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)]PF(6)](2) (1), Cr(phen)(3)]PF(6)](3) (2), CrCl(2)((t)bpy)(2)] (3), CrCl(2)(bpy)(2)]Cl(0.38)PF(6)](0.62) (4), Cr(TPP)(py)(2)] (5), Cr((t)BuNC)(6)]PF(6)](2) (6), CrCl(2)(dmpe)(2)] (7), and Cr(Cp)(2)] (8), where phen is 1,10-phenanthroline, (t)bpy is 4,4'-di-tert-butyl-2,2'-bipyridine, and TPP(2-) 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 (S(Cr) = (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 Cr(III)(phen(?))(phen(0))(2)]PF(6)](2), Cr(III)(phen(0))(3)]PF(6)](3), Cr(III)Cl(2)((t)bpy(?))((t)bpy(0))], Cr(III)Cl(2)(bpy(0))(2)]Cl(0.38)PF(6)](0.62), Cr(III)(TPP(3?-))(py)(2)], Cr(II)((t)BuNC)(6)]PF(6)](2), Cr(II)Cl(2)(dmpe)(2)], and Cr(II)(Cp)(2)], respectively, where (L(0)) and (L(?))(-) (L = phen, (t)bpy, or bpy) are the diamagnetic neutral and one-electron-reduced radical monoanionic forms of L, and TPP(3?-) is the one-electron-reduced doublet form of diamagnetic TPP(2-). Following our previous results that have shown Cr((t)bpy)(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.