“…In neopentyl complex A , the calculated Ir−Ir bond distance is 0.043 Å shorter than the crystallographic value observed in 3 ; the disagreement between model trihydride J and 4 is 0.022 Å, with the calculated bond length being too short. This level of accuracy compares favorably with bond lengths calculated between fifth-period atoms using all-electron basis sets or pseudopotentials. − Inspection of calculated intraligand and metal−ligand bond lengths indicates an overbinding tendency, with calculated bond distances being ≤0.043 Å too short on a root-mean-square basis. The calculated enthalpy change of hydrogenation reaction 1 is −212.6 kJ mol -1 ; its free energy change is −188 kJ mol -1 .…”
Two-electron mixed-valence complexes of the general formula (tfepma)(3)Ir(2)(0,II)RBr [tfepma = bis(bis(trifluoroethoxy)phosphino)methylamine, MeN[P(OCH(2)CF(3))(2)](2), and R = CH(3) (2), CH(2)C(CH(3))(3) (3)] have been synthesized and structurally characterized and their reactivity with H(2) investigated. Hydrogenation of 2 and 3 proceeds in a cascade reaction to produce alkane upon initial H(2) addition, followed by the formation of the Ir(2)(I,III) binuclear trihydride-bromide complex (tfepma)(3)Ir(2)(I,III)H(3)Br (4) upon the incorporation of a second molecule of H(2). Hydrogenation of two-electron mixed-valence di-iridium alkyl complexes is examined with nonlocal density-functional calculations. H(2) attacks the Ir(II) metal center prior to alkyl protonation to produce an eta(2)-H(2) complex. Transition states link all intermediates to a complex that has the same regiochemistry as the crystallographically determined final product. Calculated atomic charges suggest that the second H(2) molecule is homolytically cleaved within the di-iridium coordination sphere and that a hydrogen atom migrates across the intact Ir-Ir metal bond. These results are consistent with the emerging trend that two-electron mixed-valence cores manage the two-electron chemistry of substrates with facility when hydrogen is the atom that migrates between metal centers.
“…In neopentyl complex A , the calculated Ir−Ir bond distance is 0.043 Å shorter than the crystallographic value observed in 3 ; the disagreement between model trihydride J and 4 is 0.022 Å, with the calculated bond length being too short. This level of accuracy compares favorably with bond lengths calculated between fifth-period atoms using all-electron basis sets or pseudopotentials. − Inspection of calculated intraligand and metal−ligand bond lengths indicates an overbinding tendency, with calculated bond distances being ≤0.043 Å too short on a root-mean-square basis. The calculated enthalpy change of hydrogenation reaction 1 is −212.6 kJ mol -1 ; its free energy change is −188 kJ mol -1 .…”
Two-electron mixed-valence complexes of the general formula (tfepma)(3)Ir(2)(0,II)RBr [tfepma = bis(bis(trifluoroethoxy)phosphino)methylamine, MeN[P(OCH(2)CF(3))(2)](2), and R = CH(3) (2), CH(2)C(CH(3))(3) (3)] have been synthesized and structurally characterized and their reactivity with H(2) investigated. Hydrogenation of 2 and 3 proceeds in a cascade reaction to produce alkane upon initial H(2) addition, followed by the formation of the Ir(2)(I,III) binuclear trihydride-bromide complex (tfepma)(3)Ir(2)(I,III)H(3)Br (4) upon the incorporation of a second molecule of H(2). Hydrogenation of two-electron mixed-valence di-iridium alkyl complexes is examined with nonlocal density-functional calculations. H(2) attacks the Ir(II) metal center prior to alkyl protonation to produce an eta(2)-H(2) complex. Transition states link all intermediates to a complex that has the same regiochemistry as the crystallographically determined final product. Calculated atomic charges suggest that the second H(2) molecule is homolytically cleaved within the di-iridium coordination sphere and that a hydrogen atom migrates across the intact Ir-Ir metal bond. These results are consistent with the emerging trend that two-electron mixed-valence cores manage the two-electron chemistry of substrates with facility when hydrogen is the atom that migrates between metal centers.
“…The HF/BS1 and MP3/BS1 levels of theory calculate the GT transition state TS3 − 4 to be lower in energy than the CH transition state TS6 − 7 by the largest amounts (15.5 and 11.3 kcal/mol, respectively), whereas the MP2/BS1 and MP4SDQ/BS1 levels of theory give the opposite results and calculate the CH transition state TS6 − 7 to be lower in energy than the GT transition state TS3 − 4 by over 16 kcal/mol. These values for [ E ( TS6 − 7 ) − E ( TS3 − 4 )] are indicative of oscillatory behavior in the Møller−Plesset perturbation series, which has been observed previously in other transition-metal systems. , 3 Energy differences [ E ( TS6 − 7 ) − E ( TS3 − 4 )] (kcal/mol) between key transition states in the GT and CH pathways, as calculated with various DFT and WFT methods and BS1. …”
Section: Resultssupporting
confidence: 53%
“…As part of our ongoing interest in evaluating and comparing computational methods using transition metal systems, − we sought to determine whether the conclusion that the GT mechanism is energetically favored was influenced by the computational methodology. To this end, the key reaction steps that determine whether Si−C and C−H bond formation occurs via the GT mechanism or the CH mechanism were examined using a series of DFT and wave function theory (WFT) methods.…”
A series of density functional theory (DFT) and wave function theory (WFT) methods were used in conjunction with a series of basis sets to investigate the influence of the computational methodology on the relative energies of key intermediates and transition states in potential reaction pathways in ruthenium-silylene-catalyzed hydrosilylation reactions. A variety of DFT methods in a modest basis set and B3LYP calculations in a variety of basis sets calculated the key transition in the Glaser-Tilley (GT) pathway to be energetically favored. In contrast, with the smaller basis sets, the CCSD(T) method calculated the Chalk-Harrod (CH) pathway to be favored; however, CCSD(T) results extrapolated to larger basis sets favored the GT pathway.
“…40,41 In general, -bond metathesis is favored on light electrondeficient metals, whereas oxidative addition is favored on the heavy and electron-rich late transition metals. [42][43][44][45] For chromium, -bond metathesis would be expected and is explored in this work. However, the possibility that dehydrogenation takes place by means of oxidative addition of the alkane can not be ruled out and will be investigated in subsequent work.…”
Mononuclear Cr(III)-silica models have been studied by quantum chemical methods with respect to catalytic activity toward dehydrogenation of ethane. Both cluster and slab models have been developed and used to explore the conceptual model of mononuclear Cr(III) with three covalent ligands that coordinate through oxygen. The study focuses on a reaction mechanism consisting of three main reaction steps: (1) C-H activation of ethane according to -bond metathesis and accompanied by the formation of O-H and Cr-C bonds, (2) -H transfer to chromium with subsequent loss of ethene and (3) regeneration of the chromium site under evolution of H 2 . An alternative mechanism is also explored, in which C-H activation takes place at a reactive hydridochromium complex. Stationary points pertaining to these reactions have been optimized, and free energy calculations are used to identify the rate-determining steps. The influence of the local structure of the chromium surface sites is explored by means of a number of idealized surface models and electronic energy profiles for the reactions.
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