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Mössbauer and optical spectra and magnetic susceptibilities of some iron(III)-β-diketone complexes
J. inorg,nucl. Chem., 1969,Vol. 31, pp. 2853 to 2858. PergamonPress. Printed in Great Britain
M()SSBAUER AND OPTICAL SPECTRA AND MAGNETIC SUSCEPTIBILITIES OF SOME IRON(ilI)-~DIKETONE COMPLEXES C. K. M A T H E W S Radiochemistry Division, Bhabha A t o m i c R e s e a r c h Centre. T r o m b a y . Bombay-74, India
(First received 12 November 1968 in revised form 27 January 1969) A b s t r a c t - - T h e M~Sssbauer and optical spectra, and magnetic susceptibilities of the c o m p l e x e s of iron(l l l) with thenoyltrifluoroacetone and d i b e n z o y l m e t h a n e are reported. T h e nephelauxetic parameter/33~ and the ligand field p a r a m e t e r A have been d e d u c e d from the optical spectra, and the quadrupole splitting in the M 6 s s b a u e r spectra is explained in terms of the covalent effects in the metal-oxygen bonding.
FERRIC ION has a d 5 electronic configuration and hence a spherically symmetric charge distribution. It should, therefore, be characterized by the absence of any electric field gradient at the nucleus unless the unsymmetric arrangement of ions around it in the crystal lattice produces such a gradient. In fact, ionic compounds of Fe(Ill) with an octahedral symmetry around the cation do not in general exhibit quadrupole splitting in their M6ssbauer spectra[l,2], thus indicating the absence of an electric field gradient at the iron nucleus. However, the MSssbauer spectrum of ferric acetylacetonate, in which oxygen atoms are arranged octahedrally around the central iron atom[3], shows a wide absorption line [4]. To investigate whether this large line broadening is related to its chemical structure, a study of the iron(II1)/3-diketones was undertaken. The fl-diketones studied included, in addition to acetylacetone(acac), dibenzoyl methane (dbm) and thenoyltrifluoroacetone (tta). While this work was in progress Bancroft e t al.[5] published the M6ssbauer parameters of a number of iron(I lI)-/~-diketone chelates and hence this work was not extended to other/3-diketones. In addition to the MSssbauer spectra, the optical absorption spectra of the Fe(dbm)3 and Fe(tta)3 complexes have been measured in the visible, u.v. and i.r. regions and their magnetic susceptibilities determined in the temperature range from liquid nitrogen to room temperature. Barnum[6] and Jorgensen[7] have discussed the electronic absorption spectra of ferric acetylacetonate, and Cambi and Szego [8] have reported its magnetic susceptibility. S. De Benedeni, G. Lang and R. lngalls, Phys, Rev. Lett. 6, 60 ( 1961 ). P. R. Brady, J. F. D u n c a n and K. F. Mok, Proc. R. Soc. A287, 343 (1965). R. B. Roof, Jr., Acta Crystallogr. 9, 781 (1956). G. K. Wertheim, W. R. Kingston and R. H. Herber, J. chem. Phys. 37,687 (1962). G. M. Bancroft, A. G. Maddock, W. K. Ong, R. H. Prince and A. J. Stone, J. chem. Soc.. (A). 1967, 1966. 6. D . W . B a r n u m , J . inorg, nucl. Chem. 21,221 ( 1961 ). 7. C. K. Jorgensen, Acta chem. scand. 16, 2406 (1962). 8. L. Cambi and L. Szego, Chem. Ber. 64.2591 ( 1931 ). 2853 I. 2. 3. 4. 5.
2854
C . K . MATHEWS EXPERIMENTAL
Fe(tta)3 was prepared by mixing stoichiometric amounts of ferric nitrate and tta in methanol solution, with enough ammonia added to neutralize most of the acid produced. The bright red precipitate thus obtained was recrystallized from xylene and methanol. Analysis gave the following results (with the expected values in parenthesis): Fe, 7.82% (7-76%); C, 39.7% (40.0%); H, 2.3% (I .7%). Fe(dbm)3 was also prepared by a similar method. Analysis: Fe, 7.34% (7-70%); C, 74.8% (74"4%); H, 4"56% (4-39%). Fe(acac)3 was prepared by standard methods [9]. The Mrssbauer spectra were taken on a mechanical constant velocity spectrometer set up in this laboratory[10]. The source used was 5 mc of Co 57 in copper matrix. The absorbers were prepared by packing the powder sample between polythene films and using a few drops of collodion solution as an adhesive. The visible u.v. spectra were measured using a Carl Zeiss RPQ20AV spectrophotometer and the i.r. spectra with a Perkin-Elmer model 237 i.r. spectrophotometer. The magnetic susceptibility measurements were carried out on a Gouy balance. RESULTS AND DISCUSSION
1. MiJssbauer p a r a m e t e r s T h e M r s s b a u e r s p e c t r a o f F e ( d b m ) z a n d F e ( t t a ) z at 7 8 ° K a r e s h o w n in F i g . l. I n b o t h t h e s e c a s e s t h e t w o p e a k s o f t h e q u a d r u p o l e split s p e c t r a a r e s u f f i c i e n t l y
2110
";.
zro
i o
260
31, (b)
300
i
290
280
I -3"0
,
I -2"0
,
I -I'0
,
I 0 Velocity,
,
I l'O
I
i 2"0
!
I 3"0
I
mm/sec.
Fig. 1. The Mrssbauer spectra of(a) Fetdbm)~ and (b) Fe(tta)3 at 78°K. 9. W. C. Fernelius, Inorganic Syntheses, Vol. 11. McGraw-Hill, New York (1946). I 0. C. K. Mathews, Internal Rep., B.A.R.C. (Radiochem) 75 (1967).
I 4.0
iron(lll)-/3-diketone c o m p l e x e s
2855
well resolved. However, the spectrum of Fe(acach showed a broad single line absorption. The MSssbauer parameters of these compounds are tabulated in Table 1 along with those of other/3-diketone complexes available in literature. Table 1. M 5 s s b a u e r parameters of iron(l I 1)/3-diketones* Chemical shift
Compound I il
111
IV V VI ViI VII1 IX
Feltta)3t Fe(dbm}3 This workt Bancroft e t a/.[5] Fa(acac)3 T h i s work Bancroft et a/.[5] W e r t h e i m et a/.[4] Fe(hfac)3 [5] + F e ( b z a c h [5]~Fe(NO2bzac)[5]$ Fe(tfac)3 [5]$ Fe(CNacac)z[5]~Fe(Clbzac)3 [5].~
Relative to sodium nitro-prusside
Quadrupole splitting
Percentage absorption
0.78_+_0-01
0.39-+-0.02
7.4
0.67 ± 0.05 0.81 _+0.10
0.60 ± 0.02 N o t resolved
3.1 I
0-77_+0.05 0.785 _+0.03 0.61 _+0.10 0.76_+0.05 0.81_+0.15 0-66±0.15 (I.73 ± 0.06 0.75±0.10 0.82_+0-20
N o t resolved N o t resolved Not resolved 0.65_+0.10 0-33±0.30 0.76_+0.10 0.67 ± 0-10 0.71 _+0.08 0.66+_0.42
4.0 4.6 2.6 1.5 2-5 3.5 1.3 1.0
tta = thenoyltrifluoroacetone; d b m = dibenzoylmethane: acac = acetytacetone: hfac = hexafluoroacetylacetone; bzac = benzoylacetone; NO2bzac = p-nitrobenzoylacetone; tfac = trifluoroacetylacetone; C N a c a c = 3-cyanoacetylacetone: Clbzac = p-chlorobenzoylacetone. *All values are in m m / s e c . t A v e r a g e of several runs. Errors quoted are standard deviations. SAverage of the several values quoted in [5] are given. Errors have been estimated by taking into account the spread of the values.
T h e MSssbauer p a r a m e t e r s of Fe(tta)3 have not been reported before. Bancroft et a/.[5] have recently reported that the MSssbauer spectrum of Fe(dbm):~ consisted of a broad single line which could not be resolved into two c o m p o n e n t s , a p r o g r a m m e to fit a doublet having failed to converge. This is contrary to our observation of a well resolved doublet. It is worth mentioning, however, that the spectrum tended to deteriorate with time. Our observations on Fe(acac)..~ agree well with earlier work [4, 5]. 2. Visibh' a n d u.v. s p e c t r a
T h e optical absorption spectra of these c o m p o u n d s in the visible and u.v. region were measured in ethanol and chloroform. T h e s e spectra are characterized by very broad and intense bands in the ultra-violet with w e a k e r bands appearing at lower frequencies. T h e data are s u m m a r i z e d in Table 2. T h e molar extinction coefficients (e) and the half-widths (6) quoted are a p p r o x i m a t e in the case of the w e a k e r bands appearing as shoulders. T h e bands in the ultraviolet have a halfwidth of several kK, while the narrow bands have 6 --~ 0.5 kK. T t a has strong absorption bands in the ultraviolet at 267 mix (e ~- 104) and 292
C. K. M A T H E W S
2856
Table 2. Absorption bands in Fe(tta)3 and Fe(dbm)3t ,k (/z) l
8 (kK)
e
Assignments
Fe(tta)a (I) (2) (3) (4) (5) (6)
11
o(k K)
263 333 (383) (420) (490) (735)
38"0 30.0 (26.1) (23.8) (20-4) (13"6)
~ 2× - 5× < < <
105 104
v. broad v. broad
103
narrow narrow
39'7 29.0 (23.8)
~ 105 ~ 2 × 105 < 103
100
100 weak
~r --~ 7r*
Electron transfer
broad
6Ft ~ 4Fta (TsaT32) er~ --~ 4F5 (y54y3) 6F~ ~ 4F 4 (y54Ts)
v. broad v. broad
Electron transfer
narrow
6F1 "-") 4F1, 3 (T53T32)
( ~ 1-5)
Fe(DBm)3 (1) 252 (2) 345 (3) (420)
~'---~ ¢r*
(- 0.5) (4) (510)*
(19.6)
< I00
(I-1.5)
eF~ ~ 4F5 ('/5~3)
* In CHCI3. ¢Spectra measured in ethanol unless otherwise specified; bands which appear as shoulders on the side of a more intense band are given in parantheses.
m/~ (4 ~ 8 × 103)[11], and dbm has bands at 245 m/x (4 ~- 4-5× 103) and 348 m/z (e ~ 2-2 × 104)[12]. These evidently represent transitions in the rr-electron systems of these ligands. The 263 m/z band in Fe(tta)3 and the 252 m/z band in Fe (dbm)3 may be attributed to these w-electron transitions. However, in Fe(dbm)3 the molar extinction coefficient in this region is an order of magnitude higher than that in the ligand. It is possible that some electron transfer transition not considered below (such as 7r ~ y~) occurs in this region. The 7r ---> rr* transitions of the ligands occuring at lower energies (292 rn/x in tta) seem to be masked by the electron transfer bands considered below. The most intense band in the 30 kK region is assigned to the electron transfer from the filled 7r orbital of the ligand to the 3:5 orbital of the central atom (7, 13). In Fe(tta)3 there is a weak band at 13.6 kK and in both Fe(tta)3 and Fe(dbmh there is a relatively broad shoulder at about 20 kK (4 < 100). The former is attributed to the 6F1 ---> 4F4(T54-y3 ) transition and the latter to the 6F1--9 4Fs(')/54T3 ) transition, in analogy with other ferric complexes of this type [14, 15, 16]. Similarly the narrow band appearing at 23.8 kK in both the complexes must be the nFx---> 4F~.3(T~aT32) transition. The large values of the extinction coefficients may be related to the distortion from octahedral symmetry. The ligand field parameter, A, and the nephelauxetic parameter/3 can now be calculated assuming Tanabe and Sugano's determinants[15,17]. Thus nF1--> 4F1,3(T53T32 ) transition energy, assuming C----4B, one gets B = 793 cm -1 I I. 12. 13. 14. 15. 16. 17.
E. L. King and W. H. Reas, J. Am. chem. Soc. 73, 1804 ( 1951). G. S. Hammond, W. G. Borduin and G. A. Guter, J. ,4 m. chem. Soc. 81, 4682 (1959). R.J.P. Williams, J. chem. Soc. 1955, 137. C. K. Jorgensen, Progress in Inorganic Chemistry, Vol. 4, p. 73. Interscience, N e w York (1962). C. K. Jorgensen, Advances in ChemicalPhysics, Vol. V, p. 33. Interscience, N e w York (1963). M. Dvir and W. Low, Phys. Rev. 119, 1587 (1960). Y. Tanabe and S. Sugano, J. phys. Soe. Japan 9, 753,766 (1954).
lron(l l l)-fl-diketone complexes
2857
and hence /3a5= 0.73 (15) for both these complexes. Similarly from the 6F~ 4F~(~/54y3) and 6F~ ~ 4I'4(y~4y3) energy differences, A is calculated to be about 11,500 cm -1 for both Fe(tta)3 and Fe(dbmh.
3. Infrared spectra The i.r. absorption spectra show bands characteristic of "the chelate carbonyl" in the 1500-1600 cm -1 region. In the case of Fe(dbm)3 these bands appear in the 1510-1550 cm -1 region and at 1592 cm-'. In the Fe(tta)3 spectrum there are bands at 1502, 1540-1550, 1575 and 1605 cm-L
4. Magnetic susceptibility measurements The magnetic susceptibilities of the Fe(tta)3 and Fe(dbm)3 complexes were measured at several temperatures in the region 86°K to room temperature. The results are summarized in Table 3. The susceptibility values for the dbm complex obeys the Curie law while those for the tta complex obeys the Curie-Weiss law with 0 = 32°. The/..6eft values of these complexes as well as that of Fe(acac)~( p~,,rf= 5.95) [8] clearly indicate that they are spin-free. Table 3. Magnetic susceptibility data Fe(tta):~ T (OK)
~/~eff (B.M.)
86 122 188.5 228 292.7
5.20 5.44 5-67 5-75 5.83
Feldbm}:~ T ~3.eff (OK) (B.M.) 87 112.5 161 231 291. I
5.94 6.18 6.14 6.19 6.21
5. Quadrupole splitting Quadrupole splitting is usually observed in the compounds of iron(Ill) if (a) the compound is spin-paired giving the electronic ground state a F5 symmetry or (b) if the iron atom has an unsymmetric environment of ions in the crystal lattice. The magnetic susceptibility measurements clearly indicate that the fl-diketone complexes are not spin-paired. That the ground state is F~ is confirmed by the optical spectra. In ionic ferric compounds, the electric field gradient at the iron nucleus is given by (1 -'y~)qlat, where qlat is a lattice contribution and 3% is the Sternheimer antishielding factor[18]. It is difficult to explain the quadrupole splitting in the /3-diketone complexes on the basis of this ionic model. Even in ionic crystals where this lattice contribution should be larger because of appreciable distortion from cubic symmetry, the quadrupole splitting is small. For example, in ~-Fe203 where six oxygen atoms are arranged around the iron atom with appreciable departure from octahedral symmetry, the splitting is only 0.20 mm/sec[19], in the 18. R. Ingalls, Phys. Rev. 128, 1155 (1962). 19. D. E. Cox, G. Shirane, P. A. Flinn, S. L. Ruby and W. J. Takei, Phys. Rev. 132, 1547 (1963).
2858
C. K. MATHEWS
case of the fl-diketone complexes each of the six oxygen atoms surrounding the iron may be considered to have an effective charge of only - 0 . 5 e (as against - 2e in FezO3) and bulky organic groups separate the ionic charges further away. Bancroft et a/.[5] have also discarded this possibility. They have further neglected the contribution due to charges on the more distant parts of the ligand molecules on the basis of the existing experimental data and recent calculations. Covalent effects must, therefore, be considered in accounting for the observed quadrupole splitting. The fact that the nephelauxetic effect is fairly large shows that d-orbitals are involved in bonding. Nephelauxetic effect has been explained in terms of symmetry-restricted covalency and central-field covalency [ 15]. Bancroft et a/.[5] have suggested that 7r-donation from the ferric ion to the chelate ring is the principal factor determining the quadrupole splitting in this group of complexes. There is disagreement in literature on the extent of Tr-bonding in such complexes. Barnum [6] discusses this and suggests that the actual 7r-interaction in aceylacetonate complexes is a combination of two different types of interaction: the interaction of the 3'5 orbitals with both the lower, filled ~'-orbitals and the higher, empty orbitals in the ligands. Jorgensen[14] has also pointed out the significance of the low value of fl55 in Cr(acac)3 [20]. However, if all the three 3'5 orbitals are equally involved in 7r-bonding, it is not likely to lead to any large quadrupole splitting, provided the octahedral symmetry is maintained. It is interesting to note that in all these complexes the ligands are bidentate. With the same ligand bonding at two points, all the F e - - O bonds may not be equivalent. X-ray data[3] has, however, shown that these distortions are not large. While the small distortions associated with the bonding in these complexes may not explain the observed quadrupole splitting on the ionic model, the consequent distortions of the valence electrons through covalent bonding may enhance the electric field gradient. Covalent effects may thus be a major factor contributing to the quadrupole splitting observed in these complexes. Whether the splitting in Fe(acac)a is also comparable is not clear because of the line broadening and Wignall [2 l] has shown that this broadening is due to spin-spin relaxation effects. Acknowledgements-The author wishes to thank Dr. M. V. Ramaniah, Head, Radiochemistry
Division for his interest and encouragement. He is also grateful to Professor C. R. Kanekarof the Tata Institute of Fundamental Research for makingthe Gouy balance available. Thanks are due to Mr. P. K. Sukumaranfor his assistancein the M/Sssbauermeasurements. 20. L. S. Forster and K. Armond, Proc. 7th Int. Conf. Co-ordination Chemistry, Stockholm,p. 20 (1962). 21. J. W. G. Wignall,J. chem. Phys. 44, 2462 (1966).
M()SSBAUER AND OPTICAL SPECTRA AND MAGNETIC SUSCEPTIBILITIES OF SOME IRON(ilI)-~DIKETONE COMPLEXES C. K. M A T H E W S Radiochemistry Division, Bhabha A t o m i c R e s e a r c h Centre. T r o m b a y . Bombay-74, India
(First received 12 November 1968 in revised form 27 January 1969) A b s t r a c t - - T h e M~Sssbauer and optical spectra, and magnetic susceptibilities of the c o m p l e x e s of iron(l l l) with thenoyltrifluoroacetone and d i b e n z o y l m e t h a n e are reported. T h e nephelauxetic parameter/33~ and the ligand field p a r a m e t e r A have been d e d u c e d from the optical spectra, and the quadrupole splitting in the M 6 s s b a u e r spectra is explained in terms of the covalent effects in the metal-oxygen bonding.
FERRIC ION has a d 5 electronic configuration and hence a spherically symmetric charge distribution. It should, therefore, be characterized by the absence of any electric field gradient at the nucleus unless the unsymmetric arrangement of ions around it in the crystal lattice produces such a gradient. In fact, ionic compounds of Fe(Ill) with an octahedral symmetry around the cation do not in general exhibit quadrupole splitting in their M6ssbauer spectra[l,2], thus indicating the absence of an electric field gradient at the iron nucleus. However, the MSssbauer spectrum of ferric acetylacetonate, in which oxygen atoms are arranged octahedrally around the central iron atom[3], shows a wide absorption line [4]. To investigate whether this large line broadening is related to its chemical structure, a study of the iron(II1)/3-diketones was undertaken. The fl-diketones studied included, in addition to acetylacetone(acac), dibenzoyl methane (dbm) and thenoyltrifluoroacetone (tta). While this work was in progress Bancroft e t al.[5] published the M6ssbauer parameters of a number of iron(I lI)-/~-diketone chelates and hence this work was not extended to other/3-diketones. In addition to the MSssbauer spectra, the optical absorption spectra of the Fe(dbm)3 and Fe(tta)3 complexes have been measured in the visible, u.v. and i.r. regions and their magnetic susceptibilities determined in the temperature range from liquid nitrogen to room temperature. Barnum[6] and Jorgensen[7] have discussed the electronic absorption spectra of ferric acetylacetonate, and Cambi and Szego [8] have reported its magnetic susceptibility. S. De Benedeni, G. Lang and R. lngalls, Phys, Rev. Lett. 6, 60 ( 1961 ). P. R. Brady, J. F. D u n c a n and K. F. Mok, Proc. R. Soc. A287, 343 (1965). R. B. Roof, Jr., Acta Crystallogr. 9, 781 (1956). G. K. Wertheim, W. R. Kingston and R. H. Herber, J. chem. Phys. 37,687 (1962). G. M. Bancroft, A. G. Maddock, W. K. Ong, R. H. Prince and A. J. Stone, J. chem. Soc.. (A). 1967, 1966. 6. D . W . B a r n u m , J . inorg, nucl. Chem. 21,221 ( 1961 ). 7. C. K. Jorgensen, Acta chem. scand. 16, 2406 (1962). 8. L. Cambi and L. Szego, Chem. Ber. 64.2591 ( 1931 ). 2853 I. 2. 3. 4. 5.
2854
C . K . MATHEWS EXPERIMENTAL
Fe(tta)3 was prepared by mixing stoichiometric amounts of ferric nitrate and tta in methanol solution, with enough ammonia added to neutralize most of the acid produced. The bright red precipitate thus obtained was recrystallized from xylene and methanol. Analysis gave the following results (with the expected values in parenthesis): Fe, 7.82% (7-76%); C, 39.7% (40.0%); H, 2.3% (I .7%). Fe(dbm)3 was also prepared by a similar method. Analysis: Fe, 7.34% (7-70%); C, 74.8% (74"4%); H, 4"56% (4-39%). Fe(acac)3 was prepared by standard methods [9]. The Mrssbauer spectra were taken on a mechanical constant velocity spectrometer set up in this laboratory[10]. The source used was 5 mc of Co 57 in copper matrix. The absorbers were prepared by packing the powder sample between polythene films and using a few drops of collodion solution as an adhesive. The visible u.v. spectra were measured using a Carl Zeiss RPQ20AV spectrophotometer and the i.r. spectra with a Perkin-Elmer model 237 i.r. spectrophotometer. The magnetic susceptibility measurements were carried out on a Gouy balance. RESULTS AND DISCUSSION
1. MiJssbauer p a r a m e t e r s T h e M r s s b a u e r s p e c t r a o f F e ( d b m ) z a n d F e ( t t a ) z at 7 8 ° K a r e s h o w n in F i g . l. I n b o t h t h e s e c a s e s t h e t w o p e a k s o f t h e q u a d r u p o l e split s p e c t r a a r e s u f f i c i e n t l y
2110
";.
zro
i o
260
31, (b)
300
i
290
280
I -3"0
,
I -2"0
,
I -I'0
,
I 0 Velocity,
,
I l'O
I
i 2"0
!
I 3"0
I
mm/sec.
Fig. 1. The Mrssbauer spectra of(a) Fetdbm)~ and (b) Fe(tta)3 at 78°K. 9. W. C. Fernelius, Inorganic Syntheses, Vol. 11. McGraw-Hill, New York (1946). I 0. C. K. Mathews, Internal Rep., B.A.R.C. (Radiochem) 75 (1967).
I 4.0
iron(lll)-/3-diketone c o m p l e x e s
2855
well resolved. However, the spectrum of Fe(acach showed a broad single line absorption. The MSssbauer parameters of these compounds are tabulated in Table 1 along with those of other/3-diketone complexes available in literature. Table 1. M 5 s s b a u e r parameters of iron(l I 1)/3-diketones* Chemical shift
Compound I il
111
IV V VI ViI VII1 IX
Feltta)3t Fe(dbm}3 This workt Bancroft e t a/.[5] Fa(acac)3 T h i s work Bancroft et a/.[5] W e r t h e i m et a/.[4] Fe(hfac)3 [5] + F e ( b z a c h [5]~Fe(NO2bzac)[5]$ Fe(tfac)3 [5]$ Fe(CNacac)z[5]~Fe(Clbzac)3 [5].~
Relative to sodium nitro-prusside
Quadrupole splitting
Percentage absorption
0.78_+_0-01
0.39-+-0.02
7.4
0.67 ± 0.05 0.81 _+0.10
0.60 ± 0.02 N o t resolved
3.1 I
0-77_+0.05 0.785 _+0.03 0.61 _+0.10 0.76_+0.05 0.81_+0.15 0-66±0.15 (I.73 ± 0.06 0.75±0.10 0.82_+0-20
N o t resolved N o t resolved Not resolved 0.65_+0.10 0-33±0.30 0.76_+0.10 0.67 ± 0-10 0.71 _+0.08 0.66+_0.42
4.0 4.6 2.6 1.5 2-5 3.5 1.3 1.0
tta = thenoyltrifluoroacetone; d b m = dibenzoylmethane: acac = acetytacetone: hfac = hexafluoroacetylacetone; bzac = benzoylacetone; NO2bzac = p-nitrobenzoylacetone; tfac = trifluoroacetylacetone; C N a c a c = 3-cyanoacetylacetone: Clbzac = p-chlorobenzoylacetone. *All values are in m m / s e c . t A v e r a g e of several runs. Errors quoted are standard deviations. SAverage of the several values quoted in [5] are given. Errors have been estimated by taking into account the spread of the values.
T h e MSssbauer p a r a m e t e r s of Fe(tta)3 have not been reported before. Bancroft et a/.[5] have recently reported that the MSssbauer spectrum of Fe(dbm):~ consisted of a broad single line which could not be resolved into two c o m p o n e n t s , a p r o g r a m m e to fit a doublet having failed to converge. This is contrary to our observation of a well resolved doublet. It is worth mentioning, however, that the spectrum tended to deteriorate with time. Our observations on Fe(acac)..~ agree well with earlier work [4, 5]. 2. Visibh' a n d u.v. s p e c t r a
T h e optical absorption spectra of these c o m p o u n d s in the visible and u.v. region were measured in ethanol and chloroform. T h e s e spectra are characterized by very broad and intense bands in the ultra-violet with w e a k e r bands appearing at lower frequencies. T h e data are s u m m a r i z e d in Table 2. T h e molar extinction coefficients (e) and the half-widths (6) quoted are a p p r o x i m a t e in the case of the w e a k e r bands appearing as shoulders. T h e bands in the ultraviolet have a halfwidth of several kK, while the narrow bands have 6 --~ 0.5 kK. T t a has strong absorption bands in the ultraviolet at 267 mix (e ~- 104) and 292
C. K. M A T H E W S
2856
Table 2. Absorption bands in Fe(tta)3 and Fe(dbm)3t ,k (/z) l
8 (kK)
e
Assignments
Fe(tta)a (I) (2) (3) (4) (5) (6)
11
o(k K)
263 333 (383) (420) (490) (735)
38"0 30.0 (26.1) (23.8) (20-4) (13"6)
~ 2× - 5× < < <
105 104
v. broad v. broad
103
narrow narrow
39'7 29.0 (23.8)
~ 105 ~ 2 × 105 < 103
100
100 weak
~r --~ 7r*
Electron transfer
broad
6Ft ~ 4Fta (TsaT32) er~ --~ 4F5 (y54y3) 6F~ ~ 4F 4 (y54Ts)
v. broad v. broad
Electron transfer
narrow
6F1 "-") 4F1, 3 (T53T32)
( ~ 1-5)
Fe(DBm)3 (1) 252 (2) 345 (3) (420)
~'---~ ¢r*
(- 0.5) (4) (510)*
(19.6)
< I00
(I-1.5)
eF~ ~ 4F5 ('/5~3)
* In CHCI3. ¢Spectra measured in ethanol unless otherwise specified; bands which appear as shoulders on the side of a more intense band are given in parantheses.
m/~ (4 ~ 8 × 103)[11], and dbm has bands at 245 m/x (4 ~- 4-5× 103) and 348 m/z (e ~ 2-2 × 104)[12]. These evidently represent transitions in the rr-electron systems of these ligands. The 263 m/z band in Fe(tta)3 and the 252 m/z band in Fe (dbm)3 may be attributed to these w-electron transitions. However, in Fe(dbm)3 the molar extinction coefficient in this region is an order of magnitude higher than that in the ligand. It is possible that some electron transfer transition not considered below (such as 7r ~ y~) occurs in this region. The 7r ---> rr* transitions of the ligands occuring at lower energies (292 rn/x in tta) seem to be masked by the electron transfer bands considered below. The most intense band in the 30 kK region is assigned to the electron transfer from the filled 7r orbital of the ligand to the 3:5 orbital of the central atom (7, 13). In Fe(tta)3 there is a weak band at 13.6 kK and in both Fe(tta)3 and Fe(dbmh there is a relatively broad shoulder at about 20 kK (4 < 100). The former is attributed to the 6F1 ---> 4F4(T54-y3 ) transition and the latter to the 6F1--9 4Fs(')/54T3 ) transition, in analogy with other ferric complexes of this type [14, 15, 16]. Similarly the narrow band appearing at 23.8 kK in both the complexes must be the nFx---> 4F~.3(T~aT32) transition. The large values of the extinction coefficients may be related to the distortion from octahedral symmetry. The ligand field parameter, A, and the nephelauxetic parameter/3 can now be calculated assuming Tanabe and Sugano's determinants[15,17]. Thus nF1--> 4F1,3(T53T32 ) transition energy, assuming C----4B, one gets B = 793 cm -1 I I. 12. 13. 14. 15. 16. 17.
E. L. King and W. H. Reas, J. Am. chem. Soc. 73, 1804 ( 1951). G. S. Hammond, W. G. Borduin and G. A. Guter, J. ,4 m. chem. Soc. 81, 4682 (1959). R.J.P. Williams, J. chem. Soc. 1955, 137. C. K. Jorgensen, Progress in Inorganic Chemistry, Vol. 4, p. 73. Interscience, N e w York (1962). C. K. Jorgensen, Advances in ChemicalPhysics, Vol. V, p. 33. Interscience, N e w York (1963). M. Dvir and W. Low, Phys. Rev. 119, 1587 (1960). Y. Tanabe and S. Sugano, J. phys. Soe. Japan 9, 753,766 (1954).
lron(l l l)-fl-diketone complexes
2857
and hence /3a5= 0.73 (15) for both these complexes. Similarly from the 6F~ 4F~(~/54y3) and 6F~ ~ 4I'4(y~4y3) energy differences, A is calculated to be about 11,500 cm -1 for both Fe(tta)3 and Fe(dbmh.
3. Infrared spectra The i.r. absorption spectra show bands characteristic of "the chelate carbonyl" in the 1500-1600 cm -1 region. In the case of Fe(dbm)3 these bands appear in the 1510-1550 cm -1 region and at 1592 cm-'. In the Fe(tta)3 spectrum there are bands at 1502, 1540-1550, 1575 and 1605 cm-L
4. Magnetic susceptibility measurements The magnetic susceptibilities of the Fe(tta)3 and Fe(dbm)3 complexes were measured at several temperatures in the region 86°K to room temperature. The results are summarized in Table 3. The susceptibility values for the dbm complex obeys the Curie law while those for the tta complex obeys the Curie-Weiss law with 0 = 32°. The/..6eft values of these complexes as well as that of Fe(acac)~( p~,,rf= 5.95) [8] clearly indicate that they are spin-free. Table 3. Magnetic susceptibility data Fe(tta):~ T (OK)
~/~eff (B.M.)
86 122 188.5 228 292.7
5.20 5.44 5-67 5-75 5.83
Feldbm}:~ T ~3.eff (OK) (B.M.) 87 112.5 161 231 291. I
5.94 6.18 6.14 6.19 6.21
5. Quadrupole splitting Quadrupole splitting is usually observed in the compounds of iron(Ill) if (a) the compound is spin-paired giving the electronic ground state a F5 symmetry or (b) if the iron atom has an unsymmetric environment of ions in the crystal lattice. The magnetic susceptibility measurements clearly indicate that the fl-diketone complexes are not spin-paired. That the ground state is F~ is confirmed by the optical spectra. In ionic ferric compounds, the electric field gradient at the iron nucleus is given by (1 -'y~)qlat, where qlat is a lattice contribution and 3% is the Sternheimer antishielding factor[18]. It is difficult to explain the quadrupole splitting in the /3-diketone complexes on the basis of this ionic model. Even in ionic crystals where this lattice contribution should be larger because of appreciable distortion from cubic symmetry, the quadrupole splitting is small. For example, in ~-Fe203 where six oxygen atoms are arranged around the iron atom with appreciable departure from octahedral symmetry, the splitting is only 0.20 mm/sec[19], in the 18. R. Ingalls, Phys. Rev. 128, 1155 (1962). 19. D. E. Cox, G. Shirane, P. A. Flinn, S. L. Ruby and W. J. Takei, Phys. Rev. 132, 1547 (1963).
2858
C. K. MATHEWS
case of the fl-diketone complexes each of the six oxygen atoms surrounding the iron may be considered to have an effective charge of only - 0 . 5 e (as against - 2e in FezO3) and bulky organic groups separate the ionic charges further away. Bancroft et a/.[5] have also discarded this possibility. They have further neglected the contribution due to charges on the more distant parts of the ligand molecules on the basis of the existing experimental data and recent calculations. Covalent effects must, therefore, be considered in accounting for the observed quadrupole splitting. The fact that the nephelauxetic effect is fairly large shows that d-orbitals are involved in bonding. Nephelauxetic effect has been explained in terms of symmetry-restricted covalency and central-field covalency [ 15]. Bancroft et a/.[5] have suggested that 7r-donation from the ferric ion to the chelate ring is the principal factor determining the quadrupole splitting in this group of complexes. There is disagreement in literature on the extent of Tr-bonding in such complexes. Barnum [6] discusses this and suggests that the actual 7r-interaction in aceylacetonate complexes is a combination of two different types of interaction: the interaction of the 3'5 orbitals with both the lower, filled ~'-orbitals and the higher, empty orbitals in the ligands. Jorgensen[14] has also pointed out the significance of the low value of fl55 in Cr(acac)3 [20]. However, if all the three 3'5 orbitals are equally involved in 7r-bonding, it is not likely to lead to any large quadrupole splitting, provided the octahedral symmetry is maintained. It is interesting to note that in all these complexes the ligands are bidentate. With the same ligand bonding at two points, all the F e - - O bonds may not be equivalent. X-ray data[3] has, however, shown that these distortions are not large. While the small distortions associated with the bonding in these complexes may not explain the observed quadrupole splitting on the ionic model, the consequent distortions of the valence electrons through covalent bonding may enhance the electric field gradient. Covalent effects may thus be a major factor contributing to the quadrupole splitting observed in these complexes. Whether the splitting in Fe(acac)a is also comparable is not clear because of the line broadening and Wignall [2 l] has shown that this broadening is due to spin-spin relaxation effects. Acknowledgements-The author wishes to thank Dr. M. V. Ramaniah, Head, Radiochemistry
Division for his interest and encouragement. He is also grateful to Professor C. R. Kanekarof the Tata Institute of Fundamental Research for makingthe Gouy balance available. Thanks are due to Mr. P. K. Sukumaranfor his assistancein the M/Sssbauermeasurements. 20. L. S. Forster and K. Armond, Proc. 7th Int. Conf. Co-ordination Chemistry, Stockholm,p. 20 (1962). 21. J. W. G. Wignall,J. chem. Phys. 44, 2462 (1966).