Low temperature high field magnetisation studies on metalloporphyrins

Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 98, Nos 1 & 2, January 1987, pp. 53--68.

9 Printed in India.

Low temperature high field magnetisation studies on metalioporphyrins

S A M A R E S H M I T R A

Chemical Physics Group, Tata Institute of Fundamental Research, Colaba, Bombay 400 005, India

Abstract. Magnetisation studies at very high magnetic fields and low temperatures on several high-spin iron (III) and manganese (III) porphyrins are reviewed. The usefulness of these studies in understanding the properties of the ground electronic state of the metal ion is discussed.

Keywords. Magnetisation; iron porphyrins; zero-field splitting; magnetic saturation;

exchange interaction.

1. Introduction

Metalioporphyrins are a class of biologically important molecules which form the prosthetic group in heme proteins. The metal ion in the prosthetic group is the site of the biological activity in many of them, and its electronic structure determines many of their biological functions. This factor has led to considerable interest in the physico-chemical properties and electronic structure of these metalloporphyrins (Smith 1975; Dolphin 1978).

Study of magnetisation at low temperatures and high magnetic fields is a powerful method to probe the electronic structure, especially the ground state, of the metal ion in the paramagnetic hemes (Mitra 1983). At very high magnetic fields the Zeeman splitting of the ground level becomes sufficiently large to cause mixtng between the close-lying spin-multiplets and provide information about the ground state. The experiments must however be done at temperatures below 10 K when only the ground state is preferentially populated. In this article we discuss results of high-field magnetisation studies carried out at very low temperatures on some iron and manganese porphyrins.

Magnetisation (~r) of a paramagnetic ion is given by or = X- H, where X is the magnetic susceptibility. At very high-fields and at low-temperatures (i.e. in the saturation region), the magnetic susceptibility is field-strength dependent. Magnc- tisation is therefore a more appropriate quantity to define the magnetic properties in this region.

The following abbreviations of the porphyrin ligands have bccn uscd in this paper: TPP, tetraphcnylporphyrin; O E P , Oclacthylporphyrin; PP, protoporphyrin IX; D P D M E , deuteroporphyrindimcthylcslcr.

2. Experimental

Measurement of magnelisation is essentially the same as lhc measurement of magnetic susccplibility and can be performed using a Farad;!y or vibrating sample 53

54 Samaresh Mitra

magnetometer or even a SQUID susceptometer. Magnetic fields as high as 80 kOc can easily be obtained using superconducting magnets which also provide a cryostalic arrangement for measurements at different temperatures. During magnetisation measurements on polycrystalline samples some precautions must be taken. At very low temperatures the polycrystallites of anisotropic samples tend to preferentially orient in high magnetic fields, making the measurement of average magnetisation difficult. To overcome this problem measurements are made on a homogeneous mull of metalloporphyrins in diamagnetic vaseline frozen at low temperature outside the magnetic field. This ensures that the polycrystallites are randomly oriented in the vaseline mull and would remain so fixed even at very high magnetic fields. Alternatively, the measurements can be done on highly compress- ed tablets of the samples.

The magnetisation is usually expressed as reduced moment (Ix)= cr/N[3 in conformity with the Brillouin function (see later). The effective average magnetic moment/2elf reported in this article generally refers to measurements carried out at low magnetic field (H < 10 kOe).

3. Theoretical aspects

The magnetic moment of a paramagnetic ion can be given by [ 2S +1 (2S +1 / 1 ~---1 M = N g ~ S ~ coth \ - - ~ / y 2S coth

= Nut, S Bs(y),

where y = g~ S H / k T . Here the expression B ( y ) is usually called the Brillouin function. For very large values of H/T--~, ~ , ( M / N ~ ) -+ gS. Thus for g = 2 the saturation moment for S = 5/2 would be 5-0 BM, and for S = 2, 4.0 BM. A theoretical plot of the moment is shown in figure 1. This plot is strictly for free-ions, i.e., where the effects of crystal fields have been neglected. For S = 1/2, the ground state is a slain-doublet (with no orbital degeneracy), and the effect of crystal fields is negligible. Experimental results for such spin systems are found to obey the Brillouin curve very closely..However for S I> 1, a sizeable zero-field splitting of the ground state is expected because of the combined effect of crystal field and spin-orbit coupling. This zero-field splitting is known to be very large in metalloporphyrins (Mitra 1983), which would cause large deviations in the expected behaviours from those in figure 1.

The magnetisation for a S ~ > 1 spin system can be calculated by following spin-Hamiltonian formalism, which includes the effect of zero-field splitting in axial symmetry,

n = DS 2 + g[3H.8, (1)

where D is the zero-field splitting parameter. While calculating magnetisation using (1), two points must be considered, One is that at very high fields and low temperatures, magnetisation (or magnetic susceptibility) cannot be calculated using the Van Vleck equation which assumes f l H ~ k T . Instead the thermodynamic equation

Magnetisation studies on metalloporphyrins 55

<p>

2

1

s ; 2

s=1/2

A I I $ ! | I 1 I |

0 1 2 3 4 5 6 7 8 9 10

gp.~HIkT

Figure I. B r i l l o u i n f u n c l i o n plot for r n a g n e l i s a l i o n o f S = 5/2 to S = 1/2~

- \all ^{/ _}

should be used (Marathe and Mitra 1973). Further in calculating the average magnetisation the usual procedure of averaging, i.e. 6-= (~rq + 2~r• rr~ay lead to significant errors at lower temperatures and higher fields (Marathe and Mitra 1974;

Varmass and Groeneveld 1974), so, a spatial averaging technique should be used for calculating average (7 (Marathe and Mitra 1974).

4. Results and discussion

4.1 High-spin iron (11I) porphyrins

Magnetisation of several high-spin porphyrins has been reported over an extended range of temperature and magnetic field. The results are briefly discussed here.

Tetraphenylporphinato iron(Ill) ha!ides: The average magnetisation of tet- raphenylporphinato iron(]II) chloride, Fe(TPP)C], and tetraphenylporphinato iron(III) bromide, Fe(TPP)Br. has been reported over 2-20 K and 10-50 kOe

56 S a m a r e s h Mitra

(Behere and Mitra 1980a; Behere et al 1979; Birdy et al 1983). Both these molecules have similar stereochemical geometry with the iron atom being five-coordinated and lying appreciably above the mean plane of the porphyrin core (figure 2). The temperature dependence of their effective magnetic moment is very similar.

showing sharp decrease in /2crf below 60 K (figure 3). At 4-2 K the ~,,fr has the values of 4.8 and 4.6 BM, respectively, for the chloro and bromo complexes. The decrease in/2cf f at lower temperatures is characteristic of large zero-field splitting of the 6A ~ ground state of the ferric ion (figure 4). The magnetisation data at various fields are summarised in figures 5 and 6.

At 11)kOe the magnetisation varies linearly ~ith temperature. As the field increases, deviation' from linearity increases so much that for H/> 40 kOe the magnetisation reaches complete saturation below 4K. For F e ( T P P ) B r where complete saturation is achieved, the saturation moment is (p,),:, = 3-0 BM. For Fe(TPP)CI the measurements extend down to only 4.2 K and complete saturation has not been achieved though the trend is clear (figure 5). The saturation moments together with other relevant data are listed in table I. The low value of saturation moment is clearly a consequence of hlrgc zero-field splitting (ZFS).

The magnetisation has been calcuhlted on spin Hamiltonian and crystal field models. For Fe(TPP)Br. a hlrgc value of ZFS. D = 12.5 c m - i has been deduced

H

H ,2 ,3 \ \ \

H ~ 5 H

14

Figilrt, 2.

jN\ //I

3 ~ 2

H H / \H

H~ ~ H

N Mt+Iccuhu s t r u c t u r e of Fc( l ' l ' l ' I N .

Magnetisation studies on metalloporphyrins

6.0 56 5-2 4.~

4.4 /2. 4-0 6.0 5 6

5.2 4.8:

4 ~ 4 0

Figure 3.

~ Fe (TPP) Br

F e ( T P P ) Cl

I 1 I J , I I l l

I0 20 ~0 4 0 50 60 70 80 90

T

Tempcralure dependence /i~,,t for Fc(TPP)('I and I-'c(TPP)Br.

57

6A I (6)

/

/ / /

(-.

\

\

\

\

4 D

M s = + _ 5 / 2

M s : t 5 / 2 RI)

1 M,

S.O. coui')hng mixing

Fi~inre 4. /.~.'r,i~-tb,.'M splittinlg m in o n ( I l l ) pori-4Lvrir~n (d" electrum ,,.'ont'igunati~.'m with %4,

~r()lllld hI;lIe).

(i~chcrr el a/1979). The value is close to tha! deduced from high lu'llil'~cr;ll|lik" .~iHglr cl'ynlal susceptibility dala (Bchcrc el , / 1'970. P, irdy el a/ I~-}F~3). |:or I:c(Tl:'l')('l a

5~ Samare.dt Mitra 5 30

2.475

/k :::L i 65 V

0 8 2 5

5OkOe 4 0

30

2 0

I0

l I l I

7.5 15.0 22.5 3 0 0

I / T ( x l O 2)

Figure 5. Magnctisation of Fe(TPP)CI down to 4-2 K (Bcherc and Mitra ]9Nla). Solid curves are calculated for D = 8-0 cm

v a l u e D = 8-0 cm J was d e d u c e d f r o m the m a g n e t i s a t i o n study ( B e h e r e and M i t r a 1980a; Birdy et at 1983), while the single crystal susceptibility measurements yielded D = 6-0 cm - j (Behere et al 1977; B e h e r e and Mitra 1979). The discrepancy is significant and has been attributed to the possible effects of magnetic exchange interaction ( B e h e r e and Mitra .1980a).

The molecular stacking in Fe (TPP)CI is interesting in that half o f the molecules in the lattice are stacked in a Fe-CI .... CI-Fe pseudo-dimeric fashion (figure 7).

This long path appears to provide a route for occurrence of the magnetic exchange interaction through a superexchange m e c h a n i s m . A further evidence of magnetic exchange in F e ( T P P ) C I comes from measurement of single crystal magnetisation between 4--0.1 K (Neiheisel et al 1975). A fit of their data on the dimer model for effective spin S = 1/'2 (M~ = 1/2 lying lowest, D positive) gave a .value of J = - 0 . 0 7 cm -I. The magnetic interaction is, as expected, weak and antiferro- magnetic. If this value of exchange interaction is included in the theoretical fitting, D = 6.0 cm -I fits the magnetisation data of figure 5 well.

F e ( P P ) C l and F e ( D P D M E ) C I : The si0gle crystal structural data on Fe(PP)CI, which is also called hemin chloride, show that the molecule has a stereochemical structure closely related to the F e ( T P P ) X series. The iron is slightly out of the plane of the porphyrin core and is coordinated to the chlorine atom completing an approximate square pyramidal geometry around it. Relevant structural data are included in table 1. Though no structural data are available for F e ( D P D M E ) C I , the

5 5 O O I

2.475

T 1

x O

X + +

A 1.650 ::L V

0-825

M a g n e l i w t t i o n .vludi('.v o n mclallrq)orl)hyrin.v 5 9

5Ok0e 4 0 30 2 0

I0

j I I I

150 3 0 0 4 5 0 6 0 0

I/T(xI05

Figure 6. Magnetisation of Fe(TPP) Br down to 2 K (Beherc et a11979). Solid curves are calculated for D = 12-5 cm i

Table !. Magnetic and structural p a r a m e t e r s for s o m e high-spin i r o n ( I l l ) porphyrins.

Fe-X Fe-N Fe-Ct (/.t),at D ZJ

C o m p o u n d (,~) (A) (,~) (BM) (cm 1) (cm 1) R e m a r k s

8-0 a - - E x c h a n g e not considered F e ( T P P ) C I 2-t92 2-060 0-39 3-4 6-0 "-h - 0 . 0 7 E x c h a n g e included with

Z = 2 (dimer m o d e l ) F e ( T P P ) B r 2.348 2-069 0-56 3-0 12-5 "'b - - Inclusion of e x c h a n g e not

considered important 8.0 - - E x c h a n g e not considered

Fe(PP)CI - - - - 0-49 3-4

6-95 ~ - 0 - 0 8 Molecular field m o d e l 11-0 - 0 . 2 2 Molecular field model F e ( D P D M E ) C I - - - - - - 3.0

9.0 c - - Far infrared m e a s u r e m e n t

" From magnetisation; h value derived from single crystal studies as ,,Cell; ~ far infrared m e a s u r e m e n t .

s t e r e o c h e m i c a l s t r u c t u r e is e x p e c t e d t o b e s i m i l a r t o t h e a b o v e f i v e - c o o r d i n a t e d h i g h - s p i n i r o n ( I I I ) p o r p h y r i n s . T h e t e m p e r a t u r e d e p e n d e n c e o f t h e e f f e c t i v e

60 Samaresh Mitra

a

t,, / % . . l

b

C

4,7

Figure 7. Molecular stacking in somc iron(Ill) porphyrins: (a) Fc{PP)CI: (b) Fc(I)EP);

(c) Fc('IPP)CI.

average magnetic moment of these complexes is shown in figure 8. While the general nature of the ~la v s 7" curves is similar to that of F e ( T P P ) X , the magnetic moment of F e ( D P D M E ) C I decreases much faster than that of F e ( T P P ) X . For example the/2~rl for the d e u t e r o complex at 4.2 K is 4.2 BM as against 4.6 BM for Fe(PP)CI, 4.8 BM for Fc(TPP)CI and 4-5 BM for F e ( T P P ) B r . The large decrease in the /2~, for the deuterocomplex may arise from a much larger /).=value or presence of large antiferromagnetic exchange interaction.

The magnetisation of these two molecules has been reported between 2-20 K and 1(}--50 kOe (Marathe and Mitra 1983) and is summarised in figures 9 and 10.

The general features of the magnetisation of the two compounds are similar to those of F e ( T P P ) X . In both the molecules the magnetisation saturales below 4 K for H t> 40 kOe. The saturation moment for the Fe(I)PI)Mt~)CI is however much lower than that of the Fe(PP)CI (see table 1).

Equation (1) gave excellent fit to the (/~) vs l I T (and/~.ll vs T as well) data for the F e(PP)CI over the entire t e m p e r a t u r e and magnetic field range for D = 8.0 c m - i . The value of D in Fe(PP)('I has however been accurately determined by far infrared spectroscopy (Brackett el al 1971) which has yielded D = 6.95 cm-*. The discrepancy may not be large but is significant. A similar attempt to fit the data for Fe(DPI)MI~)CI was however unsuccessful. The ,fi-,.ll vs T data could only be fitted to D - - 3 0 c m ~ which is unreasonably high. T h e

Magnetisatiotl studie.~ on metallol~OtT~llvrins 61

6 . 0

5 0

4 . 0 t, Cf 6-0"

Fe ( P P ) CI

I I t 1 I 1 1 1 l I

v

s - o L

4 ' 0 1 I 1 I I 1 I I I I

0 I0 2 0 3 0 4 0 50 6 0 70 80 9 0 I 0 0

T,K ^---

F i g u r e 8. T e m p e r a t u r e d e p e n d e n c e cff,fL.tj for F e ( P P ) C I a n d F e ( D P D M E ) C I (Miirathe a n d Mitru 1983}.

magnetisation data over the entire range of magnetic fields could not be fitted at all to an 3' single value of D. While the lower field data (H -< 20 kOe) can only be fitted to an unreasonably high value of D ( ~ 30-25 c m - l ) , the 50 kOe data can be fitted to D = 11.0 cm- ~. The data at intermediate magnetic fields correspond best to values of D lying between 11.0 and 30-0 cm -I. The D value of Fe(DPDME)CI is also known accurately from the far infrared spectroscopy, giving D = 9.0 cm- (Brackett et al 1971).

It has been suggested that the above discrepancies indicate the presence of significant magnetic exchange interaction between the ferric ions in the crystal lattice (Marathe and Mitra 1983). To explain the magnetisation the exchange interaction must be included in (1). In view of the uncertainty in the structure of the Fe(DPDME)CI it is appropriate to consider the exchange interaction in a general way in molecular field framework. Equation (1) may then be modified, above the Neel temperature, as (Maral~he and Mitra 1983),

H~ = DS~ + g ~ H . S - 2 Z J ( S ) S . (2)

62 Samaresh Mitra

4

I

~ - - " o - o o _O--c_5OkOe

-

I 1 I I I

0 o-i 0.2 0-3 0 . 4 0.5

( I / T ) , K -I

Figure 9. Magnetisation of Fc(PP)CI down to 2 K (Marathe and Mitra 1983). The solid

c u r v e s a r e f o r D = 8 - 5 c m ~ w i t h o u t e x c h a n g e i n t e r a c t i o n . T h e b r o k e n c u r v e s a r e c a l c u l a t e d f o r D = 6 - 9 5 a n d Z J = - 0 . 0 8 c m

Here Z is the number of nearest equivalent neighbours interacting with an exchange interaction J, and (S) is the expectation value of the spin operator S given by the relation

(S) " e x p ( - E , / k T ) = "~ <~o, Ia[~b,> exp ( - E i / k T ) ,

i i

where E~ and qh are the eigenvalues and eigenvectors of the spin Hamiltonian H.~ in (2). Since the value of (S) depends on the it/,. which contains (S), an iterative procedure was used t o calculate (S) self-consistently. Using this procedure magnetisation has been calculated as a function of temperature and magnetic field to fit the entire set of data for Fe(PP)CI and F e ( D P D M E ) C I (Marathc and Mitra 1983). The best fit values are listed in table I. It is encouraging to find that with the inclusion of the exchange interaction the fit to the data is very good over the entire range of temperature and magnetic field, especially so, for Fc(DPDME)CI (see figures 9 and 10). The exchange energy is small, as expected; however, for deutcroporphyrin it is three times larger than that for heroin chloride. Further the discrepancy mentioned above in the value of D for hemin chloride disappears when even a very weak exchange interaction is included in fitting the data.

The existence of weak exchange interaction in these and other metalloporphyrins is interesting. Since these molecules are relatively large and do not possess a polynuclear structure, they were considered to be magnetically dilute. This does not seem to be true at very low temperatures. A further point of interest is that the magnetisation data at different fields are more sensitive to weak exchange interaction and hence better suited for its detection and evaluation.

Magnetisation studies on metalloporphyrins 63

4

Fe ( D P D M E ) CI

~ ~ ~ 0 ~ o._ c~ . . . . o - - - o - -

~ , . ~ ~ o

~o'f~ _ ~ . ~ ~ ~ o o

2 Z / / o i /

h

o Oli 0 - 2 0 3 0 . 4 0.5

( I / T ) , K "~

3OkOe 4 0

EO

IO

F i g u r e 10. , M a g n e t i s a t i o n o f F e ( D P D M T ~ ' K ' I do~vn t o 2 K ( M a r : l t h e mid M i t r a 19g3).

l ' h c solid ;rod b r o k e n ct,r~cs a r c c:tletllatcd o n e s for I ) = 1141 a n d ZJ = - (I.22 c m t. a n d D - 9.11 a m l Z J = - ( I . 2 ( I cnl t r c s p c c l k c l y .

4.2 High-spin manganese(Ill) i~orph)'rins

Manganese(Ill) porphyrins are usually high-spin (S = 2) with ~Bt ground state.

The combined effect of spin-orbit coupling aml the axial crystal field partly removes the spin-degeneracy as shown in figure II. Here D is positive when ,A,I~ = 0 lies lowest, negative when M~ = __+ 2 of rhe lowest. The magnetic properties of the manganese (!!i) l~rphyrins are, as in iron (!! I) porphyrins, largely governed by the sign and magnitude of the zero-field splitting.

Magnet isation of several manganese (Ill) porphyrins has been reported over a wide range of temperature and magnetic fields. The measurements on Mn(TPP)Ci and Mn(TPP)CI (py) extend between 2-2(I K and 10-50 kOe, and show complete saturation of the magnetisation (Behere et al 1981b). Ma~netisation measurements on Mn(TPP)OAc.H:O and Mn(OEP)CIO.~.H20 also extend over a wide range of magnetic fields and temperatures (upto 4 K) but do not show complete saturation (Kennedy and Murray 1985). Limited measurements on some other man- ganesc(Ill) porphyrins have also been recently reported (Dugad et al 1984). We discuss below some of these results in detail.

64 S a m a r e s h Mitra

M n ( T P P ) C I a n d M n ( T P P ) C I ( p y ) : C o m p l e t e s t r u c t u r a l d a t a o n t h e s e t w o c o m - p o u n d s a r e a v a i l a b l e ( T u l i n s k y ~tnd C h e n 1 9 7 7 ; K i r n e r a n d S c h e i d t 1 9 7 5 ) . I n M n ( T P P ) C I t h e m a n g a n e s e a t o m h a s a s q u a r e p y r a m i d a l s t r u c t u r e a s i n F e ( T P P ) C I w i t h t h e m a n g a n e s e a t o m b e i n g c o o r d i n a t e d t o f o u r b a s a l p y r r o l e n i t r o g e n s a n d a n a x i a l c h l o r i d e i o n . I n M n ( T P P ) C I ( p y ) h e x a c o o r d i n a t e d g e o m e t r y is c o m p l e t e d b y t h e p y r i d i n e t h r o u g h a l o n g M - N r , v b o n d . S o m e r e l e v a n t s t r u c t u r a l d a t a a r e

5B I (5)

!

/

/

/ 5D

4. _{\ -} t

\ D

T

Ms = •

Ms=_+l

Ms=O SrO. coupling mixing

Figure 1 I. Zero field splitting in manganese(Ill) porphyrins (d 4 electron configuration).

Table 2. Magnetic and structural parameters for somc high-spin mangancsc(lll) porphyrins.

Compound Fe-X Fe-N Fe-Ct (p.),~, D ZJ Remarks

- 2 - 3 - - From high temperature

Mn(TPP)C1 2.373 2-008 0-27 3-44 single crystal measurement

- 1 . 9 - 0 - 2 5 Exchange included with MF model

- 3 - 0 - - From high tcmperaturc

Mn(TPP)CI(py) 2.468 2-009 0.12 3-01 singlc crystal mettsuremcnt

- 3 . 0 - - From magnetisation measurement M n ( O E P ) O A c - - - - - - 3.0 - 1 . 9 - - Value deduced from

magnetisation J =

Mn(OEP)CIO4 - - - - - - 2-7 - 2 . 3 -0.(.17 Deduced from magnetisa-

tion

Mn(TPP)CIO4 . . . . . 2.0 - - From p.~. vs T data

Mn(TPP)(I-Melm)2CIO4 . . . . . 2-5 - - From tt~r t vs T data

Magnetisution ~tudie~ on metalloporphyrins 65 included in t~tble 2. "['he effective average magnetic moment of these two e~>mpounds is nearly constant at 4-9 BM down to about 20 K below which it decreases sharply (figure 12) (Beherc and Mitra 198(}). 3"he decrease is similar to that observed in high-spin ir()n(lll) porphyrins and has been largely ascribed to zero-field splitting. The magnetisation data are also very similar to those on Fe(TPP)X (figures 13 and ]4). The moments saturate at 3.44 and 3-01 BM for the chloro and the pyridinato compounds. The higher saturation moment for the chloro compound suggests, on simple consideration, a lower zero-field splitting. This is consistent with the ~.ff vs 7' data in figure 12 which shows that the variation in/~.,, with temperature for the Mn(TPP)CI is much less. The saturation moments in both the compounds are however much less than (/~),,,t = 4-0 BM expected from the Brillouin function for S = 2 and g = 2 (see figure l), and reflect the effect of zero-field splitting of the ground state.

The sign and magnitude of the zero-field splitting in these manganese(Ill) porphyrins has been accurately determined by single crystal magnetic susceptibility

4 9

4

"i

4

4

4 3 - - " L I. I I l , l , 1 [ i

(Z)

t ~

4 9 - 4 8 4 7 4 6 4.5 4 4

- - - C , - ^{0 K., w3, 0 9}

TPPMn CI

1 I L , L I I I l L

tO 20 30 40 50 60 70 80 90 ~00

T

Figure 12. Tempcrature depcndence of ~eff for Mn(TPP)CI and Mn(TPP)(p,.)CI

66 Samaresh Mitra

4.0

3.0

A v

2-0

1 . 0 4 -

T P P M n C {

~ t , = ~ 1 o ~ 5 0 ~,Oe

~ i z Y ~ j ' ~ - ~ ~ - 6 - - G 4 0

o - - ~ - 3 0

7// y .. o ....

~ o I

-V

O I'0 2 0 3"0 4 0 5"0 6 0 = IO

("r)

Figure 13. Magnetisation data on Mn(TPP)CI(py) between 20-2 K (Behere et al ]98]b).

Solid curves are theoretical fits for D = - 3 . 0 r -1.

TPPMn (py) Ct

4 0 3 0

IO t

0-O 0"1 0"2 0"3 0"4 0"5 O~

I T

]Figure 14. Magnetisation data on Mn(TPP)CI between 20 and 2 K. The broken and solid curves are theoretical fits without and with exchange interaction (Behere et al 1981b).

Magnettsation studies on metalloporphyrins 67 measurements (Behere and Mitra 1980b). These values are included in table 2.

Using (1) the magnetisation was filled by varying D. Excellent agreement was obtained for Mn(TPP)CI(py) for D = - 3 . 0 cm -t, but for Mn(TPP)CI no fit was possible for any value of D over the entire range of temperature and magnetic fields. As in the case of Fe(DPDME)Ci, presence of magnetic exchange was considered as the reason for this discrepancy. Including magnetic exchange within molecular field approximation, (2) gave excellent fit to the data for Mn(TPP)CI for D = - 2 . 3 + 0-5 cm-I and J Z = - 0 . 0 2 5 cm-~.

The values of D obtained from the magnetisation agree well with the single crystal values (table 2). The D is negative and small as against the large positive values for the iron(Ill) porphyrins. The origin of the negative sign and small magnitude of D in manganese(Ill) porphyrins has recently been explained on a crystal field theory (Dugad et al 1984).

5 - 0

4 . 1

4 " 2

._ 3"8

5"0

4 " 6

4 " 2

0 0 - - 0 0 0 . ~

Mn(TPP)( 1 -Melm) 2 0 0 4

I I I I

o

4 1 2

Q ^{0 0 0 0}

Mn(TPP)CIO 4

3 - 8 t I t i

2 0 2 8 3 6

T(K)

Figure 15. Temperature dependence of /2~f r I or Mn(TPP)CIO4 and Mn(TPP) (l- MeIm)2CIO4 (Dugad et al 1984).

68 Samaresh Mitra

M n ( T P P ) C I 0 4 a n d M n ( T P P ) ( C I O 4 ) ( 1 - M e l m ) z : M n ( T P P ) C I O 4 is s t r u c t u r a l l y a n a l o g o u s ( R e e d et a11978) t o F e ( T P P ) C I O 4 w h i c h s h o w s n o v e l s p i n - m i x e d g r o u n d s t a t e b e t w e e n S = 5/2 a n d 3/2 ( M i t r a et al 1983). M n ( T P P ) C I O 4 ( 1 - M e l m ) 2 is h e x a c o o r d i n a t e d a n d similar ferric p o r p h y r i n s a r e k n o w n to b e low-spin ( S c h e i d t a n d G o u t e r m a n 1983). M a g n e t i c m e a s u r e m e n t s o n t h e s e t w o m o l e c u l e s b e t w e e n 2 - 1 0 0 K a n d u p t o 15 k O e at 4.2 K ( D u g a d et al 1984) e s t a b l i s h t h e m to b e strictly h i g h - s p i n w i t h v a l u e s o f D s i m i l a r to o t h e r m a n g a n e s e ( I I I ) p o r p h y r i n s ( f i g u r e 15;

t a b l e 2). T h e e f f e c t o f axial p e r c h l o r a t e c o o r d i n a t i o n in m a n g a n e s e ( I I I ) p o r p h y r i n s a p p e a r s t o b e t h u s m i n i m a l , in c o n t r a s t to t h e d r a m a t i c e f f e c t it has o n t h e i r o n ( I I I ) p o r p h y r i n s , w h e r e it c h a n g e s t h e s p i n s t a t e o f t h e m e t a l ion.

M n ( T P P ) ( C I , B r ) H 2 0 a n d M n ( O E P ) O A c . H 2 0 : M a g n e t i s a t i o n o f s e v e r a l o f the m a n g a n e s e ( I I I ) p o r p h y r i n s o f g e n e r a l f o r m u l a M n ( P o r p h ) X . L w h e r e X = C l , B r , O A c , ClO4 a n d L = H 2 0 h a s b e e n r e p o r t e d b e t w e e n 4 . 2 - 2 5 K a n d 5--46 k O e ( K e n n e d y a n d M u r r a y 1985). T h o u g h t h e m o m e n t s d o n o t r e a c h s a t u r a t i o n at t h e l o w e s t t e m p e r a t u r e a n d h i g h e s t field o f m e a s u r e m e n t , t h e d a t a a r e a d e q u a t e to d e r i v e v a l u e s o f D. In all c a s e s a s m a l l e x c h a n g e i n t e r a c t i o n was f o u n d to b e p r e s e n t . T h e v a l u e s o f D fall w i t h i n t h e r a n g e o f o t h e r m a n g a n e s e ( I I I ) p o r p h y r i n s ( t a b l e 2).

R e f e r e n c e s

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Behere D V and Mitra S 1980a Indian J. Chem. Ai9 505 Behere D V and Mitra S 1980b Inorg. Chem. 19 992

Birdy R, Behere D V and Mitra S 1983 J. Chem. Phys. 78 1453

Brackett G C, Richards P L and Caughey W S 1971 J. Chem. Phys. 54 4383 Dugad L B, Behere D V, Marathe V R and Mitra S 1984 Chem. Phys. Lett. 104 353 Dolphin D (ed.) 1978 The porphyrirts (New York: Academic Press)

Kennedy B J and Murray K S 1985 lnorg. Chem. 24 1558 Kirner J F and Scheidt W R 1975 inorg. Chem. 14 2081 Marathe V R and Mitra S 1973 Chem. Phys. Lett. 19 140 Marathe V R and Mitra S 1974 Chem. Phys. Leu. 27 103 Marathe V R and Mitra S 1983 J. Chem. Phys. 78 915

Mitra S 1983 in Iron porphyrins (eds) A B P Lever and H B Gray (Reading, Mass: Addison-Wesley) vol. 2

Mitra S, Marathe V R and Birdy R 1983 Chem. Phys. Lett. 96 103 Neiheisel G L, Imes J L and Pratt W P 1975 Phys. Rev. Lett. 35 101

Reed C A, Chua H K and Hoard S 1978 (unpublished) quoted by W R Scheidt in Theporphyrins, vol. 3, (ed.) D. Dolphin (New York: Academic Press) 1978 p. 510

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Mass: Addison-Wesley) vol. 2

Smith K M (ed.) 1975 Porphyrins and Metalloporphyrins (Amsterdam: Elsevier) Tulinsky A and Chen B M 1977 J. Am. Chem. Soc. 99 3647

Vermass A and Groeneveld W L 1974 Chem. Phys. Lett. 27 583