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

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.

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.