Direct access to parent amido complexes of rhodium and iridium through N-H activation of ammonia.

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 taking advantage of this chemical as feedstock in catalytic organic transformations to produce higher value products, issue that has began to be explored with some degree of success. However, most of late transition metal catalyzed reactions do not occur with ammonia. Several factors have been invoked to explain this lack of reactivity: (i) the high strength of the N-H bond of ammonia (107 kcal mol) makes its activation very hard to be achieved by metal centres, (ii) the catalyst often deactivates through the formation of stable Werner ammine (M NH3) adducts, and (iii) the low acidity of ammonia prevents its participation in proton exchange reactions that could lead to N-H activation. In order to achieve a metal-mediated functionalization of ammonia, an imperative requisite should be the formation of M-NH2 bonds directly from ammonia (i.e. rather than leading to stable M NH3 adducts). [3] So far, only few early examples involving interaction of NH3 with iridium complexes followed an oxidative addition profile. This concept was elegantly illustrated by Hartwig et al., who reported on the formal oxidative addition of ammonia to an electron rich Ir pincer system, leading to the first structurally characterized terminal amido hydrido Ir complex. In spite of the beauty and simplicity of this N-H activation, uptake and homolytic breakage of ammonia by late transition metal complexes still remains a difficult goal. We believed that a good approach to induce the formation of M-NH2 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 “NH2” source to generate stable novel parent bridging and terminal amido Rh and Ir complexes under very mild conditions. We chose as metallic precursors dinuclear complexes bearing alkoxo-bridging ligands, well suited to induce N-H activation. In this way, treatment of the methoxo-bridged compounds [{M(μOMe)(tfbb)}2] (M = Rh, Ir; tfbb = tetrafluorobenzobarrelene) with gaseous ammonia in diethyl ether at atmospheric pressure, rapidly afforded the parent amido-bridged trinuclear complexes [{M(μ2NH2)(tfbb)}3] (M = Rh (1), Ir (2)), isolated in good yields. On the other hand, reactions of the cod complexes [{M(μ-OMe)(cod)}2] (cod = 1,5-cycloctadiene) with gaseous ammonia yielded dinuclear amido-bridged complexes [{M(μ-NH2)(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 [D6]benzene showed upon 10 minutes, when the equilibrium was considered to be reached, the presence of unchanged 3, the original methoxo-bridged complex [{Rh(μ-OMe)(cod)}2] and the mixed amido-alkoxo species [{Rh(cod)}2(μ-OMe)(μ-NH2)] in a 0.4/0.3/1 ratio respectively, along with released ammonia (see Supporting Information). Excess of substrates (i.e. NH3 or MeOH) are required to drive the reactions to completion either way, as observed experimentally.