Dehydrogenation of Alcohols Mediated by an Amido-Bridged Diiridium Complex: A Case of a Concerted Bimetallic Mechanism

Interest in catalytic functionalization of ammonia into higher-value nitrogen-containing products constitutes today a hot topic. 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. Therefore, it is convenient to find alternative ways to achieve the formation of [M–NH2] species directly from ammonia as a premise to accomplish its subsequent functionalization. Since oxidative addition of ammonia to a single iridium centre was unambiguously demonstrated by Hartwig et al., further research has led to the recognition that NH3 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. 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 and heterolytic activation of ammonia, a body of research that is begining to unfold the chemistry around otherwise unusual late [M–NH2] 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 and indeed very important in the context of dinitrogen fixation and reduction. However, the scarceness of methods to generate [M–NH2] 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. Herein we report on the alcohol-induced transformation of a dinuclear amido-bridged iridium complex to give higher nuclearity Ir3 and Ir4 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)( -NH2)}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 NH3 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 [Ir4(cod)4( 4-HNCH2NH)( 4-NH)] (2), along with variable amounts of orange crystals characterized as complex [Ir3(cod)3( 3-NH)2( 2-H)] (3).