Neutral bis(α-iminopyridine)metal complexes of the first-row transition ions (Cr, Mn, Fe, Co, Ni, Zn) and their monocationic analogues: Mixed valency involving a redox noninnocent ligand system

Connie C. Lu, Eckhard Bill, Thomas Weyhermüller, Eberhard Bothe, Karl Wieghardt

Research output: Contribution to journalArticle

215 Scopus citations

Abstract

A series of bis(α-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic π radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural characterizations, the neutral complexes are best described as possessing a divalent metal center and two monoanionic π radicals of the α-iminopyridine. The neutral M(L)2 compounds undergo ligand-centered, one-electron oxidations generating a second series, [(Lx)2M(THF)][B(ArF)4] [where L x represents either the neutral α-iminopyridine (L)0 and/or its reduced π radical anion (L)-]. The cationic series comprise mostly mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L) -, and the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe and Co complexes exhibit Robin-Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr, and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV-vis spectroscopy. The derealization versus localization of the ligand radical is determined only by the nature of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a monovalent metal center or [(L) 2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between high spin and intermediate spin (SFe = 2 ↔ SFe = 1), is described for the complex [(L )(L)Fe(THF)][B(ArF)4], which consequently is characterized by the overall spin equilibrium (Stot = 3/2 ↔ Stot = 1/2). The two different spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR spectroscopy, zero-field and applied-field Mössbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.

Original languageEnglish (US)
Pages (from-to)3181-3197
Number of pages17
JournalJournal of the American Chemical Society
Volume130
Issue number10
DOIs
StatePublished - Mar 12 2008

Fingerprint Dive into the research topics of 'Neutral bis(α-iminopyridine)metal complexes of the first-row transition ions (Cr, Mn, Fe, Co, Ni, Zn) and their monocationic analogues: Mixed valency involving a redox noninnocent ligand system'. Together they form a unique fingerprint.

  • Cite this