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)
