Methoxy Complexes Research Articles

Ruthenium methoxy and ruthenium vinylidene complexes featuring dithiocarbamate ligands

Treatment of [(Me3tacn)RuIII(κ2-S2CNR2)Cl]PF6 (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) with methanol in the presence of zinc powder in air afforded four ruthenium methoxy complexes [(Me3tacn)RuIII(κ2-S2CNR2)(OCH3)]PF6 (R = Me 1, Et 2, nPr 3, iPr 4). Further reactions of complexes 1–4 and phenylacetylene gave four ruthenium vinylidene complexes [(Me3tacn)(κ2-S2CNR2)Ru=C=CHPh]PF6 (R = Me 5, Et 6, nPr 7, iPr 8). Molecular structures of complexes 1, 2, 3, 6, 7 and 8 were established by single crystal X-ray diffraction analysis. Moreover, all complexes were characterized by infrared, UV–vis, fluorescence and mass spectrometry, and their electrochemical properties were also investigated. The visible-light-induced catalytic properties of complexes 5–8 for H2 evolution by water splitting were explored.

An In-Depth Computational Study of Alkene Cyclopropanation Catalyzed by Fe(porphyrin)(OCH3) Complexes. The Environmental Effects on the Energy Barriers.

Iron porphyrin methoxy complexes, of the general formula [Fe(porphyrin)(OCH3)], are able to catalyze the reaction of diazo compounds with alkenes to give cyclopropane products with very high efficiency and selectivity. The overall mechanism of these reactions was thoroughly investigated with the aid of a computational approach based on density functional theory calculations. The energy profile for the processes catalyzed by the oxidized [FeIII(Por)(OCH3)] (Por = porphine) as well as the reduced [FeII(Por)(OCH3)]− forms of the iron porphyrin was determined. The main reaction step is the same in both of the cases, that is, the one leading to the terminal-carbene intermediate [Fe(Por)(OCH3)(CHCO2Et)] with simultaneous dinitrogen loss; however, the reduced species performs much better than the oxidized one. Contrarily to the iron(III) profile in which the carbene intermediate is directly obtained from the starting reactant complex, the favored iron(II) process is more intricate. The initially formed reactant adduct between [FeII(Por)(OCH3)]− and ethyl diazoacetate (EDA) is converted into a closer reactant adduct, which is in turn converted into the terminal iron porphyrin carbene [Fe(Por)(OCH3)(CHCO2Et)]−. The two corresponding transition states are almost isoenergetic, thus raising the question of whether the rate-determining step corresponds to dinitrogen loss or to the previous structural and electronic rearrangement. The ethylene addition to the terminal carbene is a downhill process, which, on the open-shell singlet surface, presents a defined but probably short-living diradicaloid intermediate, though other spin-state surfaces do not show this intermediate allowing a direct access to the cyclopropane product. For the crucial stationary points, the more complex catalyst [Fe(2)(OCH3)], in which a sterically hindered chiral bulk is mounted onto the porphyrin, was investigated. The corresponding computational data disclose the very significant effect of the porphyrin skeleton on the reaction energy profile. Though the geometrical features around the reactive core of the system remain unchanged, the energy barriers become much lower, thus revealing the profound effects that can be exerted by the three-dimensional organic scaffold surrounding the reaction site.

NCN Nickel(II) Pincer Complexes of 5‐tert‐Butyl‐1,3‐bis(N‐substituted benzimidazol‐2′‐yl)benzenes: Solid and Solution State Behaviour

AbstractA series of square planar NCN nickel(II) pincer complexes; [Methyl(N C N)NiBr] (14), [Benzyl(N C N)NiBr] (15), [Benz°yl(N C N)NiBr] (16), and [Pentyl(N C N)NiBr] (17) [(N C N)=5‐tert‐butyl‐1,3‐bis(N‐substituted benzimidazol‐2′‐yl)phenyl] have been synthesized by the treatment of nickel(0) precursor, Ni(COD)2 [COD=1,5‐cyclooctadiene] with 5‐tert‐butyl‐1,3‐bis‐(N‐substituted‐benzimidazol‐2′‐yl)‐2‐bromobenzenes via oxidative addition in 44–74% yield. The formation of complexes 14–17 was confirmed by elemental analysis, mass spectrometry. Complexes 14 and 15 are poorly soluble in common organic solvents, however, complexes 16 and 17 are fairly soluble in CDCl3. The structure of complex 17 was further corroborated by X‐ray crystallographic studies. Interestingly, the bromide anion bonded to nickel is found to be very labile and it is easily substituted with methoxy group. The formation of the methoxy complex 18 is more facile as compared with related PCsp3P nickel pincer complex [(C6H4PPri2)2CH−NiBr] reported in literature.

Chloro and Alkyl Rare-Earth Complexes Supported byansa-Bis(amidinate) Ligands with a Rigido-Phenylene Linker. Ligand Steric Bulk: A Means of Stabilization or Destabilization?

ansa-Bis(amidinate) ligands with a rigid o-phenylene linker, C6H4-1,2-{NC(tBu)N(2,6-R2C6H3)H}2 (R = Me (1), iPr (2)), were successfully employed for the synthesis of rare-earth chloro and alkyl species. The reaction of dilithium derivatives of 1 and 2 with LnCl3 (Ln = Y, Lu) afforded the monomeric bis(amidinate) chloro lanthanide complexes [C6H4-1,2-{NC(tBu)N(2,6-R2C6H3)}2]Y(THF)(μ-Cl)2Li(THF)2 (R = Me (3), iPr (5)) and [C6H4-1,2-{NC(tBu)N(2,6-Me2C6H3)}2]LuCl(THF)2 (4). Bis(amidinate) ligands in complexes 3 and 4 are coordinated to the metal atoms in a tetradentate fashion, while the bulkier ligand in 5 is tridentate. The alkane elimination reactions of 1 and 2 with equimolar amounts of (Me3SiCH2)3Ln(THF)2 (Ln = Y, Lu) allowed us to obtain the monoalkyl complexes [C6H4-1,2-{NC(tBu)N(2,6-R2C6H3)}2]Ln(CH2SiMe3)(THF)n (Ln = Y, R = Me, n = 1 (6); Ln = Lu, R = Me, n = 1 (7); Ln = Y, R = iPr, n = 2 (8)). The kinetics of thermal decomposition of complexes 6–8 were measured, and for 6 the activation energy was obtained from the temperature dependence of the rate constants (Ea = 67.0 ± 1.3 kJ/mol). Complexes 6 and 7 turned out to be inert toward H2 and PhSiH3. Surprisingly, complex 8 was inert toward H2 and PhSiH3 but rapidly cleaved C–O bonds of DME. The reaction resulted in the formation of the methoxy complex {[C6H4-1,2-{NC(tBu)N(2,6-iPr2C6H3)}2]Y(μ2-OMe)]}2(μ2-DME) (9) and methyl vinyl ether.

Piezochromism in Nickel Salicylaldoximato Complexes: Tuning Crystal‐Field Splitting with High Pressure

The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni⋅⋅⋅Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.

C–H Bond Activation of Methane by PtII–N-Heterocyclic Carbene Complexes. The Importance of Having the Ligands in the Right Place at the Right Time

A DFT study of methane C–H activation barriers for neutral NHC–PtII–methoxy complexes yielded 22.8 and 26.1 kcal/mol for oxidative addition (OA) and oxidative hydrogen migration (OHM), respectively. Interestingly, this is unlike the case for cationic NHC–PtII–methoxy complexes, whereby OHM entails a calculated barrier of 26.9 kcal/mol but the OA barrier is only 14.4 kcal/mol. Comparing transition state (TS) and ground state (GS) geometries implies an ∼10 kcal/mol “penalty” to the barriers arising from positioning the NHC and OMe ligands into a relative orientation that is preferred in the GS to the orientation that is favored in the TS. The results thus imply an intrinsic barrier arising from C–H scission of ∼15 ± 2 kcal/mol for NHC–PtII–methoxy complexes. Calculations show the importance of designing C–H activation catalysts where the GS active species is already structurally “prepared” and which either does not need to undergo any geometric perturbations to access the methane C–H activation TS or is not...

Theoretical study on selective oxidation of olefin and alcohol with Mo–peroxo amine complex using “paired interacting orbitals (PIOs)” analysis

Oxidation mechanisms of ethylene and methanol on Mo(O)(OO) 2 (an amine) and the influence of amines on the oxidation are studied by using paired interacting orbital (PIO) analysis of the transition states (TS) of the ethylene epoxidation and of the methanol coordinated complexes, the methoxy complexes, and the hydrogen abstraction of the methanol oxidation. In the case of ethylene oxidation, we are able to recognize three important PIOs: PIO-1 which expresses the electron delocalization from the π-orbital of the ethylene to the σ*-orbital(O 3–O 4) of the Mo peroxo; PIO-2 which expresses the electron delocalization from the p-orbital of O 4 of the Mo peroxo to the π*-orbital of the ethylene; and PIO-3 which expresses the overlap repulsion between the ethylene and occupied (Mo–O 3–O 4) orbitals of the Mo peroxo. This overlap repulsion is enhanced by the coordination of higher alkyl amines to the Mo peroxo, because of the destabilization of these occupied (Mo–O 3–O 4) orbitals. In the case of methanol oxidation, the main components of the PIO-1 of the methanol coordinated complexes and the methoxy complexes are Mo-4d orbitals of the unoccupied orbitals of the Mo peroxo part, and the main components of the PIO-1 of the hydrogen abstraction are O 5-2p, O 6-2p and C-2p orbitals of SOMO of the Mo peroxo part. The energy of the MOs, that contain these Mo-4d, O-2p or C-2p orbitals are not influenced by the amine coordination on the Mo atom. Results suggest that ethylene epoxidation is hindered by the coordination of higher alkyl amines on Mo peroxo complexes, whereas methanol oxidation is not hindered by the coordination of the amines.

FT-IR study of methanol, formaldehyde and methyl formate adsorption on the surface of Mo/Sn oxide catalyst

The FT-IR spectra of the surface species arising from the adsorption of methanol, formaldehyde and methyl formate on mixed molybdenum-stannic oxide with Mo/(Mo + Sn) = 0.1 are discussed. Methanol is adsorbed mainly in the undissociated form due to hydrogen bond on a cation–anion couple with basic character. Methoxy complex is oxidized to a bidendate formate near 473 K. Hydrogen-bonded H 2CO, coordinatively bonded H 2CO, bidendate formate and polymer species are observed during the adsorption of formaldehyde at pressure 1.5 kPa. Methyl formate is adsorbed in the form of hydrogen-bonded HCOOCH 3, HCOOCH 3 coordinatively bonded, methoxy complex and bidentate formate. In contrast to MoO 3–SnO 2 catalyst with molar ratio Mo/(Mo + Sn) = 0.3, formates are more stable up to 473 K. This can be explained by the contribution of Sn ions during their interaction with CH 3OH, H 2CO and HCOOCH 3.

Ortho C−H Activation of Haloarenes and Anisole by an Electron-Rich Iridium(I) Complex: Mechanism and Origin of Regio- and Chemoselectivity. An Experimental and Theoretical Study

Reaction of (PNP)Ir(COE)+PF6- (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C−H activation takes place. No C−halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C−H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange−correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C−H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2C-C intermediate, which is converted into an η2C-H complex, both being important intermediates in the C−H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2C-H complex leads to selective activation. The unobserved C−halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C−H activation process, but it experiences a higher activation barrier. The ortho C−H activation product is also thermodynamically more stable than the C−Cl oxidative addition complex.

4-Methoxy- and 4-cyano-substituted lithium aryloxides: electronic effects of substituents on aggregation

The para-substituted lithium aryloxides [{4-NC-C6H4OLi.(Pyr)2}2.Pyr] 1a, [{4-NC-C6H4OLi.(THF)2}2] 1b, [{4-MeO-C6H4OLi.Pyr}4] 2a, [4-MeO-C6H4OLi.(THF)n] 2b, [{4-NC-2,6-(t-Bu)2-C6H2OLi.(Pyr)2}infinity] 3a, [{4-NC-2,6-(t-Bu)2-C6H2OLi.(THF)2}infinity] 3b, [{4-MeO-2,6-(t-Bu)2-C6H2OLi.Pyr}2.(Pyr)2] 4a, and [4-MeO-2,6-(t-Bu)2-C6H2OLi.(THF)n] 4b were prepared by the direct deprotonation of the corresponding phenol with an alkyllithium base (BuLi or MeLi) in the appropriate solvent, either pyridine or THF. All compounds were characterized by 1H and 13C NMR spectroscopy, and the crystal structures of 1a, 1b, 2a, 3a, 3b and 4a were elucidated. The cyano derivatives 1a and 1b adopt discrete tetrasolvated Li2O2 ring dimers whereas the methoxy analogue 2a crystallizes as a tetrasolvated molecular tetramer with a pseudo cubic Li4O4 core. The sterically encumbered cyano derivatives 3a and 3b form isostructural 1D polymeric chains of monomers via bridging of the phenolate ligands through Li...NC and Li-O contacts. In comparison, the crystal structure of the methoxy counterpart 4a is a disolvated molecular Li2O2 ring dimer. Solution NMR spectroscopic studies of 1-4 in d5-pyridine and d8-THF indicate that the methoxy complexes are more highly aggregated than the cyano derivatives, consistent with the solid-state studies. Ab initio molecular orbital calculations at the HF/6-31G* level of theory indicate that the origin of the aggregation state variations between the cyano and methoxy complexes is due to electronic effects.

Columnar liquid crystals from wedge-shaped tetrahedral copper(i) complexes

Very stable copper(I) complexes were engineered from pyridine ligands bearing on one side an iminophenyl fragment and on the other a pivotal methyl group. The phenyl group was substituted in the para position either by a methoxy group or by a phenyl ester group grafted with three paraffin chains (n = 8, 12, 16). The methoxy complex in the crystalline state was characterized by X-ray diffraction. Its molecular structure was found to correspond to a distorted tetrahedral arrangement round the copper(I) metal centre. The long-chain complexes near room temperature were found to produce columnar liquid crystals of hexagonal symmetry. The columns were found to result from the aggregation of the wedge-shaped molecules round the columnar axis with the metal-containing iminopicoline ligands crowded together in the centre and the alkyl chains pointing outwards with a disordered conformation. The degree of molecular aggregation, around 7, was found to decrease significantly on heating.

Synthesis, characterization, and acid–base properties of (N–N)PtIV(CH3)2(OH)2 − x(OCH3)x(x = 0, 1) complexes

(N–N)PtIV(CH3)2(OH)2 − x(OCH3)x (x = 0, 1; N–N = bipy, 4,4′-(CH3)2bipy, 4,4′-tBu2bipy, ArNC(CH3)–C(CH3)NAr (Ar = 3,5-(CF3)2C6H3, 3,5-(CH3)2C6H3, 2,6-(CH3)2C6H3) complexes have been prepared by oxidation of the appropriate (N–N)PtII(CH3)2 complexes using H2O2/H2O. In methanol, the hydroxo–methoxy complexes are formed (x = 1), whereas in acetone, the dihydroxo complexes are formed (x = 0). A single-crystal X-ray structure determination establishes the structure of (4,4′-(CH3)2bipy)PtIV(CH3)2(OH)2 as a dihydroxo compound with octahedral geometry, with the hydroxo ligands occupying apical coordination sites. The acidities of the protonated bipy hydroxo complexes, (4,4′-R2bipy)PtIV(CH3)2(OH)(OH2)+ cations, were determined in methanol. The acidities closely matched that of Cl2CHCOOH (pKa = 6.38 in CH3OH).

ChemInform Abstract: Asymmetric Diels-Alder Reactions with Chiral Acetylenic Carbene Complexes as Dienophiles.

A new synthetic method is developed for the asymmetric Diels-Alder reactions of acetylenic dienophiles. Acetylenic Fischer carbene complexes with chiral pyrrolidines as the heteroatom stabilizing substituent were anticipated to block three of the four possible approaches of a diene and lead to selective asymmetric cycloadditions. A series of complexes of the type (CO)5MvC((C4H7N)CR2OMe)- CuCH (MaCr, W; RaH, Me, Ph) were prepared by aminolysis of the corresponding methoxy complexes that have the terminal acetylene protected as a silane. Michael addition is completely suppressed if the alkyne is protected as a triisopropylsilyl group and after aminolylsis by 1,2-addition of a pyrrolidine the terminal alkynyl carbene complexes can be obtained predominately as E-isomers upon protodesilylation. The cycloadditions of the E-isomers (but not Z-isomers) of these complexes occur with significant asymmetric induction with 2-tri- isopropylsiloxy-1,3-pentadiene (66-73% de) but not with cyclopentadiene and a-triisopropylsiloxyvinyl cyclohexene. A model is presented to account for the observed stereoselectivities. q 2000 Published by Elsevier Science Ltd.

Asymmetric Diels–Alder Reactions with Chiral Acetylenic Carbene Complexes as Dienophiles

A new synthetic method is developed for the asymmetric Diels–Alder reactions of acetylenic dienophiles. Acetylenic Fischer carbene complexes with chiral pyrrolidines as the heteroatom stabilizing substituent were anticipated to block three of the four possible approaches of a diene and lead to selective asymmetric cycloadditions. A series of complexes of the type (CO)5MC((C4H7N)CR2OMe)CCH (M=Cr, W; R=H, Me, Ph) were prepared by aminolysis of the corresponding methoxy complexes that have the terminal acetylene protected as a silane. Michael addition is completely suppressed if the alkyne is protected as a triisopropylsilyl group and after aminolylsis by 1,2-addition of a pyrrolidine the terminal alkynyl carbene complexes can be obtained predominately as E-isomers upon protodesilylation. The cycloadditions of the E-isomers (but not Z-isomers) of these complexes occur with significant asymmetric induction with 2-triisopropylsiloxy-1,3-pentadiene (66–73% de) but not with cyclopentadiene and α-triisopropylsiloxyvinyl cyclohexene. A model is presented to account for the observed stereoselectivities.

Fragmentation pathways of [Re2(μ-OR)3(CO)6]− (R = H, Me) and ligand exchange reactions with oxygen donor ligands, investigated by electrospray mass spectrometry †

The rhenium hydroxy and methoxy carbonyl complexes [Re2(μ-OR)3(CO)6]− (R = H or Me) have been studied by negative-ion electrospray mass spectrometry (ESMS). The complexes undergo facile exchange reactions with protic compounds, including alcohols and phenols. With dimethyl malonate, ester hydrolysis occurs giving carboxylate-containing complexes, and with H2O2 or ButOOH, oxidation to ReO4− occurs. The feasibility and extent of these reactions can conveniently, rapidly, and unambiguously be determined by electrospray mass spectrometry, and is dependent on the acidity and steric bulk of the protic compound. The results also suggest that the complexes can be used as versatile starting materials for the synthesis of a wide range of analogous [Re2(μ-OR)3(CO)6]− complexes by simple reaction with an excess of the appropriate alcohol. By varying the applied cone voltage the fragmentation pathways have been investigated; the hydroxy complex undergoes dehydration followed by CO loss, whereas for the methoxy complex β-hydride elimination (and CO loss) is observed, with confirmation provided by deuterium labelling studies. Under ESMS conditions, the neutral complexes [Re2(μ-OR)2(μ-dppf)(CO)6] [R = H or Me; dppf = 1,1′-bis(diphenylphosphino)ferrocene] undergo substantial solvolysis and hydrolysis to give mainly mononuclear species; simple parent ions (e.g. [M + H]+) are not formed in appreciable abundance, probably due to the lack of an efficient ionisation pathway.

Reaction-Induced Spreading of Metal Oxides onto Surfaces of Oxide Supports during Alcohol Oxidation: Phenomenon, Nature, and Mechanisms

Reaction-induced spreading of bulk metal oxides onto surfaces of oxide supports during alcohol oxidation, a new phenomenon occurring at temperatures much lower than that required for thermal spreading, has been extensively investigated with Raman spectroscopic and fixed-bed catalytic studies. The reaction-induced spreading kinetics were accelerated by reaction of gaseous components with metal oxides to form mobile complex compounds and found to depend on temperature, gaseous component, metal oxide, and oxide support. Increasing the reaction temperature increases the metal oxide spreading rate. The efficiencies of converting three-dimensional bulk metal oxides into two-dimensional surface metal oxide species by different gaseous components are methanol ≫ ethanol > 2-butanol, water ≫ oxygen. The high reaction-induced spreading efficiency of methanol is related to the high volatility and stability of its metal−methoxy complexes. Reaction-induced spreading of CrO3, MoO3, V2O5, Re2O7, and Cr2O3 during alcohol ...

Nucleophilic Attack on Cyclooctadiene Complexes of Palladium: Stereochemistry of the Resulting Cyclooctenylpalladium Derivatives

Reaction of [PdClMe(cod)] with MeO2CC⋮CCO2Me occurs in two steps, namely, insertion of the alkyne into the Pd−Me bond followed by migratory insertion of one of the double bonds of the coordinated cyclooctadiene into the Pd−alkenyl bond thus formed. The resulting chloride-bridged, cyclooctenyl complex was converted to [Pd(acac){η1,η2-C8H12C(CO2Me)CMe(CO2Me)}], 2a, which was characterized crystallographically. The molecular structure revealed that each insertion took place in a cis manner. Reactions of [PdCl2(cod)] with a range of nucleophiles were performed, which gave analogous chloride-bridged, cyclooctenylpalladium dimers, 1, which could be converted to their acetylacetonate derivatives, 2. The solid-state structure of the methoxy complex [Pd(acac)(η1,η2-C8H12OMe)], 2b, obtained by reaction of [PdCl2(cod)] with NaOMe followed by Ag(acac), was determined. This showed that exo attack of the OMe group occurred. In contrast, the structure of the phenyl derivative [Pd(acac)(η1,η2-C8H12Ph)], 2e, produced by treatment of [PdCl2(cod)] with Ph4Sn followed by Ag(acac), indicated that the phenyl group added in an endo fashion, presumably via a phenylpalladium intermediate of the type [PdClPh(cod)]. With the larger mesityl group, the complex [PdCl(C6H2Me3)(cod)] could be isolated. Each of the complexes was fully characterized by 1H and 13C{1H} NMR spectroscopy, and the appearance of the CHPd signal was used to distinguish between the exo and endo isomers. On the basis of the crystallographic and NMR data, it was deduced that the organometallic reagents ArMgX (Ar = Ph, 2-MeC6H4, 2,4,6-Me3C6H2), Ph4Sn, and (4-MeC6H4)2Hg generate the endo products by initial attack at the metal. In contrast, reactions with NaOR (R = Me, Et), NaCH(CO2Me)2, AgOAc, and Et2NH proceed by direct attack of the nucleophile at the coordinated double bond to give the exo products.

Theoretical Study of the Oxidation of Alcohol to Aldehyde by d0 Transition-Metal−Oxo Complexes: Combined Approach Based on Density Functional Theory and the Intrinsic Reaction Coordinate Method

A combined density functional (DF) and intrinsic reaction coordinate (IRC) method has been applied to the mechanistic study of methanol oxidization to formaldehyde by the d0 transition-metal−oxo complexes MO2X2 (M = Cr, Mo, X = Cl; M = Ru, X = O). A two-step mechanism was investigated. The two steps involve addition of the methanol OH bond to an MO linkage to form a M−methoxy complex, MO2X2 + CH3OH = M(O)(OH)Cl2(OCH3) (step 1), and the elimination of formaldehyde from the M−methoxy complex to yield the final products, M(O)(OH)Cl2(OCH3) = M(OH)2X2 + CH2O (step 2). The calculated vibrational adiabatic intrinsic barriers were 23.7 kcal/mol (Cr), 16.2 kcal/mol (Mo), and 21.4 kcal/mol (Ru) for the addition process (1), as well as 23.1 kcal/mol (Cr), 33.3 kcal/mol (Mo), and 7.4 kcal/mol (Ru) for the elimination step (2). The enthalpies of the overall oxidation process were computed to be 3.1 kcal/mol (Cr), 41.9 kcal/mol (Mo), and −1.9 kcal/mol (Ru). The IRC trajectories revealed that reaction 1 is initiated by the formation of the weaker adduct CH3OH−MO2X2 between the initial reactants, whereas reaction 2 results in the strong adduct CH2O−M(OH)2X2 between final products. It is concluded that only the chromium and ruthenium oxo complexes are efficient reagents for the conversion of methanol to formaldehyde.

Synthesis and structure of trinuclear methoxy-bridged titanium(III) -magnesium complexes: [(C 5H 5− nMe n) 2Ti( μ-OMe) 2] 2Mg (n = 4 and 5)

The hydride complexes [(C 5Me 5) 2Ti(μ-H) 2] 2Mg ( 1) and [(C 5HMe 4) 2Ti(μ-H) 2] 2Mg ( 2) smoothly react with four equivalents of methanol to give the methoxy complexes [(C 5Me 5) 2Ti(μ-OMe) 2] 2Mg ( 3) and [(C 5HMe 4) 2Ti(μ-OMe) 2] 2Mg ( 4). The X-ray crystal analysis of 4 revealed a nearly linear arrangement of the metal atoms in an unsymmetrical molecule. The EPR spectra of the frozen toluene glass of 3 and 4 showed the triplet state spectra with very similar zero field splitting of rhombic symmetry. The distance between the unpaired electrons in 4 was estimated from the value of D = 0.00958 cm −1 to be 6.43 Å, in good agreement with the crystallographic Ti IIITi III distance of 6.329(1) Å. Complex 4 reacts with excess dibutyl magnesium to give a mixture of 2 and [(C 5HMe 4) 2Ti(μ-H]) 2Mg(μ-OMe)] 2 ( 6). A mixture of 4 and methylalumoxane (1 : 100) did not induce the polymerization of ethylene.

Metallcarbonyl‐Synthesen, XXI. Einfache Synthese eines präparativ nützlichen Alkoxy(carbonyl)metallats

Metal Carbonyl Syntheses, XXI. – Straightforward Synthesis of a Synthetically Useful Alkoxy(carbonyl)metallateAnionic methoxy complexes are formed upon carbonylation of ammonium perrhenate in methanol at 230–240°C. Thus, the dinuclear ionic complex [N(CH3)4][Re2(CO)6(μ‐OCH3)3] (2) results in 80% yield at CO pressures of ca. 100 bar, while the trinuclear derivative [N(CH3)4][Re3(CO)9(μ3‐OCH3)3(μ3‐OCH3)] (3) is due to lower pressure conditions (ca. 50 bar); 2 yields the bis(carbene) complex 4 of formula fac‐Re(CO)3L2I upon treatment with a bis(imidazolium) iodide, thus demonstrating that carbonylrhenium chemistry not necessarily depends on the availability of Re2(CO)10.