Michel Etienne

In this work, we report the transfer of alkyl bis- and tris(pyrazolyl)aluminates metalloligands to an electron-rich organotransition metal center. The 16-electron heterobimetallic complexes of rhodium [Rh(COD){Al(Ph₂pz)₂Me₂}] (3) and [Rh(COD){Al(Ph₂pz)₃Me}] (4) were obtained by metathesis reaction of the sodium bis- (1) and tris(pyrazolyl)aluminate (2) with [RhCl(COD)]₂. For 3, ¹H and ¹³C NMR in solution along with DFT calculations are consistent with a κ²-coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar κ²-coordination mode of the tris(pyrazolyl)aluminate to Rh(I) with a pendant pyrazolyl moiety. The attempted synthesis of aluminate-rhodium complexes with R = CF₃, ^tBu on the pyrazolate ring afforded [Rh(R₂pz)(COD)]₂ and [R₂pzAlMe₂]₂. Complexes 3 and 4 were investigated as homogeneous catalysts in the polymerization of phenylacetylene (PA). Both complexes showed enhanced catalytic activity compared to analogous rhodium poly(pyrazolyl)borates. Optimized gas-phase DFT geometries of 3, 4, [Rh(COD){B(Ph₂pz)₂Me₂}], and [Rh(COD){B(Ph₂pz)₃Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)₂{B(Ph₂pz)₂Me₂}], and [Rh(CO)₂{B(Ph₂pz)₃Me}] were employed to probe the electronic situation of the rhodium center through IR CO stretching modes. The wider bite angles and the less electron-rich rhodium center of the poly(pyrazolyl)aluminates compared with their borate analogues could be implicated in the better performance of the active catalytic species during polymerization of PA.

This paper addresses the problem of the observation of the so-called C–C agostic interactions in cyclopropyl complexes of scandium. Three new cyclopropyl complexes of scandium based on the β-diketiminato ligand were synthesized including by an intramolecular C–H bond activation reaction in one case. X-ray diffraction analysis revealed distorted cyclopropyl groups in the complexes, and the distortion could be observed in solution as well for one of the complexes thanks to the natural abundance INADEQUATE NMR spectroscopy which showed a markedly reduced J_C–C coupling constant. This signature of the C–C agostic interaction was further examined using DFT modelling which, together with NBO calculations, indicated that C–H and C–C agostic interactions are not exclusive but can complement each other, accounting for the distortions. The intramolecular C–H bond activation in the scandium biscyclopropyl complex was investigated by isotopic labeling experiments, which indicated a direct proton abstraction of the isopropyl group in the β-diketiminato ligand by a cyclopropyl group.

This paper addresses the problem of the strong and inert C–F bond activation of various fluoropyridines by zirconocene derivatives. Dicyclopropylzirconocene, [Cp₂Zr(c-C₃H₅)₂], thermally eliminates cyclopropane, giving the transient intermediate zirconabicyclobutane [Cp₂Zr(η²-c-C₃H₄)] that cleaves a C–F bond of pentafluoropyridine selectively at position 2, forming new Zr–F and C–C bonds stereoselectively to give [Cp₂ZrF{c-cis-CHCH₂CH(2-NC₅F₄)}]. DFT computations indicate the selectivity results from the initial formation of an azazirconacycle intermediate that undergoes ring opening and C–F bond cleavage. Transmetalation with a variety of cyclopropyl donors yields [Cp₂Zr(c-C₃H₅){c-cis-CHCH₂CH(2-NC₅F₄)}] with the selectivity depending on the nature of the donor. A synthetic cycle is realized when [Cp₂Zr(c-C₃H₅){c-cis-CHCH₂CH(2-NC₅F₄)}] reacts with pentafluoropyridine, yielding 2-(c-C₃H₅)NC₅F₄ and [Cp₂ZrF{c-cis-CHCH₂CH(2-NC₅F₄)}] with C–F bond activation. Attempts at converting this reaction sequence to a catalytic version failed due to either decomposition of the active species or multiple C–F bond substitutions by the transmetalating agent.

This article explores the contrasting reactivity of cyclopropyl complexes of early transition metals. [Cp*W(NO)(CH₂R)(c‐C₃H₅)] (R = SiMe₃, Ph, t‐Bu) generated in THF solution from [Cp*W(NO)(CH₂R)Cl] and [Mg(c‐C₃H₅)₂(dioxane)_x] readily rearrange to η³‐allyl derivatives [Cp*W(NO)(CH₂R)(η³‐C₃H₅)] by an intramolecular ring opening reaction. Both direct and catalyzed pathways are revealed by kinetic studies. Computational modeling indicates the ring opening reaction is preferred on thermodynamic grounds for tungsten whereas kinetic products arising from β‐H abstraction reactions of cyclopropane are observed for related zirconium and niobium complexes reported previously. The energetic accessibility and the nature of the LUMO in the tungsten complexes promote distal CC bond cleavage in the cyclopropyl ring.

We report quantitative measurements of the relative reactivities of a series of C−H bonds of gaseous or liquid C_nH_2n+2 alkanes (n=1–8, 29 different C−H bonds) towards in situ generated electrophiles (copper, silver, and rhodium carbenes), with methane as the reference. This strategy surpasses the drawback of previous model reactions of alkanes with strong electrophiles suffering from C−C cleavage processes, which precluded direct comparison of the relative reactivities of alkane C−H bonds.

We report quantitative measurements of the relative reactivities of a series of C−H bonds of gaseous or liquid C_nH_2n+2 alkanes (n=1–8, 29 different C−H bonds) towards in situ generated electrophiles (copper, silver, and rhodium carbenes), with methane as the reference. This strategy surpasses the drawback of previous model reactions of alkanes with strong electrophiles suffering from C−C cleavage processes, which precluded direct comparison of the relative reactivities of alkane C−H bonds.

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

This paper reports the synthesis and structural characterisation of a series of cyclooctatetraenyl (C₈H₈) complexes of calcium and strontium, most of them containing the bis(trimethylsilyl)amido ligand. Mixing MI₂ (M = Ca, Sr), KN(SiMe₃)₂ and K₂(C₈H₈) in thf in a 2 : 2 : 1 ratio gave the inverse sandwich amido complexes [[{(Me₃Si)₂N}M(thf)x]₂(μ-C₈H₈)] (M = Ca, x = 1; M = Sr, x = 2) (1-Ca, 1-Sr) in fair to good yields. From a 1 : 1 : 1 ratio, the mixed potassium calcium complex [[{(Me₃Si)₂N}Ca(thf)](μ-C₈H₈)K] (2) was obtained in good yield. The reaction of 2 with a 1 : 1 mixture of CaI₂ and KN(SiMe₃)₂ yielded 1-Ca confirming that the reaction could be carried out stepwise. Attempts at making heterobimetallic calcium strontium amido complexes from the reaction of 2 with Sr{N(SiMe₃)₂}₂ led to redistribution reactions which afforded the potassium strontium complex [[K{(Me₃Si)₂N}₂Sr]₂(μ-C₈H₈)] (3) among other species. Complex 3 was more conveniently synthesised in fair yield starting from a 2 : 4 : 1 molar mixture of SrI₂, KN(SiMe₃)₂ and K₂(C₈H₈), respectively. Treatment of 2 with PhCCH in benzene yielded an insoluble complex tentatively formulated as the polymeric complex [CaK(CCPh)(C₈H₈)(thf)_x]_n (4). Upon dissolution in thf, 4 underwent a redistribution reaction yielding the X-ray characterised [{Ca(μ-C₈H₈)₂}K₂(thf)₂] (5) and a homoleptic calcium alkynyl complex [Ca(CCPh)₂(thf)_x]. Several compounds were characterised by X-ray diffraction as discrete or polymeric structures where Ca⋯Cπ interactions were identified in some cases. Despite identified drawbacks, such as the lability of the coordinated thf ligands and facile redistribution reactions, this study represents the first synthetic approach to cyclooctatetraenyl heavy alkaline-earth metal amido complexes.