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Synthesis and characterization of a mononuclear iron(III) complex with a tripodal triamide ligand
Po/yhedron Vol. 12, No. 12, pp. 1553-1557, Printed in Great Britain
1993 0
$6.00 + 80 0277-5X37/93 1993 Pergamon Press Ltd
SYNTHESIS AND CHARACTERIZATION OF A MONONUCLEAR IRON(II1) COMPLEX WITH A TRIPODAL TRIAMIDE LIGAND R. SHUKLA
and P. K. BHARADWAJ*
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India and U. C. JOHRI
Department of Physics, Indian Institute of Technology, Kanpur 208016, India (Received 8 October 1992 ; accepted 26 January 1993) Abstract-A
new potentially heptadentate tripodal ligand has been synthesized by the condensation reaction between tris(2-aminoethyl)amine and acetylsalicyclic acid in a 1: 3 molar ratio. The o-acetyl linkage is cleaved by treating with KOH to form the desired ligand with the donors that include three amide nitrogens, three phenolate oxygens and one bridgehead nitrogen. Fe(acac)3 readily forms a dark red 1: 1 complex with the ligand. The complex behaves as a non-electrolyte in acetonitrile. The electronic absorption spectrum in the visible region is dominated by a strong absorption with &,,, at 425 nm, assignable to an LMCT transition from phenolate oxygen to iron(II1) in an octahedral coordination geometry. The magnetic moment value at room temperature (5.5 &pB) and the EPR spectra in the solid state and in solution (g = 4.06) are consistent with high-spin rhombically distorted octahedral iron(II1). Room-temperature MGssbauer data provide the following values : isomef shift 0.37 mm s- ’ and quadrupole splitting 0.82 mm s- I. These data are consistent with a discrete, high-spin octahedral iron(II1) complex.
Our interest in tripodal ligands stems from the fact that these ligands are capable of forming complexes with metal ions that can exhibit unusual coordination, high thermodynamic stability and kinetic inertness. ‘2’Besides, the tripodal ligands can serve as precursors to the synthesis of macrobicycles.3-5 The present paper describes the synthesis of a new, potentially heptadentate tripodal ligand having an N403 donor set and its complexing ability towards iron(II1). There has been continued interest in iron(III) chemistry with N,O-donor ligands. A large number of iron tyrosinate proteins are known where iron is bound mostly by nitrogen and oxygen atoms of the amino acid residues like histidine, tyrosine, aspartic acid, etc.“” These proteins all contain high-spin iron(III), characterized by a g = 4.3 signal and an intense absorption in the visible region (400-600 nm) with an E,,,~~ value in the range 2000* Author to whom correspondence should be addressed.
4000 M- I cm-‘,
due to a phenolate oxygen-toiron(II1) LMCT transition. Modelling these sites has been an active area of research. ’ I-’ 5The present work was undertaken for the dual purposes of modelling the iron tyrosinate active sites and probing the binding characteristics of the ligand towards a transition metal ion for the possible use of the complex as a precursor in the synthesis of various macrobicyclic compounds. 5 EXPERIMENTAL Materials
Reagent grade tris(2_aminoethyl)amine, ethylchloroformate and acetyIacetone (Merck), salicyclic acid, glacial acetic acid, acetic anhydride and ferric chloride (Glaxo, India) were used as received. All the solvents (Glaxo) were purified16 prior to use.
1553
R. SHUKLA et al.
1554
Physical measurements were made as reported earlier. ” The Miissbauer spectra were recorded with the help of a Wissel Miissbauer spectrometer (Wissel Electronik GmbH, Germany) operating in constant acceleration mode. The spectra were recorded at room temperature (300 K). Elemental analyses were performed at the Central Drug Research Institute, Lucknow, India.
Solid triethylamine hydrochloride was removed by filtration and the almost colourless filtrate evaporated to dryness to obtain a pale yellow, thick, oily liquid. This was taken in CHCl, (25 cm3) and washed four times with water. After drying the CHC13 layer the product was stripped of solvent to obtain the pure ligand (L) in 80% yield. ‘H NMR (80 MHz, CDC13, BTMSppm): 7.5 (m, 12H), aromatic ; 4.1. (m, 9H), 0COCH3 ; 3.0 (t, 12H), CH2.
Synthesis of the ligand L The ligand was synthesized by the mixed anhydride method (Fig. l).” Acetylsalicylic acidI (2 g, 11 mmol) was dissolved in THF (20 cm3) under argon and cooled in an ice-salt bath. Freshly distilled triethylamine (1.12 g, 11 mmol) diluted with THF (20 cm3) was added to the cold solution dropwise with constant stirring. After the addition was complete the solution was stirred for a further period of 20 min. Ethylchloroformate (1.2 g, 11 mmol) mixed with THF (20 cm3) was then slowly added to the cold solution in 15 min. Finally, tris(2aminoethyl)amine (0.54 gi 3.7 mmol) mixed with THF (20 cm3) was added to the well-stirred solution over a period of 45 min. After all the additions were complete the reaction mixture was stirred at 0°C for 2 h and then left at room temperature overnight.
Synthesis of the iron(II1) complex (1) A solution of [Fe(acac),]*’ (1 g, 2.8 mmol) in DMF (10 cm3) was added dropwise with constant stirring at room temperature to a solution of the ligand, L (1.42 g, 2.8 mmol), in DMF (10 cm’). The colour of the solution changed slowly to dark red. The dark red solution was allowed to reflux for l/2 h and filtered after cooling to room temperature. All of the DMF was removed from the filtrate under low pressure and the residue was dissolved in methanol (25 cm3). The red solution thus obtained was filtered and the filtrate was allowed to evaporate slowly at room temperature, affording a dark-red crystalline solid overnight; yield 47%. A small amount of the second crop of the product could be isolated on further evaporation. Found : C, 58.6 ; H, 4.8; N, 10.1; 0, 17.5. Calc. for Cz7Hz7N406Fe: C, 58.0; H, 4.9; N, 10.0; 0, 17.2%. RESULTS AND DISCUSSION
CICOOBt EtjN,tHF
I
NaOH
Fig. 1. Synthetic scheme for the tripodal ligand.
The compound is stable at room temperature in the solid state or in solution in acetonitrile, alcohols, DMF, etc. The complex behaves as a non-electrolyte in these solvents. The IR spectrum in KBr shows a strong band at 1650 cm-‘, which is redshifted by ca 35 m- ’ with respect to the free ligand, indicating that the three amide nitrogens are bonded to the metal ion. The complex shows an intense band at 425 nm (,&,,,3, for which it has a dark red colour. The electronic spectra of a number of model iron(III) phenolate complexes have been studied. ‘*-I5*2’ Hexa-coordinated (distorted octahedral) iron(III) phenolates show an intense LMCT transition in the 420-470 nm region. This transition is assigned as due to the transition from a p-orbital on the phenolate oxygen to the half-filled d-orbital on iron(III).** In the present case the peak at 425 nm is similarly assigned. For di-iron(II1) lactoferrin (Fe,lf) this band appears at 465 nm (Lax,,,).The high molar absorptivity of the transition is due to a good Fe-O overlap. The IR and conductivity data show that the amides are not deprotonated in 1. A protonated amide is a poor donor. In complex 1 the
1555
Synthesis and characterization of Fe”’ complex poor donating abilities of the amide nitrogens are compensated by stronger bonds between iron(II1) and phenolate oxygens-a fact corroborated by room-temperature Miissbauer data. This is reflected in the higher molar absorptivity for the LMCT band involving phenolate oxygens. The ligand field transitions are obscured due to the strong 42.5 nm band. The room-temperature magnetic moment value after diamagnetic corrections is 5.5 ,uJpa, which is within the normal range found’5,2’*23for octahedral high-spin iron(II1) complexes, corresponding to S = 5/2. The X-band EPR spectra of the complex in the solid state as well as in acetonitrile solution (ca 1 x lo- 3 M) were examined at room temperature (300 K) and at liquid nitrogen temperature (77 K). In the solution phase the complex does not exhibit a well resolved spectrum either at room temperature or at 77 K. However, in the solid state it shows a well resolved signal at g = 4.06. The signal at g = 4.06 is due to a transition between the middle Kramers doublet. Iron tyrosinate proteins”’ and model iron(II1) complexes ’ ‘-’ 5 exhibit similar EPR spectra, which are typical of high-spin iron(II1) complexes with rhombically distorted octahedral geometry. Cyclic voltammograms for the complex were recorded at room temperature in acetonitrile (ca 1 x lo- 3 M) at a glassy carbon electrode. When scanned in the positive direction first no welldefined peak could be observed either in the positive or in the negative side. However, when scanned in the negative direction first a broad cathodic peak appears at - 1.02 V (vs S.C.E.) and on scan reversal the corresponding anodic peak appears at -0.5 V (vs S.C.E.). If the scan continues to ca +0.6 V then no peak appears anywhere. This signifies the breakdown of the ligand-metal ensemble on . . oxidation. The couple with E1,2 at -0.76 V is attributable to the following redox process : [FeL]+e-
= [FeL]-
(1)
The ligand-based nature of the couple is ruled out because nickel(I1) and manganese(II1) complexes of L do not show any couple at this El12 value.24 The large separation of the peaks can be attributed
I
-2
I
I
I
I
-1
0
1
2
Velocity
(mm
ST
Fig. 2. Room-temperature Mossbauer spectrum for 1.
to extensive rearrangement of the product accompanying the redox process. The Miissbauer data for 1 and related systems are collected in Table 1. The complex shows (Fig. 2) an isomer shift of 0.37 mm s- ’ with respect to iron metal, suggesting more covalency in the bonding between iron(II1) and the ligands. This value lies close to that of lactoferrin25 (0.39 mm s- ‘) and transferrin26 (0.38 mm s- ‘), besides other octahedral high-spin iron(II1) complexes. For octahedral complexes where the iron(III)-ligand bonding is more ionic an isomer shift of ca 0.5 mm s- ’ with respect to the iron metal is observed. ” Complex 1 shows quite a large value for the quadrupole splitting reflecting an unsymmetrical field around the ferric ion. Similar values are obtained in the case of structurally characterized, rhombically-distorted octahedral iron(II1) complexes, as well as in cases of some iron tyrosinate proteins (Table 2). CONCLUSION A new tripodal ligand has been synthesized, which forms a rhombically-distorted octahedral complex with iron(II1). The Mijssbauer data for this complex closely resembles those of the iron tyrosinate proteins, lactoferrin and transferrin. The EPR and electronic spectral results are also very
Table 1. IR (selective) and electronic absorption spectral data Compound L 1
Selected IR bands (cm- ‘) 3300sbr;176Osbr;1685sbr 3310s br; 1630sbr
J
Electronic absorption bands A,,,,,(nm) (haX,dm3 mol- ’ cm- ‘) 425(2620); 325(sh, 5400);300( 10,900)
1556
R. SHU.KLA et al. Table 2. Mossbauer spectral data for 1 and related systems
Compound 1 FeLf FeTf HC03Free FeTf Fe(L,), *6H,O Fe(L,),* l.5H20 Fe(EHGS)
Isomer shift (Fe, mm s- ‘)
Quadrupole splitting (Es, mm s- ‘)
0.37 0.39 0.38 0.47
0.82 0.75 0.72
This work 25 26 27
0.41 0.38 0.54
0.92 0.72 1.60
28 12 lla
Ref.
Lf = lactofernin. Tf = transferqin. L, = nicotinylhydroxamic acid. L2 = 2-(5-methylpyrazol-3-yl)phenol. EHGS = N-[2-((o-hydroxyphenyl)glycino)ethyl]salicylidenimine.
similar. These results suggest that effective electronic structural analogues of the iron tyrosinate proteins can be synthesiqd from ligands incorporating a number of phenolate groups in addition to nitrogen donors. Acknowledgement-Financial jhelp from the Department of Science and Technology, New Delhi, India, is gratefully acknowledged. REFERENCES 1. R. M. Kirchner, C. Meal& M. Bailey, M. Howe, L.
P. Torre, L. J. Wilson, L. C. Andrews, N. J. Rose and R. C. Lingafelter, Caord. Chem. Rev. 1987, 77, 89. 2. K. Ragunathan and P. K. Bharadwaj, J. Chem. Sot., Dalton Trans. 1992, 2417; C. Orvig, D. J. Berg and S. J. Rettig, J. Am. Chem. Sot. 1991, 113, 2528; S. Liu, L. Gelmini, S. J. Rettig, R. C. Thompson and C. Orvig, J. Am. Chem. S c. 1992,114,6081. P 3. K. Ragunathan and P. K.,Bharadwaj, J. Chem. Sot., Dalton Trans. 1992, 1654; J. Hunter, J. Nelson, C. Harding, M. McCann and V. McKee, J. Chem. Sot., Chem. Commun. 1990, 1148. 4. J. de Mendoza, E. Mezal I. C. Rodriguez-Ubis, P. Vasquez, F. Vogtle, P. M. Windscheif, K. Rissanen, J. M. Lehn, D. Lilienbaim and R. Ziessel, Angew. Chem., Znt. Edn Engl. 1991,30,1331; T. J. McMurry, S. J. Rodgers and K. N. Raymond, J. Am. Chem. Sot. 1987,109,3451. K. Ragunathan and P. K. Bharadwaj, Tetrahedron Lett. 1992, 7581. L. Que Jr, Coord. Chem., Req. 1983, 50, 73 ; L. Que Jr, J. Chem. Ed. 1985,621 938. B. F. Anderson, H. M. Baker, E. J. Dodson, G. E. Norris, S. V. Ramball, J; M. Watersand and E. N. Baker, Proc. Natl. Acad. Sci. (USA) 1987,&i, 1769.
8. S. Bailey, R. W. Evans, R. C. Garratt, B. Gorinsky, S. Hasnain, C. Horsburg, H. Jhoti, P. F. Lindley, A. Mydin, R. Sarra and J. L. Watson, Biochemistry 1988,27,5804. 9. J. W. Whittaker, J. D. Lipscomb, T. A. Kent and E. Munck, J. Biol. Chem. 1984,259,4466. 10. D. H. Ohlendorf, J. D. Lipscomb and P. C. Weber, Nature (London) 1988, 336, 403 ; F. C. Beadley, S. Lindstedt, J. D. Lipscomb, L. Que Jr, A. L. Roe and M. Rundgren, Biochemistry 1986,261, 11693. 11. C. J. Carrano, K. Spartalian, G. V. N. A. Rao, V. L. Pecoraro and M. Sundaralingam, J. Am. Chem. Sot. 1985,107,1651; C. J. Carrano, M. W. Carrano, K. Sharma, G. Backes and J. S. Loehr, Znorg. Chem. 1990,29, 1865. 12. E. W. Ainscough, A. M. Brodie, J. E. Plowman, K. L. Brown, A. W. Addison and A. R. Gainsford, Inorg. Chem. 1980, 19, 3655. 13. L. Casella, M. Gullotti, A. Pintar, L. Messori, A. Rochenbauer and M. Gyor, Znorg. Chem. 1987,26, 1031. 14. J. W. Pyrz, A. L. Roe, L. J. Stern and L. Que Jr, J. Am. Chem. Sot. 1985,107,641. 15. M. R. McDevitt, A. W. Addison, E. Sinn and L. K. Thompson, Inorg. Chem. 1990,29,3425. 16. D. D. Perrin, W. L. F. Armarego and D. R. Pert-in, Purification of Laboratory Chemicals, 1st edn. Pergamon Press, Oxford (1966). 17. S. Mandal and P. K. Bharadwaj, Polyhedron 1992, 11, 1037. 18. S. Mandal and P. K. Bharadwaj, Polyhedron 1993, 12, 543. 19. B. R. Furniss, A. J. Hannaford, V. Rogers, P. W. G. Smith and A. R. Tatchell, Vogel’s Textbook of Practical Organic Chemistry, 4th edn, pp. 832. Longman, U.K. 20. R. B. Roof Jr, Acta Cry&. 1956,9, 781. 21. K. Ramesh and R. N. Mukherjee, J. Chem. Sot., Dalton Trans. 1992, 83.
Synthesis and characterization 22. B. P. Gaber, V. Miskowski and T. G. Spiro, J. Am. Chem. Sot. 1974,96,6868. 23. D. F. Cook, D. Cummins and E. D. McKenzie, J. Chem. Sot. Dalton Trans. 1976,1369; R. C. Burrows and J. C. Bailer Jr, J. Am. Chem. Sot. 1966, 88, 4150 ; J. C. Davis, W. J. Kung and B. A. Averill, Znorg. Chem. 1986, 25, 394; R. W. Oehmbe and J. C. Bailar Jr, J. Znorg. Nucl. Chem. 196.5,27,2209. 24. R. Shukla and P. K. Bharadwaj, unpublished results.
of Fe”’ complex
25. J. Laidriere, R. Coussement and B. J. Phys. (Paris) Colloq. 1974, 351. 26. K. Spartalian and W. T. Oosterhuis, J. 1973,59, 617. 27. C. P. Tsang, A. J. F. Boyle and E. Biochem. Biophys. Acta 1975,386, 32. 28. L. M. Epstein and D. K. Straub, Inorg. 8,453.
1557 Theuwissen, Chem. Phys. H. Morgan, Chem. 1969,
1993 0
$6.00 + 80 0277-5X37/93 1993 Pergamon Press Ltd
SYNTHESIS AND CHARACTERIZATION OF A MONONUCLEAR IRON(II1) COMPLEX WITH A TRIPODAL TRIAMIDE LIGAND R. SHUKLA
and P. K. BHARADWAJ*
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India and U. C. JOHRI
Department of Physics, Indian Institute of Technology, Kanpur 208016, India (Received 8 October 1992 ; accepted 26 January 1993) Abstract-A
new potentially heptadentate tripodal ligand has been synthesized by the condensation reaction between tris(2-aminoethyl)amine and acetylsalicyclic acid in a 1: 3 molar ratio. The o-acetyl linkage is cleaved by treating with KOH to form the desired ligand with the donors that include three amide nitrogens, three phenolate oxygens and one bridgehead nitrogen. Fe(acac)3 readily forms a dark red 1: 1 complex with the ligand. The complex behaves as a non-electrolyte in acetonitrile. The electronic absorption spectrum in the visible region is dominated by a strong absorption with &,,, at 425 nm, assignable to an LMCT transition from phenolate oxygen to iron(II1) in an octahedral coordination geometry. The magnetic moment value at room temperature (5.5 &pB) and the EPR spectra in the solid state and in solution (g = 4.06) are consistent with high-spin rhombically distorted octahedral iron(II1). Room-temperature MGssbauer data provide the following values : isomef shift 0.37 mm s- ’ and quadrupole splitting 0.82 mm s- I. These data are consistent with a discrete, high-spin octahedral iron(II1) complex.
Our interest in tripodal ligands stems from the fact that these ligands are capable of forming complexes with metal ions that can exhibit unusual coordination, high thermodynamic stability and kinetic inertness. ‘2’Besides, the tripodal ligands can serve as precursors to the synthesis of macrobicycles.3-5 The present paper describes the synthesis of a new, potentially heptadentate tripodal ligand having an N403 donor set and its complexing ability towards iron(II1). There has been continued interest in iron(III) chemistry with N,O-donor ligands. A large number of iron tyrosinate proteins are known where iron is bound mostly by nitrogen and oxygen atoms of the amino acid residues like histidine, tyrosine, aspartic acid, etc.“” These proteins all contain high-spin iron(III), characterized by a g = 4.3 signal and an intense absorption in the visible region (400-600 nm) with an E,,,~~ value in the range 2000* Author to whom correspondence should be addressed.
4000 M- I cm-‘,
due to a phenolate oxygen-toiron(II1) LMCT transition. Modelling these sites has been an active area of research. ’ I-’ 5The present work was undertaken for the dual purposes of modelling the iron tyrosinate active sites and probing the binding characteristics of the ligand towards a transition metal ion for the possible use of the complex as a precursor in the synthesis of various macrobicyclic compounds. 5 EXPERIMENTAL Materials
Reagent grade tris(2_aminoethyl)amine, ethylchloroformate and acetyIacetone (Merck), salicyclic acid, glacial acetic acid, acetic anhydride and ferric chloride (Glaxo, India) were used as received. All the solvents (Glaxo) were purified16 prior to use.
1553
R. SHUKLA et al.
1554
Physical measurements were made as reported earlier. ” The Miissbauer spectra were recorded with the help of a Wissel Miissbauer spectrometer (Wissel Electronik GmbH, Germany) operating in constant acceleration mode. The spectra were recorded at room temperature (300 K). Elemental analyses were performed at the Central Drug Research Institute, Lucknow, India.
Solid triethylamine hydrochloride was removed by filtration and the almost colourless filtrate evaporated to dryness to obtain a pale yellow, thick, oily liquid. This was taken in CHCl, (25 cm3) and washed four times with water. After drying the CHC13 layer the product was stripped of solvent to obtain the pure ligand (L) in 80% yield. ‘H NMR (80 MHz, CDC13, BTMSppm): 7.5 (m, 12H), aromatic ; 4.1. (m, 9H), 0COCH3 ; 3.0 (t, 12H), CH2.
Synthesis of the ligand L The ligand was synthesized by the mixed anhydride method (Fig. l).” Acetylsalicylic acidI (2 g, 11 mmol) was dissolved in THF (20 cm3) under argon and cooled in an ice-salt bath. Freshly distilled triethylamine (1.12 g, 11 mmol) diluted with THF (20 cm3) was added to the cold solution dropwise with constant stirring. After the addition was complete the solution was stirred for a further period of 20 min. Ethylchloroformate (1.2 g, 11 mmol) mixed with THF (20 cm3) was then slowly added to the cold solution in 15 min. Finally, tris(2aminoethyl)amine (0.54 gi 3.7 mmol) mixed with THF (20 cm3) was added to the well-stirred solution over a period of 45 min. After all the additions were complete the reaction mixture was stirred at 0°C for 2 h and then left at room temperature overnight.
Synthesis of the iron(II1) complex (1) A solution of [Fe(acac),]*’ (1 g, 2.8 mmol) in DMF (10 cm3) was added dropwise with constant stirring at room temperature to a solution of the ligand, L (1.42 g, 2.8 mmol), in DMF (10 cm’). The colour of the solution changed slowly to dark red. The dark red solution was allowed to reflux for l/2 h and filtered after cooling to room temperature. All of the DMF was removed from the filtrate under low pressure and the residue was dissolved in methanol (25 cm3). The red solution thus obtained was filtered and the filtrate was allowed to evaporate slowly at room temperature, affording a dark-red crystalline solid overnight; yield 47%. A small amount of the second crop of the product could be isolated on further evaporation. Found : C, 58.6 ; H, 4.8; N, 10.1; 0, 17.5. Calc. for Cz7Hz7N406Fe: C, 58.0; H, 4.9; N, 10.0; 0, 17.2%. RESULTS AND DISCUSSION
CICOOBt EtjN,tHF
I
NaOH
Fig. 1. Synthetic scheme for the tripodal ligand.
The compound is stable at room temperature in the solid state or in solution in acetonitrile, alcohols, DMF, etc. The complex behaves as a non-electrolyte in these solvents. The IR spectrum in KBr shows a strong band at 1650 cm-‘, which is redshifted by ca 35 m- ’ with respect to the free ligand, indicating that the three amide nitrogens are bonded to the metal ion. The complex shows an intense band at 425 nm (,&,,,3, for which it has a dark red colour. The electronic spectra of a number of model iron(III) phenolate complexes have been studied. ‘*-I5*2’ Hexa-coordinated (distorted octahedral) iron(III) phenolates show an intense LMCT transition in the 420-470 nm region. This transition is assigned as due to the transition from a p-orbital on the phenolate oxygen to the half-filled d-orbital on iron(III).** In the present case the peak at 425 nm is similarly assigned. For di-iron(II1) lactoferrin (Fe,lf) this band appears at 465 nm (Lax,,,).The high molar absorptivity of the transition is due to a good Fe-O overlap. The IR and conductivity data show that the amides are not deprotonated in 1. A protonated amide is a poor donor. In complex 1 the
1555
Synthesis and characterization of Fe”’ complex poor donating abilities of the amide nitrogens are compensated by stronger bonds between iron(II1) and phenolate oxygens-a fact corroborated by room-temperature Miissbauer data. This is reflected in the higher molar absorptivity for the LMCT band involving phenolate oxygens. The ligand field transitions are obscured due to the strong 42.5 nm band. The room-temperature magnetic moment value after diamagnetic corrections is 5.5 ,uJpa, which is within the normal range found’5,2’*23for octahedral high-spin iron(II1) complexes, corresponding to S = 5/2. The X-band EPR spectra of the complex in the solid state as well as in acetonitrile solution (ca 1 x lo- 3 M) were examined at room temperature (300 K) and at liquid nitrogen temperature (77 K). In the solution phase the complex does not exhibit a well resolved spectrum either at room temperature or at 77 K. However, in the solid state it shows a well resolved signal at g = 4.06. The signal at g = 4.06 is due to a transition between the middle Kramers doublet. Iron tyrosinate proteins”’ and model iron(II1) complexes ’ ‘-’ 5 exhibit similar EPR spectra, which are typical of high-spin iron(II1) complexes with rhombically distorted octahedral geometry. Cyclic voltammograms for the complex were recorded at room temperature in acetonitrile (ca 1 x lo- 3 M) at a glassy carbon electrode. When scanned in the positive direction first no welldefined peak could be observed either in the positive or in the negative side. However, when scanned in the negative direction first a broad cathodic peak appears at - 1.02 V (vs S.C.E.) and on scan reversal the corresponding anodic peak appears at -0.5 V (vs S.C.E.). If the scan continues to ca +0.6 V then no peak appears anywhere. This signifies the breakdown of the ligand-metal ensemble on . . oxidation. The couple with E1,2 at -0.76 V is attributable to the following redox process : [FeL]+e-
= [FeL]-
(1)
The ligand-based nature of the couple is ruled out because nickel(I1) and manganese(II1) complexes of L do not show any couple at this El12 value.24 The large separation of the peaks can be attributed
I
-2
I
I
I
I
-1
0
1
2
Velocity
(mm
ST
Fig. 2. Room-temperature Mossbauer spectrum for 1.
to extensive rearrangement of the product accompanying the redox process. The Miissbauer data for 1 and related systems are collected in Table 1. The complex shows (Fig. 2) an isomer shift of 0.37 mm s- ’ with respect to iron metal, suggesting more covalency in the bonding between iron(II1) and the ligands. This value lies close to that of lactoferrin25 (0.39 mm s- ‘) and transferrin26 (0.38 mm s- ‘), besides other octahedral high-spin iron(II1) complexes. For octahedral complexes where the iron(III)-ligand bonding is more ionic an isomer shift of ca 0.5 mm s- ’ with respect to the iron metal is observed. ” Complex 1 shows quite a large value for the quadrupole splitting reflecting an unsymmetrical field around the ferric ion. Similar values are obtained in the case of structurally characterized, rhombically-distorted octahedral iron(II1) complexes, as well as in cases of some iron tyrosinate proteins (Table 2). CONCLUSION A new tripodal ligand has been synthesized, which forms a rhombically-distorted octahedral complex with iron(II1). The Mijssbauer data for this complex closely resembles those of the iron tyrosinate proteins, lactoferrin and transferrin. The EPR and electronic spectral results are also very
Table 1. IR (selective) and electronic absorption spectral data Compound L 1
Selected IR bands (cm- ‘) 3300sbr;176Osbr;1685sbr 3310s br; 1630sbr
J
Electronic absorption bands A,,,,,(nm) (haX,dm3 mol- ’ cm- ‘) 425(2620); 325(sh, 5400);300( 10,900)
1556
R. SHU.KLA et al. Table 2. Mossbauer spectral data for 1 and related systems
Compound 1 FeLf FeTf HC03Free FeTf Fe(L,), *6H,O Fe(L,),* l.5H20 Fe(EHGS)
Isomer shift (Fe, mm s- ‘)
Quadrupole splitting (Es, mm s- ‘)
0.37 0.39 0.38 0.47
0.82 0.75 0.72
This work 25 26 27
0.41 0.38 0.54
0.92 0.72 1.60
28 12 lla
Ref.
Lf = lactofernin. Tf = transferqin. L, = nicotinylhydroxamic acid. L2 = 2-(5-methylpyrazol-3-yl)phenol. EHGS = N-[2-((o-hydroxyphenyl)glycino)ethyl]salicylidenimine.
similar. These results suggest that effective electronic structural analogues of the iron tyrosinate proteins can be synthesiqd from ligands incorporating a number of phenolate groups in addition to nitrogen donors. Acknowledgement-Financial jhelp from the Department of Science and Technology, New Delhi, India, is gratefully acknowledged. REFERENCES 1. R. M. Kirchner, C. Meal& M. Bailey, M. Howe, L.
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