Electronic and molecular structures of the members of the electron transfer series [Cr(tbpy)3]n (n = 3+, 2+, 1+, 0): an X-ray absorption spectroscopic and density functional theoretical study

Scarborough, C. C., Sproules, S. , Weyhermüller, T., DeBeer, S. and Wieghardt, K. (2011) Electronic and molecular structures of the members of the electron transfer series [Cr(tbpy)3]n (n = 3+, 2+, 1+, 0): an X-ray absorption spectroscopic and density functional theoretical study. Inorganic Chemistry, 50(24), pp. 12446-12462. (doi:10.1021/ic201123x)

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Abstract

The electron transfer series of complexes [Cr(tbpy)3]n(PF6)n (n = 3+, 2+, 1+, 0 (1–4)) has been synthesized and the molecular structures of 1, 2, and 3 have been determined by single-crystal X-ray crystallography; the structure of 4 has been investigated using extended X-ray absorption fine structure (EXAFS) analysis. Magnetic susceptibility measurements (4–300 K) established an S = 3/2 ground state for 1, an S = 1 ground state for 2, an S = 1/2 ground state for 3, and an S = 0 ground state for 4. The electrochemistry of this series in CH3CN solution exhibits three reversible one-electron transfer waves. UV–vis/NIR spectra and Cr K-edge X-ray absorption spectra (XAS) are reported. The same experimental techniques have been applied for [CrIII(tacn)2]Br3·5H2O (5) and [CrII(tacn)2]Cl2 (6), which possess an S = 3/2 and an S = 2 ground state, respectively (tacn = 1,4,7-triazacyclononane, a tridentate, pure σ-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K4[CrII(CN)6]·10H2O (S = 1) (7) and K3[CrIII(CN)6] (S = 3/2) (8) have also been recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. The molecular and electronic structures of the anionic members of the series [Cr(bpy)3]1–,2–,3– have also been calculated. It is unequivocally shown that all members of the electron transfer series 1–4 and [Cr(bpy)3]n (n = 3+, 2+, 1+, 0, 1–, 2, 3−) possess a central CrIII ion ((t2g)3, S = 3/2). The three N,N′-coordinated neutral (bpy0) ligands in the trication 1 and [CrIII(bpy)3]3+ are one-electron reduced in a stepwise fashion to localized one, two, and three π-radical anions (bpy•)1– in the dicationic, monocationic, and neutral species, respectively. Complexes 2 and [Cr(bpy)3]2+ cannot be described as low-spin CrII species; they are in fact best described as [CrIII(tbpy•)(tbpy0)2]2+ and [CrIII(bpy•)(bpy0)2]2+ species. Further one-electron reductions yield one, two, and three diamagnetic (bpy2–)2– dianions in the mono-, di-, and trianion. Thus, [CrIII(bpy2–)3]3– is a normal Werner-type CrIII (!) species. In all complexes containing (bpy•)1– ligands, the ligand spins are strongly antiferromagnetically coupled to the spins of the central CrIII ion (d3, SCr = 3/2) affording the observed ground states given above. Thus, all redox chemistry of [Cr(bpy)3]n complexes is ligand-based and documents that the ligand 2,2′-bipyridine is a redox noninnocent ligand; it exists in three oxidation levels in these complexes: as N,N′-coordinated neutral (bpy0), monoanionic π-radical (bpy•)1–, and diamagnetic dianionic (bpy2–)2–.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Sproules, Dr Stephen
Authors: Scarborough, C. C., Sproules, S., Weyhermüller, T., DeBeer, S., and Wieghardt, K.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Inorganic Chemistry
Publisher:American Chemical Society
ISSN:0020-1669
ISSN (Online):1520-510X

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