Direct Access to Parent Amido Complexes of Rhodium and Iridium through NH Activation of Ammonia

Abstract

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 [{...

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

“…The structure of complex 1 has been recently reported. [20] In the present study, X-ray diffraction analysis of complex 2 confirms that both complexes exhibit similar structural features. The main difference between them concerns their asymmetric unit contents: two and three crystallographically independent but chemically identical molecules for complex 1 and 2, respectively.…”

“…In a preliminary communication we reported on the facile and high-yield access to Rh I and Ir I parent amido complexes directly with gaseous ammonia. [20] In this way, bubbling NH 3 to ethereal solutions of the methoxo-bridged complexes Rh,Ir;cod = 1, at atmospheric pressure and low temperatures (À5 8C) afforded amido-bridged complexes of formulae (1), Ir (2)) in excellent yields (Scheme 1). Even though these reactions are reversible, the poor solubil-ity of complexes 1 and 2 in diethyl ether allowed their easy isolation.…”

“…Having in mind the success on isolating the first terminal parent amido complexes of rhodium by utilizing the electron-withdrawing di-olefin tetrafluorobenzobarrelene and IPr as a voluminous and strong s-donor N-heterocyclic carbene ligand, [20] we explored in the same way the splitting of the amido bridges in complexes 1 and 2 with IPr. When 1 was treated with two molar equivalents of IPr, a yellow microcrystalline solid was isolated in excellent yield.…”

“…Complexes 1 and 2 were prepared by following the synthetic procedure recently described. [20] Gaseous ammonia was purchased from Air Liquide LTD. All the other chemicals used in this work have been purchased from Aldrich Chemicals and were used as received. Carbon, hydrogen, and nitrogen analyses were performed with a Perkin-Elmer 2400 CHNS/ O microanalyzer.…”

“…Accordingly to this statement, we have recently disclosed a novel strategy that allows the high-yield access to parent-amido-bridged complexes of Rh I and Ir I through heterolytic splitting of gaseous ammonia under very mild conditions. [20] The ready availability of these unusual species through this protocol has allowed us to assess their reactivity towards a variety of substrates. So far, the reactivity of terminal parent amido complexes has been focused in proton exchange processes with weak acids, [11a, 12b, 21] insertions of carbon monoxide into the N À H bond, [10b] and reductive elimination of ammonia, [11b] a work that evidences a high basicity at the terminal amido nitrogen, which bears an exposed and reactive free electron pair.…”