Studies of iron(III) complexes with 4-substituted pyridine N-oxides

Studies of iron(III) complexes with 4-substituted pyridine N-oxides

4344 Notes various divalent metal ions, indicated that the M - N vibrations are to be found in the range 260200 cm -~. We have made a very rough est...

319KB Sizes 0 Downloads 3 Views

4344

Notes

various divalent metal ions, indicated that the M - N vibrations are to be found in the range 260200 cm -~. We have made a very rough estimate for the M - O force constant using the empirical equation of Cordy[12], in which the electronegativity values of Little and Jones[13] and the ionic radii of Ref. [6] have been used. These calculations gives values of the M - O vibration which is in the range of 450 cm -1. This is in close agreement with the observed 490 cm -1 band, considering the severe approximation that one makes using this equation. Based on the above calculations and the work of Larsson et al. and Clark et al. we feel that the 490 cm -1 is an M - O vibrational band. The fact that the M - O band is expected to be mainly ionic in complexes involving the lanthanide elements, a strong linear relationship between the ionic potential and the vibration position of the band should be expected. Such behaviour have been clearly demonstrated in our work, Fig. 2. The vibration band observed in the range 732 cm -~ shows an almost linear relationship with the ionic potential, Fig. 3. Gordon and MaGee [8] observed this band, moreover, they assumed it an M - O vibration. The i.r. spectra of the oxinate complexes of Cu, Fe and Co of Larsson et al. did not mention this band. The band in the region 1100 cm -1, has been assigned by Charles et al. [5] to the C - O stretching vibration frequency. This absorption band is common in the different metal oxinates. Charles et al. noted that for divalent elements, this band is shifted to lower frequency as the atomic weight increased. In the present work it is also found that the position of the C - O stretching vibration frequency is largely affected with the ionic potential of the cation, Table 1. Finally, the absorption bands in the region of C=C and C=C stretching vibration frequencies are shifted to lower frequencies (5-10 cm -~) in the spectra of the metal complexes than respective bands in the 8-hydroxyquinoline, Table 1. This shift could be explained on the assumption that on chelation, the 7r electron distribution of the quinoline ring slightly disturbed and partially contributed in chelation. H . F . ALY F . M . A B D E L KERIM* A.T. KANDIL

Department o f Nuclear Chemistry Atomic EnergyEstablishment Cairo U.A .R.

*Present address: Spectroscopy Laboratory, National Research Centre, Cairo, U.A.R. 12. W.J. Cordy, J. chem. Phys. 14, 305 (1946). 13. E. Little, Jr. and M. J. Jones, Chem. Edn. 37, 231 (1960).

J. inorg,nucl.Chem., 197I, Vol.33, pp. 4344-4348. PergamonPress. Printedin Great Britain

Studies of iron(Ill) complexes with 4-substituted pyridine N-oxides (Received 4 April 1971 )

SEVERAL Fe(llI) complexes with aromatic amine oxides have been reported and characterized in recent years[I-9]. Ferric perchlorate forms complexes of the type [FeLd(ClO4)3 with pyridine NI. J. V. Quagliano, J. Fujita, G. Franz, D. J. Phillips, J. A. Walmsley and S. Y. Tyree, J. Am. chem. Soc. 83, 3770 (1961). 2. R. L. Carlin, J . A m . chem. Soc. 83, 3773 (1961). 3. K. Issleib and A. Kreibich,Z. anorg, allg. Chem. 313, 338 (1961). 4. S. Kida, J. V. Quagliano, J. A. Walmsley and S. Y. Tyree, Spectrochim. Acta 19, !89 (1963). 5. R. Whyman, W. E. Hatfield and J. S. Paschal, Inorg. Chim. A cta 1, 113 (1967). 6. G. Schmauss and H. Specker, Z. anorg, allg. Chem. 364, 1 (1969). 7. D. X. West, T. J. Delia and T. M. Wilcox, J. inorg, nucl. Chem. 31, 3665 (1969). 8. L. C. Nathan and R. O. Ragsdale, Inorg. Chim. A cta 3,473 (1969). 9. S.A. Cotton and J. F. Gibson, J. chem. Soc. A, 2105 (1970).

Notes

4345

oxide (PNO) and its derivatives[l-3, 5, 6]. Evidence in favor of a ligand-field symmetry lower than O~, for [ML~] n+ (L = PNO and derivatives; M = 3d metal ion) complex cations was recently presented[10]. FeCI~ generally forms complexes of the type FeC13.2L with pyridine N-oxides [3,7, 9]. The PNO complex was formulated as [Fe(PNO)4CI2](FeCI4), on the basis of spectral and conductance data[9]. PNO also forms the neutral complex [Fe(PNOhCI:3], which is probably cis- octahedral [9]. In the course of our studies of metal complexes with 4-substituted pyridine N-oxides [ 11 ] and the effects of the ligand substituents on the properties of these complexes[12], we undertook a study of Fe(lll) complexes with these ligands. The results of this investigation are reported in the present communication. EXPERIMENTAL The ligands utilized were: PNO, 4-picoline N-oxide (PicNO), 4-methoxypyridine N-oxide (MPNO), 4-chloropyridine N-oxide (CPNO) and 4-nitropyridine N-oxide (NPNO). These ligands were obtained commercially and purified by recrystallization or vacuum sublimation. Complexes of the type FeCI3.2L were prepared by the method of West et al.[7]. These new complexes gave satisfactory analyses (Schwarzkopf Microanalytical Laboratory, Woodside, N.Y.) as follows: FeCI:~. 2MPNO: Fe, 13.27; CI, 26.02; C, 35.08; H, 3.42; N, 7.00; FeCI3.2CPNO: Fe, 13.17: CI, 41.96; C, 28.80; H, 2.04; N, 6-60; FeCI 3 . 2NPNO: Fe, 12.47; CI, 23.81; C, 26.93; H, 1.97; N, 12.92. The color of these complexes varies between yellow and orange. [FeLr](CIO4)3 complexes have been reported by Whyman et al., and were prepared according to their procedure [5]. I.R. spectra and magnetic measurements were obtained as previously described [11,12]. RESULTS AND DISCUSSION Pertinent i.r. data and magnetic moments of the Fe(lll) complexes are given in Table 1. The i.r. data suggest coordination of the ligand to the metal ion through the N - O oxygen. Thus, complex formation results in negative VN-oshifts [4, 13]. The two PNO complexes exhibit positive shifts of the ring out-of-plane deformation frequencies. These shifts have been attributed to lowering of the electron density of the pyridine ring[14], upon complex formation[13]. It is noteworthy that the 4-substituted pyridine N-oxides do not show similar YC-Hshifts in their ferric complexes (Table 1) (vide infra). The low frequency i.r. spectra of the FeCI 3 . 2L complexes invariably exhibit a very strong band at 379-373 cm -1 (Table 1). This band has been assigned as the i.r. active v3 mode of the tetrahedral tetrachloroferrate(lll) anion[15] in the spectrum of the complex [Fe(PNO)4CI~](FeCI4)[9]. The i.r. data reported here substantiate this assignment, and the FeCI3.2L complexes are generally formulated as [FeL4CIz](FeCIa). These complexes also show absorptions at 294-279 cm -1, which are tentatively assigned as vFe-c~in the complex cation [FeL4CI~] + (cf. ref. [9]). These assignments are considered as reasonable, in view of the occurrence of v~e-c~at 257-251 cm -j in (FeCIr) 3- [16], and the anticipated positive vFe-c~shifts, upon substitution of chloride ions with neutral ligands in the latter complex[171. The occurence of Vre-c~as a single band in [Fe(PNO)4CI2]*, led Cotton and Gibson to the assignment of a trans- octahedral structure to this complex cation[9]. The 4-substituted pyridine N-oxide analogs also do not show a second vr~_c~ band in the 295-200 cm -~ region, and are most probably transoctahedral (D4h symmetry). Tentative assignments of v~e-o for both the ferric chloride and ferric perchlorate complexes are shown in Table 1. vFe-oassignments in [FeLr](CIO4)3 (L = PNO. PicNO, CPNO) are in satisfactory 10. W. Byers, A. B. P. Lever and R. V. Parish, lnorg. Chem. 7, 1835 (1968). 11. N. M. Karayannis, S. D. Sonsino, C. M. Mikulski, M. J. Strocko, L. L. Pytlewski and M. M. Labes, Inorg. Chim. A cta 4, 141 (1970); and references therein. 12. N. M. Karayannis, M. J. Strocko, C. M. Mikulski, E. E. Bradshaw, L. L. Pytlewski and M. M. Labes, J. inorg, nucl. Chem. 33, 2691 (1971). 13. Y. Kakiuti, S. Kida and J. V. Quagliano, Spectrochim. A cta 19, 201 (1963). 14. R. D. Kross, V. A. Fassel and M. Margoshes,J.Am. chem. Soc. 78, 1332 (1956). 15. D. M. Adams, J. Chatt, J. M. Davidson and J. Gerratt, J. chem. Soc. 2189 (1963); J. S. Avery. C. D. Burbridge and D. M. L. Goodgame, Spectrochim. A cta 24A, 1721 (1968). 16. C. A. Clausen and M. L. Good, lnorg. Chem. 7, 2662 (1968). 17. R . J . H . Clark, Spectrochim. Acta 21,955 (1965).

JINC-Vol. 33 No. 12-L

4346

Notes

0

o

~,~

~

O0 r,n 0¢_ 4

%

O~ 0 o

¢'4

¢'4

¢~

E

¢q

o

"0

,o Z

0

o

¢¢1 ~ e~

e~

'r "

r~e~

r~ r~ n~

0

u.

I

I~

~

o~

o

o e~ o

o ~ 0o 0o

0o

oo

oo

u~

02

7

2

z

8 •

2

~ 0o

~ .

~ .

z

,z

~

7 ~~ .

.

z

z

°~

o CL

0 Z Z

Z

Z

Z

0

0

ZZ

© Z

Z eL

Z

Z

CL

Notes

4347

agreement with data previously reported[5, 9]. However, [Fe(NPNO)6](CIO4h shows a strong band at 446 cm -1 and no band at 424 cm -1. Whyman et al. reported ure-o at 424 cm -1 and a strong shoulder at 448 cm -1 for this complex [5]. In view of the absence of the former band in our product, which was prepared from N PNO doubly recrystallized from an ethanol-ether mixture [18], we assign the band at 446 cm -j as Ure-o- For each of the above ligands, small differences are observed between the frequencies of VVe-Oin the FeCI:~ and Fe(CIO4)3 complexes (Table 1). In the case of MPNO-transition metal perchlorate complexes, bands at 340-280 cm -1 have been quite reasonably identified as uM-o in earlier studies [5, 18]. Thus, in [Fe(MPNO)6](CIO4)3 a band at 311 cm-' was assigned as Ure-o[5]. Nevertheless, Nathan and Ragsdale recently suggested that a band located at 422-410 cm -~ in [M(M PNO)6](CIO4)2(M = Co, Ni, Zn) has all the characteristics of UM-o[8]. The data of the present work are in support of this suggestion. In fact, a strong band at 308 cm-~ is observed in the spectrum of [Fe(MPNO)~](CIO4)3,but no correspondingband (within ± ca. 10 cm --~)occurs in that of [Fe(MPNO)4CI2](FeCI4) (Table I). On the other hand, both these complexes exhibit a strong band at 440-437 cm-k which is tentatively assigned as u~e-o(u~m~-o generally occurs at higher frequencies than u~,r~-~ in compounds of the same type). The Uve-o values given in Table 1 are lowest for PNO and increase as the electron-releasing or electron-withdrawing ability of the substituent increases. Thus, UF~-OVS. trpNO[19] plots produce a V-shaped trend. A similar trend was observed in the case of transition metal perchlorate complexes with 4-substituted quinoline N-oxides[8]. The increase of the u.~t_ofrequencies in these complexes was attributed to domination of the basicity effect in the case of electron-releasing substituents (-CH:~, -OCH:3) and a predominantly 7r-bonding effect for electron-withdrawing substituents (-CI, -NO~) [8]. The V-shaped trend is produced by a combination of these effects[8]. Uncomplexed pyridine N-oxides exhibit various bands at 950-650 cm -~ which were generally attributed to "/c-a (ring out-of-plane deformations), ring skeletal and 8N-O(at 880-840 cm -~) vibrations [13,20]. However, Kawasaki et al. identified 8s-o at 462 cm -~ in PNO, and assigned the 950-650 cm -~ bands in this oxide as Yc-~ and ring skeletal vibrations [21]. For PNO nearly equal contributions from canonical structures !, 11, and I I I have been proposed [22].

O!

O

O

O-

O-

~r

Y

I1

III

In the case of 2- or 4-substituted pyridine N-oxides, electron-releasing substituents enhance the contribution of structural types 111, while electron-withdrawing groups enhance the contribution of structural types 1I [23]. The Yc-Hbands of PNO generally exhibit positive shifts upon complex formation [4, 13], which are also observed in the PNO complexes studied here (Table 1). These shifts have been attributed to lowering of the electron density of the ring, caused by a smaller contribution of structures 1I as a result of the formation of the metal-to-ligand bond[13]. As shown in Table 1, the Fe(lll) complexes of CPNO exhibit smaller 3'C-H positive shifts than those of PNO, whereas these bands in the case of N P N O complexes either show small negative shifts or remain at the same position. These trends are consistent with the anticipated increase of d~r-p ~r back-bonding with increasing electron-withdrawing character of the 4-substituent[18]. Increased 7r-bonding would enhance the contribution of structures I1 and lead to a moderation of the lowering of the ring electron density. 18. D.W. Herlocker, R. S. Drago and V. lmhof Meek, lnorg. Chem. 5, 2009 (1966). 19. J. H. Nelson, R. G. Garvey and R. O. Ragsdale, J. Heterocycl. Chem. 4, 591 (1967); and references therein. 20. H. Shindo, Chem. pharm. Bull. Tokyo 6, 117 (1958). 21. Y. Kawasaki, M. Hori and K. Uenaka, Bull. chem. Soc. Japan 40, 2463 (1967). 22. H. H. Jaffr,J. Am. chem. Soc. 76, 3527 (1954). 23. P. G. Garvey, J. H. Nelson and R. O. Ragsdale, Coord. chem. Rev. 3,375 (1968); and references therein.

4348

Notes

Rather surprisingly, PicNO and MPNO complexes exhibit similar trends in the Tc-M shifts as those observed for the C P N O and N P N O analogs (Table 1). In the cases of these two ligands, the metal-toligand bond is most probably stronger than that in the PNO analog (cf. VFe-Odata). This increase of the metal-to-ligand bond order would be, however, due to the formation of stronger o,-bonds[8], since electron-releasing substituents would increase the energy of the antibonding ~-* ligand orbital, and lead to a decreasing extent of metal-to-ligand ~'-bonding[18]. Nevertheless, the behavior of the Tc-n absorptions upon complex formation might be interpreted as implying that no significant change in the contribution of structural types Ii occurs when the free ligand is coordinated to the 3d metal ion. This would suggest that metal-to-ligand back-donation increases as the electron-releasing character of the 4-substituent increases. In view of the interesting i.r. spectra in the 3'c-n region, additional studies involving 4-substituted pyridine N-oxide complexes with both transition and non-transition metal ions are currently under way, in an attempt to interpret the trends observed in the present work. The bands assigned as 8N-O(PNO 840; M P N O 850; PicNO 855; C P N O 848; N P N O 874 cm -1) [13, 24] generally exhibit small negative shifts upon complex formation. The relative insensitivity of 8N.o during complex formation has been attributed to a possible combination of the effects of coordination on the N - O group and decrease of the N - O double bond character[25]. Finally, VN-Ooccurs generally at lower frequencies in the metal chloride complexes (Table I) than in those of metal perchlorates, as would be expected[4]. This mode is insensitive to substituent effects in the ferric complexes. In fact, its frequency varies between 1203 and 1198 cm -1 for the FeCI3 complexes and between 1210 and 1205 cm -~ for the Fe(CIO4)3 complexes. From literature data, it is apparent that VN-O is sensitive to 4-substituent effects in d o (Ti 4+, Zr~+)[26] and dS-d TM metal ion (Mn '-'+, Fe ~+, Co S÷, Ni ~+, Cu 2+, Zn2÷) [5, 10, 23] complexes, and insensitive in the cases of d~(V 4+, VOZ+)[26, 27], d2(V3+)[12], da(Cra÷)[5] and dS(Fea+)[5] metal ion complexes. A satisfactory explanation for the UN-oinsensitivity in all the above cases had not been advanced thus far. The magnetic moments of the ferric complexes are within---0.1-0.2 B.M. of the spin-only value (5.92 B.M.), while the/zen of the FeCIa complexes are lower than those of Fe(C104)a in general. in conclusion, the results of an investigation of a number of ferric chloride and perchlorate complexes with 4-substituted pyridine N-oxides, studied by means of i.r. spectroscopy, is presented, wherein bands observed in various characteristic regions of the spectrum are discussed in detail.

Department of Chemistry Drexel University Philadelphia, P e nna. 19104 U.S.A.

N. M. K A R A Y A N N I S * J. T. C R O N I N C. M. M I K U L S K I L. L. PYTLEWSKI M. M. L A B E S t

*Present address: Amoco Chemicals Corp., Naperville, Illinois, U.S.A. tPresent address: Department of Chemistry, Temple University, Philadelphia, Pennsylvania, U.S.A. 24. A. R. Katritzky and J. N. Gardner, J. chem. Soc. 2192 (1958). 25. I. S. Ahuja and P. Rastogi,J. inorg, nucl. Chem. 32, 1381 (1970). 26. F. E. Dickson, E. W. Gowling and F. F. Bentley, lnorg. Chem. 6, 1099 (1967); F. E. Dickson, E. W. Baker and F. F. Bentley, J. inorg, nucl. Chem. 31,559 (1969); idem. lnorg. & Nucl. Chem. Lett. 5, 825 (1969). 27. R. G. Garvey and R. O. Ragsdale, J. inorg, nucl. Chem. 29, 745, 1527 (1967); B. E. Bridgland and W. R. McGregor, ibid. 31, 43 (1969).