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

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

Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009 Our aim is to make molybdenum dithiolene complexes 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

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

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*

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.

The Strategy + * No reaction with Mo=O Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009

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

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

Comparison of Mo(5+/4+) Potentials in Tp*MoO(S-S) Complexes Frontiers in Metal Dithiolene Chemistry ACS 2008 Prior Work Electrochemistry

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

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.

Introduction We figured it out Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009

[M-] 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

[M-] 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!

[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

[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

Pterin-Dithiolene Model System Gordon Research Conference on Mo & W Enzymes Lucca, Italy H HMR shows loss of H7 consistent with pyran ring formation H7 N-H open pterin-dithiolene pyranopterin-dithiolene

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

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

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 A Reductive Titration of Mo(5+)- Pyranopterin Forms a Reduced Pterin Complex

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

[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

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

Mo1-O (3) Å S1-C (5) Å S2-C (5) Mo1-S (1) Å Mo1-S (1) C1-C (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

Dihedral=4.8 deg Between quinox & pyrrole Asymmetry further displayed in twist of quinoxaline

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 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

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

in Acetonitrile vs. Ag/AgCl bdt ead 2 x -SPh 2 x -SEt S4S4 dmac qdt V 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

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

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

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 )

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

 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

Bryn Mawr College, Bryn Mawr, Pennsylvania Kelly Ginion Matz Tanya Michelle Corder Belinda Leung Alison Panosian Rebecca Rothstein $$NIH-GM 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

   Fc+/Fc Mo(5+/4+)   Ligand reductions?        Electrochemistry E 1/ mV E 1/2 -96 mVE 1/ mV Potentials vs. to Ag/AgCl; internal ferrocene at mV, ACN/TEAP/Pt, 100 mV/s Mo(5+/4+) Potentials Gordon Research Conference on Mo & W Enzymes Lucca, Italy 2009