Issue 69, 2016

Structural, kinetic, and DFT studies of the transfer hydrogenation of ketones mediated by (pyrazole)pyridine iron(ii) and nickel(ii) complexes

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.

Graphical abstract: Structural, kinetic, and DFT studies of the transfer hydrogenation of ketones mediated by (pyrazole)pyridine iron(ii) and nickel(ii) complexes

Supplementary files

Article information

Article type
Paper
Submitted
17 May 2016
Accepted
27 Jun 2016
First published
29 Jun 2016

RSC Adv., 2016,6, 65205-65221

Author version available

Structural, kinetic, and DFT studies of the transfer hydrogenation of ketones mediated by (pyrazole)pyridine iron(II) and nickel(II) complexes

M. N. Magubane, G. S. Nyamato, S. O. Ojwach and O. Q. Munro, RSC Adv., 2016, 6, 65205 DOI: 10.1039/C6RA12788F

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