Download Experimental study on transition metal complexes containing N,S

157
Chapter 8
Chapter 8
Summary
The coordination chemistry of transition metals with bidentate ligands
derived from o-aminothiophenolate, o-dithiolate and o-catacholate has been studied in
this work. The ligands used were:
1) o-aminothiophenol [LNS]
2) 2-Phenylbenzothiazoline [LPh]
3) 2-mercapto-3,5-di-tert-butylaniline H2[tLNS]
4) 3,5-di-tert-butyl-1,2-benzenedithiol H2[tLSS]
5) 3,6-di-tert-butylcatacholate H2[tLCat]
Twenty-two transition metal complexes of these ligands have been
synthesized and characterized spectroscopically. Seventeen complexes among them have
been structurally characterized. These ligands were readily one-electron oxidized yielding
N,S-, S,S- and O,O-coordinated π radical anions. In this work, we have shown that the
spectroscopic oxidation states in transition metal complexes coordinated to these ligands
could be assigned by the combination of crystallographic and spectroscopic methods. The
main outcomes of the study are listed below:
Summary
158
Chapter 2
The complexes [As(Ph)4] [Co(LNS)2] (1) and [N(n-Bu)4] [Co(LNS)2] (2)
have been fully characterized structurally, electrochemically and by magnetic
measurements in order to show that these species contain a central CoIII (d6, S=1) metal
ion. The cyclic voltammograms of 1 and 2 show reversible one-electron waves
corresponding to oxidation of a ligand and irreversible waves corresponding to a one
electron reduction of the cobalt. The absorption spectra of the complexes contain intense
LMCT bands occurring in the visible region (< 700 nm). The ground state of the
complexes has been shown to be a spin triplet, in which the degeneracy is lifted by large
positive zero field splitting. A zero field splitting (+41 cm-1) for complex 2 has been
measured independently by magnetic moment measurement, variable-temperature and
variable field SQUID magnetometry, and far-infrared absorption. These studies have
clarified the ambiguity surrouding the proper formalism of the monoanionic cobalt
complex, and therefore it is possible to assign this complex properly as [CoIII(LNSIP)2]1-.
Chapter 3
The reported compounds Na[Cr(tLNS)3(OEt)3(µ-OMe)4(OHEt)4] (3),
Na[Cr(tLNS)3(OMe)3(µ-OMe)4(OHMe)3]
(4a)
and
Na
[Cr(tLNS)3(OMe)3(µ-
OMe)4(OH2)3] (4b) are archetypal CrIII cuboidal clusters with a N,S- coordination.
Methoxy bridges are an additional feature of these cuboidal complexes. The structural
features of 3, 4a, and 4b demonstrate that the ligand, 2-mercapto-3, 5-di-tert-butylaniline
exists in o-iminothiobenzosemiquinonato (1-) π–radical form, which verifies the
noninnocent nature of o-aminothiophenolato ligands. Magnetic behaviour of these
complexes show antiferromagnetic exchange coupling between three S = 1 spins, giving
a singly non-degenerate S = 0 ground state and thus making these clusters very
exceptional examples of S = 1 spin-triangles having three equal J values.
Chapter 4
In this study the crystal structure properties as well as the electrochemical,
spectroscopic and magnetic behavior of
[Ni(ddbt)] (5), [Co(Cp)2][Ni(ddbt)] (5b),
[Co(ddbt)] (6), [Co(Cp)2][Co(ddbt)] (6b) and [Zn(phbt)2] (7) have been fully
159
Chapter 8
characterized. As pointed out in the introduction, it has been verified by low temperature
crystallography of 5, 6, 5b and 6b that the metal ions in analogous tetra coordinated
cobalt and nickel complexes exist in different oxidation levels. Thus, in the neutral and
monoanionic nickel complexes 5 and 5b, the central nickel has a NiII (d8) electron
configuration, where the HOMO are predominantly ligand centered. On the other hand, in
the case of the neutral cobalt complex (6), the observed ligand bond lengths clearly
suggest a CoIII (d6) electron configuration. In the monoanion 6b, the ligand bond
distances indicate dianionic form of ligands leading to a spin triplet ground state for 6b.
The DFT calculations on the similar compounds suggested two possibilities, namely:
CoIII (d6), or CoII (d7) coordinated to a ligand radical, though the structural features of 6b
point toward CoIII (d6) configuration. The degeneracy of the ground state of 6b is lifted
by large positive zero field splitting of 36.67 cm-1, which has been measured
independently by magnetic moment measurement, variable-temperature and variable field
and far-infrared absorption.
The absorption spectra of the neutral and electrochemically-generated
species show several charge transfer bands. Intense LLCT bands occurring in the visible
region are a significant feature of the spectra of complexes containing two radical
ligands. Spectra of complexes containing only one radical ligand contain IVCT bands in
the near infrared region. The EPR parameters for 6 and 5b show that spin-orbit coupling
plays a role in the deviation of the g value from the free radical value. These radical
stabilized complexes have proven the non-innocent nature of the o-aminothiophenolate
ligands, thus disproving the proposed valence isomer structure reported in the literature
for the assignment of the oxidation level for ligands.
Chapter 5
We have shown in this chapter that oxidation states of higher than +3 for
the central chromium ion are not present in the tris-(o-benzene-dithiolato) [N(n-
t
t
Bu)4][Cr( LSS)3] (8) and tris-(o-benzocatacholato) chromium [Cr( LCat)3] (10)
[Co(Cp)2][Cr(tLCat)3] (10b) complexes. In contrast to earlier literature reports, we
Summary
160
have not found spectroscopic evidence for the occurrence of
chromium(IV),
chromium(V) and chromium(VI). The electronic structures of the resulting complexes are
often complicated and were elucidated by a combination of electronic, EPR, X-ray
crystallography and magnetic susceptibility measurements.
In the complex [N(n-Bu)4] [CrO(tLSS)2] (9), chromium was found in the
higher oxidation state of (+5) which was confirmed by the EPR parameters, and the
existence of the Cr=O bond was confirmed by infrared spectroscopy and X-ray
crystallography.
Chapter 6
In this chapter [M(tLSS)3]n (M = Mo, W; n = 0, 1) (12, 12b, 13, 13b)
complexes along with [Mo(tLNS)3] (11) have been studied and it has been found that the
oxidation state of +5 is present for the central metal ion for all these compounds. In
contrast to the previous literature reports viewing the neutral complexes as MVI (d0)
configuration, we have found that the neutral species are MV (d1), which undergo ligand
centered redox activities to produce corresponding monoanionic and monocation species
with an unchanged oxidation state of the metal ion. The electronic spectra of all
compounds contain characteristic ligand to metal charge transfer bands. The electronic
structures of these complexes were elucidated by a combination of electronic, EPR, X-ray
crystallography and magnetic susceptibility measurements.
As compared to [Cr(tLSS)3]1- (8), the electronic spectra of monoanionic
compounds 11b, 12b and 13b are quite different. The X-ray structure analyses of these
compounds also show significant differences in bond distances for 8 than other
[M(tLSS)3]1- complexes. Thus, in contrast to previous suggestions considering Cr, Mo and
W as having a similar oxidation state in a similar ligand atmosphere, we conclude that the
metal oxidation states is different in the case of 8 than other [M(tLSS)3]1- complexes (M =
Mo, W).
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Chapter 8
Chapter 7
The XAS studies on the neutral Mo and W complexes coordinated to triso-dithiolene clearly showed the presence of dithiobezosemiquinonate (1-) radical and its
absence in the monoanionic Mo complex; in agreement with the results of the other
spectroscopic studies done on these complexes (chapter 6). Comparison of the
monoanionic species of Cr and Mo reveal that the electronic structures of these species
are different: in the first case dithiobezosemiquinonate (1-) radical ligand is present and
in second case it is not.
However, the situation is a little more complicated for the [Cr(LSS)3]1- (8),
[Cr(tLCat)3] (10) and [Cr(LCat)3]1- (10b) species. While the metal K-edge for 8 appears at
0.5 eV less than the reference CrIII compound, the metal K-edges of 10 and 10b appear
almost at the same position as compared to the reference CrIII. In contrast with this, the
electronic absorption spectra of electrochemically oxidized and reduced forms of 8 and
10 are correspondingly identical (chapter 5). Thus, while the metal K-edge data for the 8
points toward a CrII assignment, IR spectroscopy, X-ray crystallography and electronic
absorption spectroscopy (chapter 5) correlate best with that of a CrIII assignment. In the
case of 9, the metal is clearly more oxidized than 8, consistent with the +5 oxidation state
of the metal and the absence of a ligand radical.