Neutral cis-Alkyl Olefin Rhodium(I) Complexes: Models of Intermediates in Late Transition Metal Olefin Polymerization with Surprising Structure

Abstract

The reaction of the γ-agostic 14 VE complex [(κ2-dtbpm)Rh(Np)] (1) with olefins opened an access to neutral, cis-alkyl olefin complexes (κ2-dtbpm)Rh(Np)(η2-olefin) (2a/b, Np = neopentyl, olefin = ethylene, methylacrylate) bearing the cis chelating bisphosphine ligand dtbpm (bis(di-tert-butylphosphino)methane, t Bu2P-CH2-P t Bu2). They represent the first structurally characterized examples of neutral Rh systems with cis bisphosphine ligation, isoelectronic to the presumed resting states in late transition metal (e.g., Pd, Ni) catalyzed polymerization reactions. Their solid-state molecular structures revealed an unexpected, non-square-planar coordination mode of the olefin moiety, with the center of the CC double bond moved 34° out of the P2RhCNp plane. In solution rapid dynamic processes interconvert equivalent structures with the olefin above or below this plane. DFT and QM/MM (ONIOM) calculations reproduce correctly the minimum geometries found in the crystal structures. They are caused by electronic effects, as shown by fragment MO (NBO analysis) and frontier MO arguments. The unusual structure motif carries over to the chloro complexes (κ2-dtbpm)Rh(Cl)(η2-olefin) (5a/b, olefin = fumaronitrile, acrylonitrile). No polymerization is observed when 2a is exposed to an excess of ethylene, indicating a rather high olefin insertion barrier for this uncharged RhI species, in agreement with DFT calculations of the transition state for ethylene insertion into the Rh−CNp bond of (κ2-dhpm)Rh(Np)(η2-C2H4) (2a*). This contrasts with the high catalytic activity of isolectronic NiII and PdII systems.

Ethylene polymerization catalyzed by a cyclophane-diimine-based Ni(II) complex, a quantum/molecular mechanic study

“…4). This property was also observed by our group in a previous chemical computational study of an analogous stericaly demanding ␣-diimine complex [18], and by Hofmann and co-workers [39]. This late group has performed a very elegant work, isolating and full characterizing a cis-alkyl ethene distorted complex, (Ä 2 -dtbpm)Rh(Np)(Á 2 -ethene), bearing a bulky, electron-rich cis-chelating bisphosphine ligand (dtbpm = bis(ditert-butylphosphino)methane; Np = neopentyl).…”

“…This late group has performed a very elegant work, isolating and full characterizing a cis-alkyl ethene distorted complex, (Ä 2 -dtbpm)Rh(Np)(Á 2 -ethene), bearing a bulky, electron-rich cis-chelating bisphosphine ligand (dtbpm = bis(ditert-butylphosphino)methane; Np = neopentyl). This complex can be considered as a neutral isoelectronic structural models ofcomplexes present on olefin polymerization reactions catalyzed by d 8 -L 2 M(alkyl) + species (M = Ni II , Pd II ) [39]. Their results are strong evidences of the non-square-planar olefin coordination occurrence.…”

“…In order to verify the electronic and steric origin of the non-square-planar coordination mode of the olefin moiety, we performed calculations with the non-substituted diimine ligand (non-bulky ligand) [39]. One can see that non-substituted diimine, bis[N-(phenyl)imino]acenaphthene, presents classical square-planar coordination mode (Fig.…”

Well‐Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

“…[10] Additionally, these systems in general still require high loadings, and activated alkenes or terminal alkynes as substrates. Small-bite-angle phosphine ligands, R 2 PCH 2 PR 2 (e.g., R= t Bu, Cy), initially developed by Hofmann et al,[11] have been shown to favor the products of reductive elimination[12] and we recently demonstrated that catalyst systems exemplified by [Rh(R 2 PCH 2 PR 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] [R= t Bu, 1 ; Ar F =3,5-C 6 H 3 (CF 3 ) 2 ] can be used at low catalyst loadings (e.g., 0.1 mol %) to couple terminal and activated internal alkenes with β-substituted aldehydes. [13] However, challenging internal alkenes are still out of reach with this system, as decarbonylation now outruns productive turnover.…”

Organometallic chemistry

“…His research interests combined all three fields, with a focus on homogeneous catalysis with transition metal complexes. Bidentate donor ligands with a small bite angle such as bis(di- tert -butylphosphino)methane (dtbpm) were a unique feature of his work [61,67,70–72 80–81 84,86,93–94 97,100–101 103,107,124,128,148–149] that ultimately enabled C–Si activation of organosilanes and the C–C activation of oxiranes by coordinatively unsaturated platinum species [66,149]. In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88].…”