Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis, EPR/DFT Studies and Catalytic Activity

Abstract

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d z 2〉 and ∣3d x 2-y 2〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4≈6≫5).

The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

“…In an extension of this work, EPR was used to investigate this systematic series of well-defined Ni(I) complexes bearing a set of easily modified NHC ligands (Scheme 1) in order to obtain insights into both the fundamental properties (structure and bonding) and catalytic behaviour of these 3-coordinate d 9 species. Using continuous wave (CW) and pulsed EPR, in combination with DFT calculations, this series of complexes enabled us to test the relationship between the NHC ring size and substituents on the electronic properties of the d 9 Ni(I) centre through perturbations to the spin Hamiltonian parameters [12].…”

“…1. The pulsed X-band and CW Q-band spectra were also recorded to supplement and facilitate analysis of the X-band data [12]. All spectra displayed a rhombic g profile, with considerably broadened line-widths arising from the large superhyperfine couplings to the 31 P (I = ) nuclei and in some cases to the 79,81 Br (I = 3/2) nuclei.…”

“…For a number of years we have exploited a number of advanced EPR and hyperfine based techniques to explore the structure-function relationships in transition metal based homogeneous catalysis, including Fe(I) for catalytic C-C bond formation [8,9], Cr(I) for ethylene oligomerisation [10], Co(II) for hydrolytic kinetic resolution [11], Ni(I) for cross coupling reactions [12] and Cu(II) for asymmetric aziridination [13]. We have demonstrated how the EPR approach can be used not only to identify the key oxidation states involved in these reactions, but also how the technique can help to unravel the complementary role of the ligands in controlling the selectivity and stability of the intermediates formed.…”

Mechanistic Implications for the Ni(I)-Catalyzed Kumada Cross-Coupling Reaction

“…Such Ni(I) complexes are frequently used as potent pre-catalysts in the Negishi coupling of alkyliodides and alkylzinc bromides [11][12][13], in the Suzuki-Miyaura reaction for the formation of C-C bonds from arylhalides and phenylboronic acid [14][15][16], as well as in Ni(I)-catalyzed Kumada coupling reactions [17,18]. Here, most commonly, bulky N-heterocyclic carbenes (NHCs) are utilized as a ligand platform for the stabilization of the monovalent nickel centers.…”

“…Here, most commonly, bulky N-heterocyclic carbenes (NHCs) are utilized as a ligand platform for the stabilization of the monovalent nickel centers. Along this line, Whittlesey and coworkers reported a series of extremely air-sensitive [Ni I (PPh3)(NHC)X] complexes (X = Br, Cl) (Scheme 1) comprising bulky NHC ligands [17]. Herein, the sterically demanding substitution at the central heteroaromatic ring is required to stabilize the low-coordinate and low-valent Ni(I)-complex and to avoid intermolecular reactions.…”