Unprecedented ROMP Activity of Low‐Valent Rhenium–Nitrosyl Complexes: Mechanistic Evaluation of an Electrophilic Olefin Metathesis System

Abstract

The reaction of [Re(H)(NO)2(PR3)2] complexes (1 a: R = PCy3; 1 b: R = PiPr3) with [H(OEt2)2][BAr(F)4] ([BAr(F)4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) in benzene at room temperature gave the corresponding cations [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b). The addition of phenyldiazomethane to benzene solutions of 2 a and 2 b afforded the moderately stable cationic rhenium(I)-benzylidene-dinitrosyl-bis(trialkyl)phosphine complexes 3 a and 3 b as [BAr(F)4]- salts in good yields. The complexes 2 a and 2 b catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights and over 80 % Z configuration of the double bonds in the chain backbone. However, these complexes do not show metathesis activity with acyclic olefins. The benzylidene derivatives 3 a and 3 b are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave the first hints of the initial formation of carbene complexes from [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) and norbornene. In a detailed mechanistic study ESI-MS/MS measurements provided further evidence that the carbene formation is initiated by a unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylide-carbene complex, capable of undergoing metathesis with alternating rhenacyclobutane formation and cycloreversion reactions ("ylide" route). However, even at an early stage the ROMP propagation route is expected to merge into an "iminate" route by attack by the ylide function on one of the N(NO) atoms followed by phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. The proposed mechanism is supported further by detailed DFT calculations.

Rhenium Nanochemistry for Catalyst Preparation

“…The molecular models of this surfaced intermediate complex have shown activity in the ring-opening polymerization [39]. Scheme 5.…”

Metal Nitrosyl Reactivity: Acetonitrile‐Promoted Insertion of an Alkylidene into a Nitrosyl Ligand with Fission of the NO Bond

“…[9] As a large stoichiometric excess of acetonitrile was added to CH 2 Cl 2 solutions of the benzylidene complexes 2 a and 2 b at À30 8C, exclusive formations of [ReA C H T U N G T R E N N U N G (N CCH 3 )- …”

“…[9] The reactions of such benzylidene complexes with acetonitrile were first thought to be related to studies of R. Beckhaus reporting the [2+2] addition reaction of the titanium vinylidene complex [Cp* 2 TiA C H T U N G T R E N N U N G (=CCH 2 )] with organonitrile compounds. [10] However, the reactions of 2 a and 2 b with acetonitrile turned out to be much more complicated, showing a sequence of elementary steps involving the coupling of the benzylidene and the NO ligand.…”

Dinitrosyl rhenium complexes for ring-opening metathesis polymerization (ROMP)

“…1). Complexes 13a,b were fully characterized by various NMR techniques and an X-ray diffraction study of 13a [53].…”

“…Since the ROMP mechanisms of complexes 1a and 1b could not be fully established on an experimental base, supporting density functional theory (DFT) computational studies were carried out with structural optimization of potential model intermediates of the ROMP catalysis with norbornene. The complete DFT analysis has been published recently [53]. The DFT calculations were carried out using PMe 3 ligands, and the relative energies are referenced to the total energy of the norbornene complex 3-Me with an orientation of the olefin perpendicular to the Re-P axis.…”