Scrutinizing Low-Spin Cr(II) Complexes

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.