Iron Dicarbonyl Complexes Featuring Bipyridine‐Based PNN Pincer Ligands with Short Interpyridine CC Bond Lengths: Innocent or Non‐Innocent Ligand?

Abstract

A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy ((1) H, (13) C, (15) N, (31) P), IR spectroscopy, cyclic voltammetry, (57) Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)2 ] (5: R-PNN=tBu-PNN=6-[(di-tert-butylphosphino)methyl]-2,2'-bipyridine, 6: R-PNN=iPr-PNN=6-[(diisopropylphosphino)methyl]-2,2'-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2'-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)2 ] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO. The analogous Ph-substituted complex 7 was synthesized by a reaction of the free ligand with iron pentacarbonyl. The low-spin complexes 5-7 (S=0) are diamagnetic and have distorted trigonal bipyramidal structures in solution, whereas in the solid state the geometries around the iron are best described as distorted square pyramidal. Compared to other structurally characterized complexes with these PNN ligands, shortened interpyridine CC bonds of about 1.43 Å were measured. A comparison with known examples, theoretically described as metal complexes bearing bipyridine π-radical anions (bpy(.) (-) ), suggests that the complexes can be described as Fe(I) complexes with one electron antiferromagnetically coupled to the ligand-based radical anions. However, computational studies, at the NEVPT2/CASSCF level of theory, reveal that the shortening of the CC bond is a result of extensive π-backbonding of the iron center into the antibonding orbital of the bpy unit. Hence, the description of the complexes as Fe(0) complexes with neutral bipyridine units is the favorable one.

Bis(6‐methylene‐2,2′‐bipyridine)phenylphosphine—A Flexible Ligand for the Construction of Trinuclear Coinage‐Metal Complexes

“…Related mono(bipyridylmethyl)diphenylphosphines were also obtained by salt metathesis from ClPR 2 and the respective lithiated methylbipyridine as shown by Milstein et al . In our hands, reactions of LiCH 2 (bipy) (bipy=6′‐2,2′‐bipyridyl) and Cl 2 PPh in THF or Et 2 O at low temperature were not successful.…”

Electronic Structure of Manganese Complexes of the Redox‐Non‐innocent Tetrazene Ligand and Evidence for the Metal‐Azide/Imido Cycloaddition Intermediate

“…However, ligand/metal charges are a continuum phenomenon, and the assignment of redoxnon-innocent behavior versus ligand p-donor/acceptor behavior is often not clear. [13] Recently, the Neidig and Milstein groups demonstrated that even seemingly definitive bond length perturbations can be misleading.…”

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…[9][10][11][12] For example, Milstein and coworkers demonstrated the role of PCP and PNP pincer ligands in catalysis and small molecule activation and proposed mechanisms in which metal-ligand cooperation plays a major role (Chart 1c). [13][14][15][16][17][18][19][20][21] In addition, Chirik and coworkers exploited the potential of base metals in catalytic transformations by using flexible and redox non-innocent ligands to overcome the limitation of first row transition metals that usually undergo single electron processes (Chart 1d). [22][23][24][25] Besides traditional phosphine and nitrogen donors, an arene is suitable for metal-ligand cooperation as well: first, the coordination mode of the arene can vary from  6 to  2 according to the degree of delocalization.…”