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Syntheses, Crystal Structures and Ligand Field Properties of Iron(II) Complexes with PNP Ligands: Origin of Large Ligand Field by a Phosphorous Donor Atom

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Abstract

New iron(II) complexes were synthesized with two tridentate hybrid ligands having phosphorous and nitrogen donor sites, in order to quantitatively estimate the difference of the ligand-field strengths of phosphorous and nitrogen donor sites in cationic metal complexes. Iron(II) complexes with bis(dimethylphosphinoethyl)amine (PNP) and 2,6-bis(diphenylphosphinomethyl)pyridine (PpyP) ligands crystallized as un-symmetric facial-[Fe(PNP)2](PF6)2·CH3NO2 and mer-[Fe(PpyP)2](CF3SO3)2, respectively, as expected from the steric congestion and from the tendency to avoid the mutual trans influence between two phosphorous donor sites. Both complexes are in the low-spin electronic state up to 400 K. The pseudo-D 4h coordination geometry of the PpyP complex made it possible to separate axial (2 × N) and equatorial (4 × P) contributions to the overall ligand-field by means of a spectrometric method: the difference in the ligand-field strengths by the equatorial Ph2P-donor sites and by the axial 2,6-disubstituted pyridine donor sites is ca. 13,200 cm−1. A significantly reduced inter-electronic repulsion parameter (425 cm−1 for both PNP and PpyP complexes) from the value of the free ion (1,060 cm−1) indicates covalent interaction between the Fe(II) and P atoms even in these cationic metal complexes. It is shown that the degree of covalency as well as the coordination bond strengths between various metal ions and phosphorous/nitrogen donor atoms is successfully explained by the relative energy levels of interacting atomic orbitals calculated on the basis of the Thomas–Fermi–Dirac potential.

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Authors and Affiliations

  1. Research Center for Materials Science, Nagoya University, Chikusa-ku, Nagoya, 464-8602, Japan

    Takuya Mabe, Hiroshi Yamaguchi, Masayuki Fujiki, Kyoko Noda & Hideo D. Takagi

  2. School of Science and Engineering, Waseda University, Tokyo, 169-8555, Japan

    Koji Ishihara

  3. Department of Chemistry, Aichi University of Education, Kariya, 448-8542, Japan

    Masahiko Inamo

  4. Department of Chemistry, Faculty of Science, Assiut University, Assiut, 71516, Egypt

    Refat Moustafa Hassan

  5. Department of Chemistry, Konan University, Kobe, 658-8501, Japan

    Satoshi Iwatsuki

  6. Department of Chemistry, Okayama University, Okayama, 700-8530, Japan

    Takayoshi Suzuki

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  1. Takuya Mabe
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  2. Hiroshi Yamaguchi
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  3. Masayuki Fujiki
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  10. Hideo D. Takagi
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Correspondence to Hideo D. Takagi.

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Mabe, T., Yamaguchi, H., Fujiki, M. et al. Syntheses, Crystal Structures and Ligand Field Properties of Iron(II) Complexes with PNP Ligands: Origin of Large Ligand Field by a Phosphorous Donor Atom. J Solution Chem 43, 1574–1587 (2014). https://doi.org/10.1007/s10953-014-0182-3

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