Hyper-Paramagnetic Molybdenum tris-Dithiolene Complexes
Sharon J. Nieter Burgmayer*, Laura Snyder, Janet Lee, Laura Picraux, Cheryl Soricelli
sburgmay@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
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
Mo
related N-heterocycle, a quinoxaline. Below the dithiolene and pterin portions
S
O
H
of the Moco ligand are highlighted since these constitute the unique pieces of
S
N
HN
our strategy to develop new Moco models.
OPO32H2N
N
N
O
H
Fig. 1
Mo
S
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:
Fig. 2
S
EC
CE
Mo
S
S
C
C
O
E
+
S2
(R)HN
a) the prototype dithiolene forming reaction through
the coupling of Mo(S4) and alkyne units
(E is an electron-withdrawing substituent.)
N
N
S
N
HN
E
Mo
S
R
N
c) the target Mo-dithiolene
model for Moco
C
C
S
Mo
S
S
R
C S S C
C
C quinox
R
S
Mo S
S
S
C C
quinox
R
2S
S
S
S S
+
R
N
d) the structural relationship
quinoxaline-substituted alkyne
used in the coupling reaction
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.
N
C C
N
• [Mo(+4)(S2PEQO)3]2- is paramagnetic
with very high magnetic moment values
(4.5 - 8.4 B. M.) that exceed expected
values expected for a d2 metal ion.
using support from:
NSF
NIH
Bryn Mawr College
2-
quinox
S
b) the dithiolene ligand of Moco in molybdenum enzymes
S S
With thanks to:
 Hannah Wilhelm
 Angelina Lucento
 Ying Hou
 Ria Sankar
 Grace Shin
PEQO
phe nyle thynylquinoxaline
(4 e q)
OR
N
[Mo(S 2PEQO) 3]2- 2 TEA +
R = phenyl
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.
[Mo(S 2AEQO) 3]2- 2 TEA +
R = acetyl
We find that:
O
* the magnetic susceptibilities measured for the paramagnetic complexes are extremely large, from 4.5 - 8. 4 B. M.
C C
• [Mo(+4)(S2AEQO)3]2- is diamagnetic,
It is for this reason we call these hyper-paramagnetic complexes.
N
like all other known Mo(+4)-tris-dithiolenes.
* the combination of quinoxaline and any aromatic group produces paramagnetic tris-dithiolene Mo(+4) complexes
S
Ar
N
* 4-toluyl causes a facile degradation of the dithiolene ligand to a thiophene.
AEQO
ace tyle thynylquinoxaline
(4 e q)
S
Paramagnetic Mo-tris-dithiolene
complexes prepared according to the reaction
Ar
An unusual electronic structure of Mo-tris-dithiolene complexes
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.
N
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.
Chem 3D views
Down C3 axis
O
Perpendicular to C3 axis
N
S
S
DIAMAGNETIC
[Mo(S2AEQO)3]2-
Mo(+4)
S
N
S
S
No crystals suitable for X-ray structure
Magnetic Character
Mo(4+) complexes
O
S
N
O
N
N
[Mo(S2PEQO)3
Fig. 4
N
]2S
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+) --> Mo(5+) --> 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.
rest potential
Electrochemical data
from cyclic voltammetry
reveal that the isolated
complexes are Mo(+4).
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.
Mo(+5)/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.
Mo(+6)/Mo(+5)
rest potential
Mo(S2diFEQO)3
Mo(+5)/Mo(+4)
2.500
Mo(+6)/Mo(+5)
Mo(+4)
complexes listed, [Mo(S2AEQO)3]2- and
[Mo(S2PEQO)3]2-, 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)3]2- , there is a preference to
place the acyl group co-planar with the dithiolene
thereby requiring rotation of quinoxline to be nonplanar with dithiolene. In contrast for
[Mo(S2PEQO)3]2-, 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.
N
S
S
S
2.000
N
N
N
N
N
S
[Mo(S2PyEQO)3
Mo(S2AEQO)3
Mo(+4)
Mo(+5)
Mo(+6)
1.500
]2-
1.000
0.500
Mo(+4)
N
S
S
N
S
N
N
N
0.000
N
300
350
400
450
500
550
600
650
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 Modithiolene unit in these molecules as
indicated by the large extinction coefficients for
thecharge-transfer absorptions in the visible region.
These tris-dithiolenes exhibit
a rich electrochemistry beyond
that of the Mo atom.
PARAMAGNETIC, eff 4.22 B. M.
S
S
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.
Mo(S2PEQO)3 spectra
N
Absorbance
The surprising difference in
magnetic character between the first two
S
What do we think?
Mo(S2PEQO)3
Electronic Spectra of Mo(+4), Mo(+5) & Mo(+6) states
S
S
Electrochemical Data
Fig. 5
PARAMAGNETIC, eff 8.4 B. M.
N
S
N
N
Fig. 3
N
700
750
800
850
The CV at right for
Mo(S2AEQO)3
has many ligand-based
reductions (E < -0.4 V) in
addition to the Mo couples.
wavelength, nm
Another possibility is dithiolene bending across
the S—S 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).
N
Electronic Sp ectral Data for Mo-tris(dithiol ene) Complexes in the
Mo(+4), Mo(+5), Mo(+6) Oxid ation States
N
S
[Mo(S2NEQO)3 ]2-
S
Dithiolene
Mo(+4)
S
N
S
S
PARAMAGNETIC, eff 4.5 B. M.
S
N
N
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.
F
N
N
PARAMAGNETIC, eff 4.6 B. M.
N
[Mo(S2FEQO)3
]2-
S
(plotted as g-value)
S
F
S
S
EPR spectrum of Mo(+4)(S2PyEQO)3,
Powder sample having eff 4.22 B. M.,
77 K
S
S
N
N
N
F
F
N
PARAMAGNETIC, eff 4.6 B. M.
N
F
S
S
[Mo(S2diFEQO)3 ]2F
Mo(+4)
F
S
S
N
S
S
N
F
F
S2C2Ph2
S2PEQO
S2NEQO
S2FEQO
S2diFEQO
QDT
S2PEQO
S2NEQO
S2FEQO
S2diFEQO
326 (34.9)
358 (39)
364 (33)
401 (54.4)
326 (37.0)
320 (29.6)
320 (32)
Mo(+5) complexe s
435 (47)
500 (37.9)
687 (23.5)
425 (10.7)
742 (17.0)
428(16.1)
719 (9), 736 (9)
740 (12)
758 (6)
S
S
N
N
N
MNT
DMAC
QDT
S2PEQO
S2NEQO
S2FEQO
S2diFEQO
391 (10.0)
356 (11.0)
396 (47)
390 (40.0)
356 (30)
326
326(36)
Mo(+4) complexe s
465 (2.6, sh) 500 (2.7)
450 ( sh)
------561 (55)
598 (15.0)
622 (9)
584 (11)
554 (11)
667 (5.6)
650 (5.8)
770 (1.4)
690 (15.0)
656 (9), 692 (8.5)
682 (10), 694 (10)
656 (10), 684 (10)
S2AEQO
S
S
MeO
OMe
S
S2PEQO
S
S
NC
CN
N
N
S2diFEQO
S
CF 3
S
S
F
CF3
S
mnt
0.0
S
bdt
F
0.4
S
N
in DMF vs. Ag/AgCl
(plotted vs gauss)
N
Me
N
0.8
S
O
S
S
N
S
N
O
dmac
qdt
N
N
S
800 (10.5)
S
N
S
O
S2NEQO
S
S
N
S
S
N
S2PyEQOS
S
Mo
F3 C
Mo(+4)
N
EPR spectrum of Mo(+4)(S2PEQO)3,
Powder sample having eff 4.6 B. M.,
77 K
nm ( x 10-3)
Mo(+6) complexe s
449 (12.2)
690 (20.9)
436 (17.8)
686 (30.0)
450 (11)
686 (13)
446 (16)
692 (20)
676 (16)
A Yardstick of
Mo(+5/+4) Potentials
For [Mo(dithiolene)3]2- S S
Complexes
- 0.4
References
- 0.8 V
1. M. Kirk, R. McNaughton, M. Helton. " The
Electronic Structure and Spectroscopy of MetalloDithiolene 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