Identification of Copper(ll) Complexes in Aqueous Solution by ~lectron-spin~esonance An Undergraduate Coordination Chemistry Experiment G. Micera, S. Deiana, L. Erre, P. Piu, and A. Panzaneiii lstituto di Chimica Generale ed lnorganica dell'universita di Sassari, 07100 Sassari, Italy This paper describes a simple experiment for studying, through ESR spectroscopy, complex species formed by Cn2+ and 2,6-dihydroxybenzoate ions in aqueous solution. The study can he illustrative of several aspects related to the inorgnnic and cwrdination rhemistry: ( I I identification o f t h r s ~ e c i e stakine Dart in c o m ~ l e xeauilibria with v a r v i n ~nH vhues; (2) ev&ation of th;ir relative stability, and, a t more advanced levels.. (3) . . introduction to the ESR snectroscoov .. of transition metal ions.
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bitals relative to the ground state. The ESR hyperfine structure is due to the interaction of the unpaired electron with the " s ~ ~ Cnuclei, U where I = 312, and has contribution from Fermi contuct, dipolar nuclear spin-electron spin, and nuclear spin-electron orbit mechanisms (2). ~
~~
Results and Discussion The spectra shown in Figures 1-3, illustrate the changes occurring in the Cu(I1)-2,6-dihydroxyhenwatesystem as pH is raised. Each copper(I1) species in solution gives rise to a Experimental typical axial spectrum consisting of an intense (normal) signal in the high-field side and a weaker (parallel) absorption in the Synthesis low-field side. The parallel signal, always split into four Diaquabis(2,6-dihydroxybenzoate~copper(l1, [Cu(H~A)z(H~0)~1.1 equally spaced hyperfine lines, shows definite shifts depending is ohtained easily according to methods described elsewhere ( 1 ) . on the nature of the complex. Briefly,an excess of Cu(N0&3Hz0 is added to an aqueous solution The signals of the solvated ion are identified easily. They (50 ml) of the acid (0.50g or 3.2.10V mol) while stirring. The solution, are the only spectral features of the solution below pH 2.5. adjusted at pH 2.5-3 by addition of sodium hydroxide, is allowed to Over the pH range 2.5-3.5, the free ion is found to he in stand at 50% for one day. Upon coaling to room temperature a green equilibrium with a complex, I, exhibiting ESR parameters crystalline product is obtained, which after filtering and air-drying quite similar to those of the single species obtained by disis satisfactorilyanalyzed for [CU(HZA)Z(H~O)~~. solving [Cu(H2A)z(Hz0)2] in dimethyl sulfoxide. I t is thus ESR Measurements complex, which in neutral suggested that I is a [CU(HZA)~] solution is transformed into 11. Basic media favor a species, Samples of the complex are dissolved in water to known 111, which is stable until pH 12.5, when signals due to concentration, and the pH of a few milliliters is adjusted to [Cu(OH)4I2- (3) are detected. selected values (ranging from 2.0 to 13.5) with HN03 or For the analogous salicylate system (41, a structure of the NaOH. Dimethyl sulfoxide is added to assure the formation type Cu(HA) is suggested for 11, also if the formulation of good glasses a t low temperatures. The final solution con[Cu(HzA)(HA)]- appears to be more reasonable. The species centrations ranee from 5 to 10 mM in 50%aaueous dimethvl I11 is instead believed to he [Cu(HA)d2- (or species formed sulfoxide. The spectrometer used in our lahorkory is a ~ a r i k upon further deprotonation of phenolic groups). Both I11 and E9 instrument equipped with a temperature control unit. Aliquots of the solutions at different pH values are introduced into quartz tubes and submitted to ESR measurements in frozen state. First-derivative spectra are obtained, which are typical of diluted c o ~ ~ e r ( I com~lexes 1) ~ o w d e r sor frozen solutions (2).
Theoretical Considerations Cu2+ has in its outermost shell nine d-electrons, a configuration which can be considered equivalent to one unpaired "hole." In square-planar or tetragonally elongated geometry (ground state dz~-y2)the g-factors deviate from the free electron value (2.0023), according to the following equations:
g, = 2.0023 (1
=)
- 2aZX
where a is the coefficient of dX2-y2in the molecular orbital containing the unpaired hole, X is the spin-orbit constant Axy and Ax, are the energy separations of the d,, and d,,,y, or-.
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' H2A and HA indicate the mono anion CeH,(OH),COO- and the di-
anion CerlslOrl)lCOO-)0-, respectively. X-ray analysis reveals that H2A is linked to the copper(l1)ion only througha carboxylate bond. 646
Journal of Chemical Education
1
2.4
2.6
3.0
2.8
3.2
3.4
H,kG
Figure 1. X-band ESR spectra (110 K) ot -10 mMaqueous solutions of [Cu(HzA)z(H20)~] at (a) pH 2.5, (b) pH 3.1, and (c)pH 4.0 diluted 1:l wilh dlmethyl sulfoxide (Ref. (n).
[ C U ( O H ) ~ ] show ~ - well-resolved spectra exhibiting copper hyperfine splitting in the normal components also. T h e observed behavior fully agrees with the stepwise complex formation scheme: $ [Cu(HzA)(HA)]-or Cu(HA) Cu2+,,iV$ [CU~HZA)~] I I1 a [CU(HA)~]~e [CU(OH)~]~111
Based on the ESR data, it is not possible to establish the remainder of ligands (water andlor dimethyl sulfoxide) in the first coordination sphere of copper(II), if the geometry of [ C U ( H A ) ~ ] and ~ - [Cu(OH)a12- apparently remains square ~lanar. 'I'he quantitative evaluation of the ESR signals could allow one to estimate the distribution ot'copper(l1) species in the pH range examined and to demonstrate the high stability of chelate complexes such as I1 and 111, thr only snecies accounting fo; the whole concentration in the Goper pH range. T h e results obtained from such experiments facilitate a thorough introduction to the ESR theory of copper(I1) complexes. Inspection of the ESR parameter^,^ Table 1, reveals that the free ion has nearly octahedral geometry, exhibiting the highest .g and thesmallest A values-mong thecomplexes examined herein. On the other hand, the changes one observes on examinine-the narametersol Ill or lCulOH1~12arr those . . expected on passing from a regular six-coordination to the sauare-olanar limit. a CuOa chromo~horeactine in the equatorial plane o f all the species. 1; is known t h a t the strenahtenine of the in-olane bond occurs a t the exoense of the &ial b o d i n g (5)and vice versa. Thus, the increase of the
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When the copper hyperfine structure is resolved, approximate @values can be calculated (see Ref. (2). ch. 9) by measuring the maonetic field at Ue center of Ue hvoerfine a m e t . When the hvoerfine sb;cture around o , is not resolved: -~ ~~, ihe field kt Ue bottom of the &mml absorption can be measwed (see Ref.(6)).The hywfline splming value (cm-') is obtained simpsy by measuring the separation Delween the innermost peaks (gauss)and multiplying by the 9121,420 conversion factor. ~~
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-
A
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in-plane a-covalency and the opening out of the 3d levels account for the decrease ofg as well as the increase of A Opportunity is provided for improving the experiment with the aid of electronic absorption data. As the pH is raised a continuousshift of theabsorption maximum to higher energy is observed, two "plateau" regions showing the swpwise lor= 740 nmJ and 111 (A,. = 645 nm), remation of I1 (A,. spectively. At a n introductorv level. both the close relationshiv hetween ESR parame& and hectronic structure of copper(11) complexes can be substantiated easily, and fundamental principles of coordination chemistry can be demonstrated. Literature Cited (1) Cariati. I?, En.. L.. Miera, G., Psnrsnelli, A,. C i i , A,. and Sironi. A.. Inorg. Chim. Aefo. 80.57 (1983). 12) Orago, R. S.. "Physical Methods in Chemistry." W. B. Saundera Co., Philadelphia, 1977. (3) Chao, Y.-Y.H..and Kearns, D. R.,J Phvs Chem., 81,666 (1917). (4) 0'Young.C. L.,Dewq.J. C.,Lilicnthal, H. %and Lippard.S.J., J A m e r Chpm Soc, 100.7291 (1978).
(4 Ammeter, J., Rist, 0.. and Gimthsrd, Hs.H.. J Chem. Phys..57,3852 (1972).
(6) Hathaway. B. J., and Billing, D.E..
Coord. Chpm. Re".. 5,143 (1970). G.. and Piu. P.. Inora Chim. Acto. 64. L213
17) Cariati, P., Deiana. S.. Erre. L.. Miera.
ESR Parameters of Copp.r(ll) Specks In the Cu(ll)-2.8Dlmethyl Sultoxide) Dlhydroxybenzoale Sysiem (Solvent: SO% .Species CU~+_,~~ 1 2 3
ICUIOH)~~~-
911
All (cm-')
gs
2.412 2.378 2.333 2.305 2.252
t34.t0-4 147.10-' 163.10-' 172.10-'
2.099 -2.08 -2.08 2.068 2.048
188.10-•
AL
(em-')
17.10-' 31.10-'
Farignala will?rawlved hypafine s w w e ms estlmatederrorsare wlmln 0.005 (g m1ues)and C10W4cm-' (A values). Ref. (4).(Not detmed. 'Ref (3).
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Figure 2. X b n d ESR spema (110 K) of -10 mtdaqueous solutions of [Cuat (a) pH 6.5. (b) pH 9.5,and ( C ) pH 12.7 diluted 1:l with dimethyi (H2A)2(H20h] sulfoxide(Ref. (71).
Figure 3. Low-fieldregion of the ESR spectra (110 K) of-10 mMaqueous solutions of [CU(H~A).(H.O)~] at (a) pH 3.1, (b) pH 4.0, (c)pH 4.5, (d) pH 5.5, (e) pH 6.2, (I) pH 9.5 diluted 1:l with dimethyl sulfoxide (Ref.(7)). Volume 61 Number 7 July 1984
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