Ligand CH Research Articles

Palladium complexes with simple iminopyridines as catalysts for polyketone synthesis.

Four iminopyridines (N-N') differing in the nature of the substituents on the iminic carbon and on the ortho positions of the aryl ring (H or CH3) on the iminic nitrogen were used for the synthesis of neutral and monocationic palladium(ii) complexes of general formulae [Pd(CH3)Cl(N-N')] and [Pd(CH3)(NCCH3)(N-N')][PF6]. The detailed NMR characterization in solution highlighted that: (i) for both series of complexes, the Pd-CH3 signal is progressively shifted to a lower frequency on increasing the number of methyl groups on the ligand skeleton; (ii) for the neutral derivatives, the chemical shift of the (15)N NMR signals, determined through {(1)H,(15)N}-HMBC spectra, is significantly affected by the coordination to palladium; (iii) the coordination induced shift (CIS) of the nitrogen atom trans to the CH3 ligand is smaller than the other. The structure in the solid state for the neutral derivatives with all the four ligands was solved, pointing out that: (iv) the Pd-C bond distance increases with the basicity of the nitrogen-donor ligand; (v) the Pd-N bond distance correlates well with the CIS value. The combining of the solution and solid state structural features allows stating that: (vi) the Pd-CH3 singlet is a good probe for the electron donor capability of the ligand; (vii) the CIS value might be used as a probe for the strength of the Pd-N bond. All monocationic complexes generated active catalysts for the CO/vinyl arene copolymerization, leading to prevailingly syndiotactic polyketones. The catalyst performances, both in terms of catalyst productivity and polymer molecular weight, correlate well with the precatalyst structural features.

Dimerization of an organoplatinum complex triggered by oxidative addition: A model for dynamic ring-opening polymerization

The bis(pyridine) ligand thiophene-2,5-dicarboxylic acid bis(N-methyl-N-4-pyridyl-amide), 1, reacts with [Pt2Me4(μ-SMe2)2], with displacement of Me2S, to give the chelate complex [PtMe2(1)], 2, which then reacts with methyl iodide to give [PtIMe3(1)], 3. Complex 3 spontaneously dimerizes to give a macrocyclic complex [{PtIMe3(μ-1)}2], 4, which exists as a mixture of anti and syn isomers 4a and 4b, as characterized by both NMR spectroscopy and X-ray structure determination. In the complex 4*, formed by oxidative addition of CD3I, the CH3 and CD3 ligands are scrambled between the axial and equatorial coordination sites at platinum(IV). The dimerization can be considered as a model for the first step in a dynamic ring-opening polymerization and occurs to give a new conformation of the flexible ligand.

Initial Growth Mechanism of Atomic Layer Deposited Hafnium Dioxide Using Cyclopentadienyl-Type Precursor: A Density Functional Theory Study

The initial growth mechanisms of atomic layer deposition of Hafnium dioxide thin films using Cp2Hf (CH3)2 precursor have been investigated. The calculations show that CH4 elimination reaction is energetically more favorable than CpH elimination reaction. As a result, the two ―CH3 ligands of Cp2Hf (CH3)2 may be dissociated prior to the two ―Cp rings. According to the activation barrier analysis, one CpH elimination may occurs sequentially following the first CH4 elimination reaction. During the pulse of Cp2Hf (CH3)2 precursor, the byproduct CH4 and a small amount of CpH are released.

Characterizing pressure-induced uranium C-H agostic bonds.

The diuranium(III) compound [UN′′2]2(μ-η6:η6-C6H6) (N′′=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory. In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal=CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal–metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.

Characterizing Pressure‐Induced Uranium CH Agostic Bonds

AbstractThe diuranium(III) compound [UN′′2]2(μ‐η6:η6‐C6H6) (N′′=N(SiMe3)2) has been studied using variable, high‐pressure single‐crystal X‐ray crystallography, and density functional theory . In this compound, the low‐coordinate metal cations are coupled through π‐ and δ‐symmetric arene overlap and show close metalCH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal–metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic‐type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms‐in‐molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.

Role of Electronegative Substituents on the Bond Energies in the Grubbs Metathesis Catalysts for M = Fe, Ru, Os

Coupled cluster theory [CCSD(T)] with the aug-cc-pVDZ/aug-cc-pVDZ-PP basis sets is used to predict the thermodynamic properties of models of the Grubbs catalyst, H2ImM(PH3)(CRR′) and H2ImM(C2H4)(CRR′) for M = Fe, Ru, and Os and CRR′ = CH2, CHF, and CF2. The PH3 and C2H4, imidazolinium carbene (H2Im), and CRR′ bond dissociation energies (BDEs) are reported. Because of low metal-carbene BDEs, the M = Fe complexes are unlikely to form, so they will not be good catalysts for olefin metathesis. The metal–carbene BDE is an important component in metathesis catalyst design and correlates with the singlet–triplet splitting in the carbene. The two metallacyclobutane intermediates (cis and trans to the imidazolinium carbene) formed by reaction of the CRR′═CRR′ with the 14-electron active species (H2ImM(CRR′)) (R and R′ either H or F) are investigated at the same level of theory. The metallacycles cis to the nitrogen heterocyclic carbene are lower in energy than the trans conformer with the exception of four M = Fe metallacycles in the gas phase and in CH2Cl2 solution at 298 K. The olefin π complex for the simplest CH2 ligand plus C2H4 reactant combination is more stable in the gas phase, but in CH2Cl2 solution at 298 K, the cis metallacycles are more stable than the π complexes or the trans metallacycles on the free energy scale as a result of the large dipole moments in the cis metallacycles. The results show that the best energy balance is achieved with M = Ru, a CH2 carbene substituent, and a C2H4 reactant. The energetics for the Grubbs catalysts are shown to differ from those of the Schrock catalysts.

A Novel Trisamidophosphine Ligand and Its Group(IV) Metal Complexes

A novel trisamidophosphine ligand scaffold has been synthesized and employed to coordinate all of the group(IV) metals. The resulting complexes feature closed half-cage geometries with the phosphorus donor and all three amides binding to the metal. Out of nine complexes of the type [PN3]MX (M = Ti, Zr, Hf; X = NMe2, OTf, CH3), the zirconium derivatives have been studied by X-ray diffraction and shown to exhibit metal–phosphorus distances that are well in range of a coordinative bond. In contrast to its heavier homologues, the titanium methyl species [PN3]TiMe is prone to thermally induced ligand CH activation with concomitant loss of methane.

Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal–Oxo Species

Abstract Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed. The activation of the O–O bond of dioxygen at the mono- and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with µ-η1:η1-O2 and µ-η2:η2-O2 modes to the corresponding dioxo complexes are discussed in detail. In considering the mechanism of methane hydroxylation, special attention to FeO+-mediated methane hydroxylation that occurs under ion cyclotron resonance (ICR) conditions is given. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+ are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process. The spin inversion from the high-spin state to the low-spin state effectively decreases the barrier height of C–H activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the C–H bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HO–M–CH3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH3 groups as ligands. Although compelling discussion to support the involvement of oxygen- and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the C–H activation process by the bare FeO+ complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.

Organoruthenium Complexes Containing New Phosphorus–Carbon and Phosphorus–Carbon–Sulfur Ligands Generated in the Coordination Sphere by Nucleophilic Addition Reactions

The reaction of the dimeric complex [RuCl(μ-Cl)(η6-C10H14)]2 (C10H14 = p-cymene) with an excess of allyldiisopropylphosphane (ADIP) leads to the complex [RuCl(η6-C10H14){κ2(P,C)-iPr2PCH2CH(iPr2PCH2CH═CH2)CH2}]+ (1+), which presents a new bidentate κ2(P,C) ligand. The same cationic complex can be prepared by nuclephilic attack of 1 equiv of the free ADIP at the coordinated κ3(P,C,C)-iPr2PCH2CH═CH2 ligand in [RuCl(η6-C10H14){κ3(P,C,C)-iPr2PCH2CH═CH2}]+ (3+). Addition of different phosphane ligands to complex 3+ allows to the synthesis of ruthenium complexes with new bidentate ligands κ2(P,C). The unusual complexes [Ru{κ3(P,C,S)-iPr2PCH2CH(SR)CH2}(η6-C10H14)]+ (11a–c+) containing new κ3(P,C,S) ligands can be obtained by the reaction of 3+ with anionic nucleophiles RS–. For comparative purposes, the indenyl complex [Ru(η5-C9H7){κ3(P,C,C)-iPr2PCH2CH═CH2}(PPh3)]+ (8+) has been also prepared and differences in the reactivities of cationic complexes 3+ and 8+ toward nucleophiles are pointed out.

Influence of magnetic field on the reaction mechanisms of plasma-assisted atomic layer deposition of Al2O3

In this paper the time-evaluation diagnostics of plasma-assisted atomic layer deposition (PA-ALD) of Al2O3 thin film were performed, in which the spectroscopic ellipsometry and mass spectrometry were used in-situ monitoring group reactions. We obtain that the nucleation density and film growth rate of PA-ALD were increased along with the magnetic field strength. The film growth in the initial cycles was switched from island mode to layer-by-layer mode. Besides, in no magnetic field deposition O radicals were consumed at the surface in whole process by combustion-like reaction with the surface CH3 ligand when the deposition was processed in O2 plasma period, whereas under the magnetic field deposition, the O radicals were consumed much quickly in the first several seconds. Additionally, in magnetic field assistance process the main by-product were C2H4, C2H2 and CH4, then releasing gases were CO2, CO and H2O. We result that the reaction mechanisms under the magnetic field assistance might be totally different from normal PA-ALD, or thermal-ALD process.

Efficient and chemoselective ethene hydromethoxycarbonylation catalysts based on Pd-complexes of heterodiphosphines o-C6H4(CH2PtBu2)(CH2PR2)

The synthesis and properties of a series of unsymmetrical diphosphines o-C6H4(CH2PtBu2)(CH2PR2) are reported where R = C6H5 (La); p-CH3C6H4 (Lb); o-CH3C6H4 (Lc); o-CH3CH2C6H4 (Ld); o-CH3OC6H4 (Le); 2,4,6-(CH3)3C6H4 (Lf); CH(CH3)2 (Lg); or PR2 = PCg (Lh) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions is catalysed by the Pd complexes of each of the ligands La–ha–h to give methyl propanoate in >99% selectivity with catalytic activities comparable to those obtained with o-C6H4(CH2PtBu2)2 (L1) or o-C6H4(CH2PCg)2 (L55). The catalysts derived from Lc, Ld and Lh are more active than the catalyst derived from La or L1; these ligands have in common, a PR2 donor that is more bulky than the PPh2 present in La. Treatment of [PtCl(CH3)(cod)] with La–ha–h gave [PtCl(CH3)(L)] as mixtures of 2 isomers 1a–h and 2a–h. The major isomer in each case was 1a–h with the CH3 ligand trans to the PtBu2 group; the diastereoselectivity of this reaction for products 1a–h ranges from 88% to over 99%. The crystal structures of 1b, 1c and 1e have been determined. Fluxionality associated with chelate ring inversion has been detected for 1c by variable temperature 31P NMR spectroscopy. The 31P NMR data are compared for the complexes [PtCl(CH3)(L)], [Pt(CH3)2(L)] and [PtCl2(L)] where L = Lh, L1 or L55. The crystal structure of [Pt(CH3)2(Lh)] (4h) has been determined and shows that PCg is sterically less demanding than PtBu2 in this complex. Treatment of 4h with HCl gave a 1:1 mixture of 1h and 2h that equilibrated over 5 h to a 70:1 mixture. Treatment of an equilibrium mixture of 1h and 2h with isotopically labelled 13CO gas gave a single acyl complex [PtCl(13COCH3)(Lh)] (5h) with retention of configuration at Pt, i.e. the 13COCH3 is trans to the PtBu2 group. Mechanisms for the CO insertion are discussed which are consistent with the observed stereochemistry. The palladium complexes [PdCl(CH3)(Lh)] (7/8) also reacted with labelled 13CO to give a single acyl species [PdCl(13COCH3)(Lh)]. Treatment of [PdCl(13COCH3)(Lh)] with MeOH rapidly gave CH313COOMe.

A theoretical study on initial growth mechanism of ZrO2 film using cyclopentadienyl-type precursor

The initial growth reaction of atomic layer deposition (ALD) of ZrO2 using Cp2Zr(CH3)2 (CpC5H5, cyclopentadienyl) as metal precursor on the hydroxylated silicon surface is investigated by using density functional theory (DFT). The ALD cycle is achieved through two types of ligand elimination reactions (i.e., CH4 and CpH elimination reactions). The possible reaction pathways are proposed in order to find the dominant initial reaction during the Cp2Zr(CH3)2 precursor pulse. DFT calculations show that the CH4 elimination reaction is energetically more favorable than CpH elimination reaction. As a result, the two CH3 ligands of Cp2Zr(CH3)2 may be dissociated prior to the two Cp rings during the metal precursor pulse. In addition, one CpH elimination may occurs sequentially following the first CH4 elimination reaction according to activation barrier analysis during the Cp2Zr(CH3)2 pulse. All the calculated results are in agreement with the experimental findings.

Surprising photochemical reactivity and visible light-driven energy transfer in heterodimetallic complexes

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been used for the synthesis of a series of novel heterodimetallic complexes starting from [Ru(bpy)(2)(dppcb)]X(2) (1; X = PF(6), SbF(6)), so-called dyads, showing surprising photochemical reactivity. They consist of [Ru(bpy)(2)](2+)"antenna" sites absorbing light combined with reactive square-planar metal centres. Thus, irradiating [Ru(bpy)(2)(dppcb)MCl(2)]X(2) (M = Pt, 2; Pd, 3; X = PF(6), SbF(6)) dissolved in CH(3)CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH(3)CN)(2)(dppcb)MCl(2)]X(2) (M = Pt, 7; Pd, 8; X = PF(6), SbF(6)). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (5/6) lead to [Ru(bpy)(CH(3)CN)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (9), where only the RuP(2)N(4) moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb)NiCl(2)]X(2) (4; X = PF(6), SbF(6)) no clean photoreaction is observed. Interestingly, this difference in photochemical behaviour is completely in line with the related photophysical parameters, where 2, 3, and 5/6, but not 4, show long-lived excited states at ambient temperature necessary for this type of photoreaction. Furthermore, the photochemical as well as the photophysical properties of 2-4 are also in accordance with their single crystal X-ray structures presented in this work. It seems likely that differences in "steric pressure" play a major role for these properties. The unique complexes 7-9 are also fully characterized by single-crystal X-ray structure analyses, clearly showing that the stretching vibration modes of the ligand CH(3)CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH(3)CN)(2)](2+) chromophore of 9 acts as efficient "antenna" for visible light-driven energy transfer to the Os-centred "trap" site, resulting in k(en) ≥ 2 × 10(9) s(-1) for the energy transfer. Since electron transfer is made possible by the use of this intervening energy transfer, in dyads like 2-4 highly reactive M(0) species (M = Pt, Pd, Ni) could be generated. These species are not stable in water and M(II) hydride intermediates are usually formed, further reacting with H(+) to give H(2). Thus, derivatives of 3, namely [M(bpy)(2)(dppcb)Pd(bpy)](PF(6))(4) (M = Os, Ru) dissolved in 1:1 (v/v) H(2)O-CH(3)CN produce H(2) during photolysis with visible light.

The effect of ionization and CH3 ligand for hydrogen storage in Co- and Ni-based organometallic compounds

Maximum capacities of the hydrogen storage in organometallic compounds consisting of Co and Ni atoms bound to CmHm ring (m = 4, 5; capped type) were, respectively, found as 3.48 and 3.49 wt % (Guo et al., Struct Chem, 2009, 20, 1107). Here, we extend this study to structures having a transition metal (TM) inserted in CmHm ring (inserted type), having TM located on either a C4H or a C5H molecule, and the CH3 ligand bound to the organometallic compounds. We find that for the CoC4H4 and NiC4H4 complexes, the capped types are 1.39 eV and 1.41 eV higher in energy than the inserted types, respectively, while the ground states for CoC5H5 and NiC5H5 complexes are found to be the capped type, which are lower than the inserted types, respectively, by 1.27 eV and 1.31 eV. The maximum capacity of hydrogen storage reached 5.13 wt % for both of CoC4H(H2)3 complex and the inserted-type CoC4H4(H2)3 complex with a reasonable binding energy (0.3–1.0 eV per H2). The positively charged C4H4 and C5H5 molecules do not only improve the capacity of hydrogen storage but also make all H2 adsorbing in molecular form and keep the adsorption energy in an ideal range. After adding the CH3 ligand to the compounds, the average adsorption energy of H2 decreased to an ideal range 0.61–0.94 eV per H2 and the stability of the compounds is also improved. Finally, we analyze the HOMO–LUMO gaps and display the kinetic stability when H2 was added to organometallic compounds. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Synthesis and X-ray Structure of a Diamagnetic Oxo-Bridged Trifluoromethyl−Chromium(V) Complex: Structural and Computational Comparisons between CF3 and CH3 Ligands in Two Different Oxidation States of Chromium

A diamagnetic oxo-bridged chromium(V) complex, [Cp*Cr(O)(CF3)]2O, has been synthesized through exposure of a hexane solution of Cp*Cr(CO)3(CF3) to air in the presence of light and characterized by a single-crystal X-ray diffraction study. DFT calculations have been employed to probe its structure and properties and to provide a comparison between CF3 and CH3 ligands in Cr(V) and Cr(II) analogues.

ChemInform Abstract: Pressure-Tuning Spectroscopy of Inorganic Compounds: A Summary of the Past 15 Years

Abstract Since its development over 40 years ago, the diamond–anvil cell has been widely used to study the high-pressure behavior of numerous materials and compounds. Pressures from a few kbar to a Mbar can easily be achieved and the transparent diamonds provide an optical window for in-situ measurements. Pressure-tuning spectroscopy allows researchers to examine the effect of decreased inter- and intramolecular and atomic distances and the resultant orbital perturbations. This review summarizes some interesting results of pressure-tuning infrared, Raman and electronic spectroscopic studies for a variety of inter- and intramolecular interactions including metal–CO π-backbonding, metal–metal interactions in dimers, clusters, and one-dimensional compounds, order–disorder transitions of CH3 and NH3 ligands and hydrogen bonding.

Synthesis and Reactivity of Coordination and Six-Membered, Cyclometalated Platinum Complexes Containing a Bulky Diimine Ligand

The reaction of [Pt2Me4(μ-SMe2)2] with ligand (9-C14H9)CH═NCH2CH2N═CH(9-C14H9) (L) gives initially the platinum(II) complex [PtMe2{(9-C14H9)CH═NCH2CH2N═CH(9-C14H9)}], 1, which contains a chelate dinitrogen ligand. Compound 1 was reacted with MeI to give the related six-coordinate platinum(IV) compound [PtMe3I{(9-C14H9)CH═NCH2CH2N═CH(9-C14H9)}], 2, by oxidative addition. Compound 1, when refluxed in toluene, gave the monometalated complex [PtMe{(9-C14H9)CH═NCH2CH2N═CH(9-C14H8)}], 3, with a six-membered, cyclometalated, anthryl ring. When compound 1 was reacted with acetyl chloride, the dicyclometalated compound [Pt{(9-C14H8)CH═NCH2CH2N═CH(9-C14H8)}], 4, with two six-membered cyclometalated anthryl rings, was formed. Similarly, cis-[PtCl2(dmso)2] was reacted with ligand L to afford the platinum(II) compound [PtCl2{(9-C14H9)CH═NCH2CH2N═CH(9-C14H9)}], 5. This compound, when left in solution at room temperature, gave the monometalated compound [PtCl{(9-C14H9)CH═NCH2CH2N═CH(9-C14H8)}], 6, but under no condition...

Effect of ligand substitution on DNA binding ability of two new square planar copper(II)–Schiff base complexes

Two terdentate NNO donor Schiff base ligands CH 3C(OH) CHC(CH 3) NCH 2C 5H 4N [ HL 1 ] and C 6H 5C(OH) CHC(CH 3) NCH 2C 5H 4N [ HL 2 ] were used to synthesize two mononuclear square planar copper(II) complexes, [Cu L 1 (CF 3COO)] ( 1) and [Cu L 2 (CF 3COO)] ( 2). The complexes were characterized by elemental analyses, FT-IR, UV–vis spectral methods and their structures were established by single crystal X-ray diffraction study. The biochemical activity of the complexes towards DNA binding were performed using absorbance and fluorescence titration methods and their mode of binding with calf thymus DNA has been established by viscosity measurement technique.

Phosphine- and thiophosphorane-amine ligands: Lithiation and coordination to Rh(I)

A series of phosphine-amine and thiophosphorane-amine ligands (PR 2(X)CH 2NHR′, X = lone pair, S, R = Ph, Cy, R′ = tBu, Ph) differing by the nature of the phosphorus and nitrogen substituents were synthesized. Their lithiation, in order to generate the corresponding amido ligand mainly led to dissociation with formation of phosphide or thiophosphinite anion and imine. DFT calculations confirmed that this pathway is in most cases thermodynamically favoured. But for a phosphine-amido anion featuring alkyl substituents on the P atom and phenyl on the nitrogen, calculations showed that the dissociation is strongly disfavoured (Δ G = +24.3 kcal mol −1). Actually, {PCy 2CH 2NPhLi} was isolated in high yield and fully characterized. This phosphine-amido ligand and the corresponding amine derivative were then coordinated to Rh(I) metal centre.

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

The cationic cluster complexes [Ru3(CO)10(mu-H)(mu-kappa2N,C-L1Me)]+ (3+; HL1=quinoxaline) and [Ru3(CO)10(mu-H)(mu-kappa2N,C-L2Me)]+ (5+; HL2=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru3(CO)10(mu-H)(mu-kappa2N,C-Ln)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N-heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris-sec-butylborohydride (K-selectride) to give neutral products that contain novel nonaromatic N-heterocyclic ligands. The following are the products that have been isolated: [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L1Me2)] (6; from 3+ and methyl lithium), [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L1HMe)] (7; from 3+ and K-selectride), [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L2Me2)] (8; from 5+ and methyl lithium), and [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L2HMe)] (11; from 5+ and K-selectride). Whereas the reactions of 3+ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N-methyl group, the reactions of 5+ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N-methyl group. The LUMOs and the atomic charges of 3+ and 5+ confirm that the reactions of these clusters with anionic nucleophiles are orbital-controlled rather than charge-controlled processes. The N-heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face-capping modes. Those of compounds 6-8 have the atoms of a ligand C=N fragment sigma-bonded to two Ru atoms and pi-bonded to the other Ru atom, whereas the ligand of compound 11 has a C-N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable-temperature 1H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above -93 degrees C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH(2) group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180 degrees rotation of the entire organic ligand over a face of the metal triangle.