A Class of Trinuclear Clusters with Carbonyl Bridging

The impetus for this work was the structure of a trinuclear complex with two carbonyl groups showing incipient triple bridging - Cp₂Rh₃(CO)₄^?. Its structure, barrier to rotation of one Rh(CO)₂^? piece vs. the rest of the molecule, and the nature of the bridging carbonyl interaction are analyzed. Isolobal analogies form an interesting connection between this complex and a bridged isomer of the recently synthesized carbene complexes, Cp₂Rh₂(CO)₂CR₂, one isomer of Cp₂Rh₃(CO)₃, and hypothetical carbyne complexes Cp₂Rh₂(CO)₂CH^+,?. A general bonding model for Cp₂Rh₂(μ-CO)₂X complexes is constructed. The model, rich in geometrical detail, allows minima for the bridging carbonyl groups bending toward and away from the bonded ligand X.     

Multinuclear NMR studies on substituted derivatives of Rh6(CO)16 in solution

Multinuclear NMR data (¹³C, ³¹P, ¹³C–{³¹P}, ¹³C–{¹⁰³Rh} and ³¹P–{¹⁰³Rh}) for a series of mono- and di-substituted derivatives of Rh₆(CO)₁₆ containing neutral two electron donor ligands [Rh₆(CO)₁₅L, (L=NCMe, py, cyclooctene, PPh₃, P(OPh)₃,1/2(μ₂,η¹:η¹-dppe)); Rh₆(CO)₁₄(LL), (LL=cis-CH₂=CMe-CMe=CH₂, dppm, dppe, (P(OPh)₃)₂)] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the ¹³CO NMR spectra of Rh₆(CO)₁₅(PPh₃) and Rh₆(CO)₁₄(dppm) clusters have been made on the basis ¹³C–{¹⁰³Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of ³J(P–C) for terminal carbonyl ligands is discussed and the values of ³J(P–C) are found to be mainly dependent on the bond angles in the P–Rh–Rh–C fragment; these data enable the fine structure of the complex multiplets in the ¹³C–{¹H} and ³¹P–{¹H} NMR spectra of Rh₆(CO)₁₄ (dppm) to be simulated. Variable temperature ¹³C–{¹H} NMR measurements on Rh₆(CO)₁₅(PPh₃) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh₃ on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ₃-CO's.     

Unsaturation in binuclear cyclopentadienylrhodium carbonyls: Comparison with their cobalt analogs

The Cp₂Rh₂(CO)_n (n = 4, 3, 2, 1) derivatives have been examined by density functional theory using the BP86 and MPW1PW91 functionals. The known tricarbonyl Cp₂Rh₂(CO)₃ is predicted to have a singly bridged structure with a predicted Rh–Rh single bond distance of ～2.70 Å in close agreement with the experimental value of 2.68 Å, determined by X-ray crystallography. In contrast to the cobalt analog, no evidence for a triply bridged Cp₂Rh₂(μ-CO)₃ structure was found. The known dicarbonyl Cp₂Rh₂(CO)₂ is predicted to have a doubly bridged structure with a predicted RhRh double bond distance of 2.58 Å in close agreement with the experimental RhRh double bond distance of 2.564 Å, found by X-ray crystallography for the permethylated derivative (η⁵-Me₅C₅)₂Rh₂(μ-CO)₂. The monocarbonyl Cp₂Rh₂(CO) is predicted to have a four-electron donor bridging carbonyl group with a Rh–O distance of ～2.5 Å and a RhRh double bond distance of ～2.54 Å. This differs from Cp₂Co₂(CO) which was previously predicted to have only a two-electron donor bridging carbonyl group with a long Co?O distance and a short CoCo distance of ～2.0 Å suggesting a formal triple bond. For Cp₂Rh₂(CO)₄ doubly bridged trans and cis isomers were found within ～1.0 kcal/mol in energy with non-bonding Rh?Rh distances of ～3.2 Å. However, these Cp₂Rh₂(CO)₄ isomers are predicted to be unstable with respect both to CO loss to give Cp₂Rh₂(CO)₃ and to fragmentation into two CpRh(CO)₂ units.     

Octahedral rhodium cluster [Rh6Cp6(μ6-C)]2+(PF6 -)2

The 86-electron dicationic octahedral rhodium cluster [Rh₆Cp₆(_μ6-C)]²⁺(PF₆ ^-)₂ containing Cp ligands and the interstitial carbon atom was synthesized by the reaction of Rh₃Cp₃(μ-CO)₃ with RhCp(C₂H₄)₂ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 395–396, February, 1999     

Rhodium carbonyl derivatives containing Rh-O-Si bonds

Summary The following mono-, di- and tetra-nuclear rhodium carbonyl derivatives with silanolato and disilanolato ligands were prepared: Rh(CO)(PR₃)₂(OSiR₃), [Rh(CO)₂(OSiR₃)]₂, Rh₂(CO)₃ (PBu₃)(OSiR₃)₂ and Rh₄(CO)₈(OSiPh₂)₂ (R = Me or Ph; R = n-Bu or Ph). The complexes may serve as models for the bonding of transition metals on silica surfaces.     

Oxidation of the [Rh(CO)2Cl2] anion in aqueous solution

The reaction of rhodium(I) carbonyl chloride, [Rh(CO)₂Cl]₂, with dichromate, cerium(IV) sulfate, hexachloroplatinic acid or p-benzoquinone in aqueous hydrochloric acid proceeds by consumption of 4 equivalents of oxidizing agent per mole or rhodium(I) in accordance with the equation Rh^I(CO)₂  4e + H₂O → Rh^III(CO) + 2H⁺ + CO₂A “cyclic” oxidation mechanism is suggested.     

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.     

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) »).     

Elektronenstruktur von Rh4(CO)12, ein Modell für linear und brückenförmig gebundenes CO an Rhodiumkatalysatoren

Electronic Structure of Rh₄(CO)₁₂, a Model for Linear- and Bridge-bonded CO on Rhodium Catalysts The electronic structure of the Rh₄(CO)₁₂ cluster containing both linear- and bridgebonded CO groups has been studied by the EHMO method and compared with that of the Ir₄(CO)₁₂ cluster with only linear-bonded CO ligands. The charge distribution shows a distinctly higher π-back donation for the bridge-bonded CO groups. This result is compared with experimental data such as bond lengths, force constants of the C? O stretching frequencies and XPS data. It allows further an interpretation of results of CO hydrogenation on supported rhodium catalysts and on rhodium model complexes.     

Organometallic chemistry and homogeneous catalysis of Rh and Rh-Co mixed metal carbonyl clusters

The reactions of hydrosilane and/or alkyne as well as isonitriles with rhodium and rhodium cobalt mixed metal carbonyl clusters, e.g., Rh₄(CO)₁₂ and Co₂Rh₂(CO)₁₂, are studied. Novel mixed metal complexes, e.g., CoRh(CO)₅ (HCCBu ⁿ ), (R₃Si)₂Rh(CO) _n Co(CO)₄, Rh(R–NC)₄Co(CO)₄, Co₂Rh₂(CO)₁₀(HCCR), and Co₂Rh₂(CO)₉(HCCBu ⁿ ), are synthesized and identified. The catalytic activities of these rhodium and rhodium-cobalt mixed metal complexes are examined in hydrosilyation, silylformylation, and novel silylcarbocyclization reactions. Possible mechanisms for these reactions are proposed and discussed.     

Dirhodium(II) dicarboxylato complexes containing carbonyl and C-bonded methoxycarbonyl ligands

Oxidation of rhodium(I) carbonyl chloride, [Rh(CO)₂Cl]₂, with copper(II) acetate or isobutyrate in methanol solutions yields binuclear double carboxylato bridged rhodium(II) complexes with RhRh bonds, [Rh(μ-OOCRκO)(COOMeκC)(CO)(MeOH)]₂, where R=CH₃ or i-C₃H₇. According to X-ray data, surrounding of each rhodium atom in these complexes is close to octahedral and consists of another rhodium atom, two oxygens of carboxylato ligands, terminal carbonyl group, C-bonded methoxycarbonyl ligand, and axial CH₃OH. Methoxycarbonyl ligand is shown to originate from CO group of the parent [Rh(CO)₂Cl]₂ and OCH₃ group of solvent. N- and P-donor ligands L (p-CH₃C₆H₄NH₂, P(OPh)₃, PPh₃, PCy₃) readily replace the axial MeOH yielding [Rh(μ-OOCRκO)(COOMeκC)(CO)(L)]₂. The X-ray data for the complex with R=i-C₃H₇, L=PPh₃ showed the same molecular outline as with L=MeOH. Electronic effects of axial ligands L on the spectral parameters of terminal carbonyl group are essentially the same as in the known series of rhodium(I) complexes (an increase of δ¹³C and a decrease of ν(CO) with strengthening of σ-donor and weakening of π-acceptor ability of L).     

Synthesis of Functionalized (η5‐Indenyl)rhodium(III) Complexes and Their Application to C−H Bond Functionalization

It has been established that reductive complexation of functionalized benzofulvenes, which are readily prepared from commercially available indene and 2‐methylindene, with RhCl₃ in ethanol affords the corresponding indenyl–rhodium(III) dichlorides bearing substituents at the 1‐ (H or CO₂Et), 2‐ (H or Me), and 3‐ [CH₂Ph or CH₂(2‐MeOC₆H₄)] positions. The indenyl–rhodium(III) complexes bearing one ethoxycarbonyl group showed higher thermal stability and regioselectivity than our previously reported Cp^ERh^III complex toward the oxidative [3+2] annulation of acetanilides with internal alkynes.     

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.     

High Nuclearity Bimetallic Rhodium–Palladium Carbonyl Cluster Complexes. Synthesis and Characterization of Rh6(CO)16[Pd(PBu t 3)]3 and Rh6(CO)16[Pd(PBu t 3)]4

The reaction of Rh₄(CO)₁₂ with Pd(PBu ^t ₃)₂ yielded the high nuclearity bimetallic hexarhodium-tripalladium cluster complex Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₃, 10, in 11% yield. Compound 10 was converted to the hexarhodium-tetrapalladium cluster Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₄, 11, in 62% yield by reaction with an additional quantity of Pd(PBu ^t ₃)₂. Both compounds were characterized crystallographically. Structurally, both compounds consist of an octahedral cluster of six rhodium atoms with sixteen carbonyl ligands analogous to that of the known compound Rh₆(CO)₁₆. Compound 10 also contains three Pd(PBu ^t ₃) groups that bridge three Rh–Rh bonds along edges of the Rh₆ octahedron to give an overall D₃ symmetry to the Rh₆Pd₃ cluster. Compound 11 contains four edge bridging Pd(PBu ^t ₃) groups distributed across the Rh₆ octahedron to give an overall D_2d symmetry to the Rh₆Pd₄ cluster. Each Rh–Pd connection in both compounds contains a bridging carbonyl ligand that helps to stabilize the bond between the Pd(PBu ^t ₃) groups and the Rh atoms. Both compounds can be regarded as Pd(PBu ^t ₃) adducts of Rh₆(CO)₁₆.     

Synthesis of neutral tri- and tetranuclear organometallic clusters of group VIII transition metals

A number of earlier unknown tri- and tetranuclear organometallic clusters of group VIII transition metals was synthesized by the addition of coordinatively unsaturated species to a single metal-metal bond. A number of novel heteronuclear clusters, CpCp ₂ ^′ Rhm₂(μ−CO)₃(μ₃−CO), (Cp′=Cp, Cp^*; M₂=Ru₂, Fe₂; RuFe); Cp₂Cp ₂ ^* Rh₂M₂(μ₃−CO)₃ (M = Ru, Fe); , Cp₃Cp^*Rh₃M(μ₃−CO)₂(μ₃−d) (M = Ru, Fe); Cp₂Cp ₂ ^*> Rh₂Co₂(μ₃−CO)₂,etc., was obtained. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 579–586, March, 1997.     

Structure of palladium(I) carbonylcarboxylate clusters [Pd<Subscript>2</Subscript>(CO)<Subscript>2</Subscript>(RCOO)<Subscript>2</Subscript>]<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> in solution: NMR and IR study

The structures of palladium carbonylcarboxylate clusters [Pd₂(CO)₂(RCOO)₂] _n (n = 2, R = CH₃, CH₂Cl, CF₃, n = 3, R = CMe₃, CHMe₂, n-C₅H₁₁) are studied in benzene and tetrahydrofuran solutions by IR and ¹H and ¹³C NMR spectroscopy. The clusters in the solid state have a planar cyclic metal framework with pairs of the carbonyl and carboxylate ligands alternately coordinated on its sides. In solutions, compounds under consideration contain one-type carbonyl ligands and one-type carboxylate ligands; their structures are similar to thaso in the solid state.     

Hydrosilylation of alkenes catalysed by rhodium complexes immobilized on silica via a pyridine group

2-(2-Trimethoxysilylethyl)pyridine, together with 3-methcryloxypropyltrimethoxysilane, was used to prepare a series of rhodium carbonyl complexes bound to silica via a pyridine group. The rhodium complex Rh₂(CO)₄Cl₂ (Rh₂) was used as the starting compound, and the immobilized complexes were prepared by four different routes which yielded both surface-bonded complexes and complexes bonded within the silicate matrix. These complexes were efficient catalysts of hydrosilylation of octene by triethxysilane. All the immobilized complexes were more than their homogeneous analogues and some could be re-used.     

Magnetic properties and EPR spectroscopy of molecular metal clusters

The magnetic properties of molecular metal cluster compounds resemble those of small metal particles in the metametallic size regime. Even-electron metal carbonyl clusters with 10 or more metal atoms are paramagnetic, because their frontier orbital separations of less than 1 eV lead to high-spin electronic configurations. The rhodium cluster [Rh₁₇S₂(CO)₃₂]^3? gives EPR below 200 K withg=2.04, the first example of this type of paramagnetism in an even-electron carbonyl cluster of this 4d metal. Its spectral parameters are compared with those of osmium carbonyl clusters and some significant differences highlighted. Attempts have also been made to generate radical cations from lower-nuclearity, diamagnetic molecular clusters such as Rh₆(CO)₁₆ by chemical oxidation in sulphuric acid. An EPR active species (g=2.09) believed to be [Rh₆(CO)₁₆]⁺ has been obtained.     

1- and 2-D NMR Studies on [Rh6C(CO)14(PPh3)]2−

A complete NMR study involving both 1D and 2D ¹³C-{¹⁰³Rh} and ³¹P-{¹⁰³Rh} HMQC measurements, on [Rh₆C(CO)₁₄(PPh₃)]^2- are reported and discussed, together with the multiple Rh quantum effects found for resonances associated with edge- and face-bridging CO's. As found in [Rh₆C(CO)₁₅]^2-, the carbonyl ligands in [Rh₆C(CO)₁₄(PPh₃)]^2- undergo CO-intermolecular exchange with ¹³CO at different rates; for the edge-bridging CO's, the lower the value of ¹J(Rh–CO), the faster the rate of intermolecular exchange with ¹³CO.     

Amide‐Functionalized Naphthyridines on a RhII–RhII Platform: Effect of Steric Crowding,Hemilability, and Hydrogen‐Bonding Interactions on the Structural Diversity and Catalytic Activity of Dirhodium(II) Complexes

Ferrocene‐amide‐functionalized 1,8‐naphthyridine (NP) based ligands {[(5,7‐dimethyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L¹H) and {[(3‐phenyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L²H) have been synthesized. Room‐temperature treatment of both the ligands with Rh₂(CH₃COO)₄ produced [Rh₂(CH₃COO)₃(L¹)] ( 1 ) and [Rh₂(CH₃COO)₃(L²)] ( 2 ) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho‐methyl group in L¹H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh^II–Rh^II} unit. The use of the more labile Rh₂(CF₃COO)₄ salt with L¹H produced a cis bis‐adduct [Rh₂(CF₃COO)₄(L¹H)²] ( 3 ), whereas L²H resulted in a trans bis‐adduct [Rh₂(CF₃COO)₃(L²)(L²H)] ( 4 ). Ligand L¹H exhibits chelate binding in 3 and L²H forms a bridge‐chelate mode in 4 . Hydrogen‐bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen‐bonding interaction, both ligands bind axially as evident from the X‐ray structure of [Rh₂(CH₃COO)₂(CH₃CN)₄(L²H)₂](BF₄)₂ ( 6 ). However, the axial ligands reorganize at reflux into a bridge‐chelate coordination mode and produce [Rh₂(CH₃COO)₂(CH₃CN)₂(L¹H)](BF₄)₂ ( 5 ) and [Rh₂(CH₃COO)₂(L²H)₂](BF₄)₂ ( 7 ). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7 , which features hemilabile amide side arms that occupy sites trans to the Rh–Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the C?H bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen‐bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.