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

Abstract

The electron transfer series of complexes [Cr((t)bpy)(3)](n)(PF(6))(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 CH(3)CN 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)·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 σ-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K(4)[Cr(II)(CN)(6)]·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 π-radical anions (bpy(•))(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(•))((t)bpy(0))(2)](2+) and [Cr(III)(bpy(•))(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(•))(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)](n) 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 π-radical (bpy(•))(1-), and diamagnetic dianionic (bpy(2-))(2-).

Origin-independent calculation of quadrupole intensities in X-ray spectroscopy

“…For instance, the analysis of the prepeak intensities in ligand K-edge XAS spectra of transition metal complexes (i.e., excitations from the ligand 1s to metal d orbitals) provides insights into the covalent contributions to metal-ligand bonding [40][41][42][43][44][45][46]. Another example is metal K-edge XAS, probing excitations from metal 1s to d orbitals, which can be used to assign coordination numbers [47][48][49] and to probe details of the metal-ligand bonding mechanisms [18,44,50]. Recently, we have demonstrated that the prepeaks in Fe K-edge XAS spectra of ferrocene derivatives are sensitive to subtle differences in the electronic structure at the iron atom, which are induced by substituents at the cyclopentadienyl rings, i.e., beyond the first coordination shell of the metal center [51].…”

Cobalt in a Bis-β-diketiminate Environment

“…For example, bipyridine can coordinate to transition metals in its neutral, monoanionic, or dianionic form. 20 Detailed X-ray structural analysis has shown that the reduction of bipyridine corresponds to a successive increase in the C1−C1′ bond length by roughly 0.05 Å for each electron added.…”

[Cr(ddpd)2]3+: A Molecular, Water‐Soluble, Highly NIR‐Emissive Ruby Analogue

“…[23] Thee nvisaged ddpd complex 1 3+ should resist ligand-centered reductions and nucleophilic attack at the metal center due to the strong electron donating power of ddpd.…”