Protonation Of Cp Research Articles

Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

Remote Oxidative Activation of a [Cp*Rh] Monohydride.

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e- . Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible RhII/I process at -0.96 V vs. ferrocenium/ferrocene (Fc+/0 ). This redox manifold was confirmed by isolation of an uncommon RhII species, [Cp*Rh(dppf)]+ , that was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, [Cp*Rh(dppf)H]+ , and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs. Fc+/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh-H moiety by 23 pKa units, a reactivity pattern confirmed by in situ 1 H NMR studies. Taken together, these results show that remote oxidation can effectively induce M-H activation and suggest that ligand-centered redox activity could be an attractive feature for the design of new systems relying on hydride intermediates.

Are cucurbiturils better drug carriers for bent metallocenes? Insights from theory.

Bent metallocenes (BM) have anti-tumor properties but they face a serious drug efficacy problem due to poor aqueous solubility and rapid hydrolysis under physiological conditions. These two problems can be fixed by encapsulating them in host molecules such as cyclodextrin (CD), cucurbituril (CB) etc. Experimentally, CD-BM, CB-BM host-guest complexes have been investigated to check the efficiency of the drug delivery and efficiency of the encapsulated drug. CB has been reported to be a better host than CD but the reasons for this has not been figured out. This can be done by finding out the mechanism of binding and the nature of the binding forces in both the inclusion complexes. This is exactly done here by performing a DFT study at BP86/TZP level on CB-BM host-guest systems. For comparison CD-BM with β-cyclodextrin as host have been studied. Four BMs (Cp2MCl2, M=Ti, V, Nb, Mo) and their corresponding cations (Cp2MCl+, Cp2M2+) are chosen as guests and they are encapsulated into cucurbit-[6]-uril (CB[6]) and cucurbit-[7]-uril(CB[7]) host systems. Computations reveal that CB[7] accommodates well the BMs over CB[6] due to their larger cavity size and also CB[7] is found to be a better host than β-cyclodextrin. BMs enter vertically rather than horizontally into the CB cavity. The reversible binding of BMs within CB[7] is controlled by various non-bonding interactions and mainly by hydrogen bonding between the portal oxygen atoms and Cp protons as revealed by QTAIM analysis. On the other hand, the interaction between the wall nitrogen atoms in CB[7] and chlorine atoms attached to the metal in BM strengthens the M-Cl bonds that prevents rapid hydrolysis of M-Cl and M-Cp bonds saving the drug. Comparatively, BMs experience less electrostatic attraction and more Pauli repulsion within β-cyclodextrin cavity and this affects the drug binding with CD. This makes β-cyclodextrin a less suitable drug carrier for BMs than CBs. Among the four BMs, niobocene binds strongly and titanocene binds weakly with CBs. EDA clearly shows that all the interactions between the guest and host are non-covalent in nature and electrostatic interactions outperform high-repulsion resulting in stable complexes. Cations form stronger complexes than neutral BMs. FMO analysis reveals that neutral BMs are less reactive compared to their cations and complexes are more reactive in CB[6] environment due to excess strain. QTAIM analysis helps to bring out the newer insights in these types of host-guest systems.

Structural and Electrochemical Consequences of [Cp*] Ligand Protonation.

There are few examples of the isolation of analogous metal complexes bearing [η5-Cp*] and [η4-Cp*H] (Cp* = pentamethylcyclopentadienyl) complexes within the same metal/ligand framework, despite the relevance of such structures to catalytic applications. Recently, protonation of Cp*Rh(bpy) (bpy = 2,2'-bipyridyl) has been shown to yield a complex bearing the uncommon [η4-Cp*H] ligand, rather than generating a [RhIII-H] complex. We now report the purification and isolation of this protonated species, as well as characterization of analogous complexes of 1,10-phenanthroline (phen). Specifically, reaction of Cp*Rh(bpy) or Cp*Rh(phen) with 1 equiv of Et3NH+Br- affords rhodium compounds bearing endo-η4-pentamethylcyclopentadiene (η4-Cp*H) as a ligand. NMR spectroscopy and single-crystal X-ray diffraction studies confirm protonation of the Cp* ligand, rather than formation of metal hydride complexes. Analysis of new structural data and electronic spectra suggests that phen is significantly reduced in Cp*Rh(phen), similar to the case of Cp*Rh(bpy). Backbonding interactions with olefinic motifs are activated by formation of [η4-Cp*H]; protonation of [Cp*] stabilizes the low-valent metal center and results in loss of reduced character on the diimine ligands. In accord with these changes in electronic structure, electrochemical studies reveal a distinct manifold of redox processes that are accessible in the [Cp*H] complexes in comparison with their [Cp*] analogues; these processes suggest new applications in catalysis for the complexes bearing endo-η4-Cp*H.

Conformationally rigid cyclic tungsten bis-alkyne complexes derived from 1,1′-dialkynylferrocenes

In exploring the conformational behavior of cyclic tungsten bis-alkyne complexes, two dialkynylamides (14a and 14c) and two dialkynylesters (14b and 14d) derived from 1,1’-ferrocenedicarboxylic acid were prepared. They were subsequently reacted with W(CO)3(dmtc)2 to yield the desired cyclic tungsten bis-alkyne complexes 8–11. In the cyclization of 14a to yield 8 a dimeric macrocyclic complex, 15, featuring two tungsten bis-alkyne complexes in the ring, also was isolated. The conformational behavior of these complexes was assessed by analysis of the 1H NMR resonances for the alkyne hydrogens, which appear around 11 ppm. The spectra for complexes 10, 11 and 15 show multiple singlets of varying integrations for these protons, while the spectra for complexes 8 and 9 show only two resonances of equal integration for the alkyne hydrogens. The spectra for 8 and 9 changed very little when examined at higher temperatures, indicating that the solution conformation is robust. A ROESY spectrum was obtained for 8. It did not show any crosspeaks between the two alkyne hydrogens. The NMR data shows that the alkyne ligands in 10, 11 and 15 are able to rotate about the tungsten-alkyne bond; these complexes adopted several different solution conformations relating to syn and anti arrangements of the alkyne ligands. In contrast, complexes 8 and 9 adopt only one solution conformation, and the alkyne ligands in these species do not rotate about the tungsten-alkyne bond. The NMR spectra for 8 and 9 also show that these complexes are asymmetric. The 1H NMR spectra for 8 and 9 show that each hydrogen atom has its own unique resonance in the 1H NMR spectrum. There are 8 resonances for the 8 Cp protons, 4 resonances for the methylene protons, 2 resonances for the alkyne protons, and in the case of 8, 2 resonances for the NH protons. The two NH protons on complex 8 were found to have widely different chemical shifts. A DMSO titration was performed and it showed that one of the two NH protons in 8 is involved in an intramolecular hydrogen bond. Given that the diester 9 adopts a similar conformation as the diamide 8, this intramolecular hydrogen bond appears to result from the conformation imposed by cyclization of the ring system. Overall, the data show that the ring system for 8 and 9 provides a unique, rigid, robust, and air stable cyclic molecule where the alkyne ligands are limited to one orientation, presumably the syn orientation. The lack of mobility for the alkyne ligands limits the cyclic molecule to only one solution conformation. Complexes 8 and 9 are the first reported examples of cyclic tungsten bis-alkyne complexes that only adopt a single, robust conformation in solution.

Protonation of Cp*Fe(η5-C6H7) as a convenient procedure for the preparation of the valuable synthon [Cp*Fe(MeCN)3

The (cyclohexadienyl)iron complex Cp*Fe(η5-C6H7) obtained by a reaction of the cation [Cp*Fe(η6-C6H6)]+ with LiAlH4 reacts with HBF4·Et2O or TfOH in MeCN to afford the acetonitrile cation [Cp*Fe(MeCN)3]+ in a high yield. The structure of complex Cp*Fe(η5-C6H7) was determined by X-ray diffraction analysis.

140 H/D Isotopomers Identified by Long-Range NMR Hyperfine Shifts in Ruthenium(III) Ammine Complexes. Hyperconjugation in Ru–NH3Bonding

(1)H NMR spectra of the paramagnetic cyanide-bridged mixed-valence compound [(η(5)-C5H5)Fe(CO)2(μ-CN)Ru(NH3)5](CF3SO3)3 (I) have been obtained in several solvents. When traces of partially deuterated water are present, instead of a single cyclopentadienyl (Cp) resonance shifted by the hyperfine interaction, numerous well-resolved resonances are observed. The spectra were simulated satisfactorily by giving the appropriate statistical weight to 140 possible H/D isotopomers formed by deuteration in the five ruthenium(III) ammine ligands. The proliferation of distinct resonances occurs because (a) the hyperfine shifts (HSs) due to each sequential deuteration in a single ammine are different and (b) while deuteration in an ammine cis to the cyanide bridge causes a downfield shift, in the trans ammine it causes an upfield shift that is nearly twice as large. All of these shifts exhibit a 1/T dependence, but temperature-independent components, due to large second-order Zeeman effects at the Ru(III) center, are also present. Combining the results of density functional theory calculations with data from metal-metal charge-transfer optical transitions and with the effect of solvent-induced NMR HSs, it is argued that Fermi contact shifts at the Cp protons are insignificant compared to those due to the dipolar (pseudocontact) mechanism. Analytical expressions are presented for the dependence of the HS on the tetragonal component of the ligand field at the Ru(III) ion. The tetragonal field parameter, defined as the energy by which the 4d(xy) orbital exceeds the mean t(2g) orbital energy, was found to be 147, 52, and 76 cm(-1), in dimethylformamide, acetone, and nitromethane, respectively. The effects of deuteration show that there is a significant component of hyperconjugation in the Ru-ammine interaction and that ND3 is a weaker π donor than NH3. A single deuteration in an axial ammine increases the tetragonal field parameter (ν) by +2.8 cm(-1), resulting in a HS of -37 ppb in the Cp proton resonance, whereas a single deuteration in an equatorial ammine decreases the field by -1.5 cm(-1) with a HS of +20 ppb, despite a nominal separation of seven chemical bonds. We analyze the origin of this remarkable sensitivity, which relies on the favorable characteristics of the Ru(III) low-spin t(2g)(5) configuration, having a spin-orbit coupling constant ζ ≈ 950 cm(-1).

Promoting Effect of Calcium Alkoxide on the Protonation of Cp Groups of Cp2TiCl2

The variable-temperature 1H NMR spectra show that protonation of Cp and CpH liberation are more rapid and more extensive after addition of calcium (CaL2) to Cp2TiCl2 in LH (LH: 2-methoxyethanol). The unstable CpTiIV intermediates can be easily reduced by Al to a stable CpTiIII moiety and isolated in the form of the [CpTi(μ,η2-L′)Cl]2 compound (L′H: 1-methoxy-2-propanol).

Dihydrogen to Dihydride Isomerization Mechanism in [(C5Me5)FeH2(Ph2PCH2CH2PPh2)]+ through the Experimental and Theoretical Analysis of Kinetic Isotope Effects

The isomerization of complex [Cp*Fe(dppe)(eta2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H double dagger = 21.6 +/- 0.8 kcal mol(-1) and DeltaS double dagger = 5 +/- 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a "direct" pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a "via Cp" pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol(-1), respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(eta2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the eta2-H2 and eta2-D2 analogues. The equilibrium constant value (K = 3.3 +/- 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD for the three isotopomers, yielding kinetic isotope effects: kHH/kHD = 1.24 +/- 0.01 and kHD/kDD = 1.58 +/- 0.01 (and, consequently, kHH/kDD = 1.96 +/- 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(eta2-XY)]+ isotopomers as well as transition states for the "direct" (TSdir) and "via Cp" (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the "direct" pathway for the present system, although the small difference in the calculated energy barriers suggests that the "via Cp" pathway may be preferred in other cases.

Cp(PMe3)2Ru(η2-HSiCl3)]+: Structure and Bonding of a Rare Cationic η2-Silane Complex

Protonation of Cp(PMe3)2RuSiCl3 (1) leads to the formation of [Cp(PMe3)2Ru(η2-HSiCl3)]+ (2+), a rare example of a cationic η2-silane complex. The η2-silane coordination in 2+ is confirmed by an X-r...

Synthesis and investigation of [Cp(PMe(3))Rh(H)(H(2))](+) and its partially deuterated and tritiated isotopomers: evidence for a hydride/dihydrogen structure.

Hydrogenolysis of [Cp(PMe(3))Rh(Me)(CH(2)Cl(2))](+)BAr'(4)(-) (4, Ar' = 3,5-C(6)H(3)(CF(3))(2)) in dichloromethane afforded the nonclassical polyhydride complex [Cp*PMe(3))Rh(H)(H(2))](+)BAr'(4)(-) (1), which exhibits a single hydride resonance at all accessible temperatures in the (1)H NMR spectrum. Exposure of solutions of 1 to D(2) or T(2) gas resulted in partial isotopic substitution in the hydride sites. Formulation of 1 as a hydride/dihydrogen complex was based upon T(1) (T(1)(min) = 23 ms at 150 K, 500 MHz), J(H-D) (ca. 10 Hz), and J(H-T) (ca. 70 Hz) measurements. The barrier (Delta G(++)) to exchange of hydride with dihydrogen sites was determined to be less than ca. 5 kcal/mol. Protonation of Cp(PMe(3))Rh(H)(2) (2) using H(OEt(2))(2)BAr'(4) resulted in binuclear species [(Cp(PMe(3))Rh(H))(2)(mu-H)](+)BAr'(4)(-) (3), which is formed in a reaction involving 1 as an intermediate. Complex 3 contains two terminal hydrides and one bridging hydride ligand which exchange with a barrier of 9.1 kcal/mol as observed by (1)H NMR spectroscopy. Additionally, the structures of 3 and 4, determined by X-ray diffraction, are reported.

Classical and non-classical iron hydrides: synthesis, NMR characterisation, theoretical investigation and X-ray crystal structure of the iron(IV) dihydride 〚Cp*Fe(dppe)(H) 2〛 +BF 4–

Protonation of Cp*Fe(dppe)H ( 1; Cp* = η 5-C 5Me 5, dppe = Ph 2PCH 2CH 2PPh 2) by HBF 4Et 2O at –80 °C in diethylether affords the dihydrogen complex 〚Cp*Fe(dppe)( η 2-H 2)〛 +BF 4 – ( 2 +BF 4 –) in 90% yield. Its PF 6 – salt analogue ( 2 +PF 6 –) is obtained in 94% yield by reaction between the 16-electron derivative 〚Cp*Fe(dppe)〛 +PF 6 – ( 3 +PF 6 –) with H 2 gas at –80 °C. The presence of a bound dihydrogen ligand in 2 + is indicated by a short T 1 minimum values consistent with a H–H distances of 0.98(1) Å. For the partially deuterated derivative 2 +–d 1, the observed J HD value of 27.0 Hz confirms the presence of the coordinated dihydrogen ligand, which displays an H–H separation of 0.97(1) Å, in complete agreement with the distance calculated using the T 1 static rotation model. Variable temperature NMR study shows the gradual, complete and irreversible transformation of the dihydrogen complex into its classical dihydride isomer trans-〚Cp*Fe(dppe)(H) 2〛 + ( 4 +). Thermal solid state reaction (–20 °C, 48 h) of 2 +BF 4 – gives quantitatively 4 +BF 4 –, whereas 4 +PF 6 – is obtained by simple contact of H 2 with a solution of 3 +PF 6 – in THF at room temperature. The crystal structure of 4 +BF 4 – has been determined and shows a transoid arrangement of hydride ligands, consistent with the formulation of 4 + as an iron(IV) dihydride. DFT calculations on both dihydride and dihydrogen isomers of 〚Cp*Fe(dppe)H 2〛 + indicate that 4 + is more stable than 2 + by 0.19 eV, while this energy difference is reversed in the case of 〚CpFe(dpe)H 2〛 + (dpe = H 2PCH 2CH 2PH 2). The preference for the dihydride form in the case of 〚Cp *Fe(dppe)H 2〛 + and of the dihydrogen one in the case of 〚CpFe(dpe)H 2〛 + is due to the larger π-donor and σ-acceptor abilities of the 〚Cp*Fe(dppe)〛 + fragment, as compared to the 〚CpFe(dpe)〛 + unit.

A novel heterobimetallic complex composed of the imide-bridged [3]ferrocenophane and the tridentate palladium(II) complex

The imide-bridged [3]ferrocenophane, 2-pyridyl-1,1′-ferrocenedicarboximide, served as a monodentate ligand to form the corresponding heterobimetallic complex 4 with the palladium(II) complex bearing the tridentate podand ligand, through displacement of the ancillary acetonitrile. The X-ray crystal-structure determination revealed that the pyridyl nitrogen of the ferrocenophane coordinates to the palladium center with distortion of the ferrocenophane moiety, which lies below the coordination plane of palladium. Such a structure is considered to be present even in solution, resulting in four nonequivalent broad signals of Cp protons in the 1H-NMR spectrum, due to the electronic environment effect of the coordination plane of palladium. A π–π stacking interaction was observed between the benzene ring of the podand moieties and the pyridine ring of the neighboring molecule in the crystal packing of 4.

Solvent-Assisted Reversible Proton Transfer within an Intermolecular Dihydrogen Bond and Characterization of an Unstable Dihydrogen Complex

A reversible protonation of transition metal hydrides by weak proton donors has been found by NMR spectroscopy. The reactants are mixed at room temperature, but the protonation is induced only at low temperature by an increase of the dielectric constant of a suitably chosen solvent. Under these conditions the decomposition via dihydrogen release is avoided. Thus, the unstable complex [Cp*RuH4(PCy3)]+ was obtained in CDF3/CDF2Cl by protonation of Cp*RuH3(PCy3) with fluorinated alcohols.

Synthesis of organometallic solids by protonation of Cp*M(CN)3n− (M = Rh, Ir, Ru)

Protonation of aqueous acetone solutions of [Cp*Ir(CN)3]– produces the two-dimensional organometallic solid (H3O)-[Cp*Ir(CN)3].Me2CO wherein 14 A diameter channels (Ir4C8N8O4) contain exchangeable solvent molecules; the analogous Ru and Rh tricyanide complexes are also susceptible to proton-induced polymerization.

Dihydrogen Complex Formation and C−C Bond Cleavage from Protonation of Cp*RuH(diene) Complexes

Reaction of Cp*RuH(COD) with HBF4·OEt2 at −76 °C produced [Cp*Ru(H2)(COD)]BF4, which transformed to [Cp*Ru(η6-1,3,5-COT)]BF4 on warming to room temperature. Reaction of Cp*RuH(NBD) with HBF4·OEt2 produced a mixture of nortricyclene and the novel bimetallic complex [(Cp*Ru)2(μ-H)(μ-C5H5−CHCH2)]BF4.

Migration Reaction of a Hypervalent Fragment: Base-Induced Migration of a Phosphorane Fragment from Iron to the Cyclopentadienyl Ring in Cp(CO)2Fe{P(OC6H4Y)(OC6H4Z)} (Y, Z = NMe, NH, O)

Iron phosphorane complexes, (Y, Z = NMe, O), react with lithium diisopropylamide (LDA) and then MeI to give showing the first example of migration of a hypervalent phosphorus fragment. The structure of one of the migration products, was determined by X-ray analysis. Spectroscopic monitoring of the migration reaction suggests that the reaction is initiated by a proton abstraction on the Cp ring by LDA, followed by migration of the phosphorane fragment to the Cp ring to give which is converted into by the treatment of alkyl halide. The reaction of containing two NH groups with LDA proceeds first with NH proton abstraction to give and then with Cp proton abstraction followed by phosphorane migration to give The addition of MeI finally led to the isolation of and

Dynamics of intramolecular hydrogen exchange in cyclic organohydroborate–zirconocene complexes

ABSTRACT: Proton and 11B NMR investigations of two zirconocene boracycloalkane derivatives A and B, as a function of temperature, identified two hydrogens bridged between boron and zirconium and a third attached only to zirconium. Observation of the 11B decoupled proton NMR lineshape of A and B (toluene-d8 solution) revealed a fast intramolecular mutual exchange of the two bridged hydrogens and a slower process which exchanges each of the latter with the single hydrogen bonded to zirconium alone. Comparison of the observed 11B decoupled proton NMR spectra with calculated lineshapes which take account of the exchange processes yielded the corresponding rates and activation parameters. Proton NMR data for A and B (with 11B coupled) indicate spin coupling between 11B and the bridge hydrogens of ca. 55 Hz, subject to averaging by 11B nuclear electric quadrupole induced relaxation; its rate increases with decreasing temperature. Thus, the coupling is averaged at the lower temperatures due to 11B relaxation and on warming becomes obscured as a result of hydrogen exchange. Analysis of these latter lineshapes taking account of exchange dynamics, 11B relaxation and the 11B proton coupling constant provided dynamic parameters closely similar to those obtained from the 11B decoupled proton NMR data. At elevated temperature (50°C), broadening of the Cp proton resonance indicates that these protons undergo slow exchange with the three hydrido hydrogens. © 1998 John Wiley & Sons, Ltd.

Protonation of Cp(CO)(PPh 3) RuCCPh: Formation of cationnic phenylacetylene and phenylvinylidene complexes and subsequent reaction with ethylene oxide to form the net [2 + 3] cycloadduct [Cp(CO)(PPH 3)Ru=CCH(Ph)CH 2CH 2O] +

Protonation of the alkynyl complex Cp(CO)(PPh 3)RuCCPh ( 1) at low temperature affords quantitatively the vinylidened complex [Cp(CO)(PPh 3)RuCCH(Ph)] + ( 3), which upon warming to room temperature forms an equilibrium with the η 2- phenylacetylene complex [Cp(CO)(PPh 3)Ru( η 2- HC CPh)] + ( 4), with the latter predominating. Subsequent reaction with ethylene oxide yields the cyclic oxacarbene complex [Cp(CO)(PPH 3)Ru=CCH(Ph)CH 2CH 2O] + ( 5), which can be regarded as the result of a net [3+2] cycloaddition reaction between 3 and ethylene oxide. Depronation of 5 affords teh corresponding neutral cyclic vinyl complex [Cp(CO)(PPH 3)RuC=C(Ph)CH 2CH 2O] + ( 6), which can in turn be protonated to regenerate 5 in a diastereoselective manner. The structures of complexes 5 and 6 were determined by X-ray crystallography.

Ligand and metal effects on the enthalpies of protonation of Cp'M(PR3)(PR'3)X complexes (M = ruthenium or osmium)

Titration calorimetry has been used to determine the enthalpies of protonation (ΔH HM ) of 22 Cp'M(PR 3 )-(PR' 3 )X complexes (Cp'=η 5 -C 5 H 5 (Cp) or η 5 -C 5 Me 5 (CP * ); M=Ru, Os; PR 3 =PPh 3 , PPh 2 Me, PPhMe 2 , PMe 3 , P(OEt) 3 , dppm, dppe, dppp; X=H, Cl, Br, I) with CF 3 SO 3 H in 1,2-dichloroethane solution at 25.0 o C to give Cp'M(PR 3 )(PR' 3 )(X)(H) + CF 3 SO 3 -