Effect of the structure of the diamine backbone of P-N-N-P ligands in iron(II) complexes on catalytic activity in the transfer hydrogenation of acetophenone.
ABSTRACT The asymmetric transfer hydrogenation of aromatic ketones can be efficiently accomplished using catalysts that are based on platinum group metals which are more toxic and less abundant than iron. For that reason the discovery of iron based catalysts for the use in this transformation is important. To address this issue, we synthesized a new series of iron(II)-based precatalysts trans-[Fe(Br)(CO)(PPh(2)CH(2)CH═NCHRCHRN═CHCH(2)PPh(2))]BPh(4) (5a-5d) containing P-N-N-P ligands with the diamines (R,R)-1,2-diaminocyclohexane (a), (R,R)-1,2-diphenyl-1,2-diaminoethane (b), (R,R)-1,2-di(4-methoxyphenyl)-1,2-diaminoethane (c), and ethylenediamine (d) incorporated in the backbone using a convenient one-pot synthesis using readily available starting materials. All of the complexes, when activated with a base, show a very high activity in the transfer hydrogenation catalysis of acetophenone, using 2-propanol as a reducing agent under mild conditions. A comparison of the TOF of complexes 5a-5d show that the catalytic activity of complexes increase as the size of the substituents in the backbone of ligands increases (d < a < b = c).
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ABSTRACT: Transition metal complexes bearing pincer ligands have been extensively studied during the past decade. One of the less-explored or perhaps even neglected areas of pincer chemistry is on the reactivity of iron complexes that incorporate these enormously popular ligand sets. Meanwhile, recent emphasis on homogeneous catalysis has been placed on the developments of catalytic reactions promoted by inexpensive and environmentally benign metals, for which iron is particularly attractive. In that regard, investigating the reactivity patterns of well-defined iron pincer complexes can potentially provide valuable guidelines for the rational design of iron catalysis. This review analyzes the challenges and successes of synthesizing iron complexes with different pincer-type ligands, and discusses the established or potential utility of these complexes for catalytic reactions.Comments on Inorganic Chemistry 03/2011; 32(2):88-112. · 1.64 Impact Factor
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ABSTRACT: With respect to its availability, low toxicity, and price iron should be one of the most used metals in homogeneous catalysis. Surprisingly, so far the application of iron is underdeveloped in comparison to other transition metals. Herein, we highlight promising attempts obtained in hydrogenation, transfer hydrogenation and hydrosilylation, which nicely illustrate the potential of iron and hopefully initialize a ferric future for catalysis.Chemical Communications 03/2011; 47(17):4849-59. · 6.38 Impact Factor
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ABSTRACT: The asymmetric reduction of ketones and imines by transfer of hydrogen from isopropanol as the solvent catalyzed by metal complexes is a very useful method for preparing valuable enantioenriched alcohols and amines. Described here is the development of three generations of progressively more active iron catalysts for this transformation. Key features of this process of discovery involved the realization that one carbonyl ligand was needed (as in hydrogenases), the synthesis of modular ligands templated by iron, the elucidation of the mechanisms of catalyst activation and action, as well as the rational synthesis of precursors that lead directly and easily to the species in the catalytic cycle. The discovery that iron, an abundant element that is essential to life, can form catalysts of these hydrogenation reactions is a contribution to green chemistry.Dalton Transactions 04/2014; · 3.81 Impact Factor