Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths

Innocent or non-innocent ligand?

Thomas Zell, Petr Milko, Kathlyn L. Fillman, Yael Diskin-Posner, Tatyana Bendikov, Mark A. Iron, Gregory Leitus, Yehoshoa Ben-David, Michael L. Neidig, David Milstein

Research output: Contribution to journalArticle

38 Citations (Scopus)

Abstract

A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe 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 FeI 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. Innocent till proved guilty! Metrical parameters for the assignment of oxidation states of bipyridine ligands are challenged. A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and fully characterized by various methods. Unusually short interpyridine C-C bonds were derived by X-ray diffraction (see scheme). The question if this is an effect of an intramolecular electron transfer or an effect of classical π-backbonding is addressed.

Original languageEnglish
Pages (from-to)4403-4413
Number of pages11
JournalChemistry - A European Journal
Volume20
Issue number15
DOIs
Publication statusPublished - Apr 7 2014

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Bond length
Iron
Ligands
Carbon Monoxide
Anions
Bearings (structural)
Negative ions
Spectroscopy
X ray diffraction
Electrons
Coordination Complexes
Metal complexes
Nuclear magnetic resonance spectroscopy
Cyclic voltammetry
Infrared spectroscopy
X ray photoelectron spectroscopy
Single crystals
Oxidation
Geometry

Keywords

  • bipyridine
  • iron
  • pi interactions
  • pincer ligands
  • redox chemistry

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths : Innocent or non-innocent ligand? / Zell, Thomas; Milko, Petr; Fillman, Kathlyn L.; Diskin-Posner, Yael; Bendikov, Tatyana; Iron, Mark A.; Leitus, Gregory; Ben-David, Yehoshoa; Neidig, Michael L.; Milstein, David.

In: Chemistry - A European Journal, Vol. 20, No. 15, 07.04.2014, p. 4403-4413.

Research output: Contribution to journalArticle

Zell, T, Milko, P, Fillman, KL, Diskin-Posner, Y, Bendikov, T, Iron, MA, Leitus, G, Ben-David, Y, Neidig, ML & Milstein, D 2014, 'Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths: Innocent or non-innocent ligand?', Chemistry - A European Journal, vol. 20, no. 15, pp. 4403-4413. https://doi.org/10.1002/chem.201304631
Zell, Thomas ; Milko, Petr ; Fillman, Kathlyn L. ; Diskin-Posner, Yael ; Bendikov, Tatyana ; Iron, Mark A. ; Leitus, Gregory ; Ben-David, Yehoshoa ; Neidig, Michael L. ; Milstein, David. / Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths : Innocent or non-innocent ligand?. In: Chemistry - A European Journal. 2014 ; Vol. 20, No. 15. pp. 4403-4413.
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abstract = "A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe M{\"o}ssbauer 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 {\AA} 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 FeI 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. Innocent till proved guilty! Metrical parameters for the assignment of oxidation states of bipyridine ligands are challenged. A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and fully characterized by various methods. Unusually short interpyridine C-C bonds were derived by X-ray diffraction (see scheme). The question if this is an effect of an intramolecular electron transfer or an effect of classical π-backbonding is addressed.",
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T1 - Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths

T2 - Innocent or non-innocent ligand?

AU - Zell, Thomas

AU - Milko, Petr

AU - Fillman, Kathlyn L.

AU - Diskin-Posner, Yael

AU - Bendikov, Tatyana

AU - Iron, Mark A.

AU - Leitus, Gregory

AU - Ben-David, Yehoshoa

AU - Neidig, Michael L.

AU - Milstein, David

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N2 - A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe 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 FeI 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. Innocent till proved guilty! Metrical parameters for the assignment of oxidation states of bipyridine ligands are challenged. A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and fully characterized by various methods. Unusually short interpyridine C-C bonds were derived by X-ray diffraction (see scheme). The question if this is an effect of an intramolecular electron transfer or an effect of classical π-backbonding is addressed.

AB - A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe 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 FeI 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. Innocent till proved guilty! Metrical parameters for the assignment of oxidation states of bipyridine ligands are challenged. A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and fully characterized by various methods. Unusually short interpyridine C-C bonds were derived by X-ray diffraction (see scheme). The question if this is an effect of an intramolecular electron transfer or an effect of classical π-backbonding is addressed.

KW - bipyridine

KW - iron

KW - pi interactions

KW - pincer ligands

KW - redox chemistry

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