1. EPR Study of Vanadyl Complexes

2. Experimental Objectives
To synthesize to vanadyl complexes
Vanadyl acetylacetonate
Bis(O,O’-diethyldithiophosphato)oxovanadium (IV)
Compare the EPR spectra of the two complexes
Observe how the variations in the chemical environment affect the spectrum of the same d1 system

3. Chemistry of Vanadium First discovered by A.M. del Rio (1801)
VOSO4
First discovered by A.M. del Rio (1801)
Rediscovered by N.G. Sefström (1830)
Named for Vanadis
Natural abundance ~0.014%
19th most abundant element
5th most abundant transition metal
Primary industrial use is in alloy steels and cast iron
Adds ductility and shock resistance
Iron alloy, ferrovanadium
Formal oxidation states -1 to +5
V2O5
Element 23, first claimed to have been discovered by A. M. del Rio in 1801, then rediscovered by N. G. Sefström in 1830, was named vanadium after Vanadis, the Scandinavian goddess of beauty, because of the richness and variety of colors in its compounds. While vanadium is widely spread with a natural abundance of ~0.014% (making it the nineteenth most abundant element and fifth most abundant transition metal), there are few concentrated deposits and pure vanadium is rare due to its reactivity toward oxygen. The primary commercial uses are in alloy steels and cast iron to which it adds ductility and shock resistance. Consequently, commercial production of vanadium is as an iron alloy, ferrovanadium. Although vanadium can possess formal oxidation states from +5 to -1, the most stable under normal conditions is +4, with +3 and +2 states also having many well-characterized compounds.
Pb5(VO4)3Cl V

4. Vanadium(IV) Chemistry is dominated by formation of oxo species
VOSO4
Chemistry is dominated by formation of oxo species
Vanadyl ion, VO2+
Often a result of hydrolysis of other VIV compounds
Usually blue to green
Form stable complexes with F, Cl, N and O ligands
Frequently 5 coordinate and square pyrimidal
Many compounds containing the vanadyl unit have two characteristic features:
EPR spectrum
Characteristic g values
51V hyperfine coupling
Strong V=O stretching band
A brief review of EPR spectroscopy…
The chemistry of VIV is dominated by the formation of the oxo species, and a wide range of compounds with VO2+ (vanadyl) groups are known. The extremely stable VO2+ complexes are the most widely studied of the vanadium(IV) complexes and are often a result of hydrolysis of other vanadium(IV) complexes. Many of compounds containing the VO2+ unit are blue to green, form stable complexes with F, Cl, O, and N donor ligands, and can be cationic, neutral, or anionic. Most often, these compounds are 5 coordinate and are almost always square pyramidal. Frequently, these vanadium(IV) complexes are studied by EPR spectroscopy as they have one unpaired electron and possess very characteristic spectra.
VO(acac)2
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1995, C51, 12-14

5. ESR Spectroscopy Electron Spin Resonance Spectroscopy
Also called EPR Spectroscopy
Electron Paramagnetic Resonance
Spectroscopy
Applicable for species with one or more
unpaired electrons
ESR measures the transition between the electron spin energy levels in a magnetic field
Transition induced by the appropriate frequency radiation
Required frequency of radiation dependent upon strength of magnetic field
Common field strengths 9.5 and 35 GHz (microwave region)
Important features of the spectrum
Proportionality factor, g
Hyperfine interactions
Electron spin resonance (ESR) spectroscopy, also referred to as electron paramagnetic resonance (EPR) spectroscopy, is a versatile, nondestructive analytical technique which can be used for a variety of applications including: oxidation and reduction processes, biradicals and triplet state molecules, reaction kinetics, as well as numerous additional applications in biology, medicine and physics. However, this technique can only be applied to samples having one or more unpaired electrons. When an electron is placed within an applied magnetic field, Bo, the two possible spin states of the electron have different energies. This energy difference is a result of the Zeeman effect. The lower energy state occurs when the magnetic moment of the electron is aligned with the magnetic field and a higher energy state where m is aligned against the magnetic field. The two states are labeled by the projection of the electron spin, MS, on the direction of the magnetic field, where MS = -1/2 is the parallel state, and MS = +1/2 is the antiparallel state. In EPR spectrometers a phase-sensitive detector is used. This results in the absorption signal being presented as the first derivative. So the absorption maximum corresponds to the point where the spectrum passes through zero. This is the point that is used to determine the center of the signal.

6. How does the spectrometer work?
Shown is a block diagram for a typical EPR spectrometer. The radiation source usually used is called a klystron. Klystrons are vacuum tubes known to be stable high power microwave sources which have low-noise characteristics and thus give high sensitivity. A majority of EPR spectrometers operate at approximately 9.5 GHz, which corresponds to about 32 mm. The radiation may be incident on the sample continuously (i.e., continuous wave, abbreviated cw) or pulsed. The sample is placed in a resonant cavity which admits microwaves through an iris. The cavity is located in the middle of an electromagnet and helps to amplify the weak signals from the sample. Numerous types of solid-state diodes are sensitive to microwave energy and absorption lines then be detected when the separation of the energy levels is equal or very close to the frequency of the incident microwave photons. In practice, most of the external components, such as the source and detector, are contained within a microwave bridge control. Additionally, other components, such as an attenuator, field modulator, and amplifier, are also included to enhance the performance of the instrument.

7. Proportionality Factor
Measured from the center
of the signal
g =
For a free electron
For organic radicals
Typically close to free-
electron value
For transition metal compounds
Large variations due to spin-orbit coupling and zero-field splitting
As mentioned earlier, an EPR spectrum is obtained by holding the frequency of radiation constant and varying the magnetic field. Absorption occurs when the magnetic field “tunes” the two spin states so that their energy difference is equal to the radiation. This is known as the field for resonance. As spectra can be obtained at a variety of frequencies, the field for resonance does not provide unique identification of compounds. The proportionality factor, however, can yield more useful information.
For a free electron, the proportionality factor is For organic radicals, the value is typically quite close to that of a free electron with values ranging from For transition metal compounds, large variations can occur due to spin-orbit coupling and zero-field splitting and results in values ranging from

8. Proportionality Factor
MoO(SCN)52-
1.935
VO(acac)2
1.968 e-
2.0023
CH3
2.0026
C14H10 (anthracene) cation
2.0028
C14H10 (anthracene) anion
2.0029
Cu(acac)2
2.13
acac = acetylacetonate

9. Hyperfine Interactions
EPR signal is ‘split’ by neighboring nuclei
Called hyperfine interactions
Provides information about number and identity of nuclei
and their distance from unpaired electron
Selection rules same as for NMR
Every isotope of every element has a ground state nuclear spin quantum number, I
has value of n/2, n is an integer
Isotopes with even atomic number and even mass number have I = 0, and have no EPR spectra
12C, 28Si, 56Fe, …
Isotopes with odd atomic number and even mass number have n even
2H, 10B, 14N, …
Isotopes with odd mass number have n odd
1H, 13C, 19F, 55Mn, …
In addition to the applied magnetic field, unpaired electrons are also sensitive to their local environments. Frequently the nuclei of the atoms in a molecule or complex have a magnetic moment, which produces a local magnetic field at the electron. The resulting interaction between the electron and the nuclei is called the hyperfine interaction. Hyperfine interactions can be used to provide a great deal of information about the sample including providing information about the number and identity of nuclei in a complex as well as their distance from the unpaired electron. This interaction expands the previous equation to:
E = gmBBoMS + aMSmI
where a is the hyperfine coupling constant and mI is the nuclear spin quantum number for the neighboring nucleus.
The rules for determining which nuclei will interact are the same as for NMR. For every isotope of every element, there is a ground state nuclear spin quantum number, I, which has a value of n/2, where n is an integer. For isotopes which the atomic and mass numbers are both even, I=0, and these isotopes have no EPR (or NMR) spectra. For isotopes with odd atomic numbers but even mass numbers, the value of n is even leading to values of I which are integers, for example the spin of 14N is 1. Finally for isotopes with odd mass numbers, n is odd, leading to fractional values of I, for example the spin of 1H is ½ and the spin of 51V is 7/2.

10. Hyperfine Interactions
Coupling patterns (or splitting) same as in NMR
More common to see coupling to nuclei with I > ½
The number of lines = 2NI + 1
Only determines the number of lines--not the intensities
Relative intensities determined by the number of
interacting nuclei
If only one nucleus interacting
All lines have equal intensity
If multiple nuclei interacting
Distributions derived based upon spin
For spin ½ (most common), intensities follow binomial distribution
It is important to note that if a signal is split due to hyperfine interactions, the center of the signal (which is used to determine the proportionality factor) is the center of the splitting pattern. So for a doublet, the center would be half way between the two signals and for a triplet, the center would be the center of the middle line.
The coupling patterns that are observed in EPR spectra are determined by the same rules that apply to NMR spectra. However, in EPR spectra it is more common to see coupling to nuclei with spins greater than ½. The number of lines which result from the coupling can be determined by the formula:
2NI + 1
where N is the number of equivalent nuclei and I is the spin. It is important to note that this formula only determines the number of lines in the spectrum, not their relative intensities.

11. Relative Intensities for I = ½1
1 : 1 2
1 : 2 : 1 3
1 : 3 : 3 : 1 4
1 : 4 : 6 : 4 : 1 5
1 : 5 : 10 : 10 : 5 : 1 6
1 : 6 : 15 : 20 : 15 : 6 : 1
Relative intensities of splitting patterns observed due to hyperfine coupling with a nucleus with I = ½. The splitting patterns are named similar to those in NMR:
2 lines = doublet
3 lines = triplet
4 lines = quartet
5 lines = quintet
6 lines = sextet
7 lines = septet

12. Relative Intensities for I = ½
Computer simulations of EPR spectra for interactions with N equivalent nuclei of spin 1/2.

13. Relative Intensities for I = 11
1 : 1 : 1 2
1 : 2 : 3 : 2 : 1 3
1 : 3 : 6 : 7 : 6 : 3 : 1 4
1 : 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1 5
1 : 5 : 15 : 20 : 45 : 51 : 45 : 20 : 15 : 5 : 1 6
1 : 6 : 21 : 40 : 80 : 116 : 141 : 116 : 80 : 40 : 21 : 6 : 1
Relative intensities of splitting patterns observed due to hyperfine coupling with a nucleus with I = 1.

14. Relative Intensities for I = 1
Computer simulations of EPR spectra for interactions with N equivalent nuclei of spin 1.

15. Hyperfine Interactions
Coupling to several sets of nuclei
First couple to the nearest set of nuclei
Largest a value
Split each of those lines by the coupling to the next closest nuclei
Next largest a value
Continue 2-3 bonds away from location of unpaired electron
If an electron couples to several sets of nuclei, then the overall pattern is determined by first applying the coupling to the nearest nuclei, then splitting each of those lines by the coupling to the next nearest nuclei, and so on.

16. Hyperfine Interactions
Example:
Pyrazine anion
Electron delocalized over ring
Exhibits coupling to two equivalent N (I = 1)
2NI + 1 = 2(2)(1) + 1 = 5
Then couples to four equivalent H (I = ½)
2NI + 1 = 2(4)(1/2) + 1 = 5
So spectrum should be a quintet with intensities 1:2:3:2:1 and each of those lines should be split into quintets with intensities 1:4:6:4:1
An example of this can be seen in the radical anion of pyrazine. Where coupling to two equivalent 14N (I = 1) nuclei gives a quintet with the relative intensities of 1:2:3:2:1 which are further split into quintets with relative intensities of 1:4:6:4:1 by coupling to four equivalent hydrogens.

17. Hyperfine Interactions
Computer simulated EPR spectrum of pyrazine radical anion
EPR spectrum of pyrazine radical anion

18. Experiment Overview Begin synthesis of VO(dtp)2
While cooling, synthesize VO(acac)2
Collect both complexes
Determine
IR spectra
EPR spectra

19. Safety/Tips
Dispose of all waste in the appropriately labeled waste containers – do not throw solutions down the drain
Wear your safety glasses at all times
If you should get any reagents on your skin, rinse with plenty of water and tell your TA
Be sure VOSO4 is completely dissolved before adding ligand
VO(acac)2 – add NaHCO3 slowly with vigorous stirring 19