Cyclometalated Ni(II) Complexes [Ni(N^∧C^∧N)X] of the Tridentate 2,6-di(2-pyridyl)phen-ide Ligand

Abstract

A series of cyclometalated Ni(II) complexes [Ni(PyPhPy)X] containing anionic N ∧ C ∧ N − tridentate ligand Py(Ph − )Py 2,6-di(2-pyridyl)benzene-1-ide, (Py(HPh)Py = 1,3-di(2-pyridyl)benzene) and X = Cl, Br, or I as coligands, were studied. All three complexes were obtained through direct C−H base-assisted nickelation from NiX 2 and Py(HPh)Py using KOAc/K 2 CO 3 in nonpolar high-boiling point solvents. While the overall molecular structures are quite similar to those of the previously studied [Ni(C ∧ N ∧ N)X] complexes with the anionic C ∧ N ∧ N tridentate − Phbpy (HPhbpy = 6-(phenyl)-2,2′-bipyridine) ligand, bond lengths in the molecular structures are slightly different. Large differences between these N ∧ C ∧ N and C ∧ N ∧ N Ni complexes which can be traced to the different orientation of the X coligand to the carbanionic phen-ide group, trans or cis, and the different ligand pattern, Py−Ph−Py versus Ph−Py−Py, were found for the UV−vis absorption spectra and the electrochemical reductions, while the oxidation potentials are very similar. Extended DFT calculations with the TPSSh functional associate the indifference of oxidation potentials with conserved energies of metal-borne HOMOs. By contrast, the diminished π-acceptor qualities of the N ∧ C ∧ N ligand translate into a destabilization of the LUMO by ca. 400 mV and a blue-shift of the leading visible transition by 80 nm in very good agreement with the experimental data.

Ni, Pd, and Pt complexes of a tetradentate dianionic thiosemicarbazone-based O^N^N^S ligand

“…28 We found excellent agreement of predicted and experimental metrics across the entire reference set (error in bond lengths <2 pm). Taken together with the general close compliance between model and experiment of planar d 8 configured complexes, [36][37][38][39] As could be concluded from strictly diamagnetic behaviour of the complexes in the 1 H NMR spectra, all complexes exhibit close-to-ideal square planar coordination of the S^N^N^O donor set. The bond length of the coordinated C-S moiety cluster at 1.78 ± 0.01 Å, indicating dominant ene-thiolate character.…”

Enhancing the Stability of Aromatic PCN Pincer Nickel Complexes by Incorporation of Pyridine as the Nitrogen Side Arm

“…However, the cyclometallated NCN pincer nickel complex 9 was successfully obtained at 75 °C (Scheme 2). [ 34 ] This is a rare example of an NCN pincer nickel complex accessible through direct C–H activation. NCN pincer nickel complexes (Figure 3, A ) were first reported by van Koten and are usually prepared either through an oxidative addition of the C–X bond of the halide functionalized NCN ligand using Ni(COD) 2 or a transmetallation of the lithiated NCN ligand with a Ni(II) precursor.…”

“…Figure 4. [ 34 ] A square‐planar geometry is adopted by the nickel center, which is common for the three complexes with different degrees of distortion, controlled by the side donor atoms of their ligand architectures. For complexes 6 and 7 , the substituents on the phosphorus donor significantly influence the bond angles and the bond lengths due to the difference in the steric hindrance between the tert ‐butyl groups and the cyclohexyl groups.…”

“…Elongation in the case of complex 9 is observed compared to the previously reported complex B (2.3989 (4) vs. 2.300 (1)) as a result of the higher trans influence of the central carbon atom of the NCN nickel complex vs. the central nitrogen atom of the CNN nickel complex. [ 34 ]…”

“…In contrast to complex 6 , the symmetric NCN complex 9 failed to support high valent nickel(III) complexes under the same reaction conditions. This is rather surprising since the Ni(II)/Ni(III) oxidation potential for 6 is 0.27 V (vs. Fc/Fc + , see supporting info for details) whereas the potential for 9 has been reported to be 0.16 V. [ 34 ] It means that although 9 is seemingly thermodynamically easier (in agreement with the expected increase in electron density) to oxidize there is a kinetic barrier towards chemical oxidation by CuBr 2 . Complex 10 is NMR silent and the EPR spectrum of the solid at room temperature is shown in the upper panel of Figure 5.…”