Non-interconverting Isomers Research Articles

Combining Ion Mobility Spectrometry, Mass Spectrometry, and Photoelectron Spectroscopy in a High-Transmission Instrument

We have developed a novel instrument that combines ion mobility spectrometry, mass spectro-metry, and photoelectron spectroscopy. The instrument couples an electrospray ion source, a high-transmission ion mobility cell based on ion funnels, a quadrupole mass filter, and a time-of-flight (magnetic bottle) photoelectron spectrometer operated with a pulsed detachment laser. We show that the instrument can resolve highly structured anion arrival time distributions and at the same time provide corresponding photoelectron spectra-using the DNA oligonucleotide ion [dC(6) - 5H](5-) as a test case. For this multianion we find at least four different, noninterconverting isomers (conformers) simultaneously present in the gas phase at room temperature. For each of these we record well-resolved and remarkably different photoelectron spectra at each of three different detachment laser wavelengths. Two-dimensional ion mobility/electron binding energy plots can be acquired with an automated data collection procedure. We expect that this kind of instrument will significantly improve the capabilities for structure determination of (bio)molecular anions in the gas phase.

Transformations and Reactions of Re2(CO)8(μ-SbPh2)(μ-H) Induced by the Addition of a Platinum(tri-t-butylphosphine) Group

Three products Re(2)[Pt(PBu(t)(3))](mu-SbPh(2))(CO)(8)(mu-H), 2, Re(2)[Pt(CO)(PBu(t)(3))]Ph(CO)(8)(mu(3)-SbPh)(mu-H), 3, and Re(2)[Pt(PBu(t)(3))](2)(CO)(8)(mu(4)-Sb(2)Ph(2))(mu-H)(2), 4, were obtained from the reaction of Re(2)(CO)(8)(mu-SbPh(2))(mu-H), 1, with Pt(PBu(t)(3))(2). Compound 3 was also obtained from 2 by further reaction with Pt(PBu(t)(3))(2). Compound 2 is a Pt(PBu(t)(3)) adduct of 1 formed by the insertion of the platinum atom into one of the Re-Sb bonds of 1 with formation of two Pt-Re bonds. Compound 3 contains an open Re(2)Pt cluster and was also obtained in a low yield by the addition of CO to 2. The addition of SbPh(3) to 2 yielded the compound Re(2)Pt(PBu(t)(3))(Ph)(CO)(8)(SbPh(3))(mu(3)-SbPh)(mu-H), 5, a SbPh(3) derivative of 3. Compound 4 can be viewed as a dimer of the fragment Re[Pt(PBu(t)(3))](CO)(4)(SbPh)(mu-H). The two halves of the molecule are held together by Pt-Sb bonds and a significant interaction directly between the Sb atoms, Sb-Sb distance, 2.9834(7) A. The Sb-Sb bonding in 4 was explained by density functional calculations. Compound 4 adds 2 equiv of CO at 1 atm/25 degrees C, one to each platinum atom, to yield the compound [Re(CO)(4)Pt(H)(CO)(PBu(t)(3))(mu(3)-SbPh)](2) which exists as a mixture of two noninterconverting isomers, cis-6 and trans-6. Both isomers of 6 were isolated and structurally characterized. Each isomer of 6 consists of a central planar Re(2)Sb(2) core composed of two Re(CO)(4) groups with two bridging SbPh ligands. There is a Pt(H)(CO)(PBu(t)(3)) group coordinated to each antimony atom of 6. In the cis-isomer both Pt(H)(CO)(PBu(t)(3)) groups lie on the same side of the Re(2)Sb(2) plane. In the trans-isomer the Pt(H)(CO)(PBu(t)(3)) groups lie on opposite sides of the Re(2)Sb(2) plane.

Ruthenium Complexes of the 1,4-Bis(diphenylphosphino)-1,3-butadiene-Bridged Diphosphine 1,2,3,4-Me4-NUPHOS: Solvent-Dependent Interconversion of Four- and Six-Electron Donor Coordination and Transfer Hydrogenation Activity

In chloroform, [RuCl2(nbd)(py)2] (1) (nbd = norbornadiene; py = pyridine) reacts with 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me4-NUPHOS) to give the dimer [Ru2Cl3(η4-1,2,3,4-Me4-NUPHOS)2]Cl (2a), whereas, in THF [RuCl2(1,2,3,4-Me4-NUPHOS)(py)2] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1 mixture of two noninterconverting isomers, the major with C1 symmetry and the minor with either Cs or C2 symmetry. A single-crystal X-ray analysis of [Ru2Cl3(η4-1,2,3,4-Me4-NUPHOS)2][SbF6] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a η4-six-electron donor, bonded through both P atoms and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-ethylenediamine (en) in THF to afford [RuCl2(1,2,3,4-Me4-NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)][Cl] (5a). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent equilibrium, and in THF 5a readily adds chloride to displace the η2-interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)][ClO4] (5b) confirmed that the diphosphine coordinates in an η4-manner as a facial six-electron donor with the η2-coordinated double bond occupying the site trans to chloride. The η4-bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the η2-coordinated double bond. Complexes 2a, 2b, 4, and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all cases, catalysts formed from precursors 2a and 2b are markedly more active than those formed from 4 and 5a.

1,3-Dipolar Cycloaddition to the Fe−OC Fragment. 19. Synthesis and Properties of Fe(CO)2(PR3)(R1NC(R2)−C(R3)O) and Fe(CO)(Ph2PCH2CH2PPh2)(R1NC(R2)−C(R3)O) and Their Reactivity toward Dipolarophiles. First X-ray Crystal Structure of the Initial Bicyclo[2.2.1] Adduct

Complexes 7ak−an, 7bk,bm [(R1NC(R2)−C(R3)O)Fe(CO)2PR3 a: R1 = tBu, R2 = H, R3 = Ph; b: R1 = Me, R2 = Ph, R3 = Ph; k: R = OMe; l: R = Ph; m: R = Et; n: R = nPr], and 7ao,bo [(R1NC(R2)−C(R3)O)Fe(CO)Ph2PCH2CH2PPh2] have been prepared by substitution of CO in the respective tricarbonyl complexes 1a,b at room temperature. Various dynamic processes have been investigated by 13C and 31P NMR. On the IR time scale, complex 7bk exists in two distinct isomeric forms. Complexes 7 have been characterized by IR, UV−vis, and 1H, 13C, and 31P NMR spectroscopy. Surprisingly, in the 1,3-dipolar cycloaddition reaction of complexes 7 with the dipolarophiles dimethyl acetylenedicarboxylate (DMAD, v), methyl propynoate (w), phenyl acetylene (x), and p-methoxyphenyl-isothiocyanate (y) the bicyclic ferra[2.2.1] complexes 8, i.e., the initial cycloadducts, are stable and isolable. With dimethyl maleate (z) it is formed reversibly. Complexes 8 containing one phosphorus ligand are formed as two non-interconverting isomers. Th...

First evidence for two different mu-eta¹-eta¹- and mu-eta¹-eta²- co-ordination modes of the P3C2Bu t2 ring of [Fe(eta5-P3C2Bu t2)(eta5-C5H5)] to a same cluster fragment: synthesis and characterisation of [Ir4(CO)10{[Fe(eta5-P3C2Bu t2)(eta5-C5H5)}] and x-ray molecular structure of the mu-eta¹-eta²- isomer

Investigation of the solution structures of [Ir4(CO)11L] (L = [Fe(h5-P3C2Bu t2)(h5-C5H 5)] (1) and [Fe(h5-P3C2Bu t2)(h5-P2C 3But3)] (2) by 13C and 31P NMR spectroscopy showed that, at 163K, 1 exists in the form of two isomers with bridged and non-bridged structures, in a 1:0.15 ratio, respectively, whereas 2 exists only in the bridged form. At RT, 1,2 shift of the h5-P3C2But 2 ring was only observed for compound 2. Where as 2 loses CO readily in solution to give [Ir4(CO)10{m-h1-h 1-[Fe(h5-P3C2 But2)(h5-P2 C3But3)}] (3), activation with Me3NO was necessary to produce [Ir4(CO)10{[Fe(h5-P 3C2But2)(h5-C5H5)}] (4), obtained in the form of two non-interconverting isomers 4a and 4b, which were not able to be separated. A single crystal X-ray diffraction study of isomer 4a established that the [Fe(h5-P3C2Bu t2)(h5-C5H 5)] ligand bridges one of the edges of the Ir4 tetrahedron, interacting via the lone electron pair of one of the adjacent P atoms and in an h2- mode via the P-P double bond of the h5-P3C2But 2 ring and that all CO ligands are terminally bonded. Variable temperature 31P{1H} NMR spectroscopy evidenced a fluxional process involving interactions between the Ir1 and Ir2 atoms and the lone pair on P1, the P1-P2 bond, and the lone pair on P2. According to multinuclear NMR, cluster 4b has similar structure to compound 3, with the h5-P3C2But 2 ring co-ordinated in a h1-h1- mode via the two adjacent P atoms, and all CO ligands bonded terminally.

Preparation and Properties of Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ester). X-ray Crystal Structures of Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OEt)2and Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OMe)(i-PrNC(H)C(O)O)

Reaction of Fe2(CO)9 at room temperature in THF or toluene with the α-imino esters (L) R1NC(R2)C(R3)O (a−i; R1 = alkyl, R2 = H, R3 = O-alkyl), results in the formation of several mono- and binuclear complexes, depending on the bulkiness of the R1 substituent. With all ligands employed, the known type of complex Fe2(CO)6(L) (6) is formed. Reaction of the less bulky R1 substituted ligands a−e (R1 = neo-Pe, i-Pr, 1-cyclohexylethyl) leads to the formation of Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OEt)2(10a−d/10‘a−d) as a mixture of two noninterconverting isomers together with the known type of complexes Fe2(CO)6(L−L) (7). Complexes 7 contain two (former) α-imino ester ligands joined at the imine carbon atoms. In the presence of water, the reaction in THF leads to the formation of Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OMe)(i-PrNC(H)C(O)O) (11/11‘). The unusual σ(N):σ(C) coordination of the newly formed ligands in 10 and 11 is confirmed by X-ray structure determinations. Thermal or photochemical activation of 7 in tol...

Spurious cooperativity in alkylated succinic acids

The proton–proton correlation, as measured by the ratio between the second and the first dissociation constants of dibasic acid, is sometimes very large and far beyond what could be explained by electrostatic theories. We propose a novel interpretation of this phenomenon based on the idea of spurious cooperativity. The general theoretical framework underlying the onset of spurious cooperativity is developed first. The basic result is that whenever a binding (or dissociating) two-site (or more) system splits into a mixture of noninterconverting isomers the binding isotherm (or the titration curve) behaves as if it is more negatively cooperative compared with the genuine cooperativities of the individual isomer. The theory is applied to a specific system of α–α′ dialkyl succinic acid. It is known that the Meso form of these alkylated derivatives show a normal correlation of the same order of magnitude as in succinic acid. On the other hand, the Racemic form of these alkylated derivatives shows anomalous strong negative correlations when the alkyl groups become large (e.g., isopropyl and tert butyl). It is shown that the theory of spurious cooperativity can explain the different behavior of the Racemic and the Meso forms, as well as the onset of anomalous strong negative correlations when the alkyl groups become large.

Mixed tetranuclear rhenium–ruthenium compounds formed by carbon–sulphur bond cleavage in reactions of [Re2(pyS)2(CO)6](pyS = pyridine-2-thionate) with [Ru3(CO)12]. Crystal structures of [ReRu3(µ4-S)(µ-C5H4N)(CO)14] and [Re2Ru2(µ-S)(µ-C5H4N)(µ-pyS)(CO)13

The dinuclear compound [Re2(pyS)2(CO)6](pyS = pyridine-2-thionate) reacts with [Ru3(CO)12] to give a series of compounds containing ReRu3, Re2Ru2, and Re3Ru groups and which are formally built up by the combination of Re(pyS)(CO)3 and Ru(CO)x(x= 2, 3 or 4) units. The compound [ReRu3(µ4-S)(µ-C5H4N)(CO)14] is formed by C–S bond cleavage and exists as two non-interconverting but inseparable isomers in solution. A single crystal selected from the crystallised mixture was found by X-ray structure determination to contain just one pure diastereomer as a mixture of enantiomers. The four metal atoms are linked by a µ4-S ligand and a disordered 2-pyridyl ligand bridges two Ru(CO)3 groups. The isomers in solution result from the interchange of Re(CO)4 and Ru(CO)4 units. Two non-interconvertible isomers of the compound [Re2Ru2(µ4-S)(µ-C5H4N)(µ-pyS)(CO)13] were separated and each structurally characterised. Their structures are extremely similar (except that one has CH2Cl2 molecules in the crystal) differing only in the orientation of the 2-pyridyl bridges. Full crystallographic characterisation depended upon the correct positioning of C and N atoms. The compound [Re3Ru(µ4-S)(µ-C5H4N)(pyS)2(CO)11] also forms separable non-interconverting isomers. One contains two doubly bridging pyS ligands and the other one doubly and one triply bridging pyS ligand. Therefore isomerism in these compounds has three separate origins.

Organic chemistry of dinuclear metal centres. Part 8. Organo–iron–ruthenium chemistry. X-Ray structure of trans-[FeRu(CO)2(µ-CO)2(η-C5H5)2

The iron–ruthenium complex [FeRu(CO)4(η-C5H5)2] is obtained in 60% yield from the reaction of Na[Fe(CO)2(η-C5H5)] with [RuI(CO)2(η-C5H5)]. In the solid state a trans-[FeRu(CO)2(µ-CO)2(η-C5H5)2] structure has been established by X-ray diffraction. Crystals are monoclinic, space group P21/c (no. 14), with Z= 2 in a unit cell for which a= 7.064(2), b= 12.518(3), c= 8.011(2)Å, and β= 106.23(2)°. The structure was solved by heavy-atom methods and refined to R 0.0275 (R′ 0.0313) for 1 532 independent intensities. The molecule is disordered about a centre of inversion at the mid-point of the metal–metal bond, each metal site being occupied by half an iron and half a ruthenium atom, with an iron–ruthenium bond length of 2.626(1)Å. In solution the cis-[FeRu(CO)2(µ-CO)2(η-C5H5)2] isomer is dominant, and shown by 13C n.m.r. to be undergoing cis⇌trans isomerisation with bridge ⇌ terminal carbonyl exchange at room temperature, but to be static at –80 °C. The complex is an excellent precursor of organo-iron–ruthenium chemistry. Treatment with alkynes R1C2R2(R1= R2= H, Me, Ph, or CO2Me; R11= Me or Ph, R2= H) under u.v. irradiation gives complexes [FeRu(CO)(µ-CO){µ-C(O)CR1CR2}(η-C5H5)2] in 20–65% yield as a result of alkyne–CO linkage. This link in the complexes derived from ethyne, propyne, and but-2-yne is broken upon protonation, generating µ-vinyl cations [FeRu(CO)2(µ-CO)(µ-CR1CHR2)(η-C5H5)2]+(R1= R2= H or Me; R1= H, R2= Me). These are attacked by hydride at the β carbon of the µ-vinyl to give µ-alkylidene complexes [FeRu(CO)2(µ-CO)(µ-CR1R2)(η-C5H5)2](R1= H, R2= Me or Et; R1= Me, R2= Et). Reaction of [FeRu(CO)(µ-CO){µ-C(O)CPhCPh}(η-C5H5)2] with Ph3PCHR or CH(CO2Et)N2 in boiling toluene also gives µ-alkylidene complexes [FeRu(CO)2(µ-CO)(µ-CHR)(η-C5H5)2](R = H, Me, or CO2Et) in good yield, through ready displacement of diphenylacetylene. The µ-CH2 complex is best obtained (75%) by treating [FeRu(CO)4(η-C5H5)2] with LiBHEt3 then water, and in a related manner sequential addition of methyl-lithium, HBF4·OEt2, and NaBH4 affords [FeRu(CO)2(µ-CO)(µ-CHMe)(η-C5H5)2]. Under u.v. irradiation alkynes react with µ-alkylidene complexes [FeRu(CO)2(µ-CO)(µ-CHR1)(η-C5H5)2] to give products of alkyne–alkylidene linking [FeRu(CO)(µ-CO)(µ-CR3CR2CHR1)(η-C5H5)2](R1= H or Me, R2= R3= H, Me, Ph, or CO2Me; R1= H or Me, R2= Me or Ph, R3= H). These exist as non-interconverting isomers in which the new C3 ligand is either bound σ to iron and σ, η2 to ruthenium or vice versa. The scope of organo-iron–ruthenium chemistry closely resembles that of the di-iron system but it is apparent that in reactivity terms there is an order: FeRu > Fe2 > Ru2.

The reaction of [Fe 2(η-C 5H 5) 2(CO) 4] with alkyl isocyanides; [Fe 2(η-C 5H 5) 2(CNR) 4] (R = Me OR Et)

In boiling xylene [Fe 2(η-C 5H 5) 2(CO) 4] reacts with RNC (R = Me, Et) to give [Fe 2(η-C 5H 5) 2(CNR) 2(μ-CNR) 2] complexes, which appear to exist as mixtures of non-interconverting isomers at room temperature and which have very basic N atoms in the μ-CNR ligands.

Photodissociation of gaseous olefinic cations

An ion cyclotron resonance spectrometer was used to study the wavelength dependence of the photodissociation of cations in propene and several isomeric butenes. Photodissociation onsets were determined and compared with electron impact and thermodynamic values. In all cases the photodissociation onsets were found to be above both the thermodynamic onset and the first excited state of the cation as determined by photoelectron spectroscopy. The onset results were compatible with a photodissociation mechanism that proceeds by initial excitation into the first excited state, followed by dissociation. A large increase in the photodissociation cross section for all the cations studied was observed at [inverted lazy s] 3.7 eV. This increase correlated with the second excited states of all the cations except 1-butene. Internal excitation of the parent cations was investigated by the addition of inert gas and by changing the electron beam energy. Photodissociation onsets were also found to be valuable in choosing between alternative structures for gas-phase cations. At least two noninterconverting isomers of C4H8+ were shown to exist.