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)

Abstract

New organonickel complexes [Ni­((R)­Ph­(R′)­bpy)­Br] carrying various substituted derivatives of the tridentate –C∧N∧N ligand 6-(phenyl-2-ide)-2,2′-bipyridine (−Phbpy) were synthesized from the precursor [Ni­(COD)2] (COD = 1,5-cyclooctadiene) and the protoligands (ligand precursors) Br–(R)­Ph­(R′)­bpy. Several synthetic routes for the protoligands were studied and compared. All new compounds have been analyzed and spectroscopically characterized. From several complexes crystal and molecular structures were obtained from XRD experiments. UV–vis absorption spectroscopy and detailed electrochemical measurements reveal the impact of the various substituents on the electronic structure of the complexes. Quantum chemical DFT calculations illustrate the composition of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) and support the assignment of the single-electron reduction and oxidation products as bpy-localized ligand radical species and transient nickel­(III) intermediates, respectively.

Direct Base-Assisted C‒H Cyclonickelation of 6-Phenyl-2,2′-bipyridine

“…However, further reaction and isolation yielded grey-green materials. This is in line with earlier reports on the title complex [29,30] and related work on the complex [Ni(Mes)(bpy)Br] (Mes = 2,4,6-trimethylphenyl; bpy = 2,2 -bipyridine), which decomposes in protic solvents yielding greenish octahedral Ni(II) species [46]. Avoiding protic solvents and working in vigorously dried solvents, we heated HPhbpy with NiBr 2 in toluene, but no yield was observed after 60 h at 130 • C ( Table 1, entry 1).…”

“…From previous work, it was known that the [Ni(Phbpy)Br] is somewhat stable and soluble in CH 2 Cl 2 ( 1 H NMR) and THF (reaction medium, crystallisation solvent). The complex has a characteristic red colour [29,30]. In the course of our investigation, we found that [Ni(Phbpy)Br] is insoluble in diethyl ether.…”

“…We worked out different procedures for the direct C-H cyclonickelation of 6-phenyl-2,2 -bipyridine (HPhbpy) to yield the previously reported red complex [Ni(Phbpy)Br] (Scheme 3) [29,30].…”

“…The yield was determined from the UV-vis absorption band of the product [Ni(Phbpy)Br] in THF (see Figure S5). The isolated material (183 mg, 0.495 mmol, 99%) was identified through its 1 H NMR spectrum [29,30].…”

“…Van Koten et al introduced two approaches for the synthesis of Ni(NˆCˆN)-pincer complexes: (a) transmetalation of a Au(I) precursor; or (b) oxidative addition of zero-valent nickel to the bromoaryl precursor [27]. Much later, Wolczanski et al directly metalated 2-phenylpyridine under harsh heating conditions (Scheme 1) [28], whereas Klein et al performed a two-step cyclometalation via Pd-directed ortho-bromination of 6-phenyl-2,2 -bipyridine followed by the oxidative addition of zero-valent nickel, akin to van Koten's approach [29,30]. Direct, base-assisted metalation with NiCl 2 was demonstrated by Gong and Song in a pincer topology [31] and, in a similar approach by Zargarian et al [32], it was found to be also feasible in macrocyclic NˆCˆNˆC or NˆCˆNˆN setups [33].…”