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Bimetallic iron-iron and iron-zinc complexes of the redox-active ONO pincer ligand.

ABSTRACT: A new bimetallic platform comprising a six-coordinate Fe(ONO)₂ unit bound to an (ONO)M (M = Fe, Zn) has been discovered ((ONO^cat)H₃ = bis(3,5-di-tert-butyl-2-phenol)amine). Reaction of Fe(ONO)₂ with either (ONO^cat)Fe(py)₃ or with (ONO^q)FeCl₂ under reducing conditions led to the formation of the bimetallic complex Fe₂(ONO)₃, which includes unique five- and six-coordinate iron centers. Similarly, the reaction of Fe(ONO)₂ with the new synthon (ONO^sq˙)Zn(py)₂ led to the formation of the heterobimetallic complex FeZn(ONO)₃, with a six-coordinate iron center and a five-coordinate zinc center. Both bimetallic complexes were characterized by single-crystal X-ray diffraction studies, solid-state magnetic measurements, and multiple spectroscopic techniques. The magnetic data for FeZn(ONO)₃ are consistent with a ground state S = 3/2 spin system, generated from a high-spin iron(ii) center that is antiferromagnetically coupled to a single (ONO^sq˙)^2- radical ligand. In the case of Fe₂(ONO)₃, the magnetic data revealed a ground state S = 7/2 spin system arising from the interactions of one high-spin iron(ii) center, one high-spin iron(iii) center, and two (ONO^sq˙)^2- radical ligands.

SUBMITTER: Wong JL

PROVIDER: S-EPMC5530940 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett ?(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

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Bimetallic iron-iron and iron-zinc complexes of the redox-active ONO pincer ligand.

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