Ammonia is a low-cost and potentially valuable building block for almost every nitrogen-containing compound required by industry. There is an obvious interest in utilizing this chemical as feedstock in catalytic organic transformations to produce higher-value products, an issue that has began to be explored with some degree of success.[1] However, most of late transition metal catalyzed reactions do not occur with ammonia. Several factors have been invoked to explain this lack of reactivity: [2] 1) The high strength of the NÀH bond of ammonia (107 kcal mol À1 ) makes its activation very difficult to achieve by metal centers; 2) the catalyst often deactivates through the formation of stable Werner ammine (M ! NH 3 ) adducts; and 3) the low acidity of ammonia prevents its participation in proton exchange reactions that could lead to NÀH activation.In order to achieve a metal-mediated functionalization of ammonia, an imperative requisite should be the formation of M À NH 2 bonds directly from ammonia (i.e. rather than leading to stable M ! NH 3 adducts).[3] So far, only few early examples involving interaction of NH 3 with iridium complexes followed an oxidative addition profile.[4] This concept was elegantly illustrated by Hartwig et al., who reported on the formal oxidative addition of ammonia to an electron-rich Ir I pincer system, leading to the first structurally characterized terminal amido hydrido Ir III complex. [5] In spite of the efficacy of this N À H activation, uptake and homolytic breakage of ammonia by late transition metal complexes still remains a difficult goal.[6] We assumed that a good approach to induce the formation of M À NH 2 bonds, circumventing the formation of Werner adducts, should rely on an appropriate design of organometallic precursors. Herein we report on a synthetic protocol that uses gaseous ammonia as "NH 2 " source to generate stable novel parent bridging and terminal amido Rh I and Ir I complexes under very mild conditions. We chose as metallic precursors dinuclear complexes bearing alkoxo-bridging ligands, well suited to induce NÀH activation. [7] In this way, treatment of the methoxo-bridged compounds [{M(m-OMe)(tfbb)} 2 ] (M = Rh, Ir; tfbb = tetrafluorobenzobarrelene) with gaseous ammonia in diethyl ether at atmospheric pressure rapidly afforded the parent amidobridged trinuclear complexes [{M(m 2 -NH 2 )(tfbb)} 3 ] (M = Rh (1), Ir (2)) which were isolated in good yields. On the other hand, reactions of the cod complexes [{M(m-OMe)(cod)} 2 ] (cod = 1,5-cycloctadiene) with gaseous ammonia yielded dinuclear amido-bridged complexes [{M(m-NH 2 )(cod)} 2 ] (M = Rh (3), Ir (4)) in excellent yields (Scheme 1). All the reactions leading to complexes 1-4 were found to be reversible. For example, monitoring by NMR spectroscopy the reaction of 3 with MeOH in a 1:1 ratio in [D 6 ]benzene showed upon 10 min, when the equilibrium was considered to be reached, the presence of unchanged 3, the original methoxo-bridged complex [{Rh(m-OMe)(cod)} 2 ] and the mixed amido-alkoxo species [{Rh(co...

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. ...

Ammonia is a low-cost and potentially valuable building block for almost every nitrogen-containing compound required by industry. There is an obvious interest in utilizing this chemical as feedstock in catalytic organic transformations to produce higher-value products, an issue that has began to be explored with some degree of success. [1] However, most of late transition metal catalyzed reactions do not occur with ammonia. Several factors have been invoked to explain this lack of reactivity: [2] 1) The high strength of the NÀH bond of ammonia (107 kcal mol À1 ) makes its activation very difficult to achieve by metal centers; 2) the catalyst often deactivates through the formation of stable Werner ammine (M ! NH 3 ) adducts; and 3) the low acidity of ammonia prevents its participation in proton exchange reactions that could lead to NÀH activation.In order to achieve a metal-mediated functionalization of ammonia, an imperative requisite should be the formation of M À NH 2 bonds directly from ammonia (i.e. rather than leading to stable M ! NH 3 adducts). [3] So far, only few early examples involving interaction of NH 3 with iridium complexes followed an oxidative addition profile. [4] This concept was elegantly illustrated by Hartwig et al., who reported on the formal oxidative addition of ammonia to an electron-rich Ir I pincer system, leading to the first structurally characterized terminal amido hydrido Ir III complex. [5] In spite of the efficacy of this N À H activation, uptake and homolytic breakage of ammonia by late transition metal complexes still remains a difficult goal. [6] We assumed that a good approach to induce the formation of M À NH 2 bonds, circumventing the formation of Werner adducts, should rely on an appropriate design of organometallic precursors. Herein we report on a synthetic protocol that uses gaseous ammonia as "NH 2 " source to generate stable novel parent bridging and terminal amido Rh I and Ir I complexes under very mild conditions. We chose as metallic precursors dinuclear complexes bearing alkoxo-bridging ligands, well suited to induce NÀH activation. [7] In this way, treatment of the methoxo-bridged compounds [{M(m-OMe)(tfbb)} 2 ] (M = Rh, Ir; tfbb = tetrafluorobenzobarrelene) with gaseous ammonia in diethyl ether at atmospheric pressure rapidly afforded the parent amidobridged trinuclear complexes [{M(m 2 -NH 2 )(tfbb)} 3 ] (M = Rh (1), Ir (2)) which were isolated in good yields. On the other hand, reactions of the cod complexes [{M(m-OMe)(cod)} 2 ] (cod = 1,5-cycloctadiene) with gaseous ammonia yielded dinuclear amido-bridged complexes [{M(m-NH 2 )(cod)} 2 ] (M = Rh (3), Ir (4)) in excellent yields (Scheme 1). All the reactions leading to complexes 1-4 were found to be reversible. For example, monitoring by NMR spectroscopy the reaction of 3 with MeOH in a 1:1 ratio in [D 6 ]benzene showed upon 10 min, when the equilibrium was considered to be reached, the presence of unchanged 3, the original methoxo-bridged complex [{Rh(m-OMe)(cod)} 2 ] and the mixed amido-alkoxo species [{...