Long-range C-H bond activation by Rh(III)-carboxylates (original) (raw)
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Rhodium-mediated activation of an alkane-type C–H bond
Chemical Communications, 2010
Abnormal C4-bonding of N-heterocyclic carbenes effectively 5 modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp 3 )-H bond, whereas no such intramolecular C-H bond breaking is observed when the carbene binds normally through the C2 carbon.
Journal of the American Chemical Society, 2013
Tp′Rh(PMe 3)(CH 3)H was synthesized as a precursor to produce the coordinatively unsaturated fragment [Tp′Rh(PMe 3)], which reacts with benzene, mesitylene, 3,3dimethyl-1-butene, 2-methoxy-2-methylpropane, 2-butyne, acetone, pentane, cyclopentane, trifluoroethane, fluoromethane, dimethyl ether, and difluoromethane at ambient temperature to give only one product in almost quantitative yield in each case. All of the complexes Tp′Rh(PMe 3)(R)H were characterized by NMR spectroscopy, and their halogenated derivatives were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The active species [Tp′Rh(PMe 3)] was also able to activate the alkynyl C−H bond of terminal alkynes to give activation products of the type Tp′Rh(PMe 3)(CCR)H (R = t-Bu, SiMe 3 , hexyl, CF 3 , Ph, p-MeOC 6 H 4 , and p-CF 3 C 6 H 4). The measured relative rhodium−carbon bond strengths display two linear correlations with the corresponding carbon−hydrogen bond strengths, giving a slope of 1.54 for α-unsubstituted hydrocarbons and a slope of 1.71 for substrates with α-substitution. Similar trends of energy correlations were established by DFT calculated metal−carbon bond strengths for the same groups of substrates.
AsiaChem Magazine
Transition metal catalyzed remote C-H activation offers a powerful tool to modify architecture of organic molecules. Remote or distal C-H activation of aromatic and aliphatic compounds presents more challenge than proximal sites due to the inaccessibility of these sites in formation of energetically favorable organometallic pre-transition states. Nonetheless, recent years have witnessed remarkable progress in reaching out and functionalizing distal C-H bonds. With the advancement of this field, synthetic modifications of organic molecules have become more step and atom economical, and have shown great promise to revolutionize synthetic organic chemistry.
Understanding C–H Bond Activation on a Diruthenium(I) Platform
Organometallics, 2013
Activation of the C−H bond at the axial site of a [Ru I −Ru I ] platform has been achieved. Room-temperature treatment of 2-(R-phenyl)-1,8-naphthyridine (R = H, F, OMe) with [Ru 2 (CO) 4 (CH 3 CN) 6 ][BF 4 ] 2 in CH 2 Cl 2 affords the corresponding diruthenium(I) complexes, which carry two ligands, one of which is orthometalated and the second ligand engages an axial site via a Ru•••C−H interaction. Reaction with 2-(2-Nmethylpyrrolyl)-1,8-naphthyridine under identical conditions affords another orthometalated/nonmetalated (om/nm) complex. At low temperature (4°C), however, a nonmetalated complex is isolated that reveals axial Ru•••C−H interactions involving both ligands at sites trans to the Ru−Ru bond. A nonmetalated (nm/nm) complex was characterized for 2-pyrrolyl-1,8-naphthyridine at room temperature. Orthometalation of both ligands on a single [Ru−Ru] platform could not be accomplished even at elevated temperature. X-ray metrical parameters clearly distinguish between the orthometalated and nonmetalated ligands. NMR investigation reveals the identity of each proton and sheds light on the nature of [Ru−Ru]•••C−H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C−H bond cleavage that involves a C(p π)−H → σ* [Ru−Ru] interaction, resulting in a Wheland-type intermediate. The heteroatom stabilization is credited to the isolation of nonmetalated complexes for pyrrolyl C−H, whereas lack of such stabilization for phenyl C−H causes rapid proton elimination, giving rise to orthometalation. NPA charge analysis suggests that the first orthometalation makes the [Ru−Ru] core sufficiently electron rich, which does not allow significant interaction with the other axial C−H bond, making the second metalation very difficult.
Journal of the American Chemical Society, 1998
Reaction of [RhClL 2 ] 2 (L ) cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp 3 -sp 3 ArCH 2 -CH 3 bond. The identity of complex Rh(Et){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η 1 -N 2 ){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } (7) with EtI. It is possible to direct the bond activation process toward the benzylic C-H bonds of the aryl-alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ( t Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH 2 -CH 3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH 2 CH 3 bond activation process. Thus, aryl-C activation by Rh(I) is kinetically preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH 2 CH 3 bond, followed by -H elimination, may be preferred over sp 2 -sp 3 C-C activation of an Ar-CH 3 group.
Organometallics, 2008
The mechanisms of two recently reported thermal transformations of NHC-triruthenium and -triosmium cluster complexes, which involve the unusual oxidative addition of two C-H bonds of an NHC N-methyl group, have been investigated by density functional theory calculations. The transformations of [M 3 (Me 2 Im)(CO) 11 ] (Me 2 Im ) 1,3-dimethylimidazol-2-ylidene; M ) Ru (1a), Os (1b)) into the ligandcapped dihydrido derivatives [M 3 (µ-H) 2 (µ 3 -κ 2 -MeImCH)(CO) 9 ] (M ) Ru (3a), Os (3b)) are mechanistically very similar, but they differ in the energy barriers of key steps. For both metal systems (M ) Ru, Os), the first step is a ligand rearrangement that moves the Me 2 Im ligand from an equatorial (in 1a and 1b) to an axial coordination position. Both C-H activation steps are oxidative addition processes, and each one is preceded by a CO elimination step that provides a coordinatively unsaturated intermediate. The first C-H oxidative addition occurs via a transition state that implies an unusual interaction of an N-methyl hydrogen atom with two metal atoms simultaneously. This transition state directly leads to an intermediate that contains an edge-bridging hydride and an edge-bridging MeImCH 2 ligand, i.e., [M 3 (µ-H)(µ-κ 2 -MeImCH 2 )(CO) 10 ] (M ) Ru (2a), Os (2b)). The second C-H oxidative addition takes place via an interaction of the unbridged metal atom with a CH 2 hydrogen atom of the bridging MeImCH 2 ligand. This gives a face-capping MeImCH ligand and a terminal hydride that subsequently rearranges to an edge-bridging position to give 3a or 3b. The activation barriers of both CO elimination steps are higher for the osmium system than for the ruthenium system. For both metal systems, the slowest step is the first CO elimination.
Structural snapshots of concerted double E–H bond activation at a transition metal centre
Nature Chemistry, 2017
Bond activation at a transition metal center is a key fundamental step in numerous chemical transformations. The oxidative addition of element-hydrogen bonds, for example, represents a critical step in a range of widely applied catalytic processes. Despite this, experimental studies characterizing intermediates along the bond activation pathway are very rare. In this work, we report on fundamental studies defining a new oxidative activation pathway: combined experimental and computational approaches yield structural snapshots of the simultaneous activation of both bonds of a β-diketiminate-stabilized GaH 2 unit at a single metal center. Systematic variation of the supporting phosphine ligands and single crystal X-ray/neutron diffraction are exploited in tandem to allow structural visualization of the activation process, from a η 2-H,H σ-complex showing little Ga-H bond activation, through species of intermediate geometry featuring stretched Ga-H and compressed M-H/M-Ga bonds, to a fully activated metal dihydride featuring a neutral (carbene-type) N-heterocyclic Ga I ligand. Main text (3050 words-not including abstract, methods, captions; 6 display items-5 figures and 1 table) Bond activation at transition metal centers is key to a wide range of societally important chemical reactions, being a fundamental step in catalytic processes underpinning synthetic, medicinal and materials science. 1 Bond cleavage can be achieved through a number of routes, with oxidative addition at late transition metal centers being among the most widely exploited and extensively studied (particularly for 'noble' metals such as ruthenium, rhodium and palladium). 2-11 Experimental and theoretical studies imply that E-H bond activation by such systems proceeds via initial formation of a η 2-E,H σ-complex, with transfer of electron density from the metal into the E-H σ* orbital bringing about bond weakening, and ultimately cleavage to give the corresponding elementyl hydride (see Fig. 1(a)). 2,9,11-18 Despite its widespread relevance, however, studies describing sequentially intercepted intermediates along this reaction trajectory for a given (single) metal system are very rare. 19,20 <Fig. 1> Simultaneous activation of two E-H bonds via an analogous η 2-H,H σ-complex leading to the formation of a metal dihydride (Fig. 1(b)) has very little precedent (even for the more polar group 13 E-H bonds which are the focus of this study), 22-26 since it requires a metal precursor with a formal electron count of ≤ 14, or an in situ source thereof. Processes for which experimental mechanistic evidence is available (e.g. the direct formation of Fischer carbene complexes [L n M=C(OR)R'] from the corresponding ether, H 2 C(OR)R') are thought to proceed via a two-step C-H oxidative addition/α-migration mechanism. 27-34 Nevertheless, such chemistry represents an attractive net transformation, given the widespread use of metal carbene and related complexes (for example in homogenous catalysis), and the importance of dehydrogenation reactions in chemical processes relevant to applications in energy, polymer synthesis, etc. 35,36 In the current contribution we demonstrate the viability of a concerted single-step double E-H activation process. Systematic variation in the ancillary metal-bound ligands and single-crystal X-ray/neutron diffraction are exploited in tandem to demonstrate sequential steps in the activation process, from a η 2-H,H σ-complex showing little E-H bond activation, through species of intermediate geometry featuring stretched E-H and compressed M-H/M-E bonds, to a fully activated metal dihydride featuring a neutral (carbene-like) element-ylidene ligand. Experimental geometric data have been analysed in the light of Atoms in Molecules and fragment orbital calculations to yield the first 'snapshot' visualization of a bond activation process of this type. The EH 2-containing system chosen for the current study is the bulky β-diketiminato supported gallane (NacNac) Dipp GaH 2 [(NacNac) Dipp = HC(MeCDippN) 2 , where Dipp = C 6 H 3 i Pr 2-2,6], 37 which has previously been shown to undergo dehydrogenation at metal carbonyl fragments to give complexes of the type M x (CO) y {Ga(NacNac) Dipp } (M = Cr, Mo, W, Fe, Co), containing the neutral two-electron (carbene analogue) gallylene ligand, :Ga(NacNac) Dipp. Spontaneous reductive loss of H 2 at M in these systems is driven by the presence of the strongly π-accepting carbonyl ligand set. 24,25 The use of ancillary phosphine donors was therefore targeted in order to stabilise higher oxidation state hydride-containing species, which might