posted on 2012-06-18, 00:00authored byChristopher C. Scarborough, Stephen Sproules, Christian J. Doonan, Karl S. Hagen, Thomas Weyhermüller, Karl Wieghardt
The oxidation state of the chromium center in the following
compounds
has been probed using a combination of chromium K-edge X-ray absorption
spectroscopy and density functional theory: [Cr(phen)3][PF6]2 (1), [Cr(phen)3][PF6]3 (2), [CrCl2(tbpy)2] (3), [CrCl2(bpy)2]Cl0.38[PF6]0.62 (4), [Cr(TPP)(py)2] (5), [Cr(tBuNC)6][PF6]2 (6), [CrCl2(dmpe)2] (7), and [Cr(Cp)2] (8), where phen is 1,10-phenanthroline, tbpy is 4,4′-di-tert-butyl-2,2′-bipyridine, and TPP2– is doubly
deprotonated 5,10,15,20-tetraphenylporphyrin. The X-ray crystal structures
of complexes 1, [Cr(phen)3][OTf]2 (1′), and 3 are reported. The X-ray
absorption and computational data reveal that complexes 1–5 all contain a central Cr(III) ion (SCr = 3/2), whereas complexes 6–8 contain a central low-spin (S = 1) Cr(II) ion. Therefore, the electronic structures
of 1–8 are best described as [CrIII(phen•)(phen0)2][PF6]2, [CrIII(phen0)3][PF6]3, [CrIIICl2(tbpy•)(tbpy0)], [CrIIICl2(bpy0)2]Cl0.38[PF6]0.62, [CrIII(TPP3•–)(py)2], [CrII(tBuNC)6][PF6]2, [CrIICl2(dmpe)2], and [CrII(Cp)2], respectively, where
(L0) and (L•)− (L =
phen, tbpy, or bpy) are the diamagnetic
neutral and one-electron-reduced radical monoanionic forms of L, and
TPP3•– is the one-electron-reduced doublet
form of diamagnetic TPP2–. Following our previous
results that have shown [Cr(tbpy)3]2+ and [Cr(tpy)2]2+ (tpy
= 2,2′:6′,2″-terpyridine) to contain a central
Cr(III) ion, the current results further refine the scope of compounds
that may be described as low-spin Cr(II) and reveal that this is a
very rare oxidation state accessible only with ligands in the strong-field
extreme of the spectrochemical series.