Cleavage of Nitrogen–Hydrogen Bonds of Ammonia Induced by Triruthenium Polyhydrido Clusters

Nakajima, Yumiko; Kameo, Hajime; Suzuki, Hiroharu

doi:10.1002/ange.200503222

Cited by 35 publications

(9 citation statements)

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“…[6,7] The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds.…”

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confidence: 99%

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

et al. 2010

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For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. [1,2] For example, stable singlet carbenes can activate CO, [3a] H 2 , [3b] and P 4 .[3c-e] Such reactions have long been known for transition metals. [4,5] However, stable singlet carbenes can also activate NH 3 ;[3b] a much more difficult task for transition metals. [6,7] The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds.[8]Herein, we report the first examples of the activation of E À H bonds (E = Si, B, P) at a single nonmetal center.On the basis of our successful results with H 2 , [3b] we began our study with the activation of SiÀH bonds. Indeed, silanes are similar to H 2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si À H s complexes, which undergo subsequent oxidative addition.[4] To test the possible activation of Si À H bonds with carbenes, we treated the cyclic (alkyl)-(amino)carbenes (CAACs) 1 a and 1 b [9] with primary, secondary, and tertiary silanes.The addition of phenylsilane to 1 a and 1 b occurred readily at room temperature, and the corresponding adducts 2 a,b were isolated in 91 and 83 % yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1 a, two diastereomers 2 a,a' were formed (in a 2:1 ratio), as shown by two singlets at d = À36.4 and À29.3 ppm in the 29 Si NMR spectrum. The 13 C NMR spectrum revealed the loss of the carbene signal and a new C À H peak at d = 63.2 (2 a) and 65.5 ppm (2 b). The 1 H NMR spectrum of the major isomer 2 a revealed a pseudotriplet at d = 4.78 ppm (SiCH) and two doublets at d = 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH 2 fragment. The structure of 2 a was confirmed by X-ray crystallography [10] (Figure 1, top), whereas the presence of a triplet at d = 4.53 ppm and a doublet at d = 4.08 ppm in the 1 H NMR spectrum confirmed the identity of adduct 2 b.CAACs 1 a,b also reacted with (EtO) 3 SiH to afford 3 a (d.r. 3:1) and 3 b in 64 and 73 % yield, respectively. However, when Ph 2 SiH 2 was used, only the less bulky carbene 1 b underwent insertion into the Si À H bond (to give 4 b in 65 % yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H 2 , [11] we found that imidazolidin-2-ylidene 5[12] also reacted at room temperature with phenylsilane to afford the Si À H insertion product 6 in 88 % yield (Figure 1, bottom).

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Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

et al. 2010

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“…[12] There are synthetic routes towards these compounds mainly based on salt metathesis reactions, [13][14][15] however they need strong bases and do not use ammonia directly for their formation. Only few examples of these elusive species have been recently reported to be prepared from ammonia without change of the redox state of the metals; they are based on deprotonative NÀH cleavage by a m 3 -oxo triruthenium cluster [16] and by a diaryl Fe complex. [17] Furthermore, it has been shown how ammonia is activated by bifunctional iridium [18] or ruthenium [19] complexes through metal-ligand cooperation.…”

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

et al. 2011

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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...

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“…[11] Oxido ligands in polyhydrido ruthenium cluster compounds can induce a comparable activation reaction. [12] A reversible activation of ammonia was observed by Milstein and co-workers on using a ruthenium complex with a pincer ligand, which acts as a cooperative ligand. [13,14] Herein, we report on the reactivity of the iridium complex [Cp*Ir(H) 4 ] (1; Cp*= 1,2,3,4,5-pentamethylcyclopentadienyl) towards NH 3 and ND 3 .…”

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confidence: 94%

Activation and Coordination of Ammonia at [Cp*Ir(H)₂]: NMR and Matrix Isolation Studies

Jungton

Herwig

Braun

et al. 2012

Chemistry A European J

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(1)H NMR exchange spectroscopy of a reaction mixture of [Cp*Ir(H)(4)] (1; Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) and ammonia suggests an exchange of hydrogen atoms between the hydrido ligands and ammonia. Treatment of 1 with ND(3) led to an H/D exchange between ND(3) and the hydrido ligands of 1. Subsequent studies showed that photolysis of 1 isolated in frozen argon matrices leads to the formation of the iridium compounds [Cp*Ir(H)(2)] (2) and [Cp*Ir(H)(3)] (4), as it was confirmed by IR spectroscopy. In the presence of water the aqua complex [Cp*Ir(H)(2)(OH(2))] (3) was generated simultaneously. Accordingly, photolysis of 1 in an argon matrix doped with ammonia gave rise to the ammine complex [Cp*Ir(H)(2)(NH(3))] (5). IR assignments were supported by calculations of the gas-phase IR spectra of 1-5 by DFT methods.

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Cleavage of Nitrogen–Hydrogen Bonds of Ammonia Induced by Triruthenium Polyhydrido Clusters

Cited by 35 publications

References 30 publications

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

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

Activation and Coordination of Ammonia at [Cp*Ir(H)₂]: NMR and Matrix Isolation Studies

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Cleavage of Nitrogen–Hydrogen Bonds of Ammonia Induced by Triruthenium Polyhydrido Clusters

Cited by 35 publications

References 30 publications

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

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

Activation and Coordination of Ammonia at [Cp*Ir(H)2]: NMR and Matrix Isolation Studies

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Activation and Coordination of Ammonia at [Cp*Ir(H)₂]: NMR and Matrix Isolation Studies