Energetics of Homogeneous Intermolecular Vinyl and Allyl Carbon−Hydrogen Bond Activation by the 16-Electron Coordinatively Unsaturated Organometallic Fragment [Tp‘Rh(CNCH₂CMe₃)]

Abstract

Reaction of the complex Tp‘Rh(CNneo)(CHCH2)Cl (neo = CH2CMe3, Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) with Cp2ZrH2 leads to the formation of Tp‘Rh(CNneo)(CHCH2)H. This complex is also formed upon photolysis of a solution of Tp‘Rh(CNneo)(PhNCNneo) containing ethylene or by thermal reaction of Tp‘Rh(CNneo)(c-hexyl)H with ethylene. The vinyl hydride complex rearranges to the more stable η2-ethylene complex with a half-life of 8 h at 22 °C. Photolysis of a solution of Tp‘Rh(CNneo)(PhNCNneo) in liquid propylene produces the allylic activation product Tp‘Rh(CNneo)(CH2CHCH2)H, which rearranges (t 1/2 = 3 days at 22 °C) to the η2-propylene complex. Allylic activation is also seen with isobutylene, but loss of olefin is observed at 22 °C in benzene solution to generate Tp‘Rh(CNneo)(Ph)H (t 1/2 = 16.6 h). Photolysis of a tert-butylethylene solution of Tp‘Rh(CNneo)(PhNCNneo) produces the trans vinyl hydride complex, which loses tert-butylethylene to generate Tp‘Rh(CNneo)(Ph)H (t 1/2 = 113 days at 22 °C). A combination of kinetic selectivity and reductive elimination experiments have allowed for calculation of relative Rh−C bond strengths for both the rhodium allyl and vinyl hydride complexes and for the inclusion of these new data in an analysis of bond strength correlations. The results show that the trend for relative Rh−C bond strengths parallels the trend of hydrocarbon C−H bond strengths, i.e., Rh−Ph > Rh−vinyl > Rh−methyl > Rh−alkyl (1°) > Rh−cycloalkyl (2°) > Rh−benzyl > Rh−allyl, but that differences in M−C bond strengths typically exceed the differences in C−H bond strengths.

Cationic iridium complexes of the Xantphos ligand. Flexible coordination modes and the isolation of the hydride insertion product with an alkene

“…In the 1 H NMR spectrum a triplet at d À18.20 (1H) with a cis 1 He 31 P coupling constant of J(PH) ¼ 15.2 Hz, a singlet at d 0.55 that integrated as 9H is assigned to the t Bu group and two methyl environments for the Xantphos ligand were observed. Signals assigned to methylene groups at d 1.93 and 1.02 (2H each) were seen, while no signals characteristic to a coordinated tbe ligand [33,34] [38]), and that the Ir1eC1eC2 angle (116.8(4) ) is also much smaller than expected for a vinyl group (w130 [38]). The hydride ligand was located in the final difference map and sits trans to the MeCN ligand.…”

Transition Metal Mediated C–H Activation and Functionalization: The Role of Poly(pyrazolyl)borate and Poly(pyrazolyl)alkane Ligands

“…The Jones group has utilized the hydridotris(3,5‐dimethylpyrazyolyl)borate ligand (Tp*) to explore stoichiometric C–H bond activations . [Tp*Rh(CNneo)(PhN=C=Nneo)] (neo = neopentyl, –CH 2 CMe 3 ) has been used to determine kinetic selectivity toward C–H bond activations of alkanes and aromatics as well as M–C bond strengths for cyclic and acyclic hydrocarbons . As shown in Scheme , the complex [Tp*Rh(CNneo)(PhN=C=Nneo)] can facilitate C–H bond activation of a number of hydrocarbon substrates under photolytic conditions through initial dissociation of the carbodiimide ligand and generation of an open coordination site for σ(C,H)‐alkane coordination prior to oxidative addition to produce [Tp*Rh(CNneo)(R)(H)].…”

“…Furthermore, this kinetic selectivity was shown in a variety of competition experiments. For example, reaction of [Tp*Rh(CNneo)(PhN=C=Nneo)] under photolytic conditions with a 1:1 mixture of benzene (C–H bond strength: 464 kJ/mol) and pentane (primary C–H bond strength: 419 kJ/mol) results in a kinetic selectivity of 4.7:1 for benzene activation in preference to pentane C–H activation to form [Tp*Rh(CNneo)(Ph)(H)] …”

“…The reactivity of [Tp*Rh(CNneo)(PhN=C=Nneo)] with different C–H bonds has been extended to include multiple hydrocarbons ranging from aromatic,, sp, sp 2 , and sp 3 ‐hybridized C–H bonds to cycloalkanes , , . Also, the effects of heteroatoms and nitrile functional groups in the hydrocarbon undergoing C–H activation have been studied .…”

Complementary Strategies for Directed C(sp3)−H Functionalization: A Comparison of Transition‐Metal‐Catalyzed Activation, Hydrogen Atom Transfer, and Carbene/Nitrene Transfer

“…[5] Ther hodium complexes ligated by Tp also show that the RhÀPh bond in 3 has abond energy 16 kcal mol À1 higher than the RhÀMe bond in 4 ( Figure 3). [6] This difference in bond energies affords ap lausible explanation for the fact that it is more difficult to activate aC(sp 3 ) À Hbond than aC(sp 2 ) À Hbond in transitionmetal catalysis.T he following gives ab rief overview of seminal reports on C(sp 3 )ÀHa ctivation catalyzed by transition-metal complexes through the formation of am etalcarbon bond and does not represent an exhaustive list of all research in this area. [7] 2.1.1.…”