Milstein and co-workers have reported that the pincer complexes trans-[Ru(H)2(PNN)(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru–H bonds to produce the six-coordinate trans-[Ru(OCH(OMe)2)(H)(PNN)(CO)] intermediate and (ii) a metal–ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH(OMe)2]− anion along with cleavage of a C–OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru(H)(PNN)(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru(H)2(PNN)(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH(OMe)2]− anion in which the C–H bond is facing the metal center, (ii) reorientation of the [OCH(OMe)2]− anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C–OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and trans-[Ru(H)(OMe)(PNN)(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal–ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru(H)2(PNN)(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru(H)(OCH(OMe)2)(PNN)(CO)] and [Ru(H)(OCH2OMe)(PNN)(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotochemical dehydrogenation of alkanes catalyzed by trans-carbonylchlorobis(trimethylphosphine)rhodium: aspects of selectivity and mechanismJohn A. Maguire, William T. Boese, and Alan S. GoldmanCite this: J. Am. Chem. Soc. 1989, 111, 18, 7088–7093Publication Date (Print):August 1, 1989Publication History Published online1 May 2002Published inissue 1 August 1989https://pubs.acs.org/doi/10.1021/ja00200a030https://doi.org/10.1021/ja00200a030research-articleACS PublicationsRequest reuse permissionsArticle Views1430Altmetric-Citations123LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
Solutions of Rh(PR3)2(CO)Cl (R = Me, Ph) are found to catalyze the rapid transfer of oxygen from amine oxides or organoselenium oxides to carbon monoxide; however, the rhodium complexes undergo no reaction with the oxides in the absence of added CO. Kinetic studies indicate that the catalytically active species is the CO-substituted complex Rh(PR3)(CO)2Cl, although it is not present in any observable concentration under the conditions of the reaction. Ir(PPh3)2(CO)2Cl also acts as an efficient catalyst precursor for the same oxygen transfer reactions, although like the rhodium complex it undergoes little or no direct reaction with the oxides. The catalytically active species is again found to be the product of substitution of a ligand (in this case, chloride) by CO: [Ir(PPh3)2(CO)3]+ in either ion-paired or unpaired states. Among substrates with weak E−O bonds (E = N, Se), reactivity correlates with substrate basicity in accord with a transition state having the character of a nucleophilic attack (at carbonyl carbon). Oxides with much stronger E−O bonds, even the highly basic triphenylarsine oxide, are much less reactive; the transition state in this case apparently involves significant E−O bond breaking and is presumably not well modeled as a simple nucleophilic attack. Pt(Ph3As)(CO)Cl2 was found to act as a good catalyst precursor for deoxygenation of arsine oxide, but this system is apparently very complex and the nature of the catalytically active species has not been elucidated.
Stratospheric volume mixing ratio profiles of chlorine nitrate (ClONO 2 ) have been retrieved from 0.01‐cm −1 resolution infrared solar occultation spectra recorded at latitudes between 14°N and 54°S by the atmospheric trace molecule spectroscopy Fourier transform spectrometer during the ATLAS 1 shuttle mission (March 24 to April 2, 1992). The results were obtained from nonlinear least squares fittings of the ClONO 2 ν 4 band Q branch at 780.21 cm −1 with improved spectroscopic parameters generated on the basis of recent laboratory work. The individual profiles, which have an accuracy of about ±20%, are compared with previous observations and model calculations.
The para-N-pyridyl-based PCP pincer ligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners (tBuPCP)IrHCl and derivatives, 2-H is highly air sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; DFT calculations of direct one-electron oxidations are in good agreement with this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (negative charge on Ir) and a p-N-pyridylidene (remote NHC) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (G° = 31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t butyl C-H bond, and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered complexes would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n alkanes. The resulting iridium alkyl complexes could undergo facile -H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation that is driven by one-electron oxidation and deprotonation, enabled by MLPT.
The para-N-pyridyl-based PCP pincer ligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners (tBuPCP)IrHCl and derivatives, 2-H is highly air sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; DFT calculations of direct one-electron oxidations are in good agreement with this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (negative charge on Ir) and a p-N-pyridylidene (remote NHC) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (G° = 31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t butyl C-H bond, and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered complexes would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n alkanes. The resulting iridium alkyl complexes could undergo facile -H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation that is driven by one-electron oxidation and deprotonation, enabled by MLPT.
The final flight of the Atmospheric Trace Molecule Spectroscopy experiment as part of the Atmospheric Laboratory for Applications and Science (ATLAS-3) Space Shuttle mission in 1994 provided a new opportunity to measure broadband (625-4800 cm(-1), 2.1-16 µm) infrared solar spectra at anunapodized resolution of 0.01 cm(-1) from space. The majority of the observations were obtained as exoatmospheric, near Sun center, absorption spectra, which were later ratioed to grazing atmospheric measurements to compute the atmospheric transmission of the Earth's atmosphere and analyzed for vertical profiles of minor and trace gases. Relative to the SPACELAB-3 mission that produced 4800 high Sun spectra (which were averaged into four grand average spectra), the ATLAS-3 mission produced some 40,000 high Sun spectra (which have been similarly averaged) with an improvement in signal-to-noise ratio of a factor of 3-4 in the spectral region between 1000 and 4800 cm(-1). A brief description of the spectral calibration and spectral quality is given as well as the location of electronic archives of these spectra.
The paranoid top executive will seek out and promote others who share his obsessions. The histrionic leader will recruit only dependent, passive and second tier managers so that he himself can make all the key decisions. All of these selection biases maximize the impact of the neurotic styles of the top executives and allow them to endure.(Kets de Vries & Miller, 1984a, p. 38)
