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Mixed-ligand complexes of copper(II) with pyridine
Polyhedron Vol. 9, No. IO, pp. 12574262, Printed in Chat Britain
0277-5387/90 53.00+.00 Pergamon Press plc
1990
MIXED-LIGAND COMPLEXES OF COPPER WITH PYRIDINE EDUARDO
ALMEIDA
NEWS*
and MORENA
PINTO PETERS
Instituto de Quimica da Universidade de SBo Paulo, Cx. Post. 20780, SBo Paulo, Brazil (Received 21 November 1988 ; accepted 15 December 1989)
Abstract-The copper(IIkpyridineazide system was studied at 1.0 M ionic strength (sodium methanesulphonate) and 25°C under conditions which favour the formation of CUN~+ and mixed species up to [Carnal+. The treatment of data led to several types of equilibrium constant (also for the binary copper(IIkpyridine complexes), spectral data and evidence that the formation of mixed complexes overcomes statistical effects.
neutralization of the acid with sodium hydroxide, followed by evaporation. The solid was recrystallized twice from water, and standardization of a 3 M solution was carried out by titration of the acid eluted from a cationic exchanger (Amberlite I-R-30). Pyridine was purified according to Vogel, lo distilled and kept in dark bottles. Aqueous solutions were prepared with water previously bubbled with nitrogen. Standardization was performed potentiometrically with hydrochloric acid. Sodium azide was standardized by boiling an aliquot with excess of 0.1 N H2S04 and back-titrating the excess with sodium hydroxide. All equilibrium studies (potentiometric or spectrophotometric) were performed at a controlled temperature, 25.0 f O.l”C. A digital potentiometer, Orion Research 801 A, was used for the pH measurements with a glass electrode combined with a reference electrode of Ag/AgCl/NaCl(sat). In the equilibrium studies the EXPERIMENTAL glass electrode was calibrated with 0.01000 M CP or AR chemicals were normally used. HC104 in 0.99 M NaClO, (conditional pH = Copper(I1) methanesulphonate was prepared by 2.000) as described in previous studies,3*” in order reaction of 2 M methanesulphonic acid with excess to measure the hydrogen ion concentration rather of copper(I1) carbonate under constant stirring for than its activity. A precision of 0.002 pH units 24 h, at room temperature. After filtering, strong was attained. acid was added in millimolar quantities to avoid The spectrophotometric equilibrium studies were hydrolysis. Standardization was carried out electrocarried out with an initial solution of pH 5.5-6, 10 gravimetrically. mM Cc,11with a C,, containing the desired fi level Sodium methanesulphonate was prepared by (ionic strength 1.0 M). A sodium azide concentration, usually lower than 1 mM, was maintained so that the transmittance obtained was never lower * Author to whom correspondence should be addressed. than 15% in the 370400 nm range. 3.00 cm3 of this
This paper is an extension of the systematic studies carried out in this laboratory for the copper(I1 j azide reaction’-’ from an analytical or inorganic viewpoint. The affinity of azide ion for copper(I1) complexed with pyridine, to form monoazide species is presented and possible analytical application discussed. All equilibrium studies were performed at 1.0 M ionic strength, held constant with sodium methanesulphonate. We also tested a new experimental condition for studying the mixed-ligand complex equilibrium using methods of calculation which made the data treatment unusually simple and accurate. Experimental values of equilibrium constants of mixed-ligand complexes are important for the development of coordination chemistry in a number of ways. In fact, there is an up-to-date trend in equilibrium studies, as has been pointed out by specialists such as Beck’ and Marcus and Eliezer.g
1257
E. A. NEVES and M. P. PETERS
1258
solution were placed in a quartz cuvette (1 cm light path) which was modified to hold a few more cm3 of added reagent. Measurements at eight different wavelengths were carried out vs the blank with the background electrolyte. The initial solution was diluted with 0.5 cm3 volumes of a blank with the same free pyridine concentration and measured. The copper(I1) and azide concentrations in this solution were decreased but the Cc,t&ratio as well as the A value were maintained. The pH was slightly changed but could be followed by using the glass electrode in a parallel experiment. The transmittance of the initial solution increased with dilution to a maximum value of 70%. This procedure has proved to be very rapid and accurate. Other instruments used were a Zeiss PM QII spectrophotometer, and the treatment of data required the use of programmable calculators (e.g. HP 97 or HP 41 CV from Hewlett Packard).
RESULTS
added to a copper(I1) concentration, C,,II, complexed with pyridine at a particular fi condition, in order to form a mixture of monoazide complexes such as CuN3+, C~(py)N3+, C~(py),N3+, etc. AS Cc,11is relatively high, the complexation with azide has no significant effect on its fi value. This K,, constant can be spectrophotometrically determined as for the CuN,+ species5*13with excess of Cu*+, with the only difference being that now copper(I1) is complexed with pyridine at a desired A level. However, this constant changes with A because the per cent distribution of the copper(I1) species is altered and each species is expected to have a different affinity for the azide ion. The free pyridine concentration used to set the copper(I1) at a fi level can be calculated iteratively by eq. (1) if the pi0 formation constants are known. The following equation can be developed with mass balance and equilibrium constants for the copper(II)/pyridine complexes : C cu” = [Cu(py)fi”] = [cu’+]+[cu(py)*+]
AND DISCUSSION
+[cu(py)22+]+[cu(py)32+]+[cu(py)42+]~
An outline of the equilibrium studies A number of spectrophotometric studies of complex formation have been shown in the literature as performed at conditions which favour the monoligand species by maintaining an excess of metal cations over the total ligand concentration. It is expected that if an excess of copper( complexed with pyridine, is in equilibrium with a lower azide concentration, a mixture of monoazide mixed complexes, Cu(pyLN3+, is formed. The composition of the excess copper(I1) is controlled by the free ligand, pyridine, and is expressed by its fi values (Bjerrum’s function), average ligand number : ~ = Bn3[PYl+28*0~Y12+3830~Y13f4B40[PY14 1 +B101pYl+~~~+B40EpY14
Each complex concentration is related to /Ilo [eq. (2)]. For the monoazide complexes in equilibrium with this Cc,11the following mass balance is applied : [Cu(py),N,+l
=
[CuN3+1
+W~Y)N~+I+...+[C~(PY)~N~+I.
.”
(5)
From eq. (3) the total concentration of the monoazido complexes is : [Cu(py)fiN3+1= CC~IIIN~-IK~,.
(6)
Each monoazido complex concentration is related to an addition constant referred to a copper(I1) cation with i pyridine ligands :
;I) where ,Bj, is the well known overall formation constant8y1*with i pyridine ligands and no azide
(4)
[Cu(PYhN3 +1 K(+N3)il
=
[cu(py),*+][N,-]
(7)
and to the equilibrium BiO=
[CutPY)i’+l
(2)
[cu2+l[py]i’
The following conditional place for a &+I >>C,, - : Cu(py),*+ +N,-
=
takes
equilibrium Cu(py),N3
+
Cu(py)i*+ +N3- _
Cu(py)iN3+.
Of course, if i = 0, K( + N3)oi is the conventional formation constant of CuN3+. The combination of eqs (5), (6) and (7) leads to
.
A conditional equilibrium constant is defined at each A value, ECu(py),N3 ‘3 [Cu(py)fiN3 + 1 &’ = [Cu(py),*+]N3- = Cc”11m3-]
(3)
and represent the average tendency of azide to be
IN3-KIGu”
= W(+N3-)oKu*+]
+K(+N3-),,[Cu(py)i*+l+...+K(+N3-)41 x [Cu(PY)4*+1W3-1.
(8)
By eliminating [N3-] and considering that the followina function can be taken for the conner(II)/ _~~ ~~~_ 11 . ,I
Mixed-ligand complexes of copper
1259
with pyridine
pyridine complexes,
[CU2f]
-----=a
c CU”
0
(9)
and [cu(PY)i2+l = a, ” c C”” b
eq. (8) can be modified to %i = a&(+N3-)ol +a,K(+N~-),,+...+a4K(+N3-)4,.
(11)
This is the final equation, and is surprisingly simple for dealing with mixed-ligand complexes. Simultaneous equations can be used to solve the K( -I-N3-)iI constants from a, and the experimental K, , data. These a, data are calculated with the free ligand which places copper(I1) as the derived A, and the /Ii0 constants : CW
=
1+B~0iP~l+~~~+840bpYl~ ai
Determination
of fiio
=
BioaOhvli.
(12)
(13)
constants
The overall formation constants in the copper(H)/ pyridine system, /Ilo, has been studied”‘* and the majority of the authors agree with the existence of four stepwise species. We obtained the formation constants at 1.0 M ionic strength, held with sodium methanesulphonate. The main reason for using this medium is the low tendency of this anion to precipitate with the Cu(py)i2+ cations. Perchlorate precipitates for some copper(H) concentrations used in the spectrophotometric study. As methanesulphonic acid is recognized as a strong one, its salts can eventually be used to adjust the ionic strength in equilibrium studies, preferably to nitrate. Experimental ri data vs the free ligand (formation curve) presented in Fig. 1 were taken from an indirect method based on the pH change in several py/Hpy+ buffers by addition of Cu2+. The experimental procedure, calculation and advantages of the method are described in detail by Neves et al. ’ ’ The Leden function, F,(X), was obtained and treated by the classical graphical method8*i2 and by the modern process of weighted siniuhaneous equations.lg The log pi0 data are given in Table 1 along with literature values for comparison. Good agreement with literature data is found at 1.0 M ionic strength and supports the use of methanesulphonate ion as the background electrolyte.
I 5.0
I 10.0
I
IS.0 [PYI x 6
I
I
20.0
25.0
Fig. 1. Formation curve of the copper(II)/pyridine complexes in 1.0 M ionic strength held constant with NaO,S-CH, at 25°C.
Spectrophotometric study The monoazido species was formerly studied under conditions of low and high ionic strength. 3*1 3 The McConnel and Davidson2’ spectrophotometric method was applied at constant pH by Saini and Ostacoii’ 3 in order to correct for HN3 formation. As this is a cumbersome process, the introduction of a correction factor, F = 1 +[H+]/ Ki, with the ionization constant (Ki) of the hydrazoic acid, directly into the equation allows us to work at variable pH, 3
G,Gl-
+&I+_ e:F E,b,
AF L
&
1 Cc,11+ CIVF ’ 8,
I
L
(14)
I
v
X Y'
where A is the absorbance and .a1the molar absorptivity of the complex (or an average value g,, for the mixture of monoazide mixed complexes). The term including E: is normally neglected, as a virtually linear relationship is maintained between “y” and “x”. However we found it convenient to find the true “y” value iteratively, and more accurate final values were obtained. The final /I, is about 10% higher than the initial value. The Cc,&,,ratio was about ten (see Experimental) in the working solution containing pyridine. In order to maintain the desired Zlevel the following total pyridine concentration is required : C,, = Cc,IIfi+ IpYl
+ + [HPY
I.
(15)
At pH 6-6.5, [Hpy+] is a small fraction of [py] as can be inferred from the pK = 5.45 (Table 1) and C&II = 10 mM. The ionization constant of hydrazoic acid,
E. A. NEVES and M. P. PETERS
1260
Table 1. Literature values for pKof Hpy+ and overall formation constants in the copper(H)/ pyridine system at 25°C in aqueous medium Medium, I; strength, M KN03, 0.50 KN09, 1.0 NaClO,, 1.O NaClO 4r 1.O PYHNO~,0.50 NaCH,S03, 1.O
PK HPY+
1
2
log Bi 3
4
5
Reference
5.45 5.5 5.75 5.22 5.42
2.52 2.59 2.46 2.86 2.41 2.551
4.38 4.64 4.41 4.78 4.29 4.474
5.69 5.93 5.68 5.45 5.687
6.54 6.54 6.52 6.03 6.592
7.0 -
14 15 16 17 18 n
0Present paper.
measured as described in ref. 3,is (3.94+0.04) x lop5 M- ’ taken at 1.0 M ionic strength held constant with sodium methanesulphonate. The Kfi, constants were obtained for Alevels from 0 to 3.5 using this spectrophotometric method, with iterative correction of the “y” term in eq. (14). Measurements were taken at eight wavelengths in the region around the maximum (370-400 nm).The standard deviation of each average result is f 1.7%. The graphical interpolations of the constants at small increments are presented in Table 2 and these were used in simultaneous equations (see next section). The constant in the absence of pyridine is the conventional /3,, but has other notations when mixed-ligand equilibria are considered :
[CuN3+1
= [Cu2+1[N,_1 = 111.5M-‘.
(16)
This value agrees, within experimental error, with that found by Maggio ef aL2’ 112 M- ‘, at the same ionic strength (held constant with sodium perchlorate). The spectra of the CuN3+ species in the UV region3 are similar to those obtained for the mixed complexes with pyridine, as the round maxima of charge-transfer are always observed. Although the K,-,constants decrease for 3 > 1, the z?, (average molar absorptivity) increases markedly with the level of A, with a discrete shift of A,,,,, to higher wavelengths. This means that the power of oscillation (N,- + Cu”) is increased under mixed-ligand conditions. From an analytical viewpoint the spectral shift to higher wavelengths favours the sensitivity of a qualitative test for the azide on the basis of mixed species formation. However, a compromise exists between the stability of the mixed complexes, its E, and the masking effect of the deeper blue colour of
the pyridine mixed complexes, when the sensitivity of the human sight is considered. In fact, the best conditions for a qualitative test of tide is found at ti = 2 for copper(I1) complexed with pyridine.22 In Table 2. K,, vs ii taken by interpolation from experimental data, and the corresponding [py] calculated from eq. (1) A
K,,
[PYIx 103
0.500 0.575 0.675 0.750 0.825 0.900 1.ooo 1.100 1.200 1.300 1.400 1.500 1.600 1.700 1.800 1.900 2.000 2.100 2.200 2.300 2.400 2.500 2.600 2.700 2.800 2.900 3.000 3.100 3.200 3.300 3.400 3.500
127.5 129.0 130.8 131.9 133.0 133.8 134.5 135.0 135.0 134.5 134.0 133.0 131.5 129.7 127.4 124.8 121.5 118.0 113.5 109.0 103.7 98.0 92.5 87.0 81.5 76.0 70.0 65.0 59.2 54.0 48.5 43.5
1.878 2.266 2.843 3.326 3.858 4.444 5.325 6.325 7.475 8.795 10.30 12.05 14.04 16.31 18.91 21.87 25.23 29.01 33.30 38.13 43.59 49.77 56.80 64.84 74.12 85.00 97.80 113.3 132.3 156.4 188.2 232.0
-
Mixed-ligand complexes of copper
with pyridine
1261
Table 3. Simultaneous equations calculated using the data of Table 2, /IO,and eqs (9), (10) and (17) 5.4061=2.7740x lo-* K(+N3-),,+8.9858x 1O-3K(+N,-)*,+6.4398x 1O-4K(+N3-)9,+2.5008x 1O-5K(+N3-)4, 7.3666=2.3283x lo-* K(+N,-),,+2.5257x 10-2K(+N3-)2,+6.C048x 1O-3K(+N3-)3,+7.7485x 10-4K(+N3-)4, 7.9089= 1.0334x lo-‘K(+N,-),,+3.3586x lo-* K(+N3&+2.3381 x 1O-4K(+NI-)9,+8.7251 x 1O-5K(+N3-)4, 7.9915= 2.6868x lo-’ K(+N,-),,+2.4666 x lo-* K(+N3-)*,+4.9948 x lo-* K(+N3-)3,+5.6737 x lo-* K(+N3-)4, SolutionK(+N,-),,
= 1.486x lo-* K(+N3-)*, = 1.375x lo2 K(+N3-)3, =7.06x 10 K(+N3-)4, = 1.19x 10
a quantitative spectrophotometric procedure higher ff levels can be maintained for higher sensitivity in the UV range at 380nm.
Table 3 with the solution for the K( +N3-)il constants. The obtained equilibrium constants are in agreement with interpolated K,-1 data within f0.7%.
Calculations of equilibrium constants from K,=,, As the K(+N,-)o, constant is known, 112 M-‘, it can be introduced into eq. (11) to decrease the number of unknowns in each simultaneous equation. A weighting factor l/&i can be introduced in order to ensure that all dependent terms are of the same magnitude. This is justifiable on the basis that the same relative uncertainty can be attributed to the experimental K,, constants and the intermediate data obtained by interpolation (Table 2). The free concentrations of pyridine at defined ri levels corresponding to the interpolated constants, calculated iteratively from eq. (l), are also included in Table 2. These Lpy]data were then used in eqs (12) and (13) to obtain the fraction of all species containing pyridine in order to prepare the simultaneous equations. In the treatment of data two possibilities were considered : whether the Cu(py),N3+ species exists or not, defining a system with four or three unknowns. The best solution is consistent with the contribution of the Cu(py),N3+ complex. Thus, a 4 x 4 matrix was built with four simultaneous equations with the available & 1 and Clidata :
Final comments on the results Figure 2 shows that the formation of Cu(py)N3+ is particularly favoured and that a marked decrease in affinity for azide occurs when more than two pyridine ligands fill coordination sites on Cu2+. Conversely, the average molar absorptivities increase in the reverse direction as already stated. Interesting relationships can also be seen in Fig. 3, where other equilibrium situations are considered, indicating the spontaneous processes. The backbonding copper(II)/pyridine favours the
= K(+N3-)11~$ cKnl -ao~~(+N3-)ol Kil nl +**.+K(+N,-)4,~$
nl
(17)
The four simultaneous equations are shown in
Fig. 2. Change of K( +N3-)i, constants vs the number of pyridine molecules coordinated to copper(I1).
Table 4. Stabilization constants X;, eq. (19), for some monoazido complexes Equilibrium 1/2 cuby)22+ 2/3 Cu(Py)2*+ 3/4 ‘WPY)~*+
+ l/2 + +
Cu(N,),=Cu@y)~,)+
l/3 CuPJ3)3- *CU@y)*(N3)+ l/4 CW3)4*--‘Cu(py),(N,)+
log J&f I)
log LYi I)
log &XI1)
0.445 0.809
0.301 0.477
0.144 0.333
0.859
0.602
0.257
E. A. NEVES and M. P. PETERS
1262
6.59
CuN;
L
2.67 \
Cu@y)N;
1.89
-CU(PYPJi
\
0.92
tCu(PYkd-
0.13
curwL& c I
5.63
Fig. 3. Spontaneous
processes of complex formation in copper(I1) systems, on the basis of positive values of log K.
azide bond more than predicted by statistical effects, and is considered as follows. When two complexes of the same cation MA,,, and MB, exchange ligands according to
(i/n) MA, + (j/n) MB n_
MAiBj
to form one mole Of MAiBj, anequilibrium
KM,,,
constant
1s
KM(ij)
=
With regard to such equilibrium, Marcus and Eliezer’ have proposed a stabilization constant K, log fL = log
KM(ij)
-log
$7
(19)
where &a, =
$. ..
A positive value for log K, indicates a true stabilization of the mixed-ligand complex, higher than predicted by statistica factors. The combination of present equilibrium data with that from the copper(II)/azide system,’ although taken at an ionic strength of 2.0 M, leads to the conclusion that there is a true stabilization of the monoazido mixed species, as shown in Table 4. Acknowledgements-The authors are greatly indebted to CNPq, FAPESP, FINEP and CAPES agencies (Brazil) for support.
REFERENCES 1. E. F. A. Neves and P. Senise, Anal. Chim. Acta 1969, 48, 177.
2. E. F. A. Neves, J. Znorg. Nucl. Chem. 1971,33,35 1. 3. P. Senise and E. F. A. Neves, J. Znorg. Nucl. Chem. 1971,33, 351. 4. P. Senise and E. F. A. Neves, J. Znorg. Nucl. Chem. 1972,34, 1923. 5. E. F. A. Neves and P. Senise, J. Znorg. Nucl. Chem. 1972,34,1915. 6. E. F. A. Neves, E. Oliveira and L. Sant’Agostino, Anal. Chim. Acta 1976,87,243. 7. E. A. Neves, E. Oliveira and Z. Santos, Talanta 1980, 21, 609. 8. M. T. Beck, Chemistry of Complex Equilibria. Van Nostrand, London (1969). 9. Y. Marcus and I. Eliezer, Coord. Chem. Rev. 1969, 4, 273. 10. A. I. Vogel, Practical Organic Chemistry, 2nd edn, p. 173. Longmans, London (1951). 11. E. A. Neves, R. Tokoro and M. E. V. Suarez, J. Chem. Res. (M), 1979,440l; (S), 1979,11,376. 12. K. B. Yatsimirskii and V. P. Vasil’ev, Instability Constant of Complex Compounds. Pergamon Press, Oxford (1960). 13. G. Saini and G. Ostacoli, J. Znorg. Nucl. Chem. 1958, 8,346. 14. R. J. Bruehlman and F. H. Verhoek, J. Am. Chem. Sot. 1948,70, 1401. 15. D. L. Leussig and R. C. Hansen, J. Am. Chem. Sot. 1957,79,4270. 16. G. Atkinson and J. E. Bauman Jr, Znorg. Chem. 1963, 2, 64. 17. V. Mihailova and M. Bonnet, Bull. Sot. Chim. Fr. 1969,12,4258. 18. J. Bjerrum, Acta Chem. Stand. 1964, 18,843. 19. E. A. Neves, N. Milcken and D. W. France, J. Znorg. Chem. 1981,43,2081. 20. H. McConnel and N. Davidson, J. Am. Chem. Sot. 1950,72,3164. 21. F. Maggio, V. Roman0 and L. Pellet-no, Ann. Chim. 1967,57, 191. 22. E. A. News and M. Peters, unpublished results.
0277-5387/90 53.00+.00 Pergamon Press plc
1990
MIXED-LIGAND COMPLEXES OF COPPER WITH PYRIDINE EDUARDO
ALMEIDA
NEWS*
and MORENA
PINTO PETERS
Instituto de Quimica da Universidade de SBo Paulo, Cx. Post. 20780, SBo Paulo, Brazil (Received 21 November 1988 ; accepted 15 December 1989)
Abstract-The copper(IIkpyridineazide system was studied at 1.0 M ionic strength (sodium methanesulphonate) and 25°C under conditions which favour the formation of CUN~+ and mixed species up to [Carnal+. The treatment of data led to several types of equilibrium constant (also for the binary copper(IIkpyridine complexes), spectral data and evidence that the formation of mixed complexes overcomes statistical effects.
neutralization of the acid with sodium hydroxide, followed by evaporation. The solid was recrystallized twice from water, and standardization of a 3 M solution was carried out by titration of the acid eluted from a cationic exchanger (Amberlite I-R-30). Pyridine was purified according to Vogel, lo distilled and kept in dark bottles. Aqueous solutions were prepared with water previously bubbled with nitrogen. Standardization was performed potentiometrically with hydrochloric acid. Sodium azide was standardized by boiling an aliquot with excess of 0.1 N H2S04 and back-titrating the excess with sodium hydroxide. All equilibrium studies (potentiometric or spectrophotometric) were performed at a controlled temperature, 25.0 f O.l”C. A digital potentiometer, Orion Research 801 A, was used for the pH measurements with a glass electrode combined with a reference electrode of Ag/AgCl/NaCl(sat). In the equilibrium studies the EXPERIMENTAL glass electrode was calibrated with 0.01000 M CP or AR chemicals were normally used. HC104 in 0.99 M NaClO, (conditional pH = Copper(I1) methanesulphonate was prepared by 2.000) as described in previous studies,3*” in order reaction of 2 M methanesulphonic acid with excess to measure the hydrogen ion concentration rather of copper(I1) carbonate under constant stirring for than its activity. A precision of 0.002 pH units 24 h, at room temperature. After filtering, strong was attained. acid was added in millimolar quantities to avoid The spectrophotometric equilibrium studies were hydrolysis. Standardization was carried out electrocarried out with an initial solution of pH 5.5-6, 10 gravimetrically. mM Cc,11with a C,, containing the desired fi level Sodium methanesulphonate was prepared by (ionic strength 1.0 M). A sodium azide concentration, usually lower than 1 mM, was maintained so that the transmittance obtained was never lower * Author to whom correspondence should be addressed. than 15% in the 370400 nm range. 3.00 cm3 of this
This paper is an extension of the systematic studies carried out in this laboratory for the copper(I1 j azide reaction’-’ from an analytical or inorganic viewpoint. The affinity of azide ion for copper(I1) complexed with pyridine, to form monoazide species is presented and possible analytical application discussed. All equilibrium studies were performed at 1.0 M ionic strength, held constant with sodium methanesulphonate. We also tested a new experimental condition for studying the mixed-ligand complex equilibrium using methods of calculation which made the data treatment unusually simple and accurate. Experimental values of equilibrium constants of mixed-ligand complexes are important for the development of coordination chemistry in a number of ways. In fact, there is an up-to-date trend in equilibrium studies, as has been pointed out by specialists such as Beck’ and Marcus and Eliezer.g
1257
E. A. NEVES and M. P. PETERS
1258
solution were placed in a quartz cuvette (1 cm light path) which was modified to hold a few more cm3 of added reagent. Measurements at eight different wavelengths were carried out vs the blank with the background electrolyte. The initial solution was diluted with 0.5 cm3 volumes of a blank with the same free pyridine concentration and measured. The copper(I1) and azide concentrations in this solution were decreased but the Cc,t&ratio as well as the A value were maintained. The pH was slightly changed but could be followed by using the glass electrode in a parallel experiment. The transmittance of the initial solution increased with dilution to a maximum value of 70%. This procedure has proved to be very rapid and accurate. Other instruments used were a Zeiss PM QII spectrophotometer, and the treatment of data required the use of programmable calculators (e.g. HP 97 or HP 41 CV from Hewlett Packard).
RESULTS
added to a copper(I1) concentration, C,,II, complexed with pyridine at a particular fi condition, in order to form a mixture of monoazide complexes such as CuN3+, C~(py)N3+, C~(py),N3+, etc. AS Cc,11is relatively high, the complexation with azide has no significant effect on its fi value. This K,, constant can be spectrophotometrically determined as for the CuN,+ species5*13with excess of Cu*+, with the only difference being that now copper(I1) is complexed with pyridine at a desired A level. However, this constant changes with A because the per cent distribution of the copper(I1) species is altered and each species is expected to have a different affinity for the azide ion. The free pyridine concentration used to set the copper(I1) at a fi level can be calculated iteratively by eq. (1) if the pi0 formation constants are known. The following equation can be developed with mass balance and equilibrium constants for the copper(II)/pyridine complexes : C cu” = [Cu(py)fi”] = [cu’+]+[cu(py)*+]
AND DISCUSSION
+[cu(py)22+]+[cu(py)32+]+[cu(py)42+]~
An outline of the equilibrium studies A number of spectrophotometric studies of complex formation have been shown in the literature as performed at conditions which favour the monoligand species by maintaining an excess of metal cations over the total ligand concentration. It is expected that if an excess of copper( complexed with pyridine, is in equilibrium with a lower azide concentration, a mixture of monoazide mixed complexes, Cu(pyLN3+, is formed. The composition of the excess copper(I1) is controlled by the free ligand, pyridine, and is expressed by its fi values (Bjerrum’s function), average ligand number : ~ = Bn3[PYl+28*0~Y12+3830~Y13f4B40[PY14 1 +B101pYl+~~~+B40EpY14
Each complex concentration is related to /Ilo [eq. (2)]. For the monoazide complexes in equilibrium with this Cc,11the following mass balance is applied : [Cu(py),N,+l
=
[CuN3+1
+W~Y)N~+I+...+[C~(PY)~N~+I.
.”
(5)
From eq. (3) the total concentration of the monoazido complexes is : [Cu(py)fiN3+1= CC~IIIN~-IK~,.
(6)
Each monoazido complex concentration is related to an addition constant referred to a copper(I1) cation with i pyridine ligands :
;I) where ,Bj, is the well known overall formation constant8y1*with i pyridine ligands and no azide
(4)
[Cu(PYhN3 +1 K(+N3)il
=
[cu(py),*+][N,-]
(7)
and to the equilibrium BiO=
[CutPY)i’+l
(2)
[cu2+l[py]i’
The following conditional place for a &+I >>C,, - : Cu(py),*+ +N,-
=
takes
equilibrium Cu(py),N3
+
Cu(py)i*+ +N3- _
Cu(py)iN3+.
Of course, if i = 0, K( + N3)oi is the conventional formation constant of CuN3+. The combination of eqs (5), (6) and (7) leads to
.
A conditional equilibrium constant is defined at each A value, ECu(py),N3 ‘3 [Cu(py)fiN3 + 1 &’ = [Cu(py),*+]N3- = Cc”11m3-]
(3)
and represent the average tendency of azide to be
IN3-KIGu”
= W(+N3-)oKu*+]
+K(+N3-),,[Cu(py)i*+l+...+K(+N3-)41 x [Cu(PY)4*+1W3-1.
(8)
By eliminating [N3-] and considering that the followina function can be taken for the conner(II)/ _~~ ~~~_ 11 . ,I
Mixed-ligand complexes of copper
1259
with pyridine
pyridine complexes,
[CU2f]
-----=a
c CU”
0
(9)
and [cu(PY)i2+l = a, ” c C”” b
eq. (8) can be modified to %i = a&(+N3-)ol +a,K(+N~-),,+...+a4K(+N3-)4,.
(11)
This is the final equation, and is surprisingly simple for dealing with mixed-ligand complexes. Simultaneous equations can be used to solve the K( -I-N3-)iI constants from a, and the experimental K, , data. These a, data are calculated with the free ligand which places copper(I1) as the derived A, and the /Ii0 constants : CW
=
1+B~0iP~l+~~~+840bpYl~ ai
Determination
of fiio
=
BioaOhvli.
(12)
(13)
constants
The overall formation constants in the copper(H)/ pyridine system, /Ilo, has been studied”‘* and the majority of the authors agree with the existence of four stepwise species. We obtained the formation constants at 1.0 M ionic strength, held with sodium methanesulphonate. The main reason for using this medium is the low tendency of this anion to precipitate with the Cu(py)i2+ cations. Perchlorate precipitates for some copper(H) concentrations used in the spectrophotometric study. As methanesulphonic acid is recognized as a strong one, its salts can eventually be used to adjust the ionic strength in equilibrium studies, preferably to nitrate. Experimental ri data vs the free ligand (formation curve) presented in Fig. 1 were taken from an indirect method based on the pH change in several py/Hpy+ buffers by addition of Cu2+. The experimental procedure, calculation and advantages of the method are described in detail by Neves et al. ’ ’ The Leden function, F,(X), was obtained and treated by the classical graphical method8*i2 and by the modern process of weighted siniuhaneous equations.lg The log pi0 data are given in Table 1 along with literature values for comparison. Good agreement with literature data is found at 1.0 M ionic strength and supports the use of methanesulphonate ion as the background electrolyte.
I 5.0
I 10.0
I
IS.0 [PYI x 6
I
I
20.0
25.0
Fig. 1. Formation curve of the copper(II)/pyridine complexes in 1.0 M ionic strength held constant with NaO,S-CH, at 25°C.
Spectrophotometric study The monoazido species was formerly studied under conditions of low and high ionic strength. 3*1 3 The McConnel and Davidson2’ spectrophotometric method was applied at constant pH by Saini and Ostacoii’ 3 in order to correct for HN3 formation. As this is a cumbersome process, the introduction of a correction factor, F = 1 +[H+]/ Ki, with the ionization constant (Ki) of the hydrazoic acid, directly into the equation allows us to work at variable pH, 3
G,Gl-
+&I+_ e:F E,b,
AF L
&
1 Cc,11+ CIVF ’ 8,
I
L
(14)
I
v
X Y'
where A is the absorbance and .a1the molar absorptivity of the complex (or an average value g,, for the mixture of monoazide mixed complexes). The term including E: is normally neglected, as a virtually linear relationship is maintained between “y” and “x”. However we found it convenient to find the true “y” value iteratively, and more accurate final values were obtained. The final /I, is about 10% higher than the initial value. The Cc,&,,ratio was about ten (see Experimental) in the working solution containing pyridine. In order to maintain the desired Zlevel the following total pyridine concentration is required : C,, = Cc,IIfi+ IpYl
+ + [HPY
I.
(15)
At pH 6-6.5, [Hpy+] is a small fraction of [py] as can be inferred from the pK = 5.45 (Table 1) and C&II = 10 mM. The ionization constant of hydrazoic acid,
E. A. NEVES and M. P. PETERS
1260
Table 1. Literature values for pKof Hpy+ and overall formation constants in the copper(H)/ pyridine system at 25°C in aqueous medium Medium, I; strength, M KN03, 0.50 KN09, 1.0 NaClO,, 1.O NaClO 4r 1.O PYHNO~,0.50 NaCH,S03, 1.O
PK HPY+
1
2
log Bi 3
4
5
Reference
5.45 5.5 5.75 5.22 5.42
2.52 2.59 2.46 2.86 2.41 2.551
4.38 4.64 4.41 4.78 4.29 4.474
5.69 5.93 5.68 5.45 5.687
6.54 6.54 6.52 6.03 6.592
7.0 -
14 15 16 17 18 n
0Present paper.
measured as described in ref. 3,is (3.94+0.04) x lop5 M- ’ taken at 1.0 M ionic strength held constant with sodium methanesulphonate. The Kfi, constants were obtained for Alevels from 0 to 3.5 using this spectrophotometric method, with iterative correction of the “y” term in eq. (14). Measurements were taken at eight wavelengths in the region around the maximum (370-400 nm).The standard deviation of each average result is f 1.7%. The graphical interpolations of the constants at small increments are presented in Table 2 and these were used in simultaneous equations (see next section). The constant in the absence of pyridine is the conventional /3,, but has other notations when mixed-ligand equilibria are considered :
[CuN3+1
= [Cu2+1[N,_1 = 111.5M-‘.
(16)
This value agrees, within experimental error, with that found by Maggio ef aL2’ 112 M- ‘, at the same ionic strength (held constant with sodium perchlorate). The spectra of the CuN3+ species in the UV region3 are similar to those obtained for the mixed complexes with pyridine, as the round maxima of charge-transfer are always observed. Although the K,-,constants decrease for 3 > 1, the z?, (average molar absorptivity) increases markedly with the level of A, with a discrete shift of A,,,,, to higher wavelengths. This means that the power of oscillation (N,- + Cu”) is increased under mixed-ligand conditions. From an analytical viewpoint the spectral shift to higher wavelengths favours the sensitivity of a qualitative test for the azide on the basis of mixed species formation. However, a compromise exists between the stability of the mixed complexes, its E, and the masking effect of the deeper blue colour of
the pyridine mixed complexes, when the sensitivity of the human sight is considered. In fact, the best conditions for a qualitative test of tide is found at ti = 2 for copper(I1) complexed with pyridine.22 In Table 2. K,, vs ii taken by interpolation from experimental data, and the corresponding [py] calculated from eq. (1) A
K,,
[PYIx 103
0.500 0.575 0.675 0.750 0.825 0.900 1.ooo 1.100 1.200 1.300 1.400 1.500 1.600 1.700 1.800 1.900 2.000 2.100 2.200 2.300 2.400 2.500 2.600 2.700 2.800 2.900 3.000 3.100 3.200 3.300 3.400 3.500
127.5 129.0 130.8 131.9 133.0 133.8 134.5 135.0 135.0 134.5 134.0 133.0 131.5 129.7 127.4 124.8 121.5 118.0 113.5 109.0 103.7 98.0 92.5 87.0 81.5 76.0 70.0 65.0 59.2 54.0 48.5 43.5
1.878 2.266 2.843 3.326 3.858 4.444 5.325 6.325 7.475 8.795 10.30 12.05 14.04 16.31 18.91 21.87 25.23 29.01 33.30 38.13 43.59 49.77 56.80 64.84 74.12 85.00 97.80 113.3 132.3 156.4 188.2 232.0
-
Mixed-ligand complexes of copper
with pyridine
1261
Table 3. Simultaneous equations calculated using the data of Table 2, /IO,and eqs (9), (10) and (17) 5.4061=2.7740x lo-* K(+N3-),,+8.9858x 1O-3K(+N,-)*,+6.4398x 1O-4K(+N3-)9,+2.5008x 1O-5K(+N3-)4, 7.3666=2.3283x lo-* K(+N,-),,+2.5257x 10-2K(+N3-)2,+6.C048x 1O-3K(+N3-)3,+7.7485x 10-4K(+N3-)4, 7.9089= 1.0334x lo-‘K(+N,-),,+3.3586x lo-* K(+N3&+2.3381 x 1O-4K(+NI-)9,+8.7251 x 1O-5K(+N3-)4, 7.9915= 2.6868x lo-’ K(+N,-),,+2.4666 x lo-* K(+N3-)*,+4.9948 x lo-* K(+N3-)3,+5.6737 x lo-* K(+N3-)4, SolutionK(+N,-),,
= 1.486x lo-* K(+N3-)*, = 1.375x lo2 K(+N3-)3, =7.06x 10 K(+N3-)4, = 1.19x 10
a quantitative spectrophotometric procedure higher ff levels can be maintained for higher sensitivity in the UV range at 380nm.
Table 3 with the solution for the K( +N3-)il constants. The obtained equilibrium constants are in agreement with interpolated K,-1 data within f0.7%.
Calculations of equilibrium constants from K,=,, As the K(+N,-)o, constant is known, 112 M-‘, it can be introduced into eq. (11) to decrease the number of unknowns in each simultaneous equation. A weighting factor l/&i can be introduced in order to ensure that all dependent terms are of the same magnitude. This is justifiable on the basis that the same relative uncertainty can be attributed to the experimental K,, constants and the intermediate data obtained by interpolation (Table 2). The free concentrations of pyridine at defined ri levels corresponding to the interpolated constants, calculated iteratively from eq. (l), are also included in Table 2. These Lpy]data were then used in eqs (12) and (13) to obtain the fraction of all species containing pyridine in order to prepare the simultaneous equations. In the treatment of data two possibilities were considered : whether the Cu(py),N3+ species exists or not, defining a system with four or three unknowns. The best solution is consistent with the contribution of the Cu(py),N3+ complex. Thus, a 4 x 4 matrix was built with four simultaneous equations with the available & 1 and Clidata :
Final comments on the results Figure 2 shows that the formation of Cu(py)N3+ is particularly favoured and that a marked decrease in affinity for azide occurs when more than two pyridine ligands fill coordination sites on Cu2+. Conversely, the average molar absorptivities increase in the reverse direction as already stated. Interesting relationships can also be seen in Fig. 3, where other equilibrium situations are considered, indicating the spontaneous processes. The backbonding copper(II)/pyridine favours the
= K(+N3-)11~$ cKnl -ao~~(+N3-)ol Kil nl +**.+K(+N,-)4,~$
nl
(17)
The four simultaneous equations are shown in
Fig. 2. Change of K( +N3-)i, constants vs the number of pyridine molecules coordinated to copper(I1).
Table 4. Stabilization constants X;, eq. (19), for some monoazido complexes Equilibrium 1/2 cuby)22+ 2/3 Cu(Py)2*+ 3/4 ‘WPY)~*+
+ l/2 + +
Cu(N,),=Cu@y)~,)+
l/3 CuPJ3)3- *CU@y)*(N3)+ l/4 CW3)4*--‘Cu(py),(N,)+
log J&f I)
log LYi I)
log &XI1)
0.445 0.809
0.301 0.477
0.144 0.333
0.859
0.602
0.257
E. A. NEVES and M. P. PETERS
1262
6.59
CuN;
L
2.67 \
Cu@y)N;
1.89
-CU(PYPJi
\
0.92
tCu(PYkd-
0.13
curwL& c I
5.63
Fig. 3. Spontaneous
processes of complex formation in copper(I1) systems, on the basis of positive values of log K.
azide bond more than predicted by statistical effects, and is considered as follows. When two complexes of the same cation MA,,, and MB, exchange ligands according to
(i/n) MA, + (j/n) MB n_
MAiBj
to form one mole Of MAiBj, anequilibrium
KM,,,
constant
1s
KM(ij)
=
With regard to such equilibrium, Marcus and Eliezer’ have proposed a stabilization constant K, log fL = log
KM(ij)
-log
$7
(19)
where &a, =
$. ..
A positive value for log K, indicates a true stabilization of the mixed-ligand complex, higher than predicted by statistica factors. The combination of present equilibrium data with that from the copper(II)/azide system,’ although taken at an ionic strength of 2.0 M, leads to the conclusion that there is a true stabilization of the monoazido mixed species, as shown in Table 4. Acknowledgements-The authors are greatly indebted to CNPq, FAPESP, FINEP and CAPES agencies (Brazil) for support.
REFERENCES 1. E. F. A. Neves and P. Senise, Anal. Chim. Acta 1969, 48, 177.
2. E. F. A. Neves, J. Znorg. Nucl. Chem. 1971,33,35 1. 3. P. Senise and E. F. A. Neves, J. Znorg. Nucl. Chem. 1971,33, 351. 4. P. Senise and E. F. A. Neves, J. Znorg. Nucl. Chem. 1972,34, 1923. 5. E. F. A. Neves and P. Senise, J. Znorg. Nucl. Chem. 1972,34,1915. 6. E. F. A. Neves, E. Oliveira and L. Sant’Agostino, Anal. Chim. Acta 1976,87,243. 7. E. A. Neves, E. Oliveira and Z. Santos, Talanta 1980, 21, 609. 8. M. T. Beck, Chemistry of Complex Equilibria. Van Nostrand, London (1969). 9. Y. Marcus and I. Eliezer, Coord. Chem. Rev. 1969, 4, 273. 10. A. I. Vogel, Practical Organic Chemistry, 2nd edn, p. 173. Longmans, London (1951). 11. E. A. Neves, R. Tokoro and M. E. V. Suarez, J. Chem. Res. (M), 1979,440l; (S), 1979,11,376. 12. K. B. Yatsimirskii and V. P. Vasil’ev, Instability Constant of Complex Compounds. Pergamon Press, Oxford (1960). 13. G. Saini and G. Ostacoli, J. Znorg. Nucl. Chem. 1958, 8,346. 14. R. J. Bruehlman and F. H. Verhoek, J. Am. Chem. Sot. 1948,70, 1401. 15. D. L. Leussig and R. C. Hansen, J. Am. Chem. Sot. 1957,79,4270. 16. G. Atkinson and J. E. Bauman Jr, Znorg. Chem. 1963, 2, 64. 17. V. Mihailova and M. Bonnet, Bull. Sot. Chim. Fr. 1969,12,4258. 18. J. Bjerrum, Acta Chem. Stand. 1964, 18,843. 19. E. A. Neves, N. Milcken and D. W. France, J. Znorg. Chem. 1981,43,2081. 20. H. McConnel and N. Davidson, J. Am. Chem. Sot. 1950,72,3164. 21. F. Maggio, V. Roman0 and L. Pellet-no, Ann. Chim. 1967,57, 191. 22. E. A. News and M. Peters, unpublished results.