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Introduction Reactions of Molybdenum Pterin-Dithiolenes Closely Related to Moco Sharon J. Nieter Burgmayer Department of Chemistry Bryn Mawr College Bryn Mawr, Pennsylvania USA The Three-Ring Circus of Pterin-Dithiolenes Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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Introduction Why are we doing this work? Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 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 pterin Electronic Buffer Oxo Gate Fold Angle Burgmayer JBIC 2004 oxidative ring opening no reduction
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Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Our aim is to make molybdenum dithiolene complexes Dithiolene model system
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Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Our aim is to make molybdenum dithiolene complexes incorporating a pterin on the dithiolene
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Dithiolene model system Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Our aim is to make molybdenum dithiolene complexes incorporating a pterin on the dithiolene at a six-coordinate oxo-Mo center
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Dithiolene model system Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Our aim is to make molybdenum dithiolene complexes incorporating a pterin on the dithiolene at a six-coordinate oxo-Mo center where the ancillary sites are occupied by Tp*
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Dithiolene model system Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 It’s easier to focus on the important aspects this way. Or like this.
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The Strategy + * No reaction with Mo=O Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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Prior Work We previously reported results on pterin-dithiolene Moco models that included two types of R-groups: 1. aryl substituents 2. -hydroxyalkyl substituents Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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1.Observations using aryl substituents [J Inorg Biochem 2007] Prior Work Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 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. 70 o, 4 h acetonitrile
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Comparison of Mo(5+/4+) Potentials in Tp*MoO(S-S) Complexes Frontiers in Metal Dithiolene Chemistry ACS 2008 Prior Work Electrochemistry
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2. Observations using -hydroxyalkyl substituents Prior Work Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Under the same reaction conditions the expected product not observed 70 o, 4 h acetonitrile Not observed The pterin-dithiolene reaction yields many Mo products
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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 Not observed ESI-MS m/z Less m/z 835 Why less Mo=O? now what’s this? Why more Mo=S ? Clearly we do not understand what’s going on in this system.
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Introduction We figured it out Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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[M-] 820.0 Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 All we had to do was “chill out”: run reaction at ambient temperature and only one species forms: the expected pterin-dithiolene
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[M-] 820.0 90 deg C, 1 hr Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Surprising stability to heat! Typical Mo=S hydrolysis during chromatography SiO 2 or Al 2 O 3 No Reaction!
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[M-] 820 [M-] 804 Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Sulfido to Oxo Exchange Promoted by Phosphine ESI-MS: clean reaction PPh 3 - SPPh 3
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[M-] 804 Phosphine induces pyranopterin formation! [M-] 802 Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 But something different happens if further Phosphine is added [M-] 804 [M-] 802 monitoring phosphine- mediated conversion by ESI-MS shows clean reaction
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Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 1 H HMR shows loss of H7 consistent with pyran ring formation H7 N-H open pterin-dithiolene pyranopterin-dithiolene
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Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Pyranopterin formation is net oxidation: 2H are lost PPh 3 + O 2 - H 2 O
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Molybdenum Redox in PyranoPterin-Dithiolenes Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 H 2 O 2 or O 2 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 KBH 4, Mo(5+) is reduced back to Mo(4+) KBH 4 Mo(5+) 560 nm Mo(4+) 630 nm
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Mo(5+) pyranopterin Mo(4+) pyranopterin Mo(4+) open pterin Mo(4+) dihydropterin All share a maximum of 375 nm Redox in PyranoPterin-Dithiolenes Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 1 2 3 4 A Reductive Titration of Mo(5+)- Pyranopterin Forms a Reduced Pterin Complex
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Pterin Redox in PyranoPterin-Dithiolenes Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 M[-]: 804 orange KBH 4 ESI-MS indicates pterin reduction to dihydropterin M[-]: 806 yellow
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[M-] 804 [M-] 786 Pterin Dithiolene model system Al 2 O 3 chromatography Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Chromatography induces a 2 nd Type of Pterin Cyclization [M-] 804 [M-] 786 A Pyrrole-Ring on Pterin
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Quinoxalyl Dithiolene model system The thermal reaction leads to hydroxyl loss by dehydration and ring cyclization.` alumina chromatography 6000 M -1 cm -1 Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 We’ve seen this Pyrrole Ring Cyclization before: in a Quinoxaline Dithiolene quinoxaline
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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 Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 X-ray Structure Provides Two Surprises
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Dihedral=4.8 deg Between quinox & pyrrole Asymmetry further displayed in twist of quinoxaline
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Dihedral angle = 14.5 deg [Mo-S1-S2/S1-S2-C1-C2] Unexpected for a Mo(4) dithiolene 2) non-planar dithiolene: folded
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Quinoxalyl Dithiolene model system HOMO localized on Mo d(xy) LUMO localized on quinoxaline 611 nm, MLCT Note: asymmetric electron density on dithiolene From the ML Kirk Lab: Isodensity Density Plots of HOMO & LUMO Top View Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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Quinoxalyl Dithiolene model system Conventional bond and charge assignmentAsymmetric electronic distribution Consistent with X-ray structure and calculations Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Dithiolene Asymmetry Suggests Electronic Delocalization and Partial Dithiolene Oxidation
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in Acetonitrile vs. Ag/AgCl bdt ead 2 x -SPh 2 x -SEt S4S4 dmac qdt 0.0- 0.6 V 0.4 0.6- 0.4 0.2 -0.2 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 Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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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 Ferrocenium hexaphosphate Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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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 system Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 As Mo(4+): blue: 631 nm As Mo(5+): cherry red: 528 nm
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reduction KBH 4 Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 The Three-Ring Circus Of Pterin-Dithiolene reduction KBH 4 oxidation (PPh 3, O 2 ) reduction KBH 4 oxidation (O 2 ) oxidation (H 2 O 2, O 2 )
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reduction KBH 4 Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Surprising Features reduction KBH 4 oxidation (PPh 3, O 2 ) reduction KBH 4 oxidation (O 2 ) oxidation (H 2 O 2, O 2 ) - No rxn w/ phosphine - Air stable - H 2 O stable -Air stable as solution (weeks) -- most stable form -Air stable as solid -Not stable in solution in air - not stable to phosphine
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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 Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Cyclizes to 6-membered Pyran ring Cyclizes to 5-membered Pyrrole ring This implies facile rotation of the pterin
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Bryn Mawr College, Bryn Mawr, Pennsylvania Kelly Ginion Matz Tanya Michelle Corder Belinda Leung Alison Panosian Rebecca Rothstein $$NIH-GM081848-01 University of New Mexico Martin L. Kirk Tony Williams Diana Habel-Rodriguez Regina Mtei Thanks to: Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 University of Pennsylvania Pat Carroll
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Fc+/Fc Mo(5+/4+) Ligand reductions? Electrochemistry E 1/2 + 53 mV E 1/2 -96 mVE 1/2 + 250 mV Potentials vs. to Ag/AgCl; internal ferrocene at + 400 mV, ACN/TEAP/Pt, 100 mV/s Mo(5+/4+) Potentials Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009
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