Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2′-Bipyridine and N,N-Diaryl-α-diimines?

Abstract

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me-, 3,5-Me-, 2,4,6-Me-, 2,6-Pr-, 3,5-(OMe)-, 2-NO-4-Me-, 4-NO-, 2-CF-, and 2-CF-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh)Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the N^N ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; N^N) and N,N-diaryl-substituted aliphatic α-diimines (N^N) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

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)

“…While the complexes show good stability toward oxygen, the problems virulent in the NMR measurements (see below) indicate some tendency to hydrolysis. NMR signals crowd in the spectral region typical of diamagnetic compounds, corroborating the expected square planar geometry with a diamagnetic character 9 of the organonickel(II) complexes [Ni((R)Ph(R′)bpy)Br]. However, some of the 1 H NMR spectra show marked line broadening, which interferes with signal assignment.…”

“…Prolonged electrolysis revealed the cleavage of the bromide coligand resulting in an uncharged radical complex with nominally a vacant coordination site. Very similar to what has been reported for related organometallic nickel complexes carrying redox-active diimine ligands and halide coligands, − the highly plausible stabilization of the resulting complexes by solvent molecules was postulated. − As reduction-induced solvolysis represents a key step in reductively activated C–C cross-coupling reactions catalyzed by Ni­(diimine) systems (Negishi or Kumada type), incremental control of the reactivity of the resulting species is of special interest. − Compared with the isoelectronic terpy , (terpy = 2,2′:6′,2″-terpyridine) system the anionic – Phbpy ligand should allow for a more flexible electronic fine-tuning through substitution on both the presumably σ-donating – Ph part and the potentially π-accepting bpy part (Chart ).…”

“…The first complex of the tridentate cyclometalating ligand 6-(phenyl-2-ide)-2,2′-bipyridine ( − Phbpy) was the Pd­(II) complex [Pd­(Phbpy)­Cl] reported in 1990 . In the following years further complexes containing the platinum group metals Ir, Pt, and Ru were obtained through orthometalation/C–H bond activation of the H–Phbpy protoligand (ligand precursor) or by transmetalation reactions. − The resulting ortho-metalated complexes have found applications as photocatalysts or triplet emitting materials. − Only in 2014, the first Phbpy complex of the base metal nickel [Ni­(II)­(Phbpy)­Br] could be obtained from the Ni(0) source [Ni­(COD) 2 ] (COD = 1,5-cyclooctadiene) through chelate supported oxidative addition of the Br–Phbpy protoligand. − Akin to the previous complexes of Ir, Pt, and Ru the nickel­(II) analogue revealed reversible one-electron oxidation and reduction. UV–vis spectroelectrochemistry allowed a preliminary assignment to Ni­(II)/Ni­(III) and (Phbpy) − /(Phbpy) 2– redox pairs .…”

Crystal structure of (E)-4-bromo-N-(pyridin-2-ylmethylene)aniline, C12H9BrN2

“…the figure). The compound exhibits an E conformation around the N2=C6 bond which is comparable to related compounds in literature [10][11][12][13][14]. The analogous compound with a methyl group instead of the bromo substituent forms an isotypic structure [15].…”

Coupling of Challenging Heteroaryl Halides with Alkyl Halides via Nickel-Catalyzed Cross-Electrophile Coupling

“…We anticipate that these catalysts will also prove useful for the growing number of reactions that use bipyridine-ligated nickel catalysts . Studies to understand why these N -cyanocarboxamidine catalysts perform better are ongoing …”