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Introduction
Reactions of Molybdenum Pterin-DithiolenesClosely Related to Moco
Sharon J. Nieter BurgmayerDepartment of Chemistry
Bryn Mawr CollegeBryn Mawr, Pennsylvania USA
The Three-Ring Circus of Pterin-Dithiolenes
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HN
N
O
H2N
M
SS
PO43-
O
HN
NH
Introduction
Why are we doing this work?
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• Much about the dithiolene chelate on Mo is fairly well understood
• The chemistry at the pterin is not, especially when attached to a dithiolene
• The two main components of Moco are the dithiolene chelate and the pterinMo
SS
HN
NH
HN
N
O
H2NPO4
3-
O
Electronic Buffer
Mo
O
S
S
Oxo Gate
Mo
O
S
S
Fold Angle
Mo
O
S
S
HN
N NH
HN
OH2N
OHOH
H
HO
HH
HN
N NH
HN
H2N
OHO OH
HN
N N
N
H2N
HO
OH
O OH - 2e , - 2H+
Burgmayer JBIC 2004
oxidativering opening
no reduction
Mo
O
SS
R1
R2
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9Our aim is to make molybdenum dithiolene complexes
Dithiolene model system
Dithiolene model systemG
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Mo
O
SS R
NN
NH
N
O NH
O
Our aim is to make molybdenum dithiolene complexes
incorporating a pterin on the dithiolene
Dithiolene model systemG
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Mo
O
SS R
NN
NH
N
O NH
O
Mo
O
SS R
NN
NH
N
O NH
O
Our aim is to make molybdenum dithiolene complexes
incorporating a pterin on the dithiolene
at a six-coordinate oxo-Mo center
Dithiolene model systemG
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Mo
O
SS R
NN
NH
N
O NH
O
N NN N Mo
NHBN
Our aim is to make molybdenum dithiolene complexes
incorporating a pterin on the dithiolene
at a six-coordinate oxo-Mo centerwhere the ancillary sites are occupied by Tp*
Dithiolene model systemG
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Mo
O
SS R
NN
NH
N
O NH
O
It’s easier to focus on the important aspects this way.
Mo
O
SS R
NN
NH
N
O NH
O
Or like this.
HN
N N
NO
NH
O
Me3C
C C R
N NN N Mo4+
NHBN
S
SS
SS
SS R
NN
NH
N
O NH
O
N NN N Mo4+
NHBN
S
Mo4+SS
SS
The Strategy
Mo4+SS
R1
R2
C C R2R1+
* No reaction with Mo=O
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Prior Work
R =
F
F
OH
CH3
OH
CH3
CH3
H
We previously reported results on pterin-dithiolene Moco modelsthat included two types of R-groups:
1. aryl substituents
2. -hydroxyalkyl substituents
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HN
N N
NO
NH
O
Me3C
C C RMo4+
S
S
SSS + Mo4+
S
S
S R
NN
NH
N
O NH
O
1. Observations using aryl substituents [J Inorg Biochem 2007]
Prior WorkG
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R =
F
F
OH
CH3
OH
CH3
CH3
H
Aryl-pterin dithiolenes complexes can be isolated with Mo(4+)=S, Mo(4+)=O, and Mo(5+)=O groups
EPR and MCD studies indicate aryl-pterin dithiolene Mo complexes have electronic characteristics almost identical to benzenedithiolate. Electrochemistry (CV) reveals that the Mo(5+)/(4+) shifts ~ +100 mV indicating that pterin-dithiolene ligand is considerably more electron withdrawing than benzenedithiolate.
Mo4+
S
S
S
NN
NH
N
O NH
O
FF
70 o, 4 hacetonitrile
HN
N N
NO
NH
O
Me3C
C C
F
FMo4+
S
S
SSS +
Comparison of Mo(5+/4+) Potentials in Tp*MoO(S-S) Complexes
NN
N N Mo
NHB
N
O
S
S
Fronti
ers
in M
eta
l D
ith
iole
ne C
hem
istr
y A
CS 2
008
Prior Work Electrochemistry
2. Observations using -hydroxyalkyl substituents
HN
N N
NO
NH
O
Me3C
C CMo4+
S
S
S
SS
Mo4+
S
S
S
NN
NH
N
O NH
O
+OH
CH3
CH3 OH
H3C CH3
Prior WorkG
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Under the same reaction conditions the expected product not observed
70 o, 4 hacetonitrile
R =
F
F
OH
CH3
OH
CH3
CH3
H
Not observed
The pterin-dithiolene reaction yields many Mo products
Wrong mass for Mo=S
Wrong mass for Mo=O
and what’s this?
ESI-MS m/z
Model System
would be expected:m/z 837
Mo4+
O
S
S
NN
NH
NO
NH
O
OH
H3CCH3
S
HN
N N
NO
NH
O
Me3C
C CMo4+
S
S
S
SS
Mo4+
S
S
S
NN
NH
N
O NH
O
+OH
CH3
CH3 OH
H3C CH3
Not observed
ESI-MS m/z
Less m/z 835
Why lessMo=O?
now what’s this?
Why more Mo=S ?
Clearly we do not understand what’s going on in this system.
Mo4+
O
S
S
NN
NH
NO
NH
O
OH
H3CCH3
S
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
H3C CH3
PPh3
S=PPh3
Introduction
We figured it out
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HN
N N
NO
NH
O
Me3C
C CMo4+
S
SS
SS
Mo4+
S
SS
NN
NH
N
O NH
O
+ OH
CH3
CH3
OH
H3C CH3
25 deg C4 hr
[M-] 820.0
[M-] 820.0
Pterin-Dithiolene Model SystemG
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9 All we had to do was “chill out”: run reaction at ambient temperature
and only one species forms:the expected pterin-dithiolene
[M-] 820.0
90 deg C, 1 hr
Pterin-Dithiolene Model SystemG
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9 Surprising stability to heat!
HN
N N
NO
NH
O
Me3C
C CMo4+
S
SS
SS
Mo4+
S
SS
NN
NH
N
O NH
O
+ OH
CH3
CH3
OH
H3C CH3
25 deg C4 hr
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
H3C CH3
Typical Mo=S hydrolysis during chromatography
SiO2 or Al2O3
No Reaction!
HN
N N
NO
NH
O
Me3C
C CMo4+
S
SS
SS
Mo4+
S
SS
NN
NH
N
O NH
O
+ OH
CH3
CH3
OH
H3C CH3
25 deg C4 hr
[M-] 820
[M-] 804
Pterin-Dithiolene Model SystemG
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9 Sulfido to Oxo Exchange Promoted by Phosphine
ESI-MS: clean reaction
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
H3C CH3
PPh3
- SPPh3
[M-] 804
Mo4+
O
SS
NN
NH
N
O NH
O
O
H3C CH3
Phosphine induces pyranopterin formation!
[M-] 802
Pterin-Dithiolene Model SystemG
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9 But something different happens if further Phosphine is added
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
H3C CH3 [M-] 804
[M-] 802
monitoring phosphine-mediated conversionby ESI-MS shows clean reaction
Pterin-Dithiolene Model SystemG
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9 1H HMR shows loss of H7 consistent with pyran ring formation
Mo4+
O
SS
NN
NH
N
O NH
O
O
H3C CH3
Mo4+
O
S
S
NN
NH
N
O NH
O
OHH3C CH3
H
H7N-H
open pterin-dithiolene
pyranopterin-dithiolene
Pterin-Dithiolene Model SystemG
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9 Pyranopterin formation is net oxidation: 2H are lost
Mo4+
O
SS
NN
NH
N
O NH
O
O
H3C CH3
Mo4+
O
S
S
NN
NH
N
O NH
O
OHH3C CH3
H PPh3
+ O2
- H2O
Molybdenum Redox in PyranoPterin-DithiolenesG
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Mo4+
O
SS
NN
NH
N
O NH
O
O
Mo5+
O
SS
NN
NH
N
O NH
O
O
H2O2 or O2
Mo(5+) pyranopterin Cherry Red
Mo(4+) pyranopterin Blue
During oxidative pyranopterin formation, Mo(4+) is oxidized to Mo(5+)This can be reversed with KBH4, Mo(5+) is reduced back to Mo(4+)
KBH4
Mo(5+)560 nm Mo(4+)
630 nm
Mo(5+)pyranopterin Mo(4+)
pyranopterin
Mo(4+)open pterin
Mo(4+)dihydropterin
All share a maximum of 375 nm
Redox in PyranoPterin-DithiolenesG
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1
2
34
AReductive Titrationof Mo(5+)-PyranopterinForms aReduced Pterin Complex
Pterin Redox in PyranoPterin-DithiolenesG
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Mo4+
O
S
S
NN
NH
N
O NH
O
OHMo4+
O
S
S
NN
NH
N
O NH
O
OH
HH
M[-]: 804orange
KBH4
ESI-MS indicates pterin reduction to dihydropterin
M[-]: 806yellow
[M-] 804
[M-] 786
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
H3C CH3
+ H+
- H2O
Pterin Dithiolene model system
Al2O3
chromatography
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9Chromatography induces a 2nd Type of Pterin Cyclization
[M-] 804
Mo4+
O
S
S
N
N
NH
N
O
NH
O
H3CCH3
[M-] 786
A Pyrrole-Ring on Pterin
Quinoxalyl Dithiolene model system
The thermal reaction leads to
hydroxyl loss by dehydration and
ring cyclization.`
N
N C CMo4+
S
SS
SS
+ OH
CH3
CH3Mo4+
S
SS
N
N
H3C CH3
H
70 deg C
chromatography
Mo4+
O
SS
N
N
H3C CH3
H
aluminachromatography
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
290 390 490 590 690 790Wavelength (nm)
Absorbance 6000 M-1 cm-1
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We’ve seen this Pyrrole Ring Cyclization before: in a Quinoxaline Dithiolene
quinoxaline
Mo1-O1 1.688(3) Å
S1-C1 1.748(5) Å
S2-C2 1.695(5)
Mo1-S1 2.416(1) Å
Mo1-S2 2.456(1)
C1-C2 1.373(7) Å
Quinoxalyl Dithiolene model system
1) asymmetric structure of dithiolene
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X-ray Structure Provides Two Surprises
Dihedral=4.8 degBetween quinox & pyrrole
Asymmetry furtherdisplayed in twist ofquinoxaline
Dihedral angle = 14.5 deg[Mo-S1-S2/S1-S2-C1-C2]
Unexpected for a Mo(4) dithiolene
2) non-planar dithiolene: folded
Quinoxalyl Dithiolene model system
HOMOlocalized on Mo d(xy)
LUMOlocalized on quinoxaline
611 nm, MLCTNote: asymmetric electron density on dithiolene
From the ML Kirk Lab: Isodensity Density Plots of HOMO & LUMO Top View
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Quinoxalyl Dithiolene model system
Mo4+
S
S
N
N
Conventional bond and charge assignmentAsymmetric electronic distribution Consistent with X-ray structure and calculations
Mo4+
S
S
N
N
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9 Dithiolene Asymmetry Suggests Electronic Delocalization andPartial Dithiolene Oxidation
in Acetonitrile vs. Ag/AgCl
bdt
ead2 x -SPh2 x -SEt
S4
S SS S
dmacS S
OMeO
MeOO
qdt
S S
N N
S
SN
N
HN
N
O
piv-HNPh
S S
S SS S
0.0 - 0.6 V 0.4 0.6 - 0.4 0.2 -0.2
N
N
SS
HO
NN
SS
Electrochemistry
• Cyclized quinoxalyl dithiolene more electron withdrawing (+ 350 mV)• Pterin more electron withdrawing ( ~ +50 mV) than quinoxaline
Electronic Picture Consistent with Mo(5+)(4+) Potential Shift
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Mo5+
O
S
SN
N
H3C CH3
H
PF6-
Oxidation to Mo(5+)
blue: 611 nm (6000 M-1 cm-1)
cherry red: 528 nm (>7000 M-1 cm-1)
Quinoxalyl Dithiolene model system
Ferroceniumhexaphosphate
Mo4+
O
S
SN
N
H3C CH3
H
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
290 390 490 590 690 790
Wavelength (nm)
Absorbance
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Mo
O
SS
NN
NH
N
O NH
O
O
H3C CH3
Similar colors of Mo(4+), Mo(5+)— similar electronic structure?
As Mo(4+): blue: 611 nm (6000 M-1 cm-1)
As Mo(5+): cherry red: 528 nm (>7000 M-1 cm-1)
Quinoxalyl Dithiolene model systemG
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Mo
O
S
S
N
N
H3CCH3
As Mo(4+): blue: 631 nm
As Mo(5+): cherry red: 528 nm
Mo4+
S
S
N
N
Mo4+
S
S
HNN
NH
O
ONH2
reductionKBH4
Pterin-Dithiolene Model SystemG
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The Three-Ring CircusOfPterin-Dithiolene
Mo4+
S
SS
NN
NH
N
O NH
O
OH
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
HH
Mo4+
O
SS
NN
NH
N
O NH
O
O
Mo5+
O
SS
NN
NH
N
O NH
O
O
Mo4+
O
SS
N
N
NH
N
O
NH
O
reductionKBH4
oxidation(PPh3, O2)
reductionKBH4
oxidation (O2)
oxidation(H2O2, O2)
Mo4+
S
SS
NN
NH
N
O NH
O
O
reductionKBH4
Pterin-Dithiolene Model SystemG
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SurprisingFeatures
Mo4+
S
SS
NN
NH
N
O NH
O
OH
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
Mo4+
O
S
S
NN
NH
N
O NH
O
OH
HH
Mo4+
O
SS
NN
NH
N
O NH
O
O
Mo5+
O
SS
NN
NH
N
O NH
O
O
Mo4+
O
SS
N
N
NH
N
O
NH
O
reductionKBH4
oxidation(PPh3, O2)
reductionKBH4
oxidation (O2)
oxidation(H2O2, O2)
- No rxn w/ phosphine
- Air stable- H2O stable
-Air stable as solution (weeks)-- most stable form
-Air stable as solid
-Not stable in solution in air- not stable to phosphine
Mo4+
S
SS
NN
NH
N
O NH
O
O
Pterin dithiolenes can be made in coupling reaction of pterin alkynes and Mo-tetrasulfides
Dithiolene model system
on a pterin dithiolene both 5- and 6- membered ring closure occurs, 5-membered closure to pyrrole favored under dehydrating conditions, 6- membered closure to pyran favored under oxidative conditions.
Conclusion
Reversble pyranopterin formation has been demonstrated on a Mo-dithiolene model. The pyranopterin cyclizes under oxidative conditions and the pyran ring is opened under reducing conditions
hydroxyl groups promote ring closure in two ways:
on a quinoxalyl dithiolene ligand only a 5-membered ring closure occurs. The resultant pyrrolo-quinoxaline is highly electron withdrawing due to electronic delocalization from the dithiolene to the heterocycle.
The quinoxalyl dithiolene ligand behaves differently from pterin dithiolene
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Mo4+
S
S
NN
NH
N
O NH
O
OH
H3C CH3
Mo4+
S
S
N
N
NH
N
O
NH
O
OH
H3C CH3
Cyclizes to6-memberedPyran ring
Cyclizes to5-memberedPyrrole ring
This implies facile rotation of the pterin
Bryn Mawr College,
Bryn Mawr, Pennsylvania
Kelly Ginion Matz Tanya Michelle CorderBelinda LeungAlison PanosianRebecca Rothstein
$$NIH-GM081848-01
University of New Mexico
Martin L. KirkTony Williams
Diana Habel-RodriguezRegina Mtei
Thanks to:G
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University of Pennsylvania
Pat Carroll
Fc+/Fc
Mo(5+/4+)
Ligand reductions?
Electrochemistry
Mo4+
S
SS
N
N
H3C CH3
H
"closed"
E1/2 + 53 mV
Mo4+
O
SS
NN
OH
H3C CH3
"open"
Mo4+
O
SS
N
N
H3C CH3
H
"closed"
E1/2 -96 mV E1/2 + 250 mV
Potentials vs. to Ag/AgCl; internal ferrocene at + 400 mV, ACN/TEAP/Pt, 100 mV/s
Mo(5+/4+) Potentials
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Italy
200
9
