Download A combined experimental and theoretical study of transition metal

Chapter 7
7.1. Conclusions
Aminothiol ligands have been extensively studied for their redox noninnocence.
However, owing to the redox noninnocence nature of the ligand, unambiguous
assignment of the metal oxidation state has always been challenging. In this thesis we
have synthesized a series of transition metal complexes and tried to address the inherent
ambiguity present in the transition metal complexes coordinated to redox noninnocent
ligands. Earlier attempts to isolate the monomeric complexes of Fe and Co with
aminothiol ligands did not work, owing to the kinetic stability of the dimer formed and
facile approach of the two monomeric units within the bonding range of Fe and Co. We
theorized that, if we could substitute the amine proton of the aminothiol ligand with a
bulky phenyl group then it might prevent dimerization, owing to the steric hindrance
imparted by the bulky phenyl groups. We have synthesized a novel pentafluorobenzene
substituted aminothiol ligand and a series of transition metal complexes with this ligand
and unambiguously assigned the oxidation state of the metal center with the aid of
various spectroscopic techniques and DFT studies.
Chapter 2
The synthesis and characterization of two new ligands (LH2 and 3LH2) are discussed in
this chapter. The molecular structure of the dimerized form of the ligand (3LH)2 is
discussed. The obtained bond distance parameters of the free ligand are informative when
observing changes upon formation of metal complexes with the ligand. Moreover, DFT
calculations were performed on the free ligand which help us to ascertain the redox active
MO of the ligand. Mulliken spin densities of the radical form of both ligands, 1LH2 and
LH2, clearly represents that the majority of spin density is located on the S atom and the
rest is dispersed throughout the ligand. The MO manifold of the quasi-restricted orbitals
of the three different forms of the ligand, namely the dianionic form, the oxidized radical
form and the iminobenzoquinone form, helps in the identification of the redox active
MO. The characteristic quinoidal distortion observed in the radical form of the ligand and
the changes in the bond distances upon the change of redox state of the ligand can also be
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Chapter 7
predicted from the MO manifold. DFT calculations display a gradual decrease in the C-S
bond distance on going from the dianionic form (L)2- to the radical form (LISQ•)1- and
finally to the iminobenzoquinone form (LIBQ)0. This is exactly what we observe
experimentally. Thus, in this chapter we have theoretically shown the structural changes
observed in the different redox forms of the ligand.
Chapter 3
The electronic structures of the [MII(L•)2]0 series with group 10 metals (M = Ni, Pd and
Pt) are discussed in this chapter. We have established that all three complexes possess a
common oxidation state of +II at the central metal center, which is substantiated with the
help
of
spectroscopy
and
DFT
studies.
The
presence
of
o-imino-thione-
benzosemiquinonate-(1-) π-radicals can be assigned crystallographically by a systematic
shortening of the C-S bonds and the characteristic quinoidal distortion of the aromatic
benzene rings. Moreover, DFT calculations also support the fact that these compounds
can be best described as strongly exchange-coupled diradicals which interact through a
diamagnetic central MII ion in a low-spin d8 configuration. The LLCT bands in the near
infrared region (800-920 nm) of the electronic absorption spectra provide additional
support for the π-radical nature of the ligands.
Interestingly, the cyclic voltammogram of the doubly oxidized species of [NiII(L•)2] gives
rise to two different electronic structure descriptions, which can be elucidated with the
help of DFT calculations. Theoretical studies indicate that the oxidation process is ligand
based. Moreover, from the calculation we can identify four occupied molecular orbitals,
which are entirely metal based and the geometry optimized structure of the two electron
oxidized species exhibit the absence of ligand based molecular orbitals. Thus, the MO
scheme coupled with the spin density map which shows no spin population neither on the
metal center nor on the ligand center supports the hypothesis that the oxidation process is
entirely ligand based. The absence of IVCT band in the UV-vis spectra also corroborates
our hypothesis.
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Chapter 7
In summary, we can infer from the DFT calculation, that upon two-electron oxidation we
have generated the species [NiII(LIBQ)2]2+ and not the mixed metal-ligand oxidized form
[NiIII(L•)(LIBQ)]2+ or the entirely metal based oxidized form [NiIV(L•)2]2+.
Chapter 4
In this chapter, we discuss the synthesis and characterization of two new iron complexes
4 and 5. Complex 4 is a square-planar four coordinate FeIII complex, which are relatively
rare in the literature. We have also reinvestigated [N(n-Bu)4][Fe(qdt)2] (6) and
[PPh4]2[Fe(qdt)2]2 (7) and, contrary to the reported literature, we found no spectroscopic
evidence in support of a low spin FeIII center. We have unambiguously shown that all
these complexes possess a central, intermediate-spin ferric ion (SFe = 3/2) and it has been
established with the help of spectroscopy and DFT studies. The dimeric dianion in 7 also
contains intermediate spin ferric ions in a square-based pyramidal FeS5 geometry as was
judged by its Mössbauer parameter which resemble closely those of 6. It is interesting to
note that all ferric complexes containing four monodentate benzenethiolate(1-)1 or two oxylyl-α,α'-dithiolate(2-)2 ligands, namely [FeIII(SR)4]- or [FeIII(S2-o-xyl)2]- species,
possess a high spin configuration (SFe = 5/2) and a tetrahedral FeS4 polyhedron. The
observed Mössbauer parameters δ = 0.13 mm s-1 and ∆EQ = 0.57 mm s-1 at 77 K are
consistent with a high spin ferric species with FeIII-S4 units distorted from Td symmetry.2
It is important to note that in these tetrahedral FeIIIS4 species, a single quadrupole doublet
is observed with a small quadrupole splitting. In contrast, in square planar 6 the ferric ion
possesses an intermediate spin ground state S = 3/2 with Mössbauer parameters δ = 0.34
mm s-1 and ∆EQ = 3.42 mm s-1. Thus, a very large quadrupole splitting is observed. In
agreement with this, the average Fe-S distance in high-spin, tetrahedral [FeIII(S2-o-xyl)2]is long at 2.268 Å 2 but somewhat shorter at 2.175 Å in intermediate spin, square planar
6.3 This is due to the fact that the strongly antibonding dx2-y2 metal orbital is halfoccupied in the tetrahedral, high-spin FeS4 species but empty in the intermediate spin,
square planar complex 6.
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Chapter 7
Chapter 5
Synthesis and characterization of two new cobalt complexes 8 and 9 with a
pentafluorophenyl substituted aminothiol ligand are discussed in this chapter. 8 is a four
coordinate, perfectly square-planar monomeric complex with aminothiol ligands.
Spectroscopic studies and DFT calculations suggest that the electronic structure of the
monoanionic square-planar complex 8 is best described as containing an intermediatespin CoIII central ion with a spin state (SCo = 1) coordinated to two dianionic ligand
moieties. However, it is arguable whether the two ligands are in nonradical dianionic
forms or if one of them is in the radical form. Similar to dithiolene ligands as well as for
N,N’-coordinated phenylene diamine ligands, assignment of the correct spectroscopic
oxidation state of cobalt as CoIII is still ambiguous and can be best represented as a
resonance hybrid of the following three canonical forms: [CoIII(L)2-(L)2-]- ↔ [CoII(L•)(L)2-]- ↔ [CoII(L)2-(L•)-]-. These resonance forms can be invoked to explain the
observation that the first SOMO orbital of the monoanion has 36% ligand character. It is
therefore not possible to determine an unequivocal dn (n = 6 or 7) electron configuration
at the metal ion and assign unambiguously a spectroscopic oxidation state of the cobalt.
However, we do note that the closed-shell-like (L)2- ligand configuration as determined
by X-ray crystallography for 8 does point more to a pronounced CoIII (d6, S = 1)
spectroscopic oxidation state than to a [CoII(L•)-(L)2-]- mixed-valent configuration.
The iodo complex 9 is described as a d6 CoIII in a square pyramidal arrangement (with an
intrinsic SCo = 1 spin state) which is coupled antiferromagnetically to two ligand radicals
giving a ground state of Ms = 0 and having an electronic structure [CoIII(L•)2I]. The
presence of diradicals is supported by the intense charge transfer band observed at 734
nm (ε ≈ 2.78 × 104 M-1 cm-1), similar to the LLCT band reported for [Co(LISQ)2I]
complex with 2,4-di-tert-butyl-6-aminothiophenol.4
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Chapter 7
Chapter 6
The synthesis and characterization of a series of transition metal complexes, 10-15 with a
novel N,S,O-donor ligand (3LH2) are discussed in this chapter. The versatility of the
ligand is underlined by its ability to form polynuclear complexes of variable structural
motifs all involving a bridging S. Moreover, it is particularly interesting to find that even
Schiff base-type ligands can be redox noninnocent. Complexes 14 and 15 are
representative examples, which were oxidized electrochemically and we observed four
and six reversible one-electron oxidations in the cyclic voltammogram indicative of
sequential ligand oxidations. g values measured from the EPR spectra of the one-electron
oxidized species of 14 and 15 indicate the presence of ligand centered radicals. In
particular, the axially symmetric g value points towards the generation of phenoxyl
radicals. Resonance Raman spectroscopy further corroborates our assignment; upon
excitation in resonance with the π-π* transition of the phenoxyl, the rR bands originating
from the modes ν7a (∼1500 cm-1; C-O stretching) and ν8a (∼1600 cm-1; C=C stretching)
are enhanced and clearly detectable. The starting material 14 does not show any bands in
the region 1500-1600 cm-1 of considerable intensity; however, the one-electron oxidized
species 14a exhibits two new bands at 1564 cm-1 and 1605 cm-1 respectively,
corresponding to ν7a and ν8a modes. Thus we have spectroscopic evidence to suggest that
a phenoxyl radical is generated upon oxidation.
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