Electronic and Molecular Structures of the Members of the Electron Transfer Series [Cr(tbpy)3]n (n=3+, 2+, 1+, 0): An X-ray Absorption Spectroscopic and Density Functional Theoretical Study

The electron transfer series of complexes [Cr((t)bpy)(3)](n)(PF6)(n) (n = 3+, 2+, 1+, 0 (1-4)) has been synthesized and the molecular structures of 1, 2, and 3 have been determined by single-crystal X-ray crystallography; the structure of 4 has been investigated using extended X-ray absorption fine structure (EXAFS) analysis. Magnetic susceptibility measurements (4-300 K) established an S = 3/2 ground state for 1, an S = 1 ground state for 2, an S = 1/2 ground state for 3, and an S = 0 ground state for 4. The electrochemistry of this series in CH3CN solution exhibits three reversible one-electron transfer waves. UV-vis/NIR spectra and Cr K-edge X-ray absorption spectra (XAS) are reported. The same experimental techniques have been applied for [Cr-III(tacn)(2)]Br-3 center dot 5H(2)O (5) and [Cr-II(tacn)(2)]Cl-2 (6), which possess an S = 3/2 and an S = 2 ground state, respectively (tacn = 1,4,7-triazacyclononane, a tridentate, pure sigma-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K-4[Cr-II(CN)(6)]center dot 10H(2)O (S = 1) (7) and K-3[Cr-III(CN)(6)] (S = 3/2) (8) have also been recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. The molecular and electronic structures of the anionic members of the series [Cr(bpy)(3)](1-,2-,3-) have also been calculated. It is unequivocally shown that all members of the electron transfer series 1-4 and [Cr(bpy)(3)](n) (n = 3+, 2+, 1+, 0, 1-, 2, 3-) possess a central Cr-III ion ((t(2g))(3), S = 3/2). The three N,N'-coordinated neutral (bpy(0)) ligands in the trication 1 and [Cr-III(bpy)(3)](3+) are one-electron reduced in a stepwise fashion to localized one, two, and three pi-radical anions (bpy(center dot))(1-) in the dicationic, monocationic, and neutral species, respectively. Complexes 2 and [Cr(bpy)(3)](2+) cannot be described as low-spin Cr-II species; they are in fact best described as [Cr-III((t)bpy(center dot))((t)bpy(0))(2)](2+) and [Cr-III(bpy(center dot))(bpy(0))(2)](2+) species. Further one-electron reductions yield one, two, and three diamagnetic (bpy(2-))(2-) dianions in the mono-, di-, and trianion. Thus, [Cr-III(bpy(2-))(3)](3-) is a normal Werner-type Cr-III (!) species. In all complexes containing (bpy(center dot))(1-) ligands, the ligand spins are strongly antiferromagnetically coupled to the spins of the central Cr-III ion (d(3), S-Cr = 3/2) affording the observed ground states given above. Thus, all redox chemistry of [Cr(bpy)3]0 complexes is ligand-based and documents that the ligand 2,2'-bipyridine is a redox noninnocent ligand; it exists in three oxidation levels in these complexes: as N,N'-coordinated neutral (bpy(0)), monoanionic pi-radical (bpy(center dot))(1-), and diamagnetic dianionic (bpy(2-))(2-).