Coupling of an N-Heterocyclic Carbene on Iron with Alkynes to Form η5-Cyclopentadienyl-Diimine Ligands

From ferrocene to decasubstituted enantiopure ferrocene-1,1'-disulfoxide derivatives

The functionalization of (R,R)-S,S'-di-tert-butylferrocene-1,1'-disulfoxide by deprotolithiation-electrophilic trapping sequences was studied towards polysubstituted, enantiopure derivatives for which the properties were determined. While the 2,2'-disubstituted ferrocene derivatives were obtained as expected, subsequent functionalization of the 2,2'-di(phenylthio) and 2,2'-bis(trimethylsilyl) derivatives occurred primarily at the 4- or 4,4'-positions. This unusual regioselectivity was discussed in detail in light of pK_a values and structural data. The less sterically hindered 2,2'-difluorinated derivative yielded the expected 1,1',2,2',3,3'-hexasubstituted ferrocenes by the deprotometallation-trapping sequence. Further functionalization proved possible, leading to early examples of 1,1',2,2',3,3',4,4'-octa, nona and even decasubstituted ferrocenes. Some of the newly prepared ferrocene-1,1'-disulfoxides were tested as ligands for enantioselective catalysis and their electrochemical properties were investigated.

Case Study of N-i Pr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2 ]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)₂] (NHC=ⁱPr₂Im^Me1^Me, Mes₂Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni₂(ⁱPr₂Im^Me)₄(μ‐(η² : η²)‐COD)] B/ [Ni(ⁱPr₂Im^Me)₂(η⁴‐COD)] B’ or [Ni(Mes₂Im)₂] 2, respectively, with alkynes afforded complexes [Ni(NHC)₂(η²‐alkyne)] (NHC=ⁱPr₂Im^Me: alkyne=MeC≡CMe 3, H₇C₃C≡CC₃H₇4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me₃SiC≡CSiMe₃7, PhC≡CMe 8, HC≡CC₃H₇9, HC≡CPh 10, HC≡C(p‐Tol) 11, HC≡C(4‐^tBu‐C₆H₄) 12, HC≡CCOOMe 13; NHC=Mes₂Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4‐^tBu‐C₆H₄) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p‐Tol) and HC≡C(4‐^tBu‐C₆H₄), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N‐ⁱPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2‐butyne, 4‐octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1‐pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1^Me is not a good catalyst. The reaction of 2 with 2‐butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)₂]. DFT calculations reveal that the differences between 1^Me and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N‐alkyl substituted NHC, to enhanced Ni‐alkyne backbonding due to a smaller C_NHC−Ni−C_NHC bite angle, and to less steric repulsion of the smaller NHC ⁱPr₂Im^Me.

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal-ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Well-Defined Heterobimetallic Reactivity at Unsupported Ruthenium-Indium Bonds

The hydride complex [Ru(IPr)₂ (CO)H][BAr^F₄ ], 1, reacts with InMe₃ with loss of CH₄ to form [Ru(IPr)₂ (CO)(InMe)(Me)][BAr^F₄ ], 4, featuring an unsupported Ru-In bond with unsaturated Ru and In centres. 4 reacts with H₂ to give [Ru(IPr)₂ (CO)(η² -H₂ )(InMe)(H)][BAr^F₄ ], 5, while CO induces formation of the indyl complex [Ru(IPr)₂ (CO)₃ (InMe₂ )][BAr^F₄ ], 7. These observations highlight the ability of Me to shuttle between Ru and In centres and are supported by DFT calculations on the mechanism of formation of 4 and its reactions with H₂ and CO. An analysis of Ru-In bonding in these species is also presented. Reaction of 1 with GaMe₃ also involves CH₄ loss but, in contrast to its In congener, sees IPr transfer from Ru to Ga to give a gallyl complex featuring an η⁶ interaction of one aryl substituent with Ru.

Unusual rearrangement of an N-heterocyclic carbeneviaa ring-opening and ring-closing process

The reaction of a pentadentate NHC ligand precursor with Ni(OAc)₂·4H₂O or Pd(OAc)₂in the presence of a base yields four-coordinate square-planar Ni(ii) and Pd(ii) complexes with an unusual ligand generatedin situ.

On the alcoholysis of alkyl-aluminum(iii) alkoxy-NHC derivatives: reactivity of the Al-carbene Lewis pair versus Al-alkyl

Alkyl-aluminum(iii) derivatives supported by a bifunctional alkoxy-NHC ligand display unusual reactivity at the carbene, yielding imidazolium-aluminate zwitterions.

N-Heterocyclic carbenes as supporting ligands in transition metal complexes of N2

Recent developments have substantially expanded the scope of N-heterocyclic carbenes (NHCs) as ancillary ligands in coordination chemistry and homogeneous catalysis. This review provides a short overview of the emerging field of NHC-supported transition metal complexes of N2 and the possibilities to catalytically activate N2 in these complexes.

A niobium-necked cluster [As3Nb(As3Sn3)]3− with aromatic Sn32−

A new Zintl cluster [As₃Nb(As₃Sn₃)]³⁻ was synthesized and characterized, in which an As₃ triangle and a bowl-type As₃Sn₃ ligand are bridged by a niobium atom. The Sn₃ ring is found to have σ-aromaticity featured by a delocalized Sn–Sn–Sn σ orbital.

25 years of N-heterocyclic carbenes: activation of both main-group element-element bonds and NHCs themselves

N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.

“Decarbonization” of an imino-N-heterocyclic carbene ligand via triple benzyl migration from hafnium

Binding of an imino-N-heterocyclic carbene to tetrabenzylhafnium triggers complete removal of the carbene carbon via three successive benzyl migrations.