J . Phys. Chem. 1988, 92, 1062-1065
1062
Elucidation of the Geometry of the Dipotassium Bls(biuretato)cuprate( II ) Dimer by Simulation of Its EPR Spectra Saba M. Mattar Department of Chemistry, University of New Brunswick, Bag Service Number 45222, Fredericton, New Brunswick, Canada EJB 6E2 (Received: July 8, 1987)
The axial interactions of the bis(biuretato)cuprate(II) complex in basic media are investigated. EPR and electronic absorption spectroscopy show that the copper-biuret complex interacts in an axial fashion with ethylene glycol but interacts only marginally with ethanol. It also undergoes dimerization to the tetrakis(biuretato)dicuprate(II) complex at 110 K. The simulation of the originally forbidden half-field transition indicates that the two copper complexes forming the dimer interact axially. In the presence of ethylene glycol the dimerization does not occur because the axial sites are blocked by the glycol.
Introduction Coppex(I1) ions and biuret, a model amide ligand, form a variety of complexes in neutral and alkaline media. The complexes in alkaline media are characterized by their intense colors ranging from red to blue, depending on their method of preparation.' Another significant property of these complexes is their ability to dissolve Their dissolution power is comparable to, if not greater than, standard inorganic solvent^.^ In order to understand the mechanism by which these complexes dissolve cellulose, the structure and bonding of the individual copper-biuret complexes and their interaction with model hydroxy ligands must be understood. As a first part of such a study, the electronic structure, bonding, and magnetic properties of the bis(biuretato)cuprate(II) dianion (Figure 1) were studied by electron paramagnetic resonance (EPR), ultraviolet, and visible spectr~scopies.~The experimental tensor components of the copper hyperfine, ligating nitrogen hyperfine, and g tensors as well as the relative orientation of their principal axes were determined by the computer simulation of the experimental EPR spectra and were then compared with those computed by the X a self-consistent field scattered wave (Xa-SW) method.4 The analytical expressions for the g-tensor components were derived in terms of molecular orbital coefficients by using second-order perturbation theory. The agreement between the theoretical and experimental g, and gyy components was very good while the g,, component was slightly underestimated. The bis(biuretato)cuprate(II) monomer was found to have strong b,, (u-type) bonding between the copper and four ligating nitrogen atoms. The bz and b3, out-of-plane (.-type) bonding were also strong not only for Cu-N bonds but between the biuretato ligand components as well. The in-plane .-type Cu-N bonding was found to be weak. It manifests itself only through the interaction of the 3dx2-,,z(Cu),4p,(Cu), and 4py(Cu) with the 2p,(N) and 2py(N) components of the a,,, b3,,, and bzU molecular orbital^.^ In this study the interaction of the bis(biuretato)cuprate(II) complex formed in basic solutions (pH 13.2) is investigated. The mode of interaction of this complex with ethanol or ethylene glycol is studied by electronic absorption and EPR spectroscopies. In these solutions axial adduct formation is found to occur while no substitution of the biuretato ligands is observed. The crystal structure of dipotassium bis(biuretato)cuprate(II) as the tetrahydrate has been established by X-ray d i f f r a ~ t i o n . ~ This complex has an elongated tetragonal structure with the copper being coordinated to four nitrogen amide nitrogen atoms from the equatorial biuret ligands. In addition, the copper atom is axially coordinated with two nitrogen atoms of neighboring
complexes. The axial interactions between neighboring monomers in the crystalline state strongly suggest that similar interactions might cause the dimerization of this complex in frozen glasses at low temperatures. In fact, such dimers were detected when the interaction of the copper complex with ethylene glycol at 110 K was studied. The theory used to simulate the EPR spectra and elucidate the structure of a dimer has been developed by Pilbrow and Smith.6 The perturbations of the electronic structure of the complex upon dimer formation were represented in terms of a Heisenberg exchange energy term and a magnetic dipole-dipole interaction term. The ordinarily forbidden AMs = r 2 transitions are weakly allowed for a dimer. With this theory, both the AMs = f 2 and AMs = f l can be simulated and compared with the observed experimental spectra.6 The theory is quite general, and there are no restrictions on the possible orientation of the two transition-metal complexes with respect to one another. In addition, the two metal complexes forming the dimer could be dissimilar.' This theory is used here to simulate the EPR spectra of the tetrakis(biuretato)dicuprate(II) dimer and elucidate its geometry. Experimental Section Biuret was purified and converted into the anhydrous form by a previously described m e t h ~ d .The ~ solid dipotassium bis(biuretato) cuprate(I1) complexes were prepared by the method of McLellan and Melson." Acculute grade sodium and potassium hydroxides (Anachemia) wqre purchased and used without further purification. Ethanol and ethylene glycol were of spectrograde quality. Solutions of the bis(biuretato)cuprate(II) dianion used for the titrations were prepared by mixing cupric hydroxide4(1.OOg) with 6 mol equiv of anhydrous biuret (9.185 g). Potassium hydroxide (23.7 mL, 5.05 M) was then added to the mixture to produce the starting stock solution. Aliquots from this stock solution were then titrated with either ethanol or ethylene glycol. The method of titration was designed so that the solutions contained exactly the same amounts of copper and biuret. The amount of titrant was the only ~ a r i a b l e .The ~ total metal ion content in the copper solutions was determined electrogravimetrically. The ultraviolet and visible electronic absorption spectra were measured by means of a standard Cary 17 double-beam spectrometer. Quartz cuvettes of 1-cm path length were used. The EPR measurements were carried out on a Varian V-4501-B spectrometer with a double rectangular cavity. The reference part of the cavity contained a solid sample of diphenylpicrylhydrazyl (DPPH) free radicals. The specific components of the spectrometer and accompanying apparatus have been described pre-
(1) Kurzer, F. Chem. Rev. 1956, 56, 95.
(2) Jayme, G.; Lang, F. Kolloid Z . 1957, 150, 5. (3) Jayme, G. Cellulose and Cellulose Derivatives; Wiley-Interscience: Toronto, 1971; Vol. 5 , Part IV, p 381. (4) Mattar, S. M. J . Phys. Chem., in press. (5) Freeman, H. C.; Smith, J. E. W. L.; Taylor, J. C. Acra Crystallogr. 1961, 14, 407.
0022-3654/88/2092-l062$01.50/0
(6) Smith, T. D.; Pilbrow, J. R. Coord. Chem. Rev. 1974, 13, 173. (7) Carr, S. G.; Smith, T. D.; Pilbrow, J. R. J. Chem. Soc., Faraday Trans. 2 1974, 497.
( 8 ) McLellan, A. W.; Melson, G. A. J. Chem. SOC.A 1967, 137. (9) Mattar, S. M. Ph.D. Thesis, McGill University, 1982.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1063
Dipotassium Bis(biuretato)cuprate(II) Dimer
4, Figure 1. Molecular structure of the bis(biuretato)cuprate(II) dianion.
1.2
0.8
F/ I/ /I
I
04
I
I
I
1
08
I
1.2
I 1.6
I
I
*
2.0
b
Figure 3. Difference absorbance, 6, for the aqueous solution of bis(biuretato)cuprate(II) complex as a function of the concentration of ethylene glycol, 6 , in mol/L.
I
I
I
1
I
400
500
600
700
800
hlnm
Figure 2. Electronic absorption spectra of a 5.0 X IOA3 M aqueous solution of bis(biuretato)cuprate(II) when titrated with ethylene glycol.
viously.lo All EPR sample tubes were checked for any interfering paramagnetic species before use. Results and Discussion The bis(biuretato)cuprate(II) was titrated with ethanol and ethylene glycol. Ethanol reacted only marginally with the bis(biuretato)cuprate(II) complex, and only minor changes were observed in the band maximum of the visible spectra. However, when the copper complex was titrated with ethylene glycol, the spectra shown in Figure 2 were obtained. The spectra show two isosbestic points at 365 and 547 nm for the same set of spectral at 504 nm, which is characteristic of the lines. The original A,, bis(biuretato)cuprate(II), shifts toward lower energies, and there is a slight decrease in the extinction coefficient. This behavior has been associated in Cu(I1)-N, complexes with the addition of a fifth axial The presence of the isosbestic points unequivocally indicates that the titration of the complex with ethylene glycol produces only two copper species in equilibrium with each other. If two chemical species are monitored by electronic absorption spectroscopy, the total optical density at a particular wavelength X is given by €(X)C
= r,(X)c,
+
€2(X)C2
(1)
where 6, and c, are the extinction coefficients and concentrations. The fractional concentrations of the two species a1and a2 are13
and
The fractional concentrations may then be used to determine the equilibrium constant for the reaction occurring during the titration. However, a 1and a2 can only be determined if the extinction coefficients A(Xj), e,(X,), and €,(A,) are determined at every particular wavelength A,. By starting with the spectrum of the pure bis(biuretato)cuprate(II), complex el(XJ) may be determined. As the titration proceeds, one also determines e(X,). The values e2(X,) are not known unless at the extreme end of the titration one obtains a solution containing the second species only. This only wcurs if the stability constant of the second species is comparable to or greater than that of the first complex. However, in this case a solution containing only the second species could not be obtained. In order to determine the tZ(X ) values difference electronic absorption spectroscopy was used.(3 In this case the second fractional concentration may be written in the form a2 = A6
(4)
The value of A is a constant during the titration, and the variable 6 is estimated by using the pure bis(biuretato)cuprate(II) as the reference in a double-beam spectrophotometer. The resulting spectrum is thus directly proportional to 6 . The substitution of al and a2in the equation of mass balance for a particular model chemical reaction results in one equation that contains the equilibrium constant K and A as two unknowns. The determination of the electronic absorption spectra at two different ligand concentrations yields a second equation which is sufficient, in principle, to determine K and A . The model chemical reaction is arbitrarily assumed, and its validity is determined by how well it fits the experimental spectra. Attempts to fit the experimental spectra in Figure 2 to a substitution reaction where the ethylene glycol displaces one or two biuretato ligands were unsuccessful. However, for the axial addition reaction K2[Cu(Bi)J ca 1
+ HOCH2CH20H b - CCY~
-
K2[Cu(Bi)2(HOCH2CH20H)](7) ca2 (10) Hocking, M. B.; Mattar, S . M. J. Magn. Reson. 1982, 47, 187. (1 1) Addison, A. W.; Carpenter, M.; Lau, L. K. M.; Wicholas, M. Inorg. Chem. 1978. 17. 1545. (12) Barbucci, R.; Fabrizzi, L.; Paoletti, P. J . Chem. Soc., Dalton Trans. 1972, 1099. (13) Vink, H . Ark. Kemi 1957, 1 1 , 9.
the equilibrium constant takes the form
K = caz/[caI(b- C L Y ~ ) ]
(8)
Here c represents the total concentration of the Cu(I1) ions, b is the concentration of ethylene glycol in solution, and Bi represents
1064 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Mattar
Magnetic Field ( m T )
Figure 5. Low-temperature (110 K) EPR spectra of bis(biuretat0)cuprate(I1) solution when titrated with ethylene glycol.
I
3
1
306 Magnetic
I
I 32 6 Field
I
I
346 mT
Figure 4. The rmm temperature spectra of the bis(biuretato)cuprate(II) solution when titrated with ethylene glycol. The labels I , 2, 3, 4, and 5 correspond to spectra containing 0.192,0.352,0.604, 1.06, and 2.1 13 M ethylene glycol, respectively. The vertical line represents the resonance field position of the DPPH reference.
the biuretato ligand. The substitution of the values of a I and a2 in eq 8 gives 6 = [l
+ K ( c + b) i{[l + K ( c + b)I2 - 4Kcb)'/*/(2KcA) (9)
The constants K and A are determined by means of a nonlinear least-squares fit using eq 9 as a model. Figure 3 shows the experimenal values of S as a function of the concentration of ethylene glycol, b. The fit between theory (solid line) and experiment (points) is excellent and indicates that an addition reaction occurs during the titration. The optimized value for the equilibrium constant is found to be 2.1 1 L/mol, which is of the same order of magnitude as that reported for the axial addition of OH- to bis(ethylenediamine)copper(II) c0mp1exes.I~ A further confirmation that ethylene glycol adds axially to the copper complex is obtained when the same solutions used for electronic absorption spectroscopy are used for EPR in the liquid state at 298 K. These spectra are shown in Figure 4. Every
individual spectrum is characteristic of a rapidly tumbling Cu(I1) complex in solution. This class of spectra is well understood. The It arises from spectrum labeled 1* has been described previ~usly.~ a resonance which is split by -198.126 MHz into four copper hyperfine lines (Icu= 3/2) from the two 63Cuand 65Cuisotopes. The hyperfine lines of all the spectra of Figure 4 are split by approximately 38.0 MHz into nine lines of intensity ratios 1:4:6:1 019:10:6:4:1, due to four nitrogen nuclei in the equatorial plane (1, = 1). The two outermost lines of unit intensity are not observed since they are lost in the envelope of those of higher intensity from the other i ~ o t o p e . The ~ loss of resolution with decreasing field is attributable to the dependence of the line widths on the Cu nuclear magnetic quantum number mI. The extent of variation of the line width as a function of mIdepends on the rate of tumbling of the molecule in solution, its microviscosity, and anisotropy in its g and A tensor component^.'^*'^ Although from the present experiments one cannot determine the sign of the isotropic hyperfine coupling constants, the 65Cu and 63Cuhyperfine splittings are computed to be negative while those for the ligating nitrogen atoms are p ~ s i t i v e . ~ As the titration proceeds, the spectra show a shift toward lower fields in comparison to the relative position of the DPPH reference. When axial ligation with the ethylene glycol occurs, g,, tends to increase while gxxand gyvremain almost unchanged.16 Thus the average g value ( g ) = (gxx + gyy + g A / 3
(10)
increases, causing the whole spectrum to shift downfield. The EPR spectra show a slight loss of resolution as the copper complex is titrated with the glycol. The original copper complex and the axial adduct are expected to have slightly different average hyperfine components. During most of the titration both complexes exist simultaneously in solution, causing their composite spectrum to be slightly less resolved. Similar shifts toward lower magnetic fields and loss of resolution were also observed by Falk et al. when titrating bis(dimethylglyoximato)copper(II) with alkali." The EPR intensities, as determined from the peak heights, remain relatively constant during the titration with ethylene glycol. This indicates that the adduct formed is also a monomer and no (14) Kivelson, D. J . Chem. Phys. 1960, 33, 1094. ( 1 5 ) Wilson, R.; Kivelson, D. J . Chem. Phys. 1966, 44, 4440. (16) Smith, D. W. J . Chem. SOC.A 1970, 3108. (17) Falk, K. E.; Ivanova, E.; Roos, B.; Vangaard, T. Inorg. Chem. 1970, 9, 556.
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1065
Dipotassium Bis(biuretato)cuprate(II) Dimer
I
I
I
1
I
110
130
150
170
19 0
M a g n e t i c Field
L
L
(mT)
Figure 6. Experimental (a) and simulated (b) AM, = 7 2 spectra of the dipotassium bis(biuretato)cuprate(II) dimer.
significant dimer or polymer species exist at room temperature. In general, these spectra confirm that during the titration no equatorial biuret ligands are replaced by the ethylene glycol. The low temperature EPR spectra in Figure 5 show a completely different behavior. Figure 5a represents the EPR spectra of the pure bis(biuretato)cuprate(II) complex at 110 K. It is characteristic of a composite spectrum of copper(I1) monomers and and arises from the partial dimerization of the monomer at low temperatures. The AMs = 7 2 dimer transitions are only weakly allowed, making their detection possible only under favorable conditions. However, their presence is unequivocal proof of dimer formation. The AMs = 7 2 spectrum at 110 K for this complex occurring around 160 mT was detected (Figure 6) and will be discussed later. As the copper complex is titrated with ethylene glycol, one observes a slight enhancement of the resolution of the monomeric component (Figure 5b). A search for the corresponding AMs = 7 2 dimer spectrum in this case gave no detectable EPR signal, indicating that the percentage of the dimeric species had decreased. Further titration with ethylene glycol resulted in the EPR spectra shown in Figure 5c,d. These spectra are very similar to that reported for Cu(I1)-hemoglobin where the copper is known to be surrounded by four nitrogen ligands in the equatorial plane.21 Thus it may be concluded that at low temperatures dimerization of the bis(biuretato)cuprate(II) occurs as demonstrated by its AMs = 7 2 EPR spectrum. As the temperature is raised the weak dimer breaks up. In the presence of ethylene glycol, the hydroxyl groups block the axial sites of the bis(biuretato)cuprate(II) complex, preventing dimerization at low temperatures. In theory, a computer simulation of the AMs = 7 2 EPR spectrum should yield the parameters r, q, and that define the relative positions of the two bis(biuretato)cuprate(II) monomers when they form a dimer. These parameters are defined in Figure 7. The EPR spectra of the AMs = 7 2 dimer transition was ~ program is very similar simulated by the program D ~ M R .This to ALLSYM and GNDIMER written by Pilbrow et al.7319,22It takes into account the effects of the 63Cuand 65Cu isotopes, solves the full quadratic equation for the resonance field positions, and (18) Toy, A. D.; Smith, T. D.; Pilbrow, J. R. Aust. J . Chem. 1973, 26, 2349. (19) Boyd, P. D. W.; Toy, D.; Smith, T. D. J . Chem. SOC., Dalton Trans. 1973, 1549. (20) Makoto, C.; Yokoi, H. J . Chem. SOC.,Dalton Trans. 1977, 2344. (21) Luoro, S. M.; Bemski, G.J . Magn. Reson. 1977, 28, 427. (22) Carr, S.G.; Smith, T. D.; Pilbrow, J. R. Supplementary publication available from the British Lending Library, Boston, Spa., U.K.; Manuscript No. 20897.
Figure 7. Parameters r, 7, and [ used to define the relative positions of two paramagnetic complexes with respect to one another when they aggregate to form a weakly coupled dimer.
includes the effects of the S1, and SlZspin components on the transition probabilities. These modifications slightly improve the simulated spectra. However, their derivation is lengthy and is fully presented el~ewhere.~ The experimental and simulated EPR spectra of the AMs = 7 2 transitions are shown in parts a and b of Figure 6, respectively. These two spectra agree within 70.4 mT. The simulated spectrum indicates that q = O.Oo, = 40.2O, and r = 4.2 A. In addition, the Heisenberg isotropic exchange interaction J was effectively zero. The g, and gVucomponents were found to be 2.050 while g,, was 2.175. The copper hyperfine tensor components A,,, Ayy, and A,, were determined to be 10.0 X lo4, 10.0 X lo4, and 195.0 X cm-', respectively. They are very similar to those found for the bis(biuretato)cuprate(II) m ~ n o m e r . ~ From the values of the angles q and t and the Cu-Cu internuclear distance, r, the separation between the two equatorial planes of the monomers is estimated to be 3.24 A. A reconstruction of a geometrical model similar to Figure 7 shows that the angle formed between the Cu-zl axis and the direction of the intermolecular Cu-N5 bond is N 14.0°. Thus the interaction between the two monomers is considered to be axial in nature. The axial distance of 3.24 A is similar to other distances found for the dimerization of two copper complexes.6 Conclusions
The interaction bis(biuretato)cuprate(II) monomer with ethylene glycol, as studied by electronic absorption spectroscopy and EPR, is found to be axial in nature. At 110 K, the monomer is found to partially dimerize to the tetrakis(biuretato)dicuprate(II) complex. The simulation of the weakly allowed AMs = 7 2 spectrum indicates that the dimer is formed by the dimerization of two bis(biuretato)cuprate(II) monomers in a very similar fashion to that found in the crystalline state.5 N o dimers are detected in the presence of ethylene glycol, indicating its interaction with the copper monomer blocks the axial sites of dimerization. Acknowledgment. I express my gratitude to Professor William C. Galley for his encouraging interest and valuable discussions. Financial assistance from La Direction General de L'enseignment Superieur du Quebec and the Department of Chemistry at McGill University, where most of this work was carried out, is acknowledged. Registry No. Bis(biuretato)cuprate(II), 6177 1-68-4; ethylene glycol, 107-21-1; ethanol, 64-17-5; tetrakis(biuretato)dicuprate(II), 112296-47-6; dipotassium bis(biuretato)cuprate(II), 15558-63-1.
