)12]2−

The reduction of Os₃(CO)₁₂ by NaBH₄ in tetrahydrofuran has been studied, and the formation of the anionic clusters [HOs₃(CO)₁₁]^?, [H₃Os₄(CO)₁₂]^? and [H₂Os₄(CO)₁₂]^2? observed. The previously unreported dianion [H₂Os₄(CO)₁₂]^2? was prepared in satisfactory yield, and characterised as the bis(triphenylphosphine)iminium salt. This compound crystallizes in the space group P$\text{1}$, with Z = 1, and cell dimensions of a 11.014(2), b 14.751(3), c 15.168(3) Å, α 123.95(2)°, β 95.77(2)°, γ 98.73(2)°. The structure was solved by a combination of multisolution sign expansion and Fourier methods, and final residuals were R 0.067 and R_W 0.066 for 5972 observed intensity data. The dianion comprises a distorted tetrahedron of osmium atoms, each metal also bonding to three terminal carbonyl ligands, which as staggered with respect to the metalmetal bonds. Unlike the cation, the cluster anion is statistically disordered between two centrosymmetrically related sites.     

A joint electrochemical/spectroelectrochemical inspection (and re-inspection) of high-nuclearity platinum carbonyl clusters

The accurate study of the electron transfer activity of the tetraanion [Pt₁₉(CO)₂₂]⁴⁻ is presented together with that of the dianion [Pt₃₈(CO)₄₄]²⁻, which was previously studied by spectroelectrochemistry but only partially examined from the electrochemical viewpoint. The main feature of the two clusters is that they undergo a sequence of close-spaced pairs of reversible one-electron processes, which are qualitatively reminiscent of those exhibited by the dianion [Pt₂₄(CO)₃₀]²⁻. In order to focus on such unique aspect of the three structurally characterised platinum clusters, we have investigated (and reinvestigated) their electrochemical and spectroelectrochemical redox properties, also reporting on the electron paramagnetic resonance (EPR) spectrum of the monoanion [Pt₂₄(CO)₃₀]⁻, which represents the first successful study of the paramagnetism of homoleptic platinum–carbonyl clusters.