Binuclear Arylnickel Complexes with Bridging Bis(arylimino)‐1,4‐pyrazine Ligands

Abstract

Binuclear organometallic nickel complexes of the type [(μ‐NN){Ni(Mes)Br}2] {NN = α‐diimine chelate ligand of the type 2,5‐bis[1‐(aryl)iminoethyl]pyrazine; Mes = mesityl = 2,4,6‐trimethylphenyl} have been prepared and characterised electrochemically and spectroscopically in detail. A combination of NMR spectroscopy and quantum chemical calculations allowed the assignment of stereoisomers and their relative stability. The long‐wavelength absorptions (600–1000 nm) assignable to charge‐transfer transitions reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low‐lying π* orbitals. The reversible reductive electrochemistry yields stable radical anionic complexes with mainly ligand‐centred spin density as shown by electron paramagnetic resonance (EPR) spectroscopy and UV/Vis spectroelectrochemistry of the free ligands and their nickel complexes in combination with DFT calculations. Preliminary investigations of the complexes as catalysts in Negishi cross‐coupling reactions gave promising results.

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?

“…Similar values have been found for other Ni-Mes complexes. [30][31][32]36 The tilt angle between the N-aryl cores and the binding plane ranges from approximately 50 to 90°w ith large angles up to 90°for the cis derivatives with the Br coligands but only 48°for cis-[(PyMA)Ni(Mes)Cl], close to the value found for the uncoordinated ligand 4-NO 2 -PyMA. A dihedral angle of about 50°has been also observed in the bpip (Chart 1) ligands, and this angle seems to represent an electronically favored orientation.…”

“…16,35 (iv) The complexes [(N^N)Ni(Mes)Br] and the binuclear derivatives [(μ-N^N){Ni(Mes)Br} 2 ] were active in Negishi-like C−C coupling reactions. 36 In this contribution, we report of the use of unsymmetric N^N′ ligands of the N-aryl-1-(pyridin-2-yl)methanimine (PyMA) type (Chart 1).…”

“…37−39 They are also related to the bridging bpip ligands, which have recently been used to generate binuclear complexes. 36 From general considerations, the pyridine moiety should be a stronger σ-donating ligand than the N-arylmethanimine function, and, in turn, the latter should be a better π acceptor. Such unsymmetric PyMA [alternatively called N-aryl-1-(pyridin-2-ylmethylene)aniline] ligands have been previously used in organonickel chemistry, 40,41 with applications in polymerization catalysis.…”

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)

“…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 .…”