Ligand Substitution of Ruthenium (II) Complexes that ...
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Effect of Ligand Substitution of Ruthenium (II) Complexes on Oxidation and MLCT Energy Gaps Dakota Tan, Megan Farley, John Hendrix, Jeffery Curtis, Lawrence Margerum Department of Chemistry, University of San Francisco ACS ® UNIVERSITY OF SAN FRANCISCO Introduction Methods Connelly et al. showed that when an electron is excited and a visible color change is seen, an electron gets delocalized from the ruthenium t2g orbital to some anti-bonding region (π*) on the substituted pyridine.1 The MO diagram (a) above shows the mixing between a d6 metal’s t2g and a ligand π*orbital. Our goal was to show that changing the substituents on the pyridine would change the energy of the π* orbital (coming from a ligand). More specifically, a more electron-withdrawing substituent on X, Y or Z of our ruthenium complex would lower in energy of the π*. (Shown by MO diagram (b)) This would also decrease the energy level of the HOMO of the complex, leading to a greater ionization energy, thus greater voltage for oxidation. A more basic polar solvent, such as DMSO, would also lower the energy of the complex HOMO. Questions to be answered: 1) How easily can each complex be oxidized? 2) How will each substituent and their position on the pyridine ligand affect energy levels? 3) Will data support previously conducted computational methods showing that this type of transition is chemically allowed? Acknowledgements Results Using three complimentary techniques (H 1 NMR, UV/Vis spectrometry, and electrochemical voltammetry) data was gathered on ruthenium (II) coordination compounds. Data was collected using the three complimentary methods and the calculated E 1/2 values, MLCT energies and H 1 NMR chemical shifts support computational methods showing the presence of d-d orbital back bonding of the substituted ruthenium complexes. The comparison of E ½ and MLCT values show that symmetry would allow for the excitation of an electron from the t2g to π * orbital would be symmetrically allowed. Computational analysis were carried out with a Gaussian program on Ru (II) complexes by Dr. Jeff Curtis in order to optimize geometry of the HOMO and LUMO states of the free ligands and predict this allowed symmetry. The 3 position on the ligand has closer proximity to ruthenium and more molecular orbital overlap. Results show that ligand substitution on that position result in lower energy in comparison to substitution on the 4 position (para to the ruthenium metal). The lower the energy the more allowed it is for the excitation of an electron into the π * orbital of the ligand. Results also show that the exchange of a bromine for a chlorine substituent does not effect the oxidation and no significantly greater red shift occurs in the UV/Vis spectra. In the field of inorganic chemistry, color is commonly associated with d–d transitions. In transition metal complexes a change in electron distribution between the metal and a ligand gives rise to charge transfer (CT) bands when performing Ultraviolet/visible spectroscopy. 2 When the ruthenium pyridine complex absorbs light, the electron is promoted from the HOMO to the LUMO. The homo has more ruthenium character from the t2g orbitals so the electron is considered to be from the ruthenium. The LUMO has more pyridine character (from the π* orbitals) so when the promotion of the e - occurs, the transition is called metal to ligand charge transfer (MLCT). Upon changing the substituents to more electron withdrawing groups, the π* orbitals lower in energy which then lower the octahedral splitting of the MO. This allows for an electron to be promoted from the HOMO to LUMO with less energy. According to E=hc/λ, the energy required to promote the e - (i.e. the absorbed light) lowers in energy from uv-vis region towards more visible (see figure 2). This shift of absorbed light causes the complex to absorb lower energy light turning the complex from yellow to red. 3) Voltammetry – 3 Electrode System 0 0.2 0.4 0.6 0.8 1 1.2 1.4 185 385 585 785 Absorbance(AU) Wavelength(nm) λ max = 408nm MLCT = 3.100 eV 1) H 1 NMR Shows us the chemical composition differences between each complex. Shifting of peaks up field or downfield was substituent dependent. 1 Chemical Redox Agents for Organometallic Chemistry, Neil G. Connelly*,† and and William E. Geiger*,‡ Chemical Reviews 1996 96 (2), 877-910 DOI: 10.1021/cr940053x 2 Chem Libre Texts. “Metal to Ligand and Ligand to Metal Charge Transfer Bands.” Chem Libre Texts, National Science Foundation , 15 Mar. 2017 3 Paris, J. P., and Warren W. Brandt. "Charge Transfer Luminescence of a Ruthenium(II) Chelate." Communications to the Editor 81 (1959): 5001-002. Ru 2+ NH 3 NH 3 NH 3 H 3 N H 3 N N X Y Z 2PF 6 - Absorbance of Ru (II) petamminepyridine Photo taken of standard solutions of Ruthenium (II) complexes in solvent. Over the last few years the field of coordination chemistry of ruthenium has grown. Ruthenium (II) and ruthenium (III) carry oxidation states that accommodate six-coordinated octahedral configurations in which attached ligands can determine its chemical and optical properties. Voltammetry such as differential pulse voltammetry (DPV) reveals the energy required to oxidize (remove an electron from) ruthenium(II). When a ligand attached to ruthenium (II) has an electron-withdrawing substituent, such as 3,5-dichloropyridine (blue), more voltage is required when compared with ruthenium with a ligand containing an electron-donating substituent, such as 4-methylpyridine (green). These samples were solvated in acetonitrile. References 2) UV/Visible Spectroscopy 4-Methyl Pyridine 4-Chloro 3-Chloro 3,5-Dichloro 3- Bromo 4-Bromo 2.70 2.75 2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 -0.10 -0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10 MLCT (eV) DPV E 1/2 (V) DPV E 1/2 and Experimental E MLCT in Acetonitrile t 2g t 2g π * π * energy level for ionization hv hv IE IE complex HOMO a) b) 3 2 4 5 6 0 2 4 6 8 10 12 14 -300 -200 -100 0 100 200 300 Current (μA) E vs. fc/fc+ (V) DPV in Acetonitrile 4MEPY AN 35CLPY AN I would like to thank my research director Dr. Lawrence Margerum and Dr. Jeffery Curtis for all their help and support throughout this project. Under whom, I have learned much about organometallics and inorganic chemistry in general. I would also like to thank Megan Farley and John Hendrix for their contributions to this research as well as the USF Chemistry Department as a whole. H 3 N H 3 N NH 3 NH 3 NH 3 Methods cont. Figure 1: General ruthenium (II) pentammine complex (left) showing general substituents changes on the X, Y, and Z positions of sixth ligand, which were pyridine analogues. Molecular orbital (MO) diagram of the ruthenium complex with (a) electron donating groups (H, methyl) and with (b) electron withdrawing groups (Cl, Br) Figure 2: UV-vis of ruthenium pentamminepyridine (left) and representative stock solutions of ruthenium complexes (right) showing color changes. Figure 3: Comparison of E ½ and MLCT energy values of all tested ruthenium (II) complexes.
Transcript of Ligand Substitution of Ruthenium (II) Complexes that ...
Effect of Ligand Substitution of Ruthenium (II) Complexes on Oxidation and MLCT Energy Gaps
Dakota Tan, Megan Farley, John Hendrix, Jeffery Curtis, Lawrence MargerumDepartment of Chemistry, University of San Francisco ACS ®UNIVERSITY OF
SAN FRANCISCO
Introduction
Methods
Connelly et al. showed that when an electron is excited and a visible color change is seen, anelectron gets delocalized from the ruthenium t2g orbital to some anti-bonding region (π*) onthe substituted pyridine.1 The MO diagram (a) above shows the mixing between a d6 metal’st2g and a ligand π*orbital. Our goal was to show that changing the substituents on thepyridine would change the energy of the π* orbital (coming from a ligand). More specifically, amore electron-withdrawing substituent on X, Y or Z of our ruthenium complex would lower inenergy of the π*. (Shown by MO diagram (b)) This would also decrease the energy level ofthe HOMO of the complex, leading to a greater ionization energy, thus greater voltage foroxidation. A more basic polar solvent, such as DMSO, would also lower the energy of thecomplex HOMO.
Questions to be answered: 1) How easily can each complex be oxidized? 2) How will each substituent and their position on the pyridine ligand affect energy levels? 3) Will data support previously conducted computational methods showing that this type of transition is chemically allowed?
Acknowledgements
Results
Using three complimentary techniques (H1 NMR, UV/Vis spectrometry, and electrochemicalvoltammetry) data was gathered on ruthenium (II) coordination compounds.
Data was collected using the three complimentary methods and the calculated E1/2 values,MLCT energies and H1 NMR chemical shifts support computational methods showing thepresence of d-d orbital back bonding of the substituted ruthenium complexes. Thecomparison of E ½ and MLCT values show that symmetry would allow for the excitation ofan electron from the t2g to π * orbital would be symmetrically allowed. Computationalanalysis were carried out with a Gaussian program on Ru (II) complexes by Dr. Jeff Curtis inorder to optimize geometry of the HOMO and LUMO states of the free ligands and predictthis allowed symmetry.
The 3 position on the ligand has closer proximity to ruthenium and more molecular orbitaloverlap. Results show that ligand substitution on that position result in lower energy incomparison to substitution on the 4 position (para to the ruthenium metal). The lower theenergy the more allowed it is for the excitation of an electron into the π * orbital of the ligand.Results also show that the exchange of a bromine for a chlorine substituent does not effectthe oxidation and no significantly greater red shift occurs in the UV/Vis spectra.
In the field of inorganic chemistry, color is commonly associated with d–d transitions. Intransition metal complexes a change in electron distribution between the metal and a ligandgives rise to charge transfer (CT) bands when performing Ultraviolet/visible spectroscopy.2When the ruthenium pyridine complex absorbs light, the electron is promoted from theHOMO to the LUMO. The homo has more ruthenium character from the t2g orbitals so theelectron is considered to be from the ruthenium. The LUMO has more pyridine character(from the π* orbitals) so when the promotion of the e- occurs, the transition is called metal toligand charge transfer (MLCT). Upon changing the substituents to more electron withdrawinggroups, the π* orbitals lower in energy which then lower the octahedral splitting of the MO.This allows for an electron to be promoted from the HOMO to LUMO with less energy.According to E=hc/λ, the energy required to promote the e- (i.e. the absorbed light) lowers inenergy from uv-vis region towards more visible (see figure 2). This shift of absorbed lightcauses the complex to absorb lower energy light turning the complex from yellow to red.
3) Voltammetry – 3 Electrode System
0
0.2
0.4
0.6
0.8
1
1.2
1.4
185 385 585 785
Abso
rban
ce(A
U)
Wavelength(nm)
λmax = 408nm
MLCT = 3.100 eV
1) H1NMRShows us the chemical composition differences between each complex. Shifting of peaks up field or downfield was substituent dependent.
1 Chemical Redox Agents for Organometallic Chemistry, Neil G. Connelly*,† and and William E. Geiger*,‡ Chemical Reviews 1996 96 (2), 877-910 DOI: 10.1021/cr940053x
2Chem Libre Texts. “Metal to Ligand and Ligand to Metal Charge Transfer Bands.” Chem Libre Texts, National Science Foundation , 15 Mar. 2017
3Paris, J. P., and Warren W. Brandt. "Charge Transfer Luminescence of a Ruthenium(II) Chelate." Communications to the Editor 81 (1959): 5001-002.
Ru2+
NH3
NH3
NH3H3N
H3N NX
Y
Z
2PF6-
Absorbance of Ru (II) petamminepyridine
Photo taken of standard solutions of Ruthenium (II) complexes in solvent.
Over the last few years the field of coordination chemistry of ruthenium has grown. Ruthenium(II) and ruthenium (III) carry oxidation states that accommodate six-coordinated octahedralconfigurations in which attached ligands can determine its chemical and optical properties.
Voltammetry such as differential pulse voltammetry (DPV) reveals the energy required tooxidize (remove an electron from) ruthenium(II). When a ligand attached to ruthenium (II)has an electron-withdrawing substituent, such as 3,5-dichloropyridine (blue), more voltage isrequired when compared with ruthenium with a ligand containing an electron-donatingsubstituent, such as 4-methylpyridine (green). These samples were solvated in acetonitrile.
References
2) UV/Visible Spectroscopy
4-Methyl
Pyridine
4-Chloro
3-Chloro
3,5-Dichloro
3- Bromo4-Bromo
2.70
2.75
2.80
2.85
2.90
2.95
3.00
3.05
3.10
3.15
-0.10 -0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10
MLC
T (e
V)
DPV E1/2 (V)
DPV E1/2 and Experimental EMLCT in Acetonitrile
t2gt2g
π*
π*
energy level for ionization
hv
hv
IE
IE
complex HOMO
a) b)
32
45
6
0
2
4
6
8
10
12
14
-300 -200 -100 0 100 200 300
Curr
ent (μA
)
E vs. fc/fc+ (V)
DPV in Acetonitrile
4MEPY AN 35CLPY AN
I would like to thank my research director Dr. Lawrence Margerum and Dr. Jeffery Curtis for alltheir help and support throughout this project. Under whom, I have learned much aboutorganometallics and inorganic chemistry in general. I would also like to thank Megan Farleyand John Hendrix for their contributions to this research as well as the USF ChemistryDepartment as a whole.
H3N
H3NNH3
NH3NH3
Methods cont.
Figure 1: General ruthenium (II) pentammine complex (left) showing general substituents changes on the X, Y, and Z positions ofsixth ligand, which were pyridine analogues. Molecular orbital (MO) diagram of the ruthenium complex with (a) electron donatinggroups (H, methyl) and with (b) electron withdrawing groups (Cl, Br)
Figure 2: UV-vis of ruthenium pentamminepyridine (left) and representative stock solutions of ruthenium complexes (right) showingcolor changes.
Figure 3: Comparison of E ½ and MLCT energy values of all tested ruthenium (II) complexes.
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