Cyclometalated Organonickel Complexes through C-H and C-X Activation
Final Report Abstract
The two essential ways to cyclometalated Ni(II) complexes with tridentate C^N^N or N^C^N ligands (prototypical C^N^N = 6-(phen-2-ide)-2,2’-bipyridine and N^C^N = 1,3-bis-(2-pyridyl)phen-2-ide), the C‒X oxidative addition and the base-assisted C‒H nickelation were explored in detail in this project. The C‒X oxidative addition was first reported in 2014 prior to the project and was further explored in the project. We first varied the X group on X‒Phbpy and found that for X = F, Cl, Br, and I the rates of oxidative addition are slightly increasing parallel to the leaving group character F < Cl < Br < I. OH, OMe and OTf failed as “leaving groups”, The corresponding protoligands did not react with Ni(0) precursors. The next aim of varying the phenyl group, the central pyridine unit and the pendant pyridine unit was motivated by the assumed localisation of the lowest unoccupied molecular orbitals (LUMO) in the bpy unit and the phenyl group contributing to the highest occupied molecular orbital (HOMO). We managed a very broad variation of all three groups: • phenyl-to-thiophenyl, substituted phenyl; • central pyridine to substituted pyridines; • peripheral pyridine to thiazole, thiophen, pyrazole - and we introduced the 8-(6-(2-bromophenyl)pyridin-2-yl)quinoline) protoligand which provides a C^N five-ring and an N^N six-ring chelate. The problem of synthesising the plethora of protoligands was solved though the very versatile Kroehnke pyridine synthesis. Further variations of the protoligands were possible through triazines as intermediates and reaction with electron-rich dienophiles in retro-Diels-Alder reactions with invers electron-demand (iDA). The impact of these variations on the electronic structure of the complexes as measured in electrochemical potentials lead to a variation of the first oxidation potential from 0 to 0.3 V (vs ferrocene/ferrocenium) and from completely reversible to irreversible. The corresponding highest occupied molecular orbitals (HOMO) show variable contributions from the metal, the phenyl core and the X coligand. For the first reduction process, variations are even bigger. The potentials vary from ‒1.6 to ‒2.1 V and again fully reversible as well as irreversible waves were observed. Irreversible waves are assigned to a CE process, which describes the splitting of the halide coligand (chemical reaction) following the electrochemical reduction. This is remarkable, as the lowest unoccupied molecular orbital (LUMO) lies largely centred on the bpy-core. DFT-calculations showed that the follow-up splitting of X‒ involves a re-arrangement of the reduced electronic state. The Ni(II) complex [Ni(PhPyQ)Br] with the ring-expanded (8-(6-(phen-2-ide)pyridin-2-yl)quinoline) ligand was synthesised to study the impact of the angle strain around Ni from of two five-ring chelates in the parent tridentate ligand Phbpy. Compared with the parent complex [Ni(Phbpy)Br], this new complex showed an obtuse C‒Ni‒N angle of about 173° instead of 165° and the four acute angles around Ni lie much closer to 90°. As a consequence, the ligand field splitting should be increased. Electrochemical studies showed a quite negative oxidation potential for both complexes shifted cathodically approximately by 0.5 V compared with the parent [Ni(Phbpy)Br] system and the DFT-calculated HOMO energy is markedly higher. Remarkable impact on the catalytic activity for Negishi-type C‒C cross-coupling reactions was found upon variation of the R-substituents of the complexes [Ni(R-Phbpy)X], while the variation of X was much less important on the overall activity in keeping with the assumption that the splitting of the X coligand generates the primary active catalytic species. The C‒H activation/metalation was successfully developed for the two prototypical C^N^N and N^C^N platforms 6-(phenyl)-2,2’-bipyridine and N^C^N = 1,3-bis-(2-pyridyl)benzene. The complexes [Ni(Phbpy)X] and [Ni(PyPhPy)X] were synthesised in excellent (Phbpy) to good (PyPhPy) yields using a base-assisted method in inert solvents. As the Py^(Br-Ph)^Py protoligand was not available, this method paved the way to a direct comparison of NiX complexes of Phbpy and PyPhPy with fascinating insight into the impact of the carbanionic group shifted from the peripheral (Phbpy) to the central (PyPhPy) position. We could already study the systems with X = Cl and Br, H and CN.
Publications
- Exploring mechanisms in Ni terpyridine catalysed C‒C cross coupling reactions – a review, Inorganics 2018, 6, 18 (1–18)
Y. H. Budnikova, D. A. Vicic, A. Klein
(See online at https://doi.org/10.3390/inorganics6010018) - Nickel-alkyl complexes with a reactive PNC-pincer ligand, Eur. J. Inorg. Chem. 2018, 2408–2418
L. S. Jongbloed, N. Vogt, A. Sandleben, B. de Bruin, A. Klein, J. I. van der Vlugt
(See online at https://doi.org/10.1002/ejic.201800168) - Nitrogen–Nitrogen Bond Formation via a Substrate-Bound Anion at a Monomolecular Nickel Platform, Organometallics 2018, 37, 521–525
M. D Kosobokov, A. Sandleben, N. Vogt, A Klein, D. A. Vicic
(See online at https://doi.org/10.1021/acs.organomet.7b00887) - On the Redox Series of Cyclometalated Nickel Complexes [Ni((R)Ph(R’)bpy)Br]+/0/‒/2‒ (H‒(R)Ph(R’)bpy = substituted 6-phenyl-2,2’-bipyridine), Organometallics 2018, 37, 3332‒3341
A. Sandleben, N. Vogt, G. Hörner, A. Klein
(See online at https://doi.org/10.1021/acs.organomet.8b00559) - Cyclometalated Ni(II) complexes [Ni(N^C^N)X] of the tridentate 2,6-di(2-pyridyl)-phen-ide ligand, Organometallics 2020, 39, 2820−2829
L. Kletsch, G. Hörner, A. Klein
(See online at https://doi.org/10.1021/acs.organomet.0c00355) - Direct Base-Assisted C‒H Cyclonickelation of 6-Phenyl-2,2’-bipyridine, Molecules 2020, 25, 997 (1‒13)
N. Vogt, V. Sivchik, A. Sandleben, G. Hörner, A. Klein
(See online at https://doi.org/10.3390/molecules25040997)
