New Complexes of Chromium(III) Containing Organic π-Radical Ligands: An Experimental and Density Functional Theory Study

Abstract

The electronic structures of a series of chromium complexes 1-7 have been experimentally investigated using a combination of X-ray crystallography, magneto- and electrochemistry, and Cr K-edge X-ray absorption and UV-vis spectroscopies. Reaction of the dimer [Cr(II)2(μ-CH3CO2)4](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr(III)(bpy(0))(bpy(•))(CH3CO2)2](0) (S = 1) (1), but in the presence of isopropylamine ((i)PrNH2) [Cr(III)(bpy(•))((i)PrNH2)2(CH3CO2)2](0) (S = 1) (2) was obtained. Both 1 and 2 contain a Cr(III) ion and a single (bpy(•))(1-) ligand, so are not low-spin Cr(II) species. One-electron oxidation of 1 and 2 yielded [Cr(III)(bpy(0))2(CH3CO2)2]PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [Cr(III)(DAD(•))3](0) (S = 0) (4) and [Cr(III)(CF3AP(•))3](0) (S = 0) (5) have been synthesized. Both complexes contain three π-radical anion ligands, which derive from one electron reduction of 1,4-bis(cyclohexyl)-1,4-diaza-1,3-butadiene and one electron oxidation of 2-(2-trifluoromethyl)-anilino-4,6-di-tert-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d(3) Cr(III) gives the observed singlet ground states. Reaction of [Cr(II)(CH3CN)6](PF6)2 with 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (PDI(0)) under anaerobic conditions affords dark brown microcrystals of [Cr(III)(PDI(0))(PDI(•))](PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [Cr(III)(PDI)2](3+/2+/1+/0), in which all one-electron transfer processes are ligand-based. By X-ray crystallography, it was shown that 6 possesses a localized electronic structure, such that one ligand is neutral (PDI(0)) and the other is a π-radical monoanion (PDI(•))(1-). Again, it should be highlighted that 6 is not a Cr(II) species. Lastly, the structure of [Cr(III)((Me)bpy(•))3](0) (S = 0) (7, (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three ((Me)bpy(•))(1-) radical anions are present. To further validate our electronic structure assignments, complexes 1-6 were investigated computationally using density functional theory (DFT) and found in all cases to contain a Cr(III) ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.

Electronic Structures of “Low‐Valent” Neutral Complexes [NiL2]0 (S = 0; L = bpy, phen, tpy) – An Experimental and DFT Computational Study

“…[a] See bond labeling scheme in Figure 6. [14] Similar linear correlations between ligand redox state and bond lengths have been documented by us [15,16,18] and others [19] for N,NЈ-coordinated (bpy) n and (tpy) n ligands. It has been shown that an experimental assignment of the ligand oxidation level in a given coordination compound can be made from high-resolution X-ray structures alone by using these correlations.…”

“…The molecular structure is tetrahedral (Θ ≈ 90°), which is presumably a consequence of the steric demands of the ligand methyl substituents. On the basis of the DFT calculations outlined below, the electronic structure of this species is best described as [ (in red) [14] and [Ni ) 1-π-radical anions [14,15] and suggest that at least one ligand π-radical is present in each complex. Indeed, the spectrum of [ …”

Determining the Electronic Structure of a Series of [(phen)3M]0 (M = Ti, V, Mo) and [(pdi)2M]n+ (M = Cr, Mo) Complexes: Coordination of Neutral Ligands vs. π‐Radical Anions

“…We have previously assigned these bands in the near‐infrared region 700–1200 nm as d→π* transitions of N , N ′, N ″‐coordinated (pdi · ) 1– π radical anions and its corresponding dianions 1. Very similar features have been reported for the spectra of [Cr III (pdi · )(pdi 0 )] 2+ ( S = 1) in 4 (350–400, 500, 750, 900 nm)7 and [Mn II (pdi · )] 0 (400, 550, ca. 1000 nm) 10.…”

“…Values are taken from ref 1a. for six [M II (pdi 0 ) 2 ] 2+ species (M = Mn, Fe, Co, Ni, Zn) and [Cr III (pdi) 2 ] 2+ from ref 7. and this work (complex 3 ) and [Cp 2 Zr IV (pdi 2– )] 0 ref 10…”

Geometric and redox flexibility of pyridine as a redox-active ligand that can reversibly accept one or two electrons

“…1 The ground state of these complexes can also have significant radical anion character on the pyridine ligands. 2–4 The strong bond between the oxidized metal and the reduced pyridine is important for enabling this transfer of electrons, because reduction of free pyridine is very difficult in the absence of a metal (−2.7 V vs. SCE). 5 We have been interested in showing the potential for the parent pyridine to accept charge, and exploring the ways in which the pyridine responds to this additional charge.…”