- Determination of Rh-C bond dissociation energy in methyl(porphyrinato)rhodium(III) complexes: A new application of photoacoustic calorimetry
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The photolysis of RhCH3(ttp) and RhCH3(tmp) (ttp = 5,10,15,20-tetratolylporphyrin, tmp = 5,10,15,20-tetramesitylporphyrin) was studied in methanol at ambient temperature: the quantum yields for photolysis were determined to be 0.51 and 0.54, respectively, and the Rh-C bond dissociation energies (227 and 219 kJ mol-1, respectively) were measured by photoacoustic calorimetry, which were larger than those of Co-C bond.
- Li, Gang,Zhang, Fei Fei,Pi, Na,Chen, Hui Lan,Zhang, Shu Yi,Chan, Kin Shing
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- Activation of unstrained aliphatic carbon-carbon bonds by a transition metal complex
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The activation of unstrained aliphatic carbon-carbon bonds by a transition metal complex was discussed. The intermolecular activation of aliphatic carbon-carbon bonds by transition metal complexes were found to be demonstrated in the reaction of nitroxides with a rhodium(II) porphyrin radical. The analysis showed that the methyl group in the nitroxide was cleaved to give Rh(tmp)Me.
- Tse, Man Kin,Chan, Kin Shing
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- Reduction of rhodium(III) porphyrin hydroxide to rhodium(II) porphyrin
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Highly reactive rhodium(III) porphyrin hydroxides were formed from the ligand substitution of rhodium porphyrin halides in benzene and were rapidly reduced to rhodium(II) porphyrins and hydrogen peroxide. Thus hydroxide acted as the reducing agent. Oxidat
- Choi, Kwong Shing,Lai, Tsz Ho,Lee, Siu Yin,Chan, Kin Shing
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- Excited state dynamics of Rh(II) tetramesityl porphyrin monomer from nanosecond transient absorption and emission spectroscopy
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The excited states of [tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), Rh(II)TMP, have been studied using nanosecond transient absorption spectroscopy, emission spectroscopy, and electrochemistry. Rh(II)-TMP has a long-lived excited state, with a 460 nm absorption band, that decays with a time constant of 180 ± 22 ns, in benzene and 205 ± 28 ns in 1,3-difluorobenzene. No transient absorption features are seen between 600 and 700 nm, but weak phosphorescence is detected at 743 nm at 77 K. Thus, the excited state has the characteristics of a triplet (π,π*) state, although a low lying, charge transfer state had been expected from iterative extended Huckel calculations. The phosphorescence is biphasic, and highly temperature dependent, consistent with a trip-quartet [4T] lowest excited state, lying not far below a trip-doublet [2T] state. Electrochemistry establishes that the Rh(II)/(I) reduction potential is more negative than -1.5 V versus SSCE while the porphyrin cation reduction potential is +0.55 V. These potentials place the lowest LMCT state at >2.05 V, well above the triplet states. In addition to elucidating the photophysics of a novel type of metalloporphyrin, these results have significant bearing on the feasibility of using Rh(II) porphyrins in photoinduced catalytic systems.
- Vitols,Friesen, Duane A.,Williams, Darryl S.,Melamed, Dan,Spiro, Thomas G.
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- Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a RhII metalloradical: A combined experimental and theoretical study
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Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [RhII(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [RhIII(tmp)Me] (3) and [RhIII(tmp)H] (5), respectively. In the presence of excess TEMPO, [RhII(tmp)] is regenerated from [RhIII(tmp)H] with formation of 2,2,6,6-tetramethyl- piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [RhIII(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80°C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the RhII radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [RhII(por)] producing the [TEMPO] +[RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to RhI with formation of [RhIII(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3, 4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the dz2 orbital of [RhI(por)]- at one of the CMe-Cring σ* orbitale of [TEMPO] +. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (S N2) pathway have barriers which are low enough to explain the experimental kinetic data.
- Kin, Shing Chan,Xin, Zhu Li,Dzik, Wojciech I.,De Bruin, Bas
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- Superoxo, peroxo, and hydroperoxo complexes formed from reactions of rhodium porphyrins with dioxygen: Thermodynamics and kinetics
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Rhodium(II) porphyrin complexes react with dioxygen to form terminal superoxo and bridged μ-peroxo complexes. Equilibrium constants for dioxygen complex formation with rhodium(II) tetramesitylporphyrin ((TMP)Rh?) and a m-xylyl-tethered dirhodium(II) diporphyrin complex (?Rh(m-xylyl)Rh?) are reported. (TMP)Rh-H reacts with oxygen to form a transient hydroperoxy complex ((TMP)Rh-OOH), which reacts on to form the rhodium(II) complex ((TMP)Rh?) and water. Kinetic studies for reactions of (TMP)Rh-H with O2 suggest a near concerted addition of dioxygen to the (TMP)Rh-H unit. Reactivity studies for mixtures of H2/O2 and CH4/O2 with the dirhodium(II) complex (?Rh(m-xylyl)Rh?) are reported. Copyright
- Cui, Weihong,Wayland, Bradford B.
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- Ligand effect on the rhodium porphyrin catalyzed hydrogenation of [2.2]paracyclophane with water: Key bimetallic hydrogenation
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Rhodium porphyrin catalyzed hydrogenation of the aliphatic carbon-carbon σ-bond of [2.2]paracyclophane with water has been examined with a variety of tetraarylporphyrins and axial ligands. Mechanistic investigations show that RhIII(ttp)H, which can be derived from the reaction of [RhII(ttp)]2 with water without a sacrificial reductant, plays an important role in promoting bimetallic reductive elimination to give the hydrogenation product.
- Tam, Chun Meng,To, Ching Tat,Chan, Kin Shing
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p. 10057 - 10063
(2017/08/09)
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- Metalloradical-Catalyzed Selective 1,2-Rh-H Insertion into the Aliphatic Carbon-Carbon Bond of Cyclooctane
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The selective aliphatic carbon-carbon activation of cyclo-octane (c-octane) was achieved via the RhII(ttp)-catalyzed 1,2-addition of Rh(ttp)H to give Rh(ttp)(n-octyl) (ttp = tetratolylporphyrinato dianion) in good yield under mild reaction conditions. This mechanism is further supported by DFT calculations. The reaction worked only with the sterically accessible Rh(ttp) porphyrin complex but not with the bulky Rh(tmp) system (tmp = tetrakismesitylporphyrinato dianion), thus showing the highly steric sensitivity of carbon-carbon bond activation by transition metal complexes. (Chemical Equation Presented).
- Chan, Yun Wai,De Bruin, Bas,Chan, Kin Shing
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p. 2849 - 2857
(2015/06/30)
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- Metalloradical-catalyzed aliphatic carbon-carbon activation of cyclooctane
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The aliphatic carbon-carbon activation of c-octane was achieved via the addition of Rh(ttp)H to give Rh(ttp)( n-octyl) in good yield under mild reaction conditions. The aliphatic carbon-carbon activation was RhII(ttp)- catalyzed and was very se
- Chan, Yun Wai,Chan, Kin Shing
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supporting information; experimental part
p. 6920 - 6922
(2010/08/06)
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- Activation of aliphatic carbon-carbon bonds of esters and amides by rhodium(II) porphyrin
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Aliphatic carbon-carbon bonds of esters and amides were activated successfully with rhodium(II) porphyrin radical to give rhodium(III) porphyrin alkyls in moderate yields.
- Zhang, Lirong,Chan, Kin Shing
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p. 2021 - 2027
(2007/10/03)
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- Aliphatic carbon{single bond}carbon bond activation of ketones by rhodium(II) porphyrin radical
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Aliphatic carbon{single bond}carbon bond activation of both enolizable and non-enolizable ketones occurred successfully with rhodium(II) porphyrin radical to give rhodium(III) porphyrin alkyls. Added Ph3P promoted the yields of products.
- Zhang, Lirong,Chan, Kin Shing
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p. 3782 - 3787
(2007/10/03)
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- Formation and reactivity of (tetraarylporphyrinato)rhodium(II) monocarbonyls: Bent RhIICO complexes that react like acyl radicals
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Reactions for a series of rhodium(II) porphyrins with CO are used in illustrating the use of ligand steric effects in both promoting and inhibiting CO coupling to form α-diketone complexes ((por)RhC(O)C(O)Rh(por)). [Tetrakis(2,4,6-trimethylphenyl)porphyri
- Wayland, Bradford B.,Sherry, Alan E.,Poszmik, George,Bunn, Andrew G.
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p. 1673 - 1681
(2007/10/02)
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- Activation of methane and toluene by rhodium(II) porphyrin complexes
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Thermodynamic and kinetic-mechanistic studies are reported for reactions of (tetramesitylporphyrinato)rhodium(II) monomer, (TMP)Rh., and (tetraxylylporphyrinato)rhodium(II) dimer, [(TXP)Rh]2, with methane that produce hydride and methyl derivatives. A RhII-RhII bond energy of ~ 12 kcal mol-1 in [(TXP)Rh]2 was determined by 1H NMR line broadening and found to dominate differences in the thermodynamic and kinetic parameters for reactions of methane with Rh(II) porphyrins. The sum of the Rh-H and Rh-CH3 energies is found to be ~ 117 kcal in both the (TMP)Rh and (TXP)Rh derivatives. Rate laws, activation parameters, and deuterium isotope effects suggest that a four-centered linear transition state (Rh-H3C-H-Rh) provides a relatively low activation enthalpy route for methane reacting with two metalloradicals. Comparative studies demonstrate that rhodium(II) porphyrins react with toluene exclusively at the benzylic C-H bond, and kinetic studies suggest that this reaction proceeds through a transition state related to that for the methane reactions. Aromatic C-H bond reactions are kinetically excluded for rhodium(II) porphyrins due to steric effects in the transition state.
- Wayland, Bradford B.,Ba, Sujuan,Sherry, Alan E.
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p. 5305 - 5311
(2007/10/02)
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