American Chemical Society
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Scrutinizing Low-Spin Cr(II) Complexes

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journal contribution
posted on 2012-06-18, 00:00 authored by Christopher 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 15 all contain a central Cr­(III) ion (SCr = 3/2), whereas complexes 68 contain a central low-spin (S = 1) Cr­(II) ion. Therefore, the electronic structures of 18 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.