172
M. Gruden et al.
labeling studies were performed (varying the amount of
16 O and
18 O, in 3:0, 2:1, 1:2,
0:3 ratios). These experiments showed further confirmation of the correctness of our
mechanistic study, with four bands shown for the four isotopologues that matched
the computational data.
Since we were dealing with high-valent (formal) oxidation states, ranging from
+II for the starting material, to +III/+ IV in the high-valent species, we also explored
the use of X-ray absorption spectroscopy (XAS) . For iron, it was shown that a change
in oxidation state is accompanied by a shift of the near-edge XAS position (XANES)
[43]. Instead for nickel, there exists an ambiguous situation where, e.g., Pirovano and
coworkers showed [44, 45] that XANES seemed to be unable to distinguish between
Ni
II and Ni
III . Nevertheless, in order to be consistent, and answer one of the questions
from a reviewer, we asked our colleague and XAS expert Moniek Tromp to perform
the experiments. As anticipated, Ni K edge XANES spectra taken of starting material
and just after adding NaOCl show a nondescript edge, i.e., no significant pre-edge
features. This indicated the presence of a near-octahedral, six-coordinate, geometry
around Ni for all species present; this is consistent with complexes 1, 3 and 3a, but
does not allow assignment to any one of them.
The final piece in the puzzle was made by EPR spectroscopy, which would allow to
(dis)prove the presence of paramagnetic mononuclear Ni
III complexes. The absence
of EPR (X-band) signals at 77 K at any time suggested that such mononuclear species
were not present at any time.
The interplay between theory and experiment for the characterization of the shortlived [(Ni
IV ) 2 (μ-O) 3 (tmtacn) 2 ]
2+ species was therefore essential for arriving at the
final result. It all started with the observation of the pronounced absorption band,
for which possible structures were determined by theory. The spin ground states of
the three isomers then led to a variety of spectroscopic measurements to (dis)prove
the proposed candidates, including isotope labeling experiments in the wet and dry
laboratory. In the end, a consistent picture emerged that showed evidence for the
existence of a high-valent Ni
IV species, in reaction conditions that appear so simple that it was surprising to see that such a transition-metal complex had not been
discovered earlier.
5 Magnetic Anisotropy in Transition-Metal Complexes
The final part of this chapter is dedicated to the investigation of magnetic anisotropy
using various spectroscopic techniques for its experimental determination in combination with quantum chemistry for its theoretical understanding. Such an approach
could lead to the definition of magnetostructural correlation essential for the rational
design of complexes with targeted magnetic properties. Nearly two decades ago,
researchers discovered that certain molecules exhibit slow relaxation of the magnetization after removal of an applied dc magnetic field [46]. Such species can thus
behave at low temperature as classical magnets but of molecular origin, and have
come to be known as single-molecule magnets (SMMs). Slow relaxation present in
M. Gruden et al.
labeling studies were performed (varying the amount of
16 O and
18 O, in 3:0, 2:1, 1:2,
0:3 ratios). These experiments showed further confirmation of the correctness of our
mechanistic study, with four bands shown for the four isotopologues that matched
the computational data.
Since we were dealing with high-valent (formal) oxidation states, ranging from
+II for the starting material, to +III/+ IV in the high-valent species, we also explored
the use of X-ray absorption spectroscopy (XAS) . For iron, it was shown that a change
in oxidation state is accompanied by a shift of the near-edge XAS position (XANES)
[43]. Instead for nickel, there exists an ambiguous situation where, e.g., Pirovano and
coworkers showed [44, 45] that XANES seemed to be unable to distinguish between
Ni
II and Ni
III . Nevertheless, in order to be consistent, and answer one of the questions
from a reviewer, we asked our colleague and XAS expert Moniek Tromp to perform
the experiments. As anticipated, Ni K edge XANES spectra taken of starting material
and just after adding NaOCl show a nondescript edge, i.e., no significant pre-edge
features. This indicated the presence of a near-octahedral, six-coordinate, geometry
around Ni for all species present; this is consistent with complexes 1, 3 and 3a, but
does not allow assignment to any one of them.
The final piece in the puzzle was made by EPR spectroscopy, which would allow to
(dis)prove the presence of paramagnetic mononuclear Ni
III complexes. The absence
of EPR (X-band) signals at 77 K at any time suggested that such mononuclear species
were not present at any time.
The interplay between theory and experiment for the characterization of the shortlived [(Ni
IV ) 2 (μ-O) 3 (tmtacn) 2 ]
2+ species was therefore essential for arriving at the
final result. It all started with the observation of the pronounced absorption band,
for which possible structures were determined by theory. The spin ground states of
the three isomers then led to a variety of spectroscopic measurements to (dis)prove
the proposed candidates, including isotope labeling experiments in the wet and dry
laboratory. In the end, a consistent picture emerged that showed evidence for the
existence of a high-valent Ni
IV species, in reaction conditions that appear so simple that it was surprising to see that such a transition-metal complex had not been
discovered earlier.
5 Magnetic Anisotropy in Transition-Metal Complexes
The final part of this chapter is dedicated to the investigation of magnetic anisotropy
using various spectroscopic techniques for its experimental determination in combination with quantum chemistry for its theoretical understanding. Such an approach
could lead to the definition of magnetostructural correlation essential for the rational
design of complexes with targeted magnetic properties. Nearly two decades ago,
researchers discovered that certain molecules exhibit slow relaxation of the magnetization after removal of an applied dc magnetic field [46]. Such species can thus
behave at low temperature as classical magnets but of molecular origin, and have
come to be known as single-molecule magnets (SMMs). Slow relaxation present in