Parent Cyclopentadienyl Ruthenium(II) Chloride Synthon: Derivatization to CpRu Amido, Imido, and Oxo Complexes

Heterobimetallic μ2-Halocarbyne complexes

The halocarbyne complexes [M(≡CX)(CO)2(Tp*)] (M = Mo, W; X = Cl, Br; Tp* = hydrotris(dimethylpyrazolyl)borate) react with [AuCl(SMe2)], [Pt(-H2C=CH2)(PPh3)2] or [Pt(nbe)3] (nbe = norbornene) to furnish rare examples of μ2-halocarbyne...

Transition Metal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L_nM-CE_x fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C₁ reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Linear Hydrocarbon Chain Growth from a Molecular Diruthenium Carbide Platform

The formation of linear hydrocarbon chains by sequential coupling of C₁ units on the metal surface is the central part of the Fischer-Tropsch (F-T) synthesis. Organometallic complexes have provided numerous models of relevant individual C-C coupling events but have failed to reproduce the complete chain lengthening sequence that transforms a linear C_n hydrocarbon chain into its C_n+1 homologue in an iterative fashion. In this work, we demonstrate stepwise growth of linear C_n hydrocarbon chains and their conversion to their C_n+1 homologues via consecutive addition of CH₂ units on a molecular diruthenium carbide platform. The chain growth sequence is initiated by the formation of a μ-η¹:η¹-C═CH₂ ligand from a C + CH₂ coupling between the μ-carbido complex [(Cp*Ru)₂(η-NPh)(μ-C)] (1; Cp* = η⁵-C₅Me₅) and Ph₂SCH₂. Then, the chain propagates via a general C═CHR + CH₂ coupling and subsequent hydrogen-assisted isomerization of the resulting allene ligand μ-η¹:η³-H₂C═C═CHR to a higher vinylidene homologue μ-η¹:η¹-C═CH(CH₂)R. By repeating this reaction sequence, up to C₆ chains have been synthesized in a stepwise fashion. The key step of this chain homologation sequence is the selective hydrogenation of the μ-η¹:η³-allene unit to the corresponding μ-alkylidene ligand. Isotope labeling and computational studies indicate that this transformation proceeds via the hydrogenation of the allene ligand to a terminal alkene form and its isomerization to the μ-alkylidene ligand facilitated by the coordinatively unsaturated diruthenium platform.

Construction of an iminoketenylidene

The new isonitrile-μ-carbido complexes [WPt(μ-C)Br(CNR)(PPh₃)(CO)₂(Tp*)] (R = C₆H₂Me₃-2,4,6, C₆H₃Me₂-2,6; Tp* = hydrotris(dimethylpyrazolyl)borate) rearrange irreversibly in polar solvents to provide the first examples of iminoketenylidene (CCNR) complexes.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide–thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium‐based technology and the standard copper‐catalyzed azide–alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

Four-Electron Reduction of Dioxygen on a Metal Surface: Models of Dissociative and Associative Mechanisms in a Homogeneous System

Two different four-electron reductions of dioxygen (O₂) on a metal surface are reproduced in homogeneous systems. The reaction of the highly unsaturated (56-electron) tetraruthenium tetrahydride complex 1 with O₂ readily afforded the bis(μ₃-oxo) complex 3 via a dissociative mechanism that includes large electronic and geometric changes, i.e., a four-electron oxidation of the metal centers and an increase of 8 in the number of valence electrons. In contrast, the tetraruthenium hexahydride complex 2 induces a smooth H-atom transfer to the incorporated O₂ species, and the O-OH bond is cleaved to afford the mono(μ₃-oxo) complex 4 via an associative mechanism. Density functional theory calculations suggest that the higher degree of unsaturation in the tetrahydride system induces a significant interaction between the tetraruthenium core and the O₂ moiety, enabling the large changes required for the dissociative mechanism.

Metal-metal multiple bond formation induced by σ-acceptor Lewis acid ligands

The reaction of [Cp*Ru(μ-NHPh)]2 (Cp* = η5-C5Me5) with Lewis acids of the type MX2 (M = Zn, Sn, Pb; X = Cl, OTf) affords Ru2 → M donor-acceptor adducts characterized as π complexes of a Ru[double bond, length as m-dash]Ru double bond with M(ii) Lewis acids. The results illustrate for the first time the ability of σ-acceptor Lewis acid ligands to induce the formation of a metal-metal multiple bond via stabilizing dative interactions.

Benzyne addition to a metal-carbon multiple bond

The linear μ-carbido complex [Rh2(μ-C)Cl2(dppm)2] (dppm = bis(diphenylphosphino)methane) reacts with a benzyne equivalent (Me3SiC6H4OTf-2/F-) to afford [Rh2(μ-CC6H4)(μ-Cl)(C6H5)Cl2(μ-dppm)2], in which the benzyne moiety adds across one of the two metal-carbon double bonds.

Heterobimetallic μ2-carbido complexes of platinum and tungsten

The tungsten–platinum μ-carbido complex [WPt(μ-C)Br(CO)₂(PPh₃)₂(Tp*)] (Tp* = hydrotris(dimethylpyrazol-1-yl)borate) undergoes facile substitution of both bromide and phosphine ligands to afford a diverse library of μ-carbido complexes.

Dimetalla-heterocyclic carbenes: the interconversion of chalcocarbonyl and carbido ligands

Different classes of dirhodium μ-carbido complexes cleave CS₂ to afford mono- and bi-nuclear CS complexes, the CSe analogues of which are also described.

Halogenation of A-frame μ-carbido complexes: a diamagnetic rhodium(ii) carbido complex

Chlorination of the new μ-carbido [Rh₂(μ-C)Cl₂(μ-dppf)₂] (dppf = 1,1′-bis(diphenylphosphino)ferrocene) affords the dirhodium(ii) complex [Rh₂(μ-C)Cl₄(μ-dppf)₂] the carbido bridge of which can only be adequately described by delocalised bonding.