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 (k_obs, 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.