COPPERNUCLEAR MAGNETIC RESONANCE STUDY OF CYANOCUPRATE (I) IONS species to the high-frequency species. This causes the cluster size to decrease with temperature. The production of more surface molecules at the expense of
4369
interior ones with temperature appears to proceed through an intermediate species which has a constant spectral contribution in the temperature range 20-80".
Copper Nuclear Magnetic Resonance Study of Cyanocuprate(1) Ions in Solution. Formation of Polynuclear Species
and of Mixed Complexes by Takeo Yamamoto,* Hiroki Haraguchi,l and Shizuo Fujiwara Department of Chemistry, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan
(Received February 66,1970)
Aqueous solutions of copper(1) cyanide with an excess of alkali cyanide were studied by means of copper43 and -65 nmr. Copper nmr signals were detected when the cyanide to copper ratio was larger than 4. The nmr line width decreased with an increase in the above ratio. It was concluded that (1) the dominant species has ~ -a, distorted tetraCu(CN)2- retains the tetrahedral symmetry in solution, (2) Cu(CN)a2-,or C U ~ ( C N ) ~ hedral structure and therefore makes little contribution to the nmr line width even though it is the largest in amount among the minor species, and (3) polynuclear species such as Cuz(CN)sa- largely determine the nmr line width. The formation of mixed complexes of Cu(1) with the cyanide ligand and several other ligands was also found to increase the nmr line width. It was indicated that the values of K m = [CU(CN)~L(~+~"] [CN-]/ [CU(CN)I~--][L~-] are in the order (NH2)tCS > SCN- > I- 2 NH3 > Br- > C1- 2 (NHZ)zCO, where K m for SCN- was estimated to be about 1 X loea.
Introduction Copper-63 and -65 nmr of solid Cu(1) compounds has been reported in copper(1) halide^^,^ and in potassium tetracyan~cuprate(I).~However, there have been few nmr studies on aqueous solutions of copper(1) S a l k 6 I n the present work, the copper nmr is studied in aqueous solutions of copper(1) cyanide in the presence of an excess of alkali cyanide. The line width, which strongly depends on the concentrations of cyanide and of copper(1) ions, is discussed in terms of the composition and the ligand exchange of the complexes in the solution. The line width is also observed to increase in the presence of other potential ligands such as thiourea, thiocyanide, iodide, ammonia, bromide, chloride, or urea. The effect can be explained in terms of the formation of weak mixed complexes Cu(CN)aL"-.
Experimental Section Commercial reagent grade chemicals were used throughout the experiment. The concentrations (in molar units) of copper and cyanide ions are denoted by m and 1, respectively. The estimated error in the ratio Z/m was j=O.O5. This value will be used in Figure 3. Nmr signals were measured by using two bridge-type
spectrometers operating at 6.1403 MHz and 15.086 MHz, respectively. The narrower signals were measured by the side band method at either 500- or 1000Hz field modulation frequency. The broader signals were recorded as the first derivative lines with 35.5-Hz modulation. For weak signals, the modulation amplitude was varied by 5-db increments and the peak-topeak intensity and line width were measured for each modulation amplitude. The intensity us. modulation amplitude and the line width vs. amplitude curves were compared with the theoretical curves calculated for the Gaussian and the Lorentzian lines,6 and the line width extrapolated to zero modulation amplitude was obtained graphically. Most of the observed line shapes
* T o whom correspondence should be addressed. (1). Department of Agricultural Chemistry, Faculty of Agriculture, University of Tokyo, Yayoi, Tokyo 113, Japan. (2) (a) S. Domngang and J. Wucher, C. R. Acad. Sci. Paris, Ser. B, 268, 1608 (1969); (b)P. K. Burkert and H. P. Fritz, 2.Naturforsch., B23, 1312 (1968). (3) P. K. Burkert and H. P. Fritz, ibid., B24, 253 (1969). (4) G. Becker, 2.Phys., 130, 415 (1951). (5) H. M. McConnell and H. E. Weaver, J . Chem. Phys., 25, 307 (1956). (6) G. V. H. Wilson, J . A p p l . Phys., 34, 3276 (1963).
The Journal of Physical Chemistry, Vol. 74, N o . 66,1970
4370 are Lorentzian within experimental error, though some of the narrower lines have shapes between Gaussian and the Lorentzian. Therefore, all the experimental lines were assumed to have the Lorentzian line shape in the following discussion. The error due to the discrepancy was included in the estimation of errors shown in Figures 1,2, and 3. The peak-to-peak separation of the first derivative lines will be referred to as the line width in the following discussion. The magnitude of the inhomogeneity of the magnetic field was estimated from the line width of 2aNasignals and was subtracted from the observed line width. The inverse of the transverse relaxation time l/T2 is given by multiplying the line width obtained above with 1/3 T . Powdered copper(1) chloride was used as an external reference in the chemical shift measurements. The viscosities of sample solutions were measured with an Ostwald viscometer at 20". The copper nmr signal was too broad to be detected (line width 2 5000 Hz) with the spectrometers used in the present study, when either potassium tetracyanocuprate(1) or sodium tetracyanocuprate(1) was dissolved in water. As alkali cyanide was added to the solution, the signal became observable at a field (8.20 0.08) X lo9 ppm lower than that of powdered copper (I) chloride. For a quantitative study, three stock solutions were made dissolving 3, 4.5, and 6 mol each of potassium cyanide, 1.5 mol each of sodium cyanide, and 1.5 mol each of copper(1) cyanide in water and diluting to 1 1. Sample solutions with different I values and/or different m values from the above solutions were made by mixing aliquots of the stock solutions with each other and/or with water. Chemical shifts were identical for all the samples within experimental error. I n Figure 1, examples of the spectra are shown for samples with m = 1.5 M. When Z/m is equal to or smaller than 4, no signal is observed. At l / m = 4.1, a broad signal becomes observable. The line becomes considerably narrow at l/m = 4.2. The line width at l/m = 6 is about 85 HE. I n Figure 2, the inverse of the 63Cunmr relaxation time, l/Tz, obtained from the line width after applying the correction for the inhomogeneity of the field, is plotted against the ratio of the total concentration of cyanide to that of copper, l/m. It is seen there that l / T z increases rapidly as l/m decreases toward 4. The dependence of the line width on the observing radiofrequency was studied by measuring several samples both at 6.1403 MHz and at 15.086 MHz. When the correction for the inhomogeneity of the magnetic field was applied, the line widths (in frequency units) were identical at the two frequencies within experimental error. The line width was found to increase with increasing temperature for all the samples studied. For example,
T. YAMAMOTO, H. HARAOUCHI,AND S.FUJIWARA M=1.5M
din= 4
A 200Hz
Figure 1. Copper-63 nmr spectra of aqueous solutions of cyanocuprate(1) ions: m, the total concentration of copper(1); 1, the total concentration of cyanide.
*
The Journal of Physical Chemistry, Vol. 74, N o . 2.5, 1970
0
% c
8cn k!
'r
'r
Or'
8
0
0
4
5
6
elm Figure 2. The inverse of the relaxation time of copper-03 us. the ligand to metal ratio in cyanocuprate(1) solutions. The perpendicular bars are the estimated errors.
the line width for a sample with m = 1.5 M , l / m = 4.1, was 230 HE at 0" and 420 Hz at 20". The effect of the viscosity of the solution was studied by changing the ratio of the concentration of sodium cyanide t o that of potassium cyanide, keeping the total cyanide concentration constant. For m = 1 M, l/m = 4.2, f o r m = 1.5 M, l / m = 4.5, and for m = 2 M,
COPPERNUCLEAR MAGNETIC RESONANCE STUDY OF CYANOCUPRATE(~) IONS
l / m = 5.0, the line width increased linearly with the viscosity. In order to find the dependence of the relaxation time on the nuclide, the relative intensities of the first derivative lines, 1 6 6 and 163, were measured for 6sCuand 63Cuat the same frequency of 6.1403 MHz. The same experimental condition was maintained except for changing the magnitude of the static magnetic field. The ratio of the two intensities, 1 6 j / 1 6 3 , was measured as it is much more sensitive to the change in the relaxation time than the ratio of the line widths themselves. The observed ratio and the estimated error were 16j/163
= 0.71 =k 0.08
The effect of other potential ligands on the line width was measured by mixing equal aliquots of an aqueous solution of 2 M cuprous cyanide and 8 M potassium cyanide and the aqueous solution of one of the following substances of concentration 2L M : lithium chloride, sodium bromide, sodium iodide, sodium thiocyanide, ammonia, urea, and thiourea. The concentration of the resulting solution was assumed to be L M in the potential ligand, m = 1 M and l/m = 5 , neglecting the small change in the liquid volume. For all the potential ligands, the line width increased with L. The results will be discussed below.
Discussion Mechanism of the Broadening. The copper chemical shift observed here agrees with the shift reported for solid potassium tetracyanocuprate(I). 4 It is evident that the sharp absorption in samples with large values of Z/m is the signal from the dominant species, Cu(CN)43-. From the rapid increase in the line width as l/m decreases toward 4, it is also clear that the broadening is associated with chemical exchange of Cu(CN)d“ with other species whose presence in the solution increases as the amount of CN- decreases. When the j t h species has a quadrupole coupling constant e2Qqj/hJan asymmetry factor r3, and a correlation time of the rotational Brownian motion roj,the intrinsic relaxation time of the species, T2j,due to the quadrupole coupling is given by7 1/Tzj = 3.95(e2&qj/hl2[l
+ ( r j 2 / 3bo, )
(1)
provided that the correlation time is sufficiently short so that (e2&qj/h)raj -4. It is seen from Figure 3 that the values for m = 1.5, The Journul of Physical Chemistry, Vol. 74, Xo, 86, 1970
02 x =t/m
- 05
1.0
20
4
Figure 3. The reduced broadening us. the ligand to metal ratio in cyanocuprate(1) solutions. The perpendicular and the horizontal bars are the estimated errors. The solid straight lines are (1/Tz - l/TZa)/qbulk = 67.0 x m-2,sx-a.0.
(6)
where Kif is a concentration equilibrium constant
Kij =
01
1.5/2 and 1.5/4 M make approximately three parallel lines,
(1/TZ - 1/T2a)/l]bulk = 57. (m)robBd* where /Lobed
= -2.5
f
0.5;
Xobsd
=
-2.0
* 0.5
The minor species which largely determine the magnitude of the reduced broadening are likely to be polynuclear species with low j/i ratios such as C U ~ ( C N ) ~ ~ - . This ion would give pcalod = -2 and Xoalcd = -3. These values are not in quantitative agreement with the observed data. The disagreement may be explained in the following way. The contribution from Cu(CN)a2- relative to the one from Cuz(CN)8- becomes important at low m and high z values, whereas the effect of higher polymers such as C U ~ ( C N ) ~as~ well as the breakdown of the approximation used in deriving eq 6 becomes appreciable at low x values. The concentration of CU(CN)~-estimated from the equilibrium constants K4 and Ks is too small to give the observed broadening, if we assume reasonable values of T ~ ,and (e2&qj/h)for this species. According to eq 3 and 6, C U ( C N ) ~ and ~ - Cuz(CN)s4- 1, Xcaled = - 1 and pealcd = - 1, Xoaled = give pcalcd -2, respectively. The disagreements between these sets of values and the observed set of values are large enough to disqualify Cu(CN)2- and Cuz(CN)a4- as species which contribute significantly to the reduced broadening. This, in turn, means that a Cu(CN)2ion does not have a planar triangle coordination of the (11) J. Brigando, Bull. SOC.Chim. Fr., 503 (1957).
(12) Cu-C-N = 3.1& l a van der Waals radius of N = 1.5 (13) R. B.Roof, Jr., A. C. Larson, and D. T. Cromer, Acta Crystallogr., Sect. B , 24, 269 (1968).
COPPERNUCLEAR MAGNETIC RESONANCE STUDYOF CYANOCUPRATE(~) IONS --q-
4373
NaSCN Nal
,*.*."'"
2 4 Ligand Concn., M
d 7
Figure 4 . The reduced broadening us. the concentration of ionic potential ligands in a solution with l / m = 5, m = 1 M.
ligands as in the case of solid potassiumodicyanocuprate(1),l2 In fact, if we assume rl = 4.6 A, Kq = 3 X lo2, and (e2Qqj/h)[1 (qj2/3)]1/2= 60 MHz14 for the fictitious planar ion Cu(CN)s2- in solution, the contribution from this species to the reduced broadening at m = 1.5 M and x = 0.5 would be 6.4 X lo3 sec-l CP-', which is more than 50 times as large as the observed. value. The Effect of Mixed Complex Formation. The above conclusion is supported by the effect of other potential ligands on the line width. The reduced broadening (l/Tz - l/Tzs)/qbulk due to several potential ligands is plotted against the Iigand concentration in Figure 4 and Figure 5 . In all the experiments, the total copper and the cyanide concentrations were 1 M and 5 M , respectively. It is seen that for each ligand the reduced broadening is approximately proportional to the concentration of the ligand. This may be explained by assuming a formation of mixed complexes
/
-
+
+
+
Cu(CN)d3L"Cu(C1\')3L('+")CN- (7) where n = 0 or 1. Then, the gradient of a line in Figure 4 or Figure 5 is K m / ( T t m r b u l k ) where K,
[CU(CN)~L(~+")--] [CN-] [Cu(CN)2-] [L"-]
=
and Tz, is the intrinsic relaxation time of copper in the mixed complex species. The values of Krn/(Tzm?lbulk) obtained by a leastsquares fitting of the experimental data are shown in Table I, together with the reported values of
Pz
=
[CuL2'-2"] [Cu+][L"-]2
There is a strong correlation between the two values for the ligands studied. This suggests that the values of Km have the same order as the values of p2 and of Km/(TZmqbulk) namely (NH2)zCS > SCN-
> I- > NH3 > Br- > C1- 2 (NH2)2C0 N
0
2
4
LigandConcn.,
6
&I
Figure 5. The reduced broadening us. the concentration of nonionic potential ligands in a solution with E/m = 5, I = 1 M .
Table I : Relative Efficiency of Mixed Ligands in Broadening the Copper Nmr Line Width, Krn/(T2mVbulk) Ligand
Km/( Tzmtlbul k)
CN (NH2)zCS SCN NH3
log 82
16-20 78 I
282 75 71 69 37 20 19
IBrc1(NHz)aCO
12.11 8.85 8.72-11.2 5.92 4,60-5.52
Cooper and Plane15 have suggested that C u ( C N ) F has a planar triangle coordination, on the basis of the absence of distinct Raman lines corresponding to mixed complexes in solutions which contain C U ( C N ) ~ ~and one of several potential ligands, including thiocyanate, as the dominant species. However, the present result indicates that mixed complexes are formed, though in rather low concentrations, in the solutions. In fact, if we assume a reasonable value of TZmqbulk = 1X sec CPfor the thiocyanate ligand [Cu(CN)3SCN3-] [Cu(CN)a2-][SCN-]
=
K4.Km'v 0.2, or K ,
N
which may be too small for the observation of the mixed complex species by the Raman method. (14) G. L. McKown and J, D. Graybeal, J. Chem. Phys., 44, 610 (1966). (15) D. Cooper and R. A. Plane, Inorg. Chem., 5 , 16 (1966).
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