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 [{Rh(co...

Mechanistic Studies on the Catalytic Oxidative Amination of Alkenes by Rhodium(I) Complexes with Hemilabile Phosphines

“…Unfortunately, we were unable to observe the amido ligand by 15 N NMR, however, it has been possible to identify the protons and carbons of the amido ligand, all of them inequivalent, through 1 H, 1 H-COSY and 1 H, 13 C-HSQC correlation experiments (Supporting information). In sharp contrast, when a solution of complex [Rh{Ph 2 P(CH 2 ) 3 OEt} 2 (HNC 5 H 10 )] + (7) in THF-d 8 was heated for 20 h the formation of 9 was never observed. This result supports the active role of piperidine in the formation of this key rhodium-amido species from 8.…”

“…However, the 31 P{ 1 H} NMR spectrum at 253 K consists of two doublet of doublets at δ 50.24 (J P-Rh = 215.0) and 30.23 ppm (J P-Rh = 161.0) with a J P-P coupling constant of 54.4 Hz (Figure 2), in full agreement with the proposed structure. The reaction of 5 with an excess of piperidine (1:40) in THF-d 8 As can be seen in Figure 3, the equilibrium is shifted to 8 at temperatures down 178 K. Increasing the temperature results in the formation of 7 as a consequence of the equilibrium displacement. The calculated equilibrium constant at 273K is K = 2.62.…”

“…The addition of one equiv. of piperidine to a solution of 5 in THF-d 8 at room temperature gave the compound [Rh{Ph 2 P(CH 2 ) 3 OEt} 2 (HNC 5 H 10 )] + (7) with κ 1 -P and κ 2 -P,O coordinated ligands (Scheme 1). Compound 7 is fluxional as it was evidenced in the 31 P{ 1 H} NMR spectrum at 298 K that showed two broad resonances at δ ≈ 51 and 33 ppm.…”

“…which could be in part, responsible for the enhanced stability of 8 over 7. When a mixture of the piperidine compounds 7 plus 8, obtained by reaction of 5 with a slight excess of piperidine (1:3) in THF-d 8 , was heated at 343 K for 2 h, the clean formation of the new specie 9 was observed. The 31 P{ 1 H} NMR at room temperature, consists in two doublet of doublets at δ 39.60 and 36.31 ppm with J P-Rh coupling constants of 165.0 and 173.1 Hz, and a J P-P of 51.3 Hz (Figure 4).…”

“…Although the N-H bond activation has been postulated as the first step in the mechanism, there are only a few examples of direct formation of amido complexes from amines, most of them in iridium complexes. 8,9 Migratory insertion of alkenes into the M-NR 2 bond is a favourable process in late transition metal complexes. In fact, theoretical calculations for the migratory insertion of the alkene in complexes [(PMe 3 ) 2 Rh(η 2 -alkene)(NH 2 )] evidenced low energy barriers, even lower than the found for the migratory insertion into a Rh-CH 3 bond.…”