Synthesis and Reactivity of a Cationic Platinum(II) Alkylidene Complex

Reversible Bimetallic Inhibition to Modulate Selectivity During Catalysis

Bimetallic complexes have demonstrated a great ability to enhance the activity of monometallic systems for bond activation and catalysis. In this work, we explore the opposite approach: using a second metal to passivate the activity of another by reversible bimetallic inhibition. To do so we have synthesized a family of nine electrophilic gold complexes of formula Au(PR3)(NTf2) ([NTf2]- = [N(SO2CF3)2]-) that can act as inhibitors in the semihydrogenation of terminal and internal alkynes catalyzed by the iconic iridium Vaska complex IrCl(CO)(PPh3)2. This behavior parallels the well-known passivation effect of lead over palladium in the heterogeneous Lindlard catalyst. Most gold fragments, except for the most hindered, form metal-only Lewis pairs upon combination with iridium, which have been fully characterized and exhibit distinct dative Ir → Au bonds. When applied to alkyne hydrogenation, these bimetallic structures have a clear tendency toward olefin formation, while the monometallic catalyst unselectively leads to overreduction products. Our computational studies not only provide a feasible mechanism for the Ir-only system, but also evince the active role of gold in passivating iridium by reversibly forming heterobimetallic structures that lead to enhanced selectivity.

Electrophilic activation of alkynes promoted by a cationic alkylidene complex of Pt(II)

Pt(II) alkylidene 1a has been reacted with terminal alkynes to afford ylide complexes 3a-d, resulting from electrophilic activation of the CC bond and its insertion into the platinacyclic fragment of 1a that contains the carbene functionality. DFT calculations indicate that the observed regioselectivity is determined by the nucleophilic attack of the alkyne to the alkylidene carbon.

Synthesis and Structures of Stable PtII and PtIV Alkylidenes: Evidence for π-Bonding and Relativistic Stabilization

Isolable cationic Pt^II and Pt^IV alkylidenes, proposed intermediates in catalytic organic transformations, are reported. The bonding in these species was probed by experimental, structural, spectroscopic, electrochemical and computational methods, providing direct evidence for π-bonding, the often-theorized relativistic stabilization of these species, and the influence of oxidation state.

Oxidation reactions of a nucleophilic palladium carbene: mono and bi-radical carbenes

A series of mono and bi-radical carbenes.

Reactivity of a gold(i)/platinum(0) frustrated Lewis pair with germanium and tin dihalides

The rich reactivity of tetrylene dihalides towards a metallic FLP: phosphine exchange reactions and formation of homo and heterobimetallic compounds.

Redox-state dependent activation of silanes and ammonia with reverse polarity (PCcarbeneP)Ni complexes: electrophilic vs. nucleophilic carbenes

A rigidified PC_alkylP ligand allowed for the synthesis and characterization of cationic and radical PC_CarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic.

Heterobimetallic Pd-K carbene complexes via one-electron reductions of palladium radical carbenes

Heterobimetallic Pd-K carbenes featuring Pd-C_carbene-K moieties were synthesized via an unprecedented sequential substitution/reduction reaction from a radical precursor, [{PC˙(sp²)P} ^tBuPdI] ([PC(sp²)P] ^tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene). Polymeric structures were observed in the solid state for the heterobimetallic compounds that can be interrupted in the presence of a donor solvent.

Platinum(0)-mediated C–O bond activation of ethers via an SN2 mechanism

DFT calculations demonstrate that Pt(0) bis(phosphine) complexes react with Ar^F–O–Me via an S_N2 mechanism to activate the O–CH₃ bond; experimental support is provided by reaction of Pt(PCy₃)₂ with 2,3,5,6-tetrafluoro-4-allyloxypyridine to form an aryloxide salt of [Pt(η³-allyl)(PCy₃)₂]⁺.

Redox-induced umpolung of transition metal carbenes

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes. Herein, we report an umpolung of a nucleophilic palladium carbene complex, [{PC(sp2)P} tBuPd(PMe3)] ({PC(sp2)P} tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene), realized by successive one-electron oxidations that generated a cationic carbene complex, [{PC(sp2)P} tBuPdI]+, via a carbene radical, [{PC˙(sp2)P} tBuPdI]. An EPR spectroscopic study of [{PC˙(sp2)P} tBuPdI] indicated the presence of a ligand-centered radical, also supported by the results of reactions with 9,10-dihydroanthracene and PhSSPh. The cationic carbene complex shows electrophilic behavior toward nucleophiles such as NaH, p TolNHLi, PhONa, and PMe3, resulting from an inversion of the electronic character of the Pd-Ccarbene bond in [{PC(sp2)P} tBuPd(PMe3)]. The redox induced umpolung is reversible and unprecedented.

Orbital-like motion of hydride ligands around low-coordinate metal centers

Hydrogen atoms in the coordination sphere of a transition metal are highly mobile ligands. Here, a new type of dynamic process involving hydrides has been characterized by computational means. This dynamic event consists of an orbital-like motion of hydride ligands around low-coordinate metal centers containing N-heterocyclic carbenes. The hydride movement around the carbene-metal-carbene axis is the lowest energy mode connecting energy equivalent isomers. This understanding provides crucial information for the interpretation of NMR spectra.

Reactivity of coordinatively unsaturated bis(N-heterocyclic carbene) Pt(II) complexes toward H(2). Crystal structure of a 14-electron Pt(II) hydride complex

The reactivity toward H2 of coordinatively unsaturated Pt(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt(NHC')(NHC)](+) (where NHC' stands for a cyclometalated NHC ligand) react very fast with H2 at room temperature, leading to hydrogenolysis of the Pt-CH2 bond and concomitant formation of hydride derivatives [PtH(NHC)2](+) or hydrido-dihydrogen complexes [PtH(H2)(NHC)2](+). The latter species release H2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH(IPr)2][SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H2 in complexes [PtH(H2)(NHC)2](+) is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (I(t)Bu). This outcome is related to the higher close-range steric effects of the I(t)Bu ligands. Accordingly, H/D exchange reactions between hydrides [PtH(NHC)2](+) and D2 take place considerably faster for IPr and IMes* derivatives than for I(t)Bu ones. The reaction mechanisms for both H2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a σ-complex assisted-metathesis mechanism in the case of I(t)Bu.

Structure of a reactive gold carbenoid

A formal gold-for-chromium transmetalation allowed the gold carbenoid species [Cy3 PAuCAr2 ]NTf2 (11) (Ar=pMeOC6 H4 -) to be obtained in crystalline form. The structure in the solid state suggests that there is only little back donation of electron density from gold to the carbene center of 11 and hence very modest AuC double-bond character; rather, it is the organic ligand framework that is responsible for stabilizing this species by resonance delocalization of the accumulated positive charge. Because 11 is capable of cyclopropanating p-methoxystyrene even at low temperature, the discussion of its structure is deemed relevant for a better understanding of the mechanisms of π-acid catalysis in general.