Kinetic and Thermodynamic Study of Syn−Anti Isomerization of Nickel Complexes Bearing Amino-Linked N-Heterocyclic Carbene Ligands: The Effect of the Pendant Arm of the NHC

Abstract

The mono-(2) and bis(amino-NHC) (3) nickel halide complexes can be prepared in moderate yield by reacting Ni(PPh 3 ) 2 X 2 with the amino pendant linked NHC 1, [C{(MES)N(CHCH)-N(CH 2 CH 2 NHtBu)}], or its lithium bromide carbene adduct 1-LiBr. The mono(amino-carbene) nickel complexes (2) are paramagnetic species bearing a tetrahedrally distorted Ni center featuring a bidentate coordination mode via NHC and amine dative bonds. In contrast, the bis(amino-carbene) nickel complexes (3) are proton NMR active, consisting of a square-planar geometry with two trans NHC moieties. Both complexes 2 and 3 contain nickel-carbene bonds that are comparable to the literature value. Two forms of isomeric products for 3 are observed in the solution structure, but only solid structural syn-trans isomers are obtained, despite numerous attempts using various solvents with different polarity. On the basis of the powder X-ray analysis and various spectroscopy and theoretical results, we have ruled out a cis-trans isomerization, but confirmed a syn-anti conformational equilibrium exhibited in the solution of 3. The variable-temperature NMR for complexes 3 are consistent with restricted rotation about the nickel-carbene carbon bond. Plots of K eq vs 1/T are obtained with calculated ΔH°of 6.03 and 7.61 kJ/mol and ΔS°of 16.1 and 24.1 J/mol 3 K in toluene-d 8 for 3-Br and 3-Cl, respectively, indicating that syn-anti conformers of 3 are more stable. The outcome of the theoretical calculation is in line with the experimental result.

Mechanistic Study of a Switch in the Regioselectivity of Hydroheteroarylation of Styrene Catalyzed by Bimetallic Ni–Al through CH Activation

“…In support of this rationale, we calculated the steric buried volume based on a known bidentate structure from our previous work, the amino-NHC nickel(II) complex 5. [14] Indeed, a bidentate feature in amino-NHCs generates a large buried volume of 48.1 %, thus making a compelling case for sterically controlled selectivity. Furthermore, the theoretical findings on cobalt-mediated hydroarylation of styrenes by Fu's group further support our observation of ligandcontrolled regioselectivity based on steric effects.…”

Ligand Dynamics and Aerobic Allylic Oxidation with Bifunctional Ni(NHC) Complexes

“…This indicates that the Ni‐NHC bond undergoes proteolysis following oxidation. A second set of signals is observed that match to the known bis‐NHC complex NiCl 2 ( 1 a ) 2 that is formed in ca. 10 % yield.…”

“…The facile dynamic processes of 2 a are in sharp contrast to NiCl(allyl)(IMes) that contains a rigorously monodentate NHC and exhibits static allyl resonances at room temperature . We ascribe the greater fluxionality found in 2 a to the known, hemilabile character of the NHC 1 a . Reversible coordination of the secondary amine may have the further consequence of inducing allyl isomerization (Scheme ).…”

“…Notable derivatives include 1 a‐b (Figure ) that contain a secondary amine on one of the N ‐imidazolylidene substituents ,. Attractive features of this ligand set include: the versatility of the structure where both the R and R’ groups are modifiable; reliable synthetic procedures are known to prepare and store the free carbene, which greatly facilitates metallation strategies; and prior studies of 1 a confirm that the secondary amine can act as a hydrogen bond donor.…”

Amine‐Linked N‐Heterocyclic Carbenes: The Importance of an Pendant Free‐Amine Auxiliary in Assisting the Catalytic Reaction

“…The short intermolecular contact in 5a was a consequence of a possible electrostatic interaction between the hydrogen atom and the lone pair on the carbene carbon of another molecule. On the other hand, the steric influence imposed by the tert ‐butyl amino side‐arm in 5b diminished the intermolecular interaction, which was somewhat longer than in 5a (2.507 Å, 3.404 Å, 170°) 5e. Based on the above observation, we speculated that the intermolecular interactions in 5a would disrupt the stability of the amino–NHC.…”