Acidity of osmium tetroxide (OsO4) towards coordination with pyridine and its derivatives

This article reports on the Lewis acidity of osmium tetroxide (OsO₄), the most reliable reagent available for the cis-hydroxylation of alkenes. Three simple and straightforward scales, donor ability of N-donors, the Hammett reaction constant and the Electrostatic-Covalent model (ECW) were used for this purpose. Equilibrium constants (K_eq) and enthalpies for the complexation of OsO₄ with pyridine and its derivatives have been evaluated by spectrophotometeric measurements. The excellent linear relationships were observed between the equilibrium constants and either the pK_a values of the N-donors or the Hammett σ constants of the substituents on the ligands. The slope of the plot of log K_eq versus ligand’s pK_a and the Hammett ρ value derived suggest the use of these parameters as an index of the Lewis acidity strength for OsO₄. The enthalpies of complex formation of N-donors with OsO₄ also fit the ECW model to predict the values of E_A and C_A parameters for OsO₄ as 2.01 and 0.25 (kcal/mol)^1/2, respectively. The parameters can be used to make an acidity scale for OsO₄. These parameters also permit the chemists and biochemists to predict and correlate quantitatively the enthalpies of OsO₄ · Lewis base interactions.     

Fluorous osmium tetraoxide (OsO4): a recoverable and reusable catalyst for dihydroxylation of olefins

A fluorous osmium catalyst was firstly developed. It had been effectively used as recoverable and reusable catalyst in the dihydroxylation of olefins.     

The OsO4-mediated oxidative cleavage of olefins catalyzed by alternative osmium sources

The OsO₄-mediated oxidative cleavage of olefins is compatible with alternative, easier-to-handle osmium sources. Four different osmium sources were employed with favorable results.     

Ethylene addition to OsO3(CH2) - A theoretical study

Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO₃(CH₂). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]_O, C addition which has a barrier of only 2.3 kcal mol⁻¹. The [3+2]_O, O addition has a slightly higher barrier of 6.5 kcal mol⁻¹. Four other reactions of OsO₃(CH₂) with C₂H₄ have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol⁻¹. Even higher barriers are calculated for the [2+2] addition to the OsO bond (32.6 kcal mol⁻¹) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol⁻¹). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol⁻¹) and for the addition reactions of the latter with C₂H₄ are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the OsO bond which has an activation barrier of 46.6 kcal mol⁻¹ and the [3+2]_O, O addition which is an endothermic process with an activation barrier of 44.3 kcal mol⁻¹.     

Reactivity of 2,3-bis(2-pyridyl)pyrazine with [Re2(CO)8(CH3CN)2]: Molecular structures of [Re2(CO)8(C14H10N4)] and [Re2(CO)8(C14H10N4)Re2(CO)8]

The reaction of the labile compound [Re₂(CO)₈(CH₃CN)₂] with 2,3-bis(2-pyridyl)pyrazine in dichloromethane solution at reflux temperature afforded the structural dirhenium isomers [Re₂(CO)₈(C₁₄H₁₀N₄)] (1 and 2), and the complex [Re₂(CO)₈(C₁₄H₁₀N₄)Re₂(CO)₈] (3). In 1, the ligand is σ,σ′-N,N′-coordinated to a Re(CO)₃ fragment through pyridine and pyrazine to form a five-membered chelate ring. A seven-membered ring is obtained for isomer 2 by N-coordination of the 2-pyridyl groups while the pyrazine ring remains uncoordinated. For 2, isomers 2a and 2b are found in a dynamic equilibrium ratio [2a]/[2b]  =  7 in solution, detected by ¹H NMR (−50 °C, CD₃COCD₃), coalescence being observed above room temperature. The ligand in 3 behaves as an 8e-donor bridge bonding two Re(CO)₃ fragments through two (σ,σ′-N,N′) interactions. When the reaction was carried out in refluxing tetrahydrofuran, complex [Re₂(CO)₆(C₁₄H₁₀N₄)₂] (4) was obtained in addition to compounds 1-3. The dinuclear rhenium derivative 4 contains two units of the organic ligand σ,σ′-N,N′-coordinated in a chelate form to each rhenium core. The X-ray crystal structures for 1 and 3 are reported.     

Ligand substitution in the heterometallic cluster Cp*IrOs3(μ-H)2(CO)10

The reactions of the heterometallic cluster Cp*IrOs₃(μ-H)₂(CO)₁₀ with phosphines, isonitriles and pyridine under TMNO activation afforded the substitution products Cp*IrOs₃(μ-H)₂(CO)_10−nL_n (n = 1, 2; L = PPh₃, P(OMe)₃, ^tBuNC, CyNC or py) in good yields. For the monosubstituted derivatives, the substitution site was exclusively at an osmium atom in an axial position for L = phosphine or phosphite. Spectroscopic evidence suggested the presence of isomers in solution for the PPh₃ derivative. In contrast, for L = isonitrile, the ligand occupied an equatorial site. In the disubstituted derivatives, the group 15 ligands were coordinated to two different osmium atoms, one each at an axial and an equatorial site. The isomerism and fluxional behaviour of some of these clusters have also been examined.     

The He(I) photoelectron spectra of OsO4, and RuO4

The photoelectron spectra of ruthenium tetroxide and osmium tetroxide excited by He(I) radiation are reported. From their interpretation it follows that the first two strong low energy transitions in the electronic absorption spectra of OsO₄, RuO₄, TcO^?₄, ReO^?₄, MoO^2?₄ and WO^2?₄ can be assigned to t₁ → 2e and 3t₂ → 2e respectively.     

Displacement of neutral nitrogen donors by chloride in [Pt(SNS)(R-py)](ClO4)2 (SNS = 2,6-bis(methylthiomethyl)pyridine; R-py = pyridines): A reactivity comparison between meta- and para-substituted pyridines

The kinetics of the process [Pt(SNS)(R-py)]²⁺ + Cl⁻ → [Pt(SNS)Cl]⁺ + R-py {SNS = 2,6-bis(methylsulfanylmethyl)pyridine; R-py = meta- or para-substituted pyridines covering a wide range of basicity} were studied in methanol at 25 °C. The reactions obey the usual two-term rate law observed in the substitution reactions of square-planar d⁸ complexes. The plots of log k₂ {k₂ = second-order rate constants} against the pK_a of the heterocycles conjugate acids highlighted a different sensitivity of the two groups of N-donors to changes in basicity, thepara-substituted pyridines (4R-py) showing a weaker dependence on pK_a than the meta-substituted (3R-py). The results have been explained on the basis of a π-acidity difference between 3R-py and 4R-py which influences the reaction ground state.     

Reaction of multifunctional molecule (Ph2P(o-C6H4)CHNCH2CH2)3N with triosmium carbonyl clusters

Reaction of (Ph₂P(o-C₆H₄)CHNCH₂CH₂)₃N with 3 equiv. of Os₃(CO)₁₀(NCMe)₂ at ambient temperature affords the triple cluster [Os₃(CO)₁₀Ph₂P(o-C₆H₄)CHNCH₂CH₂]₃N (1) through coordination of the phosphine and imine groups. Thermolysis of 1 in benzene leads to decarbonylation and C-H/C-N bond activation of the ligand to generate (μ-H)Os₃(CO)₈(μ₃-Ph₂P(o-C₆H₄)CHNCCH₂) (2). The molecular structure of 2 has been determined by an X-ray diffraction study.     

Reaction of the heteronuclear cluster Cp*IrOs3(μ-H)4(CO)9 with alkynes: An unusual case of amine activation

Reaction of the heteronuclear cluster Cp*IrOs₃(μ-H)₄(CO)₉ with alkynes is activated by excess amine to afford the butterfly clusters Cp*IrOs₃(CO)₉(RCCR′); hinge-apex isomers are formed. In the case of PhCCH, another cluster Cp*IrOs₃(CO)₉(CCHPh)₂, which contained two alkenyl moieties was also isolated.     

Pd(PBu 3 t ) Adducts of Os3(CO)12. Synthesis and Structures of Pd2Os3(CO)12(PBu 3 t )2 and Pd3Os3(CO)12 (PBu 3 t )3

Two new compounds Pd₂Os₃(CO)₁₂ , 13 and Pd₃Os₃(CO)₁₂ , 14 have been obtained from the reaction of with Os₃(CO)₁₂ at room temperature. The products were formed by the addition of two and three groups to the Os–Os bonds of Os₃(CO)₁₂. Compounds 13 and 14 interconvert between themselves by intermolecular exchange of the groups in solution. Compounds 13 and 14 have been characterized by single crystal X-ray diffraction analyses.Dedicated to Professor Brian F. G. Johnson on the occasion of his retirement – 2005.     

Reactions of the dichloroboryl complex of osmium, Os(BCl2)Cl(CO)(PPh3)2, with water, alcohols, and amines: Crystal structures of Os[B(OH)2]Cl(CO)(PPh3)2, Os[B(OEt)2]Cl(CO)(PPh3)2, and

Reaction between Os(CO)₂(PPh₃)₃ and 3,3-diphenylcyclopropene under quartz-halogen irradiation leads to C(sp²)-H bond activation and the formation of the 3,3-diphenylcyclopropenyl complex, OsH[C₃H(Ph-2)₂](CO)₂(PPh₃)₂ (1). When complex 1 is heated there is ring-opening of the cyclopropene ring and rearrangement to the 3-phenylindenyl complex, OsH[C₉H₆(Ph-3)](CO)₂(PPh₃)₂ (2). Compound 1 reacts with HCl forming the 2,2-diphenylcyclopropyl complex, OsCl[C₃H₃(Ph-2)₂](CO)₂(PPh₃)₂ (3). Reaction of either 1 or 3 with excess HCl leads to reversible formation of the hydroxycarbene complex, OsCl₂[C(OH)C₃H₃(Ph-2)₂](CO)(PPh₃)₂ (4), through protonation of the acyl group formed by a migratory insertion reaction involving a carbonyl ligand and the σ-bound 2,2-diphenylcyclopropanyl ligand. An X-ray crystal structure determination of 2 is reported.     

C-H Activation of 3,3-diphenylcyclopropene in the reaction with Os(CO)2(PPh3)3: 3,3-diphenylcyclopropenyl, 2,2-diphenylcyclopropyl, and 3-phenylindenyl complexes of osmium(II)

Reaction between Os(CO)₂(PPh₃)₃ and 3,3-diphenylcyclopropene under quartz-halogen irradiation leads to C(sp²)-H bond activation and the formation of the 3,3-diphenylcyclopropenyl complex, OsH[C₃H(Ph-2)₂](CO)₂(PPh₃)₂ (1). When complex 1 is heated there is ring-opening of the cyclopropene ring and rearrangement to the 3-phenylindenyl complex, OsH[C₉H₆(Ph-3)](CO)₂(PPh₃)₂ (2). Compound 1 reacts with HCl forming the 2,2-diphenylcyclopropyl complex, OsCl[C₃H₃(Ph-2)₂](CO)₂(PPh₃)₂ (3). Reaction of either 1 or 3 with excess HCl leads to reversible formation of the hydroxycarbene complex, OsCl₂[C(OH)C₃H₃(Ph-2)₂](CO)(PPh₃)₂ (4), through protonation of the acyl group formed by a migratory insertion reaction involving a carbonyl ligand and the σ-bound 2,2-diphenylcyclopropanyl ligand. An X-ray crystal structure determination of 2 is reported.     

Synthesis and reactivity of [ReBr2(NCCH3)2(CO)2]: A new precursor for bioorganometallic chemistry

We have synthesised (Et₄N)[ReBr₂(NCCH₃)₂(CO)₂] 1 in two steps from [ReBr₃(CO)₃]²⁻. Complex 1 is water and air stable and the two Br⁻ ligands are easily exchanged for coordinating solvent molecules such as water. The reactivity of 1 with several ligands such as imidazole (imz) and 2-picolinic acid (2-pic) are easily possible with substitution exclusively occurring in trans-position to the carbonyl groups. The resulting complexes [Re(imz)₂(NCCH₃)₂(CO)₂]⁺ and [Re(2-pic)(NCCH₃)₂(CO)₂] have been isolated and structurally characterised. The two acetonitrile ligands are strongly bound and are not substituted under any conditions. Complex 1 represents therefore the new moiety “trans,cis-[Re(NCCH₃)₂(CO)₂]⁺” which can be considered as a further building block in organometallic chemistry.     

Influence of Fe3(CO)12 on the interaction of Re2(CO)10 with hydroxylated alumina surface

The interaction of Re₂(CO)₁₀ and Fe₃(CO)₁₂, and that of Re₂Fe(CO)₁₄ with alumina were studied during thermal treatment by FT-IR spectroscopy. The interaction of Re₂Fe(CO)₁₄ with alumina results in the formation of Re-tricarbonyls as in the Re₂(CO)₁₀ + Fe₃(CO)₁₂/Al₂O₃ system, even at room temperature. In the view of this fact, the possibility of the action of reactive Fe-monocarbonyls [Fe(CO)₅, Fe(CO)₄] on the Re₂(CO)₁₀ with appearance of a Re₂Fe(CO)₁₄ as a transient intermediate on the support, cannot be excluded.     

Thermal and photochemical reactivity of Os(HNO)(CO)Cl2(PPh3)2: Evidence for photochemical HNO generation

FTIR studies of the thermal and photochemical reactions of Os(N(O)H)(CO)Cl₂(PPh₃)₂ (1) are described. Though 1 is relatively stable, it readily reacts when irradiated to form multiple products, including a metal–carbonyl species and N₂O, the decomposition product of HNO. The relative yields of products varied depending on whether or not excess CO was present. A model is presented that includes initial photochemical release of HNO from 1 as a significant but not exclusive photoreaction.     

Substitution derivatives of the heteronuclear cluster RuOs3(μ-H)2(CO)13: Monosubstituted group 15 derivatives

Tertiary group 15 ligand monosubstituted derivatives of the heteronuclear cluster RuOs₃(μ-H)₂(CO)₁₃ have been prepared and their solid state and solution structures examined. A number of isomeric structural types have been identified in solution, and these appear to be correlated to disorder in the solid state. Hydride fluxionality and restricted rotation about the metal-phosphorus bond have also been observed.     

Studies of N-heterocyclic compounds with triosmium clusters: Formation of the heterocyclic-linked dicluster compound [Os 6(μ-H)2(μ3-C6H4N3)2(CO)18]

The cluster [Os₃(CO)₁₀(MeCN)₂] reacts with indazole (C₇H₆N₂) to give two isomeric products [0s₃(μ-H)(μ-C₇H₅N₂)(CO)₁₀] in which the five-membered ring has been metallated with N-H cleavage to give an N,N-bonded isomer or with C-H cleavage to give a C,N-bonded isomer. These two isomers have very similar X-ray structures but can be clearly distinguished by ¹H NMR methods. They are shown to correspond to related clusters derived from pyrazole. Benzotriazole (C₆H₅N₃) also reacts (as shown earlier by others) to give two isomers: an N,N-bonded species [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₁₀] coordinated only through the five-membered ring and a minor C,N-bonded isomer [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₁₀], metallated at the C₆ ring and coordinated through both rings. The former isomer reacts with Me₃NO in acetonitrile to give [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₉(MeCN)] which thermally looses MeCN to produce the coupled product [Os₆(μ-H)₂(μ₃-C₆H₄N₃)₂(CO)₁₈] which was shown by X-ray structure determination to have all six nitrogen atoms coordinated to osmium, a novel situation for coordinated benzotriazole. The two Os₃ units are linked together by an OsNNOsNN ring in a boat conformation with the whole cluster adopting C₂ symmetry.     

Dimethyl carbonate: an environmentally friendly solvent for hydrogen peroxide (H2O2)/methyltrioxorhenium (CH3ReO3, MTO) catalytic oxidations

Environmentally friendly oxidations of various organic compounds with the hydrogen peroxide (H₂O₂)/methyltrioxorhenium (CH₃ReO₃, MTO) catalytic system have been described in dimethyl carbonate (DMC), a cheap commercially available and benign chemical having interesting solvating properties, low toxicity and high biodegradability. Oxidations proceeded with good conversions and in good yields. Spectrophotometric analysis demonstrated that the [CH₃ReO(O-O)₂] complex was formed in DMC and that it was stable for several days at room temperature.     

A novel arene bonding mode in the mixed-metal cluster Os4Ru(μ-H) 3(CO) 12(μ3-η-C6H5) P(OMe)3

The ionic coupling of [Os₄H₂(CO)₁₂]²⁻ with [Ru(η⁶-C₆H₆)(MeCN)₃]²⁺ affords the neutral mixed metal cluster Os₄Ru(μH)₂(CO)₁₂(η⁶-C₆H₆) 1. The reaction of 1 with trimethylphosphite leads to the initial formation of the addition product Os₄Ru(μH)₂(CO)₁₂(η⁶-C₆H₆)P(OMe)₃ 2, but this complex rearranges in solution to give Os₄Ru(μ-H)₃(CO)₁₂(μ₃-η⁶-C₆H₅)P(OMe)₃ 3. An X-ray structure of 3 shows that the metal core of the cluster is a ruthenium-spiked Os₄ tetrahedron, with one hydrogen atom from the arene having transferred to the Os₄ core, and one arene carbon bridging an Os-Os edge, while the ring as a whole remains η⁶-bound to the Ru atom.