Halide Ligands—More Than Just σ-Donors? A Structural and Spectroscopic Study of Homologous Organonickel Complexes

Abstract

The isoleptic organonickel complexes [(bpy)Ni(Mes)X] (bpy ) 2,2'-bipyridine; Mes ) 2,4,6-trimethylphenyl; X ) F,Cl, Br, or I, and for comparison X ) OMe and SCN) have been investigated by multiple spectroscopic means.Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer)obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X] + Solv a [(bpy)Ni(Mes)(Solv)]+ + X-, which occurs very fast for X ) I (k ) 0.176(4) M(-1) s(-1)) or Br but very slow for X ) Cl (k ) 5.18(5) x 10(-5) M(-1) cm(-1))or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all halide coligands but marginal pi-donation.

Mono‐ and Binuclear Arylnickel Complexes of the α‐Diimine Bridging Ligand 2,2′‐Bipyrimidine (bpym)

“…[14,16] In nitriles, DMSO and alcohols the complexes undergo the same decomposition reaction as reported for a number of monomolecular diimine complexes [(N ∧ N)NiMesX] (N ∧ N = various diimine ligands, X = halogenido). [14][15][16] This will be important for the further measurements in THF or DMF solutions (see below). 1 H NMR spectra revealed that the binuclear complex was obtained as 1.45 trans/1 cis mixtures of the two stereoisomers depicted in Scheme 1.…”

“…Based on similar results from mononuclear derivatives, we assign them to metal-centred oxidations Ni II /Ni III for the two nickel atoms. [16,17] From the wave of the binuclear complex it is not discernible if both nickel atoms are oxidised at the same potential (no electronic coupling), or if a mixed-valent state occurs (coupling of the metal centres).…”

“…[9][10][11][12][13][14][15][16][17] We have contributed to this with the investigation of a number of organonickel complexes [(N ∧ N)Ni(Mes)X] (N ∧ N = α-diimine ligands, Mes = mesityl = 2,4,6-trimethylphenyl, X = halides) with various diimine ligands. Their strucures and reactivity towards ligand exchange reactions have been studied using X-ray diffraction and absorption spectroscopy, [13,14] their photophysics and photochemistry were explored by a combination of multiple spectroscopy and quantum chemical calculations, [13,15,16] and finally, their redox chemistry was investigated, [16,17] with respect to the application of such systems in electrocatalytical C-C coupling reactions. [6] From theses studies we have a clear picture of the crucial role of the diimine ligand in these complexes.…”

Trifluoromethylation of a Well‐Defined Square‐Planar Aryl‐NiII Complex involving NiIII/CF3. and NiIV−CF3 Intermediate Species

“…[16] However, the pre-edge of 1 NO3 having an intensity of ~0.3 units is similar to the well resolved preedge of a previously reported square planar Ni II complex, [(bpy)Ni(Mes)Cl], having a Ni-C aryl bond of 1.94 Å. [17] This can be explained by the highly covalent nature of the Ni-C bond, which facilitates p-d mixing through a configuration interaction model, as previously described. [18] Furthermore, the strength of the Ni-C aryl interaction in 1 NO3 is further highlighted by the lower energy of the features due to 1sà4p + shake-down contributions which occur ~2 eV lower in energy than in either the previously reported [(bpy)Ni(Mes)Cl] complex having a Ni-C aryl bond of 1.94 Å or the [Ni(cyclam)](ClO 4 ) 2 analogue having no Ni-C bond.…”

Replacing Conventional Carbon Nucleophiles with Electrophiles: Nickel-Catalyzed Reductive Alkylation of Aryl Bromides and Chlorides

“…The role of the iodide may be to (1) help facilitate reduction of the nickel catalyst by acting as a bridging ligand with zinc, 56,18 (2) promote formation of more reactive nickelate complexes, 57,18 (3) generate a small amount of the more reactive alkyl iodide in situ, 58 and/or (4) facilitate ligand exchange reactions. 59 The addition of more sodium iodide (>25 mol %) was not helpful except in the synthesis of 3ac , which was slow to react and for which S N 2 reactions are a challenge. 60 The effect of sodium iodide on reactions to form 3b – 3d are shown below (Table 4).…”