C−H and C−CN Bond Activation of Acetonitrile and Succinonitrile by [Tp′Rh(PR₃)]

Abstract

The [Tp 0 Rh(PR 3 )] fragments, where Tp 0 = tris(3,5-dimethylpyrazolyl)borate, R 3 = Me 3 (a) or PhMe 2 (b), were shown to be applicable for the activation of the C-H and C-CN bonds of acetonitrile. The photoirradiation of Tp 0 Rh(PR 3 )H 2 (1a,b) in acetonitrile at room temperature afforded the complexes Tp 0 Rh(PR 3 )(CH 2 CN)H (3a,b), by the selective oxidative addition of the primary C-H bond of acetonitrile to the [Tp 0 Rh(PR 3 )] fragment generated by the dehydrogenation of 1. The thermal reactions of Tp 0 Rh(PMe 3 )(Ph)H (2a) or Tp 0 Rh(κ 2 -C 6 H 4 -2-PMe 2 )H (2b) in acetonitrile at 80°C also resulted in the formation of 3a and 3b, respectively, by the oxidative addition of the C-H bond to the [Tp 0 Rh(PR 3 )] fragment, generated by the reductive elimination of benzene (from 2a) or intramolecular reductive elimination of the PPhMe 2 ligand (from 2b). Heating the isolated C-H bond activated complexes 3a,b in acetonitrile at higher temperature (100°C) for longer times (≈5 days) resulted in the formation of the corresponding C-CN bond activated complexes, Tp 0 Rh(PR 3 )(CH 3 )(CN) (4a,b), respectively. The molecular structures of the C-H activated product 3a,b (kinetic product) and C-CN activated product 4a,b (thermodynamic product) were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallographic analysis. Furthermore, the secondary C-H bond activated product of succinonitrile, Tp 0 Rh(PMe 3 )[CH(CN)CH 2 CN]H (5), was isolated by the thermal reaction of 2a with succinonitrile (20 equiv) at 100°C, and the molecular structure was characterized by NMR spectroscopy and X-ray crystallographic analysis. The cleavage of CH 3 CN was also investigated using DFT models to compare thermodynamics and identify transition-state geometries.

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…DFT analysis by Jones and co-workers indicated the presence of a high-energy intermediate bearing an agostic H interaction with Rh and connecting the transition states for C−H and C−CN activation (Scheme 40). 435,436 While the barrier for C−H activation is lower, C−CN activation is favored thermodynamically. This finding is supported by experimental results, which report C−H activation via a photochemical reaction at room temperature but a thermal conversion to C−CN activation product upon prolonged heating.…”

Methyl Complexes of the Transition Metals

“…87 Cobalt(I) methyl complexes with (bis)phophinopyridine PNP pincer ligands, which undergo H radical loss, have been characterized by the group of Milstein. 88 A number of Rh(I) and Rh(III) methyl complexes -more than forty -have been structurally characterized in the last five years, being most of them supported by chelating bidentate 18,89 or tridentate 19,20,90 ligands. Two relevant examples of C-C activation reactions that generate a Rh-CH3 bond were cited above.…”

“…Two relevant examples of C-C activation reactions that generate a Rh-CH3 bond were cited above. 19,20 Square-planar Rh(I) carbonyl methyl complexes stabilized by (diisopropylphosphino)ethane or (diisopropylphosphino)methane have been recently reported to react with HSi(OEt)3 affording the corresponding silyl complex with liberation of CH4. 89f The characterization of the first methane complex 12 in 2009 by Brookhart, Goldberg, and co-workers represents, without any doubt, a landmark discovery in transition metal chemistry and was made possible thanks to a Rh(I) methyl complex supported by a bulky PNP ligand (Scheme 12, see more in the reactivity section).…”

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

“…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 . Kinetic data from alkane or aromatic reductive elimination from [Tp*Rh(CNneo)(alkyl)(H)] and oxidative addition to [Tp*Rh(CNneo)] were used to determine the free energy of the transformations (Δ G °).…”