Facile Single or Double C−H Bond Activation on a Cp* Ligand Promoted by the Presence of Alkynylphosphine Ligands

Heptadentate, Octadentate, Or Even Nonadentate? Denticity in the Unexpected Formation of an All-Carbon Donor-Atom Ligand in RhIII(Cp*)(Anthracenyl-NHC) Complexes

Investigations on incorporating an N-flanking anthracenyl moiety to [Rh(Cp*)(NHC)Cl₂] complexes surprisingly led to the formation of an intramolecular C-C bond between the Cp* and anthracenyl moieties, with additional auxiliary interactions between the metal and the anthracenyl ring system. In silico modeling supports a reaction mechanism whereby Rh(η⁴-tetramethylfulvene) intermediates undergo metallocycloaddition and the abstraction of a chlorido ligand, affording unique cationic complexes that feature Rh centers coordinated by a nonadentate ligand with exclusively carbon donor atoms. Some Rh-C interactions were extremely weak but nevertheless exhibited covalent bonding character. These weak Rh-C interactions were readily displaced by stronger electron donors, and the nonadentate ligand reverted to the heptadentate coordination mode observed in the intermediate. As far as we are aware, this study provides the first conclusive evidence of complexes bearing a single nonadentate κ⁹-coordinating ligand that features only carbon donors bound to a metal center.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non-innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C-H activation at one methyl terminus of C5Me5 in [(η-C5Me5)Rh(PMe3)2] to form a new Rh-H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C-H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E-H bonds (E=O, N), including the N-H bonds in ammonia, can be understood in terms of bimetallic frustration.

Base-Promoted, Remote C-H Activation at a Cationic (η5-C5Me5)Ir(III) Center Involving Reversible C-C Bond Formation of Bound C5Me5

C–H bond activation at cationic [(η⁵-C₅Me₅)Ir(PMe₂Ar′)] centers is described, where PMe₂Ar′ are the terphenyl phosphine ligands PMe₂Ar^Xyl₂ and PMe₂Ar^Dipp₂. Different pathways are defined for the conversion of the five-coordinate complexes [(η⁵-C₅Me₅)IrCl(PMe₂Ar′)]⁺, 2(Xyl)⁺ and 2(Dipp)⁺, into the corresponding pseudoallyls 3(Xyl)⁺ and 3(Dipp)⁺. In the absence of an external Brønsted base, electrophilic, remote ζ C–H activation takes place, for which the participation of dicationic species, [(η⁵-C₅Me₅)Ir(PMe₂Ar′)]²⁺, is proposed. When NEt₃ is present, the PMe₂Ar^Dipp₂ system is shown to proceed via 4(Dipp)⁺ as an intermediate en route to the thermodynamic, isomeric product 3(Dipp)⁺. This complex interconversion involves a non-innocent C₅Me₅ ligand, which participates in C–H and C–C bond formation and cleavage. Remarkably, the conversion of 4(Dipp)⁺ to 3(Dipp)⁺ also proceeds in the solid state.