Structural, kinetic, and DFT studies of the transfer hydrogenation of ketones mediated by (pyrazole)pyridine iron(ii) and nickel(ii) complexes12. Issue 69 (11th July 2016)

• Record Type: Journal Article
• Authors: Magubane, Makhosazane N.; Nyamato, George S.; Ojwach, Stephen O.; Munro, Orde Q.
• Is Part Of: RSC advances. Volume 6:Issue 69(2016)
• Page Start: 65205
• Page End: 65221
• Publication Date: 2016-07-11
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c6ra12788f
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : DFT simulations effectively delineate structural and electronic factors underpinning the reactivity of Fe(ii ) and Ni(ii ) complexes in ketone transfer hydrogenations. 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 complexes4 and6 afforded stable six-coordinate cationic species4a and6a 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 of7 ·4H2 O and8 ·2H2 O, in contrast, afforded tri- and bis(aqua) salts ofL3 chelated to Ni(ii ) in a 1 : 1 L : M stoichiometry, respectively. Complexes1–9 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 ) chelate5 affording the most active catalyst ( k obs, 4.3 × 10 −5 s −1 ) when the inductive phase lag was appropriately modelled in the kinetics. Iron(ii ) complex3 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 of1–9 . Catalysts derived from1 and9 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.