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| Content Provider | Royal Society of Chemistry (RSC) |
|---|---|
| Author | Nyamato, George S. Magubane, Makhosazane N. Munro, Orde Q. Ojwach, Stephen O. |
| Copyright Year | 2016 |
| Abstract | A series of iron(II) and nickel(II) complexes chelated by 2-pyrazolyl(methyl)pyridine (L1), 2,6-bis(pyrazolylmethyl)pyridine (L2), and 2,6-bis(pyrazolyl)pyridine (L3) ligands have been investigated as transfer hydrogenation (TH) catalysts for a range of ketones. Nine chelates in total were studied: [Ni(L1)Br2] (1), [Ni(L1)Cl2] (2), [Fe(L1)Br2] (3), [Ni(L2)Br2] (4), [Ni(L2)Br2] (5), [Fe(L2)Cl2] (6), [Ni(L3)Br2] (7), [Ni(L3)Br2] (8), and [Fe(L3)Cl2] (9). Attempted crystallization of complexes 4 and 6 afforded stable six-coordinate cationic species 4a and 6a with a 2 : 1 ligand : metal (L : M) stoichiometry, as opposed to the monochelates that function as precursors to catalytic species for TH reactions. Crystallization of 7·4H2O and 8·2H2O, in contrast, afforded tri- and bis(aqua) salts of L3 chelated to Ni(II) in a 1 : 1 L : M stoichiometry, respectively. Complexes 1–9 formed active catalysts for the TH of a range of ketones in 2-propanol at 82 °C. Both the nature of the metal ion and ligand moiety had a discernible impact on the catalytic activities of the complexes, with nickel(II) chelate 5 affording the most active catalyst (kobs, 4.3 × 10−5 s−1) when the inductive phase lag was appropriately modelled in the kinetics. Iron(II) complex 3 formed the most active TH catalyst without a significant inductive phase lag in the kinetics. DFT and solid angle calculations were used to rationalize the kinetic data: both steric shielding of the metal ion and electronic effects correlating with the metal–ligand distances appear to be significant factors underpinning the reactivity of 1–9. Catalysts derived from 1 and 9 exhibit a distinct preference for aryl ketone substrates, suggesting the possible involvement of π-type catalyst⋯substrate adducts in their catalytic cycles. A catalytic cycle involving only 4 steps (after induction) with stable DFT-simulated structures is proposed which accounts for the experimental data for the system. |
| Starting Page | 65205 |
| Ending Page | 65221 |
| Page Count | 17 |
| File Format | HTM / HTML PDF |
| ISSN | 20462069 |
| Volume Number | 6 |
| Issue Number | 69 |
| Journal | RSC Advances |
| DOI | 10.1039/c6ra12788f |
| Language | English |
| Publisher | Royal Society of Chemistry |
| Access Restriction | Open |
| Subject Keyword | Isopropyl alcohol Transfer hydrogenation Bromine Stoichiometry Solid angle Pyridine Ligand Ion Nickel Aryl Ketone Catalytic cycle |
| Content Type | Text |
| Resource Type | Article |
| Subject | Chemistry Chemical Engineering |
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