Exafs and spectrophotometric studies on the structure of pyridine complexes with copper(II) and copper(I) ions in aqueous solution

Poiyhedron

Vol.

I I, No.

2, pp.

169475,

1992 0

Printed in Great Britain

0277-5387/92 $5.00+.00 1992 Pergamon Press plc

EXAFS AND SPECTROPHOTOMETRIC STUDIES ON THE STRUCTURE OF PYRIDINE COMPLEXES WITH COPPER(I1) AND COPPER(I) IONS IN AQUEOUS SOLUTION KAZUHIKO

Laboratory

OZUTSUMI*

and TAKUJI

KAWASHIMA

of Analytical Chemistry, Department of Chemistry, University of Tsukuba, Tsukuba 305, Japan (Received 30 January 199 1; accepted 6 September 1991)

Abstract-The

structure of pyridine (py) complexes with copper(I1) and copper(I) ions in aqueous solution has been studied by the EXAFS (extended X-ray absorption fine structure) method. The results of the EXAFS analyses of copper(I1) solutions containing the complexes as the main species, respec[C~(PY)I? [C~(PY)ZI’+, [Cu(P~)31*+and Keel’+ tively, revealed that all copper(H) pyridine complexes have an axially-elongated octahedral structure with additional water molecules. The equatorial Cu-0 and Cu-N bond distances within the complexes are found to be slightly lengthened on increasing the number of pyridine molecules bound to the copper(I1) ion and the axial Cu-0 bond length is ‘+ . It was also shown that the tris(pyridine)copper(I) complex is lengthened in [Cue] formed in a copper(I) solution containing a large excess of pyridine. No water molecules coordinate to the copper(I) ion and thus the complex has a three-coordinate triangular structure.

Six-coordinate copper(I1) complexes are stereochemically interesting. The equatorial and axial bond distances of CuO& CuN, and CuN402 type complexes in crystals correlate asymptotically with an increase in the interatomic distances within the equatorial plane, resulting in the shortening of the bond lengths along the axial direction. ’ However, structure determination in solution is desirable since the axial bond distance may be significantly affected by various interactions in the crystal, the axial bond being essentially weak relative to the equatorial one. The structures of copper(I1) complexes with ligands such as monodentate type halide ions2-8 and ammonia,g-’ ’ and bidentate-type ethylenediamine’““2 and glycinate ions ’ 3 have been investigated by the X-ray, neutron diffraction, or EXAFS (extended X-ray absorption fine structure) method in aqueous solution. The quadridentate ligand cyclam (1,4,8,11 -tetraazacyclotetradecane) has also been investigated. I4 However, ligands so far examined were of the type including aliphatic donor atoms, except halide ions. It is known that the catalytic effect of copper ions on the redox reac-

tion for the determination of copper is well pronounced in the presence of aromatic nitrogen compounds such as pyridine (py) and 2,2’bipyridine. ’ 5,16We previously determined the structure of the copper(I1) 2,2’-bipyridine complexes in aqueous solution. I7 In the present report we describe the structure of pyridine complexes with copper(I1) and copper(I) ions in aqueous solution studied by the EXAFS method. Spectrophotometric measurements were also performed to obtain the additional information on the structure of the individual copper(I1) complexes present in aqueous solution. Since the molar absorption coefficients are constant regardless of high or low ionic medium, the species distribution in concentrated solutions used for the structure determinations is correctly estimated using the known molar absorption coefficients. EXPERIMENTAL Reagents

All chemicals used were of reagent grade. *Author to whom correspondence should be addressed. Copper(I1) nitrate and copper(I1) sulphate were 169

K. OZUTSUMI

170

and T. KAWASHIMA

recrystallized once from water. Pyridinium nitrate was prepared by mixing aqueous pyridine and nitric acid solutions and recrystallized from water. Pyridine, potassium hydroxide, potassium nitrate and hydroxylammonium sulphate were used without further purification. Preparation of solutions Eight test solutions were prepared for EXAFS Solution A was an aqueous measurements. copper(I1) nitrate solution, which contained of known structure. Solutions B-G [Cu(H&12+ were prepared by dissolving copper(I1) nitrate and pyridine in water at suitable C,,/CcU mole ratios. In order to prevent the hydrolysis of copper(I1) ions, pyridinium nitrate was added to solutions B and C. Solution H was a copper(I) solution containing a large excess of pyridine, which was prepared by dissolving copper(I1) sulphate and pyridine in water and then mixing with a large amount of hydroxylammonium sulphate to reduce the copper(I1) to copper(I). The composition of the solutions is given in Table 1. EXAFS measurements X-ray absorption spectra were measured around the Cu-Kedge at the BLlOB station of the Photon Factory, the National Laboratory for High Energy Physics. I8 The white synchrotron radiations were monochromatized by an Si(3 11) channel-cut crystal. The apparent absorbance px is given as ln(Z,/Z), where Z and Z, are X-ray intensities with and without a sample, respectively. The intensities Z, and Z were simultaneously measured by ionization chambers filled with nitrogen and nitrogen (50%) plus argon (50%) gas, respectively. A filter paper was immersed in a sample solution

Table 1. The composition (mol dm- “) of sample solutions for EXAFS measurements Solution

Cc,

Copper(I1) solution 0.995 A 1.00 B 1.00 C 0.498 D 0.523 E F 0.506 0.395 G Copper(I) solution H 0.503

Cw

C,

C&C”

and then sealed in a polyethylene bag in order to prevent the evaporation of water. The number of filter papers was chosen to attain an effective jump at the absorption edge. Analysis of EXAFS data Background absorption other than that for the K edge of copper atom was approximated by leastsquares fitting Victoreen’s formula” to the pre-edge and was subtracted from the total absorption by extrapolation. The smooth K-shell absorption p,, due to an isolated atom was evaluated by fitting a smooth curve to the observed absorption spectrum using a sixth-order polynomial function. The EXAFS modulation was then extracted and normalized by the following equation : xw

= MW --Pclwh4~)

;

where k is the photoelectron wave. vector ejected and given as {2m(E- E,)} ‘12/h. E represents the energy of the incident X-rays. E, is the threshold energy of a K-shell electron and in the present study the value was selected as the position of the halfheight of the edge-jump in each sample. The k3 * X(k) values are converted to the radial structure function F(r) as k

F(r) = (1/27~)“~ mark3.X(k) s‘h,, * W(k) - exp (- 2ikr) dk ;

4.00

1.oo 1.01 -

-

2.01 2.95 3.13 4.03 7.02 17.4 7.95

(2)

where W(k) is the window function of the Hanning type.20 A curve fitting procedure in the k-space for the refinement of structure parameters was applied to the Fourier filtered k3 * x(k)Obsdvalues to minimize the error-square sum U; u = E k6(x(k)oi,d -x(k)u1,32.

(3)

The model function x(k)cald is given by the singleelectron and single-scattering theory as2’-24 x(&cd = E (nJ(k - r,‘)} exp ( - 202k2 - 2rj/A)>Fj(7r, k) x sin (2kr,+ txj(k)) ;

2.01 2.95 1.56 2.11 3.55 6.86

(1)

(4)

where Fj(x, k) is the backscattering amplitude from each of nj scatterersj at a distance of rj from the Xray absorbing atom. aj2 is the mean square displacement of the equilibrium distance rj and I is the mean free path of the photoelectron ejected. q(k) is the total scattering phase shift experienced by the photoelectron. The values of Fj(q k) and c+(k) in eq. (4) were quoted from tables reported by Teo and Lee.25 In the fitting procedures the parameters

171

Structure of Cur’ and Cu’ pyridine complexes E, and 1 were determined from the standard sample

(an aqueous copper@) nitrate solution containing [Cu(H,O),$+) and then they were used as constants in the course of the structural analysis of unknown samples, while r, u and n values were optimized as variables. Spectrophotometric measurements

Electronic spectra of copper(I1) complexes were measured at 25°C by a UV-2000 spectrophotometer (Shimadzu) equipped with a PC-98OlVX computer(NEC), which recorded absorbance data in 1 nm intervals over the wavelength range 300-900 nm. All test solutions contained 1 mol dm- 3 KN03 as an inert constant ionic medium. A flow cell with a path length of 0.5 cm was connected to a titration vessel through Teflon and glass tubes. A solution containing copper(I1) nitrate and pyridinium nitrate was placed in a vessel and then titrated with a 0.5 mol dm- 3KOH solution. Spectrophotometric data at 40 selected wavelengths over the range 45& 850 nm were employed for the least-squares calculation to determine the molar absorption coefficients of the individual copper(I1) pyridine complexes. Absorbance data of test solutions for EXAFS measurements were obtained using a combination cell with a light path length of 0.01 cm. The data were employed for evaluating the distribution of the complexes in the test solutions by the linear least-squares method on the basis of the known molar absorption coefficients.

Cr., = [PYI~+=~ ~B,~,~Cu2+lP~ylP~ylr”~H+l~ (9) G,, = [H+l,+C~~

-&v/[H+11.

RESULTS AND DISCUSSION Electronic spectra

Figure l(A) shows measured electronic spectra. A series of mononuclear [Cu(py),J2+ (n = l-4) complexes form in aqueous solution.2”29 Hence, the spectral change was analysed on the basis of the known stability constants. The variation of spectra was satisfactorily reproduced by the stability constants with the error-square sum of 0.0133 for 2200 data points. The molar absorption coefficients of the individual copper(I1) pyridine complexes were

100 60

‘; E

E

pCu*+ + qpy + rH+ = [CUJ~~)~H,](~~+~+ (5)

A,, = XZC &,,~(~,)B,,[C~‘+IP[PYI~[H+I/ (7)

The concentrations of free Cu’+, py and H+ are related to their total concentrations, Cc”,,, C,,,, and Cn,,, in the solution 1by the mass-balance equations (8)-(10), respectively. Ccu,l

=

-O 0 5

40

20

0 500 600 700

(6)

The absorbance measured in the solution 1 at a given wavelength A,,,is expressed using the overall formation constants #IN,and the molar absorption coefficients s,&.,,,) of the [CU,(P~)$I,](~~+‘)+ complexes as eq. (7).

+ &c\l(n,)[Cu’+l,+~~(~,)[pYL.

60

N

The overall formation of the [CU,(~~)~H,]‘~~+“+ complex can be defined as eqs (5) and (6).

Bpqr= [Cu,(~~),Hd’~~+“+ItCu~+l~t~~l~[H+lr.

(10)

Kw represents the autoprotolysis constant of the . solvent. Molar absorption coefficients of individual copper(I1) pyridine complexes were determined by minimizing the error-square sum CIZ(A+,bsd A /m,calcd)’ on the basis of the formation constants reported in the literature.26

0

Analysis of spectrophotometric data

rB,,,~Cu2’1P[Py1P~H’3;

~~~‘+~,+~~~PB,,[~~~+~P[~Y~~[H+]; (8)

500 600 700

Wave1

500 600 700 800

engt h/nm

Fig. 1. (A) Measured electronic spectra of copper(I1) pyridine solutions at a fixed C,,/& mole ratio of 8 at 25°C with varying hydrogen ion concentrations in the range 450-850 nm. Absorbances are normalized with a unit concentration of copper(I1) ions. Initial volume of the test solution is 30 cm3 and initial concentrations Cc,, C,, and C, are 42.30, 337.7 and 337.7 mmol dm-‘, respectively. Volume of 0.5255 mol dmm3 KOH titrant solution added is 0 (spectrum l), 1.0 (2), 2.0 (3), 3.0 (4), 4.0 (5), 5.0 (6), 6.0 (7), 7.0 (8), 8.0 (9) 9.5 (lo), 11.0 (1 l), 12.5(12), 14.0(13), 16.0(14), 18.0cm3(15).(B)Electronic spectra of individual copper(I1) pyridine complexes in aqueous solution. The numbers represent II in [Cu(py),J2+. (C) Electronic spectra of sample solutions BH for EXAFS measurements.

K. OZUTSUMI and T. KAWASHIMA

172

determined with uncertainties of ca 2% (three standard deviations) at peak positions and are depicted in Fig. l(B) along with that of the hexaaquacopper(I1) ion. The spectral pattern for all complexes is still similar and thus the coordination structure around the copper(I1) ion in the pyridine complexes pay be axially-elongated octahedral as in the hexaaquacopper(I1) ion.3w33 Figure l(C) shows the electronic spectra of test solutions B-G for EXAFS measurements. The spectra must be represented as the sum of the molar absorption coefficients of the individual complexes taking into account the species distribution in solutions B-G. By the known molar absorption coefficients of the individual complexes shown in Fig. l(B), the distribution of the pyridine complexes in solutions B-G was estimated by the linear leastsquares calculation, The results are summarized in Table 2. It should be noted that the relative amounts of complexes in solutions B-G thus determined are appreciably different from that estimated on the basis of the stability constants in a dilute solution, confirming that these stability constants are not applicable to the concentrated solutions in the present study. Structure of copper(I1) complexes

Figure 2 depicts the EXAFS spectra weighted by k3 of sample solutions A-G, Shoulders appearing at 4.5 and 8.5 x lo-’ pm- ’ are gradually predominant and a peak around 6 x lo- ’ pm- ’ becomes well separated with increasing numbers of pyridine molecules bound to the copper(I1) ion. In Fig. 3 the Fourier transforms of the sample

Fig. 2. The EXAFS spectra measured for sample solutions A-G.

solutions A-G are shown, in which the first intense peaks at 160 pm are ascribed to the bonds between copper(I1) ions and oxygen and/or nitrogen atoms in the first coordination sphere. The second peaks around 240 pm gradually increase with the number of pyridine molecules within the complexes. Hence, these peaks are attributed to the non-bonding interactions between copper(I1) ions and carbon atoms in the pyridine molecule. The non-bonding interactions are in fact expected to appear in the same region as the crystallographic data of various halogenopyridine complexes of divalent transition metal ions such as cobalt(II), nickel(II), copper(I1) or

Table 2. Relative amounts of complexes in copper(I1) solutions for EXAFS measurements evaluated on the basis of electronic spectra of individual copper(I1) pyridine complexes shown in Fig. 1. The values in parentheses are those estimated on the basis of the stability constants Solution

cu2+

[Cu(PY)12+

[Cu(PY)z12+

(025)

0.70 (0.54)

0.30 (0.17) 0.63 (0.35)

B C D

(0.04) -

E F

-

G -

(OY2) (Oi3) -

(O-17) (0.04) -

[CNpY)312’

[Cu(PY)412+

(004, 0.37 (0.27) 0.87 (0.59)

1 (OT2) 0.13 (0.21)

(::Z) 0.32 (0.14) 0.28 (0.04)

(P2) 0.68 (0.86) 0.72 (0.96)

Structure of Cu” and Cur pyridine complexes

r/102

pin

Fig. 3. The Fourier transforms F(r) of the k3 *x(k) curves shown in Fig. 2, uncorrected for the phase shift.

zinc(II).3”37 The first peaks are practically unchanged in intensity among solutions A-G, indicating that the coordination structure around the copper(I1) ion in the pyridine complexes is similar to

173

that of [Cu(H,O)$’ . This observation is consistent with the result derived from the spectrophotometric data. The structure parameters for the copper(I1) pyridine complexes were determined by a least-squares calculation applied to the Fourier filtered k3 *x(k) values over the range 4.5 < k/lop2 pm-’ < 12.0. The Fourier filtering of the F(r) values was performed over the range 0.85 < r/10’ pm < 3.00. The axially-elongated octahedral structure for the copper(I1) complexes was adopted in the course of the calculations on the basis of the spectrophotometric results. The E, and 12values were first evaluated from an aqueous copper(I1) nitrate solution (solution A) involving a hexaaquacopper(I1) ion of known structure.3~33 The best-fit values are listed in Table 3. The r and e values obtained for the equatorial and axial Cu-0 bonds satisfactorily agree with those determined with the different data reduction procedure (pre-edge subtraction, ,uOestimation, etc.) by Nomura and Yamagu~hi.~~ Hence, the E, and 1 values are wellapproximated in the present analysis. The interatomic distances and Debye-Waller factors for the copper(I1) pyridine complexes were then refined by adopting the E, and 1 values evaluated

Table 3. Results of the least-squares refinements of structure parameters for copper(I1) and copper(I) pyridine complexes in aqueous solution” Complex Copper(U) complex 1Cu(&0)J2+ PXPY)(H,O)J*+

Interaction

* (pm)

Q (Pm)

n

cu-o, cu-o,

196(l) 227(1) 198(l) 198(3) 229(2) 292(2) 199(3) 197(3) 225(2) 296(l) 201(l) 200(3) 233(3) 298(1) 203(l) 240(2) 299(1)

6.3(4) 11.3(8) 6.3b 6.3’ 12.6(4) 8.1(S) 6.3b 6.3’ 15.2(7) 8.3(3) 6.3’ 6.3’ 16.8(17) 7.3(2) 6.1(2) 16.6(14) 6.9(4)

4b 26

202(1) 301(l)

8.0(S) 9.3(5)

2.8(3) 5.9(8)

Cu-NC,

cu-o, cu-o, cu...c Cu-N,

cu-o, cu-o, cu*.*c Cu-N,,

cu-o, cu-o, cu..-c Cu-N,

cu-o, cu . **c

;: ;: 26 f 46 36 1” ;: 46 26 8’

Copper(I) complex DaY)31+

Cu-N

cu..-c

Standard deviations of the curve fits are given in parentheses. “E,, = 8.999(l) keV and 1= 700(70)pm. bThe values were kept constant during the calculations.

174

K. OZUTSUMI

and T. KAWASHIMA

above. In the fitting procedure the contribution of minor species existing in solution to the observed k’ *X(k) values was taken into account on the basis of the distribution of the complexes given in Table 2. In the course of the refinements the coordination numbers were fixed at the given values in Table 3 based on the axially-elongated octahedral model. The Debye-Waller factors of the equatorial Cu-0 and Cu-N bonds in [C~(py)(H~0)~]*+, [C~(PY)Z(H~O)~I*+ and [CU(~~)~(H~O)~]~+ were also fixed at the value of [Cu(H20)J2+. No significant error could be introduced by the treatment since the similar cr value was obtained for the equatorial Cu-N bond in [C~(py)~(H~0)~]*‘. The solid curves calculated using the parameter values in Table 3 reproduce well the experimental points as shown in Fig. 4. Both equatorial Cu-N and Cu-0 bond distances are found to be slightly lengthened with increasing numbers of pyridine molecules bound to the copper(I1) ion. The lengthening may be caused by an increase in the electron donation of nitrogen atoms within the pyridine molecules coordinated to the central metal ion. The difference in the axial Cu-0 distances of [Cu(H20)J2+ and D(~~>4@320)21~+ is 13 pm and beyond the experimental uncertainty (f 5 pm for the axial bond”). The axial length is lengthened in the [CU(PYMH~~)~I~+ complex. One of the plausible explanations is the steric repulsion between pyridine molecules at the equatorial position and water molecules at the axial site, since pyridine molecules may

Fig. 4. The Fourier filtered k3*x(k) curves of the main peak depicted in Fig. 3. The observed values are shown by dots and calculated ones using parameter values in Table 3 by solid lines.

coordinate to copper(I1) ions with their molecular plane not parallel to the equatorial plane of the complex, as found in a crystal of the nickel(I1) pyridine complex. ’ 5 Structure of copper(I) complex

The colour of solution H is yellow, which indicates that copper(I1) is reduced to copper(I). The highest copper(I) pyridine complex is expected to be formed as a main species since a large excess of pyridine is added to solution H. The EXAFS data for the solution are also shown in Fig. 2. The pattern is very similar to that of solutions F and G except for the suppressed amplitude of the oscillation, suggesting the smaller coordination number of the copper(I) pyridine complex than that of the copper(I1) complexes. This is also clear from the 8’(r) values for solution H shown in Fig. 3, in which the intensity of the first peak at 160 pm is appreciably weaker than that of copper(I1) solutions, while the peak maximum still remains at the same position as that of the copper(I1) complexes. A least-squares calculation for the determination of the structure parameters of the copper(I) complex was performed by a similar procedure to that applied to the copper(I1) complexes. Similar E,, and slightly different 1 values were obtained for Cu,O and CuO powders. The difference in the 1 values is small and does not affect the numbers of interactions obtained. The same E,, and I values for the copper(I1) pyridine complexes were thus adopted for the copper(I) complex. In the course of the calculations the number of Cu-N and Cu. . . C interactions as well as their interatomic distances and Debye-Waller factors were refined as independent variables. The results are shown in Table 3. The numbers of Cu-N and Cu. . . C interactions were converged to almost three and six, respectively, independent of the initial values inserted. Therefore, three pyridine molecules and no water molecules coordinate to the copper(I) ion. The tris(pyridine)copper(I) complex can be described as [Cu(py),]+ with a triangular structure. The threecoordinate structure of the copper(I) 2-aminopyrazine complex has been found in the crystal.38 The Cu-N bond length within the pyridine complex is 202(l) pm and practically the same as the equatorial Cu-N distance within the tetrakis(pyridine)copper(II) complex. This is not surprising since the Cu-N distance within four-coordinate [Cu(py).,]+ in pure pyridine is 205 pm.39 AcknowledgementsThis work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 88-127). The work

Structure of Cu” and Cu’ pyridine complexes was financially supported by the Grant-in-Aid for Scientific Research No. 01470030 from the Ministry of Education, Science and Culture of Japan. Part of the cornputer calculations was carried out at the computer centre of the Institute for Molecular Science in Okazaki. REFERENCES 1. J. Gazo, I. B. Bersuker, J. Garaj, M. Kabesova, J. Kohout, H. Langfeldderova, H. Melmk, M. Serator and F. Valach, Coord. Chem. Rev. 1976,19,253. 2. J. R. Bell, J. L. Tyvoll and D. L. Wertz, .Z.Am. Chem. Sot. 1973,95, 1456. 3. D. L. Wertz and J. L. Tyvoll, J. Inorg. Nucl. Chem. 1974,36,3713. 4. M. Magini, J. Chem. Phys. 1981,74,2535. 5. M. Ichihashi, H. Wakita, T. Mibuchi and I. Masuda, Bull. Chem. Sot. Japan 1982,55,3160. 6. G. W. Neilson, J. Phys. C 1982, 15, L233. 7. P. Lagarde, A. Fontaine, D. Raoux, A. Sadoc and P. Milgliardo, J. Chem. Phys. 1980,72, 3061. 8. K. F. Ludwig Jr, W. K. Warburton and A. Fontaine, J. Chem. Phys. 1987,87,620. 9. T. Yamaguchi and H. Ohtaki, BUN.Chem. Sot. Japan 1979, 52,415. 10. M. Sano, T. Maruo and H. Yamatera, Bull. Chem. Sot. Japan 1983,56,3287. 11. M. Sano, T. Maruo, Y. Masuda and H. Yamatera, Inorg. Chem. 1984, 23,4466. 12. T. Fujita and H. Ohtaki, Bull. Chem. Sot. Japan 1983,56,3276. 13. K. Ozutsumi and H. Ohtaki, Bull. Chem. Sot. Japan 1984,57,2605. 14. H. Ohtaki and H. Seki, private communication. 15. F. S. Lopez, J. B. Nevado and A. E. Mansilla, Talanta 1984,31, 325. 16. F. Holz, Fresenius’ Z. Analyt. Chem. 1984,319, 29. 17. K. Ozutsumi and T. Kawashima, Znorg. Chim. Acta 1991,180,231. 18. M. Nomura, KEK Report 85-7. National Laboratory for High Energy Physics, Tsukuba (1985).

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