Vanadium: The Versatile Metal - American Chemical Society

Chapter 26

Electron Spin Lattice Relaxation of V(IV) Complexes in Glassy Solutions between 15 and 70 K 1

1

1

Alistair J. Fielding , Dong Bin Back , Michael Engler , Bharat Baruah , Debbie C. Crans , Gareth R. Eaton , and Sandra S. Eaton 2

2

1

Downloaded by UNIV LAVAL on July 2, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch026

1

1

Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208-2436 Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872 2

Electron spin lattice relaxation rates for four vanadium(IV) complexes: bis(acetylacetonate)oxovanadium(IV), (VO(acac) ), 1; bis(maltolato)oxovanadium(IV), (VO(maltol) ), 2; Cesium N,N'-ethylenebis(salicylideneiminato-5'-sulfonato)oxovanadium(IV), Cs [VO(salen-SO )(H O)], 3; and bis(N— hydroxyiminodiacetato)oxovanadium(IV), (Ca[V(hida) ]), 4; in 1:1 water:glycerol glasses were measured by long-pulse saturation recovery at X-band. Although these complexes have coordination spheres that vary from O to N O and N2O6, the relaxation rates differ by factors of only about 2 at 15 K and 4 at 70 K . Relaxation rates for 4, which does not contain an oxo group, are very similar to those for oxo-containing complexes. At 70 K relaxation rates decrease in the order aquo VO2+ > VO(acac) ~ VO(maltol) > [V(hida) ] > VO(salen-SO3) (H O)]2- > vanadyl porphyrin. This order correlates with decreasing flexibility of the ligands and coordination sphere. The temperature dependence of spin lattice relaxation rates was analyzed in terms of contributions from the direct process, the Raman process, and local modes. 2

2

2

3

2

2

5

4

2-

2

2

2

2

364

© 2007 American Chemical Society

In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


365 Although the coordination chemistry of vanadium(IV) complexes has been studied extensively using electron paramagnetic resonance (EPR) (/, 2), there have been few studies of the electron spin relaxation rates of these systems. Prior studies on a limited selection of complexes include primarily lowtemperature measurements in ionic lattices near 4 K (3) and N M R studies in fluid solution (4). Mechanisms of relaxation for aquo vanadyl ion (5) and vanadyl porphyrins (6) in glassy solution between about 10 and 100 K have been analyzed (7). Relaxation rates reflect the electronic structures of the paramagnetic center and the dynamic processes of these species and their environment. Knowledge of typical relaxation rates is useful in predicting observability of EPR signals and feasibility of electron-nuclear double resonance (ENDOR) experiments, and in selecting parameters for recording spectra. The difference between aquo vanadyl and vanadyl porphyrin suggests an impact of structure on relaxation. We report the relaxation rates of vanadium(IV) in complexes with diverse coordination geometries: five-coordinated VO(maltol) 2 and VO(acac) 1, six-coordinated [VO(salen-S0 )(H 0)] " 3, and eightcoordinated non-oxo amavadin analog, [V(hida) ] " 4. Our goal is to understand the relaxation processes that occur in glassy solutions at temperatures between about 15 and 70 K and to determine the extent to which coordination geometry impacts relaxation rates. 2

2

2

3

2

2

2

1

2

0H

2

3

4


366

Experimental VO(acac) , vanadyl sulfate, 3-hydroxy-2-methyl-4-pyrone, salicylaldehyde, ethylenediamine, and bromoacetic acid were purchased from Aldrich. Hydroxylamine hydrochloride was purchased from Fisher Scientific. Reagents were used as received. Deionized water was used throughout and prepared to a specific resistivity of > 18 MSI cm (Barnstead E-pure system). VO(maltol) (5), Cs [VO(salen-S0 )(H 0)] (9), and the non-oxo-vanadium(IV) complex, (Ca[V(hida) ]) (10, 77) were prepared by literature methods. Aqueous stock solutions of VO(acac) (10 m M , pH 4.2), VO(maltol) (5 m M , p H 4.6), Cs [VO(salen-S0 )(H 0)] (25 m M , pH 7.0) and Ca[V(hida) ] (25 m M , pH 5.3) were prepared by dissolving in water in a volumetric flask. The p H values of the aqueous solutions were measured at 25°C using a pH meter (Orion 420A) calibrated with three buffers of p H 4, 7 and 10. The VO(salen-S0 ) complex is assigned as the 6-coordinate water adduct on the basis of the red color of the solution (12). The aqueous solutions were combined 1:1 v:v with glycerol to make mixtures that form a good glass when cooled quickly in liquid nitrogen. C W spectra were recorded on a Varian E9 and computer simulated using the locally-written program, M O N M E R , which is based on the equations in (75). Spin-lattice relaxation rates, 1/7*1, as a function of temperature and position in the spectrum were measured by long-pulse saturation recovery on a locally constructed X-band spectrometer using a rectangular T E i cavity (14). The procedures for data acquisition and analysis are similar to those employed in a recent study of relaxation rates for Cu(II) complexes (75). Analyses of recovery curves as distributions of exponentials were performed using Brown's uniform penalty, U P E N , routines (16, 17). By minimizing the sum of the residuals on a log-log scale, a fit line for the temperature dependence of 1/7*1 for the mi = -1/2 transition was based on Eq. [1] (7). 2

2

2

3

2

2

2


2

3

2

2

2

3

0 2

1 T

T o

l

On

e

T

9D

(e

A l o c / T

A , 0 C

-l)

2

where 7* is temperature in Kelvin, A is the coefficient for the contribution from the direct process, ^Ram is the coefficient for the contribution from the Raman process, is the Debye temperature, J is the transport integral, diT

8

T

o

(e*-l)

2

A\ is the coefficient for the contribution from a local vibrational mode, and A ^ is the energy for the local mode in units of Kelvin. The expressions to describe the temperature dependence of the Raman process (75, 19) and local mode (20) were taken from the literature. QC


367

Results and Discussion The V(IV) complexes were characterized by continuous wave E P R spectra at X-band (~ 9.2 GHz) of samples in water at ambient temperature and in glassy 1:1 water:glycerol at about 120 K (Figure 1). Solvent crystallization can cause locally high concentrations of solute that broaden C W spectra and increase relaxation rates (7, 21). Therefore glycerol was added to the solutions to ensure glass formation at low temperature. However, the presence of glycerol in the solutions for low-temperature spectroscopy raises the question whether glycerol coordinated to the V(IV). A single species was observed in fluid and frozen solution E P R spectra o f VO(maltol) , [VO(salen-S0 )(H 0)] \ and [V(hida) ] \ For each of the complexes the average of anisotropic g and A values in the glassy 1:1 watenglycerol were in good agreement with the isotropic values in water so there was no evidence for complexation of glycerol. For VO(acac) two species were observed in water at room temperature, in agreement with prior studies (27), and in the glassy 1:1 watenglycerol. Comparison of average g and A values for the two species in fluid and glassy solution provide no evidence for glycerol complexation, but to determine whether the two species are the same would require E N D O R (22-24) or electron spin echo envelope modulation (ESEEM) experiments (25). 2


2

3

2

2

2

2

In the glassy solutions all orientations of the molecule with respect to the magnetic field are present so the spectra are superpositions of signals with g and A values characteristic of differing orientations. The first-derivative display emphasizes positions in the spectrum where there are abrupt changes in the amplitude of the absorption signal. These changes occur at magnetic fields that correspond to extrema in the orientation dependence of the EPR resonance. Thus "peaks" in the first derivative signal occur at magnetic fields that correspond to resonance along the principal magnetic axes, x, y, and z. The vanadium nucleus has a spin of 7/2 so for each orientation of the molecule with respect to the external field the EPR signal is split into 8 lines corresponding to mi = -7/2, -5/2, -3/2, -1/2, 1/2, 3/2, and 7/2. At X-band the nuclear hyperfme splitting is much larger than the difference in resonance field that arises from the g anisotropy. The large hyperfine splitting along the magnetic z-axis (153X10" cm" , 171 G) determines the width of the spectrum (Figure 1). The differences between g and g and between A and A are small so these peaks overlap in the center of the spectrum. The g values and hyperfine coupling constants obtained by computer simulation of the spectra are summarized in Table 1 and are in good agreement with literature values for [V(hida) ] " (11), VO(maltol) (26), and VO(acac) (27). Despite the absence of an oxo-group the rigid-lattice E P R spectrum of [V(hida) ] " (Figure 1) is very similar to those for oxo-containing vanadyl complexes. A saturation recovery curve is recorded by applying a pulse of microwaves with sufficiently large microwave magnetic field to saturate (equalize) the 4

1

x

y

x

y

2

2

2

2

2


2

368 Table 1. EPR g and A values at 120 K in glassy 1:1 watenglycerol" Sample Aquo-VO * VO(acac) VO(maltol) [V(hida)J [VO(salen-SO^H Of !

gy.A 1.978, 1.967, 1.974, 1.984, 1.974,

gx, A 1.979, 70 1.980, 54 1.977, 60 1.986, 42 1.978, 46

2

2

2

2

4

gz, A 1.933, 183 1.950, 163 1.939, 169 1.918, 153 1.955, 154 z

y

x

b

70 56 55 49 55

1

'Values of A are in units of 10" cm' . Uncertainties in g and A values are ±0.001 and ±1, respectively. EPR parameters reported in (23, 28) in water.


b

—, 2800

1

j

1

j

1

1—

3200 3600 magneticfield(gauss)

4000

2

Figure 1. X-band (9.2267 GHz) spectrum of [V(hida)rf ~ in 1:1 water:glycerol at 118 K. The dashed line is a simulation using the parameters listed in Table 1. The asterisk marks the position of the perpendicular component of the m = -1/2 transition where most of the saturation recovery curves were recorded. t

populations of the electron spin energy levels, which nulls the E P R signal. The return to equilibrium of the amplitude of the E P R signal is then recorded using very low (minimally-perturbing) microwave power (29). The time constant for return to equilibrium is the spin-lattice relaxation time, T Representative saturation recovery curves for the non-oxo complex [V(hida) ] ", recorded at the peak of the m = -1/2 line, are shown in Figure 2. Analogous to observations for Cu(II) complexes, better fits to the saturation recovery curves were obtained with a distribution of exponentials than with a single exponential (75). The dashed lines in Figure 2 are fit functions based on distributions of exponentials calculated using the uniform penalty method (16, 17). The widths of the distributions were 2 to 3 times the central value. Distributions in g and A values, h

2

2

{


369


sometimes called g-strain and >4-strain, are routinely observed for Cu(II) complexes (30, 31) and more recently for vanadyl complexes (32). These distributions reflect variations in geometry that may also impact relaxation rates. The observation of distributions in relaxation rates may be characteristic of transition metal complexes.

\

63 K

I 200

c)

'

I 400 time (jxsec)

•

I 600

'

I 800

42 K

1

I c)

1

0

0

1

I time (usee) 2000

0

S**^^

1

c)

I 3000 20 K

1 10

1

1 20

1

time (msec)

2

Figure 2. X-band saturation curves for 2.5 mM [V(hida) ] ' in 1:1 water.glycerol as a function of temperature recorded at the maximum in the absorption signal, which is on the m = -1/2 line. Note the changes in the scales of the x axes as a function of temperature. The dashed lines are fits to the data for a distribution of exponentials. 2

t

In some systems electron spin relaxation rates are strongly dependent upon the orientation of the molecule with respect to the external field (7). Relaxation


370 2

rates at 62 K for [V(hida) ] " and VO(maltol) were measured at a series of positions in the spectrum, but the variation with orientation was less than a factor of 2. This small orientation dependence of Tj is similar to what was observed previously for a vanadyl porphyrin (6). 2

2

Temperature dependence of relaxation rates The temperature dependence of the median values from the distributions of relaxation rates measured for the mi = -1/2 transition was analyzed by fitting equation [1] to the data, as shown in Figure 3 for [V(hida) ] \ At the lowest temperatures studied there was a small contribution from the direct process, which is characterized by a weak temperature dependence. This process involves a single photon and increases as the local concentration of spins increases. The dominant process between about 20 and 40 K is the two-photon Raman process which arises from the many motional modes with energies less than the Debye temperature. At higher temperature there is a contribution from an additional process that can be modeled as a local vibrational mode with an energy of about 205 K (140 cm" ). A n equally good fit to the X-band experimental relaxation rates could be obtained with a thermally-activated process instead of the local mode. However a thermally-activated process predicts relaxation rates that are dependent on microwave frequency. For a vanadyl porphyrin the relaxation rate at 80 K and 95 GHz is about the same as at 9.5 G H z which is inconsistent with a thermally-activated process (33). In view of the similarity in relaxation rates for the series of V(IV) complexes, discussed below, the additional process for [V(hida) ] " is assigned as local mode. 2


2

1

2

2

Comparison of temperature dependence of relaxation rates The relaxation rates as a function of temperature for the four V(IV) complexes in 1:1 watenglycerol are compared (Figure 4) with previously reported data for aquo V 0 in 1:1 watenglycerol and for a vanadyl porphyrin in 2:1 toluene:CHCl . Over the full temperature range there is substantial similarity in the relaxation rates for the six V(IV) complexes and in the parameters required to fit the temperature dependence (Table 2). The relaxation rates for [V(hida) ] ", which does not have an oxo group, are similar to those for the oxocontaining complexes. The shape of the curve for VOTTP-bipy at low temperature is different than for the other complexes because of a larger contribution from the direct process, which is attributed to stacking of the planar aromatic molecules. The differences between molecules are largest at higher temperatures where local modes dominate the relaxation. At about 70 K the relaxation rates decrease in the order aquo V 0 > VO(acac) - VO(maltol) > 2 +

3

2

2

2 +

2


2

371 2

2

[V(hida) ]) " > [VO(salen-S0 )(H 0)] " > vanadyl porphyrin, which is the order in which entries are listed in Table 2. Modulation of spin-orbit coupling is a significant mechanism of spin-lattice relaxation. Deviation of g values away from the free electron value (g = 2.0023) is a qualitative indicator of the magnitude of spin-orbit coupling. The g values for the V(IV) complexes are quite similar, which is consistent with the observation of similar spin-lattice relaxation rates. At 70 K the relaxation rates for all of the V(IV) complexes are slower than for a wide range of Cu(II) complexes (15). The g values for the Cu(II) complexes were in the range of 2.08 to 2.33, which is substantially further from g = 2.0023 than for V(IV), which indicates greater spin-orbit coupling and is consistent with faster relaxation for the Cu(II) complexes than for the V(IV) complexes. 2

3

2


z

Figure 3. Temperature dependence of X-band spin lattice relaxation rates for the mj = -1/2 line for [V(hida) ] ' in 1:1 water glycerol. The solid lines through the data are fits obtained using Eq. [1] and the parameters in Table 2. The contributions to the relaxation from the direct process (—), Raman process (--) and local mode (--•) are shown separately. 2

2



372

Figure 4. Temperature dependence of X-band spin lattice relaxation rates for the perpendicular lines in the spectra of aquo V&*(A), VO(acac) (•), VO(maltol) (O), [V(hida) f (+), and [VO(salen-S0 )H 0] ~ (O), in 1:1 water .glycerol and vanadylporphyrin (V) in 2:1 toluene :CHCl . The solid lines through the data are fits obtained using Eq. [1] and the parameters in Table 2. 2

2

2

2

3

2

3

Results from prior studies (7, 15) suggest that molecular flexibility contributes to differences in relaxation rates - the less flexible the molecule, the slower the relaxation. Studies presented here allow testing of this hypothesis for V(IV) complexes with different geometries and ligands. The 5-coordinate complexes investigated are VO(acac) , VO(maltol) , and vanadyl porphyrin. In addition the 6-coordinated aquo V 0 and [VO(salen-S0 )(H 0)] ' and an 8coordinated non-oxo amavadin analog [V(hida) ] " were investigated. The more flexible metal-ligand complexes aquo V 0 , VO(acac) , and VO(maltol) are compared with the more rigid [V(hida) ] ", [VO(salen«S0 )(H 0)] " and vanadyl porphyrin. The relaxation rates decrease in the order aquo V 0 > bidentate VO(acac) , VO(maltol) > polydentate [V(hida) ] " > polydentate [VO(salen2

2

2 +

2

3

2

2

2

2 +

2

2

2

2

2

3

2

2 +

2

2

2

2


373 2

S 0 ) H 0 ] " > rigid polydentate vanadyl porphyrin. The decreasing relaxation rates with increasing denticity of the ligands is consistent with slower relaxation for more rigid complexes. Within the set of polydentate ligands the slowest relaxation is observed for the porphyrin which has the rigid aromatic ligand. The similarity of relaxation rates for the non-oxo containing [V(hida) ] " and the oxo-vanadium complexes indicates that motion of the vanadyl unit is not a major determinant of relaxation rates. 3

2

2

2

Table 2. Contributions to spin-lattice relaxation at 15 to 70 K in 1:1 watenglycerol glasses 3


Sample

coord.

A

g, z

z

Siso

Direct

Raman

Local

Adir

AR

A^,

0

4.4x10 , 110 5.8xl0 , 110 7.4x10 , 110 2.2x10 , 110 1.5xl0 , 110 6.5x 10 , 100

a m >

Aloe

§D

Aquo V 0

2 +

VO(acac)

"

2

VO(maltol)

2

o

6

o

5

o

5

c

[V(hida) f

N 0 2

6

[VO(salen S0 )H 0f VOporphyrin

N G

4

2

3

2

2

N 0 4

b

a

4

1

1.933, 1.969 1.950, 1.966 1.939, 1.963 1.918, 1.964 1.955, 1.963 1.984, 1.971 1

c

163

3.5

169

0

152

2.0

154

3.0

158

8.5

1

Units are: A (10" cm' ), A*, (s' K" ), A z

183

5

5

5

5

5

4

1

R a m

(s' ), ^ (K), A

toc

(s'), A

loc

5

9.2x10 , 185 7.0x10 , 205 3.0xl0 , 205 4.5xl0 , 205 2.5x10 , 205 6.5xl0 , 350 s

5

5

5

5

(K).

b

Contributions to relaxation in 2:1 toluenexhloroform reported in (34). parameters reported in (28).

c

EPR

Acknowledgments Financial support of this work by NIH/NIBIB EB002807 (GRE and SSE) and NSF 0314719 (DCC) is gratefully acknowledged.

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Chasteen, N. D. Biol. Magn. Reson. 1983, 3, 53-119. Smith, T. S.; LoBrutto, R.; Pecoraro, V . L . Coord. Chem. Rev. 2002, 228, 1-18.


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9. 10. 11. 12.

13. 14. 15.

16. 17. 18. 19. 20. 21.

22. 23.

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Vanadium: The Versatile Metal - American Chemical Society

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