The Structure and EPR Behavior of Short Nitroxide Biradicals Containing Sulfur Atom in the Bridge

The Structure and EPR Behavior of Short NitroxideBiradicals Containing Sulfur Atom in the Bridge

Alexander I. Kokorin • Victor N. Khrustalev •

Elena N. Golubeva

Received: 11 February 2014 / Revised: 5 March 2014 / Published online: 25 March 2014

� Springer-Verlag Wien 2014

Abstract Two short nitroxide biradicals of similar composition: S(OR6)2 (1) and

O=S(OR6)2 (2), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have

been studied by electron paramagnetic resonance spectroscopy, and X-ray structural

analysis. Variations of the intramolecular electron spin exchange in the biradicals,

dissolved in toluene and ethanol, as a function of temperature were characterized by

changes in the isotropic 14N hyperfine splitting constant a, values of the exchange

integral Jj j; and compared with the X-ray structural data. Thermodynamic param-

eters of the conformational rearrangements were calculated. Geometry optimization

and spin density distribution calculations of biradicals 1 and 2 were carried out on

the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. Structural

rigidity and probable differences in biradicals behavior are discussed.

1 Introduction

Nitroxide biradicals of different composition and structure and their properties are

described in numerous articles, reviews, books and references therein [1–7]. One of

the most interesting and unsolved up to now problems relates to the effect of the

solvent on the peculiarities of the intramolecular electron spin exchange in short

A. I. Kokorin (&)

N. Semenov Institute of Chemical Physics, Russian Academy of Sciences,

Moscow, Russian Federation

e-mail: [email protected]; [email protected]

V. N. Khrustalev

A.N. Nesmeyanov Institut of Organoelement Compounds of Russian Academy of Sciences,

Vavilov st., 28, 119991 Moscow, Russian Federation

E. N. Golubeva

Chemistry Department, M. Lomonosov Moscow State University, Moscow, Russian Federation

123

Appl Magn Reson (2014) 45:397–409

DOI 10.1007/s00723-014-0528-4

Applied

Magnetic Resonance

biradicals with the indirect mechanism of the exchange. During last decades, such

systems in which two nitroxide rings are bound by a bridge with only three atoms

such as R–A–X(Y)–A–R, were under investigation [6–16]. Here A means oxygen,

O, or nitrogen, N; X = C, S, P or Si, and Y depicts other substitutes at X; R could

be piperidine, pyrrolidine or pyrroline nitroxide ring.

Among the first biradicals of this line were R6–O–S–O–R6 (1) and R6–O–S(O)–

O–R6 (2), where OR6 means 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine nitroxide

radical fragment. Their synthesis is described in detail in [1]. Electron paramagnetic

resonance (EPR) studies of 2 are published in many original articles, for example,

[17], while 1, to our knowledge, has attracted attention only once [11], in which has

been shown that 1 demonstrates fast intramolecular electron spin exchange contrary

to slow exchange in biradical R6–O–C(O)–O–R6.

Important information about physical–chemical properties of biradical molecules

follows from the knowledge of their structure, which can be obtained from X-ray

analysis. Till now, there were no published X-ray data on biradicals 1 and 2. Some

results about mean distances between unpaired electrons in frozen at 77 K toluene

and ethanol solutions were reported in Refs. [3, 18]. Further understanding of

mechanisms of intramolecular spin exchange and features of the spin density

distribution in biradicals, including the effect of solvent molecules on those,

depends on revealing of correlations between biradical structure and dynamic

properties of biradical systems with various structures.

In this paper, using X-band EPR and single-crystal X-ray structural analysis, we

describe differences in spin exchange dynamics and conformational transitions

occurring in 1 and 2, and the molecular structure of both biradicals in comparison

with the dynamic behaviour. Thermodynamic parameters of the transitions are also

reported and discussed.

2 Experimental

Biradicals R6–O–S–O–R6, 1 and R6–O–S(O)–O–R6, 2, where OR6 is 1-oxyl-

2,2,6,6-tetramethyl-4-oxypiperidine residue, used in this work, were synthesized as

described in Ref. [19]. Their colors and melting points were in good agreement with

the published data.

Toluene and ethanol were selected as solvents because of their different viscosities and

polarities. Both solvents were carefully purified according to the literature procedures

[20]. Solutions were prepared, bubbled with nitrogen for 20–25 min. 0.5 ml of a solution

was taken in a thin capillary and degassed by freeze pump cycle to remove oxygen, and

sealed off under vacuum. Radical concentrations were sufficiently low (B5 9

10-4 mol l-1) to eliminate intermolecular exchange broadening of EPR lines [21].

EPR spectra were recorded at X-band on a Bruker EMX-8 spectrometer with a

modulation frequency of 100 kHz. Temperatures were controlled (accuracy

±0.5 �C at temperatures between -20 and ?100 �C) by means of a JEOL JNM-

VT-30 temperature control system. The following parameters were experimentally

measured: the hyperfine splitting, hfs, constant on nitrogen 14N atom, a, and a value

of the exchange integral J=aj j , which is sensitive to any changes of the spin density

398 A. I. Kokorin et al.

123

distribution of the unpaired electron in the system. The exchange integral values

J=aj j were calculated in accordance with [3, 22]. EPR spectra of biradicals were

simulated with the computer program created by Dr. A. A. Shubin (Boreskov

Institute of Catalysis, Siberian Branch, Russian Academy of Sciences) and

described in detail in Ref. [22]. For these calculations, we used spin-Hamiltonian

parameters of such nitroxide radicals collected in Ref. [23].

2.1 X-Ray Structure Determination

Data were collected on a Bruker SMART APEX II CCD diffractometer [k(MoKa)-

radiation, graphite monochromator, x and u scan mode] and corrected for

absorption using the SADABS program [24]. For details, see Table 1. The structures

Table 1 Crystallographic data for 1 and 2

Biradical 1 2

Empirical formula C18H34N2O4S C18H34N2O5S

fw 374.53 390.53

T, K 100(2) 100(2)

Crystal size, mm 0.30 9 0.05 9 0.05 0.25 9 0.25 9 0.03

Crystal system Orthorhombic Monoclinic

Space group Fdd2 P21/c

a, A 14.6817(8) 7.9531(10)

b, A 47.230(3) 24.846(3)

c, A 5.7368(3) 11.2474(14)

a, � 90 90

b, � 90 109.602(2)

c, � 90 90

V, A3 3,978.0(4) 2,093.7(5)

Z 8 4

dc, g cm-3 1.251 1.239

F(000) 1,632 848

l, mm-1 0.187 0.184

2hmax, � 60 50

Index range -20 B h B 20 -9 B h B 9

-66 B k B 66 -29 B k B 29

-8 B l B 8 -13 B l B 13

No. of rflns collected 12,893 18,267

No. of unique rflns 2,877 (Rint = 0.0417) 3,662 (Rint = 0.0689)

No. of rflns with I [ 2r(I) 2,692 2,398

Data/restraints/parameters 2,877/1/118 3,662/34/235

R1; wR2 (I [ 2r(I)) 0.0314; 0.0770 0.0985; 0.2223

R1; wR2 (all data) 0.0347; 0.0786 0.1430; 0.2428

GOF on F2 1.001 1.004

Tmin; Tmax 0.946; 0.991 0.955; 0.995

The Structure and EPR Behavior of Short Nitroxide Biradicals 399

123

were solved by direct methods and refined by a full-matrix least squares technique

on F2 with anisotropic displacement parameters for non-hydrogen atoms. The

molecule of 1 is strongly disordered over two positions with the occupancies of

0.75:0.25. The absolute structure of 1 was objectively determined by the refinement

of Flack parameter, which has become equal to 0.00(6). The hydrogen atoms in both

compounds were placed in calculated positions and refined within the riding model

with fixed isotropic displacement parameters [Uiso(H) = 1.5 Ueq(C) for the CH3

groups and Uiso(H) = 1.2 Ueq(C) for the other groups]. All calculations were

carried out using the SHELXTL program [25]. Crystallographic data for 1 and 2have been deposited with the Cambridge Crystallographic Data Center.

CCDC 936471 and CCDC 936472 contain the supplementary crystallographic data

for this paper. These data can be obtained free of charge from the Director, CCDC,

12 Union Road, Cambridge CB2 1EZ, UK (Fax: ?44 1223 336033; e-mail:

[email protected] or http://www.ccdc.cam.ac.uk).

All calculation details have been already reported in detail in Refs. [16, 26].

3 Results and Discussion

3.1 EPR Spectroscopy

Typical EPR spectra of biradicals 1 and 2 at different temperatures are shown in

Figs. 1 and 2. We would like to stress that the comparison of simulated and

experimental spectra in all solvents at all temperatures used showed that they are in

a very good agreement. One can see that positions of the ‘‘exchange’’ lines in the

magnetic field are changing with temperature for 1 (Fig. 1), while they practically

do shift neither on temperature nor on the solvent in the case of 2 (Fig. 2). Indeed,

EPR spectra in Fig. 2 are rather similar for non-polar dioxane and polar CH3CN

media.

In liquid solutions with low viscosity, the dipole–dipole interaction between

unpaired electrons, the anisotropic hyperfine and Zeeman interactions are practi-

cally completely averaged to zero, and the spin-Hamiltonian H should take into

account only isotropic hyperfine and Zeeman interactions. In the case when both

radical fragments are identical and each has only one nucleus with a nonzero

nuclear spin I [9, 27]:

H ¼ gbeH0 Sð1Þz þ Sð2Þz

� �þ a Sð1Þz Ið1Þz þ Sð2Þz Ið2Þz

� �þ JSð1ÞSð2Þ: ð1Þ

Here, the spin Hamiltonian is written in frequency units: superscripts 1 and 2

symbolize different radical fragments, S(k) are electron spin operators, Sz(k) and Iz

(m) are

projections of the electron and nuclear spins correspondingly to the Z-axis, g is the

isotropic g-factor of the radical fragments, be is the Bohr magneton, H0 is the external

magnetic field, a & 3 9 108 rad s-1 denotes the 14N isotropic hfs constant, and J is

the exchange integral. For any individual conformation, one value of Jj j should

correctly describe the positions and integral intensities for all lines in the EPR

spectrum simultaneously. It was also shown that any changes in EPR spectra and


123

http://www.ccdc.cam.ac.uk

322 323 324 325 326 327

c

b

a

B, mT

Fig. 1 Experimental (solid lines) and calculated (open circles) EPR spectra of biradical 1 in toluene at293 (a), 323 (b), and 343 K (c)

323 324 325 326 327

c

b

a

B, mT

Fig. 2 Experimental (solid lines) and calculated (open circles) EPR spectra of biradical 2 in CH3CN at303 K (a), in dioxane at 293 (b), and 373 K (c)


123

conformational features of biradicals should be discussed and formulated in terms of

the ‘‘effective conformations’’; the conditions of applicability of this model were

determined in Refs. [3, 28, 29]. Further, speaking about the structure and

conformational transitions, we will mean ‘‘effective conformations’’.

Figure 3 illustrates dependences of a values on temperature for 1 and 2 dissolved

in toluene and ethanol, which decrease with the increase of T. Quantitatively this

decrement is usually characterized by parameter da/dT, equal to

-(30 ± 2) 9 10-4 G K-1 for 1 in both solvents. In case of 2, da/dT =

-(21 ± 3) 9 10-4 G K-1. These values are typical to those for piperidine

nitroxide radicals and biradicals known in the literature [7, 13, 14, 16]. Indeed,

the hfs constant a reflects the interaction of solvent molecules with[N–O• groups in

1 and 2, i.e., the a value should not depend on the structure and solvating of

functional groups in the bridge.

For both biradicals 1 and 2, positions and intensities of all lines can be described

with a certain value of Jj j, which is changing with temperature; such changes show

that both biradicals can exist in two conformations, A and B, with transitions

between them and different magnitudes of the exchange integral in each: JA and JB.

In the case of fast transitions, one can measure the effective (averaged in time) value

of J from EPR spectra:

J� ¼ JAsA þ JBsB

sA þ sB

: ð2Þ

Here JAj j\ JBj j, and parameters sA and sB are the characteristic lifetimes of

these conformations. It is known that in case of 1, JAj j = 0 at low temperatures [11,

30]. The Arrhenius plots of J�=aj j, given in Fig. 4, fit well the two-conformational

model for biradicals 1 and 2, and allow determine the differences in enthalpies, DH,

and entropies, DS, of the effective conformations, respectively [11, 31]:

-20 0 20 40 60 8014,5

15,0

15,5

16,0

a, G

T, oC

Fig. 3 a values as a function of temperature for 1 in toluene (filled circle), ethanol (open circle), and 2 intoluene (filled triangle), ethanol (open triangle)


123

ln J=aj j ¼ DS=R� DH=RT : ð3ÞCorresponding parameters measured for 1 and 2 in toluene from Fig. 4 are equal to

DH1 = 13.3 ± 1.1 kJ/mol, DS1 = 37.2 ± 4 J mol-1 K-1; DH2 = 1.2 ±

0.2 kJ mol-1, DS2 = 21 ± 3 J mol-1 K-1. These results indicate that since the

DH1 value is reasonable for typical hydrogen bonds and intramolecular rearrange-

ments in nitroxide biradicals dissolved in low-viscous solvents [32–34]. DH2 value for

2 is negligibly small, tenfold less than DH1, similar to values described in Ref. [13].

Probable explanation of such low magnitudes of DH has been recently suggested

in Ref. [25], in which, using quantum chemical and DFT calculations, authors

showed that nitroxide biradicals with acetylene groups in the bridge, such as

R6–C:C–C:C–R6, 3, and R6–C:C–p–C6H4–C:C–R6, 4, are ‘‘flexible’’, and

biradical 3 due to fast rotations even at low temperatures demonstrates average EPR

parameters. Biradical 4 is characterized by a higher rotation barrier in vacuum, and

is therefore more rigid. Experimentally measured from EPR spectra parameters

were DH3 = 0 ± 1, DH4 = -1.0 ± 0.1 kJ mol-1, DS3 = 21 ± 2,

DS4 = 10 ± 1 J mol-1 K-1 in different molecular solvents. One can assume that

in case of biradical 2 similar ‘‘flexibility’’ of the bridge connecting two R6 rings

causes the small effective value of DH2.

Structural information about both biradicals has been obtained by X-ray analysis

for the first time.

3.2 Molecular and Crystal Structures

Compounds of 1 and 2 were characterized by single-crystal X-ray diffraction study.

Their structures are shown in Figs. 5 and 6 along with the atomic numbering

schemes. Selected bond lengths and angles are listed in Table 2.

Complex 1 possesses overall C2 symmetry and, in the crystal, occupies a special

position on the twofold axis. The piperidine rings in 1 have slightly distorted chair

conformations, and adopt a gauche–gauche mutual configuration relative to the

0,0030 0,0035 0,0040

-1

0

1

2

ln |J

/a|

T–1, K

–1

Fig. 4 J=aj j value as a function of temperature for biradicals 1 (filled circle), and 2 (open circle)dissolved in toluene


123

central SO2 fragment [the C–O–S–O dihedral angles are equal to 84.34(8)�].

Moreover, the oxygen atoms of the SO2 fragment are arranged in the more sterically

favorable equatorial positions. The nitrogen atoms have a flattened trigonally

pyramidal geometry [the sums of bond angles at the nitrogen atoms are 355.8(3)�].

The N–O• bond lengths in 1 [1.288(1) A, Table 2] are in good agreement with

those in the previously studied complexes [35–39]. The intramolecular O•��O•

distance between paramagnetic centers of 9.538(2) A is almost twice longer than

the two closest intermolecular O•��O• distances of 5.373(2) A (O1��O1i [x, y,

-1 ? z]) and 5.737(2) A (O1��O1ii [� ? x, � - y, -� ? z]).

Fig. 5 Molecular structure of 1. Displacement ellipsoids are shown at the 50 % probability level.Hydrogen atoms are omitted for clarity. The labeling A denotes symmetrically equivalent atom relative tothe twofold axis

Fig. 6 Molecular structure of 2 (the disordered fragment of the less occupancy is depicted by dashedlines). Displacement ellipsoids are shown at the 30 % probability level. Hydrogen atoms are omitted forclarity


123

The crystal packing of molecules of 1 is stacking along the c axis. The molecules

are arranged at van der Waals distances.

Complex 2 occupies a common position in the crystal. In contrast to complex 1,

the molecules of 2 adopt two different conformations, with the trans-gauche and

gauche–gauche mutual configurations of the piperidine rings relative to the central

SO3 fragment [the C–O–S–O dihedral angles are equal to -169.9(2)/-70.4(2) and

127.1(3)/100.7(4)�, respectively] (Table 3). It is important to note that the trans-

gauche conformation of 2 prevails over the gauche–gauche one in the 3:1 ratio.

Similarly to 1, the piperidine rings in 2 have slightly distorted chair conformations,

the O2 and O3/O30 oxygen atoms of the SO3 fragment are arranged in the more

sterically favorable equatorial positions, and the N1 and N2/N20 nitrogen atoms

Table 2 Selected bond lengths (A) and angles (�) for biradical 1, obtained from X-ray analysis and DFT

calculation

Bond lengths X-ray data Calculated Angles X-ray data Calculated

S1–O2 1.6317(9) 1.69 O2–S1–O2A 105.24(7) 104.0

O1–N1 1.2884(13) 1.28 S1–O2–C4 118.04(8) 118.6

O2–C4 1.4692(14) 1.46 O1–N1–C2 116.16(9) 115.7

N1–C2 1.4979(15) 1.51 O1–N1–C6 115.98(9) 115.7

N1–C6 1.4988(15) 1.51 C2–N1–C6 123.69(9) 124.5

Table 3 Selected bond lengths [A] and angles [�] for biradical 2, obtained from X-ray analysis

Biradical 2

S1–O1 1.450(2) O10–S10–O2 121.6(4)

S1–O2 1.622(2) O10–S10–O30 108.8(5)

S1–O3 1.617(2) O2–S10–O30 80.6(2)

O2–C1 1.475(2) O5–N20–C12 115.9(2)

O3–C10 1.479(2) O5–N20–C13 113.5(2)

O4–N1 1.297(2) C12–N20–C13 124.6(3)

O5–N2 1.295(2) S10–O2–C1 121.3(2)

N1–C3 1.487(4) S10–O30–C10 111.3(3)

N1–C4 1.493(4) O1–S1–O2 109.9(2)

N2–C12 1.489(2) O1–S1–O3 106.8(2)

N2–C13 1.485(2) O2–S1–O3 96.3(1)

S10–O10 1.445(3) S1–O2–C1 112.2(2)

S10–O2 1.612(2) S1–O3–C10 115.8(2)

S10–O30 1.625(3) O4–N1–C3 115.4(3)

O30–C10 1.488(3) O4–N1–C4 115.9(2)

O5–N20 1.301(2) C3–N1–C4 124.6(3)

N20–C12 1.492(2) O5–N2–C12 116.5(2)

N20–C13 1.491(2) O5–N2–C13 114.2(2)

C12–N2–C13 125.2(2)


123

have a flattened trigonally pyramidal geometry [the sums of bond angles at the

nitrogen atoms are 355.9(6) and 355.9(6)/354.0(6)�, respectively]. Observation of

two conformers in the crystal of 2 confirms the suggestion about its possible

existence in two conformations in liquid solutions discussed in Refs. [13, 15, 16].

The N–O• bond lengths in 2 (1.297(2), 1.295(2) and 1.301(2) A, Table 3) also

correspond well to the values found in the related complexes [40–43]. The

intramolecular O•��O• distance between paramagnetic centers in 2 (11.358(3) A) is

substantially larger, but the closest intermolecular O•��O• distance (O4��O5i [1 - x,

1 - y, 1 - z] 4.457(3) A) is significantly smaller than those in 1.

The crystal packing of molecules of 2 is stacking along the a-axis. The molecules

are arranged at van der Waals distances.

Calculated geometry of biradical 1 (B3LYP/cc-pVdz) is shown in Fig. 7, and the

appropriate values of bond lengths and angles in 1 are listed in Table 2. One can see

that all parameters are in a good agreement with the X-ray data with only one

exception: the calculated distance S1–O2 is about 0.6 A longer comparing to

experimental one. Corresponding parameters of calculated geometry of biradical 2have been already published in Ref. [16], and the comparison with X-ray data

(Fig. 6; Table 3) revealed their reasonable agreement also.

The spin density distribution in biradical 1 in open shell singlet and triplet states

has been calculated and is given in Fig. 8, where red color depicts a positive sign of

spin density, and blue color depicts a positive sign (color figure online). It is

evidently seen that spin density in these biradicals is localized practically only on

the N and O atoms of the N–O• bond, which corresponds well with all the previous

results [1, 2, 5, 44].

Thus, it follows from the results obtained with EPR and X-ray techniques, and

quantum chemical calculations that transitions between two different conformations

of 2, characterizing by the trans-gauche and gauche–gauche mutual configurations,

are very fast with DH2 B1.2 kJ mol-1, and can be realized over very small potential

barrier even at low temperatures. In contrast to 2, changing the O=S\group to S\

Fig. 7 Calculated geometry of biradical 1 (B3LYP/cc-pVdz)


123

atom in the bridge of 1, accompanying changing of the valency of sulfur atom,

results in significant increasing of the bridge flexibility. Now, for transitions

between two conformations, 1 needs essential reorganization of the solvent net

envelope surrounding 1. This leads to noticeable increasing of the barrier and of the

DH1 � DH2 value typical to diffusion processes in low-viscous molecular solvents.

4 Conclusion

Two short nitroxide biradicals of similar composition, S(OR6)2 (1) and O=S(OR6)2

(2), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been investi-

gated by EPR spectroscopy. The crystal structures of biradicals 1 and 2 have been

determined by a single-crystal X-ray analysis for the first time. The effect of

temperature variations on the intramolecular electron spin exchange in 1 and 2dissolved in toluene, dioxane, CH3CN and ethanol has been characterized by

Fig. 8 Spin density distribution in biradical 1 in open shell singlet a and triplet b states (contour value isequal to 0.01)


123

changes in the isotropic 14N hfs splitting constant a, by values of the exchange

integral J=aj j. The EPR spectroscopy changes have been compared with the X-ray

structural data and were also confirmed by quantum chemical calculations. The

thermodynamic parameters of the conformational transitions were calculated.

Acknowledgments This work was supported by the Russian Foundation for Basic Research (project no.

12-03-00623-a). A.I.K. and E.N.G. are grateful to Dr. O.I. Gromov for his help in designing some figures

of the article, and also to Dr. A.A. Shubin for kindly providing us his program package.

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