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((t)bpy)(2)] (3), [CrCl2(bpy)(2)]Cl-0.38[PF6](0.62) (4), [Cr(TPP)(py)(2)] (5), [Cr((BuNC)-Bu-t)(6)][PF6](2) (6), [CrCl2(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 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 (S-Cr = 3/2), whereas complexes 6-8 contain a central low-spin (S = 1) Cr(11) ion. Therefore, the electronic structures of 1-8 are best described as [Cr-III(phen(center dot))(phen(0))(2)][PF6](2), [Cr-III(phen(0))(3)][PF6](3), [(CrCl2)-Cl-III((t)bpy(center dot))((t)bpy(0))], [(CrCl2)-Cl-III(bpy(0))(2)]Cl-0.38[PF6](0.62), [Cr-III(TPP3(center dot-)). (py)(2)], [Cr-II((t)BuNc)(6)][PF6](2), [(CrCl2)-Cl-II(dmpe)(2)], and [Cr-II(Cp)(2)], respectively, where (L-0) and (L-center dot)- (L = phen, (t)bpy, or bpy) are the diamagnetic neutral and one-electron-reduced radical monoanionic forms of L, and TPP3(center dot-) is the one-electron-reduced doublet form of diamagnetic TPP2-. 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.