The Dehydrogenation of Alcohols through a Concerted Bimetallic Mechanism Involving an Amido‐Bridged Diiridium Complex

Abstract

Interest in catalytic functionalization of ammonia into higher-value nitrogen-containing products constitutes today a hot topic. [1] However, productive participation of ammonia as substrate in homogeneous catalysis is normally hampered by the intrinsic high strength of the N-H bond, very difficult to activate by metal centres. [2] Therefore, it is convenient to find alternative ways to achieve the formation of [M-NH 2 ] species directly from ammonia as a premise to accomplish its subsequent functionalization. [3] Since oxidative addition of ammonia to a single iridium centre was unambiguously demonstrated by Hartwig et al., [4] further research has led to the recognition that NH 3 is able to participate in metalmediated N-H bond cleavage processes that ultimately may lead to the formation of parent amido and imido late metal species. [5] Sound advances in this field have been recently reported, in which the clever design of metallic precursors often dictates the success of these strategies. In this way, there have been disclosed efficient synthetic pathways towards these compounds that involve both homolytic [6] and heterolytic [7,8] activation of ammonia, a body of research that is begining to unfold the chemistry around otherwise unusual late [M-NH 2 ] complexes. Within this context, it is even more intriguing and challenging to achieve multiple metal-mediated ammonia activation. This phenomenon could lead to the formation of very rare late metal imido complexes, valuable intermediates in useful catalytic transformations such as nitrogen transfer reactions [9] and indeed very important in the context of dinitrogen fixation and reduction. [10] However, the scarceness of methods to generate [M-NH 2 ] complexes (preferably from ammonia) has prevented the possibility of exploring the reactivity and catalytic performance of low valent late metal parent amido complexes. It is interesting to mention in this context the key role of amido intermediates on Noyori type catalytic reactions. [11] Herein we report on the alcohol-induced transformation of a dinuclear amido-bridged iridium complex to give higher nuclearity Ir 3 and Ir 4 imido clusters, whose formation is a direct consequence of alcohol dehydrogenation.We recently reported on the heterolytic activation of ammonia mediated by the methoxo-bridged compound [{Ir(cod)( -OMe)} 2 ] (cod = 1,5-cyclooctadiene) to afford the amido dinuclear complex [{Ir(cod)( -NH 2 )} 2 ] (1). [8] As a part of our on-going research about the topic of metal-mediated ammonia activation, we found out that when bubbling a solution of [{Ir(cod)( -OMe)} 2 ] in THF with gaseous NH 3 and then allowing it to stand under a layer of hexanes for several days, the expected complex 1 was obtained in high yields, mixed however with a crop of red crystals, further identified by Xray methods as cluster [Ir 4 (cod) 4 ( 4 -HNCH 2 NH)( 4 -NH)] (2), along with variable amounts of orange crystals characterized as complex [Ir 3 (cod) 3 ( 3 -NH) 2 ( 2 -H)] (3). Figure 1. Molecular diagram of complex 2. ...

Terminal and Bridging Parent Amido 1,5‐Cyclooctadiene Complexes of Rhodium and Iridium

“…Catalytic activity of the cyclooctadiene complexes 1 and 2 in the transfer hydrogenation of acetophenone : In a preliminary communication we reported that complex 2 , at room temperature and in absence of any base, was active as homogeneous catalysts for the transfer hydrogenation of acetophenone to 1‐phenylethanol by using i PrOH as hydrogen donor. Under such mild conditions a bimetallic mechanism that allows a concerted net hydrogen transfer through a proposed eight‐membered dimetallacycle was proposed 25. However, the catalytic activity was relatively low and, in the absence of acetophenone, or when its concentration was substantially decreased, the formation of a stable imido cluster of the formula [Ir 3 (cod) 3 (μ 2 ‐H)(μ 3 ‐NH) 2 ] was observed.…”

“…Under such mild conditions a bimetallic mechanism that allows a concerted net hydrogen transfer through a proposed eight-membered dimetallacycle was proposed. [25] However, the catalytic activity was relatively low and, in the absence of acetophenone, or when its concentration was substantially decreased, the formation of a stable imido cluster of the formula [Ir 3 A C H T U N G T R E N N U N G (cod) 3 A C H T U N G T R E N N U N G (m 2 -H)A C H T U N G T R E N N U N G (m 3 -NH) 2 ] was observed.…”

“…The situation is quite different in bridging parent amido complexes, in which the lone electronic pair is usually tightly bounded in a dative fashion to a second metal 13. 16a, b, f, 18, 24 In this context, a significant contribution has been recently reported dealing with the role of [Ir‐NH 2 ‐Ir] linkages in catalytic transfer hydrogenation, enlightening a novel binuclear, outer‐sphere Noyori‐like mechanism, in which the NH 2 groups behave as non‐innocent ligands playing a key role in alcohol dehydrogenation and imido formation 25…”

“…[13, 16a, b, f, 18, 24] In this context, a significant contribution has been recently reported dealing with the role of [Ir-NH 2 -Ir] linkages in catalytic transfer hydrogenation, enlightening a novel binuclear, outer-sphere Noyori-like mechanism, in which the NH 2 groups behave as non-innocent ligands playing a key role in alcohol dehydrogenation and imido formation. [25]…”

Iridium complexes as catalysts in the hydrogen transfer of isopropanol to acetophenone: Ligand effects and DFT studies

“…In this context, it has been reported recently a special example of the metal-ligand bifunctional mechanism involved with a dinuclear amido bridged iridium(I) complex )(cod)} 2 ], in which a concerted cooperation of the metals and the amido bridges is essential for the hydrogen transfer of isopropanol to acetophenone to occur. [12] …”