Catalysts in Petroleum Refining & Petrochemicals

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1 Iridium homogeneous catalysts following outer-sphere mechanisms Luis A. Oro Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea, Universidad de Zaragoza-CSIC, Zaragoza, España / Centre of Research Excellence in Petroleum and Petrochemicals, King Fahd University of Petroleum & Minerals Visiting Chair Professor, Dhahran, 31261, Saudi Arabia. oro@unizar.es Abstract The reported mechanisms of homogeneous catalytic reactions by iridium complexes are usually of inner sphere type, implying substrate coordination to the metallic centre. However, some complexes catalyse unsaturated substrates reductions by an outer sphere bifunctional Noyori-type mechanism, or by an ionic type mechanism. Recently, we have discovered new evidences of outer sphere mechanisms in catalytic reactions mediated by iridium complexes containing nitrogen donor ligands. Thus, diiridium complexes containing pyrazolato bridging ligands are active catalysts for imine hydrogenation following a ionic mechanism, while an amido-bridged diiridium complex shows an unprecedented outer sphere bimetallic mechanism, allowing a concerted net hydrogen transfer through a proposed eight-membered dimetallacycle. Interestingly, a new air and moisture stable iridium complex with the new tridentate bis-(pyridine-2-yloxy)methylsilyl ligand is very effective for carbon dioxide catalytic hydrosilylation. The reaction is highly selective to silyl formate and proceeds efficiently under mild conditions, following an outer sphere mechanism. The usual mechanisms found in homogeneous catalysis by transition metal complexes are of inner sphere type, implying substrate coordination to the unsaturated metallic centre [1,2]. However, a groundbreaking in the field of homogeneous catalysis has been the discovery of asymmetric reduction of ketones and imines using mainly ruthenium complexes reported by Noyori and co-workers in a series of communications and reviews through A novel mechanism was proposed introducing novel concepts in homogeneous catalysis such as outer sphere mechanism and metal-ligand bifunctional catalysis. The proposed nonclassical metal-ligand bifunctional mechanism provided a new insight for hydrogenative reduction of C-X compounds (X = O, N) catalyzed by transition metal complexes possessing NH ligands, or more generally protic neutral ligands

2 LH (L = heteroatom) [3]. The unsaturated bonds can be hydrogenated without ligation to the metallic centre, where hydride and proton are simultaneously delivered from MH and NH, respectively, without a metal-substrate interaction, via a six-membered pericyclic mechanism (Figure 1). Figure 1. Metal-ligand bifunctional mechanism of hydrogenation of ketones An alternative outher sphere mechanisms for delivery of hydrogen is through ionic hydrogenation, in which a proton and a hydride are sequentially transferred to a ketone or other substrate [4]. The main difference between the two outer sphere mechanisms is that the proposed by Noyori is a concerted mechanism in contrasts with the ionic hydrogenation system, where sequential proton transfer and hydride transfer steps are observed (Figura 2). Figure 2. Ionic mechanism of hydrogenation of ketones Recently, we have discovered new evidences of outer sphere mechanisms in other types of iridium catalyzed reactions. Thus treatment of the diiridium compound [Ir 2 (µ-pz) 2 (µ-h)h 3 (NCMe)(PiPr 3 ) 2 ] (Pz = pyrazolato) with one equivalent of HBF 4 or [PhNH=CHPh]BF 4 affords efficient catalysts for the homogeneous hydrogenation of N-benzylideneaniline. The hydrogenation reaction proceeds through an ionic mechanism comprising the rate-determining substitution of the amine ligand by

3 dihydrogen, followed by fast elementary steps of proton and hydride transfer to the imine substrate [5]. This catalytic imine hydrogenation involves the participation of a single metal centre of the dinuclear compound (Figure 3). Figure 3. Catalytic imine hydrogenation cycle by [Ir 2 (µ-pz) 2 (µ-h)h 3 (NCMe)(PiPr 3 ) 2 ]. On the other hand, studying hydrogen transfer reactions from isopropanol to acetophenone catalyzed by the diiridium complex, [Ir 2 (µ-nh 2 ) 2 (COD) 2 ], (COD = 1,5-cyclooctadiene), we have recently found an unprecedented bimetallic mechanism that allows a concerted net hydrogen transfer through a proposed eight-membered dimetallacycle (Figure 4) [6]. Figure 4. Proposed eight-membered dimetallacycle transition structure for acetophenone reduction.

4 Interestingly, an air and moisture stable iridium complex with the new facial disposed tridentate bis-(pyridine-2-yloxy)methylsilyl ligand is very effective for carbon dioxide catalytic hydrosilylation [7]. The catalyst precursor was prepared as shown in Figure 5. Figure 5. Synthesis of the catalyst precursor. The reaction is very selective and proceeds effectively under mild conditions, being the first example of a solvent free synthesis and isolation, in a gram scale, of a silyl formate by iridium-catalysed reduction of CO 2 with an hydrosiloxane (Figure 6). Figure 6. Catalytic reduction of CO 2 with hydrosiloxane.

5 Theoretical calculations suggests that the CO 2 hydrosilylation reaction follows an outer sphere mechanism, where both hydride transfer from the metal and silyl transfer from the triflate to the O=C=O occurs in a concerted way. Figure 7 shows the calculated energetic profile for the catalytic hydrosilylation of carbon dioxide. Figure 7. Energetic profile (kcalmol -1 ) calculated at DFT level for the catalytic hydrosilylation of carbon dioxide. The coordination sites of the NSiN ligand are represented by L and Si. Thus, the CO 2 hydrosilylation catlytic reaction takes place in three steps: a) substitution of acetonitrile ligand by η 2 -(Si-H) coordination of the silane, b) silyl transfer from the Ir-η 2 -(Si-H) moiety to the triflate ligand and concomitant Ir-hydride bond formation and c) transfer of silyl and hydride ligands to the CO 2 in a concerted way, through an eight member cycle characterised by the transition structure TS-2. The last step implies a outer sphere double group transfer from the H-Ir-triflate-SiMe 3 moiety to the carbon dioxide substrate resulting in silyl formate releases [7]. This key step involves both nucleophilic attack of the hydride ligand to carbon atom and electrophilic attack of the silyl moiety to the oxygen atom of the O=C=O. Future challenges on iridium catalysed functionalization of carbon dioxide

6 include the development of efficient electrophiles compatible with the inclusion of appropriate nucleophiles [8]. In conclusion there is an increasing number of evidences of outer sphere reactions, catalyzed by iridium complexes, and theoretical calculations are very helpful to deeply understand the proposed mechanisms. Acknowledgments The support of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum & Minerals is highly appreciated. The author thankfully acknowledges the efforts of a group of talented collaborators cited in the bibliographic references. References 1. P. W. N. M. VAN LEEUWEN, Homogeneous Catalysis, Kluwer (Dordrecht) R. H. CRABTREE, Acc. Chem. Res., 12, , R. NOYORI, M. YAMAKAWA, S. HASHIGUCHI, J. Org. Chem., 66, , 2001; T. IKARIYA, K. MURATA, R. NOYORI, Org. Biomol. Chem., 4, , R. M. BULLOCK, Chem. Eur. J., 10, , L. A. ORO, Iridium Complexes in Organic Synthesis (Eds. L.A. Oro, C. Claver), Wiley-VCH (Weinheim), 15-38, 2009; 6. I. MENA, M. A. CASADO, V. POLO, P. GARCÍA-ORDUÑA, F. J. LAHOZ, L. A. ORO, Angew. Chem. Int. Ed., 51, , R. LAREMPUIA, M. IGLESIAS, V. POLO, P. J. SANZ MIGUEL, F. J. FERNÁNDEZ-ALVAREZ, J. J. PÉREZ-TORRENTE, L. A. ORO, Angew. Chem. Int. Ed., 51, , F. J. FERNÁNDEZ-ALVAREZ, M. IGLESIAS, L. A. ORO, V. POLO, ChemCatChem., in press (DOI: /cctc ).