Noninnocence of the Ligand Glyoxal-bis(2-mercaptoanil). The
Electronic Structures of [Fe(gma)]2, [Fe(gma)(py)]·py,
[Fe(gma)(CN)]1-/0, [Fe(gma)I], and [Fe(gma)(PR3)n] (n = 1, 2).
Experimental and Theoretical Evidence for “Excited State”
Coordination
posted on 2003-01-08, 00:00authored byPrasanta Ghosh, Eckhard Bill, Thomas Weyhermüller, Frank Neese, Karl Wieghardt
The electronic structure of the known iron complexes [Fe(gma)]2 (St = 0) (1)6 and [Fe(gma)(py)]·py (St = 1) (2)7 where H2(gma) represents glyoxal-bis(2-mercaptoanil) has been shown by X-ray
crystallography, Mössbauer spectroscopy, and density functional theory calculations to be best described
as ferric (SFe = 3/2) complexes containing a coordinated open-shell π radical trianion (gma•)3- and not as
previously reported6,7 as ferrous species with a coordinated closed-shell dianion (gma)2-. Compound 1 (or
2) can be oxidized by I2 yielding [FeIII(gma)I] (St = 1/2) (3). With cyanide anions, complex 1 forms the
adduct [(n-Bu)4N][FeIII(gma•)(CN)] (St = 1) (4), which can be one-electron oxidized with iodine yielding the
neutral species [FeIII(gma)(CN)] (St = 1/2) (5). With phosphines complex 1 also forms adducts7 of which
[FeIII(gma•)(P(n-propyl)3)] (St = 1) (6) has been isolated and characterized by X-ray crystallography. [FeII(gma)(P(n-propyl)3)2] (St = 0) (7) represents the only genuine ferrous species of the series. Density functional
theory (DFT) calculations at the BP86 and B3LYP levels were applied to calculate the structural as well as
the EPR and Mössbauer spectroscopic parameters of the title compounds as well as of the known complexes
[Zn(gma)]0/- and [Ni(gma)]0/-. Overall, the calculations give excellent agreement with the available
spectroscopic information, thus lending support to the following electronic structure descriptions: The gma
ligand features an unusually low lying LUMO, which readily accepts an electron to give (gma•)3-. The one-electron reduction of [Zn(gma)] and [Ni(gma)] is strictly ligand centered and differences in the physical
properties of [Zn(gma•)]- and [Ni(gma•)]- are readily accounted for in terms of a model that features enhanced
back-bonding from the metal to the gma LUMO in the case of [Ni(gma•)]-. In the case of [Fe(gma)(PH3)],
[Fe(gma)(py)], and [Fe(gma)(CN)]- an electron transfer from the iron to the gma LUMO takes place to give
strong antiferromagnetic coupling between an intermediate spin Fe(III) (SFe = 3/2) and (gma•)3- (Sgma =
1/2), yielding a total spin St = 1. Broken symmetry DFT calculations take properly account of this
experimentally calibrated electronic structure description. By contrast, the complexes [Fe(gma)(PH3)2] and
[Fe(PhBMA)] feature closed-shell ligands with a low-spin Fe(II) (SFe = St = 0) and an intermediate spin
central Fe(II) (SFe = St = 1), respectively. The most interesting case is provided by the one-electron oxidized
species [Fe(gma)(py)]+, [Fe(gma)I], and [Fe(gma)(CN)]. Here the combination of theory and experiment
suggests the coupling of an intermediate spin Fe(III) (SFe = 3/2) to the dianionic ligand (gma)2- formally in
its first excited triplet state (Sgma = 1) to give a resulting St = 1/2. All physical properties are in accord with
this interpretation. It is suggested that this unique “excited state” coordination is energetically driven by the
strong antiferromagnetic exchange interaction between the metal and the ligand, which cannot occur for
the closed-shell form of the ligand.