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X-ray, spectral and magnetic studies of copper(II) chelates of l -asparagine and l -glutamine
Z inorg, nucl. Chem., 1976, Vol. 38, pp. 189%1900. Pergamon Press. Printed in Great Britain
X-RAY, SPECTRAL AND MAGNETIC STUDIES OF COPPER(II) CHELATES OF L-ASPARAGINE AND L-GLUTAMINE M. N. SRIVASTAVA,t R. C. TEWARI and U. C. SRIVASTAVA Chemical Laboratories, University of Allahabad, Allahabad-211002, India and G. B. BHARGAVA and A. N. V1SHNOI Department of Physics, University of Allahabad, Allahabad-211002, India (First received 23 September 1975; in revised form 15 January 1976)
Akltraet--Copper(II) chelates of L-asparagine and L-glutamine have been studied using reflectance and UV absorption spectra, magnetic and ESR measurements and X-ray absorption edge spectrometry (XAES). Based on these measurements, a distorted octahedral structure has been assigned to these complexes. The relevant ligaud field parameters are reported. Average metaMigand bond-lengths have also been estimated using X-ray absorption fine structure.
INTRODUCTION THE STUDY o f the molecular structure of copper complexes of amino acids and their derivatives has been of interest in the field of bioinorganic chemistry [1, 2]. In particular, the identification and determination of the nature of the binding sites of Cu(II) has been the dominant theme of many current investigations [3, 4]. The chemical specificity of Cu(II) ions enables them to bind to several positions on these ligands although frequently one position is preferred from stereochemical considerations[5,6]. For example, complexes with racemic amino acids adopt a trans structure whereas those synthesized with optically active amino acids are cis isomers[2]. It has been suggested that stereoselective crystal packing effects stabilize a particular configuration [7]. Copper(II) chelates formed with L-asparagine and L-glutamine offer an interesting system for study for a number of reasons. First, their solution properties have been studied by Albert [8], Ritsma et al. [9] and Tewari and Srivastava[10] but no literature is available on their properties in the solid state; and second, these ligands play an important role in biological chemistry [ l l, 12]. In the present work, therefore, an attempt has been made to study these complexes using reflectance and UV absorption spectra, magnetic and ESR measurements and X-ray absorption spectrometry (XAES). Their preparation and characterization (from analytical and IR data) have been reported in an earlier paper[13]. The complexes are: bis(L-asparagino) copper(IlL [Cu(OOC.CH'NH2.CH2.CO. NH2)2]; and bis-(L-glutamino) copper(II), [Cu(OOC. C H-NH2CH2. C H_~.CO.N H.+.].
Table 1. Magnetic susceptibility measurements on Cu(II) chelates
Compound Cn(Aspn.)2 Cu(Glun.)2
~(~ x 10"
XMXIO 6
T(°K)
(c.g.s. units)
(c.g.s. units)
X~x 10" (c.g.s. /zofr units) (B.M.)
304 304
4.09 3.83
1,331 1,354
1,466.5 1.898 1,513.0 1.922
Spectral measurements. U.V. absorption spectra were recorded on aqueous solutions on a Cary Model-14 recording Spectrophotometer using 1 cm quartz cells. The observed bands are listed in Table 2. But the solubility of these chelates in water is rather too low, to yield good peaks in the visible region. Hence reflectance spectra of the solid complexes were recorded on "C-10" recording spectrophotometer (Russian) in the range 400-700 nm. Results are included in Table 2.
Table 2. Electronic spectra of Cu(lI) chelates Band (cm ')
Compound
Compound
16,000 42,330 (6,250) Cu(Glun.)2 50,760 (5,650) 52,230 (4,400)
Cu(Aspn.)2
Band (cm ') 16,400 42,199(5,000) 51,280 (4,800) 52.230 (4,300)
UV molar extinction coefficient values are given in brackets. Electron spin resonance. Spectra of solid samples (Fig. 1 and Fig. 2) were taken on a Bruker ModeI-B-ER 402 X-band, ESR-spectrophotometer and polycrystalline DPPH was used as a reference. The "g" factors are given in Table 3.
EXPERIMENTAL Magnetic measurements at room temperature (3I°C) were made on powder samples of the complexes employing Gouy's method. A semi-micro Sartorius balance was employed and mercury(I1) tetrathiocyanatocobaltate(II) (X~ = 16.44 × 10-4 c.g.s, units) was used as calibrant. Both complexes were found to be paramagnetic. The data are presented in Table 1.
Table 3. "g" Values of the Cu(II)-chelates
Compound Cu(Asph.)2 Cu(Glun.)~
tPostal address: Dr. M. N. Srivastava, 266 (near distillery), Mumfordganj, Allahabad-2! 1002, India. 1897
g,
g,
g,,~.
2.0503 2.2534 2.12 2.046 2.212 2.10
(A2= E3 - Eo, i.e. E(E,) - E(B,g)).
x(cm ') (Spin orbit coupling constant)
A2 (cm ')
-510 -425
20,000 18,500
M . N . SRIVASTAVAet al.
1898
X-Ray K-absorption spectra were recorded photographically employing a 400 mm Cauchois-type covered-crystal spectrograph using (100) reflection planes of mica as analyser. The radiation source was a sealed X-ray tube with tungsten target run at 20 KV-10 mA. The mica crystal was good enough to resolve the Mo/3~.3 doublet (A)t = 0,572 x. u.) in the first order and gave a dispersion of about 12 x u. per ram. On the spectrogram Wa,, WL~81 and WL#, were used as reference lines. The intensity records were obtained with the help of a Kipp and Zonen Moll microphotometer at magnification 8x and 25x. The results of measurements (made on a Hilger comparator) of the edge-shift (A) and the energies of the principal absorption maxima (EA) are presented in Table 4. Table 5 illustrates the relationship[14] between measured edge with (EW) and the coordination stoichiometry (metal-nearest neighbour ratio) of the complexes. The metal-ligand bond-lengths estimated from fine structure energy separations (B-~)[15] are given in Table 6. Figure 3 shows the trace of the microphotometer record with a magnification x25. The position of the various absorption maxima and minima have also been shown in the figure. The position of the inflection point of the absorption edge in pure metal has been taken as zero ernergy.
t.) V O.. tt3
I
\
Fig. 1. ESR spectra of copper(II) L-asparagine chelate (in solid state).
DISCUSSION The ground state of a cubic octahedrally coordinated Cu(II) is 2Eg and the only excited state is 2T2~, the energy difference being 10Dq [16]. But tetragonal distortion splits the E, and T2~ levels so that as many as three d-d transitions are possible[17].
dx2-r2">d~=
d~2-y~-~ d~r ; t.)
dx2 y2~(d,,,d~).
and
Cu (Aspn)2 A °
Cu (Glun)~ A
o
Io
J
I
I
I
F
20
30
40
50
60
Energy, eV
Fig. 2. ESR spectra of copper(lI) L-glutamine chelate (in solid state).
Fig. 3.
Table 4. Data on K-absorption edge of Cu(II) chelates Absorber Copper metal Cu(Aspn)2 Cu(Glun)z
AK (X.U.)
(X.U.)
1377.99
--
1375.53 1375.59
hA
A
(F/R)K 661.30
1375.28 1374.35
662.48 662.45
(P/R)A A(F/R)K A(p/R)A (eV) .
.
663.09 663.09
.
.
1.18 1.15
. 1.79 1.75
16.0 15.6
Table 5. K-Edge-widths and the coordination stoichiometry of the Cu(II) chelates
Compound Cu(Aspn)2 Cu(Glun.h
Ah (K - A) (X.U.) 1.25 1.24
Coordination EW stoichiometry A(v/R)K-a (eV) (Cu:N:O) 0.61 0.60
8.3 8.1
1 : 2: 4 1 : 2: 4
EA (eV)
X(X~-Xt,)
[(EWXAx)'/2]
8.6 8.6
8.4 8.3
24.3 23.7
I
1899
Magneticstudiesof copper(lI)
/
Table 6. Averagemetal-ligandbond-lengthsof the Cu(ll) chelates Compound
AE(B -/3)
i x,151,:2o r=t--A~ ) A
Cu(Aspn.)2 Cu(Glun.)2
37.1 38.4
2.01 1.97
'
/NH~
/c:o\
\
/ ¢=0\ /OH, .\\
\
I
CH2 I H&--N \0 ~
Thus the observed band envelope may not be due to a single transition but may consist of two or three symmetrical absorption bands. The expressions[18] derived for "g" values in a tetragonally distorted copper(II) molecule, which is stretched along the symmetry axis (z-axis) are g, = 2 - 8A/(E2 - Eo) and g~ = 2 - 2AI(E3 - Eo) where, E0, E_, and E~ are the energies corresponding to :A~, 2B2~ and :E~ levels. Thus E 2 - E0 is equivalent to 10 Dq and the energies corresponding to the observed .~max (1090 16,000 cm ~) may be taken as E 2 - E0. On this basis, from the observed g, values (Table 3) )t (spin orbit coupling constants) values were calculated, which from the above equation, gave E3 - E0, i.e. A2. It is of interest to note that the observed band envelopes in the reflectance spectra of copper(II) chelates engulf these transitions. The bands observed in the UV spectra of the copper chelates can be attributed to charge transfer transitions. Earlier, in the case of bis-(phenylglycinato)copper(II), Hare [19] ascribed the band observed at 39,000 cm-* to an apparent charge transfer from the carboxylate group to the metal and that observed at 52,000 cm ' to a charge transfer from amino group to metal with a possible contribution from the ~r-~r* transition of the COO group. Thus in the present case the strong broad bands observed at 42,330 cm ' in Cu(Aspn.)2 and at 42,190 cm-' in Cu(Glun.)2 correspond to a charge transfer from carboxylate group to the metal, whereas the bands observed at 50,760 cm ' and 51,280 cm ~ respectively in the two chelates may be ascribed to charge transfer transitions from the amino group to the metal. Other weaker bands at -52,000cm -1 may correspond to carboxyl 7r ~ 7r* transitions. The similarity in X-ray absorption edge data (Tables 4 and 5) and the structure of the absorption edge (Fig. 3) appear to suggest that the two chelates are essentially similar, particularly in regard to stereochemistry and coordination sites. Thus the chemical shifts and the widths of the K-absorption edges in the two chelates are nearly the same. Earlier, band-widths of the same order ( - 8 eV) were also observed on the spectra of several copper complexes involving nitrogen-oxygen ligands [20]. Further, the measured edge width gives, through the relationship[14] [~(X~ - X,.) Edge width] '/2 = constant (8.5 for copper system) (where XM and XL are respectively the electronegativitiesof the metal and donor atoms) a coordination stoichiometry-Cu: N: O : : 1 : 2: 4. It thus indicates that coordination occurs from carboxylic oxygen, amino nitrogen and carbonyl oxygen of the primary amide group of the two ligands (Fig. 4). The edge structures (Fig. 3) are visibly characterized by the presence of a small maximum at ~ 10 eV, lower energy with respect to the main edge, a splitting of the principal absorption maximum and a small hump (a') to its high energy side. The intensity of the low energy absorption suggests that it probably corresponds to a nominally forbidden transition and it may be appropriate to designate it as ls o3d,~, which would be in agreement with the hybridization (sp3d ') envisaged. The width of the
\
. . . . . .
to='. ......
\
/2
\o
....
\'\ I ~/cu2+
/2
L - Glutamine chelate
L - Asparagine chelate
Fig. 4. absorption jump of the lower energy side of the main edge (-10eV) when compared with that present on its high energy side (upto A) ( - 8 eV) suggests that the extra width of - 2 eV in the 3d- region results from 3d- splitting in the complexes. It may be interesting to note (Table 3) that the values obtained for the chelates from optical data are also 20,000 cm 1 and 18,500 cm-1, i.e. 2.4 eV and 2.2 eV respectively and are thus in close agreement with the X-ray measurements. The principal absorption maxima, appearing at -24 eV may as usual, be assigned a transition ls ~4p* (antibonding) in accordance with the bonding configuration (sp3d2). Earlier, in the case of Cu(II)-DL-prolinate dihydrate having a similar coordination stoichiometry, Cotton and Hansen[20] have observed the main peak at -20 eV. The increased energy of the 4p* (antibonding) orbitals in the chelates under study may be taken to signify an enhanced strength[21] of the metal-ligand bond in them as compared to that in CU(II)-DL prolinatedihydrate. It may be noted that the axially coordinated oxygen atoms are from water molecules in the latter complex and from amido groups in the former. Further, a comparison of the energies of the main peaks of the asparagine and glutamine chelates (Table 4) seems to agree with the fact that stronger bonds are formed with asparagine than glutamine, as is also suggested by their stability constants data[8-10]. The observed splitting of the 1s ~ 4p* peak into two (rather broad) components presumably indicates a ligand field symmetry lower than Oh for these complexes [20, 22]. A similar structure has earlier been observed by Cotton and Hansen[20] in the case of CU(II)-DL prolinate dihydrate, which has been shown to be in reasonable agreement with its D2h symmetry. A similar symmetry for the chelates under study may, therefore, be suggested particularly in view of the probable mode of tridentate coordination involved in them. The small maximum subsequent to the main peak, particularly observed in copper complexes involving ligands having 7r bonding ability, have usually been attributed[23] to transitions of the ls electron to the antibonding ~- levels of the complex. This may be important since broad and intense bands attributable to charge transfer have been observed in the UV spectra of the two complexes as discussed above. The average metal-ligand bond-lengths (Table 6) estimated from /3-/3 separations (Fig. 4) in the fine structure are found to be comparable with those obtained from X-ray diffraction analysis of bis-glycinato)copper(II) monohydrate[24] having a similar distorted octahedral structure.
Acknowledgement--Authors (M.N.S. and R.C.T.) are grateful to State C.S.I.R., U.P. Lucknow for financialassistance in the form of a scheme.
M.N. SRIVASTAVAet al.
1900 REFERENCES
1. H. C. Freeman, The Biochemistry o[ Copper (Edited by J. Peisach, P. Aisen and W. E. Blumberg), p. 77. Academic Press, New York (1966). 2. A. H. Herlinger and T. V. Long, J. Am. Chem. Soc. 92, 6481 (1970). 3. A.W. Herlinger, S. L. Wenhold and T. V. Long, 3. Am. Chem. Soc. 92, 6474 (1970). 4. R. A. Condrate and K. Nakamoto, J. Chem. Phys. 42, 2590 (1965). 5. G. L. Eichhorn, P. Clark and E. D. Becker, Biochem. 5, 245 (1966). 6. N. A. Berger and G. L. Eichhorn, Biochem. 10, 1847, 1857 (1971). 7. R. D. Gillard, R. Mason, N. C. Payne and G. B. Robertson, J. Chem. Soc. 1864 (1969). 8. A. Albert, Biochem. J. 47, 531 (1950); 50, 690 (1962). 9. J. H. Ritsma, W. A. Wiegers and F. Jellinek, Rec. Trao. Chim. 84, 1577 (1965). 10. R. C. Tewari and M. N. Srivastava, J. Inorg. NucL Chem. 35, 2441 (1973). 11. V. V. Mamalva, Biokhimia, 20, 450 (1955). 12. D. R. Curtis and K. Koizumi, J. Neurophysiol. 24, 80 (1961).
13. R. C. Tewari and M. N. Srivastava, Ind. J. Chem. 12, 527 (1974). 14. H. L. Nigam and U. C. Srivastava, J. Chem. Soc. D. (Chem. Commun.) 14, 761, 0971). 15. R. M. Levy, J. Chem. Phys. 43, 1846 (1965). 16. C. J. Ballhausen, Introduction to Ligand Field Theory. McGraw-Hill, New York (1962). 17. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 902. 2nd Edn. Interscience, New York (1966). 18. R. S. Drago, Physical Methods in Inorganic Chemistry, p. 340. Reinhold, New York (1965). 19. R. C. Hare, Visible and Ultra-Violet Spectroscopy In Spectroscopy and Structure o[ Metal Chelate Compounds (Edited by K. Nakamoto and P. J. McCarthy), p. 74. Wiley, New York 0968). 20. F. A. Cotton and H. P. Hansen, J. Chem. Phys. 25,619 (1956). 21. H. L. Nigam and U. C. Srivastava, Inorg. Chim. Acta 5, 338 (1971). 22. F. A. Cotton and C. J. Ballhausen, J. Chem. Phys. 25, 617 (1958). 23. U. C. Srivastava and H. L. Nigam, Coord. Chem. Rev. 3/4,275 (1973). 24. K. Tomita, Bull. Chem. Soc. (Japan) 34, 280 (1961).
X-RAY, SPECTRAL AND MAGNETIC STUDIES OF COPPER(II) CHELATES OF L-ASPARAGINE AND L-GLUTAMINE M. N. SRIVASTAVA,t R. C. TEWARI and U. C. SRIVASTAVA Chemical Laboratories, University of Allahabad, Allahabad-211002, India and G. B. BHARGAVA and A. N. V1SHNOI Department of Physics, University of Allahabad, Allahabad-211002, India (First received 23 September 1975; in revised form 15 January 1976)
Akltraet--Copper(II) chelates of L-asparagine and L-glutamine have been studied using reflectance and UV absorption spectra, magnetic and ESR measurements and X-ray absorption edge spectrometry (XAES). Based on these measurements, a distorted octahedral structure has been assigned to these complexes. The relevant ligaud field parameters are reported. Average metaMigand bond-lengths have also been estimated using X-ray absorption fine structure.
INTRODUCTION THE STUDY o f the molecular structure of copper complexes of amino acids and their derivatives has been of interest in the field of bioinorganic chemistry [1, 2]. In particular, the identification and determination of the nature of the binding sites of Cu(II) has been the dominant theme of many current investigations [3, 4]. The chemical specificity of Cu(II) ions enables them to bind to several positions on these ligands although frequently one position is preferred from stereochemical considerations[5,6]. For example, complexes with racemic amino acids adopt a trans structure whereas those synthesized with optically active amino acids are cis isomers[2]. It has been suggested that stereoselective crystal packing effects stabilize a particular configuration [7]. Copper(II) chelates formed with L-asparagine and L-glutamine offer an interesting system for study for a number of reasons. First, their solution properties have been studied by Albert [8], Ritsma et al. [9] and Tewari and Srivastava[10] but no literature is available on their properties in the solid state; and second, these ligands play an important role in biological chemistry [ l l, 12]. In the present work, therefore, an attempt has been made to study these complexes using reflectance and UV absorption spectra, magnetic and ESR measurements and X-ray absorption spectrometry (XAES). Their preparation and characterization (from analytical and IR data) have been reported in an earlier paper[13]. The complexes are: bis(L-asparagino) copper(IlL [Cu(OOC.CH'NH2.CH2.CO. NH2)2]; and bis-(L-glutamino) copper(II), [Cu(OOC. C H-NH2CH2. C H_~.CO.N H.+.].
Table 1. Magnetic susceptibility measurements on Cu(II) chelates
Compound Cn(Aspn.)2 Cu(Glun.)2
~(~ x 10"
XMXIO 6
T(°K)
(c.g.s. units)
(c.g.s. units)
X~x 10" (c.g.s. /zofr units) (B.M.)
304 304
4.09 3.83
1,331 1,354
1,466.5 1.898 1,513.0 1.922
Spectral measurements. U.V. absorption spectra were recorded on aqueous solutions on a Cary Model-14 recording Spectrophotometer using 1 cm quartz cells. The observed bands are listed in Table 2. But the solubility of these chelates in water is rather too low, to yield good peaks in the visible region. Hence reflectance spectra of the solid complexes were recorded on "C-10" recording spectrophotometer (Russian) in the range 400-700 nm. Results are included in Table 2.
Table 2. Electronic spectra of Cu(lI) chelates Band (cm ')
Compound
Compound
16,000 42,330 (6,250) Cu(Glun.)2 50,760 (5,650) 52,230 (4,400)
Cu(Aspn.)2
Band (cm ') 16,400 42,199(5,000) 51,280 (4,800) 52.230 (4,300)
UV molar extinction coefficient values are given in brackets. Electron spin resonance. Spectra of solid samples (Fig. 1 and Fig. 2) were taken on a Bruker ModeI-B-ER 402 X-band, ESR-spectrophotometer and polycrystalline DPPH was used as a reference. The "g" factors are given in Table 3.
EXPERIMENTAL Magnetic measurements at room temperature (3I°C) were made on powder samples of the complexes employing Gouy's method. A semi-micro Sartorius balance was employed and mercury(I1) tetrathiocyanatocobaltate(II) (X~ = 16.44 × 10-4 c.g.s, units) was used as calibrant. Both complexes were found to be paramagnetic. The data are presented in Table 1.
Table 3. "g" Values of the Cu(II)-chelates
Compound Cu(Asph.)2 Cu(Glun.)~
tPostal address: Dr. M. N. Srivastava, 266 (near distillery), Mumfordganj, Allahabad-2! 1002, India. 1897
g,
g,
g,,~.
2.0503 2.2534 2.12 2.046 2.212 2.10
(A2= E3 - Eo, i.e. E(E,) - E(B,g)).
x(cm ') (Spin orbit coupling constant)
A2 (cm ')
-510 -425
20,000 18,500
M . N . SRIVASTAVAet al.
1898
X-Ray K-absorption spectra were recorded photographically employing a 400 mm Cauchois-type covered-crystal spectrograph using (100) reflection planes of mica as analyser. The radiation source was a sealed X-ray tube with tungsten target run at 20 KV-10 mA. The mica crystal was good enough to resolve the Mo/3~.3 doublet (A)t = 0,572 x. u.) in the first order and gave a dispersion of about 12 x u. per ram. On the spectrogram Wa,, WL~81 and WL#, were used as reference lines. The intensity records were obtained with the help of a Kipp and Zonen Moll microphotometer at magnification 8x and 25x. The results of measurements (made on a Hilger comparator) of the edge-shift (A) and the energies of the principal absorption maxima (EA) are presented in Table 4. Table 5 illustrates the relationship[14] between measured edge with (EW) and the coordination stoichiometry (metal-nearest neighbour ratio) of the complexes. The metal-ligand bond-lengths estimated from fine structure energy separations (B-~)[15] are given in Table 6. Figure 3 shows the trace of the microphotometer record with a magnification x25. The position of the various absorption maxima and minima have also been shown in the figure. The position of the inflection point of the absorption edge in pure metal has been taken as zero ernergy.
t.) V O.. tt3
I
\
Fig. 1. ESR spectra of copper(II) L-asparagine chelate (in solid state).
DISCUSSION The ground state of a cubic octahedrally coordinated Cu(II) is 2Eg and the only excited state is 2T2~, the energy difference being 10Dq [16]. But tetragonal distortion splits the E, and T2~ levels so that as many as three d-d transitions are possible[17].
dx2-r2">d~=
d~2-y~-~ d~r ; t.)
dx2 y2~(d,,,d~).
and
Cu (Aspn)2 A °
Cu (Glun)~ A
o
Io
J
I
I
I
F
20
30
40
50
60
Energy, eV
Fig. 2. ESR spectra of copper(lI) L-glutamine chelate (in solid state).
Fig. 3.
Table 4. Data on K-absorption edge of Cu(II) chelates Absorber Copper metal Cu(Aspn)2 Cu(Glun)z
AK (X.U.)
(X.U.)
1377.99
--
1375.53 1375.59
hA
A
(F/R)K 661.30
1375.28 1374.35
662.48 662.45
(P/R)A A(F/R)K A(p/R)A (eV) .
.
663.09 663.09
.
.
1.18 1.15
. 1.79 1.75
16.0 15.6
Table 5. K-Edge-widths and the coordination stoichiometry of the Cu(II) chelates
Compound Cu(Aspn)2 Cu(Glun.h
Ah (K - A) (X.U.) 1.25 1.24
Coordination EW stoichiometry A(v/R)K-a (eV) (Cu:N:O) 0.61 0.60
8.3 8.1
1 : 2: 4 1 : 2: 4
EA (eV)
X(X~-Xt,)
[(EWXAx)'/2]
8.6 8.6
8.4 8.3
24.3 23.7
I
1899
Magneticstudiesof copper(lI)
/
Table 6. Averagemetal-ligandbond-lengthsof the Cu(ll) chelates Compound
AE(B -/3)
i x,151,:2o r=t--A~ ) A
Cu(Aspn.)2 Cu(Glun.)2
37.1 38.4
2.01 1.97
'
/NH~
/c:o\
\
/ ¢=0\ /OH, .\\
\
I
CH2 I H&--N \0 ~
Thus the observed band envelope may not be due to a single transition but may consist of two or three symmetrical absorption bands. The expressions[18] derived for "g" values in a tetragonally distorted copper(II) molecule, which is stretched along the symmetry axis (z-axis) are g, = 2 - 8A/(E2 - Eo) and g~ = 2 - 2AI(E3 - Eo) where, E0, E_, and E~ are the energies corresponding to :A~, 2B2~ and :E~ levels. Thus E 2 - E0 is equivalent to 10 Dq and the energies corresponding to the observed .~max (1090 16,000 cm ~) may be taken as E 2 - E0. On this basis, from the observed g, values (Table 3) )t (spin orbit coupling constants) values were calculated, which from the above equation, gave E3 - E0, i.e. A2. It is of interest to note that the observed band envelopes in the reflectance spectra of copper(II) chelates engulf these transitions. The bands observed in the UV spectra of the copper chelates can be attributed to charge transfer transitions. Earlier, in the case of bis-(phenylglycinato)copper(II), Hare [19] ascribed the band observed at 39,000 cm-* to an apparent charge transfer from the carboxylate group to the metal and that observed at 52,000 cm ' to a charge transfer from amino group to metal with a possible contribution from the ~r-~r* transition of the COO group. Thus in the present case the strong broad bands observed at 42,330 cm ' in Cu(Aspn.)2 and at 42,190 cm-' in Cu(Glun.)2 correspond to a charge transfer from carboxylate group to the metal, whereas the bands observed at 50,760 cm ' and 51,280 cm ~ respectively in the two chelates may be ascribed to charge transfer transitions from the amino group to the metal. Other weaker bands at -52,000cm -1 may correspond to carboxyl 7r ~ 7r* transitions. The similarity in X-ray absorption edge data (Tables 4 and 5) and the structure of the absorption edge (Fig. 3) appear to suggest that the two chelates are essentially similar, particularly in regard to stereochemistry and coordination sites. Thus the chemical shifts and the widths of the K-absorption edges in the two chelates are nearly the same. Earlier, band-widths of the same order ( - 8 eV) were also observed on the spectra of several copper complexes involving nitrogen-oxygen ligands [20]. Further, the measured edge width gives, through the relationship[14] [~(X~ - X,.) Edge width] '/2 = constant (8.5 for copper system) (where XM and XL are respectively the electronegativitiesof the metal and donor atoms) a coordination stoichiometry-Cu: N: O : : 1 : 2: 4. It thus indicates that coordination occurs from carboxylic oxygen, amino nitrogen and carbonyl oxygen of the primary amide group of the two ligands (Fig. 4). The edge structures (Fig. 3) are visibly characterized by the presence of a small maximum at ~ 10 eV, lower energy with respect to the main edge, a splitting of the principal absorption maximum and a small hump (a') to its high energy side. The intensity of the low energy absorption suggests that it probably corresponds to a nominally forbidden transition and it may be appropriate to designate it as ls o3d,~, which would be in agreement with the hybridization (sp3d ') envisaged. The width of the
\
. . . . . .
to='. ......
\
/2
\o
....
\'\ I ~/cu2+
/2
L - Glutamine chelate
L - Asparagine chelate
Fig. 4. absorption jump of the lower energy side of the main edge (-10eV) when compared with that present on its high energy side (upto A) ( - 8 eV) suggests that the extra width of - 2 eV in the 3d- region results from 3d- splitting in the complexes. It may be interesting to note (Table 3) that the values obtained for the chelates from optical data are also 20,000 cm 1 and 18,500 cm-1, i.e. 2.4 eV and 2.2 eV respectively and are thus in close agreement with the X-ray measurements. The principal absorption maxima, appearing at -24 eV may as usual, be assigned a transition ls ~4p* (antibonding) in accordance with the bonding configuration (sp3d2). Earlier, in the case of Cu(II)-DL-prolinate dihydrate having a similar coordination stoichiometry, Cotton and Hansen[20] have observed the main peak at -20 eV. The increased energy of the 4p* (antibonding) orbitals in the chelates under study may be taken to signify an enhanced strength[21] of the metal-ligand bond in them as compared to that in CU(II)-DL prolinatedihydrate. It may be noted that the axially coordinated oxygen atoms are from water molecules in the latter complex and from amido groups in the former. Further, a comparison of the energies of the main peaks of the asparagine and glutamine chelates (Table 4) seems to agree with the fact that stronger bonds are formed with asparagine than glutamine, as is also suggested by their stability constants data[8-10]. The observed splitting of the 1s ~ 4p* peak into two (rather broad) components presumably indicates a ligand field symmetry lower than Oh for these complexes [20, 22]. A similar structure has earlier been observed by Cotton and Hansen[20] in the case of CU(II)-DL prolinate dihydrate, which has been shown to be in reasonable agreement with its D2h symmetry. A similar symmetry for the chelates under study may, therefore, be suggested particularly in view of the probable mode of tridentate coordination involved in them. The small maximum subsequent to the main peak, particularly observed in copper complexes involving ligands having 7r bonding ability, have usually been attributed[23] to transitions of the ls electron to the antibonding ~- levels of the complex. This may be important since broad and intense bands attributable to charge transfer have been observed in the UV spectra of the two complexes as discussed above. The average metal-ligand bond-lengths (Table 6) estimated from /3-/3 separations (Fig. 4) in the fine structure are found to be comparable with those obtained from X-ray diffraction analysis of bis-glycinato)copper(II) monohydrate[24] having a similar distorted octahedral structure.
Acknowledgement--Authors (M.N.S. and R.C.T.) are grateful to State C.S.I.R., U.P. Lucknow for financialassistance in the form of a scheme.
M.N. SRIVASTAVAet al.
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