Electrochemical data

1
Hyper-Paramagnetic Molybdenum tris-Dithiolene
Complexes
Sharon J. Nieter Burgmayer, Laura Snyder,
Janet Lee, Laura Picraux, Cheryl Soricelli
sburgmay_at_brynmawr.edu
Department of Chemistry, Bryn Mawr
College, Bryn Mawr, Pennsylvania, USA 19010
University of Arizona thanks also to Arnold
Raitsmiring, EPR Facility Arpad Somogyi, Mass
Spectrometry John H. Enemark
Bryn Mawr College, Bryn Mawr, Pennsylvania using
support from NSF NIH Bryn Mawr College

• With thanks to
• Hannah Wilhelm
• Angelina Lucento
• Ying Hou
• Ria Sankar
• Grace Shin

Fig. 2
Our research group has been studying the
formation of Mo-dithiolene complexes in reactions
of a molybdenum polysulfide reagent and selected
alkynes. The long range goal of these studies was
to develop methodology for preparing dithiolene
models for the molybdenum cofactor, Moco, found
in molybdenum enzymes. Below is shown the
structure of the dithiolene ligand on Mo in Mo
enzymes and the prototype dithiolene-forming
reaction. Unique to the dithiolene ligand in
Moco is an N-heterocycle group known as a pterin.
In the preliminary stages of our model studies,
the pterin was replaced by a related
N-heterocycle, a quinoxaline. Below the
dithiolene and pterin portions of the Moco
ligand are highlighted since these constitute the
unique pieces of our strategy to develop new
Moco models.

• Mo-tris-dithiolene complexes were the expected
  products in the reaction shown in Fig. 2. The
  first investigations of the coupling reaction
  used alkynes substituted by quinoxaline. These
  reacted readily with the molybdenum polysulfide
  MoS(S9)2- to produce Mo-tris-dithiolene
  complexes. Two examples of reactive quinoxalyl-
  alkynes are shown in Fig. 2 at right.
• What was not expected were these astonishing
  results
• Mo(4)(S2PEQO)32- is paramagnetic
• with very high magnetic moment values
• (4.5 - 8.4 B. M.) that exceed expected
• values expected for a d2 metal ion.
• Mo(4)(S2AEQO)32- is diamagnetic,
• like all other known Mo(4)-tris-dithiolenes.

Here we present our recent studies to better
understand the factors that determine the unusual
paramagnetic behavior observed for our
Mo-tris-dithiolene complexes and what causes this
difference as compared to all other diamagnetic
tris-dithiolene Mo(4) complexes. Our strategy
is to synthesize new quinoxalyl-dithiolenes by
varying the other dithiolene substituent. We
speculated that an aromatic substituent was
required to produce the paramagentic
Mo(4)-tris-dithiolene complexes. We have made
complexes where the dithiolene has a quinoxaline
substituent and one of the following aromatic
groups 2- pyridyl, 2,4-difluorophenyl,
4-difluorophenyl , naphthyl and 4-toluyl. We
find that the combination of quinoxaline
and any aromatic group produces paramagnetic
tris-dithiolene Mo(4) complexes the
magnetic susceptibilities measured for the
paramagnetic complexes are extremely large, from
4.5 - 8. 4 B. M. It is for this reason we
call these hyper-paramagnetic complexes.
4-toluyl causes a facile degradation of the
dithiolene ligand to a thiophene.
Fig. 1
b) the dithiolene ligand of Moco in molybdenum
enzymes
a) the prototype dithiolene forming reaction
through the coupling of Mo(S4) and alkyne
units (E is an electron-withdrawing
substituent.)
c) the target Mo-dithiolene model for Moco
d) the structural relationship quinoxaline-substi
tuted alkyne used in the coupling reaction
What do we think? The odd paramagnetic behavior
measured for the Mo(4)-tris(dithiolene)
complexes reported here is highly unusual. All
other Mo(4)-tris(dithiolene) complexes are
diamagnetic. The subtle effect of the dithiolene
substituent, (for example, acetyl vs phenyl in
AEQO vs PEQ suggests the ground electronic state
is sensitive to conformation. Electronic effects
of the varied substituent may play a role, for
example by favoring extended dithiolene
delocalization towards the acyl rather than the
quinoxaline, but we note that fluorination of the
aromatic ring does not appear prevent
paramagentic character. The yardstick of
Mo(5) / Mo(4) reduction potentials indicates
no correlation with paramagnetic behavior. We
favor the hypothesis that coplanarity between the
quinoxaline and the dithiolene causes the
paramagnetic states by delocalizing spins on the
quinoxaline heterocycle. Sulfur and molybdenum
atomic orbitals are close in energy and allow a
high degree of covalent bondingwithin the
Mo-dithiolene unit in these molecules
as indicated by the large extinction coefficients
for thecharge-transfer absorptions in the visible
region. Another possibility is dithiolene
bending across the SS axis influences the
magnetic behavior. Dithiolene bending was
proposed to stabilize the ground state through
increased mixing of dithiolene sulfur orbitals
into Mo dz2 according to Fenske-Hall
computational analysis. (Campbell and Harris,
Inorg. Chem. 1996, 35. 3285). References
1. M. Kirk, R. McNaughton, M. Helton. " The
Electronic Structure and Spectroscopy of
Metallo-Dithiolene Complexes in Progress in
Inorganic Chemistry, Vol. 52, Stiefel, E. I.,
Ed. Wiley,111-212. 2. M. Kirk et al. Journal
of the American Chemical Society. 2001, 123,
10389. 3. S. Harris, Inorg. Chem. 1996, 35.
3285
Electrochemical Data
Fig. 5
An unusual electronic structure of
Mo-tris-dithiolene complexes must be the
fundamental cause of their unusual paramagnetism.
Prior computational work by others indicates
that substantial covalent interactions occur
between metal and dithiolene orbitals,
specifically orbitals of sulfur.
Experimentally, we have looked at the electronic
spectroscopy of the paramagnetic complexes (in
Fig. 4) and at their electrochemical behavior
through cyclic voltammetry (in Fig. 5). Cyclic
voltammetry suggested that two oxidized states,
Mo(5) and Mo(6), were accessible. Indeed, we
have oxidized all of the mo(4)-tris-dithiolene
complexes listed in Fig. 3 at left to generate
the Mo(5) and Mo(6) species using iodine. The
electronic spectra of a Mo(4) --gt Mo(5) --gt
Mo(6) series look very similar for all the
different dithiolenes and a representative
example is shown below for Mo(S2PEQO)3 2-.
Data for the other complexes are listed in the
Table at bottom. All complexes have intense
absorptions between 600-800 nm where extinction
coefficients are large, up to 30,000 M -1 cm-1.
The paramagnetic Mo(4)complexes in particular,
as compared to their normal diamagnetic
analogs, have higher extinction coefficients.
This is consistent with studies by others where a
high degree of Mo 4d orbital - S pi orbital
mixing produces large extinction coefficients for
Mo-S charge transfer transitions in this spectral
region.
Fig. 3
rest potential
Electrochemical data from cyclic
voltammetry reveal that the isolated complexes
are Mo(4). Two examples are shown at right, for
the Mo(S2PEQO)3 and Mo(S2diFEQO)3 systems. Note
that the rest potential measured at the start of
the experiment is more negative than the
Mo(5)/Mo(4) reduction wave.
Magnetic Character
Mo(S2PEQO)3
Mo(4) complexes
Paramagnetic Mo-tris-dithiolene complexes
prepared according to the reaction in Fig. 2 are
illustrated at right in Fig. 3 along with values
of their magnetic susceptibilty as measured at
room temperature using a Faraday balance. The
nomenclature for each is presented alongside a
stick diagram for the complex. No crystals
suitable for X-ray structure determination have
been obtained. The coordination geometry, based
on many other Mo(4) complexes studied, is likely
to be trigonal prismatic with a slight distortion
towards octahedral coordination. Idealized views
generated using the program Chem3D are shown on
the far right of Fig. 3. Two perspective views
are presented, one down the C3 axis and one
perpendicular to the C3 axis. The surprising
difference in magnetic character between the
first two complexes listed, Mo(S2AEQO)32- and
Mo(S2PEQO)32-, may be due to a difference in
the orientation of the quinoxaline ring with
respect to the plane of the dithiolene chelate.
The Chem3D views illustrate this. We speculate
that for the complex Mo(S2AEQO)32- , there is a
preference to place the acyl group co-planar with
the dithiolene thereby requiring rotation of
quinoxline to be non-planar with dithiolene. In
contrast for Mo(S2PEQO)32-, the preferred
quinoxaline conformation may be coplanar with
dithiolene placing the phenyl ring out of plane.
A result of preparing the series of
Mo(4)-trisdithiolenes bearing different aromatic
substituents is to demonstrate the general trend
that all complexes bearing an aromatic
substituent in addition to quinoxaline are
paramagnetic. We take that as evidence that
conformational similarity in this set accounts
for their unusual paramagnetic character. The
magnetic moment value of samples average around
4.5 B. M., a value far in excess of the expected
value for a d2 metal ion such as Mo(4). In one
case, increased purity of the compound produced
a huge moment of 8.4 B. M.
Mo(5)/Mo(4)
Mo(6)/Mo(5)
DIAMAGNETIC
Mo(S2AEQO)32-
rest potential
Mo(S2diFEQO)3
PARAMAGNETIC, ?eff 8.4 B. M.
Fig. 4
Electronic Spectra of Mo(4), Mo(5) Mo(6)
states
Mo(S2PEQO)3 2-
Mo(5)/Mo(4)
Mo(6)/Mo(5)
Mo(S2AEQO)3
These tris-dithiolenes exhibit a rich
electrochemistry beyond that of the Mo atom. The
CV at right for Mo(S2AEQO)3 has many
ligand-based reductions (E lt -0.4 V) in addition
to the Mo couples.
PARAMAGNETIC, ?eff 4.22 B. M.
Mo(S2PyEQO)3 2-
Mo(S2NEQO)3 2-
PARAMAGNETIC, ?eff 4.5 B. M.
PARAMAGNETIC, ?eff 4.6 B. M.
Mo(S2FEQO)3 2-
PARAMAGNETIC, ?eff 4.6 B. M.
Mo(S2diFEQO)3 2-