Exploring the redox states and reactivity of a vanadium bis‐ tetrazinylpyridine complex with DFT tetrazinylpyridine complex with DFT Adam M. Terwilliger (GVSU) Kenneth G. Caulton (Indiana) Kenneth G. Caulton (Indiana) Richard L. Lord (GVSU)
Exploring the redox states and p greactivity of a vanadium bis‐
tetrazinylpyridine complex with DFTtetrazinylpyridine complex with DFT
Adam M. Terwilliger (GVSU)
Kenneth G. Caulton (Indiana)Kenneth G. Caulton (Indiana)
Richard L. Lord (GVSU)
Redox‐Active Ligands
• Polypyridine ligands popular in redox catalysisPolypyridine ligands popular in redox catalysis
• Recognized for ability to “accept” an electron
• Idea: make the ligand more electron acceptingIdea: make the ligand more electron accepting by introducing additional nitrogens
Luca, O.R.; Crabtree, R.H. Chem. Soc. Rev. 2013, 42, 1440‐1459. Caulton, K.G. Eur. J. Inorg. Chem. 2012, 13, 435‐443.
Redox‐Active Ligands
• btzp + electron rich V(III) sourcebtzp + electron rich V(III) source
• Expected: (btzp)VCl3 with one ofVIII bt 0– VIII + btzp0
– VIV + btzp1–
V 2– VV + btzp2–
• Found: (btzp‐H)VCl2O
• What are the redox states?What are the redox states?
Vanadium‐Oxo Applications
• Biological reactions and enzyme inhibitionBiological reactions and enzyme inhibition
Figure from: Crans, D.C.; Smee, J.J.; Ernestas, G.; Yang, L. Chem. Rev. 2004, 104, 849‐902.
Vanadium‐Oxo Applications
• Oxidation catalysts in organic chemistryOxidation catalysts in organic chemistry
Figures from: Hirao, T. Chem. Rev. 1997, 97, 2707‐2724.
Methods
• Calculations used Gaussian09Calculations used Gaussian09
• B3LYP/LANL2DZ/6‐31G(d,p) level of theory
f i fi d b bl• Wavefunctions confirmed to be stable
• Minima verified through harmonic analysis
• Redox states were assigned by– visualizing spin densitiessua g sp de s es
– analyzing corresponding orbitals
– comparing bond lengthscomparing bond lengths
Goals
• What are the oxidation states of the metal andWhat are the oxidation states of the metal and ligands in the lowest energy spin state of [(btzp)VCl2O]0?[(btzp)VCl2O] ?
• Which N atom does H atom prefer to bind to in this complex?in this complex?
• How does the electron distribution change h h H bi d b ?when the H atom binds to btzp?
doublet(S = 1/2)
quartet(S = 3/2)( / ) ( / )
N1‐N2 1.307 1.321
N1‐C3 1.361 1.356
N2 C2 1 349 1 339N2‐C2 1.349 1.339
C2‐N3 1.346 1.356
N3‐N4 1.321 1.320
C3‐N4 1.333 1.335
Relative Free Energy
0.00 +42.49
Unpaired electron Spin density
Conclusions for [(btzp)VCl2O]0
• The spin density and SOMO show that thep yunpaired electron density is concentrated aroundthe metal center with no concentration on thebtzp ligandbtzp ligand.
• The spin density plot shows a slight excess of spin (white) at the oxygen; however, thecorresponding orbital analysis (used to generatethe SOMO) did not identify an unpaired electronon Oon O.
• This finding of one unpaired electron at the metalis consistent with VIV and btzp0.
Which N Does H Bind To?
• H atom can bind to N2, N3, N4, ,
• Proton or H(dot)?
• If H+ where does that• If H+, where does thatelectron go to?
III / ( )• VIII / btzp‐H+ (seems unlikely)
• VIV / btzp‐H0 (where is radical?)
• VV / btzp‐H– (can btzp oxidize VIV?)
• [(btzp)VCl O] + (triplet) or (singlet)[(btzp)VCl2O] + (triplet) or (singlet)
Species Spin State H PositionRelative
Species Spin State H PositionFree Energy
2S Singlet N2 –1.54
2T Triplet N2 +0.38T
3S Singlet N3 0.00
3T Triplet N3 +1.36
4 Singlet N4 +7 474S Singlet N4 +7.47
4T Triplet N4 +7.21
2 /3 l t i 3 t h ll ith• 2S/3S lowest in energy. 3S matches well with experimental structure. Is 2S artificially stabilized?
intramolecular H‐bondintramolecular H‐bond
Top‐down view ofthe optimizedstructures showingH‐bonding in 2S (left)vs. 3S (right).
What Are Redox/Spin States in 3S?
• Consistent with VIV and btzp‐H0, AF‐coupled
Conclusions for [(btzp‐H)VCl2O]0Conclusions for [(btzp H)VCl2O]
• Excellent structural agreement with expExcellent structural agreement with exp.
• Crystallography suggested anionic btzp‐H
C l l i h b i h d• Calculations show btzp‐H is uncharged
• The metal SOMO does notmix significantly with the ligand SOMO (S = 0.36); spatial separation of opposite spins is found to give a more stable electronic structure
Submitted to Acta Crystallographica C
AcknowledgementsAcknowledgements
• Prof. Caulton and his group at IU for provoking o . Cau to a d s g oup at U o p o o gour interest in this chemistry (NSF/CHE‐0822838)
• GVSU Office of Undergraduate Research and gScholarship for a Modified Student Summer Scholar Award to Adam Terwilliger
• GVSU Center for Scholarly and Creative Excellence Faculty Research Grant‐in‐Aid to Richard LordRichard Lord
• MU3C for Computational Resources (NSF/CHE‐1039925)1039925)