Toward New Organometallic Architectures: Synthesis of Carbene-Centered Rhodium and Palladium Bisphosphine Complexes. Stability and Reactivity of [PCBImPRh(L)][PF6] Pincers

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

C-H activation in bimetallic rhodium complexes to afford N-heterocyclic carbene pincer complexes

The pro-ligands 1,8-bis(di-R-phosphinomethyl)-2,3-dihydroperimidine (RH₂Pm, R = phenyl, cyclohexyl) react with [RhCl(CE)(PPh₃)₂] (E = O, S) to afford the bimetallic complexes [RhCl(CE)(μ-RH₂Pm)]₂ (E = O, S). Upon heating, these species undergo double C-H activation to afford the N-heterocyclic carbene (NHC) pincer complexes [RhCl(RPm)]. Reduction of [RhCl(CO)(μ-PhH₂Pm)]₂ with KC₈ results in the bimetallic rhodium(0) complex, [Rh(μ-CO)(PhH₂Pm)]₂, with a formal Rh-Rh bond and a hydrogen-bonding interaction between rhodium and the central methylene group (C-H⋯Rh = 2.802 Å). Upon treatment with tritylium, ferrocenium or triphenylcyclopropenium tetrafluoroborates this species undergoes double C-H activation to afford a mononuclear NHC pincer complex salt, [Rh(CO)(PhPm)]BF₄. Treatment of [RhCl(CO)(PhH₂Pm)]₂ with lithium (trimethylsilyl)acetylide provides another bimetallic species, [Rh(CCSiMe₃)(CO)(PhH₂Pm)]₂, however heating this species does not proceed cleanly to the monomeric NHC complex, [Rh(CCSiMe₃)(CO)(PhPm)] which may however be obtained from [RhCl(RPm)] and LiCCSiMe₃.

Experimental and Theoretical Study of Ni II ‐ and Pd II ‐Promoted Double Geminal C(sp 3 )−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

We report the first examples of metal‐promoted double geminal activation of C(sp³)−H bonds of the N−CH₂−N moiety in an imidazole‐type heterocycle, leading to nickel and palladium N‐heterocyclic carbene complexes under mild conditions. Reaction of the new electron‐rich diphosphine 1,3‐bis((di‐tert‐butylphosphaneyl)methyl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (1) with [PdCl₂(cod)] occurred in a stepwise fashion, first by single C−H bond activation yielding the alkyl pincer complex [PdCl(PCsp3^HP)] (3) with two trans phosphane donors and a covalent Pd−Csp3 bond. Activation of the C−H bond of the resulting α‐methine Csp3 H−M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC^NHCP)]Cl (2). Treatment of 1 with [NiBr₂(dme)] also afforded a NHC pincer complex, [NiBr(PC^NHCP)]Br (6), but the reactions leading to the double geminal C−H bond activation of the N−CH₂−N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N−CH₂−N moiety in the N−C^NHC−N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PC_carbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PC_NHCP functionalized iron complex [(PC_NHCP)FeCl₂] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PC_NHCP)Fe(H)₂N₂)] (2). Complex 2 is stable under an atmosphere of N₂ and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N₂). With benzene-d₆ as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp²)–H bond activation, which is further discussed in this study.

PSb+P Ligand: Platform for a Stibenium to Transition-Metal Interaction

The coordination chemistry of cationic divalent pnictogen ligands, such as nitrenium and phosphenium, has been well-explored in recent years. However, corresponding studies of a heavier congener, stibenium ion, are rare. To better facilitate a Sb⁺-metal interaction, a tridentate P-Sb⁺-P ligand with two phosphine buttresses was designed and synthesized, and its coordination chemistry toward late transition metals was investigated. The stibenium ligand was delivered as an activated P(SbCl)P-AgOTf complex (2) that releases AgCl and the P-Sb⁺-P ligand upon the treatment with transition metals. Reacting 2 with Rh(I) and Ir(I) metals yielded the anticipated stibenium-transition-metal complexes [(Rh(COD)Cl)₂(μ-PSb⁺P)] OTf ([3][OTf]) and [(Ir(COD)Cl)₂(μ-PSb⁺P)] OTf ([4][OTf]). The M-Sb⁺-M bridging structure was confirmed by single-crystal X-ray crystallography, and the bonding situation was examined computationally. Theoretical studies revealed the presence of three-center delocalized M-Sb⁺-M bonding interactions in [3][OTf] and [4][OTf].

New 1,2,3-triazole based bis- and trisphosphine ligands: synthesis, transition metal chemistry and catalytic studies

This paper describes a novel synthetic methodology for the preparation of 1,2,3-triazole based phosphines and their transition metal chemistry and preliminary catalytic studies.

Bis(diphenylphosphinyl)-functionalized dipyrido-annulated NHC towards copper(i) and silver(i)

Sterically rigid bis(phosphinyl)dipyrido-annulated NHC coordinates with Ag(i) and Cu(i) centres to form luminescent multinuclear complexes.

Facile synthesis of Pd(ii) and Ni(ii) pincer carbene complexes by the double C–H bond activation of a new hexahydropyrimidine-based bis(phosphine): catalysis of C–N couplings

A new hexahydropyrimidine-based NHC proligand undergoes facile double C–H bond activation to give Pd(ii) and Ni(ii) NHC pincer cationic complexes. The Pd complex catalyzes C–N cross couplings very efficiently.

Synthesis of platinum, palladium and rhodium complexes of α-aminophosphine ligands

α-Aminophosphine-type ligands are of interest as building blocks of transition metal complexes. This review focuses on the utilization of α-aminophosphines as monodentate and bidentate ligands in platinum, palladium and rhodium complexes. Besides the linear derivatives, the applications of cyclic α-aminophosphines as ligands are also summarized. Various aspects, such as synthesis, structure and applications, as well as the catalytic activity of these complexes are discussed.

Iron pyrrole-based PNP pincer ligand complexes as catalyst precursors

The structure of a pincer ligand consists of a backbone and two `arms' which typically contain a P or N atom. They are tridentate ligands that coordinate to a metal center in a meridional configuration. A series of three iron complexes containing the pyrrole-based PNP pincer ligand 2,5-bis[(diisopropylphosphanyl)methyl]pyrrolide (PNpyrP) has been synthesized. These complexes are possible precursors to new iron catalysts. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3 P,N,P′}carbonylchlorido(trimethylphosphane-κP)iron(II), [Fe(C18H34NP2)Cl(C3H9P)(CO)] or [Fe(PNpyrP)Cl(PMe3)(CO)], (I), has a slightly distorted octahedral geometry, with the Cl and CO ligands occupying the apical positions. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3 P,N,P′}chlorido(pyridine-κN)iron(II), [Fe(C18H34NP2)Cl(C5H5N)] or [Fe(PNpyrP)Cl(py)] (py is pyridine), (II), is a five-coordinate square-pyramidal complex, with the pyridine ligand in the apical position. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3 P,N,P′}dicarbonylchloridoiron(II), [Fe(C18H34NP2)Cl(CO)2] or [Fe(PNpyrP)Cl(CO)2], (III), is structurally similar to (I), but with the PMe3 ligand replaced by a second carbonyl ligand from the reaction of (II) with CO. The two carbonyl ligands are in a cis configuration, and there is positional disorder of the chloride and trans carbonyl ligands.

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

Intensive efforts for the transformation of dinitrogen using transition metal-dinitrogen complexes as catalysts under mild reaction conditions have been made. However, limited systems have succeeded in the catalytic formation of ammonia. Here we show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N₂)₂(PCP)}₂(μ-N₂)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. of ammonia are produced based on the catalyst. DFT calculations on 1 reveal that the PCP-pincer ligand serves as not only a strong σ-donor but also a π-acceptor. These electronic properties are responsible for a solid connection between the molybdenum centre and the pincer ligand, leading to the enhanced catalytic activity for nitrogen fixation.

Manganese-mediated synthesis of an NHC core non-symmetric pincer ligand and evaluation of its coordination properties

Alkylation of N-(2-pyridyl)imidazole with [Cp(CO)₂Mn(η²-P,C-Ph₂PC(H)Ph)]BF₄ followed by in situ photochemical demetallation leads to an NHC-core non-symmetric PĈN pro-ligand, whose aptitude to coordinate in a pincer mode was recognized in Rh^I, Rh^III and Ni^II complexes.

Synthesis, structure and reactivity of Pd and Ir complexes based on new lutidine-derived NHC/phosphine mixed pincer ligands

Novel proton-responsive lutidine-derived CNP ligands having a flexible Py–CH₂–NHC linkage adopt mer and fac coordination modes.