Heterolytic CH-bond activation in the synthesis of Ni{(2-aryl-κC2)pyridine-κN}2 and derivatives

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

“…The observation of high reaction temperatures suggests a high activation barrier for the direct cyclonickelation of N-donor/aryl systems. Similarly, cyclonickelation to form CˆN or NˆCˆN chelates required harsh heating [28,31,32]. High reaction temperatures seem to be the price for starting from rather unreactive Ni(II) precursors and a not very reactive C(sp 2 )-H bond.…”

“…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].…”

1H, 13C and 15N nuclear magnetic resonance coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with phenylpyridines

“…[13] The deshielding of H (6) [5,6,8 -10] and in the related Ni(II) bromide complex [Ni(2ppy)(2ppy * )Br] − H(6) coord = 0.79 ppm (in benzene-d 6 ). [41] The same phenomenon is predominant for the analogous H(6) atom in Au(III), Pd(II) and Pt(II) chloride complexes with many other 2-aryl pyridine derivatives, although the magnitudes of the H (6) coord parameters are usually smaller than in case of 2ppy (Table 2S, (6) coord = 0.08 ppm, [42] [Au(2-benzoyl-py)Cl 3 ]− H(6) coord = 0.01 ppm, [43] [Au(2-phenoxy-py)Cl 3 ] − H(6) coord = −0.06 ppm and [Au(2-phenylsulfanyl-py)Cl 3 ] − H(6) coord = −0.01 ppm, [44] trans-[Pd(2-benzyl-py) 2 Cl 2 ] − H(6) coord = 0.19 ppm, [45] trans-[Pd(2-(1-phenylethyl)-py) 2 Cl 2 ] − H(6) coord = 0.52 ppm, [46] trans-[Pd(2-(2-naphthyl)-py) 2 Cl 2 ] − H(6) coord = 0.20 ppm, [47] [Pd(2-(8-quinolyl)-py)(P(CH 2 CH 3 ) 3 (2) coord = 0.15 ppm, [37] although not for trans-[Pd(4-phenylpyrimidine) 2 Cl 2 ] (4-phenylpyrimidine being an analogue of 2ppy), where H(2) coord = 0.00 ppm. [49] The above-determined H (6) coord parameters for 2-benzoyl-py, 2-phenoxy-py and 2-phenylsulfanyl-py coordination compounds (relatively small or even negative) are more uncertain as we have compared the complexes in DMSO-d 6 to the free ligands in CDCl 3 .…”

Electrochemical Insight into Mechanisms and Metallocyclic Intermediates of C−H Functionalization

“…Nickel complexes are interesting for the possibility of accepting and donating electrons in a metal‐centered manner, forming compounds in different oxidation states, Ni(IV)/Ni(III)/Ni(II)/Ni(I)/Ni(0), and in the case of non‐innocent ligands, additional transfers of electrons can also be observed with the participation of the latter. However, there are not very many isolated and characterized nickel cycles, which are assumed in ligand‐directed reactions of C−H substitution and determine the selectivity of the reaction, [44–61] and the proven structures as intermediates of specific reactions are even fewer [44] . Electrochemical and ESR data for nickelacycles are rare and incomplete, [44–48] although electrochemical properties are very important for understanding and explaining their reactivity, selectivity, in particular, in oxidative C(sp2)−H functionalization, and in recent years researchers have been trying to fill this gap.…”