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)