title: 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-Me2-, 3,5-Me2-, 2,4,6-Me3-, 2,6-iPr2-, 3,5-(OMe)2-, 2-NO2-4-Me-, 4-NO2-, 2-CF3-, and 2-CF3-6-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-[(PPh3)2Ni(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 NPy^Nmethanimineligand, 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; NPy^NPy) and N,N-diaryl-substituted aliphatic α-diimines (Nmethanimine^Nmethanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Biewer
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Claudia
authors.initials: C
authors.lastname: Hamacher
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Andre
authors.initials: A
authors.lastname: Kaiser
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Vogt
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Aaron
authors.initials: A
authors.lastname: Sandleben
authors.affiliation: Department of Chemistry, Lehigh University , 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States.
authors.forename: Mason T
authors.initials: MT
authors.lastname: Chin
authors.affiliation: Department of Chemistry, Lehigh University , 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States.
authors.forename: Siqi
authors.initials: S
authors.lastname: Yu
authors.affiliation: Department of Chemistry, Lehigh University , 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Vicic
authors.affiliation: Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln , Greinstraße 6, D-50939 Köln, Germany.
authors.forename: Axel
authors.identifier: http://orcid.org/0000-0003-0093-9619
authors.initials: A
authors.lastname: Klein