Novel Metallomacrocyclic Carboxamide Mn(II) Complexes as Efficient Catalysts for the Transfer Hydrogenation of Ketones

Herein, we present the first metallomacrocyclic Mn(II) complexes as efficient catalysts for the transfer hydrogenation of ketones. The dinuclear Mn(II) complexes [Mn2(L1-L4)2Cl2] (Mn1-Mn4), were synthesised in good yields by reacting MnCl2 & sdot; 4H2O with the ligands N,N '-(1,4-phenylene)dipicolinamide (L1), N,N '-(1,2-phenylene)dipicolinamide (L2), N,N '-(4-methoxy-1,2-phenylene)dipicolinamide (L3) and N,N '-(4,5-dimethyl-1,2-phenylene)dipicolinamide (L4). Structural characterization of the resulting Mn(II) complexes was achieved using FT-IR spectroscopy, mass spectrometry, magnetic moment measurements, elemental analysis, and single-crystal X-ray diffraction for Mn2. The solid-state structure of complex Mn2 reveals a dinuclear Mn(II) core, in which the metal coordination sphere consists of two bidentate, K2(N-O) bridging L2 units and two chlorido ligands to give a distorted octahedral geometry. The Mn(II) complexes (Mn1-Mn4) were quite active catalysts for the transfer hydrogenation of ketones displaying turnover numbers (TON) of up to 2633, which is comparable to those of well-established Mn(I) catalysts. The new Mn(II) complexes also efficiently hydrogenated a number of aromatic, heterocyclic, functionalized and aliphatic ketones. More significantly, cheaper, and readily available hydrogen sources like ethanol also gave decent catalytic activities in the transfer hydrogenation reactions. Dinuclear Mn(II) complexes based on carboxamide ligands form efficient catalysts in the transfer hydrogenation of a wide scope of ketone substrates at very low catalyst concentrations. The current Mn(II) catalysts are relatively stable and affordable, thus offer superior alternatives to the more established Mn(I) carbonyl complexes in the transfer hydrogenation reactions.image