Published: August 08, 2011 r2011 American Chemical Society 4720 dx.doi.org/10.1021/om2005589 | Organometallics 2011, 30, 4720–4729 ARTICLE pubs.acs.org/Organometallics Iron Hydride Complexes Bearing Phosphinite-Based Pincer Ligands: Synthesis, Reactivity, and Catalytic Application in Hydrosilylation Reactions Papri Bhattacharya, Jeanette A. Krause, and Hairong Guan* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States b S Supporting Information ’ INTRODUCTION Hydrido complexes of iron are fundamentally important for their crucial roles in a wide variety of catalytic processes. 1 Studies on synthetic iron hydride complexes have shed light on the mechanisms by which nitrogenase and hydrogenase enzymes function. 2,3 In homogeneous catalysis, iron hydride species are often invoked as key intermediates in iron-catalyzed reactions. 4 Recent efforts have been made to elucidate structurereactivity relationship of well-defined iron hydride complexes with the objective to provide mechanistic bases for the rational design of iron catalysis. 5 Meanwhile, several well-characterized iron hydride complexes have been directly employed as the catalysts for a number of reactions. 6 Transition metal complexes with pincer-type ligands, especially those containing precious metals, have shown high reactivity for both stoichiometric bond activations and catalytic transformations. 7 One of the new focuses in this research area has been placed on the chemistry of 3d metals, among which iron is particularly attractive due to its relatively low cost and toxicity. 8 Although various iron compounds bearing either neutral 911 or anionic 12 pincer-type ligands have been synthesized, examples of the corresponding hydrido complexes are scarce in the literature. 10d,f,12a,12i Of the known iron pincer hydride complexes, only two have been reported to be catalytically active. The first of such compounds is Chirik’s ( iPr PNP)FeH 2 (N 2 )( iPr PNP = 2,6-( i Pr 2 PCH 2 ) 2 (C 5 H 3 N)), which can be used to catalyze the hydrogenation of olefins. 10d The second example is Milstein’s( iPr PNP)FeH(CO)Br, which has been shown as a very efficient catalyst for the hydrogenation of ketones. 10f Noticeably, in both cases the iron center is supported by a bis(phosphino)pyridine ligand that is typically synthesized from 2,6-bis(chloromethyl)pyridine and an expensive, pyrophoric sec- ondary phosphine. 13 Taking into account not only the prices of metals but also the costs of ligand synthesis, we have focused our studies on the catalysis of first-row transition metal complexes with phosphinite- based POCOP-pincer ligands as shown in Figure 1. This specific type of pincer ligands is readily accessible via PO bond forming reaction of resorcinol or other diols with relatively inexpensive ClPR 2 . 14 The synthesis of POCOP-pincer complexes of nickel, in particular, is also straightforwardly accomplished via cyclometala- tion of the pincer ligands with simple metal salts such as NiCl 2 . For catalytic applications, we and other groups have demon- strated that these nickel pincer complexes are excellent catalysts for the reduction of carbonyl functionalities, 15 Michael addition of amines and alcohols to acrylonitrile derivatives, 14g,k,m Kharasch addition of CCl 4 to alkenes, 14f,g and CS cross-coupling reac- tions. 16 In stark contrast to nickel chemistry, POCOP-pincer complexes of other first-row transition metals are exceedingly rare, largely due to the inability of common metal salts to activate the CH bonds of the pincer ligands. To date, there are only two established cobalt systems 12i,17 and one iron system; 12i however, Received: June 28, 2011 ABSTRACT: Treatment of resorcinol-derived bis(phosphinite) ligands 1,3-(R 2 PO) 2 C 6 H 4 (R = i Pr and Ph) with Fe(PMe 3 ) 4 furnishes iron POCOP-pincer hydride complexes [2,6- (R 2 PO) 2 C 6 H 3 ]Fe(H)(PMe 3 ) 2 (R = i Pr, 1a; R = Ph, 1b) with two PMe 3 cis to each other. The isopropyl complex 1a undergoes ligand substitution upon mixing with CO to give [2,6-( i Pr 2 PO) 2 C 6 H 3 ]Fe(H)(PMe 3 )(CO). The kinetic pro- duct (2a) of this process contains a CO ligand trans to the hydride, whereas the thermodynamic product (2a 0 ) has a CO ligand cis to the hydride. The displacement of PMe 3 in 2a by CO takes place at an elevated temperature, resulting in the formation of [2,6-( i Pr 2 PO) 2 C 6 H 3 ]Fe(H)(CO) 2 (3a). These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups, and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl substrates after ligand (PMe 3 , or CO in the case of 3a) dissociation from the iron center.