Binuclear rhodium carbonyl halide complexes containing the 1,1-bis(diphenylphosphino)ethene ligand,and the crystal structure of [Rh2Cl2(μ-CO){Ph2PC(CH2)PPh2}2]

Treatment of [RhCl(CO)₂]₂ with 1,1-bis(diphenylphosphino)ethene (dppee) yields the cationic binuclear rhodium complex [Rh₂(μ-Cl)(CO)₂(μ-CO)(dppee)₂]⁺ which may be isolated as the [RhCl₂(CO)₂]⁻ salt at low temperature (230 K) but which readily forms the Cl⁻ salt on allowing the solution to warm to room temperature. On bubbling nitrogen through a solution of this cationic complex at room temperature, the monocarbonyl species, [Rh₂Cl₂(μ-CO)(dppee)₂] is obtained. The crystal structure of this complex has been determined. The crystals are orthorhombic, space group Pnca, a = 29.98(3), b = 23.70(2), c = 14.78(3) Å, Z = 8. Using 3019 unique reflections the structure was refined to R = 0.091. The rhodium-rhodium distance is 2.650(14) Å. Direct rhodium NMR data are reported for these complexes.     

Low temperature matrix photochemistry of M2(CO)4(μ-S-t-Bu)2 and [M(CO)2Cl2] anions, where M = Rh and Ir

Photolysis of M₂(CO)₄(μ-S-t-Bu)₂, where M = Rh or Ir, in Nujol matrices at ca. 90 K results in simple CO loss to form a tricarbonyl intermediate analogous to that observed for Rh₂(CO)₄(μ-Cl)₂. Photolysis of the anions, [M(CO)₂Cl₂]¹⁻, where M = Rh or Ir, in inert ionic matrices at ca. 90 K, results in CO-loss to form an intermediate analogous to that formed by Rh(CO)₂(i-Pr₂HN)Cl. Finally, photolysis of trans-Ir(CO)(PMe₃)₂Cl in a Nujol matrix at ca. 90 K gives rise to a new species whose carbonyl band is shifted slightly down in energy as has been observed for trans-Rh(CO)(PMe₃)₂Cl. In all cases the iridium compounds behave similarly to the rhodium species although the photon energy for iridium photochemistry is typically above that of the rhodium compounds.     

Metal cluster topology. 1. Osmium carbonyl clusters

Important theoretical approaches to metal cluster bonding including the Wade-Mingos skeletal electron pair method, the Teo topological electron count, the King-Rouvray graph theory derived method, and Lauher's extended Hückel calculations are shown to agree in their apparent skeletal electron counts for the most prevalent metal cluster polyhedra including the tetrahedron, the trigonal bipyramid (both ordinary and elongated), square pyramid, octahedron, bicapped tetrahedron, pentagonal bipyramid, and capped octahedron. The graph theory derived method is used to treat osmium carbonyl clusters containing from five to eleven osmium atoms. In this connection most osmium carbonyl clusters can be classified into the following types: (1) Clusters exhibiting edge- localized bonding containing multiple tetrahedral chambers (e.g., Os₅(CO)₁₆, Os₆(CO)₁₈, H₂Os₇(CO)₂₀ and HOs₈(CO)₂₂⁻); (2) Capped octahedral clusters derived from osmium carbonyl fragments of the type Os_6+p(CO)_19+2p (p = 0, 1, 2, and 4) (e.g., Os₆- (CO)₁₈²⁻, Os₇(CO)₂₁, Os₈(CO)₂₂²⁻, and H₄Os₁₀- (CO)₂₄²⁻). Other more unusual osmium carbonyl clusters such as the planar Os₆(CO)₁₇ [P(OCH₃)₃]₄, the Os₉ cluster [Os₉(CO)₂₁C₃H₂R]⁻, and the Os₁₁ cluster Os₁₁C(CO)₂₇²⁻ can also be treated satisfactorily by these methods. The importance of the number of ligands around isoelectronic Os_n systems in determining the cluster polyhedron is illustrated by the different cluster polyhedra found for each member of the following isoelectronic pairs: HOs₆- (CO)₁₈⁻/H₂Os₆(CO)₁₈. Os₇(CO)₂₁/H₂Os₇(CO)₂₀, Os₈(CO)₂₂²⁻/HOs₈(CO)₂₂⁻. The tendency for osmium carbonyl clusters frequently to form polyhedra exhibiting edge-localized rather than globally delocalized bonding relates to the facility for osmium carbonyl vertices to contribute more than three internal orbitals to the cluster bonding. In this way Wade's well-known analogy between boron hydride clusters and metal clusters, which assumes exactly three internal orbitals for each vertex atom, is frequently no longer followed in the case of osmium carbonyl clusters.     

Synthesis and characterization of a fluxional Re(I) carbonyl complex fac-[Re(CO)3(dpop′)Cl] with the nominally tri-dentate ligand dipyrido(2,3-a:3′,2′-j)phenazine (dpop′)

A new Re(I) carbonyl complex [Re(CO)₃(dpop′)Cl] with nominally N-donor tri-dentate heterocyclic ligand dipyrido(2,3-a:3′,2′-j)phenazine (dpop′) was prepared and characterized. The ligand complexes in a bi-dentate mode undergoing fluxional behavior in room temperature solution. VT NMR results show ΔG³ of 61.1 kJ mol⁻¹ for [Re(CO)₃(dpop′)Cl] is smaller than for comparable tpy related complexes. The electronic absorption spectrum shows solvent dependent MLCT energies at 483 and 368 nm in dichloromethane. A single irreversible Re centered oxidation at +1.29 V and a semi-reversible dpop′ centered reduction at −0.71 V are observed by cyclicvoltammetry. Electrolysis of [Re(CO)₃(dpop′)Cl] at −1.0 V produces complete loss of dpop′ from the metal.     

Equilibria in N-heterocyclic complexes. Part 45. Ligand electron densities in coordinated pyridine

Results of INDO calculations on the species pyridine (py), (pyH)⁺, [py-CH₃]⁺, [Fe(NH₃)_x(py)_6−x]²⁺, [Fe(NH₃)₅(py)]³⁺, [Fe(CN)₅(py)]³⁻, and [Co(CN)₅(py)]²⁻ are presented and discussed, comparing quaternization and coordination.     

Kinetics of CO substitution in Co4(CO)12 and Rh4(CO)12

The kinetics of rapid CO substitution by PPh₃ in Co₄(CO)₁₂ and Rh₄(CO)₁₂ have been examined by stopped-flow and low temperature FT-IR methods. In Co₄(CO)₁₂ rapid (k_obs ∼ 1.8 s⁻¹) substitution of CO occurs after a 1–15 s induction period at 28 °C in C₆H₅Cl solvent by a catalytic process. Addition of PPh₃ to Rh₄(CO)₁₂ yields Rh₄(CO)₁₁(PPh₃) according to a predominantly second order rate law k₁[Rh₄- (CO)₁₂] + k₂[Rh₄(CO)₁₂][PPh₃] with k₁ = 25 ± 11 s⁻¹ and k₂ = 2.97 ± 0.27 X 10⁴ M⁻¹ s⁻¹ at 28 °C. Substitution of a second CO ligand also occurs rapidly with k₁ = 0.15 ± 0.09 s⁻¹ and k₂ = 6.54 ± 0.07 X 10² M⁻¹ s⁻¹ at 28 °C. The reactivity of Rh₄(CO)₁₂ toward associative substitution is 10⁴– 10¹¹ faster than for the Co and Ir analogues, In Rh₄(CO)₁₁(PPh₃) the increase in CO substitution rates over Co and Rh analogues is 10²–10⁷. The ordering of associative substitution rates Co << Rh >>> Ir in these clusters exaggerates the trend seen in mononuclear metal complexes.     

Octahedral (cyclopentadienyl)rhodium clusters [Rh6Cp6(μ6-C)] and [Rh6Cp6(μ3-CO)2]: Synthesis, structures and electrochemistry

The 86-electron dicationic octahedral rhodium clusters containing Cp (Cp = C₅H₅) ligands and either an interstitial carbon atom, [Rh₆Cp₆(μ₆-C)]²⁺ ([1]²⁺), or two carbonyl groups, [Rh₆Cp₆(μ₃-CO)₂]²⁺ ([2]²⁺), were synthesized in low yields by reactions of Rh₃Cp₃(μ-CO)₃ with RhCp(C₂H₄)₂ or [RuCp^∗(MeCN)₃]⁺ (Cp^∗ = C₅Me₅), respectively. The structures of [1]²⁺ and [2]²⁺ were determined by X-ray diffraction. Their electrochemical behavior proved that they possess a rather extended electron transfer activity. In accordance with DFT calculations, the nearly octahedral structure of [1]²⁺ and [2]²⁺ is retained both upon oxidation (2+/3+) and the first reduction (2+/+); however, the second reduction (+/0) results in the breaking of one (for [1]⁰) or two (for [2]⁰) Rh-Rh bonds. In the case of the related Dahl’s nickel cluster Ni₆Cp₆ the nearly octahedral structure is retained upon all redox steps (3+/2+/+/0/−/2−).     

Structure of binuclear Rh(II) carboxylato complexes with 2,2-bipyridine and an unprecedented rhodium wire with Rh4 core

Structures of rhodium(II) binuclear complexes [Rh₂(OOCCH₃)₂(bpy)₂(H₂O){(CH₃)₂CHOH}][B(C₆H₅)₄]₂ · H₂O (1), [Rh₂Cl₂(OOCCH₃)₂(bpy)₂] · 2H₂O (2), [Rh₂Br₂(OOCCH₃)₂(bpy)₂] · 3H₂O (3), and [Rh₂I₂(OOCCH₃)₂(bpy)₂] (4), as well as an unprecedented wire with infinite Rh-Rh chain, {[Rh₄(μ-OOCH)₄(bpy)₄](BF₄)}_n · 0.5nC₄H₈O₂ (5), have been determined and discussed. Mass spectra of complexes [Rh₂(OOCMe)₂(bpy)₂(H₂O)₂](MeCOO)₂ and [Rh₂(OOCMe)₂(phen)₂(H₂O)₂](MeCOO)₂ have showed stability of polynuclear cations with rhodium in oxidation states in the range +1.25 to +1.75.     

3,5-Bis(diphenylphosphinoethyl)pyrazolate ligand (PNNP) and its dirhodium complexes: Comparison with related quadridentate dinucleating diphenylphosphinomethyl (PNNP) and phthalazine derivatives (PNNP)

Dirhodium carbonyl complex with the 3,5-bis(diphenylphosphinoethyl)pyrazolato ligand (PNNP^C2), [(μ-κ²:κ²-PNNP^C2)Rh₂(CO)₃]BF₄, is prepared and its reactivity is studied as compared with the previously reported 3,5-bis(diphenylphosphinomethyl)pyrazolate (PNNP), [(μ-κ²:κ²-PNNP){Rh(CO)₂}₂]BF₄, and 1,4-bis(diphenylphosphinomethyl)phthalazine (PNNP^Ph) derivatives, [(μ-κ²:κ²-PNNP^Ph){Rh(CO)₂}₂](BF₄)₂. The three quadridentate ligands are different in the size of the central ring and the charge; six-membered ring/neutral (PNNP^C2) vs. five-membered ring/mono-negative (PNNP) vs. six-membered ring/neutral (PNNP^Ph). The number of the carbonyl ligands (n) in the dirhodium carbonyl complexes, [(μ-PNNP)Rh₂(CO)_n](BF₄)_x, is dependent on the dinucleating ligand: n = 2 (PNNP^Ph), 3 (PNNP^C2) and 4 (PNNP^Py). The three dirhodium carbonyl complexes serve as 4e-acceptors, and their reactivities turn out to be very similar as can be seen from formation of the analogous, unique tetranuclear μ₄-acetylide ([(μ-PNNP)₂{Rh(CO)}₄(μ₄-CC-R)](BF₄)_x) and μ₄-dicarbide complexes ([(μ-PNNP)₂{Rh(CO)}₄(μ₄-C₂)](BF₄)_x).     

Synthesis and characterisation of ferrocene-containing β-diketonato complexes of rhodium(I) and rhodium(III)

The synthesis of new β-diketonato rhodium(I) complexes of the type [Rh(FcCOCHCOR)(CO)₂] and [Rh(FcCOCHCOR)(CO)(PPh₃)] with Fc=ferrocenyl and R=Fc, C₆H₅, CH₃ and CF₃ are described. ¹H, ¹³C and ³¹P NMR data showed that for each of the non-symmetric β-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, K_c, which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. Δ_rG, Δ_rH and Δ_rS values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and K_c exists. The relationship between RhP bond lengths, d(RhP), and ³¹P NMR peak positions as well as coupling constants ¹J(³¹P¹⁰³Rh) has been quantified to allow calculation of approximate d(RhP) values. Variations in d(RhP) for [Rh(RCOCHCOR′)(CO)(PPh₃)] complexes have also been related to the group electronegativities (Gordy scale) of the terminal β-diketonato R groups trans to PPh₃. A measure of the electron density on the rhodium centre of [Rh(RCOCHCOR′)(CO)(PPh₃)] may be expressed in terms of the IR carbonyl stretching wave number, ν(CO), the sum of the group electronegativities of the R and R′ groups, (χ_R+χ_R′), or the observed pK_a^′ values of the free β-diketones RCOCH₂COR^′. An empirical relationship between ν(CO) and either pK_a^′ or (χ_R+χ_R′) has also been quantified.     

A restricted polyhedral rearrangement of an aminosubstituted 12-vertex ferratricarbollide

The oxidation of anion [7,8-CH₂OCH₂-7,8-C₂B₉H₁₀]⁻ with aqueous FeCl₃ gives the 10-vertex nido-carborane 5,6-CH₂OCH₂-5,6-C₂B₈H₁₀ in 23% yield. Its interaction with Bu^tNC in the presence of proton sponge gives the tricarbollide anion [7,8-CH₂OCH₂-9-Bu^tNH-7,8,9-C₃B₈H₈]⁻ (44% yield) having a short linkage between carbon atoms. Further photochemical reaction of this anion with [CpFe(C₆H₆)]⁺ is accompanied by room-temperature polyhedral rearrangement giving ferratricarbollide 1-Cp-2,3-CH₂OCH₂-9-Bu^tNH-1,2,3,9-FeC₃B₈H₈ (5) in 89% yield. The process involves the migration of the amino-substituted carbon atom, while the separation of two other carbons (observed for the non-linked analogue) is restricted by the CH₂OCH₂ bridge. DFT calculations of the hypothetical non-rearranged isomer 1-Cp-2-Bu^tNH-1,2,3,4-FeC₃B₈H₁₀ revealed its strongly distorted geometry with the C2-C3 distance (2.347 Å) being clearly non-bonding, thus explaining the mild conditions of the polyhedral rearrangement. The structure of 5 was confirmed by X-ray diffraction.     

Reactions of 2-thiopyridone and related N-, S- and C-methylated derivatives with [Rh2Cl2(CO)4]: crystal and molecular structure of fac-[Rh(MeC5H3NS)3] containing 6-methyl-2-thiopyridonato ligands

[Rh₂Cl₂(CO)₄] reacts with the ligands L (2-pyridone, 2-thiopyridone, and the isomers 6-methyl-2-thiopyridone, 2-methylmercaptopyridine, and N-methylthiopyridone) to give initially, when L/Rh = 1, the bridged-cleaved compounds cis- [RhCl(CO)₂L]. Further additions of 2-methyl- mercaptopyridine, N-methylthiopyridone, or 2-pyridone caused no further change, but 2- thiopyridone and 6-methyl-2-thiopyridone gave new cis-dicarbonyl species (L/Rh = 2) and eventually monocarbonyl species (L/Rh > 3). All these solutions are air-sensitive and air oxidation of a solution of [Rh₂Cl₂(CO)₄] with an excess of 6-methyl-2- thiopyridone gave fac-[Rh(MeC₅H₃NS)₃] the X-ray structure of which shows three equivalent chelating 6-methyl-2-thiopyridonato ligands.     

A rare example of a di-cationic hydrido carbonyl tetra-nuclear cluster, [H2Rh2Pt2(CO)7(PPh3)3]

Addition of excess CF₃CO₂H (HTFA) to [Rh₂Pt₂(CO)₇(PPh₃)₃], I, under nitrogen results in the formation of a salt (X²⁺ Y²⁻), which contains only the second example of a di-cationic carbonyl hydride tetra-nuclear cluster, [H₂Rh₂Pt₂(CO)₇(PPh₃)₃]²⁺, X²⁺, and a presently partially characterized polymetallic anion Y²⁻. The di-cation X²⁺ has been characterized by mass spectrometry and a variety of multinuclear NMR methods. Since there is no difference in the electron count for I and X²⁺, it is probable that both I and X²⁺ adopt similar butterfly metallic frameworks with a Rh-Rh hinge; in X²⁺, there are two bridging hydrides to the same wing-tip Pt but the phosphine site occupancies on the Rh₂Pt₂-framework in I and X²⁺ are different.     

Enlarging the family of ferrocenylphosphine dinuclear rhodium complexes: synthesis and X-ray structure of a novel “A-frame”-type trimetallic Rh/Fe/Rh complex

The symmetrically substituted ligand 1,1^′-bis[di(5-methyl-2-furyl)phosphino]ferrocene (1) has been obtained from the bromophosphine BrP(Fu^Me)₂ and the dilithioferrocene/TMEDA adduct. The quantitative addition of this ferrocene derivative to the tetracarbonyl dimer [(CO)₄Rh₂{μ-(S^tBu)₂}] leads, through decarbonylation, to the dinuclear rhodium complex [(CO)₂Rh₂{μ-(S^tBu)₂}{μ-P,P-Fc[P(Fu^Me)₂]₂}] (2) in high yield. A X-ray structure [orthorhombic, space group P2₁2₁2₁; a=11.2982(2) Å, b=13.3165(3) Å, c=27.2687(7) Å] and the solution multinuclear NMR characterization are reported, which show that the rare “quasi-closed bridging” A-frame structure of the complex is rather similar to the one reported for [(CO)₂Rh₂{μ-(S^tBu)₂}{μ-P,P-dppf}] in solid state. However, in solution the furyl-containing ferrocenylphosphine complex presents a greater fluxionality, together with an electronic environment at phosphorus very different from the dppf analogue (δ_P=−10 and 27 ppm, respectively).     

Carbonyl trichlorostannate complexes of platinum: chemistry and 119Sn, 195Pt,and 13C NMR spectroscopy

The reactions of the carbonyl anion [PtCl₃(CO)]^- with SnCl₂ in the presence of CO in both methylene chloride and acetone are reported. In the former solvent, only Pt^II-SnCl₃ species are formed. These have been identified by ¹³C, ¹¹⁹Sn and ¹⁹⁵Pt NMR measurements as cis-[PtCl₂(SnCl₃)(CO)]^-, (I), trans- [PtCl(SnCl₃)₂(CO)]^-, (II), and [Pt(SnCl₃)₄(CO)]^2-, (III). Salts of these complexes have been isolated. In contrast, when acetone is the solvent, reduction of the platinum occurs to give two new complexes. On the basis of NMR measurements, we assign one of these as the Pt^I dimer [Pt₂(SnCl₃)₄(CO)₂]^2-, (IV), and the other as a platinum triangle (VI) containing terminal CO ligands and two types of Sn ligand. The Pt^II compound (IV) can also be generated by treating a CH₂Cl₂ solution of trans-[PtCl(SnCl₃)_2- (CO)]^-, (II), with dihydrogen. NMR spectroscopic data, including those from measurements on samples of the complexes containing ¹³C-enriched CO, are reported and discussed.     

Generation of trans-dioxoruthenium(VI) porphyrins: A photochemical approach

trans-Dioxoruthenium(VI) porphyrin complexes have been developed as one of the best-characterized model systems for heme-containing enzymes. Traditionally, this type of compounds can be prepared by oxidation of ruthenium(II) precursors with peroxyacids and other terminal oxidants under different conditions, depending on the porphyrin ligands. In this work, a new photochemical generation of trans-dioxoruthenium(VI) porphyrins has been developed by extension of the known photo-induced ligand cleavage reactions. Refluxing ruthenium(II) carbonyl porphyrins [Ru^II(Por)(CO)] in carbon tetrachloride afforded dichlororuthenium(IV) complexes [Ru^IV(Por)Cl₂]. Facile exchange of the counterions in [Ru^IV(Por)Cl₂] with Ag(ClO₃) or Ag(BrO₃) gave the corresponding dichlorate [Ru^IV(Por)(ClO₃)₂] or dibromate [Ru^IV(Por)(BrO₃)₂] salts. Visible-light photolysis of the photo-labile porphyrin-ruthenium(IV) dichlorates or dibromates resulted in homolytic cleavage of the two O-Cl or O-Br bonds in the axial ligands to produce trans-dioxoruthenium(IV) species [Ru^VI(Por)O₂] bearing different porphyrin ligands.     

Electrochemical oxidation of Mo(CO)4(LL) and Mo(CO)3(LL)(CH3CN): Generation, infrared characterization, and reactivity of [Mo(CO)4(LL)] and [Mo(CO)3(LL)(CH3CN)] (LL = 2,2′-bipyridine, 1,10-phenanthroline and related ligands)

Mo(CO)₄(LL) complexes, where LL = polypyridyl ligands such as 2,2′-bipyridine and 1,10-phenanthroline, undergo quasi-reversible, one-electron oxidations in methylene chloride yielding the corresponding radical cations, [Mo(CO)₄(LL)]⁺. These electrogenerated species undergo rapid ligand substitution in the presence of acetonitrile, yielding [Mo(CO)₃(LL)(CH₃CN)]⁺; rate constants for these substitutions were measured using chronocoulometry and were found to be influenced by the steric and electronic properties of the polypyridyl ligands. [Mo(CO)₃(LL)(CH₃CN)]⁺ radical cations, which could also be generated by reversible oxidation of Mo(CO)₃(LL)(CH₃CN) in acetonitrile, can be irreversibly oxidized yielding [Mo(CO)₃(LL)(CH₃CN)₂]²⁺ after coordination by an additional acetonitrile. Infrared spectroelectrochemical experiments indicate the radical cations undergo ligand-induced net disproportionations that follow first-order kinetics in acetonitrile, ultimately yielding the corresponding Mo(CO)₄(LL) and [Mo(CO)₂(LL)(CH₃CN)₃]²⁺ species. Rate constants for the net disproportionation of [Mo(CO)₃(LL)(CH₃CN)]⁺ and the carbonyl substitution reaction of [Mo(CO)₃(LL)(CH₃CN)₂]²⁺ were measured. Thin-layer bulk oxidation studies also provided infrared characterization data of [Mo(CO)₄(ncp)]⁺ (ncp = neocuproine), [Mo(CO)₃(LL)(CH₃CN)]⁺, [Mo(CO)₃(LL)(CH₃CN)₂]²⁺ and [Mo(CO)₂(LL)(CH₃CN)₃]²⁺ complexes.     

Synthesis and redox properties of dinuclear rhodium(II) carboxylates with 2,6-di-tert-butylphenol moieties

By exploiting the peculiar reactivity of [Rh₂(μ-O₂CBu^t)₄(H₂O)₂] (1) the examples of dinuclear rhodium(II) carboxylates containing N-donor axial ligands (2, 3) [Rh₂(μ-O₂CBu^t)₄L₂] [where L = benzonitrile (2), 3,5-di-tert-butyl-4-hydroxybenzonitrile (3)] were synthesized and characterized by elemental analysis, IR, multinuclear NMR spectroscopy, MALDI-TOF mass spectrometry. It was found by X-ray diffraction that pairs of 3 in crystals are associated through H atoms of phenol groups to produce a dimer of dimers. The chemical oxidation of dirhodium complexes with 2,6-di-tert-butyl-4-cyanоphenol pendants studied by means of ESR method in solutions leads to the formation of phenoxyl radicals 3′ in dirhodium system. The ESR data show the interaction of the unpaired electron with ligand nuclei (¹H, ¹⁴N) and ¹⁰³Rh. The stability of radical complexes with phenoxyl fragments in axial position is influenced by the temperature. The enthalpy of the 3′ decomposition followed by the formation of cyanophenoxyl radical as 20 ± 1 kJ/mol was estimated. Redox transformations in dirhodium system including both metal and axial ligands were investigated by electrochemistry. CV experiments confirm the assumption of the metal oxidation (Rh^II→Rh^III) as the first step following by the oxidation of the ligand.     

Oxidative addition of methyl iodide to [Rh(CO)2I]2: synthesis, structure and reactivity of neutral rhodium acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2

Reaction of [Rh(CO)₂I]₂ (1) with MeI in nitrile solvents gives the neutral acetyl complexes, [Rh(CO)(NCR)(COMe)I₂]₂ (R=Me, 3a; ^tBu, 3b; vinyl, 3c; allyl, 3d). Dimeric, iodide-bridged structures have been confirmed by X-ray crystallography for 3a and 3b. The complexes are centrosymmetric with approximate octahedral geometry about each Rh centre. The iodide bridges are asymmetric, with Rh-(μ-I) trans to acetyl longer than Rh-(μ-I) trans to terminal iodide. In coordinating solvents, 3a forms mononuclear complexes, [Rh(CO)(sol)₂(COMe)I₂] (sol=MeCN, MeOH). Complex 3a reacts with pyridine to give [Rh(CO)(py)(COMe)I₂]₂ and [Rh(CO)(py)₂(COMe)I₂] and with chelating diphosphines to give [Rh(Ph₂P(CH₂)_nPPh₂)(COMe)I₂] (n=2, 3, 4). Addition of MeI to [Ir(CO)₂(NCMe)I] is two orders of magnitude slower than to [Ir(CO)₂I₂]⁻. A mechanism for the reaction of 1 with MeI in MeCN is proposed, involving initial bridge cleavage by solvent to give [Rh(CO)₂(NCMe)I] and participation of the anion [Rh(CO)₂I₂]⁻ as a reactive intermediate. The possible role of neutral Rh(III) species in the mechanism of Rh-catalysed methanol carbonylation is discussed.     

Synthetic, electrochemical and structural aspects of a series of ferrocene-containing dicarbonyl β-diketonato rhodium(I) complexes

Treatment of [Rh(β-diketonato)(cod)] with CO resulted in better yields of [Rh(FcCOCHCOR)(CO)₂] than by treating [Rh(Cl)(CO)₂]₂ with FcCOCH₂COR, R = CF₃ (Hfctfa), CH₃ (Hfca), Ph (Hbfcm, Ph = phenyl) and Fc (Hdfcm, Fc = ferrocenyl). The single crystal structure of the fctfa rhodium(I) complex [C₁₆H₁₀F₃FeO₄Rh], monoclinic, C 2/c(15), a = 13.266(3) Å, b = 19.553(3) Å, c = 13.278(3) Å, β = 100.92(2)°, Z = 8 showed both rotational and translational displacement disorders for the CF₃ group. An electrochemical study revealed that the formal reduction potential, E⁰′, for the electrochemically reversible one electron oxidation of the ferrocenyl group varied between 0.304 (for the fctfa complex) and 0.172 V (for the dfcm complex) versus Fc/Fc⁺ in a manner that could be directly traced to the group electronegativities, χ_R, of the R groups on the β-diketonato ligands, as well as to the values of the free β-diketones. Anodic peak potentials, E_pa,Rh, for the dominant cyclic voltammetry peak associated with rhodium(I) oxidation were between 0.718 (bfcm complex) and 1.022 V (dfcm complex) versus Fc/Fc⁺. Coulometric experiments implicated a second, much less pronounced anodic wave for the apparent two-electron Rh^I oxidation that overlaps with the ferrocenyl anodic wave and that the redox processes associated with these two Rh^I oxidation waves are in slow equilibrium with each other.