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Structure[edit]

Fig. 1 Fe (II) PNP ligand

Iron (II) PNP is a type of pincer ligand, a chelating agent that tightly binds to three other structures. Since the structure is essentially a neutral pyridine structure and flanking phosphines coordinating with an iron center, the PNP in Iron (II) PNP represents the phosphate-nitrogen-phosphate coordination to Iron (Fig. 1). The pincer ligand electron-donating group and its substituents can be ethers, phosphines, amines, phosphites, thioethers, arsines, selenoethers, and N-heterocyclic carbenes[1]. In the case of PNP pincer ligands, the electron-donating groups are phosphines. In solid state, PNP Ligands can have two different conformations in which the PNP is bound to the iron center in a meridional geometry or in a facial geometry. Although both conformations are present, PNP ligands are typically in the meridional geometry a majority of the time.[2]

Synthesis[edit]

Fig. 2 Synthesis of Fe (II) PNP

In order to synthesize an iron PNP pincer ligand, a PNP ligand must be constructed first. This is done by taking 2,6-diaminopyridine and adding 2 equivalents of a tri-substituted phosphine group with chlorine and two bulky R groups, such as phenol, isopropyl, or tert-butyl.[1] Also within this reaction mixture two equivalents of a strong base, preferably triethyl amine or n-butyl lithium, and toluene or THF are added to the mixture, which can take upwards of 16 hours to complete. The reaction is performed at temperatures ranging from -78ºC to 80ºC, depending upon which solvents are being used. This causes a phosphine group to attach to each of the amino groups on the pyridine. Next, the N,N’-bis(di-R-phosphino)-2,6-diaminopyridine is then  added to iron (II) chloride.[3] The reaction is done in a THF solvent, and stirred for 12 hours at room temperature to complete the reaction (Fig. 2).[4] This leaves the iron bound to the nitrogen on pyridine, as well as the two phosphorous groups.  The product of these reactions generally result in a red or yellow powder, which can be obtained simply through filtration, and washed with an n­-pentane solution. If purification is necessary, the solvent can be dissolved in CH­­2Cl2. Then, a chromatography column can be used by extracting the pure iron complex with an ethyl acetate and petroleum ether mixture, with ratios that vary depending upon which R groups are bound. The product is then recrystallized through the use of the n-pentane or an n­­-hexane mixture.

Uses[edit]

Fig. 3 Iron (II) PNP can act as a CO sponge

Iron centered PNP ligands with a double halogenated complex (most likely Cl and/or Br) serve as efficient CO sponges. This ideal molecule is reactive in both a solid state and in solution and can thus serve as a catalyst within both phases[5]. The addition of CO to the PNP-complex causes a spin state change, which causes a color change from a light yellow to a deep red. This change of color can quite possibly serve as a sensor in environments where CO might be a present2[6]. If an Fe-PNP complex has successfully captured CO twice, it can begin a catalytic function of H2 bond cleavage in the presence of Zinc (Fig. 3)[7]. This H2 capture can serve as an effective catalyst in converting ketones to alcohols and CO2 to a formate salt4[8].

  1. ^ a b Benito-Garagorri, David; Kirchner, Karl. "Modularly Designed Transition Metal PNP and PCP Pincer Complexes based on Aminophosphines: Synthesis and Catalytic Applications". Accounts of Chemical Research. 41 (2): 201–213. doi:10.1021/ar700129q.
  2. ^ Fillman, Kathlyn L.; Bielinski, Elizabeth A.; Schmeier, Timothy J.; Nesvet, Jared C.; Woodruff, Tessa M.; Pan, Cassie J.; Takase, Michael K.; Hazari, Nilay; Neidig, Michael L. "Flexible Binding of PNP Pincer Ligands to Monomeric Iron Complexes". Inorganic Chemistry. 53 (12): 6066–6072. doi:10.1021/ic5004275.
  3. ^ Holzhacker, Christian, Berthold Stöger, Maria Deus Carvalho, Liliana P. Ferreira, Ernst Pittenauer, Günter Allmaier, Luis F. Veiros, Sara Realista, Adrià Gil, Maria José Calhorda, Danny Müller, and Karl Kirchner. "Synthesis and Reactivity of TADDOL-based Chiral Fe( Ii ) PNP Pincer Complexes-solution Equilibria between κ 2 P,N- and κ 3 P,N,P-bound PNP Pincer Ligands." Dalton Trans. 44.29 (2015): 13071-3086.
  4. ^ Zhao, Hua, Xiaoyan Li, Shumiao Zhang, and Hongjian Sun. "Synthesis and Characterization of Iron, Cobalt, and Nickel [PNP] Pincer Amido Complexes by N-H Activation." Zeitschrift Für Anorganische Und Allgemeine Chemie Z. Anorg. Allg. Chem. 641.14 (2015): 2435-439.
  5. ^ Carbonyl Complexes Featuring Small to Bulky PNP Pincer Ligands – Facile Substitution of κ2P,N-Bound PNP Ligands by Carbon Monoxide. Glatz, M., Holzhacker, C., Bichler, B., Mastalir, M., Stöger, B., Mereiter, K., Weil, M., Veiros, L. F., Mösch-Zanetti, N. C. and Kirchner, K. (2015), FeII  Eur. J. Inorg. Chem., 2015: 5053–5065. doi:10.1002/ejic.201500646
  6. ^ Carbon Monoxide Ligand-Exchange Reaction of Triruthenium Cluster Complexes Induced by Photosensitized Electron Transfer:  A New Type of Photoactive CO Color Sensor. Mitsunari Itou,†, Yasuyuki Araki,†, Osamu Ito,*,† and, and Hiroaki Kido‡ Inorganic Chemistry 2006 45 (16), 6114-6116 DOI: 10.1021/ic060751n
  7. ^ Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal–Ligand Cooperation. Bernhard Bichler, Christian Holzhacker, Berthold Stöger, Michael Puchberger, Luis F. Veiros, and Karl Kirchner. Organometallics 2013 32 (15), 4114-4121. DOI: 10.1021/om400241x
  8. ^ Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Chem. Eur. J. 2012, 18, 7196−7209
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