Structural study by EPR of promazine cation radical

Journal of Molecular Structure, 98 (1983) 165-174 Elsevier Scientific Publishing Company, Amsterdam

STRUCTURAL

F. LOPEZ Instituto

STUDY BY EPR OF PROMAZINE

RUPEREZ,

J. C. CONESA

10 May 1982;

in final form

in The Netherlands

CATION RADICAL

and J, SORIA*

de Catblisis y Petroleoquimica,

(Received

-Printed

C.S.I.C., 2 August

Serrano,

119 -Madrid

(6) (Spain)

1982)

ABSTRACT A study by EPR of the PRMZ’ cation radical in solution at different temperatures and in the solid state (single crystal and powder) has been made. The set of experimental data provided by the spectra has allowed calculation of some parameters of the phenothiazinic nucleus and the side chain conformation. The calculated dihedral angles formed by the p-protons in the favoured conformation are t?b = 89 = 60”. INTRODUCTION

The phenothiazine molecule and its derivatives have attracted great interest because of their pharmacological properties, which are strongly dependent on their side chains. The cation radicals of these compounds, which have been suggested as participants in the pharmacological action [l],do not always produce well resolved EPR spectra; the size and conformation of the side chain influence appreciably the resolution of the spectra for the samples in solution. The attempts made to gain information on the structure of the molecules from the EPR spectra have been limited, in many cases, by the complexity of the spectra and by the restriction of working only with the samples in solution and at room temperature.

In the present work, we have studied a compound, the promazine cation radical (Scheme l), under different experimental conditions. The study with

*To

whom

correspondence

0022-2860/83/0000-0000/$03.00

should

be addressed. G 1983

Elsevier

Scientific

Publishing

Company

166

the sample in solution at variable temperature, including the frozen state to register the strongly immobilized spectrum, can provide information on the hyperfine structure tensor anisotropy, particularly on that corresponding to the nitrogen heterocyclic atom. The single crystal and powder spectra can provide information on the g-tensor. All these data have been used to obtain an estimation of some molecular parameters and some conformational characteristics of the side chain in sulphuric acid solution. EXPERIMENTAL

The sample of PRMZ in neutral form was supplied by Rhodia and the cation was obtained following the method of Merkle and Disher [ 21. For its study in solution, the solid sample was dissolved in dilute sulphuric acid (25% by volume), at a concentration of 1 mg ml-‘. The EPR spectra were obtained with a JEOL-PE-3X spectrometer, fitted with a variable temperature controller system JES-UT-3A. On line with the spectrometer a Digital Computer Controls D-116E minicomputer could digitize the spectra and use them for the simulation procedure. The spectra were obtained in X-band. A Mn:MgO standard was used to calculate the g-values. The spectra for different orientations of the single crystal, in relation to the external magnetic field, were obtained using a goniometer and a hexahedral sample holder that allowed the rotation of the single crystal in mutually perpendicular planes. Since the small size of the PRMZ’ single crystal did not allow placement in a known position relative to its crystallographic axes, the sample holder edges were considered as an arbitrary axis system (1, 2, 3) and the single crystal, although forming unknown angles with it, is jointly affected by the rotations. The effective g was measured by rotating the sample holder around each axis at intervals of 10”. These data were used by a computer program to calculate the main values of the g-tensor. The experimental g*-effective values were fitted to a sinusoidal curve using the mean squares method, independently for each axis 1, 2 and 3. From each one of the fitted g’& (0) curves, the amplitude, the minimum value and its position along the &axis (first or second quadrant) were obtained. Here the angle 0 indicates, for each axis, the sample holder rotation inr+elation to an initial orientation having an edge parallel to the magnetic field H [ 3, 41. Using the equations that relate, for each axis, the former parameters with different g$ values, the components of the g* symmetric tensor were calculated [ 51. The diagonalization of that This procedure eliminated the intensor led to the values g,,, g,,, and g,,. fluence on the g-values of small errors in the initial orientation of the sample holder edge in relation to the magnetic field, fi, direction.

RESULTS

PRMZ' in solution The EPR spectrum of the PRMZ’ in solution at 295 K is better resolved at its low field half side than at the high field half, probably because of a progressive line broadening. This effect can be due to an incomplete averaging of the g factor or the hfsc anisotropy, related to the solvent viscosity and the size and conformation of the molecule. Because of its better resolution, only the low field half of the spectra has been simulated in most cases. The experimental and simulated spectra at 295 K are presented in Figs. la and lb. The splitting scheme considered in the simulation takes into account the coupling of the unpaired electron with: (a) the heterocyclic nitrogen nucleus; (b) the two @-protons of the side chain; (c) the four proton pairs of the lateral rings. The simulation parameters are given in Table 1 .These spectra show seven groups of lines centered at the positions represented in Fig. lc, as can be expected for side chains with equivalent P-protons [ 61. The assignation of the hfsc to the different lateral ring protons has been made on the basis of the results obtained by Sullivan and Bolton for the lo-methylphenothiazine cation radical [ 71. (0)

Fig. 1. (a), (b) EPR experimental and simulated (c) Main hyperfine splitting diagram.

spectra

of PRMZ’ in solution

at 300 K.

168 TABLE

1

EPR parameters

of PRMZ’

in solution

at room

temperature ~(H.,,,)

*Hpp

g-value

0.80

0.40

0.40

2.0053

(0.5) 0.89

(0.5) 0.39

T(K)

a(N)

a(W)

a(%,)

W,,,)

a(%)

300a RTb RTC

7.08 6.95 7.15

3.52 3.58 3.61

1.96 2.0 1.98

0.92 1.05 0.89

aOur work.

bFenner

and Mockel

[9].

‘Clarke

2.0052

et al. [ 81.

The P-protons equivalency can be due to a symmetric disposition of these protons with respect to the 2 axis of the N(lO) 2p(n) orbital in a preferred conformation of the side chain or to motional averaging (conformation exchange) effect of the several hfsc’s, corresponding to different conformations. In order to differentiate between these two possibilities, the effect of the temperature on the spectra has been studied between 233 and 353 K. The experimental and simulated spectra at three temperatures are presented in Fig. 2, and the corresponding simulation parameters in Tables 1 and 2. The different spectra can be simulated keeping the hfsc constant, but adjusting the linewidth. The variation of the linewidth thus optimised with temperature is shown in Fig. 3.

Fig. 2. Experimental and simulated 273 K, (c) at 233 K.

spectra

of PRMZ’

in solution,

(a) at 333

K, (b) at

169 TABLE

2

Linewidth

values at different

T(K) A&p

(G)

temperatures

353

333

300

273

253

233

1.06

0.86

0.40

0.48

0.58

0.86

PRMZ’ single crystal Using a PRMZ’ single crystal, the effective g values obtained by rotating the sample around the three axes of the system (1, 2, 3) are represented in Fig. 4. The relation between the crystallographic axis system and the axis system of the g tensor cannot be determined because of the arbitrary orientation of the single crystal within its holder as a consequence of its very small size. However, the variation of the effective g values with the angle 0 allows calculation of the principal values g,, g,,, and g,, of the g tensor: which are presented in Table 3. For each orientation, the signal has a Lorentzian line shape, without hfs, an indication of the strong exchange interactions between the PRMZ’ radicals. The linewidth changes with the angle also in a sinusoidal form, with values between 1.30 and 1.85 Gauss.

233

273

713

3”:

0

30

60

‘7 G

TIK.

Fig. 3. Peak to peak width vs. absolute acid solution. Fig. 4. Angular tem

variation

123

i ‘>I:

w

, 6--,

temperature

of the g-effective

for EPR spectra

values for different

ofPRMZ+

in sulphuric

axes of the (1, 2, 3) sys-

170 TABLE 3 EPR parameters

of PRMZ’ in frozen

Parameter

Sol. at 77K

&x

gYY

gz* AfH (G)

and solid state Powder

Single crystala

2.0072 2.0060

2.0073

2.0072

2.0058

2.0060

2.0022

2.0022

2.0022

3.5 1.8

AN (G)

AZ(G)

1.8

A:(G)

I17.5

~f$p (xx) G) AHpp (YY1 (G) AHpp (2~) (G) aImprecision

solution

4.0 2.8

0.90

4.8

0.90

0.90

of g values ? 0.0002.

Polycrystalline

sample

The powder spectrum of this sample presents a g tensor with orthorhombit symmetry. The experimental and simulated spectra are shown in Fig. 5 and the EPR parameters in Table 3. Frozen solution

In order to avoid the effects of the exchange interactions on the hfs, the frozen solution was studied at 77 K. According to theoretical calculations for the 14N nucleus, we can take A:= > 0 and due to the cylindrical symmetry of the 2p(n) orbital, we can assume A:* 9 AZ -v A;,,. Considering the hyperfine coupling of the lateral and @-protons as isotropic and supposing that the effect of these protons is included in the linewidth of the peaks, the separation between the extreme peaks in the spectrum of Fig. 6a will be 2A,“, s 35.1 gauss; thus AZ L 17.5 gauss. As AT = i(ATx + At, + AZ) and A; = 7.08 gauss, we obtain AZ% -v Af;: 2 1.8 gauss. Using the g values calculated from the single crystal spectrum and these hfsc values and assuming that the axis systems of AN and g tensors coincide, we have simulated the spectrum of Fig. 6b. The values of the parameters used for the simulation are presented in Table 3. DISCUSSION

The EPR parameters obtained for PRMZ’ in solution at 295 K are similar to those obtained by other authors [ 8, 91 but our study at different tem-

171

i’

*P

i

-

2.0073

Fig. 5. EPR

experimental

and simulated

spectra

of PRMZ’

2a:”

in a polycrystalline

Fig. 6. (a) EPR experimental spectrum of PRMZ’ in sulphuric simulated spectrum, (c) hyperfine splitting pattern.

acid solution

sample. at 77 K, (b)

peratures shows, moreover, that the hyperfine coupling constants do not change significantly with the temperature and the variations observed in the spectra are only due to changes in the linewidth. As indicated in Fig. 3, AH,, changes with temperature in a different way above and below 295 K. The drop of the AHHpp value between 233 and 295 K can be explained by considering that the linewidth varies proportionally to q/T, where 9 is the solvent viscosity. This dependence is related to a spin-orbit relaxation mechanism [lo]. The increase of AHDpbetween 295 and 353 K is more difficult to explain. It can be due to the contribution of several slightly different conformations, but a more likely rationalization can be the existence of a relaxation mechanism of the spin-rotational type [ 111, producing a linewidth linear dependency of T/v. The study of the PRMZ’ in liquid solution, solid state and frozen solution provides a set of experimental data that allows us to calculate some parameters of the phenothiazinic nucleus and the side chain conformation.

172

Structural

aspects of the phenothiazinic

nucleus

It is known that by comparison of the theoretical values of the isotropic and anisotropic coupling constants with the corresponding experimental values, it is possible to estimate the s character, c,2, and the p character, ci, of the atomic orbital containing the unpaired electron. On the basis of these C2

values, the hybridization ratio X2 = 2 is obtained and estimations of some c: structural parameters can be made [ 121. Thus, using the EPR results, we have obtained the following values for the nitrogen heterocyclic atom N( 10) 3=_ ’

12 =

APdT 7.08 = _ Ak,

552

= 0.013

10.4 = 34

= 0.306

C2

-F- = 23.54 c,”

where At is the experimental isotropic value of the hfsc of the N(lO) atom, T,, = A,Nz- A: is the anisotropic 2 component of the hyperfine tensor AN, obtained from the spectra in fluid solution (A:) and frozen solution at 77 K (A:); Ai, and 2Bo have been taken from the literature [ 31 and correspond to theoretical values of isotropic and anisotropic coupling constants, assuming that the unpaired electron occupies the pure 2s or 2p orbitals respectively. The value obtained for ci is also an estimation of 71density of spin pN, on the nitrogen heterocyclic atom N(lO), that can be compared with the results obtained by MO theoretical studies by Sullivan and Bolton [ 71 for lo-methyl phenothiazine (pN = 0.295). A possible interpretation of the data may assume that the radical is locally planar at nitrogen and the apparent value of c,” is due to spin polarization effects. However, Singhabhandhu et al. [ 131 have shown by X-ray diffraction studies that the phenothiazine cation radical possesses a folding angle at the nitrogen of 172”. In the case of PRMZ’ the existence of steric repulsions between the 1 and 9 hydrogen! of the heterocycle and the fl-protons could lead to an additional folding around the N-S axis [ 141. In any case the EPR data cannot provide the value of the folding angle directly because A: can include both contributions of sp3 hybridization and of spin polarization. Conformational

aspects of the side chain

The analysis of the results obtained from the variable temperature spectra shows that, on the range explored, g and hfsc values are independent of T and thus no interconversion exists between noticeably different conforma-

173

tions. Therefore, the P-protons are completely equivalent and the a/In value (ePn, = afln,) can be used to calculate the dihedral angle 8, formed by the pprotons with the Z-axis of the 2p(77) orbital of the heterocyclic nitrogen. From McConnell’s equation, a&.r = pN B cos20 + I!$,, and neglecting the small correcting term B, but accepting the additional condition 0 1 + 0 2 = 120” we obtain

B =

$

[(@H,

+@Hz ) + (@H,

’ uPH2)1Rl

N

By substitution in eqn. (1) of the experimental values aon, = @n, 1 3.52 Gauss and pN = 0.31 previously obtained, the value B = 46 has been calculated which can be compared with the value B = 49 corresponding to the lomethyl phenothiazine cation radical [ 81. Moreover, the resulting 0 angle values 0 1 = O2 = 60” show that the side chain of the PRMZ’ in solution adopts one conformation A (Fig. 7a) with /3H symmetrically placed in relation to the 2 axis of the 2p(n) orbital of the N(lO) atom. Although the published crystallographic data for promazine do not allow the torsion angle 7 around the N(lO)-C(ll) bond to be obtained and, consequently, the dihedral angles 19~and 19*,there are in the literature different r values obtained for chlorpromazine (promazine with a chlorine atom substitu. ent in position 2). Accepting a structural analogy between both molecules, we have considered a possible second conformation C (Fig. 7b) with 7c = -34.5” [ 1.51. Taking the same values of B and PN for C, McConnell’s equation predicts (I&r, 1 0.09 gauss and aon, 2 11.50 gauss. The simultaneous presence of A and C could possibly be observable in the spectra, particularly at high temperature. The absence of such an effect may indicate that conformation C is not favoured for PRMZ’ in solution. This situation can be explained as follows. The loss of an electron because of the oxidation of the molecule will imply some modification of the sp3 hybridization state towards partial sp2 character at the N(lO) atom which must produce on the cation radical some approximation to planarity. This variation could eliminate some steric restrictions between the side chain and the ring system so that the P-protons could be lodged at the same side of the folded plane, and symmetrically with respect to the 2axi.s of the 2p(n) orbital of N(10). This

Fig. 7. (a) Conformation (A) of the side chain with T* = 0” or 180”; of the side chain with 7c = -34.5”, ref. 15.

(b) conformation

(C)

174

rationalization is in agreement with the theoretical predictions of Coubeils and Pullman for the promazine neutral molecule [16] in relation to the influence of degree of folding on the conformation of the side chain. REFERENCES 1 A. E. Szent-Gyorgi, I. Isenberg and G. Karreman, Science, 130 (1964) 1191. 2 F. H. Merkle and C. A. Disher, J. Pharm. Sot., 53 (1964) 965. 3 J. E. Wertz and J. R. Bolton, ESR Elementary Theory and Practical Application, McGraw-Hill, New York, 1972, p. 208. 4 Ch. P. Poole Jr. and H. A. Farach, The Theory of Magnetic Resonance, Wiley-Interscience, New York, 1972, Chap. 5. 5 F. Lopez Ruperez, J. C. Conesa and J. Soria, Org. Magn. Resonance, in press. 6 F. Lopez Ruperez, J. C. Conesa and J. Soria, J. Chem. Sot. Perkin Trans. 2, in press. 7 P. 0. Sullivan and J. R. Bolton, J. Magn. Reson., 1 (1969) 356. 8 D. Clarke, B. C. Gilbert, P. Manson and C. M. Kirk, J. Chem. Sot. Perkin Trans. 2, 10 (1978) 1103. 9 H. Fenner and H. Mockel, Tetrahedron Lett., 33 (1969) 2815. 10 G. K Fraenkel, J. Phys. Chem., 71 (1967) 139. 11 R. Wilson and D. Kivelson, J. Chem. Phys., 44 (1966) 154; P. W. Atkins and D. Kivelson, J. Chem. Phys., 44 (1966) 169; J. R. Thomas, J. Am. Chem. Sot., 88 (1966) 2064. 12 P. W. Atkins and MC. R. Symons, the Structure of Inorganic Radicals, Elsevier, Amsterdam, 1967. 13 A. Singhabhandhu, P. D. Robinson, J. H. Fang and W. E. Geiger, Inorg. Chem., 14 (1975) 318. 14 P. Marsau, Acta Crystallogr. Sect. B, 27 (1971) 42. 15 J. J. H. McDowell, Acta Crystallogr. Sect. B, 25 (1969) 2175. 16 J. L. Coubeils and B. Pullman, Theor. Chim. Acta, 24 (1972) 35.